Thermodynamic and kinetic studies of the equilibration reaction between the sulfur and carbon bonded forms of a cobalt(III) complex with the ligands 1,4,7-triazycyclononane and 1,4-diaza-7-thiacyclodecane
The new cyclic thioether 1,4-diaza-7-thiacyclodecane, dathicd, has been synthesized and used for the prepn. of the sulfur- and carbon-bonded cobalt(III) complexes: [Co(tacn)(S-dathicd)]Cl3.5H2O and [Co(tacn)(C-dathicd)](ClO4)2 (tacn, 1,4,7-triazacyclononane; C-dathicd, 1,4-diamino-7-thiacyclodecan-8-ide anion). A thermodn. and kinetic study of the equilibration between these coordination compds. has been performed using UV-VIS absorption spectroscopy, IE-HPLC and 13C NMR ([OH-]=10-5-1.0 M, T=25.0 DegC, I=1.0 M). In basic soln. Co(tacn)(S-dathicd)3+ deprotonates at one of the coordinated amine groups and the base dissocn. const. has been detd. to Kb(NH)=0.311(32) M. The equil. const. for the reaction of Co(tacn)(S-dathicd)3+ with hydroxide ions to give Co(tacn)(C-dathicd)2+ has been detd. to Kf=1.8x105 M-1 at 25 DegC. 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 estd. that the carbanion reacts with water 270 times faster than it is captured by cobalt(III).
Acta Chemica Scandinavica, 1996, Vol 34, Issue 10, p. 853-861
Entropy; Equilibrium; Exchange reaction (deuterium; Heat of reaction; Kinetics; Thermodynamics (thermodn. and kinetics of equilibr; thermodn. and kinetics of equilibration between su