Spectral Analysis of the Uranyl Squarate and Croconate System

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Spectral analysis of the uranyl squarate and croconate system: Evaluating the differences between solution and solid-state phases. M. Basile, D. K. Unruh, L. Streicher, and T. Z. Forbes Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00838 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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Crystal Growth & Design

For submission to Crystal Growth and Design

Spectral analysis of the uranyl squarate and croconate system: Evaluating differences between the solution and solid-state phases. M. Basile, D.K. Unruh, L. Streicher, and T. Z. Forbes* Department of Chemistry, University of Iowa, Iowa City, IA, 55242 * Corresponding author: [email protected] Abstract The design of hybrid materials relies on an understanding of the structural building units present in the reaction, but identifying these building units in solution can be hampered by difficulties in the interpretation of the characteristic spectroscopic signals. The importance of speciation and intermolecular interactions in identifying building units was explored in the uranyl squarate and croconate system with the isolation and characterization of six compounds: (USq1) [C4H12N2][(UO 2)(C4O4)2(H2O)]·(H2O), (USq2) [C5H6N][(UO2)(C4O4)(µ2-OH)]·2H2O, (USq3) [C2H10N2]2[(UO2)6(C4O4)3(µ3-O)2(µ2-OH)6],

(UCr1)

[C4H12N2]2[(UO 2)(C5O5)3(H2O)]·3H2O,

(UCr2) [C5H6N]4[(UO 2)4(C5O5)4(µ 2-OH)4(H2O)4], and (UCr3) [C2H10N2]5[(UO2)6(C5O5)6(µ 3O)2(µ 2-OH)6(H2O)4]. These compounds are built upon the traditional monomeric, dimeric, and trimeric uranyl oligomers that occur upon hydrolysis of the metal center. A combined solid and solution spectroscopic approach was used to understand the impact of uranyl oligomerization, ligand coordination and intermolecular interactions. The v1(UO22+) Raman shift and the UVVisible spectrum for each solid uranyl-squarate and uranyl-croconate compound reveals important information regarding the impact of π-π interactions on the chemical properties of uranyl hybrid materials. 1 ACS Paragon Plus Environment


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Introduction

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Controlling the molecular building blocks of hybrid materials is one major goal of

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crystal engineering, but in many systems this is difficult to achieve because of flexible

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chelation of the ligand and metal hydrolysis. 1-7

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development of hybrid materials is typically through the choice of ligand, where specific

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functional groups and sterics can aid in metal chelation, direct dimensionality, or enhance

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supramolecular interactions. 2, 4, 5, 8 More variability is observed in the metal centre where

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flexibility in the coordination number can lead to changes in molecular geo metry or 9

The greatest level of control in the

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chelation denticity. 6,

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centres, which can cause variations in ligand complexation or dimensionality of the

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synthesized co mplex. 7

In addition, hydrolysis processes can form multinuclear metal

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Spectroscopic tools can be used to provide enhanced insight into the molecular

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building units present in the initial synthetic conditions, but the crystallization process can

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lead to difficulties correlating the solution species with the resultant solid-state phase. For

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instance, crystallization of aluminium phosphate molecular sieves is a well -studied

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processes that has previously been explored using X-ray diffraction/scattering and

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spectroscopic techniques. 10-12 An investigation by Fan et al., demonstrated how UV Raman

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spectroscopy can provide a very detailed analysis of the initial nucleation process. 12 In this

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system, specific changes in the spectrum can be assigned to bands associated with isolated

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AlO 6 octahedra, PO 43- tetrahedra, and the organic templating agent. The crystallization

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pathway can be followed as the initial isolated species are consumed and well -defined new

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bands appear that can be paired with the resulting solid state material.

However, the

2 ACS Paragon Plus Environment

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organic templating agent undergoes less well defined spectral shifts as it interacts with the

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inorganic components and beco mes entrapped within the solid -state lattice.

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intramolecular interactions, such as van der Waals forces, hydrogen -bonding, or pi-pi

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interactions, can occur either in the solution or solid state phases. These unexpected shifts

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in the characteristic bands can further complicate identification and analysis of the

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molecular building units. This can be a significant issue for hybrid materials, where the se

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interactions are expected to be stronger and influence the spectral shifts to a greater degree.

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Design difficulties are particularly intricate in uranyl hybrid materials owing to the

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variability in coordination geo metry, ligand denticity, and metal hydrolysis all of which can

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occur within one system. 13-18 Four, five, or six equatorial ligands can coordinate about the

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nearly linear uranyl ((U(VI)O 2)2+ ) cation, leading to the formation of several possible

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coordination geo metries, including square, pentagonal, or hexagonal bipyramids. 19 Organic

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molecules with carboxylate functional groups are the most common linkers for uranyl

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hybrid materials and can chelate in mono dentate or bidentate modes, either to one metal

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centre or in a bridging fashion. 13, 14 Adding to this complexity is the susceptibility of the

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uranyl cation to hydrolyze, typically forming multinuclear secondary building units when

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the pH of the solution is greater than three leading to the formation of uranyl monomer,

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dimers, trimers and larger oligomeric groups. 13, 16 This flexibility in the design features has

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resulted in the synthesis of hundreds of compounds, but predicting the formation of specific

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compounds, or even specific building units within this system is still difficult. Thus,

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additional information regarding the structural features of the uranyl ligand building unit,

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particularly in solution or before crystallization, would lead to a more directed and targeted

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system.

These



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Raman spectroscopy can be an important tool to explore aqueous uranyl chemistry

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because the presence of a strong symmetric stretching band (ν 1) associated with the uranyl

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cation that is located at 870 cm -1 for the pentaaquauranyl ([(UO 2)(H 2 O)5]2+ ) complex. 20

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This band typically red shifts up to 70 cm-1 upon hydrolysis or ligand complexation and has

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previously been used to identify specific uranyl species present aqueous solutions and

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throughout the crystallization process. 20-26 We have previously utilized Raman

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spectroscopy to understand the major species in solution within uranyl citrate, where there

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was a direct correlation between the bands in the solid -state material and the solution

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phase. 23 However, analysis of the spectral features is not always straightforward as the

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exact band shifts are not always known or there can be changes during the crystallization

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process. Therefore, it is important to understand when variations in the solution and solid -

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state spectral features could occur to accurately identify building units for a variety of

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uranyl hybrid systems.

