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

Abstract

The rate constants for bond shift in dicyclooctatetraenylmethane (1), dicyclooctatetraenyldimethylsilane (2), and their dianions (1²⁻ and 2²⁻) in [²H₈]THF, have been determined from the temperature dependence of their ¹³C NMR linewidths. The corresponding parameters for intramolecular electron and cation transfer (charge transfer) between the dinegative and neutral rings have been measured by ¹³C 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 ¹³C NMR chemical shifts and ab initio molecular orbital calculations at 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 1²⁻–2K⁺. A significant portion of the remainder is attributed to a greater inter-ring through-space interaction in 1²⁻, although mediation by the cation and/or through-bridge interactions probably also contribute to some extent. A temperature-dependent differential ¹³C NMR line broadening is observed for the dianion ring carbons of the dipotassium salts. Possible mechanisms for this counterion-specific line broadening, which occurs only for carbons with large HOMO coefficients, are discussed.