Crystallographic characterization of (C5H4SiMe3)3U(BH4)

The structure of Cp′3U(BH4), (C5H4SiMe3)3U(BH4), at 112 K has triclinic (P ) symmetry. It is of interest with respect to borohydride coordination modes.


Chemical context
Actinide borohydrides have been of interest since the 1940s, owing to their potential volatility and applied use in vapor deposition technologies for the production of thin films (Hoekstra & Katz, 1949;. Uranium borohydride compounds are structurally interesting because the (BH 4 ) À ligand can coordinate large electropositive cations (such as uranium) in several modes. For example, 1 , 2 , and 3 U-(BH 4 ) binding has previously been reported (Ephritikhine, 1997). Borohydrides can also achieve high coordination numbers with uranium, e.g. the oligomeric 14-coordinate U(BH 4 ) 4 (Bernstein et al., 1972). Although several cyclopentadienyl uranium borohydrides have been crystallographically characterized (Ephritikhine, 1997), the structure of Cp 0 3 U(BH 4 ) (Cp 0 = C 5 H 4 SiMe 3 ), made in 1992 (Berthet & Ephritikhine, 1992), has not been reported. Our interest in Cp 0 uranium chemistry Windorff et al., 2017) prompted us to determine the coordination mode of (BH 4 ) À within the tris-cyclopentadienyl uranium platform using single-crystal X-ray diffraction. Toward this end, we developed new synthetic routes to the Cp 0 3 U(BH 4 ) compound.
The Cp 0 3 U(BH 4 ) compound was originally synthesized by reacting Cp 0 3 UH with H 3 B-PPh 3 (Berthet & Ephritikhine, 1992). Our attempts to repeat this procedure in toluene and diethyl ether solvents were unsuccessful, potentially because we were uncertain about the details of the reaction. However, we were successful in synthesizing Cp 0 3 U(BH 4 ) from Cp 0 3 UH with H 3 B-PPh 3 in hot THF solvent. We also observed Cp 0 3 U(BH 4 ) could be prepared in high yield (96%) by reacting ISSN 2056-9890 Cp 0 3 UI with NaBH 4 in the presence of 15-crown-5. When this reaction was carried out in toluene at room temperature, the I À ligand was substituted by the (BH 4 ) À anion. Another method we developed for synthesizing Cp 0 3 U(BH 4 ) involved reacting U(BH 4 ) 4 with KCp 0 (3 equiv.) in diethyl ether. This reaction, where (BH 4 ) À was substituted by (Cp 0 ) À , also proceeded in high yield (89%). X-ray quality crystals of Cp 0 3 U(BH 4 ) formed at 253 K overnight from diethyl ether solutions.
Of our two synthetic routes, we preferred making Cp 0 3 U(BH 4 ) from Cp 0 3 UI over U(BH 4 ) 4 because the U(BH 4 ) 4 starting material was more challenging to isolate in a chemically pure form. Another interesting comparison between the two synthetic methods involved the substitution chemistry. The (Cp 0 ) À anion displaced (BH 4 ) À from U(BH 4 ) 4 and (BH 4 ) À displaced I À in Cp 0 3 UI. Hence, we qualitatively concluded that the stability of the U-X bond for molecular compounds dissolved in organic solvents was largest for (Cp 0 ) À , intermediate for (BH 4 ) À , and lowest for I À . The generality of this conclusion is limited, and we acknowledge the solubility of the other reaction products (such as NaI) might significantly influence the substitution chemistry on uranium.

Figure 1
Structure of Cp 0 3 U(BH 4 ) with atomic displacement parameters drawn at the 50% probability level. Boron-bound hydrogen atoms are represented as isotropic circles. All carbon-bound hydrogen atoms are omitted. Structure of Cp 0 3 UCl with atomic displacement parameters drawn at the 50% probability level, as reproduced from the published CIF (Windorff et al., 2017); the isomorphous thorium complex, Cp 0 3 ThCl, is also known (Ré ant et al., 2020). Hydrogen atoms are omitted for clarity.

Figure 4
Structure of Cp 0 3 UI with atomic displacement parameters drawn at the 50% probability level, as reproduced from the published CIF (Windorff et al., 2017). Hydrogen atoms are omitted for clarity.

Figure 3
Structure of Cp 0 3 UCH 3 /Cl with only the (CH 3 ) À ligand shown, with atomic displacement parameters drawn at the 50% probability level, except the -CH 3 unit, which has been plotted as an isotropic sphere, as reproduced from the published CIF (Windorff et al., 2017), see the manuscript for further details. Hydrogen atoms are omitted for clarity.

