UNDERSTANDING AN(IV)-ORGANIC INTERACTIONS VIA THE …

UNDERSTANDING AN(IV)-ORGANIC INTERACTIONS VIA THE SOLID-STATE

STRUCTURAL CHEMISTRY OF TH(IV) AND U(IV)-CARBOXYLATES ISOLATED

FROM AQUEOUS SOLUTION

A Thesis

submitted to the Faculty of the

Graduate School of Arts and Sciences

of Georgetown University

in partial fulfillment of the requirements for the

degree of

Master of Science

in Chemistry

By

Nicole Vanagas, B.S.

Washington, DC

June 9, 2016

ii

Copyright 2016 by Nicole Vanagas

All Rights Reserved

iii

UNDERSTANDING AN(IV)-ORGANIC INTERACTIONS VIA THE SOLID-STATE

STRUCTURAL CHEMISTRY OF TH(IV) AND U(IV)-CARBOXYLATES ISOLATED FROM

AQUEOUES SOLUTON

Nicole Vanagas, B.S.

Advisor: Karah E. Knope, Ph.D.

ABSTRACT

Anthropogenic releases of actinides to the environment from activities such as actinide mining,

weapons testing, nuclear accidents, and past inadequate disposal strategies underscore the need to

develop a fundamental understanding of actinide chemistry under environmentally relevant

conditions. Generally, the overall chemical behavior of the actinides in environmental systems is

governed by processes such as precipitation, complexation, sorption, and colloid formation, all of

which may be affected by metal-ligand interactions. In addition, the oxidation state of the metal

center is of considerable consequence to the fate of such contaminants. Tetravalent actinides were

previously thought to be insoluble and immobile, however, unexpected mobility of these ions have

been found, leading to an increased interest in understanding the behavior of the 5f metal ions in

the environment. For example, previous studies have probed such processes and found the

unexpected mobility of U(IV) and Pu(IV) via their association with a colloidal phase. In this work,

actinide-carboxylate donor interactions are examined with an eye towards understanding how the

nature of small organic molecules affects speciation of thorium(IV) and uranium(IV) in aqueous

systems. Prior work of thorium(IV)-carboxylates has shown prevalence of hexanuclear hydro(oxo)

bridged clusters. In this work, however, Th(IV) complexed by furan-dicarboxylate yielded

unprecedented bridged molecular units that may further point to the directing effects of –COO-

functionalized ligands. Beyond the formation of this unique dimer, hexameric and chain-like

iv

structures were synthesized comprised of Th(IV) or U(IV) ions. Varying the metal-to-ligand ratio,

pH, and temperature were systematically studied to elucidate their effects on the structural

chemistry of the resulting Th(IV) and U(IV) carboxylate complexes.

v

The research and writing of this thesis

is dedicated to everyone who helped along the way.

Many thanks,

Nicole Vanagas

vi

Table of Contents

Introduction ..................................................................................................................................... 1

Actinides in the Environment ..................................................................................................... 1

An-Ligand interaction ................................................................................................................. 1

Accessible Oxidation State ......................................................................................................... 2

An(IV) vs An(VI) ....................................................................................................................... 3

Our Interest: An(IV)-Small Organic Molecule Complexation ................................................... 4

Experimental Methods .................................................................................................................... 7

Synthesis ..................................................................................................................................... 7

X-Ray Structure Determination ................................................................................................ 11

Infrared and Raman Spectroscopy ............................................................................................ 15

Thermogravimeteric Analysis ................................................................................................... 15

Elemental Analysis ................................................................................................................... 15

Results ........................................................................................................................................... 17

Structure Descriptions ............................................................................................................... 17

Powder X-Ray Diffraction ........................................................................................................ 24

Infrared and Raman Spectroscopy ............................................................................................ 25

Thermogravimetric Analysis .................................................................................................... 28

Discussion ..................................................................................................................................... 30

Conclusions ................................................................................................................................... 33

Future Works ............................................................................................................................ 33

Conclusion ................................................................................................................................ 34

Endnotes ........................................................................................................................................ 36

Appendix ....................................................................................................................................... 39

Powder X-ray Diffraction Data ................................................................................................. 39

Infrared Spectroscopy ............................................................................................................... 43

Raman Spectroscopy ................................................................................................................. 45

Thermoagravimetric Analysis Plots .......................................................................................... 49

vii

List of Figures

Figure 1. Representative example of humic acid. …………….…………..……………………... 2

Figure 2: Common coordination chemistry An(IV) (a) and An(VI) (b)..……...……. …...……... 3

Figure 3. Ligands of study: 2-furoic acid, 2,5-furan-dicarboxylic acid and 4-hdroxybenzoic acid

(left to right)…………………………..…………………………………………………………... 4

Figure 4. Diagram of 1 showing the coordination environment of the chain……….…………... 17

Figure 5. Hexanuclear unit of 2 without hydrogens shown for clarity……………...…………... 18

Figure 6. Illustration of 3 without hydrogens shown for clarity…………...……………………. 19

Figure 7. Illustration of 5 showing the coordination environment of the dimer.........…………... 20

Figure 8. Representation of 6 showing the Th(IV) bridged through the carboxylate to form a 3D

chain.………………………………………………………………………………………..…... 21

Figure 9. Illustration of 7 hexanuclear cluster…….…………………………………………... 22

Figure 10. Diagram of compound 8.............................................................................................. 23

Figure 11. Raman spectra of SXRD 2 (blue) and FDCA (red)…………………...……………... 27

Figure 12. Raman Spectra of compound 5 crystal (blue), solution speciation (red), and FDCA

dissolved in H2O (green)…….…………………………………………………………………... 28

Figure 13: Thorium(IV) molecular clusters that have been isolated from aqueous solution (Dimer

= (a), Hexamer = (b))……………………..……………………………………………………... 31

Figure 14. Proposed ligands: Picolinic acid, 2,6-pyridine-dicarboxylic acid, and 2,4-pyridine-

dicarboxylic acid (Left to Right).………………………………………………………………... 33

viii

Figure A1. PXRD of sample from which compound 1 was isolated comparing bulk phase (red)

and calculated pattern for 1 (black)….…………………………………………………………... 39

Figure A2. PXRD showing calculated pattern for 2 (black) and bulk phase (red) showing single

crystal representative of bulk………………..…………………………………………………... 39


crystal representative of bulk.….………………………………………………………………... 40


crystal not representative of bulk.……………………..…………………………………….…... 40

Figure A5. Comparison of PXRD calculated patter 5 (back) and bulk phase (red) showing

agreement between the calculated pattern and bulk….………………………………………...... 41


crystal is representative of bulk however, there are peaks unaccounted for such as at 28°..…........41

Figure A7. Comparison of PXRD calculated pattern 7 (black) and bulk phase (red) shows

agreement between powders suggests single crystal is representative of bulk phase.…………... 42

