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Anions Stabilize Each Other inside Macrocyclic Hosts

Profile image of Arkajyoti SenguptaArkajyoti Sengupta

2016, Angewandte Chemie International Edition

Abstract

Contrary to the simple expectations from Coulombs law, Weinhold proposed that anions can stabilize each other as metastable dimers, yet experimental evidence for these species and their mutual stabilization is missing. We show that two bisulfate anions can form such dimers, which stabilize each other with self-complementary hydrogen bonds, by encapsulation inside a pair of cyanostar macrocycles. The resulting 2:2 complex of the bisulfate homodimer persists across all states of matter, including in solution. The bisulfate dimers OH•••O hydrogen bonding is seen in a 1 H NMR peak at 13.75 ppm, which is consistent with borderline-strong hydrogen bonds. The fundamental role of Coulombs law [1] is so ingrained in all areas of chemistry, biology, and physics that it is the starting point for formulating hypotheses on the behavior of matter. It governs the bonding in ionic solids, the salt-bridges that guide protein structure and function, and the movement of ions in electric fields. We rely daily upon the notion that like charges repel (Figure 1 a) and opposite charges attract (Figure 1 b). For this reason, it came as some surprise when Weinholds recent 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. In solution, where solvation overtakes ionic bonding, dimer salts do not appear to survive. Supramolecular encapsulation offers a way to stabilize unstable species in solution. [11] The only reports of complexes of anion dimers are with H 2 PO 4 À and are based on the observation of a 2:1 ratio of phosphate and receptor. Nevertheless, there are no direct observations of anion dimers in solution. With good reason, the authors of those works, Reinhoudt, [5a, 12] Kubo, [13] Fabbrizzi, [5b, 14] Sessler, [15] and Tomisič, [16] took care to circum

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