The Influence of the Second and Outer Coordination Spheres on Rh(diphosphine)2 CO2 Hydrogenation Catalysts

A series of [Rh­(PCH2XRCH2P)2]+ complexes was prepared to investigate second and outer coordination sphere effects on CO2 hydrogenation catalysis, where X is CH2 (dppp) or X–R is N–CH3, N–CH2COOH (glycine), N–CH2COOCH3 (Gly-OMe), or N–CH2C­(O)­N–CH­(CH3)­COOCH3 (GlyAla-OMe). All of these complexes were active for CO2 reduction to formate, with the N–CH3 derivative offering an 8-fold enhancement over the dppp derivative, which is consistent with increased electron density around the metal. Despite the increase in rate with the addition of the pendant nitrogen, the addition of electron withdrawing amino acids and dipeptides to the amine resulted in complexes with reductions in rate of 1 to 2 orders of magnitude, most consistent with a change in pKa of the pendant amine, resulting in lower activity. Collectively, the data suggest multiple contributions of the pendant amine in this catalytic system.