Equilibrium studies on mixed ligand complex formation of Co(II), Ni(II), Cu(II) and Zn(II) with N-(2-hydroxybenzyl)-l-histidine (H2hb-l-his) and typical N,N donor ligands: Crystal structure of [Ni(hb-l-his)(bipyridine)] · H2O complex

Equilibrium study on complex formation of Co(II), Ni(II), Cu(II) and Zn(II), hereafter M(II), with the quadridentate (O−, N, O−, N) donor ligand, N -(2-hydroxybenzyl)-l-histidine (H2hb-l-his, hereafter H2L), in the absence and in the presence of typical (N, N) donor bidentate ligands, 1,10 phenanthroline(phen), 2, 2′-bipyridine(bipy), ethylenediamine(en), hereafter B, in aqueous solution at 25 ± 1 °C was done at a fixed ionic strength, I = 0.1 mol dm−3(NaNO3) by combined pH-metric, UV–Vis and EPR measurements provide evidence for the formation of mononuclear and dinuclear binary and mixed ligand complexes of the types: M(L), View the MathML sourceM(L)22-, M2(L)2+, M2(H−1L)+, M(L)(B), (B)M(H−1L)M(B)+. The imidazole moiety of the ligand is found to act as a bridging bidentate ligand in the dinuclear M2(L)2+, M2(H−1L)+ and (B)M(H−1L)M(B)+ complexes, using its N3 atom and N1–H deprotonated moiety. Stability constants of the complexes provide evidence of discrimination of Cu(II) from the other M(II) ions by this ligand. Solid complexes: [Ni(L)(H2O)2] (1), [Cu(L)(H2O)] (2), and [Ni(L)(bipy)] · H2O (3) have been isolated and characterized by various physicochemical studies. Single crystal X-ray diffraction of the ternary complex, 3, shows an octahedral [(O−,N,N,O−)(N,N)] geometry with extensive π–π stacking of the aromatic rings and H-bonding with imidazole (N1–H), secondary amino N-atom, the lattice H2O molecule, and the carboxylate and phenolate O-atoms.