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To further explore the spectroscopic changes that can occur during the

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crystallization of uranyl hybrid materials, we synthesized and characterized uranyl squarate

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and croconate co mpounds and related aqueous solutions. Highly symmetrical oxocarbon

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dianions, Cn On 2- (n = 3, 4, 5, 6 for deltate, squarate, croconate and rhodizonate,

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respectively), belong to an interesting class of monocyclic ligands that possess electronic

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delocalization over the entire molecule and can engage in significant intermolecular

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interactions in both solution and solid state materials. 27 Our initial interest in the oxocarbon

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anions was piqued by the synthesis of the unusual porous uranyl squarate metal organic

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framework developed by Rowland and Cahill 28 and their reported building units based upon

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hydrothermal reaction conditions. 29 Herein, we build upon this previous study with the


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syntheses and structural characterization of the uranyl building units within the squarate

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((USq1)

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OH)]·2H2 O, and (USq3)24 [C 2 H 10 N2] 2 [(UO2 )6 (C4 O4 )3 (µ 3 -O)2 (µ2 -OH)6]) and croconate

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((UCr1) [C 4 H12 N2] 2 [(UO 2 )(C5 O5 )3 (H2 O)]·3H 2 O, (UCr2) [C 5 H6 N] 4 [(UO 2 )4 (C 5 O5)4 (µ 2 -

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OH)4 (H2 O)4] and (UCr3) [C 2 H10 N 2] 5 [(UO 2 )6 (C5 O5 )6 (µ 3 -O)2 (µ 2 -OH)6 (H2 O)4]) system.

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The solid state and solution phases were further characterized by Raman spectroscopy to

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explore variations in the chemical signatures associated with the uranyl solution and solid

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state phases. UV-Vis spectroscopy was also utilized as a complimentary spectroscopic

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technique to further understand the building units present in the uranyl squarate and

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croconate systems.

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Experimental Methods

[C 4 H12 N 2][(UO 2 )(C4 O4 )2 (H2 O)]·(H2 O),

(USq2)

[C 5 H6 N][(UO 2 )(C 4 O 4 )(µ 2 -

81 82

Synthesis. Uranyl Nitrate UO2(NO3)2·6H2O (Flinn Scientific Inc.), squaric acid C 4H2O4 (Sigma

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Aldrich), croconic acid C5H2O5 (TCI America), piperazine C4H10N2·6H2O (Alfa Aesar), pyridine

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C5H5N (Fisher Chemicals), and ethylenediamine C 2H8N2 (Sigma Aldrich) were reagent grade and

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used without further purification. Millipore-filtered ultrapure water (18.2 MΩ∙cm) was provided

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by a ThermoScientific Barnstead EasyPure II which was used in all the syntheses. CAUTION:

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(UO2)(NO3)2·6H2O contains radioactive 238U, which is an alpha emitter, and like all radioactive

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materials must be handled with care. These experiments were conducted by trained personnel in

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a licensed research facility with special precautions taken towards the handling, monitoring, and

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disposal of radioactive materials.

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In general, crystals were formed from combining uranyl nitrate hexahydrate with squaric

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or croconic acid and adding an organic base to increase the pH to 2-4. Piperazine, pyridine, and 5 ACS Paragon Plus Environment


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ethylenediamine could all be used to increase the alkalinity of the solution to the targeted pH range.

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Crystallization occurred under ambient conditions after two to 16 days of aging. The optimized

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synthetic conditions for crystallization of all six compounds are summarized in Table 1 while the

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more detailed information regarding the experimental syntheses is described in the supporting

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information section.

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Table 1. Synthetic conditions for the preparation of [C4H12N2][(UO2)(C4O4)2(H2O)]·H2O (USq1), [C5H6N][(UO2)(C4O4)(µ2-OH)]·(H2O)2 (USq2), [C2H10N2]2[(UO 2)6(C4O4)3(µ 3-O)2(µ2OH)6] (USq3), [C4H12N2]2[(UO2)(C5O5)3(H2O)]·3H2O (UCr1), [C5H6N]4[(UO2)4(C5O5)4(µ2OH)4(H2O)4] (UCr2) and, [C2H10N2]5[(UO2)6(C5O 5)6(µ3-O)2(µ2-OH)6(H2O)4] (UCr3) crystals. The final concentration (mM) and moles (mol) are provided for the individual components. USq1

USq2

USq3

UCr1

UCr2

UCr3

Uranyl Nitrate (mM; µmol)

20; 60

20; 20

20; 20

20; 40

20; 40

20; 40

Squaric Acid (mM; µmol)

20; 60

20; 60

20; 10

—

—

—

Croconic Acid (mM; µmol)

—

—

—

20; 80

20; 40

20; 40

Water (mL; mol)

—

4; 0.22

8.5; 0.47

2; 0.11

4; 0.22

3; 0.17

Pyridine (v/v%; µmol)

—

10; 0.25

—

—

—

—

Piperazine (M; µmol)

0.5; 55

—

0.5; 17

0.5; ~95

0.5; ~50

—

Ethylenediamine (v/v%; µmol)

—

—

—

—

—

10; ~60

pHf

2.3

4.7

4.4

3.0

3.6

4.2

6

16

4

2

7

8

Solution color

Red

Orange

Orange

Orange

Orange

Yellow

Crystal Color

Red

Yellow

Yellow

Red

Orange

Red

Percent Yield

70

50

80

55

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2σ(I)] R Indices (all data)

USq1

USq2

USq3

UCr1

UCr2

618.30 P-1 7.396(2) 9.560(3) 12.260(4) 70.84(1) 87.82(1) 81.25(1) 809.3(4) 2 2.28 10.102 580 1.759-26.295 -9 < h < 9 -11 < k < 11 -15 < l < 15 19667/3250 [Rint = 0.0331] 1.107 R1 = 0.0129 wR2 = 0.0332 R1 = 0.0134 wR2 = 0.0334

