A Keggin-type polyoxometalate-based metal-organic complex as a highly efficient heterogeneous catalyst for the selective oxidation of alkylbenzenes

Maneuvering the Electronic State and Active Site of Assembled-Gold Nanoclusters through Polyoxometalate Implantation for Heterogeneous Green-Light Photocatalysis

Gold nanoclusters (AuNCs) exhibit unique molecule-like optical and electronic properties, making them promising candidates for photocatalysis. However, their application as primary photocatalysts in heterogeneous systems is limited by rapid electron recombination, small size, and high solubility. To overcome these limitations, we developed an approach combining AuNCs assemblies with electron trap centers to enhance charge separation and electron transfer. Using a depletion-driven assembly method, Keggin-type polyoxometalates (POMs), Na10SiW9O34 (SiW9), were uniformly embedded within spherical assemblies of glutathione-protected AuNCs, forming gold superclusters (AuSCs). The resulting AuSCs@SiW9 exhibited complete photoluminescence quenching, enhanced metallicity, and stabilized photogenerated electrons via SiW9, enabling their use as primary photocatalysts. The AuSCs@SiW9 efficiently catalyzed the functionalization of terminal aryl alkyne with N-bromosuccinimide (NBS), achieving a tribromoketones yield of 94%, significantly outperforming AuSCs with lower or higher metallicity (38 and 65%, respectively). Mechanistic studies revealed that the improved gold metallicity in AuSCs@SiW9 promotes charge transfer complex formation with NBS, while SiW9 stabilizes photogenerated electrons, enhancing electron density under light irradiation. The AuSCs@SiW9 exhibited strong visible-light absorption, photostability, and solvent dispersibility, enabling recyclability for up to five cycles and scalability for broader applications. This strategy positions gold as a viable primary photocatalyst, expanding its potential in heterogeneous photocatalysis toward synthesizing small molecules.

Solventless Heterogeneous Catalysis with [P2Mo5O23]6--Based Complexes: Achieving High-Efficiency and Rapid Oxidation of C-H Bonds

The oxidation of C-H bonds holds paramount importance in the realms of organic synthesis and industrial chemistry. However, conventional methods are fraught with environmental concerns and economic drawbacks. Consequently, heterogeneous catalysis and solvent-free reactions have emerged as the focal points of research. This work reports two polyoxometalates-based complexes incorporating the [P2Mo5O23]6- ({P2Mo5}) unit as catalysts for solvent-free C-H bond oxidation: (CuIL2)4[H2(P2Mo5O23)]·H2O (1) and (HL)6(P2Mo5O23)·H2O (2) (L = benzimidazole), which exhibit similar stacking modes but obvious structural differences: ligands L coordinated with Cu ions to form {CuIL2} subunits in complex 1, while complex 2 has fully protonated ligands L. Notably, complex 1 demonstrated exceptional heterogeneous catalytic activity compared to complex 2, efficiently oxidizing diphenylmethane (DPM) in the absence of the solvent and achieving conversion rates as high as 99.5% and selectivity for benzophenone (BP) of 100% within 6 h. The reasons for their different catalytic performances were unveiled. More importantly, even when the catalytic reaction was scaled up by 10-fold (with the amount of substrate DMP reaching 10 mmol), complex 1 still maintained high catalytic activity, highlighting its substantial potential for industrial applications.

The applications of Keggin-based metal-organic compounds in sensing and catalysis

Environmental pollution and energy problems caused by excessive use of fossil fuels deviate from the theme of green and sustainable development. It is very promising to detect small molecules or catalyze the conversion of pollutants to obtain renewable energy by using photoelectric technology. Therefore, there is an urgent requirement to develop materials with low detection limits and high catalytic performance. Keggin polyoxometalate-based metal-organic compounds (POMOCs) hold great promise for sensing, and catalytic applications due to their controllable structure, remarkable reversible multi-electron transfer capability and multi-component synergistic activity. In this review, the applications of Keggin POMOCs in photocatalytic/electrocatalytic conversion of energy materials and the detection of metal ion/inorganic molecule are introduced. The different mechanisms of Keggin POM units and MOF units in sensors and catalysis are discussed. Additionally, the prospects of the Keggin POMOCs as electrode materials or catalysts for enhancing the performance of sensors and catalysts are discussed, which will provide a platform for further development of advanced Keggin POMOC material-based sensors and catalytic systems.

