Effect of hydrogen bonding on the coordination: Part 2. Semi-coordination in trans-di(salicylato)bis(1,3-diaminopropane-N,N’)copper(II)

The crystal and molecular structure of trans-di(ortho-hydroxobenzoato-O)bis(1,3-diaminopropane-N,N′)copper(II) [ortho-hydroxobenzoate=salicylate hereafter] determined by single crystal X-ray diffraction methods at T=193(2) K. The structure is composed of monomeric complex units with the central copper(II) ion occupying a centre of symmetry. The coordination polyhedron can be described as axially elongated distorted octahedron with the axial Cu–O bond length of 2.559(3) Å indicating semi-coordination. There is notable asymmetry in the coordinated 1,3-diaminopropane molecule, which forms a six-membered chelate ring displaying Cu–N1–C1 and Cu–N2–C3 angles of 121.5(2) and 117.4(2)°, respectively. This deviation may be correlated to differences in hydrogen bonding between the amino groups and the non-coordinated oxygen atoms of the salicylate anions. The coordinated salicylate anion shows considerable declination of 28.9°, which may be correlated to intramolecular hydrogen bonding. The electronic properties of benzoate anions were studied by Natural Bond Order (NBO) analyses for optimised model compounds at the MP2/6-311+G*//MP2/6-311+G* level of theory. The results suggest that semi-coordination is mainly characterised by concomitant electrostatic attraction of the charges at the central copper(II) cation and repulsion between an electron lone pair of a ligand atom and the electron lone pair at the copper(II) 3dz2 atomic orbital.