Figure A8. PXRD of calculated pattern 8 (black) and bulk phase (red) showing single crystal is

representative of bulk phase.……………………………………………………...……………... 42

Figure A9. IR spectrum of Compound 2.……………………………………..…….…………... 43

Figure A10. IR spectrum of Compound 5.…………………………………………….………... 43

Figure A11. IR spectrum of Compound 7…………………………….………………………... 44

Figure A12. IR spectrum of Compound 8………………………………………………………..44

Figure A13. Raman spectra of Compound 1.……………………………….…………………... 45

Figure A14. Raman Spectra of Compound 2.…………………………………………….……... 45

ix

Figure A15. Raman Spectra of Compound 3.………………………………….………………... 46

Figure A16. Raman Spectra of Compound 4.……………………………………….…………... 46

Figure A17. Raman Spectra of Compound 5.…………………………………………………... 47

Figure A18. Raman Spectra of Compound 6………………………………………………..…... 47

Figure A19. Raman Spectra of Compound 7.………………………………….………………... 48

Figure A20. Raman Spectra of Compound 8.……………………………………………….…... 48

Figure A21. TGA of Compound 5…………………….………………………………………... 49

x

List of Tables

Table 1. Accessible oxidation states with most common in

red…………………………………………………………….…………………………………... 3

Table 2. Crystallographic Data and Structure Refinement for 1 -8……………………………... 13

1

Introduction

Actinides in the Environment

The unintended release of actinides into the environment has become a pervasive dilemma

in recent years arising from actinide mining, weapons testing, nuclear accidents, and inadequate

disposal strategies through the years.1 Locations such as the Nevada Test Site, Rocky Flats, Lake

Karachi, and Hanford, are sites where large quantities of actinides have been anthropogenically

introduced and are being studied to understand the impact of these heavy elements in the

environment.2 Current estimates predict that in the U.S. alone, the amount of contaminated soil

ranges from 73 million to 200 million m3, moreover, thousands of metric tons of additional nuclear

waste are generated each year from spent fuel and byproducts of refinement.3 This adds to the

inventory of waste in the U.S. for which no clear disposal strategy has been identified. While the

desire exists from the government, industry, and citizens to address the proper disposal of nuclear

material, no such consensus exists to date.4 Catastrophic contamination events of the recent past

coupled with growing concerns over end disposal strategies for our increasing inventory of spent

nuclear fuel has created a pressing need to further develop our fundamental understanding of how

actinides behave under geological conditions.1c The chemical behaviors of the actinides in such

environments are governed by various competing processes, including complexation,

precipitation, colloid formation, and sorption.1c, 2c, 3

An-Ligand interaction

The aforementioned processes are related to An-ligand interaction and accessible oxidation

states of the actinides. The mobility of the actinides has been attributed, in part, to their association

with metal hydroxide and/or organic colloidal phases.5 Naturally occurring organics, such as

2

humic and fulvic substances

(Figure 1), are compositionally

and structurally complex,

consisting of a wide range of

functional groups with effective

actinide complexing strength.6

Due to this complexing power,

it is widely recognized that such

organic substances are capable

of effective actinide transport in

the environment. The mechanisms, of this transport, particularly of the tetravalent actinides,

remains elusive despite the numerous studies focused on this process. These shortcomings

necessitate a deeper, fundamental understanding of the chemical behavior of actinides under the

influence of complex organic ligands. In this context, actinide-small organic ligand interactions

containing environmentally relevant functional groups were investigated as models for actinide-

organic colloid complexation.

Accessible Oxidation State

The processes of complexation, sorption, colloid formation, and precipitation are largely

dependent on the oxidation state of the metal ion. The early actinides have a number of accessible

oxidation states as seen in Table 1, ranging from trivalent up to heptavalent oxidation states.1c For

uranium in particular, most work has focused on the hexavalent state as it is the most mobile and

soluble form.1a, 1b, 3, 7 Because of these qualities, hexavalent uranium has served as the choice

Figure 1. Representative example of humic acid. Adapted

from Stevenson, F.J., HUMUS CHEMISTRY, 1994.6

3

oxidation state studied to gain an

understanding of how actinides

behave in the environment.10 The

tetravalent oxidation state of the

actinides is largely unexplored by

comparison, however, recent

studies have shown an

unexpected mobility of this

oxidation state, highlighting our limited understanding of the overall chemical behavior of these

metal ions in geochemical and biochemical surroudings.1, 3, 7b, 8 In light of these findings, a deeper

investigation into the lesser examined oxidation state is warranted in complex chemical systems.

An(IV) vs An(VI)

Interest lies with the

tetravalent and hexavalent

actinide oxidation states due to

their accessibility, as well as

being the most commonly

found in the environment. It is

interesting to note that the

coordination chemistry varies

drastically between the

tetravalent and hexavalent

III IV V VI VII

Thorium

X

Protactinium

X X

Uranium X X X X

Neptunium X X X X X

Plutonium X X X X X

Table 1. Accessible oxidation states with most common

in red.

(b)

(a)

Figure 2: Common coordination chemistry An(IV) (a)

and An(VI) (b). (Green = An(IV), Yellow = An(VI), Red =

Oxygen, and Blue = Nitrogen)

4

oxidation states of the actinides.3, 7a For example, U(VI) exits as the uranyl (UO22+) cation,

containing short U=O bond lengths between 1.7 -1.8 Å.8a These oxo atoms are nominally terminal

and as such subsequent coordination occurs in the equatorial plane giving a six to eight coordinate

metal center. Common coordination geometries for U(VI) are shown in Figure 2b; in contrast,

U(IV) does not possess this characteristic uranyl functionality, and, consequently, has longer U-O

bond lengths ranging from 2.04 – 2.57 Å, and a metal center that is eight to twelve coordinate as

shown in Figure 2a.8a These coordination geometries greatly dictate actinide-ligand interactions

and underscore differences in the complexes and species formed by U(VI) as compared to U(IV)

Though hexavalent actinides, and more specifically uranium, are generally the more soluble and

mobile form of the actinides, unexpected mobility of tetravalent uranium in the environment has

prompted further studies in recent years.1a, 1b, 7

Our Interest: An(IV)-Small Organic Molecule Complexation

Studies have shown an unexpected mobility of the actinides leading to a need for a fundamental

understanding of how actinides behave in the environment.1, 5b The goal of this research is to

examine how the nature of the organic ligand affects speciation, complexation, and precipitation

of the metal center as a means of understanding the chemical behavior of actinides under the

influence of complex organic ligands.

In this work, thorium(IV) and

uranium(IV)-small organic ligand

interactions were investigated as

models for actinide-organic colloid

complexation. Ligands such as 2-

Figure 3. Ligands of study: 2-furoic acid, 2,5-furan-

dicarboxylic acid and 4-hdroxybenzoic acid (left to

right).

5

furoic acid, 2,5-furan-dicarboxylic acid, and 4-hydroxybenzoic acid (Fig. 3) were studied as a basis

for understanding small organic interactions. Starting with these small organic ligands, the

functionality, carboxylate loading, flexibility, pKa, and complexing strength of the organic ligand

was systematically tuned, such that the effects of these changes on metal ion speciation could be

examined. Such characteristics of the organic ligands, as well as solution conditions, are expected

to influence the structure of the solid state species.

The actinide-organic interactions were investigated using spectroscopic methods including

Raman and infrared spectroscopy (IR), as well as, powder and single crystal X-ray diffraction.

These techniques allow for examination of the inorganic species formed and how the functionality

of the organic ligand influences the structural chemistry of the actinide ions in both solution and

solid state. The relationship between the solution- and solid-state complexes can be correlated

using the aforementioned spectroscopic techniques in an effort to identify factors that drive the

formation of the products yielded.

