Polyoxometalate for catalyzing knoevenagel reaction, preparation method and application thereof

Abstract

This invention discloses a polyoxometalate that catalyzes the Knoevenagel reaction, its preparation method, and its applications. The chemical formula of the polyoxometalate that catalyzes the Knoevenagel reaction is: H3[K(H2O)6][K2(H2O) 10 ]2[Cu(en)2(H2O)2][Cu(en)2(Nb6O 19 Cu(en))2·24H2O, where en = ethylenediamine; the polyoxometalate crystals catalyzing the Knoevenagel reaction belong to the triclinic crystal system with space group P-1; the polyoxometalates catalyzing the Knoevenagel reaction prepared in this invention exhibit excellent heterogeneous catalytic performance in the highly efficient Knoevenagel condensation reaction, showing high yield and high stability under isothermal conditions at a 1:1 substrate ratio and over a wide substrate range. The superior catalytic ability of the polyoxometalates catalyzing the Knoevenagel reaction can be attributed to the presence of dense and effective basic sites on the surface of the heteropolycationic chain, and the increased contact frequency between the organic-inorganic hybrid catalyst and the substrate.

Description

Technical Field

[0001] This invention belongs to the technical field of organic catalytic materials and crystal materials, specifically relating to a polyoxometalate that catalyzes the Knoevenagel reaction, its preparation method, and its application. Background Technology

[0002] One of the long-term goals of green chemistry is efficient synthesis with low energy consumption and low pollution. Obtaining easily produced, efficient, and sustainable catalysts has always attracted researchers. Knoevenagel condensation is a base-driven reaction, crucial in the pharmaceutical, plastics, and rubber industries, for example, for the synthesis of coumarins and their derivatives, and is also an important strategy in organic synthesis. While conventional organic amines such as triethylamine and piperidine can be effective homogeneous catalysts in some cases, they can be difficult to separate and lead to additional pollution in the strongly basic environment of organic amines. Therefore, it is important to prepare efficient and easily prepared heterogeneous catalysts that are easily separable and environmentally friendly for these condensation reactions. Polyoxometalates (POMs) are a class of stable, discrete, structurally well-defined oxide cluster anions, mainly composed of high-valence transition metals (such as W...). 6+ Mo 6+ V 5+ 、Nb 5+ and Ta 5+ This has stimulated research in heterogeneous catalysis across a wide range of fields. Polyoxoniobates (PONbs) are considered ideal base heterogeneous catalysts for catalyzing Knoevenagel condensation. The abundant exposed oxygen atoms in PONbs endow them with excellent basicity, which is beneficial for removing active protons from organic substrates during base catalysis. Furthermore, various transition metal heterocomplexes modified with organic ligands exhibit even stronger catalytic performance. Transition metal heteroatoms (such as Cu...) 2+ It can be easily introduced into POMs and then modified with organic ligands to produce novel and highly efficient organic-inorganic hybrid multi-oxygen metal catalysts.

[0003] Based on the above requirements, this invention designs and successfully incorporates Cu(en) complex units (en = ethylenediamine) into polyniobate via coordination. The resulting copper-containing heteropolyoxyniobate crystals possess strong... Its base catalytic properties and excellent substrate compatibility make it a potential application in the field of organocatalysis. Summary of the Invention

[0004] To address the above problems, this invention provides a polyoxometalate that catalyzes the Knoevenagel reaction, its preparation method, and its applications.

