Bond shift and charge transfer dynamics in methylene- and dimethylsilyl-bridged dicyclooctatetraene dianions

The rate constants for bond shift in dicyclooctatetraenylmethane (1), dicyclooctatetraenyldimethylsilane (2), and their dianions (1(2-) and 2(2-)) in [H-2(8)]THF, have been determined from the temperature dependence of their C-13 NMR linewidths. The corresponding parameters for intramolecular electron and cation transfer (charge transfer) between the dinegative and neutral rings have been measured by C-13 NMR spin saturation transfer experiments for the dipotassium salts of 1 and 2. Selected structural features of the neutral compounds and the dianions are discussed on the basis of C-13 NMR chemical shifts and ab initio molecular orbital calculationsat the HF/6-31G* and HF/3-21G(*) levels of theory. Energy contributions to the ring flattening in the bond shift process are calculated by molecular mechanics methods. The measured rate constants for both bond shift and charge transfer are larger for the methylene-bridged dianion. Approximately half of this difference is due to the greater ease of "gating" (i.e., ring flattening and distortion to the bond shift transition state) in 1(2-)-2K(+). A significant portion of the remainder is attributed to a greater inter-ring through-space interaction in 1(2-), although mediation by the cation and/or through-bridge interactions probably also contribute to some extent. A temperature-dependent differential C-13 NMR line broadening is observed for the dianion ring carbons of the dipotassium salts. Possible mechanisms for this counterion-specificline broadening, which occurs only for carbons with large HOMO coefficients, are discussed.