Bis-imidazole ligand, synthesis method thereof, metal catalyst prepared from bis-imidazole ligand and application of metal catalyst

Abstract

The invention provides a bis-imidazole ligand, a synthesis method thereof, a metal catalyst prepared from the bis-imidazole ligand and application of the metal catalyst, and relates to the technical field of catalyst preparation. According to the invention, dicyclohexylphosphine with poor stability is substituted by imidazole to design and synthesize the bis-imidazole ligand, the stability of the ligand is greatly improved, the ligand is easy to use and store, the synthesis method of the bis-imidazole ligand is simple, the ligand synthesis only needs two-step reaction, and the total yield can reach 53%. The coordination mode of the dicarbene improves the stability and catalytic activity of the nickel catalyst, and can improve the conversion number of carboxylation reaction of carbon dioxide and ethylene.

Description

Technical Field

[0001] The invention relates to the technical field of preparation of catalysts for synthesizing acrylic acid from carbon dioxide, and in particular to a bisimidazole ligand, a metal catalyst, and a synthesis method and application thereof. Background Art

[0002] As an important basic raw material and intermediate in modern chemical industry, acrylic acid has broad application prospects in the coating, textile, building materials and other industries. At present, the main production methods of acrylic acid are acetylene carbonylation and propylene oxidation, the latter of which is the mainstream method for acrylic acid production. Propylene oxidation to produce acrylic acid has significant disadvantages. The reaction is highly exothermic, and the reaction is operated within the explosion limit of propylene, which is dangerous. The conversion rate of the reaction is very high, almost reaching 100%, but the selectivity is not very high, there is a problem of over-oxidation side reaction, and a large amount of carbon dioxide and carbon monoxide are produced as by-products. This is very unfavorable in the context of emission reduction. At present, a new method for synthesizing acrylic acid is attracting people's attention, that is, the carboxylation reaction of carbon dioxide and 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 the catalytic cycle, related research is still in the laboratory exploration stage and is far from reaching the scale of industrial production.

[0004] Due to its structural diversity, good modifiability and strong coordination ability, nitrogen heterocyclic carbene has become one of the most important ligands for preparing transition metal complexes. NHC itself and its transition metal complexes are also widely used as excellent catalysts in the field of modern organic synthetic chemistry. CN107298691A discloses a preparation method and application of a lead complex based on a rigid biimidazole ligand. The preparation method has the advantages of simple synthesis, easy crystallization, high yield, good reproducibility, etc., and can be used as a green luminescent material. CN105854947A discloses a chiral pyridine biimidazole ligand transition metal complex catalyst and a preparation method thereof. The chiral pyridine biimidazole ligand used can change the electronic properties and steric hindrance of the ligand by functional group modification, and as a catalyst precursor, it has moderate catalytic activity and stereoselectivity for the asymmetric hydroboration of alkenyl borane. It has the advantages of simple preparation method, cheap and easy to obtain raw materials, environmental friendliness, mild reaction conditions, high yield, and simple synthesis operation. In 2019, a Japanese research team reported an alkylphosphine-substituted imidazole ligand (Chem. Eur. J. 2019, 25, 13504-13508), which can generate a carbene intermediate under the action of a strong base to form a metal nickel catalyst with a phosphine-carbene bidentate coordination with nickel metal. This carbene-coordinated nickel catalyst has good stability and catalytic activity. The conversion number can reach up to 450 in the reaction of carbon dioxide and ethylene carboxylation to synthesize acrylic acid, which is the highest conversion number of carbon dioxide carboxylation without the participation 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 complicated, the route is cumbersome, and the total yield of the synthesis is only 39.3%. The stability of the ligand will be greatly improved after the poorly stable dicyclohexylphosphine is replaced with imidazole. However, the existing dicarbene ligand is not suitable for carbon dioxide carboxylation reaction. Summary of the invention

[0005] In view of the deficiencies of the prior art, the present invention provides a biimidazole ligand used in the reaction of synthesizing acrylic acid from carbon dioxide, a synthesis method thereof, a metal catalyst coordinated therewith, and a method for preparing acrylic acid by carboxylation reaction of carbon dioxide and ethylene. The synthesis method of the biimidazole ligand designed and developed by the present invention is simple, the ligand synthesis only requires two steps of reaction, the total yield can reach 53%, the coordination mode of the bicarbene improves the stability and catalytic activity of the nickel catalyst, and can improve the conversion number of the carboxylation reaction of carbon dioxide and ethylene; compared with the ligand used in the current carbon dioxide carboxylation reaction, the stability of the ligand is greatly improved after the poorly stable dicyclohexylphosphine is replaced by imidazole, and it is easy to use and store.

