ROLE OF SOLVATION AND CHOICE OF ANION IN CONTROLLING THE THERMODYNAMIC STABILITY OF THE CYCLOOCTATETRAENE AND ANTHRACENE DIANIONS

The enthalpies of reaction for the alkali metal salts of the cyclooctatetraene dianion and sodium salt of the anthracene dianion with water were measured calorimetrically. These enthalpies were used in a thermochemical cycle with the heats of solution of the dianions in tetrahydrofuran (THF) and in hexamethylphosphoramide (HMPA) to evaluate the relative importance of solvation upon the thermodynamic stability of these dianions. It was found that the thermodynamic stability of the cyclooctatetraene dianion neutral ion aggregate (COT2-, 2M+) in THF and the solvated ions (COT2- + 2M+) in HMPA relative to the gas-phase ions (COT2-g + 2M+g) follow the order Li > Na > K > Rb > Cs. This is attributed to the ion aggregate and cation enthalpies of solvation, which may vary as much as 110 kcal/mol with the choice of cation. In HMPA the single ion solvation enthalpies have been evaluated for the series of cations. The disodium salt of anthracene reacts more than twice as exothermically as does the COT salt. © 1979, American Chemical Society. All rights reserved.