Structural and equilibrium studies on mixed ligand complexes of Co(II), Ni(II), Cu(II) and Zn(II) with N-(2-benzimidazolyl)methyliminodiacetic acid and typical N, N donor ligands

Mixed ligand complexes: [Co(L)(bipy)] · 3H2O (1), [Ni(L)(phen)] · H2O (2), [Cu(L)(phen)] · 3H2O (3) and [Zn(L)(bipy)] · 3H2O (4), where L2− = two –COOH deprotonated dianion of N-(2-benzimidazolyl)methyliminodiacetic acid (H2bzimida, hereafter, H2L), bipy = 2,2′ bipyridine and phen = 1,10-phenanthroline have been isolated and characterized by elemental analysis, spectral and magnetic measurements and thermal studies. Single crystal X-ray diffraction studies show octahedral geometry for 1, 2 and 4 and square pyramidal geometry for 3. Equilibrium studies in aqueous solution (ionic strength I = 10−1 mol dm−3 (NaNO3), at 25 ± 1 °C) using different molar proportions of M(II):H2L:B, where M = Co, Ni, Cu and Zn and B = phen, bipy and en (ethylene diamine), however, provides evidence of formation of mononuclear and binuclear binary and mixed ligand complexes: M(L), M(H−1L)−, M(B)2+, M(L)(B), M(H−1L)(B)−, M2(H−1L)(OH), (B)M(H−1L)M(B)+, where H−1L3− represents two –COOH and the benzimidazole N1–H deprotonated quadridentate (O−, N, O−, N), or, quinquedentate (O−, N, O−, N, N−) function of the coordinated ligand H2L. Binuclear mixed ligand complex formation equilibria: M(L)(B) + M(B)2+ ⇌ (B)M(H−1L)M(B)+ + H+ is favoured with higher π-acidity of the B ligands. For Co(II), Ni(II) and Cu(II), these equilibria are accompanied by blue shift of the electronic absorption maxima of M(II) ions, as a negatively charged bridging benzimidazolate moiety provides stronger ligand field than a neutral one. Solution stability of the mixed ligand complexes are in the expected order: Co(II) < Ni(II) < Cu(II) > Zn(II). The Δ log KM values are less negetive than their statistical values, indicating favoured formation of the mixed ligand complexes over the binary ones.