Dual Role of the 1,2,3-Triazolium Ring as a Hydrogen-Bond Donor and Anion–π Receptor in Anion-Recognition Processes
Several bis(triazolium)-based receptors have been synthesized as chemosensors for anion recognition. The central naphthalene core features two aryltriazolium side-arms. NMR experiments revealed differences between the binding modes of the two triazolium rings: one triazolium ring acts as a hydrogen-bond donor, the other as an anion–π receptor. Receptors 92+⋅2BF4− (C6H5), 112+⋅2BF4− (4-NO2C6H4), and 132+⋅2BF4− (ferrocenyl) bind HP2O73− anions in a mixed-binding mode that features a combination of hydrogen-bonding and anion–π interactions and results in strong binding. On the other hand, receptor 102+⋅2 BF4− (4-CH3OC6H4) only displays combined Csp2H/anion–π interactions between the two arms of the receptors and the bound anion rather than triazolium (CH)+⋅⋅⋅anion hydrogen bonding. All receptors undergo a downfield shift of the triazolium protons, as well as the inner naphthalene protons, in the presence of H2PO4− anions. That suggests that only hydrogen-bonding interactions exist between the binding site and the bound anion, and involve a combination of cationic (triazolium) and neutral (naphthalene) CH donor interactions. Theoretical calculations relate the electronic structure of the substituent on the aromatic group with the interaction energies and provide a minimum-energy conformation for all the complexes that explains their measured properties.