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-, BArF-, and B(C6F5)4-] salts to form ion quadruples (IQs) and higher aggregates (HAggs) in [D6]benzene is investigated by means of diffusion NMR spectroscopy. 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=5×10-5 M and show a limited tendency to further self-aggregate, reaching a maximum average aggregation number (N=VH/$V_\rmH^\rm0IP $, where VH=measured hydrodynamic volume and $V_\rmH^\rm0IP $=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≥1BArF; HAggs: 1BTol4>1BPh4> 1NTf2>1B(C6F5)4> 1BArF>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. The self-aggregation tendency of [N(CH3)2(C18H37)2]X ion pairs to form ion quadruples (IQs) and higher aggregates (HAggs) in [D6]benzene strongly depends on the nature of X- according to diffusion NMR studies. Salts with small anions such as PF6- show a marked tendency to form IQs and low propensity to further self-aggregate, and the opposite is true for salts with large (hydrogenated) anions like BPh4- (cf. left- and right-hand plots of aggregation number N versus C in the picture). Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.