Reactivity of Titanium Oxo Ethoxo Cluster [Ti16O16(OEt)32]. Versatile Precursor of Nanobuilding Block-Based Hybrid Materials

Structural Chemistry of Titanium (IV) Oxo Clusters, Part 2: Clusters without Carboxylate or Phosphonate Ligands

Homometallic titanium oxo clusters (TOC) are one of the most important groups of metal oxo clusters. In a previous article, TOC structures with carboxylato and phosphonato ligands were reviewed and categorized. This work is now extended to clusters with other ligands. Comparison of the different cluster types shows how the interplay between condensation of the titanium polyhedra by means of bridging oxygen atoms and the coordination characteristics of the ligands influences the cluster structures and allows working out basic construction principles of the cluster core.

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.

Monolithic Titanium Alkoxide Networks for Lithium-Ion Conductive All-Solid-State Electrolytes

Reticular chemistry provides opportunities to design solid-state electrolytes (SSEs) with modular tunability. However, SSEs based on modularly designed crystalline metal-organic frameworks (MOFs) often require liquid electrolytes for interfacial contact. Monolithic glassy MOFs can have liquid processability and uniform lithium conduction, which is promising for the reticular design of SSE without liquid electrolytes. Here, we develop a generalizable strategy for the modular design of noncrystalline SSEs based on a bottom-up synthesis of glassy MOFs. We demonstrate such a strategy by linking polyethylene glycol (PEG) struts and nanosized titanium-oxo clusters into network structures termed titanium alkoxide networks (TANs). The modular design allows the incorporation of PEG linkers with different molecular weights, which give optimal chain flexibility for high ionic conductivity, and the reticular coordinative network provides a controlled degree of cross-linking that gives adequate mechanical strength. This research shows the power of reticular design in noncrystalline molecular framework materials for SSEs.

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.

Synthesis, Structure, and Light Absorption Behaviors of Prismatic Titanium-Oxo Clusters Containing Lacunary Lindqvist-like Species

Exploring new structural types of polyoxotitanium clusters (PTCs), especially those containing classical polyoxometalates structures, has always been the focus of research in the field of metal-oxo clusters. In this work, we present the synthesis and characterization of three prismatic PTCs: namely, Ti₈(μ₂-O)₃(μ₄-O)₂(OnPr)₆(HOnPr)₂(L1)₈ (PTC-237; H₂L1 = 3,5-di-tert-butylcatechol), Ti₁₂(μ₂-O)₆(μ₃-O)₈(OnPr)₆(L2)₁₂(L3)₂ (PTC-238; HL2 = 1-adamantanecarboxylic acid, HL3 = 2-picolinic acid), and [Ti₁₈(μ₂-O)₄(μ₃-O)₁₆(μ₅-O)₂(OiPr)₁₈(L3)₈](L3)₂ (PTC-239). Single-crystal X-ray diffraction analyses indicate that the construction of these prismatic PTCs is based on a stepwise interlayer assembly of {Ti₃} and {Ti₄} substructures. The diameters of their core skeletons are in the range between 0.9 and 1.3 nm. In particular, lacunary Linqvist-like {Ti₄} and {Ti₅} building units are found to exist in the structures of PTC-237 and PTC-239. According to the solid-state UV-vis diffuse reflectance measurements, the absorption band of 3,5-di-tert-butylcatecholate-functionalized PTC-237 shifts toward the visible-light region, giving a smaller optical band gap of 1.56 eV in comparison to PTC-238 (3.36 eV) and PTC-239 (3.25 eV).

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.

A Cyclic Titanium-Oxo Cluster with a Tetrathiafulvalene Connector as a Precursor for Highly Efficient Adsorbent of Cationic Dyes

Titanium-oxo clusters (TOCs) have been studied for applications in catalysis, energy storage and transfer, light emission, and so on; however, use of TOCs for the selective adsorption of dyes has not yet been reported. Herein, a TOC compound formulated as [Ti₆O₃(OⁱPr)₁₄(TTFTC)]₄ (1, TTFTC = tetrathiafulvalene-tetracarboxylate) was successfully prepared and crystallographically characterized. Compound 1 has a cyclic structure assembled by four Ti₆ clusters and four rodlike TTFTC connectors. Red compound 1 self-condenses to form a black polymeric organic-inorganic hybrid material (denoted as B-1), which was characterized by various techniques. B-1 is an amorphous TiO material that is formed by the irregular condensation of 1 by the removal of alkoxyl groups. B-1 exhibits high dye adsorption efficiency toward cationic dyes with a q_e value of 651.3 mg/g at 298 K for methylene blue (MB). Moreover, B-1 can be used to selectively remove MB not only from mixed cationic-anionic dye solutions but also from some mixed cationic dyes, which is related to their structures. Kinetic, isotherm, and thermodynamic studies demonstrated that the pseudo-second-order kinetic model and Freundlich model show a good fit to the experimental data. The adsorption process involves an exothermic and entropy decreasing process. In addition, dye-adsorbed B-1 can be further used as a photocurrent-responsive material. The work opens up a new field for the application of TOCs in the selective adsorption and removal of dyes.

