Aggregation of titanium‐oxo clusters

Compared with nanoparticle‐aspect relatives titanium dioxide (TiO2), titanium‐oxo clusters (TOCs) are atomically structural‐determined and can be further precisely modified through coordination and supramolecular chemistry. Another parallel research direction is titanium‐based metal‐organic frameworks, and those based on TOC have attracted particular attention because of their high optical performances resulting from the cluster aggregation effect. Though challenging, assembling macro‐materials from specific clusters helps establish the assembly chemistry of clusters and incorporates porous and flexible characteristics into a single bulk material. Although separate reviews are reported in these two branches, no comprehensive review is available to highlight the bridges between them. Herein, we review and summarize the development and progress of new aggregation of TOCs, from intramolecular unique cluster aggregation to hierarchical intermolecular aggregation via covalent forces, coordination bonds, and non‐covalent forces using the specific clusters as precursors. We hope this review fills the gap in the methodology of assembling particular‐aggregated TOCs and their derived frameworks, providing general guidance to researchers interested in this area.


INTRODUCTION
[3] The structure can be precisely confirmed to benefit from the high crystallinity and stands a chance of revealing the realistic catalytic reaction mechanism at the atomic level. [4]As a structural mode of titanium dioxide (TiO 2 ), titanium-oxo clusters (TOCs), also denoted as polyoxo-titanium clusters (PTCs) or polyoxotitanates (POTs), can act as a platform to study the catalytic performance permitted by their accessible structure. [5]8][9][10][11][12] Till now, great advances have been made in constructing TOC with comparable size to TiO 2 such as Ti 44 [13] and Ti 52 . [14]One of the biggest challenges in TOC synthesis is the water sensitive of Ti ions, which may cause drastic hydrolysis and structure corruption.This problem can be solved by introducing hydrophobic ligands to construct a protective surface. [6]Design methodology and synthetic routes advance inspire the extension of TOCs to framework materials.For instance, titanium-based metalorganic frameworks (Ti-MOFs) are the most famous and most investigated among TOC-frameworks such as MIL-125, [15] NH 2 -MIL-125, [16] MOF-901, [17] MOF-902, [18] NTU-9, [19] MUV-11, [20] MIL-101(Ti), [21] FIR-125, and FIR-126. [22]everal reviews on two branches of TOCs and titaniumbased frameworks have been reported thus far (Scheme 1).C. Sanchez summarized titanium-alkoxy clusters with different nuclearities and described the correlative structures of building units and clusters after expanding in 2011. [23]n 2014, Coppens et al. discussed the structures and photochemical properties of TOCs with nuclearities of 11 and larger. [4]It follows two critical reviews on metal-doped and ligand-modified TOCs by D. S. Wright et al. [24,25] In 2018, our group summarized TOCs from their synthesis, structures, and properties. [26]Because of the importance of heterometallic doping and ligand modification in modulating TOC properties, [27][28][29] four different groups focused on this area and published reviews about various doping metals and functional ligands during 2020-2021.For example, our group summarized concentrates on doped TOCs, [30] while T. Gunji, [31] U. Schubert, [32] and J. Dai [33] discussed phosphorus-substituted ligands, bi-/tridentate ligands, and photoactive ligands, respectively.Besides, Y. F. Wang's group discussed supramolecular interactions, mainly about the host-guest systems in TOC structure. [34]Moreover, the synthesis of TOCs is also of great concern.We published a review in 2022 discussing new synthetic methods. [35]s for titanium-based frameworks, researchers are more interested in their applications, especially photocatalytic applications.In 2017, H. L. Nguyen discussed the synthesis, structures, and applications of Ti-MOFs. [36]Y. L. Zhao, [37] T.-F.Liu, [38] and X. Peng [39] summarized the photocatalysis behaviors of Ti-MOFs and derived materials, and related reviews were published in 2018 and 2020.Y. Q. Peng and co-workers introduced the biomedical applications of Ti-MOFs, such as antibacterial treatments and cancer therapy, in 2022. [40]Very recently, H.-L. Jiang et al. discussed Ti-MOFs' applications in thermal catalysis, photocatalysis, and electrocatalysis. [41]t should be noted that the developments of these two branches of TOCs and Ti-MOFs are closely related and complement each other.On the one hand, prepared TOCs can be further extended into Ti-MOFs to obtain new MOF structures.On the other hand, the building blocks of numerous Ti-MOFs are oxo-cluster like Ti 8 O 8 (OH) 4 in MIL-125. [15]Many researchers have noticed this overlap, and several reviews contain both TOC and Ti-MOF.In Sanchez's review, they illustrated the construction strategy to transform TOCs into inorganic-organic hybrids. [23]Serre et al. summarized TOCs and Ti-MOFs assembled from different types of ligands in 2017. [42]In 2020, Q. C. Zhang discussed Ti-MOFs' properties from the viewpoint of TOC units they were obsessed. [43]n 2021, Y. F. Wang introduced the supramolecular chemistry of TOCs and mentioned TOC-based supramolecular frameworks. [34]Although the overlap has been noticed, the published reviews discuss TOCs and Ti-MOFs individually based on coordination numbers, ligands, and nuclearities.There is still room for us to summarize them from a new perspective.
Compared with the serendipitous one-step synthesis of Ti-MOF, the pre-designed cluster can be used to achieve directional material assembly.However, there is no systematic summary of this aspect.Herein, we expect to build a bridge between the two branches of TOC and Ti-MOF.In this review, we focus on the aggregation features of TOCs developed in recent years, including two aspects: (1) summarize the intramolecular aggregation of TOC with novel configurations that arose recent years, such as ball-like, polyhedron-like, and wheel-like clusters (Table 1); (2) Then, based on the understanding of structural characteristics, we chose some representative TOCs as examples to introduce the methodology to expand TOCs into framework materials by controlling intermolecular aggregation through coordination, covalent, and non-covalent interactions.Intramolecular and intermolecular aggregations are strategies employed to prepare intramolecular cages (peculiar in unique shapes) or intermolecular frameworks (permanent pore architecture), respectively.Several related works are highlighted to discuss fascinating construction and functionalization methods.We hope this review can fill the gap between TOCs and their framework materials and give up-to-date guidance to readers interested in this area.

SYNTHETIC STRATEGIES
IVB metal cations have the nature of a high charge-radius ratio compared with others in the same period.Thus, they are a group of hard acids that strongly interact with hard bases such as, O 2− according to the Hard-Soft Acid-Base (HSAB) theory. [91]This phenomenon has an even stronger reflection on Ti(IV), which has the smallest radius among IVB cations.Rapid hydrolysis is one of the biggest challenges during TOC synthesis.Therefore, stepwise synthetic strategies are employed to avoid hydrolysis and promote crystallization.

