An ultrasensitive photoelectrochemical assay for tumor necrosis factor-alpha based on hollow CdS cubes as a signal generator and NiCo2O4-Au as a signal extinguisher.

Sensitive detection of tumor necrosis factor-alpha (TNF-α) in human serum is beneficial for finding cancer patients early due to overexpressed TNF-α being related to some cancers. Here, a photoelectrochemical (PEC) aptasensor was constructed for ultrasensitive TNF-α assay based on the signal generator of hollow CdS cubes (H-CdS) and the signal extinguishing activity of NiCo2O4-Au. In this work, compared with traditional solid CdS, H-CdS could greatly promote the PEC signal because its hollow structure could accelerate the separation of photogenerated charges, which also possesses abundant active sites and high light absorption capability. Moreover, H-CdS can be prepared facilely with Cd-based Prussian blue analogs as the precursor. Meanwhile, NiCo2O4-Au was fabricated and utilized as a signal extinguisher. In the presence of TNF-α, NiCo2O4-Au could be introduced onto the H-CdS modified electrode, producing competitive consumption of the electron donor effect, the p-n semiconductor quenching effect, and the mimetic enzymatic catalytic precipitation effect, which all can significantly reduce the PEC signal. Based on the signal extinguishing activity of NiCo2O4-Au and the signal generator of H-CdS, TNF-α can be detected sensitively with a lower detection limit (0.63 fg mL-1) and a wide linear range (1 fg mL-1- to 1 ng mL-1), which may have a potential application in the PEC bioanalysis field and the disease diagnostics field.

Visible-Light-Responsive 2D Photocatalysts Assembled by Evans-Showell-Type POMs and Metalloviologen Frameworks for Sulfide-Sulfoxide Transformation.

Sulfide-sulfoxide aerobic photo-oxidation is of great interest in organic and medicinal chemistry; however, developing efficient and facile heterogeneous photocatalytic systems without additional additives remains challenging. Herein, we intentionally designed and synthesized two polyoxometalate (POM)-based metalloviologen frameworks, formulated as [MII(4-PBPY)2(H2O)][MII(H2O)4][CoIII2MoVI10H4O38]·nH2O (M = Cu, n = 10 for 1; M = Co, n = 11 for 2), prepared by the mild one-step synthesis strategy and characterized in detail. X-ray single-crystal diffraction analysis shows that they present a two-dimensional layered structure formed by two parallel metalloviologen layers pillared by dimeric Evans-Showell-type POMs. The connection of POM to the metalloviologen framework enables easier flow of electrons to the POM port, which can theoretically further induce O2 to generate reactive oxygen species (O2•-) to oxidize substrates to form target products. As expected, both 1 and 2 exhibit outstanding photocatalytic activity in the oxidation of sulfides. Within 6 h, methyl phenyl sulfide can be quantitatively converted into methyl phenyl sulfoxide. The in-depth mechanism reveals that there is also a synergistic energy-transfer pathway in the catalytic system in addition to the electron-transfer pathway. In addition, the corresponding catalytic activity and structure can be well maintained after at least 10 cycle experiments.

Visible-Light-Driven Oxidation of Amines to Imines in Air Catalyzed by Polyoxometalate-Tris(bipyridine)ruthenium Hybrid Compounds.

The development of visible-light photocatalysts for the selective oxidative coupling of amines to imines is an area of great interest. Herein, four hybrid compounds based on polyoxometalate anions and tris(bipyridine)ruthenium cations, Ru(bpy)3[M6O19] (M = Mo, W) 1-2, [Ru(bpy)3]2[Mo8O26] 3, [Ru(bpy)3]2[W10O32] 4, are prepared and characterized by X-ray diffraction (single-crystal and powder), elemental analysis, energy-dispersive X-ray spectroscopy (EDS) analysis, infrared (IR) spectroscopy, and solid diffuse reflective spectroscopy. Single-crystal structural analysis indicates that polyoxometalate anions and tris(bipyridine)ruthenium cations interact with each other through extensive hydrogen bonds in these compounds. These hybrid species with strong visible-light-harvesting abilities and suitable photocatalytic energy potentials show excellent photocatalytic activity and selectivity for the oxidation of amines to imines at room temperature in air as an oxidant. Among them, compound 1 with the [Mo6O19]2- anion has the highest catalytic activity, which can swiftly convert >99.0% of benzylamine into N-benzylidenebenzylamine with a selectivity of 98.0% in 25 min illumination by a 10 W 445 nm light-emitting diode (LED). Its turnover frequency reaches 392 h-1, which is not only better than the homogeneous catalyst [Ru(bpy)3]Cl2 but also much superior to those achieved over most of reported heterogeneous catalysts. Moreover, it shows a wide generality for various aromatic amines, accompanied by the advantages of good recyclability and stability. The photocatalytic oxidation mechanism of amines to the corresponding imines over polyoxometalate-based hybrid compounds was fully investigated.

