Uranyl Coordination by the 14-Membered Macrocycle

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Uranyl Coordination by the 14-Membered Macrocycle Dibenzotetramethyltetraaza[14]annulene Elizabeth A. Pedrick, Mikiyas K. Assefa, Megan E. Wakefield, Guang Wu, and Trevor W. Hayton* Department of Chemistry and Biochemistry, University of California Santa Barbara, Santa Barbara, California 93106, United States S Supporting Information *

ABSTRACT: Reaction of [UO2(N(SiMe3)2)2(THF)2] with 1 equiv of dibenzotetramethyltetraaza[14]annulene (tmtaaH2) affords the uranyl complex [UO2(tmtaaH)(N(SiMe3)2) (THF)] (1) (THF = tetrahydrofuran) as red blocks in 83% yield. Similarly, thermolysis of a mixture of [UO2(N(SiMe 3 ) 2 ) 2 (THF) 2 ] and 2 equiv of tmtaaH 2 affords [UO2(tmtaaH)2] (2), which can be isolated as red-orange crystals in 67% yield after workup. Both 1 and 2 were fully characterized, including analysis by X-ray crystallography. The tmtaaH ligands in 1 and 2 are only coordinated to the uranium center via one β-diketiminate fragment, while the protonated β-diketimine portion of the ligand remains uncoordinated. Reaction of [UO2(N(SiMe3)2)2(THF)2] with 1 equiv of Li2(tmtaa) in C6H6 results in the formation of [Li(THF)]2[UO2(N(SiMe3)2)2(tmtaa)] (3), which can be isolated in 55% yield as a red-brown crystalline solid. The tmtaa ligand in complex 3 supports a dative interaction between an oxo ligand in the uranyl fragment and a lithium cation, suggesting that tmtaa could be a useful ligand for developing the oxo ligand functionalization chemistry of the uranyl ion.

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INTRODUCTION The rigid trans-dioxo stereochemistry of the uranyl ion (UO22+), coupled with the limited degrees of freedom of many macrocyclic ligands, presents interesting opportunities for materials chemistry,1−3 separation science,4−6 oxo ligand functionalization,7−10 and fundamental studies of uranyl structure and bonding.11 For example, because the uranyl ion cannot simultaneously bind to all four of its carboxylate arms, coordination of 1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴tetraacetic acid (DOTA) to UO22+ results in formation of a layered material, wherein each DOTA ligand binds to four different uranium centers.1 Coordination of uranyl to 1,4,7,10tetraazacyclotetradecane-N,N′,N″,N‴-tetraacetic acid (TETA) and ethylenediaminetetraacetic acid (EDTA) result in similar layered structures.2,3 Likewise, Arnold and co-workers have exploited the presence of an unoccupied metal binding site in the uranyl polypyrrolic macrocycle complex [UO2(THF)(H2L)] to effect a variety of elegant oxo functionalizations.7−10,12,13 In these examples, the macrocyclic ligand enforces the close approach of an incoming metal ion to one of the oxo ligands of the uranyl moiety, which primes the uranyl fragment for further reactivity.9,10,12 More recently, we have begun to explore the degree to which a macrocylic ligand can affect uranyl stereochemistry.11 In particular, we found that coordination of UO22+ to a 12-membered pyridinophane ligand, HN4 (2,11-diaza[3,3](2,6)pyridinophane), resulted in formation of the unusual 8-coordinate uranyl complexes, [UO2X2(HN4)] (X = Cl, OTf). These complexes are notable, because they feature the smallest O−U−O angles yet observed (ca. 163°) for the uranyl moiety, a consequence of the steric clash between the uranyl oxo ligands and the macrocycle backbone. © 2017 American Chemical Society