One-pot solvothermal synthesis
A solvothermal method is one of the most common techniques employed in the synthesis of crystalline materials.
Glassy bottles are commonly used as containers but can only be utilized in mild conditions due to their low mechanical strength and cap material.For example, the temperature should be set under 120 • C, and the pressure should not be high.Reaction kettles made from Teflon are required if the reaction condition is harsh.[94] This strategy has proved to be a simple but effective way widely accepted by laboratories worldwide.A glove box with inert gas protection is required if the reagents are O 2 -sensitive or have excessively high water affinity.
It is worth noting that hydrolysis of Ti precursors is the main process to assemble Ti ions into cluster form, and water is an indispensable reactant to provide the μ-O moiety in TOC structure. [14]Esterification is a reversible reaction that can produce water smoothly; thus, it can be utilized as a water source in TOC synthesis.This strategy is known as coordination-delayed-hydrolysis (CDH).Readers can refer to the review reported by our group in 2022. [35]Basically, this method requires alcohol as a solvent and carboxylic acid as a regulator.For instance, isopropanol and formic acid can be used as a CDH system.The esterification provides molecular water needed during the whole synthesis.With the steady hydrolysis of Ti precursors, TOC cores are produced.][97] Based on different growing conditions, the dosage of alcohol and carboxylic acid is supposed to be optimized to fit the true growth situation of TOCs.
In many circumstances, the combination of multiple synthetic strategies should be considered when an independent strategy cannot obtain crystals suitable for single-crystal X-ray diffraction (SCXRD).For instance, evaporative crystallization is effective when the solution is clear but no single crystal is formed.This phenomenon is because the TOC concentration in solution is probably too low or the solubility is too high to produce single crystals.TOC can reach saturation and crystallize from the liquid phase through evaporating solvent. [98]A B L E 1 Titanium-oxo clusters with special aggregation shape: formula, nuclearity, space group, and CCDC/ICSD number.a

Two-step synthesis
The growth of single crystals is often serendipitous when based on one-pot synthesis.Post-synthetic-modification (PSM) toward as-prepared building blocks or post-processing of the precipitation or powder produced during one-pot synthesis is needed.This kind of stepwise approach, denoted as two-step synthesis, has the advantages of being controllable and flexible.Typically, TOCs are dissolved in or immersed in a ligand solution to conduct ligand exchange.The concentration of ligand solution should be high to provide the driving force during the shifting of coordination-equilibrium.
In 2012, Coppens et al. successfully obtained various ligandsmodified Ti 17 -oxo clusters utilizing this method. [56]Our group synthesized a series of phenol-based TOCs via PSM in 2019. [96]The as-prepared TOC substrates are mixed with phenol and heated in the oven.The temperature is higher than 43 • C, at which phenol solids will be molten and act as a solvent, offering enough concentration for ligand exchange.Thus, this method is also denoted as phenol-thermal synthesis.In addition to PSM, Gunji and his co-workers concentrated the white precipitation that appeared during synthesis and then stirred and heated until the precipitation dissolved and crystallized. [99] I G U R E 1 Representative core structure of (A) polyhedron-like, (B) ball-like and (C) wheel-like TOCs.Color codes: green, Ti; red, O. (D) Representative heterometallic TOC wheels.Reproduced with permission. [83,86,88,89,90]Copyright 2020, Elsevier B.

TOCS AGGREGATED IN A SPECIAL CONFIGURATION
Compared with randomly arranged classic clusters, TOCs with special aggregation have the unique advantage of high structure-regularity.Ti ions in special aggregation can be classified into limited chemical environments, which offer convenience in investigating their behaviors in assembly and applications.In addition, they usually exhibit cage structures with cavities inside, providing excellent platforms for various host-guest systems.By employing intramolecular aggregation, TOCs with special configurations such as ball-like, polyhedron-like and wheel-like can be readily prepared.

Ball-like aggregated TOCs
TOCs with ball-like aggregated shapes are one of the most classic members of the whole TOC family.Multiple Ti, O, and ligands coordinate with each other and form a ball-like molecular cage with a cavity inside.This kind of aggregated structure is widely observed in titanium-alkoxide clusters.In this section, we summarized a series of particular-aggregated TOCs, from low-nuclearity Ti 4 to high-nuclearity Ti 42 , and discussed their constitutions and structures.Several basic types of TOC cores are shown in Figure 1.

Ti 17-19 -oxo cluster
The Ti 17 /Ti 18 family is an essential branch in ball-like aggregated clusters, exhibiting a more rounded Keggin structure than Ti 12 (Figure 3A).In 1999, C. Sanchez et al. synthesized a titanium-alkoxide cluster with a Ti 17 core, denoted as Ti 17 O 24 (O i Pr) 20 . [53]Ti 17 core comprises four types of Ti ions: a four-coordinated Ti ion at the center, four fivecoordinated and four six-coordinated Ti ions in the middle of the outer shell, and eight six-coordinated Ti ions at the bottom and the top (Figure 3B).The accessibility of ligand-exchange is closely related to coordination environments, where five-coordinated Ti ions are relatively easy to perform ligand modification.Inspired by dye-sensitized solar cell (DSSC) and to enhance the photoactivity, a series of chromophore ligands were employed to assemble Ti 17 -L clusters (Figure 3C). [56,58,59]For example, Coppens et al. chose isonicotinic acid, catechol, [54] and p-nitrophenyl acetylacetone [55] to increase the utilization of light in Ti 17 .Despite direct ligand exchange, J. Dai et al. utilized transition metals as bridging units to introduce chromophores. [57]The F I G U R E 2 Crystal structure of (A) Ti 11 O 13 O i Pr 18 , titanium atoms are represented by small filled spheres, oxide oxygens by large open spheres, and alkoxide oxygens by large shaded spheres.Reproduced with permission. [45]opyright 1993, American Chemical Society.Color codes: green, Ti; red, oxygen; gray, C. (E) Photocurrent using various catalysts in ref. [46].Conditions: 1.5 mM in isopropanol, bias = 0.7 V.A high-pressure xenon lamp was used to simulate solar irradiation.No photocurrent was measurable by visible light irradiation (>400 nm).Reproduced with permission. [46]Copyright 2016, American Chemical Society.(F) The stacking of [Ti 12 + Ti 6 ] in one unit cell.Reproduced with permission. [52].Copyright 2023, The Royal Society of Chemistry.
photoactive TOCs can be further used in the conversion of sunlight to electricity.Ball-like Ti 18 -oxo clusters have a similar structure to Ti 17 .The first ball-like example is Ti 18 O 22 (O n Bu) 26 (acac) 2 (acac = acetylacetonates) synthesized by C. Sanchez's group in 1991. [60]In 1996, C. F. Campana et al. obtained another Ti 18 structure using HO t Bu. [61] Coppens et al. assembled Ti 18 with an acetate ligand in 2010. [62]These TOCs are both generated with the same Ti 18 core.In 2021, a novel [Ti 18 O 27 (PhCOO) 24 (en) 9 ] cage constructed from benzoic acid and ethylenediamine was prepared by X.-M.Zhang. [63]he direct input of nitrogen moieties into TOC skeleton remarkably enhanced the photoactivity of Ti 18 , reducing the band gap to 2.4 eV.Very recently, our group obtained Ti 19 O 29 (phen) 2 (O i Pr) 18 , a new member of the ball-like TOC family, through ionothermal synthesis (Figure 3A). [59]lthough it has been more than 30 years since the first discovery of ball-like TOCs, the exploration is still on its way.3D) with a ball-like cage structure. [48]The Ti 22 cage was constructed with meticulous control of reaction time.3E), [64] obtained by reacting Ti(O i Pr) 4 and HO i Pr with the assistance of formic acid.In the preparation of Ti 42 , the reaction time is four days, which is adequate for forming a large-sized TOC core.Ti 42 is also the first example of a fullerene-like TOC with similar symmetry (I h ) and linking mode to C 60 .Remarkably, carboxylic acid added to the reaction system acts as a regulator, effectively controlling the aggregation rate and morphology of TOCs.The impact of acid modulators was also demonstrated in the synthesis of Ti 44 in 2019. [13]

