Base-catalysed hydrolysis of cis-(imidazolato)-, cis-(benzimidazolato)- and cis-(N-methylimidazole)-bis-(ethylenediamine)halogenocobalt(III) cations. A comparison of the reactivities of the deprotonated imidazole and benzimidazole complexes

Abstract

The base hydrolysis of the complexes cis-[Co(en)₂B(X)]ⁿ⁺[en = ethylenediamine; B = imidazole (Him), N-methylimidazole (mim), or benzimidazole (Hbzim); X = Cl or Br] has been investigated at 20–35 °C and [OH^–]_T=(4.7–99.7)× 10^–3 mol dm^–3(I= 0.10 mol dm^–3) under which conditions the co-ordinated imidazole and benzimidazole undergo complete NH deprotonation. The activation parameters (85 ⩽ΔH‡/kJ mol^–1⩽ 97, +84 ⩽⩽ΔS‡/J K^–1 mol^–1⩽+97), and the rate dependence on the leaving groups and the non-labile amine ligands are consistent with a S_N1 CB mechanism. The electron-displacement properties of the N-co-ordinated imidazolate (im) and benzimidazolate (bzim) ions appear to enhance the pK_NH of the co-ordinated ethylenediamine, the effect being relatively more significant for the former anion. Analysis of the activation entropy data in terms of the dissociative activation model for the conjugate bases, cis-[Co(en)(en – H)B(X)]ⁿ⁺[B = im or bzim (n= 0); mim (n= 1)] indicates that the configurational rearrangement at the cobalt(III) centre most likely occurs in the transition state of the actual acta of the substitution for the less reactive chloro complexes. The presumed trigonal-bipyramidal intermediate is efficiently scavenged by azide.