Copyright 2019, Patrick Larson

Catalysis in Metal-organic Frameworks Using Pendant and Linker Hydroxyl Sites

by

Patrick Larson, BS

A Dissertation

In

Inorganic Chemistry

Submitted to the Graduate Faculty

of Texas Tech University in

Partial Fulfillment of

the Requirements for

the Degree of

DOCTOR OF PHILOSOPHY

Approved

Anthony Cozzolino

Chair of Committee

Clemens Krempner

Michael Findlater

Mark Sheridan

Dean of the Graduate School

December 2019

Copyright 2019, Patrick Larson

Texas Tech University, Patrick Larson, December 2019

ii

ACKNOWLEDGEMENTS

The author would like to acknowledge Kendall Larson for graphic design

assistance and constant moral support. In addition, the author would like to

acknowledge collaborators Bo Zhao for SEM-EDAX analysis, the Hope-Weeks group

for TGA assistance, the Fatib-Khatib group for BET assistance, the Findlater group for

synthetic assistance and catalytic studies, and the Wylie group for SS MAS NMR

studies. The author would also like to acknowledge fellow group members Miranda

Andrews, Shiva Moaven, Babak Tahmouresilerd, and Jinchun Qiu as fellow graduate

students providing support. The author would also like to acknowledge the

undergraduate and high school students involved in the projects, Joseph Cheney,

Garrett Toyofuku, and Siddharth Srinivasan for their assistance in experimental work

across the variety of projects.


iii

TABLE OF CONTENTS

ACKNOWLEDGEMENTS .................................................................................. ii

ABSTRACT ......................................................................................................... vii

LIST OF TABLES ............................................................................................. viii

1. CATALYSIS IN METAL-ORGANIC FRAMEWORKS ............................. 1

1.1 Metal-organic frameworks .......................................................................... 1

1.1.1 MOF SBUs ......................................................................................... 2

1.1.2 MOF Pores ......................................................................................... 3

1.1.3 Isoreticular design of MOFs............................................................... 6 1.1.4 Stability of MOFs ............................................................................... 8

1.1.5 Defects in MOFs ................................................................................ 9

1.2 Current approaches to catalysis in MOFs ................................................. 10

1.2.1 PSM of linkers in MOFs .................................................................. 11

1.2.2 Post-synthetic exchange of motifs in MOFs .................................... 11 1.2.3 Coordinatively unsaturated sites in MOFs ....................................... 12 1.2.4 Grafting catalytic sites onto the SBU of MOFs ............................... 13

1.2.5 Encapsulation of a catalyst in MOFs ............................................... 14

1.3 Considerations for catalysis in MOFs ....................................................... 15

1.4 The approach of this work ........................................................................ 16

1.5 Reactions employed to probe catalyst activity .......................................... 17

1.5.1 Considerations for catalysts ............................................................. 18

1.6 Purpose, scope and overview of the dissertation ...................................... 19

2. SINGLE-SITE CATALYSIS IN MOFS ....................................................... 20

2.1 Selection of MOFs .................................................................................... 20

2.1.1 UiO-6X series................................................................................... 21 2.1.2 MIL-53 ............................................................................................. 21

2.2 MPV reductions ........................................................................................ 23

2.2.1 Al@MOFs ........................................................................................ 25

2.2.2 Range of metallation in Al@MOFs ................................................. 27

2.2.3 Stability of Al@MOFs upon metallation ......................................... 28 2.2.4 Vibrational spectroscopy as evidence of metallation ....................... 31 2.2.5 NMR studies of Al@MOFs ............................................................. 34 2.2.6 MPV reduction split test of Al@MOFs ........................................... 37 2.2.7 Reuse and recovery of Al@MOF catalysts ...................................... 38


iv

2.2.8 Substrate scope of Al@MOF catalysts ............................................ 40

2.2.9 Native MOF reactivity for MPV reduction ...................................... 42

2.2.10 Testing defect-free MOFs for MPV reductions ............................. 42

2.3 Knoevenagel condensations in UiO-66 and MIL-53 ................................ 43

2.3.1 Catalysis using Zn@MOFs .............................................................. 43 2.3.2 Vibrational spectroscopy: evidence of metallation ......................... 44 2.3.3 Native MOF reactivity for Knoevenagel condensation ................... 45

2.3.4 Split test Knoevenagel condensation using Zn@MOFs .................. 46

2.4 Experimental ............................................................................................. 46

2.4.1 General experimental methods ......................................................... 46 2.4.2 Synthesis of UiO-66HCl .................................................................... 49 2.4.3 Synthesis of UiO-66AcOH .................................................................. 51

2.4.4 Synthesis of UiO-67 ......................................................................... 52 2.4.5 Synthesis of MIL-53as ...................................................................... 53

2.4.6 Synthesis of MIL-53RT ..................................................................... 56 2.4.7 Synthesis of DUT-5 ......................................................................... 57

2.4.8 Synthesis of MOF-74 ....................................................................... 58 2.4.9 General metallation of MOFs using AlMe3 ..................................... 59 2.4.10 General metallation of MOFs using ZnEt2..................................... 59

2.4.11 Initial catalysis screening for MPV reduction ................................ 60 2.4.12 General MPV reduction experiment .............................................. 61

2.4.13 Catalyst screening for Knoevenagel condensation ........................ 61

2.5 Conclusions ............................................................................................... 62

2.6 Future work ............................................................................................... 62

3. TOWARDS SELF-ASSEMBLED MULTIDENTATE SITES (SAMS) .... 64

3.1 Bridging motifs in MOFs .......................................................................... 64

3.2 Knoevenagel condensations ...................................................................... 66

3.2.1 Vibrational evidence of metallation with Zn ................................... 66

3.2.2 Knoevenagel condensation split test ................................................ 67 3.2.3 Stability of the catalysts ................................................................... 68

3.3 MPV reductions ........................................................................................ 71

3.3.1 Range of metallation in Al@MOF-OH ............................................ 71 3.3.2 Vibrational evidence of metallation of Al@MOF-OH .................... 74

3.3.3 NMR studies of metallation of Al@MOF-OH ................................ 76 3.3.4 MPV reduction split test of Al@MOF-OH ...................................... 83 3.3.5 Stability of the Al@MOF-OH catalysts ........................................... 84 3.3.6 Comparison with single-site catalyst in Al@MOF .......................... 87

3.4 Experimental ............................................................................................. 89


v

3.4.1 Synthesis of 2-hydroxyterephthalic acid via Sandmeyer reaction ... 89

3.4.2 Synthesis of UiO-66-OH .................................................................. 90

3.4.3 Synthesis of MIL-53-OH ................................................................. 91 3.4.4 General metallation of MOFs using AlMe3 ..................................... 92 3.4.5 General metallation of MOFs using ZnEt2....................................... 92 3.4.6 Catalytic procedure for MPV reduction ........................................... 93 3.4.7 MPV reduction split test .................................................................. 93

3.4.8 Knoevenagel condensation catalysis ................................................ 93 3.4.9 Knoevenagel condensation split test ................................................ 94

3.5 Conclusions ............................................................................................... 94

3.6 Future work ............................................................................................... 94

4. TRIPODAL LIGAND COORDINATION CHEMISTRY ......................... 96

4.1 Tripodal amides ......................................................................................... 96

4.1.1 Vanadium trisamide complexes ....................................................... 96

4.2 Synthesis and characterization of tripodal amide complexes ................... 99

4.2.1 Tripodal amides ................................................................................ 99

4.2.2 Lithiation of tripodal trisamide compounds ..................................... 99 4.2.3 Towards a vanadium TMIM complex............................................ 102 4.2.4 Characterization of VTMIM·LiCl·3THF ....................................... 105

4.2.5 Towards a deuterium-labeled H3TMIM ......................................... 108

4.3 Experimental ........................................................................................... 108

4.3.1 Synthesis of tris(3-methylindol-2-yl)methane (H3TMIM)............. 109 4.3.2 Synthesis of d1-triethylorthoformate .............................................. 109

4.3.3 Synthesis of d1-tris(3-methylindol-2-yl)methane (TMIM)140 ........ 109 4.3.4 Lithiation of H3TMIM with LDA .................................................. 111

4.3.5 Metathesis of Li3TMIM·5THF with VCl3(THF)3 .......................... 111 4.3.6 Crystallographic data ..................................................................... 112

4.4 Conclusions ............................................................................................. 113

4.5 Future work ............................................................................................. 113

5. IRON(BIAN) COMPUTATIONAL STUDY .............................................. 115

5.1 Fe(BIAN) complexes .............................................................................. 115

5.2 DFT calculations ..................................................................................... 116

5.2.1 Geometry optimizations ................................................................. 117

5.2.2 Hammett correlation....................................................................... 119 5.2.3 Energy comparisons ....................................................................... 120

5.3 Experimental ........................................................................................... 122


vi

5.4 Conclusions ............................................................................................. 122

5.5 Future work ............................................................................................. 123

6. SUMMARIES AND CONCLUSIONS ........................................................ 124

REFERENCES .................................................................................................. 126


vii

ABSTRACT

Metal-organic frameworks (MOFs) were investigated as supports for single-

site catalysts. Al and Zn were covalently bound in MOFs for MPV reductions and

Knoevenagel condensations, respectively. Different binding motifs were considered,

including inherent hydroxyl sites in the MOFs, and phenolic sites in linkers in the

MOFs. The length of the linkers used in the MOFs and corresponding size of the pore

was varied in an attempt to determine the effect of pore size on reactivity. Metallation

of these MOFs gave catalysts that were shown to outperform the native MOF in each

case. The Al metallated MOFs were readily recycled with little loss in activity more.

The Zn metallated MOFs deactivated through leaching of the catalyst into solution.

Hypothesized self-assembled multidentate sites within the MOFs were also

investigated.

A tripodal trisamide ligand for early transition metal complexes was

synthesized. After deprotonation of the ligand, a vanadium complex was formed by a

salt metathesis reaction. This example, VTMIM⸱LiCl⸱5THF (where TMIM3- is

tris(methylindolyl)methane) is shown to be one of only a few examples of an

octahedral V(III) tris amide complex, and may prove useful for small molecule

activation. Computational studies using density functional theory were performed to

provide insight on the redox noninnocence of a series of FeBIAN complexes.


viii

LIST OF TABLES

1.1 Selected MOFs and their physical properties.............................................. 4

2.1 MPV Reduction of cyclohexanone in isopropyl alcohol (%

conversion) comparison of metallated vs. native MOF (80 °C) ................ 25

2.2 Ratio of aluminum to initial hydroxyl sites (Al:OH) in

Al@UiO-67 as determined by SEM-EDX ................................................ 28

2.4 Percent conversion (and TON) for MPV reduction of select

substrates at 80 °C over 1 hour ................................................................. 41

3.3 27Al NMR relative amounts of the tetrahedral and 5-coordinate

Al sites to the octahedral Al sites .............................................................. 77

5.1 Selected bond lengths and angles for FeBIAN-benzene

FeBIAN-toluene and FeBIAN-trifluorotoluene ...................................... 115

5.2 Isodesmic reaction of different arene complexes of FeBIAN ................. 118


ix

LIST OF ACRONYMS

ALD : Atomic layer deposition 13

BIAN : Bis(arylimino)‐acenaphthene 19

CUS : Coordinatively unsaturated site 12

DFT : Density functional theory 19

EXAFS : X-ray absorption fine structure 65

H2BDC : 1,4-benzenedicarboxylic acid 2

H2BPDC : 4,4ʹ-biphenyldicarboxylic acid 2

HSAB : Hard/soft acid/base 8

ICP : Inductively coupled plasma 27

MOF : Metal-organic framework 1

MPV : Meerwein-Ponndorf-Verley 17

PSM : Post-synthetic modification 10

PXRD : Powder X-ray diffraction 6

SBU : Secondary building unit 1

SEM-EDX : Scanning electron microscopy-energy-dispersive X-ray 27

SS MAS : Solid-state magic angle spinning 34

SS NMR : Solid state nuclear magnetic resonance 27

TGA : Thermogravimetric analysis 6

XANES : X-ray absorption near edge spectroscopy 65


x

LIST OF FIGURES

1.1 Depiction of a generic MOF, with the Lewis acidic secondary

building unit (SBU), the Lewis basic organic linker, the pore

volume, and pore aperture diameter shown. ............................................... 1

1.2 Two of the SBUs that are used in this dissertation, a) the Zr6

cluster present in the UiO series from two different perspectives

as well as b) the octahedral Al–O–Al SBU of MIL-53. .............................. 3

1.3 Some representative MOF pores, including a) the discrete pore

of UiO-66, b) the continuous pore of MIL-53, and c) the

discrete pore of MOF-5. .............................................................................. 3

1.4 Examples of isoreticular synthesis of MOFs including a)

original framework, b) different linker, c) different metal in

SBU, and d) longer linker leading to larger MOF. ..................................... 7

1.5 Isoreticular MOF-5 containing up to 8 linkers in varying ratios

while still forming the same lattice structure43 ........................................... 7

1.6 Examples of common defects in MOFs include: a) missing

linker defects, b) excess linker in the pores, and c) missing SBU

or missing node defects. .............................................................................. 9

1.7 Depiction of different strategies for incorporating active

catalytic sites into MOFs that can be used include: post-

synthetic metal exchange (blue), post-synthetic linker exchange

(yellow), post-synthetic metallation of a linker (red), grafting a

catalytic site on to the SBU (green), and encapsulation of a

catalyst (navy). .......................................................................................... 11

1.8 Introduction of titanium via: grafting to give Ti-UiO-66, post-

synthetic exchange at the SBU to give UiO-66-Cat-Ti(Ex), and

post-synthetic metallation to give UiO-66-Cat-Ti.66 ................................. 14

1.9 Cobalt catalyst in a MOF to prevent oligomerization and

deactivation.79............................................................................................ 16

1.10 Possible models for incorporation of a metal (denoted Mʹ) post-

synthetically include a) metallation of bridging hydroxyls in the

SBU (where M denotes a metal atom in the SBU), b)

metallation of single-site phenolic sites on the linker, and c)

metallation of adjacent phenolic sites that form a self assembled

multidentate site (SAMS). ......................................................................... 17

2.1 Representative single-sites (hydroxyl sites shown in blue) for

introduction of catalysts, showing a) the μ3-OH site in the

octahedral pore of UiO-67, b) the pore aperture diameter in the


xi

tetrahedral pore of UiO-67 and c) the μ2-OH site in the 1D pore

of DUT-5.91 ............................................................................................... 20

2.2 Three of the most common phases for MIL-53, a breathable

MOF. The phases can be distinguished by the Al–Al–Al angle,

from the as synthesized (109.2°), the high temperature form

(104.9°), and the low temperature form (137.4°). ..................................... 22

2.3 Generic MPV reduction mechanism through direct transfer

hydrogenation. ........................................................................................... 24

2.4 MPV reduction of cyclohexanone in the presence of isopropyl

alcohol. ...................................................................................................... 25

2.5 Catalytic reduction of cyclohexanone using varying amounts of

added aluminum in MOFs (10 mol% activated MOF used in

each case). Lines are provided for visualization.91 ................................... 28

2.6 PXRD patterns of UiO-66HCl, Al@UiO-66HCl, and Al@UiO-

66HCl post MPV reduction reactions. ........................................................ 29

2.7 PXRD patterns of MIL-53as, Al@MIL-53as, and Al@MIL-53as

post MPV reduction reactions. .................................................................. 30

2.8 PXRD of a) UiO-67 and b) DUT-5 materials (from bottom to

top) compared with the predicted patterns, after activation, after

treatment with AlMe3, after subsequent washing with isopropyl

alcohol, and after subsequent catalytic experiment.91 ............................... 31

2.9 FTIR spectra of µ3-OH stretch at 3677 cm-1 of UiO-66,

Al@UiO-66, and Al@UiO-66 post MPV reduction catalysis. ................. 32

2.10 FTIR of UiO-67 (black) showing loss of O-H str upon

introduction of AlMe3 (blue) and subsequent introduction of

isopropyl alcohol (orange).91 ..................................................................... 33

2.11 FTIR of DUT-5 (black), after introduction of AlMe3 (blue) and

subsequent washing with isopropyl alcohol (orange).91 ........................... 33

2.12 Raman spectra of UiO-67 (black), after introduction of AlMe3

(blue), and subsequent addition of isopropyl alcohol (orange).91 ............. 34

2.13 SS 27Al NMR of a) activated DUT-5 b) Al@DUT-5 and c)

Al@UiO-67 (* denotes background signal from the rotor).91 .................. 36

2.14 Split test reduction of cyclohexanone in the presence of wet or

dry isopropyl alcohol and metallated MOFs a) Al@UiO-67 and

b) DUT-5.91 ............................................................................................... 38

2.15 Catalytic MPV reduction of cyclohexanone using [email protected] ............. 39

2.16 The targeted Knoevenagel Condensation, of benzaldehyde and

malononitrile to give 2-benzylidenemalononitrile. ................................... 43


xii

2.17 Knoevenagel condensation of benzaldehyde and malononitrile

in DMSO, with sampling every 1 hour over 5 reaction cycles. ................ 44

2.18 FTIR spectra of the µ3-OH stretch of UiO-66 (black), Zn@UiO-

66 (orange) and Zn@UiO-66 after 3 reaction cycles (blue). .................... 45

2.19 Knoevenagel condensation of benzaldehyde and malononitrile

in DMSO at room temperature split test studies using

Zn@MOFs, with filtering occurring at 10 minutes. Shown here

are a) Zn@UiO-66, b) Zn@MIL-53as and Zn@MIL-53RT. ...................... 46

2.20 PXRD patterns of UiO-66, Zn@UiO-66, and Zn@UiO-66 post

Knoevenagel condensation catalysis. ........................................................ 50

2.21 FTIR spectra of UiO-66, Al@UiO-66, and Al@UiO-66 post

MPV reduction catalysis. .......................................................................... 50

2.22 FTIR spectra of UiO-66, Zn@UiO-66, and Zn@UiO-66 post

Knoevenagel condensation catalysis. ........................................................ 51

2.23 PXRD pattern of UiO-66AcOH. .................................................................. 52

2.25 PXRD patterns of MIL-53as, Al@MIL-53as, and Al@MIL-53as

post MPV reduction catalysis.................................................................... 54

2.26 PXRD patterns of MIL-53as, Zn@MIL-53as. ............................................ 55

2.27 FTIR spectra of MIL-53as.......................................................................... 55

2.28 PXRD patterns of MI-53 RT, Al@MIL-53 RT, and Al@MIL-

53 post MPV reduction. ............................................................................ 56

2.29 PXRD patterns of MIL-53 RT, Zn@MIL-53 RT, and Zn@MIL-

53 RT post Knoevenagel condensation. .................................................... 57

2.30 FTIR of DUT-5 and Al@DUT-5. ............................................................. 58

2.31 PXRD pattern of MOF-74 and Al@MOF-74 post MPV

reduction catalysis. .................................................................................... 59

2.32 Typical 1H NMR spectra of reduction of cyclohexanone in

isopropyl alcohol. ...................................................................................... 61

3.1 Three different metallation models, a) metallation of bridging

hydroxyl groups inherent in the SBU, b) metallation of a single

phenolic hydroxyl site, and c) metallation of adjacent phenolic

sites leading to a chelated metal site. ........................................................ 64

3.2 Synthesis of a cross-linked MOF using a cross-linked linker that

bisects the MOF pore. ............................................................................... 65

3.3 Infrared spectra of Zn@UiO-66-OH, showing metallation

(orange). After Knoevenagel condensation catalysis(blue). ..................... 66


xiii

3.4 FTIR of MIL-53-OH (black), Zn@MIL-53-OH (orange) and

Zn@MIL-53-OH after 3 reaction cycles (blue). ....................................... 67

3.5 Split test of Zn@MOF-OH after filtering out the solid materials

at the 10 minute mark, with a) Zn@MIL-53-OH, and b)

Zn@UiO-66-OH. ...................................................................................... 68

3.6 PXRD patterns of UiO-66-OH, Zn@UiO-66-OH, and

Zn@UiO-66-OH after Knoevenagel condensation catalysis. The

peak at 7.38° corresponding to the 111 plane. .......................................... 69

3.7 The (111) plane of UiO-66. ....................................................................... 70

3.8 PXRD of MIL-53-OH, Zn@MIL-53-OH and Zn@MIL-53-OH

after Knoevenagel Condensation. ............................................................. 71

3.9 Range of metallation of Al@UiO-66-OH from 0.25 to 2.5 eq.

(with the ratio AlMe3:OH) 80 °C over 6 hours, showing ideal

amount of metallation. .............................................................................. 72