[0005] The present invention adopts the following technical solution:

[0006] A polyoxometalate that catalyzes the Knoevenagel reaction, wherein the chemical formula of the polyoxometalate that catalyzes the Knoevenagel reaction is:

[0007] H3[K(H2O)6][K2(H2O) 10 ]2[Cu(en)2(H2O)2][Cu(en)2(Nb6O 19 Cu(en))2]·24H2O, where en = ethylenediamine; the polyoxometalate crystal catalyzing the Knoevenagel reaction belongs to the triclinic crystal system, space group P-1; the polyoxometalate catalyzing the Knoevenagel reaction is a one-dimensional chain structure, characterized by its asymmetric unit containing a Lindqvist unit [Nb6O] 19 ] 8- An attached [Cu(en)] 2+ A complex and a bridging [Cu(en)2] 2+ Unit; wherein, two {CuNb6} units are further interconnected to form a centrally symmetric {CuNb6}2 dimer through a distorted octahedron centered on Cu1; Cu2 atoms, as bridging units, connect two μ2 oxygen atoms of two adjacent {CuNb6}2 dimers and four N atoms of two en ligands respectively, forming a one-dimensional organic-inorganic hybrid {Cu4Nb6}∞ nanowire along the

[110] direction; the {Cu4Nb6}∞ nanowire based on 1D clusters generates a two-dimensional supramolecular layer extending on the ab plane through hydrogen bonding and electrostatic interaction of hydrated potassium ions, and the two-dimensional supramolecular layer forms a three-dimensional supramolecular framework by a potassium ion and a complex centered on Cu3 as counterions.

[0008] Preferably, the cell parameters of the polyoxometalate catalyzing the Knoevenagel reaction are: α=90.693(2)°, β=100.806(2)°, γ=110.263(2)°.

[0009] A method for preparing a polyoxometalate that catalyzes the Knoevenagel reaction specifically includes the following steps:

[0010] S1. Weigh out the niobium precursor K7HNb6O in sequence. 19 • 13H2O, copper acetate monohydrate, chromium acetate monohydrate, and sodium pyrophosphate decahydrate; the molar ratio of the niobium precursor, copper acetate monohydrate, chromium acetate monohydrate, and sodium pyrophosphate decahydrate is 1:1:1:1, and the amount of niobium precursor is 2 mmol.

[0011] S2. Ultrapure water and ethylenediamine are added sequentially and stirred until homogeneous. The polyoxometalate catalyzing the Knoevenagel reaction is prepared by a solvothermal reaction. The molar ratio of the niobium precursor to ultrapure water is 1:100. The volume ratio of ultrapure water to ethylenediamine is 10:1. The reaction temperature of the solvothermal reaction is 120°C and the reaction time is 72 hours.

[0012] An application of a polyoxometalate for catalyzing the Knoevenagel reaction, wherein the polyoxometalate for catalyzing the Knoevenagel reaction is used as an organic catalytic material for catalyzing the Knoevenagel reaction.

[0013] By adopting the above technical solution, the present invention has the following advantages compared with the prior art:

[0014] 1. The polyoxometalates prepared in this invention exhibit excellent heterogeneous catalytic performance in the highly efficient Knoevenagel condensation reaction, demonstrating high yield and high stability under isothermal conditions at a 1:1 substrate ratio and over a wide substrate range. The superior catalytic ability of the polyoxometalates catalyzing the Knoevenagel reaction can be attributed to the dense and effective basic sites on the surface of the heteropolycation chains and the increased contact frequency between the organic-inorganic hybrid catalyst and the substrate. The maintenance of cycle stability is due to the one-dimensional cluster aggregates generated by long-range ordered electrostatic interactions. This not only enriches the structure of PONbs but also opens new avenues for the application of polyoxometalates (HPONb1) catalyzing the Knoevenagel reaction as potent basic catalysts.

