Anion dimer captured within cyanostar macrocycles

Opposite charges attract and like charges repel, Coulomb’s law says. But anions can pair up and stabilize each other, according to research led by Amar H. Flood of Indiana University, Bloomington (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201608118). Although indirect evidence had previously pointed to the existence of dihydrogen phosphate dimers, direct evidence was lacking. Combining macrocyclic cyanostars and bisulfate, HSO4^–, Flood and coworkers expected to create a complex with one bisulfate anion sandwiched in the center of two cyanostars, as already documented for perchlorate. Instead, they observed a 2:2:2 complex incorporating a pair of bisulfate anions hydrogen-bonded to each other, nestled in the centers of two stacked cyanostars and capped by tetrabutylammonium cations. The anion dimer is stabilized by hydrogen-bonding interactions to the cyanostar’s cyanostilbene-based C–H groups. The complex persists in both crystalline form and in solution, as documented by X-ray crystallography and ¹H NMR. The system could point to new ways to recognize and sequester ions, the researchers say.