Thermodynamic Studies and Hydride Transfer Reactions from a Rhodium Complex to BX₃ Compounds

This study examines the use of transition-metal hydride complexes that can be generated by the heterolytic cleavage of H2 gas to form B−H bonds. Specifically, these studies are focused on providing a reliable and quantitative method for determining when hydride transfer from transition-metal hydrides to three-coordinate BX3 (X = OR, SPh, F, H; R = Ph, p-C6H4OMe, C6F5, tBu, Si(Me)3) compounds will be favorable. This involves both experimental and theoretical determinations of hydride transfer abilities. Thermodynamic hydride donor abilities (ΔG°H−) were determined for HRh(dmpe)2 and HRh(depe)2, where dmpe = 1,2-bis(dimethylphosphinoethane) and depe = 1,2-bis(diethylphosphinoethane), on a previously established scale in acetonitrile. This hydride donor ability was used to determine the hydride donor ability of [HBEt3]− on this scale. Isodesmic reactions between [HBEt3]− and selected BX3 compounds to form BEt3 and [HBX3]− were examined computationally to determine their relative hydride affinities. The use of these scales of hydride donor abilities and hydride affinities for transition-metal hydrides and BX3 compounds is illustrated with a few selected reactions relevant to the regeneration of ammonia borane. Our findings indicate that it is possible to form B−H bonds from B−X bonds, and the extent to which BX3 compounds are reduced by transition-metal hydride complexes forming species containing multiple B−H bonds depends on the heterolytic B−X bond energy. An example is the reduction of B(SPh)3 using HRh(dmpe)2 in the presence of triethylamine to form Et3N−BH3 in high yields.