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‐1 H ‐imidazol‐4‐yl)]‐2‐propyl}‐ N ‐(1‐methyl‐1 H ‐imidazol‐2‐ylmethyl)aminobutyl)] contains two equivalent Cu A centers bound by tridentate arms and a Cu B 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 Cu A centers of the complex and only weakly to the Cu B center. The EPR spectra showed the existence of a strong coupling between one of the two Cu A centers and the Cu B center, which appears to be mediated by a hydroxido‐bridging ligand. Further information about metal binding was obtained by analyzing the NMR spectra of the trinuclear Zn II 3 –PHI complex, which serves as an analogue of the extremely oxygen sensitive Cu I 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 Cu n O 2 intermediate should be involved.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)