Unveiling the Potential of p-Block Bismuth in Homogeneous Electrocatalytic Proton Reduction: Secondary Coordination...

The development of sustainable hydrogen evolution reaction (HER) based on earth-abundant molecular electrocatalysts requires strategies that enable controlled access to reactive low-valent intermediates while overcoming conventional activity-overpotential scaling relationships. Although transition metal complexes have been extensively explored, the potential of main-group systems remains largely untapped due to challenges in stabilizing reactive low-valent states and the lack of predictive catalyst design frameworks integrating electronics, geometry, and secondary sphere effects. Herein, we report a series of NCN pincer-supported Bi(III) organometallic catalysts [(L¹)BiCl₂] (1), [(L²)BiCl₂] (2), [(L³)BiCl₂] (3), [(L⁴)BiCl₂] (4), [(L⁵)BiCl₂] (5), [(L⁶)BiCl₂] (6), [(L⁷)BiCl₂] (7)) that promote HER through in situ electrochemical access of active Bi(I) intermediates. Mechanistic investigations combining potential-pK_a analysis, Tafel slope measurements, in situ electrochemical studies, detailed electrokinetic study exploring foot-of-the-wave analysis (FOWA), controlled potential electrolysis, and density functional theory calculations support a Bi^I/Bi^III redox cycle-mediated proton-coupled electron transfer (PCET) pathway involving transient bismuth hydride intermediates that are challenging to isolate under conventional chemical conditions. Systematic modulation of ligand electronics, geometry, and secondary coordination sphere features reveals a prominent role in modulating reactivity and further highlights that incorporation of a pendant -NH functionality acts as an intramolecular proton-assistance that significantly enhances catalytic activity, relative to analogues lacking pendant -NH proton functionality in the ligand backbone. Notably, secondary-sphere interactions from the ligand backbone enable these catalysts to circumvent traditional activity-overpotential scaling relationships, exhibiting enhanced performance at low overpotential that rivals state-of-the-art transition metal systems. These findings establish fundamental design principles for redox-active main-group electrocatalysis and expand the accessible redox space of the periodic table by leveraging p-block redox chemistry toward practical applications.