Crown ether bridged homo- and heterodinuclear copper(II) and nickel(II) cyclidene complexes: Interaction with anions

Homodinuclear (3CuCu, 3NiNi) and heterdionuclear (3CuNi) cyclidene complexes linked by two 1,10-diaza-18-crown-6 ethers were synthesized and their properties characterized. For the 3CuCu in the negative potential range the mixed-valence state CuI–CuII was observed and the comproportionation constants were determined. The redox potentials of MII/III showed a good correlation with DN of the solvent. In acetonitrile solution containing 0.1 mol dm−3 Bu4NPF6 reversible MII/III redox processes were observed. However, in the presence of stronger ion pairing anions (e.g. BF4− and ClO4−) the “potential inversion” occurred. Redox potentials of dinuclear complexes were compared with mononuclear cyclidene complexes. The influence of selected anions (Cl− and NO3−) on the redox process MII/III was studied. The effect of Cl− anion was different for 3CuCu and 3NiNi. In 3CuCu coordination of two chloride anions took place after oxidation of copper centers. In the 3NiNi complex two Cl− anions were coordinated to one of the nickel(II) centers facilitating oxidation, at different potentials, of both nickel(II) cations. The behaviour of heteronuclear complex with Cl− anions was similar to 3NiNi. All dinuclear complexes interacted with NO3− anions and the observed potential shifts were larger for nickel(II) than for copper(II) cations.