Two Mn(II)-Bridged Silverton-Type [UMo12O42]8- Polyoxometalates with Catalytic Activity for the Synthesis of Pyrazoles.

Two Mn(II)-bridged Silverton-type {UMo12O42}-based polyoxomolybdates with different three-dimensional structures, Na6(H2O)12[Mn(UMo12O42)] (NaMn) and (NH4)2[K2Na6(µ4-O)2(H2O)1.2Mn(UMo12O42)]·4.6H2O (KMn), were hydrothermally synthesized and further characterized, demonstrating a feasible strategy for the assembly of Silverton-type polyoxomolybdates. Additionally, NaMn is demonstrated to be a good heterogeneous catalyst in the condensation cyclization reaction of hydrazines and 1,3-diketones, and a range of valuable pyrazoles were produced in up to 99% yield.

Polyoxometalate-Cyclodextrin-Based Cluster-Organic Supramolecular Framework for Polysulfide Conversion and Guest-Host Recognition in Lithium-sulfur Batteries.

Developing polyoxometalate-cyclodextrin cluster-organic supramolecular framework (POM-CD-COSF) still remains challenging due to an extremely difficult task in rationally interconnecting two dissimilar building blocks. Here we report an unprecedented POM-CD-COSF crystalline structure produced through the self-assembly process of a Krebs-type POM, [Zn2 (WO2 )2 (SbW9 O33 )2 ]10- , and two ß-CD units. The as-prepared POM-CD-COSF-based battery separator can be applied as a lightweight barrier (approximately 0.3 mg cm-2 ) to mitigate the polysulfide shuttle effect in lithium-sulfur batteries. The designed Li-S batteries equipped with the POM-CD-COSF modified separator exhibit remarkable electrochemical performance, attributed to fast Li+ diffusion through the supramolecular channel of ß-CD, efficient polysulfide-capture ability by the dynamic host-guest interaction of ß-CD, and improved sulfur redox kinetics by the bidirectional catalysis of POM cluster. This research provides a broad perspective for the development of multifunctional supramolecular POM frameworks and their applications in Li-S batteries.

Efficient Oxygen Evolution Reaction on Polyethylene Glycol-Modified BiVO4 Photoanode by Speeding up Proton Transfer.

The rate-determining step of the oxygen evolution reaction based on a semiconductor photoanode is the formation of the OO bond. Herein, polyethylene glycol (PEG)-modified BiVO4 photoanodes are reported, in which protons can be transferred quickly due to the high proton conductivity of PEG, resulting in the acceleration of the OO bond formation rate. These are fully demonstrated by different kinetic isotope effect values. Moreover, the open-circuit voltage (Uoc ) further illustrates that PEG passivates the surface states and surface charge recombination is reduced. The composite photoanode can achieve a maximum photocurrent density of 3.64 mA cm-2 at 1.23 V compared to 1.04 mA cm-2 for pure BiVO4 , and an onset potential of 170 mV, which is a 230 mV negative shift compared to pure BiVO4 . This work provides a new strategy to accelerate water oxidation kinetics for photoanodes by speeding up the transfer of the proton and the OO bond formation rate.

Recent Advances of Anderson-Type Polyoxometalates as Catalysts Largely for Oxidative Transformations of Organic Molecules.

Anderson-type ([XM6O24]n-) polyoxometalates (POMs) are a class of polymetallic-oxygen cluster inorganic compounds with special structures and properties. They have been paid extensive attention by researchers now, due to their chemical modification and designability, which have been widely applied in the fields of materials, catalysis and medicine. In contemporary years, the application of Anderson-type POMs in catalytic organic oxidation reaction has gradually shown great significance for the research of green catalytic process. In this paper, we investigate the application of Anderson-type POMs in organic synthesis reaction, and these works are summarized according to the different structure of POMs. This will provide a new strategy for further investigation of the catalytic application of Anderson-type POMs and the study of green catalysis.

