Hydrogen Evolution Mediated by Cobalt Diimine‐Dioxime Complexes: Insights into the Role of the Ligand Acid/Base Functionalities.

Abstract

The benchmarking of the performance for H 2 evolution of cobalt diimine-dioxime catalysts is provided based on a comprehensive study of their catalytic mechanism. The latter follows an ECE'CC pathway with intermediate formation of a Co(II)-hydride intermediate and second protonation possibly at a basic site of the ligand, acting as a proton relay. This suggests an intramolecular coupling between the hydride and protonated ligand as the proton concentration-independent ratedetermining step controlling the turnover frequency for H 2 evolution.

Deciphering Reversible Homogeneous Catalysis of the Electrochemical H2 Evolution and Oxidation: Role of Proton Relays and Local Concentration Effects

“…Such a positive positioning of the standard potential of the Ni III H/Ni II H couples (in both protonation states of the relay) compared to the Ni II /Ni I couple is in full agreement with previous calculation on cobalt-based H2 evolution catalysts: the standard potentials of the Co III H/Co II H couples of cobaloximes [24] and cobalt diimine-dioxime complexes [25] were also found positive by 100 and 130 mV to the respective Co II /Co I couples.…”

Deciphering Reversible Homogeneous Catalysis of the Electrochemical H2 Evolution and Oxidation: Role of Proton Relays and Local Concentration Effects**

“…Such a positive positioning of the standard potential of the Ni III H/Ni II H couples (in both protonation states of the relay) compared to the Ni II /Ni I couple is in full agreement with previous calculation on cobalt-based H 2 evolution catalysts: the standard potentials of the Co III H/Co II H couples of cobaloximes [24] and cobalt diimine-dioxime complexes [25] were also found positive by 100 and 130 mV to the respective Co II /Co I couples.…”

Electrocatalytic reduction of protons to dihydrogen by the cobalt tetraazamacrocyclic complex [Co(N4H)Cl2]+: mechanism and benchmarking of performances

“…The rate-determining step has been confirmed as the intramolecular H 2 evolution step, surmised to be coupled to the second protonation, thereby regenerating a protonated ligand and thus acting as a proton relay in catalysis, 32 which, interestingly, is at variance with the behaviour of the dioxime bridge in cobalt diimine–dioxime complexes. 48 Catalytic Tafel plots could be derived to enable the benchmarking of the H 2 evolution performance of Cat1 alongside other highly efficient catalysts and confirm its place on the podium.…”

“…Its overpotential requirement, estimated to be ∼400 mV at the catalytic half-wave potential (and corresponding to the inflexion point in the red catalytic Tafel plot on Fig. 6 ) is ∼100 mV higher than that of cobaloxime, cobalt diimine–dioxime 48 or DuBois' complexes. Importantly, the high TOF max value also places Cat1 in an intermediate position between cobaloximes and DuBois' complexes, the two champion H 2 -evolving molecular catalyst series identified so far in non-aqueous solvents.…”