Figure 5
Structure of Cp 0 3 U( 1 -CH CH 2 ) with atomic displacement parameters drawn as isotropic spheres, as reproduced from the CIF (Schock et al., 1988). Hydrogen atoms are omitted for clarity.  Table 1.
An unusual feature of the Cp 0 3 U(BH 4 ) structure is that all three of the trimethylsilyl groups are oriented in a single direction towards the (BH 4 ) À unit. This orientation has not been observed in other Cp 0 3 U(anion) and Cp 0 3 U(dianion)UCp 0 3 structures, which are shown in Figs. 2-12. The closest comparison is with the Cp 0 3 UCl structure (Windorff et al., 2017), where all three trimethylsilyl groups are oriented towards the Cl À unit, but twisted down and away from the chloride towards the meridian. The Structure of Cp 0 3 ThCH 3 with atomic displacement parameters drawn at the 50% probability level, as reproduced from the published CIF (Wedal et al., 2019). Hydrogen atoms are omitted for clarity.

Figure 8
Structure of Cp 0 3 ThBr with atomic displacement parameters drawn at the 50% probability level, as reproduced from the published CIF in the P3 space group (Windorff et al., 2017); there is a second report of the same molecule in the P2 1 /c space group, featuring the same ligand orientation (Wedal et al., 2019). Hydrogen atoms are omitted for clarity.

Figure 11
Structure of (Cp 0 3 U) 2 (-CCO) with atomic displacement parameters drawn at the 50% probability level and disorder in the (-CCO) 2À unit displayed in one configuration, as reproduced from the published CIF (Tsoureas & Cloke, 2018). The molecule contains a plane of symmetry, and the unit cell contains two half molecules with the same orientation. For clarity, only one full molecular unit is depicted and hydrogen atoms are omitted for clarity. trimethylsilyl groups are oriented away from the [Si(SiMe 3 ) 3 ] 1À unit. Since Cp 0 3 U(BH 4 ) has the smallest monoanion of the Cp 0 3 U(anion) complexes and the correspondingly smallest (Cp 0 centroid)-U-(Cp 0 centroid), and the largest (Cp 0 centroid)-U-X angles, the orientation of the silyl groups could occur due to steric factors. However, it is also possible that some dispersion forces between the (BH 4 ) À and the trimethylsilyl groups could contribute to the orientation (Liptrot et al., 2016). It is interesting to note that in the Cp 0 3 ThX series where X = Cl (Ré ant et al., 2020), Br (Windorff et al., 2017), and CH 3 (Wedal et al., 2019), all three trimethylsilyl groups are oriented towards the anion, but twisted down and away from the anion towards the meridian as in Cp 0 3 UCl.

Supramolecular features
There are no major supramolecular features to report. The molecules pack in an alternating 180 rotation from one another within the unit cell and stack 'head to tail' between the unit cells.