Recent work in actinide chemistry, in the absence of a complexing organic ligand, has shown

the formation of dimeric hydroxyl bridged units.9 These dimeric units are prevalent in acidic

aqueous solutions.9-10 The dimer has been found with both Th(IV)9 and Pu(IV)10 metal centers in

acidic solutions containing chloride or nitrate. Due to the charge density of the An(IV) ion, even

under these acidic conditions, hydrolysis and condensation reactions proceed to form the hydroxyl-

bridged species.10

In the presence of carboxylates, a range of structural motifs that include monomers7a, dimers9-

10, hexamers7a, 8a, 11, and chain-like7a, 11c, 12 structures have been determined. In contrast to the dimer

previously mentioned, those of the carboxylate ligand systems are commonly monomers,

6

hexamers, and chain-like structures. Hexanuclear Th(IV) molecular clusters and chain formation

have been previously isolated with terephthalates11c, 13, formate11b, acetate8a, chloroacetate8a, and

glycine.11a, 11e Much of this work on the complexation of U(IV) and organics has been performed

under non-aqueous conditions.11d, 13-14 Studies on such systems or those utilizing mixed

aqueous/non-aqueous solvents with organic ligands prompts the question to what extent do non-

aqueous systems effectively mimic or model aqueous systems.8a, 11b, 15

Our research presented herein investigates small organic carboxylates, and their complexation

to actinide ions in aqueous solutions. While related work studying carboxylates, is present in

current literature, none focus on the aqueous chemistry of U(IV)-organic interactions.11d, 14

Additionally, it is recognized that the behavior of actinide-ligand interactions in non-aqueous

systems differs significantly from aqueous systems, thus driving part of our research in our pursuit

to understand the formation of actinide(IV)-organics in environmentally relevant water.5b, 7b, 8a, 11b,

15 The small organic molecules used in this study (4-hydroxybenzoic acid, 2-furoic acid, and 2,5-

furan-dicarboxylic acid) are systematically altered to contain additional functional groups, and the

behavior of the actinide-organic interactions are anticipated to provide a better understanding of

actinide-ligand interactions in aqueous systems.

7

Experimental Methods

Synthesis

Caution: 232Th and 238U are alpha-emitting radionuclides and standard precautions for handing

radioactive materials should be followed when working with the quantities used in the syntheses

that follow.

All starting materials were commercially available and used without any further purification,

with the exception of UCl4 which was synthesized using published procedures.16 UCl4 was

synthesized by adding UO3 (5 g, 15.5 mmol) and an excess of hexacloropropene (50 mL, 354

mmol) into a two necked round bottom flask. The flask was connected to a reflux condenser

attached to a Schlenk line, under a N2 atmosphere. The mixture was heated to 190 °C and refluxed

overnight (18 hours). During the first two hours of the reflux, the reaction is watched to make sure

a vigorous reaction does not occur. The reaction is then left to reflux overnight. The next day, the

reaction was cooled, filtered, and a green solid was isolated. The green solid was washed with

excess purified dicloromethane (100 mL) under nitrogen, and dried under vacuum overnight. (6.50

g, yield = 98.0% based on uranium metal center)

Compound 1, Th(2-FA)4, was synthesized at room temperature. ThCl4 (0.0348 g, 0.15 mmol)

and 2-furoic acid (2-FA) (0.0336 g, 0.30 mmol) in 1.5 g (83 mmol) of deionized water were placed

into a 10 mL vial that was capped and left on the benchtop at ambient temperature. After five days,

crystallization was observed. Colorless, rod-like crystals and a white microcrystalline powder,

unreacted 2-FA, were then filtered and washed with water and ethanol. After a final wash with

ethanol, the samples were left out on the benchtop to air dry. (0.113 g, estimated yield based upon

8

visual inspection = 33% based on thorium) Raman from single crystal: 1587, 1480, 1419, 1382,

1204, 1151, 1077, 1019, 940, 880 and 812 cm-1.

Compound 2, UIV6O6(OH)2(H2O)6(2-FA)10(2-HFA)2•14(H2O), was synthesized by placing

UCl4 (0.0481 g, 0.202 mmol) and 2-furoic acid (2-FA) (0.046 g, 0.404 mmol) in 2 mL (83 mmol)

of deionized water into a 10 mL vial (pH = 2.00). The vial was capped under N2 (g) and placed in

a benchtop glove box. After three days, crystallization was observed. Formation of green needle-

like crystals and a white microcrystalline powder were then observed. These were filtered, and

washed with water and ethanol. After a final wash with ethanol, the crystals were left out on the

benchtop to air dry and found to be stable at ambient conditions. (0.016 g, yield = 33% based on

Uranium) FTIR: 3444, 1585, 1555, 1532, 1480, 1417, 1424, 1368, 1223, 1197, 1138, 1078, 1013,

930, 888, 822, 769, 677, 605, 588, 526 and 470 cm-1. Raman of single crystal: 1579, 1476, 1412,

1153, 1080, 1013, 935, 884, 795 and 475 cm-1. EA: calc (obs): C: 22.05% (22.09%); N: 0.0%

(0.0%); H: 2.81% (2.18%).

Compound 3, UIV(2-FA)4, was synthesized by placing UCl4 (0.0481 g, 0.202 mmol) and 2-

furoic acid (2-FA) (0.046 g, 0.404 mmol) in 2 mL (111 mmol) of deionized water into a 23 mL

Teflon-lined Parr bomb (pH = 2.00) while under nitrogen in a benchtop glove box. The reaction

vessel was sealed and heated statically in an isothermal oven at 120 °C. After 3 days, the reaction

vessels were removed from the oven, placed on the benchtop, and cooled to room temperature over

4 h. The mother liquor was decanted and green crystals and a white microcrystalline powder, the

latter being unreacted 2-FA, were obtained. The green block crystals were washed with water and

ethanol. After a final wash with ethanol, the crystals were left out on the benchtop to air dry and

found to be stable in air. (0.021 g, estimated yield based on visual inspection = 44% based on

9

uranium) Raman of single crystal: 1586, 1483, 1438, 1411, 1375, 1228, 1999, 1145, 1079, 1012,

934, 885, 845 and 810 cm-1.

Compound 4, UVIO2(2-FA)3, was synthesized by adding UCl4 (0.0357 g, 0.15 mmol) and 2-

furoic acid (2-FA) (0.0336 g, 0.30 mmol) in 1.5 mL (83 mmol) of deionized water and 20 μL (0.25

mmol) pyridine placed into a 10 mL vial under ambient conditions, capped (pH = 2.00), and left

on the benchtop. After 5 days, crystallization was observed, the yellow block crystals and white

microcrystalline powder were then filtered and washed with water and ethanol. After a final wash

with ethanol, the crystals were left out on the benchtop to air dry. The UCl4 used in this synthesis

was found to have an acyl chloride impurity due to incomplete washing of the UCl4. (0.048 g, yield

based upon visual inspection = 85% based on uranium metal center) Raman of single crystal: 1650,

1479, 1441, 1431, 1390, 1298, 1238, 1146, 1080, 1028, 932, 886, 845, 769, 737, 606 and 571 cm-

1.