3.10 Range of metallation of Al@MIL-53-OH from 0.25 to 2.5 eq.

(with the ratio AlMe3:OH) at 80 °C over 6 hours showing ideal

amount of metallation. .............................................................................. 73

3.11 MPV reduction of cyclohexanone in isopropyl alcohol using

Al@MOF-OH vs. MOF-OH at 80 °C over 6 hours. ................................. 74

3.12 FTIR spectra of the phenolic OH stretch (~3250 cm-1, v br.)

and the µ3-OH (3674 cm-1, s) stretch in UiO-66-OH, Al@UiO-

66-OH and Al@UiO-66-OH post MPV reduction. .................................. 75

3.13 FTIR spectra of of the µ2-OH (3250 cm-1, v. br.) and phenolic

OH (3684 cm-1, s) sites in MIL-53-OH, Al@MIL-53-OH, and

Al@MIL-53-OH post MPV reduction. ..................................................... 76

3.14 27Al SS MAS of a) UiO-66-OH after introduction of AlMe3 an

subsequent washing with hexanses and isopropyl alcohol and b)

the same material that has been subjected to reaction conditions

for the MPV reduction of cyclohexanone. (* denotes

background signal of the rotor) ................................................................. 77

3.15 13C SS MAS NMR of a) UiO-66-OH, b) Al@UiO-66-OH, and

c) Al@UiO-66-OH post MPV reduction conditions. ............................... 79

3.16 27Al SS MAS NMR of a) MIL-53-OH, b) Al@MIL-53-OH, and

c) Al@MIL-53-OH post MPV reduction. ................................................. 81

3.17 13C SS MAS NMR of MIL-53-OH, b) Al@MIL-53-OH, and c)

Al@MIL-53-OH post MPV reduction. ..................................................... 82

3.18 Split test of Al@MIL-53-OH (black) and Al@UiO-66-OH

(red), with filtering at the 1 hour mark (empty circles)............................. 84


xiv

3.19 PXRD patterns of UiO-66-OH, Al@UiO-66-OH, and Al@UiO-

66-OH and after MPV reduction conditions ............................................. 85

3.20 PXRD patterns of MIL-53-OH, Al@MIL-53-OH, Al@MIL-53-

OH post MPV reduction ........................................................................... 86

3.21 Comparison of PXRD pattern of Al@MIL-53-OH post with the

predicted pattern for MIL-53 (102.6°). ..................................................... 87

3.22 Representation of the tetrahedral pore aperture of UiO-66, with

yellow spheres depicting some possible regions in which the

catalyst could bind to a phenolic site ........................................................ 89

3.23 FTIR spectra of UiO-66-OH ..................................................................... 91

3.24 FTIR spectra of MIL-53-OH ..................................................................... 92

4.1 Treatment of V(N[tBu]Ar)3 with O2 to give the V(V) peroxo

species, showing an example of small molecule activation ...................... 97

4.2 Acid-catalyzed synthesis of H3TMIM from 3-methylindole and

triethylorthoformate .................................................................................. 99

4.3 Incomplete crystal structure of TMIM treated with n-BuLi,

showing the lithium activating THF to give an unbound lithium

ethoxide in the structure .......................................................................... 100

4.4 FTIR comparison of Li3TMIM·5THF with H3TMIM ............................ 101

4.5 Crystal structure of Li3TMIM·5THF ...................................................... 102

4.6 Synthetic procedure of lithiation of TMIM and subsequent

metallation using VCl3·3THF to give VTMIM·LiCl·5THF ................... 103

4.7 Crystal structure of VTMIM·LiCl·3THF................................................ 104

4.8 Wide window 1H NMR of VTMIM·LiCl·3THF in C6D6 ....................... 105

4.9 Qualitative Evan's method of VTMIM complex in, showing the

shift of hexamethyldisiloxane in C6D6 .................................................... 106

4.10 EPR spectra of VTMIM·LiCl·3THF in C6D6-benzene, showing

that the complex is EPR silent ................................................................ 107

4.11 FTIR spectra of VTMIM·LiCl·3THF and Li3TMIM·5THF ................... 108

4.12 1H NMR spectra of d1-H3TMIM in CDCl3 ............................................. 110

4.13 FTIR comparison of d1-H3TMIM with H3TMIM ................................... 111

5.1 Some previously used redox noninnocent ligands .................................. 115

5.2 Reduction of a Fe(0) complex144 ............................................................. 115

5.3 Previously investigated BIAN complexes containing Fe144.................... 116


xv

5.4 Valence tautomers of a Fe bismino complex with a) Fe(0) with

a neutral ligand, b) Fe(I) antiferromagnetically coupled with a

monoanionic ligand, and c) Fe(II) with a dianionic ligand144 ................. 117

5.5 Geometry optimized HOMO of a) FeBIAN-benzene, b)

FeBIAN-toluene, and c) FeBIAN-trifluorotoluene, depicting the

singlet optimized HOMO, broken symmetry singlet α, and

broken symmetry singlet β HOMO144 ..................................................... 118

5.6 Hammett Correlation of HOMO energy and σπ from electron

donating to electron withdrawing (from left: aniline, toluene,

cumene, benzene, trifluorotoluene)144 ..................................................... 120


1

CHAPTER 1

CATALYSIS IN METAL-ORGANIC FRAMEWORKS

1.1 Metal-organic frameworks

Metal-organic frameworks (MOFs) are coordination polymers with an open

framework containing potential voids. These are typically permanently porous

crystalline materials consisting of Lewis acidic metals or metal nodes and Lewis basic

multitopic organic linkers,1 often carboxylates2–4 or amines (Figure 1.1).5–8 MOFs are

crystalline and ordered materials that exhibit some of the highest known surface

areas.9 The ability to combine different metals with different multitopic linkers has led

to the isolation of tens of thousands of different MOFs.10 This synthetic flexibility

allows for tailoring of the MOF structure to a wide variety of applications, including

catalysis,11–13 gas sorption,14–16 and conductivity.17–19

Figure 1.1 Depiction of a generic MOF, with the Lewis acidic secondary building unit

(SBU), the Lewis basic organic linker, the pore volume, and pore aperture diameter

shown.

In a typical MOF (Figure 1.1), the metal ion or cluster makes up the secondary

building unit (SBU), which is connected using multitopic, or containing multiple

Lewis basic sites, organic linkers. The linkers used in this thesis are ditopic,

containing two bidentate carboxylate groups, however there are many examples of

tritopic, as well as tetratopic linkers.9,20

The choice of linker used in the formation of a MOF is critical. In order to

form an ordered network, the linker that is used must align to form the geometry


2

repeatedly to give a phase-pure material. While there are notable examples of more

flexible linkers,21,22 typically linkers are chosen that contain rigid, planar aromatic or

alkyne groups as well as ligating geometries that are linearly opposed to one another.

A commonly employed linker is 1,4-benzenedicarboxylic acid (H2BDC) and its

extended analog, 4,4ʹ-biphenyldicarboxylic acid (H2BPDC).

Compound 1.1 H2BDC (1,4-benzenedicarboxylic acid)

Compound 1.2 H2BPDC (4,4-biphenyldicarboxylic acid)

1.1.1 MOF SBUs

MOFs can be formed with a variety of metal SBUs. The geometry and

orientation of the metal clusters can depend on the synthetic procedure as well as the

linkers incorporated in the MOF. Often-observed SBUs include the M6O4(OH)4 SBU

in the UiO-series as well as the M3(μ3-O) cluster used in MIL-101. Another common

SBU is a rod-like metal-oxide SBU as in the MIL-53 series or the MOF-74 series.

This type of SBU leads to pore channels, as the pore in the third dimension is limited

by the shorter axis of the SBU. Examples of these MOFs are shown in Table 1.1. The

stability and activity of the SBU depends on the topology of the framework and the

strength of the interactions in the SBU. Coordinatively unsaturated metal sites, which

may be formed on activation, or removal of coordinating solvents within the MOF,

may be susceptible to nucleophilic attack. This susceptibility of the SBU towards

nucleophilic attack contributes to the instability observed in MOF-5.23


3

Figure 1.2 Two of the SBUs that are used in this dissertation, a) the Zr6 cluster present

in the UiO series from two different perspectives as well as b) the octahedral Al–O–Al

SBU of MIL-53.

1.1.2 MOF Pores

MOFs can contain either 1D connected pores or 3D continuous discrete pores.

In the case of continuous pores, the MOF forms channels through which diffusion is

facilitated in 1D channels. MOFs containing 3D interconnected pores may have

multiple directions through which substrates can enter or exit the pore. Some common

MOF pores as well are displayed in Table 1.1.

While MOFs can be considered relatively rigid structures, a number of MOFs

exhibit “breathing” behavior, in which the dimensions and geometry of the MOFs can

change upon response to external stimuli.4,24,25 A more in-depth discussion as well as

an example of breathing behavior in MOFs is provided on page 21.

Figure 1.3 Some representative MOF pores, including a) the discrete pore of UiO-66,

b) the continuous pore of MIL-53, and c) the discrete pore of MOF-5.


4


5


6

The stability of MOFs towards thermal decomposition or oxidation are

typically assessed through thermogravimetric analysis (TGA), with powder X-ray

diffraction (PXRD), vibrational spectroscopy, and surface area analysis as supporting

evidence. TGA is used to determine the temperature at which a MOF decomposes, as

well as the products that are formed as a result. The decomposition temperature and

the products formed can vary depending on whether the analysis is performed under

an atmosphere of air or an inert gas, such as nitrogen. Decomposition of the MOF in

nitrogen can offer insight into the upper limit of framework stability, which may help

determine the activation procedure for removing solvent from the framework. On the

other hand, depending on the reaction, the decomposition of the framework in oxygen

may offer information on the oxidative stability of the framework. PXRD and surface

area analyses are used to confirm the crystallinity of the MOF and the surface area,

respectively, before and after activation or being subjected to reaction conditions.

The synthetic procedure used, as well as the handling of the MOF can

contribute to the stability or lack thereof. An example of this was shown in the case of

UiO-67, a larger analog of UiO-66, a MOF that is typically considered to be stable

towards a wide variety of conditions. The Farha group found that if the MOF was

treated with water before activation, the MOF lost its crystallinity over time.33 Further

studies from Farha and co-workers showed that the collapse of the framework was due

to capillary force as solvent left the framework.34 Later studies from Lillerud and co-

workers, however, show that a more stable UiO-67 analog can be synthesized by using

bulkier linkers.35 By using 4,4ʹ-binaphthyldicarboxylic acid instead of H2BPDC in the

synthesis of their MOF, they were able to synthesize UiO-67-BN, which showed

exceptional stability towards hydrolysis, preventing loss of crystallinity through

capillary force.

1.1.3 Isoreticular design of MOFs

Different materials that share the same or similar structural topology are

isoreticular to one another. Isoreticular design allows materials sharing the same net or


7

relative geometry to be formed using different linkers or metals.36,37 This allows the

MOF to be tuned to the specific need, by: 1) introducing new functionality into the

framework by using functionalized linkers (Figure 1.4b),38,39 2) changing the metal to

alter the characteristics of the framework (Figure 1.4c),40,41 or 3) making the structure

larger or smaller by changing the length of the linker (Figure 1.4d).2,42

Figure 1.4 Examples of isoreticular synthesis of MOFs including a) original

framework, b) different linker, c) different metal in SBU, and d) longer linker leading

to larger MOF.

Figure 1.5 Isoreticular MOF-5 containing up to 8 linkers in varying ratios while still

forming the same lattice structure43


8

An example of introduction of functional groups was shown by the Yaghi

group,37,43 in which they synthesized MOFs containing 8 different linkers (Figure 1.5).

These groups can lead to incompatibilities with the synthetic method, so procedures

may require adaptation. An example of this is MIL-53, which can be synthesized with

Al in hydrothermal conditions with no additives, whereas the Cr3+ analog requires the

addition of HF.24

The expansion of pores by changing the length of the linker was shown in the

case of the MOF-74 family of materials, which have been synthesized using at least 11

linkers.44 These materials range in pore size from 14 Å up to 98 Å pore apertures. The

Yaghi group was able to achieve this by varying the number of phenyl rings between

the ligating groups of the linkers. The results are MOFs sharing the same topology,

with varying size of pores.

1.1.4 Stability of MOFs

Although MOFs contain a network of coordination bonds, which form a

crystalline, ordered network, MOFs have been shown to degrade both chemically

(through alcoholysis, hydrolysis) and thermally. Early MOFs were shown to undergo

decomposition upon loss of solvent, and it was not until the synthesis of MOF-5 that

MOFs were considered to be structurally stable in the absence of guest molecules.26

To this point, the choice of MOF can be more complex than simply considering

surface areas. Typically, MOFs that are formed from stronger Lewis acid-base

interactions based on hard/soft acid/base (HSAB) theory tend to be more resistant to

degradation. This means that more acidic cations (Zr4+, Ti4+, Al3+, Cr3+) are needed to

bind strongly to the basic coordinating groups (RCOO-, RO-). Less acidic, and softer

cations (Zn2+, Cd2+, Cu2+) tend to interact more weakly with the same anions, forming

MOFs that are typically more susceptible to hydrolysis or alcoholysis. UiO-66, a

zirconium framework with an ideal formula of Zr6O4OH4BDC6, has been shown to

maintain crystallinity under a pH range from 1-14.45 In addition, more complex motifs

can be explored in order to make typically less stable MOFs more stable to external


9

conditions, including the use of diamine linkers to coordinatively saturate less stable

MOFs.6,7 Some common examples of MOFs and their stability toward hydrolysis are

shown in Table 1.1. Except for MOF-5, all of these MOFs are considered to be stable

under a variety of conditions, and have been used for catalytic studies due to their

exceptional stability. The majority of these MOFs also contain hydroxyl sites inherent

in the MOF that may possibly be functionalized.

1.1.5 Defects in MOFs

While previously prepared MOFs are identified by comparison with predicted

PXRD patterns, many MOFs may have defects that may play important roles in the

behavior of the MOF while keeping the same structure and sharing the same unit cell

with a known material.46–49 Some examples of defects in MOFs that can play a role in

catalysis include missing linkers,48 missing metal ions or clusters, and excess linkers

in the MOF (Figure 1.6).4

Figure 1.6. Examples of common defects in MOFs include: a) missing linker defects,

b) excess linker in the pores, and c) missing SBU or missing node defects.

As MOFs can often contain not only Lewis acidic coordinatively unsaturated

sites in the MOF, but also Brønsted acidic sites in the SBU (Figure 1.2), the catalytic

behavior of the MOF can be more ambiguous. An example of this was the activity of

UiO-66 in the Fischer esterification of carboxylic acids with methanol.50 The SBU of

the MOF was modeled computationally to show that understanding the anions that

charge balance the Zr6 cluster are vital to understanding the structure and catalytic

behavior of the MOF. These defects arise from either an absence of SBU or linker in

the framework, and can be targeted by using specific acidic modulators in the

syntheses of the frameworks. The amount and nature of the defects varies with the

choice of modulator, and also the relative amount added. Lillerud et al. varied the


10

temperature of the solvothermal synthesis of UiO-66 to better understand the

formation of defects in the MOF.49 By using a much higher reaction temperature (220

°C vs. 100 °C), they were able to synthesize UiO-66 containing fewer defects. The

defect-free UiO-66 also had notably lower surface areas. Further, defects can also

provide opportunities for post-synthetic modification. The synthetic approach taken to

provide or limit the amount of defects in the MOF being synthesized is an important

choice in determining the properties of the material to be used.

Defects have also been shown to lead to improved catalytic activity. In Llabres

i Xamena, et al.’s work, zirconium MOFs were used for the MPV reduction of

cyclohexanone, and defects were intentionally designed through the linkers used.51

They compared the use of MOFs containing different coordination environments of

the Zr6O4(OH)4 cluster commonly used for synthesizing MOFs. They found that by

incorporating a variety of trimesate-based linkers, they could rationally introduce

defects in the MOF, leading to better conversion. They were even able to observe a

diastereomeric ratio of 62:40 in the reduction of estrone (approximate dimensions:

11.2 Å × 6.2 Å × 4.2 Å), a relatively large substrate for catalysis in the pore of a MOF,

in an achiral MOF. They found that the much larger MOF-808 (pore diameter of 14 Å)

yielded better conversion of the bulkier substrate than UiO-66 (pore diameter of 6 Å),

showing that smaller MOFs may have limitations corresponding to the smaller pore

aperture diameter.

1.2 Current approaches to catalysis in MOFs

MOFs are excellent platforms for catalysis because of the variety of strategies

that are available for introducing catalytically active sites. Common approaches

include post-synthetic modification (PSM) of the MOF, coordinatively unsaturated

sites, encapsulation of materials, and grafting functionality into the SBU (Figure 1.7).


11

Figure 1.7. Depiction of different strategies for incorporating active catalytic sites into

MOFs that can be used include: post-synthetic metal exchange (blue), post-synthetic

linker exchange (yellow), post-synthetic metallation of a linker (red), grafting a

catalytic site on to the SBU (green), and encapsulation of a catalyst (navy).

1.2.1 PSM of linkers in MOFs

PSM of a linker is an approach that has been used to incorporate functionality

into MOFs that would ordinarily be synthetically inaccessible. This is accomplished

via organic transformations or metallation of a functional group on the linker. An

example employing organic transformations includes the work of the Cohen group in

which MIL-53 with amine groups were post-synthetically modified using

condensation with a ketone to yield MOFs containing amides.52 These MOFs were

found to be active for the alcoholysis of epoxides. While post-synthetic modification

of the linker allows for synthesis of the MOF topology using a simpler linker, the

transformations may damage the framework, leading to loss of crystallinity in the

MOF.REF

1.2.2 Post-synthetic exchange of motifs in MOFs

Due to synthetic constraints or stability issues, the use of certain linkers or

metals for the preparation of MOFs is not always possible. By soaking MOFs in

saturated solutions,53,54 the existing linkers or metals can be exchanged post-


12

synthetically while retaining the same topology. An example of this is the synthesis of

more stable frameworks sharing the same geometry by exchanging the metal, as in the

case of HKUST-1.55 This approach has also been used to synthesize mixed metal

MOFs, some of which have been shown to have beneficial properties over a MOF

containing only one metal in the SBU.56,57 The Dinca group showed the use of post-

synthetic metal exchange in MFU-4l (Zn3Cl4(BTDD)3), where H2BTDD = bis(1H-

1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin).58 By exchanging only 10% of the

Zn2+ sites for Ni2+, they were able to produce a MOF capable of the dimerization of

ethylene, yielding 96.2% 1-butene, with a turnover frequency (TOF) of 41500 mol/hr,

without leaching the catalytic site into solution.

Post-synthetic linker exchange allows for incorporation of linkers that would

be otherwise inaccessible, or allow a mixed linker approach that may not be

achievable otherwise. The limitations of post-synthetic exchange need to be

considered, however. The structural integrity of the framework can be compromised

by the reaction conditions of the exchange. As exchange experiments may require

boiling a MOF in a saturated solution of the material to be incorporated, the stability

of the MOF under these condition has to be considered.59 In addition, the amount of

linker or metal exchanged is not usually predictable, and highly stable metal SBUs

such as the zirconium or aluminum SBUs may not easily undergo metal exchange due

to the stronger M-carboxylate bond compared to softer metals or softer linkers.

1.2.3 Coordinatively unsaturated sites in MOFs

Coordinatively unsaturated sites (CUSs) are metal sites in the SBU that can be

formed either through constrained geometry or removal of coordinating solvents

through activation. This contrasts with homogeneous catalysts, in which a

coordinating solvent may be much more difficult to remove. Because the coordination

environment at the metal is less than full, the metal site can be considerably more

reactive than a saturated metal site. Co-MOF-74 was shown to have coordinatively

unsaturated sites able to oxidize CO at low temperatures, showing that these


13

unsaturated metals sites can be more catalytically active than the same MOF

containing either Mg2+ or Zn2+.60 Another example of coordinatively unsaturated sites

was shown in the case of Mn3[(Mn4Cl)3(BTT)8(CH3OH)10]2.61 This framework has

several coordinatively unsaturated metal sites upon activation, and it was shown to be

effective in catalyzing both the cyanosilylation of aromatic aldehydes and ketones as

well as Mukaiyama-aldol reactions.

