ABSTRACT
The rigid duplex cyclodextrin 6 composed of two alpha-cyclodextrin macrocycles connected with two disulfide bonds in "transannular" (C6(I), C6(IV)) positions was prepared from partially debenzylated alpha-cyclodextrin 1 in four steps in 73% overall yield. In the last key step involving oxidative coupling of the thiol 5, predominance of the target duplex 6 can be attained under conditions of thermodynamic control. The structure of duplex cyclodextrin was established by MS as well as 2-D NMR methods and confirmed by a single-crystal X-ray analysis. The ability of the duplex cyclodextrin 6 to bind alpha,omega-alkanediols (C9-C14) and 1-alkanols (C9 and C10) was studied by isothermal titration calorimetry in aqueous solutions. The stability constants of the complexes gradually increase with the alkyl chain length and reach an unprecedently high value of K = 8.6 x 10(9) M(-1) for 1,14-tetradecanediol. It was found that the doubly bridged dimer 6 exhibits higher binding affinity toward the series of alpha,omega-alkanediols than the singly bridged analogue 10 by about 2 orders of magnitude in K (M(-1)) or 3.1-3.3 kcal/mol in deltaG(o), the enhancement being due to enthalpic factors. Theoretical calculations using DFT-D methods suggest that the enthalpic contribution stems from dispersion interactions.
Subject(s)
Disulfides/chemistry , alpha-Cyclodextrins/chemistry , Alcohols/chemistry , Crystallography, X-Ray , Dimerization , Models, Molecular , Molecular Conformation , Thermodynamics , alpha-Cyclodextrins/chemical synthesisABSTRACT
Three crystal structures of a ditopic cyclophane ligand (L) in which two 1,5,8,12-tetraamine molecules have been attached through methylene spacers to the ortho positions of a benzene ring are reported. The first one (1) corresponds to the tetraprotonated free macrocycle (H4L4+) having two tetrachlorozincate(II) counteranions (C24H54O2N8Cl8Zn2, a = 9.1890(2) A, b = 14.0120(3) A, c = 15.3180(3) A, alpha = 89.2320(7) degrees , beta = 82.0740(6) degrees , gamma = 83.017(1) degrees , Z = 2.00, triclinic, P); the second one (2) is of a binuclear Cu2+ complex having coordinated chloride anions and perchlorate counteranions (C24H58O14N8Cl4Cu2 a = 9.9380(2) A, b = 30.2470(6) A, c = 53.143(1) A, orthorhombic, F2dd, Z = 18), and the third one (3) corresponds to an analogous Zn2+ complex that has been crystallized using triflate as counteranion (C26H(51.2)O(6.6)N8Cl2F6S2Zn2 a = 8.472(5) A, b = 9.310(5), c = 13.745(5) A, alpha = 84.262(5) degrees , beta = 77.490(5) degrees , gamma = 73.557(5) degrees , triclinic, P, Z = 2). The analysis of the crystallographic data clearly shows that the conformation of the macrocycle and, in consequence, the overall architecture of the crystals are controlled by the anions present in the moiety, pi-pi-stacking associations, and hydrogen bonding interactions. The protonation and stability constants for the formation of the Cu2+ and Zn2+ complexes in aqueous solution have been determined potentiometrically in 0.15 mol dm(-3) NaClO4 at 298.1 K. Intramolecular hydrogen bonding defines the protonation behavior of the compound. Positive cooperativity is observed in the formation of the Cu2+ complexes.
ABSTRACT
[structure: see text] The interaction of a macrocycle containing three trans-(1R,2R)-diaminocyclohexane units connected by p-xylene spacers with the isomers 1,3,5-benzenetricarboxylic (1,3,5-BTC), 1,2,4-benzenetricarboxylic (1,2,4-BTC), and 1,2,3-benzenetricarboxylic (1,2,3-BTC) acids and their relevant anions is studied by means of potentiometric and NMR analysis. The interaction is highest for the isomer 1,3,5-BTC, which perfectly fits within the macrocyclic cavity of the host species. The studies have been extended to the triacid 1,3,5-benzenetriacetic observing in this case the effect of a size mismatch between host and guest species.
Subject(s)
Cyclohexylamines/chemistry , Models, Chemical , Tricarboxylic Acids/chemistry , Binding, Competitive , Hydrogen-Ion Concentration , Models, Molecular , Molecular Structure , Tricarboxylic Acids/chemical synthesisABSTRACT
D4 -symmetric chiral hydrogen-bonded cyclotetramers (see the structure in the picture) are present in the self-assembled achiral title compound in the solid state. The unilayered network set up from the chiral "square" blocks is achiral as a consequence of the crystal symmetry.
