Effect of metal ionic radius and chelate ring alternation motif on stabilization of trivalent nickel and copper in binuclear complexes with double cis-oximato bridges

Abstract

Oxime ligands are able to form stable binuclear species with copper(II) ions in aqueous solution. They also have a strong tendency to decrease the M^n+/(n−1)+ redox potentials of the central ions. Ligands possessing the hydroxyimino groups together with other powerful σ-donor groups can be very efficient chelating agents able to facilitate the stabilisation of high oxidation states of 3d-metals. Here we report the synthesis, structural characterization and redox behaviour of mononuclear and binuclear complexes based on hydroxyiminoamide tetradentate open-chain ligands. In all mononuclear anionic complexes the central atom is situated in a square-planar surrounding of four nitrogen atoms. This pseudo-macrocyclic conformation is due to the presence of short intramolecular hydrogen bonds uniting the cis-oximate oxygen atoms. The square-planar surrounding of the strong σ-donors facilitates efficient stabilization of the trivalent state of copper and nickel ions. In cyclic voltammetry studies the quasi-reversible processes M²⁺→M³⁺ can be observed. In the binuclear complexes the coordinatively saturated octahedral ion M′ is bound to the two oxygen atoms of the bridging oximate groups and the four nitrogen atoms of the tetradentate ligand tren. Two metal ions (M and M′) are linked by the double cis-oximate bridge and are incorporated in a six-membered bimetallic chelate ring. Metallamacrocycle formation leads to certain changes in the structural parameters of the binuclear complexes as compared to those observed in the mononuclear species. Also the study of the electrochemical activity of binuclear complexes has shown important differences in their redox behaviour as compared to their mononuclear precursors.