Spectroscopic and Energetic Properties of Thorium (IV) Molecular

Article pubs.acs.org/JPCA

Spectroscopic and Energetic Properties of Thorium(IV) Molecular Clusters with a Hexanuclear Core Monica Vasiliu,† Karah E. Knope,‡ L. Soderholm,‡ and David A. Dixon*,†,‡ †

Chemistry Department, The University of Alabama, Shelby Hall, Box 870336, Tuscaloosa, Alabama 35487-0336, United States Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

‡

S Supporting Information *

ABSTRACT: The spectral and energetic properties of three polynuclear thorium(IV) molecular complexes Th6(OH)4O4(H2O)6(HCOO)12·nH2O (1), Th 6 (OH) 4 O 4 (H 2 O ) 6 (CH 3 CO O) 1 2 ·nH 2 O (2 ), and Th6(OH)4O4(H2O)6(ClCH2COO)12·4H2O (3) have been studied. Each complex has a hexanuclear core with six 9coordinate Th(IV) cations bridged by four μ3-hydroxo and four μ3-oxo groups. The +12 core is stabilized by twelve bridging carboxylate functionalized organic acid (formate, acetate, and chloroacetate) units. The calculated 1H NMR chemical shifts for the four μ3-hydroxo, water, and formate protons are reported and compared to the experimental values. The vibrational frequencies were calculated to aid in the assignment of the observed Raman bands. The Mulliken and NBO (natural bond orbital) charges are calculated for the Th clusters. The Th atoms are positive and the bridging O and O(H) are negative. The analysis of the calculated highest-occupied and lowest-unoccupied molecular orbitals (HOMO and LUMO) is reported. The average water complexation energies, the gas phase, the aqueous and dimethylsulfoxide (DMSO) acidities were predicted, and the Th clusters are found to be mild to strong acids in gas phase yet they behave as weak acids in solution.

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INTRODUCTION Metal ions of high-charge density, acidity, and electronegativity are susceptible to hydrolysis and condensation reactions, an understanding of which is critical for accurately predicting the aqueous behavior of such ions in solution.1−3 The condensation of hydrolyzed metal ions to form well-defined clusters has been demonstrated, as exemplified by the family of widely diverse polyoxometalates that form upon condensation of high-valent, transition-metals.4,5 For lower-valent, softer ions these reactions are less well understood but are often assumed to result in amorphous, chemically ill-defined oxyhydroxides or hydrous oxides.2 Recent work on trivalent Bi3+ and 4f ions is providing a much different picture, in which control of reaction conditions can result in the formation of chemically well-defined small metal oxide/hydroxide clusters, captured and crystallized for structural studies by the addition of supporting organic ligands.6−8 Fewer examples are available for tetravalent ions,9−13 possibly reflecting the importance of metal ion charge density or hardness as it relates to the olation and oxolation reactions underpinning oligomer formation.2 As the softest of the tetravalent ions, and without valence electrons, the 5f ion Th4+ provides an opportunity to further study condensation reactions with a moderately acidic metal ion.14 Recent work is suggesting that the formation of well-defined Th clusters may be more widespread than has been historically appreciated.10,12,13,15−20 Yet, the energetic and mechanistic drivers that are central to the controlled synthesis of well-defined metal oxide/hydroxide clusters from solution are poorly understood. © 2012 American Chemical Society

To enable a new class of directed synthetic approaches to targeted materials, it is necessary to simultaneously develop a theoretical approach to understanding, and eventually predicting the size, structure, and molecular/energetic properties of small metal aggregates and the solution conditions that favor their formation. Limiting such an approach has been the paucity of information on metal-ion correlations and the associated energetics in solution over the critical distance range of about 5 to 30 Å. Recent experimental efforts to combine solution structural probes is allowing further developments of this approach, notably EXAFS (extended X-ray absorption fine structure)9,12,13,18,21 and HEXS (high-energy X-ray scattering) with structural studies on single-crystal precipitates from the same solutions.10,22−25 The Fourier transform of solution HEXS data provides one-dimensional ion−ion correlation information that can be quantitatively modeled with constraints imposed by the crystal structure of the precipitated aggregates. We have recently shown that polynuclear Th4+ complexes, formed in aqueous solution at room temperature via hydrolysis and subsequent condensation reactions, can be isolated by the appropriate addition of an organic acid.19 Common to these isolates is a cationic [Th6(μ3-O)4(μ3-OH)4]12+ core encapsulated by the requisite organic-acid anions (formate, acetate, chloroacetate) to give a neutral species. The clusters precipitate Received: April 11, 2012 Revised: May 21, 2012 Published: May 22, 2012 6917

dx.doi.org/10.1021/jp303493t | J. Phys. Chem. A 2012, 116, 6917−6926

The Journal of Physical Chemistry A

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quantitatively from aqueous solution, forming crystals of Th6(OH)4O4(H2O)6(HCO2)12·nH2O (1), Th6(OH)4O4(H2O)6(CH3CO2)12·nH2O (2), and Th6(OH)4O4(H2O)6(ClCH2CO2)12·4H2O (3), the structures of which were determined using single crystal X-ray diffraction. Density functional theory (DFT) calculations with the hybrid B3LYP exchange correlation functional and the local SVWN5 functional were applied to analyze the structures and to predict the location of the μ3-OH groups. There are 6 ways to distribute the 4 protons over the 8 oxygen sites in the Th6O88+ core. The lowest energy isomer for 1, which is lower in energy than the other isomers by more than 13 kcal/mol, has the protons arranged in a “tetrahedral” orientation with two in the upper hemisphere and two in the lower hemisphere. This orientation maximally separates the 4 positive charges (see Figure 1).19 The clusters are formally composed of Th4+ ions

