A bis-imidazole ligand, a synthetic method thereof, a metal catalyst prepared therefrom, and applications thereof

Abstract

This invention provides a bisimidazole ligand, its synthesis method, the metal catalyst prepared from it, and its applications, relating to the field of catalyst preparation technology. This invention designs and synthesizes a bisimidazole ligand by replacing the poorly stable dicyclohexylphosphine with imidazole, greatly improving the ligand's stability, ease of use, and storage. The synthesis method of the bisimidazole ligand is simple, requiring only two reaction steps, with an overall yield of up to 53%. The dual-carbene coordination mode improves the stability and catalytic activity of the nickel catalyst, and can increase the conversion number of the carbon dioxide-ethylene carboxylation reaction.

Description

Technical Field

[0001] This invention relates to the field of catalyst preparation technology for the synthesis of acrylic acid from carbon dioxide, and particularly to a diimidazole ligand, a metal catalyst, its synthesis method, and its application. Background Technology

[0002] Acrylic acid, as an important basic raw material and intermediate in modern chemical industry, has broad application prospects in coatings, textiles, building materials, and other industries. Currently, the main production methods for acrylic acid are the acetylene carbonylation method and the propylene oxidation method, with the latter being the mainstream method. The oxidation of propylene to produce acrylic acid has significant drawbacks: the reaction is highly exothermic and operates within the explosive limits of propylene, making it hazardous. While the conversion rate is very high, almost reaching 100%, the selectivity is not very high, and there are problems with excessive oxidation side reactions, producing large amounts of carbon dioxide and carbon monoxide as byproducts. This is highly detrimental in the context of emission reduction. Currently, a new method for synthesizing acrylic acid is attracting attention: the carboxylation reaction of carbon dioxide with ethylene to produce acrylic acid.

[0003] Since the 1970s, scholars from various countries have conducted extensive research on the carboxylation reaction of carbon dioxide and olefins. However, due to factors such as low yields or difficulty in closing catalytic cycles, the relevant research is still in the laboratory exploration stage and has not yet reached the scale of industrial production.

[0004] Nitrogen heterocyclic carbenes, due to their structural diversity, good modifiability, and strong coordination ability, have become one of the most important ligands for the preparation of transition metal complexes. NHC itself and its transition metal complexes are also widely used as excellent catalysts in modern organic synthetic chemistry. CN107298691A discloses a method for preparing lead complexes based on rigid bisimidazole ligands and their applications. The preparation method has advantages such as simple synthesis, easy crystallization, high yield, and good reproducibility, and can be used as a green luminescent material. CN105854947A discloses a chiral pyridine bisimidazole ligand transition metal complex catalyst and its preparation method. The chiral pyridine bisimidazole ligand used can have its electronic properties and steric hindrance modified by functional group modification. As a catalyst precursor, it exhibits moderate catalytic activity and stereoselectivity for the asymmetric hydroboration of alkenylboranes. It has the advantages of simple preparation method, inexpensive and readily available raw materials, environmental friendliness, mild reaction conditions, high yield, and simple synthetic operation. In 2019, a Japanese research team reported an alkylphosphine-substituted imidazole ligand (Chem. Eur. J. 2019, 25, 13504-13508). Under strong base conditions, it can generate a carbene intermediate to form a phosphine-carbene bidentate nickel catalyst. This carbene-coordinated nickel catalyst exhibits good stability and catalytic activity, achieving a maximum conversion number of 450 in the carboxylation of carbon dioxide with ethylene to synthesize acrylic acid, which is currently the highest conversion number for carbon dioxide carboxylation reactions without the involvement of reducing metals. However, this type of ligand also has many problems. The dicyclohexylphosphine substituent in the ligand has poor stability and is easily oxidized. In addition, the synthesis method of this ligand is complex and cumbersome, with an overall yield of only 39.3%. Replacing the unstable dicyclohexylphosphine with imidazole will significantly improve the stability of the ligand. However, existing double carbene ligands are not suitable for carbon dioxide carboxylation reactions. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a bisimidazole ligand for the synthesis of acrylic acid from carbon dioxide, its synthesis method, the metal catalyst for coordination with it, and a method for preparing acrylic acid through the carboxylation reaction of carbon dioxide and ethylene. The bisimidazole ligand synthesis method developed in this invention is simple, requiring only two reaction steps, with an overall yield of up to 53%. The dicarbene coordination mode improves the stability and catalytic activity of the nickel catalyst, increasing the conversion number of the carbon dioxide-ethylene carboxylation reaction. Compared to the ligands currently used in carbon dioxide carboxylation reactions, replacing the unstable dicyclohexylphosphine with imidazole significantly improves ligand stability, making it easier to use and store.

