Metal oxide cluster-anion (polyoxometalate, or POM) protecting ligands, [α-PW11O39]7– (1), modify the rates at which 14 nm gold nanoparticles (Au NPs) catalyze an important model reaction, the aerobic (O2) oxidation of CO to CO2 in water. At 20 °C and pH 6.2, the following stoichiometry was observed: CO + O2 + H2O = CO2 + H2O2. After control experiments verified that the H2O2 product was sufficiently stable and did not react with 1 under turnover conditions, quantitative analysis of H2O2 was used to monitor the rates of CO oxidation, which increased linearly with the percent coverage of the Au NPs by 1 (0–64% coverage, with the latter value corresponding to 211 ± 19 surface-bound molecules of 1). X-ray photoelectron spectroscopy of Au NPs protected by a series of POM ligands (K+ salts): 1, the Wells–Dawson ion [α-P2W18O62]6– (2) and the monodefect Keggin anion [α-SiW11O39]8– (3) revealed that binding energies of electrons in the Au 4f7/2 and 4f5/2 atomic orbitals decreased as a linear function of the POM charge and percent coverage of Au NPs, providing a direct correlation between the electronic effects of the POMs bound to the surfaces of the Au NPs and the rates of CO oxidation by O2. Additional data show that this effect is not limited to POMs but occurs, albeit to a lesser extent, when common anions capable of binding to Au-NP surfaces, such as citrate or phosphate, are present.

Photocatalytic organic synthesis needs photocatalysts to initiate the reactions and to control the reaction paths. Available photocatalytic systems rely on electron transfer or energy transfer between the photoexcited catalysts and the substrates. We explore a concept based on the photopromoted catalyst coupling to the substrate and the phototriggered catalyst regeneration by elimination from the catalytic cycle. A catalytic amount of elementary I2 is applied as both a visible light photocatalyst and a π Lewis acid, enabling the direct activation of alkyne C≡C bonds for electrophilic cyclization reactions, one of the most important reactions of alkynes. Visible light is crucial for both the iodocyclization of the propargyl amide and the deiodination of the intermediate. Singlet oxygen is found to play a key role in the regeneration of I2. This system shows good functional group compatibility for the generation of substituted oxazole aldehydes and indole aldehydes. Hence, this study provides a readily accessible alternative catalytic system for the construction of heterocycle aldehyde derivatives by sunlight photocatalysis.Keywords: alkyne; iodine; photocatalysis; photoredox; visible light;

This work focuses on a new strategy to overcome the overoxidation in heterogeneous TiO2 photocatalysis and to realize high-efficiency photosynthesis. We demonstrate that TiO2 photocatalysis can integrate C–C and C═O formation in a tandem manner to achieve efficient oxidative cyclization for the syntheses of aryltetralones. This protocol does not need any additive besides the inexpensive and/or recyclable TiO2, O2, and (solar) light. High yields with excellent diastereoselectivities are obtained for a wide scope of electron-rich substrates. Our findings demonstrate that in contrast to the conventional overoxidation, as long as the radical cations possess sufficient reactivity toward nucleophilic addition, single-electron transfer processes in TiO2 photocatalysis can be developed into a powerful tool to construct C–C bonds and even strained carbon rings.Keywords: lignan; photocatalysis; photoredox; synthesis; titanium dioxide;

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, 17O NMR, and vibrational spectroscopy. Furthermore, this study provides new titanium oxide candidates for surface modifications and homogeneous photocatalysis.

During solvothermal alcoholysis of a mixture of TiI4 and Ti(OiPr)4, a {I@Ti22} cage cluster encapsulating an OH and iodide guests is crystallized. The {I@Ti22} host–guest cluster surface is postfunctionalizable with catecholate and carboxylate ligands. The synthetic details, structural characterization, spectroscopic properties of the obtained cages clusters are provided. The present study provides candidates for modeling ligand exchange and electron-hole transfer at the titanate nanoparticle surface, and meanwhile offers new opportunities for understanding the TiO2 nanocrystalline formation in solvothermal processes.

For sol–gel synthesis of titanium oxide, the titanium(IV) precursors are dissolved in water to form clear solutions. However, the solution status of titanium(IV) remains unclear. Herein three new and rare types of titanium oxo clusters are isolated from aqueous solutions of TiOSO4 and TiCl4 without using organic ligands. Our results indicate that titanium(IV) is readily hydrolyzed into oxo oligomers even in highly acidic solutions. The present clusters provide precise structural information for future characterization of the solution species and structural evolution of titanium(IV) in water and, meanwhile, are new molecular materials for photocatalysis.

Herein we report the syntheses and the X-ray structure of [Cs8X27]19- (X = Cl, Br) clusters, the first binary cluster anions isolated in bulk crystal structures. They were obtained by electrostatic capture and face-directed recognition of the prenucleation [CsmCln](n−m)– clusters from water solutions, using [M4(OH)8(OH2)16]8+ (M = ZrIV or HfIV) as the counter cations. These compounds have been thoroughly characterized with a variety of techniques including vibrational spectroscopy and superionic conductivity analysis. This work not only provides structural models for a better understanding of the nucleation of binary materials but also shows that magic number binary clusters adopting a cubic lattice structure do form, in agreement with the time-honored theoretical and spectroscopic predictions.

