Calixpyrrole chemistry: a study of a new ditopic receptor highlighting some fundamental concepts in assessing thermodynamic selectivity

Abstract

Following the synthesis and characterization of meso-tetramethyl tetrakis (4-phenoxy acetone) calix[4]pyrrole, 1, the solution properties of this receptor in various solvents were investigated. Particular emphasis is placed on the selection of the solvent in assessing thermodynamic selectivity in ion complexation studies involving calixpyrrole receptors. Thus, statements recently made in the literature are addressed.

The interaction of 1 with anions and cations was assessed through ¹H NMR in CD₃CN at 298 K. Among anions, significant chemical shift changes were observed in the pyrrolic protons by the addition of halide salts to 1 in the deuterated solvent. The sequence observed in the Δδ values is that observed in the transfer Gibbs energies of these anions from a dipolar aprotic (acetonitrile) to a protic medium (representative of 1) based on the Ph₄AsPh₄B convention. As far as metal cations in CD₃CN are concerned, the most significant changes are observed (relative to those of the ligand) in the protons of the phenoxy acetone functionality upon the addition of the mercury(II) salt to 1. Conductance measurements reveal the formation of 1 : 1 complexes with these ions. Thermodynamic data derived from titration calorimetry are reported and compared with available data for analogous ligands. A quantitative evaluation of the thermodynamic stability was carried out. The applications of these ligands as anchor groups in oligomeric frameworks are discussed. Final conclusions are given.