Water-Soluble and Ultrastable Ti4L6 Tetrahedron with Coordination Assembly Function

Acyl-anchored metal-organic cages with interior cryptand-like recognition sites for selective removal of radioactive strontium(II)

Tailored design of organic linkers or metal nodes can introduce desirable functionalities into metal-organic cages (MOCs), significantly expanding their potential applications. In this study, we present a viable approach for engineering acyl-type metal nodes to create interior oxygen-rich sites within MOCs, enabling specific recognition of metal ions, including radioactive contaminants, while maintaining the structural integrity of the MOCs. A novel MOC featuring a uranyl-sealed calix[4]resorcinarene (C[4]R)-based multisite cavity, referred to as UOC, is synthesized as a prototype. In UOC, peroxide-bridged dimeric uranyl units at both ends of the coordination cage provide abundant oxygen sites for coordination, forming a cryptand-like cavity that facilitates efficient recognition and encapsulation of Sr2+ due to the size-matching effect. Additionally, hydrophobic binding cavities at both ends of UOC promote the co-inclusion of two different guest species. Inspired by the strong binding affinity of UOC for Sr2+, it is employed as a solid adsorbent for capturing low concentrations of Sr2+ in strontium-contaminated simulated groundwater. A removal efficiency of 99.7% for Sr2+ at an initial concentration as low as 0.013 mmol L-1 is achieved, demonstrating its significant potential for the selective removal of trace amounts of radioactive 90Sr2+. This work demonstrates the feasibility of incorporating acyl-type metal nodes into MOCs for the recognition and encapsulation of metal ions, offering a novel strategy for developing new MOC-based functional materials.

Metallo-Supramolecular Helicates as Surface-Enhanced Raman Scattering (SERS) Substrates with High Tailorability

The exploration of novel functionalized supramolecular coordination complexes (SCCs) can enable new applications in domains that include purification and sensing. In this study, employing a coordination-driven self-assembly strategy, we designed and prepared a series of benzochalcogenodiazole-based metallohelicates as high-efficiency charge-transfer surface-enhanced Raman scattering (SERS) substrates, expanding the range of applications for these metallohelicates. Through structural modifications, including the substitution of single heteroatoms on ligands, replacement of coordinating metals, and alteration of ligand framework linkages, the Raman performance of these metallohelicates as substrates were systematically optimized. Notably, the SERS enhancement factors (EFs) of the metallohelicate-based SERS substrates were significantly enhanced to levels as high as 1.03×107, which rivals the EFs of noble metals devoid of "hot spots". Additionally, the underlying Raman enhancement mechanisms of these metallohelicates have been investigated through a combination of control experiments and theoretical calculations. This study not only demonstrates the utility of metallohelicates as SERS substrates but also offers insights and materials for the development of high-efficiency new charge-transfer SERS substrates.

Selective aqueous anion recognition in an anionic host

Water-soluble Fe4L4 4- cages can be synthesized in a multicomponent self-assembly process exploiting functionalized trigonal ligands, FeII salts, and water-soluble sulfonated formylpyridine components. The cages are soluble in purely aqueous solution and display an overall 4- charge, but are capable of binding suitably sized non-coordinating anions in the host cavity despite their anionic nature. Anions such as PF6 - or AsF6 - occupy the internal cavity, whereas anions that are too small (BF4 -) or too large (NTf2 -) are not encapsulated. The external anionic charge and sterically blocked ligand cores limit the exchange rate of bound anions, as no exchange is seen over a period of weeks with the anion-filled cages, and internalization of added PF6 - by an empty cage takes multiple weeks, despite the strong affinity of the cavity for PF6 - ions. In the future, this recognition mechanism could be used to control release of anions for environmental applications.

Advancements in calixarene-protected titanium-oxo clusters: from structural assembly to catalytic functionality

This review explores calixarenes, a prominent family of third-generation supramolecules celebrated for their distinct hollow, cavity-shaped structures. These macrocycles are intricately assembled by linking multiple phenolic units orthogonally through methylene (-CH2-), sulfur (-S-), or sulfonyl (-SO2-) bridges. This structural framework plays a pivotal role in the intricate assembly of nanoclusters, significantly advancing the field of cluster chemistry. A key focus of current research is the remarkable ability of calixarenes to stabilize titanium-oxo clusters. Our review details the application of calixarenes in constructing titanium-oxo cluster structures, emphasizing how these clusters, when encapsulated within calixarenes, exploit flexible coordination sites for structural modifications and serve as foundational units for more complex assemblies. Additionally, we investigate how these calixarene-stabilized metal-oxo clusters function as versatile scaffolds for catalytically active metal ions, facilitating the creation of bimetallic nanoclusters. These clusters not only exhibit unique structural diversity but also demonstrate exceptional catalytic efficiency. This review aims to inspire ongoing exploration and innovation in the use of calixarenes for the synthesis and application of advanced cluster materials.

