Compartmentalization of the aqueous space within a cell is necessary for life. In similar fashion to the nanometer-scale compartments in living systems, synthetic water-soluble coordination cages (WSCCs) can isolate guest molecules and host chemical transformations. Such cages thus show promise in biological, medical, environmental, and industrial domains. This review highlights examples of three-dimensional synthetic WSCCs, offering perspectives so as to enhance their design and applications. Strategies are presented that address key challenges for the preparation of coordination cages that are soluble and stable in water. The peculiarities of guest binding in aqueous media are examined, highlighting amplified binding in water, changing guest properties, and the recognition of specific molecular targets. The properties of WSCC hosts associated with biomedical applications, and their use as vessels to carry out chemical reactions in water, are also presented. These examples sketch a blueprint for the preparation of new metal-organic containers for use in aqueous solution, as well as guidelines for the engineering of new applications in water.

Design and Applications of Water-Soluble Coordination Cages.

Metal–organic frameworks (MOFs) have emerged as a significant class of porous crystalline materials constructed from metal nodes and multidentate linkers. Many research studies have shown the promising applications of MOFs in gas adsorption and separation, electrocatalysis, photocatalysis, biomedicine etc., mainly due to the advantages of high porosity, large surface areas and easily tunable optical and electronic structures. Particularly, light-sensitive Ti-oxo clusters in Ti-based MOFs result in promising photocatalytic activity. Recently, a few Ti–carboxylate MOFs based on infinite Ti-oxo chains and sheets were reported, which open a new horizon for the photocatalytic activity of Ti-MOF chemistry. This review highlights some typical Ti-MOFs based on discrete Ti-oxo clusters and recent progress in the fabrication of Ti-MOFs based on infinite Ti-oxo chains and sheets. Moreover, facile modification methods for improving the photocatalytic activity under visible light irradiation are exemplified in detail. We hope that the photocatalytic applications of Ti-MOFs in photocatalytic H2 generation, photocatalytic reduction of CO2, dye photodegradation, photocatalytic alcohol oxidation and photocatalytic polymerization will benefit researchers who are interested in this field.

From isolated Ti-oxo clusters to infinite Ti-oxo chains and sheets: recent advances in photoactive Ti-based MOFs

Recent advances in POM-organic frameworks and POM-organic polyhedra

In this study, an electrochemically self-doped WO3/TiO2 nanotubes (R-WO3/TNTs) composite film was developed for the photocatalytic degradation of waste gas. The doping of oxygen vacancies (OVs) into the heterojunction of WO3/TNTs was conducted by a simple electrochemical approach, by which the quantity and distribution of OVs on surface as well as bulk were tailored with respect to the applied cathodic potential and the duration of the treatment. With an increase of applied cathodic potential as well as the duration, the quantity of OVs on WO3/TNTs was observed to be consistently raised, while those presented in the surface were raised initially and further showed a plateau trend as the duration extended at a fixed potential. The incorporation of OVs into WO3/TNTs enhanced the charge-transport resistance and reduced the electron-hole recombination, thereby showed an enhanced photocatalytic performance. The R-WO3/TNTs prepared by the electrochemical polarization process at −1.4 V (vs SCE) exhibited a 12 times higher photo-current density and obviously, an enhanced efficiency in the photocatalytic degradation of VOCs with a prolonging photostability compared to that of the pristine WO3/TNTs, under a simulated solar light irradiation. The single-time purification trial demonstrated the effectiveness of the R-WO3/TNTs composite in treating the VOCs for the practical application.

Electrochemically self-doped WO3/TiO2 nanotubes for photocatalytic degradation of volatile organic compounds

Metal–organic cages (MOCs) are discrete molecular assemblies formed by coordination bonds between metal nodes and organic ligands. The application of MOCs has been greatly limited due to their poor stability, especially in aqueous solutions. In this work, we thoroughly investigate the stability of several Zr-MOCs and reveal their excellent stability in aqueous solutions with acidic, neutral, and weak basic conditions. In addition, we present for the first time a process-tracing study on the postassembly modification of one MOC, ZrT-1-NH2, highlighting the excellent stability and versatility of Zr-MOCs as a new type of molecular platform for various applications.

