Salen ligand-based tetranuclear cobalt complex, preparation method and application thereof

Abstract

The invention belongs to the technical field of carbon dioxide catalytic conversion, and provides a tetranuclear cobalt complex based on a Salen ligand, a preparation method and an application thereof. The complex is a 2,2'-[1,2-ethylenediylbisimine)]bis[6-(2-tert-butyl)-4-(4-pyridyl)phenol] bis-Schiff base tetranuclear cobalt complex, and the molecular formula is: C 137 H 148 Co4N 16 O8, which is prepared by: mixing the ligand H2L, the cobalt salt and the mixed solvent in a certain proportion, adjusting the pH value to be greater than 8, placing in a container, evacuating and sealing in the container, reacting at a temperature above 75°C, cooling to room temperature after stopping the reaction, collecting crystals, and drying; the molar ratio of the Schiff base ligand H2L to the cobalt salt is 1:1. The complex of the present invention exhibits high catalytic activity, selectivity and stability in carbon dioxide reduction as a homogeneous molecular catalyst, and has potential catalytic application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of carbon dioxide catalytic conversion, and in particular to a tetranuclear cobalt complex based on a Salen ligand, and a preparation method and application thereof. Background Art

[0002] With the continuous development of the economy and society, the greenhouse effect caused by the burning of fossil fuels and other reasons has become increasingly severe. Carbon dioxide is the main factor causing the greenhouse effect. Therefore, capturing and converting carbon dioxide into carbon-containing reduction products such as carbon monoxide, formic acid, and methane is of great significance to alleviating the energy crisis, improving the environment, and achieving sustainable economic and social development.

[0003] The thermal stability of carbon dioxide molecules is very high (only 1.8% decomposed at 2000°C), and its C=O bond length is short and the bond energy is large (803kJ / mol). These properties determine that it is not easy to be activated at room temperature. In order to promote the activation and conversion of carbon dioxide, an effective catalyst must be introduced to promote the reduction of carbon dioxide. Metal complex catalysts have clear catalytic sites and can be controlled by ligands and metal centers, becoming a hot topic in current research. For example, the invention patent with publication number CN113135960A discloses a catalyst with good photocatalytic carbon dioxide reduction performance. The catalyst is a binuclear rhenium complex constructed with dipyridyloxyanthene as a ligand, and its structure is divided into cis and trans, as shown in the following formula (1). The invention uses a binuclear rhenium complex as a catalyst. After 15 hours of illumination in a homogeneous system, it catalyzes the reduction of carbon dioxide into carbon monoxide and hydrogen. Its catalytic conversion number TON values ​​are 1320 and 27, respectively, and the selectivity is 98% and 2%, respectively. Although noble metal complexes (such as Re, Ru and Ir) show excellent catalytic CO2 reduction effects, they are expensive and not conducive to large-scale application. In recent years, researchers have been committed to developing non-noble metal complex catalysts (Fe, Co, Ni, Cu, Mn, etc.) with abundant reserves on the earth to build an efficient and low-cost catalytic CO2 reduction system.

[0004]

[0005]

[0006] The selection of catalyst plays a decisive role in the photocatalytic carbon dioxide reduction system. Based on the considerations of catalyst activity, selectivity, stability and green economy, it is hoped that the selected catalyst can have excellent catalytic performance in aqueous system and under visible light irradiation. At present, there has been no research and report on the photocatalytic carbon dioxide reduction of tetranuclear cobalt complex catalyst with 2,2'-[1,2-ethylenediamine)]bis[6-(2-tert-butyl)-4-(4-pyridyl)phenol]bis-Schiff base as ligand. Summary of the invention

[0007] The object of the present invention is to provide a tetranuclear cobalt complex based on Salen ligand, a preparation method and application thereof in view of the above-mentioned problems.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0009] On the one hand, the present invention provides a tetranuclear cobalt complex based on a Salen ligand, wherein the complex is a 2,2'-[1,2-ethylenediamine)]bis[6-(2-tert-butyl)-4-(4-pyridyl)phenol]bis-Schiff base tetranuclear cobalt complex 1, and the molecular formula is: C 137 H 148 Co4N 16 O8, the structural formula is as follows:

