A Pseudotetrahedral Uranium(V) Complex - ACS Publications

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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A Pseudotetrahedral Uranium(V) Complex Aaron M. Tondreau,† Thomas J. Duignan,†,§ Benjamin W. Stein,† Valerie E. Fleischauer,‡ Jochen Autschbach,§ Enrique R. Batista,*,† James M. Boncella,*,† Maryline G. Ferrier,† Stosh A. Kozimor,*,† Veronika Mocko,† Michael L. Neidig,‡ Samantha K. Cary,† and Ping Yang*,† †

Los Alamos National Laboratory, MS J514, Los Alamos, New Mexico 87545, United States Department of Chemistry, University of Rochester, Rochester, New York 14627, United States § Department of Chemistry, University at Buffalo, State University of New York, Buffalo, New York 14260-3000, United States Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 6, 2018 at 02:09:47 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: A series of uranium amides were synthesized from N,N,N-cyclohexyl(trimethylsilyl)lithium amide [Li][N(TMS)Cy] and uranium tetrachloride to give U(NCySiMe3)x(Cl)4−x, where x = 2, 3, or 4. The diamide was isolated as a bimetallic, bridging lithium chloride adduct ((UCl2(NCyTMS)2)2-LiCl(THF)2), and the tris(amide) was isolated as the lithium chloride adduct of the monometallic species (UCl(NCyTMS)3-LiCl(THF)2). The tetraamide complex was isolated as the four-coordinate pseudotetrahedron. Cyclic voltammetry revealed an easily accessible reversible oxidation wave, and upon chemical oxidation, the UV amido cation was isolated in near-quantitative yields. The synthesis of this family of compounds allows a direct comparison of the electronic structure and properties of isostructural UIV and UV tetraamide complexes. Spectroscopic investigations consisting of UV−vis, NIR, MCD, EPR, and U L3-edge XANES, along with density functional and wave function calculations, of the fourcoordinate UIV and UV complexes have been used to understand the electronic structure of these pseudotetrahedral complexes.

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INTRODUCTION Homoleptic uranium tetraamides have been known since the 1950s1 and were used early on in situ as precursors to other complexes.2 Edelstein was able to characterize these early complexes, later reporting the first monometallic, pseudotetrahedral uranium tetrakis(diphenylamide).3 Several groups have reported recent examples of similar amido compounds (Figure 1). Arnold reported a four-coordinate uranium complex supported by a pair of chelating ferrocenyl diamides,4 a structural motif further investigated by Diaconescu.5 The groups of Arnold6 and Schelter7 have recently reported tetrahedral uranium amido complexes of bis(silyl)amide ligands. The Schelter group has also contributed a variety of homoleptic fluorinated(diaryl)amide UIV complexes,8 as well as a variety of silylated amide complexes.9 Uranium complexes in the +5 oxidation state have historically been more difficult to characterize than those in the +4 or +6 state,10 an observation that arises from the fact that aqueous UVO2+ complexes are known to disproportionate to UIV and UVIO22+ species. Progress in non-aqueous uranium synthesis has seen the number of UV species increase considerably. 11 Many reported UV compounds assume coordination numbers of five or higher or are supported by Cp or COT ligands and their derivatives (Cp = cyclopentadienyl; COT = cyclooctatetraenyl).12 Isolation of fourcoordinate UV compounds has been known since the 1970s, © XXXX American Chemical Society

with complexes bearing multiple U−E bonds, (N(SiMe3)2)3U(E) (E = O or NR).13 Cummins has also reported fourcoordinate uranium complexes derived from a nitride moiety.14 Herein, we report a series of uranium amides composed of the bis-, tris-, and tetraamide species synthesized using cyclohexyl(trimethylsilyl)lithium amide and uranium tetrachloride. The UIV tetra-amido complex was isolated as the base-free pseudotetrahedral complex. Cyclic voltammetry (Figure S5) revealed a reversible oxidation wave at mild potentials, and upon chemical oxidation, the cationic UV tetra(amide) was isolated in near-quantitative yields. A spectroscopic investigation into this complex revealed an EPR-active, S = 1/2 ground state in which the electron is localized in an f orbital. This report provides further characterization of the historically significant UIV tetraamide class of compounds and allows a direct comparison of the electronic structures and properties of the isostructural, pseudotetrahedral UIV and UV tetraamide complexes.

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RESULTS AND DISCUSSION Synthesis and Characterization of Uranium Amide Complexes. The synthesis of the desired U(Cl)4−x(NCyTMS)x complexes was accomplished with the Received: January 30, 2018

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DOI: 10.1021/acs.inorgchem.7b03139 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

determine the structure of 1 (Figure 2a). Both uranium centers in the solid-state structure are six-coordinate and are bridged through three chlorine atoms.

