Steric-Hindrance Effect of a Substituent in the Self-Assembly Process of Copper(II) Complexes with Quadridentate Schiff-Base Ligands Involving a 2-Substituted-Imidazole Moiety

Four copper(II) complexes with unsymmetrical quadridentate ligands involving an imidazole moiety, [CuHLⁿ]-ClO₄ (1 - 4; n=1 - 4), have been prepared and characterized, where HL¹ = N-salicylidene-N'-(2-methylimidazol-4-ylmethylidene)-1,3-propanediamine, HL² =N-salicylidene-N'-(2-phenylimidazol-4-ylmethylidene)-1,3-propanediamine, HL³ = N-acetylacetonylidene-N'-(2-methylimidazol-4-ylmethylidene)ethylenediamine, HL⁴ = N-acetylacetonylidene-N'-(2-phenylimidazol-4-ylmethylidene)ethylenediamine, respectively. The corresponding deprotonated complexes 1′-4′ were obtained when 1 - 4 were treated under an alkaline condition. Deprotonation of the imidazole moiety of the copper-(II) complex motivates a self-assembly process which is formed by an axial coordination of the imidazolate nitrogen atom of a molecular unit to the copper(II) ion of another unit. 1′-3′ assumes a self-assembled imidazolate-bridged infinite-chain structure {Cu(II)-Im}_n in the solid state, while the polymeric species dissociates to monomeric species in the solution state. 4′ consists of a discrete deprotonated monomeric species, even in the solid state. The physical measurements, including reflectance and absorption spectra, cryomagnetic measurements, and single-crystal X-ray analyses, revealed that the self-assembly behavior for the series of copper(II) complexes depends both on the steric effect of the substituent of 2-substituted-4-formylimidazole moiety and the ligand field strength of the equatorial quadridentate ligand.