Combining Ti4(embonate)6 anionic cages and π-conjugated coordination cations for highly effective optical limiting

The integration of anionic Ti4L6 (L = embonate) cages and π-conjugated coordination cations into ordered structures can produce high-performance nonlinear optical (NLO) materials. More specifically, by employing Ti4L6 cages for assembly with mixed N,N-chelated and P,P-chelated type conjugated organic ligands and Ag+ ions, three cage-based structures have been synthesized and structurally characterized. Among them, an ion pair structure with strong π-π accumulation exhibits a significant third-order NLO response, and an excellent optical limiting effect has been experimentally verified. This work provides a promising material for NLO applications.

Unlocking High Porosity: Post-Synthetic Solvothermal Treatment of Cu-Paddlewheel Based Metal-Organic Cages

Metal-organic cages (MOCs) have garnered significant attention due to their unique discrete structures, intrinsic porosity, designability, and tailorability. However, weak inter-cage interactions, such as van der Waals forces and hydrogen bonding can cause solid-state MOCs to lose structural integrity during desolvation, leading to the loss of porosity. In this work, a novel strategy to retain the permanent porosity of Cu-paddlewheel-based MOCs, enabling their use as heterogeneous catalysts is presented. Post-synthetic solvothermal treatments in non-coordinating solvents, mesitylene, and p-xylene, effectively preserve the packing structures of solvent-evacuated MOCs while preventing cage agglomeration. The resulting MOCs exhibit an exceptional N2 sorption capacity, with a high surface area (SBET = 1934 m2 g-1 for MOP-23), which is among the highest reported for porous MOCs. Intriguingly, while the solvothermal treatment reduced Cu(II) to Cu(I) in the Cu-paddlewheel clusters, the MOCs with mixed-valenced Cu(I)/Cu(II) maintained their crystallinity and permanent porosity. The catalytic activities of these MOCs are successfully examined in copper(I)-catalyzed hydrative amide synthesis, highlighting the prospect of MOCs as versatile reaction platforms.

Circular Heterochiral Titanium-Based Self-Assembled Architectures

Circular trinuclear helicates have been synthesized from a bis-biphenol strand (LH4), titanium isopropoxide, and various diimine ligands. These self-assembled architectures constructed around three TiO4N2 nodes have a heterochiral structure (C1 symmetry) when 2,2'-bipyridine (A), 4,4'-dimethyl-2,2'-bipyridine (B), 4,4'-bromo-2,2'-bipyridine (C), or 4,4'-dimethyl-2,2'-bipyrimidine (D) is employed. Within these complexes, one nitrogen ligand is endo-positioned inside the metallo-macrocycle, whereas the other two diimine ligands point outside the helicate framework. This investigation highlights that the nitrogen ligand which does not participate in the helicate framework of the complex controls the overall symmetry of the helicate since the 2,2'-bipyrimidine chelate (F) ends in the formation of a homochiral aggregate (C3 symmetry). The lack of symmetry found in the solid state for the trinuclear species ([Ti3L3(B)3], [Ti3L3(C)3], and [Ti3L3(D)3]) is observed for these complexes in solution (dichloromethane or chloroform). Remarkably, the 2,2'-bipyrazine ligand (ligand E) ends in the formation of a hexameric aggregate formulated as [Ti6L6(E)6], whereas the use of 4,4'-dimethyl-2,2'-bipyrimidine (ligand D) permits to generate the dinuclear complexes ([Ti2L(D)2(OiPr)4] and [Ti2L2(D)2]) in addition to the trimeric structure [Ti3L3(D)3]. The behavior of [Ti3L3(A)3] in solution, on the other hand, is unique since an equilibrium between the homochiral and the heterochiral form is reached within 17 days after the complex has been dissolved in dichloromethane (C3-[Ti3L3(A)3]/C1-[Ti3L3(A)3] ratio = 0.3). In chloroform, the heterochiral form of [Ti3L3(A)3] is stable for the same period of time, evidencing the dependence of this stereochemical transformation toward the solvent medium. The thermodynamic and kinetic parameters linked to this stereochemical equilibrium have been obtained and point to the fact that the transformation is intramolecular and not induced by the presence of external ligands. The thermodynamic constant of the C1-[Ti3L3(A)3]/C3-[Ti3L3(A)3] equilibrium is found to be K = 0.34 ± 10%. Further evidence to rationalize this solvent-induced symmetry switch is obtained via a DFT calculation and classical molecular dynamics. In particular, this computational investigation elucidates the reason why the stereochemical transformation of a heterochiral architecture into a homochiral structure is possible only for a trinuclear assembly containing ligand A.

