H₂ Evolution from Electrocatalysts with Redox-Active Ligands: Mechanistic Insights from Theory and Experiment vis-à-vis Co-Mabiq

Abstract

Electrocatalytic hydrogen production via transition metal complexes offers a promising approach for chemical energy storage. Optimal platforms to effectively control the proton and electron transfer steps en route to H2 evolution still need to be established, and redox-active ligands could play an important role in this context. In this study, we explore the role of the redox-active Mabiq (Mabiq = 2–4:6–8-bis­(3,3,4,4-tetramethlyldihydropyrrolo)-10–15-(2,2-biquinazolino)-[15]-1,3,5,8,10,14-hexaene1,3,7,9,11,14-N6) ligand in the hydrogen evolution reaction (HER). Using spectro-electrochemical studies in conjunction with quantum chemical calculations, we identified two precatalytic intermediates formed upon the addition of two electrons and one proton to [CoII(Mabiq)­(THF)]­(PF6) (CoMbq ). We further examined the acid strength effect on the generation of the intermediates. The generation of the first intermediate, CoMbq-H1 , involves proton addition to the bridging imine-nitrogen atom of the ligand and requires strong proton activity. The second intermediate, CoMbq-H2 , acquires a proton at the diketiminate carbon for which a weaker proton activity is sufficient. We propose two decoupled H2 evolution pathways based on these two intermediates, which operate at different overpotentials. Our results show how the various protonation sites of the redox-active Mabiq ligand affect the energies and activities of HER intermediates.

Ligand-Based Redox: Catalytic Applications and Mechanistic Aspects

“…Catalyst 120 performs an electrocatalytic HER reaction at −1.32 V, in the presence of slightly acidic anilinium salt. 114 The reduction of catalyst in this case is entirely ligand centered, forming Co II (Mabiq • ) 0 (121) and leaving the oxidation state of Co II unchanged. Through spectro-electrochemical experiments, the second reduction of the complex was also assigned to be ligand centered, forming [Co II (Mabiq •• )] − , followed by protonation to form neutral Co II (Mabiq-H 2 ) (122) (Figure 46).…”

“…A few such examples are Ni-porphyrin [(Ni II (Porp)] (117) by Nocera 113 and Co-Mabiq (120) by Hess. 114 Catalyst 117 in the presence of a weak acid and at a potential of −1.91 V uptakes 2e − /H + to form a stable Niphlorin intermediate (Ni I Phl) − (118) (Figure 45). In an excess of acid, the reversibility of 118 is lost and instead of producing hydrogen as observed with the Ni-hangman catalyst, 118 undergoes another 2e − /3H + reduction to protonate the ligand, producing neutral Ni-isobacteriochlorin (Ni I iBC) (119).…”

“…Through spectro-electrochemical experiments, the second reduction of the complex was also assigned to be ligand centered, forming [Co II (Mabiq •• )] − , followed by protonation to form neutral Co II (Mabiq-H 2 ) (122) (Figure 46). 114 The adjacent carbons possessing hydrogens can liberate dihydrogen under a low overpotential, thus proving the ligand-dominated redox processes to be an effective strategy for HER electrocatalysts.…”

“…Similarly, several other hydrogen evolution electrocatalysts are reported which exploit redox noninnocence of the ligand to store electrons and/or protons on the ligand backbone to drive the catalysis. A few such examples are Ni-porphyrin [(Ni II (Porp)] ( 117 ) by Nocera and Co-Mabiq ( 120 ) by Hess . Catalyst 117 in the presence of a weak acid and at a potential of −1.91 V uptakes 2e – /H + to form a stable Ni-phlorin intermediate (Ni I Phl) − ( 118 ) (Figure ).…”

Electrocatalytic CO2 Reduction and H2 Evolution by a Copper (II) Complex with Redox-Active Ligand

“…Our group also explored the crucial effect of the redox-active ligand bis(imino)pyridine (PDI) on the electrocatalytic activity for CO 2 reduction [15]. Hess and co-workers investigated the electrocatalytic H 2 evolution by a Co (II) complex bearing macrocycle ligand, and they found that the redox-active ligand could modulate the energy and activity of HER [16]. Marinescu's group prepared a cobalt (II) complex with thiolate ligand, and they found that protonation of the redox non-innocent ligand could influence the electrocatalytic reactivity for syngas generation [17].…”