Highly Efficient Synthesis of p-Benzoquinones Catalyzed by Robust Two-Dimensional POM-Based Coordination Polymers.

Selective oxidation of alkyl-substituted phenols offers efficient access to p-benzoquinones (BQs) that serve as key components for synthesizing biologically active compounds, but rational manufacture of efficient recyclable catalysts for such a reaction remains a severe challenge. Herein, two crystalline 2D polyoxometalate-based coordination polymers (POMCPs), formulated as H3[CuI3(L)3]2[PM12O40]·xH2O (M = Mo, x = 4 for 1; M = W, x = 6 for 2; and HL = 4-(1H-tetazol-5-yl)pyridine), are prepared by a mineralizer-assisted one-step synthesis strategy and explored as heterogeneous catalysts for p-BQs synthesis. Both compounds have been characterized through elemental analysis, EDS analysis, infrared spectroscopy, UV-vis diffuse reflectance spectrum, EPR, XPS, BET, single-crystal, and powder X-ray diffraction. Single-crystal X-ray diffraction analysis indicates that both 1 and 2 exhibit an interesting 2D sheet structure composed of 2-connected Keggin type anions [PM12O40]3- and hexa-nuclear {CuI6(HL)6} cluster-based metal-organic chains via Cu···O interactions. When used as catalysts, POMCPs 1 and 2 have excellent catalytic activities in the selective oxidation of substituted phenols to p-BQs with H2O2. Notedly, in the model reaction from 2,3,6-trimethylphenol (TMP) to the vitamin E key intermediate trimethyl-p-benzoquinone (TMBQ), the catalytic activities expressed by turnover frequency (TOF) of 1 and 2 can reach an unprecedented 2400 and 2000 h-1, respectively, at close to 100% TMBQ yield. The truly heterogeneous nature, stability, and structural integrity of both catalysts were ascertained by FTIR, PXRD techniques, and the following cycles. Mechanism studies reveal that both catalysts can involve a dual reaction pathway through a heterolytic oxygen atom transfer mechanism and homolytic radical mechanism. Moreover, the 2D POMCPs with highly accessible bilateral active sites and efficient mass transfer efficiency possess superior catalytic performance to their analogous 3D species.

2D Hybrid Architectures Constructed from Two Kinds of Polyoxovanadates as Efficient Heterogeneous Catalysts for Cyanosilylation and Knoevenagel Condensation.

The design of heterogeneous catalysts for highly efficient catalysis for cyanosilylation reactions and Knoevenagel condensations is greatly significant, due to the important application of their products in industry. Herein, five hybrid compounds constructed from two types of polyoxovanadates, [MV12O38]12- (M = Ni, Mn) and [V10O28]6-, were successfully synthesized as heterogeneous catalysts for both reactions, which are {(dpdo)[Ln2(H2O)9(dpdo)][Ln(H2O)5]2[Ln(H2O)4]2[V10O28][NiV12O38]·nH2O} (1, Ln = La, n = 23; 2, Ln = Ce, n = 27; 3, Ln = Pr, n = 27; dpdo = 4,4'-bipyridine N,N'-dioxide) and {(dpdo)[Ln2(H2O)9(dpdo)][Ln(H2O)5]2[Ln(H2O)4]2[V10O28][MnV12O38]·27H2O} (4, Ln = La; 5, Ln = Pr). These compounds were characterized by IR spectroscopy, elemental analysis, TG analysis and X-ray diffraction (single crystal and powder) etc. Compounds 1-5 have similar 2D hybrid structures, in which isopolyvanadate [V10O28]6- and heteropolyvanadate [MV12O38]12- are linked together by hydrated Ln3+ cations and Ln-dpdo coordination complexes. Polyoxovanadates [V10O28]6- and [MV12O38]12- both originate from the transformation of the [MV13O38]7- raw material. This kind of extended structure containing isopolyvanadate and heteropolyvanadate has never been reported hitherto. Because of the Lewis acidity of the Ln3+ cation and the Lewis basicity of the polyoxovanadates, all five compounds exhibit excellent catalytic performance in cyanosilylation and Knoevengel condensation reactions. Especially, compound 3, with a Pr3+ cation, displayed the best catalytic results. Furthermore, these catalysts exhibit a truly heterogeneous nature and good recyclability.

Multiunit Catalysts with Synergistic Reactivity: Three-Dimensional Polyoxometalate-Based Coordination Polymers for Highly Efficient Synthesis of Functionalized p-Benzoquinones.