[0006] One of the purposes of the present invention is to provide a bisimidazole ligand.

[0007] The second object of the present invention is to provide a method for synthesizing the biimidazole ligand.

[0008] The third object of the present invention is to provide a metal catalyst made from the bisimidazole ligand.

[0009] A fourth object of the present invention is to provide an application of the metal catalyst.

[0010] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted:

[0011] In a first aspect, the present invention provides a bisimidazole ligand having a structure as shown in the following formula I:

[0012]

[0013] Mes stands for mesitylene.

[0014] In a second aspect, the present invention provides a method for synthesizing the above-mentioned bisimidazole ligand, comprising the following steps:

[0015]

[0016] S1: in the presence of ammonium acetate and acetic acid, mesitylene, 2,3-butanedione and paraformaldehyde are heated to react, and after the reaction, the pH value of the system is adjusted to 9, and after purification, a mesitylene-substituted imidazole derivative is obtained;

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

[0018] In some embodiments, in step S1, the molar ratio of mesitylidine, 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 mesitylidine to ammonium acetate is 1:1 to 1:1.5.

[0020] In some embodiments, in step S1, the molar ratio of mesitylidine 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 embodiments, the purification in step S1 comprises the following steps:

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

[0024] In some embodiments, step S2 comprises:

[0025] The mesitylene-substituted imidazole derivative prepared in S1 is dissolved in anhydrous tetrahydrofuran, and then sodium hydride is slowly added in batches. After the reaction, dibromomethane is dropped into the reaction system, and the mixture is stirred at room temperature overnight. The precipitate is 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 synthesis method of the biimidazole ligand comprises the following steps:

[0029] (1) adding mesitylidine, 2,3-butanedione, paraformaldehyde, ammonium acetate and acetic acid into a round-bottom flask and heating for reaction, then cooling the temperature to room temperature and adjusting the pH value of the reaction system to 9 with 1N potassium carbonate solution;

[0030] (2) extracting the mixture obtained in step (1) with ethyl acetate three times (100 mL×3), mixing the obtained organic phases, washing with saturated brine and then drying with anhydrous sodium sulfate, filtering and distilling the clarified ethyl acetate solution under reduced pressure to remove the solvent, and purifying the obtained brown viscous oil by column chromatography to obtain a mesitylene-substituted imidazole derivative;

[0031] (3) The mesitylene-substituted imidazole derivative obtained in step (2) is dissolved in anhydrous tetrahydrofuran to obtain a tetrahydrofuran solution of the mesitylene-substituted imidazole derivative (with a concentration of 0.2 mol / L), and then sodium hydride is slowly added in batches. After reacting for half an hour, dibromomethane is dripped into the reaction system. Finally, the mixture is stirred at room temperature overnight, and the precipitate is filtered out to obtain the bisimidazole ligand.

[0032] In a third aspect, the present invention provides a metal catalyst prepared from the above-mentioned bisimidazole ligand by the following method:

[0033] In a glove box, 0.1 mmol of the above-mentioned bisimidazole ligand was put into a 20 mL Shrek tube, and then 2 mL of anhydrous solvent was added and stirred thoroughly to make it evenly dispersed. 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 a metal compound was added and stirred for 2 hours to obtain a 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] In a fourth aspect, the present invention provides a use of the above metal catalyst in the preparation of acrylic acid by 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, filling it with carbon dioxide until the pressure of the reactor is 1.0-2.0 MPa, and then filling the reactor with ethylene to a specified pressure of 4.0-5.0 MPa for reaction.

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

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

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

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

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

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

[0044] Technical effects:

[0045] 1. Replacing the poorly stable dicyclohexylphosphine with imidazole greatly improves the stability of the ligand and makes it easy to use and store.

[0046] 2. The synthesis method of the biimidazole ligand designed and developed by the present invention is simple, and the ligand synthesis only requires two steps of reaction, and the total yield can reach 53%.

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

[0048] The present invention has been described in detail above, but the above embodiments are only illustrative in nature and are not intended to limit the present invention. In addition, this article is not limited by any theory described in the above prior art or invention content or the following examples. DETAILED DESCRIPTION

[0049] The present invention is further described below in conjunction with examples. It should be noted that the following examples are provided for illustrative purposes only and do not constitute a limitation on the scope of protection claimed for the present invention.

[0050] Unless otherwise specified, the raw materials, reagents, methods, etc. used in the examples are conventional raw materials, reagents, methods in the art.

[0051] TON (Turnover Number): indicates 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 capacity of the catalyst.

