Induced Aggregation, Solvent Regulation, and Supracluster Assembly of Aluminum Oxo Clusters.

ConspectusRecent years have witnessed the development of cluster materials as they are atomically precise molecules with uniform size and solution-processability, which are unattainable with traditional nanoparticles or framework materials. The motivation for studying Al(III) chemistry is not only to understand the aggregation process of aluminum in the environment but also to develop novel low-cost materials given its natural abundance. However, the Al-related clusters are underdeveloped compared to the coinage metals, lanthanides, and transition metals. The challenge in isolating crystalline compounds is the lack of an effective method to realize the controllable hydrolysis of Al(III) ions. Compared with the traditional hydrolysis of inorganic Al(III) salts in highly alkaline solutions and hydrolysis of aluminum trialkyl compounds conducted carefully in an inert operating environment, we herein developed an effective way to control the hydrolysis of aluminum isopropanol through an alcoxalation reaction. By solvothermal/low melting point solid melting synthesis and using "ligand aggregation, solvent regulation, and supracluster assembly" strategies, our laboratory has established an organic-inorganic hybrid system of aluminum oxo clusters (AlOCs). The employment of organic ligands promotes the aggregation and slows the hydrolysis of Al(III) ions, which in turn improves the crystallization process. The regulation of the structure types can be achieved through the selection of ligands and the supporting solvents. Compared with the traditional condensed polyoxoaluminates, we successfully isolated a broad range of porous AlOCs, including aluminum molecular rings and Archimedes aluminum oxo cages. By studying ring expansion, structural transformation, and intermolecular supramolecular assembly, we demonstrate unique and unprecedented structural controllability and assembly behavior in cluster science. The advancement of this universal synthetic method is to realize materials customization through modularly oriented supracluster assembly. In this Account, we will provide a clear-cut definition and terminology of "ligand aggregation, solvent regulation, and supracluster assembly". Then we will discuss the discovery in this area by using a strategy, such as aluminum molecular ring, ring size expansion, ring supracluster assembly, etc. Furthermore, given the internal and external pore structures, as well as the solubility and modifiability of the AlOCs, we will demonstrate their potential applications in both the solid and liquid phases, such as iodine capture, the optical limiting responses, and dopant in polymer dielectrics. The strategy herein can be applied to extensive cluster science and promote the research of main group element chemistry. The new synthetic method, fascinating clusters, and unprecedented assembly behaviors we have discovered will advance Al(III) chemistry and will also lay the foundation for functional applications.

Assembly of Homochiral Aluminum Oxo Clusters for Circularly Polarized Luminescence.

Chiral aluminum oxo clusters (cAlOCs) are distinguished from other classes of materials on account of their abundance in the earth's crust and their potential for sustainable development. However, the practical synthesis of cAlOCs is rarely known. Herein, we adopt a synergistic coordination strategy by using chiral amino acid ligands as bridges and auxiliary pyridine-2,6-dicarboxylic acid as chelating ligands and successfully isolate an extensive family of cAlOCs. They integrate molecular chirality, absolute helicity, and intrinsic hydrogen-bonded chiral topology. Moreover, they have the structural characteristics of one-dimensional channels and replaceable counteranions, which make them well combined with fluorescent dyes for circularly polarized luminescence (CPL). The absolute luminescence dissymmetry factor (glum) of up to the 10-3 order is comparable to several noble metals, revealing the enormous potential of cAlOCs in low-cost chiral materials. We hope this work will inspire new discoveries in the field of chirality and provide new opportunities for constructing low-cost chiral materials.

Designing External Pores of Aluminum Oxo Polyhedrons for Efficient Iodine Capture.

Although metal-organic polyhedra (MOPs) expansion has been studied to date, it is still a rare occurrence for their porous intermolecular assembly for iodine capture. The major limitation is the lack of programmable and controllable methods for effectively constructing and utilizing the exterior cavities. Herein, the goal of programmable porous intermolecular assembly is realized in the first family of aluminum oxo polyhedrons (AlOPs) using ligands with directional H-bonding donor/acceptor pairs and auxiliary alcohols as structural regulation sites. The approach has the advantage of avoiding the use of expensive edge-directed ditopic and face-directed tritopic ligands in the general synthesis strategy of MOPs. Combining theoretical calculations and experiments, the intrinsic relationship is revealed between alcohol ligands and the growth mechanism of AlOPs. The maximum I2 uptake based on the mass gain during sorption corresponds to 2.35 g g-1, representing the highest reported I2 sorption by an MOP. In addition, it can be easily regenerated and maintained the iodine sorption capacity, revealing its further potential application. This method of constructing stable and programmable porous materials will provide a new way to solve problems such as radionuclide capture.

