Structural analyses of two new highly distorted octahedral copper(II) complexes with quinoline-type ligands; Hirshfeld, AIM and NBO studies

The structural features of two new copper(II) nitrato complexes with 3-bromoquinoline, [Cu(3BrQ)(2)(NO3)(2)] (1), and 6-methylquinoline, [Cu(6MeQ)(2)(NO3)(2)] (2), were investigated using X-ray single crystal structure determination, combined with Hirshfeld topology analysis of the molecular packing. Both complexes consist of two quinoline ligands and two bidentate nitrate ions hexa-coordinating the central Cu2+ atom. The major difference is that the Br-substituted quinoline ligands adopt trans positions, while the methyl-substituted ligands are in cis positions to one another. The packing arrangements of both ligands contain one-dimensionally infinite strings of complex molecules due to pi-pi stacking of neighboring ligand molecules. These strings are held together by C-H/O, Br-H/O and H-H/O contacts. The nature and strength of the Cu-N and Cu-O bonds were investigated using quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. Based on the QTAIM topological parameters, the Cu-N and Cu-O bonds have covalent nature where shorter Cu-O/Cu-N bonds are more covalent. Natural charges indicated a transfer of negative charge from the ligand groups to the Cu atom. The 6MeQ ligand transferred a larger amount of electron density to the Cu metal than 3BrQ. NBO analysis of the Cu2+ antibonding orbitals included in the metal-ligand interactions have mixed d(x2-y2) and s-atomic orbital characters for 1, while in 2 they have mixed s and p-characters. (C) 2017 Elsevier Ltd. All rights reserved.