An Effective Strategy To Construct Novel Polyoxometalate-Based Hybrids by Deliberately Controlling Organic Ligand Transformation In Situ

Recent advances in oxidative catalytic applications of polyoxovanadate-based inorganic-organic hybrids

Polyoxovanadates (POVs) have received widespread attention in catalytic applications due to their various structures and remarkable redox properties. By introducing a second transition metal, POV-based inorganic-organic hybrid (POVH) catalysts show increasing stability and more catalytic active sites compared with pure POVs. In this perspective article, POVH materials as oxidative catalysts have been classified into two main categories according to the interactions between transition metal-complex units and POV clusters: (i) hybrids with metal-organic units act as isolated cations and (ii) hybrids with an organic ligand coordinate to the second transition metal, which is further linked to a POV cluster via oxygen bridges directly or indirectly to give zero-, one-, two- or three-dimensional supramolecular structures. The oxidative conversion of organic compounds, including thiophene derivatives, thioethers, alkanes, alcohols, and alkenes, and oxidative detoxification of a sulfur mustard simulant or degradation of lignin, along with the oxidative photo/electrocatalytic transformation of organic compounds catalyzed by POVH materials, are discussed in detail. Furthermore, the challenges and prospects toward the development of POVH catalysts are explored briefly from our perspectives.

Hepta-Zr-Incorporated Polyoxometalate Assembly

Driven by the synergistic-directing effect of the lacunary fragments, [B-α-GeW₉O₃₄]^10- and [B-α-GeW₁₁O₃₉]^8-, an unprecedented hepta-Zr-substituted polyoxometalate (POM) assembly K₂Na₆H₁₀(Hpy)₃[SbZr₇O₆(OH)₄(B-α-GeW₉O₃₄)₂(B-α-GeW₁₁O₃₉)₂]·28H₂O (1) was made under hydrothermal condition and structurally characterized. Of which, a unique hepta-Zr cluster, [SbZr₇O₆(OH)₄]¹⁵⁺ core, was built by two trilacunary [B-α-GeW₉O₃₄]^10- fragments and two monolacunary [B-α-GeW₁₁O₃₉]^8- fragments and further arranged in a mode of a vertical cross and formed a pseudo-tetrahedron geometry. Compound 1 features the first Zr₇-cluster-substituted POM. Moreover, 1 is an effective heterogeneous catalyst for the catalytic oxidation of sulfides into the corresponding sulfones with H₂O₂, manifesting distinguished conversion, excellent yield, and desired recyclability.

Hybrid Cu-Containing Compounds Based on Lacunary Strandberg Anions: Synthesis under Mild Conditions, Crystal Structure, and Magnetic Properties

A one-pot reaction of a copper source (metallic powder Cu⁰ or Cu²⁺ salts) and bpy (bpy = 2,2'-bipyridine) in the presence of (NH₄)₂HPO₄ and (NH₄)₆Mo₇O₂₄·4H₂O yields heterometallic hybrid compounds of the general type {[Cu(bpy)_n(H₂O)_m]_p[P₂Mo_xO_y]}. The structures exhibit a number of phosphomolybdate POMs including not only a common Strandberg anion [P₂Mo₅O₂₃]^6- but also its unprecedented bi- and trilacunary derivatives [P₂Mo₃O₁₈]^8- and [P₂Mo₂O₁₅]^8-. The structural determinants including the metal source (copper powder vs copper salts), counterion of the salts, and stoichiometry of the reagents were examined. An ex situ EPR study revealed the formation of different Cu^II complexes in the reaction mixture depending on the copper precursor. The obtained compounds have been found to possess selectivity toward the sorption of methylene blue in a mixture of organic dyes. DC magnetic measurements of 1-3 indicate rather strong antiferromagnetic metal-metal exchange interactions. Compound 1 exhibits field-induced slow magnetic relaxation in AC magnetic measurements, which is a rarely observed phenomenon among Cu(II) complexes.

Two Cu(i)-based inorganic–organic complexes assembled with polyoxometalate and thiacalix[4]arene for efficient catalytic reactions

Two Cu(i)-based inorganic–organic complexes were synthesized, which can be used as high-efficiency heterogeneous catalysts for ODS and AAC reactions, respectively.

