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

Abstract

Heterodinuclear di-µ-phenoxo-M ^II Co ^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 (CH ₂ ) _m (m = 2 or 3) and (CH ₂ ) _n (n = 3 or 4) at the imino nitrogens. The crystal structure of [CuCo(L ^2,3 )(dmf) ₂ (H ₂ O)][ClO ₄ ] ₂ (dmf = dimethylformamide) has been determined. The copper ion resides at the N ₂ O ₂ 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 ^II Co ^II → Ni ^I Co ^II → Ni ^I Co ^I . The Ni ^I Co ^II and Ni ^I Co ^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 ^I Co ^II and then to Cu ^I Co ^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.