Jonathan Graeupner1, Ulrich Hintermair2, Daria L Huang1, Julianne M Thomsen1, Mike Takase1, Jesús Campos1, Sara M Hashmi3, Menachem Elimelech3, Gary W Brudvig1, Robert H Crabtree1. 1. Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States. 2. Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States ; Centre for Sustainable Chemical Technologies, University of Bath, Bath BA2 7AY, UK. 3. Department of Chemical and Environmental Engineering, Yale University, 9 Hillhouse Avenue, New Haven, Connecticut 06520, United States.
Abstract
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.
A series of Cp*IrIII dimers have been synthesized to elucidate the mechanistic viability of radical oxo-coupling pathways in iridium-catalyzed n class="Chemical">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 twometals 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.
Authors: Somnath Maji; Laura Vigara; Francesca Cottone; Fernando Bozoglian; Jordi Benet-Buchholz; Antoni Llobet Journal: Angew Chem Int Ed Engl Date: 2012-04-30 Impact factor: 15.336
Authors: Nathan D Schley; James D Blakemore; Navaneetha K Subbaiyan; Christopher D Incarvito; Francis D'Souza; Robert H Crabtree; Gary W Brudvig Journal: J Am Chem Soc Date: 2011-06-15 Impact factor: 15.419