Synthesis and structural investigation of mononuclear penta- and hexa-coordinated Co complexes of 8-hydroxyquinoline derived ligands

Abstract

This article describes the synthesis and structural property investigation of two new mononuclear Co(II) complexes of 8-hydroxyquinoline derived flexible tridentate N,O,N'- (LI) and tetradentate N,O,O',N'-ligands (LII). The reactions were carried out between CoCl2.6H2O and the respective ligands in methanol to obtain the complexes CoLICl2 and CoLIICl2 in good yields. The complexes are characterized by elemental analysis, mass, IR spectroscopy, and single-crystal X-ray diffraction. The molecular structure of CoLICl2 shows distorted trigonal bipyramidal coordination geometry with the structural index parameter τ = 0.83. The apical positions are occupied with the quinoline O- and one terminal Cl-atom, whereas the equatorial positions are held by the two N-atoms and the second Cl-atom. In contrast, CoLIICl2 has a slightly distorted octahedral structure with all the donor atoms of the chelating ligand LII holding the equatorial positions with the two-terminal Cl-atoms at the apical positions. Electronic structure calculations are used to evaluate the geometrical, electronic, and magnetic properties of these complexes. Non-periodic calculations used to simulate molecules indicate a strong dπ-pπ bonding between Co and ligands. The crystal packing from both the solid-state structures revealed further stability of the molecule via weak non-covalent interactions through π.…π stacking between quinoline rings and C–H.…Cl hydrogen bonding. A detailed comparison between the geometric parameters of CoL1Cl2 and CoLIICl2 are reported. In addition, the comparison of selected structural parameters between a range of transition metal quinolates from the literature are given. Periodic calculations to simulate crystals using B3LYP hybrid functional predicts both the crystalline structures to be magnetic insulators.