Electron-self-exchange kinetics of the cyclooctatetraene/cyclooctatetraene radical-anion couple

Rate constants k(hom) and k(het) are reported for the homogeneous electron-self-exchange and the heterogeneous electrochemical electron-transfer reactions, respectively, of the cyclooctatetraenel cyclooctatetroene(-) (COT/COT-) redox couple. In acetonitrile, the values k(hom)(298K)=(5 +/- 3) x 10(5) m(-1)s(-1) and k(het)(295K) = 8x 10(-3) cm s(-1) are found, whereas slightly faster rates are obtained in dimethylformamide, namely, k(hom) (298 K) = (1.6 +/- 0.6) x 10(6) m(-1) s(-1) and k(het) (295 K) = 2 x 10(-2) cm s(-1). The khom rates are obtained from electron spin resonance (ESR) line broadening where as the k(het) rates are measured at a mercurized Pt electrode by using Nicalson's method. The slowness of both electron-transfer reactions is caused by the high inner-sphere reorganization energy that results from the inevitable conformational change that takes place upon going from the tub-like COT molecule to the planar COT- onion. The rates are well-understood in terms of Marcus theory, including an additional medium inner-sphere mode which is responsible for the flattening of COT.