Ligand binding, conformational and spectroscopic properties, and biomimetic monooxygenase activity by the trinuclear copper-PHI complex derived from L-histidine

The trinuclear copper(II) complex derived from the octadentate N-donor ligand PHI [piperazine-l,4-bis(4-{N-[1-acetoxy-3-(l-methyl-1H-imidazol-4-yl)]-2-propyl)}-N-(1-methyl-1H-imidazol-2-ylmethyl)aminobutyl)] contains two equivalent CuA centers bound by tridentate arms and a CuB center bound by a central didentate residue. The conformational and ligand binding properties of the complex were extensively studied by various spectroscopic techniques (UV/Vis, CD, NMR, EPR) to probe its behavior in solution. Studies on the binding properties of the complex performed with the azide anion as a probe showed that the ligand preferably binds to the CuA centers of the complex and only weakly to the CuB center. The EPR spectra showed the existence of a strong coupling between one of the two CuA centers and the CuB center, which appears to be mediated by a hydroxide-bridging ligand. Further information about metal binding was obtained by analyzing the NMR spectra of the trinuclear Zn3-PHI complex, which serves as analogue of the extremely oxygen sensitive CuI3-PHI complex. The latter complex does not form a stable dioxygen adduct at low temperature, but exhibits an interesting monooxygenase activity. This was studied at low temperature using p-chlorophenolate as a substrate; the formation of 4-chlorocatechol in sizeable yield indicates that some of the very reactive Cun O2 intermediate should be involved.