Figure 1. Previously synthesized four-coordinate monomeric UIV amido complexes.

addition of x = 2, 3, or 4 equiv of lithium cyclohexyl(trimethylsilyl)amide (NCyTMS) to a solution of uranium tetrachloride (UCl4) in THF. Addition of 2 equiv of LiNCyTMS in THF generated a bridging, bimetallic lithium chloride adduct ((UCl 2 (NCyTMS) 2 ) 2 -LiCl(THF) 2 ) [1 (Scheme 1a)]. Compound 1 is furnished as an emerald green crystalline material in good yields (∼80%). The 1H NMR spectrum of 1 is indicative of a paramagnetic complex, composed of broad singlet resonances over a 60 ppm range. A solid-state structure was obtained to unambiguously Figure 2. Solid-state structures of (a) 1 and (b) 2 are presented with 50% probability ellipsoids. All hydrogen atoms and the carbon atoms from two lithium-coordinated THF molecules have been omitted from each structure for the sake of clarity.

Scheme 1. Outlined Syntheses of Complexes (a) 1, (b) 2, and (c) 3 with Isolated Yields Given

The addition of 3 equiv of Li-NCyTMS led to the isolation of the tris(amide) uranium(IV) chloride. The UCl(NCyTMS)3-LiCl(THF)2 complex was obtained as the monomeric lithium chloride adduct [2 (Scheme 1b)]. Compound 2 was isolated as a pale green crystalline material in good yield. The 1H NMR spectrum of 2 indicated a paramagnetic complex, consisting of broad singlet resonances over a 50 ppm range. The solid-state structure was obtained (Figure 2b) and shows that 2 is the monomeric LiCl adduct. The uranium adopts a distorted trigonal bipyramidal structure; the axial ligands consist of a chlorine atom and an [NCyTMS] ligand, while the Cl(1)−U(1)−N(2) angle deviates from linearity [170(37)°]. The equatorial plane consists of the two remaining amides and the second chlorine atom. The lithium is bound between the two chlorine atoms. The addition of 4 equiv of Li-NCyTMS to UCl4 in THF led to the isolation of an off-white tacky product identified as U(NCyTMS)4 [3 (Scheme 1c)]. The 1H NMR spectrum of 3 consisted of broad peaks over a 60 ppm range as expected for a B


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Inorganic Chemistry paramagnetic metal complex. A single-crystal X-ray diffraction structure was obtained and unambiguously demonstrates that 3 is the pseudotetrahedral amido (Figure 3). Compound 3 represents one of a handful of crystallographically characterized four-coordinate monomeric uranium complexes.

Scheme 2. Synthesis of 4 from 3 with Isolated Yields Given

crystals were obtained from both THF/hexane and fluorobenzene/hexane solutions. In both cases, the solid-state structure contained four-coordinate uranium centers with no evidence of THF coordination. Complex 4 crystallized with two molecules in the asymmetric unit cell in both cases (see the Supporting Information for full structural information). Complex 4 maintains the pseudotetrahedral geometry of the uranium center. The metrical parameters of 3 and 4 are presented for comparison (Table 1). Upon oxidation, the Table 1. Summary of Geometrical Data for Complexes 3 and 4a

Figure 3. Solid-state structure of 3 with 50% probability ellipsoids. Hydrogen atoms have been omitted for the sake of clarity.

4b

3

The oxidation of compound 3 was investigated for several reasons. An f1 system is a desirable starting point for a spectroscopic investigation because of its relatively simple electronic structure.11c,15 The potential to thoroughly investigate a four-coordinate, pseudotetrahedral UV metal center could provide fundamental insight into the ground-state electronic structure of these complexes. Although pseudotetrahedral tetraamides in the pentavalent oxidation state have been generated using cyclic voltammetry, there have been no reports of an isolable product.7 Complex 3 removes any ambiguity associated with the presence of redox-active ligands as the amides are strictly “innocent” donor ligands. The electrochemical data obtained for 3 revealed a reversible wave at −0.37 V versus ferrocene/ferrocenium (Fc/Fc+) in the CV (see the Supporting Information). Neither the UV/VI nor the UIII/IV redox couples were found within the solvent window [α,α,α-tri(fluoro)toluene]. This is in contrast to U[N(TMS)2]4 in which the UIV/III couple was observed at −2.05 V versus Fc/ Fc+. The UV/IV couple of U[N(TMS)2]4 was found at 0.49 V versus Fc/Fc+ in acetonitrile, indicating a UIV complex that is much more stable than 3, although a direct comparison of the potentials in two different solvents renders any differences debatable. The reversibility of this wave at this accessible potential suggested that a ferrocenium salt would be adequate for generating the chemically oxidized species. The addition of 1 equiv of [Cp2Fe][BArF24] {Cp2Fe = ferrocene; BArF24 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate} to a fluorobenzene solution of 3 resulted in an immediate change in color from light yellow to a deep purple/ black. The product, [U(NCyTMS)4][BArF24] (4), was isolated as a dark purple crystalline material by precipitation from the mother liquor with hexane in near-quantitative yields (Scheme 2). The low solubility prevented a 1H NMR spectrum from being recorded in a noncoordinating solvent such as toluene or benzene. THF readily dissolved 4, and the 1H NMR spectrum obtained in THF-d8 revealed broad peaks over a relatively small 20 ppm range. The aryl resonances attributed to the BArF24 counterion appeared at 7.80 and 7.60 ppm. To ensure no coordination of THF to the uranium center, X-ray quality single