Compound 5, Th2(FDC)4(H2O)10•2(H2O), was synthesized at room temperature. ThCl4 (0.224

g, 0.60 mmol) and 2,5-furan-dicarboxylic acid (FDC) (0.187 g, 1.2 mmol) in 6 g (333 mmol) of

deionized water were placed into a 10 mL vial and capped (pH = 1.39). After five days,

crystallization was observed. The colorless block crystals and white microcrystalline powder,

unreacted FDC, were then filtered and washed with water and ethanol. After a final wash with

ethanol, the crystals were left out on the benchtop to air dry. (0.180 g, yield = 80% based on

thorium) FTIR: 3400 (O-H), 1585 (C=Oasym), 1384 (C=Osym), 1230, 1200, 1164, 1029, 976, 782,

608, 523 and 486 cm-1. Raman of single crystal: 1659, 1583, 1531, 1400, 1369, 1315, 1204, 1040,

961, 897, 818, 538 and 495 cm-1. EA: calc (obs): C: 22.85% (22.37%); N: 0.0% (0.02%); H: 2.22%

(2.57%).

10

Compound 6, Th(FDC)2(H2O)2, was synthesized hydrothermally. Thorium chloride (0.224 g,

0.60 mmol), 2,5-furan-dicarboxylic acid (FDC) (0.187 g, 1.2 mmol) and deionized water (5 g, 278

mmol) were placed into a 23 mL Teflon-lined Parr bomb (pH=1.39). The reaction vessel was then

sealed, and heated statically in an isothermal oven at 140°C. After three days the reaction vessel

was removed from the oven, placed on the benchtop, and cooled to room temperature over four

hours. The mother liquor was decanted and colorless crystals and white microcrystalline powder

were obtained. The colorless rectangular crystals and white microcrystalline powder, were

collected and washed with distilled water, and ethanol. The sample was then allowed to air-dry at

room temperature. Percent yield could not be determined as a pure phase has not yet been isolated,

however the estimated yield based upon visual inspection is 0.174 g, yield = 78 %. Raman of single

crystal: 2381, 2023, 1643, 1242, 1087, 996, 818 and 552 cm-1.

Compound 7, Th6O4(OH)4(4-HBA)12(H2O)6•12(H2O), was synthesized at room temperature.

A stock solution of 0.1M ThCl4 was prepared by dissolving 1.12 g ThCl4 in 30 mL deionized

water. The thorium stock solution (1.5 mL, 0.15 mmol) was added to a glass vial with 4-

hydroxybenzoic acid (4-HBA) (0.025 g, 0.18 mmol), and pyridine (16.9 μL, 0.21mmol), then left

capped at room temperature for three days. Immediately following the addition of pyridine, a

cloudy white precipitate formed, but gradual and complete dissolution was observed followed by

the formation of colorless block-like crystals. Crystals were washed with water and dried. (0.022

g, yield = 63% based on Thorium) FTIR: 3400 (O-H), 2427, 1924, 1773, 1605, 1546 (C=Oasym),

1493, 1407 (C-Cstretch), 1381 (C=Osym), 1273, 1233, 1167 (C-OHstretch), 1092, 1026, 855, 789, 743,

700671, 641, 618, 575, 526 and 407 cm-1. Raman of single crystal: 1598, 1521, 1428, 1392, 1275,

11

1173, 1146, 872, 856, 705 and 640 cm-1. EA: calc (obs): C: 28.76% (28.57%); N: 0.0% (0.0%);

H: 3.20% (2.93%).

Compound 8, UIV6O4(OH)4(4-HBA)12(H2O)6•12(H2O), was synthesized at room temperature.

A stock solution of 0.1M UCl4 was prepared by dissolving 1.14 g UCl4 in 30 mL deionized water

in the glove box. UCl4 stock solution (1.5 mL, 0.15 mmol) was added to a glass vial with 4-HBA

(0.025 g, 0.18 mmol) and pyridine (16.9 μL, 0.21mmol). Immediately following the addition of

pyridine, a cloudy white precipitate formed, but gradual and complete dissolution was observed.

The vial was allowed to rest capped at room temperature for 3 days in the glove box and formation

of green block-like crystals was observed. The green block crystals were washed with water and

dried under air. The crystals were found to be stable under ambient conditions. (0.007 g, yield =

20% based on Uranium) FTIR: 3440 (O-H), 2095, 1638 (C=Oasym), 1477, 1411 (C=Osym), 1302,

1269, 1233, 1171 (C-OHstretch), 1141, 1095, 1006 and 595 cm-1. Raman of single crystal: 1596,

1521, 1407, 1272, 1146, 873, 816 and 636 cm-1. EA: calc (obs): C: 27.63% (27.71%); N: 0.0%

(0.0%); H: 3.55% (2.91%).

X-Ray Structure Determination

Single crystals were selected from the bulk samples and mounted on MiTeGen micromounts in

mineral oil. Reflections were collected at 100 K on a Bruker D8 QUEST diffractometer equipped

with a CMOS detector or Bruker DUO diffractometer equipped with an APEXII CCD detector

using Mo-Kα radiation (λ = 0.71073 Å). The data was integrated and corrected for absorption

using SAINT17 and a multi-scan technique in SADABS18, included in APEX2 crystallographic

software.19 The structures were solved using SHELXT and refined by full-matrix least-squares on

F2 using the SHELXL software in WinGX. Compound 2 was found to be a twinned crystal

12

requiring the use of TWINABS command in order to separate the component reflections.20

Crystallographic data for compounds 1-8 is provided in Table 2. Preliminary refinements of

compounds 1, 3 and 4 revealed the coordination and are reported however, weak diffraction,

disorder in the crystal structure, or both, have precluded a full refinement up to this point.

All non-hydrogen atoms were located using difference Fourier maps and were ultimately refined

anisotropically. Hydrogen atoms of the bound and outer-sphere solvent water molecules in 2, and

5-7 were found during refinement.

13

Table 2. Crystallographic Data and Structure Refinement for 1 -8.

1 2 3 4

Formula Th(2-FA)4 UIV6O6(OH)2(H2O)

6(2-FA)10(2-

HFA)2•14(H2O)

UIV(2-FA)4 UVIO2(2-FA)3

MW 647.3 1514.39 682.324 603.894

Temperature

(K)

100 102 102 100

λ (Mo Kα) 0.71073 0.71073 0.71073 0.71073

Crystal System Orthorhombic Triclinic Orthorhombic Monoclinic

Space Group Pca21 P-1 Pbcm C2

a ( Å) 22.0127(35) 13.6331(9) 4.715 10.412(13)

b ( Å) 4.7626(8) 14.3408(9) 21.961 15.962(19)

c ( Å) 19.4471(31) 20.6797(14) 19.388 7.276(9)

α ( °) 90 85.026(2) 90 90

β ( °) 90 84.864(2) 90 117.77(2)

γ ( °) 90 79.214(2) 90 90

V ( Å3) 2038.79(6) 3945.6(4) 2007.55 1844.65

Z 4 4 4 4

Rint 0.0962 N/A 0.0496 0.0107

R1 [1>2σ(I)] 0.0735 0.0759 0.1019 0.0580

wR2 0.2087 0.1175 0.2891 0.1496

GooF 1.035 1.066 2.498 1.245

14

Table 2. (cont’d)

*Note: The Rint for 3 is not reported as the crystal used for SCXRD was a 2 component twin and

as such a Rint could not be calculated.

