Experimental and computational studies of the binding properties of lower rim tetra- and di-substituted calix[4]arene amide derivatives with metal ions

Abstract

<p>Experimental and theoretical binding studies of representative alkali, alkaline earth, transition, heavy metal and lanthanide cations by tetra- and di-substituted calix[4]arene amide derivatives (diethyl amide <b>1a</b>–<b>c</b> and morpholide amide <b>2a</b>–<b>c</b>) in the cone conformation were carried out. Binding was assessed by extraction experiments of the metal picrates from water to dichloromethane and proton NMR titrations. Density functional theory calculations were also performed to determine the binding energy of the complexes formed and to analyse the host–guest interaction modes. In the cases of ligands <b>1b</b> and <b>2c</b> with Na⁺ and Ag⁺ picrates, the extraction energy was also determined using the polarisable continuum model. The results are discussed in terms of the nature of the amide residue and the substitution pattern (1,3 vs. 1,2). Both tetra-amide derivatives are good extractants, showing preference for Na⁺, Ca²⁺, Ag⁺ and Pb²⁺ cations, mainly di-ethylamide <b>1a</b>. Concerning di-amide derivatives, the relative position of the substituents seems to be more important than the nature of the amide group in the extraction process. Proton NMR studies indicate the formation of 1:1 complexes between the amides and the cations studied, and DFT data show that all ligands form the most stable complexes with La³⁺.</p> <p>The binding properties of <b>1a-c</b> and <b>2a-c</b> towards several metal cations were determined by UV spectrophotometry, proton NMR spectrometry and DFT methods.</p