Synthesis, characterization and potent superoxide dismutase-like activity of novel bis(pyrazole)-2,2′-bipyridyl mixed ligand copper(II) complexes

Eleven new complexes of Cu(II) chloride and nitrate with bis(pyrazol-1-yl) propane and bis[2-(pyrazol-1-yl) ethyl] ether ligands were prepared and characterized by spectral and electrochemical methods. X-Ray crystal structure determination of bis[2-(3,5-dimethylpyrazol-1-yl) ethyl] etherdinitratocopper revealed a hepta-coordinated structure with the bis(pyrazole) ligand coordinated in a tridentate NNO-fashion and both of the nitrate ions in a bidentate fashion. Reaction of Cu(II) nitrate complexes with 2,2'-bipyridyl led to the displacement of one of the nitrate ions into the outer sphere and the formation of mixed-ligand complexes. Mixed-ligand bipyridyl Cu(II) complexes demonstrated the highest superoxide dismutase (SOD)-like activity in a chemical superoxide anion-generating system, with IC50 values in the low micromolar range. Density functional theory calculations showed that introduction of a bipypidyl ligand into the complexes dramatically lowered the lowest unoccupied molecular orbital (LUMO) energy level, which explains the increased SOD-like activity of these complexes compared to non-bipy species. These bipy complexes were also effective scavengers of reactive oxygen species generated by phagocytes (human neutrophils and murine bone marrow leukocytes) ex vivo. Thus, these bipy mixed-ligand complexes represent a promising class of SOD mimetics for future development.