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

Abstract

Conspectus Atomically 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.

Platonic and Archimedean solids in discrete metal-containing clusters

“…220 Recently, Jian Zhang and Lei Zhang team applied an effective coordination-delayed-hydrolysis strategy for the synthesis and structural modulation of atomically precise titanium-oxo clusters. 2,68,221 Although there have been many examples of the construction of giant Ti-O clusters (with nuclearity of 30 or greater), with up to 52 metal-core, 222 there is only one Ti-O cluster (Ti 42 ) with polyhedral feature. Through the reaction of Ti(O i Pr) 4 with formic acid under solvothermal conditions, this group isolated the first fullerene-like Ti-O cage, H 6 [Ti 42 (m 3 -O) 60 (O i Pr) 42 (OH) 12 )] (Ti 42 , Fig.…”

Calix[4]arene-Functionalized Titanium-Oxo Compounds for Perceiving Differences in Catalytic Reactivity Between Mono- and Multimetallic Sites

“…Beyond that, the intrinsic ultraviolet photosensitivity of TiO 2 also seriously weakens the utilization efficiency of sunlight in photocatalytic reactions. Recently, titanium-oxo compounds (TOCs) with similar compositions to TiO 2 have developed rapidly. − Compared with TiO 2 , TOCs have more diverse structures, and their intrinsic properties, such as stability, semiconductor-like behaviors, energy levels, and light absorption capacity, can be effectively regulated through modification with different functionalized organic ligands. ,− In particular, as an aggregate containing multiple metal ions, TOCs can create multiple active metal sites via rationally regulating the coordination environment of metal ions, − thus providing more opportunities for the catalytic activity improvement and application of photocatalytic reactions.…”

Divalent Heterometal Doped Titanium-Oxide Cluster Polymers: Structures, Photoresponse, and Photocatalysis

“…Titanium-oxide clusters (TOCs), − an important and highly active subgroup of MOCs, are a family of discrete, molecular titanium oxides with sizes comparable to TiO 2 nanoparticles (NPs). They have been extensively synthesized in the last ten years, because of their role as molecular mimics of TiO 2 NPs and their promising potential for sunlight harvesting. , On the surfaces of TOCs, there are several functionalizable coordination sites and exchangeable ligands. ,− Thus, they might be employed as fundamental structural units in the one-pot or post-functionalization approach to developing hierarchical architectures.…”