Synthesis, structural characterization, and electrocatalytic hydrogen generation activity of fac-tris(β-oxodithioester-O^S)cobalt(III) complexes

Abstract

With the growing interest in cobalt-based complexes for electrocatalytic applications, we synthesized four homoleptic facial-octahedral Co(III) β-oxodithioester complexes (1-4) with the general composition [Co(L)3], where L represents 3-(methylthio)-1-(furan-2-yl)-3-thioxo prop-1-en-1-olate (L1), 3-(methylthio)-1-(thiophen-2-yl)-3-thioxoprop-1-en-1-olate (L2), 3-(methylthio)-1-(pyridin-3-yl)-3-thioxoprop-1-en-1-olate (L3), and 3-(methylthio)-1-phenyl-3-thioxoprop-1-en-1-olate (L4). The complexes were fully characterized using elemental analysis, FTIR, UV-Vis, 1H and 13C{1H} NMR spectroscopy. Single-crystal X-ray diffraction confirmed a distorted octahedral geometry around each cobalt center, with three ligands coordinated in an O^S-chelating mode. Complexes were evaluated for their electrocatalytic activity towards proton reduction. Electrochemical studies were carried out in dimethylformamide (DMF) with trifluoroacetic acid (TFA) serving as the proton donor. The findings indicated that all complexes exhibit electrocatalytic hydrogen evolution activity. Notably, complex 1 demonstrates superior catalytic activity among the four complexes, achieving an overpotential of 580 mV, high turnover frequencies (~103 s-1) and Faradaic efficiency (89%). The UV-Vis spectral and rinse test studies together with controlled potential electrolysis (CPE) indicated that all complexes are stable under acidic conditions, maintaining their molecular integrity throughout the catalytic process. Furthermore, computational and spectroelectrochemical studies support an ECEC mechanism for proton reduction by β-oxodithioester cobalt complexes, where the second protonation step is rate-determining. This is also supported by foot-of-wave analysis (FOWA) analysis with the second protonation step as a rate-determining step (k1>k2). This step generates a Co(II)-H intermediate, which subsequently leads to hydrogen evolution.