1.2.4 Grafting catalytic sites onto the SBU of MOFs

Grafting onto the SBU is a specific case of post-synthetic modification. Many

MOFs contain hydroxyl groups that bridge metal centers in the SBU. These groups

can be functionalized using other reagents. Successful application of this approach

emerge from the Lin group, who reported functionalized zirconium based UiO MOFs

with cobalt or magnesium reagents for catalysis.62,63 They showed that the Brønsted

acidic hydroxyl sites of UiO-68 could be deprotonated with n-BuLi, followed by

metallation with CoCl2 or FeBr2·2THF to give a covalently bound Co2+ or Fe2+ metal

site. Alternatively, treatment of the same hydroxyl sites of UiO-68 with Me2Mg gave a

covalently bound Mg site grafted onto the SBU of the MOF. The Farha group has also

used atomic layer deposition (ALD) to incorporate aluminum,31 vanadium, 64 and

zinc,65 into MOFs. By treating MOFs with commercially available gas-phase

organometallic metal precursors, post-synthetic modification of the MOFs can occur at

a single layer. This allows for single-site catalysts to be incorporated within specific

chemical environments. In these examples, the metals are grafted onto the SBU of the

MOFs at hydroxyl sites that are formed in the synthesis of the MOF.

An example of a comparison of the same metal in unique domains throughout

the MOF was shown in the case of Ti-functionalized UiO-66.66 By incorporating Ti4+

in three different methods, it was demonstrated that the method of incorporation is

important, while also showing the variety of domains in a typical MOF. Ti4+ was

grafted on to the SBU, was post-synthetically modified at a catechol in the linker, and


14

post-synthetically exchanged for Zr4+ in the framework. Notably, the post-synthetic

exchange of Ti4+ for Zr4+ is a rare case of metal exchange at a robust SBU.

Figure 1.8 Introduction of titanium via: grafting to give Ti-UiO-66, post-synthetic

exchange at the SBU to give UiO-66-Cat-Ti(Ex), and post-synthetic metallation to

give UiO-66-Cat-Ti.66

1.2.5 Encapsulation of a catalyst in MOFs

The void space of a MOF can be used to encapsulate a catalyst, either after or

during MOF synthesis.67,68 The available space for reaction would commensurately

also be less. Examples of this include the incorporation of metal nanoparticles in

MOFs,69,70 as well as the encapsulation of enzymes in MOFs for catalysis.71,72

Farha et al. recently encapsulated an enzyme in the pores of a MOF, PCN-

128y.71 The design of the MOF pore was critical in this work, as the larger hexagonal

pores were just large enough to hold the enzyme without significant leaching into

solution. In addition, the second triangular pore size was large enough to allow


15

substrate to enter, with channels in the MOF allowing the substrate to interact with the

enzyme. This enzyme-supported MOF also showed increased activity over the enzyme

in solution alone.

1.3 Considerations for catalysis in MOFs

When designing a catalyst, a number of factors need to be taken into

consideration. Homogeneous catalysts offer a level of control and knowledge about

the catalyst, allowing for unambiguous structural information about the catalyst. In

addition, homogeneous catalysts can be more readily designed to be chiral, allowing

for the possibility of asymmetric catalysis. Being able to form valuable chiral reaction

products from achiral starting materials is a significant end goal for designing most

catalysts.73 Homogeneous catalysts may require expensive ligands or precious metals,

however, and can be sensitive towards a variety of conditions, causing the catalyst to

be poisoned over time. Because homogeneous catalysts are dissolved in the reaction

mixture, catalyst reuse and recovery is also a consideration, and homogeneous

catalysts are often more suitable for batch systems as opposed to reactions in flow.74,75

Heterogeneous catalysts offer a very robust platform, allowing the catalyst to

be recycled easily and over a number of reaction cycles.62 Due to the unique methods

of characterization of heterogeneous materials, the design of the catalyst and the

mechanism can, at times, be more ambiguous than a homogeneous catalyst. MOFs

have an advantage over other solid materials such as zeolites, in that they encompass a

vast number of geometries, whereas zeolites only have 248 known structures

according to the database of zeolite structures.76 The higher surface areas and synthetic

flexibility available to MOFs make them ideal as supports for catalysts over traditional

heterogeneous catalysts such as zeolites or other solid state materials that do not have

the same tunability.

MOFs have been used to site-isolate catalysts in order to improve upon the

activity of homogeneous analogs or provide catalysts not available otherwise. For

example, treatment of a UiO-derived MOF containing a salicylaldimine motif in the


16

linker with iron (II) chloride gave rise to an iron catalyst in a MOF.77 Attempts to

prepare the homogeneous counterpart yielded a tetrameric species that was inactive

towards the reaction. In another study, the Lin group post-synthetically modified

amine-functionalized UiO-69 to give an α, β diketiminate, a bidentate ligating group

that they were able to functionalize with cobalt which gave an active catalyst for the

hydrogenation of 1-octene. This contrasts with the homogeneous analog, which

showed no activity at catalyst loadings of 0.0001 mol% and 6 days reaction time.78

The Lin group also incorporated cobalt in a zirconium framework using a bipyridyl

analog linker and observed unprecedented catalytic activity for several reactions

(Figure 1.9). By incorporating their single-site catalyst inside the MOF using bipyridyl

ligating groups, they prevented the disproportionation of the catalyst into catalytically

inactive nanoparticles and the dimeric analog.79

Figure 1.9 Cobalt catalyst in a MOF to prevent oligomerization and deactivation.79

1.4 The approach of this work

The goal of the work described in this dissertation is to design catalysts using

MOFs that will site isolate molecular catalysts. By adapting a homogeneous catalyst

into a heterogeneous environment, catalyst decomposition through aggregation can be

prevented,80 and in doing so provide a stable catalyst for easy recycling. This is

approached using two different methods: post-synthetically metalating ligating sites


17

that are inherent in MOFs,64 or by incorporating linkers containing functional groups

that can be metallated. It is hypothesized that adjacent linkers have the possibility of

acting as self-assembled multidentate ligating groups.

Figure 1.10 Possible models for incorporation of a metal (denoted Mʹ) post-

synthetically include a) metallation of bridging hydroxyls in the SBU (where M

denotes a metal atom in the SBU), b) metallation of single-site phenolic sites on the

linker, and c) metallation of adjacent phenolic sites that form a self assembled

multidentate site (SAMS).

The strategy of the work in this dissertation is to incorporate Lewis acidic

metal catalysts into MOFs using organometallic reagents containing strongly basic

groups at acidic sites inherent in the MOF. This would yield a covalently bound single

metal site within the MOF (Figure 1.10a). MOFs containing functional groups

introduced through the linker were used, providing more available acidic sites for

incorporation of metal catalysts. (Figure 1.10b) In the process of doing so, it was

hypothesized that adjacent linkers could act as self-assembled multidentate ligating

groups for the incorporation of chelated single-site metal catalysts (Figure 1.10c).

Through these moieties, decomposition mechanisms relating to aggregation may be

prevented, providing a more robust catalyst, while taking advantage of the benefits of

recovery and reuse that stem from the heterogeneous MOF material.

1.5 Reactions employed to probe catalyst activity

Reductions make up a significant portion of industrial reactions.74,81 The

Meerwein-Ponndorf-Verley (MPV) reaction, a reduction of ketones or aldehydes to

alcohols in the presence of an alcohol as both solvent and reductant,82–84 was


18

considered in the context of MOFs. The MPV reduction is an atom economical

reaction using a green solvent and reductant, with a benign byproduct in acetone. In

addition, the MPV reduction is specific for ketones and aldehyde; other reducible

groups are not affected.

A typical catalyst for this reduction, aluminum isopropoxide, is cheap, with

high abundance and low toxicity. This contrasts with other metals used for reductions

or hydrogenations, such as palladium or platinum, which are significantly more rare

and more expensive. Aluminum isopropoxide, however, is susceptible to hydrolysis,

and aggregates over time, leading to deactivation of the catalyst.

The Knoevenagel condensation is a condensation of a ketone or aldehyde with

a molecule containing a methylene and two electron withdrawing groups. This forms a

new C–C bond, giving an alkene. The strengths of the electron withdrawing groups are

important in making the methylene protons sufficiently acidic for the reaction to

proceed. The reaction can be either Lewis acid or Lewis base catalyzed, with Zn being

a common choice for Lewis acid and amines being used as Lewis bases.85–90

1.5.1 Considerations for catalysts

Using MOFs in heterogeneous catalysis requires considering the conditions

that the MOFs will be subjected to. In order to reduce reaction times and increase

catalyst loading, stability under a range of temperatures and a variety of solvents is

important. For example, the MPV reduction is typically performed in the presence of

isopropyl alcohol at elevated temperatures, so the MOF must be thermally stable and

stable toward alcoholysis. In addition, the MPV reduction would ideally be performed

using wet isopropyl alcohol, therefore the MOF must also be stable towards

hydrolysis.

Examples of common MOFs made from readily accessible starting materials

that meet these criteria include the UiO-6X series (Zr4+/Hf4+), the MIL-53 series

(Al3+/Cr3+/Fe3+), and MOF-74 (Mg2+/Cr2+/Fe2+). In addition, MOFs were targeted that

would be amenable to the addition of molecular Al3+ through AlMe3. MOFs that can


19

undergo metalation under these conditions must have available functional groups (a

hydroxy or amine group, for example) or have isoreticular analogs containing

available functional groups. Both the UiO-66 and MIL-53 series of frameworks can be

synthesized with isoreticular analogs amenable for metallation. MOF-74 does not have

any available hydroxyl sites for metallation. Redox inactive metals were chosen for

constructing the SBUs to avoid competing reaction pathways. For catalysis to occur

inside a MOF, the MOF must have a high surface area and a relatively large or open

pore for substrates to interact with the catalyst. This means that extra attention must be

paid to MOFs containing variable or smaller pores, or constricted pore apertures as

this can lead to inefficient diffusion of substrates.

1.6 Purpose, scope and overview of the dissertation

Chapters 2 and 3 describes the introduction of Lewis acidic metal sites in

MOFs for MPV reductions and Knoevenagel Condensations. Both Al and Zn were

introduced into MOFs for these reactions. Their use and recovery was investigated, as

well as the coordination of the catalyst in the MOFs.

In addition to catalysis in MOFs, homogeneous complexes containing first row

transition metals were investigated. Complimentary to the heterogeneous catalysis

previously discussed, a series of ligand scaffolds for homogeneous catalysts were

investigated. Synthetically accessible tripodal amides were targeted for the scaffolds

for these complexes. Advances in the synthesis and characterization of these tripodal

amide complexes are described in Chapter 4.

Lastly, Chapter 5 details a computational study in the redox behavior

surrounding a series of Fe(BIAN) complexes were interrogated using density

functional theory (DFT) calculations. The redox noninnocence of the BIAN

complexes was modeled to supplement the somewhat ambiguous experimental

evidence. In addition, the electron donating or withdrawing effects were modeled to

better understand the driving force of the catalyst in the hopes of eventually designing

a better catalyst.


20

CHAPTER 2

SINGLE-SITE CATALYSIS IN MOFS

Portions of this chapter have been adapted with permission from “Anchored

Aluminum Catalyzed Meerwein–Ponndorf–Verley Reduction at the Metal Nodes of

Robust MOFs” Inorg. Chem. 2018, 57 (12), 6825–6832.

https://doi.org/10.1021/acs.inorgchem.8b00119. Copyright 2018, American Chemical

Society.

2.1 Selection of MOFs

The MPV reduction of cyclohexanone as well as the Knoevenagel

condensation of malononitrile and benzaldehyde were evaluated using several

different MOFs. Three different MOF families were used: UiO-66/UiO-67, MIL-

53/DUT-5, and MOF-74. UiO-66/UiO-67 and MIL-53/DUT-5 were targeted due to

the presence of acidic sites within the MOF (as depicted in Figure 2.1). These sites are

formed during the solvothermal synthesis of the MOFs and can be post-synthetically

modified. Organometallic reagents containing strong bases were chosen to deprotonate

and install the metal at the acidic sites, with the liberation of methane or ethane. It has

been shown that these sites can be functionalized using Me2Mg as well as AlMe3.63

MOF-74 served as a MOF containing no available hydroxyl sites.

Figure 2.1 Representative single-sites (hydroxyl sites shown in blue) for introduction

of catalysts, showing a) the μ3-OH site in the octahedral pore of UiO-67, b) the pore

aperture diameter in the tetrahedral pore of UiO-67 and c) the μ2-OH site in the 1D

pore of DUT-5.91


21

2.1.1 UiO-6X series

The UiO-6X series of MOFs are zirconium or hafnium containing MOFs that

have a M6 (M=Zr or Hf) Zr6 cluster SBU. The cluster has been studied for a variety of

applications, but the main attraction of its use is the amphoteric nature of the SBU, as

well as its robust nature. The UiO-6X series has been known to have defects in the

MOF depending both on synthesis conditions and post-synthetic treatment of the

MOF.49,92,93 This can manifest as missing linker defects, as well as missing node

defects. The idealized M6 cluster is 12-coordinate, with a generic formula

M6O4(OH)4L6 (where L is a dicarboxylate linker). As the MOF needs to be charge

balanced, defects can affect the Zr6 cluster, leading to an increase in CUSs, as well as

a variety of acidic and basic sites in the M6 cluster.

The nature of the defects in UiO-66 has been debated extensively, and multiple

charge-balancing anionic sites have been ruled out, including formate,94 and chloride49

sites, while hydroxyl sites have been shown to be the most likely option. The specific

Bronsted acidity of the acidic sites in the M6 cluster was shown by the Farha group

through titrations.95 They showed that the μ3-OH was the most acidic species, whereas

M-OH species caused by missing linker defects would be the least acidic. Further

modeling showed that the M-OH species have dynamic behavior, being shared across

multiple Zr sites.96

2.1.2 MIL-53

MIL-53 was chosen as a robust aluminum framework with exceptional

stability towards hydrolysis. The 1D pore channel of MIL-53 offers a useful contrast

to the 3D connected tetrahedral and octahedral pores of UiO-66.

MIL-53 presents several challenges; upon synthesis of MIL-53 through a

typical procedure, the framework contains excess linker in the pores.4 The method of

activation to remove the linker in the pores is critical. The MOF also exhibits

“breathing” behavior, meaning that the pore size and shape changes as a response to

external stimuli. As there are many phases available, it is appropriate to label the pore


22

that has been formed using the Al–Al–Al angle of the MOF (Figure 2.2). In the first

synthesis of MIL-53, the MOF exhibited a small pore denoted MIL-53109.2, which,

upon activation (removal of excess linker and solvent), gives a larger, open pore

denoted MIL-53104.9. In the presence of water, the MOF takes the narrow pore form, as

hydrogen bonding stabilizes the MOF in a more closed pore phase, denoted MIL-

53137.4.

Figure 2.2 Three of the most common phases for MIL-53, a breathable MOF. The

phases can be distinguished by the Al–Al–Al angle, from the as synthesized (109.2°),

the high temperature form (104.9°), and the low temperature form (137.4°).

Two examples of MIL-53 were chosen for metallation and subsequent

catalysis, with two uniques methods of synthesis and activation. MIL-53as, synthesized

hydrothermally at 220 °C over 3 days, was chosen as an unactivated MIL-53 analog,

and MIL-53RT, MIL-53 synthesized in DMF at room temperature, represents a large


23

pore MIL-53. MIL-53 has also recently been shown to be metallated with cobalt after

treatment with LiCH3Si(CH3)3 followed by introduction of CoCl2.97 MIL-53(Al)-CoCl

was then treated with NaBEt3H to give MIL-53(Al)-CoH, a material containing a

Co(II) hydride capable of catalyzing the hydroboration of alkynes or nitriles as well as

the hydrosilylation of esters. The MIL-53(Al)-CoH catalyst even outperformed a much

larger MOF analog, UiO-68-CoH. The Lin group attributed the increased activity to an

increased bond order in the Co-H bond of Al2-O-CoH in MIL-53(Al)-CoH as opposed

to the Co-H bond Zr3-O-CoH in UiO-68-CoH. This suggests that not only is pore

aperture important in catalytic performance, but also the bond strength of the

covalently bound catalyst within the MOF.

2.2 MPV reductions

As described on page 17, MPV reductions are transfer hydrogenations of

ketones or aldehydes with isopropyl alcohol as a solvent and reductant. The

mechanism for this reaction98,99 is typically considered to go through a cyclic

transition state. Computational and experimental evidence suggests that this is done

through an outer sphere mechanism, through a chelated, planar, six-membered ring

transition state. Hydrogen transfer occurs directly from the alkoxy group to the

carbonyl carbon. Recent studies of the mechanism of the reaction in heterogeneous

ZrO2 showed that the heterogeneous mechanism varies from the mechanism of a

homogeneous single-site Zr4+.100,101 While the single-site Zr4+ coordinates both the

carbonyl and the alcohol, the heterogeneous mechanism operates through adjacent

Zr4+ atoms.


24

Figure 2.3 Generic MPV reduction mechanism through direct transfer hydrogenation.

For the MPV reduction of cyclohexanone, the MOFs were treated with

trimethylaluminum followed by subsequent introduction of isopropyl alcohol with

concomitant liberation of methane (observed occasionally through effervescence) to

give aluminum isopropoxide functionalized MOFs (denoted Al@MOFs). As the

hydroxyl sites inherent in the UiO-66/67 MOFs are quite acidic, with pKa’s around

3.5-3.9, the MOFs would be metallated assuming sufficient space for introduction.95,96

This single-site aluminum isopropoxide should have the possibility for catalytic

activity for the MPV reduction of ketones and aldehydes.

Initial reactivity for the MPV reduction was tested using the reduction of

cyclohexanone to cyclohexanol in the presence of excess isopropyl alcohol as both

solvent and reductant (Figure 2.4).


25

Figure 2.4 MPV reduction of cyclohexanone in the presence of isopropyl alcohol.

2.2.1 Al@MOFs

Initial studies of Al@UiO-66 showed good conversions over 96 hours at room

temperature, with no reactivity from the non-metallated control (Table 2.1). In order to

improve activity, the reaction was run at 80 °C, offering better conversions over 12

hours, but the native MOF also showed reactivity as has been previously reported for

Zr MOFs.12,51

Table 2.1 MPV Reduction of cyclohexanone in isopropyl alcohol (% conversion)

comparison of metallated vs. native MOF (80 °C).

MOF Reaction Time (h) Cat. Loading

(%)a

No Al (%

Conversion)

Al (% Conversion)

UiO-66 12 2.5 6 25

UiO-66 12 25 59 99

UiO-66 96b 25 0 86

a Catalyst loading determined by eq. Al incorporated in the MOF for Al@MOF, and eq. of M in the

MOF b Reaction was run at rt


26

Table 2.1 Continued

UiO-66AcOH 24 25 46 33

MIL-53as 12 25 0 0

MIL-53RT 12 2.5 0 0

UiO-67 1 25 9 66

DUT-5 1 25 3 94

Mg-MOF-74 168 25 0 0

While MIL-53 contains μ2-hydroxyl sites that could be functionalized, both

Al@MIL-53RT and Al@MIL-53as did not facilitate the reduction of cyclohexanone.

This could mean that the pore opening is not available for incorporation of metals, that

the MOF pore is too congested to allow efficient diffusion of substrate and reductant,

or that the specific reagent is not amenable for metallation of MIL-53 materials.

Considering MIL-53 has been metallated using a metathesis reaction with CoCl2 and

lithiated hydroxyl sites within the MOF, it is unlikely that the pore opening is the issue

in the case of MIL-53RT.97

UiO-67 was chosen as a larger analog of UiO-66, and DUT-5 as a larger

analog of MIL-53. Subjecting the two larger MOFs to identical catalytic conditions

(Table 2.1) revealed that these frameworks allow for superior catalytic activity

(shorter times, with high conversions) when treated with AlMe3. In addition, the native

MOFs are not active under these conditions, meaning that the catalytic activity can be

attributed to the incorporated aluminum. Due to the enhanced reactivity of the larger

frameworks, subsequent studies mainly focused on UiO-67 and DUT-5.

In order to test a MOF in which a covalently bound catalyst would not be

likely, Mg-MOF-74 was considered as a robust MOF that did not contain available

hydroxyl sites to be functionalized. The MOF showed no catalytic activity for this

reaction, even at 80 °C over 1 week, whether AlMe3 had been introduced or not. This

suggests that the site responsible for catalysis is not simply an encapsulated aluminum

species, but rather a covalently bound aluminum species.


27

2.2.2 Range of metallation in Al@MOFs

Once the introduced aluminum was deemed responsible for catalysis, catalyst

loading was evaluated with regards to maximizing catalyst efficiency. By introducing

the proper amount of catalyst, the catalyst site density could be balanced with the

internal space of the MOF for efficient diffusion of reagents.

Aluminum ratios of the metallated MOFs were analyzed using both scanning

electron microscopy coupled with energy-dispersive X-ray absorption spectroscopy

(SEM-EDX) and solid state nuclear magnetic resonance (SS NMR) spectroscopy.

