Which Oxidation State Leads to O–O Bond Formation in Cp*Ir(bpy)Cl-Catalyzed Water Oxidation, Ir(V), Ir(VI), or Ir(VII)?

Abstract

Density functional calculations are used to revisit the reaction mechanism of water oxidation catalyzed by the Cp*Ir(bpy)Cl (Cp* = pentamethylcyclopentadienyl, bpy = 2,2′-bipyridine) complex. One of the experimentally suggested active species [(bpy)Ir(H2O)2(HCOO)Cl]+ can undergo very facile intramolecular formate oxidation at higher oxidation state even though it can also promote O–O bond formation. Therefore, [(bpy)Ir(H2O)2(CH3COO)Cl]+ is here proposed to be the most likely precatalyst as acetate was also experimentally observed after Cp* oxidation. O–O bond formation takes place at the high formal oxidation states of IrVI and IrVII, rather than that of IrV, as suggested before. Three sequential proton-coupled electron transfer oxidations result in the formation of a highly oxidized intermediate, [(bpy)IrVIO(OH)(CH3COO)Cl]+. From this formal IrVI intermediate, O–O bond formation takes place by a water attack on the IrVI=O moiety assisted by the acetate ligand, which abstracts a proton during the attack. The barrier was calculated to be very facile, being 14.7 kcal/mol, in good agreement with experimental kinetic results, which gave a barrier of around 18 kcal/mol. The attack leads to the formation of an IrIV-peroxide intermediate, which undergoes proton-coupled electron transfer to form an IrIII–O2 intermediate. Finally, O2 can be released, coupled with the binding of another water molecule, to regenerate the catalytic IrIII species. Water oxidation at IrVII has a slightly higher barrier, but it may also contribute to the activity. However, water oxidation at IrV has a significantly higher barrier. Acetate oxidation by C–H activation was found to have a much higher barrier, suggesting that [(bpy)Ir(H2O)2(CH3COO)Cl]+ is a remarkably stable catalyst. The possible catalytic species [(bpy-dc)IrIII(H2O)3Cl]2+ without acetate coordination has also been considered and also gave a reasonably feasible barrier for the water oxidation. O–O bond formation at IrVII is slightly preferred compared with at IrVI, which is different from the case with acetate.