Ligand-field control in the self-assembly of polymeric metal complexes: copper(II) complexes with quadridentate Schiff-base ligands involving an imidazole moiety

Abstract

Ligand-field control in the self-assembly of polymeric metal complexes has been studied. A series of seven ‘ligand complexes’[Cu(HL)]⁺ containing an imidazole moiety have been prepared, where H₂L are the quadridentate Schiff-base ligands of the 1 : 1 : 1 condensation products of 4-formylimidazole, a diamine (ethylenediamine, 1,3-diaminopropane or trans-cyclohexane-1,2-diamine), and a salicylaldehyde derivative (salicylaldehyde, 5-bromosalicylaldehyde, 5-methoxysalicylaldehyde, 3-methoxysalicylaldehyde, o-hydroxyacetophenone) or benzoylacetone. The single-crystal X-ray analyses of three complexes have confirmed that the co-ordination geometries are essentially square planar. From the deprotonation behaviour of the imidazole proton and the d–d band maxima, the complexes can be classified into two types; type A complexes composed of salicylaldehyde derivatives and type B complexes composed of benzoylacetone. Under alkaline conditions, the type A complexes easily produce a self-assembly process giving insoluble imidazolate-bridged polymeric species and their potentiometric pH titrations are irreversible in the forward and reverse titrations. The variable-temperature magnetic data of the deprotonated type A complexes are well reproduced using the Ising model based on H=–2ΣJ_ijS_iS_j, and J values of –8.6, –1.8, –5.2, –13.0 and –4.5 cm^–1 are obtained. On the other hand, under alkaline conditions, the type B complexes give mononuclear species soluble in common organic solvents and act as good ‘ligand complexes’. The potentiometric pH titration curves are almost the same in the forward and reverse titrations, indicating that protonation and deprotonation were reversible. The type A complexes exhibiting weaker ligand-field strength in the equatorial ligand are ‘self-assembly complexes’ and the type B complexes with stronger fields are ‘ligand complexes’.