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-OCH(2)COR(1)) (R = H or Bu(t)) with alkyl ketone residues (R(1) = 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 (1)H NMR and X-ray diffraction analysis, and by molecular mechanics calculations. Pentamer 3 (R = R(1) = Bu(t)) possesses a symmetrical cone conformation in solution and a very distorted cone conformation in the solid state. Pentamer 5 (R = H, R(1) = 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.