[0006] One of the objectives of this invention is to provide a diimidazole ligand.

[0007] The second objective of this invention is to provide a method for synthesizing the diimidazole ligand.

[0008] A third objective of this invention is to provide a metal catalyst made from this diimidazole ligand.

[0009] The fourth objective of this invention is to provide an application of this metal catalyst.

[0010] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0011] In a first aspect, the present invention provides a diimidazole ligand having the structure shown in Formula I:

[0012]

[0013] Mes stands for mesitylene.

[0014] Secondly, the present invention provides a method for synthesizing the above-mentioned diimidazole ligand, comprising the following steps:

[0015]

[0016] S1: Tris-methylamine, 2,3-butanedione and paraformaldehyde were heated and reacted in the presence of ammonium acetate and acetic acid. After the reaction, the pH of the system was adjusted to 9 and purified to obtain a tris-methylamine-substituted imidazole derivative.

[0017] S2: The mesitylene-substituted imidazole derivative obtained in S1 undergoes a substitution reaction with dibromomethane to yield the bisimidazole ligand.

[0018] In some embodiments, in step S1, the molar ratio of tricresylamine, 2,3-butanedione and paraformaldehyde is 1:1:1.5 to 1:1:2.5.

[0019] In some embodiments, in step S1, the molar ratio of mesitylene to ammonium acetate is 1:1 to 1:1.5.

[0020] In some embodiments, in step S1, the molar ratio of mesitylene to acetic acid is 1:3 to 1:8.

[0021] In some embodiments, in step S1, the reaction temperature is 130-160°C and the reaction time is 10-15 hours.

[0022] In some implementations, the purification step S1 includes the following steps:

[0023] The mixture after the reaction was extracted three times with ethyl acetate. The resulting organic phases were mixed and washed with saturated brine and dried with anhydrous sodium sulfate. After filtration, the clear organic phase was distilled under reduced pressure to remove the solvent. The resulting brown viscous oil was purified by column chromatography to obtain a mestriylbenzene-substituted imidazole derivative.

[0024] In some implementations, step S2 includes:

[0025] The mesitylene-substituted imidazole derivative obtained in S1 was dissolved in anhydrous tetrahydrofuran, and then sodium hydride was slowly added in batches. After the reaction, dibromomethane was added dropwise to the reaction system, and the mixture was stirred overnight at room temperature. The precipitate was filtered out to obtain the bisimidazole ligand.

[0026] In some embodiments, in step S2, the molar ratio of the mesitylene-substituted imidazole derivative to dibromomethane is 1:0.4 to 1:0.6.

[0027] In some embodiments, in step S2, the molar ratio of the mesitylene-substituted imidazole derivative to sodium hydride is 1:1.1 to 1:1.5.

[0028] Specifically, the method for synthesizing the diimidazole ligand includes the following steps:

[0029] (1) Add mesitylene, 2,3-butanedione, paraformaldehyde, ammonium acetate and acetic acid to a round-bottom flask and heat to react. Then, after cooling to room temperature, adjust the pH of the reaction system to 9 with 1N potassium carbonate solution.

[0030] (2) The mixture obtained in step (1) was extracted three times with ethyl acetate (100 mL × 3). The resulting organic phases were mixed and washed with saturated brine and then dried with anhydrous sodium sulfate. After filtration, the solvent was removed by vacuum distillation of the clear ethyl acetate solution. The resulting brown viscous oil was purified by column chromatography to obtain a mestriylbenzene-substituted imidazole derivative.

