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 combined with Hirshfeld topology analysis of 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 a trans position, while the methyl-substituted ligands are in cis position to another. The packings of both ligands contain one-dimensionally infinite strings of complex molecules due to π-π 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 Cu-atom. The 6MeQ ligand transferred larger amount of electron density to Cu-metal than 3BrQ. NBO analysis of the Cu2+ antibonding orbitals included in the metal-ligand interactions have mixed dx2-y2 and s-atomic orbital characters for 1 while in 2 have mixed s and p-characters.