Limor Frish

Compensation of steric demand by cation–π interactions, cobaltocenium cations as guests in tetraurea calix[4]arene dimers

The affinities of ferrocene (2) and the cobaltocenium cation (3+), which have roughly the same size and differ in their charge, towards the inner cavity of the dimeric capsule formed by tetraurea calix[4]arene (1) were studied in C2D4Cl2 solutions. While 3+, which occupies more than 75% of the internal volume of the dimer, is readily encapsulated this is not the case for 2. This is probably due to cation-pi interactions, which operate only between 3+ and the aromatic rings of the calix[4]arene dimer. We found that the affinity of the cobaltocenium cation is higher than that of the tropylium cation (4+) and is only 2-3 times less than that of the tetraethylammoniun cation (5+). From the variable temperature 1H NMR spectra of this capsule, the free energy of activation at 298 K (deltaGdouble dagger(298K)) for the reorientation of the hydrogen bonded belt between the two parts of the dimer could be determined by total line shape analysis for the aromatic protons of the calixarene. The value of 14.3 +/- 0.2 kcal mol(-1) for the dimeric capsules of 3+ PF6- is very similar to the free activation energy found for dimeric capsules of 1 with 4+ PF6- and 5+ PF6- in C2D4Cl2. It becomes significantly lower, if PF6- is replaced by BF4-. We also found that ten times more DMSO is needed to disrupt the capsule 1 x 3+ x 1 than the corresponding 1 x 1 dimer containing benzene as guest. This demonstrates again the importance of the cation-pi interactions for the stability of such hydrogen-bonded dimeric capsules.

A pulsed gradient spin echo NMR study of guest encapsulation by hydrogen-bonded tetraurea calix(4)arene dimers

Diffusion coefficients, as obtained by the PGSE NMR technique, were used to probe encapsulation of guests in self-assembled tetraurea calix[4]arene dimers in organic solvents and to demonstrate their destruction by DMSO.

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.

Fluorinated smart micelles as enzyme-responsive probes for 19F-magnetic resonance

Labeling of smart PEG-dendron hybrids with fluorinated groups transformed their assemblies into enzyme-responsive micellar probes for 19F-magnetic resonance (MR). Two distinct labeling approaches were used to study the ability of these smart hybrids to be turned OFF at the assembled micellar state and to turn ON their 19F-MR signal upon enzymatic activation.

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, CDCl3 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 (ΔG‡298), 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 dimers 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–π interactions in such systems.

NMR Diffusion Measurements in Chemical and Biological Supramolecular Systems

Nature self-assembles molecules into large supramolecular aggregates which are capable of performing specific biological functions [1]. Therefore it is not surprising that supramolecular chemistry has become an important theme of chemistry in recent years [2]. One of the main problem of supramolecular sciences is the elucidation of the structure of the supramolecular aggregate that prevails in solution. Consequently, new analytical methods that may assist these structural determinations are required, especially those techniques that may provide some information regarding the mutual interactions between the different molecular components of the supramolecular system. In this context it seems to us that the diffusion coefficient, as measured by the pulsed gradient spin echo (PGSE) NMR technique [3], may provide such information.