Extraction and complexation of alkali and alkaline earth metal cations by lower-rim calixarene diethylene glycol amide derivatives
Novel calixarene derivatives, 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis(N-(2-(2-methoxyethoxy)ethyl)carbamoyl methoxy)calixarene (1) and 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetra(N,N-bis(2-(2-methoxyethoxy)ethyl)carbamoyl methoxy)calixarene (2), were prepared by introducing diethylene glycol subunits at the lower calixarene rim. The complexation affinities of these compounds towards alkali and alkaline earth metal cations were studied at 25 °C in acetonitrile and methanol by means of spectrophotometric, conductometric, and potentiometric titrations. The stability constants of the corresponding complexes with 1 : 1 (cation : ligand) stoichiometry were determined (in some cases only estimated), and their values obtained by different methods were in good agreement. The complexes with secondary-amide derivative (1) were observed to have much lower stabilities than those with tertiary-amide derivative (2). This was presumably mostly caused by the presence of intramolecular NH⋯OC hydrogen bonds in the former case. It was found that solvent significantly affected the stability of the complexes; the prepared calixarenes showed considerably higher affinities for cations in acetonitrile than in methanol. Generally, the ligands studied showed better binding abilities for alkaline earth cations than for alkali metal cations. The extraction of metal picrates from water to dichloromethane by the complexation of metal ions with ligands 1 and 2 was also investigated. In accordance with the complex stabilities, all cations were extracted fairly well (in some cases even excellently) with the tertiary-amide derivative, whereas this was not the case in the extraction experiments with the other ligand studied.