Hybrid polyoxometalate materials for photo(electro-) chemical applications

As one of the most prosperous classes of cluster-based materials reported to date, polyoxo-titanium clusters (PTCs) have been closely related to many photo-activities that broadly impact not only chemical but also energy and environmental sciences. In contrast to the well-developed polyoxometalates like polyoxotungstates and polyoxomolybdates, there is still large room for the development of PTCs. The exploration of crystalline PTC materials originates from the molecular model of technically important TiO2 materials but has been greatly hindered by their daunting and challenging synthesis. This review firstly summarizes the conventional and latest successful synthetic strategies applied to improve the poor degree of control of crystallization of PTCs. And attributed to the synthetic progress achieved in this area, there is a growing number of PTCs with diverse structures known to us, also enabling us to study their bandgap engineering and light absorption behaviours at the molecular level. In addition, exploitation of their applications in many fields is also under way.

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

We report a 3.6 nm Ti52–oxo cluster with precise atomic structure, which presents a largest size record in the family of titanium–oxo clusters (TOCs). The crystal growth of such large Ti52 is based on a stepwise interlayer assembly approach from Ti6 substructures. The possible growth mechanism of Ti52 could be deduced from crystal structures of two substructures, Ti6 and Ti17, which were also synthesized under similar conditions as Ti52. Moreover, these TOCs show cluster-size-dependent photocatalytic hydrogen evolution activities with Ti52 giving a H2 production rate up to 398 μmol/h/g, which is also the highest record in the family of TOCs. This work not only represents a milestone in constructing large TOCs with comparable sizes as TiO2 nanoparticles but also brings significant advances in improving photocatalytic behaviors of TOCs.

A 3.6 nm Ti52-Oxo Nanocluster with Precise Atomic Structure.

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 (ie, 19573 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

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

Phase isomerism is a common and important phenomenon in inorganic materials, but the molecular isomerism of their nanocluster models is still rare. In this work, we report the first pair of isomeric titanium-oxo clusters. Through combining pentagonal {Ti(Ti5 )} building units in corner-sharing or edge-sharing forms, the two obtained Ti20 -oxo clusters take vertical and horizontal core configurations, respectively, which are both functionalized by the same organic ligands. More interestingly, the vertical type Ti20 -oxo core contains an identical {Ti8 O14 } moiety as anatase, making it the first molecular model of anatase TiO2 . As a result, the vertical type cluster exhibits better photocatalytic H2 evolution activity, higher photocurrent response and faster charge transfer to external acceptors than its horizontal isomer. Thus, this work provides a cluster isomer method to understand the structure-property relationship of import titanium oxide materials.

Isomerism in Titanium‐Oxo Clusters: Molecular Anatase Model with Atomic Structure and Improved Photocatalytic Activity

Attention has been paid to titanium–oxo clusters (TOCs) modified with functional molecules, because they can be considered as model systems for dye-sensitized titanium oxides in terms of their information in structures and electron transfer. We select 9-anthracenecarboxylate (9-AC) as a photoactive ligand and prepare two model compounds, [Ti6O6(OiPr)6(9-AC)6] (1) and [Ti6O4(OiPr)6(cat)4(9-AC)2] (2) (where cat = catecholate). Structures of the TOCs and the dye–TOC linkage are characterized by single-crystal analysis. Solvent-induced fluorescence change is observed for the cluster solution, and the fluorescence can be turned off by irradiating and on by oxygen bubbling. Photoinduced Ti(III) is responsible for the fluorescence extinction. The photocurrent conversion property of the clusters is examined by use of a three-electrode cell with cluster-coated indium tin oxide (ITO) electrodes. The results indicate that 9-AC is an effective photosensitizer and cluster 1 shows higher photocurrent intensity for its ...