Capped Keggin Type Polyoxometalate-Based Inorganic-Organic Hybrids Involving In Situ Ligand Transformation as Supercapacitors and Efficient Electrochemical Sensors for Detecting Cr(VI)

To construct polyoxometalate-based complexes as electrode materials for supercapacitors and electrochemical sensors, we intentionally used in situ ligand transformation during the reaction. Two complexes based on polyoxometalates capped by zinc ions, H{Zn₄(DIBA)₄[(DIBA)(HPO₂)]₂(α-PMo^VI₈Mo^V₄O₄₀Zn₂)} (1) and [ε-PMo^V₈Mo^VI₄O₃₇(OH)₃Zn₄(HDBIBA)₂]·6H₂O (2) [DIBA = 3,5-di(1H-imidazol-1-yl)benzoic acid, and DBIBA = 3,5-bis(1H-benzoimidazol-1-yl)benzoic acid], have been prepared successfully. The DIBA and DBIBA ligands were generated in situ from initial materials 3,5-di(1H-imidazol-1-yl)benzonitrile and 3,5-di(1H-benzoimidazol-1-yl)benzonitrile. The three-dimensional structure of 1 consisted of two-dimensional interpenetrating layers and polyoxometalate-based chains composed of bicapped α-PMo₁₂Zn₂ polyoxoanions and phosphite-modified DIBA ligands. In 2, a kind of tetracapped ε-PMo₁₂Zn₄ polyoxoanion exists, which was further linked by DBIBA ligands into a one-dimensional chain. Two complexes could be employed as not only electrode materials for supercapacitors with specific capacitances of 171.17 F g^-1 for 1 and 146.77 F g^-1 for 2 at 0.5 A g^-1 but also efficient electrochemical sensors for detecting Cr(VI) with excellent limits of detection of 0.026 μM for 1 and 0.035 μM for 2, which represents a hopeful approach for exploiting polyoxometalate-based complexes as supercapacitor and electrochemical sensor materials.

Ten polytorsional-amide-induced helical-based coordination polymers with difunctional electrochemical activities

Ten polytorsional-naphthalene-amide-based CPs constructed from different carboxylates and metal ions were obtained and exhibited difunctional electrochemical activities.

Assembly, structures and properties of polyoxometalate-based supramolecular complexes involving in situ transformation of single-branch N-donor cyano ligands

A series of polyoxometalate-based supramolecular complexes were prepared with in situ transformation of single-branch N-donor cyano ligands, exhibiting diverse structures and good electrocatalytic and photocatalytic properties.

Two rare {M2(MoO4)2} n chain-containing molybdate-based metal-organic complexes with a bis-pyrazole-bis-amide ligand: fluorescent sensing and photocatalysis performance

By introducing a bis-pyrazole-bis-amide ligand, N,N′-bis(1H-pyrazole-4-carboxamide)-1,4-benzene (L), two molybdate-based metal–organic complexes containing {M₂(MoO₄)₂}_n (M = Co, Zn), [Co₂L₂(MoO₄)₂]·H₂O (1), [Zn₂L₂(MoO₄)₂]·H₂O (2), have been prepared under hydrothermal/solvothermal conditions. X-ray diffraction analyses reveal that both 1 and 2 are isostructural. An interesting structural feature is that a kind of {M₂(MoO₄)₂}_n chain could be found in 1 and 2, although different raw materials [Mo₇O₂₄]⁶⁻ and [PMo₁₂O₄₀]³⁻ anions were utilized. Then these chains are further linked by L ligands into a two dimensional (2D) structure. The title complexes represent the first examples containing {MoO₄} units and pyrazole-/or amide-derivative ligands. Complexes 1 and 2 exhibit distinct performances due to different metal centers, with 2 acting as a fluorescent sensor for Fe³⁺, MnO₄⁻, CrO₄²⁻ and Cr₂O₇²⁻, but 1 being a better photocatalyst towards degradation of cationic dyes methylene blue (MB) and neutral red (NR).

Two molybdate-based complexes containing {M₂(MoO₄)₂}_n (M = Co or Zn) chains were obtained, which demonstrated different properties: excellent photocatalytic degradation performance of cationic dyes for 1 and fluorescence sensing behavior for 2.

