Kinetics of reaction of imidazole, glycine, and L-histidine with the aqua-pentacyanoferrate(II) ion

Abstract

The properties and reactivity of the pentacyanoferrate(II) complexes of imidazole (imH), glycinate (glyO), and L-histidine (his) are reported. The rates of formation, starting from [Fe(CN)₅(OH₂)]^3–{generated by aquation of [Fe(CN)₅(NH₃)]^3–}, are typically of first order with respect to the concentration of the ligands, with k_f= 240, 28.0, and 320 dm³ mol^–1 s^–1, respectively. The kinetics of dissociation of the complexes, measured in the presence of an excess of dimethyl sulphoxide, show saturation behaviour with respect to this reactant. The limiting rates of dis-sociation are 1.33 × 10^–3(imH), 2.67 × 10^–3(glyO^–), and 5.3 × 10^–4 s^–1(his) at 25 °C and I= 0.100 mol dm^–3(Li[ClO₄]). L-Histidine, in zwitterionic form, reacts with [Fe(CN)₅(OH₂)]^3– to yield two isomers in solution. The predominant form contains iron co-ordinated to the hindered N³ nitrogen of imidazole in the L-histidine ligand. This isomer is labile and dissociates rapidly, k= 0.109 s^–1 to form the inert and stable N¹ isomer. Similar behaviour, but involving the three N-co-ordinating sites of histidine, is observed in alkaline solutions. The strain energy associated with substitution adjacent to the iron centre is estimated as 3,4 kcal mol^–1 A correlation of the limiting rates of dissociation with the ligand-field energies in the complexes is presented.