Studies on kinetics and mechanism of imidazole substitution from cis-equatorial-[Cr(S-pdtra)(Him)]0 complex in acidic and alkaline aqueous solutions [S-pdtra = (S)-propylenediaminetriacetate]†

Abstract

Aquation of the cis-equatorial-[Cr(S-pdtra)(Him)]⁰ complex [S-pdtra = (S)-propylenediaminetriacetate] has been studied within the pH range 0–14. Liberation of imidazole in acidic medium leads to the cis-eq-[Cr(S-pdtra)(H₂O)]⁰ complex. Deprotonation of coordinated imidazole in alkaline solutions stimulates a reversible one-end dechelation of S-pdtra, which precedes substitution of imidazole; a characteristic bell-shaped k_obsvs. [H⁺] dependence is observed. The values of ΔS ^‡ = –52.8 J mol^–1 K^–1, ΔH ^‡ = 84.2 kJ mol^–1 and ΔV ^‡ = –10.1 cm³ mol^–1 for aquation in acidic medium are consistent with an I_a mechanism, whereas substitution of the imidazolate from the hydroxo complex with a tetradentate S-pdtra proceeds via an I_d mechanism on the basis of ΔS ^‡ = 49.6 J mol^–1 K^–1 and ΔH ^‡ = 117.5 kJ mol^–1. One-end dechelation of the S-pdtra ligand from the complex with imidazolate (pH > 13) is characterised by k₂₉₈ = 6.43 × 10^–3 s^–1, ΔS ^‡ = –57.2 J mol^–1 K^–1 and ΔH ^‡ = 68.5 kJ mol^–1.