Metal ClustersJoel M. SmithBaran Group Meeting

6/17/17

Definitions

Etymology of “Cluster”Derived from the Middle English “cluster” and Old English “clyster” meaning “bunch.” Also from Proto-Germanic klas meaning “to clump, lump together.” Related to icelandic “cluster; bunch of grapes.”

Rhodium (II) Acetate, $153/g

(Aldrich)

Metal Cluster History [Clustory?]

Focus of This Presentaion

Further Resources not fully in the scope of this GMBimetallic Lewis Acids (Qin GM 2015) Transition Metal Photochemistry (Farmer GM 2015)Metal Pairs (Cornella 2016) Organometallic Oddities (O’Malley 2003)

1

Co2(CO)8, $ 7.6/g (Aldrich)(exists as bridged an non-bridged isomers)

Honey Nut Clusters(crispy wheat & rice flakes with delicious honey nut

flavored clusters)

(Currently unavailable from Amazon)

The Cloisters Museum(Upper Manhattan, NYC)

Fullerene - C60$134/g (Aldrich)

12th century - Historically Metal cluster was calo- mel (Hg2Cl2). Later used to treat syphilis and Laxative for George III.1826 - First polyoxometallate discovered as PMo12O40

-4 anion by Berzelius.

1908 - Miolatti-Rosenheim Hypothesis postulated ab- out the structure of 12-molybdophosphoric acid as a polyoxometalate cluster.1928 - Linus Pauling proposes structure that accurately accounts for structure of polyoxotungstate anion1934 - J. F. Keggin obtains first crystal structure of poly- oxo-metallate. Now referred to as Keggin stucture.1953 - X-Ray Evidence for Metal-to-Metal Bonds in Cupric and Chromous Acetate (Schoening)1964 - F. A. Cotton (Wilkinson Student) discovers Re2Cl8-2 which contained metal-metal bond.Early 1960s - Cotton coins terms cluster when referring to chemical entities that contain one or more metal-metal bond.

– Homo and Heterometallic Dinuclear metal clusters– Polyoxometallate complexes– Biologically relevant metal clusters and their related congeners.– Higher order Metal clusters– Future of Metal Clusters in Synthetic Chemistry and Catalysis

Preparation of Metal Clusters

Teflon Reaction Vessel keeps sample pressurized

Autoclave used for heating (usually used

for sterilization at 121 °C)

variousmetalsalts

+heat

solventtime

(days)

metal clustercrystals

Solution synthesis– Reactions are usually carried out in flasks, filtered, and product is crystallized.– Sometimes crystallization times are on the order of weeks.– Examples of clusters generated by this method are polyoxometalates, iron-oxo clusters, etc.

various metal salts+

ligands (if needed)+ solvent

temperaturetime

metal clustercrystals

crystallization

Cluster (Chemistry) - ensemble of atoms ormolecules intermediate in size between a moleculeand a bulk solid. The can be made up of diverse nuclearities and stoichiometry. (Mingos and Wales1990, Introduction to Cluster Chemistry)

Examples: Fullerene, Diborane, Water

Metal Cluster: (1) Compounds that contain metal-metal bonds. (2) A compound that containsa group of two or more atoms where direct andsubstantial metal bonding is present. (HuheeyInorganic Chemistry Huheey, JE,Harper and Row, New York)

Examples: Co2(CO)8, Rh2(OAc)4, Stryker’s reagent.

Hydrothermal Synthesis– Crystallization of substances from high temperature conditions. Usually, the reactions are run in aqueous media.–Most reactions are run in steel pressure vessels in an autoclave. on the order of days.– The first documented growth of a crystal was by Karl Emil von Schafhäutl with the synthesis of quartz crystal. Robert Bunsen also what a major developer of the technique. – The production of quartz was done by this method after a whortage occured during WWII.

Metal ClustersJoel M. SmithBaran Group Meeting

6/17/17

Preparation of Metal Clusters (continued)

2

Solid-state synthesis– Reactions are usually carried out in alumina or silver boats at very high temperatures.– Usually reaction times are on the order of days to weeks– This method of synthesis is typically used in the preparation of unligated metal salts.– Products are typically characterized by powder x-ray diffraction analysis.

variousmetalsalts

+high temperature(500 – 1200 °C)

time

metal cluster powder

Reductive methods under CO atmosphere– Reactions usually start with an air stable oxidized form with stoichiometric metallic reductant– Some methods employ 1 atm of CO, while others require harsher pressures.– Most methods for preparation require inert atmosphere or “filtering under CO.”– Products are typically characterized by elemental analysis and x-ray diffraction.

