Enantioselective Tail-to-Head Terpene Cyclizations by Optically Active Hexameric Resorcin[4]arene Capsule Derivatives

Molecular capsules enable the conversion of substrates inside a closed cavity, mimicking to some extent enzymatic catalysis. Chirality transfer from the molecular capsule onto the encapsulated substrate has been only studied in a few cases. Here we demonstrate that chirality transfer is possible inside a rather large molecular container of approximately 1400 Å3 . Specifically, we present 1) the first examples of optically active hexameric resorcin[4]arene capsules, 2) their ability to enantioselectively catalyze tail-to-head terpene cyclizations, and 3) the surprisingly high sensitivity of enantioselectivity on the structural modifications.

Just Add Water: Modulating the Structure-Derived Acidity of Catalytic Hexameric Resorcinarene Capsules

The hexameric undecyl-resorcin[4]arene capsule (C11R6) features eight discrete structural water molecules located at the vertices of its cubic suprastructure. Combining NMR spectroscopy with classical molecular dynamics (MD), we identified and characterized two distinct species of this capsule, C11R6-A and C11R6-B, respectively featuring 8 and 15 water molecules incorporated into their respective hydrogen-bonded networks. Furthermore, we found that the ratio of the C11R6-A and C11R6-B found in solution can be modulated by controlling the water content of the sample. The importance of this supramolecular modulation in C11R6 capsules is highlighted by its ability to perform acid-catalyzed transformations, which is an emergent property arising from the hydrogen bonding within the suprastructure. We show that the conversion of C11R6-A to C11R6-B enhances the catalytic rate of a model Diels-Alder cyclization by 10-fold, demonstrating the cofactor-derived control of a supramolecular catalytic process that emulates natural enzymatic systems.

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.

Assembly of C-propyl-pyrogallol[4]arene with bipyridine-based spacers and solvent molecules

Co-crystallization of C-propyl-pyrogallol[4]arene (PgC3) with 4,4′-bipyridine (bpy) in ethanol afforded a multi-component complex (PgC3) · 3(bpy) ·(EtOH) (1) that consists of a one-dimensional brick-wall framework, which was formed by four pyrogallol[4]arene molecules and two juxtaposed bpy molecules, entrapping two other bpy molecules as guests within each cavity. Heating a mixture of PgC3 and trans-1,2-bis-(4-pyridyl)ethylene (bpe) in an ethanol-water mixed solvent allowed the isolation of a multi-component complex (PgC3) ·(bpe) · 2(EtOH) ·(H2O) (2), which has a two-dimensional wave-like polymer structure with the bpe molecules embedded in the wave trough between two PgC3 molecules. Single-crystal X-ray crystallography was utilized to investigate the hydrogen bonding networks of the multi-component complexes 1 and 2.

Water plays a dynamical role in a hydrogen-bonded, hexameric supramolecular assembly

The hexameric resorcin[4]arene supramolecular assembly has attracted significant interest as a self-assembled capsule that exhibits dynamic host-guest chemistry. Many studies have been carried out to investigate the structure and thermodynamics of the assembly, but considerably less is known about its dynamical properties. Here, molecular dynamics simulations are used to investigate the timescales of water encapsulation in this assembly in wet chloroform. We have previously shown [A. Katiyar et al., Chem. Commun. 2019, 55, 6591-6594] that at low water content there are three distinct populations of water molecules present, while at higher water content an additional population, long water chains interacting with the assembly, appears. The relative free energies of these different water positions are calculated and time correlation functions are used to determine the timescales for interconversion between the populations. This analysis demonstrates that the water molecules are in rapid exchange with each other on timescales of tens of ps to a few ns, and suggests that water molecules might be acting as a critical component in the guest exchange mechanism.

Self-Assemblies of Zinc Complexes for Aggregation-Induced Emission Luminogen Precursors

Positional isomers of zinc-nitrobenzoate complexes possessing pyridine -3-(or-4-) carboxamide are used for a comparative theoretical and experimental study to understand their utility as model complexes to understand the role of metal-to-ligand charge transfer in aggregation-induced emission (AIE). Among the five different model zinc complexes, four of them are non-ionic, and one is an ionic complex. The frontier molecular energy levels of different combinations of the positional isomeric complexes and the absorption maximum were ascertained by density functional theory calculations. The PolyQ value obtained from solid samples of each complex is different. Shifts in the emissions to higher wavelengths than the expected emission for the S₁ to S₀ transition were observed due to aggregations. The highest value of PolyQ among the complexes was 13.56% observed for emission at 439 nm (λ_ex = 350) of the non-ionic complex, namely, (di-aqua)bis(pyridine-3-carboxamide)di(2-nitrobenzoato)zinc(II) monohydrate. Close resemblance in emission lifetime decay profiles of the solid samples of those complexes and the respective solutions of those complexes in dimethyl sulfoxide with or without water showed a common trend, suggesting aggregation-induced emission in each case. Aggregation-induced emission caused by adding water in dimethyl sulfoxide solution of each complex showed an initial increase without a shift in the emission wavelength followed by a quenching with a shift of the respective emission peak to a short wavelength. Dynamic light scattering studies showed an increase in the average particle sizes upon an increase in the concentration of water. This indicated initial participation of water molecules to form aggregates with the complexes, favoring an increase in the AIE intensity. Aggregation of each complex changes with the concentration of water, and an increase in the concentration of water beyond a characteristic limit causes lowering of the emission intensity to the short wavelength.

Anion-mediated encapsulation-induced emission enhancement of an IrIII complex within a resorcin[4]arene hexameric capsule

The anions of the Ir complex salts control the thermodynamic stability and photoluminescence properties of the host–guest complex.