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.