Process-Tracing Study on the Postassembly Modification of Highly Stable Zirconium Metal–Organic Cages

We have successfully constructed a tetrahedral Ti4L6 cage with calixarene-like coordination-active vertices. It further features high solubility and stability in H2O and DMF/H2O solution, affording an interesting stepwise assembly function with other metal ions. Through trapping of different amounts of Co or Ln ions, the Ti4L6 tetrahedra can be organized into various dimensional architectures, including a Ti4L6-Co3 cage, a Ti4L6-Ln2 cage, a Ti4L6-Ln2 chain, and a three-dimensional Ti4L6-Ln framework. An unusual mixed-valence phenomenon was observed in the Ti4L6 cage, whose Ti3+/4+ compositions were adequately identified by electron spin resonance and X-ray photoelectron spectroscopy analyses. More remarkably, the calixarene-like oxygen vertices of the Ti4L6 cage can also be used for the recognition of C3-symmetric dye molecules through N–H···O hydrogen bonding. Accordingly, driven by visible light, selective and efficient homogeneous photodecomposition of acid blue 93 and alkali blue 4B were successfully ...

Water-Soluble and Ultrastable Ti4L6 Tetrahedron with Coordination Assembly Function

Many metal-organic cages (MOCs) and a few hydrogen-bonded organic cages (HOCs) have been investigated, but little is reported about cooperative self-assembly of MOCs and HOCs. Herein, we describe an unprecedented MOC&HOC co-crystal composed of tetrahedral Ti4 L6 (L=embonate) cages and in-situ-generated [(NH3 )4 (TIPA)4 ] (TIPA=tris(4-(1H-1,2,4-triazol-1-yl)phenyl)amine) cages. Chiral transfer is observed from the enantiopure Ti4 L6 cage to enantiopure [(NH3 )4 (TIPA)4 ] cage. Two homochiral supramolecular frameworks with opposite handedness (PTC-235(Δ) and PTC-235(Λ)) are formed. Such MOC&HOC co-crystal features high stability in water and other solvents, affording single-crystal-to-single-crystal transformation to trap CH3 CN molecules and identify disordered NH4 + cations. A tablet pressing method is developed to test the third-order nonlinear optical property of KBr-based PTC-235 thin film. Such a thin film exhibits an excellent optical limiting effect.

Combining a Titanium-Organic Cage and a Hydrogen-Bonded Organic Cage for Highly Effective Third-Order Nonlinear Optics.

Synergetic optical resolution and chiral amplification of tetrahedral Ti4L6 cages by enantiopure coordination cations have been realized in this work. Anionic ΔΔΔΔ-[Ti4L6] and ΛΛΛΛ-[Ti4L6] cages (L...

Optical Resolution of the Water-Soluble Ti4(embonate)6 Cages for Enantioselective Recognition of Chiral Drugs

The cooperative assembly of a tetrahedral (Ti4L6)8– (L = embonate) cage and a penta-nuclear [Zn5(OH)3(H2O)5]7+ cluster unit results in an unprecedented cage-cluster-based metal–organic framework th...

Ti4(embonate)6 Based Cage-Cluster Construction in a Stable Metal–Organic Framework for Gas Sorption and Separation

Herein we report an efficient strategy to construct cage-based metal–organic frameworks (MOFs) via the assembly of Ti4L6 (L = embonate) cages and N-contained ligands with Mn2+ and Zn2+ ions, respec...

Guang-Hui Chen

Papers

Water-Soluble and Ultrastable Ti4L6 Tetrahedron with Coordination Assembly Function

Combining a Titanium-Organic Cage and a Hydrogen-Bonded Organic Cage for Highly Effective Third-Order Nonlinear Optics.

Optical Resolution of the Water-Soluble Ti4(embonate)6 Cages for Enantioselective Recognition of Chiral Drugs

Ti4(embonate)6 Based Cage-Cluster Construction in a Stable Metal–Organic Framework for Gas Sorption and Separation

Ti4(embonate)6 Cage-Ligand Strategy on the Construction of Metal-Organic Frameworks with High Stability and Gas Sorption Properties.