515.22 P-1 8.684(4) 8.716(4) 9.696(4) 63.54(1) 79.93(1) 77.51(1) 638.9(5) 2 2.678 12.747 472 2.356 -25.534 -10 < h < 10 -10 < k < 10 -11 < l < 11 14,577/2357 [Rint = 0.0581] 1.085 R1 = 0.0236 wR2 = 0.0642 R1 = 0.0236 wR2 = 0.0645

4168.6 I-43m 16.9843(12) 16.9843(12) 16.9843(12) 90 90 90 4899.4(1) 2 2.826 19.837 3520 2.398-25.326 -20 < h < 20 -20 < h < 20 -20 < h < 20 52,767/870 [Rint = 0.0467] 1.213 R1 = 0.0259 wR2 = 0.0838 R1 = 0.0278 wR2 = 0.0854

920.54 Pnma 7.6105(2) 24.8221(7) 15.1521(4) 90 90 90 2862.36(13) 4 2.136 5.773 1792 2.689-27.101 -9 < h < 9 -31 < k < 31 -19 < l < 19 25,092/2930 [Rint = 0.0488] 1.033 R1 = 0.0199 wR2 = 0.0416 R1 = 0.0247 wR2 = 0.0431

1498.77 P-1 9.5681(3) 10.1169(4) 12.5613(5) 101.486(1) 101.189(1) 100.462(1) 1137.84(7) 1 2.187 7.220 716 1.704-25.678 -11 < h < 11 -12 < k < 12 -15 < l < 15 21,636/43240 [Rint = 0.0172] 1.018 R1 = 0.0143 wR2 = 0.0377 R1 = 0.0149 wR2 = 0.0380

UCr3 1192.24 P21/m 8.8817(8) 15.8893(15) 9.9559(10) 90 94.865(4) 90 1400.0(2) 2 2.828 17.391 1036 2.053-25.349 -10 < h < 10 -18 < k < 19 -11 < l < 11 19,076/2662 [Rint = 0.0447] 1.018 R1 = 0.0277 wR2 = 0.0548 R1 = 0.0446 wR2 = 0.0593

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Chemical Characterization. Purity of the bulk material was determined using powder X-ray

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diffraction (pXRD). Large single crystals were ground using mortar and pestle and a slurry mount

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was prepared using acetone on zero background Si wafer. PXRD patterns were collected from 5

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to 60° 2θ with a step size of 0.02° 2θ and a count time of 1 s/step on a Bruker D-8 ADVANCE

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diffractometer equipped with Cu Kα radiation and a LynxEye solid state detector.

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Raman spectra for single crystals of the model uranyl compounds with squarate or

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croconate were acquired on either a Thermo Nicolet Almega XR High-Performance

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Dispersive Spectrometer or a Snowy Range Raman Spectrometer. Both instruments are

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equipped with a 780 nm laser source fro m 90 to 1300 cm -1 for a total of 128 scans at an

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integration time of one second per scan.

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Aqueous uranyl species within the solution squarate or croconate systems were

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investigated using Raman spectroscopy at pH values between 2.00 and 5.25. The uranyl to

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ligand ratio was varied for the squarate solutions, but only one ratio could be measured for

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the uranyl croconate system. Solution analyses were performed on a SnRI High-Resolution

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Sierra 2.0 Raman spectrometer equipped with 785 nm laser energy and 2048 pixels TE -

146

cooled CCD. The laser power was set to the maximum output value of 15 mW and the

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system was configured to acquire data by the Orbital Raster Scanning mode, giving the

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highest achievable spectral resolution of 2 cm-1. Experiments were conducted in the dark

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to mitigate fluorescence interferences.

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integration time of 60 seconds and automatically reiterated ten times in Multi-Acquisition

151

mode. The average of the ten Raman spectra acquired for a sample is reported as the final

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Raman spectru m. Broad bands observed in the spectra obtained from the aqueous solution

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suggests the presence of multiple species. To correctly determine the species present in

Each solution sample was irradiated for an



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solution, the background was subtracted, and multiple peaks were fit using the peak

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analysis protocol in the Origin 9.1 software (OriginLab, Northampton, MA). The Raman

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cross-section for all uranyl species can be assumed to be similar, therefore the relative

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abundance of each species can be estimated using the peak height of the Gaussian function.

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FWHM values are expected to be approximately 13 -15 cm-1 for all uranyl co mplexes,

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except for the pentaaquauranyl species, which can be observed with a FWHM closer to 10

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cm-1. Additional information regarding data analysis of the solution phase can be found in

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Lu et al. 34

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Solution and diffuse reflectance UV-Vis spectra were collected on Agilent Cary 5000 UV-

163

vis-NIR spectrophotometer. Solid-state samples were ground and mounted to the holder and

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spectra were collected from 200-800 nm at intervals of 1 nm and a 600 nm·min-1 scan rate. The

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raw diffuse reflectance data was converted to absorbance using Kubelka-Munk function.35 Given

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the low yields of UCr3, the solid-state UV spectrum could not be obtained for this sample. Spectra

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of the aqueous uranyl squarate and croconate solutions (10 mM; pH 2) were also collected on the

168

same instrument using similar scan parameters.

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Results and Discussion

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Structural Descriptions. All six compounds discussed in this work contain UO 7 pentagonal

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bipyramids with two short U≡Oyl axial bonds and five longer U−O equatorial bonds forming the

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nearly linear uranyl, [O≡U≡O]2+, moiety. Five O atoms, occupying the pentagonal vertices about

173

the U(VI) equatorial plane, correspond to ligated waters, bridging oxo/hydroxo groups, or ligand

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coordination from squarate or croconate depending on the structure. In view of this structural series

175

as a whole, equatorial U–O bond lengths span the ranges of 2.226(7)-2.490(2) Å (USq1, USq2,

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USq3) and 2.203(6)–2.531(18) Å (UCr1, UCr2, UCr3). Each oxocarbon dianion possesses a 10 ACS Paragon Plus Environment

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negative two charge that is typically delocalized over the entire (CnOn)2- unit, where n = 4 in

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squarate and n = 5 in croconate, resulting in a symmetric and aromatic ligand in both cases.

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Carbon-carbon bonds within squarate range from 1.437(4) to 1.484(6) Å with the biggest

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difference observed in USq1. More variations are observed for the C-C bonds within the croconate

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compounds with bond distances ranging from 1.419(4) to 1.492(4) Å. The unique coordination

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environments of the three uranyl squarates and the three uranyl croconates are described in detail

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as follows.