Article

EXPERIMENTAL AND COMPUTATIONAL DETAILS

Raman Spectroscopy. Raman spectra were collected on powder samples of 1 to 3 on a Renishaw inVia Raman Microscope with an excitation line of 532 nm. NMR Spectroscopy. Approximately 5 mg of 1, 2, and 3 were dissolved in 0.6 mL of d6-DMSO (∼concentrations of 3.8 mM for 1, 3.5 mM for 2, and 3.0 mM for 3). 1H NMR spectra were collected on an Oxford 500 MHz NMR using dimethylsulfoxide (DMSO) as the internal reference, set to 2.5 ppm relative to TMS.26 To confirm that the cluster remained intact upon dissolution, High Energy X-ray Scattering (HEXS) data were collected for 1 dissolved in DMSO. Data were collected at the Advanced Photon Source (APS), Argonne National Laboratory on beamline 11-ID-B. The incident beam of 91 keV corresponds to a wavelength of 0.13702 Å. HEXS data were also obtained from carefully prepared background solutions, which after subtraction from the sample data provide scattering patterns containing only correlations involving Th. Samples were loaded in Kapton capillaries sealed with epoxy plugs and further contained as required for actinide samples. Scattered intensity was measured using an amorphous silicon flat panel X-ray detector mounted in a static position (2θ = 0°) providing detection in momentum transfer space Q (Å−1) up to 32 Å−1. Data were treated as described previously.27,28 Electronic Structure Calculations. The geometries and vibrational frequencies of 1 to 3 and the corresponding structures 4, 6, and 7 where the six solvating water molecules have been removed, from 1, 2, 3, respectively, the Th6(OH)4(O)412+ core terminated with six oxygen atoms (5), and negatively charged species derived from removing a proton from 1 to 7 were optimized using DFT.29 DFT was chosen as the computational method because of the large size of the system under consideration and the types of properties to be calculated. Although high level of correlated molecular orbital methods may provide improved results, they are far too computationally expensive to apply to these systems especially for geometry optimizations, frequency calculations, and NMR chemical shift calculations (The wave functions for these can be difficult to converge just at the self-consistent field level and the NMR calculations took almost two months on an 8-core node with a large memory for just a few protons on 3). In addition, the DFT approach used herein does provide reasonable estimates of the structures and vibrational properties of thorium oxides,30 as well as the energetics of a range of chemical reactions.31,32 The DFT calculations were done with the B3LYP exchange correlation functional33,34 and the DZVP basis set35 for H, C, and O (polarized double-ζ on the C and O and double-ζ on the H) and the Stuttgart large core effective core potential and basis set for Th.36 The f orbitals in the Th basis set were initially excluded because of issues with wave function convergence. Single point energies were calculated with the addition of the two innermost contracted f orbitals to the Th. These calculations were done with the Gaussian09 program system.37 The NMR chemical shifts for the protons bonded to the O atoms in the Th6O8 core were calculated with the ADF program system38,39 with the BLYP functional40 and the TZ2P basis set in ADF.39 The NMR calculations were done in the gauge invariant atomic orbital (GIAO) approach41 at the DFT level based on the developments in the Ziegler group.42−45 Scalar relativistic effects were included at the two-component zero-order regular approximation (ZORA) level for the NMR

Figure 1. Illustration of Th6(μ3-OH)4(μ3-O4)(H2O)6(HCO2)12 (1). Removal of the six solvating water molecules generates Th6(μ3OH)4(μ3-O4)(HCO2)12 (4), and exchange of the twelve formate complexing anions by six ThO groups results in Th6O6(OH)4O4 (5).

with a closed shell electron configuration of [Rn]0. This makes the computations substantially easier than computing the structures and energies of actinide complexes with open shell, multiple f occupancies, and provides a method for testing our computational approaches for f-elements. Presented herein is an in-depth study on the hexametal clusters, executed through a comparison of predicted and experimental NMR and Raman spectra. The computational results afford the assignment of the NMR and Raman spectra obtained experimentally and a discussion of the gas phase acidity and energetics of water complexation of the clusters is also provided. Comparison of these experimental and computational results demonstrates the utility of coupling these techniques to understand the spectroscopic and energetic properties of polynuclear clusters. The current complementary approach to quantifying the structure, energetics, and properties of metal-oxide/hydroxide clusters represents our initial steps to develop an understanding of the reactivities of such species, which can then be targeted to enable the design of directed syntheses. 6918



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calculations.45−47 Proton chemical shifts in ppm were predicted relative to TMS as the standard. The acidities of various compounds in solution were evaluated using the self-consistent reaction field approach48 with the COSMO parametrization49−51 using the ADF code.38,52,53

