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

Abstract

In this work, two novel dinuclear cobalt complexes, [Co II (hbqc)-(H 2 O)] 2 (Co-Cl) and [Co II (hbqn)(H 2 O)] 2 (Co-NO 2 ), featuring benzimidazole derived redox-active ligand have been synthesized to investigate their catalytic activities toward electrocatalytic proton reduction (where hbqc is 2. The electrochemical responses in 95/5 (v/v) DMF/H 2 O with the addition of 24 equiv of AcOH as a proton source manifest high catalytic activity for proton reduction to H 2 . The catalytic reduction event yields H 2 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 NO 2 -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 NO 2 -substituted Co-NO 2 -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 2 is more efficient toward molecular hydrogen formation reaction than Co-Cl.

Bioinspired Diiron Complex with Proton Shuttling and Redox-Active Ligand for Electrocatalytic Hydrogen Evolution

“…Furthermore, crossing in the traces of cyclic voltammogram was noted during the reverse scan when the concentration of HDBU + exceeds 30 mM. A similar curve crossing was noted in the CV traces of [Co(hbqc)(H 2 O)] 2 (where hbqc is the 2-{[6-chloro-2-(8-hydroxyquinolin-2-yl)-1 H -benzimidazol-1-yl]methyl}quinolin-8-ol) dinuclear cobalt complex at higher concentration of AcOH . This phenomenon generally occurs due to the formation of a catalytic intermediate which has relatively positive resting potential and is not stable in the cathodic region or there is deposition of catalyst due to the side reactions at high concentration of acid .…”

“…Additionally, the Fe–Fe distance was increased from 3.09 to 3.31 Å, and the ∠Fe–O–Fe bond angle was reduced to 112.1°. A similar increase in the metal-to-metal separation was also observed in the [Co(hbqc)(H 2 O)] 2 dinuclear cobalt complex after protonation of bridged oxygen . In addition to this, average Fe–O and Fe–N amide bond lengths increased from 1.72 and 2.01 Å to 2.00 and 2.13 Å, respectively.…”

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

“…In summary, Int-1 catalyzed a feasible path for stepwise two-proton and two-electron addition reactions by ECEC and EECC (Scheme 4), but EECC was the most favourable. 41 After the electron-addition reaction, electrons were delocalized among Cu and ligand centres, and the hydrogenation reaction proceeded via the formation of two O–H bonds. After the hydrogenation reaction, one H atom migrated from the O centre to the Cu centre to make a Cu–H bond ( Int-5b ).…”

An oxo‐acetato‐bridged trinuclear cobalt(III) cluster‐persuaded cobalt oxide active electrocatalyst for efficient hydrogen evolution activity

“…Notably, numerous dinuclear cobalt complexes stand as excellent electrocatalysts over several mononuclear cobalt complexes for efficient proton reduction with a high TOF and high molecular stability under electrochemical conditions. [53][54][55][56] Surprisingly, electrocatalytic hydrogen generation with trinuclear cobalt complexes is rarely reported. Eventually, we did not find a single example of a tricobalt(III) complex that shows an efficient HER in alkaline conditions.…”