3−4

bond

Exp.

ΔDFT

Exp.

ΔDFT

Exp.

DFT

N−U N−C N−Si

2.258 1.479 1.728

−0.022 −0.002 0.035

2.189 1.488 1.782

−0.018 0.009 0.017

0.069 −0.009 −0.054

0.072 −0.002 −0.036

a All reported values in angstroms. ΔDFT is the difference between the experimental bond length and the optimized geometry. The last two columns are the bond length differences between 3 and 4. bThere are two unique molecules in the crystal structure, and the experimental values are the average of both.

uranium−nitrogen bond distances contract roughly 0.069 Å, indicating a stronger uranium−nitrogen interaction. This is consistent with a stronger uranium−nitrogen bond that arises from the better energy match between the UV and the nitrogen ligand. Density functional theory (DFT) calculations were performed on 3 and 4 to compare the optimized and experimental solid-state structures. These calculations reproduced the bond lengths and angles accurately. For complex 3, the optimized U−N bond lengths were 0.022 Å (average) shorter than the experimentally observed values. Accordingly, the N−Si bond lengths were overestimated to be 0.035 Å longer than the observed values. The optimized U−N bond lengths of 4 were 0.018 Å shorter, and the N−Si bond lengths were overestimated by 0.017 Å relative to the solid-state values. In the solid state, the U−N bond contracts by 0.069 Å upon oxidation to 4, with the theoretical contraction of 0.072 Å being in excellent agreement. The calculated orbital manifold (Figure 4) illustrates several characteristics of these complexes. The U−N bonding occupied ligand MOs contain substantial metal−ligand σ and π character, as both 3 and 4 have Nalewajski−Mrozek bond orders16 of >1 (1.36 and 1.54, respectively). The uranium spin populations for 3 and 4 are 2.20 and 1.28, respectively, indicating that the additional electron is indeed localized on the uranium center. The spin populations on nitrogen are −0.05 for 3 and −0.06 for 4. A natural bond orbital (NBO) analysis was performed to further characterize the U−N interactions in 3 and 4.17 The C


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Inorganic Chemistry

Figure 4. Molecular orbital (MO) diagram of complexes 3 and 4 with selected inset orbital iso-surfaces (value = ±0.03) from density functional theory calculations. Uranium is colored light blue, nitrogen dark blue, silicon beige, and carbon gray; hydrogens were omitted. The orbital energies are colored according to the legend on the right. The U 5f manifold is localized and isolated, whereas the U 6d manifold undergoes significant mixing in both occupied and virtual orbitals.

NBO algorithm identifies two U−N NBOs (for each N) with occupanices of >1.90. One NBO is predominantly N sp2 in nature with a small admixture of hybridized U 6d and 5f character. The second NBO contains primarily N 2p character, again mixing with a hybrid of U 6d and 5f character. While the nature of the NBOs is similar between 3 and 4, the contributions from U in said NBOs are different. In 3, the N sp2 NBO contains ∼8% U and the N p NBO contains ∼10% U character. In 4, these contributions increase to ∼11% U and 18% U, respectively, reflecting the strengthened U−N interaction observed in 4. In the ground states, the orbitals with unpaired electrons are mainly f orbital localized, with both 3 and 4 containing only 1− 5% d orbital mixing. Of note, too, are the virtual orbitals of significant 6d character lying above the unoccupied 5f manifold. There exists a large density of states above the LUMO; the density of states above the 5f manifold becomes very large, quickly approaching a continuum of diffuse virtual orbitals. These computed characteristics are observed in the spectroscopy that will be discussed presently, which lends validity to the accuracy of the calculations. EPR Spectroscopy. A UV amido with a noncoordinating counterion supported by a bis(1,1′-(di(trialkyl-silyl)amideferrocenyl)) ligand scaffold has previously been reported by Diaconescu.5a However, the electronic structure was determined to consist of a UIV metal center with FeII/FeIII centers in rapid exchange on the ligands. This manifested in the EPR spectrum as a rather high giso value of >2.13, which is more consistent with an iron-centered radical rather than an f1 radical on uranium. These results contrast with those obtained for complex 4, which does not have redox activity observed in the ligand and affords an instance of an unambiguous lowcoordinate UV EPR spectrum. The EPR spectrum of complex 4 was obtained in a THF glass at 10 K (Figure 5). The X-band EPR spectrum shows a rhombic system with g1 = 2.99, g2 = 1.46, and g3 = 1.07 giving an average giso value of 1.84. This is consistent with an S = 1/2 system, shifted from the spin only value of 2.0023 due to spin− orbit coupling of the single unpaired electron. A low giso value (giso < ge) has been observed in a number of other UV systems.18 Theoretical magnetic data for UV were obtained with a multireference wave function calculation and are reported along with the experimental data and the principal axis system of the

