Diphenoxo-bridged NiCo and CuCo complexes of macrocyclic ligands: synthesis, structure and electrochemical behaviour

Heterodinuclear di-mu-phenoxo-(MCoII)-Co-II (M = Ni or Cu) complexes have been derived from phenol-based dinucleating ligands (L-m,L-n)(2-), comprising of two 2,6-di(iminomethyl)-4-methylphenolate entities linked by two lateral chains (CH2)(m) (m = 2 or 3) and (CH2)(n) (n = 3 or 4) at the imino nitrogens. The crystal structure of [CuCo(L-2,L-3)(dmf)(2)(H2O)][ClO4](2) (dmf = dimethylformamide) has been determined. The copper ion resides at the N2O2 site formed by the ethylene lateral chain and assumes a square-pyramidal geometry together with a dmf oxygen at the apex. The Co at the site of the trimethylene lateral chain assumes a pseudo-octahedral geometry together with a dmf and a water molecule at the axial positions. The Cu ... Co separation doubly bridged by the phenolic oxygens is 2.998(2) Angstrom. The NiCo complexes are paramagnetic (S-Nl = 0), whereas the CuCo complexes show a strong antiferromagnetic interaction between the two metal ions. Cyclic voltammograms of the NiCo complexes show two quasi-reversible couples attributable to the stepwise reductions: (NiCoII)-Co-II --> (NiCoII)-Co-I --> (NiCoI)-Co-I. The (NiCoII)-Co-I and (NiCoI)-Co-I complexes were prepared in solution by coulometry and characterized by visible spectroscopy. Similarly, the CuCo complex of (L-3,L-3)(2-) is reduced stepwise to (CuCoII)-Co-I and then to (CuCoI)-Co-I. On the other hand, the CuCo complexes of (L-2,L-3)(2-) and (L-2,L-4)(2-) showed unusual electrochemical behaviour at the electrode suggesting a scrambling of site exchange of the metal ions.