Two Polyoxometalate‐Based Hybrid Compounds Modified by Iron Schiff Base Complexes: Syntheses, Crystal structures, Cyclic Voltametric Studies and Nonlinear Optical Properties

Copper-containing POM-based hybrid P2Mo22V4Cu4 nanocluster as heterogeneous catalyst for the light-driven hydroxylation of benzene to phenol

The current traditional phenol production process has many shortcomings, and the efficient and clean photocatalytic one-step oxidation to phenol is gradually attracting attention. Heteropolyacids (PMo10V2) with high-density Lewis acid active sites and excellent photoelectron transfer ability are ideal choices for catalytic reactions. In this study, a copper-modified isolated dimeric hybrid nanocluster, [Cu(pyim)2]2[Cu(pyim)2(P2MoVI20MoV2VIV4O82)]2·(H2O) (pyim = [2-(pyridin-2-yl)imidazole]), was synthesized by a convenient hydrothermal method. The structural analysis demonstrated that the compound was composed of metal-organic complexes containing pyim ligands, Keggin-type heteropolyacids, and transition metal copper ions. Remarkably, this not only solves the difficulty that the heteropolymeric acid cannot be recovered by dissolving in the solvent but also introduces the copper atom as a second active center. The catalyst exhibited a benzene conversion of 15.6% and a selectivity of 85.2% in a mixed solution of acetonitrile and acetic acid under optimal reaction conditions. After four catalytic cycles, the PXRD pattern proved that the catalyst was still stable. This study provides a good idea for photocatalytic reactions and other environmental applications.

An iron-containing POM-based hybrid compound as a heterogeneous catalyst for one-step hydroxylation of benzene to phenol

It is a major challenge to perform one-pot hydroxylation of benzene to phenol under mild conditions, which replaces the environmentally harmful cumene method. Thus, finding highly efficient heterogeneous catalysts that can be recycled is extremely significant. Herein, a (POM)-based hybrid compound {[FeII(pyim)2(C2H5O)][FeII(pyim)2(H2O)][PMoV2MoVI9VIV3O42]}·H2O (pyim = 2-(2-pyridyl)benzimidazole) (Fe2-PMo11V3) was successfully prepared by hydrothermal synthesis using typical Keggin POMs, iron ions and pyim ligands. Single-crystal diffraction shows that the Fe-pyim unit in Fe2-PMo11V3 forms a stable double-supported skeleton by Fe-O bonding to the polyacid anion. Remarkably, due to the introduction of vanadium, Fe2-PMo11V3 forms a divanadium-capped conformation. Benzene oxidation experiments indicated that Fe2-PMo11V3 can catalyze the benzene hydroxylation reaction to phenol in a mixed solution of acetonitrile and acetic acid containing H2O2 at 60 °C, affording a phenol yield of about 16.2% and a selectivity of about 94%.

Two bimetal-doped (Fe/Co, Mn) polyoxometalate-based hybrid compounds for visible-light-driven CO2 reduction

Two polyoxometalate (POM)-based hybrid compounds have been successfully designed and constructed by the hydrothermal method with molecular formulas [K(H₂O)₂FeII0.33Co_0.67(H₂O)₂(DAPSC)]₂{[FeII0.33Co_0.67(H₂O)(DAPSC)]₂[FeII0.33Co_0.67(H₂O)₄]₂[Na₂FeIII4P₄W₃₂O₁₂₀]}·21.5H₂O (1), and [Na(H₂O)₂FeII0.33Mn_0.67(H₂O)₂(DAPSC)]₂{[FeII0.33Mn_0.67(H₂O)(DAPSC)]₂[FeII0.33Mn_0.67(H₂O)₄]₂[Na₂FeIII4P₄W₃₂O₁₂₀(H₂O)₂]}·24H₂O (2) (DAPSC = 2,6-diacetylpyridine bis-(semicarbazone)), respectively. Structural analysis revealed that 1 and 2 consisted of metal-organic complexes containing DAPSC ligands with dumbbell-type inorganic clusters, iron-cobalt (iron-manganese) and some other ions. By utilizing a combination of strongly reducing {P₂W₁₂} units and bimetal-doped centres the CO₂ photoreduction catalytic capacity of 1 and 2 was improved. Notably, the photocatalytic performance of 1 was much better than that of 2. In CO₂ photoreduction, 1 exhibited CO selectivity as high as 90.8%. Furthermore, for 1, the CO generation rate reached 6885.1 μmol g^-1 h^-1 at 8 h with 3 mg, and its better photocatalytic performance was presumably due to the introduction of cobalt and iron elements to give 1 a more appropriate energy band structure. Further recycling experiments indicated that 1 was a highly efficient CO₂ photoreduction catalyst, which could still possess catalytic activity after several cycles.

