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

Abstract

The complexation of Ni^II 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 S [> with combining macron]_aq°(H⁺) and S[> with combining macron]_aq°(Ni²⁺) data yielded the values S[> with combining macron]_aq°(H2L2+)–S[> with combining macron]_aq°(L)= 110, S[> with combining macron]_aq°(L)–S[> with combining macron]_aq°(HL⁺)=–116 and S[> with combining macron]_aq°([NiL]²⁺)–S[> with combining macron]_aq°(L)=–294 J K⁻¹ mol⁻¹, which presumably reflect the varying solvent-ordering effects of L, HL⁺, H₂L₂+ and [NiL]²⁺.