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Uranyl Squarates. The primary building unit in USq1 is a mono meric species with a 1:2

185

U(VI):Sq ratio and the extended structure contains a 1-D chain, with each squarate

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molecule linked to the neighboring uranyl pentagonal bipyra mid through bridging μ 2-1,2-

187

bis(monodentate) O ato ms (Fig. 1). Thus, each uranyl polyhedron is connected to four

188

squarate molecules that both extend along the length of the chain and crosslink two chains

189

to form an infinite, ladder-like array. Squarate ligands from adjacent one-dimensional

190

chains are involved in π···π interactions with centroid-to centroid distances of 3.35 and

191

3.55 Å which constructs a pseudo-infinite sheet array in two-dimensions. The individual

192

monomeric species has a negative two charge that is balanced in the crystalline -phase

193

compound by piperazinium cations lying about independent inversion cent ers. Water

194

molecules are similarly present in the interstitial spaces, leading to a molecular formula of

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[C 4 H12 N 2][(UO 2)(C 4 O 4) 2(H2 O)]·H 2 O for the neutral USq1 compound.



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(a)

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(b)

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Figure 1. (a)Thermal ellipsoid representation (50%) of the USq1 1:4 U:Sq species. The U, O, and C atoms are depicted by the yellow, red, and gray spheres, respectively. Hydrogen atoms were omitted for clarity. Symmetry code for equivalent atoms: (ii) −x+1, −y+1, −z+1 (iii) x−1, y, z. (b) Rendered polyhedral representation of USq1 illustrating the ladder-like array of uranyl monomers chelated by squarate ligands infinitely in one-dimension. The U(VI) is positioned in the center of each solid, yellow polyhedron with an oxygen atom located a t each vertex. The black and red wires represent carbon and oxygen bonds, respectively.

206

ratio (Fig. 2). Three of the O atoms bound through the equatorial plane are affiliat ed with

207

three discrete squarate molecules that ligate one U(VI) metal center in a monodentate

208

fashion.

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oligo merization of two uranyl polyhedra through two µ 2 -OH groups on a shared edge. The

210

extended sheet structure is assembled with bridging μ 3-1,2,3-tris(monodentate) chelation

211

by squarate oxo-groups linking the isolated uranyl dimers into two-dimensions. In this

212

system, the centroid-to-centroid distances between the squarate ligands are 5.14, 5.30, and

213

6.14 Å, which suggests insignificant π- π interactions. Overall, the molecular formula of

214

USq2 compound is [C 5 H6 N][(UO 2)(C 4 O4)(µ 2-OH)]·2H2 O with interstitial pyridinium

A dimeric unit is identified as the primary species for USq2 with a 2:6 U(VI):Sq

Inner coordination sphere about the metal center is completed upon


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cations to maintain electroneutrality and water molecules that aid the crystallization

216

process. (a)

(b)

217 218 219 220 221 222 223 224

Figure 2. (a)Thermal ellipsoid representation (50%) of the USq2 2:6 U:Sq species. The U, O, and C atoms are depicted by the yellow, red, and gray spheres, respectively. Hydrogen atoms were omitted for clarity. Symmetry code for equivalent atoms: (i) −x+1, −y+1, −z (ii) −x+1, −y, −z+1 (iii) −x+1, − y+1, − z+1. (b) Perspective view of the USq2 sheet array of uranyl dimers chelated by squarate ligands infinitely in two-dimensions. The U(VI) is positioned in the center of each solid, yellow polyhedron with an oxygen atom located at each vertex. The black and red wires represent carbon and oxygen bonds, respectively.

225 226

As previously noted, USq3 was originally crystallized by Rowland and Cahill using

227

NH4 OH, whereas piperazine was utilized as the base in this study. 28 USq3 contains a

228

trimeric species with a 3:6 U(VI):Sq ratio where one crystallographically unique U(VI)O22+

229

is coordinated about the equatorial plane through one μ 3-O bridge, two μ 2-OH bridges, and

230

two O atoms from μ 4-1,2,3,4-tetrakis(monodentate) squarate ligation (Fig. 3). Hydrolysis

231

of the UO 22+ cation promotes the formation of μ 3-O and μ 2-OH bridges that link the

232

individual polyhedra into a trimeric species with shared edges. An infinite framework is

233

assembled through three-dimensional coordination provided by the bridging μ 4-1,2,3,4-



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Page 14 of 38

234

tetrakis(monodentate) chelation of the squarate oxo-groups. Rigidity of the planar, sp 2

235

hybridized squarate ligand governs the extent of structural corrugation as conveyed by the

236

array of open channels lined with puckered uranyl trimers. The individual trimeric species

237

has a negative sixteen charge that is balanced in the crystalline -phase co mpound by eight

238

piperazinium cations, resulting in a molecular formula of [C 2 H10 N2]2[(UO 2)6(C 4O 4)3(µ 3-

239

O)2(µ 2-OH)6] for USq3. Centroid-to-centroid distances are 6.01 Å, which are again too far

240

for π- π stacking to be relevant in the crystalline lattice.

241

(a)

(b)

242 243 244 245 246 247 248

Figure 3. (a) Thermal ellipsoid representation (50%) of the USq3 3:6 U:Sq species. The U, O, and C atoms are depicted by the yellow, red, and gray spheres, respectively. Hydrogen atoms were omitted for clarity. (ii) y, -z+1, -x+1 (iii) x, -z+1, -y+1 (v) -z+1/2, -y+3/2, x-1/2. (b) The structural bonding motif observed in the USq3 framework of uranyl trimers chelated infinitely in three-dimensions through squarate ligands coordination. The U(VI) is positioned in the center of each solid, yellow polyhedron with an oxygen atom located at each vertex. The black and red wires represent carbon and oxygen bonds, respectively.

249 250

Four additional solid state compounds have been previously characterized and

251

reported from the pure uranyl squarate system.