[0031] (3) Dissolve the mesitylene-substituted imidazole derivative obtained in step (2) in anhydrous tetrahydrofuran to obtain a tetrahydrofuran solution of the mesitylene-substituted imidazole derivative (concentration of 0.2 mol / L). Then slowly add sodium hydride in batches. After reacting for half an hour, add dibromomethane dropwise into the reaction system. Finally, stir overnight at room temperature. After filtering out the precipitate, the bisimidazole ligand is obtained.

[0032] Thirdly, the present invention provides a metal catalyst prepared from the above-mentioned diimidazole ligand by the following method:

[0033] In a glove box, 0.1 mmol of the above-mentioned imidazole ligand was placed into a 20 mL Shrek tube, and then 2 mL of anhydrous solvent was added. The mixture was stirred thoroughly to disperse it evenly. Then, 0.11 mmol of potassium tert-butoxide was slowly added. After stirring for 30 minutes, the reaction system turned into a yellow solution. Finally, 0.1 mmol of the metal compound was added and stirred for 2 hours to obtain the metal catalyst.

[0034] In some embodiments, the anhydrous solvent is anhydrous tetrahydrofuran or anhydrous toluene.

[0035] In some embodiments, the metal compound is bis-(1,5-cyclooctadiene)nickel or (1,5-cyclooctadiene)palladium(II) dichloride.

[0036] Fourthly, the present invention provides an application of the above-mentioned metal catalyst in the preparation of acrylic acid by the carboxylation reaction of carbon dioxide and ethylene.

[0037] In some embodiments, the application includes the following steps: adding a β-H elimination promoter, a metal catalyst, and a solvent into a reactor, sealing it, purging it with carbon dioxide to a reactor pressure of 1.0-2.0 MPa, and then purging the reactor with ethylene to a specified pressure of 4.0-5.0 MPa to carry out the reaction.

[0038] The reaction is a heterogeneous reaction carried out in a batch reactor.

[0039] Preferably, the β-H elimination promoter is one of sodium tert-butoxide, sodium phenolate, and sodium acetate, and more preferably sodium tert-butoxide.

[0040] Preferably, the molar ratio of the metal catalyst to the β-H elimination promoter is 1:100-1:500, more preferably 1:200-1:300.

[0041] Preferably, the solvent is one of tetrahydrofuran, anisole, toluene, dimethyl sulfoxide, and N-methylpyrrolidone, with tetrahydrofuran being the most preferred.

[0042] Preferably, the carbon dioxide has a purity of 90%-100%; the ethylene has a purity of 90%-100%.

[0043] Preferably, the reaction temperature is 80-160℃, more preferably 130-140℃; the reaction time is 12-24h, more preferably 15-18h.

[0044] Technical effects:

[0045] 1. Replacing the unstable dicyclohexylphosphine with imidazole greatly improves ligand stability, making it easier to use and store.

[0046] 2. The method for synthesizing diimidazole ligands designed and developed in this invention is simple, requiring only two reaction steps, with an overall yield of up to 53%.

[0047] 3. The dual-carbene coordination mode improves the stability and catalytic activity of nickel catalysts, and can increase the conversion number of the carbon dioxide-ethylene carboxylation reaction.

[0048] The present invention has been described in detail above; however, the above embodiments are merely illustrative in nature and are not intended to limit the invention. Furthermore, this document is not limited to the foregoing prior art or the invention itself, or to any theory described in the following embodiments. Detailed Implementation

[0049] The present invention will be further described below with reference to the embodiments. It should be noted that the following embodiments are provided for illustrative purposes only and do not constitute a limitation on the scope of protection of the present invention.

[0050] Unless otherwise specified, the raw materials, reagents, and methods used in the embodiments are all conventional raw materials, reagents, and methods in the art.

[0051] TON (Turnover Number): Represents the number of catalytic reactions or the number of target products generated per unit active site under certain conditions. It measures the catalytic efficiency and capability of a catalyst.

