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 1H NMR spectroscopy and UV–Vis spectrophotometry in dimethyl sulfoxide at 298.2 K. The cis isomer undergoes solvolysis by consecutive first-order reactions, k1=5.61 × 10−4 and k2=5.35 × 10−4 s−1, each with steric retention. The measured solvolysis rate (single step reaction) for the trans isomer is k=1.54 × 10−5 s−1. The solvent exchange rates have been measured by 1H NMR spectroscopy in CD3CN at 298.2 K: kex(cis)=kct + kcc=2.0 × 10−5 and kex(trans)=ktc + ktt=4.56 × 10−6 s−1. From these data, the measured cis–trans isomerisation rate (1.71 × 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; kcis=1.3 × 10−4 and ktrans=2.7 × 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)2X2]n+ complexes are found to be consistently much more reactive than their [Co(en)2X2]n+ analogues.