Steric and Lewis Basicity Influence of the Second Coordination Sphere on Electrocatalytic CO Reduction by Manganese Bipyridyl Complexes.

This study aims to provide a greater insight into the balance between steric (bpy vs (Ph)bpy vs mesbpy ligands) and Lewis basic ((Ph)bpy vs (MeOPh)bpy vs (MeSPh)bpy ligands) influence on the efficiencies of the protonation-first vs reduction-first CO reduction mechanisms with [Mn(Rbpy)(CO)(CHCN)] precatalysts, and on their respective transition-state geometries/energies for rate-determining C-OH bond cleavage toward CO evolution. The presence of only modest steric bulk at the 6,6'-diphenyl-2,2'-bipyridyl ((Ph)bpy) ligand has here allowed unique insight into the mechanism of catalyst activation and CO binding by navigating a perfect medium between the nonsterically encumbered bpy-based and the highly sterically encumbered mesbpy-based precatalysts. Cyclic voltammetry conducted in CO-saturated electrolyte for the (Ph)bpy-based precatalyst confirms that CO binding occurs at the two-electron-reduced activated catalyst in the absence of an excess proton source, in contrast to prior assumptions that all manganese catalysts require a strong acid for CO binding.