Resorcarene-based receptor: Versatile behavior in its interaction with heavy and soft metal cations

Standard solution Gibbs energies, Delta(s)G degrees, of the resorcarene-based receptor 5,11,17,23-ethylthiomethylated calix[4]resorcarene, 1, (characterized by H-1 NMR and X-ray diffraction studies) in its monomeric state (established through partition experiments) in various solvents are for the first time reported in the area of resorcarene chemistry. Transfer Gibbs energies of I from hexane (reference solvent) to other medium are calculated. Agreement between Delta(t)G degrees (referred to the pure solvents) and standard partition Gibbs energies, Delta(p)G degrees (solvent mutually saturated) is found. Cation-ligand interactions were investigated through 1H NMR (CD3CN and CD3OD) and conductometric titrations in acetonitrile and methanol. 1H NMR data revealed the sites of interaction of 1 with the metal cation. The composition of the metal-ion complexes (Ag+ and Pb2+ in acetonitrile and Ag+ and Cull in methanol) was established through conductometric titrations. Thus, complexes of 1: 1 stoichiometry were formed between I and Ag+ and Pb2+ in acetonitrile and Cu2+ in methanol. However, in moving from acetonitrile to methanol, the composition of the silver complex was altered. Thus, two metal cations are hosted by a unit of the ligand. As far as Cull and I in acetonitrile is concerned, conductance data suggest that metalates are formed in which up to four units Of Cull are taken up per unit of resorcarene. The contrasting behavior of I with Cull in acetonitrile relative to methanol is discussed. As far as mercury (11) is concerned, the unusual jump in conductance observed in the titration of Hg2+ with I in acetonitrile and methanol after the formation of a multicharged complex (undefined composition) is attributed to the presence of highly charged smaller units (higher mobility) resulting from the departure of pendant arms from the resorcarene backbone. Isolation of these species followed by X-ray diffraction studies corroborated this statement. The thermodynamic characterization of metal-ion complexes of Ag+ and Pb2+ in acetonitrile and Cu2+ and Ag+ in methanol is reported. Final conclusions are given.