The influence of the second and outer coordination spheres on Rh(diphosphine)₂ CO₂ hydrogenation catalysts

A series of [Rh(PCH₂X^RCH₂P)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 pK_a of the pendant amine, resulting in lower activity. Collectively, the data suggest multiple contributions of the pendant amine in this catalytic system.