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RESULTS AND DISCUSSION The Fourier transformed HEXS data obtained from a solution of 1 dissolved in DMSO after subtraction of solvent−solvent interactions are presented in the Supporting Information as Figure SI-1. Peaks in the pair-distribution function are observed at approximately 2.46, 3.93, and 5.55 Å, consistent with Th−O, adjacent Th−Th, and apical Th−Th distances, respectively, of the intact cluster core.19 NMR Spectroscopy. The NMR spectra are shown in Figure 2. The proton chemical shifts are given as follows in ppm with some peaks assigned. For 1: 2.49(DMSO), 3.32 (H2O), 6.16 (-OH), and 8.30 (HCO2−). For 2: 1.78, (CH3) 1.89, (CH3), 2.49 (DMSO), 3.30 (H2O), 5.97, 6.50, 7.01, 8.31, and 11.94. For 3: 2.49 (DMSO), 3.30 (H2O), 3.97 (CH2-Cl), and 6.97 (-OH). The spectrum for DMSO-d6 is shown in the Supporting Information. The spectrum shows two peaks, one for the exchanged protons on the DMSO at 2.49 ppm and one for the H2O impurity in DMSO at 3.33 ppm in excellent agreement with the reported values.26 These spectral positions in DMSO without any thorium complex are very close to the positions in the NMR spectrum when the complex is present. Thus, it is difficult to determine the peak position of any H2O bound to the complex in DMSO as it is likely overlapped by the large peak resulting from the presence of the H2O impurity in the DMSO, as well as excess lattice water that is present in the crystalline material. For our discussion of the comparison between theory and experiment, we have assumed that the peak for the H2O in the complex lies under that of the excess H2O in the system. We also cannot use the integrated intensity of the water peak for comparisons for the same reasons. The calculated NMR chemical shifts of 1 to 3 are given in Table 1. Because the experiment was done in DMSO, shifts in their positions are expected relative to the calculated values, which are predicted in the absence of solvent. The spectrum for 1 is very clean with just the DMSO, H2O, complex OH, and formate peaks present. The calculated chemical shift for the formate protons in 1 is 0.5 ppm downfield from the experimental value at 8.30 ppm. For the OH in 1, our calculated chemical shift is downfield by about 1 ppm as compared to the experimental value. The ratio of the integrated intensities for the formate to OH peak is 3.21 as compared to the expected ratio of 3.0 (12H/4H), which confirms our assignment. In contrast, our average calculated chemical shift for the H2O protons is upfield by ∼1 ppm as compared to the experimental value assumed to be hidden by the large “free” H2O peak. Thus, the calculations predict the correct ordering of the chemical shifts but the splitting of the OH and H2O peaks are too small. For 2, the calculated average chemical shift for the methyl group acetate protons is 2.0 ppm in excellent agreement with the experimental value of 1.78 ppm. Substitution of the H of the formate with −CH3 to make the acetate leads to a small downfield shift in the predicted OH value. There are two possible experimental peaks that can be assigned to the −OH protons. We assign the peak at 5.97 ppm to the OH protons on the basis of the calculated position as well as the experimental and calculated values obtained for 1. The ratio of the integrated

Figure 2. 1H NMR spectrum for 1(A), 2 (B), and 3 (C) in d6-DMSO.

intensities of the peak at 1.78 ppm to that at 5.97 ppm is 8.5, close to the expected ratio of 9.0 (36H/4H). For the H2O protons in 2, as seen for 1, our calculated average chemical shift of 4.5 ppm is upfield by about 1 ppm compared to that observed experimentally. These results suggest that the peak at 1.89 ppm may be due to some other species with a CH3 group, most likely acetic acid. The peak at 11.94 ppm is likely to be the 6919



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Table 1. Calculated 1H-NMR Chemical Shifts in ppm with ADF (BLYP/TZ2P)a molecule 1 2 3 4 5 ThO(OH)2 Th(OH)4 ThO2(H2O) Th2O2(OH)4 Th2O2(OH)4 Th2O3(OH)2 Th2O4(H2O)

OH δ (ppm)

organic H 8.8/8.30 (HCO2−) 2.0/1.78 (CH3CO2−) 4.1/3.97 (ClCH2CO2−) 8.6 (HCO2−)

