Study of strong O-H ... O- hydrogen bond interactions in binuclear Zn hydroxyquinoline carboxylic acid complex ...

Investigation of strong hydrogen bonds in crystal engineering is extensively used for the preparation of intriguing novel pharmaceutical co-crystals and ionic compounds. Strong O-H...O-hydrogen bonds in phenolates were recently recognized as a promising supramolecular synthon for the preparation of novel ionic co-crystals, and phenolates as promising catalysts and reducing agents. Investigations of complex compounds containing unco-ordinated phenolate are exceptionally rare and this particular research area is yet unexplored. As part of our continuous investigations in the supramolecular chemistry of coordination compounds, we have prepared a novel Zn(II) binuclear complex with 4-hydroxyquinoline-2-carboxylic acid as a ligand (4hqc). The compound was characterized by IR-spectroscopy, and molecular and crystal structure was determined by the single-crystal X-ray diffraction method. The compound is a binuclear Zn complex, with each Zn(II) ion coordinated by one 4hqc ligand and two water molecules in the distorted square pyramidal arrangement. The central Zn2(& mu;-O)2 unit is planar and Zn ions are bridged by monodentatelly coordinated carboxylate oxygen atoms. In the crystal, the discrete binuclear units are primarily connected by strong O-H...O- hydrogen bond interactions that involve deprotonated hydroxyl oxygen atoms and coordinated water molecules. The 3D structure is additionally stabi-lized by offset face-to-face & pi;...& pi; interactions. Additional structural analysis has shown that geometrical param-eters of O-H...O-hydrogen bonds are very similar to those in phenolate co-crystals, thus confirming the presence of phenolate in the complex. The calculated lattice energy of the compound was found to be unusually high (-505.56 kJ/mol), and O-H...O-bond energies were estimated to be in the range from -55.0 kJ/mol to -62.5 kJ/mol. By comparison to previously reported phenolate coordination compounds, it was proposed that such compounds could be synthesized in mildly alkaline conditions, ligand: metal stoichiometric ratio 1:1, and with ligands bearing carboxylate, imine, or pyridine groups separated from phenol group by hydrogen atom or other bulkier groups.