Light-Induced Efficient Hydroxylation of Benzene to Phenol by Quinolinium and Polyoxovanadate-Based Supramolecular Catalysts.

Direct Hydroxylation of benzene to phenol with high yield and selectivity has been the goal of phenol industrial production. Photocatalysis can serve as a competitive method to realize the hydroxylation of benzene to phenol owing to its cost-effective and environmental friendliness, however it is still a forbidding challenge to obtain good yield, high selectivity and high atom availability meanwhile. Here we show a series of supramolecular catalysts based on alkoxohexavanadate anions and quinolinium ions for the photocatalytic hydroxylation of benzene to phenol under UV irradiation. We demonstrate that polyoxoalkoxovanadates can serve as efficient catalysts which can not only stabilize quinolinium radicals but also reuse H2 O2 produced by quinolinium ions under light irradiation to obtain excellent synergistic effect, including competitive good yield (50.1 %), high selectivity (>99 %) and high atom availability.

Selective Oxidation of Anilines to Azobenzenes and Azoxybenzenes by a Molecular Mo Oxide Catalyst.

Aromatic azo compounds, which play an important role in pharmaceutical and industrial applications, still face great challenges in synthesis. Herein, we report a molybdenum oxide compound, [N(C4 H9 )4 ]2 [Mo6 O19 ] (1), catalyzed selective oxidation of anilines with hydrogen peroxide as green oxidant. The oxidation of anilines can be realized in a fully selectively fashion to afford various symmetric/asymmetric azobenzene and azoxybenzene compounds, respectively, by changing additive and solvent, avoiding the use of stoichiometric metal oxidants. Preliminary mechanistic investigations suggest the intermediacy of highly active reactive and elusive Mo imido complexes.

Polyoxometalate-Based Photoactive Hybrid: Uncover the First Crystal Structure of Covalently Linked Hexavanadate-Porphyrin Molecule.

Polyoxometalates (POMs)-porphyrin hybrids can serve as multifunctional materials with fascinating photocatalytic and photovoltaic properties. However, most previous POM-porphyrin hybrids are synthesized relied on electrostatic interactions to form ion pairs, which is not stable enough and subject to leaching and poor electronic communication. To our knowledge, no specific crystalline structure of direct covalently tris-functionalized POM-porphyrin hybrids has been identified. Herein, we discover an unprecedented polyoxometalates (POMs)-based photoresponsive cluster, {V6O13[ZnC61H58N5O4]2}2- (denoted as V6-Zn-2Por), which can be synthesized by covalently grafting two tris-functionalized Zn-porphyrin ligands onto Lindqvist-type hexavanadate cluster using decavanadates (TBA)3[H3V10O28] (denoted as V10, TBA = tetrabutylammonium cation) as precursor. Additionally, using tetraphenyl phosphonium as counterion, for the first time, a high-quality single crystal structure of the hybrid hexavanadate-porphyrin molecule is uncovered. Interestingly, the fluorescence emission spectra show that the fluorescence intensity of the organic-inorganic hybrid is partly quenched compared to pristine porphyrins, indicating possible energy/electron transfer between POMs cluster and porphyrin under light irradiation. Their UV-vis diffuse reflectance spectra show an extended absorption in the visible-light range. Finally, the as-prepared photoresponsive hexavanadate-porphyrin molecule is proved to exhibit effective photocatalytic activity toward removal of rhodamine B (an organic dye) under visible-light illumination.

Effects of different 2A peptides on transgene expression mediated by tricistronic vectors in transfected CHO cells.