Cavity-Directed Synthesis of Labile Polyoxometalates for Catalysis in Confined Spaces

The artificial microenvironments inside coordination cages have gained significant attention for performing enzyme-like catalytic reactions by facilitating the formation of labile and complex molecules through a "ship-in-a-bottle" approach. Despite many fascinating examples, this approach remains scarcely explored in the context of synthesizing metallic clusters such as polyoxometalates (POMs). The development of innovative approaches to control and influence the speciation of POMs in aqueous solutions would greatly advance their applicability and could ultimately lead to the formation of elusive clusters that cannot be synthesized by using traditional methods. In this study, we employ host-guest stabilization within a coordination cage to enable a novel cavity-directed synthesis of labile POMs in aqueous solutions under mild conditions. The elusive Lindqvist [M6O19]2- (M=Mo or W) POMs were successfully synthesized at room temperature via the condensation of molybdate or tungstate building blocks within the confined cavity of a robust and water-soluble Pt6L4(NO3)12 coordination cage. Importantly, the encapsulation of these POMs enhances their stability in water, rendering them efficient catalysts for environmentally friendly and selective sulfoxidation reactions using H2O2 as a green oxidant in a pure aqueous medium. The approach developed in this paper offers a means to synthesize and stabilize the otherwise unstable metal-oxo clusters in water, which can broaden the scope of their applications.

Synthesis and Structure of High-Nuclearity Carboxylate-Modified Heteropolyoxovanadate Serving as a Heterogeneous Catalyst for Selective Oxidation of Alkylbenzenes

A high-nuclearity carboxylic-modified heteropolyoxovanadate, Na2K10H15[P8VIV24(tart)15(H2O)15(OH)O51]·58H2O [1, tart = C4H2O6], has been successfully synthesized by a conventional aqueous method under mild conditions. The crystallographic study reveals that compound 1 crystallizes in the tetragonal I41/a space group and is composed by a trilayer saddle-like polyoxoanion {P8V24}. Two {V3(tart)(H2O)O11} as linking units bridge the top {P4VIV9(tart)7(H2O)4(OH)O23} and the bottom {P4VIV9(tart)6(H2O)9O22} layers via tartrate ligands and {PO4} tetrahedra, resulting in a 24-nuclearity POV skeleton structure. More interestingly, compound 1 serves as a heterogeneous catalyst for the selective oxidation of diphenylmethanes with 96.2% conversion and 93.6% selectivity under the optimized conditions.

A Metal-Organic Complex Constructed from Co(II), Azo-amide-pyridyl and Benzenetricarboxylate Mixed Ligands: Efficient Catalysis for Selective Oxidation of Benzyl Alcohols to Benzyl Acids

By using one-step hydrothermal synthesis, a novel metal-organic complex containing Co(II), the azo-amide-pyridyl ligand (E)-4,4'-(diazene-1,2-diyl)bis(N-(pyridin-3-yl)benzamide (DABA) and benzenetricarboxylate was synthesized, with a molecular formula of [Co2 (DABA)0.5 (MTC)(μ3 -OH)(H2 O)2 ] ⋅ 2H2 O (namely 1, DABA=(E)-4,4'-(diazene-1,2-diyl)bis(N-(pyridin-3-yl)benzamide, H3 MTC=1,2,4-benzenetricarboxylic acid) which was characterized by single crystal X-ray diffraction, PXRD, IR spectroscopy, TGA, and XPS. In the structure of complex 1, tetranuclear Co(II) clusters were connected by MTC to form a 2D bilayer structure and further constructed a 3D structure with DABA ligand. Complex 1 was used as an efficient heterogeneous catalyst for the oxidation of benzyl alcohol, and the conversion rate of benzyl alcohol reached 98.6 % and the selectivity of benzoic acid reached 94.8 %. In addition, complex 1 can be reused 5 times without significant loss of activity. The oxidation of benzyl alcohol with different substituents also showed satisfactory conversion and selectivity, indicating that complex 1 had good catalytic performance.