Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Three new macrocyclic cages 1, 2 and 3, able to selectively encapsulate the lithium ion, were studied. The synthesis and characterization of 2 and 3 are reported. The basicity behaviour of the three cages was investigated by spectrophotometry and by 1H and 13C NMR spectroscopy. The protonation constants were determined by potentiometric methods in aqueous solution (I = 0.15 mol dm-3 NMe4Cl, T = 298 ± 0.1 K) and were found to be logK1 = 11.67(5), 10.77(7), 11.17(7); logK2 = 7.76(5), 5.7(1), 5.6(1); logK3 = 1.3(1), 2.4(1), 1.9(1) for 1, 2, and 3, respectively. The lithium equilibrium complex formation Li+ + L = LiLH-1 + H+ was monitored by 13C and 7Li NMR spectroscopy, as well as by spectrophotometric and potentiometric techniques; log values of -9.1(1) and -8.0(1) were found for 1 and 3, respectively. For 2, precipitation of the lithium complex occurs. UV/Vis studies in aqueous solution show a significant shift towards high energy of the λmax in the lithium complex, indicating the potential for application of these ligands in colourimetric analysis. The crystal structure of 1 was examined by single crystal X-ray diffraction. Data collection was performed on two samples, namely 1a and 1b, obtained from two different crystallization procedures. Crystals of 1a are monoclinic, space group P21/n, while 1b crystallized in an orthorhombic space group Pc21b. The cell parameters are a = 12.423(3), b = 14.378(9), c = 28.436(7) Å, β = 93.74(2)° for 1a, and a = 10.674(4), b = 14.408(2), c= 17.034(2) Å for 1b. Molecular simulations were carried out on both conformational isomers 1a and 1b, with results indicating that the two isomers have essentially the same flexibility.
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