Uranium(IV) Chloride Complexes: UCl62– and an Unprecedented U

Jul 31, 2017 - ABSTRACT: The room temperature synthesis and structural characterization of two. U(IV) compounds isolated from acidic aqueous solution ...
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Uranium(IV) Chloride Complexes: UCl62− and an Unprecedented U(H2O)4Cl4 Structural Unit Jennifer N. Wacker,† Monica Vasiliu,‡ Kevin Huang,⊥ Ryan E. Baumbach,⊥,§ Jeffery A. Bertke,† David A. Dixon,‡ and Karah E. Knope*,† †

Department of Chemistry, Georgetown University, 37th and O Streets NW, Washington, D.C. 20057, United States Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States ⊥ National High Magnetic Field Laboratory, Tallahassee, Florida 32310, United States § Department of Physics, Florida State University, Tallahassee, Florida 32310, United States ‡

S Supporting Information *

ABSTRACT: The room temperature synthesis and structural characterization of two U(IV) compounds isolated from acidic aqueous solution is reported. Evaporation of a U(IV)/HCl solution containing pyridinium (HPy) yielded (HPy)2UCl6 (1), yet in the presence of an organic carboxylate U(H2O)4Cl4·(HPy·Cl)2 (2) is obtained. The structures have been determined by single crystal X-ray diffraction and characterized by Raman, IR, and optical spectroscopies. The magnetism of both compounds was also investigated. The structure of 1 is built from UCl62− anionic units, pervasive in descriptions of the aqueous chemistry of tetravalent uranium, and is found to undergo a phase transition from C2/m to P1̅ upon cooling. By comparison, the structure of 2 contains a neutral U(IV)-aquo-chloro complex, U(H2O)4Cl4, for which there is no literature precedence. Density functional theory calculations were performed to predict the geometries, vibrational frequencies, and relative energetics of the UCl62− and U(H2O)4Cl4 units. The energetics of the reaction of U(H2O)4Cl4 to form the dianion are predicted to be exothermic in the gas phase and in aqueous solution. The predicted energetics coupled with no previous solid state reports of a U(IV)-aquo-chloro complex may point toward the importance of hydrogen bonding and other supramolecular interactions, prevalent in the structures of 1 and 2, on the stabilization and/or crystallization of the U(H2O)4Cl4 structural unit.

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molecular units of the 5f elements.12−14 Despite extensive variation in syntheses, counterions, and ionic strength, An(VI) and An(IV) chlorides are shown to commonly adopt two distinct structural units: AnO2Cl42− and AnCl62−, respectively.8,10−18 AnO2Cl42− compounds such as UO2Cl42− and PuO2Cl42− have been considerably characterized in solution and the solid state.15−18 AnCl62− (An = Th, U, Pu, Np) and transition metal (TM) complexes,19 TMCl62−, have likewise been investigated extensively.20−24 Derivatives of actinide chlorides are also utilized as starting materials to facilitate nonaqueous transuranic chemistry.25,26 In addition, uranium halides such as UCl62− have more recently played an important role in understanding covalency in actinide compounds.27,28 The experimental breadth of these compounds is supported, and indeed complemented, in the sizable thermodynamic stability of these structural units.29 Given the expansive reach of actinide-halide complexes, a warranted exploration of the potential effects of supramolecular interactions on the identity of the metal center has been pursued. More specifically, in drawing on recent work highlighting the importance of second coordination sphere molecules on the stabilization of actinide and other metal ion

INTRODUCTION Actinide−ligand interactions underpin a number of technologically and environmentally relevant processes including separations chemistries, waste remediation strategies, and the environmental transport of heavy elements.1−3 In all of these cases, an understanding of the conditions over which different actinide species are stable in solution or observed in the solid state is critically important to the development of reliable models that accurately predict the chemical behavior of these radionuclides. It is well established that coordinating ligands may direct the formation of metal−ligand complexes and thereby affect the identity (e.g., nuclearity) of the metal ion.4−6 More recently, supramolecular and second coordination sphere interactions have likewise been shown to play an important role in the isolation and crystallization of actinide complexes as well as metal oxide clusters.7,8 Indeed, across many avenues of chemistry, the tunable nature of noncovalent interactions such as hydrogen bonding and halogen−halogen interactions has been utilized to access a rich catalog of supramolecular networks via both directing and stabilizing effects,9−11 yet the impact that these nonbonding interactions may have on metal ion speciation is not well understood. Within actinide chemistry, actinide halides have long received much attention due to their relevance in commercial applications and are arguably some of the most well studied © 2017 American Chemical Society

Received: May 19, 2017 Published: July 31, 2017 9772

DOI: 10.1021/acs.inorgchem.7b01293 Inorg. Chem. 2017, 56, 9772−9780

Article

Inorganic Chemistry complexes and clusters,7,8 we have been examining the effects of H-bond donors such as carboxylic acids on the composition and identity of U(IV)-chloride complexes. Herein, we report the synthesis and structural characterization of two novel U(IV) compounds, one built from anionic UCl62− structural units and the other from an unprecedented U(H2O)4Cl4 complex, isolated from acidic aqueous solution.