252

monomeric uranyl pentagonal bipyramid chelated by four monodentate squarate molecules

253

and one ligated water and can be considered the building block for USq1. 36 Additional

The Wilson complex consists of a


Page 15 of 38

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254

linkages between the Wilson complex and neighbouring building units create a 3 -D

255

framework topology. A similar mono meric uranyl building unit with a U(VI):Sq ratio of

256

1:4 is also observed in (UO 2)2(C 4 O4) 5·6NH4·4H2 O, but the ligated water is replaced by an

257

additional linkage to the neighbouring squarate molecules. 29 This results in the formation

258

of a 2-D sheet topology with ammonium cations and water molecules located in the

259

interstitial regions. Hydrolysis of the uranyl cation results in the formation of extended

260

chain structures for (UO 2)2(OH)2(H2O) 2(C 4 O4) and (UO 2)(C 4O 4)(OH) 2 ·2NH4. 28, 29 In the

261

case of (UO 2)2(OH)2 (H2 O)2(C 4 O4), a single µ 2-OH links the uranyl pentagonal bipyramids

262

into an infinite chain that then further link through the squarate molecules to create a 2 -D

263

array. The infinite chain topology in (UO2)(C 4 O4)(OH)2·2NH 4 is formed through two µ 2-

264

OH bridges between uranyl polyhedra and the squarate ligands (similar to USq2) located

265

on the exterior of the chain and engage in π···π interactions within the crystalline lattice.

266

Hydrothermal conditions (90 °C) were utilized to form previously reported comp ounds,

267

although peaks associated with (UO 2)(C 4 O4)(OH) 2·2NH 4 has been observed in the powder

268

diffractogram of material formed at room temperature. Overall, the fundamental building

269

units in the current study and previously reported uranyl squarate compounds is

270

monomeric, dimeric, and trimeric uranyl species.

271

Uranyl Croconates. A discrete mononuclear [O≡U≡O]2+ unit is observed in UCr1 and bound to

272

two O atoms from one bidentate croconate ligand, two O atoms from two monodentate croconate

273

ligands, and one O atom from one ligated water molecule to create a U(VI):Cr ratio of 1:3. Stacking

274

π-interactions are found between the monodentate croconate ligands parallel to the [100] plane

275

with intercentroid distances of 3.83 Å and 4.45 Å for the bidentate croconate ligands and

276

neighboring uranyl monomers, respectively. Two piperazinium cations offset the negative four



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Page 16 of 38

277

charge on the monomeric uranyl units and create a hydrogen bonding network with interstitial

278

water groups in the crystalline lattice for a final formula of [C4H12N2]2[(UO 2)(C5O5)3(H2O)]·3H2O.

279

(a)

(b)

280 281 282

Figure 4. (a) Thermal ellipsoid representation (50%) of the (UCr1) 1:3 U:Cr species. Symmetry code for equivalent atoms: (i) x, -y+3⁄2 , z. (b) Rendered polyhedral view down the [001] plane of the extended UCr1

283 284 285 286

structure assembled by π···π interactions of the ligated croconate molecules. The U(VI) is positioned in the center of each solid, yellow polyhedron with an oxygen atom located at each vertex and the black and red wires represent carbon and oxygen atoms, respectively. Interstitial water molecules and piperazinium cations were omitted for clarity.

287 288

The structural building unit of UCr2 contains a 2:4 U(VI):Cr species with two uranyl

289

polyhedra sharing an edge through two μ 2-OH bridges, resulting in a dinuclear oligomer with U···U

290

distances of 3.861 Å (Fig. 5). Four croconate molecules function exclusively as chelators by either

291

monodentate or bidentate ligation to complete the equatorial coordination sphere about the two

292

(UO2)2+ centers, with four O atoms from two bidentate croconate ligands and two O atoms from

293

two monodentate croconate ligands. Each discrete dimer carries a negative four charge overall

294

which is electroneutralized by two piperazinium cations that occupy the interstitial crystalline void

295

and contribute to the intermolecular hydrogen bonds via interactions between uranyl μ 2-OH

296

bridges and lattice waters within the neighbouring dimeric units. Additional π-π interactions occur


Page 17 of 38

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297

between neighbouring croconate ligands with centroid-to-centroid distances of 3.64 and 3.97 Å.

298

The resulting molecular formula for UCr2 is [C4H12N2]3[(UO2)2(C5O5)4(µ2-OH)2]·6H2O.

299

(a)

(b)

300 301 302 303 304 305

Figure 5. (a) Thermal ellipsoid representation (50%) of the (UCr) 2:4 U:Cr species. Symmetry code for equivalent atoms: (i) -x, -y, -z+1, (b) Discrete dinuclear units of UCr2 where the U(VI) is positioned in the center of each solid, yellow polyhedron with an oxygen atom positioned at each vertex and the black and red wires are the carbon and oxygen atoms, respectively, of the croconate ligands.

306

In UCr3, the pentagonal equatorial plane about each trinuclear [O≡U≡O]2+ unit is coordinated

307

by one O atom from one monodentate croconate ligand, one O atom from a ligated hydroxyl group,

308

two O atoms from two μ 2-OH bridges, and one O atom from one μ3-O group. The trinuclear 3:4

309

U(VI):Cr oligomers are linked by μ2-bridging croconate ligands forming a one-dimensional chain

310

extending infinitely down the [001] axis. Interstitial ethylenediammonium cations and water

311

molecules are located between the chains and π- π interactions are observed between the croconate

312

anions with intracentroid distances of 4.60 and 4.81 Å.



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(a)

Page 18 of 38

(b)

313 314 315 316 317 318 319

Figure 6. (a) Thermal ellipsoid representation (50%) of the ( UCr3) 3:4 U:Cr species. Symmetry code for equivalent atoms: (i) x, -y+3/2, z (ii) x, y, z+1 (iii) x, -y+3/2, z+1 (b) Perspective view of the UCr3 infinite chain of uranyl trimers linked via croconate chelation where the U(VI) is positioned in the center of each solid, yellow polyhedron with an oxygen atom located at each vertex. The black and red wires represent carbon and oxygen bonds, respectively.