Example 1: RhCl3 + 2Cu + 4CO + NaCl Na[Rh(CO)2Cl2] + 2Cu(CO)Cl

4Na[Rh(CO)2Cl2] + 6CO + 2H2O Rh4(CO)12 + 2CO2 + 4NaC + 4HCl– After the transformation, CO atmosphere must be present during all filtration. This is not only to prevent the complex from decomposition, but also “to prevent the precipitation of the insoluble CuCl by decomposition of Cu(CO)Cl.

Example 2: MnCl2 + 2Na + 2benzophenoneCO (200 atm)

Mn2(CO)10

– Many heterometallic metal clusters can catalyze reactions such as hydroformylation, hydrogenation, desulfurization olefin isomerization and related reactions.– For an extensive survey of these reactions, see Chem. Rev. 2015, 115, 28.– For a few examples, see below:

isomerization of 1,4-COD to 1,3-COD isomerization of 1-octene to other isomers

M = Mo, conversion of butane to ethane hydrogenation of naphthalene to cis-decalin

The Chemistry of Metal-Carbonyl ClustersExamples of metal carbonyl clusters and their geometry:

Fe3(CO)12

Fe

Fe

Fe

CO

CO

OC COCO

COOCOC

OC CO

CO

CO

Mn MnOC

OC

CO

COOC

COOCCO

OCOC

Mn2(CO)10 Co4(CO)12

CoCoCo(CO)2

Co

CO

CO

OC

OCCO

CO

CO

OCCOOC

Rh4(CO)12

RhRhRh(CO)2

Rh

CO

CO

OC

OCCO

CO

CO

OCCOOC

Ru

Ru

Ru

OC CO

CO

CO

CO

CO

OC

OCOC CO

COOC

Ru3(CO)12

Tc TcOC

OC

CO

COOC

COOCCO

OCOC

Re ReOC

OC

CO

COOC

COOCCO

OCOC

Os

Os

Os

OC CO

CO

CO

CO

CO

OC

OCOC CO

COOC

Os3(CO)12

Tc2(CO)10

Re2(CO)10Most metal carbonyl clusters can form higher order clusters (e.g. Co4(CO)12) that are useful in catalytic processes (vide infra). They are gen-erally soluble in organic solvent.

Metal ClustersJoel M. SmithBaran Group Meeting

6/17/17

3

Mo Mo

O O

iPr

iPriPr

4

Dalton Trans., 2011, 40, 9358.

RCCl4 +R

ClCl3C

cat. (3 mol %)

CDCl3

Cl3CCl

61% yield

CCl3Cl

77% yield

ClCCl3

59% yield

BuRCl4 +Bu

ClCl2RC

cat. (3 mol %)

CDCl3R = CO2Et, 69%; D, 25% COSPr, 32%

Mo Mo

N N 4

HAr Ar

CO2CH3

cat. (0.5 mol %)

CO2CH3BrPMMA (PDI = 1.4)

PhMe-d8, 50 °C, 12 h E1/2 Mo+4/Mo+5 = –0.308 Vversus FeCp2

Reactions promoted/catalyzed by metal carbonyl clustersMn2(CO)10

4Mn(CO)3Mn2(CO)10

tetramerization of acetylene: J. Am. Chem. Soc., 1960, 82, 4209.

product can be hydro-genated or oxidizedwith Mn still ligated.THF, 150 °C

Atom transfer reactions: TL, 2002, 43, 2535.

O

Mn2(CO)10

DCM, hv, BrCCl3NaOH, H2O, BTAC

(99% yield) O

Br

CCl3

Hydrogenation Reaction with (µ-H2)Os3(CO)10J. Am. Chem. Soc., 1976, 98, 1056

EtO2C CO2Et

EtO2C CO2Et

catalystH2

isolable by chromatography

PhCO2Me

CO2MeCo2(CO)8

DCM, rt

PhCO2Me

CO2Me

Co(CO)3Co

Sc(OTf)3RCHO

DCM, rt

O

RMeO2C CO2Me

Ph

Co2(CO)8

30 –95 % yield

Chem. Commun. 2009, 7339.

