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

Heterodinuclear di-μ-phenoxo-M^IICo^II (M = Ni or Cu) complexes have been derived from phenol-based dinucleating ligands (L^m,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^2,3)(dmf)₂(H₂O)][ClO₄] ₂ (dmf = dimethylformarmide) 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) Å. The NiCo complexes are paramagnetic (S_NI = 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: Ni^IICo^II → Ni^ICo^II → Ni^ICo^I. The Ni^ICo^I and Ni^ICo^I complexes were prepared in solution by coulometry and characterized by visible spectroscopy. Similarly, the CuCo complex of (L^3,3)^2- is reduced stepwise to Cu^ICo^II and then to Cu^ICo^I. On the other hand, the CuCo complexes of (L^2,3)^2- and (L^2,4)^2- showed unusual electrochemical behaviour at the electrode suggesting a scrambling or site exchange of the metal ions.