Evaluating the impacts of amino acids in the second and outer coordination spheres of Rh-bis(diphosphine) complexes for CO2 hydrogenation

Abstract

To explore the influence of a biologically inspired second and outer coordination sphere on Rh-bis(diphosphine) CO₂ hydrogenation catalysts, a series of five complexes were prepared by varying the substituents on the pendant amine in the P(Et)₂CH₂N^RCH₂P(Et)₂ ligands (P^EtN^RP^Et), where R consists of methyl ester modified amino acids, including three neutral (glycine methyl ester (GlyOMe), leucine methyl ester (LeuOMe), and phenylalanine methyl ester (PheOMe)), one acidic (aspartic acid dimethyl ester (AspOMe)) and one basic (histidine methyl ester (MeHisOMe)) amino acid esters. The turnover frequencies (TOFs) for CO₂ hydrogenation for each of these complexes were compared to those of the non-amino acid containing [Rh(depp)₂]⁺ (depp) and [Rh(P^EtN^MeP^Et)₂]⁺ (NMe) complexes. Each complex is catalytically active for CO₂ hydrogenation to formate under mild conditions in THF. Catalytic activity spanned a factor of four, with the most active species being the NMe catalyst, while the slowest were the GlyOMe and the AspOMe complexes. When compared to a similar set of catalysts with phenyl-substituted phosphorous groups, a clear contribution of the outer coordination sphere is seen for this family of CO₂ hydrogenation catalysts.