Copper(II) Complexes of Phenolate-Bridged Dimeric Cyclam Ligands: Synthesis, Coordination and Electrochemical Study

Bis(macrocyclic) complexes are investigated as artificial analogues of natural dinuclear metalloenzymes. Two ligands containing two cyclam rings connected through a p-cresol or 4-nitrophenol spacer were synthesized and their complexing properties were investigated. Cu^II complexes were prepared and their properties were studied by a combination of spectral, diffraction and electrochemical methods. The phenolate group is not or only weakly coordinated to the central Cu^II ion and the two macrocycles behave mostly as two independent complexing units. All the determined solid-state structures showed cyclam adopting trans-III geometry, however, solution study indicated the presence of more isomers in the solution whose abundance changes with pH. Electrochemical reduction of the complexes is irreversible and results in the reductive decomplexation to amalgamated Cu metal. The amalgamated metal is re-oxidized to Cu^II during the anodic scan of CV and the metal ion is re-coordinated. The ligand containing the nitro group shows an irreversible four-electron reduction of the nitro group. The ligands and the complexes undergo also a slow degradation when incubated in the aqueous solutions for a prolonged time. The degradation relies on the “retro-Mannich” cleavage of the methylene bridge connecting the cyclam ring with the phenol moiety.