Aluminum Molecular Ring Meets Deep Eutectic Solvents: Adaptive Assembly and Optical Behavior.

Different from the previous neutral reaction solvent system, this work explores the synthesis of Al-oxo rings in ionic environments. Deep eutectic solvents (DESs) formed by quaternary ammonium salts hydrogen bond acceptor (HBA) and phenols hydrogen bond donor (HBD) further reduce the melting point of the reaction system and provide an ionic environment. Further, the quaternary ammonium salt was chosen as the HBA because it contains a halogen anion that matches the size of the central cavity of the molecular ring. Based on this thought, five Al8 ion pair cocrystals were synthesized via "DES thermal". The general formula is Q+ ⊂ {Cl@[Al8(BD)8(µ2-OH)4L12]} (AlOC-180-AlOC-185, Q+ = tetrabutylammonium, tetrapropylammonium, 1-butyl-3-methylimidazole; HBD = phenol, p-chlorophenol, p-fluorophenol; HL = benzoic acid, 1-naphthoic acid, 1-pyrenecarboxylic acid, anthracene-9-carboxylic acid). Structural studies reveal that the phenol-coordinated Al molecular ring and the quaternary ammonium ion pair form the cocrystal compounds. The halogen anions in the DES component are confined in the center of the molecular ring, and the quaternary ammonium cations are located in the organic shell. Such an adaptive cocrystal binding pattern is particularly evident in the structures coordinated with low-symmetry ligands such as naphthoic acid and pyrene acid. Finally, the optical behavior of these cocrystal compounds is understood from the analysis of crystal structure and theoretical calculation.

Meltable Aluminum Molecular Rings with Fluorescence and Nonlinear Optical Properties.

Crystal-liquid-glass, which combines the tunable properties of crystalline compounds with the processability of glasses, has emerged as a new class of materials for fabricating bulk-shapable devices in real applications. Inspired by the characteristics of deep eutectic solvent (DES) mixtures involving significant depressions in melting points compared to their neat constituent components, in this study, we designed and synthesized the first examples of meltable aluminum oxo clusters (AlOCs) via lattice doping with DESs at the molecular level. The abundant and strong hydrogen bonding between the aluminum molecular ring, DES components, and lattice solvents is postulated to be the root that affords melting point depressions and, thus, "melting" clusters. We prepared a transparent bubble-free glass film under autogenous pressure using a hot-press method. These cluster-based films exhibited luminescent and nonlinear optical properties similar to those of pristine crystalline compounds. Our study belongs to the interdisciplinary disciplines of chemistry and physics. It not only breaks the limitations of crystalline glass on metal and ligand types but also acts as a general guide for extending the range of meltable crystalline materials.

Application of Hard and Soft Acid-base Theory to Construct Heterometallic Materials with Metal-oxo Clusters.

Heterometallic cluster-based materials offer the potential to incorporate multiple functionalities, leveraging the aggregation effects of clusters and translating this heterogeneity and complexity into unexpected properties that are more than just the sum of their components. However, the rational construction of heterometallic cluster-based materials remains challenging due to the complexity of metal cation coordination and structural unpredictability. This minireview provides insights into a general synthetic strategy based on Hard and Soft Acids and Bases (HSAB) theory, summarizing its advantages in the designed synthesis of discrete heterometallic clusters (intracluster assembly) and infinite heterometallic cluster-based materials (intercluster assembly). Furthermore, it emphasizes the potential to exploit the intrinsic properties of mixed components to achieve breakthroughs across a broad range of applications. The insights from this review are expected to drive the progress of heterometallic cluster-based materials in a controllable and predictable manner.

Designed Synthesis of an Aluminum Molecular Ring Based Rotaxane and Polyrotaxane.