[0052] The TON calculation method for the carbon dioxide carboxylation reaction is: number of moles of sodium acrylate produced / number of moles of catalyst added.

[0053] Example 1

[0054] In a 100 mL round-bottom flask, 10 mmol of mesitylene, 10 mmol of 2,3-butanedione, 15 mmol of paraformaldehyde, 10 mmol of ammonium acetate, and 2.0 mL of acetic acid were added. The reaction temperature was then raised to 130 °C and reacted for 10 hours. After the reaction was completed, the temperature was lowered to room temperature and the pH of the reaction system was adjusted to 9 with 1 N potassium carbonate solution. The mixture was extracted three times with ethyl acetate (100 mL × 3). The resulting organic phases were mixed, washed with saturated brine, and dried over anhydrous sodium sulfate. After filtration, the solvent was removed by vacuum distillation of the clear ethyl acetate solution. The resulting brown viscous oil was purified by column chromatography to obtain a mesitylene-substituted imidazole derivative with a yield of 57%. The prepared mesitylene-substituted imidazole derivative (5.7 mmol) was dissolved in anhydrous tetrahydrofuran, and then sodium hydride (6.3 mmol) was slowly added in batches. After reacting for half an hour, dibromomethane (2.3 mmol) was added dropwise to the reaction system. Finally, the mixture was stirred overnight at room temperature. The precipitate was filtered off to obtain the bisimidazole ligand, with an overall yield of 36%.

[0055] Example 2

[0056] In a 100 mL round-bottom flask, 10 mmol of mesitylene, 10 mmol of 2,3-butanedione, 25 mmol of paraformaldehyde, 15 mmol of ammonium acetate, and 2.0 mL of acetic acid were added. The reaction temperature was then raised to 130 °C and reacted for 10 hours. After the reaction was completed, the temperature was lowered to room temperature and the pH of the reaction system was adjusted to 9 with 1 N potassium carbonate solution. The mixture was extracted three times with ethyl acetate (100 mL × 3). The resulting organic phases were mixed, washed with saturated brine, and dried over anhydrous sodium sulfate. After filtration, the solvent in the clear ethyl acetate solution was removed by vacuum distillation. The resulting brown viscous oil was purified by column chromatography to obtain a mesitylene-substituted imidazole derivative with a yield of 60%. The prepared mesitylene-substituted imidazole derivative (6.0 mmol) was dissolved in anhydrous tetrahydrofuran, and then sodium hydride (6.6 mmol) was slowly added in batches. After reacting for half an hour, dibromomethane (2.4 mmol) was added dropwise to the reaction system. Finally, the mixture was stirred overnight at room temperature. The precipitate was filtered off to obtain the bisimidazole ligand with a yield of 38%.

[0057] Example 3

[0058] In a 100 mL round-bottom flask, 10 mmol of mesitylene, 10 mmol of 2,3-butanedione, 25 mmol of paraformaldehyde, 10 mmol of ammonium acetate, and 2.0 mL of acetic acid were added. The reaction temperature was then raised to 130 °C and reacted for 10 hours. After the reaction was completed, the temperature was lowered to room temperature, and the pH of the reaction system was adjusted to 9 with 1 N potassium carbonate solution. The mixture was extracted three times with ethyl acetate (100 mL × 3). The resulting organic phases were mixed, washed with saturated brine, and dried over anhydrous sodium sulfate. After filtration, the solvent was removed by vacuum distillation of the clear ethyl acetate solution. The resulting brown viscous oil was purified by column chromatography to obtain a mesitylene-substituted imidazole derivative with a yield of 61%. The prepared mesitylene-substituted imidazole derivative (6.1 mmol) was dissolved in anhydrous tetrahydrofuran, and then sodium hydride (6.7 mmol) was slowly added in batches. After reacting for half an hour, dibromomethane (2.5 mmol) was added dropwise to the reaction system. Finally, the mixture was stirred overnight at room temperature. The precipitate was filtered off to obtain the bisimidazole ligand, with an overall yield of 39%.

