Electrocatalytic Oxidation of Formate by [Ni(P^R₂N^R′₂)₂(CH₃CN)]²⁺ Complexes

Abstract

[Ni(P(R)(2)N(R')(2))(2)(CH(3)CN)](2+) complexes with R = Ph, R' = 4-MeOPh or R = Cy, R' = Ph , and a mixed-ligand [Ni(P(R)(2)N(R')(2))(P(R''(2))N(R'(2)))(CH(3)CN)](2+) with R = Cy, R' = Ph, R'' = Ph, have been synthesized and characterized by single-crystal X-ray crystallography. These and previously reported complexes are shown to be electrocatalysts for the oxidation of formate in solution to produce CO(2), protons, and electrons, with rates that are first-order in catalyst and formate at formate concentrations below ∼0.04 M (34 equiv). At concentrations above ∼0.06 M formate (52 equiv), catalytic rates become nearly independent of formate concentration. For the catalysts studied, maximum observed turnover frequencies vary from <1.1 to 15.8 s(-1) at room temperature, which are the highest rates yet reported for formate oxidation by homogeneous catalysts. These catalysts are the only base-metal electrocatalysts as well as the only homogeneous electrocatalysts reported to date for the oxidation of formate. An acetate complex demonstrating an η(1)-OC(O)CH(3) binding mode to nickel has also been synthesized and characterized by single-crystal X-ray crystallography. Based on this structure and the electrochemical and spectroscopic data, a mechanistic scheme for electrocatalytic formate oxidation is proposed which involves formate binding followed by a rate-limiting proton and two-electron transfer step accompanied by CO(2) liberation. The pendant amines have been demonstrated to be essential for electrocatalysis, as no activity toward formate oxidation was observed for the similar [Ni(depe)(2)](2+) (depe = 1,2-bis(diethylphosphino)ethane) complex.

Dynamic Processes in Biology, Chemistry, and Materials Science: Opportunities for UltraFast Transmission Electron Microscopy - Workshop Summary Report

“…A molecular understanding of these natural mechanisms will guide the synthesis of effective molecular catalysts (http://iic.pnnl.gov/), which currently are unable to catalyze CO 2 reduction to formate, to create mimics that like FDH H are able to catalyze carbon fixation 43,44 . Under biological conditions, FDH H exists as part of a formate-hydrogen lyase complex that decomposes formate to hydrogen and carbon dioxide under anaerobic conditions in the absence of exogenous electron acceptors 45 .…”

“…An inorganic sulfur (S i ) ligand is bound to Mo (at least in some states), and may participate in modifying the reactivity of the active site. The reduction mechanism involves SeCys 140 and His 141 in the rate-limiting step of proton abstraction and the Mo-molybdopterin, Lys 44 , and the [4Fe-4S] cluster in promoting electron transfer (where the [4Fe-4S] cluster is thought to permit a sequential ping-pong one-electron transfer mechanism) 46 . However, while the function of the Mo-molybdopterin cofactor is clear, the precise role of the active site Se remains uncertain 46,47,51 .…”

Compact X-ray Light Source Workshop Report

“…This information is critical to their redesign to promote synthetic biology applications associated with enhanced carbon capture and formation of biofuels. Likewise, a molecular understanding of these natural mechanisms will guide the synthesis of effective molecular catalysts, which currently are unable to catalyze CO 2 reduction (carbon fixation) that is catalyzed readily by FDH H (Reda et al 2008;Galan et al 2011). Under biological conditions, FDH H exists as part of a formate hydrogen lyase complex that decomposes formate to hydrogen and CO 2 under anaerobic conditions in the absence of exogenous electron acceptors.…”

Electrocatalytic Oxidation of Formate with Nickel Diphosphine Dipeptide Complexes: Effect of Ligands Modified with Amino Acids

“…[11] Mechanistic studies of this system [11,12] suggest that after the binding of formate, there is a rate-limiting proton transfer concomitant with a two-electron transfer process accompanied by the loss of CO 2 . This process is thought to involve proton ab-straction by the pendant amine rather than a direct hydride transfer, because the pendant amines are necessary for catalysis and the rates of catalysis become faster with increasing basicity of the pendant amine (Scheme 1).…”

“…Proposed transition state for the electrocatalytic oxidation of formate. [11,12] The molybdenum-and iron-containing formate dehydrogenase enzyme ([Se]FDH H ), isolated from E. coli grown under anaerobic conditions, is capable of interconverting formate and CO 2 at rates of up to 2800 s -1 for formate oxidation. [13] The mechanism proposed for the catalytic system above is analogous to many of the mechanisms proposed for formate dehydrogenase in which two electrons are transferred to the metal in the active site and the proton, H + , is transferred to an adjacent heteroatom.…”