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

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 H₂L^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 (pK_a 6.8 and 7.0 respectively) compared to phenol (pK_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.