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-1,4-bis(4-{N-[1-acetoxy- 3-(1-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 theCuB center, which appears to be mediated by a hydroxidobridging ligand. Further information about metal binding was obtained by analyzing the NMR spectra of the trinuclear ZnII 3–PHI complex, which serves as an analogue of the extremely oxygen sensitive CuI 3–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 CunO2 intermediate should be involved.