The rational design of highly efficient catalysts for the synthesis of functionalized p-benzoquinones (p-BQs) is of great significance for the manufacture of bioactive compounds. Herein, two 3D crystalline polyoxometalate-based coordination polymers (POMCPs) are used as heterogeneous catalysts for the synthesis of p-BQs, which are H[CuII(ttb)(H2O)3]2[CuII(ttb)Cl]2[PW12O40]·4H2O (1) (Httb = 1-(tetrazol-5-yl)-4-(triazol-1-yl)benzene) and [ClCu6I(trz)4][ClCu5I(trz)4]2[CuII(H2O)][PW12O40] (2) (trz = 1,2,4-triazole). Both compounds were characterized by elemental analysis, IR, XPS, solid diffuse reflective spectroscopy, TG analysis, and single-crystal X-ray diffraction. In 1, Keggin anions [PW12O40]3- locate in 1D square channels constructed from wave-like Cu-ttb layers to form a 3D POMCP by coordinating to Cu ions, and in 2, [PW12O40]3- anions situate in eight-membered Cu-trz channels via Cu···O interactions to yield a 3D POMCP structure. The catalytic activities of 1 and 2 have been evaluated in the selective oxidation of alkylphenols/alkoxybenzenes/methylnaphthalene, especially in the oxidation reaction of 2,3,6-trimethylphenol (TMP) to 2,3,5-trimethyl-p-benzoquinone (TMBQ, vitamin E key intermediate), with H2O2 as oxidant. By using catalysts 1 and 2 under optimal conditions, the yield of TMBQ can reach 99% and 96% within 10-20 min, respectively. Both catalysts demonstrated high turnover frequencies (300 h-1 for 1 and 600 h-1 for 2) and the truly heterogeneous nature. 1 and 2 catalyzed the synthesis of p-BQs on the basis of effective cooperative catalytic activities by POMs and metal nodes.

Synergistic Effect between Different Coordination Geometries of Lanthanides and Various Coordination Modes of 2-Picolinic Acid Ligands Tuning Three Types of Rare 3d-4f Heterometallic Tungstoantimonates.

Three types of N-heterocyclic aromatic acid decorated 3d-4f heterometallic Keggin-type tungstoantimonates: Na4[Ln(H2O)5]2[Fe4(H2O)2(pic)4(B-ß-SbW9O33)2]·26H2O [Ln3+ = La3+ (1), Pr3+ (2), Nd3+ (3), Sm3+ (4), Eu3+ (5)], Na6H4[Fe2W4O9(H2O)2(Hpic)4 (B-ß-SbW9O33)2][Ln(H2O)8]2[Fe4W2O7(H2O)4(pic)2(Hpic)2(B-ß-SbW9O33)2]·38H2O [Ln3+ = Gd3+ (6), Dy3+ (7)], and Na2H2{[Ln(H2O)6]2[Fe4(H2O)2(Hpic)2(pic)2(B-ß-SbW9O33)2]}2·44H2O [Ln3+ = Ho3+ (8), Er3+ (9), Hpic = 2-picolinic acid] have been prepared. 1-5 comprise a quadripic-inserted Krebs-type [Fe4(H2O)2(pic)4(B-ß-SbW9O33)2]10- moiety supported by two [Ln(H2O)5]3+ groups on both sides where it can be considered that four pic ligands replace eight aqua ligands located on the original Krebs-type [Fe4(H2O)10(B-ß-SbW9O33)2]10- fragment to form the [Fe4(H2O)2(pic)4(B-ß-SbW9O33)2]10- moiety. Remarkably, the quadripic-inserted subunits are further concatenated through the coordination role of the pic ligands to create a 3-D heterometallic framework. In contrast, the molecular units of 6-7 contain two kinds of non-Krebs-type quadripic-inserted [Fe2W4O9(H2O)2(Hpic)4(B-ß-SbW9O33)2]6- and {[Ln(H2O)8]2[Fe4W2O7(H2O)4(pic)2(Hpic)2(B-ß-SbW9O33)2]}4- moieties. The molecular units of 8-9 contain two identical quadripic-inserted Krebs-type {[Ln(H2O)6]2[Fe4(H2O)2(Hpic)2(pic)2(B-ß-SbW9O33)2]}2- moieties, and both display a 1-D heterometallic double chain. For all we know, 1-9 stand for the first 3d-4f heterometallic tungstoantimonate hybrids functionalized by pic ligands. Particularly, the solid-state NIR photoluminescence (PL) spectrum in the range of 800-1450 nm of 3 and the solid-state visible PL spectra in the range of 500-750 nm of 4, 5, 7, and 8 at room temperature display the featured fluorescence emission bands stemming from Ln3+ cations. In the PL emission procedures of 5 and 7, energy transfer from [B-ß-SbW9O33]9- fragments and pic ligands to Ln3+ ions has been observed. Additionally, the correlated color temperatures of 4, 5, 7, and 8 are indexed to 2731, 2020, 4557, and 1685 K, respectively.

