Thermodynamic and kinetic studies of the equilibration between the sulfur-and carbon-bonded forms of the cobalt(III) complex with the ligands 1,4,7-triazacyclononane and 1,4-diaza-7-thiacyclodecane

Abstract

The new cyclic thioether 1,4-diaza-7-thiacyclodecane, dathicd, has been synthe-sized and used for the preparation of the sulfur- and carbon-bonded cobalt (III) complexes: [Co(tacn)(S-dathicd)]Cl3·5H2O and [Co(tacn)(C-dathicd)](ClC4)2 (tacn, 1,4,7-triazacyclononane; C-dathicd, 1,4-diamino-7-thiacyclodecan-8-ide anion). A thermodynamic and kinetic study of the equilibration between these coordination compounds has been performed using UV-VIS absorption spectroscopy, IE-HPLC and 13C NMR ([OH-]=10-5-1.0 M, T=25.0°C, I=1.0 M). In basic solution Co(tacn)(S-dathicd)3+ deprotonates at one of the coordinated amine groups and the base dissociation constant has been determined to Kb(NH)=0.311(32) M. The equilibrium constant for the reaction of Co(tacn)(S-dathicd)3+ with hydroxide ions to give Co(tacn)(C-dathicd)2+ has been determined to Kf=1.8×105 M-1 at 25°C. The kinetic 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. It was shown that Co(tacn)(S-dathicd)3+ exchanges one methylene proton, and only one, prior to the formation of the alkyl complex. The subsequent reaction of the partially C-deuterated (-N-CH2-CH2-CHD-S-) cobalt-sulfur complex to form the alkyl complex gave 100% loss of deuterium. It is concluded that the labile methylene proton is bound to the carbon atom which in the alkyl complex is bound to cobalt(III). From the kinetic data it is estimated that the carbanion reacts with water 270 times faster than it is captured by cobalt (III). © Acta Chemica Scandinavica 1996.