Complex formation between nickel(II) and 2-(2-aminoethyl) benzimidazole: a kinetic and equilibrium study

Abstract

The reversible complex formation between 2-(2-aminoethyl) benzimidazole (AEB) and nickel(II) was studied by stopped flow spectrophotometry at I = 0.30 mol dm^-3. Both the neutral and monoprotonated form of AEB reacted to give the NiAEB²⁺ chelate. At 25 °C, the rates and activation parameters for the reactions Ni^II + AEB k_f^L→ NiAEB²⁺ and Ni^II + AEBH⁺ k_f^HL → NiAEB²⁺ + H⁺ are k_f^L(dm^-3mol^-1 s^-1) = (2.17 ±0.24) × 10³, ΔH ^≠ (kJ mol^-1) = 40.0 ±0.8, ΔS^≠ (JK^-1 mol^-1) = - 47 ±3 and k_f^HL (dm³ mol^-1s^-1) = 33 ±10, ΔH^≠ (kJ mol^-1) = 42.0 ±2.7, ΔS^≠ (JK^-1mol^-1) = - 72 ±9. The dissociation of NiAEB²⁺ was acid catalysed and k obs for this process increased linearly with [H⁺] in the 0.01-0.15 mol dm-3 (10-30 °C) range with k H(dm³ mol^-1s^-1) (25 °C) = 329 ±6, ΔH^≠ (kJ mol^-1) = 40 ±2 and ΔS^≠(JK^-1mol^-1) = - 61 ±8. The results also indicated that the formation of NiAEB²⁺ involves a chelation-controlled, rate-limiting process. Analysis of the ΔS° data for the acid ionisation of AEBH₂²⁺ and the formation of NiAEB²⁺ showed that the bulky AEBH⁺ ion has a solvent structure breaking effect as compared to AEB [^-S°_aq(AEBH⁺) -^-S°_aq(AEB) = 69 JK^-1 mol^-1], while AEBH₂²⁺ is a solvent ordering ion relative to NiAEB²⁺ [^-S°_aq(NiAEB²⁺) - ^-S°_aq(AEBH₂²⁺ ) = 11 JK^-1mol^-1].