Amine groups alter product selectivity and rate of catalytic hydride transfer reactions

Abstract

Primary amines are common functional groups in the reaction environment surrounding an (electro)catalyst, and this includes catalysts ranging from metalloenzymes surrounded by amino acids, to electrocatalysts operating in amine industrial sorbents for CO₂ capture and conversion. This report explores the behavior of amine functional groups at the surface of an electrocatalyst. The possible effects of those amine secondary coordination sphere (SCS) groups on a CO₂ electro-reduction mechanism include stabilization of intermediates and positioning substrate near the active site. Two different clusters were synthesized: [PPN][Fe₄N(CO)₁₁(Ph₂PCH₂CH₂NH₂)] (PPN-1) has one amine, and [PPN][Fe₄N(CO)₁₀(Ph₂PCH₂CH₂NH₂)₂] (PPN-2) has two covalently appended amine functional groups (PPN: bis(triphenylphosphine)iminium). Infra-red spectroscopic studies show a direct reaction of each cluster with CO₂ to afford an SCS carbamate functional group, and cyclic voltammetry investigations reveal a variety of roles for the amine SCS groups in the mechanism of catalyst hydride formation and hydride transfer (HT) to CO₂. The most prominent effect of the amine functional group is stabilization of the intermediate hydride to lower formate yield. With PPN-1, these combined effects serve to shut down HT to CO₂. With PPN-2, the combined effects result in some loss of selectivity, so that formate and H₂ mixtures (6 : 1) are obtained.