Figure 5. Experimental (blue) EPR spectrum and simulation (red) of complex 4 obtained as a 1:1 THF:2-(methyl)THF glass at 10 K. Cavity impurities result in the small signals observed at ∼1600 and 3300 G.

spin−orbit ground state used in the computation (see the Supporting Information). A slight overestimation along the primary axis and underestimation along the other two axes lead to an isotropic g factor in good agreement with the experimental measurement (Table 2). The electronic structure responsible for the theoretical g factors is described in Figure 6. Table 2. g1, g2, g3, and Isotropic g Factor (giso = gave) Values for the Rhombic EPR Spectrum of 4, Along with the CASPT2(1,7)-Computed Values giso g1 g2 g3

experimental

theoretical

1.84 2.99 1.46 1.07

1.81 3.22 1.33 0.89

Electronic Spectroscopy. The ultraviolet−visible (UV− vis), near-infrared (near-IR), and magnetic circular dichroism (MCD) spectra of 3 and 4 were obtained to further elucidate the nature of the ground-state electronic structures of these two complexes. Compounds 3 and 4 represent a unique opportunity to study isostructural, low-coordinate complexes of +4 and +5 uranium. As compound 3 is colorless to pale yellow and 4 appears blue/black, there should be a striking D


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Inorganic Chemistry

Figure 6. Iso-surfaces (±0.03) of spin−orbit natural orbitals in 4 of appreciable occupation (>0.001) in the CASPT2(1,7) calculation.19 Atoms are colored the same as in Figure 4. Non-integer occupations are indicative of the multiconfigurational character of the complex. The active orbitals consist of roughly equal occupations of 5fφ orbitals with minor occupation of the 5fδ orbital due to spin−orbit coupling. The low symmetry distorts the orbital shapes slightly. The other three orbitals in the active space are unoccupied. Orbital populations sum to the one unpaired 5f electron in 4.

difference in the absorption spectrum of the two. Compound 4 contains broad absorptions between 11000 and 40000 cm−1, while compound 3 displays absorption in the higher-energy region of the spectrum (Figure 7). A comparison is drawn between the experimental UV−vis spectrum and the calculated excitations using the transition dipole method. While the absolute energies are lower than those observed experimentally, the relative onset of the UV−vis absorption band of 4 is red-shifted compared to that of 3. Attempts to simulate the NIR spectra were less successful because of the f−f nature of the transitions in this energy range. The near-IR spectrum of 4 contains a transition that is reproduced in the MCD spectrum at ∼6500 cm−1. These transitions have been observed in the NIR spectrum of several UV systems, at roughly the same energy and intensity. They have been assigned to f−f transitions. The broader features, observed at a slightly higher energy, have been assigned as nitrogen lone pair to uranium transitions. From the calculations, the ligand-to-metal electron donation increases the bond order and gives rise to these broader features of the NIR spectrum. These features are present in the NIR spectrum for both UIV and UV. Significantly, these transitions are present in the MCD spectrum (Figure 8), demonstrating the involvement of the metal center. U L3-Edge XANES. Consistent with previous U L3-edge measurements,20 the background-subtracted and normalized spectra from 3 contained a single peak near 17175 eV that was superimposed on a steplike absorption threshold (Figure 9). Because the absorption energy for this peak is characteristic of the effective nuclear charge on uranium,18 it provides a metric for evaluating the uranium oxidation state. Hence, the inflection point was determined graphically, where the second derivative of the data equaled zero. For 3, this value was at 17170.0 eV (in

Figure 7. (a) UV−vis and (b) NIR spectra for complexes 3 and 4 obtained in THF at 20 °C. (c) Simulated UV−vis spectra for 3 and 4.