[0010]

[0011] The tetranuclear cobalt complex belongs to the tetragonal system, P42 / n space group, and its unit cell parameters are: α=90°, β=90°, γ=90°;

[0012] In one aspect, the present invention provides a method for preparing a tetranuclear cobalt complex, comprising the following steps:

[0013] The Schiff base ligand H2L, cobalt salt and mixed solvent are mixed in a certain proportion, the pH value is adjusted to be greater than 8, the mixture is placed in a container, evacuated and sealed in the container, reacted at a temperature above 75°C, the reaction is stopped and then cooled to room temperature, crystals are collected and dried to obtain the obtained product; wherein the mixed solvent is composed of DMF and CH3OH, and the molar ratio of the Schiff base ligand H2L to the cobalt salt is 1-1.2; the Schiff base ligand H2L is 6,6-((1E, 1'E)-(ethane-1,2-diethyldi(azamethylene))bis(2-(tert-butyl)-4-(pyridin-4-yl)phenol).

[0014] In the present invention, preferably, the volume ratio of DMF and CH3OH in the mixed solvent is 1:2.

[0015] In the present invention, preferably, the reaction time is 36-48h.

[0016] In the present invention, preferably, the synthesis steps of the Schiff base ligand H2L are as follows:

[0017] (1) o-tert-butylphenol is placed in a reaction container, anhydrous acetonitrile and DMF are added, and the mixture is stirred in an ice-water bath; then N-bromosuccinimide is weighed and added, and the mixture is stirred at room temperature; the mol / mol ratio of o-tert-butylphenol to N-bromosuccinimide is 1:1.1; after the reaction is completed, water is added to quench the mixture, and the mixture is extracted with ethyl acetate, and the solvent is evaporated to obtain a crude product;

[0018] (2) The crude product of the previous step is placed in a reaction vessel, anhydrous acetonitrile is added, stirred to dissolve, paraformaldehyde and anhydrous magnesium chloride are added, stirred, triethylamine is added, and heated to reflux for 24-60 hours; wherein the molar ratio of paraformaldehyde: anhydrous magnesium chloride: o-tert-butylphenol is 6-8:2.2-2.3:1; after the reaction is completed, the reactants are poured into ice water, the pH is adjusted to 1-2 with concentrated HCl, allowed to stand, extracted with ether, washed with water, and dried to obtain a yellow solid 5-bromo-3-tert-butyl-2-hydroxybenzaldehyde;

[0019] (3) 5-bromo-3-tert-butyl-2-hydroxybenzaldehyde, pyridine-4-boric acid, palladium acetate, triphenylphosphine, and potassium carbonate were added to a mixed solution of 1,4-dioxane and water, and nitrogen was added under vacuum, and the mixture was refluxed for three days; wherein the molar ratio of 5-bromo-3-tert-butyl-2-hydroxybenzaldehyde, pyridine-4-boric acid, palladium acetate, triphenylphosphine, and potassium carbonate was 15-16:15-17:0.3-0.4:1-1.1:72-75; after the reaction was completed, 1,4-dioxane was removed by vacuum rotation, the organic layer was extracted with ethyl acetate, washed with water, and separated by spin drying column to obtain yellow solid 3-tert-butyl-2-hydroxy-5-(4-pyridine)benzaldehyde;

[0020] (4) 3-tert-Butyl-2-hydroxy-5-(4-pyridine)benzaldehyde and ethylenediamine were ultrasonically stirred in a methanol solution at a molar ratio of 2:1 to obtain a yellow solid Schiff base ligand H2L.

