Factors determining ion-pair formation. Studies of the radical anions of 2,5-bis(tricyanovinyl)furan and 2,5-bis(tricyanovinyl)thiophene

Abstract

The specificity of ion-pair formation of the radical anions of 2,5-bis(tricyanovinyl)furan (1) and 2,5-bis(tricyanovinyl)thiophene (2) has been studied using solvents covering a broad range on the polarity scale and Li⁺, Na⁺, K⁺ and Cs⁺ as the counter-ions. The EPR/ENDOR spectra show that 1˙^– and 2˙^– can exist as the free radical anions in which the tricyanovinyl groups have a syn/anti conformation, but under appropriate conditions 2˙^– can also exist in a syn/syn conformation in an ion-pair with the metal cation. This is rationalised in terms of a selectivity for the formation of ion pairs depending on the size of the alkali-metal cation and of the cavity presented by the proximate pair of cyano groups.