Synthesis of cobalt(ii) phenolate selenoether complexes to mimic hydrogenase-like activity for hydrogen gas production

Abstract

Selenium-derived electrocatalysts have been well explored for electrocatalytic hydrogen evolution reactions to mimic hydrogenase-like activity; however, the stability of these synthetic mimics is yet to be enhanced. In this study, we report the synthesis and characterization of a series of 1,10-phenanthroline-cobalt(ii) phenolate selenoether complexes using 1,10-phenanthroline and 6-nitro-1,10-phenanthroline-Co(ii)-dichloride and substituted bis-selenophenolate ligands. The synthesized cobalt(ii) phenolate selenoether complexes have been characterized by CHN analysis, mass spectrometry, single crystal XRD, and UV-visible absorption spectroscopy. These complexes show electrocatalytic proton reduction from acetic acid at an overpotential of 0.45-0.56 V vs. Fc+/Fc and surpass previously reported selenium and sulfur-containing electrocatalysts. Furthermore, gas analysis from control potential electrolysis confirms that the cobalt(ii) selenoethers act as electrocatalysts to produce H2 with a faradaic efficiency of 43-83% and show a turnover number of 3.24-58.60 molcat−1. The hydrogen evolution reaction (HER) was probed using deuterated acetic acid, which demonstrates an inverse kinetic isotopic effect (KIE) and is consistent with the formation of metal hydride intermediates. Furthermore, control experiments (post-dip analysis and multiple CV studies) have been performed to support the catalysis being due to a homogeneous process. Acid titration using UV-visible spectroscopy reveals that protonation is the prior step for electrocatalysis and assists in the formation of a cobalt hydride intermediate, which upon reaction with a proton generates hydrogen gas.