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Table 1. Crystallographic Structure Refinement Details for 1 (100 K, 296 K) and 2 (100 K) formula MW (g/mol) T (K) crystal system space group λ (Å) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z ρ (mg/m3) μ (mm−1) R1 wR2 GOF CCDC

EXPERIMENTAL SECTION

Syntheses. Caution: 238U is an α emitter and standard precautions for handling radioactive materials should be followed when performing the syntheses that follow. Uranium tetrachloride (UCl4) was synthesized according to the procedure outlined by Kiplinger et al.30 Chemicals used for the preparation of UCl4 including UO3 (International BioAnalytical Industries, Inc.), hexachloropropene (Alfa Aesar), and dichloromethane (Fisher Chemicals) were used as received. Chemicals used for the syntheses of compounds 1 and 2 including pyridine (Alfa Aesar), hydrochloric acid (Fisher Chemical), and L-serine (Fisher Scientific) were used as received from commercial suppliers. Aqueous solutions were prepared using nanopure water (≤0.05 μS) purified by a Millipore Direct-Q 3UV water purification system. (HPy)2UCl6 (1). An aqueous solution of U(IV) in HCl was prepared in a 3 mL shell vial by dissolving UCl4 (0.05 g, 0.13 mmol) into 1 M HCl (0.5 mL). Pyridine (20 μL, 0.25 mmol) was added and the solution was left to completely evaporate under a nitrogen atmosphere. After approximately 14 days, large green crystals formed. Elemental Analysis: Found: C, 19.25; H, 2.4; N, 4.65. Calc. for UCl6N2C10H12: C, 19.7; H, 2.0; N, 4.6%. Yield based on uranium: 0.058 g, 98.3%. U(H2O)4Cl4·(HPy·Cl)2 (2). The synthesis that resulted in the formation of 1 was repeated in the presence of L-serine. An aqueous solution of U(IV) in HCl was prepared in a 3 mL shell vial by dissolving UCl4 (0.05 g, 0.13 mmol) into 1 M HCl (0.5 mL). Pyridine (20 μL, 0.25 mmol) and L-serine (0.01 g, 0.10 mmol) were added, and the resulting solution was left to evaporate under a nitrogen atmosphere. After approximately 12 days, just prior to complete evaporation, small, green, block-like crystals of 2, along with a few crystals of 1, and a minor unidentified phase, were observed with a yield of 13.4% for 2. However, if the solution was disturbed just prior to crystallization (approximately 60 μL of solution), the yield of 2 was noticeably higher: 0.012 g, 51.4% based on U; the rest of the U remains in solution. This observation points to the fact that the complex in 2 is a significant product of the reaction, yet its isolation is somewhat delicate. As expressed through the thermodynamic favorability of compound 1 (see below), it is not surprising that the reaction ultimately ends in nearly complete conversion to the UCl62− unit upon complete evaporation of the solution. X-ray Structure Determination. Single crystals of 1 and 2 were isolated from the bulk and mounted on MiTeGen micromounts in mineral oil. Single crystal X-ray diffraction studies were performed on a Bruker Quest D8 diffractometer equipped with an IμS X-ray source (Mo Kα radiation; λ = 0.71073 Å) and a CMOS detector. Data for single crystals of 1 were collected at 100 and 296 K as the compound was found to undergo a phase transition from C2/m to P1̅ upon cooling. Compound 2 exhibited the same cell parameters and space group at 100 and 296 K, and therefore only the structure from the 100 K data collection is reported. Unit cells were identified using APEX2/ Cell_Now for 1 (100 K) and APEX2 for 1 (296 K) and 2.31,32 The data were integrated and filtered for statistical outliers using SAINT32 and corrected for absorption using SAINT/SADABS/TWINABS v2014/2.32 No decay corrections were applied. All H atoms were included as riding idealized contributors unless otherwise noted below. H atoms located in the difference map were assigned as 1.5 times carrier Ueq, while all calculated H atoms were assigned as 1.2 times carrier Ueq. Crystallographic details for 1 (100 K), 1 (296 K), and 2 are provided in Table 1. We note that the relatively higher wR2 for 2 as compared to 1 is due to weaker data at high angles. Further as 2 does not undergo a phase transformation, unlike compound 1, it is not the