[0015] 2. The polyoxometalate salts prepared in this invention, used as organic catalysts for the Knoevenagel reaction, can be applied to catalyze the Knoevenagel reaction. The Knoevenagel condensation reaction plays a crucial role in establishing C-C bonds (especially in alkaline environments), promoting key reactions between active methylene compounds and carbonyl compounds. While traditional organic amines (such as triethylamine and piperidine) can serve as highly efficient homogeneous catalysts, their strong basicity poses challenges to separation and potential environmental pollution. Therefore, developing a highly efficient, easily separable, and environmentally friendly heterogeneous catalyst for these condensation reactions is particularly important. Polymetallic oxide salts (POMs) have attracted widespread attention as heterogeneous base catalysts for promoting the Knoevenagel condensation reaction. The abundant oxygen atoms on the polyanionic surface not only possess strong Boucher basicity, capable of capturing protons in the methylene substrate and activating them as nucleophiles, but also increase the contact frequency with the methylene substrate, further increasing the number of active nucleophiles. There are currently no reports on the efficient catalysis of Knoevenagel condensation by POM catalysts at room temperature, mainly because promoting the contact between the organic substrate and the catalyst in heterogeneous catalysis is challenging. Various transition metal heterocomplexes modified with organic ligands exhibit stronger catalytic performance. Transition metal heteroatoms (such as Cu...) 2+ It can be easily introduced into POMs and then modified with organic ligands to produce novel and highly efficient organic-inorganic hybrid multi-oxygen metal catalysts. Attached Figure Description

[0016] Figure 1 The image shows a crystal of the polyoxometalate obtained in this invention for catalyzing the Knoevenagel reaction.

[0017] Figure 2 The coordination mode diagram and three-dimensional crystal structure framework diagram of the polyoxometalate for catalyzing the Knoevenagel reaction prepared in this invention are shown.

[0018] Figure 3 This is a coordination mode diagram of some metal ions in this invention;

[0019] Figure 4 The XRD pattern of the polyoxometalate obtained by catalyzing the Knoevenagel reaction in this invention;

[0020] Figure 5 The TG spectrum of the polyoxometalate obtained by catalyzing the Knoevenagel reaction in this invention;

[0021] Figure 6The image shows the effect of screening the polyoxometalate obtained in this invention to catalyze the Knoevenagel reaction to determine the optimal conditions for catalyzing the reaction of benzaldehyde and ethyl cyanoacetate.

[0022] Figure 7 The diagram shows the effect of the polyoxometalate obtained in this invention on the reaction of different aldehydes with ethyl cyanoacetate under optimal conditions for the Knoevenagel reaction.

[0023] Figure 8 This is a diagram showing the effect of the polyoxometalate obtained in this invention on the reaction of different aldehydes with malononitrile under optimal conditions for the Knoevenagel reaction.

[0024] Figure 9 The catalytic effect of the polyoxometalate obtained in this invention for the Knoevenagel reaction is shown in the figure after the substrate amount is increased to the gram level.

[0025] Figure 10 This is a schematic diagram of the catalytic reaction mechanism of the polyoxometalate obtained in this invention for the Knoevenagel reaction. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0027] See Figures 1 to 10 .

[0028] H3[K(H2O)6][K2(H2O) 10 ]2[Cu(en)2(H2O)2][Cu(en)2(Nb6O 19 Preparation of Cu(en)2]·24H2O

[0029] 2 mmol of niobium precursor (K7HNb6O) 19 ·13H2O), 2 mmol of copper acetate monohydrate, 2 mmol of chromium acetate monohydrate, and 2 mmol of sodium pyrophosphate decahydrate were added to a mixed solution of 5 mL ultrapure water and 0.5 mL ethylenediamine and mixed thoroughly. The resulting mixture was then transferred to a 100 mL stainless steel reactor and stirred for 1 hour before being placed in an oven for a solvothermal reaction. After maintaining the temperature at 120 °C for 72 hours, the mixture was cooled to room temperature to obtain H3[K(H2O)6][K2(H2O)]. 10 ]2[Cu(en)2(H2O)2][Cu(en)2(Nb6O 19Cu(en)2]·24H2O (a polyoxometalate that catalyzes the Knoevenagel reaction), abbreviated as HPONb1.

[0030] Crystal photographs of the polyoxometalates that catalyze the Knoevenagel reaction, such as... Figure 1 As shown.

[0031] The polyoxometalate samples catalyzing the Knoevenagel reaction were characterized by X-ray single-crystal diffraction, and the results are as follows: Figure 2 As shown. The crystal belongs to space group P-1, and its unit cell parameters are: α = 90.693(2)°, β = 100.806(2)°, γ = 110.263(2)°. The specific cell parameters are shown in Table 1.