situ calibration to an yttrium foil at 17038.4 eV). To better compare our experimental data with previous U L3-edge XANES measurements,18a,b the data from 3 were also evaluated against a convenient UVIO22+ external standard, specifically Cs2UO2Cl4. The inflection point for 3 was 2.5 eV lower in energy than that of Cs2UO2Cl4, which was consistent with uranium in the +4 oxidation state.18a,b Obtaining U L3-edge XANES spectra from complex 4 was more challenging than obtaining those of 3, because of the instability associated with the complex. For example, we measured XANES spectra from 4 multiple times over the course of three separate experimental campaigns. Average spectra from each campaign showed inflection points that had energies higher than that of 3, as expected for 4 having a UV center. Additionally, the spectra were quite stable and showed no evidence of sample decomposition because of radiation E


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Inorganic Chemistry

17171.3 eV, is most representative of 4 and accordingly was selected for display in Figure 9. This spectrum suggests 4 is best described as having uranium in the +5 oxidation state. We are confident in this assignment, especially when the XANES data are evaluated within the context of the UV−vis and EPR measurements described above. Additional confidence in the interpretations of the U L3-edge XANES data was obtained by simulating the U L3-edge XANES spectra using DFT calculations (Figure 10). To assist

Figure 8. NIR MCD spectrum of 4 obtained at 5 K and −1.5, 1.5, 3.5, and 7 T in a 3:1 THF:2-(methyl)THF glass.

Figure 10. Experimental (top) and theoretical (bottom) U L3-edge XANES spectra. Theoretical spectra were shifted −797 eV to facilitate comparison.

comparisons of the calculated spectra with that obtained experimentally, the computational results were shifted by a constant −797 eV and the oscillator strengths were Gaussianbroadened to be more representative of the experimental peak shapes. The differences in the energies of the onset of the spectral peaks are 2.11 eV experimentally and 1.22 eV theoretically, determined by the positions of the inflection points in the spectra. However, because of the continuum of virtual states within this energy window, a clear explanation of the broadening of the UV spectrum could not be resolved.

Figure 9. U L3-edge XANES data for 3, 4, and the Cs2UO2Cl4 external standard (top) and second derivatives of the XANES spectra (bottom).

damage during data acquisition. However, between each experimental campaign, the inflection points varied substantially, −0.7, −1.5, and −2.0 eV relative to Cs2UO2Cl4. The largest inflection point difference from Cs2UO2Cl4 (−2.0 eV) was reminiscent of uranium in the +4 oxidation state. The smallest difference (−0.7 eV) indicated uranium was in the +5 oxidation state. We speculated that the low-energy inflection point value at 17170.9 eV (2.0 eV lower than that of Cs2UO2Cl4) resulted from decomposition of UV to UVI during shipping of the samples from our laboratory to the synchrotron. Support for this hypothesis was garnered from an experiment in which 4 was purposefully decomposed. For example, collecting a U L3edge XANES measurement on a sample of 4 that had been exposed to air for several hours generated a spectrum with an inflection point that was 1.2 eV lower than that of the Cs2UO2Cl4 standard. Comparing this result with our three measurements on unadulterated samples of 4 indicated that one measurement was taken on a sample of 4 that contained substantial UV decomposition (−2.0 eV from Cs2UO2Cl4), one sample had moderate amounts of UV decomposition (−1.5 eV from Cs 2 UO 2 Cl 4 ), and one had small amounts of U V decomposition (−0.7 eV from Cs2UO2Cl4). The latter, at

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SUMMARY AND CONCLUSIONS Herein, we present the synthesis of a series of uranium amide complexes. The stepwise addition of Li-NCyTMS to UCl4 resulted in the formation of U(NCySiMe3)x(Cl)4−x, where x = 2, 3, or 4. This work was a unique opportunity to make a direct comparison of the electronic properties of the isostructural UIV and UV tetraamide complexes. Tetraamide 3 was investigated with UV−vis, NIR, MCD, U L3-edge XANES, and theoretical methods. The series of results were used to assign the electronic structure of four-coordinate 3 as an f2 system. The oxidation of 3 with [Cp2Fe][BArF24] gave 4 in high yields. Complex 4 represents a rare low-coordinate, cationic UV species. Spectroscopy of complex 4 was performed, and the results were correlated to computation results. The rhombic EPR spectrum was reproduced well with a CASPT2(1,7) calculation, and the results allow for the assignment of 4 as an f1 system. These results help to describe more fully a historically F