5 6 7 8

Formula Th2(FDC)4(H2O)10

•2(H2O)

Th(FDC)2(H2O)2 Th6O4(OH)4(4-

HBA)12(H2O)6•12(H2O)

UIV6O4(OH)4(4-

HBA)12(H2O)6•12(H2O)

MW 1296.57 1152.45 3276 1754.51

Temperature

(K)

100 100 100 102

λ (Mo Kα) 0.71073 0.71073 0.71073 0.71073

Crystal System Monoclinic Orthorhombic Triclinic Hexagonal

Space Group P21/n Pbcn P-1 R-3c

a (Å) 11.1232(12) 6.4740(6) 17.760(2) 21.3406(12)

b (Å) 7.4457(8) 18.9460(18) 17.789(2) 21.3406(12)

c (Å) 20.956(2) 10.7011(10) 17.786(2) 38.358(2)

α (°) 90 90 74.218(2) 90

β (°) 97.726(3) 90 74.249(2) 90

γ (°) 90 90 74.214(2) 120

V ( Å3) 1719.8(3) 1312.6(2) 5084.0(4) 15128.7(19)

Z 2 2 2 12

Rint 0.0342 0.0543 0.0382 0.0424

R1 [1>2σ(I)] 0.0258 0.0161 0.0473 0.0519

wR2 0.0483 0.0373 0.1626 0.1607

GooF 1.052 1.140 1.050 1.15

15

Powder X-ray diffraction (PXRD) data was collected for compounds 1-8 using a Rigaku Ultima

IV diffractometer (Cu Kα λ = 1.542 Å, 2θ = 3 – 40°). Agreement between the calculated and

observed patterns (see Figures A1-A8) confirms that the single crystals used for structure

determination were representative of the bulk sample.

Infrared and Raman Spectroscopy

Infrared (IR) spectra of compounds 2, 5 and 7-8 were collected on a Perkin Elmer FTIR Spectrum

2 system. The samples were diluted with dried KBr and pressed into a pellet. Scans were collected

over 4000 – 400 cm-1 with 12 scans and 2 cm-1 resolution. The data were acquired using Spectrum

Quant software program. Raman spectra of single crystals of compounds 1 - 8 were collected on a

HORIBA LabRAM HR Evolution Raman Microscope with an excitation line of 532 nm.

Thermogravimeteric Analysis

TGA was collected on a TGA Q50 system. Compounds 2, 5 and 7-8 were determined to be pure

phases by powder and elemental analysis, however, only the thermal behavior of compound 5 is

reported due to problems with instrumentation. A 1.5 mg sample was weighed out onto a platinum

pan held at 30 °C for 30 minutes to dry off excess water. The sample was heated up to 600 °C at 5

°C/min under flowing N2 (g). The software TA universal analysis was used to collect and analyze

the data.

Elemental Analysis

Elemental Analysis (EA) was collected on a Perkin Elmer II Series II CHNS/O Analyzer 2400

system. Compounds 2, 5 and 7-8 look to be pure by powder and fell within the accepted error

range. Further purification of the other samples from unreacted organic acids is required prior to

16

bulk analysis. Compounds were weighed into small tin capsules between a mass of 1.5 – 2.0 mg.

Each sample was run in triplicate with blanks and an acetanilide standard. The sample was

determined for its percentage of carbon, nitrogen, and hydrogen.

17

Results

Structure Descriptions

The binding of the carboxylate to the Th(IV) center, results in four distinct local

coordination environments for the four compounds. Compounds 1-8 are built from eight to ten

coordinate An(IV) (An = Th or U) metal centers ligated by carboxylate donor ligands. Despite

relatively similar binding modes of the carboxylates about the An(IV) metal centers, compounds

1-8 adopt different overall

structural units, including

dimeric, hexameric

clusters, and carboxylate

bridged chains.

The structure of

compound 1, Th(2-FA)4,

is shown in Figure 4. Thus

far, only weakly

diffracting crystals have

been isolated; nonetheless, the local coordination environment about the Th(IV) metal center

and the overall connectivity is determined. The structure is built from one crystallographically

unique Th(IV) metal center and two unique 2-FA units. Each of the Th(IV) metal centers exhibit

square antiprismatic geometry. Th(IV) metal centers are each being bound to eight oxygen

atoms which are part of the bridging carboxylates linking to the adjacent thorium metal. Bond

length and angles are not discussed in further detail owing to the disorder of the crystal structure.

Figure 4. Diagram of 1 showing the coordination environment

of the chain. Hydrogen atoms and free floating waters have been

omitted for clarity. (Blue = Thorium, Red = Oxygen, Black =

Carbon)

18

Compound 2, UIV6O6(OH)2(H2O)6(2-FA)10(2-HFA)2•14(H2O), contains carboxylate

groups coordinated to UIV centers in both chelating and bridging modes, resulting in the

formation of the hexamer shown in Figure 5. The cluster core is arranged with six U(IV) metal

centers bridged through eight

μ3-oxo/hydroxo groups.There

are ten bridging bidentate 2-FA

ligands and two carboxylates

that bind to the U(IV) metal

center through one oxygen

while the other oxygen is

protonated. Overall, the U(IV)

adopts a square antiprism

geometry. The average UIV-O

bridging carboxylate bond

lengths are 2.40(2) Å, and the

μ3-O/OH are 2.34(3) Å, which agree well with previously reported UIV hexameric clusters.8a

Each uranium metal center is eight coordinate, bound to either four bridging bidentate ligands

or three bridging bidentate ligands and one bidentate ligand that has one oxygen bound and the

other oxygen remains unbound. The unbound oxygen on the carboxylate has a U-O bond

distance of 3.42(2) Å, consistent with it being unbound.

Figure 5. Hexanuclear unit of 2 without hydrogens

shown for clarity. (Green = U(IV), Red = Oxygen, Black

= Carbon)

19

Compounds 3 and 4 refinements are still in progress, due to weak diffraction and high

degree of disorder, however, the coordination chemistry was determined from this data and further

supported by other spectroscopic methods. Compound 3, UIV(2-FA)4, assumes a chain-like

structure with the carboxylate moieties bridging the U(IV) metal centers (Figure 6). The crystals

formed from the aqueous reactions were highly disordered. Yet, similar crystals could be

synthesized using dimethylformamide as the solvent instead of water, yielding crystals with a

higher level of order. The chemical structure is arranged with the U(IV) metal center bridged by

the carboxylate oxygen atoms. The UIV-O bond lengths fall between 2.26-2.35(10) Å while the

UIV---UIV distances between adjacent uranium ions is 4.72(5) Å.8a

Compound 4, UVIO2(2-FA)3, similarly forms a chain-like structure, with U(IV) bridged

through carboxylate oxygen atoms. The chain has the characteristic uranyl ion bridged by three 2-

furoic acid ligands. Considerable disorder precludes a high degree of certainty in the structural

model, as seen by the high R factors; as such no figure is presented for this structure. Consequently,

Figure 6. Illustration of 3 without hydrogens shown for clarity. (Green = U(IV), Red = Oxygen,

Black = Carbon)

20

the structural information of the crystal cannot reliably be discussed until a better refinement is

achieved. Other types of characterization methods were used to help support the coordination

chemistry of compound 4 as well as the hexavalent state of uranium.

Compound 5, Th2(FDC)4(H2O)10•2(H2O), adopts the molecular structure shown in Figure 7.

The structure is built from two unique thorium (IV) metal centers, four furan-dicarboxylates, and

ten water molecules. Overall, the

Th(IV) exhibits a monocapped

square antiprismatic geometry

resulting from each thorium metal

center coordinating to nine oxygen

atoms from five bound water

molecules and one oxygen each from

the four carboxylates on FDC. The

C-O bond distances of the

carboxylate show a bond length of

1.25-1.26(2) Å. The average

thorium-oxygen bond distances for coordinated water molecules and bidentate carboxylate groups

are 2.52(5) Å and 2.40(3) Å, respectively, showing good agreement with known values.8a The Th-

--Th distances are 6.57(2) Å.