Multicistronic vectors can increase transgene expression and decrease the imbalance of gene expression in the Chinese hamster ovary (CHO) cell expression system. Small, self-cleaving 2A peptides have a high cleavage efficiency and are essential for constructing high-expression multicistronic vectors. In this study, we investigated the effects of two different 2A peptides on transgene expression in CHO cells via their mediating action on tricistronic vectors. The enhanced green fluorescent protein (eGFP) and red fluorescent protein (RFP) genes were linked by the porcine teschovirus-1 (P2A) and Thosea asigna virus (T2A) peptides in a multicistronic vector. We transfected CHO cells with these vectors and screened for the presence of blasticidin-resistant colonies. Flow cytometry and real-time quantitative PCR (qPCR) were used to detect the expression levels of eGFP and RFP and the copy numbers of stably transfected cells. The results showed that P2A could enhance eGFP and RFP expression by 1.48- and 1.47-fold, respectively, compared to T2A. The expression levels of the genes were not proportional to their copy numbers. In conclusion, we found that P2A can effectively drive transgene expression in CHO cells and a potent 2A peptide can be used for recombinant protein production in the CHO cell system.

A Series of Weakley-type Polyoxomolybdates: Synthesis, Characterization, and Magnetic Properties by a Combined Experimental and Theoretical Approach.

Using DCC as the dehydrating agent, a series of Weakley-type polyoxomolybdates [Bu4N]3{Ln[Mo5O13(OMe)4(NO)]2} (Ln = Tb, Dy, Ho, Er) were synthesized in a one-pot reaction and structurally characterized by elemental, IR, UV-vis analysis, PXRD, and single-crystal X-ray diffraction. Furthermore, the static and dynamic measurements were utilized to investigate their magnetic performances. Typically, slow relaxation of magnetization was observed for Dy analogues with an energy barrier for the reversal of the magnetization of 50 K, which is the highest barrier height observed on the polyoxomolybdates-based single-molecule magnets (SMMs). For a deep understanding of the appearance of the SMM behavior on Weakley-type polyoxomolybdates series, ab initio calculations on {Dy[Mo5O13(OMe)4(NO)]2}3- have been conducted.

Effect of Cation-π Interaction on Macroionic Self-Assembly.

A series of rod-shaped polyoxometalates (POMs) [Bu4 N]7 [Mo6 O18 NC(CH2 O)3 MnMo6 O18 (OCH2 )3 CNMo6 O18 ] and [Bu4 N]7 [ArNMo6 O17 NC(CH2 O)3 MnMo6 O18 (OCH2 )3 CNMo6 O17 NAr] (Ar=2,6-dimethylphenyl, naphthyl and 1-methylnaphthyl) were chosen to study the effects of cation-π interaction on macroionic self-assembly. Diffusion ordered spectroscopy (DOSY) and isothermal titration calorimetry (ITC) techniques show that the binding affinity between the POMs and Zn2+ ions is enhanced significantly after grafting aromatic groups onto the clusters, leading to the effective replacement of tetrabutylammonium counterions (TBAs) upon the addition of ZnCl2 . The incorporation of aromatic groups results in the significant contribution of cation-π interaction to the self-assembly, as confirmed by the opposite trend of assembly size vs. ionic strength when compared with those without aromatic groups. The small difference between two aromatic groups toward the Zn2+ ions is amplified after combining with the clusters, which consequently triggers the self-recognition behavior between two highly similar macroanions.

Transition-Metal-Controlled Inorganic Ligand-Supported Non-Precious Metal Catalysts for the Aerobic Oxidation of Amines to Imines.

Most state-of-art transition-metal catalysts usually require organic ligands, which are essential for controlling the reactivity and selectivity of reactions catalyzed by transition metals. However, organic ligands often suffer from severe problems including cost, toxicity, air/moisture sensitivity, and being commercially unavailable. Herein, we show a simple, mild, and efficient aerobic oxidation procedure of amines using inorganic ligand-supported non-precious metal catalysts 1, (NH4 )n [MMo6 O18 (OH)6 ] (M=Cu2+ ; Fe3+ ; Co3+ ; Ni2+ ; Zn2+ , n=3 or 4), synthesized by a simple one-step method in water at 100 °C, demonstrating that the catalytic activity and selectivity can be significantly improved by changing the central metal atom. In the presence of these catalysts, the catalytic oxidation of primary and secondary amines, as well as the coupling of alcohols and amines, can smoothly proceed to afford various imines with O2 (1 atm) as the sole oxidant. In particular, the catalysts 1 have transition-metal ion core, and the planar arrangement of the six MoVI centers at their highest oxidation states around the central heterometal can greatly enhance the Lewis acidity of catalytically active sites, and also enable the electrons in the center to delocalize onto the six edge-sharing MO6 units, in the same way as ligands in traditional organometallic complexes. The versatility of this methodology maybe opens a path to catalytic oxidation through inorganic ligand-coordinated metal catalysis.

