Two Squares in a Barrel: An Axially Disubstituted Conformationally Rigid Aliphatic Binding Motif for Cucurbit[6]uril.

Novel binding motifs suitable for the construction of multitopic guest-based molecular devices (e.g., switches, sensors, data storage, and catalysts) are needed in supramolecular chemistry. No rigid, aliphatic binding motif that allows for axial disubstitution has been described for cucurbit[6]uril (CB6) so far. We prepared three model guests combining spiro[3.3]heptane and bicyclo[1.1.1]pentane centerpieces with imidazolium and ammonium termini. We described their binding properties toward CB6/7 and α-/ß-CD using NMR, titration calorimetry, mass spectrometry, and single-crystal X-ray diffraction. We found that a bisimidazolio spiro[3.3]heptane guest forms inclusion complexes with CB6, CB7, and ß-CD with respective association constants of 4.0 × 104, 1.2 × 1012, and 1.4 × 102. Due to less hindering terminal groups, the diammonio analogue forms more stable complexes with CB6 (K = 1.4 × 106) and CB7 (K = 3.8 × 1012). The bisimidazolio bicyclo[1.1.1]pentane guest forms a highly stable complex only with CB7 with a K value of 1.1 × 1011. The high selectivity of the new binding motifs implies promising potential in the construction of multitopic supramolecular components.

Assembling a supramolecular 3D network with tuneable mechanical properties using adamantylated cross-linking agents and ß-cyclodextrin-modified hyaluronan.

Hydrogels based on the supramolecular host-guest concept can be prepared if at least one constituent is a polymer chain modified with supramolecular host or guest (or both) units. Low-molecular-weight multitopic counterparts can also be used, however, guest molecules in the role of cross-linking agents are seldom reported, although such an approach offers wide-ranging possibilities for tuning the system properties via easily achievable structural modifications. In this paper, a series of adamantane-based star-like guest molecules was used for cross-linking of two types of ß-cyclodextrin-modified hyaluronan (CD-HA). The prepared 3D supramolecular networks were characterised using nuclear magnetic resonance, titration calorimetry and rheological measurements to confirm the formation of the host-guest complexes between adamantane moieties and ß-cyclodextrin units, including their typical properties such as self-healing and dynamic nature. The results indicate that the nature of the cross-linker (amides versus esters) has a greater impact on mechanical properties than the length of the guest's arms. In addition, the results show that the length of the HA polymer chain is more important than the degree of modification with supramolecular units. In conclusion, it was proven that the modular concept employing low-molecular-weight cross-linking guests is valuable for the formulation of supramolecular networks, including hydrogels.

Modes of Micromolar Host-Guest Binding of ß-Cyclodextrin Complexes Revealed by NMR Spectroscopy in Salt Water.

Multitopic supramolecular guests with finely tuned affinities toward widely explored cucurbit[n]urils (CBs) and cyclodextrins (CDs) have been recently designed and tested as functional components of advanced supramolecular systems. We employed various spacers between the adamantane cage and a cationic moiety as a tool for tuning the binding strength toward CB7 to prepare a set of model guests with KCB7 and Kß-CD values of (0.6-5.0) × 1010 M-1 and (0.6-2.6) × 106 M-1, respectively. These accessible adamantylphenyl-based binding motifs open a way toward supramolecular components with an outstanding affinity toward ß-cyclodextrin. 1H NMR experiments performed in 30% CaCl2/D2O at 273 K along with molecular dynamics simulations allowed us to identify two arrangements of the guest@ß-CD complexes. The approach, joining experimental and theoretical methods, provided a better understanding of the structure of cyclodextrin complexes and related molecular recognition, which is highly important for the rational design of drug delivery systems, molecular sensors and switches.

A Photochemical/Thermal Switch Based on 4,4'-Bis(benzimidazolio)stilbene: Synthesis and Supramolecular Properties.

Stilbene derivatives are well-recognised substructures of molecular switches based on photochemically and/or thermally induced (E)/(Z) isomerisation. We combined a stilbene motif with two benzimidazolium arms to prepare new sorts of supramolecular building blocks and examined their binding properties towards cucurbit[n]urils (n=7, 8) and cyclodextrins (ß-CD, γ-CD) in water. Based on the 1 H NMR data and molecular dynamics simulations, we found that two distinct complexes with different stoichiometry, i. e., guest@ß-CD and guest@ß-CD2 , coexist in equilibrium in a water solution of the (Z)-stilbene-based guests. We also demonstrated that the bis(benzimidazolio)stilbene guests can be transformed from the (E) into the (Z) form via UV irradiation and back via thermal treatment in DMSO.