[0059] Example 4

[0060] In a 100 mL round-bottom flask, 10 mmol of mesitylene, 10 mmol of 2,3-butanedione, 25 mmol of paraformaldehyde, 10 mmol of ammonium acetate, and 5.0 mL of acetic acid were added. The reaction temperature was then raised to 130 °C and reacted for 10 hours. After the reaction was completed, the temperature was lowered to room temperature and the pH of the reaction system was adjusted to 9 with 1 N potassium carbonate solution. The mixture was extracted three times with ethyl acetate (100 mL × 3). The resulting organic phases were mixed, washed with saturated brine, and dried over anhydrous sodium sulfate. After filtration, the solvent was removed by vacuum distillation of the clear ethyl acetate solution. The resulting brown viscous oil was purified by column chromatography to obtain a mesitylene-substituted imidazole derivative with a yield of 65%. The prepared mesitylene-substituted imidazole derivative (6.5 mmol) was dissolved in anhydrous tetrahydrofuran, and then sodium hydride (7.2 mmol) was slowly added in batches. After reacting for half an hour, dibromomethane (2.6 mmol) was added dropwise to the reaction system. Finally, the mixture was stirred overnight at room temperature. The precipitate was filtered off to obtain the bisimidazole ligand, with an overall yield of 42%.

[0061] Example 5

[0062] In a 100 mL round-bottom flask, 10 mmol of mesitylene, 10 mmol of 2,3-butanedione, 25 mmol of paraformaldehyde, 10 mmol of ammonium acetate, and 5.0 mL of acetic acid were added. The reaction temperature was then raised to 160 °C and reacted for 15 hours. After the reaction was completed, the temperature was lowered to room temperature, and the pH of the reaction system was adjusted to 9 with 1 N potassium carbonate solution. The mixture was extracted three times with ethyl acetate (100 mL × 3). The resulting organic phases were mixed, washed with saturated brine, and dried over anhydrous sodium sulfate. After filtration, the solvent in the clear ethyl acetate solution was removed by vacuum distillation. The resulting brown viscous oil was purified by column chromatography to obtain a mesitylene-substituted imidazole derivative with a yield of 68%. The prepared mesitylene-substituted imidazole derivative (6.8 mmol) was dissolved in anhydrous tetrahydrofuran, and then sodium hydride (7.5 mmol) was slowly added in batches. After reacting for half an hour, dibromomethane (2.3 mmol) was added dropwise to the reaction system. Finally, the mixture was stirred overnight at room temperature. The precipitate was filtered off to obtain the bisimidazole ligand, with an overall yield of 44%.

[0063] Example 6

[0064] In a 100 mL round-bottom flask, 10 mmol of mesitylene, 10 mmol of 2,3-butanedione, 25 mmol of paraformaldehyde, 10 mmol of ammonium acetate, and 5.0 mL of acetic acid were added. The reaction temperature was then raised to 160 °C and reacted for 15 hours. After the reaction was completed, the temperature was lowered to room temperature, and the pH of the reaction system was adjusted to 9 with 1 N potassium carbonate solution. The mixture was extracted three times with ethyl acetate (100 mL × 3). The resulting organic phases were mixed, washed with saturated brine, and dried over anhydrous sodium sulfate. After filtration, the solvent in the clear ethyl acetate solution was removed by vacuum distillation. The resulting brown viscous oil was purified by column chromatography to obtain a mesitylene-substituted imidazole derivative with a yield of 68%. The prepared mesitylene-substituted imidazole derivative (10.2 mmol) was dissolved in anhydrous tetrahydrofuran, and then sodium hydride (10.2 mmol) was slowly added in batches. After reacting for half an hour, dibromomethane (2.7 mmol) was added dropwise to the reaction system. Finally, the mixture was stirred overnight at room temperature. The precipitate was filtered off to obtain the bisimidazole ligand, with an overall yield of 53%.

