From atomic to molecular anions: a neutral receptor captures cyanide using strong C-H hydrogen bonds.

The multifaceted character of cyanide as an acceptor of hydrogen bonds from a receptor has been examined for the first time using electronic-structure theory and spectroscopic measurements (UV/Vis and NMR titrations). Motivated by the similar size and charge of the cyanide pseudohalide and the monoatomic chloride ion, strong interactions of cyanide with a rigid macrocyclic triazolophane receptor have been predicted by theory and confirmed by experimental findings. It was found that both anions bind with similar strength in the gas phase (computed) and in the solution phase (experimental) via C-H hydrogen bonds. Theoretical calculations predict that the heterodiatomic cyanide prefers to bind in the plane of the macrocycle along the north-south axis. Examination of the possible binding modes reveal low computed barriers for in-plane rotation. The predicted model is consistent with the experimental data. Overall, the binding of a molecular anion within the cavity of a triazolophane receptor has been characterized where the computed and experimental binding energies are consistent with the classification of cyanide as a pseudohalide in the context of supramolecular chemistry.