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 char ged 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 pai r 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 activa tion barrier decreases further for the Li salt when the temperature is lowered, which is accounted for by a shift in the tight/loose ion pai r equilibrium. However, the Li system is predominantly a tight ion pai r 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 als o been obtained for the bond shift in the neutral ring. The free entha lpy 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. Co ncentration 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-). Th e NMR chemical shifts and gross charges from HF/6-31G//3-21G(*) ab in itio MO calculations are in accord with a ground state structure that has the two extra charges localized at one of the cyclooctatetraenyl r ings.