Reactivity patterns and comparisons in three classes of synthetic copper-dioxygen {Cu2-O2} complexes: Implication for structure and biological relevance

Article abstract of DOI:10.1021/ja00014a027

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 °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-O₂coordination), [{Cu-(TMPA)}₂(O₂)]²⁺ (5, a trans μ-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 nulceophilic peroxo group, but 6 behaves differently, possessing a nonbasic or electrophilic peroxodicopper(II) moiety. Thus, reaction of PPh₂with 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 nearstoichiometric 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 °C, which decompose to carbonate 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 μ-η²:η²-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.