While inductively coupled plasma (ICP) coupled with atomic emission spectroscopy

or mass spectrometry should offer both relative and exact metal populations, the

samples did not provide reproducible results.

The Al:OH ratio (based on the amount of aluminum added and the initial

number of OH sites in the idealized formula) was varied from 0.8 to 2.3 for UiO-67.

EDX analysis revealed that, although the initial Al:OH ratio was retained after rinsing

the framework with hexanes, the ratio decreased following washings with isopropyl

alcohol (Table 2.2). This likely results from the alcoholysis of weakly bound, or

encapsulated Al species.

Table 2.2 Ratio of aluminum to initial hydroxyl sites (Al:OH) in Al@UiO-67 as

determined by SEM-EDX.

Al:OH ratio added EDX ratio post hexanes EDX ratio post iPrOH

2.3 2.3 1.5

1.1 1.1 0.9

0.8 0.8 0.8

Interestingly, no change in Al incorporation was observed when 0.8 equiv of

aluminum was added per hydroxyl, suggesting that all the Al is covalently bound.

EDX cannot distinguish between the post-synthetic Al and the inherent Al in MIL-53

and DUT-5, so for the purpose of comparing all the catalysts, the amount of AlMe3

initially added is used hereafter to represent the catalyst loading as this represents the


28

maximum possible catalyst loading. Subjecting these systems to the catalytic

conditions revealed no significant enhancement to the conversion beyond a ratio of

0.75 Al:OH for Al@UiO-67 and 0.5 Al:OH for Al@DUT-5 (Figure 2.5).

Figure 2.5 Catalytic reduction of cyclohexanone using varying amounts of added

aluminum in MOFs (10 mol% activated MOF used in each case). Lines are provided

for visualization.91

This lack of enhancement beyond these levels may be attributed to removal of

loosely or unbound aluminum upon washing with isopropyl alcohol, leading to a

leveling of catalyst loading. Subsequent reactions are carried out under initial catalyst

loadings of 1.0 Al:OH for Al@UiO-67 and 0.5 Al:OH for Al@DUT-5.

2.2.3 Stability of Al@MOFs upon metallation

The stability of the MOFs upon metallation and subsequent manipulations was

evaluated using PXRD. UiO-66 shows no significant change in crystallinity upon

metallation and subsequent addition of isopropyl alcohol (Figure 2.6). MIL-53as and

MIL-53RT PXRD, shows no loss in crystallinity upon metallation and shows little

change after being subjected to MPV reductions.


29

Figure 2.6 PXRD patterns of UiO-66HCl, Al@UiO-66HCl, and Al@UiO-66HCl post

MPV reduction reactions.


30

Figure 2.7 PXRD patterns of MIL-53as, Al@MIL-53as, and Al@MIL-53as post MPV

reduction reactions.

For the larger MOFs, while the PXRD patterns of the optimally metallated

materials after hexanes washes shows a small decrease in crystallinity in the larger

UiO-67 (Figure 2.8a) and DUT-5 (Figure 2.8b), exhibit almost no decrease in

crystallinity. As there is little to no change in crystallinity, the MOFs retain their

structure and are not physically degraded throughout the MPV reduction.


31

Figure 2.8 PXRD of a) UiO-67 and b) DUT-5 materials (from bottom to top)

compared with the predicted patterns, after activation, after treatment with AlMe3,

after subsequent washing with isopropyl alcohol, and after subsequent catalytic

experiment.91

2.2.4 Vibrational spectroscopy as evidence of metallation

FTIR of Al@UiO-66 shows that the µ3-OH stretch at 3677 cm-1 is lost upon

introduction of AlMe3 and subsequent treatment with isopropyl alcohol. This suggests

that the Al that is introduced is covalently bound to the inherent hydroxyl sites in the

MOF. In addition, the carbonyl stretch associated with the strongly bound DMF in the

MOF is not affected by metallation but is lost after being subjected to the MPV

reduction conditions, suggesting that the DMF is labile under the reaction conditions


32

in isopropyl alcohol. Al@MIL-53RT shows only a slight shift of the µ2-OH stretch

(from 3668 cm-1 to 3683 cm-1) in the FTIR spectra, suggesting that there has not been

a significant amount of covalently bound AlMe3 incorporated.

Figure 2.9 FTIR spectra of µ3-OH stretch at 3677 cm-1 of UiO-66, Al@UiO-66, and

Al@UiO-66 post MPV reduction catalysis.

The FTIR spectrum of UiO-67 after activation contains a peak at 3674 cm−1

attributed to the μ3-OH (Figure 2.10)42. This peak disappears following treatment of

the activated framework with AlMe3 (in an Al:OH ratio of 1:1), supporting that this is

the site of metalation. Weaker Al–O stretches are most likely obscured in the

fingerprint region of the FTIR spectra. Subsequent treatment with isopropyl alcohol,

which could lead to alcoholysis of the Al site, does not result in the reappearance of

the OH stretch at 3674 cm−1. This is consistent with the acidity of the μ3-OH sites.95

The FTIR spectrum of metallated DUT-5, on the other hand, shows little noticeable

change upon metalation when monitoring the OH stretch at 3696 cm−1 (Figure 2.11).4

The lack of change in each spectrum with regards to new Al–O stretches may be


33

attributed to the overlap of the vibrational modes of the newly formed Al–O bonds

with M–O bonds native to the MOF.

Figure 2.10 FTIR of UiO-67 (black) showing loss of O-H str upon introduction of

AlMe3 (blue) and subsequent introduction of isopropyl alcohol (orange).91

Figure 2.11 FTIR of DUT-5 (black), after introduction of AlMe3 (blue) and

subsequent washing with isopropyl alcohol (orange).91

The Raman spectra for the metallated MOFs also shows notable shifts to lower

energy for the peaks at ∼1480 cm-1 of UiO-67 and DUT-5. These peaks likely

correspond to a combination of C–O and C–C stretches that are affected by changes at

the cluster, which is consistent with metalation of the bridging hydroxide. These peaks


34

shift in response to dehydroxylation (calculated A1 mode for UiO-67 shifts from 1473

to 1485 cm−1)42 (Figure 2.12).

Figure 2.12 Raman spectra of UiO-67 (black), after introduction of AlMe3 (blue), and

subsequent addition of isopropyl alcohol (orange).91

2.2.5 NMR studies of Al@MOFs

Incorporation of aluminum through post-synthetic modification was probed via

solid state magic angle spinning (SS MAS) 27Al NMR. Figure 2.13a shows the initial

Al environments in DUT-5. Here, the primary coordination environment is octahedral

(−50 to 20 ppm).4,102 Small populations of 5- and tetrahedral aluminum can also be

observed at 33 and 66 ppm, respectively. Figure 2.13b depicts the Al resonances in

DUT-5 after metalation. A significant increase in octahedral Al can be observed at ∼6

ppm as well as a modest increase in 5- and tetrahedral Al. Fitting of the spectra reveals

that the total increase is 50%. This equates to an Al:OH ratio of 0.5, consistent with

the initial loading of AlMe3 during the post-synthetic modification. Separately, Figure

2.13c depicts the 27Al NMR for UiO-67 following treatment with AlMe3 and isopropyl

alcohol. Here it can be seen that the post-synthetically introduced Al in Al@UiO-67

occupies very similar chemical environments to the post-synthetically introduced Al in

Al@DUT-5. In all cases, narrow resonances associated with encapsulated and,

therefore, freely rotating Al species are absent, again indicating that the Al is

covalently bound to the framework.


35

Table 2.3 27Al SS MAS NMR of MOFs, comparing relative Al ratios.

Catalyst Td Al/Oh Al 5-coordinate Al/Oh Al

MIL-53as 0 0.011

Al@MIL-53as 0 0.012

Al@MIL-53as post 0 0.010

DUT-5 0.015 0.011

Al@DUT-5 0.023 0.042

Al@DUT-5 post 0.027 0.028

Al@UiO-66HCl - 0.066

Al@UiO-66HCl post - 0.081

Al@UiO-67 0.069 0.194

Al@UiO-67 post 0.023 0.041


36

Figure 2.13 SS 27Al NMR of a) activated DUT-5 b) Al@DUT-5 and c) Al@UiO-67 (*

denotes background signal from the rotor).91


37

2.2.6 MPV reduction split test of Al@MOFs

To verify that Al was not leaching and acting as a homogeneous catalyst,103 a

split test was performed on the metallated MOFs (Figure 2.14). In a split test, identical

reactions are set up, with filtering of the solids after a set amount of time in half of the

reactions. In all cases, no further reactivity was observed after separating the MOF

from the reaction mixture. A side-by-side comparison of the nonseparated mixture

shows continued reactivity which is consistent with the catalyst being heterogeneous.

This supports the conclusion that incorporated Al is covalently bound, as suggested by

the vibrational spectroscopy in 2.2.4.


38

Figure 2.14 Split testc reduction of cyclohexanone in the presence of wet or dry

isopropyl alcohol and metallated MOFs a) Al@UiO-67 and b) DUT-5.91

2.2.7 Reuse and recovery of Al@MOF catalysts

One of the major advantages of a heterogeneous catalyst can be the ease of

recovery and reuse. To test this, the reaction time was maintained at 1 h to allow

differences to be observed. While the larger MOFs demonstrate high initial reactivity,

UiO-67 shows a 22% decrease in conversion after 5 reaction cycles. Continuation of

the reaction for a total of 20 h on the fifth cycle revealed quantitative conversion.

c Solids were filtered after centrifugation at t=10 minutes


39

DUT-5, on the other hand, shows an 83% loss in conversion (Figure 2.15) after 4

reaction cycles. Furthermore, continuation of the reaction for 20 h on the fourth cycle

only resulted in a yield of 76%. None of the MOFs showed any significant loss of

crystallinity after 5 reaction cycles when compared to the post-synthetically modified

samples (Figure 2.8). This suggests that the loss of reactivity in the case of DUT-5 is

not due to framework decomposition or pore collapse. One possible reason for the

decline in reactivity is the presence of adventitious water in the reaction mixture and

washes between recycles. As aluminum alkoxides are prone to hydrolysis, this seems

to be the most likely scenario.

Figure 2.15 Catalytic MPV reduction of cyclohexanone using [email protected]

Dry isopropyl alcohol is typically one of the requirements for homogeneous

MPV catalysis, as even small amounts of water lead to the hydrolysis and deactivation

of the Lewis acid catalysts.80 Wet isopropyl alcohol was used in order to evaluate the

impact of increased amounts of water on the catalytic reduction of cyclohexanone at

80 °C. With Al@UiO-67, a lower yield as compared to the first cycle with dry

isopropyl alcohol was observed. Notably, it was on par with the yield of the second

cycle with dry isopropyl alcohol, indicating that water might indeed be responsible for

the initial decrease in yield from cycle 1 to 2 with dry isopropyl alcohol. Further

recycles with wet isopropyl alcohol revealed small decreases in conversion similar to

that observed with dry isopropyl alcohol. These small subsequent decreases in

reactivity can be attributed to attrition of the catalyst through manipulation. The last

reaction cycle of Al@UiO-67 with wet isopropyl alcohol was continued for a total of


40

20 h, and quantitative conversion was observed. A more drastic decrease in reactivity

was observed upon recycling with wet isopropyl alcohol using DUT-5. The initial

cycle only yielded 81% of the product after an hour, 12% less than when the reaction

was performed with dry isopropyl alcohol. Subsequent recycles demonstrated a rapid

loss in activity, indicating that the catalytic Al sites in Al@DUT-5 are more sensitive

to water than the Al sites in Al@UiO-67. A split test was performed to determine if

catalyst leaching was occurring in the presence of water. Again, no additional

conversion was observed after hot filtration (Figure 2.14), implying that if the water is

leading to leaching of the aluminum, then it is in an inactive form. It is more likely

that the water is leading to hydrolysis at the more basic alkoxide to give a bound

aluminum hydroxide, rather than hydrolysis at the less basic bridging oxido which

would give molecular aluminum species in solution.

2.2.8 Substrate scope of Al@MOF catalysts

Five additional substrates were evaluated with UiO-67 and DUT-5 catalysts in

order to determine the substrate scope of the catalysts. Table 2.4 depicts these and

compares the results with the catalytic activity of in situ prepared Al(OiPr)3, a more

active form of the catalyst than isolated Al(OiPr)3.104 Valeraldehyde and hexanal

showed conversion as expected, whereas dodecanal was not reduced under the same

conditions. These observations contrast with the results obtained using Al(OiPr)3

which catalyzed the reduction of all three. This suggests that, even though conversion

was expected, the larger substrate, dodecanal, was prevented from interacting with the

catalyst as a consequence of increased size (Table 2.4). One of two cases are

anticipated: 1) the reagent is too large to enter the framework or 2) the reagent is too

bulky to enter the coordination sphere of the catalyst. In addition, benzyl ketones were

not as easily reduced in the frameworks, suggesting the added bulk impacts the

kinetics. This can result from either slower diffusion or congestion at the catalytically

active site. Lastly, in the cases of the smaller aldehydes in both metallated and non-

metallated UiO-67, a side reaction was observed, leading to a mixture of aldol


41

condensation products, which was not observed in the case of metallated DUT-5. It

should also be noted that if the reaction mixture was left at elevated temperatures for

24 h instead, the metallated UiO-67 gave only the expected alcohol, with no

byproducts, whereas the non-metallated case gave a mixture of products, with little

preference for the MPV product. This suggests that the Zr SBU of the MOF may

interact differently from the incorporated Al depending on the substrate.

Table 2.4 Percent conversion (and TON) for MPV reduction of select substrates at 80

°C over 1 hour.

Substrate Structure AlMe3 AlMe3 Al@DUT-5 Al@UiO-67

Cyclohexanone

95 (41) 96 (42) 93 (40) 66 (29)

Pentanal

72 (31) 82 (36) 96 (42) 16 (7)

Hexanal

71 (31) 81 (35) 82 (36) 65 (28)

Dodecanal

57 (25) 69 (30) 4 (2) 0

Benzaldehyde

99 (43) >99 (43) 70 (30) 81 (35)

Acetophenone

51 (22) 67 (29) 16 (7) 15 (7)

Benzophenone

74 (32) 86 (36) 6 (3) 9 (4)


42

2.2.9 Native MOF reactivity for MPV reduction

MIL-53 proved to be innocent under these reaction conditions, although DUT-

5, the larger isostructural MOF formed from BPDC2- instead of BDC2- was

catalytically active for the MPV reduction of cyclohexanone in isopropyl alcohol.

The Lewis acidic sites inherent in the UiO-6X series proved to be catalytically

active for the MPV reduction.12 The smaller UiO-66HCl showed similar reactivity

(59% conversion) over 12 hours at 80 °C to the metallated MOF. As the inherent

reactivity of the MOF could not be completely decoupled from the activity of

aluminum sites added at these temperatures, larger MOFs were considered. The larger

UiO-67 showed almost no reactivity (9% conversion) at 60 minutes at 80 °C,

compared to conversions greater than 50% under similar conditions in the case of the

metallated MOF.

The smaller MOF, UiO-66HCl, also exhibits recyclability, showing similar

conversions even after the fifth reaction cycle at 80 °C for 12 hours. The inherent

metal sites of the MOF do not show the same decomposition of a molecular catalyst

incorporated into the MOF, and the MOF retains its crystallinity even after multiple

recycles, showing that the native MOF is a viable catalyst for this reaction. The

activity, however, of the non-metallated MOFs is less than the metallated MOF in

each case. Conversely, both DUT-5 and MIL-53as and MIL-53RT were not active over

the same timeframe at elevated temperatures.

2.2.10 Testing defect-free MOFs for MPV reductions

As defects are vital to understanding the behavior of catalysis in MOFs, UiO-

66 containing fewer defects was synthesized using acetic acid as the modulator in the

synthetic procedure. This material was dubbed UiO-66AcOH and was subjected to the

same reaction conditions as UiO-66HCl (Table 2.1). UiO-66AcOH showed reduced

activity compared to Al@UiO-66HCl and metallation with AlMe3 did not afford a

dramatic increase in activity. This shows that although the defects contribute to the


43

catalytic activity of the MOF, even a low-defect material is still catalytically active.

This less active material was not considered for further studies.

2.3 Knoevenagel condensations in UiO-66 and MIL-53

For the Knoevenagel condensation of malononitrile and benzaldehyde (Figure

2.16), the MOFs were treated with Et2Zn (denoted Zn@MOFs), which would yield a

single-site Zn species expected to be analogous to previous Zn catalysts used for the

reaction.105,106 Many of the current homogeneous approaches for Knoevenagel

condensations are base-catalyzed, but there has recently been a number of reports of

heterogeneous zinc-catalyzed Knoevenagel condensations.85,105,107,108 A number of

MOFs have been considered which contain Zn, and it is suspected that the Zn-O

moiety is responsible for the Knoevenagel condensation.109 The formation of

covalently bound single-site Zn-O species within the MOF was hypothesized to be

active for this reaction, and more acidic sites within the MOF would more strongly

bind Zn.

Figure 2.16 The targeted Knoevenagel Condensation, of benzaldehyde and

malononitrile to give 2-benzylidenemalononitrile.

2.3.1 Catalysis using Zn@MOFs

Both UiO-66 and MIL-53as exhibited improved reactivity upon addition of

zinc. Catalytic activity in the case of Zn@MIL-53as dropped off sharply, losing all of

the activity associated with the added zinc after only 3 recycles. UiO-66 treated with

Et2Zn showed initial activity that decreased steadily. This can be attributed to the zinc

being easily washed away. The more open Zn@MIL-53RT shows higher activity,

nearing 100% yield, however on recycling the material, the activity dropped to 11%.

This suggests the initial activity is due to the more open activated pore, but either any


44

introduced Zn is quickly lost or the catalytically active sites in the MOF are

coordinated by adventitious solvent in the reaction, deactivating sites in the MOF.

Figure 2.17 Knoevenagel condensation of benzaldehyde and malononitrile in DMSO,

with sampling every 1 hour over 5 reaction cycles.

2.3.2 Vibrational spectroscopy: evidence of metallation

Upon metallation of the MOFs with Et2Zn, there were a number of changes to

the infrared spectra. UiO-66 showed a loss of the μ3-OH stretch, which is consistent

with metallation at the bridging hydroxyls in the SBU as seen in Figure 2.18. After

subjecting the metallated MOF to the reaction conditions, the hydroxyl stretch is still

not as apparent, suggesting that the MOF is still metallated. After the reaction

conditions, the carbonyl stretch associated with the strongly bound DMF in the MOF

is also absent.


45

Figure 2.18 FTIR spectra of the µ3-OH stretch of UiO-66 (black), Zn@UiO-66

(orange) and Zn@UiO-66 after 3 reaction cycles (blue).

Both MIL-53as and MIL-53RT show no change by FTIR upon introduction of

Et2Zn, and remain constant after being used for Knoevenagel Condensation reactions,

suggesting there is not a significant amount of Zn covalently bound in the MOF.

2.3.3 Native MOF reactivity for Knoevenagel condensation

UiO-66 was shown to be active for the Knoevenagel Condensation of

malononitrile and benzaldehyde. The MOF showed some loss in catalytic activity on

subsequent recycles. This, coupled with the retention of the crystallinity in the MOF,

suggests that DMSO or water in the reaction mixture coordinates to the active site

responsible for catalysis throughout the reaction, leading to a less catalytically active

MOF (Figure 2.17). Both non-metallated MOF phases for MIL-53 that were used for

catalysis showed only a small amount of background reactivity that remained constant.

This shows that any activity above this small reactivity in the case of the metallated

MIL-53 materials is due to the added Zn.


46

2.3.4 Split test Knoevenagel condensation using Zn@MOFs

All of the MOFs treated with Et2Zn exhibited catalytic activity even after

removal of the MOF through filtration (Figure 2.19). As the natant solution showed

activity, catalytically active Zn is leaching into solution. For the Zn that is

incorporated, either the hydroxyl site is not accessible, or the Zn-O bond is sufficiently

weak enough to be susceptible by removal through washing. This led us to consider

incorporating a different motif in the MOF in the hope of a more robust Zn catalyst

(See Page 64)

Figure 2.19 Knoevenagel condensation of benzaldehyde and malononitrile in DMSO

at room temperature split test studies using Zn@MOFs, with filtering occurring at 10

minutes. Shown here are a) Zn@UiO-66, b) Zn@MIL-53as and Zn@MIL-53RT.

2.4 Experimental

Warning! Trimethyl aluminum and diethyl zinc are pyrophoric, reacting

violently with moisture. Use must be done under inert conditions and care must be

taken to quench any unused materials.

