Dimeric versus polymeric coordination in copper(ii) cationic complexes with bis(chelating) oxime and amide ligands

Abstract

A series of copper(II) complexes with earlier reported ligands N,N′-bis(2-hydroxyiminopropionyl)-1,2-diaminoethane (H₂pen) and 1,3-diaminopropane (H₂pap) and novel open chain oxime and amide ligands, N,N′-bis(2-hydroxyiminopropionyl)-1,5-diaminopentane (H₂papt), 1,6-diaminohexane (H₂pah) and 1,7-diaminoheptane (H₂pahp), were prepared and characterized by a variety of spectroscopic methods and X-ray structure analysis. Although a pH-potentiometric study revealed only monomeric species in solution, formation of both binuclear dimeric and 1D-polymeric topologies was found in solid state. In all complexes ligands provide bis-bidentate coordination to Cu²⁺ ions via the nitrogen atoms of the oxime groups and the oxygen atoms of the amide groups to form CuN₂O₂ planar fragments. Compound [Cu(Hpap)(ClO₄)(H₂O)]_x was isolated in both dimeric (x = 2) and polymeric (x = n) state, representing an example of supramolecular isomerism. All the complexes are additionally stabilized by short intramolecular hydrogen bonds N–O–H⋯O–N between cis-arranged oximato-groups. Stabilization is more effective in the case of dimeric complexes compared to 1D-polymeric chain, which appears to be the reason of preferable dimerization versus polymerization in the studied set of compounds. The effect of polymethylene linker length on the conformation of the dimers is described.