Role of Organic Fluoride Salts in Stabilizing Niobium Oxo-Clusters Catalyzing Epoxidation

We present here that easily available organic salts can stabilize/modify niobium (Nb) oxo-clusters. The as-synthesized Nb oxo-clusters have been characterized by various methods. These Nb oxo-clusters were catalytically active for the epoxidation of allylic alcohols and olefins with H₂O₂ as an oxidant. Notably, Nb-OC@TBAF-0.5 appeared as highly dispersed nanosized particles and showed the highest catalytic activity, which can be attributed to the following reasons on the basis of characterization. First, the strong coordination of fluorine ions with Nb sites and the steric protection with bulky organic cations led to high stabilization and dispersion of the oxo-clusters in the course of the reaction. Second, a hydrogen-bond interaction between the coordinated fluorine atom and the -OH group of allylic alcohol favored the epoxidation reaction. Third, the electron density of Nb sites decreased due to the strong electron-withdrawing ability of F^- adjacent to Nb sites, thus promoting the electrophilic oxygen transfer to the C═C bond.

Calixarene-Protected Titanium-Oxo Clusters and Their Photocurrent Responses and Photocatalytic Performances

Three photosensitive tert-butylcalix[n]arene (TBC[n], n = 4, 6, 8)-protected titanium-oxo clusters (TOCs), formulated as [Ti₄(μ₃-O)₂(TBC[4])₂(OⁱPr)₄(DEF)₂]·DEF (1, TBC[4]-Ti₄, DEF = N,N-diethylformamide), [Ti₄(μ₄-O)TBC[6](OCH₃)₉]·H₂O (2, TBC[6]-Ti₄), and [Ti₄(μ₃-O)₂(OⁱPr)₄TBC[8](DEF)₂]·DEF (3, TBC[8]-Ti₄), were successfully synthesized and characterized. Because of the generation of charge transfer from TBC[n] to the TiO core, the three TBC[n]-decorated TOCs show a broadened visible-light absorption and narrowed optical band gap based on the UV-visible spectra and density functional theory calculations. The corresponding photosensitive electrodes prepared using these three TOCs exhibit stable photocurrent responses. Furthermore, their photocatalytic performances for hydrogen evolution and methylene blue degradation were evaluated, and all of the materials display excellent photocatalytic activity and stability. The calixarene-Ti coordination is therefore an effective strategy to enlarge the visible-light absorption band of Ti-O materials and improve their photoelectric/photocatalytic performances.

Supramolecular Chemistry of Titanium Oxide Clusters

Titanium oxide clusters (TOCs) have been emerging as a new type of inorganic molecular entities of supramolecular chemistry. Herein, a perspective on the structures and functionalities of TOCs over the past three decades is given and the paramount roles of TOCs in serving supramolecular chemistry are demonstrated. Four types of supramolecular assemblies based on TOCs are reviewed, namely, TOC hosts for ion inclusion, mechanically interlocked molecular systems built from cyclic TOCs, reactivities of surface sites toward ligand exchange, and hierarchical structures of TOCs. The principles and advantages of TOCs toward each application are fully discussed, along with structural analyses. Following this path, more functional TOC-based supramolecular systems may be designed and synthesized in the future, which, in turn, will certainly enhance research into both supramolecular and coordination chemistry of titanium.

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).

Titanium phosphonate oxo-alkoxide “clusters”: solution stability and facile hydrolytic transformation into nano titania

Oligonuclear Ti(iv) oxo-alkoxide-phosphonate complexes, produced by reaction of tBuPO(OH)₂ with Ti(OR)₄, are easily topotactically hydrolyzed forming intricate nanostructures.

