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.