Tropylium cation capsule of hydrogen-bonded tetraurea calix 4 arene dimers

The interaction between tropylium salts and tetraurea calix[4]arene derivatives (such as 1 and 2) was studied in solution using 1D, 2D, diffusion, VT NMR and UV-visible spectroscopy. It was found that tropylium salts form charge transfer complexes with both the monomers and dimers of the tetraurea calix[4]arene derivatives depending on the experimental conditions. Compound 1 increases dramatically the solubility of tropylium salts in apolar solvents such as C2D4Cl2, CDC,(3) and CD2Cl2 by forming the molecular capsule 1.C7H7+.1, In contrast to the benzene capsule of 1, in 1.C7H7+. 1 the hydrogen bonds in the equatorial region that hold together the two parts of the dimer change their directionality faster than the NMR time scale (at 400 MHz) at temperatures higher than 298 K. Interestingly, the free energy barrier for this dynamic process at 298 K (DeltaG(298)(double dagger)), depends on the nature of the counter-anion. Free energies of activation of 14.3+/-0.2 kcal mol(-1) and 12.6+/-0.2 kcal mol(-1) were found by total lineshape analysis for the dimeric capsules of C7H7+PF6- and C7H7+BF4-, respectively. The affinity of the tropylium cation toward the dimer's cavity is much higher than that of neutral organic guests. Although exact quantitative values are not available due to the low solubility of tropylium salts in apolar solvents, a rough estimation in CD2Cl2 shows that the tropylium cation affinity is several orders of magnitude higher than that of benzene, which is known to be a good guest. These results show that once the steric requirements are met, electronic effects may serve as an additional driving force for the formation of such molecular capsules demonstrating the importance of cation-pi interactions in such systems

NMR diffusion spectroscopy for the characterization of multicomponent hydrogen-bonded assemblies in solution

NMR diffusion measurements on 10 different multicomponent hydrogen-bonded assemblies, viz. the three single rosettes SR1–SR3 ( 13· 2a3, 13· 2b3, 13· 2c3) the double rosettes DR1–DR5 ( 3a3· 2a6, 3b3· 2b6, 3c3· 2a6, 3d3· 2a6, 3e3· 2a6), and DR6 ( 4a3· 16), and the tetrarosette TR ( 53· 2a12) are described. Some of the above rosettes have been previously identified as well-defined assemblies (viz. SR1, DR1–DR3, and TR) using established characterization techniques (1H NMR spectroscopy, X-ray diffraction, and MALDI-TOF MS after Ag+-labeling). The diffusion coefficients of these assemblies were studied and used as a reference for the identification of three new assemblies ( DR4–DR6), the characterization of which could not be established unequivocally using other characterization tools. A good correlation was found between the experimental and calculated diffusion coefficients when DR1 was used as a reference. A relatively good correlation was obtained between the effective hydrolytic radii calculated from the diffusion data and those extracted from gas phase-minimized structures with SR1 and DR2 being exceptions. The diffusion measurements show that assembly DR4 is a thermodynamically stable species, while assemblies DR5 and DR6 are less stable and only present to a minor extent. \u

Self-Assembled Ionophores from Isoguanosine: Diffusion NMR Spectroscopy Clarifies Cation's and Anion's Influence on Supramolecular Structure

Cation-templated self-assembly of the lipophilic isoguanosine (isoG 1) with different monovalent cations (M+=Li+, Na+, K+, NH4+, and Cs+) was studied in solvents of different polarity by using diffusion NMR spectroscopy. Previous studies that did not use diffusion NMR techniques concluded that isoG 1 forms both pentamers (isoG 1)5M+ and decamers (isoG 1)10M+ in the presence of alkali-metal cations. The present diffusion NMR studies demonstrate, however, that isoG 1 does not form (isoG 1)5M+ pentamers. In fact, the diffusion NMR data indicates that both doubly charged decamers of formula (isoG 1)102 M+ and singly charged decamers, (isoG 1)10M+, are formed with lithium, sodium, potassium, and ammonium tetraphenylborate salts (LiB(Ph)4, KB(Ph)4, NaB(Ph)4 and NH4B(Ph)4), depending on the isoG 1:salt stoichiometry of the solution. In the presence of CsB(Ph)4, isoG 1 affords only the singly charged decamers (isoG 1)10Cs+. By monitoring the diffusion coefficient of the B(Ph)4- ion in the different mixtures of solvents, we also concluded that the anion is more strongly associated to the doubly charged decamers (isoG 1)102 M+ than to the singly charged decamers (isoG 1)10M+. The (isoG 1)102 M+ species can, however, exist in solution without the mediation of the anion. This last conclusion was supported by the finding that the doubly charged decamers (isoG 1)102 M+ also prevail in 1:1 CD3CN:CDCl3, a solvent mixture in which the B(Ph)4- ion does not interact significantly with the self-assembled complex. These diffusion measurements, which have provided new and improved structural information about these decameric isoG 1 assemblies, demonstrate the utility of combining diffusion NMR techniques with conventional NMR methods in seeking to characterize labile, multicomponent, supramolecular systems in solution, especially those with high symmetry