Electrocatalytic hydrogen evolution by a dinuclear copper complex and mechanistic elucidation through DFT studies.

A novel dinuclear copper complex, [CuII2(L1)2] (L1 = 2-{[2-(8-hydroxyquinolin-2-yl)-1H-benzimidazol-1-yl]methyl}quinolin-8-ol) was synthesised and characterised through various spectroscopic techniques. This dinuclear complex (as an electrocatalyst) was employed to examine the catalytic ability towards an electrochemical hydrogen evolution reaction (HER). Redox studies in 95/5 (v/v) DMF/H2O with the addition of 30-equivalent AcOH (acid source) led to higher catalytic activities for the HER. The evolved H2, as the resultant product, was detected and confirmed from gas chromatography to afford a faradaic efficiency of 93% at an applied potential of -1.9 V vs. SCE. Based upon measurements of open-circuit potential and electrocatalytic responses, the mechanistic route for the reduction process using [CuII2(L1)2] was elucidated. Density functional theory studies reveal that through a concerted proton-coupled electron transfer (PCET) path, the HER proceeded via the formation of a Cu-H bond with a low activation energy for the dehydrogenation reaction.

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

In this work, two novel dinuclear cobalt complexes, [CoII(hbqc)(H2O)]2 (Co-Cl) and [CoII(hbqn)(H2O)]2 (Co-NO2), 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-NO2-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-NO2 is more efficient toward molecular hydrogen formation reaction than Co-Cl.