Polyhedron-like aggregated TOCs
Ti 4 tetrahedron is the tiniest polyhedron in the TOC family, synthesized by Raymond et al. in 1998 [101] and 2005. [102]In 2017, our group considered the configuration and proposed that ligands with twofold symmetry could act as structuredirecting agents (SDAs) to induce tetrahedron formation. [65]hus, embonic acid with twofold symmetry was employed, and a series of Ti 4 L 6 tetrahedra was successfully prepared (Figure 4A).Embonic acid maintains the tetrahedron backbone and provides unsaturated coordination oxygen sites on the vertexes of Ti 4 L 6 , constructing calixarene-like cavities that could readily capture other metal ions that could extend to 3D frameworks. [12]Ti 4 L 6 also assembles a tetrahedron cage with abundant host-guest interacting sites that could be utilized in loading guest molecules. [66]Furthermore, the ligands could be changed to chiral ligands to prepare Ti 4 L 6 with all metal centers having the same chiral configuration that could be further applied to chiral recognition and separation. [67]he next polyhedron with a regular shape is the Ti 8 coordination cube.The first TOC with a Ti 8 cubic core is [Ti 8 O 12 (H 2 O) 24 ]Cl 8 synthesized by M. G. Reichmann et al. in 1987. [68]In 2018, our group used a ligand-directing In the polyhedral view, the isopropyl groups are selectively omitted.The inset shows the binding mode of catecholate-O to Ti.Copyright 2016, American Chemical Society. [48](E) Molecular structure of cluster Ti 42 O 60 (O i Pr) 42 (OH) 12 .The seven-coordinate Ti atoms, five-coordinate Ti atoms, and O atoms are presented in blue, green, and red, respectively.H and C atoms have been omitted for clarity.Copyright 2016, American Chemical Society. [64]rategy and selected 2,6-dihydroxybenzoate SDA to promote Ti ions aggregating in cubic form (Figure 4B). [69]The cubic cage inside the as-prepared Ti 8 L 12 is available for various guest molecules.By changing the reaction medium, we proved that the captured guests were closely related to the solvent (Figure 4C). [50]Compared with ball-like TOCs, the formation of polyhedrons largely relies on SDAs due to the relatively long distance between Ti ions.Well-designed ligands with elaborate spatial arrangements that can induce specific placement of Ti ions are the key to assembling TOC polyhedrons.

Wheel-like aggregated TOCs
Since the discovery of crown ether, the molecular wheel has become an important branch of supramolecular chemistry and has attracted considerable attention.The rich host-guest chemistry in molecular wheels stems from their designable cavity size and active sites, which can be further utilized to recognize and capture specific guest molecules.Compared with a traditional organic molecular wheel like crown-ethers, hybrid metal-organic clusters more easily combine the advantages of organic and inorganic moieties to provide more active sites.The pre-designed structures with targeted functions are also more accessible via coordination chemistry methods.

Ti 8 -oxo wheel and its host-guest system
The first wheel-like TOC is  [70] In Ti 8 -oxo wheels, neighboring Ti ions Assembly of the tetrahedral M 4 L 6 cage with calixarene-like coordination-active vertices for potential metal ion trapping.Reproduced with permission. [65]Copyright 2017, American Chemical Society.(B) Structure of the free Ti 8 L 12 cube in PTC-97.(C) 2,6-Dihydroxybenzoic acid-stabilized titanium-organic cages and related supramolecular derivatives.Reproduced with permission. [50]Copyright 2020, American Chemical Society.
When Wallbridge conducted the research on SCXRD of asprepared Ti 8 -oxo wheels, two toluene molecules were found being captured in the wheel-like cavity. [70]In 2008, D. B. Dell'Amico et al. also found the captured diethylammonium cations in Ti 8 O 8 (O 2 CNEt 2 ) 16 . [72]These guests all interact with the inner oxygen site of the Ti 8 wheel, inspiring further development in the Ti 8 -G (G refers to guest molecules) host-guest system.For instance, Winpenny focused on the assembly of Ti 8 rotaxane interlock system.They extended the length of guest molecules to realize rotaxanes that multi-Ti 8 wheels crossed by one guest.With the success of preparing n Pr 2 NH 2 @Ti 8 in 2017 (Figure 5A), [76] they obtained a series of [n]rotaxane by employing long-chain diamine as templates in 2018. [84]Large groups were modified at each end of the diamine as stoppers.Heterometallic doping was employed to increase the host-guest interactions, which will be further discussed in Section 3.4.In 2021, Y. F. Wang's group constructed both catenane and rotaxane via Ti 8 O 8 (SO 4 ) 16 and polyamines (Figure 5E). [106]A cyclic polyamine, C 30 N 8 was employed to prepare Ti 8 ⋅C 30 N 8 cate-nane, the first organic-inorganic catenane synthesized under aqueous conditions.They noticed that the hydrophilic alkylammonium groups always thread Ti 8 -wheel from the center, while hydrophobic benzene rings arranged both the upper and lower sides of Ti 8 , indicating the higher affinity between Ti 8wheel and cationic ammonium groups.Wang  Because of the crown-ether-like structure in Ti 8 -wheel, U. Schubert et al. investigated the ability of loading metal ions in Ti 8 O 8 (OMc) 16 .Sr@Ti 8 O 8 (OMc) 16 and Pb@Ti 8 O 8 (OMc) 16  were successfully prepared through the coordination between metals and open oxygen sites at the center of Ti 8 , which makes Ti 8 O 8 (OMc) 16 a potential absorbent for water purification. [75]In 2021, considering the poor stability of Ag nanoclusters, our group introduced Ag nanoclusters into Ti 8wheels to stabilize them and combine both the advantages of the two clusters. [77]Ag nanoclusters were incorporated via the coordination of unsaturated oxygen and carboxylate sites, the same as Sr and Pb mentioned before, resulting in the formation of Ag 5 @Ti 8 composite structures in MeCN.When the solution changed to uncoordinated toluene, an unexpected Ti 16 -wheel was obtained.The Ti 16 wheel is composed of two Ti 8 wheels packed in a "face-to-face" mode.The doublelayed Ti 16 -wheel provides a deeper cavity that can contain Ag nanoclusters with higher nuclearities.Ag 14 @Ti 16 stabilized by 28   19 .Reproduced with permission. [78]Copyright 2015 and 2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. [76](E) Synthesis and structures of the [Ti 8 ⋅C 30 N 8 ] hopf link, rotaxane [Ti 8 ⋅C 8 N 4 Py 2 ], and pseudorotaxane [2Ti 8 ⋅spermine].Hydrogen atoms are omitted for clarity.Reproduced with permission. [106]Copyright 2021, American Chemical Society.(F) Side and top structural views of Cs@Ti 12 Ser 6 .Reproduced with permission. [80]Copyright 2017, American Chemical Society.system.One is through electrostatic interaction, moieties carrying opposite charges in guests contact closely with the host-backbone.Another interaction is via coordination bonds, which are mainly found in metal guests.These two patterns are also frequently observed in other TOC-wheel systems.

