PNNL

Abstract

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. These hydrides can be generated from hydrogen gas in the presence of a strong base such as potassium t-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 H2 is favored by diphosphine ligands with electron donating substituents and large chelate bites. In the present study, the driving force for oxidative addition of H2 follows the order [Rh(dmpe)2](CF3SO3) > [Rh(depe)2](CF3SO3) > [Rh(dppe)2](CF3SO3) with [Rh(dmpe)2](CF3SO3) binding H2 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-(Et2PCH2)2C6H4) binds H2 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 H2. Acknowledgment: This research was supported by the Director’s Discretionary Research and Development Program of the National Renewable Energy Laboratory, and in part, by the Chemical Sciences Program of the Office of Basic Energy Sciences of the Department of Energy. The Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.