Mononuclear cobalt complex based on naphthyl ligand and its preparation method and application

Abstract

The present invention discloses a mononuclear cobalt complex based on a naphthyl ligand, and its preparation method and application. The preparation method of the mononuclear cobalt complex is as follows: take the ligand N,N'-bis(naphthalene)-4-nitro-1,2-phenylenediamine, a divalent cobalt salt and a mixed solvent and place them in a reaction vessel, seal the reaction vessel or adjust the pH value of the system to alkaline and then seal the reaction vessel, place it under heating conditions to react, cool the reactants, collect the precipitate, and obtain; the mixed solvent is a composition composed of methanol and DMF or acetonitrile in a volume ratio of 1:1. When the mixed solvent is a composition of methanol and acetonitrile, first adjust the system to alkaline and then seal the reaction vessel. The applicant's test results show that when using a 0.05μM cobalt complex crystal form to photocatalyze CO2 reduction, 2.22μmol of CO and 0.08μmol of H2 can be generated, and the selectivity for CO is as high as 97%, TON CO The value reaches 8868.

Description

Technical Field

[0001] The present invention relates to a metal complex, in particular to a mononuclear cobalt complex based on a naphthyl ligand, and a preparation method and application thereof. Background Art

[0002] With the growing demand for energy and the increasingly urgent need for environmentally friendly energy sources, the application of photocatalytic technology in the energy sector has become increasingly important. Due to the highly stable structure of CO₂, it is difficult to reduce to other high-value carbon-containing compounds. Furthermore, the reduction process is complex, resulting in a wide variety of products. Therefore, the development of highly active and selective catalysts has become crucial for the development of photocatalytic CO₂ reduction. The synthesis of efficient and low-cost complex catalysts using organic ligands and non-precious metal ions has become a hot topic in recent years in homogeneous photocatalytic CO₂ reduction research.

[0003] There has been extensive research on the application of cobalt metal complexes in photocatalytic CO2 reduction. For example, the invention applications previously filed by the inventor team of this application, with publication numbers CN115872897A and CN116874390A, disclose two structurally similar mononuclear cobalt complexes: a Schiff base cobalt complex and a mononuclear cobalt complex based on a salen ligand. The structure of the Schiff base cobalt complex is shown in the following formula (1), and the structure of the mononuclear cobalt complex based on a salen ligand is shown in the following formula (2):

[0004]

[0005] The Schiff base cobalt complex exhibited high catalytic activity and selectivity in a homogeneous photocatalytic system. Specifically, at a concentration of 0.1 μM, photocatalytic CO2 reduction produced 2.63 μmol of CO, with a catalytic conversion number of TON. CO The value is 5260, and the selectivity for CO is 90%. The difference between the mononuclear cobalt complex based on the Salen ligand and the above-mentioned Schiff base cobalt complex is that the substituent at the 4-position on the benzene ring of phenylenediamine is replaced by a fluorine group instead of a nitric acid group. According to the experimental results, under the same conditions (the choice of catalyst concentration, photosensitizer, sacrificial agent and light source, etc. are all the same), the amount of CO produced by the base-catalyzed CO2 reduction is only 1.69 μmol, and the catalytic conversion number is TON. COThe value dropped to 3380, and the selectivity for CO was only 86%. It can be seen that even a change in the substituent groups on the main structure can have a significant impact on the catalytic activity of the complex, but this effect is not something that can be reasonably anticipated by those skilled in the art based on existing knowledge. Therefore, how to select and regulate the ligand and then combine it with cobalt metal to synthesize a photocatalyst with high catalytic activity requires creative work. To date, there are no reports of cobalt metal complexes synthesized with N,N'-bis(naphthalene-4-nitro-1,2-phenylenediamine as an organic ligand showing significant catalytic activity for homogeneous photocatalytic CO2 reduction. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a mononuclear cobalt complex based on a naphthyl ligand which can be used as a homogeneous molecular catalyst in photocatalytic CO2 reduction and exhibits high activity, high selectivity and high stability, as well as a preparation method and application thereof.

