Binding Studies of a Luminescent Pt(II) Organometallic Calix[4]pyrrole with Halide Salts

We report the synthesis of a luminescent Pt(II) complex, <b>PtL</b> ^<b>2</b> , based on a "two-wall" aryl-extended calix[4]pyrrole (C[4]P). We characterize its binding properties as a receptor of methyl trioctyl ammonium chloride (<b>MTOA·Cl</b>) in a dichloromethane solution. To this end, we performed ¹H NMR and UV-vis spectroscopic titrations. The singular luminescent properties of <b>PtL</b> ^<b>2</b> allowed the use of highly sensitive emission spectroscopy to monitor the binding. The binding affinity of <b>PtL</b> ^<b>2</b> for <b>MTOA·Cl</b> is 2-fold smaller than that of the C[4]P precursor. This difference is attributed to dissimilar anion-π interactions and binding geometries in the ion-paired complexes of the two receptors, <b>(Cl⊂PtL</b> ^<b>2</b> <b>/C[4]P)@MTOA</b>. Isothermal titration calorimetry (ITC) experiments revealed subtle differences in enthalpy and entropy. The entropic term, although negative, was very small, suggesting the relevance of the dissociation/association processes of the salt and the ion-paired complexes. We investigated the binding selectivity of <b>PtL</b> ^<b>2</b> toward a series of halides in acetone solution favoring ion-pair dissociation. <b>PtL</b> ^<b>2</b> binds chloride selectively and with a high affinity, producing mainly an anionic complex.