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(CH(3))(2)(C(18)H(37))(2)]X [1X; X(-)=BF(4) (-), PF(6) (-), OTf(-), NTf(2) (-), BPh(4) (-), BTol(4) (-), BAr(F-), and B(C(6)F(5))(4) (-)] salts to form ion quadruples (IQs) and higher aggregates (HAggs) in [D(6)]benzene is investigated by means of diffusion NMR spectroscopy. The experimental results indicate that salts containing small anions (1BF(4), 1PF(6), 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=V(H)/V(H)(0IP), where V(H)=measured hydrodynamic volume and V{H}{0IP}=hydrodynamic volume of the ion pair) of about 6-8 (C=0.050-0.100 M). Salts with larger counterions [1BPh(4), 1BTol(4), 1BAr(F), and 1B(C(6)F(5))(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). 1NTf(2) behaves in an intermediate fashion. The self-aggregation tendency of salts is quantified by formulating the dependence of V(H) on C by means of the equations of indefinitive aggregation models. The following rankings for the formation of IQs and HAggs are obtained: IQs: 1BF(4)≈1PF(6)≈1OTf> 1NTf(2)>1B(C(6)F(5))(4)≥1BPh(4)≥1BTol(4)≥1BAr(F); HAggs: 1BTol(4)>1BPh(4)> 1NTf(2)>1B(C(6)F(5))(4)> 1BAr(F)>1BF(4)≈1PF(6)≈1OTf. Interionic NOE NMR studies and DFT calculations were conducted in order to determine the relative anion-cation orientation in the self-aggregating units.