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

Abstract

The synthesis and characterisation of cis- and trans-[Co(tmen) 2(NCCH 3) 2](ClO 4) 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, k 1=5.61 × 10 −4 and k 2=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 CD 3CN at 298.2 K: k ex( cis)= k ct + k cc=2.0 × 10 −5 and k ex( trans)= k tc + k tt=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 CH 3CN (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(NCCH 3) 2](ClO 4) 3 have also been determined spectrophotometrically and by NMR; k cis =1.3 × 10 −4 and k trans =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) 2X 2] n+ complexes are found to be consistently much more reactive than their [Co(en) 2X 2] n+ analogues.