It was over two centuries ago that Coulomb first published his theory that like charges repel each other. However, recently, the concept of anti-electrostatic hydrogen bonding laid the theoretical groundwork for the existence of anion–anion dimers. Coulomb repulsion is not the only challenge to the formation of these dimers in solution: Solvation can also separate the anions.
Amar H. Flood and colleagues, Indiana University, Bloomington, USA, could show that within a so-called cyanostar macrocycle pair (C5-symmetric macrocycles), two hydrogen-bonding HSO4− ions can form an anion–anion dimer (pictured). The complex persists in solution. 1H NMR spectroscopy results provide evidence for the bisulfate dimer’s OH⋅⋅⋅O hydrogen bonding with a peak at 13.75 ppm. The dimers in this bisulfate homodimer stabilize each other with self-complementary hydrogen bonds by encapsulation inside a pair of cyanostar macrocycles
The results demonstrate the effectiveness of a supramolecular strategy for the stabilization of reactive species in “hostile” environments, and represent the first conclusive observation of a hydrogen-bonded pair of anions.
- Anions Stabilize Each Other inside Macrocyclic Hosts,
Elisabeth M. Fatila, Eric B. Twum, Arkajyoti Sengupta, Maren Pink, Jonathan A. Karty, Krishnan Raghavachari, Amar H. Flood,
Angew. Chem. Int. Ed. 2016.