3.3.2
Ti 9,10 -oxo wheel Ti 9 -oxo wheel was first discovered by U. Schubert et al. in 2015. [78]They explored the influence of Ti-precursor: L ratio on TOC aggregating in HO i Pr and found that when HOMc: Ti(O i Pr) 4 = 4:1 or 6:1, Ti structure is more likely a derivative structure of Ti 8 because a mono-TiO hexahedron was replaced by a bi-nuclear TiO unit (Figure 5B).In 2017, Winpenny et al. synthesized wheel-like Ti 9 and Ti 10 (Figure 5C,D). [76]To control the aggregation size of Ti-oxo wheels, a series of amines from small to big were utilized.For example, the employing of i Pr 2 NH resulted in Ti 9 O 10 ( t BuCOO) 17 , while N,N-dicyclomethylamine led to the formation of Ti 10 O 11 ( t BuCOO).Probably, amines serve as templates to control the nuclearities of rings because SCXRD clearly displays amines at the center of the ring.Very recently, our group incorporated Cu 4 clusters into Ti 9 ring to prepare an enzyme-like Cu 4 @Ti 9 structure. [79]The abovementioned work further elucidates the fascinating host-guest chemistry in wheel-like clusters.

Ti 12 -oxo wheel
In 2018, Y. F. Wang's group constructed a series of Ti 12 -oxo clusters in water using amino acids (AA) or SO 4 2− as stabilizing ligands. [80]Ti 12 occupies a double-layer structure with two Ti 6 wheels packed in "face-to-face" mode as that of Ti 16 .Two different bonding sites are observed in the Ti 12 backbone, which are site-a at the center of the "pocket" and site-b outside the Ti 6 -plane (Figure 5F).The diameter of the Ti 6wheel subunit is 5.4 Å, smaller than Ti 8 reported by Schubert et al.Thus, Ti 12 host structure may exhibit stronger interactions with metal ions that have smaller sizes than Sr 2+ and Pb 2+ .Alkali metals such as K + , Rb + and Cs + were successfully loaded into Ti 12 -wheel by directly reacting TiCl 4 , metal halides and AAs in water.The details of prepared structures are shown in Table 1.In addition, they observed the rapid displacement of Rb + in Rb@Ti 12 Ser 6 (Ser = serine) in CsCl aqueous solution, indicating the reversible adsorption of guest cations in Ti 12 host-guest system.

Ti 20 -oxo wheel
In  6A). [81]12 O 2 2− and 8 HO 2 − moieties were found in Ti 20 structure, which is rarely reported in TOC structure.Among these (hydro)peroxo groups, three kinds of coordination patterns were identified via SCXRD, which are μ-η 1 :η 2 , μ-η 2 :η 2 and η 2 fashions.The former two kinds of (hydro)peroxo groups take vital parts in connecting neighboring [Ti 5 (μ-O) 2 (HO 2 ) 2 (O 2 ) 3 (R,R-tart) 3 ] 2− subunits to form Ti 20 framework.The η 2 -O 2 2− mainly makes a contribution in catalysizing oxygen transfer reaction, for it only coordinates with single Ti ion and is more coordination-reversible compared with other (hydro)peroxo groups (Figure 6B).The function of tartrate ligand is different from the ligands discussed in Ti 8 -Ti 12 .It prefers chelating with one Ti ion rather than connecting with two different ions, thus its effect is more likely to be stabilizing Ti 20 core.More importantly, the precursor used in this work is TiCl 4 , which might directly influence Ti-peroxo complex formation because Ti(OR) 4 precursors would bring alkoxide in which could greatly ocupy the coordination sites.
permanent porosity due to their fascinating packing mode, which can be employed in gas adsorption/separation or catalysis after pore engineering (Figure 6D-F).In 2018, Ti 32 O 16 (L) 16 (gly) 32 (glyH) 16 ⋅ 12 (glyH 2 )⋅ 29 (IPA) (L = 4-aminobenzoic acid, glyH 2 = ethylene glycol, IPA = isopropanol) with the same Ti 32 core but different ligands prepared by S. A. Wang et al. [82] An interesting phenomenon is that leaving solvent molecules in the framework will lead to the transformation of packing modes.
With nuclearities increasing, one of the challenges Ti-oxo wheels face is the collapse of the inner cavity.Thus, highnuclearity TOC wheels tend to aggregate in multi-cyclic type rather than a single circle.For example, as previously discussed Ti 8 , Ti 9 and Ti 10 exhibit an "ideal" cyclic shape with all Ti ions at the edge of it.However, in Ti 32 -oxo wheels, sixteen six-coordinated Ti ions are anchored at the outer shell of Ti 16 circle as reinforcement to strengthen the whole wheel framework.The same "reinforces" are also found in Ti 20wheel, leading to a significant decrease in diameter compared with "ideal" Ti-oxo wheels.In heterometallic Ti-oxo wheels, the nuclearities of which may be extremely high, "reinforce" Ti/M-patch are even more common.It will be discussed in the following section.
Judging from the TOCs in different intramolecular aggregations, we suppose that each type requires ligands as SDAs to regulate the formation of specific shapes.For instance, alcohols serve as both solvents and ligands in ball-like TOCs synthesis that can be regarded as SDAs to induce spherical configuration.It is more evident in polyhedron-like TOCs, SDAs are essential due to the long distance and weak interactions of neighboring Ti ions.As for cyclic TOCs, aromatic carboxylic acids are beneficial for assembling near-planar wheels.09]

