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

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 H-2 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-N-6) 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) (Co-Mbq). We further examined the acid strength effect on the generation of the intermediates. The generation of the first intermediate, Co-Mbq-H-1, involves proton addition to the bridging imine-nitrogen atom of the ligand and requires strong proton activity. The second intermediate, Co-Mbq-H-2, acquires a proton at the diketiminate carbon for which a weaker proton activity is sufficient. We propose two decoupled H-2 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.

Automated summary

This study explores the role of redox-active ligands in the hydrogen evolution reaction, identifying two precatalytic intermediates and proposing two decoupled H-2 evolution pathways based on these intermediates. The results demonstrate how protonation sites of the redox-active ligand affect the energies and activities of HER intermediates.