Solution-phase counterion effects in supramolecular and mechanostereochemical systems

Abstract

The self-assembly of molecular components into complex superstructures involves the subtle interplay of various noncovalent forces. Charged species are often utilised in self-assembly processes as a result of the favorable π-π, cation-π, electrostatic, and hydrogen bonding interactions that form between these species. Although the counterions associated with these charged species can exert significant effects on the synthesis, stability, and operation of superstructures in solution, rarely are the counterions considered, leading to misinterpretations and misunderstandings of the studied systems. In this tutorial review, we discuss a variety of solution-phase counterion effects, from the fundamental origins to innovative ways in which these effects are exploited for useful functions.

A Halide‐for‐Anion Swap Using an Anion‐Exchange Resin (A– Form) Method: Revisiting Imidazolium‐Based Anion Receptors and Sensors

“…However, the solvent mixture CH 3 CN/CH 3 OH (9:1) permitted counteranion exchange of 1·2Cl to afford 1·PF 6 , 1·BF 4 , and 1·CF 3 SO 3 in nearly quantitative yields. For the new targeted imidazolium salt, 1·Ph 4 B, the hydrophobic nature of the tetraphenylborate anion was evident because when a solution of 1·Cl in CH 3 CN/CH 3 OH (9:1) was passed through an AER (Ph 4 B -form) a precipitate was formed inside the column and the hydrophobic ion pair 1·Ph 4 B was obtained in just 79 % yield after additional solvent was passed through the resin. A more lipophilic solvent mixture, CH 3 CN/CH 3 OH/CH 2 Cl 2 (4.5:0.5:5), produced the chloride-to-tetraphenylborate exchange quantitatively (see Scheme 1, Table S2 in the Supporting Information).…”

“…[13] They described the transformation of 1·Cl into salts 1·A (A = PF 6 , BF 4 , and CF 3 SO 3 ), following classical counteranion exchange with inorganic salts (MA) in yields from 70 to 89 % (see Tables S1 and S2 in the Supporting Information). [13] We first examined the transformation of 1·2Cl into 1·PF 6 and 1·BF 4 4 , and 1·CF 3 SO 3 in methanol, the AER (A -form) method in acetonitrile was then assayed, but the starting imidazolium salt, 1·2Cl, was insoluble in this solvent. However, the solvent mixture CH 3 CN/CH 3 OH (9:1) permitted counteranion exchange of 1·2Cl to afford 1·PF 6 , 1·BF 4 , and 1·CF 3 SO 3 in nearly quantitative yields.…”

“…[13] We first examined the transformation of 1·2Cl into 1·PF 6 and 1·BF 4 4 , and 1·CF 3 SO 3 in methanol, the AER (A -form) method in acetonitrile was then assayed, but the starting imidazolium salt, 1·2Cl, was insoluble in this solvent. However, the solvent mixture CH 3 CN/CH 3 OH (9:1) permitted counteranion exchange of 1·2Cl to afford 1·PF 6 , 1·BF 4 , and 1·CF 3 SO 3 in nearly quantitative yields. For the new targeted imidazolium salt, 1·Ph 4 B, the hydrophobic nature of the tetraphenylborate anion was evident because when a solution of 1·Cl in CH 3 CN/CH 3 OH (9:1) was passed through an AER (Ph 4 B -form) a precipitate was formed inside the column and the hydrophobic ion pair 1·Ph 4 B was obtained in just 79 % yield after additional solvent was passed through the resin.…”

“…Notably, the synthesis of this type of bis(imidazolium) [1 4 ]cyclophanes has exploited their ability to bind chloride anions and permitted the examination of anion templates within simple bis(imidazolium) systems. [5,17,18] Finally, hydrophobic systems can be represented by dicationic calix [4]arenes 6·2Br [19] and 7·2Br. The chloride-for-anion swap of the open-chain compounds 2·2Cl and 3·2Cl was first examined in methanol by using the AER (A -form) conveniently loaded with assorted anions and the resulting compounds 2·2A and 3·2A were obtained in yields ranging from 28 to 100 %.…”

A Versatile Axle for the Construction of Disassemblage Rotaxanes

“…Furthermore, blocked-axle 7 was investigated as the chloride salt, which will likely exist in biological environments. In CDCl3, the strong ion pairs greatly weakens pseudorotaxane 8 (KA = 17 ± 0.6 M −1 ) when compared to the typical association observed in the millimolar range for the complex of DB24C8 with R2NH2 + PF6 − salts [24][25][26]. Tighter association was observed in DMSO-d6 (KA = 250 ± 10 M −1 ).…”