An NMR Study on the Oxidative Degradation of Cp*Ir-Catalysts for Water Oxidation: Evidence for a Preliminary Attack on the Quaternary Carbon of the–C–CH3 Moiety

The reaction between H2O2 and two water oxidation catalysts {[Cp*Ir(H2O)3](NO3)2 (1, Cp* = pentamethylcyclopentadienyl) and [Cp*Ir(bzpy)(NO3)] (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* CCH3 moiety. It was shown that 1 underwent double functionalization of the CCH3 moiety of Cp* leading to the formation of C(OR)CH2OR (R = H or OH) in a strictly analogous manner to that previously observed for the reaction of 1 with Ce4+. 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 CCH3 moiety [C(OR)CH3], 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.