Mechanically interlocked molecules, such as rotaxanes, have drawn significant attention within supramolecular chemistry. Although a variety of macrocycles have been thoroughly explored in rotaxane synthesis, metal-organic macrocycles remain relatively under-investigated. Aluminum molecular rings, with their inner cavities and numerous binding sites, present a promising option for constructing rotaxanes. Here, we introduce an innovative "ring-donor⋅⋅⋅axle-acceptor" motif utilizing Al8 molecular rings, enabling the stepwise assembly of molecules, complexes, and polymers through tailored coordination chemistry. This novel approach can not only be applied to macrocycle-based systems like catenanes but also enhance specific functionalities progressively.

Cluster-Based Crystalline Materials for Iodine Capture.

The treatment of radioactive iodine in nuclear waste has always been a critical issue of social concern. The rational design of targeted and efficient capture materials is of great significance to the sustainable development of the ecological environment. In recent decades, crystalline materials have served as a molecular platform to study the binding process and capture mechanism of iodine molecules, enabling people to understand the interaction between radioactive iodine guests and pores intuitively. Cluster-based crystalline materials, including molecular clusters and cluster-based metal-organic frameworks, are emerging candidates for iodine capture due to their aggregative binding sites, precise structural information, tunable pores/packing patterns, and abundant modifications. Herein, recent progress of different types of cluster materials and cluster-dominated metal-organic porous materials for iodine capture is reviewed. Research prospects, design strategies to improve the affinity for iodine and possible capture mechanisms are discussed.

Supramolecular Chemistry of Windmill-like Al10 Clusters: Adjustable Guests and Hydrogen-Bonding Networks for Enhanced Optical Limiting.

The interest in cluster chemistry lies not only in the development of new types of geometric structures but also in the higher-level connectivity and assembly of clusters at the supramolecular level. Here, we report a novel windmill-like Al10 cluster and consider this geometrically unique cluster as an anionic node assembled together with different cationic guests such as imidazolium and guanidinium. These guests with different hydrogen-bond angles can help to obtain a series of diverse hydrogen-bonding networks and then manipulate the stacking mode of hosts and guests. Furthermore, we realized a supramolecular approach to fine-tune the optical limiting properties of the cluster. This work not only enriches the host-guest chemistry of ionic windmill-like clusters but also opens up more possibilities for aluminum oxo cluster-based hydrogen-bonded frameworks.

Supracluster Assembly of Archimedean Cages with 72 Hydrogen Bonds for the Aldol Addition Reaction.

Clusters that can be experimentally precisely characterized and theoretically accurately calculated are essential to understanding the relationship between material structure and function. Here, we propose the concept of "supraclusters", which aim to connect "supramolecules" and "suprananoparticles" as well as reveal the unique assembly behavior of "supraclusters" with nanoparticle size at the molecular level. The implementation of supraclusters is full of challenges due to the difficulty in satisfying the ordered connectivity of clusters due to their abundant and dispersed hydrogen bonding sites. By solvothermal synthesis under a high catechol (H2 CATs) content, we successfully isolated a series of triangular {Al6 M3 } cluster compounds possessing brucite-like structural features. Interestingly, eight {Al6 M3 } clusters form 72-fold strong hydrogen bonding truncatedhexahedron Archimedean {Al6 M3 }8 supracluster cage (abbreviated as H-tcu). Surprisingly, the solution stability of the H-tcu was further proved by electrospray ionization mass spectrometry (ESI-MS) characterization. Therefore, it is not difficult to explain the reason for assembly of H-tcu into edge-directed and vertex-directed isomers. These porous supraclusters can be obtained by scale-up synthesis and exhibit a noticeable catalysis effect towards the condensation of acetone and p-nitrobenzaldehyde. As an intermediate state of supramolecule and suprananoparticle, the supracluster assembly can enrich the cluster chemistry and bring new structural types.

Record Aluminum Molecular Rings for Optical Limiting and Nonlinear Optics.

