Encapsulated Guest Molecules in the Dimer of Octahydroxypyridine[4]arene

Rigid, biconical hydrogen-bonded dimers that strongly encapsulate cationic guests in solution and the solid state

The octol of a new rigid, tetraarylene-bridged cavitand was investigated for self-assembly behaviour in solution. ¹H and DOSY NMR spectroscopic experiments show that the cavitand readily dimerizes through an unusual seam of interdigitated hydrogen-bonds that is resistant to disruption by polar co-solvents. The well-defined cavity encapsulates small cationic guests, but not their neutral counterparts, restricting the conformation of sequestered tetraethylammonium in solution and the solid state.

A robust, dimeric capsule forms quantitatively in low-polarity solvents via a seam of 8 hydrogen bonds. The resulting electron-rich cavity selectively binds small organic cations over neutral counterparts.

Solution NMR of synthetic cavity containing supramolecular systems: what have we learned on and from?

NMR has been instrumental in studies of both the structure and dynamics of molecular systems for decades, so it is not surprising that NMR has played a pivotal role in the study of host-guest complexes and supramolecular systems. In this mini-review, selected examples will be used to demonstrate the added value of using (multiparametric) NMR for studying macrocycle-based host-guest and supramolecular systems. We will restrict the discussion to synthetic host systems having a cavity that can engulf their guests thus restricting them into confined spaces. So discussion of selected examples of cavitands, cages, capsules and their complexes, aggregates and polymers as well as organic cages and porous liquids and other porous materials will be used to demonstrate the insights that have been gathered from the extracted NMR parameters when studying such systems emphasizing the information obtained from somewhat less routine NMR methods such as diffusion NMR, diffusion ordered spectroscopy (DOSY) and chemical exchange saturation transfer (CEST) and their variants. These selected examples demonstrate the impact that the results and findings from these NMR studies have had on our understanding of such systems and on the developments in various research fields.

Anion-driven encapsulation of cationic guests inside pyridine[4]arene dimers

Pyridine[4]arenes have previously been considered as anion binding hosts due to the electron-poor nature of the pyridine ring. Herein, we demonstrate the encapsulation of Me₄N⁺ cations inside a dimeric hydrogen-bonded pyridine[4]arene capsule, which contradicts with earlier assumptions. The complexation of a cationic guest inside the pyridine[4]arene dimer has been detected and studied by multiple gas-phase techniques, ESI-QTOF-MS, IRMPD, and DT-IMMS experiments, as well as DFT calculations. The comparison of classical resorcinarenes with pyridinearenes by MS and NMR experiments reveals clear differences in their host–guest chemistry and implies that cation encapsulation in pyridine[4]arene is an anion-driven process.

Thermodynamically driven self-assembly of pyridinearene to hexameric capsules

Pyridinearene macrocycles have previously shown unique host-guest properties in their capsular dimers including endo complexation of neutral molecules and exo complexation of anions. Here, we demonstrate for the first time the formation of hydrogen bonded hexamer of tetraisobutyl-octahydroxypyridinearene in all three states of matter - gas phase, solution and solid-state. Cationic tris(bipyridine)ruthenium(ii) template was found to stabilize the hexamer in gas phase, whereas solvent molecules do this in condensed phases. In solution, the capsular hexamer was found to be the thermodynamically favoured self-assembly product and transition from dimer to hexamer occurred in course of time. The crystal structure of hexamer revealed 24 N-HO direct intermolecular hydrogen bonds between the six pyridinearene macrocycles without any bridging solvent molecules. Hydrogen bond patterns correlate well with DFT computed structures. Thus, all structural chemistry methods (IM-MS, DOSY NMR, DFT, X-ray crystallography) support the same structure of the hexameric capsule that has a diameter of ca. 3 nm and volume of 1160 Å3.

Diffusion NMR for the characterization, in solution, of supramolecular systems based on calixarenes, resorcinarenes, and other macrocyclic arenes

The use of diffusion NMR in studying calixarenes and other arene-based supramolecular systems is described, emphasizing the pivotal role played by the calixarene community in transforming the methods into a routine tool used in supramolecular chemistry.

Simultaneous endo and exo Complex Formation of Pyridine[4]arene Dimers with Neutral and Anionic Guests

The formation of complexes between hexafluorophosphate (PF₆^- ) and tetraisobutyloctahydroxypyridine[4]arene has been thoroughly studied in the gas phase (ESI-QTOF-MS, IM-MS, DFT calculations), in the solid state (X-ray crystallography), and in chloroform solution (¹ H, ¹⁹ F, and DOSY NMR spectroscopy). In all states of matter, simultaneous endo complexation of solvent molecules and exo complexation of a PF₆^- anion within a pyridine[4]arene dimer was observed. While similar ternary complexes are often observed in the solid state, this is a unique example of such behavior in the gas phase.