Heterometallic Ti-oxo wheels
Heterometallic doping is an effective method to modulate the properties of TOCs.The direct incorporation of a second or even more metals into the original cluster skeleton can change the electronic structure and construct impurity energy levels within the valence band (VB) and conduction band (CB), achieving the engineering toward the band gap. [26]The utilization of specific light regions can thus be improved.Our group previously published a review about heterometallic-TOCs in 2020, summarizing recent advances in synthesizing different metal-doped TOCs and their properties.The potential applications enhanced by metal-doping such as photoand electrocatalysis were also discussed. [30]As for wheellike TOCs, the metal-doping strategy is also considerable to rational control their host-guest and catalytic performances.Similar to ligand modification, new charge transfer bands are created such as ligand-to-core charge transfer (LCCT) and metal-core charge transfer (MCCT), which lower the band gap and enhance the ability to utilize sunlight, giving TOCs a great enhancement in energy science. [9]Generally, the metals reported in recent literature are mostly concentrated on a fraction of transition (Cr, Mn, Fe, Co, Ni, Cu), lanthanide (Eu, Gd) and main-group metals (Al, Ga, In).To avoid confusion, the following section will be discussed in the sequence of Ti-nuclearities.

Ti 6 M-oxo wheels
In 2021, our group adopted a classic Co-doping strategy and constructed a Co-doped [TiCo 2 (μ 2 -O i Pr)(O i Pr) 2 (Dmg) 3 ] 6 , denoted as Ti 6 Co 12 . [83]Judging from the experimental results, the performance of heterometallic TOCs is closely related to the nature of the doping metal itself.Co-TOCs have a better photocurrent response performance than other transition metal-doped TOCs.The successful preparation of Ti 6 Co 12 has a close association with the directing effect of hydroximate ligands.According to HSAB, Ti 4+ is a hard acid, which is more sensitive to hard base such as O 2− and can selectively coordinate with O 2− , while Co 2+ has a higher affinity to N sites.Ti(O i Pr) 4 precursor is also essential to form a complete wheel structure.When the precursor is changed to TiCl 4 , CoTi 2 Cl 6 (Dmg) 3 complex is formed because of the absence of O i Pr bridging units.Like some other TOC wheels, a 1D channel is observed in the Ti 6 Co 12 due to the stacking of rings.

Ti 7-9 M-oxo wheels
Winpenny et al. found that the addition of M(III) salts in synthesizing Ti 8 -Ti 10 wheel could result in the formation of Ti 7 M, Ti 8 M, and Ti 9 M heterometallic rings. [76,84]When monocarboxylic acids are chosen as ligands, a big challenge is that they are more likely to coordinate with only Ti 4+ according to HSAB and produce pure TOC structures.
Considering the difference in coordination abilities between Ti and other metals, we proposed that reactions preferably proceeded under relatively high temperatures to level diverse metals.For example, in Winpenny's work, all the Ti x M wheels were synthesized at 140 or 160  [86] This is because eight Ti ions are all chelated by salicylic outside the Eu-wheel as "reinforce", two neighboring Eu ions share one μ 2 -carboxyl group of L to form wheel-structure.The difference between Ti and Eu has already been embodied via different coordinated modes.As mentioned before, the incorporation of hetero-metals would remold the host-guest interactions.Taking Ti 8 as an example, Wang et al. in 2018 found that ammonium groups were always arranged inside Ti 8 in catenane system, indicating the electrostatic interaction between negative Ti 8wheel and positive -NH 3 + . [106]The close combination also be observed in Ti 8 -rotaxane reported by Winpenny.These results illustrate that the introduction of low-valence metals such as Fe(III), Ga(III) and Co(II) will give additional negative charge to Ti-wheels, modulating the electronic structure and enhancing the interactions between wheels and positive guests.and Ti 12 In 6 O 18 (OOCC 6 H 5 ) 30 [88] with similar Ti 12 M wheel structures in 2017, denoted as Ti 12 Cr 6 and Ti 12 In 6 , respectively.They are prepared at 180 • C to equalize the coordination abilities of Ti and Cr/In to passivate the affinity between Ti and carboxylates.Ti 12 M 6 exhibits a double core-shell ring structure constructed from TiO 6 and CrO 6 octahedra (Figure 7A).The relatively large Ti 12 M 6 wheel cannot form a single ring with all atoms at the edge of the circle due to the poor rigidity, a six-membered Ti-O ring is formed inside a twelve-membered Ti 6 In 6 wheel.Both Ti 6 and Ti 6 In 6 wheels can be considered as "reinforce" to each other, producing enhanced rigidity and stability.Typical titaniumalkoxide clusters usually show absorption at the ultraviolet (UV) region, the corresponding UV light only contains 5% energy of solar light.The incorporation of Cr and In significantly broadens the absorption band of TOC from UV to even near-infrared region (Figure 7D,E).Photocatalytic performance such as dye degradation and H 2 evolution under visible light are greatly improved, proving the enhancement in utilizing visible light.