Various Cd(ii) coordination polymers induced by carboxylates: multi-functional detection of Fe3+, anions, aspartic acids and bovine serum albumin

By adjusting carboxylates, six Cd(ii) coordination polymers based on a naphthalene-methylene mixed-bridged-amide ligand, [Cd(L)(DNBA)2] (1), [Cd2(L)2(BDC)2(H2O)2]·2H2O (2), [Cd(L)(1,4-CHDA)] (3), [Cd(L)(HIPA)(H2O)]·H2O (4), [Cd(L)(MIP)]·H2O (5), and [Cd(L)(PMA)0.5(H2O)]·H2O (6) [L = N,N'-bis(4-methylenepyridin-4-yl)-1,4-naphthalene dicarboxamide, HDNBA = 3,5-dinitrobenzoic acid, H2BDC = 1,4-benzenedicarboxylic acid, 1,4-H2CHDA = 1,4-cyclohexanedicarboxylic acid, H2HIPA = 5-hydroxyisophthalic acid, H2MIP = 5-methylisophthalic acid and H4PMA = pyromellitic acid] have been synthesized under hydrothermal conditions. 1 shows a 4-c sql coplanar structure. 2 exhibits a 2-fold vertical interpenetrating structure based on wave-like 4-c sql layers. 3 shows a (3,5)-c pnh network containing a unique μ3-L. 4 features a 4-c sql wave-like network. 5 and 6 exhibit 3D structures with 6-c pcu and 4-c mog topologies. The number of carboxyl groups and functional group positions of the carboxylates have an important influence on the structures of the title complexes. The fluorescent responses of 1-6 towards Fe3+, anions, aspartic acid and bovine serum albumin were investigated. Among them, 4 shows sensitivity and selectivity (KSV = 1.16 × 104 L mol-1 for Fe3+, 1.03 × 104 L mol-1 for CrO42-, 1.08 × 104 L mol-1 for Cr2O72-, 1.17 × 104 L mol-1 for MnO4- and 1.05 × 104 L mol-1 for aspartic acid).

Modular Synthesis of Highly Porous Zr-MOFs Assembled from Simple Building Blocks for Oxygen Storage

The last decade has witnessed significant advances in the scale-up synthesis of metal-organic frameworks (MOFs) using commercially available and affordable organic linkers. However, the synthesis of MOFs using elongated and/or multitopic linkers to access MOFs with large pore volume and/or various topologies can often be challenging due to multistep organic syntheses involved for linker preparation. In this report, a modular MOF synthesis strategy is developed by utilizing the coordination and covalent bonds formation in one-pot strategy where monoacid-based ligands reacted to form ditopic ligands, which then assembled into a three-dimensional MOF with Zr₆ clusters. Chemical stability of the resulting materials was significantly enhanced through converting the imine bond into robust linkage via cycloaddition with phenylacetylene. Oxygen storage capacities of the MOFs were measured, and enhanced volumetric O₂ uptake was observed for the stabilized MOF, NU-401-Q.

Constructing new metal-organic frameworks with complicated ligands from "One-Pot" in situ reactions

Metal-organic frameworks (MOFs) have emerged as one of the most fascinating libraries of porous materials. In spite of their myriad merits, practical application of most MOFs is restricted due to their high preparation cost because of the complicated organic ligands involved. To address this limitation, we propose to use simple and cheap organic precursors to synthesize MOFs with complicated ligands via "one-pot" in situ reactions of these precursors along with the formation of new MOFs. In this work, we have carefully screened several organic reactions, through which target ligands were generated in situ from easily available reactants during the MOF construction. With this "one-pot" approach, the fabrication of a series of novel MOFs by integrating the organic covalent bond and the coordinate bond has thus been realized through the judicious selection of organic reactions, which effectively simplifies the MOF synthesis process and thus reduces the cost.

A series of Keggin- and Wells-Dawson-polyoxometalate-based compounds constructed from oxygen-functional imidazole derivatives

Through the use of two kinds of imidazole derivatives containing different O-functional units, three new Keggin- and one Wells-Dawson-polyoxometalate (POM)-based compounds, namely [Cu2La4(HPW12O40)] (1), [CuLa4(H2O)2(HPMo12O40)] (2), [Cu2La7(H2O)(H2P2W18O62)]·La(3), [Ag2(Lb4)(HPMo12O40)]·Lb·H2O (4)(La= (4-(2,5-dihydro-1H-imidazol-1-yl)benzaldehyde), and Lb=(4-(2,5-dihydro-1H-imidazol-1-yl)benzoic acid), were synthesized and characterized using single-crystal X-ray diffraction, elemental analyses, and infrared spectroscopy. The polyoxoanions occupy holes framed by two symmetrically oriented CuLa2 units in 1. In compound 2, the square-planar CuLa4 units form a two-dimensional (2D), supramolecular grid through π···π stacking interactions. The POMs occupy vacancies in this grid. In compound 3, the CuLa3 subunits connect the neighboring P2W18 units and an “S”-type chain is formed. Through hydrogen bonding interactions, the chains are connected to form a layer. In compound 4, the POM anions are linked by AgLb2 subunits to form a chain. All the chains are tied together to construct a layer structure with weak hydrogen bonding interactions between the terminal carboxylic acid functional groups of ligands Lb. The electrocatalytic and photocatalytic activities of all these four compounds were also studied.

