Homoleptic Co-II, Ni-II, Cu-II, and Zn-II Complexes Based on 8-Hydroxylquinoline Schiff Base Derivative: a Combined Synthetic, Spectral, Structural, and Magnetic Study

A series of four new homoleptic Co-II, Ni-II, Cu-II and Zn-II complexes (1-4) of the type M(HL)(2), where HL- stands for the deprotonated form of the tridentate O,N,N-donor Schiff base ligand obtained by condensation between 8-hidroxy-2-quinolinecarboxaldehyde and 2-aminophenol, have been prepared in yields ranging from 70 to 85 %. The five compounds were characterized using elemental analysis, FT-IR, ESI-MS, UV/Vis, and H-1 and C-13 NMR spectroscopy (for the diamagnetic species H2L and Zn-II complex 4). The molecular structures of H2L and its Ni-II and Zn-II complexes 2 and 4, respectively, were further confirmed by single-crystal X-ray diffraction methods. X-ray crystal structure determination revealed that in complex 2, the six-coordinate Ni-II metal ion adopts a N4O2 distorted octahedral geometry with the two tridentate HL- ligands coordinated in a perfect orthogonal fashion. In complex 4, however, the Zn-II metal center is pentacoordinate and lies in a distorted N3O2 trigonal bipyramidal environment, one HL- ligand acting as an O,N-bidentate ligand. In both cases, a free hydroxyl unit remains available for supramolecular interactions. Hirshfeld surface (HS) analysis along with 2D fingerprint plots were employed to consider the intermolecular forces, including hydrogen bonds and pi-pi stacking interactions, and their quantification in the crystal lattice. SQUID magnetometry experiments showed high spin Co-II (S = 3/2), Ni-II (S = 1) and Cu-II (S = 1/2) isolated spins at room temperature with no interaction between magnetic centers on cooling.