Thermodynamic and kinetic studies of the equilibration between the sulfur- and carbon-bonded forms of a cobalt(III) complex with the ligands 2-aminoethyl-3-aminopropyl-sulfide and 1,1,1-tris(aminomethyl)ethane

Abstract

A thermodynamic and kinetic study of the equilibration between the Co-S bonded complex Co(tame)(S-aeaps)3+ and the Co-C bonded complex Co(tame)(C-aeaps)2+ is reported (tame=1,1,1-tris(aminomethyl)ethane, aeaps=2-aminoethyl-3-aminopropylsulfide=3-thiabexane-1,6-diamine and C-aeaps=1,6-diamin-3-thia-hexan-4-ide anion) Kinetic and thermodynamic data have been obtained using UV-VIS spectroscopy, IE-HPLC and 13C NMR ([OH-]=4×10-5-1.3 M, T=25.0-100°C, I=1.0M). The kinetic data have been interpreted in terms of different reactivity of ion pairs with perchlorate and hydroxide, respectively. Rate constants and activation parameters for the formation of the cobalt-alkyl complex from ion pairs with perchlorate and hydroxide, respectively (composite terms), and for the re-formation of the cobalt-sulfur complex from ion pairs with perchlorate are given, together with thermodynamic data. The data have been interpreted in terms of the intermediate formation of a carbanion, and this was strongly supported by 13C NMR CH/CD-exchange studies. The Co(tame)(acaps)3+ cation exchanges one C-H proton, and only one, prior to the formation of the cobalt-alkyl complex. The subsequent reaction of the partially C-deuterated (-N-CH2-CH2-CHD-S-) cobalt-sulfur complex to form the cobalt alkyl complex gave complete loss of deuterium. From the kinetic data it is estimated that the carbanion reacts with water 170 times faster than it is captured by cobalt (III). © Acta Chemica Scandinavica 1996.