1 (100 K)

1 (296 K)

2 (100 K)

C10H12Cl6N2U 610.95 100(2) triclinic P1̅ 0.71073 7.3318(5) 7.9387(6) 8.2528(8) 81.573(3) 66.790(2) 83.400(2) 435.85(6) 1 2.328 10.217 0.0129 0.0312 1.118 1523702

C10H12Cl6N2U 610.95 296(2) monoclinic C2/m 0.71073 13.0357(4) 8.6252(3) 7.9764(2) 90 97.8830(10) 90 888.36(5) 2 2.328 10.025 0.0184 0.0490 1.143 1523703

C10H20Cl6N2O4U 683.01 100(2) orthorhombic Pnma 0.71073 13.6596(9) 12.0741(8) 12.1593(8) 90 90 90 2005.4(2) 4 2.262 8.909 0.0343 0.0875 1.308 1523704

source of the higher wR2. Despite weaker data, the data are still acceptable with a wR2 value below three times the R1 value. Compound 1 (100 K). The crystal exhibited non-merohedral twinning. Two distinct cells were identified which are related by the twin law (−0.997−0.005−0.812), (0.008−0.999−0.070), (−0.009− 0.001 0.996). A structural model consisting of one-half of the UCl62− anion and one pyridinium cation per asymmetric unit was developed. The U and Cl positions were located in the initial structural solution of the Fourier map. Subsequent refinement allowed the remaining C and N atoms to be located in the Fourier map. The pyridinium N−H H atom was located in the difference map and the distance was allowed to freely refine. Compound 1 (296 K). A structural model consisting of one-half of the UCl62− anion and one-half of a pyridinium cation per asymmetric unit was developed. The U and Cl positions were located in the initial structural solution of the Fourier map. Subsequent refinement allowed the remaining C and N atoms to be located in the Fourier map. The cation is disordered over a symmetry site and thus was modeled accordingly. The pyridinium N−H hydrogen atom could not be located in the difference map and thus was placed in a calculated position with a distance of 0.86 Å (esd 0.02). Compound 2. A structural model consisting of one-half of the U(H2O)4Cl4 unit, one free Cl anion, and two half pyridinium cations per asymmetric unit was developed. The U and Cl positions were located in the initial structural solution of the Fourier map, and subsequent refinement allowed the remaining C, N, and O atoms to be located. One of the pyridinium rings is disordered over a symmetry site, and therefore the pyridinium atoms were restrained to behave isotropically. Water H atoms were located in the difference map. The O−H distances were restrained to be 0.88 Å (esd 0.02). The U−H distances were restrained to be similar (esd 0.01 Å). The pyridinium N−H atoms could not be located in the difference map and thus were placed in calculated positions with a N−H distance of 0.88 Å (esd 0.02). Powder X-ray Diffraction. Powder X-ray diffraction data of 1 (Supporting Information) were measured using a Rigaku UltimaIV Xray diffractometer with Cu Kα (λ = 1.542 Å) radiation. Data were collected at room temperature using a scintillation counter detector from 3−40° 2Θ. The bulk powder pattern of the reaction product from which 2 was isolated was measured using a Bruker APEX DUO with a CCD detector using Cu Kα (λ = 1.542 Å) radiation. Data were collected at room temperature. 9773