[0065] Example 7

[0066] In a 100 mL round-bottom flask, 10 mmol of mesitylene, 10 mmol of 2,3-butanedione, 25 mmol of paraformaldehyde, 10 mmol of ammonium acetate, and 5.0 mL of acetic acid were added. The reaction temperature was then raised to 160 °C and reacted for 15 hours. After the reaction was completed, the temperature was lowered to room temperature, and the pH of the reaction system was adjusted to 9 with 1 N potassium carbonate solution. The mixture was extracted three times with ethyl acetate (100 mL × 3). The resulting organic phases were mixed, washed with saturated brine, and dried over anhydrous sodium sulfate. After filtration, the solvent in the clear ethyl acetate solution was removed by vacuum distillation. The resulting brown viscous oil was purified by column chromatography to obtain a mesitylene-substituted imidazole derivative with a yield of 68%. The prepared mesitylene-substituted imidazole derivative (6.8 mmol) was dissolved in anhydrous tetrahydrofuran, and then sodium hydride (10.2 mmol) was slowly added in batches. After reacting for half an hour, dibromomethane (4.1 mmol) was added dropwise to the reaction system. Finally, the mixture was stirred overnight at room temperature. The precipitate was filtered off to obtain the bisimidazole ligand, with an overall yield of 48%.

[0067] Each embodiment prepared the diimidazole-nickel catalyst according to the following method:

[0068] In a glove box, 0.1 mmol of diimidazole ligand was placed in a 20 mL Shrek tube, and then 2 mL of anhydrous tetrahydrofuran was added. The mixture was stirred thoroughly to disperse the ligand evenly. Then, 0.11 mmol of potassium tert-butoxide was slowly added. After stirring for 30 minutes, the reaction system turned into a yellow solution. Finally, 0.1 mmol of bis-(1,5-cyclooctadiene)nickel was added and the mixture was stirred for 2 hours to obtain the diimidazole-nickel catalyst.

[0069] Examples 8-14 (solvent types and β-H elimination promoters are shown in Table 1)

[0070] In a 150 ml high-pressure reactor, under nitrogen protection, the 0.1 mmol of the imidazolium-nickel catalyst prepared in Example 6, 10 mmol of the β-H elimination promoter, and 35 ml of solvent were added sequentially. The reactor was sealed, and the gas inside was replaced with carbon dioxide three times. Then, carbon dioxide was added until the reactor pressure reached 1 MPa, followed by the addition of ethylene until the reactor pressure reached 5 MPa. The temperature was slowly increased to 80 °C under temperature control, and the reaction was carried out for 12 h. After cooling to room temperature, the reactor was removed, and the solvent was removed by vacuum distillation. The resulting pale yellow solid was dissolved in 10 mL of deuterium water internal standard solution by thorough shaking, and then analyzed by 1H NMR spectroscopy to calculate the TON of the reaction.

[0071] Table 1

[0072]

[0073]

[0074] Examples 15-22 (Sodium tert-butoxide addition, reaction temperature, and reaction time are shown in Table 2)

[0075] In a 150 ml high-pressure reactor, under nitrogen protection, 0.1 mmol of the imidazolium-nickel catalyst prepared in Example 6, sodium tert-butoxide, and 35 ml of tetrahydrofuran were added sequentially. The reactor was sealed, and the gas inside was replaced with carbon dioxide three times. Carbon dioxide was then introduced until the reactor pressure reached 1 MPa, followed by the introduction of ethylene until the reactor pressure reached 5 MPa. The temperature was slowly increased to the specified temperature and reacted for a predetermined time under temperature control. After cooling to room temperature, the reactor was removed, and the solvent was removed by vacuum distillation. The resulting pale yellow solid was dissolved in 10 mL of deuterium water internal standard solution by thorough shaking, and then analyzed by 1H NMR spectroscopy to calculate the reaction TON.