consistent with our other results showing these waters of solvation do not significantly affect the geometry/electronic structure of the cluster core. The chemical shifts for the formate protons after removal of the H2O solvating the Th to form 4 also show essentially no change (8.6 ppm). Exchanging the 12 formate counterions by 6 terminal ThO groups leads to structure 5 and an increase in the chemical shift (shielding) for the μ3-OH protons. Additional data in the Supporting Information on the other isomers of 4 and 5 shows that lowering the symmetry from the Td symmetry of the lowest energy isomer of 4 and 5 to lower symmetry (4B and 5B, C2h; 5C, C4v; 4D and 5D, C3v; 5E, C2; and 4F and 5F, Cs) changes the OH chemical shift by up to ±0.8 ppm. The proton chemical shifts for the μ3-OH protons can be compared to those in some simple model compounds with Th in the +4 oxidation state (Table 1). The calculated chemical shifts of the OH protons in Th(OH)4, ThO(OH)2, and Th2O2(OH)4 show that the chemical shifts for the bridging OH groups in 1 to 3 are shielded when there is less direct interaction with a Th4+. The chemical shifts of the μ3-OH protons of 1 to 3 are about 1 ppm less than the chemical shifts of the protons in ThO2(OH)4 where two of the OH groups are bridging the thorium atoms. The proton of the bare formate anion is calculated to be at 10.5 ppm. Thus, the formate protons in our thorium clusters are deshielded as compared to that of the isolated formate anion. The chemical shift of the protons in an isolated water molecule is calculated to be 0.2 ppm. The calculated chemical shifts of the water protons in our Th compounds are shielded as compared to the isolated water molecule. Similar to the large clusters, the chemical shift of the water protons in Th2O4(H2O) are shielded as compared to the water molecule. Raman Spectroscopy. The experimental Raman spectra are shown in Figure 3 and the peaks in cm−1 are given in Table 2. As shown in Table 2, the calculated vibrational frequencies can be used to assign the observed Raman bands for 1−3. The highest predicted modes are the 4 μ3-OH stretches. The H2O symmetric and asymmetric stretches split by about 100 cm−1 with the symmetric stretch lower as would be expected from comparison with pure H2O.54 Scaling factors for the OH stretches of 0.965 for the H2O asymmetric O−H stretch and 0.974 for the H2O symmetric O−H stretch are obtained by taking the ratio of the observed/calculated values54 for H2O giving an average value of 0.97 for the μ3-OH stretch. Scaling is a simple way to approximate the effect of anharmonicity on the O−H stretches as the calculated values are harmonic frequencies. The scaled values are also given in the Table 2. The calculated values suggest the presence of substantial excess H2O in the experimental spectra as the broad peaks shifted to the red are consistent with extensive hydrogen binding networks in H2O. We used a scaling factor of 0.979 for the C−H stretch determined from the observed/calculated values for CH4.54 The scaled calculated C−H stretching frequencies for 1 are in good agreement with the experiment. The calculated values for symmetric and asymmetric CO2 stretches of the bidentate carbonates in 1 are consistent with the experimental values. For comparison, the calculated values for the asymmetric and symmetric CO2 stretches of free HCO2− are 1705 and 1350 cm−1. The bidentate binding to the Th6O88+ core decreases the splitting of the two C−O stretches by decreasing the value of the asymmetric stretch and increasing that for the symmetric stretch. A band at ∼750 cm−1 is predicted for the mixing of the

H2O δ (ppm)

5.1/6.16 4.9/5.97

∼4.4/3.32 ∼4.5/3.3

5.0/6.97

∼3.9/3.3

5.0 6.3 2.9 1.5 5.7 0.9 (terminal) 6.2 (bridge) 4.3 (terminal) 0.7, 1.3 3.0, 2.3

a

Values after the slash for 1, 2, and 3 are the assigned experimental values.

OH of the acetic acid, even though the ratio of intensities is too large (5.4 instead of 3.0). The use of integrated intensities here is complicated by the overlap of the intense peak at 1.78 ppm with the peak at 1.89 ppm. The former has too low an intensity and the latter is too large consistent with our assignment. The very small peaks at 6.50, 7.01, 8.31 ppm can be assigned to impurities. The peaks relevant to the Th complex are all at least 3 times larger than the impurity peaks. For the chloroacetate derivative (3), the average calculated CH2Cl proton peak is predicted to be very close to the experimental value. The −OH 1H NMR chemical shift is predicted to be 5.0 ppm. There is no upfield peak in the 1H NMR spectrum within ∼1 ppm to assign to this proton, so the difference between the calculated value and the experimental value (observed at approximately 6.97 ppm) is ∼2 ppm. This difference is somewhat larger than found for 1 and 2. The ratio of the integrated peak intensities for the peak at 3.97 ppm to the one at 6.97 ppm is 7.6 is somewhat higher than the predicted ratio of 6.0 (24H/4H) but is still consistent with the assignment. For the H2O protons in 3, the calculated average chemicals shift of 3.9 ppm is upfield by less than 1 ppm compared to experiment. The calculations predict that the R group on the RCO2− has very little effect on the OH chemical shift and only a small effect on the average value for the H2O molecules bound to the Th. The largest difference in the calculated shifts from experiment is 2 ppm for the OH protons in 3. The overall agreement suggests that ZORA is doing a reasonable job of predicting the chemical shifts on a proton on an oxygen bonded to a Th. The calculations show excellent agreement with experiment for the prediction of the chemical shifts of the organic protons (HCO2−, CH3CO2−, ClCH2CO2−) on the complexing anions. These are further from the Th atoms and not directly bonded to them so any effects from the Th are expected to be small. The NMR chemical shifts for protons in a number of additional thorium complexes were also calculated (Table 1) at the optimized B3LYP geometries. The chemical shifts for the μ3-OH protons in 3 are predicted to be 5.1 ppm. Removal of the H2O solvating the Th in 1 giving structure 4 leads to essentially no change in the chemical shift of the OH, 6920



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Figure 3. Raman spectra for 1 (A), 2 (B), and 3 (C) shown over 200−3200 cm−1.