[0032] Table 1. Cell parameters of polyoxometalates catalyzing the Knoevenagel reaction.

[0033]

[0034]

[0035] a R1=Σ||Fo|-|Fc|| / Σ|Fo|, b wR2=[Σw(Fo 2 -Fc 2 ) 2 / w(Fo) 2 ] 1 / 2 w = 1 / [σ 2 (Fo 2 )+(xP) 2 +yP],P=(Fo 2 +2Fc 2 ) / 3, where x=0.0450, y=410.1602.

[0036] The structures of polyoxometalate samples catalyzing the Knoevenagel reaction were elucidated using SHELX and OLEX. The secondary building blocks of this compound include... Figure 2 As shown in (b), the coordination mode adopted by Cu(en)x (x=1,2) is as follows: Figure 3 As shown.

[0037] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A polyoxometalate that catalyzes the Knoevenagel reaction, characterized in that, The chemical formula of the polyoxometalate that catalyzes the Knoevenagel reaction is: H3[K(H2O)6][K2(H2O) 10 ]2[Cu(en)2(H2O)2][Cu(en)2(Nb6O 19 Cu(en))2]·24H2O, where en = ethylenediamine; the polyoxometalate crystal that catalyzes the Knoevenagel reaction belongs to the triclinic crystal system, space group . P-1 The polyoxometalate catalyzing the Knoevenagel reaction is a one-dimensional chain structure characterized by its asymmetric unit containing a Lindqvist unit [Nb6O]. 19 ] 8- An attached [Cu(en)] 2+ A complex and a bridging [Cu(en)2] 2+ Unit; wherein, two {CuNb6} units are further interconnected to form a centrally symmetric {CuNb6}2 dimer through a distorted octahedron centered on Cu1; Cu2 atoms, as bridging units, connect two μ2 oxygen atoms of two adjacent {CuNb6}2 dimers and four N atoms of two en ligands respectively, forming a one-dimensional organic-inorganic hybrid {Cu4Nb6}∞ nanowire along the [110] direction; the {Cu4Nb6}∞ nanowire based on 1D clusters generates a two-dimensional supramolecular layer extending on the ab plane through hydrogen bonding and electrostatic interaction of hydrated potassium ions, and the two-dimensional supramolecular layer forms a three-dimensional supramolecular framework with a potassium ion and a complex centered on Cu3 as a counterion.

2. The polyoxometalate for catalyzing the Knoevenagel reaction as described in claim 1, characterized in that, The cell parameters of the polyoxometalate that catalyzes the Knoevenagel reaction are: a = 13.1904(13) (Å), b =14.1814(13) (Å), c = 15.3343(14) (Å), α = 90.693(2) °, β = 100.806(2) °, γ =110.263(2) °.

3. A method for preparing a polyoxometalate for catalytic Knoevenagel reaction as described in any one of claims 1-2, characterized in that, Specifically, the following steps are included: S1, the niobium precursor K7HNb6O was weighed in turn 19 • 13H2O, copper acetate monohydrate, chromium acetate monohydrate and sodium pyrophosphate decahydrate; the molar ratio of the niobium precursor, copper acetate monohydrate, chromium acetate monohydrate and sodium pyrophosphate decahydrate added was 1:1:1:1, and the amount of the niobium precursor was 2 mmol; S2. Ultrapure water and ethylenediamine are added sequentially and stirred until homogeneous. The polyoxometalate catalyzing the Knoevenagel reaction is prepared by a solvothermal reaction. The molar ratio of the niobium precursor to ultrapure water is 1:

100. The volume ratio of ultrapure water to ethylenediamine is 10:

1. The reaction temperature of the solvothermal reaction is 120°C and the reaction time is 72 hours.

4. The application of a polyoxometalate for catalyzing the Knoevenagel reaction as described in any one of claims 1-2, characterized in that: The polyoxometalate used to catalyze the Knoevenagel reaction is an organic catalytic material applied to the catalysis of the Knoevenagel reaction.

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