2,3-Dibutoxynaphthalene-based tetralactam macrocycles for recognizing precious metal chloride complexes

Molecular recognition and photoprotection of riboflavin in water by a biomimetic host

A water-soluble tetralactam macrocycle with 2,6-diethoxynaphthalene groups as side walls is able to strongly bind riboflavin (K_a >10⁷ M^-1) in water through hydrogen bonding and the hydrophobic effect. The encapsulated riboflavin can be stabilized by the host against photo-degradation under UV-vis irradiation, which may be harnessed to extend the shelf life of riboflavin.

Supramolecular Gold Stripping from Activated Carbon Using α-Cyclodextrin

We report the molecular recognition of the Au(CN)₂^- anion, a crucial intermediate in today's gold mining industry, by α-cyclodextrin. Three X-ray single-crystal superstructures-KAu(CN)₂⊂α-cyclodextrin, KAu(CN)₂⊂(α-cyclodextrin)₂, and KAg(CN)₂⊂(α-cyclodextrin)₂-demonstrate that the binding cavity of α-cyclodextrin is a good fit for metal-coordination complexes, such as Au(CN)₂^- and Ag(CN)₂^- with linear geometries, while the K⁺ ions fulfill the role of linking α-cyclodextrin tori together as a result of [K⁺···O] ion-dipole interactions. A 1:1 binding stoichiometry between Au(CN)₂^- and α-cyclodextrin in aqueous solution, revealed by ¹H NMR titrations, has produced binding constants in the order of 10⁴ M^-1. Isothermal calorimetry titrations indicate that this molecular recognition is driven by a favorable enthalpy change overcoming a small entropic penalty. The adduct formation of KAu(CN)₂⊂α-cyclodextrin in aqueous solution is sustained by multiple [C-H···π] and [C-H···anion] interactions in addition to hydrophobic effects. The molecular recognition has also been investigated by DFT calculations, which suggest that the 2:1 binding stoichiometry between α-cyclodextrin and Au(CN)₂^- is favored in the presence of ethanol. We have demonstrated that this molecular recognition process between α-cyclodextrin and KAu(CN)₂ can be applied to the stripping of gold from the surface of activated carbon at room temperature. Moreover, this stripping process is selective for Au(CN)₂^- in the presence of Ag(CN)₂^-, which has a lower binding affinity toward α-cyclodextrin. This molecular recognition process could, in principle, be integrated into commercial gold-mining protocols and lead to significantly reduced costs, energy consumption, and environmental impact.

Macrocyclic and acyclic supramolecular elements for co-precipitation of square-planar gold(iii) tetrahalide complexes

Non-covalent interactions control the solid-state packing of AuX₄⁻ anions (yellow circles) co-precipitated with different supramolecular elements.

Naphthobox: a selective molecular box for planar aromatic cations

A molecular box with an electron-rich cavity, namely naphthobox, was contructed and showed selective binding to planar aromatic cations.

Adsorptive Separation of Benzene, Cyclohexene, and Cyclohexane by Amorphous Nonporous Amide Naphthotube Solids

Benzene hydrogenation is an important industrial process. The reaction is incomplete, resulting in a mixture of benzene, cyclohexane, and/or cyclohexene that have to be separated before any further reactions. The currently used extractive and azeotropic distillations are operationally complex and energy intensive. Adsorptive separation provides an alternative energy-efficient method. However, the separation of the ternary mixture by adsorptive separation has not yet been reported. In the present research, we report two macrocyclic hosts with hydrogen-bonding sites in their cavities that are able to separate the ternary mixture of benzene, cyclohexene, and cyclohexane. N-H⋅⋅⋅π interactions were found to play a key role in the selective separation. In addition, fast adsorption, high loading ratios, and easy recycling are achieved with the present system, which is promising for practical applications.

Conformationally adaptive macrocycles with flipping aromatic sidewalls

Conformationally adaptive macrocycles possess multiple well-defined conformations through quickly flipping their aromatic sidewalls. The macrocycles combine the binding power of all the conformations. Upon binding a guest, one or a combination of conformations are selected to achieve the maximized binding affinity. In addition, the complex conformational network is responsive to changes in temperature or solvent. It has been demonstrated that these macrocycles have unique properties in chirality sensing, stimuli-responsive self-assembly, and molecular switches. In this tutorial review, we summarize recent advances on conformationally adaptive macrocycles with an emphasis on our own research. We believe that this class of macrocycles will have a bright future in supramolecular chemistry and beyond.