Intramolecular electron transfer and cation migration in the dianion of dicyclooctatetraenyldimethylsilane studied by dynamic C-13 NMR spectroscopy

The thermally activated two-electron and cation transfer from the charged to the neutral cyclooctatetraenyl ring in the title dianion (1(2-)) with alkali-metal cations in [H-2(8)] tetrahydrofuran solutions has been studied by C-13 NMR Spin saturation transfer. The free energy of activation for this process at 47 degrees C decreases in the order 1(2-)/ Na+ >K+ > Li+. This is explained by the existence of tight ion pair structures for the Na and K salts, while the Li system has a certain number of loose ion pairs coexisting with tight ion pairs. A stronger interaction between the dianion ring and Na+, as compared with K+, is supported by C-13 NMR chemical shifts of the dianion ring. The activation barrier decreases further for the Li salt when the temperature is lowered, which is accounted for by a shift in the tight/loose ion pair equilibrium. However, the Li system is predominantly a tight ion pair at -3 degrees C or lower temperatures as shown by Li-7 NMR chemical shifts, but it is concluded that the looser ion pairs undergo exchange considerably faster than the tight ion pairs. Rate constants have also been obtained for the bond shift in the neutral ring. The free enthalpy of activation for the bond shift decreases in the cation order Li+ >Na+ >K+ and the polarization of the double bonds in the neutral ring increases in the same order as probed by C-13 NMR chemical shifts. Concentration studies and C-13 NMR T-1 values of the Li and K salts show that the exchange of electrons and cations is intramolecular and that the aggregation state is the same for the Li and K salts of 1(2-). The NMR chemical shifts and gross charges from HF/6-31G*//3-21G(*) ab initio MO calculations are in accord with a ground state structure that has the two extra charges localized at one of the cyclooctatetraenyl rings.