Ti 14 M-oxo wheels
In 2021, our group incorporated Al into TOC wheel and prepared Ti 14 Al 7 O 21 L 35 (L = benzoic acid), denoted as Ti 14 Al 7 . [89]A remarkable core-shell odd-membered wheel structure is assembled at 160 • C. Ti 7 wheel-like core lies inside the overall structure with a Ti 7 Al 7 ring outside.Two rings are closely connected by μ 3 -O, leading to a donut-like wheel.The core-shell double ring is pervasive among giant TOC wheels, further demonstrating the importance of "reinforcing" moieties in preventing collapse (Figure 7B).The desirable catalytic performance of Ti 14 Al 7 in photo-induced H 2 evolution was investigated and found that under UV-vis light irradiation, the H 2 production rate could reach about 402.88 μmol g −1 L −1 .In 2022, H 7 Ti 7 Cr 14 O 21 Bz 35 ⋅4H 2 O⋅6CH 3 OH (denoted as Ti 7 Cr 14 , bz = benzoic acid) with the same wheel-like core was obtained by Y. F. Wang's group. [85]A sevenmembered Ti-ring lies in the middle of the core with a fourteen-membered Cr-ring at the outer shell concentrically.Ti 7 Cr 14 is synthesized under 100 • C, which are an exception in the assembling of monocarboxylate-based heterometallic TOC-wheels.This may probably be because of the adjacent position of Ti and Cr in the periodic table of elements.Besides, the outer Cr 14 wheel is less interlaced than Ti 7 Al 7 , which may also be assigned to the relatively low temperature.Ti 7 Cr 14 has a different component from Ti 12 Cr 6 , which can be assigned to different reaction conditions.Cr incorporation greatly enhanced the photoactivity under visible light, as demonstrated by UV-vis spectra (Figure 7F).In 2022, another type of Ti 14 -wheel, denoted as Ti 14 Ln 22 , was synthesized by X.-J.Kong et al. through ligand modulation (Figure 7C). [90]Due to the directing effect of salicylic acid, the reaction temperature is 80 • C the same as they employed in preparing Ti 8 Ln 22 in 2018.The UV-vis behavior is well promoted by doping metals (Figure 7G).In summary, heterometallic TOC wheels can be constructed mainly via ligand modulation and temperature directing.12]

High-nuclearity TOCs aggregated in classic form
The breakthrough of cluster nuclearities and the synthesis of TOC with comparable size as TiO 2 nanoparticles have been a great challenge so far.Successful preparation cases of large TOCs have significance in exploring their aggregation F I G U R E 7 (A) Crystallographic structure of Ti 12 In 6 (blue octahedron: TiO 6 , brown octahedron: InO 6 , and gray: carbon).Green sphere represents the cavity in the cluster.Hydrogen atoms and guest molecules are omitted for clarity.Copyright 2017, The Royal Society of Chemistry. [88](B) Ball-and-stick representation of the [Ti 14 A l7 ] heterometallic ring."Reinforce" unit is marked with dotted circle.Reproduced with permission. [89]Copyright 2021, Elsevier B.V. (C) Representation of the ball-and-stick model of Ti 14 Ln 22 .Copyright 2021, Wiley-VCH GmbH. [90]UV-vis spectra of (D) Ti 12 In 6 , (E) Ti 12 Cr 6 , (F) Ti 7 Cr 14 and (G) Ti 14 Ln 22 .Copyright 2017, The Royal Society of Chemistry [88] ; 2017, American Chemical Society [87] ; 2022, American Chemical Society [85] and 2021, Wiley-VCH GmbH, [90] respectively.mechanisms and investigating controllable synthesis.Here, TOCs with nuclearities of more than 30 are summarized because they usually exhibit cluster-size-dependent activities.In 2012, P. Coppens et al. synthesized Ti 34 -oxo clusters using dimethylamino benzoate, which can be regarded as a combination of two Ti 17 -oxo clusters. [56]In 2016, our group prepared the largest TOC with 52 Ti ions by controlling the chelating ligand and reactant concentrations.The stepwise assembly from Ti 6 , via Ti 17 toward Ti 52 and their sizedependent behaviors in photocatalytic H 2 evolution were also reported. [14]In 2019, our group obtained Ti 44 -oxo clusters by controlling the amount of acid modulators. [13]

TOC-DERIVED FRAMEWORK MATERIALS
In many cases, isolated TOCs do not have permanent pore structures, limiting their application in gas separation and photocatalysis.Moreover, it is of great value to prepare frameworks with a well-maintained predesigned cluster structure rather than an unexpected one.To assemble isolated TOCs into infinite networks, intermolecular assembly forces should be first considered, including coordination, non-covalent and covalent (including polymerization) way (Figure 8).Considering that the frame material has already been described in part in other reviews, we focus here on the assembly work based on pre-designed clusters.

Coordination TOC-frameworks
The key to realizing further coordination assembly is to design appropriate coordination-active sites and linking units to connect as-prepared TOC blocks.N-substituted carboxylic acids are favorable for connecting moieties with different coordination abilities.In this part, several specific TOCprecursors are selected as examples to discuss the stepwise assembling.

Ti 6 P 2 -derived frameworks
Clusters based on the Ti 6 P 2 core prepared by our group are an excellent platform to investigate intermolecular aggregation because they are highly tunable (Figure 9A). [113] I G U R E 8 Schematic illustration of stepwise assembling from TOCs to frameworks.Functional groups such as ─NH 2 , ─N─H⋅⋅⋅N─, ─CHO, and ─CCl 3 shown in the figure are for illustration only.They represent a series of connecting active groups rather than specific ones.
After incorporating metal ions and N-substituted ligands as linkages, PTC-15, 16 and 17 with framework structures are prepared.These results stimulate us to explore other N-substituted phenyl carboxylates and functional bridging metal units.In 2017, we employed isonicotinic acid, nicotinic acid, pyridine-3-sulfonic and 2-pyrazine-carboxylic acid as labile ligands and copper halides to connect neighboring TOCs. [114]In addition to the linkage, the incorporation of copper moieties also greatly lowers the band gap of original TOCs, promoting their performances in catalyzing H 2 evolution under visible light irradiation (Figure 9B).

Ti 4 L 6 -derived frameworks
The calixarene-like cavities constructed by uncoordinated oxygen sites in the Ti 4 L 6 cage can be utilized to assemble frameworks.When Tb and Eu ions are employed, PTC-105 with a 1D zig-zag chain structure and PTC-106 with a 3D framework are successfully prepared (Figure 9C). [65]The corresponding metal salts can be changed to other Ln salts, leading to the formation of PTC-105(Ln) or PTC-106(Ln).Another example of preparing Ti 4 L 6 cage-based frameworks is the synthesis of PTC-235, where supramolecular interactions such as hydrogen-bond and π-π interactions are used to assemble the framework structure. [12]The preparation of cage-based MOFs enhances the porosity, stability, and hostguest interaction (especially with gas molecules) of Ti 4 L 6 .In 2020, we examined the gas sorption properties of PTC-220, a Ti 4 L 6 framework connected by Zn. [115] PTC-220 obsesses a permanent porosity with a BET surface area of 439 m 2 g −1 .In  9D).By employing TOC frameworks in gas separation, energy consumption can be greatly reduced because of the high temperature and pressure required in industrial separation.In summary, precise design of connecting units that not only ensure the formation of TOCs but also provide expanding sites is necessary.
which leads to an uncontrollable aggregating rate when constructing framework structures.From this point of view, our group first employed Ti 44 , an extremely large TOC, as a precursor to synthesize FIR-125, 126, and 127 (Figure 9E). [22]During synthesis, the Ti Remarkably, high-quality FIR-125 single crystals can also be prepared using other large TOCs, such as Ti 32 -oxo wheels and ball-like Ti 42 -oxo clusters, further demonstrating the importance of large TOC precursors in crystal growth.

