Lower rim calix(4)arene ketone derivatives and their interaction with alkali metal cations. Structural and thermodynamic (solution and complexation) characterisation of the tetraphenyl ketone derivative and its sodium complex

A modified method for the synthesis of 5,11,17,23-tetra-tert-butyl-25,26,27 ,28-tetra(benzoyl)methoxycalix(4) arene (1) and 5,11;17,23-tetra-tert-butyl -25,26,27,28-tetra(acetoyl)methoxycalix(4)arene (2) using 18-crown-6 as pha se transfer catalyst is. reported. Ligand 1 was structurally (H-1 NMR and X -ray crystallography) and thermodynamically characterised. An account of th e steps required for the formulation of an equation representative of the c omplexation process involving metal cations and macrocycles is given using as an illustrative example the interaction of 1 with the Na(I) cation. Thus , detailed thermodynamics of this system are first reported taking into acc ount the solution properties of the reactants and the product in various so lvents with the aim of assessing quantitatively the medium effect on the co mplexation process. The sodium: monoacetonitrile : 1 complex was isolated a nd its molecular structure was determinated from X-ray diffraction data. Th e calix now adopts a more symmetric cone conformation than the free ligand and its hydrophobic cavity is filled with an acetonitrile solvent molecule. The hydrophilic pocket hosts the sodium canon which is in an eight-fold co ordination with the. four ethereal oxygen atoms [average d(Na-O) = 2.45(4) Angstrom] and the four carbonyl oxygen atoms [average d(Na-O) = 2.51(4) Ang strom] forming the corners of a distorted Archimedean square antiprism. The key structural features relevant to the complexation process are discussed . H-1 NMR and conductance measurements have been used to assess the interac tion of 1 and 2 with alkali metal cations. Final conclusions are given. Att ention is drawn about the relevance of detailed thermodynamic studies on ca tion complexation processes involving calixarene derivatives. This is discu ssed within the context of selectivity and the factors controlling it in so lution processes.