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-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* –C–CH₃ moiety. It was shown that 1 underwent double functionalization of the –C–CH₃ 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 –C–CH₃ 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.