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-II(hbqc)(H2O)](2) (Co-Cl) and [Co-II(hbqn)(H2O)](2) (Co-NO2 ), featuring benzimidazolederivedredox-active ligand have been synthesized to investigate their catalyticactivities toward electrocatalytic proton reduction (where hbqc is 2-{[6-chloro-2-(8-hydroxyquinolin-2-yl)-1H-benzimidazol-1-yl]methyl}quinolin-8-oland hbqn is 2-{[6-nitro-2-(8-hydroxyquinolin-2-yl)-1H-benzimidazol-1-yl]methyl}quinolin-8-ol). The electrochemicalresponses in 95/5 (v/v) DMF/H2O with the addition of 24equiv of AcOH as a proton source manifest high catalytic activityfor proton reduction to H-2. The catalytic reduction eventyields H-2 at an applied potential of -1.9 V vs SCE.A faradaic efficiency of 85-89% was obtained from gas chromatographyanalysis. A series of experiments performed concluded the homogeneousbehavior of these molecular electrocatalysts. Between the two complexes,the Cl-substituted analogue, Co-Cl, has an increasedoverpotential of 80 mV compared to its NO2-substitutedcounterpart, exhibiting lesser catalytic activity toward the reductionprocess. The high stability of electrocatalysts under the electrocatalyticconditions was established, as no noticeable degradation of catalystswas observed throughout the process. All these measurements were exploitedto elucidate the mechanistic route by these molecular complexes forthe reduction process. The mechanistic pathways were suggested tobe operational with EECC (E: electrochemical and C: chemical). Theoverall reaction energy by NO2-substituted Co-NO2 -catalyzed reaction is more exogenic than Cl-substituted Co-Cl-catalyzed reaction; the corresponding reaction energiesare -88.9 and -85.1 kcal mol(-1). Thecomputational study indicates that Co-NO2 ismore efficient toward molecular hydrogen formation reaction than Co-Cl. Two novel dinuclear cobaltcomplexes, [Co-II(hbqc)(H2O)](2) (Co-Cl) and[Co-II(hbqn)(H2O)](2) (Co-NO2 ), featuring benzimidazole-derived redox-activeligand have been synthesized and employed for electrocatalytic protonreduction. Density functional theory studies have investigated theEECC-based mechanism for the hydrogen evolution reaction.

Automated summary

Two novel dinuclear cobalt complexes featuring redox-active ligand have been synthesized and investigated for their catalytic activities in electrocatalytic proton reduction. The experimental results demonstrate high catalytic activity for both complexes and the catalytic activity depends on the nature of substituents on the ligand. Gas chromatography analysis and computational studies provide insights into the mechanistic pathways and the energy changes in the molecular hydrogen formation reaction.