As part of our efforts to explore the ability of macrocylic ligands to modify uranyl stereochemistry, we have continued to search for promising macrocyclic ligands that can perturb the uranyl O−U−O angle. One ligand that could be well-suited for this purpose is the 14-membered tetra(aza)macrocycle dibenzotetramethyltetraaza[14]annulene (tmtaaH2). Many octahedral transition-metal complexes of (tmtaa) 2− are k n o w n , 1 4 − 1 8 i n c lu d i n g c i s - [ T i C l 2 ( t m t a a ) ] , 1 9 c i s [ZrCl 2 (tmtaa)], 16 cis-[Zr(CH 2 Ph) 2 (tmtaa)], 14 cis-[Zr(η2-tBuNC(CH2Ph))2(tmtaa)],14 cis-[(tmtaa)Ti(OH2)]2+,15 and cis-[NbCl2(tmtaa)],17 and in each of these cases the metal ion sits above the binding pocket of the tetradentate tmtaa ligand. As a result, the tmtaa ligand enforces a cis stereochemistry of the remaining two ligands. The zirconium benzyl complex cis-[Zr(CH2Ph)2(tmtaa)]14 is particularly illustrative in this context, because Zr4+ (0.72 Å) has a similar ionic radius to U6+ (0.73 Å),20 suggesting that the two oxo ligands in the putative [UO2(tmtaa)] complex should also exhibit cis stereochemistry. Herein, we report our attempts to ligate (tmtaa)2− to the uranyl ion in an effort to promote the trans/cis isomerization of its two oxo ligands.

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RESULTS AND DISCUSSION Addition of 1 equiv of tmtaaH2 to an Et2O solution of [UO 2 (N(SiMe 3 ) 2 ) 2 (THF) 2 ], in an effort to form cis[UO2(tmtaa)], results in immediate formation of a deep redorange solution, from which [UO2(tmtaaH)(N(SiMe3)2)(THF)] (1) could be isolated as a deep red crystalline solid in 83% yield (Scheme 1). Complex 1 is the result of partial Received: March 21, 2017 Published: May 15, 2017 6638

DOI: 10.1021/acs.inorgchem.7b00700 Inorg. Chem. 2017, 56, 6638−6644

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Inorganic Chemistry Scheme 1. Synthesis of Complexes 1 and 2

protonation of [UO2(N(SiMe3)2)2(THF)2], as it still contains one silylamide ligand and one β-diketimine moiety. Its deep red color, which is much different than the yellow color expected for a uranyl complex, is likely due to a ligand-to-metal charge transfer band. For comparison, the closely related uranyl βdiketiminate complex [UO2(Ar2nacnac)Cl]2 (Ar2nacnac = (2,6-iPr2C6H3)NC(Me)CHC(Me)N(2,6-iPr2C6H3) is deep green-brown.21 The 1H NMR spectrum of 1 in C6D6 exhibits a sharp NH resonance at 13.13 ppm, which is consistent with our proposed formulation. We also observe eight aryl C−H resonances, ranging from 8.20 to 6.72 ppm, suggestive of a C1 symmetric complex, which is consistent with the structure observed in the solid state (see below). Additionally, resonances at 4.90 and 4.63 ppm are assignable to the two unique γ-CH environments. Interestingly, the spectrum also features two resonances at 0.75 and 0.46 ppm, in a 1:1 ratio, which are assignable to two magnetically inequivalent SiMe3 groups, suggesting that there is limited rotation about the U− Namide bond in solution. This phenomenon has been observed previously in other silylamide complexes of uranium(VI).22,23 Finally, complex 1 exhibits a UO νsym mode at 805 cm−1 in its Raman spectrum, which is comparable to other uranyl complexes with multiple anionic N-donor coligands, such as [UO2(N{SiMe3}2)3]− (805 cm−1)24 and [UO2(NCN)2(THF)] (NCN = Me3Si(N)CPh(N)SiMe3) (803 cm−1).25 This value is much smaller than that typically observed for uranyl (e.g., 870 cm−1 for UO22+(aq)),26 which is consequence of the anionic nature of the tmtaa ligand and its (relatively) strong donor ability. Complex 1 crystallizes in the triclinic space group P1̅, and its solid-state molecular structure is shown in Figure 1. The uranium center in complex 1 features an octahedral geometry and is ligated to two oxo ligands, a THF ligand, a silylamide ligand, and two nitrogen atoms of the tmtaaH ligand. The U− Ooxo bond lengths (1.787(5) and 1.789(4) Å) and the O−U−O angle (174.0(2)°) are typical of those observed for the UO22+ moiety,21,27,28 indicating that coordination of the tmtaaH ligand to the uranium center has not perturbed the UO22+ fragment. The tmtaaH ligand in 1 is only coordinated to the uranium center with two of its nitrogen atoms, from one of its βdiketiminate fragments. The remaining two nitrogen atoms, in the protonated β-diketimine portion of the ligand, remain uncoordinated. As a result of this coordination mode, the tmtaa ligand framework adopts a rather severe saddle-shape conformation, similar to that observed in cis-[MoO2(acac) (tmtaaH)]29 and [Rh(tmtaaH)(CO)2].30 Finally, the U−

