Probing the Viability of Oxo-Coupling Pathways in Iridium-Catalyzed Oxygen Evolution

A series
of Cp*IrIII dimers have been synthesized to elucidate the
mechanistic viability of radical oxo-coupling pathways in iridium-catalyzed
O2 evolution. The oxidative stability of the precursors
toward nanoparticle formation and their oxygen evolution activity
have been investigated and compared to suitable monomeric analogues.
We found that precursors bearing monodentate NHC ligands degraded
to form nanoparticles (NPs), and accordingly their O2 evolution
rates were not significantly influenced by their nuclearity or distance
between the two metals in the dimeric precursors. A doubly chelating
bis-pyridine–pyrazolide ligand provided an oxidation-resistant
ligand framework that allowed a more meaningful comparison of catalytic
performance of dimers with their corresponding monomers. With sodium
periodate (NaIO4) as the oxidant, the dimers provided significantly
lower O2 evolution rates per [Ir] than the monomer, suggesting
a negative interaction instead of cooperativity in the catalytic cycle.
Electrochemical analysis of the dimers further substantiates the notion
that no radical oxyl-coupling pathways are accessible. We thus conclude
that the alternative path, nucleophilic attack of water on high-valent
Ir-oxo species, may be the preferred mechanistic pathway of water
oxidation with these catalysts, and bimolecular oxo-coupling is not
a valid mechanistic alternative as in the related ruthenium chemistry,
at least in the present system.