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Inorganic Chemistry

mirrors. A single energy was selected from the white beam with a liquid N2-cooled double-crystal monochromator utilizing Si[220] (φ = 0°) crystals and passed through a Rh harmonic rejection mirror. All spectra were measured in transmission mode. Samples for XANES measurements were prepared by diluting an appropriate amount of analyte in boron nitride (BN, dried under vacuum at 150 °C) to give an edge jump of approximately 1. Samples were pressed into an eight-slot aluminum holder, sealed with kapton tape, and then loaded into a secondary container to provide radiological containment. Samples were heat-sealed into mylar bags, and the bags were not opened until immediately prior to measurements. Samples were removed from the mylar bag and rapidly loaded into a liquid nitrogen cryostat for measurements. XANES spectra were measured in transmission mode utilizing nitrogen-filled ion chambers positioned above and downstream from the cryostat. Spectra were calibrated versus in situ measurement of an yttrium calibration foil (17038.4 eV). Data were reduced using the ATHENA software package.27 Computational Details. Kohn−Sham density functional theory (KS-DFT) calculations were performed using a scalar relativistic zeroth-order regular approximation (ZORA) Hamiltonian28 and the Perdew, Burke, and Ernzerhof (PBE)29 functional as implemented in the Amsterdam Density Functional (ADF2016) package.30 Geometry optimizations employed TZP Slater-type basis sets with small cores.31 Simulated U L3-edge XANES data were computed using all-electron TZP basis sets and the transition dipole method. Larger TZ2P basis sets31 as well inclusion of a continuum solvation model32 had little effect on the properties studied. Relativistic ab initio wave function calculations were performed for 4 with a developer’s version of the Molcas program version 8.19 These calculations used the second order Douglas−Kroll−Hess scalar relativistic Hamiltonian33 and ANO-RCC Gaussian-type basis sets34 contracted to TZP quality (DZ for H) (U = 26s23p17d13f5g3h/ 9s8p6d4f2g, N = 14s9p4d3f2g/4s3p2d, Si = 17s12p5d4f2g/4s3p, C = 14s9p4d3f2g/3s2p, and H = 8s4p3d1f/2s). A state-averaged Complete Active Space Self Consistent Field (CASSCF) wave function35 was constructed with an active space of one unpaired f electron in the seven 5f orbitals [CAS(1,7)]. Dynamic correlation effects were included via second-order perturbation theory (CASPT2). Spin− orbit (SO) coupling was included through state interaction between the spin-free states of the CASPT2 wave function using the Restricted Active Space State Interaction (RASSI) program.36 Magnetic data were calculated according to ref 37. Natural orbitals for the electron density of the spin−orbit wave functions were generated according to ref 19. Caution! Depleted uranium (primary isotope 238U) is a weak α-emitter (4.197 MeV) with a half-life of 4.47 × 109 years; only persons trained to handle such material should perform work and only in an adequately prepared laboratory setting. Synthesis of Complexes. Synthesis of {Bis(N,N-cyclohexyl(trimethylsilyl)amide)dichlorouraniumIV}2 LiCl-(THF)2 (((UCl2(NCyTMS)2)2-LiCl(THF)2), 1). UCl4 (0.380 g, 1.00 mmol) was added to a 20 mL scintillation vial along with 10 mL of THF while being stirred. To this stirring solution was added LiN(TMS)Cy (0.355 g, 2.00 mmol) as a solid at room temperature over the course of 10 min. The reaction mixture was stirred for 30 min, and the volatiles were removed. The residue was extracted with hexane and filtered on a glass frit. A green solid was left on the frit, and immediately a bright green precipitate began to form from the filtrate. The filtrate was cooled to −30 °C overnight, and the bright green precipitate was collected via decanting and washing with 5 mL of cold N-hexane. Volatiles were removed under reduced pressure to yield 0.264 g of a bright green microcrystalline solid identified as 1. The green material from the frit was washed through with a 50:50 hexane/ether mixture, and the green solution was cooled to −30 °C overnight. The green solids were collected and dried on a glass frit yielding 0.270 g of a bright green microcrystalline solid, bringing the total yield to 0.534 g (72%). Analysis calcd for C44H96Cl5LiN4O2Si4U2: C, 35.57%; H, 6.51%; N, 3.77%. Found: C, 26.27%; H, 5.39%; N, 3.51%. 1H NMR (C6D6): δ 21.04 (255 Hz), 15.85 (365 Hz), 14.27 (950 Hz), 4.74

significant uranium structural motif and the corresponding isostructural cationic species.