320

Only one uranyl croconate compound containing a 1-D chain topology has been previously

321

reported in the literature by Brouca-Cabarrecq et al.37 K2(UO 2)(C5O5)2(H2O) is composed of

322

monomeric uranyl polyhedra with four croconate ligands and a terminal water around the

323

equatorial plane. The individual building units are linked through bridging croconate ligands to

324

form chains down the [001] direction. The negative charge on the uranyl coordination polymer is

325

compensated by the presence of K+ cations located within the interstitial regions of the crystalline

326

lattice. Additional π-π interactions take place between the croconate ligands on neighbouring

327

chains, with intercentroid distances of 3.26 Å. As with the uranyl squarate system, monomeric,

328

dimeric, and trimeric building units have been identified for the croconate system.

329


Page 19 of 38

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330

Solid-State Raman Spectroscopy. Raman spectroscopy was used to identify characteristic

331

bands for the solid-state U(VI) squarate and croconate compounds.

332

stretching vibration of the pentaaquauranyl complex, [(UO 2)(H 2 O)]2+ , is generally observed

333

at 870 cm-1 and will red shift to lower frequencies due to O≡U≡O bond weakening upon

334

ligand complexation within the equatorial plane. 20 The characteristic symmetric stretching

335

vibration of the uranyl cation is observed in USq1, USq2, and USq3 at 829 cm-1, 834 cm-

336

1

337

for hydrolyzed uranyl oligomers due to the presence of µ 2-OH and µ 3-O bridges that can

338

donate more electron density to the U(VI) metal. 20,

339

compound displays the largest red shift in the v1 band, despite the absence of µ 2-OH and

340

µ 3-O groups. This is counterintuitive as the most hydrolyzed species (USq2 and USq3) are

341

expected to have the largest red shift of the uranyl symmetric stretching mode. The position

342

of the symmetric stretching band does correlate with the smallest centroid -to-centroid

343

distance, suggesting that the π stacking could influence the Raman signal in the solid

344

state. 39-41

The symmetric

, and 847 cm-1, respectively (Table 3; Fig. S8). Larger red shifts are commonly observed

21, 23, 25, 38

In this system, the USq1

345

All three compounds display the symmetric ring stretching (νs; 742 – 753 cm-1) and

346

the symmetric ring bending (δ s; 658 – 670 cm-1) modes affiliated with the sp2 hybridized

347

squarate molecule. 39,

348

number and the shape of the vs(C-C) band. If the ligand is a perfectly symmetric square,

349

only a single vs(C-C) band will be present as a result of C -C bond length equalization by

350

the ideal electronic delocalization throughout the oxocarbon ring. 39, 44, 45 Both USq2 and

351

USq3 display relatively narrow vs(C-C) bands at 1128 cm -1 and 1131 cm-1, respectively,

352

implying a more highly symmetric squarate rings with enhanced electronic delocalization

42, 43

The degree of squarate hybridization can be inferred by the



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Page 20 of 38

353

are present. The bimodal vs(C-C) band observed in USq1, 1129 cm-1 and 1141 cm-1,

354

suggests lower ring symmetry and decreased sp2 squarate character in the crystalline state.

355

Again, strong π···π interactions between adjacent squarate rings (3.32 Å) are observed in

356

USq1 that may also contribute to the differences in the vs(C-C) band shape and symmetry. 40,

357

42, 43

358

stretching (νs) and ring bending (δ s) modes observed in USq1.

These interpretations would further explain the less intense squarate symmetric ring

359 360 361

Table 3. Summary of the observed solid state Raman bands for uranyl squarate and croconate compounds. Raman Mode (cm-1)

USq1

USq2

USq3

UCr1

UCr2

UCr3

829

834

847

848

833

822

(Ring twisting)

818

812

806

νs(C=C stretching)

1103

1101

1109

vs(UO 22+)

vs(ring stretching)

742

750

753

647

644

646

δs (ring bend)

659

665

670

527

527

525

-

643

641

569 556

568 557

582 554

1131

1249 1234 1211

-

-

δoop(out of plane CO bending) vs(C-C stretch)

1129 1141

1128

362 363

Uranyl croconate compounds display a different trend in regards to the uranyl symmetric

364

stretching band and ligand modes (Table 3; Fig. S9). The monomeric form (UCr1) is observed at

365

848 cm-1 and the band further red-shifts to 833 and 822 cm-1 upon oligomerization to the dimeric

366

(UCr2) and trimeric (UCr3) species, respectively. This is similar to previously reported values

367

for hydrolyzed species where a red-shift occurs upon the formation of u2-OH and u3-O groups.20,

368

23, 38

369

units, potentially negating any relative impact of these intramolecular interactions.

In all three cases, the intracentroid distance suggests π interactions between the neighboring


Page 21 of 38

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370

Similar to the squarate system, all three compounds display the symmetric ring stretching

371

(νs; 644 – 647 cm-1) and the symmetric ring bending (δs; 525 – 527 cm-1) modes.46 Peak splitting

372

for the out-of-plane C=O bend is observed for the solid-state uranyl croconate compounds, which

373

is also noted in transition metal (Zn, Ni, Fe, Co, Cu) croconate coordination polymers. 47 Multiple

374

peaks are also found for other vibrational bands, which has been attributed to lowering the

375

symmetry of the croconate molecule from D5h to C2v and is corroborated by the carbon-carbon

376

distances obtained by X-ray diffraction.46 A high background that is likely due to fluorescence is

377

observed in the case of UCr2 and UCr3; thus, the C-C stretching bands at approximately 1240

378

cm-1 cannot be identified in either case.

379

While a potential correlation can be established between the Raman bands and π stacking,

380

it is also important to consider other factors, including the number of equatorial ligands and other

381

intermolecular forces. Positioning of the Raman band is thought to be greatly influenced by the

382

number of equatorial ligands and the ability of these molecules to donate electron density to the

383

metal center.20, 22 The U:ligand ratio differs between the uranyl squarate and croconate system,

384

with more ligands coordinated to the metal centre in the uranyl squarate system. This does not

385

seem to impact the dimeric unit where in both cases the band occurs at approximately 834 cm -1,

386

nor does the increased number of ligands lead to a larger red shift. Further investigation of the

387

extended lattice for all compounds indicates that there are no significant H-bonding interactions

388

or other intramolecular interactions with the uranyl oxo groups that could potentially influence the

389

Raman band position. Therefore, the influence of π stacking in this system needs to be explored

390

further with other spectroscopic techniques.