DOI: 10.1021/acs.inorgchem.7b01293 Inorg. Chem. 2017, 56, 9772−9780

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Inorganic Chemistry Raman/IR/UV−vis-NIR Spectroscopy. Raman spectra were recorded for single crystals of 1 and 2 using a Horiba LabRAM HR Evolution Raman spectrometer with an excitation line of 532 nm. Data were collected at room temperature over Δν 100−4000 cm−1 using circularly polarized radiation. In 1, the baseline contains more noise compared to that of 2 due to the reduced laser power (1% vs 3.2%) that was required to prevent oxidation of the crystal. Noise in the baseline of the spectrum of 1 is also attributed to oil left on the crystal from unit cell determination via X-ray diffraction prior to collection of the Raman spectrum. Infrared spectra were obtained using a PerkinElmer Spectrum 2 FTIR spectrometer. Data were collected at room temperature. Powdered samples of both 1 and 2 were placed directly on the FTIR-ATR stage, and spectra were collected over Δν 400−4000 cm−1. UV−vis-NIR spectra for 1 and 2 were collected using a CRAIC 20/30 PV Technologies microspectrophotometer. Single crystals of 1 and 2 were placed on a quartz slide in oil and data were collected from 320−1100 nm Magnetic Measurements. Magnetization measurements were performed for unaligned clusters of crystals using a Quantum Design VSM Magnetic Properties Measurement System (MPMS), reaching temperatures from 300 K down to 1.8 K with an applied magnetic field of 5000 Oe. Computational Details. Electronic structure studies were performed in an effort to better understand the formation and stability of 2 relative to 1. The geometries were initially optimized at the density functional theory (DFT) level33 with the hybrid B3LYP exchangecorrelation functional34,35 with the DFT-optimized DZVP2 basis set36 for H, O, and Cl atoms and the cc-pVDZ-PP basis set for U.37 The geometries were further optimized at the DFT/B3LYP level with the aug-cc-pVnZ38 basis sets for H and O, aug-cc-pV(n + d)Z for Cl,39 and cc-pVnZ-PP basis sets for U (n = D, T). Vibrational frequencies were calculated to show that the structures were minima. The DFT calculations were done using the Gaussian09 program system.40 Single point R/UCCSD(T) calculations were performed with the same correlation consistent basis sets for n = D and T with MOLPRO.41−49 The solvation free energies in water at 298 K were calculated at the gas phase geometries at the B3LYP level using the self-consistent reaction field (SCRF) approach50 with the COSMO parameters51,52 as implemented in Gaussian 03.53 The aqueous Gibbs free energy (free energy in aqueous solution), ΔGaq, was calculated from eq 1, ΔGaq = ΔGgas + ΔGsolv

Figure 1. Packing diagram of 1 (collected at 100 K) showing UCl62− anionic units and charge balancing pyridinium cations. Dotted black lines highlight H-bonding interactions. Only those nitrogen---chloride distances within 0.2 Å of the sum of the van der Waals radii for N and Cl are depicted (U = dark green, Cl = light green, N = blue).

CHPy---CHPy 3.731(1) Å. Further, the displacement angle of the aromatic rings was calculated using an approach described previously by Janiak,56 and the angle between the vector normal to the plane of the pyridinium ring and the vector formed through the respective centroids was found to be 21.8°. The centroid−centroid distances and the slip angles are consistent with weak π−π stacking interactions.56 The structure of 1 collected at 296 K likewise consists of mononuclear UCl62− anionic units that are charge balanced by pyridinium cations (Figure 2). Such a phase transition has been

(1)

where ΔGgas is the gas phase free energy and ΔGsolv is the aqueous solvation free energy. A dielectric constant of 78.39 corresponding to that of bulk water was used. The solvation energy is reported as the electrostatic energy (polarized solute−solvent). The energetics of reaction (eq 2) were calculated at the B3LYP and CCSD(T) levels with the aug-cc-pVDZ/cc-pVDZ-PP(U) basis set. 3

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3

U(H 2O)4 Cl4 + 2Cl− → UCl 6 2 − + 4H 2O

Figure 2. Illustration of 1 (collected at 296 K) highlighting the hydrogen bonding interactions (black dotted lines) between the UCl62− anionic units and the pyridinium cations. Only those Hbonding interactions within 0.2 Å of the sum of the van der Waals radii for N and Cl are depicted. Disorder of the pyridinium rings is not shown for clarity.

(2)

RESULTS Structure Descriptions. The structure of 1 obtained at 100 K is built from U(IV) metal centers that are bound to six chloride ions to form mononuclear, UCl62− anionic units that are charge balanced by pyridinium cations (Figure 1). U−Cl bond distances range from 2.599(7) to 2.661(6) Å, consistent with previously reported UCl62− compounds.10−18 As shown in Figure 1, 1 contains weak hydrogen bonding interactions54,55 between the hydrogen bound to N1 of the pyridinium with chlorides (Cl1−Cl3) of the UCl62− structural unit. Nitrogen to chloride distances range from 3.297(2) to 3.390(2) Å. Moreover, offset π−π stacking interactions between the pyridinium cations are present in the structure. The π−π interaction distances were found by measuring the linear distance between the calculated centroids (CHPy) that correspond to the center of gravity of the aromatic rings:

observed for other AnCl62− containing compounds.57−59 However, as compared to the structure obtained at 100 K, differences in the bond distances and supramolecular interaction distances are observed. U−Cl bond distances range from 2.605(2) to 2.644(2) Å. Weak hydrogen bonding interactions54,55 between the hydrogen bound to N1 of the pyridinium with chlorides Cl2 and Cl3 of the UCl62− structural unit are similarly present in the structure. As shown in Figure 2, the pyridinium cations form bifurcated hydrogen bonding interactions with Cl2 and Cl3 ions to yield one-dimensional chains with N1---Cl2 and N1---Cl3 distances of 3.383(1) Å and 3.304(1) Å respectively. The structure of 2 consists of U(IV) metal centers that are eight coordinate, bound to four chloride ions and four water molecules to form an overall charge neutral complex as shown in Figure 3. Bond distances for U−O(H2) and U−Cl are 9774