Bridging the Homogeneous and Heterogeneous Catalysis by Supramolecular Metal-Organic Cages with Varied Packing Modes

Integrating the advantages of homogeneous and heterogeneous catalysis has proved to be an optimal strategy for developing catalytic systems with high efficiency, selectivity, and recoverability. Supramolecular metal-organic cages (MOCs), assembled by the coordination of metal ions with organic linkers into discrete molecules, have performed solvent processability due to their tunable packing modes, endowing them with the potential to act as homogeneous or heterogeneous catalysts in different solvent systems. Here, the design and synthesis of a series of stable {Cu3} cluster-based tetrahedral MOCs with varied packing structures are reported. These MOCs, as homogeneous catalysts, not only show high catalytic activity and selectivity regardless of substrate size during the CO2 cycloaddition reaction, but also can be easily recovered from the reaction media through separating products and co-catalysts by one-step work-up. This is because that these MOCs have varied solubilities in different solvents due to the tunable packing of MOCs in the solid state. Moreover, the entire catalytic reaction system is very clean, and the purity of cyclic carbonates is as high as 97% without further purification. This work provides a unique strategy for developing novel supramolecular catalysts that can be used for homogeneous catalysis and recycled in a heterogeneous manner.

Integrated Anionic Zirconium-Organic Cage and Cationic Boron-Imidazolate Cage for Synergetic Optical Limiting

Making oppositely charged metal-organic cages (MOCs) into a tightly ordered structure may bring interesting functions. Herein, we report a novel structure composed of anionic (Zr4 L6 )8- (L=embonate) tetrahedral cages and in situ-formed cationic [Zn4 (Bim)4 ]4+ (Bim=[BH(im)3 ]- ; im=imidazole) cubic cages. Chiral transfer is observed from enantiopure (Zr4 L6 )8- cage to enantiopure [Zn4 (Bim)4 ]4+ cage. A pair of enantiomers (PTC-373(Δ) and PTC-373(Λ)) are formed. PTC-373 exhibits high chemical and thermal stabilities, affording an interesting single-crystal-to-single-crystal transformation. More importantly, the combination of ionic pair cages significantly enhances its third-order nonlinear optical property, and its thin-film exhibits an excellent optical limiting effect.

All-catecholate-stabilized black titanium-oxo clusters for efficient photothermal conversion

The controlled synthesis of titanium-oxo clusters (TOCs) completely stabilized by organic dye ligands with high stability and superior light absorption remains a significant challenge. In this study, we report the syntheses of three atomically precise catechol (Cat)-functionalized TOCs, [Ti2(Cat)2(OEgO)2(OEgOH)2] (Ti2), [Ti8O5(Cat)9(iPrO)4(iPrOH)2] (Ti8), and [Ti16O8(OH)8(Cat)20]·H2O·PhMe (Ti16), using a solvent-induced strategy (HOEgOH = ethylene glycol; iPrOH = isopropanol; PhMe = toluene). Interestingly, the TiO core of Ti16 is almost entirely enveloped by catechol ligands, making it the first all-catechol-protected high-nuclearity TOC. In contrast, Ti2 and Ti8 have four weakly coordinated ethylene glycol ligands and six weakly coordinated iPrOH ligands, respectively, in addition to the catechol ligands. Ti16 is visually evident in its distinctively black appearance, which belongs to black TOCs (B-TOCs) and exhibits an ultralow optical band gap. Furthermore, Ti16 displays exceptional stability in various media/environments, including exposure to air, solvents, and both acidic and alkaline aqueous solutions due to its comprehensive protection by catechol ligands and rich intra-cluster supramolecular interactions. Ti16 has superior photoelectric response qualities and photothermal conversion capabilities compared to Ti2 and Ti8 due to its ultralow optical band gap and remarkable stability. This discovery not only represents a huge step forward in the creation of all-catecholate-protected B-TOCs with ultralow optical band gaps and outstanding stability, but it also gives key valuable mechanistic insights into their photothermal/electric applications.

A cryptand-like Ti-coordination compound with visible-light photocatalytic activity in CO2 storage

A cryptand-like Ti-coordination compound, namely Ti12Cs, comprising two Ti6-salicylate cages and hosting two Cs+ ions, was synthesized by the solvothermal method. It exhibits strong visible-light absorption with an absorption band edge of 652 nm, attributed to the electron transition from salicylate ligands to Ti ions. Electrochemical impedance, visible-light transient photocurrent response, and photoluminescence spectra confirm that Ti12Cs has excellent visible-light response and charge-separation properties. Ti12Cs can be used as a heterogeneous and recyclable photocatalyst for CO2/epoxide cycloaddition, with high utilization efficiency of visible-light under mild conditions. The mechanism investigation points to a synergistic effect of photocatalysis and Lewis acid catalysis.

Bioinspired Polyoxo-titanium Cluster for Greatly Enhanced Solar-Driven CO2 Reduction

Developing artificial enzymes with excellent catalytic activities and uncovering the structural and chemical determinants remain a grand challenge. Discrete titanium-oxo clusters with well-defined coordination environments at the atomic level can mimic the pivotal catalytic center of natural enzymes and optimize the charge-transfer kinetics. Herein, we report the precise structural tailoring of a self-assembled tetrahedral Ti4Mn3-cluster for photocatalytic CO2 reduction and realize the selective evolution of CO over specific sites. Experiments and theoretical simulation demonstrate that the high catalytic performance of the Ti4Mn3-cluster should be related to the synergy between active Mn sites and the surrounding functional microenvironment. The reduced energy barrier of the CO2 photoreduction reaction and moderate adsorption strength of CO* are beneficial for the high selective evolution of CO. This work provides a molecular scale accurate structural model to give insight into artificial enzyme for CO2 photoreduction.

