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-p receptor. Receptors 9(2)+center dot 2BF(4)(-) (C6H5), 11(2)+center dot 2BF(4)(-) (4-NO2-C6H4), and 13(2+)center dot 2BF(4-) (ferrocenyl) bind HP2O73- anions in a mixed-binding mode that features a combination of hydrogen-bonding and anion-pi interactions and results in strong binding. On the other hand, receptor 10(2)+center dot 2BF(4)(-) (4-CH3O-C6H4) only displays combined Csp(2)-H/anion-pi interactions between the two arms of the receptors and the bound anion rather than triazolium (CH)(+)center dot center dot center dot anion hydrogen bonding. All receptors undergo a downfield shift of the triazolium protons, as well as the inner naphthalene protons, in the presence of H(2)PO(4)(-)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) C-H 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.