Tetrathiafulvalene-Calix[4]pyrroles: Synthesis, Anion Binding, and Electrochemical Properties

The syntheses of monotetrathiafulvalene-calix[4]pyrrole 5 and bistetrathiafulvalene-calix[4]pyrrole
6, prepared from the acid-catalyzed condensation of monopyrrolo[3,4-d]tetrathiafulvalene (MPTTF, 7) with
acetone in the presence of tripyrrane 8 and dipyrromethane 9, respectively, are described. Compound 5
and the previously reported tetrathiafulvalene-calix[4]pyrrole 4 both adopt a 1,3-alternative conformation in
the solid state, as determined from X-ray crystallographic analysis. The anion binding properties of the
tetrathiafulvalene-calix[4]pyrroles 5 and 6, as well as those of the parent meso-octamethylcalix[4]pyrrole
(1), were investigated in acetone using 1H NMR spectroscopic and isothermal titration calorimetry (ITC)
techniques and, within the error limits of the methods, were generally found to give concordant results. On
the basis of the results of the ITC studies carried out in 1,2-dichloroethane, increasing the number of
tetrathiafulvalene units annulated to the calix[4]pyrrole system serves to enhance the anion binding affinities
substantially but at the price of lowered selectivity. Cyclic voltammetry (CV) studies, carried out in 1,2-dichloroethane, provided evidence of an anion-dependent electrochemical response with Cl- and Br- ions.
This response was particularly dramatic in the case of the monotetrathiafulvalene-calix[4]pyrrole 5, with a
ΔEmax of −145 mV being seen after the addition of approximately 1 equiv of Cl- ion.