4.1.4
Other TOC-precursors used in framework synthesis Q. C. Zhang et al. [43] and Y. Liu et al. [120] have summarized Ti-MOFs using TOCs as precursors.To avoid repetition and meanwhile make this review more integrated, here we only discuss representative precursors.Ti 6 O 6 L 6 is an effective precursor to resist severe hydrolysis of Ti ions and slower crystallization to obtain single crystals.4-aminobenzoate (AB) is widely used as a ligand to stabilize the Ti 6 O 6 core before MOF synthesis.Ti 6 O 6 (AB) 6 may decompose after assembling Ti-MOFs and then transform into other Ti-O cores, which is observed in much published literature.If the Ti 6 O 6 core is designed to be preserved as a subunit, ligands with uncoordinated nitrogen sites should be considered, such as nicotinic acid, isonicotinic acid, pyrazole-based carboxylates, and so on.Z. M. Sun et al. synthesized a series of N-heterocyclic ligand-modified Ti 6 O 6 clusters and extended them into MOFs via incorporating Cu n I n species. [121,122]In 2021, C. Liu et al. chose Ti 8 O 5 (OEt) 18 L 2 as a precursor to construct frameworks containing original TOC units. [123]hey also employed N-heterocyclic carboxylates to bridge different metal units.In general, TOC precursors can serve as not only building units but also simply Ti sources.The crystal quality is also optimized by TOC precursors.

Covalent TOC-frameworks
Covalent organic frameworks (COFs) are a new class of porous materials with permanent pore structures and ordered attaching units. [124]Most COFs are built up with imine linkage from the condensation of amine and aldehyde.Therefore, a rational design and precise arrangement of linking units such as amino and aldehyde groups in favorable steric configurations could extend TOCs to COF structure.In addition, TOCs have a relatively stable interaction between Ti and O due to their high charge-radius ratios.Thus, they are preferable to serve as a reliable building block in constructing COFs.This inspiration was first achieved by Yaghi's group in 2016. [17]An isolated Ti 6 O 6 cluster with a 4-aminobenzoic acid ligand was synthesized and then employed as a TOC precursor.After reacting with benzene-1,4-dialdehyde, MOF-901 with a Ti 6 O 6 core connected by a Schiff-base structure was assembled.Utilizing the same methodology, MOF-902 with a similar TOC-based COF structure was obtained in 2017 (Figure 10A). [18]Although it is hard to prepare Copyright 2016, American Chemical Society; [17] 2017, American Chemical Society; [18] 2022, Wiley-VCH GmbH [125] and 2022, Nature Publishing Group. [126](B) Transient photocurrent response of MCOF-Ti 6 BTT (blue), MOF-901 (red), Ti 6 -NH 2 (black).Copyright 2022, Wiley-VCH GmbH. [125](C) Transient photocurrent response of MCOF-Ti 6 Cu 3 , MOF-901 and FDM-71-ABC.Reproduced with permission. [126]Copyright 2022, Nature Publishing Group.(D-F) TiO cluster structure, the condensed structure of the Ti-PMMA inthe bulk state, and the demonstration of the TiO cluster as quasi-spherical initiator for ATRP polymerization and the formation of the dimer.Color codes: gray octahedron, TiO 6 ; green sphere, C; gray sphere, H.Copyright 2019, Society of Plastics Engineers. [127]ngle crystals for structural determination, covalent bondconnected TOC frameworks have higher stability advantages than traditional coordination compounds.Besides, the construction of frameworks via amino-aldehyde condensation is more purposeful.It should be noted that amino-aldehyde condensation is more directional than coordination.Theoretically, frameworks can be formed between any two moieties with amino or aldehyde groups, as long as their steric configuration is suitable for condensation.Yaghi's work provides a handy route to assemble composite materials from preconceived building blocks with immobilized amino or aldehyde groups.
In 2022, Y.-Q.Lan's group employed classic Ti 6 O 6 (AB) 6 to condensate with benzotrithiophene tricarbaldehyde (BTT). [125]The obtained TOC-COF is denoted as MCOF-Ti 6 BTT.BTT is a well-designed oxidation group that significantly promotes electron-hole separation/transfer of Ti 6 O 6 core via LMCT, tuning the bandgap and facilitating the utilization of visible light.MCOF-Ti 6 BTT has the same TOC core as MOF-901 but a two times higher photocurrent response, indicating the synergistic effect of a well-designed BTT unit (Figure 10B).After that, Lan et al. optimized the constitution and synthesized MCOF-Ti 6 Cu 3 -ABC with three active components. [126]Cu(NO 3 ) 2 ⋅3H 2 O is first reacted with pyrazolate-4-carboxaldehyde (1H-PyCA) to form Cu 3 units.Then, Ti 6 O 6 (AB) 6 is added to condensate with Cu 3 , giving a metal-covalent organic framework.MCOF-Ti 6 Cu 3 -ABC obsesses a fascinating structure with a separated oxidation (Ti 6 O 6 ) and reduction (Cu 3 ) group, resulting in a higher photocurrent response (Figure 10C) and excellent photocatalytic performance in the CO 2 reduction reaction (CO 2 RR).Meanwhile, the water oxidation reaction is also occurring at the other part of COFs (TOC units), exhibiting great potential in realizing artificial photosynthesis.As they proposed before, Schiff-base structure provides good stability, no obvious production loss is found during four consecutive cycles.

Hybrid TOC-organic copolymer
The synthesis of organic-inorganic hybrids is in great demand in material science.As an accessible inorganic nano building block with a well-defined structure, TOCs can be designed as precursors by incorporating active groups through ligand engineering.Most reported works employed as-designed TOCs in atom transfer radical polymerization (ATRP), which required immobilized C-X bonds such as -CCl ] to initiate ATRP of n-butyl acrylate, leading to a star-shaped hybrid titanium poly(n-butyl acrylate). [128]][131] In and successfully synthesized star-like hybrids through polymerizing methyl methacrylate on the surface of TOCs (Figure 10D-F). [127]rom the examples discussed before, it is obvious that each method needs precise ligand engineering, which can be concerned with the incorporation of SDAs.For intermolecular aggregation, functional terminal groups modulate different connecting modes, such as coordination, non-covalent, and covalent (including polymerization) ways.Fortunately, maneuverable ligand modification via coordination chemistry ways is one of the most fascinating natures of TOCs, making it an ideal ingredient to further assemble into framework materials.

