Vladimir Sindelar

Bambus[6]uril

A macrocyclic hexamer, the structure of which is reminiscent of part of the bamboo-plant stem (see picture), was directly prepared by the condensation of a glycoluril derivative and formaldehyde. The macrocycle bound halide anions with high affinity and selectivity.

A Bambusuril Macrocycle that Binds Anions in Water with High Affinity and Selectivity

Synthetic receptors that function in water are important for the qualitative and quantitative detection of anions, which may act as pollutants in the environment or play important roles in biological processes. Neutral receptors are particularly appealing because they are often more selective than positively charged receptors; however, their affinity towards anions in pure water is only in range of 1-10(3)  L mol(-1) . The anion-templated synthesis of a water-soluble bambusuril derivative is shown to be an outstanding receptor for various inorganic anions in pure water, with association constants of up to 10(7)  L mol(-1) . Furthermore, the macrocycle discriminates between anions with unprecedented selectivity (up to 500 000-fold). We anticipate that the combination of remarkable affinity and selectivity of this macrocycle will enable the efficient detection and isolation of diverse anions in aqueous solutions, which is not possible with current supramolecular systems.

Tight inclusion complexation of 2,7-dimethyldiazapyrenium in cucurbit[7]uril

The dicationic guest 2,7-dimethyldiazapyrenium is bound inside the host cucurbit[7]uril, forming a very stable inclusion complex in which the host undergoes structural distortions to accommodate the guest.

Anion-Free Bambus[6]uril and Its Supramolecular Properties

Methods for the preparation of anion-free bambus[6]uril (BU6) are presented. They are based on the oxidation of iodide anion, which is bound inside the macrocycle, utilizing dark oxidation by hydrogen peroxide or photooxidation in the presence of titanium dioxide. Anion-free BU6 was found to be insoluble in any of the investigated solvents; however, it dissolves in methanol/chloroform (1:1) or acetonitrile/water (1:1) mixtures in the presence of the tetrabutylammonium salt of a suitable anion. The association constants with halide ions, BF(4)(-), NO(3)(-), and CN(-), were measured by (1)H NMR spectroscopy. The highest association constant (8.9×10(5) M(-1)) was found for the 1:1 complex of BU6 with I(-) in acetonitrile/water mixture. A number of crystal structures of BU6 complexes with various anions were obtained. The influence of the anion size on the macrocycle diameter is discussed together with an unusual arrangement of the macrocycles into separate layers.

Electron density shift in imidazolium derivatives upon complexation with cucurbit[6]uril.

In this study, we have investigated the supramolecular interaction between series of 1-alkyl-3-methylimidazolium guests with variable alkyl substituent lengths and cucurbit[6]uril (CB6) in the solution and the solid state. Correct interpretation of (1)H NMR spectra was a key issue for determining the binding modes of the complexes in solution. Unusual chemical shifts of some protons in the (1)H NMR spectra were explained by the polarization of the imidazolium aromatic ring upon the complexation with the host. The formation of 1:1 complex between 1-ethyl-3-methylimidazolium and CB6 is in disagreement with previously reported findings describing an inclusion of two guest molecules in the CB6 cavity.

Cucurbiturils Substituted on the Methylene Bridge

Cucurbit[6]uril (CB[6]) with a substituent attached solely to one methylene bridge was prepared for the first time. The monosubstituted CB[6] undergoes self-assembly to form a cyclic tetramer in the solid state. The affinity of the monosubstituted CB[6] to a series of alkylammonium salts was measured revealing a minor effect of the substituent on the binding properties of the macrocycle.

Remarkable Salt Effect on Stability of Supramolecular Complex between Modified Cucurbit[6]uril and Methylviologen in Aqueous Media

The supramolecular complex formed by partial inclusion of methylviologen in MeCB6 was described both in solution and in the solid state. The association constant of the complex was determined using (1)H NMR and UV-vis spectrophotometric titration. An extraordinary 2000-fold drop in the association constant of the complex was observed when pure water was replaced by 50 mM NaCl solution.

Reasons why aldehydes do not generally participate in cucurbit[n]uril forming reactions.

The reaction of substituted glycolurils or a glycoluril dimer with a dialdehyde (o-phthalaldehyde) delivers S-shaped dimers and an S-shaped tetramer selectively. A combination of X-ray crystallography, PM3 calculations, and product resubmission experiments establish that the S-shaped isomers are thermodynamically more stable than the C-shaped diastereomers which we attribute to the conformational preferences of the newly formed benzo bicyclo[3.3.2]decane ring system. The preferential formation of S-shaped subunits is one reason why o-phthalaldehyde and possibly other aldehydes do not usually participate in CB[n] forming reactions. We also present evidence that points toward an equilibrium between glycoluril monomer + phthalaldehyde and S-shaped dimer + water that responds to concentration over the 1-50 mM range. This result suggests a second reason, insufficient reactivity (e.g., low equilibrium constant), why o-phthalaldehyde and possibly other aldehydes do not participate in CB[n] forming reactions. Delineation of the reasons why some aldehydes fail in these reactions is the first step toward devising methods to overcome these limitations.

Bambusuril as a One-Electron Donor for Photoinduced Electron Transfer to Methyl Viologen in Mixed Crystals

Methyl viologen hexafluorophosphate (MV2+·2PF6-) and dodecamethylbambus[6]uril (BU6) form crystals in which the layers of viologen dications alternate with those of a 1:2 supramolecular complex of BU6 and PF6-. This arrangement allows for a one-electron reduction of MV2+ ions upon UV irradiation to form MV+• radical cations within the crystal structure with half-lives of several hours in air. The mechanism of this photoinduced electron transfer in the solid state and the origin of the long-lived charge-separated state were studied by steady-state and transient spectroscopies, cyclic voltammetry, and electron paramagnetic resonance spectroscopy. Our experiments are supported by quantum-chemical calculations showing that BU6 acts as a reductant. In addition, analogous photochemical behavior is also demonstrated on other MV2+/BU6 crystals containing either BF4- or Br- counterions.

Glycoluril Dimers Bearing Hydrogen Atoms on Their Convex Face and Their Self-Assembly in the Solid State

A selective method for the synthesis of 1,6-substituted glycolurils has been developed. The glycolurils have been used for the synthesis of methylene-bridged glycoluril dimers bearing hydrogen atoms on their convex face. Depending on the side walls of the dimers, different modes of self-assembly in the solid state have been described using X-ray crystallography.