J. Am. Chem. Soc. 2016, 7492.

FeCl3 + MeMgBrTHF

MeMgBrPh

Br

PhMet = 40 s (5% yield)

t = 120 s (6% yield) PhMe t = 50 s

(99% yield)

– This study further validated a study done in the past by Kochi (see GM).

– This complex has been shown to be stable in solution, but the addition of ligands like TMEDA (known to enhance cross-coupling) disturbs the stability producing mononuclear species.

Metal ClustersJoel M. SmithBaran Group Meeting

6/17/17

Recent Reactions of Polyoxometalate Clusters

4

TBA4(W10O32)

H2O, 32 °C, 2–10 h

Photochemical reactions with POMs: J. Am. Chem. Soc. 1995, 117, 4704.

Me

Me

OHOOHO Me

Me

Me Me

OHO Me

Me

OH

Me

O97 % yield 90 % yield 85 % yield 81 % yield

C-H Oxidation: Nat. Chem. 2010, 2, 478.

Bu4N (!-HPV2PW10O40)H2O2 (1 equiv)

CH3CN-tBuOH, 1 –, 1–4 h

R2R1

HR2R1

OH

R3 R3

OH OH OHH

H

OH HO

HOOH

92% yield 75% yield 37% yield 93% yield 51% yield

50% yield 81% yield

Bu4N (!-HPV2PW10O40) [x-ray]

– Reactivity of this Vanadium POM cluster reminiscent of DMDO/TFDO– Selectivity Governed by catalyst size and not a radical mech!

R2R1

HR2R1

CHO

R3 R3

Polyoxometalates: Some of the First-discovered Metal Clusters

Lindquist Hexamolybdate structure Decavanadatedate structure

Dawson Structure (P2M18O62)-6 Keggin Structure (PW12O40)-3

Top View Preyssler Structure Side View (EuP5W30O110)-12

– Most POMs are synthesized from their parent oxides and structure is controlled by altering pH. – POMs are non-toxic but have interesting anti-HIV and anti-cancer properties.

Metal ClustersJoel M. SmithBaran Group Meeting

6/17/17

Reactions of Polyoxometalates

5

TBA4(W10O32)550 W Hg lamp (Pyrex filter)

CO (~ 9mM)

CH3CN, 25 °C, 16 h

carbonylation reactions with POMs: J. Am. Chem. Soc. 1995, 117, 4704.

MeCHO

54 turn. (68 % sel.)

18 turn. (69 % sel.)

12 turn. (85 % sel.) 2 turn.

C-H alkylation/alkenylation: J. Am. Chem. Soc. 1993, 115, 12212.

TBA4(W10O32), hv, C2H4 or C2H2

CH3CN, rt

R2R1

HR2R1

CHO

R3 R3

CHO CHO

MeCHO Ph

or

2 – 5% conversion

H TBA4(W10O32), hv, C2H4 or C2H2

CH3CN, rt+

85% 5%

(W10O32)-4

CO2MeCO2Me

Dehydrogenation: J. Am. Chem. Soc. 1990, 112, 6585

TBA4(W10O32), hv

CH3CN, 25 °CR2R1

H

R2R1R3R3

MeO2C CO2Me

CO2MeCO2Me

NCHO CO2Me

CO2Me

O

OCO2Me

CO2Me

C6H13

O

CO2Me

CO2Me CO2Me

CO2Me

CN

58% yield 61% yield 52% yield 90% yield 62% yield

TBA4(W10O32), sunlightMichael acceptor

CH3CN, 25 °C,

O

O

O CN

O

CNO

O

58% yield 73% yield 55% yield

R Bu4N (!-HPV2PW10O40) (0.05 mol %)H2O2 (1 equiv)

CH3CN-tBuOH, 1 –, 1–4 h

R OH

Arene C-H Oxidation: Angew. Chem., Int. Ed. 2012, 51, 7275.

OMe

OH

OMeMeO

OH MeO

OH OH

HO

OH

63% yield 93% yield 85% yield 46% yield 82% yield

R H5PV2PMo10O40 (2 equiv)

50% H2SO4rt – 90 °C

R CHO

Electrochemical Benzylic Oxidation: J. Am. Chem. Soc. 2015, 137, 5916.