Buildup of Redox-Responsive Hybrid from Polyoxometalate and Redox-Active Conducting Oligomer: Its Self-Assemblies with Controllable Morphologies.

A redox-responsive hybrid of polyoxometalate and conducting oligomer including its self-assemblies with controllable morphologies are reported. To this end, a hybrid molecule, containing a Lindqvist hexamolybdate as the polar head group and N,N'-bis(4'-amino-2,6-dimethylphenyl)-1,4-quinonediimine as the redox-responsive and aggregating group, is prepared. This hybrid exhibits redox-responsive behavior with controllable assembling morphological transition from spherical vesicles to short cylindrical vesicles. Besides, the hybrid-based self-assemblies are transferred to the surface, thus the surface wettability can be well-tuned owing to the morphological transitions of the self-assemblies. By marrying conducting materials with polyoxometalate chemistry, this research opens a new horizon of polyoxometalate-based self-assembled systems with potential applications in functional materials.

Experimental evidence of charge transfer in a functionalized hexavanadate: a high resolution X-ray diffraction study.

A high resolution X-ray diffraction study has been carried out on [(C4H9)4N]2 [V6O13{(OCH2)3CCH2OCCH2CH3}2] (V6-C3) at 100 K. The V6 core possesses a negative charge, leading to a strong polarization of the anion. A nucleophilic region localized near the organic moiety and an electrophilic region in the vicinity of the V6 core provide an overall description of charge-transfer behavior.

An Efficient Iron(III)-Catalyzed Aerobic Oxidation of Aldehydes in Water for the Green Preparation of Carboxylic Acids.

The first example of a heterogeneous iron(III)-catalyzed aerobic oxidation of aldehydes in water was developed. This method utilizes 1 atmosphere of oxygen as the sole oxidant, proceeds under extremely mild aqueous conditions, and covers a wide range of various functionalized aldehydes. Chromatography is generally not necessary for product purification. Its operational simplicity, gram-scale oxidation, and the ability to successively reuse the catalyst, make this new methodology environmentally benign and cost effective. The generality of this methodology gives it the potential to be used on an industrial scale.

Synthesis and Crystallization Behavior of Surfactants with Hexamolybdate as the Polar Headgroup.

Alkyl chains with different lengths were covalently grafted onto the surface of hexamolybdate through the postfunctionalization protocol of polyoxometalates. The obtained compounds represent typical structures of the so-called giant surfactants. Unexpectedly, those surfactants with hexamolybdates as polar headgroups are able to crystallize, while single-crystal X-ray diffraction reveals that the crystallization behavior of the surfactants is highly dependent on the length of the alkyl chains. For surfactants with comparatively short alkyl chains (C6 and C10), the alkyl chains prefer to interact with tetrabutylammonium, the countercation of hexamolybdate. However, the alkyl chains tend to pack with each other to form a domain of alkyl chains in the surfactant with a longer alkyl chain (C18). The possible mechanism is that a long alkyl chain cannot be fully compatible with the short chain (C4) of tetrabutylammonium.

Step-by-step strategy from achiral precursors to polyoxometalates-based chiral organic-inorganic hybrids.