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

[0053] Example 1

[0054] In a 100 ml round-bottom flask, add mesitylene (10 mmol), 2,3-butanedione (10 mmol), paraformaldehyde (15 mmol), ammonium acetate (10 mmol) and acetic acid (2.0 mL), then raise the reaction temperature to 130 ° C for 10 hours; after the reaction, lower the temperature to room temperature and adjust the pH value of the reaction system to 9 with 1N potassium carbonate solution, extract with ethyl acetate three times (100 mL × 3), mix the obtained organic phases, wash with saturated brine and then dry with anhydrous sodium sulfate, filter and distill the clarified ethyl acetate solution under reduced pressure to remove the solvent, and the obtained brown viscous oil is purified by column chromatography to obtain mesitylene-substituted imidazole derivatives 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 half an hour of reaction, dibromomethane (2.3 mmol) was dropped into the reaction system. Finally, the mixture was stirred at room temperature overnight. After filtering out the precipitate, the bisimidazole ligand was obtained, and the total yield was 36%.

[0055] Example 2

[0056] In a 100 ml round-bottom flask, add mesitylene (10 mmol), 2,3-butanedione (10 mmol), paraformaldehyde (25 mmol), ammonium acetate (15 mmol) and acetic acid (2.0 mL), then raise the reaction temperature to 130 ° C for 10 hours; after the reaction, lower the temperature to room temperature and adjust the pH value of the reaction system to 9 with 1N potassium carbonate solution, extract with ethyl acetate three times (100 mL × 3), mix the obtained organic phases, wash with saturated brine and then dry with anhydrous sodium sulfate, filter and distill the clarified ethyl acetate solution under reduced pressure to remove the solvent, and the obtained brown viscous oil is purified by column chromatography to obtain mesitylene-substituted imidazole derivatives 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 half an hour of reaction, dibromomethane (2.4 mmol) was dropped into the reaction system. Finally, the mixture was stirred at room temperature overnight. After filtering out the precipitate, the bisimidazole ligand was obtained with a yield of 38%.

[0057] Example 3

[0058] In a 100 ml round-bottom flask, add mesitylene (10 mmol), 2,3-butanedione (10 mmol), paraformaldehyde (25 mmol), ammonium acetate (10 mmol) and acetic acid (2.0 mL), then raise the reaction temperature to 130 ° C for 10 hours; after the reaction, lower the temperature to room temperature and adjust the pH value of the reaction system to 9 with 1N potassium carbonate solution, extract with ethyl acetate three times (100 mL × 3), mix the obtained organic phases, wash with saturated brine and then dry with anhydrous sodium sulfate, filter and distill the clarified ethyl acetate solution under reduced pressure to remove the solvent, and the obtained brown viscous oil is purified by column chromatography to obtain mesitylene-substituted imidazole derivatives 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 half an hour of reaction, dibromomethane (2.5 mmol) was dropped into the reaction system. Finally, the mixture was stirred at room temperature overnight. After filtering out the precipitate, the bisimidazole ligand was obtained, and the total yield was 39%.

[0059] Example 4

[0060] In a 100 ml round-bottom flask, add mesitylene (10 mmol), 2,3-butanedione (10 mmol), paraformaldehyde (25 mmol), ammonium acetate (10 mmol) and acetic acid (5.0 mL), then raise the reaction temperature to 130 ° C for 10 hours; after the reaction, lower the temperature to room temperature and adjust the pH value of the reaction system to 9 with 1N potassium carbonate solution, extract with ethyl acetate three times (100 mL × 3), mix the obtained organic phases, wash with saturated brine and then dry with anhydrous sodium sulfate, filter and distill the clarified ethyl acetate solution under reduced pressure to remove the solvent, and the obtained brown viscous oil is purified by column chromatography to obtain mesitylene-substituted imidazole derivatives 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 half an hour of reaction, dibromomethane (2.6 mmol) was dropped into the reaction system. Finally, the mixture was stirred at room temperature overnight. After filtering out the precipitate, the bisimidazole ligand was obtained, and the total yield was 42%.

[0061] Example 5

[0062] In a 100 ml round-bottom flask, add mesitylene (10 mmol), 2,3-butanedione (10 mmol), paraformaldehyde (25 mmol), ammonium acetate (10 mmol) and acetic acid (5.0 mL), then raise the reaction temperature to 160 ° C for 15 hours; after the reaction, lower the temperature to room temperature and adjust the pH value of the reaction system to 9 with 1N potassium carbonate solution, extract with ethyl acetate three times (100 mL × 3), mix the obtained organic phases, wash with saturated brine and then dry with anhydrous sodium sulfate, filter and distill the clarified ethyl acetate solution under reduced pressure to remove the solvent, and the obtained brown viscous oil is purified by column chromatography to obtain mesitylene-substituted imidazole derivatives 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 half an hour of reaction, dibromomethane (2.3 mmol) was dropped into the reaction system. Finally, the mixture was stirred at room temperature overnight. After filtering out the precipitate, the bisimidazole ligand was obtained, and the total yield was 44%.