One-Pot and Postsynthetic Phenol-Thermal Synthesis toward Highly Stable Titanium-Oxo Clusters

The synthetic approach plays a crucial role for the exploration and optimization of functional materials. As the molecular models of titanium dioxide, polyoxo-titanium clusters have undergone rapid development over past decade. Unfortunately, many of them are unstable, especially in aqueous environments, greatly limiting their applications in catalysis and environmental fields. In this work, we report a novel phenol-thermal approach toward the construction of highly stable polyoxo-titanium clusters. In addition to the traditional one-pot procedure, the phenol-thermal synthesis can also be used as a postsynthetic pathway to modify the alcohol terminated titanium-oxo clusters. During the modification in phenol, Ti-O core structures consisting entirely of 6-coordinated Ti^IV centers can be retained. Nevertheless, isopropanol terminated 5-coordinated Ti^IV centers are not stable and reconstructed to 6-coordinated Ti^IV centers during the phenol-thermal modification to form new Ti-O clusters. Physical attribute studies confirm that the obtained phenolic clusters generally display much better stability and stronger visible light absorption than isopropanol stabilized clusters with identical or similar cores. Therefore, phenol can not only offer a suitable solution environment for the construction of new cluster structures but also provide robust protection for the cluster cores and also an efficient method to enhance their visible light responses.

From a bulk solid to thin films of a hybrid material derived from the [Ti10O12(cat)8(py)8] oxo-cluster and poly(4-vinylpyridine)

Homogeneous coloured hybrid materials are prepared from the [Ti₁₀O₁₂(cat)₈(py)₈] oxo-cluster and poly(4-vinylpyridine). SiO₂ surfaces are functionalized with thin films of this material.

Titanium-Carboxylate Metal-Organic Framework Based on an Unprecedented Ti-Oxo Chain Cluster

Titanium(IV)-based metal-organic frameworks (Ti-MOFs) have received significant attention in recent years due to their numerous photocatalytic applications. We herein prepare the single-crystalline Ti-carboxylate MOF (DGIST-1) composed of an unprecedented Ti-oxo chain cluster and the porphyrinic ligand, TCPP (tetrakis(4-carboxyphenyl)porphyrin). Preformed Ti-oxo clusters were used as Ti⁴⁺ sources to avoid the spontaneous hydrolysis and condensation reactions of traditional Ti-alkoxide precursors, thus, enabling the formation of the highly crystalline Ti-MOF. The successfully activated DGIST-1 exhibited a higher surface area (i.e., 1957.3 m²  g^-1 ) than previously reported Ti-MOFs due to its high crystallinity. Furthermore, the visible-light-responsive photocatalytic activity of DGIST-1 was confirmed by the simultaneous generation of singlet oxygen (¹ O₂ ) and superoxide (^. O₂ ^- ) species, in addition to the highly efficient and selective oxidation of benzyl alcohol to benzaldehyde.

Synthetic investigation, structural analysis and photocatalytic study of a carboxylate-phosphonate bridged Ti18-oxo cluster

By the combination of carboxylate and phosphonate ligands, we herein report a nanosized H₂[Ti₁₈(μ₃-O)₁₄(μ₂-O)₆(O₃P-Phen)₂(PA)₁₆(OPri)₁₄] (PTC-51) (HPA = propionic acid, O₃P-Phen = phenyl phosphonate), whose structure can be observed as the linkage of independent phosphonate supporting and carboxylate bridging hexanuclear Ti(iv) clusters. We also demonstrate a concise crystalline phase diagram, which shows the important effect of phosphonate and carboxylate ligand ratio during synthetic reactions. The photocatalytic H₂ evolution activities of the obtained polyoxotitanate materials have been studied, with carboxylate-Ti₆ presenting the maximum H₂ production rate up to 267.78 μmol g^-1 h^-1.

Synthetic strategies, diverse structures and tuneable properties of polyoxo-titanium clusters

A review of polyoxo-titanium clusters (PTCs), with an emphasis on synthetic methodologies, diverse structures, tuneable optical properties and potential applications.