Achieving High-Efficient Photoelectrocatalytic Degradation of 4-Chlorophenol via Functional Reformation of Titanium-Oxo Clusters

Rational design of crystalline catalysts with superior light absorption and charge transfer for efficient photoelectrocatalytic (PEC) reaction coupled with energy recovery remains a great challenge. In this work, we elaborately construct three stable titanium-oxo clusters (TOCs, Ti₁₀Ac₆, Ti₁₀Fc₈, and Ti₁₂Fc₂Ac₄) modified with a monofunctionalized ligand (9-anthracenecarboxylic acid (Ac) or ferrocenecarboxylic acid (Fc)) and bifunctionalized ligands (Ac and Fc). They have tunable light-harvesting and charge transfer capacities and thus can serve as outstanding crystalline catalysts to achieve efficient PEC overall reaction, that is, the integration of anodic organic pollutant 4-chlorophenol (4-CP) degradation and cathodic wastewater-to-H₂ conversion. These TOCs can all exhibit very high PEC activity and degradation efficiency of 4-CP. Especially, Ti₁₂Fc₂Ac₄ decorated with bifunctionalized ligands exhibits better PEC degradation efficiency (over 99%) and H₂ generation than Ti₁₀Ac₆ and Ti₁₀Fc₈ modified with a monofunctionalized ligand. The study of the 4-CP degradation pathway and mechanism revealed that such better PEC performance of Ti₁₂Fc₂Ac₄ is probably due to its stronger interactions with the 4-CP molecule and better ^•OH radical production. This work not only presents the effective combination of organic pollutant degradation and simultaneously H₂ evolution reaction using crystalline coordination clusters as both anodic and cathodic catalyst but also develops a new PEC application for crystalline coordination compounds.

From an aluminum oxo cluster to an aluminum oxo cluster organic cage

Presented herein is an example of the conversion of an aluminum oxo cluster (AlOC) to an aluminum oxo cluster organic cage (AlOCOC). We successfully synthesized the first example of an aluminum cluster-based organic cage-Al₁₂ tetrahedral cage via an Al₃ cluster. The use of 4-pyrazolecarboxylic acid plays an important role in the construction of the organic cage. Due to the presence of partially deprotonated ligands, the hydrogen-bonding interactions between the discrete tetrahedra generate porous supramolecular structures. Considering the high porosity and the abundant N-H sites, we further investigated the performance of the material towards iodine capture.

Enhancing Third-Order Nonlinear Optical Property by Regulating Interaction between Zr4(embonate)6 Cage and N, N-Chelated Transition-Metal Cation

Herein, the combination of anionic Zr₄L₆ (L = embonate) cages and N, N-chelated transition-metal cations leads to a series of new cage-based architectures, including ion pair structures (PTC-355 and PTC-356), dimer (PTC-357), and 3D frameworks (PTC-358 and PTC-359). Structural analyses show that PTC-358 exhibits a 2-fold interpenetrating framework with a 3,4-connected topology, and PTC-359 shows a 2-fold interpenetrating framework with a 4-connected dia network. Both PTC-358 and PTC-359 can be stable in air and other common solvents at room temperature. The investigations of third-order nonlinear optical (NLO) properties indicate that these materials show different degrees of optical limiting effects. It is surprising that increasing coordination interactions between anion and cation moieties can effectively enhance their third-order NLO properties, which can be attributed to the formation of coordination bonds that facilitate charge transfer. In addition, the phase purity, UV-vis spectra, and photocurrent properties of these materials were also studied. This work provides new ideas for the construction of third-order NLO materials.

Metal Organic Polygons and Polyhedra: Instabilities and Remedies

The field of coordination chemistry has undergone rapid transformation from preparation of monometallic complexes to multimetallic complexes. So far numerous multimetallic coordination complexes have been synthesized. Multimetallic coordination complexes with well-defined architectures are often called as metal organic polygons and polyhedra (MOPs). In recent past, MOPs have received tremendous attention due to their potential applicability in various emerging fields. However, the field of coordination chemistry of MOPs often suffer set back due to the instability of coordination complexes particularly in aqueous environment-mostly by aqueous solvent and atmospheric moisture. Accordingly, the fate of the field does not rely only on the water solubilities of newly synthesized MOPs but very much dependent on their stabilities both in solution and solid state. The present review discusses several methodologies to prepare MOPs and investigates their stabilities under various circumstances. Considering the potential applicability of MOPs in sustainable way, several methodologies (remedies) to enhance the stabilities of MOPs are discussed here.

