Enzyme-Inspired Design of CoII-Based H2 Generation Catalysts: A Toolbox with Guiding Principles Revealed by a Systematic DFT Study

Abstract

Inspired by the proton relay pathway, as found in the hydrogenase enzymes, herein we report a series of highly promising pentadentate CoII-[N2Nring3]-based H2 generation molecular catalysts using experimentally easily accessible, redox-innocent oxamide and pyridine-/N-methyl imidazole-rich motifs. These newly designed catalysts can display proton relay using the donor nitrogen atom of the pendant arm in the catalytically active CoI state. The ligand framework contains three tether chains connecting the oxamide to pyridine/N-methyl imidazole rings. The length of the tethers and position of attachment of the third tether are found to have a significant impact on the stability of the CoII catalyst. The electronic property of the pendant ligand can be modulated by judicious choice of the functional group such that the proton relay mechanism can be triggered at acidic, basic, and neutral pH of the medium. Moreover, it has been found that electron donating functional groups can avoid formation of an unproductive ligand-reduced intermediate which can contribute to catalyst deactivation. Detailed DFT analysis revealed a set of much-needed guiding principles that can be used by an experimental chemist as a toolbox for synthesis of novel H2 generation catalysts that can successfully demonstrate the biomimetic proton relay pathway while minimizing the decomposition of the metal catalysts. © 2021 Wiley-VCH GmbH