[0021] The tetranuclear cobalt complex of the present invention can be applied to the photocatalytic reduction of carbon dioxide. In a specific application, the photocatalytic system includes a light source, a photosensitizer, a catalyst, a sacrificial agent and a solvent, wherein the catalyst is the above-mentioned cobalt complex, and the selection of the photosensitizer, the sacrificial agent and the solvent is the same as that in the prior art, the photosensitizer is preferably [Ru(phen)3](PF6)2, [Ru(phen)3]Cl2 or [Ru(bpy)3]Cl2, more preferably [Ru(phen)3](PF6)2; the sacrificial agent is preferably triethanolamine (TEOA) or triethylamine (TEA), and the solvent is preferably a mixed solution of water and acetonitrile. In the photocatalytic system, the concentration of the photosensitizer is preferably 400-500 μM, the concentration of the catalyst is preferably 0.05-1 μM, and the concentration of the sacrificial agent is preferably 0.30-0.35M.

[0022] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0023] 1. The present invention provides a tetranuclear cobalt complex obtained from a Salen ligand with a novel structure. The complex, as a homogeneous molecular catalyst, exhibits high catalytic activity, selectivity and stability in carbon dioxide reduction and has potential catalytic application prospects.

[0024] 2. The present invention also provides a method for preparing a tetranuclear cobalt complex based on a Salen ligand, which is simple, easy to obtain and highly efficient in synthesis. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The infrared spectrum of the final product obtained by the present invention.

[0026] Figure 2 The crystal structure diagram of the final product obtained by the present invention.

[0027] Figure 3 This is a graph showing the product changing over time after 10 hours of illumination reaction under LED light, adding an equal amount of photosensitizer and continuing the reaction for another 10 hours. DETAILED DESCRIPTION

[0028] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] The present invention provides a tetranuclear cobalt complex based on Salen ligands, wherein the complex is a 2,2'-[1,2-ethylenediamine)]bis[6-(2-tert-butyl)-4-(4-pyridyl)phenol]bis-Schiff base tetranuclear cobalt complex 1, and the molecular formula is: C 137 H 148 Co4N 16 O8, the structural formula is as follows:

[0030]

[0031] The tetranuclear cobalt complex belongs to the tetragonal system, P42 / n space group, and its unit cell parameters are: α=90°, β=90°, γ=90°; The metal center cobalt ion of the complex is divalent, and the complex consists of four Schiff base ligands L 2- and four cobalt ions to form a ring-shaped tetranuclear structure, each Co(II) ion is bound by a ligand L 2- Two nitrogen atoms and two oxygen atoms and another ligand L2- The pyridine nitrogen atom is coordinated to form a five-coordinate configuration. Figure 2 , which is the crystal structure diagram of the tetranuclear cobalt complex.

[0032] The synthesis route of the tetranuclear cobalt complex is: mixing the ligand H2L, cobalt salt and mixed solvent in a certain proportion, adjusting the pH value to be greater than 8, placing in a container, evacuating and sealing the container, reacting at a temperature above 75°C, stopping the reaction and cooling to room temperature, collecting crystals, and drying to obtain the tetranuclear cobalt complex; wherein the mixed solvent is composed of DMF and CH3OH, and the molar ratio of the Schiff base ligand H2L to the cobalt salt is 1:1.

[0033] During the specific preparation, the amount of the solvent is determined according to the amount of the ligand, and is generally suitable for dissolving the raw materials participating in the reaction. Specifically, 0.025 mmol of the ligand is used as a reference, and the amount of the mixed solvent used to dissolve all the raw materials is 2 to 3 mL.

[0034] During the specific preparation, an alkaline substance is used to adjust the pH value of the system to alkaline, and the alkaline substance can be a conventional choice in the prior art, preferably triethylamine. The pH value of the system is preferably adjusted to ≥ 8, and more preferably the pH value of the system is adjusted to 8.5-9.5.

[0035] In the specific preparation, the mixed solution obtained after adjusting the pH value is usually placed in a reaction kettle, sealed and then placed under heating conditions for reaction. The reaction is preferably carried out under conditions of ≥75°C, and more preferably under conditions of 80-90°C. When the reaction is carried out under conditions of 80-90°C, the reaction time is usually controlled to be about 36-48h.