Functionalized Zirconium Organic Frameworks as Fluorescent Probes for the Detection of Tetracyclines in Water and Pork

The overuse of tetracyclines (TCs) in livestock breeding may cause a series of health and environmental problems. It is necessary to develop more accurate, convenient, and rapid sensing methods toward TCs, but it is still very challenging. In this work, three isostructural zirconium organic frameworks (Zr-MOFs) have been investigated as probes for the fluorescent sensing of TCs in water. By varying the functional group at the central benzene core, their sensing performances toward TCs can be modified. Under optimized conditions, the limit of detection can be as low as 0.08 nM in a wide detection range of 0-147 μM with high sensitivity and selectivity. These Zr-MOFs can also be applied in the detection of TCs in real pork samples with satisfying reliabilities and correctness. This work provides a new method for the design and optimization of fluorescent sensors toward TCs.

Coordination-Delayed-Hydrolysis Method for the Synthesis and Structural Modulation of Titanium-Oxo Clusters

ConspectusAtomically precise titanium-oxo clusters (TOCs) are the structure and reactivity model compounds of technically important TiO₂ materials, which could help build structure-property relationships and achieve property modulation at the molecular level. However, the traditional formation of TOCs has relied on the poorly controllable hydrolysis of titanium alkoxide in the solvent for a long time, limiting the development of TOC structural chemistry to a great extent. In addition, easily hydrolyzable alkoxy groups would be still coordinated on the surface of the TOCs generated by this method, making the clusters sensitive and unstable to the moisture. To achieve controllable preparation of TOCs, we believe it is crucial to attenuate the hydrolysis of titanium ions in the formation process of a cluster. To this end, we have recently applied an effective coordination-delayed-hydrolysis (CDH) strategy for TOC synthesis, which provides powerful tools for tuning their structures.In this Account, at the beginning, a brief introduction to the coordination-delayed-hydrolysis strategy is supplied, and its predominant features for constructing novel TOCs are highlighted. In subsequent sections, we discuss how the applied chelating organic/inorganic ligands (named hydrolysis delayed ligands) influence the hydrolysis process of Ti⁴⁺ ions to form a large family of TOCs with various nuclearities and core structures. Various hydrolysis delayed ligands have been explored, ranging from common O-donor ligands (carboxylate, phenol, or sulfate) to rarely used N-donor ligands (pyrazole) or bifunctional O/N-donor ones (quinoline, oxime, or alkanolamine). Breakthroughs in the symmetry, configuration, and cluster nuclei of TOCs have been accordingly achieved. Then, we show that this CDH method can be used to tune the surface structure of TOCs by modifying functional organic ligands. As a result, the physicochemical properties of TOCs, especially optical band gaps, can be optimized, and their stability under ambient conditions is significantly improved. In addition, we illustrate that the reversible bonds between hydrolysis delayed ligands and Ti ions further allows us to introduce active heterometal ions or clusters upon or inside the Ti-O cores to prepare heterometallic TOCs with unprecedented structures and properties. In particular, noble metal (Ag ions or clusters) has been incorporated into Ti-O clusters for the first time. As a summary, the coordination-delayed-hydrolysis strategy has realized the controllable hydrolysis of Ti⁴⁺ ions to some extent, breaking through the limitations of traditional synthesis methods and producing fruitful results in the field of titanium-oxo clusters. It is believed that this CDH method would also be effective for synthesizing oxo clusters of other easily hydrolyzed metal ions (Al³⁺, Sn⁴⁺, In³⁺, etc.) to afford significant contribution for the cluster community.

RhB-Embedded Zirconium-Biquinoline-Based MOF Composite for Highly Sensitive Probing Cr(VI) and Photochemical Removal of CrO42-, Cr2O72-, and MO

How to accurately detect and efficiently sweep Cr(VI) from contaminated water has come into focus. Zirconium-based metal-organic frameworks (MOFs) play vital roles in water environmental chemistry due to excellent hydrolysis-resistant stability. However, as photochemical probes and photocatalysts, poor performances in detection sensitivity, selectivity, and photosensitiveness limit sole Zr-MOFs' applications. So, it is urgent to quest valid strategies to break through the dilemmas. Embedding luminous dyes into MOFs has been considered one of the most feasible avenues. Herein, a dual-emissive RhB@Zr-MOF with orange-yellow fluorescence has been assembled by in situ-encapsulating rhodamine B (RhB) into a zirconium-biquinoline-based MOF. Actually, within RhB@Zr-MOF, the aggregation fluorescence quenching (ACQ) effect of RhB molecules was effectively avoided. Notably, RhB@Zr-MOF exhibits a rapid fluorescence quenching response toward Cr(VI) ions with high selectivity, sensitivity, and anti-interference abilities. More interestingly, unlike the most widely reported fluorescence resonance energy transfer (FRET) between MOFs and encapsulated guest modules, photoinduced electron transfer from RhB to Zr-MOF has been confirmed by modeling the ground state and excited states of RhB@Zr-MOF using density functional theory (DFT) and time-dependent DFT (TD-DFT). The effective electron transfer makes RhB@Zr-MOF more sensitive in probing Cr₂O₇^2- and CrO₄^2- ions with ultralow detection limit (DL) values of 6.27 and 5.26 ppb, respectively. Prominently, the detection sensitivity based on DL values has been increased about 6 and 9 times, respectively, compared with pristine Zr-MOF. Moreover, rather negative CB and positive VB potentials make RhB@Zr-MOF have excellent photochemical scavenging ability toward Cr(VI) and MO.

