Precatalyst Evolution in PBiP-Pd Complexes for Electrocatalytic Proton Reduction

Main-group metals as supporting ligands for transition metals offer potential for bimetallic synergistic effects. We investigated a bismuth-palladium system utilizing a PBiP pincer ligand (BiPdCl) as precatalysts for electrocatalytic proton reduction with thiophenol. Rinse tests revealed the formation of a surface-bound active species, likely comprised of Pd-based heterogenous materials. NMR and UV-visible spectroscopic studies unveiled that in diluted solutions of polar coordinating solvents, the chloride trans to the bismuthane readily dissociates, affording the solvent-coordinated species (BiPdsolvent) as the predominant solution species. Following thiol– solvent ligand exchange, an equilibrium mixture of the solvent-bound species and the thiolate complex (BiPdS) forms. The crystal structures of BiPdS and an acetonitrile-coordinated model complex (BiPdAcN) shows the thermodynamic trans influence of the bismuthane ligand. Structural analysis, corroborated by computational investigations, suggests that the reduction of BiPdsolvent is primarily ligand-based than Pd-centered, hinting at a Bi(III)–Pd(0) oxidation states rather than the Bi(I)–Pd(II) model suggested by the Lewis structure of BiPdsolvent. The electron-rich Pd and electron-poor Bi centers explain its electrochemical decomposition similar to other Pd(0) species. Our findings shed light on the electronic properties and reactivities of the studied Bi–Pd compounds, offering an example of ambiguous oxidation states within this system.