REACTIVITY PATTERNS AND COMPARISONS IN 3 CLASSES OF SYNTHETIC COPPER DIOXYGEN (CU2-O2) COMPLEXES - IMPLICATION FOR STRUCTURE AND BIOLOGICAL RELEVANCE

We have recently characterized three classes of peroxodicopper(II) complexes, which are formed reversibly from the reaction of Cu(I) precursors (1-3) with O2 at -80-degrees-C in solution. Here, we detail and compare the reactivities of [Cu2(XYL-O-)(O2)]+ (4, a phenoxo-bridged peroxodicopper(II) species having terminal Cu-O2 coordination), [{Cu-(TMPA)}2(O2)]2+ (5, a trans mu-1,2-peroxo-bridged complex with a tetradentate ligand on each copper(II) atom), and (Cu2(N4)(O2)]2+ (6, which contains a bridging peroxo moiety with tridentate groups at both copper atoms). Complexes 4 and 5 possess a basic or nucleophilic peroxo group, but 6 behaves differently, possessing a nonbasic or electrophilic peroxodicopper (II) moiety. Thus, reaction of PPh3 With 4 and 5 readily causes the stoichiometric displacement of the bound O2 ligad, producing Cu(I)-PPh3 complexes. With 6, slow but complete oxygen atom transfer occurs, giving triphenylphosphine oxide. Protonation (or acylation) reactions are particularly striking, as addition of HBF4 or HPF6 to 4 and 5 gives near-stoichiometric yields of H2O2 (from excess H+; iodometric titration), but 6 is relatively insensitive to protons. Carbon dioxide reacts with 4 and 5 to give peroxycarbonato complexes at -80-degrees-C, which decompose to carbonato compounds; 6 does not react with CO2. All three complexes 4-6 react with sulfur dioxide to give sulfato products. Trityl cation (Ph3C+) reacts with all the complexes to give benzophenone, but the relative yields again support the notion that the peroxo group in 6 is a poorer nucleophile. 2,4-Di-tert-butylphenol acts as a protic acid toward 4 and 5, but in the presence of 6, hydrogen atom abstraction leads to oxidatively coupled biphenol products. The reactions of 4-X-C6H4MgBr (X = CH3, F) with 4-6 produce mixtures of 4-X-C6H4OH and substituted biphenyls; product ratios again support the view that 6 is a better one-electron oxidant and electrophilic reagent. The relationship of the observed reactivity patterns and structures of 4-6 is discussed, and it suggested that the mu-eta-2:eta-2-binding proposed for 6 confers its unique reactivity. The relationship of the structure and reactivity of 6 to a related and previously described monooxygenase model system is discussed, as well as the relevance to the active site chemistry of copper proteins involved in O2 utilization.