[0036] The 2,2'-[1,2-ethylenediamine]bis[6-(2-tert-butyl)-4-(4-pyridyl)phenol] Schiff base ligand H2L can be referred to (Marcello B.Solomon, Bun Chan, P. Kubiak, Katrina A. Jolliffe and Deanna M. D'Alessandro, Dalton Trans 2019, 48, 3704–3713.) can be prepared, or it can be designed and synthesized by yourself.

[0037] The preparation method of the tetranuclear cobalt complex is introduced below through specific examples.

[0038] Example 1

[0039] The preparation method of the tetranuclear cobalt complex based on Salen ligand comprises the following steps:

[0040] 1. Synthesis of ligand H2L;

[0041] (1) o-tert-butylphenol (15.0 g, 0.1 mol) was placed in a reaction vessel, 50 mL of anhydrous acetonitrile and 1 mL of DMF were added, and the mixture was stirred under an ice-water bath; N-bromosuccinimide (19.8 g, 0.11 mol) was weighed and added, and the mixture was stirred at room temperature; after the reaction was completed, water was added to quench the mixture, and the mixture was extracted with ethyl acetate, and the solvent was evaporated to obtain a crude product;

[0042] (2) The crude product of the previous step was placed in a reaction vessel, anhydrous acetonitrile was added, and the mixture was stirred to dissolve. Paraformaldehyde (22.3 g, 0.791 mol) and anhydrous magnesium chloride (26.60 g, 0.221 mol) were added and stirred, and triethylamine was added. The mixture was heated to reflux for 48 h. After the reaction was completed, the reactant was poured into ice water, the pH was adjusted to 1 with concentrated HCl, the mixture was allowed to stand, and the mixture was extracted with ether, washed with water, and dried to obtain a yellow solid 5-bromo-3-tert-butyl-2-hydroxybenzaldehyde.

[0043] (3) 5-bromo-3-tert-butyl-2-hydroxybenzaldehyde (4 g, 0.0156 mol), pyridine-4-boric acid (2 g, 0.0163 mol), palladium acetate (70 mg, 0.312 mmol), triphenylphosphine (280 mg, 1.067 mmol), potassium carbonate (10 g, 73.5 mmol) were added to a mixed solution of 50 mL 1,4-dioxane and 20 mL water, evacuated and nitrogen was added, and refluxed for three days; after the reaction was completed, 1,4-dioxane was removed in vacuo, the organic layer was extracted with ethyl acetate, washed with water, and separated by spin column to obtain yellow solid 3-tert-butyl-2-hydroxy-5-(4-pyridine)benzaldehyde;

[0044] (4) 3-tert-Butyl-2-hydroxy-5-(4-pyridine)benzaldehyde and ethylenediamine were ultrasonically stirred in a methanol solution at a molar ratio of 2:1 to obtain a yellow solid Schiff base ligand H2L.

[0045] 2. Synthesis of tetranuclear cobalt complexes:

[0046] Schiff base ligand H2L (0.0130 g, 0.025 mmol) and Co(NO3)2·6H2O (0.0073 g, 0.025 mmol) were placed in a glass tube with one end sealed and about 20 cm long. A mixed solvent consisting of 1 mL of methanol and 2 mL of DMF was added. The pH value of the system was adjusted to 9 with triethylamine (15 μL). After that, the system was vacuumed and the glass tube was sealed at high temperature. The sealed glass tube was placed in an oven at 80°C for 48 hours. After the reaction was stopped, the temperature was slowly lowered to room temperature. Black block crystals were observed to precipitate at the bottom of the glass tube. The crystals were collected and dried.