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EXPERIMENTAL SECTION

Materials and Methods. All manipulations were performed in a nitrogen-filled MBraun glovebox or by using standard Schlenk-line techniques. All solvents were freshly distilled over appropriate drying agents and collected for further use. THF, 2-methyltetrahydrofuran, diethyl ether, toluene, and n-hexane were dried on molecular sieves and shaved sodium before being used. THF-d8 and C6D6 were purchased from Cambridge Isotope Laboratories and dried over 4 Å molecular sieves. Compounds U(Cl)4,21 N,N-cyclohexyl(TMS)lithium amide,22 and ferrocenium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([Cp2Fe][BArF24])23 were synthesized according to published procedures. 1 H NMR spectra were recorded on a Bruker 400 MHz spectrometer operating at 400.132 MHz. All 1H NMR spectra were recorded using the 1H (residual) shifts of the solvent as a secondary standard. Paramagnetically shifted peaks are listed with the peak width at halfheight (hertz). Infrared spectra were collected on a PerkinElmerSpectrum 2000 FT-IR-Raman spectrometer. All samples for EPR spectroscopy were prepared in an inert atmosphere glovebox equipped with a liquid nitrogen fill port to enable sample freezing to 77 K within the glovebox. EPR samples were prepared using a 1:1 THF/2-MeTHF solution and loaded into 4 mm OD Suprasil quartz EPR tubes from Wilmad Labglass. X-Band EPR spectra were recorded on a Bruker EMXplus spectrometer equipped with a model 4119HS cavity and an Oxford ESR-900 helium flow cryostat. The instrumental parameters employed for all samples were as follows: T = 10 K, power of 1 mW, time constant of 0.01 ms, modulation amplitude of 8 G, 9.40248 GHz, and modulation frequency of 100 kHz. All samples for MCD spectroscopy were prepared in an inert atmosphere glovebox equipped with a liquid nitrogen fill port to allow sample freezing to 77 K within the glovebox. Solid mull MCD samples were prepared with paratone oil in copper cells fitted with quartz disks and a 2.5 mm gasket. NIR MCD experiments were conducted using a Jasco J-730 spectropolarimeter and a liquid nitrogen-cooled InSb detector. The spectral range accessible with this NIR MCD setup is 2000−600 nm. UV−vis MCD spectra were collected using a Jasco J715 spectropolarimeter and a shielded S-20 photomultiplier tube. Both instruments utilize a modified sample compartment incorporating focusing optics and an Oxford Instruments SM4000-7T superconducting magnet/cryostat, permitting measurements from 1.6 to 290 K with magnetic fields of ≤7 T. A calibrated Cernox sensor directly inserted into the copper sample holder is used to measure the temperature of the sample to ±0.001 K. All MCD spectra were baseline-corrected against zero-field scans. Single crystals suitable for X-ray diffraction were coated with Nparatone oil (dried under reduced pressure overnight at 100 °C) in a drybox, mounted on a nylon loop, and then transferred to the goniometer head of a Bruker X8 APEX2 diffractometer equipped with a molybdenum X-ray tube (λ = 0.71073 Å) or to a Bruker D8 Venture instrument equipped with a molybdenum X-ray tube (λ = 0.71073 Å). Preliminary data revealed the crystal system. A hemisphere routine was used for data collection and determination of lattice constants. The space group was identified, and the data were processed using the Bruker SAINT+ program and corrected for absorption using SADABS.24 The structures were determined using direct methods (SHELXS) completed by subsequent Fourier synthesis and refined by full-matrix least-squares procedures.25 Olex2 software was used as the graphical interface.26 A solvent mask was used to remove a highly disordered fluorobenzene molecule from the unit cell of 4. The U L3-edge X-ray absorption near-edge spectra (XANES) were measured under dedicated operating conditions (3.0 GeV, 5%, 500 mA) on end station 11-2 at the Stanford Synchrotron Radiation Lightsource (SSRL). This beamline, which was equipped with a 26pole, 2.0 T wiggler, utilized a liquid nitrogen-cooled double-crystal Si[220] monochromator and employed collimating and focusing G