[0076] Table 2

[0077]

[0078] Comparative Examples 1-3 (see Table 3 for comparison of ligand types)

[0079] In a 150 mL high-pressure reactor, under nitrogen protection, bis-(1,5-cyclooctadiene)nickel (0.1 mmol), bisphosphine ligand (0.11 mmol), sodium tert-butoxide (20 mmol), and THF (35 mL) were added sequentially. The reactor was sealed, and the gas inside was replaced with carbon dioxide three times. Carbon dioxide was then introduced until the reactor pressure reached 1 MPa, followed by the introduction of ethylene until the pressure reached 5 MPa. The temperature was slowly increased to 140 °C under temperature control, and the reaction was carried out for 15 h. After cooling to room temperature, the reactor was removed, and the solvent was removed by vacuum distillation. The resulting pale yellow solid was dissolved in 10 mL of deuterium water internal standard solution by thorough shaking, and then analyzed by 1H NMR spectroscopy to calculate the reaction TON.

[0080] Table 3

[0081]

[0082] The above embodiments are merely illustrative of the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and substance defined by the claims of the present invention; and such modifications or substitutions are still within the scope defined by the claims of the present invention.

Claims

1. A diimidazole ligand, characterized in that, The diimidazole ligand has the structure shown in Formula I: Formula I, Mes represents mesitylene.

2. A method for synthesizing the diimidazole ligand according to claim 1, characterized in that, Includes the following steps: S1: Tris-methylamine, 2,3-butanedione and paraformaldehyde were heated and reacted in the presence of ammonium acetate and acetic acid. After the reaction, the pH of the system was adjusted to 8-9 and purified to obtain a tris-methylamine-substituted imidazole derivative. S2: The mesitylene-substituted imidazole derivative obtained in S1 undergoes a substitution reaction with dibromomethane to yield the bisimidazole ligand.

3. The synthesis method according to claim 2, characterized in that, In step S1, the molar ratio of tricresylamine, 2,3-butanedione, and paraformaldehyde is 1:1:1.5 to 1:1:2.5; In step S1, the molar ratio of mesitylene to ammonium acetate is 1:1 to 1:1.5; In step S1, the molar ratio of tricresylamine to acetic acid is 1:3 to 1:8; In step S1, the reaction temperature is 130-160℃ and the reaction time is 10-15 hours.

4. The synthesis method according to claim 2, characterized in that, Step S2 includes: The mesitylene-substituted imidazole derivative obtained in S1 was dissolved in anhydrous tetrahydrofuran, and then sodium hydride was slowly added in batches. After the reaction, dibromomethane was added dropwise to the reaction system, and the mixture was stirred overnight at room temperature. The precipitate was filtered out to obtain the bisimidazole ligand.

5. The synthesis method according to claim 4, characterized in that, In step S2, the molar ratio of the mesitylene-substituted imidazole derivative to dibromomethane is 1:0.4 to 1:0.6; In step S2, the molar ratio of the mesitylene-substituted imidazole derivative to sodium hydride is 1:1.1 to 1:1.

5.

6. A metal catalyst, characterized in that, The diimidazole ligand according to claim 1 is prepared by the following method: Anhydrous solvent was added to the bisimidazole ligand and stirred thoroughly to disperse it evenly. Then potassium tert-butoxide was slowly added and stirred until the reaction system turned into a yellow solution. Finally, a metal compound was added and stirred to obtain a metal catalyst. The anhydrous solvent is anhydrous tetrahydrofuran or anhydrous toluene; The metal compound is bis-(1,5-cyclooctadiene)nickel.

7. The application of the metal catalyst according to claim 6 in the preparation of acrylic acid by the carboxylation reaction of carbon dioxide and ethylene.

8. The application according to claim 7, characterized in that, The application includes the following steps: adding β-H elimination promoter, metal catalyst and solvent into a reaction vessel, sealing it, purging carbon dioxide to a pressure of 1.0-2.0 MPa in the reaction vessel, and then purging the reaction vessel to a specified pressure of 4.0-5.0 MPa with ethylene to carry out the reaction.

9. The application according to claim 8, characterized in that, The β-H elimination promoter is one of sodium tert-butoxide, sodium phenolate, and sodium acetate. The molar ratio of the metal catalyst to the β-H elimination promoter is 1:100-1:500; The solvent is one of tetrahydrofuran, anisole, toluene, dimethyl sulfoxide, and N-methylpyrrolidone; The reaction temperature is 80-160℃; the reaction time is 12-24h.

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