Three Polyoxometalate-Based Ag–Organic Compounds as Heterogeneous Catalysts for the Synthesis of Benzimidazoles

Abstract

Three new POM-based compounds, with formulae [Na 0 . 6 3 Ag 3 (Htba) 2 . 3 7 (tba) 0 . 6 3 (H 2 O) 2 (PMo 1 2 O 4 0 )]•4H 2 O (Ag 3 PMo), [Ag 4 (Htba) 4 (H 2 O) 2 (PMo 12 O 40 )](NO 3 )•H 2 O (Ag 4 PMo), and [Ag 3 (Htba) 2 (tba)(PW 12 O 40 ) 0.5 ](NO 3 ) 0.5 •13H 2 O (Ag 3 PW), were prepared with a 3-(4H-1,2,4-triazol-4-yl)benzoic acid (Htba) ligand, Keggin-type anions ([PMo 12 O 40 ] 3− / [PW 12 O 40 ] 3− ), and a silver ion (Ag + ). The structural features of these compounds are particularly different from the multinuclear subunits, which are [Ag 3 (tba) 3 ] clusters in Ag 3 PMo, [Ag 4 (tba) 3 ] chains in Ag 4 PMo, and [Ag 3 (tba) 3 ] 2 clusters in Ag 3 PW, connected by multidonor atom tba ligands and Ag + ions. Meanwhile, in these compounds, polyanions act as polydentate ligands to link adjacent Ag-tba metal−organic units and expand their spatial dimensions. These compounds, as heterogeneous catalysts, exhibit high stability and excellent catalytic activity to construct benzimidazoles. Ag 3 PMo could efficiently catalyze the condensation of benzene-1,2diamines and benzaldehydes and produce benzimidazoles in good yields. In addition, Ag 3 PMo could be reused up to 7 times and was suitable for gram-scale reactions.

Synthesis, Characterization, and Catalytic Performance of a New Heterobimetallic Y/Tb Metal–Organic Framework with High Catalytic Activity

“…Metal–organic frameworks (MOFs) are hybrid compounds between organic and inorganic materials . They are formed by metal ion nodes linked by multidentate organic ligands forming a three-dimensional assembly that includes porous channels of large superficial area. − The intrinsic porosity of these MOFs together with their easy tunability and functionalization have made them attractive for multiple applications, especially in heterogeneous catalysis. , Some examples are the binuclear Ni­(II)-based metal–organic framework or those based on polyoxometalate-based Ag–organic compounds that employ 1,3,5-tri­(4-carboxylphenyl)­benzene (H 3 btb) and 3-(4 H -1,2,4-triazol-4-yl)­benzoic acid (Htba) as ligands, respectively, that were applied in the synthesis of benzimidazoles and pyrazoles . In addition, MOF compounds have garnered significant attention from the scientific community as potential functional materials. , When a second metal is incorporated into the nodes of the framework, then bimetallic architectures are generated, which could exist as “solid solution” or “core–shell” structures .…”

“… 5 , 6 Some examples are the binuclear Ni(II)-based metal–organic framework 7 or those based on polyoxometalate-based Ag–organic compounds that employ 1,3,5-tri(4-carboxylphenyl)benzene (H 3 btb) and 3-(4 H -1,2,4-triazol-4-yl)benzoic acid (Htba) as ligands, respectively, that were applied in the synthesis of benzimidazoles and pyrazoles. 8 In addition, MOF compounds have garnered significant attention from the scientific community as potential functional materials. 9 , 10 When a second metal is incorporated into the nodes of the framework, then bimetallic architectures are generated, which could exist as “solid solution” or “core–shell” structures.…”

Polyoxometalate-mediated syntheses of three structurally new silver clusters

“…To date, a large number of Ag clusters directed by POM templates have been successfully prepared and reported. 38–47 For instance, in 2014, Wang's group synthesized a novel Ag 70 alkynyl cluster by adopting lacunary POM [ A-α - PW 9 O 34 ] 9− in a distinctive ionic liquid environment. 40 Subsequently, they prepared another unique Ag 41 cluster containing a di-lacunary [ α - SiW 10 O 37 ] 10− as an anionic template and a saturated [ β - SiW 12 O 40 ] 4− as the counter anion, in which both the internal [ α - SiW 10 O 37 ] 10− template and the external [ β - SiW 12 O 40 ] 4− counter-anion are in situ derived from the precursor [ γ - SiW 10 (H 2 O) 2 O 34 ].…”

An Unusual Crystalline Porous Framework Constructed from Multitypes of Cages with Proton Conduction Property and Heterogeneous Catalytic Activity for Pyrazole Synthesis

“…Polyoxometalates (POMs), with their diverse shapes, sizes, charges, and metallic compositions, have emerged as promising materials in various research fields, including catalysis, materials science, pharmaceuticals, analytical chemistry, electrochemistry, and magnetism. − The attention from the research community has been particularly drawn to the different structural configurations and stacking modes of POMs, encompassing POM cages, , POM macrocycles, , inorganic and organic framework-based POMs, − POM organic polyhedra, and solid composite nanoclusters based on POMs . Among these, the cage structures, due to their beautiful structural features and captivating pore architectures, have been extensively investigated .…”