The kinetics of the base hydrolysis of trans-Dichloro- and trans-Dibromo-(meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene)cobalt(III) perchlorate

Abstract

The kinetics of the successive base hydrolysis of both halogen ligands in trans-[Co(hctd)Cl₂]⁺ and trans-[Co(hctd)Br₂]⁺ (hctd = meso- 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) have been investigated at 25°C in formic acid-NaOH buffers (pH range 3.0-3.7) and 2,6-lutidine-perchloric acid buffers (pH range 6.4-7.4). Hydrolysis reactions in each buffer system, consistent with release of first and second coordinated halogen respectively, follow rate expressions of the type k_obs = k_aq+ k_OH]OH^-], with base hydrolysis rates k_OH? = 7.8(±0.8) x 10⁸ dm³ mol^-1 s^-1 and 1.9(±0.3)x10⁹ dm³ mol^-1 s^-1 respectively for first chloride and bromide hydrolysis, and k_OH? = 7.3(±0.4)x10⁴ dm³ mol^-1 s^-1 and 2.4(±0.2)x10⁵ dm³ mol^-1 s^-1 for second chloride and bromide hydrolysis respectively. Comparison with the analogous complex trans-[Co(dtcd)Cl₂]⁺ (dtcd = meso-5,12-dimethyl- 1,4,8,11-tetraazacyclotetradeca-4,11-diene) shows a ninefold acceleration for base hydrolysis of the first chloride in the hctd complex over the dtcd complex, but an 80-fold rate retardation for base hydrolysis of the second chloride in the hctd complex compared with the dtcd complex, interpreted in terms of steric effects arising from gem- dimethyl substitution in the hctd ligand.