Flexibility, association constants, and salt effects in organic ion pairs: How single bonds affect molecular recognition

Abstract

Association free energies ΔG in water from NMR shift titrations with ten ion pairs from open-chain α,ω-dianions and α,ω-dications show a dependence on ionic strength that can be fitted to the Debye-Huckel equation. In spite of the anisotropic nature of the ions, the D - H coefficient m obtained in each case is close to the theoretical value m = 4.07 for the dication/dianion combination. The ΔG values of the ten complexes and six literature values are all around 16 kJmol-1 after extrapolation to ionic strength I = 0.00, yielding a constant increment of 8kJmol-1 for each cation-anion interaction. Although the number n of flexible single bonds in the ten complexes differs from 6 to 13, the difference between the strongest and the weakest complexes (ΔG = -16.3 and - 12,6 kJ mol-1, respectively) is surprisingly small. A fairly linear correlation between ΔG and n is observed, which yields an energetic disadvantage of only ΔΔG = 0.5 kJ mo1-1 for one single bond. This value is close to numbers obtained recently with similar hydrogen-bonded complexes in chloroform, and shows that until now the importance of conformational preorganization for effective molecular recognition has been overestimated.