bending of the μ3-OH hydrogen and the CO2 bends of the carbonate anions. Additional μ3-OH bends are predicted at ∼710 cm−1. Similar frequencies are observed experimentally. There are no metal oxygen bands predicted above 560 cm−1. This is consistent with calculations on 5 with six terminal Th O bonds. The ThO bond stretching frequencies are in the range of 740 to 760 cm−1 (see Supporting Information). The comparison of the predicted vibrational transitions for 2 and 3 are very similar to what is observed for 1 and the assignments are similar as well (see Table 2). Charge and Orbital Distributions. The NBO (natural bond orbital) charges55−60 are given in Table 3 for 1, 2, and 3 and for the corresponding compounds 4, 6, and 7 where the 6 waters binding to the Th have been removed. The

corresponding Mulliken charges are given in the Supporting Information and the discussion below focused on the NBO charges. The Th atoms are positive and the bridging O and O(H) groups are negative. The charge separation is predicted to be larger for the Mulliken charges. There is very little change for the core charges when the R group on the RCO2− ligands are changed from −H to −CH3 or −CH2Cl. The Th in 1, 2, and 3 has a charge of ∼+2.06 e. The bridging O is is more negative than the oxygen in the bridging OH group. Removal of the 6 H2O molecules from 1, 2, and 3 bonded to the Th atoms to generate 4, 6, and 7, respectively, leads to the following changes. The Th becomes more positive by ∼0.12 e. The charges on the bridging O and O(H) atoms do not change much. The substitution of the 12 anionic ligands in 4 by 6 O2− 6921



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Table 2. Comparison of the Experimental and Average Calculated Vibrational Spectra for 1, 2, and 3 calculated frequency (cm−1)a

exp. frequency (cm−1)

IR intensity (km/mol)a

Raman activity (Å4/amu)a

4 6 6 12

45 149 52 144

55 16 79 324

μ3-OH stretch H2O asym str H2O sym str C−H stretch, formate

6 5 7

48 1443 2

3 6 0

H2O bend CO2 asym str + H2O bend CO2 asym str

12 12

82 166

8 4

C−H in plane bend CO2 sym str

12

0

1

C−H out of plane bend

12 8

19 108

1 0

μ3-OH bend + CO2 bend μ3-OH bend

number

assignment

1 3806 3780 3680 3027

(3692)b (3648)b (3584)b (2962)c

1690 1675 1604−1630 1400−1410 1390−1400 1060 750 710

3105, 2967, 2881, 2778 2123 1718 1624, 1606 1606, 1563, 1555, 1495 ∼1400 shoulder 1374 1296 1064, 1038, 1004 814 754 711

2 3815 (3700)b 3757 (3644)b 3654 (3544)b 3177(3110)c−3055(2991)c 1696 1650−1580 1502−1457 1386−1380 1067 1039 947 745−666

3017, 2691, 1718 1658, 1546, 1356

2988, 2938 2795, 1615, 1598 1512, 1460, 1429

1041 945 875, 806 750, 706

4 6 6 36

37 184 68 11

6 12 36 12 12 12 12

48 478 183 25 5 14 1

μ3-OH stretch H2O asym str H2O sym str CH3 stretch, acetate additional observed bands H2O bend CO2 asym str CH3 bends CO2 sym str CH3 bend CH3 bend CH3 bend

154

μ3-OH bend + H2O bend

2 4 4 6 24

151 38 197 88 5

18

322

H2O asym str μ3-OH stretch H2O asym str H2O sym str CH2 stretch, chloroacetate glass slide H2O bend + CO2 asym str

24

225

CO2 sym str + CH2 bends

12 12

109 0

CH2 bends CH2 bends

24 12 8

3 80 73

CH2 bends + CO2 bend C−Cl stretch μ3-OH bend

18 3

3820 3809 3781 3680 3184

(3705)b (3695)b (3667)b (3570)b (3117)−3104 (3039)c

1714−1615 1469−1446 1292 1204 948 778 750−723 a

3620 3560 3007, 2797, 1735, 1595, 1497, 1400, 1270, 1183 1150, 945 793 706

2999, 2953 2656, 2505, 2429, 2255, 1876 1639 1551, 1540 1432 1345 1259 1129, 1064

Average. bValues in parentheses are scaled from the experiment for H2O. cValues in parentheses are scaled from the experiment for CH4.

of the clusters 1 to 3 thus exhibit approximate Td symmetry. The HOMOs of the clusters are approximately triply degenerate and are delocalized over the complex with contributions on the bridging O atoms and on the acetates (see Figure 4 for 1, the others are in the Supporting Information). The LUMO is not degenerate and is mostly localized on the Th. The HOMO-1 is also approximately triply degenerate and is, in general, close in energy to the HOMO.

bonded to the Th to generate 5 leads to larger changes in the charges. The Th becomes much less positive with a decrease of ∼0.4 e. The bridging O becomes more negative by 0.09 e. A much larger positive change is calculated for the bridging O(H) group with an increase of 0.36 e. The core of the [Th6O8H12+][RCO2−]12 clusters has Td symmetry when there are no waters present and when the symmetry of the R group is not included. The orbital properties 6922



