Binary and ternary complexes of nickel(II) with 2-aminomethylbenzimidazole and salicylaldehyde: kinetic and equilibrium studies

Abstract

The complexation of NiII with 2-aminomethylbenzimidazole (L) has been investigated at 20–40 °C, I= 0.30 mol dm–3. Both monoprotonated and unprotonated ligands bind the metal ion to form [NiL]2+. and the rate and activation parameters for the formation and acid-catalysed dissociation of this chelate are calculated. In the presence of Salicylaldehyde (Hsal), a mixed-ligand complex. [NiL(sal)]+, is also formed as an intermediate which further condenses to the Schiff-base complex, [NiL′]+[HL′=N-(benzimidazol-2-ylmethyl)salicylideneimine]. The kinetics of the fast ternary complex formation and its slow intramolecular transformation to the Schifi base complex have been investigated at 25 °C. The presence of L in the co-ordination sphere of Ni2+ enhances the dissociation of [NiL(sal)]+ to [NiL]2+ and sal– with respect to [Ni(Sal)]+, as evidenced by the stability constants of [NiL(sal)]+ and [Ni(sal)]+. Calculations based on the values of ΔS° for the ionisation of H2L2+ and the formation of [NiL]2+combined with Saq°(H+) and Saq°(Ni2+) data yielded the values Saq°(H2L2+)–Saq°(L)= 110, Saq°(L)–Saq°(HL+)=–116 and Saq°([NiL]2+)–Saq°(L)=–294 J K–1 mol–1, which presumably reflect the varying solvent-ordering effects of L, HL+, H2L2+ and [NiL]2+.