Figure 1. Solid-state molecular structure of [UO2(tmtaaH)(N(SiMe3)2)(THF)] (1) with 50% probability ellipsoids. All hydrogen atoms were omitted for clarity. Complex 1 crystallizes with two independent molecules in the asymmetric unit; only one is shown here. Selected bond lengths (Å) and angles (deg): U1−O2 = 1.787(5), U1−O1 = 1.789(4), U1−N5 = 2.307(5), U1−N1 = 2.390(5), U1−N2 = 2.416(5), U1−O3 = 2.470(5), O2−U1−O1 = 174.0(2).

NtmtaaH bond lengths in 1 (2.390(5) and 2.416(5) Å) are comparable to the U−N bond lengths in other uranyl βdiketiminate complexes, such as [UO2(Ar2nacnac)Cl]2 (average U−N = 2.40 Å),21 [UO2(Ar2nacnac)(acac)] (U−N = 2.419(5) and 2.409(5) Å),31 [UO 2(Ar2 nacnac)(dbm)] (U−N = 2.402(5) and 2.388(5) Å),31 and [Li(MeIm)][UO(μ-O) (Ar 2 nacnac)(μ-N,C-C 4 H 5 N 2 ) 2 ] (U−N = 2.446(5) and 2.459(5) Å).32 In an attempt to drive the protonolysis reaction to completion and coordinate the other two nitrogen atoms of the tmtaa ligand to the uranium center, a C6D6 solution of 1 was heated to 90 °C for 24 h. A 1H NMR spectrum of this solution reveals the presence of a new uranium-containing product, [UO2(tmtaaH)2] (2), along with complex 1 (Figure S3). These two complexes are present in an ∼2:3 ratio, respectively. [UO2(tmtaaH)2] appears to be formed via an apparent ligand scrambling reaction. However, we also observe the formation of considerable amounts of HN(SiMe3)2, suggesting that 1 may also decompose under the reaction conditions. Importantly, we do not observe any resonances that could be assigned to the desired product, cis-[UO2(tmtaa)], 6639

DOI: 10.1021/acs.inorgchem.7b00700 Inorg. Chem. 2017, 56, 6638−6644

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Inorganic Chemistry

brown crystals of [Li(THF)]2[UO2(N(SiMe3)2)2(tmtaa)] (3) in 55% yield (Scheme 2).

even after prolonged heating, and despite the presence of both an acidic proton and an internal base (e.g., [N(SiMe3)2]−) in complex 1, which highlights the thermodynamic stability of the trans-UO22+ fragment. Complex 2 can be rationally prepared by thermolysis (50 °C) of a mixture of 2 equiv of tmtaaH2 and 1 equiv of [UO2(N(SiMe3)2)2(THF)2] in toluene. When generated in this manner, 2 can be isolated as a red-orange crystalline solid in 67% yield (Scheme 1). Its 1H NMR spectrum in C6D6 features a characteristic NH resonance at 13.58 ppm, demonstrating that only partial deprotonation of the tmtaaH2 ligand has occurred. Complex 2 also features four aryl CH resonances, at 8.21, 7.24, 6.93, and 6.77 ppm, in a 1:1:1:1 ratio, indicating the presence of a mirror plane that relates the two halves of the tmtaaH ligand. We also observe the presence of two γ-CH proton resonances, at 4.99 and 4.69 ppm, in a 1:1 ratio. Finally, complex 2 exhibits a peak at 805 cm−1 in its Raman spectrum, which we tentatively assigned to the UO νsym mode. This value is identical to that observed for 1. Complex 2 crystallizes in the triclinic space group P1̅, and its solid-state molecular structure is shown in Figure 2. The

Scheme 2. Synthesis of Complex 3

Complex 3 crystallizes in the orthorhombic space group Pnma, as the C6D6 solvate, 3·2C6D6 (Figure 3). Similar to

Figure 2. Solid-state molecular structure of [UO2(tmtaaH)2] (2), with 50% probability ellipsoids. All hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and angles (deg): U1−O1 = 1.752(7), U1−N2 = 2.389(9), U1−N1 = 2.350(9), O1−U1−O2 = 180.