Water-Soluble Self-Assembled Cage with Triangular Metal-Metal-Bonded Units Enabling the Sequential Selective Separation of Alkanes and Isomeric Molecules

The selective separation of structurally similar aliphatic/aromatic hydrocarbons is an essential goal in industrial processes. In this study, we report the synthesis of a water-soluble (Tr₂M₃)₄L₄ (Tr = cycloheptatrienyl ring; M = metal; L = organosulfur ligand) molecular cage (1) via self-assembly of the water-soluble acceptor tripalladium sandwich species [(Tr₂Pd₃)(CH₃CN)][NO₃]₂ and the attachment onto L of solubilizing methoxyethoxy appendants to be utilized in an energy-friendly alternative approach to the separation of structurally similar molecules under ambient conditions. Cage 1, comprising a hydrophobic inner cavity, exhibited good solubility and stability in aqueous media. It also demonstrated excellent performance in the sequential separation of alkanes (C6-C9), xylene, and other disubstituted benzene isomers and cis/trans-decalin.

Strategies for the Construction of Functional Materials Utilizing Presynthesized Metal-Organic Cages (MOCs)

Metal-organic cages (MOCs) that assemble from metal ions or metal clusters and organic ligands have attracted the interest of the scientific community because of their various functional coordination cavities. Unlike metal-organic frameworks (MOFs) with infinite frameworks, MOCs have discrete structures, making them soluble and stable in certain solvents and facilitating their application as starting reagents in the further construction of single components or composite materials. In recent years, increasing progress has been made in this field. In this review, we introduce these works from the perspective of design strategies, and focus on how presynthesized MOCs can be used to construct functional materials. Finally, we discuss the challenges and development prospects in this field.

Ancillary ligand-regulated Ti(IV)-based metallocalixarene coordination cages for photocatalytic H2 evolution

High-valence Ti(IV)-based metallocalixarene coordination cages that are linked by oriented ancillary ligands are unknown so far. Herein, the first family of tunable calixarene-based coordination cages of Ti(IV) with a framework formula [Ti₁₂(OⁱPr)₁₂(TBC[4])₆L₆] have been assembled from six {Ti₂(OⁱPr)₂(TBC[4])}²⁺ nodes and six pyridinedicarboxylic ligands. Furthermore, the {Ti12L6} cage showed strong photocatalytic H₂ evolution activity, and DFT studies were performed to explore its electronic structure.

Synthesis of a Zr4(embonate)6-cobalt based superstructure for photocatalytic hydrogen production

Herein, we report an efficient method to construct cage-based MOF materials with exposed metal active sites for catalysis. By employing Zr₄L₆ (L = embonate) cages as precursors for assembly with N-containing ligands and Co²⁺ ions, a new Zr₄L₆-Co based chain structure (PTC-318) has been generated through two-step reactions. Interestingly, in the absence of a photosensitizer, PTC-318 exhibits notable photocatalytic activity for H₂ evolution under visible-light irradiation.

Metal-organic cage incorporating thin-film nanocomposite membranes with antifouling properties

We report the antifouling properties of thin-film nanocomposite (TFN) membranes containing two water-stable metal-organic cages (MOCs). The MOC-containing TFN membranes possess excellent antifouling properties against positively-charged foulants and protein (BSA, up to 99.7% water permeability retention) and achieve up to 100% water permeability recovery.

"Three-in-One" Structural-Building-Mode-Based Ti16-Type Titanium Oxo Cluster Entirely Protected by the Ligands Benzoate and Salicylhydroxamate

Titanium oxo clusters (TOCs) with accurate molecular structures have potential applications in photocatalysis, such as photocatalytic degradation, hydrogen production, and water oxidation. The hydrolytic stability and light absorption ability of TOCs have important impacts on photocatalysis, where the selection of peripheral organic ligands plays a significant role. In this regard, salicylhydroxamic acid (abbreviated as H₃L) attracts our attention, acting as a ligand for its multidentate and dye-functional features, which can increase the hydrolytic stability and broaden light absorption for TOCs. Herein, two TOCs were solvothermally synthesized and structurally characterized using H₃L, formulated as [Ti₈(μ₂-O)₂(μ₃-O)₂(OⁱPr)₁₂(L)₄]·2CH₃CN (1) and [Ti₁₆(μ₂-O)₁₀(μ₃-O)₄(PhCOO)₁₄(L)₆(HL)₂]·4CH₃CN·2ⁱPrOH (2). Complex 2 was obtained by adding excessive benzoic acid over the reaction system of 1, resulting in enhanced hydrolytic stability via the replacement of all alkoxy ligands by multidentate ligands for protection. Interestingly, for the first time, the "three-in-one" structural building mode with {Ti₆} + {Ti₄} + {Ti₆} by the common subunits in 2 was observed among all reported TOCs. Moreover, complex 2 can strongly absorb visible light reaching up to 700 nm and exhibit obvious activity for the photodegradation of methyl orange.