CONCLUSION AND OUTLOOK
This review summarizes recent progress in the new aggregation of TOCs.The synthetic strategies have been rapidly developed, increasing exponentially in fascinating aggregate modes, including intramolecular and intermolecular ways.(1) TOCs are formed via intramolecular aggregation.Different aggregation types lead to different TOC shapes, such as ball-like, polyhedron-like, and wheel-like.
(2) Aggregation modes between TOC molecules can be mainly divided into three types, involving coordination, noncovalent, and covalent (including polymerization initiated by TOCs); (3) Each aggregation mode requires unique SDAs.For intramolecular aggregation, alkoxides, embonic acid/2,6dihydroxybenzoate, and aromatic monocarboxylates can act as SDAs to assemble Ti ions into ball-like, polyhedron-like and wheel-like structures, respectively.For intermolecular aggregation, connecting groups on the terminal of TOCs are SDAs.Special-aggregated TOCs have fascinating properties due to their unique structures.Ball-like TOCs are the earliest to be discovered among them.They are excellent platforms for establishing a comprehensive understanding of structureproperty relationships in TOCs because of their highly investigated structures.As for TOC-polyhedrons and wheels, the cavities constructed by specific arrangements of Ti ions are extraordinary.They provide abundant active sites to construct a controllable host-guest system.The heterometallic TOC wheel is an important part because the incorporation of different metal ions can significantly improve its proper-ties in various applications.After intermolecular aggregation into 2D/3D frameworks, one of the most attractive improvements is the construction of pore architecture.The aperture significantly enhanced host-guest interactions, providing the possibility of utilizing TOC frameworks in gas separation and reticular chemistry.One of the most attractive properties of Ti-based materials is their photochemical activity.Benefiting from atomically precise and controllable structures, TOCs in various aggregation modes can act as excellent platforms to investigate correlative photo-involved applications.It is widely accepted that coordination chemistry based on coordination bonds is highly flexible and that it is possible to combine multiple active sites to assemble functional composites, greatly expanding the potential applications of TOCs.In addition to photo applications, more and more functional ligands are incorporated into TOCs to enhance their performances in applications such as electrocatalysis, chiral recognition/resolution and biomedical treatment.For example, except for the TOC precursors discussed in this review, Ti 8 -oxo rings can also be utilized as precursors to build frameworks for improving the photoactivities, which have been proven to be vital in MIL-125.Ti 8 with various functional groups such as isonicotinic acid, 4-aminobenzoate, or other active sites can be employed to prepare frameworks with excellent pore architecture.Researchers can always optimize the existing structure and construct more active materials to realize higher performance in the past, present, and future.However, a great challenge is that although many works have been reported in property-directed material synthesis, it remains difficult to accurately prepare pre-designed functional structures and predict architectures and properties before synthesis.The future direction of development V.; 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim; 2017, The Royal Society of Chemistry; 2021, Elsevier B.V. and 2021, Wiley-VCH GmbH, respectively.
respectively, Figure 2A,C,D).Ti 11 O i Pr 18 is a lacunary structure with a [TiO] defect occurring at the middle Ti 6 layer.The other two structures are formed by changing μ 2 -O to μ 2 -O i Pr.The photocurrent response of Ti 11 O i Pr 18 is significantly higher than that of other [Ti 12 ] (B) Ti 12 O 16 O i Pr 16 , (C) Ti 12 O 15 O i Pr 17 , and (D) Ti 12 O 14 O i Pr 18 .

F
I G U R E 3 (A) Timeline for synthetic development in ball-like Ti 17-19 -oxo clusters.Color codes: green, Ti; red, oxygen; gray, C. (B-C) Combined polyhedral and ball-and-stick views of the Ti 17 L n (n = 1, 2, 4, and 8) clusters and UV-vis spectra of the Ti 17 L n solutions.The substituents of the catecholate ligands are represented by cyan balls.Ct = pyrocatechol.For clarity, the isopropyl groups are omitted.Reproduced with permission. [58]Copyright 2019, Wiley-VCH Verlag GmbH&Co.KGaA, Weinheim.(D) (Left) Wireframe and (right) combined ball-and-stick and polyhedral visualization of Ti 22 O 31 (OH)(O i Pr) 23 .Color codes: blue, Ti; green, the Oin atom; red, bridging-O; pink, alkoxide-O; gray, C; deep purple, I.The protons of the two green Oin atoms are omitted.

S
C H E M E 1 A summary of reviews discussing advances in TOC and Ti-based frameworks.
. Wang's group synthesized Ti 22 O 31 (OH)(O i Pr) 23 (denoted as Ti 22 , Figure Impressive progress has been reported in assembling balllike TOCs with nuclearities of more than 20.In 2016, Y. F The reactions of Ti(O i Pr)4, TiI 4 , and HO i Pr under 100 • C at different times lead to different product formation.A tri-nuclear Ti 3 (O i Pr) 11 formed after heating in the oven for one or two days.If the reaction time was prolonged to two days, the Ti 12 O i Pr 18 discussed above was obtained after cooling down to room temperature or standing at room temperature for one day.This phenomenon also corresponds to the nature of Ti 12 O i Pr 18 , which is easy to crystallize in complicated TOC systems.Ti 22 crystallized from the solution after removing Ti 12 O i Pr 18 .The formation of Ti 22 implies the essential impact of reaction time on TOC aggregation.Large TOCs with high nuclearities usually need slow hydrolysis of Ti ions to produce ordered long-range crystal structures.Wanget al. also investigated the alkoxidesubstituted condition and obtained two ligand-modified Ti 22 structures, denoted as Ti 22 O 30 (OH) 2 (O i Pr) 20 (DTBC) 2 and Ti 22 O 31 (OH) 2 (O i Pr) 18 ( t BuCOO) 9 , respectively.
Crystals up to 100 μm are prepared through this method.
3 and C-Br groups on TOCs as initiators.This interaction is also a covalent bond but results in a distinct network compared with COFs: (1) Reaction mechanisms are different.TOC-organic hybrids are prepared by ATRP, while COFs are constructed by condensations; (2) Linking numbers are different.Hybrid copolymers have much more linking numbers than COFs; (3) The rigidity of frameworks is different.Polymerization results in a flexible network rather than rigid frameworks.In 2011, C. Sanchez et al. employed [Ti 16 O 16 (OCH 2 CH 3 ) 26 (OCH 2 CCl 3 ) 6 addition to the [Ti 16 O 16 (OCH 2 CH 3 ) 26 (OCH 2 CCl 3 ) 6 ] used by Sanchez, P. C. Yin et al. selected (Ti 6 O 4 (BrC[CH 3 ] 2 COO) 8 (O i Pr) 8