Ligand-Mediated Hydrogen Evolution by Co(II) Complexes and Assessment of the Mechanism by Computational Studies

In this work, two novel dinuclear cobalt complexes, [Co]₂ (Co-Cl) and [Co]₂ (Co-NO), featuring benzimidazole derived redox-active ligand have been synthesized to investigate their catalytic activities toward electrocatalytic proton reduction (where hbqc is 2-{[6-chloro-2-(8-hydroxyquinolin-2-yl)-1H-benzimidazol-1-yl]methyl}quinolin-8-ol and hbqn is 2-{[6-nitro-2-(8-hydroxyquinolin-2-yl)-1H-benzimidazol-1-yl]methyl}quinolin-8-ol). The electrochemical responses in 95/5 (v/v) DMF/H₂O with the addition of 24 equiv of AcOH as a proton source manifest high catalytic activity for proton reduction to H₂. The catalytic reduction event yields H₂ at an applied potential of -1.9 V vs SCE. A faradaic efficiency of 85-89% was obtained from gas chromatography analysis. A series of experiments performed concluded the homogeneous behavior of these molecular electrocatalysts. Between the two complexes, the Cl-substituted analogue, Co-Cl, has an increased overpotential of 80 mV compared to its NO₂-substituted counterpart, exhibiting lesser catalytic activity toward the reduction process. The high stability of electrocatalysts under the electrocatalytic conditions was established, as no noticeable degradation of catalysts was observed throughout the process. All these measurements were exploited to elucidate the mechanistic route by these molecular complexes for the reduction process. The mechanistic pathways were suggested to be operational with EECC (E: electrochemical and C: chemical). The overall reaction energy by NO₂-substituted Co-NO-catalyzed reaction is more exogenic than Cl-substituted Co-Cl-catalyzed reaction; the corresponding reaction energies are -88.9 and -85.1 kcal mol^-1. The computational study indicates that Co-NO is more efficient toward molecular hydrogen formation reaction than Co-Cl.