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DOI: 10.1039/A805334K (Paper) Reference Section for: J. Chem. Soc., Dalton Trans., 1999, 91-96

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[hair space])-propylenediaminetriacetate][hair space]†

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Janusz Chatłas, Sumio Kaizaki, Ewa Kita, Przemysław Kita, Narumi Sakagami and Rudi van Eldik

Abstract

Aquation of the cis-equatorial-[Cr(S-pdtra)(Him)]0 complex [S-pdtra = (S[hair space])-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 kobsvs. [H+] dependence is observed. The values of ΔS[hair space] ‡ = –52.8 J mol–1 K–1, ΔH[hair space] ‡ = 84.2 kJ mol–1 and ΔV[hair space] ‡ = –10.1 cm3 mol–1 for aquation in acidic medium are consistent with an Ia mechanism, whereas substitution of the imidazolate from the hydroxo complex with a tetradentate S-pdtra proceeds via an Id mechanism on the basis of ΔS[hair space] ‡ = 49.6 J mol–1 K–1 and ΔH[hair space] ‡ = 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[hair space] ‡ = –57.2 J mol–1 K–1 and ΔH[hair space] ‡ = 68.5 kJ mol–1.

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