ABSTRACT
A systematic study of a dual-host system exhibiting pairwise anion/cation separations has been performed for CsNO3 extraction. Tripodal triamides 1-4 and 9 derived from condensation of hexanoic (for 1), octanoic (for 2), decanoic (for 3), lauric (for 4), and p-tert-butylbenzoic (for 9) acid with tris(2-aminoethyl)amine (tren) were used together with tetrabenzo-24-crown-8, a well-known Cs+ cation receptor. By using 5 mM crown ether in the organic phase and 10 mM CsNO3 with 0.1 mM HNO3 in the aqueous phase, tripods 1, 2, and 9 enhance CsNO3 extraction by factors of 2.4, 1.7, and 4.4, respectively (for 50 mM amide concentration), while the corresponding monoamide controls 5-8 derived from n-propylamine (5, 6) or N,N'-dimethylethylenediamine (7, 8) and hexanoic (5, 7) or octanoic (6, 8) acid derivatives gave no significant enhancement under the same conditions. This behavior may be ascribed to nitrate complexation by the triamides, which lowers the overall thermodynamic barrier for the salt transfer to the organic phase. The nitrate binding was confirmed by 1H NMR titration of receptor solutions, using tetrabutylammonium nitrate. Association constants for the formation of the anion-nitrate complexes were found to vary between 33 and 52 M-1 for the more soluble triamides. The synergistic effects for CsNO3 extraction are in reasonable agreement with the values predicted theoretically from the measured association constants. Electrospray ionization mass spectrometry confirmed the predominant formation of 1:1 tripod-nitrate complexes. Monoamide controls gave no evidence of anion complexation.
ABSTRACT
The structures of a 14-crown-4 ether containing both benzo and cyclohexano substituents, 2,6,13,17-tetraoxatricyclo-[16.4.0.0(7, 12)]docosa-1(18),19,21-triene, C(18)H(26)O(4), and its lithium complex, [2,6,13,17-tetraoxatricyclo[16.4.0.0(7,12)]docosa-1(18),19, 21-triene-kappa(4)O](thiocyanato-N)lithium(I), [Li(NCS)-(C(18)H(26)O(4))], are presented. The conformation of the free crown, (I), is not preorganized for cation binding, as its donor dipoles are oriented towards opposite sides of the crown ring. The Li(+)-crown complex, (II), exhibits two formula units in the asymmetric unit. The binding conformation observed in (II) does not completely reorient the dipoles to one point, resulting in a long Li-O bond length [2.068 (5) and 2.073 (5) A].