Hybrid dimers based on metal-substituted Keggin polyoxometalates (metal = Ti, Ln) for cyanosilylation catalysis.

Six novel organic-inorganic hybrid polyoxometalate derivatives, K[(H2O)4(3-Hpic)2Ce][(H2O)5(3-Hpic)2Ce][PW10Ti2O40]·11H2O 1, K[(H2O)4(3-Hpic)2Nd][(H2O)5(3-Hpic)2Nd][PW10Ti2O40]·12H2O 2, K[(H2O)4(3-Hpic)2Sm][(H2O)5(3-Hpic)2Sm][PW10Ti2O40]·8H2O 3, H[(H2O)7(4-Hpic)La][(4-Hpic)2LaPW11O39]·14.5H2O 4, H[(H2O)7(4-Hpic)Ce][(4-Hpic)2CePW11O39]·7.5H2O 5, and K(4-Hpic)0.5[(H2O)6(4-Hpic)Nd][(4-Hpic)2NdPW11O39]·10H2O 6, (3-Hpic = 3-picolinic acid; 4-Hpic = 4-picolinic acid) have been synthesized and characterized by elemental analysis, IR, solid state UV-vis spectroscopy, thermal gravimetric analysis, powder X-ray diffraction and single-crystal X-ray diffraction. Isostructural compounds 1-3 are hybrid dimers that consist of two Ti-substituted Keggin-type polyoxoanions [PW10Ti2O40]7- and four Ln-3-Hpic coordination groups. The strong hydrogen bonds in 1-3 can make these dimers yield a 1D supramolecular chain. Compounds 4-6 are also dimers constructed from two lanthanide-substituted Keggin-type polyoxoanions [PW11LnO39]4- and four 4-Hpic ligands, which are further linked by two Ln-4-Hpic groups to form a bi-supporting structure. Then these subunits are joined together by strong hydrogen-bonding interactions between polyoxoanions and coordinated water molecules to produce a 2D supramolecular framework. All these compounds as heterogeneous Lewis acid-base catalysts show high activity and high selectivity for the cyanosilylation of different carbonyl compounds under solvent-free conditions, and can be recycled without any obvious inactivation.

Evans-Showell-Type Polyoxometalates Constructing High-Dimensional Inorganic-Organic Hybrid Compounds with Copper-Organic Coordination Complexes: Synthesis and Oxidation Catalysis.

Four new hybrid architectures containing a [Co2Mo10H4O38]6- polyoxoanion, (en)[Cu3(ptz)4(H2O)4][Co2Mo10H4O38]·24H2O (1), (Hbim)2[{Cu(bim)2(H2O)2}2{Co2Mo10H4O38}]·5H2O (2), H2[Cu(dpdo)3(H2O)4][{Cu2(dpdo)3(H2O)4(CH3CN)}2{Co2Mo10H4O38}2]·9H2O (3), and (H2bpp)4[{Cu(H2O)2}{NaCo2Mo10H4O38}2]·10H2O (4), where ptz = 5-(4-pyridyl)-1H-tetrazole, en = ethylenediamine, bim = benzimidazole, dpdo = 4,4'-bipyridine-N,N'-dioxide, and bpp = 1,3-bis(4-pyridyl)propane, have been prepared and characterized through elemental analysis, thermogravimetric analysis, IR spectroscopy, and powder and single-crystal X-ray diffraction. Compound 1 shows a 3D host-guest framework composed of 3D Cu-ptz as the host and Evans-Showell-type polyoxoanion [Co2Mo10H4O38]6- as the guest. Compound 2 is constructed from [Co2Mo10H4O38]6- polyoxoanions and Cu-bim coordination complexes to form a 2D covalent layer. Compound 3 also exhibits a 2D hybrid network based on [Co2Mo10H4O38]6- polyoxoanions linked by Cu-dpdo coordination groups. Compound 4 is a 1D double-chain structure composed of [Co2Mo10H4O38]6- polyoxoanions joined together by Na+ and Cu2+ cations. As far as we know, compound 1 is the first host-guest compound with an Evans-Showell-type polyoxometalate as the guest, and compounds 2 and 3 are the first 2D inorganic-organic hybrid architectures constructed from Evans-Showell-type polyoxometalates. Compounds 1-4 are redox catalysts that heterogeneously prompt sulfide and alcohol oxidation with excellent efficiency.