2.4.1 General experimental methods

Zirconium tetrachloride (98.0%, EMD Millipore), Aluminum nitrate

nonahydrate (98.0%, J.T. Baker Chemical Co.), terephthalic acid or H2BDC (99.0%,

TCI), 2,5-dihydroxyterephalic acid or H2BDC-(OH)2 (97%, Alfa Aesar), 4,4ʹ-biphenyl

dicarboxylic acid or H2BPDC (95%, Ark Pharm) trimethylaluminum (25 w/w% in

hexanes, Alfa Aesar), cyclohexanone (99+%, Alfa Aesar), hexanal (>95%, TCI),

dodecanal (95%, stab. Alfa Aesar), benzaldehyde (99+%, Alfa Aesar), benzophenone


47

(99%, Alfa Aesar), acetophenone (99%, Alfa Aesar), DMF or n,n-dimethylformamide

(99.9%, EMD Millipore), hydrochloric acid (36.5-38% BDH), and deuterated

chloroform (D, 99.8% Cambridge Isotope Laboratories, inc.) were used as purchased

without further purification. Dry isopropyl alcohol (99.8% Extra Dry, Acros Organics)

and hexanes stored over activated molecular sieves, with wet isopropyl alcohol

(99.5%, BDH) being used as purchased without further purification. Air sensitive

manipulations were performed in an N2 purged inert atmosphere box (LC Technology

Solutions Inc.). The 1H NMR spectra were obtained on a JOEL ECS 400. All

measurements were carried out at 298 K and NMR chemical shifts are given in ppm.

The 1H NMR spectra were referenced to the residual protonated solvent for 1H. All

FTIR spectra were obtained using a Nicolet iS 5 FT-IR spectrometer equipped with a

transmission FTIR attachment or a diamond-ATR attachment. MOF samples were

alternatively pressed into KBr pellets using a 2 ton press or mounted on the diamond

ATR attachment. Raman spectra were acquired with a StellarNet Inc Raman system

feature a 1065 nm laser and a Raman-HR-TEC-IG thermo electrically cooled

spectrometer with a Raman probe. Spectra were acquired under in nitrogen filled vials

at a laser power of 6 and sampling for 30 seconds. For each sample, 5 scans were

averaged together to give the final spectra. Thermogravimetric analysis (TGA) profiles

were collected on a Shimadzu TG-50 under air from 25 to 800 °C at 5 °C min−1 in a

platinum pan, with a dwell time of 60 minutes at 100 °C. Nitrogen sorption

measurements were performed at 77 K on a Quantichrome Autosorb iQ gas sorption

analyzer. Approximately 50 mg of the MOFs were added to a preweighed 6 mm

sample cell. All samples were activated under vacuum at 200 °C for 13 hours under

vacuum. The sample weight was then collected to accurately depict the activated

weight. The activated MOFs had weights of approximately 40 mg, which were used as

the final weight of the material. Analysis time of 20 hours and 15 minutes. Brunauer-

Emmett-Teller surface areas were calculated using the DFT method in the

Quantachrome ASiQwin software. SEM-EDAX were collected on a Zeiss crossbeam

540 with Oxford EDS system after mounting the materials on an aluminum stub with


48

carbon tape, followed by sputter coating with less than 10 nm Au/Pd for conductivity.

Materials were sampled in 3 different areas (with and without grinding the sample to

compare homogeneity of metallation) and the relative metal values averaged to give a

more complete metal ratio. EDX was performed on UiO-67 samples to determine

relative ratio of Al:Zr.

Solid-state NMR (SSNMR) spectra were acquired on a 600 MHz Agilent DD2

spectrometer (Agilent Technologies, Santa Clara, CA and Loveland, CO) with an

Agilent FastMAS SSNMR probe equipped with S6 a 1.6 mm stator. All samples were

activated on a vacuum line. All post reaction samples were washed several times with

the reaction solvent in order to remove excess homogenous species. All samples were

packed in 1.6 mm SSNMR rotors, also purchased from Agilent Technologies. Samples

were sealed with silicone rubber spacers. The FastMAS probe was placed in 1H and

13C two channel configuration. The 13C channel was retuned for 27Al experiments. The

magic-angle spinning (MAS) speed was maintained at 38,000 ± 5 Hz using an Agilent

MAS controller. The variable temperature (VT) setpoint was maintained at 10 °C,

thus, assuming no dielectric heating, the true sample temperature was 22 ± 2 °C. 1H

90° pulses were calibrated to 1.3 μs and 13C pulses were calibrated to 1.6 μs. The

optimal 27Al excitation pulse was found to be 0.6 μs at the same output power as the

13C 90° pulse. Three experiments were acquired on all samples. The first was 1H direct

polarization (DP). 1H DP spectra detect all observable 1H resonances. In SSNMR, 1H

linewidths exhibit tremendous variability based upon the mobility of each site. More

immobile sites produce very broad lines because of strong 1H1H dipolar couplings.

However, internal sample motions or isotropic mobility of small molecules, including

residual solvent, can produce very narrow lines. In the second experiment, 13C

resonances were detected using adiabatic 1H13C cross polarization (CP).110 Here, 13C

signal is increased by transferring 1H polarization to 13C using through-space dipolar

couplings. The result is greater signal to noise at the expense of losing mobile 13C

resonances that lack strong 1H13C through-space dipolar couplings. After CP

polarization transfer, 140 kHz of optimized SPINAL 1H decoupling was applied to


49

dramatically improve the 13C spectral resolution.111 27Al spectra were acquired by DP

using an empirically optimized maximal excitation pulse. 140kHz of SPINAL

decoupling were also acquired during acquisition.

2.4.2 Synthesis of UiO-66HCl

This material was prepared as previously reported.112 To a 1000 mL screw-

topped vial, 17.2 mmol (4.0 g) ZrCl4 and 31 mL concentrated HCl were suspended in

150 mL N,N-dimethylformamide (DMF). This mixture was sonicated for 20 minutes

before addition of 24 mmol (3.98 g) H2BDC and 300 mL DMF. This mixture was then

sonicated for 20 minutes before being left in a chemical oven overnight at 80 °C. After

allowing the vial to cool in ambient conditions, the reaction mixture was filtered over

a fine frit. The material was washed with 20 mL of DMF three times, followed by 20

mL of ethanol three times to give fine white powder. This material was further

activated at 90 °C under high vacuum to give the activated MOF. Phase identification

was performed using PXRD (Figure 2.6), with functional group identification

performed using FTIR (Figure 2.22)


50

Figure 2.20 PXRD patterns of UiO-66, Zn@UiO-66, and Zn@UiO-66 post

Knoevenagel condensation catalysis.

Figure 2.21 FTIR spectra of UiO-66, Al@UiO-66, and Al@UiO-66 post MPV

reduction catalysis.


51

Figure 2.22 FTIR spectra of UiO-66, Zn@UiO-66, and Zn@UiO-66 post

Knoevenagel condensation catalysis.

2.4.3 Synthesis of UiO-66AcOH

Preparation of the material was adapted from a known synthetic procedure.112

To a 1000 mL screw-topped vial, 4.0 mmol (0.93 g) ZrCl4 and 68.7 mL concentrated

acetic acid were suspended in 250 mL DMF. This mixture was sonicated for 20

minutes before the slow addition of 4 mmol (0.67 g) H2BDC in 250 mL DMF. This

mixture was then stirred until homogeneous before being left in a chemical oven for

24 hours at 120 °C. After allowing the vial to cool in ambient conditions, the reaction

mixture was filtered over a fine frit. The material was washed with 20 mL of DMF

three times, followed by 20 mL of ethanol three times to give fine white powder. This

material was further activated at 90 °C under high vacuum to give the activated MOF.

PXRD confirms phase purity (Figure 2.23).


52

Figure 2.23 PXRD pattern of UiO-66AcOH.

2.4.4 Synthesis of UiO-67


To a 1000 mL screw-topped vial, 17.2 mmol (4.0 g) ZrCl4 and 22.6 mL concentrated

HCl were suspended in 200 mL N,N-dimethylformamide (DMF). This mixture was

sonicated for 20 minutes before addition of 12.2 mmol (2.95 g) 4,4ʹ-

biphenyldicarboxylic acid and 480 mL DMF. This mixture was then sonicated for 20

minutes before being left in a chemical oven overnight at 80 °C. After allowing the

vial to cool in ambient conditions, the reaction mixture was filtered over a fine frit.

The material was washed with 20 mL of DMF three times, followed by 20 mL of

ethanol three times to give fine white powder. This material was further activated at 90

°C under high vacuum to give the activated MOF. See Figure 2.8a for phase

confirmation and Figure 2.24 for FTIR spectra.


53

.

Figure 2.24 FTIR of UiO-67 act and Al@UiO-67.

2.4.5 Synthesis of MIL-53as

This material was prepared as previously reported.4 To a 100 mL Teflon-lined

autoclave, 0.845 g 3.5 mmol AlCl3·6H2O, 0.275 g 1.7 mmol H2BDC and 30 mL water

were added. The autoclave was tightly screwed shut and the reaction mixture was left

in a chemical oven at 220 °C for 72 hours. Upon cooling, the reaction mixture was

filtered and subsequently washed with over 100 mL water and dried in air. The

material was further dried in a muffle furnace at 200 °C for 3 days to give the MOF as

a white powder. See Figure 2.25 for phase identification and Figure 2.27 for FTIR

spectra.


54

Figure 2.25 PXRD patterns of MIL-53as, Al@MIL-53as, and Al@MIL-53as post MPV

reduction catalysis.


55

Figure 2.26 PXRD patterns of MIL-53as, Zn@MIL-53as.

Figure 2.27 FTIR spectra of MIL-53as.


56

2.4.6 Synthesis of MIL-53RT


A solution of H2BDC (0.688 g, 4.14 mmol) and NaOH (0.358, 9 mmol) was prepared

in 6 mL deionized H2O (322 mmol). A second solution was prepared by dissolving

AlCl3·6H2O (0.966 g, 4.14 mmol) in 4 mL of deionized H2O and was added to the first

solution dropwise over 10 minutes under stirring. After 24 hours of stirring at room

temperature, the solution was acidifed and then washed several times with deionized

water (0.560 g). (as-synthesized MIL-53RT) The synthesized MOF was activated at

330 °C over 3 days to give a beige powder, MIL-53RT. PXRD confirms phase purity.

Figure 2.28 PXRD patterns of MI-53 RT, Al@MIL-53 RT, and Al@MIL-53 post

MPV reduction.


57

Figure 2.29 PXRD patterns of MIL-53 RT, Zn@MIL-53 RT, and Zn@MIL-53 RT

post Knoevenagel condensation.

2.4.7 Synthesis of DUT-5


To a 500 mL round bottom flask, 5.4 mmol (1.31 g) H2BPDC dissolved in 200 mL

DMF at 120 °C. A solution of 7.0 mmol (2.63 g) Al(NO3)3·9H2O in 50 DMF was then

added, and the reaction mixture was allowed to stir under reflux for 24 hours.38 After

allowing the mixture to cool, the material was filtered to give an off-white powder

which was washed with DMF and EtOH before activation at 90 °C under high

vacuum. See Figure 2.8b for phase identification and Figure 2.30 for FTIR spectra.


58

Figure 2.30 FTIR of DUT-5 and Al@DUT-5.

2.4.8 Synthesis of MOF-74

This material was prepared as previously reported.32 To a solution of 45:3:3

mL of DMF:EtOH:H2O, 0.565 mmol (0.112 g) 2,5-dihyroxy-1,4-benzenedicarboxylic

acid and 1.85 mmol (0.475 g) Mg(NO3)2·3H2O were added. This mixture was

sonicated for 20 minutes before decanting into 5 20 mL scintillation vials. These vials

were then heated in a chemical oven under static conditions at 125°C for 21 hours.

After cooling to room temperature, the mother liquor of the vials was decanted and

replaced with methanol before transferring the vials into centrifuge tubes. Over the

course of 2 days, the methanol solution was decanted and replaced with fresh

methanol a total of 5 times. After evacuating the materials to dryness, the MOFs were

activated under high vacuum at a temperature of 250 °C for 6 hours. PXRD confirms

phase purity (Figure 2.31).


59

Figure 2.31 PXRD pattern of MOF-74 and Al@MOF-74 post MPV reduction

catalysis.

2.4.9 General metallation of MOFs using AlMe3

To a 2 dram vial, 150 mg of activated MOF was added with a stir bar. 2 mL of

hexanes were added and the vial was cooled to -35 °C. A dilute AlMe3 solution was

added drowise at -35 °C and the slurry was allowed to stir for 24 hours. The slurry was

then centrifuged and the solution was decanted. The materials were then washed with

2 mL of hexanes over 4 hours before centrifugation and decanting the solution. This

was repeated for a total of 3 washes. To prepare the catalysts, the materials were then

washed 3 times with isopropyl alcohol, following the above procedure.

2.4.10 General metallation of MOFs using ZnEt2

To a 2 dram vial, 150 mg of activated MOF was added with a stir bar. 2 mL

hexanes were added and the vial was cooled to -35 °C. Dropwise addition of 1 mL

neat Et2Zn solution at -35 °C and the slurry was allowed to stir for 24 hours. The


60

slurry was then centrifuged and the solution was decanted. The materials were then

washed with 2 mL of hexanes over 4 hours before centrifugation and decanting the

solution. This was repeated for a total of 3 washes.

2.4.11 Initial catalysis screening for MPV reduction

For catalytic MPV reduction experiments described in Section 2.2.1, 2 dram

vials fitted with a Teflon-lined cap were charged with a stir bar and 0.4 mmol of the

unactivated MOF (molecular formula based on 1 metal per unit). The MOFs were

thermally activated at 200 °C overnight and slowly brought into a N2 atmosphere

glovebox uncapped and covered with a watch glass to prevent loss of MOF. After

bringing the MOFs into the glovebox, 2 mL of hexanes was added to each vial,

followed by 1.0 mL of 1.0 M AlMe3 (this is in excess of the hydroxyl sites in the

MOFs). The materials were allowed to stir for 4 h before centrifuging and decanting

off the solvent. The MOFs were washed 3 times in total with 2 mL of hexanes. For

each wash, the MOFs were allowed to stir for 4 h before centrifuging and decanting

the solvent. After 3 washes with hexanes, the materials were washed 3 times with

isopropyl alcohol using the same procedure in order to remove any soluble aluminum

species. In an inert atmosphere, 1.53 mL of isopropyl alcohol was added, followed by

0.40 mL (4 mmol) cyclohexanone. The vials were capped and taped before allowing to

stir under the reaction conditions specified. At repeating intervals, the vials were

centrifuged briefly before sampling the reaction (one drop) in an inert atmosphere. The

reaction was monitored via 1H NMR in CDCl3. A typical NMR is provided in Figure

2.32)


61

Figure 2.32 Typical 1H NMR spectra of reduction of cyclohexanone in isopropyl

alcohol.

2.4.12 General MPV reduction experiment

In an inert atmosphere, to a 2 dram vial with a Teflon-lined cap, 25 mg of the

MOF catalyst was added with a stir bar. 1.53 mL of isopropyl alcohol was added,

followed by 0.78 mL of a solution of 5.1 M mesitylene (as an internal standard) in

cyclohexanone. The vials were capped and taped before allowing to stir at 80 °C over

the reaction time. At repeating intervals, the vials were centrifuged briefly before

sampling the reaction (one drop) in an inert atmosphere. The reaction was monitored

via 1H NMR in CDCl3.

2.4.13 Catalyst screening for Knoevenagel condensation

25 mg of each MOF was suspended in 1.0 mL of solution that was 1.1 M

malononitrile and 1.0 M dimethylsulfone (as an NMR standard) in reagent grade

DMSO with a stir bar. 0.1 mL benzaldehyde was added and the vials were allowed to

stir at room temperature for the reaction time. After centrifugation of the reaction

mixture, 1 drop of the reaction mixture was sampled at intervals via 1H NMR.

Conversion rates were calculated using the aldehyde proton singlet at 10.01 ppm


62

compared to the methyl singlet at 2.97 ppm for dimethylsulfone. Yields were

calculated using the alkenyl proton of the product at 7.79 ppm compared to the methyl

singlet of dimethylsulfone at 2.95 ppm.

2.5 Conclusions

A series of robust MOFs were metallated using Et2Zn and AlMe3. Both MIL-

53 and UiO-66 were used, and multiple forms of MIL-53 were used, due to the

dynamic nature of the MIL-53 pore. While the metallated MOFs were shown to be

more active for Knoevenagel condensation test reactions initially, subsequent recycles

shows that the catalyst lost activity. A split test of the catalyst after 10 minutes showed

that the active Zn catalyst leached into solution, suggesting a more acidic site may

provide a more robust anchored catalyst. The increased Lewis acidity of Al, on the

other hand, yielded catalysts for the MPV reduction of ketones and aldehydes which

were superior over the inherent MOF. This improvement was less pronounced in the

case of the smaller MOFs, UiO-66 and MIL-53, suggesting that diffusion is a more

significant issue. The larger UiO-67 and DUT-5 provided more amenable platforms

for the incorporation of an Al catalyst for the MPV reduction of ketones and

aldehydes. Al@UiO-67 was also shown to be recyclable without significant loss in

activity, even in wet isopropyl alcohol.

2.6 Future work

Larger MOFs should lead to increased activity. In addition, larger MOFs

should allow for larger, more meaningful substrates for drug design. This study lays

the groundwork for future explorations of metal catlaysts installed at these sites. Ideal

targets include homogeneous catalysts that are prone to bimolecular decomposition or

aggregation pathways.

The capability for site isolation in a MOF is ideal for separating two

components that would otherwise meet, meaning that a frustrated Lewis pair

(FLP)114,115 is possible.116,117 As FLPs require proximity of a Lewis basic and Lewis


63

acidic component, a fixed FLP in the linker of the MOF would allow catalysis without

quenching the activity.


64

CHAPTER 3

TOWARDS SELF-ASSEMBLED MULTIDENTATE SITES

(SAMS)

In contrast to much of the current literature as well as the works of Chapter 2

(Figure 3.1a), the use of multidentate sites that are self-assembled during the synthesis

of a MOF for the isolation of a chelated catalyst (Figure 3.1c) was investigated. Due to

the spatial arrangement of linkers around the SBU in some MOFs, this allows for

adjacent linkers or functional sites to be in close enough proximity for post-synthetic

modification attachment to two or more sites. This would offer a catalyst that was

chelated by the MOF, leading to a more stable catalyst. As a proof of concept,

phenolic hydroxyl groups were incorporated into the linkers to form these self-

assembled multidentate sites (Figure 3.1b).

Figure 3.1 Three different metallation models, a) metallation of bridging hydroxyl

groups inherent in the SBU, b) metallation of a single phenolic hydroxyl site, and c)

metallation of adjacent phenolic sites leading to a chelated metal site.

UiO-66-OH and MIL-53-OH were prepared and activated (as previously

reported) as hydroxyl-containing MOFs as a contrast to the previously studied UiO-66

and MIL-53.91 Confirmation of phase purity was performed using PXRD. These

materials were then metallated and considered for both Knoevenagel condensations as

well as MPV reductions.


65

3.1 Bridging motifs in MOFs

MOFs have been used to bridge multidentate ligands or catalysts within the

pore of the framework. While this leads to decreased surface area in the framework,

there are notable examples of improved activity or physical properties through

bridging adjacent linkers in a MOF. Some of the most prominent reports of bridged

catalysts in MOFs include the use of adjacent linkers bridged, as well as motifs

bridging adjacent SBUs.

The Cohen group was able to synthesize a MOF containing chemically

crosslinked linkers in the case of their IRMOF-3-AMnXL (where AM=amide,

n=number of methylene units, and XL=crosslinked). The linkers were synthesized

using two equivalents of 2-amino-1,4-benzene dicarboxylic acid (H2BDC-NH2) and

diacid chlorides to give diamide-bridged tetracarboxylates. They showed that the

length of the bridged linkers determined whether the diamide was able to bisect the

MOF pore or if the linkers were restricted to bridging adjacent positions (Figure 3.2).36

Figure 3.2 Synthesis of a cross-linked MOF using a cross-linked linker that bisects the

MOF pore.

An example of a catalyst bridging adjacent SBUs is the ALD of a nickel metal

oxo species in the SBUs of MOF NU-1000.118 Through DFT calculations, X-ray

absorption near edge spectroscopy (XANES), X-ray absorption fine structure

(EXAFS), and pair distribution function analysis (PDF) the Farha group confirmed

that the data best fit a model in which the Ni clusters bridged adjacent SBUs of the

MOF through the smallest pores of the MOF. The selective incorporation of the metal

clusters at a less accessible site over more dispersed incorporation along the larger

hexagonal channel suggests that MOFs are dynamic enough to change shape in order


66

to accommodate a more stable catalyst. In addition, the contraction of the MOF in the

c-axis suggests that the MOFs can undergo strain and distortion in order to support a

more stable catalytic species.