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Inorganic Chemistry (12.3 Hz), 3.94 (34 Hz), 2.25 (235 Hz), −0.02 (221 Hz), −1.98 (26 Hz), −3.46 (79 Hz), −4.94 (100 Hz), −12.77 (158 Hz), −15.14 (200 Hz), −22.36 (528 Hz). Synthesis of Tris(N,N-cyclohexyl(trimethylsilyl)amide)chlorouraniumIV LiCl-(THF)2 (UCl(NCyTMS)3-LiCl(THF)2, 2). UCl4 (0.380 g, 1.00 mmol) was added to a 20 mL scintillation vial along with 10 mL of THF while being stirred. To this stirring solution was added LiN(TMS)Cy (0.533 g, 3.00 mmol) as a solid at room temperature over the course of 10 min. The reaction mixture was stirred for 30 min, and the volatiles were removed. The residue was extracted with hexane and filtered over Celite. The solution was cooled to −30 °C overnight, and a bright green precipitate formed. The solid was collected, and volatiles were removed under reduced pressure to yield 0.740 g (78%) of a pale green crystalline solid identified as 2. Analysis calcd for C35H76Cl2LiN3O2Si3U: C, 43.29%; H, 7.89%; N, 4.33%. Found: C, 41.21%; H, 7.48%; N, 4.71%. The resultant EA numbers are consistent with a mono-THF adduct (C, 41.42%; H, 7.62%; N, 4.67%), results that are consistent with prolonged removal of volatiles under reduced pressure of the isolated product. 1H NMR (C6D6): δ 90.21 (255 Hz), 16.58 (38 Hz), 15.24 (38 Hz), 6.46 (d, JHH = 12 Hz), 6.21 (q, JHH = 13 Hz), 4.81 (23 Hz), 3.85 (46 Hz), 3.48 (42 Hz), 1.57 (12 Hz), −4.99 (11 Hz), −9.51 (15 Hz). Synthesis of Tetra(N,N-cyclohexyl(trimethylsilyl)amide)uraniumIV (U(NCyTMS)4, 3). UCl4 (0.380 g, 1.00 mmol) was added to a 20 mL scintillation vial along with 10 mL of THF while being stirred. To this stirring solution was added LiN(TMS)Cy (0.710 g, 4.00 mmol) as a solid at room temperature over the course of 10 min. The reaction mixture was stirred for 12 h, and the volatiles were removed. The residue was extracted with toluene and filtered over Celite. The solution was concentrated, layered with N-hexane, and held at −30 °C overnight. The solid was collected on a glass frit and rinsed with cold hexane, and volatiles were removed under reduced pressure to yield 0.772 g (84%) of a pale yellow, nearly colorless crystalline solid identified as 3. Analysis calcd for C36H80N4Si4U: C, 47.03%; H, 8.77%; N, 6.09%. Found: C, 46.57%; H, 8.45%; N, 5.93%. 1H NMR (C6D6): δ 9.24 (760 Hz), 8.19 (98 Hz), 2.47 (q, JHH = 13 Hz), 2.21 (39 Hz), 0.12 (d, JHH = 13 Hz), −2.36 (18 Hz), −4.87 (23 Hz), −5.10 (85 Hz), −32.75 (1200 Hz). Synthesis of [Tetra(N,N-cyclohexyl(trimethylsilyl)amide)uranium V ][tetrakis[3,5-bis(trifluoromethyl)phenyl]borate] ([U(NCyTMS)4][BArF24], 4). Compound 3 (0.200 g, 0.218 mmol) was dissolved in roughly 10 mL of fluorobenzene in a 20 mL scintillation vial. As a solid, [Cp2Fe][BArF24] (0.225 g, 0.210 mmol) was added at once. The solution immediately turned dark purple. The solution was allowed to stir for roughly 10 min and was then filtered through Celite, and the solution was cooled to −30 °C overnight. Large, dark crystals were isolated by decanting the mother liquor, washing the dark solid with hexane (2 × 10 mL), and removing the volatiles under reduced pressure to a constant mass. This yielded 0.355 g (92%) of a dark purple/black solid identified as 4. Analysis calcd for C68H92BF24N4Si4U: C, 45.82%; H, 5.20%; N, 3.14%. Found: C, 45.63%; H, 5.05%; N, 3.16%. 1H NMR (THF-d8): δ 7.83 (12 Hz), 7.64 (5.5 Hz), 4.72 (1000 Hz), 2.63 (65 Hz), −2.12 (354 Hz), −4.51 (618 Hz).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected]. [email protected]. [email protected]. [email protected].

ORCID

Jochen Autschbach: 0000-0001-9392-877X Enrique R. Batista: 0000-0002-3074-4022 James M. Boncella: 0000-0001-8393-392X Maryline G. Ferrier: 0000-0003-0081-279X Michael L. Neidig: 0000-0002-2300-3867 Samantha K. Cary: 0000-0003-0398-7106 Ping Yang: 0000-0003-4726-2860 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.M.T., T.J.D., M.G.F., and B.W.S. thank the Glenn T. Seaborg Institute for support from a Seaborg Research Fellowship. Portions of this work (S.K.C.) were supported by a LANL Darleane Christian Hoffman Distinguished fellowship. M.L.N. gratefully acknowledges support from the U.S. Department of Energy, Office of Science, Early Career Research Program, via Grant DE-SC0016002. E.R.B. and P.Y. were supported by the Heavy Element Chemistry program at LANL sponsored by the U.S. Department of Energy Office of Basic Energy Sciences. J.A. acknowledges financial support from the U.S. Department of Energy, Office of Basic Energy Sciences, Heavy Element Chemistry program, via Grant DE-SC0001136 (formerly DEFG02-09-ER16066). Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DEAC02-76SF00515. A.M.T. thanks Alejandro G. Lichtscheidl for assistance with electrochemistry and general helpful discussions. Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under Contract DE-AC52-06NA25396.

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03139. Additional spectra, discussion, and crystallographic tables (PDF) Accession Codes

CCDC 1589613−1589616 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 H


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Inorganic Chemistry

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A Pseudotetrahedral Uranium(V) Complex - ACS Publications

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