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 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_H^OIP, where VH=measured hydrodynamic volume and equation image=hydrodynamic volume of the ion pair) of about 6–8 (C=0.050–0.100 M). Salts with larger counterions [1BPh₄, 1BTol₄, 1BAr^F, 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.