Effect of metal coordination and intra-molecular H-bond on the acidity of phenolic proton in a set of structurally characterized octahedral Ni(II) complexes of l-histidine derivative

Abstract

Four different mononuclear octahedral Ni(II) complexes with protonated and deprotonated form of the same ligand have been synthesized by controlling reaction conditions and structurally characterized. The complexes are [Ni(HL l-his )(benzoate)(MeOH)] ( 1), [Ni(HL l-his )(SCN)(MeOH)] ( 2), [Ni(HL l-his ) 2] ( 3) and [Ni(L l-his )(imidazole) 2] ( 4) where H 2L l-his is ( S)-2-(2-hydroxybenzylamino)-3-(1H-imidazol-4-yl)-propionic acid. The ligand behaves as a monobasic tetradentate ligand in 1 and 2, monobasic tridentate ligand in 3 and dibasic tetradentate ligand in 4. Ni(II) coordinated phenolic proton of the ligand in the complexes 1– 2 shows strong intra-molecular H-bonding with benzoate in 1 and lattice water in 2, whereas 3 shows intermolecular H-bonding between uncoordinated phenols with neighbouring carboxylate. The pH titration of the complexes revealed that metal coordination and H-bond in complexes 1 and 2 considerably lowers the acidity of ligand phenol (p K a 6.8 and 7.0 respectively) compared to phenol (p K a 10). The complex 4 does not show any proton loss due to the absence of phenolic proton. All the complexes show extensive H-bonded network in the crystals including narrow (7.8 × 5.2 Å) water filled one dimensional channel in 2.