Designable assembly of atomically precise Al4O4 cubane supported mesoporous heterometallic architectures

Heterometallic cluster-based framework materials are of interest in terms of both their porous structures and multi-metallic reactivity. However, such materials have not yet been extensively investigated because of difficulties in their synthesis and structural characterization. Herein, we reported the designable synthesis of atomically precise heterometallic cluster-based framework compounds and their application as catalysts in aldol reactions. By using the synergistic coordination protocol, we successfully isolated a broad range of compounds with the general formula, [Al4M4O4(L)12(DABCO)2] (L = carboxylates; DABCO = 1,4-diazabicyclo[2.2.2]-octane; M2+ = Co2+, Mn2+, Zn2+, Fe2+, Cd2+). The basic heterometallic building blocks contain unprecedented main-group γ-alumina moieties and surrounding unsaturated transition metal centers. Interestingly, the porosity and interpenetration of these frameworks can be rationally regulated through the unprecedented strategy of increment of the metal radius in addition to general introduction of sterically bulky groups on the ligand. Furthermore, these porous materials are effective catalysts for aldol reactions. This work provides a catalytic molecular model platform with accurate molecular bonding between the supporters and catalytically active metal ions.

An ultrastable Ti-based metallocalixarene nanocage cluster with photocatalytic amine oxidation activity

Polyhedral metallocalixarene nanocage clusters based on pure Ti(IV) ions are to our knowledge unknown hitherto. Herein we report the first Ti(IV)-based metallocalixarene nanocage cluster by assembling a [Ti₁₃O₁₄] cage with six t-butylcalix[4]arene molecules. Notably, the cluster exhibits extraordinary stability in high-concentration acid/alkali solutions and can act as a stable photocatalyst to catalyze the oxidation of ammonia to imines.

Inorganic acid influenced formation of Ti26 and Ti44 oxysulfate clusters with toroidal and capsule structures

We herein report the discovery of inorganic toroidal and capsule titanium oxysulfate clusters by ionothermal synthesis. The ratio between geometrically different anions (tetrahedral SO₄^2-vs. pseudo-tetrahedral PO₃^3-) shows an interesting influence on cluster structure formation.

Post-synthetic modifications of metal–organic cages

Metal-organic cages (MOCs) are discrete, supramolecular entities that consist of metal nodes and organic linkers, which can offer solution processability and high porosity. Thereby, their predesigned structures can undergo post-synthetic modifications (PSMs) to introduce new functional groups and properties by modifying the linker, metal node, pore or surface environment. This Review explores current PSM strategies used for MOCs, including covalent, coordination and noncovalent methods. The effects of newly introduced functional groups or generated complexes upon the PSMs of MOCs are also detailed, such as improving structural stability or endowing desired functionalities. The development of the aforementioned design principles has enabled systematic approaches for the development and characterization of families of MOCs and, thereby, provides insight into structure-function relationships that will guide future developments.

Metal-Organic Polyhedra as Building Blocks for Porous Extended Networks

Metal-organic polyhedra (MOPs) are a subclass of coordination cages that can adsorb and host species in solution and are permanently porous in solid-state. These characteristics, together with the recent development of their orthogonal surface chemistry and the assembly of more stable cages, have awakened the latent potential of MOPs to be used as building blocks for the synthesis of extended porous networks. This review article focuses on exploring the key developments that make the extension of MOPs possible, highlighting the most remarkable examples of MOP-based soft materials and crystalline extended frameworks. Finally, the article ventures to offer future perspectives on the exploitation of MOPs in fields that still remain ripe toward the use of such unorthodox molecular porous platforms.

Self-Assembly of Chiral Ferrocene-Functionalized Polyoxotitanium Clusters for Photocatalytic Selective Sulfide Oxidation

Here, we systematically studied the self-assembly behavior of chiral polyoxytitanium clusters for the first time. Through the cooperative assembly of ferrocenecarboxylic acid and ketoxime ligands, we successfully incorporated the planar chirality of ferrocene (Fc) into the layered {Ti₅} building blocks. The resulting {Ti₅Fc} clusters can be used as structural units to assemble into large ordered structures in various ways; either a pair of {Ti₅Fc} enantiomers are bridged by organic adhesive to form sandwich structures or two homochiral {Ti₅Fc} units participate in the assembly to form the large clusters. Depending on the assembly modes, the chirality of {Ti₅Fc} can be transferred to large nanoclusters or disappear to form mesostructures. The difference of the assembly modes between the {Ti₅Fc} units can also tune the photoelectric activity of the resulting clusters, which has been verified by using {Ti₁₀Fc-6/7} as catalysts for photocatalytic selective sulfide oxidation. This work not only is an important breakthrough in the study of the self-assembly of chiral nanoclusters but also provides an important reference for understanding of chiral transfer on the nanoscale.