Figure 7. Illustration of 5 showing the coordination

environment of the dimer. Hydrogen atoms and free

floating waters have been omitted for clarity. (Blue =

Thorium, Red = Oxygen, Black = Carbon)

21

Compound 6, Th(FDC)2(H2O)2, adopts the 3D structure depicted in Figure 8. The chain

structure can be seen in the bridging of the FDC to four different Th(IV) metal centers. The chains

are connected along the [010] plane via bridging carboxylate units through the FDC that extend

infinitely along [001] to form 2D sheets (seen in Figure 8a). These sheets further connect along

[100] resulting in a 3D structure shown in Figure 8b. The structure is built from one

crystallography unique Th(IV) metal center and two unique FDC units. Each of the Th(IV) metal

centers exhibit square prismatic geometry. The Th(IV) metal centers are each coordinated to eight

oxygen atoms from six monodentate carboxylate groups and two bound water molecules. The Th-

O bond for the carboxylate and bound water molecules are 2.40(5) and 2.42(12) Å, respectively.

These lengths have known literature values where the carboxylate has shorter bond lengths than

that of the bridging water molecules.8a The interatomic thorium distances are 5.50(2) Å, which are

larger than the average reported chain-like Th---Th internuclear distances, but are not uncommon

to previously reported chains.8a, 11b, 12, 21

Figure 8. Representation of 6 showing the Th(IV) bridged through the carboxylate to form

a 3D chain. Hydrogen atoms and free floating waters have been omitted for clarity. (Blue =


22

Compound 7, Th6O4(OH)4(4-HBA)12(H2O)6•12(H2O), is assembled from hexameric

clusters built from thorium and 4-HBA as shown in Figure 9. In the cluster, the core is arranged

with six Th(IV) ions bridged

through μ3-oxo/hydroxo groups,

thus forming a [Th6(OH)4O4]12+

core. There are six bridging 4-

HBA ligands and six chelating 4-

HBA bidentate ligands. The

average Th-O bond lengths of the

bridging carboxylate (2.46(3) Å)

and chelating carboxylate (2.60(8)

Å), bound water (2.52(3) Å), and

μ3-O/OH (2.38(3) Å) agree well

with previously reported

hexameric clusters, as well as the

Th---Th distances (5.57(8) Å).8a, 11a, 11b Each thorium metal center is nine coordinate and is bound

to one chelating ligand, two bridging ligands, two bound water molecules, and four bridging μ3-

O/OH forming a monocapped square antiprism geometry.

Figure 9. Illustration of 7 hexanuclear cluster. (Blue =


23

Compound 8, UIV6O4(OH)4(4-HBA)12(H2O)6•12(H2O), forms a hexameric cluster with

uranium and 4-HBA seen in Figure 10. The cluster is built from relatively similar to that observed

in 7. The core is composed of six U(IV) metal centers and six bound μ3-oxygen atoms. Four

chelating bidentate

carboxylates and eight

bridging carboxylates

complete the coordination

spheres of the U(IV) metal

centers, with average UIV-O

bond lengths of the bridging

carboxylates at 2.38(5) Å,

chelated carboxylates at

2.56(10) Å, bound water

molecules 2.88(14) Å and

μ3-O/OH lengths of

approximately 2.35(5) Å.

The UIV---UIV distances are

3.85(3) Å. Each uranium metal center is eight coordinate and is bound to oxygen atoms from one

chelating carboxylate, three bridging carboxylates, and four bound μ3-O/OH forming a square

antiprism geometry. The chelation of the carboxylate reveals dissimilar UIV-O bond lengths.

Figure 10. Diagram of compound 8. Hydrogens and free floating

waters were not shown for clarity. (Green = U(IV), Red = Oxygen,

Black = Carbon)

24

Powder X-Ray Diffraction

Powder X-ray diffraction (PXRD) was used to determine bulk phase purity. Using PXRD, it

was found for compounds 1-8 that there is agreement between the calculated powder pattern from

the single crystal data and that for the bulk phase suggesting that the crystals used for the structure

determination are representative of the bulk (See Figures A1-A8). The bulk phases of compounds

1, 3 and 5-6 have peaks that are not accounted for by the calculated powder pattern from the single

crystals. The remaining peaks matched calculated powder pattern of the starting ligand indicating

the bulk phase contained unreacted starting ligand. Compounds 2, 5 and 7-8 were isolated as a

pure phase, which is seen by the agreement between the calculated powder pattern and the data

collected for the bulk phase. The solubility of 2-FA and FDCA is low in aqueous media, thereby

preventing complete dissolution of the ligand and significantly complicating the purification of the

product formed. Yet, the compounds synthesized with the uranium metal center can by

mechanically separated due to the color of the crystals.

A systematic study was set up wherein metal to ligand ratios, pH, and temperature was varied

in order to determine the prevalence of the structural units found. The metal to ligand ratios of 1:1,

1:2, 1:4, and 1:8 were utilized to investigate the effects of excess ligand on product formation. It

was found for compound 5 that the different metal to ligand ratios all show formation of the dimer

in the bulk sample. Most of the PXRD peaks can be accounted for by the dimer, according to

PXRD, however, some signals are present which are in poor agreement with this structure,

suggesting the presence of an impurity which appears to be unreacted FDCA. By careful analysis

of the PXRD spectra obtained, the metal to ligand ratio of 1:1 and 1:2 produce the highest pure

dimeric product. A pH series was also carried out, which revealed formation of the same dimer.

25

The dimeric formation occurs from a pH of 1 to 6. Above this pH, a gel phase was formed

suspected to be the formation of thorium hydroxide.

The influence of temperature was additionally probed for the products; it was found that at a

temperature of 120 °C and above, formation of compound 6 was seen. By PXRD, the bulk of the

sample is found to be compound 6, with unaccounted peaks showing excess FDCA. It was found

that at temperatures below 120 °C, the formation of the dimer (5) is seen as well. Above a

temperature of 120 °C, only the formation of chain and excess FDCA can be seen.

Infrared and Raman Spectroscopy

The IR spectra of 2, 5, and 7-8 (Figures A9-A12) show bands consistent with complexation of

an electrophilic metal with the carboxylate, as well as the bonding seen in the crystal structure.

The difference in the carboxylate asymmetric and symmetric stretch can be used as a mean of

correlating bonding modes. In these compounds, the binding modes involve unidentate, bidentate

(chelating), and/or bridging leading to observable spectroscopic differences. Within transition

metal complexes with unidentate bonding modes, the asymmetric carboxylate stretch ranges from

1600 to 1745 cm-1, the symmetric carboxylate stretch from 1240 to 1376 cm-1, and the difference

between ranges from 228 to 470 cm-1.22 The difference in the carboxylate antisymmetric and

symmetric stretch for chelating binding mode and bridging are smaller than the unidentate binding

mode.22-23 The asymmetric carboxylate stretch for the chelating binding mode ranges from 1507

to 1610 cm-1, the symmetric carboxylate stretch from 1377 to 1465 cm-1, and the difference

between ranges from 42 to 190 cm-1, which is smaller than stretches seen for unidentate.22 Bridging

binding mode has the asymmetric carboxylate stretch range from 1548 to 1621 cm-1, the symmetric

26

carboxylate stretch from 1387 to 1440 cm-1, and the difference between ranges from 120 to 200

cm-1.22-23

Due to the many vibrations of the ligand, bound water molecules, and uncoordinated water

molecules, the frequencies in the IR spectrum cannot be confidently determined without further

theoretical studies for all of the compounds. For all the compounds the suspected asymmetric and

symmetric stretches are suggested to fall between 1600-1400 cm-1. The compounds have different

binding modes and if a computational study was done, the peaks would then be able to be

determined. It would be expected that the differences in the carboxylate asymmetric and symmetric

stretch correlate to the binding modes seen in the crystal structure. Compound 2 has monodentate

and bridging modes while compound 5 has only bridging binding modes, while chelating and

bridging modes are seen in compounds 7 and 8. The difference in the carboxylate asymmetric and

symmetric stretch for the monodentate mode would be expected to be significantly greater than

the chelating or bridging modes.