[0047] Example 2

[0048] The preparation method of the tetranuclear cobalt complex based on Salen ligand comprises the following steps:

[0049] 1. Synthesis of ligand H2L;

[0050] (1) o-tert-butylphenol (15.0 g, 0.1 mol) was placed in a reaction vessel, 50 mL of anhydrous acetonitrile and 1 mL of DMF were added, and the mixture was stirred under an ice-water bath; N-bromosuccinimide (19.8 g, 0.11 mol) was weighed and added, and the mixture was stirred at room temperature; after the reaction was completed, water was added to quench the mixture, and the mixture was extracted with ethyl acetate, and the solvent was evaporated to obtain a crude product;

[0051] (2) The crude product of the previous step was placed in a reaction vessel, anhydrous acetonitrile was added, and the mixture was stirred to dissolve. Paraformaldehyde (16.9 g, 0.6 mol) and anhydrous magnesium chloride (26.5 g, 0.22 mol) were added and stirred, and triethylamine was added. The mixture was heated to reflux for 24 h. The molar ratio of paraformaldehyde: anhydrous magnesium chloride: o-tert-butylphenol was 6:2:1. After the reaction was completed, the reactants were poured into ice water, the pH was adjusted to 1.5 with concentrated HCl, the mixture was allowed to stand, extracted with ether, washed with water, and dried to obtain a yellow solid 5-bromo-3-tert-butyl-2-hydroxybenzaldehyde.

[0052] (3) 5-bromo-3-tert-butyl-2-hydroxybenzaldehyde (3.8 g, 0.015 mol), pyridine-4-boric acid (1.8 g, 0.015 mol), palladium acetate (67.3 mg, 0.3 mmol), triphenylphosphine (262.4 mg, 1.067 mmol), potassium carbonate (9.8 g, 72 mmol) were added to a mixed solution of 50 mL 1,4-dioxane and 20 mL water, evacuated and nitrogen was added, and refluxed for three days; after the reaction was completed, 1,4-dioxane was removed in vacuo, the organic layer was extracted with ethyl acetate, washed with water, and separated by spin drying column to obtain yellow solid 3-tert-butyl-2-hydroxy-5-(4-pyridine)benzaldehyde;

[0053] (4) 3-tert-Butyl-2-hydroxy-5-(4-pyridine)benzaldehyde and ethylenediamine were ultrasonically stirred in a methanol solution at a molar ratio of 2:1 to obtain a yellow solid Schiff base ligand H2L.

[0054] 2. Synthesis of tetranuclear cobalt complexes:

[0055] Take Schiff base ligand H2L (0.0312g, 0.06mmol) and Co(NO3)2·6H2O (0.0146g, 0.05mmol) and place them in a glass tube with a length of about 20cm and one end sealed. Add a mixed solvent consisting of 2mL methanol and 2mL DMF, and then adjust the pH value of the system to 8.5 with triethylamine. Then, evacuate and seal the glass tube at high temperature. Place the sealed glass tube in an oven at 80℃ and react for 36h. After stopping the reaction, slowly cool it to room temperature. It can be observed that black block crystals are precipitated at the bottom of the glass tube. Collect the crystals and dry them.

[0056] Example 3

[0057] The preparation method of the tetranuclear cobalt complex based on Salen ligand comprises the following steps:

[0058] 1. Synthesis of ligand H2L;

[0059] (1) o-tert-butylphenol (15.0 g, 0.1 mol) was placed in a reaction vessel, 50 mL of anhydrous acetonitrile and 1 mL of DMF were added, and the mixture was stirred under an ice-water bath; N-bromosuccinimide (18 g, 0.1 mol) was weighed and added, and the mixture was stirred at room temperature; after the reaction was completed, water was added to quench the mixture, and the mixture was extracted with ethyl acetate, and the solvent was evaporated to obtain a crude product;

[0060] (2) The crude product of the previous step was placed in a reaction vessel, anhydrous acetonitrile was added, and the mixture was stirred to dissolve. Paraformaldehyde (22.6 g, 0.8 mol) and anhydrous magnesium chloride (27.70 g, 0.23 mol) were added and stirred, and triethylamine was added. The mixture was heated to reflux for 60 h. The molar ratio of paraformaldehyde: anhydrous magnesium chloride: o-tert-butylphenol was 8:2.3:1. After the reaction was completed, the reactants were poured into ice water, the pH was adjusted to 2 with concentrated HCl, the mixture was allowed to stand, extracted with ether, washed with water, and dried to obtain 5-bromo-3-tert-butyl-2-hydroxybenzaldehyde as a yellow solid.

