Effect of chain length on the glyme-separated ion-pair formation in bolaform salts of α,ω-(9-fluorenyl)polymethylenes

Formation constants of tetraglyme (glyme 5) separated ion pairs of bolaform electrolytes of the type Na⁺,^–Fl(CH₂) _n Fl^–, Na⁺ (Fl^– denoting a fluorenyl carbanion) were measured spectrophotometrically in tetrahydrofuran (THF) and tetrahydropyran (THP) at 25°C as a function of the chain lengthn, withn=2, 3, 4, or 6. The ratiosK ₁/K ₂ of the first and second glyme ion-pair separation steps were found to be equal to the statistical factor 4 in all cases except in THP for the compoundn=2, where the ratio is 15. Values ofK ₁ as well as K ₂ increase considerably with chain length, the former by a factor 19 asn increases from 2 to 6. The ion-pair separation with glyme 5 is easier in THP than in THF. The results are discussed in terms of differences in external cation solvation and the possible involvement of cyclized structures or curled conformations as a result of interactions between the terminal ion pairs.