Assembly of Cyclic Ferrocene-Sensitized Titanium-Oxo Clusters with Excellent Photoelectrochemical Activity

The development of crystalline titanium-oxo clusters has made great progress in recent years. However, the geometric assembly of titanium-oxo clusters is still very challenging. Herein, we report the assembly of...

Construction and two-dimensional assembly of double-shell Na@Sn6L6@Sn3L3 clusters through tetrahedral citrate ligands

Herein we report a double-shell Na@Sn6L6@Sn3L3 cluster and their further assembly into 2D layer, which belongs to rare Sn-oxo coordination cage based extended structure. Tetrahedral citrate ligands with multiple coordination...

Bottom-up synthesis strategy of a two-dimensional {Fe5} cluster-based coordination polymer: stepwise formation of a {Fe5} cluster and its dimension augmentation

A 2D cluster-based coordination polymer of {Fe5}-2D was developed by hierarchical dimension augmentation from its 1D coordination polymer of {Fe5}-1D and discrete 0D cluster of {Fe5}-0D. And a possible bottom-up synthesis mechanism was proposed.

Tunable Third-Order Nonlinear Optical Effect via Modifying Ti4(embonate)6 Cage-Based Ionic Pairs

Benefiting from the strong inherent π-conjugation properties, the integration of Ti4L6 (L = embonate) cages and various N, N-chelated transition-metal cations into tightly packed structures accurately lead to the high-performance...

Metal-Directed Self-Assembly of {Ti8L2} Cluster-Based Coordination Polymers with Enhanced Photocatalytic Alcohol Oxidation Activity

Cooperative assembly of the neutral cluster {Ti₈O₅(OEt)₁₈L₂} (L = pyrazine-2,3-dicarboxylic acid) with different metal units of Mn(NO₃)₂, CuCl₂, Zn(OEt)₂, Cd(NO₃)₂, Ce(NO₃)₃, Lu(NO₃)₃, and Lu(NO₃)₂(OEt), or the [Cu₂I₂] cluster, generates a family of titanium-oxygen cluster (TOC)-based coordination polymers. These one-dimensional (1D) linear structures contain the same {Ti₈L₂} cluster but with variable bridging metal units. The regulation of the heterometal not only affects the chain geometries of the {MTi₈} but also affects the way the 1D chains are stacked in the crystal lattice. Investigation of the catalytic activities toward alcohol oxidation demonstrated the synergetic effect of combining the metal site and the photosensitive {Ti₈L₂} cluster in the tailored structure. Under light illumination, the {MTi₈} with dual catalytic sites shows greatly enhanced catalytic activity in the selective oxidation of alcohols to aldehydes. Because the compositions and structures of {MTi₈} are highly tunable, this work spotlights the potential of utilizing such metal-bridged multidimensional Ti-oxo materials for cooperative photoredox catalysis for organic transformation.

Permanent Porosity in Hydroxamate Titanium-Organic Polyhedra

Following the synthesis of hydroxamate titanium-organic frameworks, we now extend these siderophore-type linkers to the assembly of the first titanium-organic polyhedra displaying permanent porosity. Mixed-linker versions of this molecular cage (cMUV-11) are also used to demonstrate the effect of pore chemistry in accessing high surface areas of near 1200 m²·g^-1.

Cd-Doped Polyoxotitanium Nanoclusters with a Modifiable Organic Shell for Photoelectrochemical Water Splitting

Incorporating heterometal and chromogenic groups into the titanium oxo cluster (TOC) nanomaterials is one of the effective strategies for the development of new high-performance photoelectrically active materials. In this Article, we report the structures and photoelectrochemical (PEC) performances of a family of TOCs, including pure [Ti₁₂O₈(OEt)₁₆L₈] ({Me-Ti₁₂}) and six Cd-doped clusters formulated as [H₄Cd₂Ti₁₀O₈(OEt)₁₆(L)₈(H₂O)₂] ({Cd₂Ti₁₀}; L = salicylic acid and their derivatives). The six Cd-doped clusters are isostructural, containing the same {Cd₂Ti₁₀O₈} core, but are protected by salicylic ligands modified with different functional groups. The compositions, structures, and solution stability of these clusters have been studied in detail by single-crystal X-ray diffraction and electrospray ionization mass spectrometry measurements. The embedding of heterometallic Cd(II) and chemical modification of organic protective shells can effectively regulate the PEC water oxidation activity of those clusters, with {F-Cd₂Ti₁₀} having the highest turnover number of 518.55 and the highest turnover frequency of 172.85 h^-1. Our work highlights the potential of using TOCs that do not contain noble metals as water oxidation catalysts, and their catalytic activity can be regulated by structural modification.