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

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(H 2 O) 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(H 2 O) 2 (CH 3 COO)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 Ir VI and Ir VII, rather than that of Ir V, as suggested before. Three sequential proton-coupled electron transfer oxidations result in the formation of a highly oxidized intermediate, [(bpy)Ir VI O(OH)(CH 3 COO)Cl] + . From this formal Ir VI intermediate, O–O bond formation takes place by a water attack on the Ir VI =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 Ir IV -peroxide intermediate, which undergoes proton-coupled electron transfer to form an Ir III –O 2 intermediate. Finally, O 2 can be released, coupled with the binding of another water molecule, to regenerate the catalytic Ir III species. Water oxidation at Ir VII has a slightly higher barrier, but it may also contribute to the activity. However, water oxidation at Ir V has a significantly higher barrier. Acetate oxidation by C–H activation was found to have a much higher barrier, suggesting that [(bpy)Ir(H 2 O) 2 (CH 3 COO)Cl] + is a remarkably stable catalyst. The possible catalytic species [(bpy-dc)Ir III (H 2 O) 3 Cl] 2+ without acetate coordination has also been considered and also gave a reasonably feasible barrier for the water oxidation. O–O bond formation at Ir VII is slightly preferred compared with at Ir VI, which is different from the case with acetate.