Kinetic studies of the base hydrolysis of cobalt(III) chloropentamine complexes containing (triamine)(diamine) chromophores

Abstract

The rates of base hydrolysis of a series of [CoCl(triamine)(diamine)]²⁺ complexes have been measured using stopped-flow techniques. The triamines used were tacn (1,4,7-triazacyclononane) or dien (3-azapentane-1,5-diamine) and the diamines were en (1,2-diaminoethane), tn (1,3-diaminopropane) or amp (2-aminomethylpyridine). Studies have also been carried out on a corresponding macrobicyclic pentamine 1,5,8,12,15-pentaazabicyclo[10.5.2]nonadecane (L¹) where the geometry of the ion is square pyramidal around the cobalt centre. Kinetic parameters [k_OH/dm³ mol^–1 s^–1 at 25 °C (I/mol dm^–3), ΔH^‡/kJ mol^–1, ΔS^‡/J K^–1 mol^–1] in the ligand order (triamine)(diamine) are: (tacn)(en), 9.66 (0.1), 3.66 (1.0), 96.6, 90; (tacn)(tn), 40.6 (0.1), 15.8 (1.0), 94.4, 94; (tacn)(amp), 154 (0.1), 66.7 (1.0), 83.3, 69; ufac-I-(dien)(amp), 334 (0.1), 183 (1.0), 79.6, 66; ufac-II-(dien)(amp), 762 (0.1), 359 (1.0), 73.3, 50; L¹, 3 × 10³(1.0). Many of the complex ions conform in some measure to the requirements for ready hydrolysis. However, in the last case identified as a result of the initial geometry of the ion, the rate is somewhat lower than anticipated possibly owing to the increased strain involved in the attainment of the trigonal-bipyramidal five-co-ordinate intermediate.