Hydride transfer from rhodium complexes to triethylborane

The hydrides HRh(depe)(2) and HRh(dmpe)(2) (depe = Et2PCH2CH2PEt2, dmpe = Me2PCH2CH2PMe2) have thermodynamic hydride donor abilities comparable to LiHBEt3, as indicated by their ability to transfer a hydride ligand to Et3B to sequentially form [Et3BHBEt3](-) and [HBEt3](-). HRh(depe)(2) and HRh(dmpe)(2) can be generated from [Rh(dmpe)(2)](CF3SO3) and [Rh(depe)(2)](CF3SO3) and hydrogen gas in the presence of a strong base such as potassium tert-butoxide or lithium diisopropylamide. This reaction proceeds through the oxidative addition of hydrogen to form the [H2Rh(diphosphine)(2)](CF3SO3) complexes, followed by deprotonation. The oxidative addition of H-2 is favored by diphosphine ligands with electron-donating substituents and large chelate bites. In the present study, the driving force for oxidative addition of H-2 follows the order [Rh(dmpe)(2)](CF3SO3) > [Rh(depe)(2)](CF3SO3) > [Rh(dppe)(2)](CF3SO3) with [Rh(dmpe)(2)]( CF3SO3) binding H-2 more strongly than [Rh(dppe)(2)](CF3SO3) (dppe =Ph2PCH2CH2PPh2) by at least 2.7 kcal/mol. The effect of the chelate bite size is larger. [H2Rh(depx)(2)](CF3SO3) (depx = 1,2-(Et-2-PCH2) 2C6H4) binds H-2 more strongly than [Rh(depe)(2)](CF3SO3) by 12 kcal/mol. An understanding of both hydrogen activation and hydride donor abilities is important for developing powerful hydride donors from H-2.