A novel hexanuclear titanium(iv)-oxo-iminodiacetate cluster with a Ti6O9 core: single-crystal structure and photocatalytic activities

A new family of hexanuclear titanium(iv)-oxo-carboxylate cluster K7H[Ti6O9(ida)6]Cl2·13H2O {Ti6O9} has been synthesized via the H2O2-assisted reaction between TiCl4 and iminodiacetate ligands. This cluster was fully characterized by single-crystal X-ray diffraction and a wide range of analytical methods, including FT-IR, UV/vis spectroscopy as well as electrochemistry and thermogravimetric analysis. As a new type of carboxylate substituted Ti-oxo-cluster, the structural motif of the {Ti6O9} cluster consists of one symmetric {Ti6O6} hexagonal prism with two staggered triangular {Ti3O3} subunits linked by three μ2-O bridges. The {Ti6O9} polyanions are linked by K(+) cations to form a novel 3D architecture. The structural information and stability of the {Ti6O9} polyanion in aqueous solution were thoroughly investigated by solid-state/solution NMR, ESI-MS spectroscopy. Moreover, this Ti-oxo cluster exhibits remarkable potential as a visible-light homogeneous photocatalyst for degradation of rhodamine B (RhB). Finally, a proposed peroxotitanium(iv)-mediated photocatalytic pathway involved is illustrated by spectroscopic data.

Two titanium(iv)-oxo-clusters: synthesis, structures, characterization and recycling catalytic activity in the oxygenation of sulfides

Two titanium(iv)-oxo-clusters were synthesized and can be used as homogeneous recycling catalysts in the oxygenation of sulfides.

Titanium coordination compounds: from discrete metal complexes to metal–organic frameworks

Recent advances in titanium based MOFs and relevant titanium molecular compounds will be discussed in this review. Particular attention will be devoted to their promising photoredox properties.

Synthesis and photocatalytic H2 evolution properties of four titanium-oxo-clusters based on a cyclohex-3-ene-1-carboxylate ligand

This work reveals an interesting Ti–Cec system with various structures, where PTC-91 and PTC-94 have good photocatalytic activity and stability in water-splitting hydrogen evolution.

Structure identification of Ti(iv) clusters in low-temperature TiO2 crystallization: creating high-surface area brush-shaped rutile TiO2

We demonstrate for the first time a controlled reaction of Cl-coordinating Ti(iv) clusters in low-temperature hydrolysis (27 °C) to create large-surface area rutile TiO₂ crystals.

Construction of molecular rectangles with titanium–oxo clusters and rigid aromatic carboxylate ligands

Four supramolecular rectangles have been successfully constructed using {Ti₅O₇} clusters as vertical edges and bridging aromatic carboxylates as horizontal edges.

Water-Soluble Pentagonal-Prismatic Titanium-Oxo Clusters

By using solubility control to crystallize the prenucleation clusters of hydrosol, a family of titanium-oxo clusters possessing the {Ti18O27} core in which the 18 Ti(IV)-ions are uniquely connected with μ-oxo ligands into a triple-decked pentagonal prism was obtained. The cluster cores are wrapped by external sulfate and aqua ligands, showing good solubilities and stabilities in a variety of solvents including acetonitrile and water and allowing their solution chemistry being studied by means of electrospray ionization mass spectroscopy, (17)O NMR, and vibrational spectroscopy. Furthermore, this study provides new titanium oxide candidates for surface modifications and homogeneous photocatalysis.

Novel properties and potential applications of functional ligand-modified polyoxotitanate cages

Functional ligand-modified polyoxotitanate (L-POT) cages of the general type [TixOy(OR)z(L)m] (OR = alkoxide, L = functional ligand) can be regarded as molecular fragments of surface-sensitized solid-state TiO2, and are of value as models for studying the interfacial charge and energy transfer between the bound functional ligands and a bulk semiconductor surface. These L-POTs have also had a marked impact in many other research fields, such as single-source precursors for TiO2 deposition, inorganic-organic hybrid material construction, photocatalysis, photoluminescence, asymmetric catalysis and gas adsorption. Their atomically well-defined structures provide the basis for the understanding of structure/property relationships and ultimately for the rational design of new cages targeting specific uses. This highlight focuses on recent advances in L-POTs research, with emphasis on their novel properties and potential applications.

A new cadmium-doped titanium–oxo cluster with stable photocatalytic H2 evolution properties

A new Cd-doped titanium–oxo cluster (TOC) with good H₂ evolution ability and high catalytic stability has been prepared and characterized.

Precursor directed synthesis--"molecular" mechanisms in the Soft Chemistry approaches and their use for template-free synthesis of metal, metal oxide and metal chalcogenide nanoparticles and nanostructures

This review provides an insight into the common reaction mechanisms in Soft Chemistry processes involved in nucleation, growth and aggregation of metal, metal oxide and chalcogenide nanoparticles starting from metal-organic precursors such as metal alkoxides, beta-diketonates, carboxylates and their chalcogene analogues and demonstrates how mastering the precursor chemistry permits us to control the chemical and phase composition, crystallinity, morphology, porosity and surface characteristics of produced nanomaterials.