Role of α-hydroxycarboxylic acids in the construction of supramolecular assemblies of nickel(ii) complexes with nitrogen donor coligands

The topology of the supramolecular self assembly structures of six nickel(ii) complexes containing nitrogen donor coligands is described, and the structural features induced by the deprotonated α-hydroxy acids (mandelic and benzilic) used in the complex synthesis are compared. In all the complexes, namely [(6-Me₃-TPA)Ni(MA)]B(Ph)₄ (1), [(6-Me ₃-TPA)Ni(R(-)-MA)]B(Ph)₄ (2R), [(6-Me₃-TPA) Ni(BA)]B(Ph)₄ (3), [(AP)₂Ni(MA)₂] (4), [(AP)₂Ni(S(+)-MA)₂] (5S) and [(AP)₂Ni(BA) ₂] (6) (where 6-Me₃-TPA = tris(6-methyl-2-pyridylmethyl) amine, AP = 2-aminopyridine, MA = racemic mandelate, R(-) and S(+) are enantiomeric mandelates, BA = benzilate anion), the metal ion exhibits a distorted octahedral environment with mandelate or benzilate anions acting as chelating agents through the hydroxyl group and a carboxylate oxygen. The differences in the crystal structures of 2R and 5S (containing enantiomerically pure mandelate anions) are compared with those of 1 and 4, obtained with a racemic mixture of the deprotonated acid. All the nickel(ii) complexes form infinite one-dimensional chains built up by hydrogen bonds. The mononuclear building blocks, 1-3, are assembled head-on, with a very similar topology, through H-bond interactions between the hydroxycarboxylate groups of adjacent units, while in neutral complexes 4-6 the cooperative presence of solvent molecules leads to different connections between the complexes. In addition, intramolecular π-π stacking interactions between pyridine rings of the ligand and the phenyl of mandelate or benzilate anions are observed in 1, 3 and 6.