[0061] (3) 5-bromo-3-tert-butyl-2-hydroxybenzaldehyde (4.1 g, 0.016 mol), pyridine-4-boric acid (2.1 g, 0.017 mol), palladium acetate (89.7 mg, 0.312 mmol), triphenylphosphine (288 mg, 1.1 mmol), and potassium carbonate (10.2 g, 73.5 mmol) were added to a mixed solution of 50 mL 1,4-dioxane and 20 mL water, and nitrogen was added under vacuum and refluxed for three days; wherein the molar ratio of 5-bromo-3-tert-butyl-2-hydroxybenzaldehyde, pyridine-4-boric acid, palladium acetate, triphenylphosphine and potassium carbonate was 16:17:0.4:1.1:75; after the reaction was completed, 1,4-dioxane was removed in vacuo, the organic layer was extracted with ethyl acetate, washed with water, and separated by spin drying to obtain a yellow solid 3-tert-butyl-2-hydroxy-5-(4-pyridine)benzaldehyde;

[0062] (4) 3-tert-Butyl-2-hydroxy-5-(4-pyridyl)benzaldehyde and ethylenediamine were ultrasonically stirred in a methanol solution at a molar ratio of 2:1 to obtain a yellow solid Schiff base ligand H2L.

[0063] 2. Synthesis of tetranuclear cobalt complexes:

[0064] The ligand H2L, cobalt salt and mixed solvent are mixed in a certain proportion, the pH value is adjusted to be greater than 8, the mixture is placed in a container, evacuated and sealed in the container, reacted at a temperature above 75°C, the reaction is stopped and then cooled to room temperature, the crystals are collected and dried; wherein the mixed solvent is composed of DMF and CH3OH mixed in a volume ratio of 2:1, and the molar ratio of the Schiff base ligand H2L to the cobalt salt is 1:1.

[0065] Schiff base ligand H2L (0.0390g, 0.075mmol) and Co(NO3)2·6H2O (0.0219g, 0.075mmol) were placed in a glass tube with a length of about 20cm and sealed at one end. A mixed solvent consisting of 1mL methanol and 2mL DMF was added, and the pH value of the system was adjusted to 9.5 with triethylamine. After that, the system was evacuated and sealed at high temperature. The sealed glass tube was placed in an oven at 80℃ for 60h. After the reaction was stopped, the temperature was slowly lowered to room temperature. Black block crystals were observed to precipitate at the bottom of the glass tube. The crystals were collected and dried.

[0066] The products obtained in the examples were characterized:

[0067] (1) Infrared spectrum, its spectrum is as follows Figure 1 shown.

[0068] IR(KBr,cm -1 ):2939vs,1681w,1598vs,1440s,1381m,1337m,1287m,1223m,1169m,1085w,1051w,997w,942w,818m,789m,665w,607m,523m,443w.

[0069] (2) Crystal structure analysis:

[0070] A black block crystal of appropriate size was selected and placed on Agilent's SuperNova single crystal diffractometer. The initial crystal structures of the products obtained in this example were solved by the SHELXS-97 direct method, and the geometric hydrogenation, non-hydrogen atomic coordinates and anisotropic thermal parameters were refined by the full matrix least squares method using SHELXL-97. The obtained crystallographic and structural refinement data are shown in Table 1 below, and some bond length and bond angle data are shown in Table 2 and Table 3 below, respectively. The chemical structure of the obtained crystal is shown in Figure 2 As shown, the obtained black crystals are determined to be the target product of the present invention.