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Table 3. Calculated NBO Chargesa for 1 to 7

a

Table 4. Average Calculated Orbital Energies in eV for 1 to 7

cluster

Th

O

O(H)

cluster

HOMO-1a

HOMOa

LUMOb

LUMO+1a

1 2 3 4 5 6 7

2.06 2.06 2.07 2.18 1.80 2.18 2.20

−1.11 −1.10 −1.11 −1.12 −1.23 −1.12 −1.12

−1.02 −1.02 −1.02 −1.04 −0.68 −1.03 −1.04

1 2 3 4 5 6 7

−7.89 −7.37 −8.04 −8.22 −7.45 −7.53 −8.36

−7.57 −7.23 −7.93 −7.93 −7.37 −7.41 −8.27

−1.34 −0.92 −1.67 −1.74 −2.63 −1.14 −2.13

−1.06 −0.65 −1.39 −1.40 −1.63 −0.86 −1.85

a

Units of electrons, e.

All of the orbtials except for the HOMO for 5 are approximately triply degenerate. See Supporting Information for exact values. bThe LUMOs are all nondegenerate.

The LUMO+1 is also triply degenerate. Using an approximate version of Koopmann’s theorem for the DFT orbital energies (Table 4),61 we predict that 2 has the lowest IP and 3 the highest IP of 1, 2, and 3. The calculated LUMOs are negative suggesting that the isolated gas phase complexes should be able to readily bind an additional electron, although the DFT LUMO is not strictly correlated with the electron affinity.61 These types of potentially reasonably large electron affinities are similar to what is observed in transition metal oxide clusters.51,62−67 It is not surprising that the gas phase cluster would bind an electron as the LUMO is delocalized over the strongly positive core and is surrounded by the negative carboxylate groups. Compound 2 is predicted to have the least negative LUMO and 3 the most negative LUMO following the HOMO energy trends. Removal of the H2O ligands from 1, 2, and 3 leads to both the HOMO and LUMO becoming more negative so the compounds should be less easily oxidized. Substitution of 6 O2− for 12 formate ligands leads to a less negative HOMO and a more negative LUMO so the first excitation energy (HOMO−LUMO GAP) should decrease. Energetic Quantities. The average water complexation energies for 1, 2, and 3 were calculated from the values shown in eqs 1 to 3 with the ΔH298 and ΔG298 in kcal/mol.

7 + 6H 2O → 3 (ΔH298 = −102.3, ΔG298 = − 41.1)

The respective average ΔH298 values for binding one water for 1, 2, and 3, are −14.6, −13.4, and −17.0 kcal/mol, and the respective average ΔG298 values are −5.0, −3.6, and −6.8 kcal/ mol. The binding energies depend slightly on the nature of the anionic ligand, and the water is most strongly bonded to the Th for the chloroacetate ligand. An important thermodynamic quantity is the acidity of the complex. The gas phase acidity is defined as the free energy for the removal of a proton from the structure to form an anion (AH → A− + H+). The acidities of 1−7 were calculated. We first calculated the energies of the three isomers that can be generated by removing a proton from 1, 4, and 5 leading to three different anions, A−, B−, and C−, as shown in Figure 5 for 5-H+. The most stable anion is generated by removing a proton from the most stable tetra-protonated structure leading to a structure with the protons separated by as much as possible in the anion (Table 5). When the positive charge in the core is balanced by the bridging HCO2− groups, the most stable anion has the protons separated as much as possible just as in the neutral. In contrast for 5-H+, anion structure B− is very close in energy to the most stable anion structure (A−). The gas phase acidity of 1 is predicted to be 308 kcal/mol (Table 6). Substitution of the H in the HCO2− ligands by CH3 leads to a less acidic compound by 8 kcal/mol and substitution of H by CH2Cl leads to a more acidic compound by 6 kcal/ mol. For comparison, the gas phase acidity of H3PO4 is 322 kcal/mol and that for H2SO4 is 305 kcal/mol.68 Removal of the

4 + 6H 2O → 1 (ΔH298 = −87.5, ΔG298 = − 30.1)

(1)

6 + 6H 2O → 2 (ΔH298 = −80.4, ΔG298 = − 21.4)

(3)

(2)

