Cation Complexation by Chemically Modified Calixarenes. 11. Complexation and Extraction of Alkali Cations by Calix[5]- and -[6]arene Ketones. Crystal and Molecular Structures of Calix[5]arene Ketones and Na⁺ and Rb⁺ Complexes

A series of four calix[5]arenes and three calix[6]arenes (R-calixarene-OCH₂COR¹) (R = H or Bu^t) with alkyl ketone residues (R¹ = Me or Bu^t) on the lower rim have been synthesized, and their affinity for complexation of alkali cations has been assessed through phase-transfer experiments and stability constant measurements. The conformations of these ketones have been probed by ¹H NMR and X-ray diffraction analysis, and by molecular mechanics calculations. Pentamer 3 (R = R¹ = Bu^t) possesses a symmetrical cone conformation in solution and a very distorted cone conformation in the solid state. Pentamer 5 (R = H, R¹ = Bu^t) exists in a distorted 1,2-alternate conformation in the solid state, but in solution two slowly interconverting conformations, one a cone and the other presumed to be 1,2-alternate, can be detected. X-ray structure analysis of the sodium and rubidium perchlorate complexes of 3 reveal the cations deeply encapsulated by the ethereal and carbonyl oxygen atoms in distorted cone conformations which can be accurately reproduced by molecular mechanics calculations. The phase-transfer and stability constant data reveal that the extent of complexation depends on calixarene size and the nature of the alkyl residues adjacent to the ketonic carbonyls with tert-butyl much more efficacious than methyl.