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

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.

Rational Design Rules for Molecular Water Oxidation Catalysts based on Scaling Relationships

“…To allow this final step, the complexes should at least have sufficient space around the oxygen radical to allow this reaction, and one may also facilitate this step by preorganizing the two metal sites. Along these lines, many dinuclear complexes have been prepared to bring the two metal centers in close proximity, but oftentimes these perform worse than their mononuclear analogues . It is likely that these dinuclear complexes are too rigid because the M−OH 2 , M−OH, and M=O intermediates should all be accommodated and require different optimal metal–metal distances.…”

“…Along these lines, many dinuclear complexes have been prepared to bring the two metal centers in close proximity,b ut oftentimes these performw orse than their mononuclear analogues. [25] It is likely that thesed inuclear complexes are too rigid because the MÀOH 2 ,M ÀOH, and M=Oi ntermediates should all be accommodated and require different optimal metal-metal distances. In addition, the M-O-O-M dihedral angle during the OÀOb ond formation is important because the cis-like structure (M-O-O-M % 08)i sa nticipated to be much higher in energy then the trans (M-O-O-M % 1808), in analogyt ot hat reported for hydrogen peroxide.…”

Electrochemical and Kinetic Insights into Molecular Water Oxidation Catalysts Derived from Cp*Ir(pyridine‐alkoxide) Complexes

“…Mononuclear iridium catalysts in particular have come to the fore since the first report by Bernhard and co‐workers 10 years ago . Since then a wide number of molecular iridium precursors have been reported, with half sandwich iridium compounds showing the highest activities ,. Although the exact nature of the active species is still a matter of debate, it has been shown that the Cp*Ir III complexes are precursors which undergo oxidative activation with loss of the Cp* ligand, either chemically, or electrochemically, before entering catalysis .…”

N,N,O Pincer Ligand with a Deprotonatable Site That Promotes Redox‐Leveling, High Mn Oxidation States, and a Mn2O2 Dimer Competent for Catalytic Oxygen Evolution

“…With these bio‐inspired principles in mind, we report a new di‐µ‐oxo dimanganese complex of the pincer 2‐([2,2′‐bipyridin]‐6‐yl)propan‐2‐ol (bipyalkH), which acts as a strong π‐donor upon deprotonation of the alcohol. This ligand incorporates the pyridine alkoxide “pyalk” motif that we previously showed can stabilize high oxidation states in Rh and Ir and promote water‐oxidation catalysis, both with chemical oxidants and electrochemically, with Ir and Cu . We also have previously reported oxygen evolution from our low‐valent precatalyst Mn(bipyalkH)Cl 2 ( 1 , shown in Figure ) driven by KHSO 5 with retention of activity for > 160 h in unbuffered solutions.…”