An NMR Study of the Oxidative Degradation of Cp*Ir Catalysts for Water Oxidation: Evidence for a Preliminary Attack on the Quaternary Carbon Atom of the –C–CH 3 Moiety

The reaction between H 2 O 2 and two water oxidation catalysts {[Cp*Ir(H 2 O) 3 ](NO 3 ) 2 ( 1 , Cp* = pentamethylcyclopentadienyl) and [Cp*Ir(bzpy)(NO 3 )] ( 2 , bzpy = 2‐benzoylpyridine)} was studied by means of in situ 1D‐ and 2D‐NMR experiments in order to elucidate if catalyst degradation proceeds through the initial functionalization of a quaternary carbon atom (C‐attack) or by hydrogen abstraction (H‐attack) of the Cp* –C–CH 3 moiety. It was shown that 1 underwent double functionalization of the –C–CH 3 moiety of Cp* leading to the formation of –C(OR)–CH 2 OR (R = H or OH) in a strictly analogous manner to that previously observed for the reaction of 1 with Ce 4+ . On the contrary, two new intermediates associated with the oxidative degradation of 2 , which are functionalized only at the quaternary carbon atom(s) of the –C–CH 3 moiety [–C(OR)–CH 3 ], were intercepted and characterized by NMR spectroscopy. This indicates that the oxidative degradation of water oxidation catalysts, featuring the –Cp* ancillary ligand, likely starts with preferential C‐attack.