The vibrations from lattice water and coordinated are expected to be seen at 3400 and 3000 cm-

1, respectively. The bound water molecules can be seen in all the IR spectra in a broad peak

centered around 3400 cm-1. However, because of this broad peak the uncoordinated water

molecule cannot clearly be seen but is expected around 3000 cm-1. The stretches of C-H bonds

from the ring are expected around 500 – 900 cm-1, while metal-oxygen bonds are expected to be

seen around 800 and 1000 cm-1. Compound 4 is believed to have characteristic uranyl stretches

appearing around 934 and 754 cm-1 that match reported literature values.15b, 24 The C=C bond is

likely around 1500 cm-1, and the C-C stretching frequency about 1200 cm-1.25 For all compounds,

the vibrations are only arbitrarily assigned due to overlapping frequencies.

27

The Raman

spectra of all compounds

(Figures A13-A20) are

similarly indicative of

metal bound carboxylate

with strong bands

centered around 1500 cm-

1. This intense peak

supports the idea that the

carboxylate is

deprotonated and

complexed to the metal center.26 Figure 11 shows the disappearance of the peak at 1700 cm-1,

which is associated with the protonated 2-furoic acid.26 As with the IR, the peaks from the Raman

spectra are arbitrarily assigned due to expected overlay with the frequencies. The peaks from the

Raman spectra are expected and seem to correspond to those of the IR. Theoretical studies must

be done in order to determine the ligand vibrations but it can be assumed the carboxylate

asymmetric and symmetric shifts of all the compounds should correspond to the different binding

modes. The U(IV)-oxygen bond peaks are suspected to be around 538 and 495 cm-1 for compound

2-3, and 8. Compound 4, which possesses a U(VI) center, has U(VI)-O bond peaks around 737

and 886 cm-1, which corresponds to known literature values.24b For all compounds the frequencies

for coordinated water is expected to be a large peak around 3400 cm-1.

Figure 11. Raman spectra of SXRD 2 (blue) and FDCA (red).

28

Raman spectroscopy

was also used in an

effort to correlate

solution speciation and

solid speciation.

Solution Raman was

taken after the starting

materials were placed

into a vial and left

capped for a day. Figure

12 shows the

comparison of Raman

of compound 5 SXRD, the solution from which 5 was isolated and FDCA dissolved in water.

When comparing the Raman speciation of the solution (1525.19 cm-1) with that of the ligand in

H2O (1516.55 cm-1), a shift in the peaks attributed to the carboxylate modes is observed. This shift

is consistent with metal-ligand complexation. Moreover, the solution spectrum compares

complexation of the metal by the ligand in solution. However, given the complexity of the Raman

spectrum as well as overlapping bands and lack of complementary data, we are hesitant to

comment further on the identity of the complex in solution.

Thermogravimetric Analysis

The thermal behavior of compound 5 was examined. TGA shows the degradation of the sample

as seen in the step-wise weight loss that occurs as the sample is heated up to 700 °C. The

Figure 12. Raman Spectra of compound 5 crystal (blue), solution

speciation (red), and FDCA dissolved in H2O (green).

29

degradation of the dimer occurs first with the loss of water to yield Th2(FDC)4 at 150 °C with a

weight loss of 83.29% observed (calc: 83.33%). The final degradation involves FDC to give

(ThO2)2 at an observed weight loss of 40.68% (calc: 40.74%) and a temperature of 450 °C (See

A21). These finding were further supported by PXRD.

30

Discussion

Compounds 1–8 have shown that the change in the ligand leads to the formation of different

types of solid state structural units, such as hexamers, chains, and dimers. The formation of the

dimeric complex 5 exhibits surprising coordination compared to other know complexes of Th(IV)

carboxylates, which are commonly monomeric, hexameric, or linear chains; indeed, a survey of

the Cambridge Crystallographic Database shows that the majority of actinide complexes assumes

one of these three structural motifs. There are few examples of thorium dimers complexed by

hydroxides, amides, perchlorates, or butoxide groups8a, 9, but none thus far show the use of

carboxylate functionality. While the nuclearity of these complexes are similar, the differences in

the structure of the organic make a direct comparison to compound 5 inadequate arising from the

differences in steric hindrance, electronic structure, and pKa of the binding ligand. To the best of

our knowledge, the dimer is the first known structure that is bridged through the ligand instead of

through a single carboxylate to form an isolated molecular unit.

In order to understand the formation of the different complexes, a systematic study of

temperature, time, metal to ligand ratio, and pH was performed. Variations in metal to ligand ratios

over a pH of 1-6 were found to have no effect on the formation of the dimer in 5. Yet an increase

in temperature led to the formation of compound 6. Solution-state studies would aid immensely in

understanding the formation of dimer, whereas a chain is seen for compound 1 and a hexamer is

seen for compound 2. However, because of the low solubility of the ligand, it is not possible to

perform such studies. Rather, this study continues and investigates how and why the stable dimeric

species forms. Compound 1, 3, 4 and 6 form chains while 2, 7 and 8 form hexamers with 2-FA,

FDCA, and 4-HBA, respectively, which is a well-established coordination mode for the actinides.

31

However, compound 5 exhibits surprising coordination compared to the other Th(IV) carboxylates

formed.

Our results have shown that the composition and structure of the ligand has a major effect on

the molecular units formed. Despite the similar carboxylate functionalities of 2-furoic acid, furan-

2,5-dicarboxylic acid, and hydroxybenzoic acid ligand systems, they give rise to decidedly

different molecular clusters and extended networks as observed in the solid state. Adding an

additional carboxylate

group to 2-FA to get

FDCA, gives dimeric

units as opposed to

chains. Within the 4-

HBA ligand system,

similarities are seen in the

structural unit formed by

Th(IV) and U(IV), with both forming a hexanucler unit. More research needs to be done in order

to understand fully the directing effects of the organic and the interesting formation of the dimeric

species of compound 5. Compounds 2 and 8 containing 2-furoic acid and 4-HBA, respectively,

are built from related structural units. Varying synthetic conditions as mentioned, has little effect

on the speciation of the An unit observed in the solid state. Isolation of these clusters presented

here do show that the organic highlights the prevalence of these structural units within carboxylate

ligand systems. As shown in Figure 13, FDCA results in formation of dimeric species under acidic

conditions at room temperature, while under the same conditions, 4-HBA gives rise to the

Figure 13: Thorium(IV) molecular clusters that have been isolated

from aqueous solution (Dimer = (a), Hexamer = (b)).

