Interconversion of conjugated and nonconjugated polyolefin cobalt complexes in two oxidation states: an electrochemical and NMR study

Abstract

The kinetics of interconversion between the conjugated (1,3) and nonconjugated (1,5) isomers of cyclooctatetraene (cot) complexes of CpCo [Cp = C5H5] 1 and *CpCo [*Cp = C5Me5] 2 have been studied. Dynamic NMR studies were employed for neutral CpCo(cot) and gave a rate of 3 s-1 at 370 K, extrapolated to 4 X 10^-3 s-1 at 298 K. The permethylated complex isomerizes more slowly, k = 3 X 10^-4 s-1 at 370 K. However, fast fourier transform ac polarography results show that the monoanions of CpCo(cot) and *CpCo(cot) isomerize at the same fast rate, 2 X 10^-3 s-1 at 298 K. An explanation is offered in terms of the lifting of symmetry restrictions to the simplest molecular isomerization motion in the anion radicals. The dibenzocyclooctatetraene complex CpCo(1,5-dbcot) (3) reduces to a stable anion which shows no tendency to isomerize. This is explained by a likely change in electronic ground states of [CpCo(cot)]- and [CpCo(dbcot)]-, a conclusion supported by ESR results.