Anion-Dependent Tendency of Di-Long-Chain Quaternary Ammonium Salts to Form Ion Quadruples and Higher Aggregates in Benzene

The self-aggregation tendency of [N(CH3)2(C18H37)2]X [1X; X^-= BF4^-, PF6^- , OTf^-, NTf2^-, BPh4^-, BTol4^- , BAr^{F-}, and B(C6F5)4^-] salts to form ion quadruples (IQs) and higher aggregates (HAggs) in [D6]benzene is investigated by means of diffusion NMR spec- troscopy. The experimental results indicate that salts containing small anions (1BF4, 1PF6, and 1OTf) are present in solution as IQs even at the lowest investigated concentration of C = 5x10^5 m and show a limited tendency to further self-aggregate, reaching a maximum average aggregation number (N = VH/VH^{0IP}, where VH = measured hydrodynamic volume and VH^{0IP} = hydrodynamic volume of the ion pair) of about 6-8 (C = 0.050- 0.100 M). Salts with larger counterions [1BPh4, 1BTol4, 1BArF, and 1B(C6F5)4] form instead ion pairs at low concentration but steadily self-aggregate (especially the non-fluorinated ones) on increasing their concentration up to N values exceeding 50 (C = 0.030-0.050 M). 1NTf2 behaves in an intermediate fashion. The self-aggregation tendency of salts is quantified by formulating the dependence of VH on C by means of the equations of indefinitive aggregation models. The following rankings for the formation of IQs and HAggs are obtained: IQs: 1BF4 ~ 1PF6 ~ 1OTf > 1NTf2 > 1B(C6F5)4 >= 1BPh4 >= 1BTol4 >= 1BAr^F; HAggs: 1BTol4 > 1BPh4 > 1NTf2 > 1B(C6F5)4 > 1BAr^F > 1BF4 ~ 1PF6 ~ 1OTf. Interionic NOE NMR studies and DFT calculations were conducted in order to determine the relative anion- cation orientation in the self-aggregating units.