Figure 3. Solid-state molecular structure of [Li(THF)]2[UO2(N(SiMe3)2)2(tmtaa)]·2C6D6 (3·2C6D6), with 50% probability ellipsoids. All hydrogen atoms, THF carbon atoms, SiMe3 carbon atoms, and the benzene solvate molecules were removed for clarity. Selected bond lengths (Å) and angles (deg): U1−O1 = 1.80(2), U1−O2 = 1.77(2), U1−N1 = 2.34(2), U1−N3 = 2.51(2), Li1−O1 = 1.95(5), Li1−N2 = 2.09(4), Li1−O3 = 1.89(5), Li2−N2 = 2.00(4), Li2−N3 = 2.23(4), Li2−O4 = 1.92(6), O1−U1−O1* = 175.9(9).

uranium ion in complex 2 exhibits an octahedral geometry, and its two tmtaaH ligands coordinate to the uranium center via an identical binding mode as was observed for 1. The U−NtmtaaH bond lengths in 2 (2.350(9) and 2.389(9) Å) are comparable to those observed in 1. The U−O bond length (1.752(7) Å) and O−U−O angle (180°) are also similar to those observed for 1, as well as other uranyl complexes,21,27,28 again indicating coordination of the tmtaaH ligands has not perturbed the UO22+ fragment. The unoccupied binding pocket of the tmtaaH ligands in complexes 1 and 2 brings to mind the seminal uranyl polypyrrolic macrocycle complex reported by Arnold and Love in 2004 (i.e., the uranyl “pacman” complex).7 The open binding site in this complex can be populated with a variety of metal cations, including Fe, Mn, Co, Ge, Sn, Pb, and Li.8,10,13 Given this precedent, we rationalized that metal cations could also be installed in the open binding pockets found in 1 and 2. Access to such a structure was conveniently achieved by direct reaction of [UO2(N(SiMe3)2)2(THF)2] with 1 equiv of Li2(tmtaa) in C6H6, which resulted in the deposition of red-

complexes 1 and 2, the uranium center in 3 only interacts with two nitrogen atoms of the tmtaa ligand. Additionally, however, one lithium ion (Li1) interacts with the remaining nitrogen atoms, along with a THF ligand, and an oxo ligand from the uranyl moiety, in a manner that is reminiscent of the polypyrrolic uranyl complex developed by the Arnold group.9,10 Complex 3 features a second Li cation (Li2) that interacts with all four nitrogen atoms of the tmtaa ligand but on the opposite face to that of Li1. A similar Li-tmtaa binding mode was observed in [Li(THF)][Ce(tmtaa)2].33 The U− Ntmtaa bond length in 3 (2.51(2) Å) is longer than the U−Ntmtaa bond lengths observed in 1 and 2, which is likely a consequence of the formal dianionic charge of the [UO 2 (N(SiMe3)2)2(tmtaa)]2− fragment. The U−O bond lengths (1.80(2) and 1.77(2) Å) and O−U−O angle (175.9(9)°) are typical of the uranyl(VI) moiety. The two U−O bond lengths are statistically identical, due to their relatively large uncertainties, which prohibits us from stating whether the 6640