DOI: 10.1021/acs.inorgchem.7b01293 Inorg. Chem. 2017, 56, 9772−9780

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Inorganic Chemistry

substantially stabilized in aqueous solution and that these energetics are not strongly basis set or method dependent. Two isomers of U(H2O)4Cl4 were calculated, and the lowest energy structure is that with the four Cl ligands moved just out of a plane with three H2O molecules below the plane and one above it, consistent with that observed in the crystal structure of 2 (Figure 3). The second structure has one Cl axial and its position in the approximate plane is replaced by one H2O and is 1.7 kcal/mol higher in energy at the B3LYP level (Supporting Information). The optimized geometry for UCl62− has a bond distance of 2.69 Å, which is 0.08−0.09 Å longer than experiment consistent with errors due to use of the B3LYP functional and the fact that the isolated gas phase dianion will have longer distances than in the crystal. The calculated U−Cl bond distances in 2 ranges from 2.62 to 2.66 Å, which is roughly 0.06 Å shorter than experimental bond lengths. Consistent with this difference and the use of the B3LYP functional, the calculated U−O(H2) bond distances are longer than experiment by 0.12−0.25 Å. DFT calculations with the local SVWN functional61,62 give a U−Cl bond distance of 2.62 Å for 1 in excellent agreement with experiment. For 2, the local DFT U−Cl bond distances are predicted to be in the range of 2.55−2.64 Å, about 0.1 Å less than experiment. However, the calculated U−O(H2) bond distances are now in better agreement with experiment ranging from 2.44 to 2.64 Å. The energetics of the reaction of U(H2O)4Cl4 to form the dianion (eq 2) were predicted to be endothermic in terms of enthalpy at 298 K in the gas phase but are predicted to be exothermic in the gas phase and in aqueous solution with ΔG(gas) = −14 kcal/mol and ΔG(aq) = −10 kcal/mol at the B3LYP level and with ΔG(gas) = −5 kcal/mol and ΔG(aq) = −1 kcal/mol at the CCSD(T) level. The results clearly show that in aqueous solution the complex observed in 2 (eq 2) will release water to generate the dianion observed in 1, found experimentally, even without the presence of the noncoordinating countercations. Vibrational Spectroscopy. The calculations also enabled us to predict the vibrational spectra and IR/Raman intensities of 1 and 2 and to compare the results to the experimental findings shown in Figure 4. IR data collected experimentally indicated several similarities between 1 and 2 at lower wavenumbers (Δν 600−1650 cm−1), which is expected due to pyridinium being present in both compounds. However, not

Figure 3. Packing diagram of 2 highlighting the supramolecular network that is formed via hydrogen bonding (dotted black lines) between the U(H2O)4Cl4 units and the outer coordination sphere pyridinium and chloride ions. Offset π−π stacking interactions between the pyridinium rings are also present in the structure. Only those H-bonding interactions within 0.2 Å of the sum of the van der Waals radii for the respective donor/acceptor atoms are shown. Disorder of the pyridinium rings is not shown for clarity (U = dark green, Cl = light green, O = red, N = blue).

2.399(6)−2.464(7) Å and 2.658(2)−2.726(2) Å, respectively. Two pyridinium cations and two chloride anions exist in the outer coordination sphere per structural unit. Weak to moderate hydrogen bonding54,55 interactions are present in the structure of 2 (Figure 3). As in both structures of 1, 2 contains weak NH---Cl hydrogen bonding. However, those intermolecular interactions between the pyridinium N and bound water molecule afford the shortest interaction with a N11---O3(H2) donor−acceptor distance of 2.934(13) Å. Hbonding further occurs between the outer coordination sphere chloride ions and the metal-bound water molecules yielding O2---Cl3 and O3---Cl3 distances of 3.035(6) and 3.051(6) Å, respectively. Selected donor−acceptor distances are given in Table 2. Other weak intermolecular interactions including Table 2. Selected Nonbonding Distances (Å) and Angles (deg) in 2 interaction

distance (Å) (D---A)

angle (deg) (∠D−H---A)

O(1)H---Cl(3) O(2)H---Cl(1) N(11)H---O(3) N(11)H---Cl(2) C(HPy)---C(HPy′)

3.035(2) 3.122(2) 2.934(2) 3.456(2) 3.589(1)

