Effect of ion pairing on charge transfer in the bicyclooctatetraenyl dianion

The rate constants for bond shift (k(BS)) and intramolecular charge transfe r (k(CT)) in alkali metal salts of bicyclooctatetraenyl dianions (1(2-)) in THF-da were determined by dynamic NMR spectroscopy. The values of kBS and k(CT) increase in the order K < Na I Rb < Cs and K < Rb < Na i Cs much less than Li, respectively. It is concluded that the K, Rb. and Cs salts react as contact ion pairs (CIPs) while the Li and Na salts react as solvent-sepa rated ion pairs (SSIPs). Increases in k(BS) and k(CT) are observed in 1(2-) /2K(+) on going to more polar solvents (NH3/CH3NH2 and THF-d(8)/DMEU). Comp arison with kinetic studies of four other dianions has led to the conclusio n that 1(2-)/2Na(+) is the first example of a Na salt of a dianion to under go charge transfer by SSIPs. This is attributed to the difficulty in formin g a Na-bridged CIP transition structure in 1(2-)/2Na(+) that can compete wi th an SSIP transition structure.