Synthesis, crystal structure, antioxidation and DNA binding properties of binuclear Ho(III) complexes of Schiff-base ligands derived from 8-hydroxyquinoline-2-carboxyaldehyde and four aroylhydrazines

X-ray crystal and other structural analyses indicate that Ho(III) and every newly synthesized ligand can form a binuclear Ho(III) complex with a 1:1 metal-to-ligand stoichiometry by nine-coordination at the Ho(III) center. Every ligand acts as a dibasic tetradentate ligand, binding to Ho(III) through the phenolate oxygen atom, nitrogen atom of quinolinato unit and the CN group, ⁻O-CN- group (enolized and deprotonated from OC-NH- group) of the aroylhydrazine side chain. One DMF molecule is binding orthogonally to the ligand-plane from one side to the metal ion, while another DMF and nitrate (bidentate) are binding from the other. Dimerization of this monomeric unit occurs through the phenolate oxygen atoms leading to a central planar four-membered (HoO)₂ ring. Investigations of DNA binding properties show that all the ligands and Ho(III) complexes can bind to Calf thymus DNA through intercalations with the binding constants at the order of magnitude 10⁵-10⁶ M⁻¹, but Ho(III) complexes present stronger affinities to DNA than ligands. All the ligands and Ho(III) complexes may be used as potential anticancer drugs. Investigations of antioxidation properties show that all the ligands and Ho(III) complexes have strong scavenging effects for hydroxyl radicals and superoxide radicals but Ho(III) complexes show stronger scavenging effects for hydroxyl radicals than ligands.