Experimental and Computational Mechanistic Studies Guiding the Rational Design of Molecular Electrocatalysts for Production and Oxidation of Hydrogen

Abstract

Understanding how to control the movement of protons and electrons is crucial to the design of fast, efficient electrocatalysts for H2 production and oxidation based on earth-abundant metals. Our work seeks to address fundamental questions about proton movement. We have demonstrated that incorporating a pendant amine functioning as a proton relay in the second coordination sphere of a metal complex helps proton mobility, resulting in faster and more energy-efficient catalysts. Proton-transfer reactions can be rate-limiting and are influenced by several factors, such as pKa values, steric effects, hydrogen bonding, and solvation/desolvation of the exogenous base and acid employed. The presence of multiple protonation sites introduces branching points along the catalytic cycle, making less productive pathways accessible or leading to the formation of stable off-cycle species. Using ligands with only one pendant amine mitigates this problem and results in catalysts with high rates for production of H2, although generally at higher overpotentials. For H2 oxidation catalysts, iron complexes with a high H2 binding affinity were developed. However, these iron complexes had a pKa mismatch between the protonated metal center and the protonated pendant amine, and consequently intramolecular proton movement was slow. Taken altogether, our results demonstrate the necessity of optimizing the entire catalytic cycle because optimization of a specific catalytic step can negatively influence another step and not necessarily lead to a better catalytic performance. We discuss a general procedure, based on thermodynamic arguments, which allows the simultaneous minimization of the free-energy change of each catalytic step, yielding a nearly flat free-energy surface, with no large barriers due to energy mismatches from either high- or low-energy intermediates.

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

“…Bidirectional catalysts typically operate at low overpotentials, in this case possibly facilitated by concerted PCET. 375,376 It was later found that the HER activity of the Ni complexes is greatly enhanced by using a “P 2 N” ligand, a seven-membered analogue of “P 2 N 2 ” ligands bearing only one NR′ substituent. When R = R′ = Ph, [Ni(P 2 N) 2 ] mediates the HER from protonated N,N-dimethylformamide (p K a = 6.1) extremely rapidly (TOF > 100 000 s −1 at −1.13 vs Fc +/0 ), albeit at a high overpotential ( η = 625 mV).…”

“…The high efficiency of H 2 ases requires that each catalytically active state be of similar free energy 375 and exist in an almost flat free energy landscape in which kinetic barriers to H 2 oxidation or evolution are minimized. 141 Reproducing these rates in synthetic systems will necessitate more rigid complexes, and no doubt motivate the synthesis of new polydentate ligands.…”

Energy Production and Storage Promoted by Organometallic Complexes

“…Hydrogenases are enzymes that can reversibly oxidize hydrogen with high efficiencies in nature. Inspired by these enzymes Bullock, DuBois and collaborators, developed complexes based on Ni, Fe, and Mn for the electrocatalytic oxidation of molecular hydrogen. All complexes are based on a P R 2 N R′ 2 cyclic 1,5‐diaza‐3,7‐diphosphacyclooctane ligand (R, R′ represent alkyl or aryl groups), that has pendent amines in close proximity to a vacant coordination site on the metal, leading to bifunctional activation of H 2 during the heterolytic cleavage of the H–H bond, mimicking the structure of the [Fe‐Fe] hydrogenase enzymes.…”

Catalysts by Design: The Power of Theory

“…12 These catalysts are bioinspired in that they incorporate the concept of a proton relay positioned near the metal center from hydrogenase enzymes but do not mimic the specific structure of the active site. The popular Ni(P 2 N 2 ) 2 hydrogen-evolving catalysts, as depicted in Figure 1d, have been particularly successful in terms of high turnover frequencies at relatively modest overpotentials.…”

“…The popular Ni(P 2 N 2 ) 2 hydrogen-evolving catalysts, as depicted in Figure 1d, have been particularly successful in terms of high turnover frequencies at relatively modest overpotentials. 12 Theoretical calculations have provided guidance for the modification of the ligands to alter the p K a ’s of the pendant amine and the metal center, as well as the reduction potentials, in ways that enhance the turnover frequency.…”