Role of Redox Active Ligand of a Cobalt-Mabiq Complex in the Hydrogen Evolution Reaction

Abstract

In nature, hydrogen is generated by a family of enzymes known as the hydrogenases, which use Fe and/or Ni as cofactors. Inspired by metalloenzyme cofactors, many earth-abundant hydrogen evolution reaction (HER) catalysts have been developed over the last decade, but their HER activity, chemical stability under operation, and overpotential could still be significantly improved. The H2 evolution mechanisms of many molecular catalysts are based on metal hydride formation, however, the role of redox-active ligands has attracted attention recently. Such systems may operate via alternative H2 evolution pathways involving mainly ligand-assisted and ligand-based routes,1, 2 in which the ligands act as H-atom or hydride reservoirs. Electron or proton storage by redox-active ligands could therewith lead to improved HER kinetics and decreased overpotentials.We will present our studies on the H2 evolution activity and possible HER pathways of a cobalt complex, containing a redox active macrocyclic biquinazoline ligand (Mabiq).3, 4 In the literature an electron transfer series of Co-Mabiq complexes exemplifies ligand-centered reduction among the formally ‘low valent’ compounds.5 The role of the ligand in H2 the evolution activity of a Co-Mabiq complex is the main focus of our study. The electrocatalytic behavior of [CoII(Mabiq)(THF)](PF6) (CoMbq) was studied in organic media (acetonitrile) using para-cyanoanilinium as the proton source.4 Besides the confirmed H2 evolution by CoMbq, the combined on-line electrochemical mass spectrometry (OEMS) and rotating ring disk electrode (RRDE) techniques denote a pre-catalytic process that involves formation of a protonated, two electron reduced intermediate (Figure1). Our results further indicate a competing deactivation pathway that is either time dependent or turnover dependent.The potential intermediate Co-Mabiq complexes in various oxidation states5 each exhibit distinct absorption spectroscopic features. Hence, in our study, bulk electrolysis and UV-Vis absorption spectrometry were further carried out to characterize the active and inactive CoMbq intermediates. Based on these findings, modifications to the ligand backbone may prevent the formation of the inactive intermediates that lead to inhibition of the deactivation pathway, and may therewith enable a prolonged activity for H2 evolution by the CoMbq complex. The combined results of our studies provide an insight into the role of Mabiq and redox-active ligands in general in the HER pathway while also providing a comprehensive toolbox of electrochemical and spectroscopic techniques to allow a deep understanding for this class of molecular H2 evolution catalysts.