160 146 140 131 19.2

offset π−π stacking are present in 2 and serve to further connect the U(H2O)4Cl4 units into an extended supramolecular network. The CHPy---CHPy centroid distance and slip angle between the pyridinium rings are 3.589(1) Å and 19.2°, respectively. Computational Investigation of Structure and Energetics of Formation. The electronic structure of UCl6− and UCl62− have been examined in detail using high level electronic structure methods where it has been shown that the electronic structure of UCl62− is quite complicated.60 Our approach involves using DFT benchmarked by coupled cluster CCSD(T) calculations. We first showed that the UCl 62− ion is

Figure 4. Overlay of Raman (bottom) and infrared (top) spectra; 1 (blue), 2 (red). 9775

DOI: 10.1021/acs.inorgchem.7b01293 Inorg. Chem. 2017, 56, 9772−9780

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Inorganic Chemistry surprisingly, peaks attributed to vibrational modes of H2O are present in 2, but not 1, including a band at 538 cm−1. Computational studies predicted this stretch to be a H2O bend at 539 cm−1. Additional H2O bends in 2 were predicted at 1597 and 1615 cm−1, observed experimentally at 1602 and 1616 cm−1 respectively. Most important is the signature symmetric and/or asymmetric stretches of water present in 2, centered at 3424 cm−1 (see Supporting Information for full spectrum). The most intense H2O symmetric and asymmetric stretches were predicted to be at 3780 and 3855 cm−1 respectively for the isolated molecule. This clear difference between the calculated and observed vibrations can be attributed to the extensive network of hydrogen bonding present in the solid, just as observed in bulk water.60−64 Distinct Raman peaks were more easily identifiable in comparison to IR stretches. For 1, the Raman U−Cl stretches are calculated to be in the range of 210−275 cm−1 with the most intense symmetric stretch band at 272 cm−1. Experimentally, U−Cl stretches were observed between 228−305 cm−1 with the most intense Raman band at 305 cm−1. For the isolated molecule observed in 2, the unscaled H2O O−H stretches are predicted to be in the range of 3700−3900 cm−1. Experimentally, the H2O O−H stretches are not observed in this range, although there is a broad band centered at 3350 cm−1 that may be attributed to the H2O O−H vibrations whose features are shifted due to the extensive hydrogen bonding network in the solid63−67 (see Supporting Information for full spectrum). Computational results predicted the four H2O H− O−H bends in the region of 1595−1630 cm−1. The peaks at 1606 and 1630 cm−1 in 2 could be associated with this bend, but these peaks are also observed in 1 and are also consistent with pyridinium ν(C = N) stretches that are typically observed in the range of 1610−1640 cm−1.68 The H2O group inversion motions are calculated in the range of 400−600 cm−1 with weak Raman intensities, which are observed experimentally at 393 and 609 cm−1. The U−O(H2) and U−Cl stretches are all predicted to be less than 310 cm−1. The most intense Raman band is predicted to be a mixed U−O(H2) plus U−Cl stretch at 308 cm−1; the most intense experimentally observed Raman peak is at 257 cm−1. The remaining mixed U−O(H2) and U− Cl stretches are calculated to be in the region of 220−290 cm−1 and have low Raman intensities; experimentally the range is larger extending from 185−353 cm−1. Overall, the computationally predicted ranges are in agreement with U-ligand vibrations seen experimentally. Earlier studies have also defined the range between 50 and 350 cm−1 as UCl62− lattice and internal vibrations, consistent with experimental results reported in this work.58,69−72 Previous reports of similar AnCl62− compounds chargedbalanced by pyridinium derivatives have indicated the difficulty in experimentally identifying metal−ligand peaks due to the high relative intensity of the pyridinium stretches.73 Similarly, the most intense Raman bands in the spectra of 1 and 2 are associated with pyridinium at 1008 cm−1, 1027 cm−1, and 3090 cm−1, which are consistent with literature values for pyridine in 10% HCl.68 Peak-shifts indicate pyridinium’s involvement as a Brönsted acid within the crystal structure.68 UV−vis-NIR Absorption Spectroscopy. The optical absorption spectra of 1 and 2 contain peaks that correspond to characteristic U(IV) f−f transitions and exhibit features similar to those that have been previously reported for tetravalent uranium.74−78 As shown in Figure 5, absorption bands can be attributed to transitions from the 3H4 ground

Figure 5. Overlay of UV−vis-NIR spectra with corresponding transitions; 1 (blue), 2 (red).

