The hydrides HRh(depe) 2 and HRh(dmpe) 2 (depe = Et 2PCH 2CH 2PEt 2, dmpe = Me 2PCH 2CH 2PMe 2) have thermodynamic hydride donor abilities comparable to LiHBEt 3, as indicated by their ability to transfer a hydride ligand to Et 3B to sequentially form [Et 3BHBEt 3] - and [HBEt 3] -. HRh(depe) 2 and HRh(dmpe) 2 can be generated from [Rh(dmpe) 2](CF 3SO 3) and [Rh(depe) 2](CF 3SO 3) 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 [H 2Rh(diphosphine) 2](CF 3SO 3) 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](CF 3SO 3) > [Rh(depe) 2](CF 3SO 3) > [Rh(dppe) 2](CF 3SO 3) with [Rh(dmpe) 2]-(CF 3SO 3) binding H 2 more strongly than [Rh(dppe) 2](CF 3SO 3) (dppe = Ph 2PCH 2CH 2PPh 2) by at least 2.7 kcal/mol. The effect of the chelate bite size is larger. [H 2Rh(depx) 2](CF 3SO 3) (depx = 1,2-(Et 2-PCH 2) 2C 6H 4) binds H 2 more strongly than [Rh(depe) 2](CF 3SO 3) by 12 kcal/mol. An understanding of both hydrogen activation and hydride donor abilities is important for developing powerful hydride donors from H 2.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry