Carboxylate anions binding and sensing by a novel tetraazamacrocycle containing ferrocene as receptor

Abstract

A tetraazamacrocycle containing ferrocene moieties has been synthesized and characterized. The tetraprotonated form of this compound was evaluated as a receptor (R) for anion recognition of several substrates (S), Cl⁻, PF₆⁻, HSO₄⁻, H₂PO₄⁻ and carboxylates, such as p-nitrobenzoate (p-nbz⁻), phthalate (ph²⁻), isophthalate (iph²⁻) and dipicolinate (dipic²⁻). ¹H NMR titrations in CD₃OD indicated that this receptor is not suitable for recognizing HSO₄⁻ and H₂PO₄⁻, but weakly binds p-nbz⁻, and strongly interacts with ph²⁻, dipic²⁻, and iph²⁻ anions forming 1 : 2 assembled species. The largest β₂ binding constant was determined for ph²⁻, followed by dipic²⁻ and finally iph²⁻. The effect of the anionic substrates on the electron-transfer process of the ferrocene units of R was evaluated using cyclic voltammetry (CV) and square wave voltammetry (SWV) in methanol solution and 0.1 mol dm⁻³ (CH₃)₄NCl as the supporting electrolyte. Titrations of the receptor were undertaken by addition of anion solutions in their tetrabutylammonium or tetramethylammonium forms. The protonated ligand exhibits a reversible voltammogram, which shifts cathodically in the presence of the substrates. The data revealed kinetic constraints in the formation of the receptor/substrate entity for dipic²⁻, ph²⁻ and iph²⁻ anions, but not for p-nbz⁻. In spite of the slow kinetics of assembled species formation with the ph²⁻ substrate, this anion provides the largest redox-response when the supramolecular entity is formed, followed by dipic²⁻, iph²⁻ and finally p-nbz⁻ anions. This trend is in agreement with the ¹H NMR results and the values of the binding constants. Single crystal X-ray structures of the receptor with PF₆⁻, ph²⁻, iph²⁻ and p-nbz⁻ were carried out and showed that supermolecules with a RS₂ stoichiometry are formed with the first three anions, but RS₄ with p-nbz⁻. In all cases the binding occurs outside the macrocyclic cavity via N-H⋯OC hydrogen bonds for carboxylate anions and N–H⋯F hydrogen bonds for the PF₆⁻ anion, which is in agreement with the solution results. The macrocyclic framework adopts different conformations in order to interact with each substrate having Fe⋯Fe intramolecular distances ranging from 10.125(14) to 12.783(15) Å.