DOI: 10.1021/acs.inorgchem.7b00700 Inorg. Chem. 2017, 56, 6638−6644

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Inorganic Chemistry

indirectly with the 1H resonance of SiMe4 at 0 ppm, according to IUPAC standard.42,43 IR spectra were recorded on a Mattson Genesis FTIR spectrometer. Elemental analyses were performed by the Microanalytical Laboratory at UC Berkeley. Raman Spectroscopy. Raman spectra were recorded on a LabRam Aramis microRaman system (Horiba Jobin Yvon) equipped with 1200 grooves/mm holographic gratings, and Peltier-cooled CCD camera. The 633 nm output of a Melles Griot He−Ne laser was used to excite the spectra, which were collected in a backscattering geometry using a confocal Raman Microscope (high stability BX40) equipped with Olympus objectives (MPlan 50×). Sample preparation was performed inside the glovebox: Pure crystalline solid samples were placed between a glass microscope slide and coverslip, sealed with a bead of silicone grease, and removed from the glovebox for spectral acquisition. X-ray Crystallography. The solid-state molecular structures of complexes 1−3·2C6D6 were determined similarly with exceptions noted in the following paragraph. Crystals were mounted on a cryoloop under Paratone-N oil. Data collection was performed on a Bruker KAPPA APEX II diffractometer equipped with an APEX II CCD detector using a TRIUMPH monochromator with a Mo Kα Xray source (α = 0.710 73 Å). Data for 1−3·2C6D6 were collected at 100(2) K, using an Oxford nitrogen gas cryostream system. A hemisphere of data was collected using ω scans with 0.5° frame widths. Frame exposures of 5 and 10 s were used for 1 and 2, respectively, while frame exposures of 20 s were used for 3·2C6D6. Data collection and cell parameter determinations were conducted using the SMART program.44 Integration of the data frames and final cell parameter refinement was performed using SAINT software.45 Absorption correction of the data was performed using the multiscan method SADABS. 46 Subsequent calculations were performed using SHELXTL.47 Structure determination was done using direct or Patterson methods and difference Fourier techniques. All hydrogen atom positions were idealized, and rode on the atom of attachment. Structure solution, refinement, graphics, and creation of publication materials were performed using SHELXTL.47 The crystal for complex 2 was twinned, which resulted in weak highangle diffractions. As a consequence, it was necessary to refine O1 isotropically. Additionally, because of disorder within the SiMe3 fragments of complex 3, the EADP command was applied to the carbon atoms of the methyl groups, while the Si−C bond lengths were constrained with the SADI command. Additionally, both oxygen atoms in the uranyl fragment were constrained with the EADP command. A summary of crystallographic data for 1−3·2C6D6 is presented in Table S1. Caution! Depleted uranium (isotope 238U) is a weak α-emitter with a half-life of 4.47 × 109 years. Manipulations and reactions should be performed in a f ume hood or inert atmosphere glovebox in a laboratory equipped with α- and β-counting equipment. [UO2(tmtaaH)(N(SiMe3)2)(THF)] (1). To a bright orange solution of [UO2(N(SiMe3)2)2)(THF)2] (157 mg, 0.214 mmol) in Et2O (3 mL) was added dropwise a yellow solution of tmtaaH2 (73 mg, 0.21 mmol) in Et2O (3 mL), which resulted in an immediate color change to deep red-orange. The solution was allowed to stir for 15 h at room temperature, whereupon the now deep red solution was filtered through a diatomaceous earth column supported on glass wool (0.5 cm × 2 cm). The red filtrate was concentrated in vacuo (ca. 2 mL), and subsequent storage of this solution at −25 °C for 24 h resulted in the deposition of deep red crystals (101.5 mg). Further concentration in vacuo and storage of supernatant at −25 °C for 24 h resulted in the deposition of a second crop of deep red crystals (total yield: 151.3, 83% yield). Anal. Calcd C32H49N5O3Si2U: C, 45.43; H, 5.84; N, 8.28. Found: C, 45.70; H, 5.79; N, 8.02%. 1H NMR (C6D6, 25 °C, 400 MHz): δ 13.13 (s, 1H, NH), 8.20 (d, JHH = 8 Hz, 1H, aryl CH), 7.33 (t, JHH = 8 Hz, 1H, aryl CH), 7.27 (d, JHH = 7 Hz, 1H, aryl CH), 6.99 (t, JHH = 7 Hz, 1H, aryl CH), 6.95 (m, 2H, overlapping aryl CH), 6.86 (d, JHH = 7 Hz, 1H, aryl CH), 6.72 (d, JHH = 8 Hz, 1H, aryl CH), 4.90 (s, 1H, γ-CH), 4.63 (s, 1H, γ-CH), 4.18 (br m, 4H, THF), 1.88 (s, 3H, CH3), 1.85 (s, 3H, CH3), 1.80 (overlapping singlets, 6H, CH3), 1.48 (br m, 4H, THF), 0.75 (s, 9H, SiMe3), 0.46 (s, 9H, SiMe3). IR (KBr

