Study of strong O[sbnd]H⋯O− hydrogen bond interactions in binuclear Zn hydroxyquinoline carboxylic acid complex

Abstract

Investigation of strong hydrogen bonds in crystal engineering is extensively used for the preparation of intriguing novel pharmaceutical co-crystals and ionic compounds. Strong OH⋯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 uncoordinated 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(µ-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 OH⋯O− hydrogen bond interactions that involve deprotonated hydroxyl oxygen atoms and coordinated water molecules. The 3D structure is additionally stabilized by offset face-to-face π⋯π interactions. Additional structural analysis has shown that geometrical parameters of OH⋯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 OH⋯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.