Crystalline cluster materials, a class of functional motif aggregations, provide a great opportunity for tuning the properties stemming from the flexible and accurate variation of inorganic and organic compositions. In this study, we demonstrate the effects of functional ligand and ring size regulation on the structures and third-order nonlinear optical (NLO) properties. Revealed by the single-crystal X-ray analysis results, aluminum molecular ring expansion is achieved by 2×9 and 3×6 strategies. In terms of the given organic shells, we further tuned the aluminum molecular ring sizes from 3.0 nm to 1.7 nm. The picosecond Z-scan measurements results revealed that the third-order NLO performances do not only depend on the general conjugate interactions but are also related to hydrogen bonding, polarizability, and ring sizes. The large nonlinear absorption coefficient and onset prove that the observed samples are promising candidates for the field of nonlinear optics.

Mesoporous Assembly of Aluminum Molecular Rings for Iodine Capture.

The effective capture and storage of radioiodine are of worldwide interest for sustainable nuclear energy. However, the direct observation of ambiguous binding sites that accommodate iodine is extremely rare. We presented herein a crystallographic visualization of the binding of iodine within mesoporous cages assembled from aluminum molecular rings. These nanocages are formed through π-π interactions between adjacent aluminum molecular rings. Compared with the general nanotubes arrangement, the supramolecular nanocage isomer exhibits better iodine adsorption behavior. The robust molecular nanocages demonstrate a high iodine vapor saturation uptake capacity of 50.3 wt % at 80 °C. Furthermore, the resulting adsorbent can be recycled. Single-crystal X-ray diffraction reveals binding sites of molecular I2 within the pores of the phenyl-based linkers stabilized by the strong I···π interactions. These compounds represent an excellent model to deduce the trapping mechanism of guest molecules interacting with the host. In addition, this work develops a promising cluster-based aluminum material as iodine adsorbents.

Synthesis, Structures, and Fluorescence Properties of Dimeric Aluminum Oxo Clusters.

Aluminum is an important component for luminescence. However, the fluorescent aluminum complex with unambiguous structural information is still limited. Herein, we report a series of fluorescence aluminum oxo clusters (AlOCs). By introducing an additional coordination site to the aromatic conjugation ligand, cluster nuclearity increment and fluorescence variation are observed. Al8(OH)2(µ4-O)2(1-NA)2(OEt)16 (AlOC-41, 1-NA = 1-naphthoic acid, OEt = ethanol) is made up of two tetrahedral subunits. By introducing an additional coordination site to the aromatic conjugation ligand, we isolate a high nuclearity compound Al10(µ3-O)2(3-HNA)2(OEt)22 (AlOC-47, 3-HNA = 3-hydroxy-2-naphthoic acid). Correspondingly, their luminescence performance is different (blue fluorescence in AlOC-41 and green in AlOC-47). Present herein is a platform to illustrate the relationship between synthesis, structure, and fluorescence properties.

Designable Assembly of Aluminum Molecular Rings for Sequential Confinement of Iodine Molecules.

Although numerous adsorbent materials have been reported for the capture of radioactive iodine, there is still demand for new absorbents that are economically viable and can be prepared by reliable synthetic protocols. Herein, we report a coordination-driven self-assembly strategy towards adsorbents for the sequential confinement of iodine molecules. These adsorbents are versatile heterometallic frameworks constructed from aluminum molecular rings of varying size, flexible copper ions, and conjugated carboxylate ligands. Additionally, these materials can quickly remove iodine from cyclohexane solutions with a high removal rate (98.8 %) and considerable loading capacity (555.06 mg g-1 ). These heterometallic frameworks provided distinct pore sizes and binding sites for iodine molecules, and the sequential confinement of iodine molecules was supported by crystallographic data. This work not only sets up a bridge between molecular rings and infinite porous networks but also reveals molecular details for the underlying host-guest binding interactions at crystallographic resolution.

Designable Al32 -Oxo Clusters with Hydrotalcite-like Structures: Snapshots of Boundary Hydrolysis and Optical Limiting.

The hydrolysis of earth-abundant AlIII has implications in mineral mimicry, geochemistry and environmental chemistry. Third-order nonlinear optical (NLO) materials are important in modern chemistry due to their extensive optical applications. The assembly of AlIII ions with π-conjugated carboxylate ligands is carried out and the hydrolysis and NLO properties of the resultant material are studied. A series of Al32 -oxo clusters with hydrotalcite-like cores and π-conjugated shells are isolated. X-ray diffraction revealed boundary hydrolysis occurs at the equatorially unsaturated coordination sites of AlIII ions. Charge distribution analysis and DFT calculations support the proposed boundary substitution. The Al32 -oxo clusters possess a significant reverse saturable absorption (RSA) response with a minimal normalized transmittance up to 29 %, indicating they are suitable candidates for optical limiting (OL) materials. This work elucidates the hydrolysis of AlIII and provides insight into layered materials that also have strong boundary activity at the edges or corners.