U1−O1 bond length is perturbed by Li coordination. The Li1− O1 distance in 3 (1.95(5) Å) is comparable to other UVIO··· Li interactions.10,26,34−38 For example, the UVIO···Li distances in the uranyl piperidine complex, [{Li(DME)}2Cl][Li(DME)][UO2(NC5H10)3]2, range from 1.89(1) to 1.94(1) Å.35 The 1H NMR spectrum of complex 3 in toluene-d8 is consistent with its solid-state structure. In particular, it features four aryl proton resonances, ranging from 7.82 to 6.79 ppm, in a 1:1:1:1 ratio, and two γ-proton resonances, at 4.78 and 4.32 ppm, in a 1:1 ratio. Additionally, we observe two sharp singlets at 0.70 and 0.25 ppm, in a 1:1 ratio, which are assignable to the proton environments of the N(SiMe3)2 ligands, indicative of impaired rotation about the U−N bond. Finally, we do not observe an NH resonance in this spectrum. The 7Li{1H} NMR spectrum of 3 in toluene-d8 features two broad resonances at 1.67 and −0.34 ppm, in a 1:1 ratio, suggesting that complex 3 retains its solid-state structure in solution. We also recorded a Raman spectrum of complex 3; however, we could not confidently make a UO νsym assignment because of the observation of several broad overlapping bands in the UO stretching region.

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SUMMARY Reaction of [UO2(N(SiMe3)2)2(THF)2] with 1 or 2 equiv of tmtaaH2 results in formation of [UO2(tmtaaH)(N(SiMe3)2)(THF)] and [UO2(tmtaaH)2], respectively. The tmtaaH ligands in both complexes are only coordinated to the uranium center via one β-diketiminate fragment, while the protonated βdiketimine portion of the ligand remains uncoordinated. Importantly, we have observed no evidence for the formation of cis-[UO2(tmtaa)] in these investigations, even after prolonged thermolysis of [UO2(tmtaaH)(N(SiMe3)2)(THF)]. This observation demonstrates that elimination of HN(SiMe3)2 does not provide the required driving force to effect the desired trans/cis oxo isomerization. The isolation of complexes 1 and 2 also highlights several other major challenges that need to be overcome in the effort to successfully isolate a cis-uranyl complex, such as ligand flexibility, along with unwanted ligand binding modes, which can result in unanticipated reaction products. Finally, the isolation of [Li(THF)]2[UO2(N(SiMe3)2)2(tmtaa)], which features a dative interaction between a uranyl oxo ligand and a lithium cation, suggests that tmtaa could be a useful ligand for further developing the oxo ligand functionalization chemistry of the uranyl ion.

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EXPERIMENTAL SECTION

General. All reactions and subsequent manipulations were performed under anaerobic and anhydrous conditions under an atmosphere of nitrogen. Hexanes, Et2O, THF, and toluene were dried using a Vacuum Atmospheres DRI-SOLV solvent purification system and stored over 3 Å molecular sieves for 24 h prior to use. C6D6 and toluene-d8 were dried over activated 3 Å molecular sieves for 24 h before use. [UO 2 Cl 2 (THF) 2 ] 2 , 27 [UO 2 (N(SiMe 3 ) 2 (THF) 2 ], 39 tmtaaH2,40 and Li2(tmtaa)41 were prepared according to literature procedures. All other reagents were purchased from commercial suppliers and used as received. NMR spectra were recorded on an Agilent Technologies 400-MR DDR2 400 MHz spectrometer, a Varian UNITY INOVA 500 MHz spectrometer, or a Varian Unity Inova AS600 600 MHz spectrometer. 1 H and 13C{1H} NMR spectra are referenced to external SiMe4 using the residual protio solvent peaks as internal standards (1H NMR experiments) or the characteristic resonances of the solvent nuclei (13C NMR experiments). 7Li{1H} NMR spectra were referenced 6641

