Binuclear copper-dioxygen complexes: Characterization, reactivity and mechanistic studies

Bioinspired copper model complexes that react with dioxygen (O₂) provide opportunities to examine biological reactivity at a small molecule level of detail. Biological structural information combined with appropriate ligand design has proven sufficient to create copper-dioxygen (Cu/O₂) complexes that mimic biological sites. Traditionally, this modeling approach has been limited to the spectroscopic and structural characterization of novel Cu/O₂ species. This thesis describes the discovery and reactivity of an unprecedented bis(μ-oxo)dicopper(III) species ( O) and its relationship with an isomeric μ-η 2:η2-peroxodicopper(II) species (P), the latter of which is observed in biology. Reactions of a family of simple Cu(I)-peralkylated diamine complexes with O₂ generate a series of novel 2:1 Cu:O₂ complexes at 193 K in aprotic solvents, best described as bis(μ-oxo)dicopper(III) species. These complexes have been characterized by various spectroscopic and physical methods including low-temperature x-ray crystallography. Over 15 different O were identified, and detailed mechanistic studies on the formation, thermal decay and reactivity with alcohols are presented. Comparative studies on alcohol oxidation by various O emphasize the requirement for substrate binding to the Cu center for reactivity. In certain cases, reactions of Cu(I)-peralkylated diamine complexes with O₂ generate equilibrium mixtures of μ-η 2:η2-peroxodicopper(II) and bis(μ-oxo)dicopper(III) isomers. The equilibrium is most sensitive to ligand structure, but also depends on temperature, anions and the polarity of solvents. The kinetics and thermodynamics of the equilibrium between P and O were investigated. The existence of a mixture of μ-η2:η2-peroxodicopper(II) and bis(μ-oxo)dicopper(III) isomers provides an opportunity to compare the reactivity of the isomers with externally added substrates. In one particular case, the use of 2-MeTHF as the solvent, causes the rate of isomer interconversion between P and O to be slow relative to rates of reaction with externally added substrates. For several different reactions, P is consistently the more reactive isomer at parity of ligand. Further, formation studies under these slow equilibrium conditions suggest an alternate mechanism, wherein O is not necessarily formed through the intermediacy of P. The implications of this result on the O-O bond formation and cleavage pathways are discussed.