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 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 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 dx2-y2 and s-atomic orbital characters for 1, while in 2 they have mixed s and p-characters.