Gas-phase fragmentation of 1-adamantylbisimidazolium salts and their complexes with cucurbit[7]uril studied using selectively 2 H-labeled guest molecules.

RATIONALE: Bisimidazolium salts (BIMs) represent an interesting family of ditopic ligands that are used in the construction of supramolecular systems with hosts based on cyclodextrins or cucurbit[n]urils. Understanding the fragmentation mechanism of individual BIMs and how this mechanism changes after complexation with cucurbit[n]urils can bring new insight into the intrinsic host-guest relationship, thereby allowing utilization of mass spectrometry to describe binding behavior. METHODS: Selectively 2 H-labeled bisimidazolium salts were prepared and fully characterized by spectroscopic methods. All MSn experiments were conducted in the positive-ion mode using an electrospray ionization (ESI) ion-trap mass spectrometer. The structures of the proposed fragments were supported by theoretical optimizations performed at the B3LYP/6-31G(d) level of density functional theory (DFT) using the Spartan'14 program. RESULTS: Using selectively deuterium-labeled isotopologues of two adamantylated bisimidazolium salts and DFT calculations, we describe the fragmentation pathways of bisimidazolium salts. The release of two important adamantane moieties, [C11 H17 ]+ and C11 H16 , from M2+ was determined, although the former was strongly preferred. In contrast, when M2+ was complexed with CB7, the neutral loss of the C11 H16 fragment was favored. The fragmentation pattern strongly depended on the steric hindrance of the M2+ guest against slippage of the CB7 unit over the guest molecular axle. CONCLUSIONS: The structures of two adamantane-based fragments and the mechanisms of their formation were rationalized. Two distinct geometric arrangements for the adamantane cage inside the CB7 cavity were hypothesized to explain the differences in the fragmentation patterns for guests with minimal, moderate, and high steric hindrance. This finding brings new insight into the understanding of intrinsic behavior of the adamantane-based guests inside the CB7 cavity.

Cubane Arrives on the Cucurbituril Scene.

Cubane, an intriguing chemical curiosity first studied in the early 1960s, has become a valuable structural motif and has recently been involved in the structures of a great number of prospective compounds. The first dicationic supramolecular guest 5 is prepared and derived from a 1,4-disubstituted cubane moiety, and its binding behavior toward cucurbit[n]urils (CBn) and cyclodextrins (CD) is studied. The bisimidazolium salt 5 forms 1:1 inclusion complexes with CB7, CB8, and ß-CD with the respective association constants (6.7 ± 0.5) × 1011 M-1, (1.5 ± 0.2) × 109 M-1, and <102 M-1 in water. The solid-state structures of the 5@CB7 and 5@CB8 complexes are also reported.

Rotaxanes Capped with Host Molecules: Supramolecular Behavior of Adamantylated Bisimidazolium Salts Containing a Biphenyl Centerpiece.

Bisimidazolium salts with one central biphenyl binding site and two terminal adamantyl binding sites form water-soluble binary or ternary aggregates with cucurbit[7]uril (CB7) and ß-cyclodextrin (ß-CD) with rotaxane and pseudorotaxane architectures. The observed arrangements result from cooperation of the supramolecular stopper binding strength and steric barriers against free slippage of the CB7 and ß-CD host molecules over the bisimidazolium guest axle.

Di-µ-chlorido-bis-[diacetonitrile-chlorido-oxidovanadium(IV)].

The title compound, [V(2)Cl(4)O(2)(CH(3)CN)(4)], is a centrosymmetric dinuclear V(IV) complex associated with four mol-ecules of acetonitrile. The coordination around both V(IV) atoms is essentially square-planar, involving three Cl atoms and one O atom [maximum deviation = 0.017 (3) Šfor the O atom]. The augmented octahedral coordination of the metal atom is completed by the N atoms of acetonitrile ligands. The V(IV) atoms are linked by two Cl atoms, acting as bridging atoms. The crystal studied was a non-merohedral twin with a ratio of the two twin components of 0.8200 (3):0.1800 (3). Although Cl and O atoms are present as potential acceptors in the title compound, no hydrogen bonds were observed in the crystal structure.