Using two types of triol ligands, several novel asymmetrically triol-functionalized Anderson organic hybrids have been efficiently synthesized in high purity and good yields via a convenient two-step esterification reaction. These organic-inorganic hybrids are chiral and can be spontaneously resolved with suitable solvents. Their molecular and crystal structures have been confirmed by single-crystal X-ray diffraction studies. Stable solid-state chirality of the corresponding enantiopure crystals has also been confirmed definitively by CD spectra. Interestingly, these organic-inorganic hybrids possess a layer-by-layer structure, forming solvent-accessible nanoscale chiral channels via a 1D infinite helical chain substructure. TGA measurements indicated that the species of the central heteroatoms significantly effects the stability of these compounds.

Trifluoromethylation of tetraphenylborate counterions in cationic iridium(III) complexes: enhanced electrochemical stabilities, charge-transport abilities, and device performance.

Trifluoromethylation of tetraphenlyborate counterions was successfully used to improve the electrochemical stabilities and device performance of cationic iridium(III) complexes. Melioration of the thermal, photoluminescent, electrochemical, and electrophosphorescent characteristics was achieved. Interionic hydrogen bonds were first found between the aromatic hydrogen atoms in the ancillary ligands of cations and the fluorine atoms in the trifluoromethyl groups of the anions. The strong impact of the counterions on the charge transport in the devices was investigated. A compound with two trifluoromethyl groups in the tetraphenlyborate ion shows the highest photoluminescent efficiency, the best electrochemical stability, and the greatest performance in green-blue-emitting devices, with a high current efficiency of 12.4 cd A(-1) and an emission peak at λ=480 nm. The efficiencies achieved are the highest reported for OLEDs with ionic complexes emitting in the blue-green region.

Aliphatic organoimido derivatives of polyoxometalates containing a bioactive ligand.

A series of aliphatic organoimido derivatives of hexamolybdate based on amantadine, namely (nBu4 N)2 [Mo6 O18 (NC10 H15 )] (1), (nBu4 N)2 {cis-[Mo6 O17 (NC10 H15 )2 ]} (2), (nBu4 N)2 {trans-[Mo6 O17 (NC10 H15 )2 ]} (3), and (nBu4 N)2 [Mo6 O16 (NC10 H15 )3 ] (4), was synthesized in reasonable yield by dehydration with N,N'-dicyclohexylcarbodiimide (DCC). They were characterized by IR and UV/Vis spectroscopy, elemental analysis, ESI mass spectrometry, and single-crystal X-ray structure analysis. The spectral and structural similarities and differences between monosubstituted, cis-disubstituted, and trans-disubstituted organoimido derivatives were elucidated and may provide guidance for related work on organoimido-functionalized Lindqvist-type polyoxometalates. In addition, trans-disubstituted and polysubstituted derivatives containing aliphatic organoimido ligands have not yet been reported, and the crystal structure of the trans isomer may lead us to a deeper understanding of disubstituted derivatives. Furthermore, proliferation and morphology of MCF-7 cells were studied with compound 1. The present results show that the DCC-dehydrating protocol could be an efficient approach to covalently graft bioactive ligands such as amantadine onto POMs and enhance their application in clinical cancer treatment.

Spontaneous stepwise self-assembly of a polyoxometalate-organic hybrid into catalytically active one-dimensional anisotropic structures.

An inorganic-organic hybrid surfactant with a hexavanadate cluster as the polar head group was designed and observed to assemble into micelle structures, which further spontaneously coagulate into a 1D anisotropic structure in aqueous solutions. Such a hierarchical self-assembly process is driven by the cooperation of varied noncovalent interactions, including hydrophobic, electrostatic, and hydrogen-bonding interactions. The hydrophobic interaction drives the quick formation of the micelle structure; electrostatic interactions involving counterions leads to the further coagulation of the micelles into larger assemblies. This process is similar to the crystallization process, but the specific counterions and the directional hydrogen bonding lead to the 1D growth of the final assemblies. Since most of the hexavanadates are exposed to the surface, the 1D assembly with nanoscale thickness is a highly efficient heterogeneous catalyst for the oxidation of organic sulfides with appreciable recyclability.