32

formation of the hexameric species. Efforts to correlate solution species with solid state using

spectroscopic methods were unsuccessful. The limited solubility of the ligands attributed in part

to gathering solution state information and helped direct a new path forward with a soluble ligand,

pyridine-dicarboxylic acids.

33

Conclusions

Future Works

Looking to the future,

there are a few different

directions that this project

could proceed in. It is of great

interest to continue

developing our

understanding of the directing effects of the nature of the ligand as well as make a correlation

between that of the solid state and the solution species. The relationship between the solution

species and solid state can be correlated in an effort to identify factors that drive the formation of

a particular species and examine the conditions over which various, structural units are stable. By

changing the functionality of the ligand being worked with, we hope to increase the complexity of

the organic and see how this, in turn, affects the coordination chemistry. Proposed ligands of study

(Figure 14) are picolinic acid, 2,6-pyridine-dicarboxylic acid, and their derivatives. These ligands

are soluble in water and allow for a more in-depth examination of actinide solution behavior. The

speciation and structural chemistry observed in the solid state compared with that observed in

solution will allow for us to identify the conditions over which the species form and are stable in

solution before crystallization occurs, as well as to be used to develop an understanding of more

general phase formation.

As this research moves into more soluble systems, it is hoped that efforts can be made for

directed growth on carboxylate functionalized self-assembled monolayer surfaces (SAMs). SAMs

Figure 14. Proposed ligands: Picolinic acid, 2,6-pyridine-

dicarboxylic acid, and 2,4-pyridine-dicarboxylic acid (Left

to Right).

34

can be used as polyelectrolyte surfaces, which will give us a better comparison to humic acid. The

goal is to develop structural models and trends that may be applied to understanding complexation

of humic acid in environmental systems. As we progress to systems representative of the

environment, the chemistry becomes inherently more complex as a result of the increased

propensity for redox reactions, hydrolysis and condensation, and the presence smaller

concentrations of actinides in the water, as well as competing ligand complexation and spectator

ions. In these systems, each of these problems will have to be carefully controlled in order to

understand how each has an effect on the actinide-ligand coordination. By carefully controlling

the addition of species found under environmentally relevant conditions, the effect of each species

can be understood, helping to gain a fundamental understanding of actinide-organic complexation

in the environment.

Conclusion

The systematic studies of all the compounds have led to various structural units with great

interest in the formation of 5, the dimer. There are no examples of dimeric formation with

carboxylates complexed with thorium. The various structural units show that the complexity and

pKa of the ligand influence the structural units seen. More research is needed to gain an

understanding on how functionality, carboxylate loading, flexibility, and complexing strength of

the organic contribute to the various structural formations and speciation of the metal center.

Proposed ligands such as 2,6-pyridine-dicarboxylic acid and its derivatives will look allow us to

look at these influences as well as correlate between the solution speciation and solid state. The

relationship between the solution species and solid state can then be correlated in an effort to

identify key factors that drive the formation of a particular solution species or solid state phase.

35

Studying the solution and solid state allows for the determination of how the nature of the organic

ligand and solution conditions affect the coordination modes, nuclearity, and overall speciation of

the actinide-organic complexes. Solution state, then, provides additional information of the

conditions over which certain species form and are stable and how the solution state compares to

the solid state. As the interactions of the actinides and small organic models are studied and

understood, more complex organics will be studied leading to a much needed fundamental

understanding on how actinides-organic complex and behave in the environment. This

understanding will then help to develop more effective remediation strategies for when nuclear

accidents and other anthropogenically events occur as well as how to deal with actinide waste that

already contaminates our environment.

36

Endnotes

1. (a) Clark, D. L.; Janecky, D. R.; Lane, L. J., Science-Based Cleanup of Rocky Flats.

Physics Today 2006, 34-40; (b) Novikov, A. P.; al, e., Colloid Transport of Plutonium in the Far-

Field of the Mayak Production Association, Russia. Science 2006, 314, 638-641; (c) Maher, K.;

Bargar, J. R.; Brown, G. E., Jr., Environmental speciation of actinides. Inorg Chem 2013, 52 (7),

3510-32.

2. (a) Plaue, J. C., K.R., Actinide speciation in Environmental Remediation. Journal of

Nuclear Science and Technology 2002, 39, 461-465; (b) Stuckless, J. S. L., R.A., The Road to

Yucca Mountain. Environmental & Engineering Geoscience 2016, 22, 1-28; (c) Kersting, A. B.,

Plutonium Transport in the Environment. Inorg Chem 2013, 52, 3533-3546.

3. Nitsche, H.; Silva, R. J., Environmental Actinide Chemistry. MRS Bulletin 2001, 707-

713.

4. (a) Graetz, G., Uranium mining and First Peoples: the nuclear renaissance confronts

historical legacies. Journal of Cleaner Production 2014, 84, 339-347; (b) Science, O. o. Basic

Research Needs for Environmental Management; 2015; pp 3-104.

5. (a) Colloids: Carriers of actinides into the environment. Los Alamos Science 2000, 26,

490-491; (b) Zanker, H. H., C., Colloid-borne forms of tetravalent actinides. Journal of

Contaminant Hydrology 2014, 157, 87-105; (c) Mibus, J. S., S.; Pfingsten, W.; Nebelung, C.;

Bernhard, G., Migration of uranium(IV)/(VI) in the presence of humic acids in quartz sand: A

laboratory column study. Journal of Contaminant Hydrology 2007, 89, 199-217.

6. (a) Ghabbour, E. A.; Davies, G., Humic Substances: Structures, Models and Functions.

Royal Society of Chemistry (Britain): 2001; (b) Wood, S. A., The role of humic substances in the

transport and fixation of metals of economic interest (Au, Pt, Pd, U, V). Ore Geology Reviews

1996, 11, 1-31; (c) Ghabbour, E. A. D., G., Humic Substances: Structures, Models and

Functions. Royal Society of Chemistry: 2001; (d) Stevenson, F. J., Humus Chemistry: Genesis.

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39

Appendix

Powder X-ray Diffraction Data

Figure A1. PXRD of sample from which compound 1 was isolated comparing bulk phase

(red) and calculated pattern for 1 (black). Impurity can be seen at 30° however, majority of bulk

phase representative of the single crystal.

Figure A2. PXRD showing calculated pattern for 2 (black) and bulk phase (red) showing

single crystal representative of bulk.

40


single crystal representative of bulk. Disagreement of peaks suggests single crystal does not

represent bulk phase.


single crystal not representative of bulk. There are peaks which are not accounted for by the

calculated pattern.

41

Figure A5. Comparison of PXRD calculated patter 5 (back) and bulk phase (red) showing

agreement between the calculated pattern and bulk. The single crystal is representative of the

bulk phase.


single crystal is representative of bulk however, there are peaks unaccounted for such as at

28°.

42

Figure A7. Comparison of PXRD calculated pattern 7 (black) and bulk phase (red) shows

agreement between powders suggests single crystal is representative of bulk phase.

Figure A8. PXRD of calculated pattern 8 (black) and bulk phase (red) showing single crystal

is representative of bulk phase.

43

Infrared Spectroscopy

Figure A9. IR spectrum of Compound 2.


44



45

Raman Spectroscopy

Figure A13. Raman spectra of Compound 1.

Figure A14. Raman Spectra of Compound 2.

46



47



48



49

Thermoagravimetric Analysis Plots

Figure A21. TGA of Compound 5.

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