391

Solid-State UV-Vis Spectroscopy. U(VI) solids are generally yellow, but the crystals obtained in

392

the uranyl squarate and croconate systems ranged from yellow to dark red; thus, the solid-state 21 ACS Paragon Plus Environment


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Page 22 of 38

393

UV-Vis spectra were collected to provide additional spectroscopic signals for these complexes.

394

Within the high-energy region, bands are expected from both the intermolecular transitions within

395

the uranyl ion and π-π* transitions for the squarate ligand (Fig. 7a).27, 48, 49 For comparison, the

396

spectra of uranyl nitrate and squaric acid are shown. Within the uranyl nitrate spectrum, a broad

397

band is observed between 200-350 nm, whereas some narrower peaks are observed at 205 and 310

398

nm for squaric acid. Bands for USq3 are similar for other reported uranyl hybrid materials, 50-52

399

suggesting that the uranyl intermolecular transitions dominate this region, with additional

400

contribution from squarate at approximately 300 nm. However, for USq2 there are two distinct

401

peaks at 240 and 284 nm and similar features are observed for USq2 that are likely related to the

402

metal ligand complex. A hypsochromic shift from the squarate band could suggest a decrease in

403

the delocalization of the squarate dianion.27 Evidence of a more localized system is indeed

404

observed in the crystallographic data, which finds unequal C-C bond distances in USq1 (Table

405

S2), and peak splitting of the bands in the Raman spectrum (Fig. S5).


Page 23 of 38

(a)

USq1 USq2

Kubelka-Munk (a.u.)

USq3 Uranyl nitrate hexahydrate Squaric Acid

200

300

406

400 500 Wavelength (nm)

(b)

600

700

UCr1 UCr2

Croconic Acid Uranyl nitrate hexahydrate

Kubelka-Munk (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200 407 408

300

400 500 Wavelength (nm)

600

700

Figure 7. Solid-state UV-Vis spectra for uranyl (a) squarate and (b) croconate compounds. 23 ACS Paragon Plus Environment


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Page 24 of 38

409 410

Further differences in the spectra can be seen in the near-UV and visible regions, with

411

notable changes in the area where vibronic transitions associated with the uranyl moiety are

412

typically located (Fig. 7a). Typically, multiple bands associated with vibronic transitions are

413

observed between 330-500 nm as seen for the uranyl nitrate spectrum and previously reported

414

uranyl hybrid materials.53 Some of those vibronic transitions are maintained in USq2 and USq3,

415

but the USq1 spectrum features just a broad band and a bathochromic shift extending to 625 nm.

416

Spectra for the uranyl croconate compounds are similar to USq1, with a dominant peak in

417

the UV region and a broad peak at lower energies associated with a LMCT feature (Fig. 7b). The

418

major peak in the UV region observed for UCr1 and UCr2 is located at 280 nm and a less intense

419

shoulder at 250 nm. Croconic acid exhibits two bands at 263 nm and 435 nm that are associated

420

with π-π* transitions and can be attributed to Jahn-Teller distortion of the first excited state.27 It is

421

unclear whether the major peak in UCr1 and UCr2 is associated with an enhanced π-π* transition

422

for the croconate anion, intermolecular transition within the uranyl ion, or a combination of both.

423

Based upon previous Monte Carlo simulations, the association of a Li + cation with the croconate

424

ion leads to a hypsochromic shift that again may be due to disruption of the delocalized electrons

425

within the ring.46 However, the direct coordination of the croconate dianion may influence the

426

electronic structure of the uranyl moiety leading to an enhanced absorption. We also observe that

427

the vibronic transitions for the uranyl cation are not noticeable within the spectra and the

428

absorption band exhibits a bathochromic shift from 500 nm to 600 and 625 nm for UCr1 and

429

UCr2, respectively.

430

Compounds that exhibit a bathochromic shift of the LMCT feature have been previously

431

shown to produce red or orange crystals. This was also observed by Silver et al., 54 where a bright 24 ACS Paragon Plus Environment

Page 25 of 38

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432

red uranyl formohydroxomate crystal was produced by room temperature evaporation of an

433

aqueous solution. The red colour of these crystals was attributed to the absorbance of all the short

434

visible wavelengths by the compound. The spectrum contained a broad band without any apparent

435

vibronic coupling features suggesting significant changes to the electronic structure of the uranyl

436

moiety. Silver et al. were able to link some of the unique chemical properties of the uranyl

437

formohydroxomate compound to a relatively bent uranyl unit (173.5(4) °) that altered the LMCT

438

electronic transition state.54

439

The compounds presented here do not display significant changes in uranyl bond lengths

440

or angles, but we can potentially correlate to the differences in π-stacking within the solid-state

441

compound. Of note is the significant π-π interactions that occur for USq1 (3.22 Å) compared to

442

that of USq2 (4.30 Å) and USq3 (8.44 Å) and the changes in the colour from red, orange, and

443

yellow, respectively. Accompanying these intermolecular forces is the change in the electronic

444

spectra of these compounds, most significant is the loss of the vibronic features and the

445

bathochromic shift of the bands associated with the uranyl moiety. Significant π stacking is also

446

observed in all three uranyl croconate compounds and this is again reflected in the red shift in the

447

electronic spectrum and the red colour of the crystals.

448

Harrowfield et al. noted the importance of π-stacking on the chemical properties of uranyl

449

hybrid materials with a detailed investigation of the solid-state emission for uranyl pyridine

450

dicarboxylate (PDC) complexes.55 Within the isostructural [UO2(HPDC)2]∙4H2O and

451

X2[UO2(PDC)2]∙3-4H2O (X = Rb & Cs), there are stacked arrays of pyridine rings with relatively

452

short planar distances (3.4 Å).

453

luminescence, but the solid-state material produces bright green luminescence under 354 nm light.

454

Harrowfield et al., suggested that the pyridine dicarboxylate ligands were acting as an antenna for

The solutions that produced these crystals show negligible



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Page 26 of 38

455

energy absorption and transfer through π-stacking within the extended lattice.55 This study

456

highlights the potential importance for these extended π-π interactions on the electronic properties

457

of uranyl hybrid materials and changes in the spectral features upon crystallization.