[0071] Table 1 Crystallographic data of cobalt complex 1 of the present invention

[0072]

[0073]

[0074] Table 2 Partial bond length data of cobalt complex 1 of the present invention

[0075]

[0076] Table 3 Partial bond angle data of cobalt complex 1 of the present invention (°)

[0077]

[0078] The tetranuclear cobalt complex prepared by the present invention has the activity of catalytic reduction of carbon dioxide and can be used as a homogeneous catalyst to perform photocatalytic reduction of carbon dioxide in an aqueous system. The specific test method is as follows:

[0079] (1) Materials used:

[0080] Photosensitizer: [Ru(phen)3](PF6)2;

[0081] Catalyst: a cobalt complex prepared according to an embodiment of the present invention (hereinafter referred to as cobalt complex 1);

[0082] Sacrificial agent: TEA;

[0083] LED light source: wavelength 450nm, light intensity 100mW·cm -2 , the irradiation area is 0.8cm 2 ;

[0084] Other instruments and reagents: 15-20 mL quartz reactor, carbon dioxide gas, rubber tube, analytical balance, stirrer and gas chromatograph.

[0085] (2) Photocatalytic experimental steps:

[0086] The molecular formula of cobalt complex 1 is C 137 H 148 Co4N 16 O8, weigh 2 mg of photosensitizer [Ru(phen)3](PF6)2, add 4 mL of acetonitrile and 1 mL of distilled water, transfer 210 μL of sacrificial agent TEA and 0.25 μM complex 1 into a quartz glass tube, seal it tightly with a rubber tube, and pass carbon dioxide gas for 15 min. -2 , irradiation area is 0.8cm 2 Stir and irradiate under LED light for 10 hours. After completing the above operations, take 0.5 mL of gas sample and inject it into gas chromatograph for carbon monoxide detection.

[0087] Reaction conditions: At 25°C, LED light (450nm, 100mW·cm -2 , illumination time 10h, illumination area 0.8cm 2 ).

[0088] The experiment was carried out in 3 groups: No. 1: the catalyst was cobalt complex 1 (0.25 μM); No. 2: no catalyst was added; No. 3: no carbon dioxide, but N2 atmosphere.

[0089] The results obtained are: 0.25 μM of cobalt complex 1 produces 3.54 μmol of carbon monoxide, the TON value is 704, and the selectivity is 83.2% (see Table 4, No. 1). When no catalyst is added to the reaction system (see Table 4, No. 2), it is found that the catalytic system does not produce carbon monoxide. In addition, the experiment using nitrogen instead of carbon dioxide (see Table 4, No. 3) also does not produce carbon monoxide. The experimental results show that the catalytic product carbon monoxide comes from the photocatalytic reduction of carbon dioxide by complex 1.

[0090] Table 4 Experimental data of photocatalytic carbon dioxide reduction of cobalt complex 1 of the present invention

[0091]

[0092] (3) After the cobalt complex 1 (0.25 μM) was photocatalytically reacted for 10 h, a new photosensitizer [Ru(phen)3](PF6)2 (0.4 μM) was added. The product curve with time is shown in the figure. Figure 3 As shown in the figure, the first photocatalytic reaction produces 3.54 μmol of carbon monoxide. After 10 hours of reaction, a new photosensitizer is added. After another 10 hours of reaction, 2.96 μmol of carbon monoxide can be produced. This shows that the stability of the catalyst is relatively high.

[0093] From the photocatalytic results, it can be seen that the cobalt complex of the present invention has good photocatalytic carbon dioxide reduction performance as a homogeneous catalyst in an aqueous system with a visible light LED lamp as a light source, and the complex has high stability and high selectivity for the product carbon monoxide.

[0094] The above description is a detailed description of the preferred feasible embodiments of the present invention, but the embodiments are not intended to limit the scope of the patent application of the present invention. All equivalent changes or modified changes completed under the technical spirit suggested by the present invention should fall within the patent scope covered by the present invention.

