Acetic Acid Mediated Synthesis of Phosphonate-Substituted Titanium Oxo Clusters

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.

Titanium-Oxo Clusters with Bi- and Tridentate Organic Ligands: Gradual Evolution of the Structures from Small to Big

Homometallic titanium oxo clusters are one of the most important groups of metal oxo clusters, with more than 300 examples characterized by X-ray structure analyses. Most of them are uncharged and are obtained by partial hydrolysis and condensation of titanium alkoxo derivatives. The cluster cores, ranging from 3 to >50 titanium atoms, are stabilized by organic ligands. Apart from residual OR groups, carboxylato and phosphonato ligands are most frequent. The article critically reviews and categorizes the known structures and works out basic construction principles by comparing the different cluster types.

Facile and scalable synthesis of Ti6Mn2 oxo-cluster nanocrystals with flower-like morphology and excellent photocatalytic properties

Three new phenylphosphonate (PhPO3) and salicylate (Sal) substituted heterometallic titanium-oxo clusters (ST-M), namely [Ti6Mn2O4(OEt)4(PhPO3)2(Sal)6(EtOH)2] (ST-Mn), [Ti6Zn2O4(OEt)4(PhPO3)2(Sal)6(EtOH)2(H2O)2] (ST-Zn) and [Ti6Tb2O4(OiPr)4(PhPO3)2(Sal)6Cl2(iPrOH)2]·iPrOH (ST-Tb), were synthesized. The most impressive structural feature of these compounds is the calixarene-type carboxylate endpoint in the Ti6-oxo core, which can readily capture metal ions with different radii and coordination geometries, for example, Mn2+, Zn2+ and Tb3+ ions in this case, to generate neutral mixed-metal Ti6M2-oxo clusters. Flower-like ST-Mn (denoted as FST-Mn) nanocrystals formed by nanosheets can be facilely obtained in gram-scale quantities from a simple solution reaction using Triton X-100 as a conditioning agent. The catalytic activity of these materials was evaluated by photocatalytic degradation of rhodamine B (RhB), among which FST-Mn showed the highest catalytic activity. Under visible light, 93.0% of RhB (50 ppm, 100 mL) was decomposed within 30 min catalyzed by FST-Mn without adding hydrogen peroxide, which is quite preeminent among Ti based catalysts. The catalytic decomposition efficiency under the same experimental conditions was 80.0% for ST-Mn, 24.6% for ST-Zn and 17.6% for ST-Tb. Investigation of the degradation mechanism showed that h+ and ·OH were the dominant active species in the decomposition of RhB. Moreover, the reusability and stability of FST-Mn were also verified.

Synthesis and structural characterization of a dumbbell-like phenylphosphonate-stabilized Ti7-oxide cluster

In recent years, crystalline polyoxotitanium clusters (PTCs) have attracted increasing attention as a new kind of promising crystalline material. In this work, a PTC stabilized by phenylphosphonate ligands, i.e. hexa-μ-isopropanolato-hexaisopropanolatodi-μ₃-oxido-hexa-μ₃-phenylphosphonato-heptatitanium tetrahydrate, [Ti₇(μ₃-O)₂(O₃P-Phen)₆(OⁱPr)₁₂]·4H₂O [PTC-54; H₂O₃P-Phen is phenylphosphonic acid, C₆H₅PO(OH)₂, and HOⁱPr is isopropanol, C₃H₇OH], was obtained successfully through a facile one-step solvothermal reaction. The dumbbell-like core structure of PTC-54 can be described as two trinuclear {Ti₃(μ₃-O)} building units combined together by six phenylphosphonate ligands and one additional Ti atom, giving rise to a Ti₇ cluster. Adjacent molecules of PTC-54 are further connected through π-π interactions between the phenyl groups to form a supramolecular one-dimensional chain.

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.

Ligand dependent assembly of trinuclear titanium-oxo units into coordination tetrahedra and capsules

Trinuclear Ti₃(μ₃-O) units have been applied as building units for the construction of different coordination cages such as {Ti₆} clusters, {Ti₉Cu} tetrahedrons and {Ti₁₈} capsules.

Structures and photophysical performances of (fluoro)salicylate stabilized polyoxo-titanium clusters

The introduction of fluorine in salicylic acid yielded different polyoxo-titanium clusters with distinct photophysical properties.

A 4-dimethylaminobenzoate-functionalized Ti6-oxo cluster with a narrow band gap and enhanced photoelectrochemical activity: a combined experimental and computational study

The D–π-A structured dye 4-dimethylaminobenzoate (DMABA) functionalized Ti₆-oxo cluster exhibits a remarkably reduced band gap of ∼2.5 eV.

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.

Deep eutectic-solvothermal synthesis of titanium-oxo clusters protected by π-conjugated chromophores

Deep eutectic-solvothermal synthesis has been employed to prepare titanium-oxo clusters with π-conjugated chromophoric shells and different photocatalytic H₂ production activities.

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.

Role of functionalization: strategies to explore potential nano-bio applications of magnetic nanoparticles

Strategies to bridge the gap between magnetic nanoparticles for their nano bio applications.

Phosphonate-substituted zirconium oxo clusters

ABSTRACT

The phosphonate-substituted zirconium oxo clusters Zr6O2(OBu)12(O3PPh)4 and Zr7O2(OiPr)12(O3PCH2CH2CH2Br)6, with octahedrally coordinated Zr atoms, were synthesized by reaction of zirconium alkoxides with phosphonic acid bis(trimethylsilyl) esters. The basic structural motif are Zr3O(µ2-OR)3(OR)3 units which are connected in different ways.

GRAPHICAL ABSTRACT

On the Question of Site-Selective Ligand Exchange in Carboxylate-Substituted Metal Oxo Clusters

Reaction of [Ti₄Zr₄O₆(OBu)₄(OMc)₁₆] (OMc = methacrylate) with acetylacetone (acacH) resulted in dissection of the cluster and formation of [Ti(OBu)₂(acac)₂] and the smaller cluster [Ti₂Zr₄O₄(OMc)₁₆]. In contrast, the same reaction with [Zr₆O₄(OH)₄(OOCR)₁₂]₂·6RCOOH (R = Et, CH₂CH=CH₂) led to site-selective substitution of two carboxylate ligands and formation of isostructural [Zr₆O₄(OH)₄(OOCR)_12–x(acac)_x]₂·6RCOOH (x ≤ 1).

Titanium oxo/alkoxo clusters with both phosphonate and methacrylate ligands

ABSTRACT

The clusters Ti5O(OiPr)11(OMc)(O3PR)3 (OMc = methacrylate; R = Et, CH2CH2CH2Br) and Ti10(OiPr)16(OMc)4(O3PCH2CH=CH2)10 were obtained when Ti(OiPr)4 was reacted with the corresponding bis(trimethylsilyl) phosphonate and methacrylic acid. Oxo clusters of the composition Ti6O4(OiPr)10(OMc)2(O3PR)2, with a variety of groups R (Et, Ph, CH=CH2, CH2Ph, CH2CH=CH2, CH2CH2CH2Br, CH2CH2CN, CH2C(O)Me, CH2CH2OC(O)C(Me)=CH2), were formed instead, when a stoichiometric amount of water was added to the reaction mixture.

GRAPHICAL ABSTRACT