[0063] Example 6

[0064] In a 100 ml round-bottom flask, add mesitylene (10 mmol), 2,3-butanedione (10 mmol), paraformaldehyde (25 mmol), ammonium acetate (10 mmol) and acetic acid (5.0 mL), then raise the reaction temperature to 160 ° C for 15 hours; after the reaction, lower the temperature to room temperature and adjust the pH value of the reaction system to 9 with 1N potassium carbonate solution, extract with ethyl acetate three times (100 mL × 3), mix the obtained organic phases, wash with saturated brine and then dry with anhydrous sodium sulfate, filter and distill the clarified ethyl acetate solution under reduced pressure to remove the solvent, and the obtained brown viscous oil is purified by column chromatography to obtain mesitylene-substituted imidazole derivatives 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 half an hour of reaction, dibromomethane (2.7 mmol) was dropped into the reaction system. Finally, the mixture was stirred at room temperature overnight. After filtering out the precipitate, the bisimidazole ligand was obtained, and the total yield was 53%.

[0065] Example 7

[0066] In a 100 ml round-bottom flask, add mesitylene (10 mmol), 2,3-butanedione (10 mmol), paraformaldehyde (25 mmol), ammonium acetate (10 mmol) and acetic acid (5.0 mL), then raise the reaction temperature to 160 ° C for 15 hours; after the reaction, lower the temperature to room temperature and adjust the pH value of the reaction system to 9 with 1N potassium carbonate solution, extract with ethyl acetate three times (100 mL × 3), mix the obtained organic phases, wash with saturated brine and then dry with anhydrous sodium sulfate, filter and distill the clarified ethyl acetate solution under reduced pressure to remove the solvent, and the obtained brown viscous oil is purified by column chromatography to obtain mesitylene-substituted imidazole derivatives 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 half an hour of reaction, dibromomethane (4.1 mmol) was dropped into the reaction system. Finally, the mixture was stirred at room temperature overnight. After filtering out the precipitate, the bisimidazole ligand was obtained, and the total yield was 48%.

[0067] Each embodiment prepares the bisimidazole-nickel catalyst according to the following method:

[0068] In a glove box, 0.1 mmol of the bisimidazole ligand was placed in a 20 mL Shrek tube, and then 2 mL of anhydrous tetrahydrofuran was added and 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 bis-(1,5-cyclooctadiene) nickel was added and stirred for 2 hours to obtain a bisimidazole-nickel catalyst.

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

[0070] In a 150ml autoclave, under nitrogen protection, the bisimidazole-nickel catalyst (0.1mmol) prepared in Example 6, β-H elimination promoter (10mmol), solvent (35ml) were added in sequence, the autoclave was sealed, the gas in the autoclave was replaced 3 times with carbon dioxide, carbon dioxide was added until the pressure of the autoclave was 1MPa, and then ethylene was added until the pressure of the autoclave was 5MPa. The temperature was slowly raised to 80°C by a temperature controller, the reaction was carried out for 12h, cooled to room temperature, the autoclave was unloaded, the solvent was removed by vacuum distillation, the obtained light yellow solid was added with 10mL of deuterated water internal standard solution, and the reaction TON was calculated after sufficient shaking and dissolution by nuclear magnetic resonance hydrogen spectrum analysis.

[0071] Table 1

[0072]

[0073]

[0074] Example 15-22 (the amount of sodium tert-butoxide added, the reaction temperature and the reaction time are shown in Table 2)

[0075] In a 150ml autoclave, under nitrogen protection, the bisimidazole-nickel catalyst (0.1mmol) prepared in Example 6, sodium tert-butoxide, and tetrahydrofuran (35ml) were added in sequence, and the autoclave was sealed. The gas in the autoclave was replaced with carbon dioxide three times, and then carbon dioxide was added to the autoclave until the pressure was 1MPa, and then ethylene was added to the autoclave until the pressure was 5MPa. The temperature was slowly raised to the specified temperature by a temperature controller and reacted for a predetermined time, cooled to room temperature, unloaded, and the solvent was removed by vacuum distillation. The obtained light yellow solid was added with 10mL of deuterated water internal standard solution and fully shaken to dissolve, and then the reaction TON was calculated by nuclear magnetic resonance hydrogen spectrum analysis.

