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 kf= 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.