Diffusion NMR of molecular cages and capsules

In the last decade diffusion NMR and diffusion ordered spectroscopy (DOSY) have become important analytical tools for the characterization of supramolecular systems in solution. Diffusion NMR can be used to glean information on the (effective) size and shape of molecular species, as well as to probe inter-molecular interactions and can be used to estimate the association constant (Ka) of a complex. In addition, the diffusion coefficient, as obtained from diffusion NMR, is a much more intuitive parameter than the chemical shift for probing self-association, aggregation and inter-molecular interactions. The diffusion coefficient may be an even more important analytical parameter in systems in which the formed supramolecular entity has the same symmetry as its building units, when there is a large change in the molecular weight, where many molecular species are involved in the formation of the supramolecular systems, and when proton transfer may occur which, in turn, may affect the chemical shift. Some of the self-assembled molecular capsules and cages prepared in the last decade represent such supramolecular systems and in the present review, following a short introduction on diffusion NMR, we survey the contribution of diffusion NMR and DOSY in the field of molecular containers and capsules. We will first focus on the role played by diffusion NMR in the field of hydrogen bond driven self-assembled capsules. We then survey the contributions of diffusion NMR and DOSY to the study and characterization of metal-ligand cages and capsules. Finally, we describe a few recent applications of diffusion NMR in the field of hydrophobic, electrostatic and covalent containers.

Recent advances in hydrogen-bonded hexameric encapsulation complexes

Basic research in the chemistry of hexameric resorcin[4]arenes and pyrogallol[4]arenes during the last decade is reviewed. Applications of NMR methods to determine solution structures, host guest properties and exchange dynamics are discussed. The scientific issue is the behavior of molecules in small spaces; the challenge is to translate this information to practical applications in, say, catalysis or transport.

Octahydroxypyridine[4]arene Self-Assembles Spontaneously To Form Hexameric Capsules and Dimeric Aggregates

In recent years it has been observed that resorcin[4]arenes and pyrogallol[4]arenes form hydrogen-bonded hexameric capsules in nonpolar solvents. In the present study we have used NMR spectroscopy, with an emphasis on diffusion NMR, to investigate the self-assembly and the aggregation mode of solutions of octahydroxypyridine[4]arene (1 b) in chloroform. In spectroscopic studies, the hexameric capsule of C-undecylresorcin[4]arene (2 b) was used as a reference compound and in some cases also as an internal reference. The current diffusion NMR spectroscopy study shows, in contrast to a previous report, that compound 1 b self-assembles spontaneously into hexameric and dimeric aggregates in solutions in chloroform. The (1)H NMR and diffusion NMR spectroscopic studies on a solution of 1 b in CHCl(3) show the presence of new upfield-shifted peaks, which diffuse with the same diffusion coefficient as the hexameric peaks in the spectrum. Therefore, these new upfield-shifted peaks were attributed to encapsulated CHCl(3) molecules. Interestingly, diffusion NMR measurements showed that the addition of trifluoroacetic acid (6.7 equiv), which had no effect on the hexameric capsules of 2 b, led to the disassembly of the hexamer and the dimer of 1 b into its monomers. Therefore, we conclude that compound 1 b self-assembles spontaneously into hexameric capsules in nonpolar organic solvents, as do resorcin[4]arenes 2 b and 2 c and pyrogallol[4]arenes 3 a and 3 b.

Diastereochemical differentiation of beta-amino acids using host-guest complexes studied by Fourier transform ion cyclotron resonance mass spectrometry

Host-guest complexes where tetraethyl resorcarene was the host molecule were used to study the stereoselectivity of diasteromeric pairs of di-endo- and di-exo-2,3-disubstituted norbornane and norbornene amino acids by ion-molecule reactions and collision-induced dissociation with electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). Both methods showed stereoselectivity for the diastereomeric pairs. Particularly high selectivity was achieved for di-endo- and di-exo-2,3-disubstituted norbornane amino acids with ion-molecule reactions. Also, ab initio and hybrid density functional theory calculations were performed to study the different structures of the host-guest complexes. Hydrogen bonding was crucial for the calculated lowest energy structures, and sterical considerations satisfactorily explained the ion-molecule reaction results.

Calixarenes as hosts for ammonium cations: a quantum chemical study and mass-spectrometric investigations

Host-guest complexes of tetramethylcavitand with different ammonium cations were investigated by using a quantum chemical method at the density functional level (BP86, B3 LYP). The NH4+ cation is strongly bound to the host. Increasing methyl substitution at the cation decreases its inclination towards the complex formation. The calculated data are in line with results from electrospray ionization mass spectrometry (ESI-MS) experiments. They reveal stable aggregates only for the NH4+ cation and for the primary alkylammonium cations.