| Summary: | In this work, we report the synthesis and characterization of a mono-nuclear “two wall” aryl-extended calix[4]pyrrole receptor (<b>2Au</b>) decorated with an acetylide-gold(I)-PTA complex at its upper rim. We describe the <sup>1</sup>H NMR titration experiments of <b>2Au</b> and its “two wall” aryl-extended calix[4]pyrrole synthetic precursors: the non-symmetric mono-iodo-mono-ethynyl <b>2</b> and the symmetric bis-iodo <b>3</b> with TBACl in dichloromethane and acetone solution. In acetone solution, we use isothermal titration calorimetry (ITC) experiments to thermodynamically characterize the formed 1:1 chloride complexes and perform pair-wise competitive binding experiments. In both solvents, we measured a decrease in the binding constant of the mono-nuclear <b>2Au</b> complex for chloride compared to the parent mono-iodo-mono-ethynyl <b>2</b>. In turn, receptor <b>2</b> also shows a reduction in binding affinity for chloride compared to its precursor bis-iodo calix[4]pyrrole <b>3</b>. The free energy differences (∆G) of the 1:1 chloride complexes cannot be exclusively attributed to their dissimilar electrostatic surface potential values either at the center of the <i>meso</i>-phenyl wall or its <i>para</i>-substituent. We conclude that solvation/desolvation processes play an important role in the stabilization of the chloride complexes. In acetone solution and in the presence of TBACl, <b>6Au</b>, a reference compound for the acetylide Au(I)•PTA unit, produces a bis(alkynyl)gold(I) anionic complex [<b>7Au</b>]<sup>−</sup>. Thus, the observation of two separate sets of signals for the bound aromatic calix[4]pyrrole protons, when more than 1 equiv. of the salt is added, is assigned to the formation of the chloride complexes of <b>2Au</b> and of the “in situ” formed calix[4]pyrrole anionic dimer [<b>8</b><b>Au</b>]<sup>−</sup>. Finally, preliminary data obtained in cell viability assays of <b>2Au</b> and <b>6Au</b> with human cancer cells lines assign them with moderate activities showing that the calix[4]pyrrole unit is not relevant.
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