Bromination of imidazole complexes of pentaammine-cobalt(III). Synthesis, structure and reactivity

Abstract

The preparations and properties (¹H, ¹³C NMR spectra; pK_a data) of various bromoimidazole complexes of pentaamminecobalt(III) are reported. Improved methods are given for synthesising and isolating related methyl imidazole derivatives. Hydrolysis of [Co(NH₃)₅(tbim)]²⁺(tbim = 2,4,5-tribromo-imidazolate) to give [Co(NH₃)₅(OH₂)]³⁺ or [Co(NH₃)₅(OH)]²⁺ and 2,4,5-Htbim follows the rate equation k_obs=(k₁[H⁺]+k₂K_a[OH^–])/([H⁺+K_a) with k₁= 2.07 × 10^–4 s^–1, k₂= 0.11 dm³ mol^–1 s^–1 and k_a= 1.49 × 10^–2 mol dm^–3 at 250°C and I= 1.0 mol dm^–3(NaClO₄); k₁ corresponds to the spontaneous hydrolysis of [Co(NH₃)₅(Htbim)]³⁺, and k₂ to the OH^–-catalysed reaction of its conjugate base. Isomerisation of [Co(NH₃)₅(4-Hbim)]³⁺(4-Hbim = 4-bromoimidazole) to its 5-bromoimidazole congener follows the rate equation (in D₂O at 70°C)k_obs=kK_a[OD^–]/(K_w+K_a[OD^–]) with k= 1.67 × 10^–4 s^–1 and pK_a= 6.86; k corresponds to the spontaneous reaction of the deprotonated [Co(NH₃)₅(4-bim)]²⁺ ion and a π-bonded, or solvent-cage-trapped, transition state is suggested. A crystal structure analysis of the dimer [(H₃N)₅Co(im)Co(NH₃)₅]Br₅·3H₂O (im = imidazolate) has been carried out: space group P2₁/n, a= 12.946(4), b= 14.469(5), c= 14.022(4)Å, β= 107.18(2)°, Z= 4, 2191 reflections, R= 0.0576. Less accurate structures of four other complexes have been determined for the purpose of identifying the positions of bromine substitution.