Titanium–Oxo Cluster with 9-Anthracenecarboxylate Antennae: A Fluorescent and Photocurrent Transfer Material

Two 9-anthracenecarboxylate (9-AC) coordinated heterometallic TOC compounds, [Ln2Ti10O14(ClO4)2(OiPr)14(9-AC)2(CH3CN)2]·2H2O, Ln = Nd (1) and Eu (2), along with two benzoate (bza) analogues, [Ln2Ti10O14(ClO4)2(OiPr)14(bza)2(HOiPr)2], Ln = Nd (3) and Eu (4), were prepared by one step in situ solvothermal synthesis, and characterized by single crystal analysis. To date, only about ten lanthanum-titanium oxo clusters have been reported. Except for two Ti28 clusters, the compounds are all small clusters (Ti ≤ 4). The cluster structures of 1–4 adopt a Ti10Ln2 core structure with pseudo-Ci symmetry, which is similar to the fundamental structure of Ti12 clusters. Furthermore, except for their structures, the properties of lanthanum-titanium oxo clusters have never been studied. Herein, the fluorescence properties of these compounds are studied in detail. The metal centered emission of Eu(III) is completely quenched by 9-AC due to the lower triplet energy of the coordinated ligand. Enhanced ligand centered fluorescence is found for 2 in comparison with that of 1.

Fluorescence and energy transfer properties of heterometallic lanthanide-titanium oxo clusters coordinated with anthracenecarboxylate ligands.

Visible-light-driven photocatalytic reductive azaarylation has been widely used to construct the important imine-containing azaarene derivatives. In addition to the direct use of various commercially available cyanoazaarenes as feedstocks, the synthetic advantages include precise regioselectivity, high efficiency, mild reaction conditions, and good functional group tolerance. However, although many efficient reductive azaarylation methods have been established, the example of an enantioselective manner is still unmet, which most likely can be ascribed to the highly reactive radical coupling as the key step of forming stereocenters. Exploring the feasibility of enantiocontrol thus constitutes an attractive but highly challenging task. Here, we demonstrate that chiral hydrogen-bonding/photosensitizer catalysis is a viable platform as it enables the realization of the first enantioselective manifold. A variety of acyclic and cyclic enones as the reaction partners are compatible with the dual catalyst system, leading to a wide array of valuable enantioenriched azaarene variants with high yields and ees. Regulating the types of chiral catalysts represents one of the important manners to success, in which several readily accessible Cinchona alkaloid-derived bifunctional catalysts are introduced in asymmetric photochemical reactions.

Catalytic Asymmetric Reductive Azaarylation of Olefins via Enantioselective Radical Coupling.

Recently, discrete large Tn clusters have become particularly attractive because they can be used as starting materials not only for constructing porous chalcogenides but also for solution processing of semiconductor film materials with electronic and optical properties. However, the discrete large clusters of main-group metal chalcogenides are difficult to stabilize because of their high negative charge. We report herein a new crystal compound co-assembled by discrete T4 [Cu4In16S35H4]14–and cubic [Cu12S8]4– clusters. It is uncommon that the negatively charged CuS cluster acts as a template to sustain a hexagonal cavity formed by negatively charged T4 clusters. Its degeneration product shows an excellent near-infrared absorption property and photocurrent responsive properties.

Co-assembled T4-Cu4In16S35 and Cubic Cu12S8 Clusters: A Crystal Precursor for Near-Infrared Absorption Material

Chiral hydrogen-bonding (H-bonding) catalytic asymmetric conjugate addition to activated olefins has been widely used to access enantioenriched molecules containing stereocentres at the β-position of the olefin activating groups. Herein, we report the first highly enantioselective radical-based manifold. Under a dual organocatalyst system involving a chiral phosphoric acid and DPZ as the photoredox sensitizer, transformations of N-arylglycines, in which aryls with CF3 substituents are introduced, with alkenyl azaarenes afforded valuable hydroaminoalkylation adducts with satisfactory results. In addition to the diversity of azaarenes, the method can be used to construct aryl-, alkyl- and silyl-substituted stereocentres. Control experiments and density functional theory calculations were performed to elucidate a plausible reaction mechanism and the origin of stereoselectivity, wherein nonclassical H-bonding interactions were found to assist chiral catalysts in offering sufficient enantiocontrol.

Xiao-Wei Zhao

Papers

Titanium–Oxo Cluster with 9-Anthracenecarboxylate Antennae: A Fluorescent and Photocurrent Transfer Material

Fluorescence and energy transfer properties of heterometallic lanthanide-titanium oxo clusters coordinated with anthracenecarboxylate ligands.

Catalytic Asymmetric Reductive Azaarylation of Olefins via Enantioselective Radical Coupling.

Co-assembled T4-Cu4In16S35 and Cubic Cu12S8 Clusters: A Crystal Precursor for Near-Infrared Absorption Material

Asymmetric Hydroaminoalkylation of Alkenylazaarenes via Cooperative Photoredox and Chiral Hydrogen-Bonding Catalysis.