In this paper, using a simple method, 17 isostructural polyoxotitanates (POTs) were synthesized, including the pristine [Ti12O16(OiPr)16], the monodefected [Ti11O13(OiPr)18], and the heterometal-doped [Ti11O14(OiPr)17(ML)] (M = Mg, Ca, Zn, Cd, Co, or Ni; L = Cl, Br, I, or NO3). The electronic structures of these POTs were determined by UV–vis spectroscopy and DFT calculations. Upon UV irradiation of the POTs, electron spin resonance showed the formation of TiIII under anaerobic conditions and superoxide (O2•–) in the presence of O2. The photoactivities of the POTs were then probed with TiIII production and short-circuit photocurrent experiments. The photophysical processes were studied using steady-state and transient photoluminescence. The results show that within the very similar structures, the deexcitation processes of the photoexcited POTs can be greatly affected by the dopants, which result in enhanced or decreased photoactivities. Co and Ni doping enhances the absorption of the visible light accompanied by serious loss of UV photoactivities. On the other hand, a Ti vacancy (in [Ti11O13(OiPr)18]) does not reduce the band gap of a POT but improves the UV photoactivities by serving as surface reaction site. The POTs were then used as molecular models of titanium oxide nanoparticles to understand some important issues relevant to doped titanate, i.e., coordination environment of the dopant metal, electronic structure, photoactivities, and photophysical processes. Our present findings suggest that for solar energy harvesting applications of titanium oxides like photocatalysis and solar cells substitution of titanium atoms by transition metal ions (like Co and Ni) to extend the absorption edges may not be an efficient way, while loading of Ti vacancies is very effective.

A novel titanium(IV) oxo cluster comprised solely of Ti, O, and H atoms, [Ti6(Oμ)8(OtH2)20]8+ (Ti6) was synthesized in high yield via controlled hydrolysis and condensation of TiX4 (X = Cl, Br) in the presence of TBAX (TBA = tetrabutylammonium; X = Cl, Br) from water, while reactions of TiI4 and TBAI yielded [Ti8O12(OH2)24]8+ (Ti8). The structures and compositions of the clusters were determined by single-crystal X-ray crystallography, powder X-ray diffraction, inductively coupled plasma atomic emission spectrometry, and energy-dispersive spectrometry. Ti6 is comprised of six-coordinated titanium(IV) atoms bridged with μ2-O atoms, structurally similar to a typical Lindqvist polyoxometalate. On the basis of a structural comparison of Ti6 and Ti8, density functional theory calculations, and spectroscopic analysis, it is evident that both clusters are stabilized by halide counteranions via the formation of hydrogen bonds. This study not only presents the second example of a titanium(IV) isopolyoxocationic cluster isolated from water but also suggests that counteranions are generally important for synthesizing molecular fragments of titanium oxides.

•A tetratitanium oxide cluster functionalized with catecholate ligands was synthesized.•Significant ligand-to-metal (LMCT) charge transfer between the sensitizer and the titanium-oxo framework was characterized.•The charger transfer mechanism was discussed.Understanding the charge transfer processes at the dye/TiO2 interface is often modeled using catechol/TiO2 systems. However, it indeed require precise structural information. In order to simulate the dye/TiO2 systems at a molecular level, herein we report the synthesis and structure of a novel titanium oxo-cluster [Ti4O(OiPr)6(DTBC)4] (1) functionalized with 3,5-di-tert-butylcatechol (DTBC). UV–Vis spectroscopy, electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations show that 1 exhibits property of Ti(III) which is caused by substantial charge transfer from the π electrons of the benzene rings to Ti3d orbitals. The present precise structure and charge transfer mechanism help to better understand chromophore binding to semiconductor surfaces and charge transfer at the dye/TiO2 interfaces.A tetratitanium oxide cluster functionalized with catecholate ligands was synthesized. The spontaneous charge transfer from the π-orbitals of benzene rings to the Ti3d orbitals is responsible for the visible absorption and the formation of Ti(III) under ambient conditions.

The highly efficient and chemoselective α,α′-bis-substitution of alkanones is important in organic synthesis. Herein, a dimeric titanium cluster, Ti2Cl2(OPri)6·2HOPri (Ti2), is used in the Claisen–Schmidt condensation reaction, for the selectively activation of symmetrical ketones containing α,α′-methylene groups and production of α,α′-bis-substituted alkanones in high efficiency and chemoselectivity. The high efficiency and chemoselectivity can be extended to a variety of typical alkanones and aromatic aldehydes. Both of the oxo-bridged dimeric motif of Ti2 and the ionic Ti–Cl bond are responsible for the high efficiency and chemoselectivity.

Electrocatalysis by polyoxometalate (POM)-monolayer protected gold nanoparticles is herein demonstrated using a newly discovered phenomenon that makes it possible to observe the electrochemistry of dilute aqueous solutions of these colloidal nanostructures. To preserve the integrity of the gold nanoparticles’ electrostatically-stabilized POM-monolayer structures, deposition and drying of the POM-protected metal(0) NPs on the electrode surface must be avoided. Overcoming this constraint, we here show that POM-monolayer protected gold nanoparticles can be induced to reversibly associate with electrode surfaces, resulting in dramatic current amplification and well behaved, quasi-reversible cyclic voltammetric behavior at remarkably small electrolyte concentrations, thus making it possible to investigate electrocatalysis by dilute aqueous solutions of POM-protected gold NPs.