Spectroscopic Features
The fully defined Cp 0 3 U(BH 4 ) compound was also characterized by 1 H, 11 B{ 1 H}, 13 C{ 1 H}, and 29 Si{ 1 H} multi-nuclear NMR spectroscopy. It was of particular interest to examine the 29 Si{ 1 H} spectrum for comparison with previous studies of silicon-containing paramagnetic uranium complexes (Windorff & Evans, 2014). The 1 H NMR spectrum in C 7 D 8 was in good agreement with the literature (Berthet & Ephritikhine, 1992). 11 B{ 1 H}, 13 C{ 1 H}, and 29 Si{ 1 H} spectra were also obtained in both C 7 D 8 and C 6 D 6 , as well as different field strengths, 500 vs 600 MHz for 1 H, to see if any significant solvent or field effects were present. Since the spectra were not dependent on solvent or field strength, only the spectra obtained in C 6 D 6 in a 600 MHz field will be discussed here. See Section 6 for full details.
In general, the resonances attributable to the Cp 0À ligands are sharp ( 1/2 < 50 Hz) and paramagnetically shifted over a range of 9.6 to À22.6 ppm, in the 1 H NMR spectrum, and a 29 Si{ 1 H} resonance at À57.4 ppm was observed, typical of other tetravalent uranium complexes (Windorff & Evans, 2014). The resonances attributable to the (BH 4 ) À unit showed considerably more shifting and broadening, resonating at À59.5 ( 1/2 = 300 Hz) and 79.6 ( 1/2 = 240 Hz) in the 1 H and 11 B{ 1 H} spectra, respectively. Since the (BH 4 ) À ligand exhibited a single 1 H NMR resonance whereas two distinct hydride environments are present in the solid state, it appears that the complex is fluxional in solution. This is in line with previous studies (Ephritikhine, 1997  Structure of (Cp 0 3 U) 4 (-L) where L = a complex organic structure containing a central cyclobutene-1,3-dione ring, with atomic displacement parameters drawn at the 50% probability level, as reproduced from the published CIF (Tsoureas & Cloke, 2018). Hydrogen atoms and disorder in the -SiMe 3 groups are omitted for clarity.
6. Synthesis and crystallization 6.1. General considerations All manipulations and syntheses described below were conducted with the rigorous exclusion of air and water using glovebox techniques under an argon atmosphere. Solvents (THF, Et 2 O, toluene, hexane, and pentane) were sparged with UHP argon (Praxair) and dried by passage through columns containing a copper(II) oxide oxygen scavenger (Q-5) and molecular sieves prior to use or stirred over sodium benzophenone ketyl, briefly exposed to vacuum several times to degas and distilled under vacuum. All ethereal solvents were stored over activated 4 Å molecular sieves. Deuterated solvents (Cambridge Isotopes) used for nuclear magnetic resonance (NMR) spectroscopy were dried over sodium benzophenone ketyl, degassed by three freeze-pump-thaw cycles, and distilled under vacuum before use. The 1 H, 11 B{ 1 H}, 13 C{ 1 H} and 29 Si{ 1 H} NMR spectra were recorded on a GN 500, Cryo 500 or Bruker Avance 600 spectrometer operating at 500.2 MHz, 160.1 MHz, 125.8 MHz, and 99.1 MHz for the 500 MHz spectrometers, respectively, and 600.1 MHz, 192.6 MHz, 150.9 MHz and 119.2 MHz for the 600 MHz spectrometer, respectively, at 298 K unless otherwise stated. The 1 H and 13 C{ 1 H} NMR spectra were referenced internally to solvent resonances, 11 B and 29 Si{ 1 H} NMR spectra were referenced externally to BF 3 (Et 2 O) and SiMe 4 , respectively, the 29 Si{ 1 H} spectra were acquired using the INEPT pulse sequence. The 15-crown-5 (Aldrich) reagent was dried over activated molecular sieves and degassed by three freezepump-thaw cycles before use. The NaBH 4 (Aldrich) reagent was placed under vacuum (10 À3 Torr) for 12 h before use. The following compounds were prepared following literature Solid NaBH 4 (15 mg, 0.40 mmol) was added to a C 7 D 8 (toluene-d 8 , 0.6 mL) solution of Cp 0 3 UI (37 mg, 0.048 mmol) in a J-Young NMR tube, an excess of 15-crown-5 (1 drop) was added and the tube was sealed and removed from the glovebox and vortexed (30 s). The NaBH 4 was not fully soluble in C 7 D 8 even in the presence of 15-crown-5. After 18 h, NMR spectroscopy showed complete conversion to Cp 0 3 U(BH 4 ). The sample was brought back into the glovebox and the volatiles were removed under reduced pressure. The product was then extracted into Et 2 O, filtered away from white insoluble solids [presumably Na(15-crown-5)I and excess NaBH 4 ] and the volatiles were removed under reduced pressure to give Cp 0 3 U(BH 4 ) (30 mg, 96%) as a wine-red solid. After stirring the mixture for an additional 12 h, volatiles were removed under reduced pressure, and the product was extracted into hexane leaving white solids behind (presumably KBH 4 ). Removal of the volatiles under reduced pressure gave Cp 0 3 U(BH 4 ) (496 mg, 89%) as a dark wine-red solid. X-ray quality crystals were grown from a concentrated ether solution at 253 K.
rendering of the molecule can be found at the following web address: https://submission.iucr.org/jtkt/serve/z/Utgd9EjfTrqJ-VoXA/zz0000/0/. C-H bond distances were constrained to 0.95 Å for cyclopentadienyl C-H moieties, and to 0.98 Å for aliphatic CH 3 moieties, respectively. Methyl torsion angles were not refined but constrained to be staggered. The borohydride H atoms were located from a difference-Fourier map and their positions were freely refined. U iso (H) values were set to a multiple of U eq (C/B) with 1.5 for CH 3 and BH 4 and 1.2 for C-H units, respectively. Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.