Copper ion is a vital constituent of metalloprotein active sites, where biological roles include electron-transfer, nitrogen oxide reduction and O2-processing. The latter includes O2-transport, oxygenase activity (i.e., O-atom(s) insertion) and O2-reduction to H2O2 or water accompanied by substrate dehydrogenation. Functions include pigment production, neurotransmitter and hormone biosynthesis, conversion of methane to methanol, oxidative cleavage of recalcitrant polysaccharides as well as scavenging of reactive oxygen species (ROS).
A major theme of our long-term research program has been ligand (L) design, systematic ligand variation and the use of cryogenic solution handling, enabling the generation and investigation of (L)CuIn/O2(g) (n = 1, 2) derived species. Through such approaches, one may identify factors such as donor atom type or number, coordination geometry, metal complex redox potential, and second coordination sphere composition, those which define Cu-protein active site structure and function.
Prior to our research efforts, no synthetically derived well-characterized CuIn-(O2(g)) species existed. The use of tripodal tetradentate N4 ligands leads to the generation of superoxo-copper(II) {(ligand)CuII(O2•–)} complexes and/or peroxo-dicopper(II) analogs which have been characterized structurally/spectroscopically and have been examined with respect to scope of reaction. Binucleating ligands hold two copper(I) ions may undergo reversible O2(g)-binding and/or ‘activation’ of the bound peroxo (O22–) ligand leading to hydroxylation of unactivated arene C–H bonds, chemistry which has relevance to the tyrosinase enzyme reaction mechanism. Study of phenolato-bridged dicopper complexes leads to new kinds of superoxo, peroxo or hydroperoxo dicopper(II) complexes which can be reversibly interconverted by employing oxidants/reductants and/or acids-bases. Thermodynamic interrelationships could then be elucidated. Most recently we described peroxo-dicopper(II) complex nucleophilic oxidative aldehyde deformylation chemistry, in a dioxygenase reaction.
Results obtained from the present research presented provides insights into biological copper ion mediated O2-processing and thus also possibly can apply to practical organic oxidation chemistry and/or energy related fuel-cell technologies.