DOI: 10.1021/acs.inorgchem.7b00700 Inorg. Chem. 2017, 56, 6638−6644

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Inorganic Chemistry pellet, cm−1): 1620(s), 1595(m), 1547(s), 1508(m), 1466(s), 1444(sh m), 1437(m), 1381(s), 1365(s), 1288(m), 1271(m), 1257(sh m), 1252(m), 1240(m), 1196(sh w), 1186(m), 1157(w), 1105(w), 1068(w), 1039(w), 1014(m), 928(s), 891(m), 883(m), 875(m), 868(m), 845(sh s), 835(s), 793(m), 773(w), 742(s), 692(w), 667(w), 661(w), 650(w), 640(w), 611(w), 575(w), 559(w), 536(w), 532(w), 511(w), 482(w), 474(w), 457(w), 424(w), 410(w). Raman (neat solid, cm−1): 1581(m), 1577(m), 1539(vs), 1501(s), 1494(s), 1486(s), 1467(m), 1437(m), 1340(m), 1325(m), 1278(m), 1267(m), 1220(m), 1023(w), 1011(w), 929(w), 863(w), 830(m), 805(m, U O νsym), 746(w), 653(w), 597(w), 541(w), 471(w), 379(w), 321(w). [UO2(tmtaaH)2] (2). To a stirring orange solution of [UO2(N(SiMe3)2)2)(THF)2] (137 mg, 0.186 mmol) in toluene (3 mL) was added dropwise a yellow solution of tmtaaH2 (128 mg, 0.372 mmol) in toluene (4 mL), which resulted in an immediate color change to red-orange. The reaction mixture was then sealed under vacuum in a Schlenk flask equipped with a Kontes Rotoflow valve. Heating of this solution to 50 °C for 18 h resulted in a color change to red-brown, concomitant with the deposition of a red-orange powder. When cooled to room temperature, the solution was decanted away from the red-orange solid, and the solid was washed quickly with Et2O (2 mL). The solid was subsequently dried in vacuo (118.9 mg, 67% yield). Xray quality crystals were grown by storage of a concentrated toluene solution of 2 at −25 °C for 24 h. Anal. Calcd C44H46N8O2U: C, 55.23; H, 4.58; N, 11.71. Found: C, 55.62; H, 4.56; N, 11.63%. 1H NMR (C6D6, 25 °C, 400 MHz): δ 13.58 (s, 2H, NH), 8.21 (d, JHH = 8 Hz, 4H, aryl CH), 7.24 (t, JHH = 7 Hz, 4H, aryl CH), 6.93 (t, JHH = 8 Hz, 4H, aryl CH), 6.77 (d, JHH = 8 Hz, 4H, aryl CH), 4.99 (s, 2H, γ-CH), 4.69(s, 2H, γ-CH), 1.90 (overlapping singlets, 24H, CH3). IR (KBr pellet, cm−1): 1618(m), 1593(w), 1560(sh m), 1545(s), 1512(m), 1464(m), 1446(w), 1435(w), 1380(m), 1362(s), 1290(w), 1263(w), 1225(w), 1192(sh vw), 1178(w), 1153(w), 1107(m), 1045(w), 1018(m), 1003(w), 941(sh vw), 920(m), 906(vs), 868(sh w), 862(m), 852(sh w), 829(m), 808(m), 793(m), 752(m), 742(s), 729(s), 688(w), 667(w), 648(w), 606(w), 573(m), 559(w), 538(m), 523(w), 503(w), 484(w), 471(m), 459(w), 434(w), 420(w), 411(w), 403(w). Raman (neat solid, cm−1): 1581(s), 1536(s), 1502(s), 1464(s), 1430(m), 1325(s), 1286(s), 1223(m), 1148(m), 1032(w), 925(w), 834(m), 805(m, UO νsym), 734(w), 653(w), 593(w), 554(m), 463(w), 361(w), 326(w), 223(w), 191(w), 113(m). Synthesis of [Li(THF)]2[UO2(N(SiMe3)2)2(tmtaa)] (3). A redbrown solution of Li2(tmtaa) (31.2 mg, 0.0876 mmol) in benzene (1 mL) was layered onto an orange solution of [UO 2 (N(SiMe3)2)2(THF)2] (64.1 mg, 0.0872 mmol) in benzene (1 mL) at room temperature. The reaction mixture was allowed to stand at room temperature for 4 h, whereupon red-brown crystals of 3·2C6H6 precipitated from solution. The crystals were isolated by decanting the supernatant, and the solid was washed with benzene (1 mL) and dried under reduced pressure (60.1 mg, 55% yield). X-ray quality crystals of 3·2C6D6 were grown by allowing an equimolar mixture of [UO2(N(SiMe3)2)2(THF)2] and Li2(tmtaa) in C6D6 to stand at room temperature for 24 h. Anal. Calcd for C42H74Li2N6O4Si4U·2C6H6: C, 51.99; H, 6.95; N, 6.74. Found: C, 51.66; H, 6.54; N, 6.74%. 1H NMR (toluene-d8, 25 °C, 400 MHz): δ 7.82 (d, JHH = 8 Hz, 2H, aryl CH), 7.22 (t, JHH = 8 Hz, 2H, aryl CH), 7.02 (t, JHH = 8 Hz, 2H, aryl CH), 6.79 (d, JHH = 8 Hz, 2H, aryl CH), 4.78 (s, 1H, γ-CH), 4.32 (s, 1H, γCH), 3.17 (br s, 8H, THF), 2.01 (s, 6H, CH3), 1.85 (s, 6H, CH3), 1.26 (br s, 8H, THF), 0.70 (s, 18H, SiMe3), 0.25 (s, 18H, SiMe3). 7Li{1H} NMR (toluene-d8, 25 °C, 155 MHz): δ 1.67 (s, 1Li), −0.34 (s, 1Li). 13 C{1H} NMR (toluene-d8, 25 °C, 150 MHz): δ 170.76 (β-C), 166.89 (β-C), 145.86 (ipso-C), 145.14 (ipso-C), 131.47 (aryl CH), 124.02 (aryl CH), 103.91 (γ-C), 101.84 (γ-C), 68.50 (br s, THF), 25.80 (Me), 25.74 (br s, THF), 23.46 (Me), 10.11 (SiMe3), 9.14 (SiMe3). Two aryl CH resonances were not observed, probably as a result of overlap with the solvent resonances. IR (KBr pellet, cm−1): 3051(w), 2980(sh), 2954(m), 2893(m), 1622(m), 1591(w), 1547(sh), 1533(s), 1510(s), 1464(vs), 1434(sh), 1396(vs), 1367(sh), 1325(w), 1267(sh), 1249(s), 1241(s), 1186(m), 1157(w), 1113(m), 1059(w), 1049(m), 1018(m), 925(vs), 885(s), 847(vs), 793(m), 775(m), 762(s), 746(s), 690(m), 663(m), 607(m), 565(w), 521(w). Raman (neat solid, cm−1):

1592(m), 1517(vs), 1506(vs), 1441(m), 1352(s), 1345(s), 1298(sh), 1278(m), 1223(w), 1176(w), 1160(w), 1104(vw), 1043(sh), 1023(m), 999(sh), 933(m), 859(vw), 825(sh), 788(w), 746(w), 665(vw), 601(m), 558(w), 532(w), 515(sh), 484(m), 475(m), 467(m), 427(vw), 352(w).

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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.7b00700. Crystallographic details (CIF) Experimental procedures and spectral data for complexes 1−3 (PDF) Accession Codes

CCDC 1547996−1547998 contains 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

Trevor W. Hayton: 0000-0003-4370-1424 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences, Biosciences, and Geosciences Division under Contract No. DE-SC0001861. This research made use of the 400 MHz NMR Spectrometer of the Chemistry Department, an NIH SIG (1S10OD012077-01A1). M.E.W. would like to thank the EUREKA program at the California NanoSystems Institute for a summer internship.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00700 Inorg. Chem. 2017, 56, 6638−6644