Figure 4. HOMO and LUMO orbitals for Th6(OH)4O4(H2O)6(HCOO)12 (1). 6923



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H2O molecules on the Th in 1 to generate 4 makes the compound more acidic by 10 kcal/mol. Removal of the waters on 2 makes 6 more acidic by only 3 kcal/mol, but removal of the waters on 3 makes 7 more acidic by 16 kcal/mol. 7 is predicted to be an extremely strong gas phase acid, comparable to (CF3SO2)2NH which has an acidity of 285 kcal/mol.68 Replacing the formate counterions in 4 by 6 O2− bonded to the Th to form 5, makes 5 more acidic in the gas phase by 26 kcal/ mol, and its acidity is comparable to those of the hydroxylated Group 6 transition metal oxides.69 The acidities in aqueous and DMSO solution were calculated with respective dielectric constants of 79.39 and 46.7. The solution acidities were calculated relative to the following experimental values, pK a (CH 3 COOH) = 4.75 and pK a (C6H5OH) = 10.0 in water, and pKa(CH3COOH) = 12.3 and pKa (C6H5OH) = 18.0 in DMSO. The two sets of pKa’s for each solvent agree to within ≤0.6 pKa unit for DMSO and ≤0.2 pKa for H2O. Compounds 1 to 3 are predicted to be very weak acids in aqueous solution with pKa’s in the range of 13.2−16.2 (CH3CO2H) and 13.4−16.4 (C6H5OH). The aqueous solution acidities of 1 to 3 follow the order of the gas phase values, but the range of pKa units of 3 (4.1 kcal/mol) is much smaller than the range of the gas phase acidities of 14 kcal/mol. Removal of the water molecules bonded to the Th in 1, 2, and 3 makes the respective 4, 6, and 7 much more acidic in aqueous solution, and 7 is predicted to be a moderate acid. Although exchanging the 12 HCO2− groups in 4 for 6 O2− groups bonded to Th to generate 5 makes 5 a far stronger gas phase acid, 5 is actually predicted to be a weaker acid in aqueous solution as compared to 4 by almost 4 pKa units. The acidities in DMSO follow the same trends as in water.

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CONCLUSIONS The three polynuclear thorium(IV) molecular complexes, previously synthesized and structurally characterized, Th6(OH)4O4(H2O)6(HCOO)12·nH2O (1), T h 6 ( O H ) 4 O 4 ( H 2 O ) 6 ( CH 3 C O O ) 1 2 · n H 2 O ( 2 ) , a n d Th6(OH)4O4(H2O)6(ClCH2COO)12·4H2O (3), have been studied with respect to their spectral and energetic properties. The calculated 1H NMR chemical shifts for the four μ3hydroxo, water and formate protons are reported and compared to the experimental values and with other small mono or binuclear Th clusters. There is relatively good agreement with experiment taking into account that our calculated 1H NMR chemical shifts are predicted in the absence of solvent. There is no change in the 1H NMR chemical shifts when the solvating water is removed from 1 to give structure 4 consistent with our other results that these waters of solvation do not significantly affect the geometry/electronic structure of the core. The chemical shifts for the formate protons after removal of the H2O solvating the Th to form 4 also show essentially no change. The calculated frequencies are in reasonable agreement with the experimentally observed Raman bands and can be used to assign the vibrational transitions. The Th atoms are positively charged and the bridging O and O(H) are negatively charged from the Mulliken and NBO (natural bond orbital) charge calculations as expected. From the analysis of the calculated highest-occupied and lowest-unoccupied molecular orbitals (HOMO and LUMO) energies, we predict that 2 has the lowest IP and 3 the highest IP among these three polynuclear thorium(IV) molecular complexes. The water binding energies depend slightly on the nature of the anionic ligand (formate, acetate, or chloroacetate). These Th clusters

Figure 5. Optimized structures for the three isomers of 5 with loss of a proton. Th6(OH)3O5O6−.

Table 5. Calculated Relative Energies of the in kcal/mol for 1-H+, 4-H+, and 5-H+ isomer −

A B− C−

1-H+

4-H+

5-H+

0.0 11.7 17.1

0.0 13.3 20.0

0.0 0.2 6.8

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Table 6. Gas Phase Acidities and pKa Values in Water and DMSO ADF Relative to CH3COOH and C6H5OH reaction 1 2 3 4 5 6 7

→ → → → → → →

Th6(OH)3O5(H2O)6(HCOO)12− + H+ Th6(OH)3O5(H2O)6(CH3COO)12− + H+ Th6(OH)3O5(H2O)6(CH2ClCOO)12− + H+ Th6(OH)3O5(HCOO)12− + H+ Th6(OH)3O5O6− + H+ Th6(OH)3O5(CH3COO)12− + H+ Th6(OH)3O5(CH2ClCOO)12− + H+

