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

Abstract

We report the synthesis of a luminescent Pt(II) complex, PtL2, 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 (MTOA·Cl) in a dichloromethane solution. To this end, we performed 1H NMR and UV–vis spectroscopic titrations. The singular luminescent properties of PtL2 allowed the use of highly sensitive emission spectroscopy to monitor the binding. The binding affinity of PtL2 for MTOA·Cl 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, (Cl⊂PtL2/C[4]P)@MTOA. 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 PtL2 toward a series of halides in acetone solution favoring ion-pair dissociation. PtL2 binds chloride selectively and with a high affinity, producing mainly an anionic complex.