Stepwise Coordination Assembly Approach toward Aluminum-Lanthanide-based Compounds.

Present herein is a stepwise assembly method toward aluminum-lanthanide-based (Al-Ln) compounds. From the perspective of charge balance, polyanions are necessary to bind with Ln ions. However, the synthesis of polyanions aluminum compounds remains quite challenging. Herein, two Al4 polyanions [Al4(L)4(Cat)2]·4Hdma (AlOC-13, H3L= 2,3-dihydroxybenzoic acid, Cat = catechol, and dma = dimethylamine) and [Al4(L)4(HL)2(DMF)2]·4Hdma·0.5DMF·0.5H2O (AlOC-14, DMF = N,N-dimethylformamide) were successfully obtained under solvothermal conditions. Catechol and Hdma were generated from the in situ decarboxylation of H3L ligand and decomposition of DMF, respectively. AlOC-13 is qualified for further coordination assembly for the available vacancy coordination sites, good water solubility, and scale-up synthesis. The assembly of Al4 polyanions with equivalent Ln ions afforded a series of zigzag chain structures [LnAl4(L)4(Cat)2(DMF)2(H2O)3]·Hdma (Ln = Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) (AlOC-13-Ln). Moreover, the magnetic behavior and photoluminescence of the series of AlOC-13-Ln were also studied. AlOC-13-Dy shows obvious antiferromagnetic behavior, while AlOC-13-Tb exhibits excellent green characteristic luminescence. This study not only paves the way toward anionic aluminum clusters but also reveals their potential application in water treatment of cationic metal ions capture.

Synthesis of Ag-Doped Polyoxotitanium Nanoclusters for Efficient Electrocatalytic CO2 Reduction.

Ag-Ti nanocomposite materials have drawn increasing research attention because of their superior catalytic properties. However, the preparation of a crystalline Ag-Ti material is an important challenge in synthetic chemistry. Herein, we report a family of atomically precise Ag-doped polyoxotitanium nanoclusters (PTCs) (PTC-253-PTC-256) with a size of 19.56 × 19.02 Å. Each Ag-PTC is made up of a tiny Ag2 kernel and a double-decker Ti12 nanowheel as well as an organic protective shell. Hence, they can be regarded as Ag2@Ti12@(L)14(OMe)n unique triple core-shell structures. Notably, the peripheral organic shell can be modified with different benzoate derivatives. With precise atomic information, these compounds can be used as ideal molecular models of Ag-Ti nanocomposite materials for studying the growth or reaction mechanism via theoretical calculations. Meanwhile, a PTC-255-modified electrode presents efficient electrocatalytic CO2 reduction activity with a Faradaic efficiency (FE) of 29.4%. This work demonstrates that Ag-doped crystalline PTC materials are promising candidates for application to the electrocatalytic CO2 reduction reaction (CO2RR).

Designable Aluminum Molecular Rings: Ring Expansion and Ligand Functionalization.

Presented herein are the AlIII molecular ring architectures from 8-ring to 16-ring. Although there are numerous reported cyclic coordination compounds based on transition metals, gallium, or lanthanides, the Al versions are less developed due to the fast hydrolysis nature of Al3+ ion. With the assistant of monohydric alcohols, a series of atomic precisely Al molecular rings based on benzoates are synthesized. The ring expansion of these Al-rings from 8-ring to 16-ring is related to the monohydric alcohol structure-directing agents. Moreover, the organic ligands on the Al-rings can be modified by using various benzoate derivatives, which lead to tunable surface properties of the Al-rings from hydrophilicity to ultra-hydrophobicity. Importantly, 4-aminobenzoic acid bridged 16-ring is soluble in organic solvents and exhibits high solution stability revealed by mass spectroscopy. Ligand substitution also can be performed between these Al-rings, which reveal controllable ligand functionalization of these Al-rings.

Induced Aggregation and Synergistic Coordination Strategy in Cluster Organic Architectures.