CHO

I

CHO

O2N

CHO

Me

CHO

Me

CHO

F

97% (96) 45% (98) 99% (94) 99% (89) 99% (98)

H5PV2PMo10O40 (2.3 mol %)Pt electrode (1.5 V)

50% H2SO470 °C, 10 h

CHO

65% yield (95% selectivity)

– Protonated POM accepts electron from arene produc-ing radical cation.

– Radical cation under-goes proton transfer to form benzylic radical.

– Benzylic radical is further oxidized and captured by water.

– Benzylic alcohol is further oxidized by POMto aldehyde.

– Overoxidation (<1%) is not observed.

Metal ClustersJoel M. SmithBaran Group Meeting

6/17/17

Biochemical Metal Clusters

6

Iron Sulfur Clusters

Fe SFe

S

RS

RS

SR

SR

[2Fe–2S]+2 [2Fe–2S]+

Fe SFe

SRS SR

S SFeSR

[3Fe–4S]+ [3Fe–4S]0

[3Fe–4S]- [3Fe–4S]-2

Fe

SS

S

FeFe

Fe

SRS

SR

SRRS

[4Fe–4S]+3 [4Fe–4S]+2

[4Fe–4S]+ [4Fe–4S]0

Fe

SS

S

FeFe

Fe

SRS

S

RS

Fe

S

S

FeFe

Fe

SSR

S[8Fe–8S]+5 [8Fe–8S]+4

[8Fe–7S]+3 [8Fe–7S]+2

– Iron-sulfur clusters are important co-factors in metalloenzymes that participate in substrate binding, electron transfer, and regulation of gene expression.– Clusters do not form spontaneously in nature but rather require complex biosynthesis to form.– 120 distinct types of enzymes that contain Fe-S clusters.– Postulated to have an important role in the origin of life.

Fe8(µ4-O)4(µ-pz)12Br4

E1/2(1) –0.33 V E1/2(2) –0.62 V E1/2(3) –0.89 E1/2(4) –1.16

Fe

SS

S

FeFe

Fe

SRS

SRRS

Mechanism of Aconitase utilizes an Iron-Sulfur Cluster

OHOOH

O

CO2-

H-O2C

BEnz

Fe

SS

S

FeFe

Fe

SRS

SRRS

OH

OH

CO2-

CO2-+

H2OCO2

-

CO2-

-O2C

OHisocitrate

cis-Aconitate

-O2C

citrate

– Citric acid is converted to isocitrate through cis-aconitate.– The dehydration is facilitated by the iron-sulfur cluster acting as a Lewis acid. – This is one of the few mechanism where the iron cluster does not change oxidation state. Most react in electron transfer processes like the one below.

Fe SFe

S

RS

RS

N

N

N

NO

OH

R

MeOOMe

Hemee-

H BEnz O

O

R

MeOOMeUbiquinol Ubiquinone

Fe SFe

S

RS

RS

N

N

N

NH

– Iron-sulfur clusters play a role in mitochondria function through electron transport.

– This octairon oxo cluster mimics the iron sulfur clusters found in vivo. The CV trace shows that there are four reversible single electron transfer events, owing to the stability of these strutures even when placed under electronic stress.

Electron Transfer Between Metal Clusters: J. Am. Chem. Soc. 2014, 136, 15873.

– Ligated clusters are soluble in organic solvent.

– Reversible electron transfer for both complexes confirms stability over wide potential range.

– HOMO/LUMO gap promotes electron transfer between clusters.

Metal ClustersJoel M. SmithBaran Group Meeting

6/17/17

7

Metal Clusters with underdeveloped reactivity

Cu(OAc)2. + K2Sb2(tartrate)2

1. NaOAc/AcOH

2. K2CO3

Photocatalytic Water splitting by cobalt cluster (Eur. J. Inorg. Chem. 2016, 3253:

Inorg. Chem. 2016, 55, 540.

NiCl2. +N

NOH

EtOH

KOH

CoCl2. +N

NOH

EtOH

KOH

Dalton Trans. 1985, 737.

Nature, 2014, 515, 545.

[W18O54(SeO3)2]-4

– POM structure was designed for flash memory devices.

– Several reduction events for the tungsten cage (gray area)– Orange area is the oxidation of the Se dopant.