Calix[4]pyrrole Based Receptors for the Recognition of Ion Pairs

The work contained in this thesis deals with the synthesis of supramolecular receptors based on calix[4]pyrrole architectures. The aim of the thesis was to obtain synthetic receptors able to bind charged species (ion pairs) in non-polar media, capable of mimicking the binding properties of naturally occurring molecules such as proteins. In this thesis, the formation of supramolecular complexes with synthetic receptors is investigated by means of NMR and ITC experiments. The relative stability was also studied in the gas-phase by means of ESI-MS, ion mobility and collision induced dissociation experiments.

We disclose the synthesis of a two-wall bis(calix[4]pyrrole) receptor bearing triazole units as linkers. The receptor was obtained by the copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) of two terminal alkyne and azide calix[4]pyrrole units. A charged ditopic guest, a therephthalate salt, was employed as the template in order to favor the formation of the macrocyclic structure over polymeric products. The receptor was obtained in a remarkable 30%. We also report the formation of a tetrameric receptor formed by four calix[4]pyrrole units as a byproduct. We discuss briefly the mechanism of formation of both receptors, supporting our conclusions on HPLC studies of the reaction.

We then explored the binding properties of the receptor towards chloride and cyanate salts of tetraalkylammonium cations. We obtained 1:2 receptor:ion-pair complexes with tetrabutylammonium cyanate and chloride, as well as with methyltrioctylammonium chloride. The complexes were formed by the establishment of hydrogen-bonding interactions with the NH core of the calix[4]pyrrole. The obtained complexes displayed different binding geometries and cooperativity that were dependent on the cation.

The same receptor was later employed for the construction of pseudorotaxane assemblies. For this, we used a lineal N-oxide as the axle, able to interact with the receptor by the establishment of hydrogen-bonding interactions with the calix[4]pyrrole NHs. The formation of the [2]-pseudorotaxane complexes was templated by the same ion-pairs previously studied. The four-particle aggregates were obtained as two distinct isomeric forms depending on the binding of the anion and the axle on the two inequivalent binding sites of the receptor. We report also the synthesis of a [2]rotaxane receptor by a stoppering procedure and its capabilities to bind various ion-pairs.

Finally, we also present the synthesis and binding studies of a strapped calix[4]pyrrole macrocycle decorated with a bis-amide unit as the strap. The receptor was obtained through the acid-catalyzed condensation of the precursor bis-dipyrromethane. We report also the formation of thermodynamically and kinetically stable complexes with various ion pairs and describe the formation of pseudorotaxane assemblies with the aforementioned axle.

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