Developments in strategies for the synthesis of oxo-hydroxo lanthanide (Ln) clusters during the past few decades have resulted in abundant relevant crystalline materials that exhibit attractive structures and interesting properties. The combination of these oxo-hydroxo Ln clusters and organic ligands has led to the formation of new extended arrays of Ln cluster organic frameworks (LnCOFs). In contrast to metal-organic frameworks, the incorporation of particular characteristics of clusters provides the opportunity to develop performances not available in single-metal compounds. Even with steady advances in oxo-hydroxo Ln clusters, progress in LnCOFs is less developed. To obtain LnCOFs, one premise is to induce the oxophilic Ln ions undergoing aggregation. Meanwhile, the organic ligands should have extra coordination sites for further expansion. Multidentate organic ligands like pyrazinecarboxylic acid and pyridinecarboxylic acid containing O and N donors will meet these two requirements. Their carboxyl groups will induce the aggregation of Ln ions, while the N donors can serve as potential extension sites. To make more open frameworks or if the oxo-hydroxo Ln clusters fail to be congregated or connected, then a second ligand is necessary. The introduction of the suitable second ligand may occupy a partial coordination sphere of Ln ions and ultimately benefit the connection process. In this Account, we introduce the origin and evolution of the induced aggregation and synergistic coordination strategy. According to the attributes of the organic ligands in the documented LnCOFs, we classify them into linear and nonlinear groups in the second and third parts. From the aspect of ligand-induced aggregation, isonicotinic acid (HIN) and lengthened 4-(4-pyridyl)benzoic acid (HPBA) ligands as well as their nonlinear analogues are settled as typical models. From the aspect of synergistic coordination, chelating ligands like 1,2-benzenedicarboxylic acid (1,2-H2BDC) and acetic acid (HOAc) play significant roles. Moreover, three types of synergistic coordination are discussed in detail: synergistic coordination between two types of organic ligands, synergistic coordination between organic and inorganic ligands, and simultaneous synergistic coordination of aforementioned two types. From the aspect of LnCOF products, in addition to traditional pure LnCOFs, new types of heterometallic frameworks containing two types of cluster building units have been developed. Although this Account focuses on the nuclearity and coordination aspects of LnCOFs, we anticipate that it will stimulate more efforts in the further study of their properties beyond the exploratory synthesis. More importantly, synergistic coordination may be applied to other systems and inspire crystal design and targeted assembly of new functional materials.

Amino-Polyalcohol-Solvothermal Synthesis of Titanium-Oxo Clusters: From Ti6 to Ti19 with Structural Diversity.

As is known, amino-polyalcohol ligands are usually used as sacrificial agents in photocatalytic reactions. And polyoxo-titanium clusters (PTCs) are molecular modes of titanium dioxide, one of the most important photocatalysts. In order to help understand the intrinsic coordination feature of amino-polyalcohol toward titanium-oxo species, we carried out the research of constructing PTCs with amino-polyalcohol ligands. In view of the low melting point of amino-polyalcohol, they have been directly applied as reaction mediums. Under this new synthetic method, a series of PTCs have been successfully obtained, namely, Ti6(µ2-O)2(OCH2CH2O)2(O iPr)4(dea)6·HO iPr (deaH2 = diethanolamine, PTC-171), Ti9(µ2-O)2(µ3-O)4(tea)2(O iPr)8(dea)5(teaH3 = triethanolamine, PTC-172), Ti11(µ3-O)10(µ4-O)(O iPr)14(dea)4 (PTC-173), Ti19(µ2-O)6(µ3-O)12(dea)18Cl4 (PTC-174), and Ti19(µ2-O)6(µ3-O)12(dea)18(NO3)4·(H2O)6 (PTC-175). Their structures are determined by single-crystal X-ray diffraction analysis. It is worth noting that PTC-174 and PTC-175 are not only the first Ti19 examples of crystalline PTCs, but also currently the largest PTCs compounds in the amino-polyalcohol system. Moreover, the solid-state UV-vis spectra of these PTCs were recorded. And their applications in photocurrent responses were also investigated. This work provides an interesting method for the preparation of amino-polyalcohol base PTCs and would also benefit the mechanism interpretation of the photocatalytic processes of titanium oxide materials.