[0076] Table 2

[0077]

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

[0079] In a 150ml autoclave, under nitrogen protection, add bis-(1,5-cyclooctadiene) nickel (0.1mmol), bisphosphine ligand (0.11mmol), sodium tert-butoxide (20mmol), THF (35ml) in sequence, seal the autoclave, replace the gas in the autoclave with carbon dioxide three times, then fill it with carbon dioxide until the pressure of the autoclave is 1MPa, and then fill it with ethylene until the pressure of the autoclave is 5MPa. The temperature is slowly raised to 140℃ by a temperature controller, react for 15h, cool to room temperature, unload the autoclave, remove the solvent by vacuum distillation, add 10mL of deuterated water internal standard solution to the obtained light yellow solid, shake it thoroughly to dissolve it, and then perform nuclear magnetic resonance hydrogen spectrum analysis to calculate the reaction TON.

[0080] Table 3

[0081]

[0082] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents, without departing from the spirit and essence of the claims of the present invention; and these modifications or replacements are still within the scope defined by the claims of the present invention.

Claims

1. A biimidazole ligand, characterized in that: The biimidazole ligand has a structure shown in the following formula I:

2. A method for synthesizing the biimidazole ligand according to claim 1, characterized in that: The following steps are involved: S1: in the presence of ammonium acetate and acetic acid, mesitylene, 2,3-butanedione and paraformaldehyde are heated to react, and after the reaction, the pH value of the system is adjusted to 8-9, and after purification, a mesitylene-substituted imidazole derivative is obtained; S2: The mesitylene-substituted imidazole derivative obtained in S1 undergoes a substitution reaction with dibromomethane to obtain a bisimidazole ligand.

3. The synthesis method according to claim 2, characterized in that In step S1, the molar ratio of mesitylidine, 2,3-butanedione and paraformaldehyde is 1:1:1.5 to 1:1:2.5; Preferably, in step S1, the molar ratio of mesitylidine to ammonium acetate is 1:1 to 1:1.5; Preferably, in step S1, the molar ratio of mesitylidine to acetic acid is 1:3 to 1:8; Preferably, in step S1, the reaction temperature is 130-160° C., 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 prepared in S1 is dissolved in anhydrous tetrahydrofuran, and then sodium hydride is slowly added in batches. After the reaction, dibromomethane is dropped into the reaction system, and the mixture is stirred at room temperature overnight. The precipitate is filtered out to obtain a bisimidazole ligand. Preferably, in step S2, the molar ratio of the mesitylene-substituted imidazole derivative to dibromomethane is 1:0.4 to 1:0.6; Preferably, in step S2, the molar ratio of the mesitylene-substituted imidazole derivative to sodium hydride is 1:1.1 to 1:1.

5.

5. A metal catalyst, characterized in that The bisimidazole ligand according to claim 1 is prepared by the following method: Anhydrous solvent is added to the bisimidazole ligand, and the mixture is stirred thoroughly to make it dispersed evenly. Potassium tert-butoxide is then slowly added. After stirring, the reaction system turns into a yellow solution. Finally, a metal compound is added and stirred to prepare a metal catalyst.

6. The metal catalyst according to claim 5, characterized in that The anhydrous solvent is anhydrous tetrahydrofuran or anhydrous toluene.

7. The metal catalyst according to claim 5, characterized in that The metal compound is bis-(1,5-cyclooctadiene) nickel or (1,5-cyclooctadiene) palladium (II) dichloride.

8. Use of the metal catalyst according to any one of claims 5 to 7 in the preparation of acrylic acid by carboxylation reaction of carbon dioxide and ethylene.

9. The use according to claim 8, characterized in that: The application comprises the following steps: adding a β-H elimination promoter, a metal catalyst and a solvent into a reaction kettle, sealing the reaction kettle, charging the reaction kettle with carbon dioxide until the pressure is 1.0-2.0 MPa, and then charging the reaction kettle with ethylene to a specified pressure of 4.0-5.0 MPa for reaction.

10. The use according to claim 9, characterized in that: The β-H elimination promoter is one of sodium tert-butoxide, sodium phenolate and sodium acetate, preferably sodium tert-butoxide; Preferably, the molar ratio of the metal catalyst to the β-H elimination promoter is 1:100-1:500, preferably 1:200-1:300; Preferably, the solvent is one of tetrahydrofuran, anisole, toluene, dimethyl sulfoxide, and N-methylpyrrolidone, preferably tetrahydrofuran; Preferably, the reaction temperature is 80-160°C, preferably 130-140°C; the reaction time is 12-24h, preferably 15-18h.

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