Quantifying U-H covalency in a series of uranyl borohydrides using 1H NMR spectroscopy.

The reaction of [UO2(OTf)2(Cy3PO)2] (1) with 2.4 equiv. of NaBH4 in THF affords [UO2(κ2-BH4)2(Cy3PO)2] (2) as yellow blocks in moderate isolated yield. Similarly, reaction of [UO2Cl2(THF)3] with 1 equiv. of [K(DME)2(H2-9-BBN)]2 ([H2-9-BBN]- = dihydrido 9-boratabicyclo-[3.3.1]nonane) in Et2O yields [UO2(κ2-H2-9-BBN)2(Et2O)2] (3) in moderate yields. Subsequent reaction of 3 with 1.8 equiv. of Cy3PO in toluene results in formation of [UO2(κ2-H2-9-BBN)2(Cy3PO)2] (4). Complexes 1-4 were characterized by X-ray crystallography, multinuclear NMR spectroscopy, and IR spectroscopy. The 1H NMR spectra for complexes 2, 3, and 4 exhibit significantly downfield shifted resonances for their hydride environments. This deshielding is due to strong spin-orbit effects arising from orbital interactions between the uranium center and the borohydride ligands. Density functional theory (DFT) calculations accurately reproduced the experimental chemical shifts and revealed that the uranium contribution to the U-H-B molecular orbitals ranged from 11-14%, depending on the complex, of which approximately 40% was due to the 5f orbitals.