Claims

1. A tetranuclear cobalt complex based on a Salen ligand, characterized in that: The complex is 2,2'-[1,2-ethylenediamine)]bis[6-(2-tert-butyl)-4-(4-pyridyl)phenol]bis-Schiff base tetranuclear cobalt complex 1, and the molecular formula is: C 137 H 148 Co4N 16 O8, the structural formula is as follows:

2. The tetranuclear cobalt complex according to claim 1, characterized in that: The tetranuclear cobalt complex belongs to the tetragonal system, P42 / n space group, and its unit cell parameters are: α=90°, β=90°, γ=90°; 3. The method for preparing a tetranuclear cobalt complex according to claim 1 or 2, characterized in that: The following steps are involved: The Schiff base ligand H2L, cobalt salt and mixed solvent are mixed in a certain proportion, the pH value is adjusted to be greater than 8, the mixture is placed in a container, evacuated and sealed in the container, reacted at a temperature above 75°C, the reaction is stopped and then cooled to room temperature, crystals are collected and dried to obtain the obtained product; wherein the mixed solvent is composed of DMF and CH3OH, and the molar ratio of the Schiff base ligand H2L to the cobalt salt is 1 to 1.2; the Schiff base ligand H2L is 6,6-((1E, 1'E)-(ethane-1,2-diethyldi(azamethylene))bis(2-(tert-butyl)-4-(pyridin-4-yl)phenol).

4. The method for preparing a tetranuclear cobalt complex according to claim 3, characterized in that: The volume ratio of DMF to CH3OH in the mixed solvent is 1-2.

5. The method for preparing the tetranuclear cobalt complex according to claim 3, characterized in that: The reaction time is 36-48h.

6. The method for preparing a tetranuclear cobalt complex according to claim 3, characterized in that: The synthesis steps of the Schiff base ligand H2L are as follows: (1) o-tert-butylphenol is placed in a reaction container, anhydrous acetonitrile and DMF are added, and the mixture is stirred in an ice-water bath; then N-bromosuccinimide is weighed and added, and the mixture is stirred at room temperature; the mol / mol ratio of o-tert-butylphenol to N-bromosuccinimide is 1:1.1; after the reaction is completed, water is added to quench the mixture, and the mixture is extracted with ethyl acetate, and the solvent is evaporated to obtain a crude product; (2) The crude product of the previous step is placed in a reaction vessel, anhydrous acetonitrile is added, stirred to dissolve, paraformaldehyde and anhydrous magnesium chloride are added, stirred, triethylamine is added, and heated to reflux for 24-60 hours; wherein the molar ratio of paraformaldehyde: anhydrous magnesium chloride: o-tert-butylphenol is 6-8:2.2-2.3:1; after the reaction is completed, the reactants are poured into ice water, the pH is adjusted to 1-2 with concentrated HCl, allowed to stand, extracted with ether, washed with water, and dried to obtain a yellow solid 5-bromo-3-tert-butyl-2-hydroxybenzaldehyde; (3) 5-bromo-3-tert-butyl-2-hydroxybenzaldehyde, pyridine-4-boric acid, palladium acetate, triphenylphosphine, and potassium carbonate were added to a mixed solution of 1,4-dioxane and water, and nitrogen was added under vacuum, and the mixture was refluxed for three days; wherein the molar ratio of 5-bromo-3-tert-butyl-2-hydroxybenzaldehyde, pyridine-4-boric acid, palladium acetate, triphenylphosphine, and potassium carbonate was 15-16:15-17:0.3-0.4:1-1.1:72-75; after the reaction was completed, 1,4-dioxane was removed by vacuum rotation, the organic layer was extracted with ethyl acetate, washed with water, and separated by spin drying column to obtain yellow solid 3-tert-butyl-2-hydroxy-5-(4-pyridine)benzaldehyde; (4) 3-tert-Butyl-2-hydroxy-5-(4-pyridine)benzaldehyde and ethylenediamine were ultrasonically stirred in a methanol solution at a molar ratio of 2:1 to obtain a yellow solid Schiff base ligand H2L.

7. Use of the tetranuclear cobalt complex according to claim 1 or 2 in photocatalytic reduction of carbon dioxide.

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