ΔH298K gas

ΔG298K gas

pKa(H2O) CH3COOH

pKa(H2O) C6H5OH

pKa(DMSO) CH3COOH

pKa(DMSO) C6H5OH

315.4 322.9 306.3 308.6 281.4 317.0 294.9

308.4 315.7 302.2 298.2 272.5 312.1 286.4

14.1 16.2 13.2 5.8 9.5 10.9 4.4

14.2 16.4 13.4 5.9 9.6 11.1 4.6

21.5 25.6 21.7 12.5 14.2 19.1 11.2

22.1 26.1 22.2 13.1 14.8 19.6 11.7

(5) Polyoxometalate Chemistry from Topology via Self-Assembly to Applications; Pope, M. T., Müller, A., Eds.; Kluwer: Dordrecht, The Netherlands, 2001. (6) Mehring, M. Coord. Chem. Rev. 2007, 251, 974−1006. (7) Calvez, G.; Daiguebonne, C.; Guillou, O.; Pott, T.; Meleard, P.; Le Dret, F. C. R. Chim. 2010, 13, 715−730. (8) Zheng, Z. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner Jr, K. A., Bunzli, J.-C. G., Pecharsky, V. K., Eds.; North-Holland, Elsevier B.V.: Amsterdam, The Netherlands, 2010; Vol. 40. (9) Rothe, J.; Walther, C.; Denecke, M. A.; Fanghaenel, T. Inorg. Chem. 2004, 43, 4708−4718. (10) Soderholm, L.; Almond, P. M.; Skanthakumar, S.; Wilson, R. E.; Burns, P. C. Angew. Chem., Int. Ed. 2008, 47, 298−302. (11) Walther, C.; Rothe, J.; Brendebach, B.; Fuss, M.; Altmaier, M.; Marquardt, C. M.; Büchner, S.; Cho, H.-R.; Yun, J.-I.; Seibert, A. Radiochim. Acta 2009, 97, 199−207. (12) Takao, K.; Takao, S.; Scheinost, A. C.; Bernhard, G.; Hennig, C. Inorg. Chem. 2012, 51, 1336−1344. (13) Takao, S.; Takao, K.; Kraus, W.; Emmerling, F.; Scheinost, A. C.; Bernhard, G.; Hennig, C. Eur. J. Inorg. Chem. 2009, 32, 4771− 4775. (14) Rand, M.; Fuger, J.; Grenthe, I.; Neck, V.; Rai, D. Chemical Thermodynamics of Thorium; Elsevier: Amsterdam, The Netherlands, 2008. (15) Toraishi, T.; Farkas, I.; Szabo, Z.; Grenthe, I. J. Chem. Soc., Dalton Trans. 2002, 3805−3812. (16) Toraishi, T.; Grenthe, I. Dalton Trans. 2003, 1634−1640. (17) Wilson, R. E.; Skanthakumar, S.; Soderholm, L. Mater. Res. Soc. Symp. Proc. 2007, 986, 183−188. (18) Torapava, N.; Persson, I.; Eriksson, L.; Lundberg, D. Inorg. Chem. 2009, 48, 11712−11723. (19) Knope, K. E.; Wilson, R. E.; Vasiliu, M.; Dixon, D. A.; Soderholm, L. Inorg. Chem. 2011, 50, 9696−9704. (20) Knope, K. E.; Vasiliu, M.; Dixon, D. A.; Soderholm, L. Inorg. Chem. 2012, 51, 4239−4249. (21) Rothe, J.; Denecke, M. A.; Neck, V.; Muller, R.; Kim, J. I. Inorg. Chem. 2002, 41, 249−258. (22) Wilson, R. E.; Skanthakumar, S.; Burns, P. C.; Soderholm, L. Angew. Chem., Int. Ed. 2007, 46, 8043−8045. (23) Wilson, R. E.; Skanthakumar, S.; Sigmon, G.; Burns, P. C.; Soderholm, L. Inorg. Chem. 2007, 46, 2368−2372. (24) Skanthakumar, S.; Antonio, M. R.; Wilson, R. E.; Soderholm, L. Inorg. Chem. 2007, 46, 3485−3491. (25) Wilson, R. E.; Skanthakumar, S.; Cahill, C. L.; Soderholm, L. Inorg. Chem. 2011, 50, 10748−10754. (26) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J. Org. Chem. 1997, 62, 7512−7515. (27) Skanthakumar, S.; Soderholm, L. Mater. Res. Soc. Symp. Proc. 2006, 893, 411−416. (28) Soderholm, L.; Skanthakumar, S.; Neuefeind, J. Anal. Bioanal. Chem. 2005, 383, 48−55. (29) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (30) Andrews, L.; Gong, Y.; Liang, B.; Jackson, V. E.; Flamerich, R.; Li, S.; Dixon, D. A. J. Phys. Chem. A 2011, 115, 14407−14416.

are found to be mild to strong acids in the gas phase but are predicted to behave as weak acids in aqueous or DMSO solution.

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ASSOCIATED CONTENT

S Supporting Information *

Complete references 37 and 52. Total energies in a.u. with the exception of S, in cal/mol K at B3LYP/DZVP for H, C, and O and Stutt (large core-Th(f)) for Th. The single point energies in hartree from B3LYP/DZVP for H, C, and O and Stutt (large core and the addition of the innermost two contracted f orbitals to the Th) for Th. Frequencies (cm−1), IR Intensities (km/ mol) and Raman Activities (Å4/amu) (only for Structures A). Additional 1H NMR chemical shifts for higher energy 4 and 5 thorium cluster isomers. Additional experimental Raman transitions. Additional orbital energies. Mulliken charges. Cartesian coordinates in angstroms. HEXS data for 1 dissolved in DMSO. Proton NMR spectrum of DMSO-d6. Optimized structures for Th6(OH)3O5(H2O)6(HCOO)12 (−1) [1−] and Th6(OH)3O5(HCOO)12 (−1) [4−] isomers and for the small Th cluster structures. Pictures of the molecular orbitals. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed in part at the Argonne National Laboratory, operated by UChicagoArgonne LLC for the United States Department of Energy under contract number DEAC02-06CH11357. The work was supported by a DOE Office of Basic Energy Sciences, Single-Investigator and Small-Group Research Project. D.A.D. is indebted to the Robert Ramsay Endowment of the University of Alabama for partial support. The authors thank Dr. John V. Muntean (Argonne National Laboratory) for his assistance in collecting the NMR data.

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REFERENCES

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Spectroscopic and Energetic Properties of Thorium (IV) Molecular

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