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

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.