Base catalysed substitution reactions of octahedral cobalt(III) complexes: Mechanism switching by changing the leaving group

Abstract

The rates of base catalysed hydrolysis of six asym-[Co(dmptacn)X] n+ and six asym-[Co(dmpmetacn)X] n+ complexes ( X = Br - , NO 3 - , MeSO 3 - , CF 3 SO 3 - , Me 2 SO, SO 4 2 - ) have been studied to explore the role of the leaving group and the formal charge on these complex ions in determining the effective site of deprotonation, at one of the four distinct α-CH 2 pyridyl sites or at an NH centre. The work corroborates the new pseudo-aminate mechanism established for the chloro derivatives. Deuterium exchange and proton NMR experiments have shown that the site of deprotonation is leaving group dependent, the first time this long-standing speculative idea has been proven. This work has also shown that the effect of improving the leaving group can override rate limiting deprotonation to the point where NH deprotonation, via the normal S N1CB mechanism, operates rather than the rate limiting pseudo-aminate mechanism. The steric course of substitution has been determined and the reaction via NH deprotonation, enforced by employing an exceptionally good leaving group such as the triflate ion, has uncovered a non-retentive pathway leading to the previously unknown (and unstable) sym isomeric product. The steric course of substitution is shown to be mechanism dependent. For reaction via α-CH 2 deprotonation, which of the two arms is utilised proved quite leaving group dependent, although the specific proton on a particular ‘arm’ was always the same, consistent with the requirements of this new mechanism.