state to a variety of excited states. Compound 1 exhibits transitions at 405, 452, 484, 528 nm (3H4 → 3P2/1I6), 596, 634, 660, 670 nm (3H4 → 3P1/3P0/1D2/1G4), 774 nm (3H4 → 3H6), 1057 and 1085 nm (3H4 → 3F4/3F3). Compound 2 has noticeably fewer absorption bands in comparison to that of 1. The peaks at 441, 499, 562, 611, 636, 672, the broad band centered at 875 and 923, and 1099 nm correspond to transitions to 3P2, 1I6, 3P1, 3P0, 1G4, 1D2, 3H6, and 3F3/3F4 respectively. Differences in the absorption spectra of 1 and 2 (Figure 5) can be attributed to differences in coordination number and crystal field effects that are known to give rise to considerable differences in the spectra of U(IV) compounds.79 Such is the case reported here wherein the U(IV) metal centers in 1 are octahedrally coordinated and exhibit transitions similar to previously reported [UCl6]2− ions.78 Conversely in 2 the U(IV) metal centers are eight coordinate and yield absorption bands analogous to previously reported uranium compounds that are coordinated to eight or more ligands.74,75 Magnetic Properties. The magnetic susceptibility for 1 and 2 are displayed in Figure 6 as 1/χ and effective magnetic moment μeff = χT as a function of temperature, where 1 and 2 are represented by the red triangles and black circles, respectively. For 2, the linear temperature dependence of the inverse magnetic susceptibility is typical for Curie−Weiss

Figure 6. Effective moment and inverse magnetic susceptibility of 1 and 2 in an applied field of 5000 Oe. Inset: Isothermal magnetization at T = 1.8 K. 9776

DOI: 10.1021/acs.inorgchem.7b01293 Inorg. Chem. 2017, 56, 9772−9780

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Inorganic Chemistry behavior and was fit to the Curie−Weiss equation 1/χ = C/(T − ΘCW) in the temperature range of 300 K down to 100 K (Supporting Information, Figure S12). From the fit we obtain a Curie−Weiss temperature ΘCW = −384 K and an effective magnetic moment μeff = 3.8 μB, experimentally close to the expected value of 3.58 μB calculated from Russell−Sanders coupling for a 3H4 ground state and in good agreement with previously reported investigations.80−86 However, the ground state of 2 appears to be nonmagnetic, as seen in μeff decreasing with decreasing temperature. This may be due to crystalline electric field (CEF) splitting as has been observed in Pr and U containing materials.87,88 Unlike sample 2, the magnetization of 1 behaves as a Pauli paramagnet with mostly temperature independent behavior. For 1, the modified Curie−Weiss fit was performed using the equation 1/(χ − χ0) = C/(T − ΘCW) and found a significantly reduced μeff = 0.8 μB/U(IV), much lower than the expected 3.58 μB/U(IV) for magnetic U.

Moreover, isolation of 2 was found to occur in the presence of L-serine, which suggested that the carboxylic acid might have a role in stabilizing and/or precipitating this novel compound. Our computational studies indicated a distinct favorability of the UCl62− complex; the U(H2O)4Cl4 structural unit was found to release water to form UCl62− with an aqueous Gibbs free energy of ΔG(aq) = −10 kcal/mol at the B3LYP level and ΔG(aq) = −1 kcal/mol at the CCSD(T) level. Despite the energetic favorability of UCl62−, U(H2O)4Cl4 is still observed. It should be noted that at increased concentrations of pyridinium, absent L-serine, 2 has been observed. Yet whereas the synthesis with L-serine reproducibly yields 2, compound 2 has only been observed on one occasion in those systems absent a carboxylic acid. These results may collectively point to the importance of H-bonding donors on the stabilization of the U(H2O)4Cl4 unit observed in 2. In other words, the supramolecular interactions present in 2 may provide an extended network capable of stabilizing and/or crystallizing the unprecedented U(IV)-aquochloro complex. However, it is interesting to note that that efforts to prepare 2 using other the amino acids including Lalanine, L-cysteine, L-proline, L-lysine, and L-threonine (a close structural analogue of L-serine), resulted only in the formation of 1. The role that these interactions, and particularly L-serine, play in stabilizing and/or crystallizing the U(H2 O) 4 Cl4 structural unit and more generally affecting U(IV) solid state speciation remains unclear; however, given the novelty of the structural unit isolated in this work it warrants further investigations.

