Solvent exchange, solvent interchange, aquation and isomerisation reactions of cis- and trans-[Co(tmen)2(NCMe)2]3+ in water, Me2SO and MeCN:: kinetics and stereochemistry

The synthesis and characterisation of cis- and trans-[Co(tmen)(2)(NCCH3)(2)](ClO4)(3) are described. Solvolysis rates have been measured by both H-1 NMR spectroscopy and UV-Vis spectrophotometry in dimethyl sulfoxide at 298.2 K. The cis isomer undergoes solvolysis by consecutive first-order reactions, k(1) = 5.61 X 10(-4) and k(2) = 5.35 x 10(-4) s(-1), each with steric retention. The measured solvolysis rate (single step reaction) for the trans isomer is k = 1.54 x 10(-5) s(-1). The solvent exchange rates have been measured by H-1 NMR spectroscopy in CD3CN at 298.2 K: k(ex)(cis) = k(ct) + k(cc) = 2.0 x 10(-5) and k(ex)(trans) = k(1c) + k(tt) = 4.56 x 10(-6) s(-1). From these data, the measured cis-trans isomerisation rate (1.71 X 10(-6) s(-1)) and equilibrium position in CH3CN (17% trans), the steric course for substitution in the exchange processes has been determined: trans reactant -69% trans product; cis reactant -99% cis product. Aquation rates for cis- and trans-[Co(tmen)(2)(NCCH3)(2)](ClO4)(3) have also been determined spectrophotometrically and by NMR; k(cis) = 1.3 x 10(-4) and k(trans) = 2.7 x 10(-5) s(-1). In both cases the steric course for the primary aquation step is indeterminate because the subsequent steps are faster. Where data are available, the [Co(tmen)(2)X-2](n+) complexes are found to be consistently much more reactive than their [Co(en)(2)X-2](n+) analogues. (C) 2003 Elsevier B.V. All rights reserved.