3.2 Knoevenagel condensations

MOFs synthesized using 2-hydroxy-1,4-benzenedicarboxylic acid (H2BDC-

OH) were treated with Et2Zn and used for the condensation of benzaldehyde and

malononitrile. The phenolic groups were hypothesized to provide a more robust Zn

catalyst chelated within the MOF over the MOFs without phenolic OH sites.

3.2.1 Vibrational evidence of metallation with Zn

FTIR spectra of the metallated MOFs shows the effect that metallation has on

the MOFs, and spectra were also collected after the MOFs were subjected to the

reaction conditions. In the case of UiO-66-OH, metallation occurs at the phenolic OH

sites as indicated by the loss of the very broad O–H stretch centered at ~3250 cm-1.

Metallation also occurs at the bridging μ3-OH sites, as seen by the loss of the weak O–

H stretch at 3640 cm-1. After the reaction, both of these OH sites are still not present,

suggesting that some of the Zn is still covalently bound in the MOF (Figure 3.3).


67

Figure 3.3 Infrared spectra of Zn@UiO-66-OH, showing metallation (orange). After

Knoevenagel condensation catalysis(blue).

This contrasts with the MIL-53-OH treated with Et2Zn, which shows only

metallation occurring at the phenolic site (Figure 3.4). This suggests that either the µ2-

OH site is not available to be metallated by Et2Zn, or that any weakly bound diethyl

zinc is washed away in subsequent washes of hexanes.


68

Figure 3.4 FTIR of MIL-53-OH (black), Zn@MIL-53-OH (orange) and Zn@MIL-53-

OH after 3 reaction cycles (blue).

3.2.2 Knoevenagel condensation split test

Split test of UiO-66-OH and MIL-53-OH treated with zinc showed that

filtration of the MOF had little effect on the catalysis occurring in solution. This

suggests that the catalytically active zinc species is leaching into solution, and that any

initial activity is short lived upon subsequent recycles. While the phenolic sites may

offer more sites for metallation, more oxophilic metals are more ideal for introduction

into the phenolic sites within the MOF. The split test data is corroborated by

microwave plasma atomic emission spectroscopy (MP AES) data showing a loss of Zn

after being subjected to the reaction conditions (Table 3.1). The relatively low amount

of Zn incorporated in Zn@MIL-53-OH agrees with the µ2-OH stretch being

unchanged in the case of Zn@MIL-53-OH. On the other hand, even after the reaction,

the phenolic stretch is not prominently seen via FTIR (Figure 3.4). This does not

correspond to a high incorporation of Zn by MP AES, however.


69

Figure 3.5 Split test of Zn@MOF-OH after filtering out the solid materials at the 10

minute mark, with a) Zn@MIL-53-OH, and b) Zn@UiO-66-OH.

Table 3.1 MP AES of MOFs treated Zn@MOF-OH and Zn@MOF-OH post reaction.

Catalyst Zn:M ratio Zn:M post reaction

UiO-66-OH 0.3 0.08

MIL-53-OH 0.12 0.06

3.2.3 Stability of the catalysts

PXRD patterns of the materials were collected upon synthesis, after treating

with Et2Zn and after undergoing 3 reaction cycles. Zn@UiO-66-OH showed a loss in

electron density in the (111) plane which corresponds to the peak at 7.38° of the

PXRD pattern (Figure 3.7). The (111) plane corresponds to a plane that bisects linkers

extending from the 3-fold symmetry of the SBU. This corresponds to a distortion of

the electron density in the framework, which is caused by introduction of 0.3

equivalents of Zn with respect to Zr in the MOF. After the reaction, the amount of Zn

decreases to 0.08 eq. of Zn with respect to Zr, and the PXRD shows that the electron

density of the (111) plane is regained, suggesting that the loss of Zn in the framework

causes the distortion to be undone. The change in the framework on addition of Zn

corroborates the changes seen by FTIR, which do not show the phenolic and µ2-OH

stretches, even though the ratio of Zn:Zr is only 0.08 post reaction (Table 3.1). After

the reaction, the material appeared to retain the overall structure of the MOF, but the

material also appeared to be significantly less crystalline than before metallation.


70

Figure 3.6 PXRD patterns of UiO-66-OH, Zn@UiO-66-OH, and Zn@UiO-66-OH

after Knoevenagel condensation catalysis. The peak at 7.38° corresponding to the 111

plane.

Figure 3.7 The (111) plane of UiO-66.


71

MIL-53-OH undergoes a number of changes its structure as seen by PXRD

after metallation, and a number of shifts are seen after the reaction as well. MIL-53-

OH, which, after activation, was isolated as the lt or low temperature phase, has an

Al–Al–Al angle of 125.8° (Figure 2.2). After introduction of Zn, there are a number of

shifted peaks in the PXRD pattern, showing that the material is affected by

metallation. The pattern still represents the MIL-53 (125.8°) phase, but after the

reaction, reflections continue to shift, even though the amount of incorporated Al

decreased from 0.12 eq. to 0.06 eq. Zn:Al in the MOF (Table 3.1).

Figure 3.8 PXRD of MIL-53-OH, Zn@MIL-53-OH and Zn@MIL-53-OH after

Knoevenagel Condensation.

3.3 MPV reductions

Due to the Zn catalyst leaching from the MOFs, Al was considered for the

same MOFs. As a more oxophilic metal, Al would more strongly bind in the MOF and

be less subject to leaching. In addition, it was hypothesized that the phenolic sites may

offer more strongly bound catalysts due to adjacent phenolic sites chelating the


72

catalyst over the µx-OH sites in the MOF. The possibility of a multidentate site formed

from adjacent phenolic sites may lead to a more robust catalyst as well.

3.3.1 Range of metallation in Al@MOF-OH

The initial Al:OH ratio (based on the initial number of OH sites in the

idealized formula) was varied from 0.25 eq. to 2.5 eq. These materials were then used

in the reduction of cyclohexanone in isopropyl alcohol at 80 °C over 6 hours. The

amount of catalyst in each case was normalized based on the theoretical amount of

aluminum added. The most effective metallation for Al@UiO-66-OH was found to be

0.25 eq. of AlMe3 added for every theoretical OH in the MOF (Figure 3.9). On

recycling, the material does appear to lose some activity. MP AES studies show that

the ratio of Al:Zr after metallation is 0.48, corresponding to 0.29 eq. of Al:OH in the

framework. After the reaction, this value does not decrease, with a relative ratio of

0.51 Al:Zr or 0.30 Al:OH in the framework (Table 3.2). Further catalytic studies and

characterization used material with this relative ratio of metallation.


73

Figure 3.9 Range of metallation of Al@UiO-66-OH from 0.25 to 2.5 eq. (with the

ratio AlMe3:OH) 80 °C over 6 hours, showing ideal amount of metallation.d

Table 3.2 MP AES of Al@UiO-66-OH and Al@UiO-66-OH, comparing Al

incorporated to Zr and OH sites in the MOF

Material Al:Zr Al:OH

Al@UiO-66-OH 0.48 0.29

Al@UiO-66-OH post 0.51 0.30

d Amount of catalyst was normalized by amount of Al added


74

Figure 3.10 Range of metallation of Al@MIL-53-OH from 0.25 to 2.5 eq. (with the

ratio AlMe3:OH) at 80 °C over 6 hours showing ideal amount of metallation.e

The same was the case for Al@MIL-53-OH, suggesting that 0.25 eq. AlMe3

for every theoretical OH in the MOF is an ideal ratio of metallation for these materials

when doing this reaction (Figure 3.10). Further catalytic studies and characterization

used material with this relative ratio of metallation. It was hypothesized that the larger

amounts of added aluminum may be restricting diffusion of substrates in pores, as the

ligating groups on the linkers are positioned into the pore aperture. Metallation of

these sites would likely have a large effect on diffusion of reagents into and out of the

pore of the MOF.

e Amount of catalyst was normalized by amount of Al added


75

Figure 3.11 MPV reduction of cyclohexanone in isopropyl alcohol using Al@MOF-

OH vs. MOF-OH at 80 °C over 6 hours.

3.3.2 Vibrational evidence of metallation of Al@MOF-OH


FTIR. Al@UiO-66-OH shows a distinct loss of absorbance in the µ3-OH stretch after

metallation (Figure 3.12), and still shows the very broad phenolic OH stretch,

suggesting that metallation has occurred at the µ3-OH site instead of the dinosaurlic

sites within the MOF. After the reaction, the µ3-OH stretch is still not showing,

suggesting that the MOF is still metallated at the same position, and the reaction does

not wash the Al sites away.


76

Figure 3.12 FTIR spectra of the phenolic OH stretch (~3250 cm-1, v br.) and the µ3-

OH (3674 cm-1, s) stretch in UiO-66-OH, Al@UiO-66-OH and Al@UiO-66-OH post

MPV reduction.

Al@MIL-53-OH, on the other hand, does not show the same definitive

evidence of metallation (Figure 3.13). There is some broadening in the phenolic OH

site consistent with incorporation of Al. After the reaction, the broadening appears to

have lessened, suggesting that the binding of the Al in the MOF has changed.


77

Figure 3.13 FTIR spectra of of the µ2-OH (3250 cm-1, v. br.) and phenolic OH (3684

cm-1, s) sites in MIL-53-OH, Al@MIL-53-OH, and Al@MIL-53-OH post MPV

reduction.

3.3.3 NMR studies of metallation of Al@MOF-OH


SS MAS NMR. Shifts in the 13C peaks as well as additional peaks in the 27Al spectra

are expected if metallation has occurred within the framework. Al@UiO-66-OH

shows the appearance of Al peaks in the 27Al spectra. These peaks include broad peaks

at 58 and 31 ppm corresponding to tetrahedral and 5-coordinate Al, respectively

(Figure 3.14a). In addition, a significant peak at 0 ppm shows the existence of

octahedral Al. This contrasts with Al@UiO-66, which does not show the existence of

tetrahedral Al, suggesting that Al@UiO-66-OH may initially have more active Al

sites. After 5 reaction cycles, the 27Al spectrum shows loss of the tetrahedral Al at 58

ppm, coinciding with a relative increase of the octahedral Al at 4 ppm (Figure 3.14b).


78

Figure 3.14 27Al SS MAS of a) UiO-66-OH after introduction of AlMe3 an subsequent

washing with hexanses and isopropyl alcohol and b) the same material that has been

subjected to reaction conditions for the MPV reduction of cyclohexanone. (* denotes

background signal of the rotor)

On introduction of AlMe3, Al@UiO-66-OH shows a new peak in the 13C

spectrum, corresponding to the carboxylate C nearest the phenol (Figure 3.15b). This


79

new peak is not observed in the case of Al@UiO-66, suggesting metallation has

affected the carboxylate carbon nearest the phenol. After the reaction, this carbon is

not as prominent, suggesting that the reactions affected the catalyst (Figure 3.15c).

This is consistent with the change in the 27Al NMR spectra, suggesting the chemical

environment of the Al has changed, affecting the 13C spectrum.


80

Figure 3.15 13C SS MAS NMR of a) UiO-66-OH, b) Al@UiO-66-OH, and c)

Al@UiO-66-OH post MPV reduction conditions.

27Al NMR of Al@MIL-53-OH shows both the tetrahedral (67 ppm) and 5-

coordinate Al (33 ppm) in Al@MIL-53-OH (Figure 3.16b). The ratio of tetrahedral Al


81

sites to octahedral sites This contrasts with Al@MIL-53, which shows no tetrahedral

Al sites by 27Al (Figure 3.16). This corresponds with the higher initial activity in

Al@MIL-53-OH over Al@MIL-53. After the reaction, however, the material shows

only a small amount of 5-coordinate Al compared to octahedral Al (0.04 eq.). This

agrees with the higher initial activity, but steeper drop off in activity on subsequent

recycles of Al@MIL-53-OH. Al@MIL-53-OH initially has higher relative populations

of tetrahedral Al (0.12 eq./Oh) over Al@UiO-66-OH (0.02 eq./Oh). After the reaction,

however, Al@UiO-66-OH has a much higher relative ratio of 5-coordinate Al (0.18

eq./Oh) over Al@MIL-53-OH post reaction (0.04 eq./Oh).

Table 3.3 27Al NMR relative amounts of the tetrahedral and 5-coordinate Al sites to

the octahedral Al sites.

Catalyst Td Al/Oh Al 5-coordinate Al/Oh Al

MIL-53-OH - 0.03

Al@MIL-53-OH 0.12 0.28

Al@MIL-53-OH post - 0.04

Al@UiO-66-OH 0.02 0.36

Al@UiO-66-OH post - 0.18


82

Figure 3.16 27Al SS MAS NMR of a) MIL-53-OH, b) Al@MIL-53-OH, and c)

Al@MIL-53-OH post MPV reduction.


83

Figure 3.17 13C SS MAS NMR of MIL-53-OH, b) Al@MIL-53-OH, and c) Al@MIL-

53-OH post MPV reduction.

13C of the material shows that the two carboxylate carbons in the MIL-53-OH

are not equivalent (Figure 3.17a), but after introduction of Al, Al@MIL-53-OH shows


84

that both of the carboxylate carbons become much closer in chemical shift (Figure

3.17b). Unexpectedly, there is no shift in the phenolic C, which seems to contradict

the vibrational evidence of metallation at the phenolic OH site (Figure 3.13). After the

reaction, the carboxylate C again separates, showing the chemical shifts are no longer

equivalent (Figure 3.17c).

3.3.4 MPV reduction split test of Al@MOF-OH

MPV reductions were carried out in side-by-side trials. Upon filtering of the

MOFs, no further catalysis was observed, confirming that the active catalyst is not

leaching into solution. This suggests that any loss in activity can be attributed to

deactivation of the catalytically active species, not due to leaching, as was suspected

from the FTIR spectra of Al@MIL-53-OH (Figure 3.13).


85

Figure 3.18 Split test of Al@MIL-53-OH (black) and Al@UiO-66-OH (red) f, with

filtering at the 1 hour mark (empty circles)

3.3.5 Stability of the Al@MOF-OH catalysts

PXRD patterns of the materials were collected to test the stability of the MOF

towards metallation and the MPV reduction. In the case of Al@UiO-66-OH treated

with AlMe3, there was little loss in crystallinity, and the material showed no evidence

of any phase-impurities that might be the result of catalyst decomposition after the

reaction, showing that the MOF retains its crystallinity. Under the reaction conditions,

the material is not degraded, suggesting any loss of activity is not due to collapse of

the framework.

f There was a higher amount of catalyst added in the Al@UiO-66-OH split, however the activity did not

continue after filtering the material


86

Figure 3.19 PXRD patterns of UiO-66-OH, Al@UiO-66-OH, and Al@UiO-66-OH

and after MPV reduction conditions

On the other hand, Al@MIL-53-OH exhibited some shifts in the PXRD, and

showed further changes after reaction cycles (Figure 3.20). This shows that

metallation and subsequent reaction cycles has an effect on the framework, and may

be responsible for some of the loss in catalytic activity. After the reaction, a number of

the reflections in the PXRD are shifted significantly, suggesting the reflections are

showing, at least in part, a different phase. The new phase appears to match most

closely with the 102.6° MIL-53 phase, suggesting that the pore is in a much more

open arrangement. This suggests that incorporation of Al into the MOF is having an

effect on the pore size and shape upon introduction.


87

Figure 3.20 PXRD patterns of MIL-53-OH, Al@MIL-53-OH, Al@MIL-53-OH post

MPV reduction


88

Figure 3.21 Comparison of PXRD pattern of Al@MIL-53-OH post with the predicted

pattern for MIL-53 (102.6°).

3.3.6 Comparison with single-site catalyst in Al@MOF

Metallation of the MOFs containing the phenolic sites may have led to more

robust catalysts over the MOFs synthesized with terephthalic acid instead. While the

MOF-OH variants are more active over their corresponding analogs (Table 3.4),

subsequent recycles were not as active. This is most likely due to the diffusion issues,

as a catalyst introduced at the phenolic sites would likely block the pores of the MOF,

preventing substrate from traveling through the MOF. This is corroborated by surface

area analysis,g which shows a decrease in surface area from UiO-66-OH (1006 m2/g)

to Al@UiO-66-OH (627 m2/g), which suggests that not only has a significant amount

g Although the ideal comparison for surface area analysis would be m2/mol of material, the

amount of MOF in each case was 25 mg and the molar mass of the catalysts are not precisely

known.


89

of Al been incorporated, but the surface area has been affected, consistent with a

blocking of the pores in the MOF (Figure 3.22).

Table 3.4 Comparison of Al@MOFs vs Al@MOF-OH MPV reduction of

cyclohexanone in isopropyl alcohol over 12 hour

MOF Reaction Time

(h)

Cat. Loading

(mg)

No Al Alh

UiO-66HCl 12 25 7 31

UiO-66-OH 12 25 21 45

MIL-53as 12 25 0 0

MIL-53RT 12 25 6

MIL-53-OH 12 25 2 78

h The amount of Al added for each MOF was based on the metallation studies on page 68. For UiO-66

and UiO-66-OH, the ratio of (Al added):Zr was 0.5 eq., and for MIL-53as MIL-53RT and MIL-53-OH,

the ratio of (Al added):OH was 0.25 eq.


90

Figure 3.22 Representation of the tetrahedral pore aperture of UiO-66, with yellow

spheres depicting some possible regions in which the catalyst could bind to a phenolic

site

3.4 Experimental

Warning! Trimethylaluminum and diethylzinc are pyrophoric, reacting

violently with moisture. Use must be done under inert conditions and care must be

taken to quench any unused materials. The Sandmeyer intermediate, a diazonium salt

should be considered shock sensitive. The intermediate should also be allowed to

properly vent while decomposing as N2 is released in the reaction. Addition of sodium

nitrite and hydrochloric acid will liberate NOx, which should be considered toxic.

3.4.1 Synthesis of 2-hydroxyterephthalic acid via Sandmeyer reaction

The procedure was adapted from a literature procedure.39 To a 1 L round

bottom flask, 44.2 mmol (8.01 g) of H2BDC-NH2 was suspended in 120 mL deionized

water. 101 mmol (4.04 g) of sodium hydroxide dissolved in 4 mL deionized water was


91

added to dissolve the starting material to give a yellow solution. To this solution, 67.0

mmol (4.62) g sodium nitrite aqueous solution was added. After allowing the material

to stir, the mixture was cooled to 5 °C and cooled 48 mL 18.6% hydrochloric acid was

added dropwise, giving an orange solution and releasing brown gas. After stirring at

10-15 °C for 4 hours, 2 spatula tips of CuSO4 were added to the slurry. The diazonium

salt was then allowed to decompose at 85 °C with stirring overnight, giving a thick,

off-white hygroscopic slurry. This material was filtered over a frit and washed

excessively with water to give the crude product. Subsequent drying in a vacuum oven

equipped with a cold trap to collect any excess hydrochloric acid gave 2-

hydroxyterephthalic acid as a beige or off-white solid. The identity of the product was

confirmed with known chemical shifts.39 7.18 g (89.7% yield). 1H NMR (400 MHz,

d6-DMSO, δ): 7.87 (d, J1=7.8 Hz, Ar-H), 7.43 (dd, J1=9.6 Hz, J2=1.4 Hz), Ar-H), 7.41

(d, J2=1.4 Hz, Ar-H)

3.4.2 Synthesis of UiO-66-OH

The procedure was adapted from a literature procedure.112 To a large screw

topped vial, 17.2 mmol (4.0 g) ZrCl4 and 31 mL concentrated HCl were suspended in

150 mL DMF. This mixture was sonicated for 20 minutes before addition of 24 mmol

(3.98 g) 1,4-benzenedicarboxylic acid and 300 mL DMF. This mixture was then

sonicated for 20 minutes before being left in a chemical oven overnight at 80 °C. After

allowing the vial to cool in ambient conditions, the reaction mixture was filtered over

a fine frit. The material was washed with 20 mL of DMF three times, followed by 20

mL of ethanol three times to give an off-white powder. This material was further

activated at 120 °C under high vacuum for 6 hours to give the activated MOF. See

Figure 3.19 for phase identification and Figure 3.23 for FTIR


92

Figure 3.23 FTIR spectra of UiO-66-OH

3.4.3 Synthesis of MIL-53-OH

The procedure was adapted from a literature procedure.119 Dissolve 21 mmol

aluminum nitrate nonahydrate in 30 mL deionized water. To this solution, 10.2 mmol

2-hydroxyterephthalic acid in 30 mL DMF was added slowly. Heat was ramped to 90

°C and left at 90 °C for 40 hours. Upon cooling, the mixture was filtered over a fine

frit to give an off-white powder, which was washed with DMF three times. A Soxhlet

extraction using EtOH was performed for 36 hours on the powder, which was placed

in a fritted cup. The material was then activated at 120 °C and high vacuum for 6

hours to give the MOF, a beige powder. See Figure 3.8 for phase identification and

Figure 3.24 for FTIR.


