Dioxygen Activation by a Macrocyclic Copper Complex Leads to a Cu₂O₂ Core with Unexpected Structure and Reactivity

We report the Cu^I/O₂ chemistry of complexes derived from the macrocylic ligands 14-TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and 12-TMC (1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane). While [(14-TMC)Cu^I]⁺ is unreactive towards dioxygen, the smaller analog [(12-TMC)Cu^I(CH₃CN)]⁺ reacts with O₂ to give a side-on bound peroxo-dicopper(II) species (^SP), confirmed by spectroscopic and computational methods. Intriguingly, 12-TMC as a N4 donor ligand generates ^SP species, thus in contrast with the previous observation that such species are generated by N2 and N3 ligands. In addition, the reactivity of this macrocyclic side-on peroxo-dicopper(II) differs from typical ^SP species, because it reacts only with acid to release H₂O₂, in contrast with the classic reactivity of Cu₂O₂ cores. Kinetics and computations are consistent with a protonation mechanism whereby the TMC acts as a hemilabile ligand and shuttles H⁺ to an isomerized peroxo core.