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

Abstract

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.

Diffusion, Ion Pairing and Aggregation in 1‐Ethyl‐3‐Methylimidazolium‐Based Ionic Liquids Studied by 1H and 19F PFG NMR: Effect of Temperature, Anion and Glucose Dissolution

“…[24] Values of N are reportedi nT able 1a nd are comparable in order of magnitude with those reported in the literature for other ionic species. [39] At this point it is important to note the significantly highera ggregation number,w ith N > [OAc] ionicl iquid [16] and is also in agreement with the hypothesis suggested by Remsing et al, [15] who studied the same liquid. The resultsr eported herein give further strength to this hypothesis by providing an actual quantitative analysis.…”

“…Values of N are reported in Table and are comparable in order of magnitude with those reported in the literature for other ionic species . At this point it is important to note the significantly higher aggregation number, with N >1, of the [OAc] − anion compared to [EMIM] + cation in both the neat [EMIM][OAc] and [EMIM][OAc] in glucose.…”

Applications of NMR diffusion methods with emphasis on ion pairing in inorganic chemistry: a mini‐review

“…In an extensive study over several decades, Macchioni and co‐workers have shown that, even at modest concentration, as well as in relatively polar alcohol solutions, various salts aggregate. A useful example involves the self‐aggregation tendency of the long chain ammonium salts, [N(CH 3 ) 2 (C 18 H 37 ) 2 ]X, X − = BF 4 − , PF 6 − , CF 3 SO 3 − , NTf 2 − , BPh 4 − , B(p‐Tol) 4 − , BArF − and B(C 6 F 5 ) 4 − , 9 , to form ion quadruples (IQs) and higher aggregates in deuterated benzene . The experimental diffusion results indicate that the salts containing the small anions BF 4 − , PF 6 − and CF 3 SO 3 − are present in solution as IQs even at the lowest investigated concentration, 5 × 10 −5 M. A relatively small selection of an extensive data collection is given in Table .…”

“…C, concentration in mM; D, D‐value for the cation or anion; V, hydrodynamic volume for the cation or anion; N + and N − , the maximum aggregation number for the cation or anion, respectively …”

Characterization of Halogen Bonded Adducts in Solution by Advanced NMR Techniques

“…More quantitative information can be useful in determining the molecular weight distribution for polymers [78], the purity and composition of functionalized carbon nanotubes [79], the formation of ion pairs and quadruples [80][81][82][83], or, in the case of neutral species, of supramolecular adducts [84,85].…”