458 459

Solution Raman and UV-Vis Spectroscopy. The aqueous phases for the uranyl squarate

460

and croconate system were characterized through spectroscopic techniques to investigate

461

the relationship between the solution and solid-state materials. The major spectral bands

462

at pH 2 and 2.5 in the uranyl squarate system are located at 870 and 858 cm -1, which

463

corresponds to the pentaaquauranyl complex and likely the USq1 mono mer (Table 3; Fig.

464

S10) . The Wilson co mplex (U:Sq ratio of 1:4) was expected based upon the precipitated

465

product reported by Rowland and Cahill 29 and the crystallization of the USq1 phase, but

466

the band at 829 cm-1 for the solid state USq1 is noticeably absent. Solution kinetics in

467

uranyl systems can be quite slow (on the order of days for equilibrium to occur between

468

uranyl hydrolysis and carbonato complexes at pH 6); 34 thus, additional spectra of these

469

solutions were acquired 3 weeks after the initial experiments (Fig. S11). Both the bands

470

for the pentaaquauranyl species and the unresolved complex remain and no ingrowth of the

471

peak associated with the Wilson complex is observed, suggesting that the band at 858 cm -

472

1

is the stable form in solution.

473

Differences in the Raman bands present in solution at pH 2 and 2.5 and the resulting

474

solid state phase could be due to several possible reasons. Previous studies by Nguyen-

475

Trung et al., 20 have reported spectral assign ment of the [(UO 2)2(OH)] 3+ species at 860 ± 1

476

cm-1, which was observed in solution under acidic conditions (pH 0.24 to 5.63). A second

477

possibility is the formation of a uranyl complex with a metal-to-ligand ratio much lower


Page 27 of 38

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478

than the USq1 mono mer, such as a 1:1 uranyl squarate species. A 1:1 uranyl oxalate

479

complex has also been previously reported at 858 cm -1 and the electron donating ability

480

between squarate and oxalate is likely to be similar, suggesting an analogous red shift for

481

the spectral bands between the two species. 38 Another possibility that the electron donating

482

ability of the ligand is weakened due to less π-π stacking in solution, which would result in

483

a different band present in solution.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

484 485

Page 28 of 38

Table 3. Solution phase Raman spectroscopy for the uranyl squarate and croconate solutions containing a 1:1 metal to ligand ratio from pH 2 to 5.25. The values in bold represent the dominate uranyl species present in solution.

-1 1:1 Raman Peak/cm U:Sq Peak Area/%

pH 2.00

pH 2.50

pH 3.00

pH 3.75

pH 4.50

pH 5.25

870.4 858.4

870.3 857.8

870.5 858.1 846.7

870.2 858.2 846.7

857.9 846.4 834.5

846.9 833.9

50.6

27.9

13.5

20.5

26.3

49.4

72.1

58.2

28.3

6.1

39.9

54.0

59.1

20.4

73.7

486

-1 1:1 Raman Peak/cm U:Cr Peak Area/%

pH 2.00

pH 2.50

pH 3.00

pH 3.50

pH 4.00

869.9 858.1

869.7 858.0

870.2 858.5

869.9 858.3

870.1 857.7 834.9

59.3

44.5

41.6

36.6

19.4

40.7

55.5

58.4

63.4

53.0

27.6

487


Page 29 of 38

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At pH 3.75, the ingrowth of the stable USq3 aggregate, [(UO 2)3(C 4O4 )6]6- at 847 cm-

488 489

1

490

USq2 precursor, [(UO 2)2(C 4O4 )6]8- (834 cm-1) also forms at pH 4.50 and governs the pH

491

5.25 solution while coexisting with the minor USq3 trimeric species. This suggests that an

492

equilibrium exists between the dimeric and trimeric hydrolysis products, similar to the

493

previously studied uranyl citrate system. 23 Unlike the uranyl citrate system, the trimeric

494

form dominates at a lower pH values and decreases with increasing basicity of the solution.

495

Presence of the dimeric form at higher pH values agrees well with the presence of a 1 -D

496

chain observed by Rowland and Cahill 29 when the pH>7. Hydrolysis will continue to be

497

favoured under these conditions, which likely leads to the release of the squarate ligands

498

and the formation of the extended 1-D coordination polymer. Lastly, it is important to note

499

that there is a direct correlation between the species identified in the solid and in the solution

500

phase; thus, the monomeric USq1 phase is the only band that is impacted in this case.

is observed and remains the dominant solution species through pH 4.50. The aqueous

501

Further investigation on metal to ligand ratios for the uranyl squarate system foun d

502

that only slight variations occurred in the relative abundance of the identified complexes

503

(Tables S4-S6). This would imply that the solution dynamics are only mildly affected by

504

variations in ligand-to-metal concentration ratios and that solution pH has a larger influence

505

on uranyl squarate species formation. Relative abundance of the trimeric species increases

506

with higher concentrations of the squarate molecule, suggesting that the presence of the

507

ligand is an important driver in its formation. The band at 858 cm-1 is still present in the

508

more acidic conditions even with variation in the metal to ligand ratios, again suggesting

509

that increasing the ligand concentration does not result in the formation of the 1:4 Wilson

510

species.



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Page 30 of 38

511

Spectra of the uranyl croconate in solution at a 1:1 ratio indicates that again there is not a

512

direct comparison to the bands identified in the solid-state compounds (Table 3; Fig. S12). At pH

513

2.0, signal associated with the pentaaquauranyl complex (870 cm-1) is present in solution with the

514

secondary peak located at 858 cm-1 at a ratio of 3:2. Again, the identity of uranyl complex with a

515

signal at 858 cm-1 is unknown, but is much closer to the band associated with the uranyl croconate

516

monomer (847 cm-1). These two bands continue to dominate the spectra up until pH 4, but there

517

is decreasing concentrations of the pentaaqua complex with increasing pH. At pH 4.0 the band a

518

858 cm-1 still dominates, but now the band at 834 cm-1 is present, suggesting hydrolysis of the

519

U(VI) and the formation of an oligomer. In this case, the observed band is similar to that of the

520

uranyl dimer and there is no evidence of the trimeric species in this solution. The uranyl trimer

521

can be crystallized out under these conditions, but the yields are quite low (

Spectral Analysis of the Uranyl Squarate and Croconate System

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