Versatile carboxylate-directed structures of ten 1D → 3D Ni(ii) coordination polymers: fluorescence behaviors and electrochemical activities

Ten 1D → 3D Ni(ii) coordination polymers based on a phenyl/methylene-mixed-bridged ligand have been obtained and structurally directed by carboxylates; they show different fluorescence behaviors and electrochemical activities.

Various types of isopolymolybdate-based metal–organic complexes formed in different conditions: synthesis, structures, luminescence, electrochemical, and photocatalytic performances

A series of isopolymolybdate-based complexes of different metals and ligands were prepared, which exhibited different luminescence behaviors, current sensing activities, and photocatalytic MB performances.

Capturing the Organic Species Derived from the C-C Cleavage and in Situ Oxidation of 1,2,3,4-Tetra(pyridin-4-yl)cyclobutane by [CuCN] n-Based MOFs

The solvothermal cycloreversion and in situ oxidation of 1,2,3,4-tetra(pyridin-4-yl)cyclobutane (tpcb) within [CuCN] _n-based MOFs were investigated. The radical mechanism for the cycloreversion of tpcb ligands was supported by capturing a 1,3-butadiene species 1,2,3,4-tetra(4-pyridyl)-1,3-butadiene (tpyb) into {[Cu₁₈(μ-CN)₁₈(tpcb)₄(tpyb)₂]·H₂O} _n in the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). Without TEMPO, a furan-based ligand 2,3,4,5-tetra(4-pyridyl)furan (tpyf) was generated within {[Cu₄(μ-CN)₄(tpyf)]·4MeCN} _n via the C-C cleavage, followed by in situ oxidation in acidic condition.

Amperometric sensing and photocatalytic properties under sunlight irradiation of a series of Keggin–AgI compounds through tuning single and mixed ligands

In this work, we synthesized eight compounds based on Keggin anions and Ag^I ions by introducing L^a and L^b.

Efficient and robust photocatalysts based on {P2W18} modified by an Ag complex

Ag⁺ and the flexible ligand bimb were introduced into Dawson phosphomolybdate systems as linkage units to induce two fascinating 3-D inorganic–organic networks, which exhibit excellent electro- and photo-catalytic behavior.

Construction of d10 metal coordination polymers based on in situ formed 3,5-di(1H-1,2,4-triazol-1-yl)benzoic acid from different precursors: influence of in situ hydrolysis reactions on assembly process

Seven new d¹⁰ coordination polymers based on in situ formed HDTBA from two different precursors were obtained. The influence of in situ reaction on the assembly process was explored.

Three novel and various isopolymolybdate-based hybrids built from the carboxyl oxygen atoms of in situ ligands: substituent-tuned assembly, architectures and properties

Three isopolyoxometalate-based hybrids assembled from in-situ generated ligands reveal that the N-donor groups of ligands may play key roles in the self-assembling process and various structures of the title compounds.

Two unusual 3D honeycomb networks based on Wells–Dawson arsenomolybdates with d10 transition-metal-pyrazole connectors

Two unusual 3D honeycomb networks based on Wells–Dawson arsenomolybdates were first reported, which have highly efficient catalytic ability for the degradation of organic dyes under UV irradiation and obvious electrocatalytic activities for the reduction of H₂O₂.

A series of organic–inorganic hybrid compounds formed by [P2W18O62]6−and several types of transition metal complexes

A series of new organic–inorganic hybrid compounds based on [P₂W₁₈O₆₂]⁶⁻and several types of transition metal complexes have been synthesized and characterized.

Using a flexible bis(pyrazol) ligand to construct four new Keggin-based compounds: syntheses, structures and properties

Four new Keggin-based compounds were constructed. A discrete mono-supporting structure and a supramolecular chain were obtained. Compounds 3 and 4 were formed in one pot containing either bi-nuclear cycles and nested cycle subunits.

A series of organic–inorganic hybrid materials consisting of flexible organic amine modified polyoxomolybdates: synthesis, structures and properties

A series of hybrid materials consisting of Tris[(2-pyridyl)methyl]amine modified polyoxomolybdates were obtained and some complexes have efficient photocatalytic activities to degrade pararosaniline hydrochloride dye molecules.