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.

Graphical abstractIn presence of strongly coordinating (N, N) bidentate ligands, the benzimidazole N3-atom of N-(2-benzimidazolyl)methyliminodiacetic acid (H2L) remains uncoordinated in the square pyramidal mixed ligand complexes, Cu(L)(N,N), but it coordinates Co(II), Ni(II) and Zn(II) in their analogous complexes having octahedral geometry. Benzimidazolate bridged binuclear complexes, (N,N)M(H−1L)M(N,N)+ (1), (M = Co(II), Ni(II), Cu(II) and Zn(II)) are indicated in solution around pH 7–9.Figure optionsDownload full-size imageDownload as PowerPoint slide