93

Figure 3.24 FTIR spectra of MIL-53-OH

3.4.4 General metallation of MOFs using AlMe3

To a 2 dram vial, 150 mg of MOF was added with a stir bar. 2 mL hexanes

were added and the vial was cooled to -35 °C. Dropwise addition of AlMe3 solution at

-35 °C and the slurry was allowed to stir for 24 hours. The slurry was then centrifuged

and the solution was decanted. The materials were then washed with 2 mL of hexanes

over 4 hours before centrifugation and decanting the solution. This was repeated for a

total of 3 washes. To prepare the catalysts, the materials were then washed 3 times

with isopropyl alcohol, following the above procedure.

3.4.5 General metallation of MOFs using ZnEt2

To a 2 dram vial, 150 mg of MOF was added with a stir bar. 2 mL hexanes

were added and the vial was cooled to -35 °C. Dropwise addition of 1 mL 1M Et2Zn

solution at -35 °C and the slurry was allowed to stir for 24 hours. The slurry was then

centrifuged and the solution was decanted. The materials were then washed with 2 mL


94

of hexanes over 4 hours before centrifugation and decanting the solution. This was

repeated for a total of 3 washes.

3.4.6 Catalytic procedure for MPV reduction

For catalytic MPV reduction experiments 2 dram vials fitted with a Teflon-

lined cap were charged with a stir bar and 25 mg of the catalyst. To each vial, 1.53

mmol isopropyl alcohol and 0.78 mL of a solution containing 5.1 M mesitylene (as an

internal standard) in cyclohexanone was added. Reactions were run at 80 °C, capped

and taped, in tandem side-by-side. The reaction vials were sampled at regular intervals

and yield of reaction was calculated via analysis through 1H NMR of 1 drop of

reaction mixture in CDCl3.

3.4.7 MPV reduction split test

In 2 dram vials, 25 mg of the catalyst was suspended with a stir bar. To each

vial, 1.53 mmol isopropyl alcohol and 0.78 mL of a solution containing 5.1 M

mesitylene (as an internal standard) in cyclohexanone was added. Reactions were run

at 80 °C, capped and taped, in tandem side-by-side. After 1 hour, one of the reaction

vials was filtered through a glass filter into a separate 2 dram vial, which was used for

subsequent sampling of the reaction. The reaction vials were sampled at regular

intervals and yield of reaction was calculated via analysis through 1H NMR of 1 drop

of reaction mixture in CDCl3.

3.4.8 Knoevenagel condensation catalysis


vial, 1.0 mL of a solution containing 1.0 M dimethylsulfone (as an internal standard),

1.1 M malononitrile in DMSO and 0.10 mL benzaldehyde were added. Reactions were

run at rt and sampled after 1 hr, with yield of reaction was calculated via analysis

through 1H NMR of 1 drop of reaction mixture in CDCl3.


95

3.4.9 Knoevenagel condensation split test


vial, 1.0 mL of a solution containing 1.0 M dimethylsulfone (as an internal standard),

1.1 M malononitrile in DMSO and 0.10 mL benzaldehyde were added. Reactions were

run at rt in tandem side-by-side. After 10 minutes, one of the reaction vials was

filtered through a glass filter into a separate 2 dram vial, which was used for

subsequent sampling of the reaction as the split test. The reaction vials were sampled

at regular intervals and yield of reaction was calculated via analysis through 1H NMR

of 1 drop of reaction mixture in CDCl3.

3.5 Conclusions

Although it was not able to be proven that adjacent linkers in these MOFs

arrange to form self-assembled multidentate sites, phenolic sites in MOFs were

metallated and shown to be more efficient catalysts over the inherent MOF and the

MOFs with the same structures only containing OH sites inherent to their preparation.

SS MAS NMR studies showed the existence of tetrahedral and 5-coordinate Al, which

is likely more reactive over the octahedral Al species present after reaction. After

recycling the materials, the yields of the reaction fell drastically. As the catalysts do

not leach active Al sites into solution, the Al species are deactivating by some other

mechanism. The MOFs metallated with Et2Zn were shown to incorporate Zn initially,

with 0.3 Zn sites per Zr site in the case of Zn@UiO-66-OH, and 0.12 Zn sites per Al

site in the case of Zn@MIL-53-OH. The catalytically active Zn sites were shown to

leach the catalytic species into solution over time, however, meaning that the active

sites were not strongly bound within the framework.

3.6 Future work

Formation of a MOF containing a chiral linker as well as adjacent hydroxyl

sites would allow enantioselective catalysis at a SAMS. This catalyst would likely be

more robust in comparison to homogeneous analogs. A synthetically accessible series

of compounds containing chiral amino acids have been targeted as a cheap source of


96

chirality. Preparation of MOFs would offer a divergent approach towards chirality in

MOFs, and would allow for chiral functionalized linkers in a framework. It is

hypothesized that these MOFs would be useful for asymmetric catalysis as well as

chiral separations.

A larger linker containing hydroxyl sites of the correct geometry may lead to

better activity. As shown in the previous section, the larger MOFs should offer better

reactivity and allow for larger substrates, making the catalyst more general. Single

crystals of a MOF containing phenolic OH sites would likely lead to structural

evidence of the existence or lack thereof of a self-assembled multidentate sites using

this method. The biphenyl derivative containing phenolic hydroxyls at the 2, 2ʹ

locations has been synthesized as used in the synthesis of an isoreticular analog of

MOF-74.

Although the phenolic groups did not appear to strongly bind Zn species, a

softer Lewis base such as an amine could be considered for binding Zn, as amide-

bound Zn species have been shown to catalyze the Knoevenagel Condensation of

aldehdyes with methylene compounds.120 Likely, a MOF containing amine-containing

linkers would be more ideal for Knoevenagel condensations, both as a basic site

within the MOF as well as a better ligand for incorporating Zn. In addition, less

electron withdrawing methylenes could be investigated as a more difficult substrate to

catalyze.


97

CHAPTER 4

TRIPODAL LIGAND COORDINATION CHEMISTRY

4.1 Tripodal amides

Amides (NRRʹ-) have been used as ligand scaffolds for a variety of

applications. As amides can strongly coordinate and can be modulated by the other

substituents, they offer a useful ligating strategy. Through ligand design, the steric as

well as the electronic properties of the ligand can be tailored for the complex to impart

beneficial reactivity.

Small molecule activation is the functionalization of simple, often relatively

unreactive substrate (CO2, N2, CH4, etc.), often in an effort to produce a value-added

product.121 One approach to small molecule activation is the utilization low coordinate

metal complexes in low oxidation states supported by electron-rich ligands.122 The

ability to activate small molecules can be enhanced by adding a strong reductant.

Reduction of these complexes can yield a number of possible motifs, with varying

utility. A notable example of this was the work done by the Schrock group in using a

tetradentate triamidoamine ligand ([HIPTN3N]3-) coordinated to molybdenum to

reduce N2 to ammonia at room temperature.123 In addition, they were able to isolate

multiple intermediates in the multiple electron and proton events in the catalytic cycle

of nitrogen. The bulky substituents of [HIPTN3N]3- prevented the dimerization of a

metal nitride to give a bimetallic species, while also increasing the solubility of the

complex. While this was a landmark accomplishment, the use of a costly ligand

scaffold, as well as requiring an expensive proton source in {2,6-lutidinium}{BAr’4}

means that there is still work to be done towards practical catalysts for small molecule

activation.

4.1.1 Vanadium trisamide complexes

A variety of complexes containing several nitrogen bound motifs have been

formed. Reaction of VCl3(THF)3 with neopentyllithium in a nitrogen atmosphere


98

yielded a N2 bridged dimeric species, with both V atoms being tetrahedral.124 The

organometallic dimeric V(III) complex shows short V–N distances (1.7248 Å), but

upon introduction of a proton source, N2 is lost and neopentane is produced. As the V–

C bond is weak, a stronger V–N bond may form a more robust complex. Work from

the Kajita group showed that while bulky substituents on the amido nitrogen atoms of

a ligand prevent ligands from coordinating in the axial position, a less sterically

hindered ligand allowed for in situ coordination of dinitrogen.125

Oxygen has also been a target for small molecule activation using vanadium

amide complexes, as in the case of a V(III) trisanilide complex, V(N[tBU]Ar)3 (Figure

4.1).126 In the presence of O2, the complex led to an η1 VV peroxo complex. In the

presence of excess V(N[tBu]Ar)3 led to a V≡O species.

Figure 4.1 Treatment of V(N[tBu]Ar)3 with O2 to give the V(V) peroxo species,

showing an example of small molecule activation


99

Figure 4.2 Some examples of V(III) complexes containing amide ligands including a)

[HIPTN3N]VTHF,127 b) ([(CH3)3Si]NPh)3V(THF)128 c) [N3N]V,129 d) (R2N)3V130

Another tripodal V complex was synthesized using the [HIPTN3N]3- ligand,

(HIPT=3,5-(2,4,6-i-Pr3C6H2)2C6H3)), as seen in Figure 4.2a.127 Using a very bulky

ligand, the Schrock group evaluated the reduction of nitrogen for V complexes, as the

same ligand was successful when coordinated with Mo. The N–V–N bond angles of

the ligand are almost planar, averaging 118.05°. They were able to synthesize the

[VN2]-, V=NH and V(NH3) complexes containing the HIPTN3N ligand. While it is

expected that these complexes would be important to the reduction of dinitrogen,

attempts to form ammonia were unsuccessful. Another vanadium trisamide complex

was prepared when a vanadium precursor (VCl3(THF)3) was introduced to three

equivalents of a deprotonated bulky silylamide ligand, the slightly distorted tetrahedral

vanadium complex ([(CH3)3Si]NPh)3V(THF) was recovered (Figure 4.2b), with THF

coordinating at the apical position.131 On the other hand, the treatment of the same

vanadium precursor with a tripodal trisamide ligand yielded a dimeric, diamagnetic

complex bridged with a nitrogen molecule.

The [N3N]V complex (Figure 4.2c) synthesized by reduction of [N3N]VCl by

Na/Hg amalgam in toluene yielded a trigonal monopyramidal complex in which the

vanadium only slightly deviates from the amido plane by 0.21 Å.129 This complex, and

many vanadium amide species, were shown to be highly susceptible to oxidation or

hydrolysis, even in a drybox by trace amounts of oxygen or water. This can lead to

oxidation and the formation of a dioxido dimeric species. A more useful case for

nitrogen fixation was shown by the Gambarotta group in the synthesis of a series of

V(III) complexes containing amide ligands (Figure 4.2d).130 The isopropyl and


100

cyclohexyl analogs of the complex were shown to dimerize, bridged by a molecule of

N2 end-on.

4.2 Synthesis and characterization of tripodal amide complexes

4.2.1 Tripodal amides

The condensation of 3-methylindole with triethylorthoformate yielded tris(3-

methyleneindoline)methane (H3TMIM) (Figure 4.3). This tripodal ligand were

considered in contrast to some of the tetradentate tripodal ligands (Figure 4.2).125,127,132

In some of these cases, the bottom of the cavity in which the V sits is occupied by a

neutral coordinating donor. As such, the geometry of the complexes containing

TMIM3- would be expected to be less planar over some of the current examples of

tripodal vanadium complexes.

Figure 4.3 Acid-catalyzed synthesis of H3TMIM from 3-methylindole and

triethylorthoformate

4.2.2 Lithiation of tripodal trisamide compounds

Tripodal amide precursors were attempted to be deprotonated using lithium

reagents. While n-butyllithium deprotonated the H3TMIM ligand scaffold as shown in

the incomplete crystal structure (Figure 4.4), the butyl carbanion acted as a

nucleophile, which reacted with THF in solution was activated to give a lithium


101

alkoxide.133 As this material would likely cause complications in metathesis reactions

to give the deprotonated TMIM3-, a different lithiating reagent was investigated.

Figure 4.4 Incomplete crystal structure of TMIM treated with n-BuLi, showing the

lithium activating THF to give an unbound lithium ethoxide in the structure

Lithium diisopropylamide (LDA) was considered as a non-nucleophilic

lithiating reagent. H3TMIM was successfully deprotonated with LDA, without the

undesired lithium alkoxide side products to give Li3TMIM·5THF (Figure 4.7). One of

the lithium atoms sits at the center of the tripodal ligand, with the other two in between

the amides. Five molecules of THF coordinate the lithium atoms to provide a

tetrahedral environment for each lithium atom. Isolation of Li3TMIM was not trivial

however, and the solubility of the final product varied greatly depending on the

amount of THF in the material when attempting recrystallization. FTIR of the

Li3TMIM⸱5THF shows almost complete loss of the N-H stretch at 3397 cm-1.

Similarly, 1H NMR of the material in d6-DMSO shows no N-H protons (Figure 4.6),

and the peaks of the NMR are significantly broadened and shifted from the 1H NMR

of H3TMIM in d6-DMSO.


102

Figure 4.5 FTIR comparison of Li3TMIM·5THF with H3TMIM

Figure 4.6 1H NMR of Li3TMIM·5THF in d6-DMSO


103

Figure 4.7 Crystal structure of Li3TMIM·5THF

4.2.3 Towards a vanadium TMIM complex

With Li3TMIM·5THF in hand (Figure 4.8), metathesis reactions with

VCl3·3THF were investigated, which may precipitate lithium chloride out of solution.

The vanadium complex proved to be highly moisture and air sensitive, and at each

point in the synthesis, the material needed to be kept as dry as possible. Repeated

synthetic procedures showed that even with dry solvents and careful manipulations in

the nitrogen-filled glovebox, the recovered product always showed evidence of the

protonated N–H stretch via FTIR centered at 3409 cm-1. The reaction was performed

in both THF and acetonitrile insoluble. Analysis of the reaction mixture in THF

showed the more promising results by 1H NMR as the acetonitrile solution showed

unreacted Li3TMIM·5THF. Crystals were grown out of a THF/hexanes/toluene

mixture of the VTMIM complex. Single crystal X-ray analysis of the crystals showed

that the VTMIM complex co-crystallized with an equivalent of LiCl and 3 THF

molecules. The V metal center is octahedral with the chloride and 2 molecules of THF

completing the coordination sphere.


104

Figure 4.8 Synthetic procedure of lithiation of TMIM and subsequent metallation

using VCl3·3THF to give VTMIM·LiCl·5THF


105

Figure 4.9 Crystal structure of VTMIM·LiCl·3THF

The V–N bonds are 2.000(1) Å, 2.063(1) Å, and 2.084(1) Å, averaging to be

2.049 Å. This is somewhat longer than the bond lengths of many vanadium (III) amide

complexes, suggesting the tripodal ligand is not as strongly bound as other tripodal

ligands. The vanadium shows only a slight deviation from an octahedral coordination

geometry, with the average N–V–N bond angle being 87.45 degrees. The V-OTHF

distances are 2.076 (8) and 2.1198 (8). Many V(III) amide complexes are trigonal

pyramidal,124,126,131 and there are only a few examples of V(III) octahedral amide

complexes,134–137 which contained porphyrinic ligands or were in a geometry closer to

trigonal prismatic. In the V(III) tripodal tetradentate porphyrinate VOEPG complex

(where OEPG=octaethylphyrinogen), the average V–N bonds are comparable,

averaging to be 2.062 Å, and the V–OTHF distances (2.075(5) Å and 2.059(5) Å) were

shorter, suggesting the THF will be more easily removed in the case of

VTMIM·LiCl·3THF.


106

4.2.4 Characterization of VTMIM·LiCl·3THF

1H NMR of the material in C6D6 shows that the VTMIM complex is

paramagnetic, and as such, a wider 1H NMR window was used. Chemical shifts

ranged from ~18 ppm to -18 ppm, and the peak widths were broadened, indicating

paramagnetic behavior. A qualitative Evans’ method NMR of the material in a

solution of C6D6 and hexamethyldisiloxane (Figure 4.11) shows that the material

contains unpaired electrons. Together with the EPR spectra, the material is confirmed

to be V3+. This shows that at least some of the material has not undergone oxidation to

V5+. quantitative Evan’s method studies were not completed at this time. Switching to

a deuterated NMR solvent that better dissolved the final product would likely make

these studies more feasible.

Figure 4.10 Wide window 1H NMR of VTMIM·LiCl·3THF in C6D6


107

Figure 4.11 Qualitative Evan's method of VTMIM complex in, showing the shift of

hexamethyldisiloxane in C6D6

Electron paramagnetic resonance (EPR) spectroscopy was performed on the

recovered product (Figure 4.12), and the material showed no signal indicative of an

odd number of unpaired electrons. This is consistent with either a V3+ or V5+ complex.

The sensitivity of EPR to odd unpaired electrons means that the material has not

oxidized to V4+, as the material is expected to be redox active. FTIR of the material

(Figure 4.13) shows that the material has not completely hydrolyzed, yet there still

appears to be an N–H stretch at 3409 cm-1.


108

Figure 4.12 EPR spectra of VTMIM·LiCl·3THF in C6D6-benzene, showing that the

complex is EPR silent


109

Figure 4.13 FTIR spectra of VTMIM·LiCl·3THF and Li3TMIM·5THF

4.2.5 Towards a deuterium-labeled H3TMIM

Deuterium-labeled TMIM would offer another spectroscopic handle for

reaction monitoring. Through the use of 2H NMR, the reaction mixture could be

sampled and analyzed without the use of deuterated NMR solvents. To this end,

deuterated triethylorthoformate was synthesized using a known literature procedure.138

The condensation of d1-triethylorthoformate and skatole yielded d1-H3TMIM in low

yields.

4.3 Experimental

Warning! N-butyllithium (nBuLi) and lithium diisopropylamide (LDA) are

pyrophoric, reacting violently with moisture. Use must be done under inert conditions

and care must be taken to quench any unused materials.


110

4.3.1 Synthesis of tris(3-methylindol-2-yl)methane (H3TMIM)

This synthetic procedure was adapted from a known literature procedure.139

3.93 g (30 mmol) 3-methylindole dissolved in minimal methanol (less than 6 mL) in a

50 mL Erlenmeyer flask with stirring to give a yellow solution. To this solution, 1.7

mL (13.0 mmol) triethylorthoformate was added. After warming the solution to

approximately 50 °C, 3 drops of sulfuric acid are added, causing the solution to go

from yellow to dark yellow and finally orange. After 15 minutes, the reaction mixture

yielded a dark violet color. After scratching the bottom of the Erlenmeyer flask, more

precipitate formed. The solid was collected via vacuum filtration over a filter to give a

pale green powder, which was washed with methanol to give the pure product. The

identity of H3TMIM was confirmed against known literature values.139 3.193 g (78.7%

yield)

4.3.2 Synthesis of d1-triethylorthoformate

This material was adapted from a previous literature procedure.138 In a

glovebox, a 250 mL Schlenk flask was charged with 14.3 g (210 mmol) sodium

ethoxide and 30 mL dry diethyl ether. After cooling the reaction to 0 °C and under a

flow of nitrogen, 5 mL (62.5 mmol) deuterated chloroform was added via needle

through a septum. The reaction mixture was then allowed to stir for 12 hours. The

reaction mixture was quenched over an aqueous sodium bicarbonate solution and the

mixture was extracted twice with hexanes. The organic extracts were dried over

magnesium sulfate and filtered before removing the hexanes under reduced pressure to

yield a mixture that was 48% d1-triethylorthoformate as a yellow oil (as determined

via 1H NMR). This material was used without further purification.

