Anions Stabilize Each Other inside Macrocyclic Hosts

Abstract

Contrary to the simple expectations from Coulombs law,Weinhold proposed that anions can stabilize each other as metastable dimers,y et experimental evidence for these species and their mutual stabilization is missing. We showt hat two bisulfate anions can form such dimers,w hichs tabilizee ach other with self-complementary hydrogen bonds,b ye ncapsulation inside apair of cyanostar macrocycles.The resulting 2:2 complex of the bisulfate homodimer persists across all states of matter,i ncluding in solution. The bisulfate dimersO H···O hydrogen bonding is seen in a 1 HNMR peak at 13.75 ppm, which is consistent with borderline-strong hydrogen bonds.The fundamental role of Coulombsl aw [1] is so ingrained in all areas of chemistry,b iology,a nd physics that it is the starting point for formulating hypotheses on the behavior of matter.Itgoverns the bonding in ionic solids,the salt-bridges that guide protein structure and function, and the movement of ions in electric fields.W er ely daily upon the notion that like charges repel (Figure 1a)a nd opposite charges attract ( Figure 1b). Fort his reason, it came as some surprise when Weinholdsr ecent theoretical work [2] and its commentary [3] suggested that anions can attract, instead of repel, each other to form dimers. [4] Propositions that anions can stabilize each other are rare and the only unequivocal evidence for this phenomenon is in the solid state. [5] We found over 80 examples of HSO 4 À dimers (see the Supporting Information, Table S1), [6] as well as chains of HSO 4 À , [7] water-bridged sulfates, [8] and oligomers [9] and cyclic forms of hydrogen-bonded phosphates. [10] However, many forces other than anti-electrostatic hydrogen bonding, [2] for example,ionic bonding,can stabilize such anion dimers in crystals.Insolution, where solvation overtakes ionic bonding, dimer salts do not appear to survive.S upramolecular encapsulation offers aw ay to stabilize unstable species in solution. [11] Theonly reports of complexes of anion dimers are with H 2 PO 4 À and are based on the observation of a2:1 ratio of phosphate and receptor.N evertheless,t here are no direct observations of anion dimers in solution. With good reason, the authors of those works,R einhoudt, [5a, 12] Kubo, [13] Fabbrizzi, [5b, 14] Sessler, [15] and Tomisič, [16] took care to circumscribe these claims with caveats such as "tentativelySupportinginformation (experimentald etails, determination of binding constants, X-ray data collection and DFT calculation details) and the ORCID identification number(s) for the author(s) of this article can be found under: http://dx.

Ion Pairing and Co‐facial Stacking Drive High‐Fidelity Bisulfate Assembly with Cyanostar Macrocyclic Hosts

“…This situation is attributed to a paucity of representative examples in solution and difficulties in characterizing which of the possible assemblies are present. We address this shortcoming by taking advantage of the recently reported bisulfate anions that dimerize upon complexation with complementary cyanostar ( CS ) receptors . The generation of signatures for the bisulfate dimers help in investigating the driving forces underpinning their formation.…”

“…Nevertheless, these hydrogen bonds are still not able to stabilize the dimer globally on account of Coulomb repulsions . Although these repulsions are weakened by solvation, computations show the dimers are still not stable in solution . Stabilization of the dimers by complexation with receptors had always been feasible but evidence has been hard to find.…”

“…We discovered that HSO 4 − dimers could be stabilized inside macrocyclic cyanostars (Figure a) using a supramolecular strategy . The crystal structure shows π‐stacked cyanostars encapsulating the [HSO 4 ⋅⋅⋅HSO 4 ] 2− anion dimer with tetrabutylammonium cations (NBu 4 + ) capping each end of the assembly.…”

“…In solution, a mix of species with a range of receptor–anion stoichiometries is seen (Figure b). In addition to the 2:1 sandwiches characteristic of simpler anions (ClO 4 − ), the unexpected 3:2 trimer and 2:2 dimer complexes were formed. The dimer complex had partial ion pairing best described as 2:2: x ( x= 0, 1, 2) with x being a variable quantity of the NBu 4 + cation.…”

“…We hypothesized that low polarity solvents should favor ion pairing between the 2:2 complex and counter cations to provide critical stabilization of the bisulfate dimer inside the macrocycles. Without ion pairing, it is not clear if the 2:2 complexes are stable . Instead repulsions between anions may need to be offset by increasing the positive electrostatic character of the receptor cavity.…”