Activation of dioxygen and nitric oxide by an yttrium-tricopper complex: Different modes of metal-metal cooperativity

Enzymes responsible for chem. reactions involving the transfer of multiple electrons often rely on multimetallic active sites to facilitate these transformations. The relevance of many of these reactions - such as O_2 redn., H_2O oxidn., and CO_2 redn. - to the field of energy science has inspired significant efforts to elucidate the mechanisms of these processes. Synthetic multimetallic model complexes have received considerable attention for the insight they can provide on the mechanism of biol. systems, including on the interactions between metal centers in multimetallic active sites and their effects on reactivity. An yttrium-tricopper complex supported by a multinucleating ligand framework, designed to model the active site of the multicopper oxidase enzymes, displayed different modes of metal-metal cooperativity as a function of substrate. Cooperative activation of O_2 by the three Cu centers on a single complex was obsd., yielding a [Cu_3(μ_3-O)_2] product in which the O-O bond had been fully cleaved. Reaction with nitric oxide (NO), on the other hand, involved cooperation of four metal centers on two distinct [YCu_3] units - two Cu and two Y centers - and resulted in reductive coupling of two NO mols. to form a hyponitrite (N_2O_2^(2-)) moiety. The synthesis, properties, and further reactivity of these species will be discussed