| Polyoxovanadates-based metal-organic frameworks are a class of crystalline materials with well-defined structures consisting of polyoxovanadates and metal-organic frameworks.They combine the structural features and performance advantages of polyoxovanadates and metal-organic frameworks and are ideal candidates for highly active and stable heterogeneous catalysts.Moreover,their well-defined structures facilitate the investigation of catalytic reaction mechanisms at the molecular level.In recent years,polyoxovanadates-based metal-organic frameworks have demonstrated high catalytic activity and stability in a variety of organic transformation reactions,but there are still some problems such as catalytic activity and selectivity need to be improved,catalytic reaction conditions are not mild enough,and the role of the active sites in the reaction is ambiguous that should be solved.Based on this,the aim of this paper is to design and synthesize polyoxovanadates-based metal-organic framework catalysts with high catalytic activity under mild conditions,and to explore the role of active sites in depth through combining experiments with theoretical calculations.The specific work is as follows:1.Three polyoxovanadates based metal-organic frameworks,[Ni(bix)2]{V4O11}(1),[Cu(bix)2]{V4O11}(2)and[Co2H2(bix)4]{V8O23}(3)(bix=1,4-bis(imidazol-1-ylmethyl)benzene)were synthesized.Compounds 1 and 2 are isomorphic,with the {V4O12} clusters in the structure being corner-sharing linked to form novel tandem-like vanadium cluster chains,while the {V4O12} clusters in compound 3 are corner-sharing linked to form dimers.In compounds 1-3,the tandem-like vanadium cluster chains or {V4O12} dimers form three-dimensional frameworks through the linkage of transition metal ions and bix ligands.Using hydrogen peroxide as the oxidant,compounds 1-3 can efficiently oxidize many types of sulfides compounds under mild conditions and are highly stable,and remain highly active after multiple cycles of reaction.Mechanistic studies have shown that the{V4O12} cluster is the active center catalyzing this reaction,where the Vv interacts with hydrogen peroxide to produce the active peroxide species to oxidize the sulfides to sulfones.In combination with density functional theory(DFT)calculations,we found that as the number of {V4O12} clusters in tandem increases,the LUMO energy level of the compound decreases,which can match the energy level of hydrogen peroxide more effectively,thus speeding up the production of the active peroxide species and allowing the reaction to proceed rapidly.This is the first time that the effect of the types of polyoxovanadates tandem on the catalytic performance of such reactions has been investigated by a combination of theoretical calculations and experiments.2.A polyoxovanadates-dinuclear cobalt metal-organic framework,[Co(bix)(H2O)]{V2O6}(4),was synthesized.The {V2O6} clusters and dinuclear cobalt units in compound 4 are connected to form a chain-like structure and further linked by bix ligands to construct a two-dimensional lamellar structure.The coordination water molecules of CoⅡ were removed by high temperature activation and the obtained compound is named compound 4’,which has both a Lewis base active {V2O6} cluster and a Lewis acid active CoⅡ center.Compound 4’ can efficiently catalyze a series of C-C coupling reactions of benzaldehyde with substituents and malononitrile under solvent-free conditions and has excellent recyclability.Mechanistic studies show that the open CoⅡ site activates the oxygen atom of the carbonyl group on benzaldehyde,the｛V2O6} cluster plucks the hydrogen proton of the methylene group on malononitrile,and finally the two reactants dehydrate to form the target product.Further DFT calculations show that the dinuclear cobalt unit,as a Lewis acid site,also acts to stabilize the alcoholic hydroxyl anion intermediate produced during the reaction,and this role of the Lewis acid site in such reactions has been identified for the first time.3.A polyoxovanadates-dinuclear copper metal-organic framework,[Cu(bix)]{V2O6}(5),was synthesized.In compound 5,the {V2O6} cluster and the dinuclear copper unit are interleaved to form a chain-like structure,which is further connected by the bix ligands to form a two-dimensional layered structure.Compound 5 has a similar structure to compound 4,and the main difference between them is that the CuⅡ center in compound 5 is in the unsaturated 5-coordination mode,so high temperature activation is not required to obtain unsaturated transition metal centers.Using hydrogen peroxide as the oxidant,compound 5 catalyzed the oxidation of 2,3,6-trimethylphenol to 2,3,5-trimethyl-p-benzoquinone within 10 minutes(>99%yield),and the compound 5 shows good catalytic activity for phenol derivatives with different substituents.The mechanism study shows that the CuⅡ and {V2O6} clusters in compound 5 are the catalytic active centers,and they co-catalyzed the oxidation of the substrate through two paths,making the reaction proceed rapidly.DFT calculations show that the dinuclear copper site of compound 5 has a weaker adsorption capacity to the substrate than the mononuclear copper site,the dinuclear cobalt of compound 4’ and mononuclear cobalt site,which makes the dinuclear copper site not easy to be poisoned and keep the high activity of catalyzing substrate transformation.4.A metal-organic framework containing polyoxovanadates and mixed-valent copper ions,[CuⅠ2CuⅡ(bix)2]{V4O12}(6),was synthesized.The {V4O12} cluster in compound 6 is linked to Cu+ and Cu2+ ions to form a two-dimensional wavy lamellar structure,and the Cu2+/Cu+ ions on the layer are further linked to the bix ligands to form a three-dimensional framework.Under mild reaction conditions,compound 6 can activate the sp3 C-H bonds with high activation energy,oxidizing a series of benzylic compounds to the corresponding ketones with good catalytic activity and selectivity.The catalytic active sites are identified as {V4O12} clusters,CuⅠ and CuⅡ,and the radicals generated are tBuOO· and tBuO· by control experiments,radical quenching experiments and EPR spectroscopy analysis.DFT calculations indicate that there exists synergistic catalysis between CuⅠ/{V4O12} sites and CuⅡ/{V4O12} sites for 6 as the catalyst in the oxidation of benzylic compounds,where the {V4O12} cluster mainly provides deprotonation and oxidation sites,and the CuⅠ site plays a role in the reduction of the oxidant,while the CuⅡ site plays a role in the adsorption of the oxidant.This work provides the first in-depth study of the mechanism of catalytic benzylic C-H bonds oxidation by combining experiments with theoretical calculations among the catalytic studies of polyoxometalates-based metal-organic frameworks. |
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