4.3.3 Synthesis of d1-tris(3-methylindol-2-yl)methane (TMIM)139

0.687 g (5.24 mmol) 3-methylindole dissolved in minimal methanol (less than

6 mL) in a 50 mL Erlenmeyer flask with stirring to give a yellow solution. To this

solution, 0.30 mL (1.79 mmol) triethylorthoformate was added. After warming the


111

solution to approximately 50 °C, 3 drops of sulfuric acid are added, causing the

solution to go from yellow to dark yellow and finally orange. After 15 minutes, the

reaction mixture yielded a dark violet color. After scratching the bottom of the

Erlenmeyer flask, more precipitate formed. The solid was collected via vacuum

filtration over a filter to give a pale green powder, which was washed with methanol to

give the product. 0.090 g (12.7%yield). FTIR in (Figure 4.15) shows purity, although

no C-D bond is observed. 1H NMR (δ in CDCl3): 7.72 ppm (s 3H, NH), 7.59 ppm (dd

J1=1.6 Hz, J2=6.8 Hz, 3H Ar-H), 7.27 ppm (dt, J1=J3=1.6 Hz, J4=6.8 Hz, 3H, Ar-H),

7.18 ppm (dtd, J1=1.6 Hz, J2=6.8 Hz, J3= 1.6 Hz, J4=6.8 Hz 6H, Ar-H), 2.19 ppm (s,

9H, CH3)

Figure 4.14 1H NMR spectra of d1-H3TMIM in CDCl3


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Figure 4.15 FTIR comparison of d1-H3TMIM with H3TMIM

4.3.4 Lithiation of H3TMIM with LDA

2.00 g (2 mmol) H3TMIM was dissolved in 2 mL THF. 10 mL of a 2 M LDA

solution in THF was added dropwise with stirring. The reaction was stirred overnight,

yielding a yellow solution. Removal of THF in vacuo yielded a taffy-like sludge,

dependent on the amount of THF remaining in the solid. Recrystallization of the

solution in diethyl ether with a minimal amount of THF yielded large yellow crystals

of Li3TMIM·5THF. See Figure 4.5 for FTIR spectra. See Figure 4.6 for 1H NMR

identification (δ in d6-DMSO): 7.12 ppm (d, J1=7.6 Hz, Ar-H, 3H), 7.02 ppm (d J2=7.6

Hz, Ar-H, 3H), 6.45 ppm (dd, J1=J2=6.8 Hz, Ar-H, 6H), 6.08 (s, C-H 1H), 2.42 ppm

(s, CH3, 9H)

4.3.5 Metathesis of Li3TMIM·5THF with VCl3(THF)3

44 mg (0.06 mmol) VCl3(THF)3 was suspended in 2 mL dry THF with stirring,

as the vanadium reagent exhibits low solubility in THF, giving a red solution. After


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cooling in a cold well, a cold solution of 100 mg (0.05 mmol) of Li3TMIM·5THF in 2

mL THF was added dropwise with stirring and allowed to warm to room temperature.

The reaction mixture immediately darkened to give a purple solution upon addition of

the Li3TMIM·5THF solution. The reaction was allowed to stir overnight. THF was

then removed in vacuo to give a dark purple residue. The material was dissolved in

toluene and filtered through a dried glass filter. Removal of toluene followed by

4.3.6 Crystallographic data

Table 4.1 Crystallographic data for Li3TMIM an dVTMIM⸱LicL⸱THF3

Compound Li3TMIM VTMIM⸱LiCl⸱THF3

Crystal color Colorless Blue

Crystal Habit Block Hexagonal plate

Empirical Formula C48 H62 Li3 N3 O5 C47 H54 Cl Li N3 O3 V

Formula Weight 781.82 802.26

Temperature 100(2) K 100(2) K

Wavelength 1.54178 Å 1.54178 Å

Crystal System Monoclinic Monoclinic

Space Group P21/c P21/c

Unit Cell Dimensions a=16.4687(1) Å, α=90 °

b=11.6775(1) Å, β=107.298(1) °

c=23.6198(1) Å, γ=90 °

a=16.1623(1) Å, α=90 °

b=13.5760(1) Å, β=112.836(1) °

c=20.7977(1) Å, γ=90 °

Volume 4336.95(5) A3 4205.73(5) Å3

Z 4 4

Calculated Density 1.197 Mg/m3 1.267 Mg/m3

Absorption Coefficient 0.591 mm-1 2.895 mm-1

F(000) 1680 1696


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Table 4.1 Continued

Crystal Size 0.254 x 0.201 x 0.155 mm 0.170 x 0.136 x 0.034 mm

Theta Range for Data Collection 2.810 to 77.425 ° 2.967 to 77.476 °

Limiting Indices -20≤h≤19, -14≤k≤14, -26≤l≤29 -20≤h≤20, -17≤k≤16, -26≤l≤26

Reflections Collected/Unique 61949/9089 [R(int)=0.0254] 134462/8905 [R(int)=0.0318]

Completeness to Theta=67.679 ° 100.0% 100.0%

Refinement Method Full-matric least-squares on F2 Full-matric least-squares on F2

Data/Restraints/Parameters 9089/300/629 8905/70/528

Goodness-of-fit on F2 0.988 1.062

Final R indices [I>2sigma(I)] R1=0.0365, wR2=0.0945 R1=0.0304, wR2=0.0809

R Indices (all data) R1=0.0391, wR2=0.0966 R1=0.0310, wR2=0.0813

Largest diff. peak and hole 0.252 and -0.223 e. Å-3 0.334 and -0.417 e. Å-3

4.4 Conclusions

A lithiated TMIM3- complex was prepared that was found to be a suitable

starting material for metathesis reactions with vanadium. A vanadium(III) complex

was structurally characterized, but further characterization is needed. d1-H3TMIM was

also synthesized in low yield, as a ligand containing a spectroscopic handle for ease of

identification.

4.5 Future work

Further characterization of the VTMIM·LiCl·3THF will be continued. Evan’s

method of the complex can be completed using 2H NMR in THF with a known

quantity of C6D6. Further elemental analysis is also required. Other metals could be

considered for metal complexes containing the TMIM ligand. In addition, there are a

variety of chemical transformations that could be attempted. Oxidation may be

attempted using a variety of oxygen atom transfer (OAT) agents to likely yield a V(V)

oxo complex.140 Introduction of KC8 or another reductant in the presence of N2 may


115

lead to a bound N2 complex,125 which could be used to investigate nitrogen fixation

and subsequent functionalization.

Further work could also explore the design of the ligand to provide modulation

of the steric profile either on the aromatic ring or at the methylene carbon, causing the

amides to have a different bite angle around the central metal. Ligands containing

different moieties at the “tail” of the ligand may offer a change in the cavity beneath

the V, which may lead to a change in coordination geometry of the amides. Electronic

modulation of the tripodal amide through incorporation of electron withdrawing or

donating groups could also be investigated.


116

CHAPTER 5

IRON(BIAN) COMPUTATIONAL STUDY

5.1 Fe(BIAN) complexes

This chapter is, in part, adapted from Synthesis, Characterization,

Electrochemical Properties and Theoretical Calculations of (BIAN) Iron Complexes.

Polyhedron 2019, 159, 365–374. Due to price as well as the relatively low abundance

of the more expensive transition metals which are used for many catalytic reactions,

first row transition metals have been considered as a cheaper, more abundant option

for catalysis.

Figure 5.1 Some previously used redox noninnocent ligands

Bis(imino)acenaphthenes (BIANs) have been used as a ligand scaffold for a

variety of complexes.141–143 Because it contains a conjugated system as well as

coordinating bisimines, BIAN offers a redox-noninnocent system for catalysis. Redox-

noninnocent ligands are ligands that contain functionalities that can be oxidized or

reduced in addition to any chemistry that may occur at the metal site. Some examples

include those shown in Figure 5.1

An example of redox noninnocence is shown in Figure 5.2. Instead of electron

reduction or oxidation events occurring exclusively at the metal center, the formal

charge on one or more areas of the ligating group are oxidized or reduced.


117

Figure 5.2 Reduction of a Fe(0) complex144

As multiple electron processes can occur at the ligand, the limitation of single

electron transfers at iron can be overcome. As electron density is delocalized in the

ligand, the ligand acts as an electron reservoir, allowing access to multiple electron

processes. Some previously used BIANs are shown in Figure 5.3.

Figure 5.3 Previously investigated BIAN complexes containing Fe144

A series of Fe(BIAN) complexes was synthesized and fully characterized, and

was found to be successful in the catalytic hydrosilylation of aldehydes and ketones as

well as ring opening polymerizations. The bond lengths of the crystal structures

showed an elongation of the C–N bond from the diimine from 1.281 Å to 1.336(5) Å,

as well as a shortening of the C–C bond in the diimine backbone from 1.529 to

1.422(5) Å. This contrasts with the previously reported bond lengths for BIAN-FeCl2,

which shows C–C and C–N more similar to the ligand itself. This suggests that the

ligand is not innocent in this case, and that Fe in this case is a formal Fe(I) species.

This does not correlate with the diamagnetic behavior of the complexes, which implies

that one method may not be absolute in determining the formal oxidation state of the

Fe and the ligand. Previous work has also indicated that bond distances may not be


118

absolute in determining the formal charge of the species.145 Due to this ambiguity,

theoretical calculations were attempted to provide insight into the catalysts’ behavior.

5.2 DFT calculations

Density functional theory (DFT) calculations offer models that can be

applicable to larger systems that is accurate while being efficient. These models

provide insight into the experimental behavior of the complexes. As DFT models the

electron density with regards to the lowest energy conformation of the system being

studied, it can be used to supplement experimental data to better understand the

electronic behavior of the system.

5.2.1 Geometry optimizations

To further investigate both the diamagnetic character of the complexes and

rationalize the bond lengths of the crystal structures, geometry optimizations were

performed at the B3LYP def2-TZVP level of theory on coordinates obtained from the

crystal structures146–149 As expected of FeBIAN-COD, the complex converged to a

triplet state, which is consistent with the paramagnetic character observed by 1H NMR

and magnetic susceptibility (μeff = 2.72 μB). Given the diamagnetic character of the

other complexes observed experimentally, one of two possibilities seemed likely: an

Fe(0) metal center complexed by a neutral ligand (Figure 5.4a) or an Fe(I) metal

center antiferromagnetically coupled with a monoanionic ligand (Figure 5.4b).

Figure 5.4 Valence tautomers of a Fe bismino complex with a) Fe(0) with a neutral

ligand, b) Fe(I) antiferromagnetically coupled with a monoanionic ligand, and c)

Fe(II) with a dianionic ligand144

Optimizations were performed both as spin restricted singlets and spin

unrestricted broken symmetry (BS) triplets, and in each of the investigated systems,


119

the broken symmetry triplet converged to an antiferromagnetically coupled broken

symmetry singlet designated BS(1,1) rather than to the singlet or the broken symmetry

triplet. These calculations gave J-coupling values ranging from 3000 to 4000 cm−1,

suggesting that the complexes are strongly antiferromagnetically coupled.150 The spin

up (α) and spin down (β) Kohn–Sham orbitals were shown to be unique to one

another, consistent with a triplet as opposed to a symmetry-restricted singlet (Figure

5.5). In all cases, the BS α-HOMO is primarily localized on the iron center while the

β-HOMO is located on the ligand, consistent with the idea of an antiferromagnetically

coupled Fe(I) and BIAN−. The BS complexes are lower in energy in each case than a

symmetry-restricted closed-shell singlet state or a triplet state. Although the spin-

restricted singlet solutions converged to give metrics closer to the crystal structures in

each case, both calculations reproduced the bond lengths reasonably well (Table 5.1),

reinforcing the work from Neidig and Milstein145 that bond lengths alone are not

sufficient to determine oxidation states in these complexes. For example, Mössbauer

studies have also proven instructive in assigning oxidation states,148,151–154 however,

given that Mössbauer parameters have previously been reported for complex FeBIAN-

benzene compound141,155–157 and all new structures exhibited similar structural and

spectroscopic features, examination using this technique was not pursued in the

present work.


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Figure 5.5 Geometry optimized HOMO of a) FeBIAN-benzene, b) FeBIAN-toluene,

and c) FeBIAN-trifluorotoluene, depicting the singlet optimized HOMO, broken

symmetry singlet α, and broken symmetry singlet β HOMO144

Table 5.1 Selected bond lengths and angles for FeBIAN-benzene FeBIAN-toluene and

FeBIAN-trifluorotoluene

Experimental DFT – singlet DFT – BS(1,1)

FeBIAN-Benzene

C–C (Å) 1.407(2) 1.408 1.427

C–N (Å) 1.339(2) and 1.341(2) 1.344 and 1.345 1.334 and 1.335

N–Fe (Å) 1.901(1) and 1.905(1) 1.920 and 1.925 1.974 and 1.983

Fe–Centroid (Å) 1.538 1.608 1.608

Fe–arene tilt (°) 89.45 89 88.732


121

Table 5.1 Continued

FeBIAN-Toluene

C–C (Å) 1.400(4) 1.408 1.427

C–N (Å) 1.343(3) 1.344 and 1.346 1.334 and 1.335

N–Fe (Å) 1.899(2) and 1.903(2) 1.921 and 1.925 1.977 and 1.983

Fe–Centroid (Å) 1.536 1.619 1.637

Fe–arene tilt (°) 89.92 87.86 88.36

FeBIAN-trifluorotoluene

C–C (Å) 1.411(2) 1.414 1.429

C–N (Å) 1.335(2) and 1.340(2) 1.339 1.333 and 1.334

N–Fe (Å) 1.905(1) and 1.910(1) 1.924 and 1.925 1.977

Fe–Centroid (Å) 1.539 1.611 1.64

Fe–arene tilt (°) 88.76 88.37 88.87

5.2.2 Hammett correlation

The relative energies of the SOMOs of the complexes increase from

trifluorotoluene to benzene to toluene. The change in the energy of the frontier orbitals

correlates well with the Hammett parameters, σ, for the functional group on the arene

(the average of σpara and σmeta was used to give σπ) as indicated in Figure 5.6. Past

studies have used a variety of σ values, including σ+, σp, σm, however a better

correlation was observed with the energy of the α SOMO when taking the average of

the σp and σm (R2 = 0.992) as opposed to just the σm (R2 = 0.914) or σp (R2 = 0.967). In

this case, the strong correlation with the average of the meta and para parameters

suggested that the metal–arene interaction will have both inductive and resonance

effects contributing to the interaction. This progression in orbital energy agrees with

the oxidation potentials determined in the voltammetry studies and suggest that the

oxidation occurs initially at the ligand. The relative ease of oxidation, as compared to


122

FeBIAN-benzene or BIAN is consistent with the oxidized electron residing in a

negatively charged BIAN framework.

Figure 5.6 Hammett Correlation of HOMO energy and σπ from electron donating to

electron withdrawing (from left: aniline, toluene, cumene, benzene,

trifluorotoluene)144

5.2.3 Energy comparisons

Experimentally, both the catalytic hydrosilylation and ring-opening

polymerization reactions employing FeBIAN-toluene and FeBIAN-benzene,

respectively, initiate at elevated temperatures.141,155–157 This suggests that the first step

may be the dissociation of the arene and the initiation temperature should be related to

the bond dissociation energy for Fe–Ar. In order to evaluate this, the energy of the

isodesmic reaction involving the replacement of the benzene in FeBIAN-benzene with

an alternate arene was calculated (Table 5.2). This reaction, using the benzene

complex as the standard, offers a simple comparison of the relative energies of the

various complexes. The toluene complex is calculated to be the most energetically

favorable, suggesting that it would maximize the stability of the complex and therefore

have the highest initiation temperature for the catalytic reaction. This contrasts with

the trifluorotoluene complex, which is the least favorable of the complexes and

therefore will be the easiest reaction to initiate (occurs at lowest temperature). This is

consistent with the what might be proposed from the Hammett parameters.


123

Table 5.2 Isodesmic reaction of different arene complexes of FeBIAN

Arene ΔErxn (kcal)

-0.087

2.538

2.891

0.005

Larger substituents do not follow the trend as might be expected; rather, the

bulkier substituents are predicted to disfavor exchange more strongly than their less

bulky counterparts. This is notably seen with the isopropyl or dimethylamino

substituents on FeBIAN-cumene and FeBIAN-dimethylaniline, respectively. While

the cumene complex should lie close to the toluene complex electronically, it is

predicted to readily be replaced by benzene instead. On the other hand,

dimethylaminobenzene in FeBIAN-dimethylaniline would be predicted to be electron

rich and bind tightly, yet it exchanges with benzene. As these arenes have similar


124

steric profiles, it is likely that sterics play a significant role in destabilizing these

complexes towards arene loss (Table 5.2).

5.3 Experimental

Calculations were performed using the ORCA 3.0 quantum chemistry program

package from the development team at the Max Planck Institute for Bioinorganic

Chemistry.158 Initial geometry optimizations were performed with the LDA and GGA

functionals of Becke and Perdew (BP-LDA, BP86) starting from the crystal structure

geometry.159,160 All geometry optimizations were carried out using the def2-TZV(2pf)

basis set for the iron atoms, the def2-TZV basis set for the hydrogen atoms, and the

def2-TZV(d) basis set for all others.161,162 Spin-restricted Kohn–Sham determinants

were chosen to describe the closed shell wavefunctions, employing the RI

approximation163 and the tight SCF convergence criteria provided by ORCA. All

geometry optimizations were followed by frequency calculations using the analytical

frequencies option in ORCA to ensure that there are no negative frequencies

indicating a global energy minimum. Spin-unrestricted calculations were performed

on triplet states with and without the broken-symmetry formalism150,164,165 employing

the RI approximation (RIJCOSX)163,166,167 with the BP86 functional as well as the

hybrid functional composed of Becke (B88)160 with 20% HF for the exchange and

Lee, Yang and Parr (LYP)168 for the correlation (B3LYP). Gabedit was used for

visualization.169

5.4 Conclusions

A series of FeBIAN-arene complexes were modeled computationally to

determine the effect of the arene on the reducing behavior of the FeBIAN complexes.

The theoretical model agrees well with experimental work, showing that the electron

withdrawing or donating nature of the arene affects temperature at which the arene

would dissociate. This directly affect the catalytic activity towards hydrosilylation and

ring opening polymerization reactions. In addition, the ligand noninnocence is


125

modeled, showing that a broken symmetry triplet is most favored, meaning that a

strongly antiferromagnetically coupled Fe(I)L(-1) is the most likely species.

5.5 Future work

Different FeBIAN catalysts could be considered for evidence of broken

symmetry or ligand noninnocence. Further experimental work to see if the

computational work on the Mössbauer studies agree with the calculated values. Better

understanding of the steric effect on the arenes, as Hammett parameters only looked at

the electronic effects, could lead to designing a better catalyst. The Hammett

parameter study was also limited to complexes containing coordinated phenyl

derivatives, and a larger scope of complexes may lead to better understanding of these

complexes.


126

CHAPTER 6

SUMMARIES AND CONCLUSIONS

The goal of this research presentation was to investigate the incorporation of

metal sites in MOFs for catalysis. This was done in an attempt to provide a more

stable catalyst and prevent aggregation of the catalysts which can occur in solution. A

series of synthetically accessible robust MOFs containing structural and charge-

balancing hydroxyl sites inherent to the MOF were used. MOFs were treated

alternatively with two different metals, Al and Zn, and were considered for catalysis.

MPV reductions were used to probe the activity of the Al-treated MOFs, and

Knoevenagel Condensations were used to probe the activity of the Zn-treated MOFs.

Zn was chosen as a less oxophilic metal to contrast with the more oxophilic Al. The Al

treated MOFs were shown to outperform the native MOF, and the materials were

shown to be recyclable. This, coupled with vibrational spectroscopy and SS NMR

results showed that the MOF had been metallated at the µx-OH sites within the

framework with Al. This is corroborated by the Mg-MOF-74 treated with Al which

showed no catalytic activity, as Mg-MOF-74 contains no available µx-OH sites. In

addition, the larger MOFs were shown to be significantly more active for this reaction,

suggesting that diffusion of reagents through the framework is an important factor for

this catalysis. The larger MOFs were also shown to be able to be recycled with

minimal loss in activity for Al@UiO-67.

MOFs were then prepared that had OH sites introduced on the linkers served as

phenolic containing analogs of the previously considered MOFs (UiO-66-OH and

MIL-53-OH). As these MOFs has more possible binding motifs over UiO-66 and

MIL-53, metallation of these sites was investigated. These materials were

characterized using FTIR, SS MAS NMR, and PXRD. Their reactivity was probed

with MPV reductions and Knoevenagel condensations. The self-assembly of adjacent

linkers to form SAMS was unable to be proven, but further studies may still be able to

show that this motif is possible.


127

A tripodal amide ligand was synthesized, deprotonated, and used to form a

vanadium (III) complex which exhibited octahedral coordination geometry. This

complex will be further investigated for small molecule activation. A series of

FeBIAN complexes were modeled computationally in order to gain some insight to

the experimentally observed activity and oxidation state of the complexes. The

Hammett parameter was compared to the oxidation potential of the complexes and

may offer a method of understanding and predicting the reactivity of similar

complexes.


128

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