Molecular Electrocatalysts for Oxidation of Hydrogen Using Earth-Abundant Metals: Shoving Protons Around with Proton Relays

Abstract

Sustainable, carbon-neutral energy is needed to supplant the worldwide reliance on fossil fuels in order to address the persistent problem of increasing emissions of CO2. Solar and wind energy are intermittent, highlighting the need to develop energy storage on a huge scale. Electrocatalysts provide a way to convert between electrical energy generated by renewable energy sources and chemical energy in the form of chemical bonds. Oxidation of hydrogen to give two electrons and two protons is carried out in fuel cells, but the typical catalyst is platinum, a precious metal of low earth abundance and high cost. In nature, hydrogenases based on iron or iron/nickel reversibly oxidize hydrogen with remarkable efficiencies and rates. Functional models of these enzymes have been synthesized with the goal of achieving electrocatalytic H2 oxidation using inexpensive, earth-abundant metals along with a key feature identified in the [FeFe]-hydrogenase: an amine base positioned near the metal. The diphosphine ligands P(R)2N(R')2 (1,5-diaza-3,7-diphosphacyclooctane with alkyl or aryl groups on the P and N atoms) are used as ligands in Ni, Fe, and Mn complexes. The pendant amines facilitate binding and heterolytic cleavage of H2, placing the hydride on the metal and the proton on the amine. The pendant amines also serve as proton relays, accelerating intramolecular and intermolecular proton transfers. Electrochemical oxidations and deprotonations by an exogeneous amine base lead to catalytic cycles for oxidation of H2 (1 atm) at room temperature for catalysts derived from [Ni(P(Cy)2N(R')2)2](2+), Cp(C6F5)Fe(P(tBu)2N(Bn)2)H, and MnH(P(Ph)2N(Bn)2)(bppm)(CO) [bppm = (PAr(F)2)2CH2]. In the oxidation of H2 catalyzed by [Ni(P(Cy)2N(R')2)2](2+), the initial product observed experimentally is a Ni(0) complex in which two of the pendant amines are protonated. Two different pathways can occur from this intermediate; deprotonation followed by oxidation occurs with a lower overpotential than the alternate pathway involving oxidation followed by deprotonation. The Mn cation [Mn(P(Ph)2N(Bn)2)(bppm)(CO)](+) mediates the rapid (>10(4) s(-1) at -95 °C), reversible heterolytic cleavage of H2. Obtaining the optimal benefit of pendant amines incorporated into the ligand requires that the pendant amine be properly positioned to interact with a M-H or M(H2) bond. In addition, ligands are ideally selected such that the hydride-acceptor ability of the metal and the basicity of a pendant are tuned to give low barriers for heterolytic cleavage of the H-H bond and subsequent proton transfer reactions. Using these principles allows the rational design of electrocatalysts for H2 oxidation using earth-abundant metals.

Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides

“…In terms of the second coordination sphere, the presence of Brønsted basic sites within a model complex can have a strong influence on the electrocatalytic mechanism and potential at which catalysis occurs. 281 Basic residues are protonated in acidic media, resulting in the catalyst bearing less negative charge and giving rise to milder reductions relative to those of more anionic species. For example, the mechanism proposed for HOTs reduction 282 catalyzed by [(OC) 3 Fe(adt R )Fe(CO) 3 ] ([ 19 ], R = i Pr, CH 2 CH 2 OCH 3 ) begins with protonation of the amino group to form [ 19 H] + (Figure 26).…”

“…The present class of compounds combines Ni with amines reminiscent of adt 2− , the latter being positioned far enough away such that Ni–N bonding is unfavorable, yet close enough to rapidly relay H + to and from the metal. 281 …”

Electrocatalytic H2 Evolution by the Co‐Mabiq Complex Requires Tempering of the Redox‐Active Ligand

“…In such mechanisms, the metal site is not the exclusive protonation site, or is not involved at all. Amine and thiol groups, including in the hydrogenase active sites, readily take up protons that subsequently can be transferred to a metal‐hydride to effect H 2 release . Furthermore, redox‐active ligands can store both protons and electrons .…”

Cyclopentadienyl Cobalt Complexes as Precatalysts for Electrocatalytic Hydrogen Evolution

“…low overpotential) is a key challenge. Significant advances towards this goal have been made over the past decade …”