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DISCUSSION Despite the prevalence of An(IV)-chlorides built from AnCl62−, a few solid state examples of An-aquo-halide compounds such as that reported here have been identified in the solid state. Thorium(IV) has been shown to form [Th(H2O)10]Br4 and [Th(μ2-OH)2Cl2(H2O)12]Cl4·2H2O from acidic HBr and HCl solutions, respectively.89,90 It is worth highlighting that Kiplinger et al. isolated a Th(H2O)4Cl4 species by refluxing Th(NO3)4·(H2O)5 in 12 M HCl, followed by recrystallization in tetrahydrofuran or 1,4-dioxane.91 By comparison, the U(IV) complex reported here was isolated in ∼5 M acid at room temperature. Further, the metal bound chloride ions and water molecules in 2 adopt a different arrangement in that four chloride ligands are moved just out of a plane with three water molecules below and one above the plane. This is in contrast to the arrangement observed for Th(H2O)4Cl4 that exhibits a distorted square antiprism coordination geometry with two planes, each formed by two water molecules and two chloride ions, offset from one another. Related Pu(III) compounds including cis-Cs[PuIIICl4(H2O)4] and transCs5[PuIIICl4(H2O)4]Cl4·2H2O have also been reported.92 U(IV)-aquo-chloro species are notably absent from descriptions of U(IV)-chloride solid state structural chemistry. However, related species have been observed using extended X-ray absorption fine structure spectroscopy (EXAFS) in aqueous U(IV) chloride solutions.93 In solutions with 3−9 M [Cl−], it was shown that with increasing chloride concentration, chloride ions displaced water molecules from the inner coordination sphere of the metal to form three U(IV)-aquochloro species including U(H2O)8Cl3+, U(H2O)6−7Cl22+, and U(H2O)5Cl3+. Consistent with the latter two solution species proposed by Hennig et al., the U(IV) center in 2 is eight coordinate. However, the neutral U(H2O)4Cl4 complex isolated in this work is compositionally different and has not been proposed previously. To the best of our knowledge, this species has yet to be reported in descriptions of U(IV) aqueous chemistry and may have significant implications for our understanding of U(IV) speciation under acidic, aqueous conditions, particularly for those systems in which H-bond donor “spectators” are present. Given the extensive hydrogen bonding and π−π stacking present in both phases of 1 and 2, it is hypothesized that supramolecular or outer coordination sphere interactions may facilitate the crystallization of these species, particularly the U(IV)-aquo-chloro complex observed in 2, and allow for its subsequent observation in the solid state.

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CONCLUSION The synthesis, structure, and optical and magnetic behavior of two U(IV) compounds isolated from acidic aqueous solution are reported. U(H2O)4Cl4·(HPy·Cl)2 represents a novel U(IV)-aquo-chloro complex that, to the best of our knowledge, is the first observation of a U(IV)-aquo-chloro complex in the solid state. Computational studies confirmed the energetic favorability of the UCl62− unit, as observed here in 1 at both 100 and 296 K. The predicted energetics of the reaction of U(H2O)4Cl4 to form the UCl6 dianion coupled with no previous reports of a U(IV)-aquo-chloro complex may point toward the importance of hydrogen bonding and thus supramolecular interactions in the stabilization of the U(H2O)4Cl4 structural unit. These results may lend further evidence to the importance of outer coordination sphere interactions on the relative stability and formation of aqueous metal ion species. However, the role that outer coordination sphere interactions play in stabilizing this structural unitor more generally directing speciationis still unclear. Yet, given the importance of the identity of the metal ion in understanding aqueous behavior, these results warrant further investigation into the stability of U(IV)-aquo-halides and related species.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01293. Powder X-ray diffraction patterns; experimental and theoretical Raman and IR spectra; figures of different calculated isomers; relative energies for isomers of U(H2O)4Cl4; calculated frequencies (cm−1), IR inten9777

DOI: 10.1021/acs.inorgchem.7b01293 Inorg. Chem. 2017, 56, 9772−9780

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Inorganic Chemistry sities (km/mol), and Raman activity (Å4/amu); reaction energies (kcal/mol); and optimized Cartesian coordinates in Å for the lowest energy isomers (PDF)

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Accession Codes

CCDC 1523702−1523704 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

David A. Dixon: 0000-0002-9492-0056 Karah E. Knope: 0000-0002-5690-715X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Clare Boothe Luce Foundation (K.E.K., J.N.W.), the Georgetown Espenscheid Fellowship (J.N.W.), and the National Science Foundation under grants NSF CHE-1429079 and NSF CHE-1337975. The authors would like to thank Dr. Faye J. Rubinson for valuable assistance with Raman data collection. The computational work (D.A.D.) was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Heavy Element Chemistry Program through a subcontract from Argonne National Laboratory. D.A.D. also thanks the Robert Ramsay Fund at the University of Alabama. R.E.B. and K.H. were supported as part of the Center for Actinide Science and Technology (CAST), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award Number DE-SC0016568.

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DOI: 10.1021/acs.inorgchem.7b01293 Inorg. Chem. 2017, 56, 9772−9780

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DOI: 10.1021/acs.inorgchem.7b01293 Inorg. Chem. 2017, 56, 9772−9780