Two New Compounds Based on Bi-Capped Keggin Polyoxoanions and Cu-Bpy Cations Contain Both CuII and CuI Complexes: Synthesis, Characterization and Properties

Two inorganic-organic hybrid complexes based on bi-capped Keggin-type cluster, {([Cu^II(2,2'-bpy)₂]₂[PMo^VI₈V^V₂V^IV₂O₄₀(V^IVO)₂])[Cu^I(2,2'-bpy)]}∙2H₂O (1) and {[Cu^II(2,2'-bpy)₂]₂[SiMo^VI_8.5Mo^V_2.5V^IVO₄₀(V^IVO)₂]}[Cu^I_0.5(2,2'-bpy)(H₂O)_0.5] (2) (bpy = bipyridine), had been hydrothermally synthesized and structurally characterized by elemental analysis, FT-IR, TGA, PXRD and X-ray single-crystal diffraction analysis. Compound 1 consists of a novel 1-D chain structure constructed from [Cu^I(2,2'-bpy)]⁺ unit linking bi-supported POMs anion {[Cu^II(2,2'-bpy)₂]₂[PMo^VI₈V^V₂V^IV₂O₄₀(V^IVO)₂]}^-. Compound 2 is a bi-capped Keggin cluster bi-supported Cu-bpy complex. The main highlights of the two compounds are that Cu-bpy cations contain both Cu^I and Cu^II complexes. Furthermore, the fluorescence properties, the catalytic properties, and the photocatalytic performance of compounds 1 and 2 have been assessed, and the results show that both compounds are active for styrene epoxidation and degradation and adsorption of Methylene blue (MB), Rhodamine B (RhB) and mixed aqueous solutions.

Two Ni-Substituted Trilacunary Keggin-Type Polyoxometalates: Syntheses, Crystal Structures, NLO Studies, and Magnetic Properties

Two Ni-substituted polyoxometalates (NiSPs), [Ni₆(Py)₆(H₂O)₅(μ₃-OH)₃(PW₉O₃₄)]₂·10H₂O (1), [Ni₇(Py)₆(Im)(H₂O)₅O(WO₄)(μ₃-OH)₃(H₂PW₉O₃₄)]·3H₂O (2) (Py = pyridine, Im = imidazole), were successfully hydrothermally synthesized. Compounds 1 and 2 have significantly different configurations by introducing different amounts of imidazole ligands. For compound 1, two malposed {Ni₆(Py)₆PW₉} units that are face to face are bridged by two Ni-O-W bonds to constitute an isolated dimeric structure. Differently, the {Ni₇(Py)₆(Im)PW₉}₂ dimer in compound 2 connects with four adjacent dimers by four {WO₄} groups in an interesting two-dimensional (2-D) arrangement. The magnetism of compounds 1 and 2 was studied, and magnetic test results demonstrated that both compounds have ferromagnetic interactions between the nickel centers. Meanwhile, the third-order nonlinear optical (NLO) measurements indicated that compound 1 can serve as potential nonlinear optical materials.

A Purely Inorganic Quasi-Keggin Polyoxometalate for Photocatalytic Conversion of Carbon Dioxide to Carbon Monoxide

A pure inorganic cluster, H₄₇ Na₂ Co₄ Mo₂₄ (PO₄ )₁₁ O₇₂  ⋅ 15H₂ O (denoted as {Co₄ Mo₂₄ }), has been successfully synthesized by hydrothermal method. Notably, the assembly of a central {Co₂ PO₄ } tetrahedron and four peripheral {Co[P₄ Mo₆ ]} fragments gives rise to a rare "quasi-Keggin" structure of {Co₄ Mo₂₄ }, in which Co linkers continue to bridge adjacent substructures, resulting in the generation of 3D framework with large cavities. Benefitting from the combination of strong reductive {P₄ Mo₆ } units and Co active centers, the photocatalytic system with {Co₄ Mo₂₄ } as heterogeneous catalyst exhibits excellent activity for CO₂ conversion to CO, offering the CO formation rate of 1848.3 μmol g^-1 h^-1 with high selectivity of 97.0 %. Besides, thermogravimetric and X-ray diffraction analysis confirm that {Co₄ Mo₂₄ } can maintain stable during the photocatalytic reaction process.

Two three-dimensional polyanionic clusters [M(P4Mo6)2] (M = Co, Zn) exhibiting excellent photocatalytic CO2 reduction performance

Two captivating {P4Mo6}-based compounds, formulated as (H2bbi)2{[Co2(bbi)][Co2.33(H2O)4][H9.33CoP8Mo12O62]}·4H2O (1) and (H2bbi){[Zn(Hbbi)]2[Zn0.75(bbi)][K2Zn(H2O)4][H8.5ZnP8Mo12O62]} (2) [bbi = 1,1'-(1,4-butanediyl)bis(imidazole)], were successfully synthesized under hydrothermal conditions. Structural analysis demonstrates that compounds 1 and 2 are constructed from hourglass-shaped structures [M(P4Mo6O31)2]n- (M = Co, Zn), which are all made up of molybdophosphates and one transition metal ion as the central connecting node. Compounds 1 and 2 feature three-dimensional (3D) frameworks, which are all connected to form a 3D structure by metal ions and bbi ligands. More interestingly, compound 1 exhibits higher catalytic activity than 2 in CO2 photoreduction due to the suitable energy band structure of Co species in {P4Mo6} clusters. The CO yield was 3261 μmol g-1 with high selectivity in 8 h for compound 1 in photocatalytic CO2 reduction, which is highly promising in the photocatalytic field. Additionally, the photoluminescence properties of 2 were investigated.