[0007] In the first aspect, the present invention provides a mononuclear cobalt complex based on a naphthyl ligand, specifically N,N'-bis(naphthaleneylidene)-4-nitro-1,2-phenylenediamine cobalt complex, whose chemical formula is C 28 H 17 CoN3O4, the structural formula is shown in the following formula (I):

[0008]

[0009] In a second aspect, the present invention provides a method for preparing the above-mentioned mononuclear cobalt complex based on a naphthyl ligand, comprising: placing a ligand having a structure represented by the following formula (II), a divalent cobalt salt, and a mixed solvent in a reaction vessel, sealing the reaction vessel or adjusting the pH value of the system to alkaline and then sealing the reaction vessel, subjecting the sealed reaction vessel to a reaction under heating conditions, cooling the reactants, and precipitating, collecting the precipitate to obtain the target product; wherein the mixed solvent is a composition composed of methanol and N,N-dimethylformamide or acetonitrile in a volume ratio of 1:1. When the mixed solvent is a composition composed of methanol and acetonitrile in a volume ratio of 1:1, it is necessary to first adjust the pH value of the system to alkaline and then seal the reaction vessel;

[0010]

[0011] In a third aspect, the present invention provides a crystal form of a mononuclear cobalt complex based on a naphthyl ligand, which belongs to the monoclinic system, the P21 / c space group, and the unit cell parameters are: α=90.00°,β=94.080(5)°,γ=90.00°. The smallest asymmetric unit of the crystal consists of a crystallographically independent Co(Ⅱ) ion and an N,N'-bis(naphthalene-deoxy)-4-nitro-1,2-phenylenediamine. The central Co(Ⅱ) ion is coordinated with the phenolic hydroxyl group and two N atoms on the ligand to form a N2O2-type four-coordinate planar structure.

[0012] In a fourth aspect, the present invention provides a method for preparing the above-mentioned crystal form, comprising: placing a ligand having a structure represented by the following formula (II) and a divalent cobalt salt in a reaction vessel, adding methanol and acetonitrile, and then adjusting the pH value of the system to alkaline. Thereafter, the reaction vessel is sealed and placed under heating conditions for reaction. The reactants are cooled, crystals precipitate, and the crystals are collected to obtain the target product; wherein the volume ratio of methanol to acetonitrile is 3:1;

[0013]

[0014] The applicant discovered through experiments that, when other conditions remain constant, the composition and ratio of the mixed solvent can affect the properties of the resulting target product. In the aforementioned method for preparing a mononuclear cobalt complex, the mixed solvent is a combination of methanol and N,N-dimethylformamide or acetonitrile in a 1:1 volume ratio. This method produces a powdered target product. In this method, when the mixed solvent is a combination of methanol and N,N-dimethylformamide, there is no need to use a base to adjust the pH of the system. In the aforementioned method for preparing a crystalline form, methanol and acetonitrile are also used as solvents. A 3:1 volume ratio of methanol to acetonitrile can produce a crystalline target product.

[0015] In the above-mentioned method for preparing a mononuclear cobalt complex or a crystalline form, an alkaline substance is used to adjust the pH of the system to alkaline. The alkaline substance can be a conventionally selected one in the prior art, preferably a triamine. The pH of the system is preferably adjusted to ≥8.0, and more preferably to 8.5-10.0. The reaction is typically carried out at ≥50°C, preferably at 60-85°C. When the reaction is carried out at 60-85°C, the reaction time is typically controlled to be 48-96 hours.

[0016] In the above-mentioned method for preparing a mononuclear cobalt complex or a crystal form, the reaction vessel is a sealed container, such as a thick-walled glass tube or a lined autoclave. Vacuuming is usually performed before sealing.

[0017] In the preparation method of the mononuclear cobalt complex or the preparation method of the crystal form described in the present invention, the ligand represented by the structure of formula (II) is N,N'-bis(naphthalene)-4-nitro-1,2-phenylenediamine, which can be prepared with reference to the literature (Tohidiyan, Z., Sheikhshoaie I., Khaleghi M., Mague JT, A novel copper(II) complex containing atetradentate Schiff base: Synthesis, spectroscopy, crystal structure, DFT study, biological activity and preparation of its nano-sized metaloxide. J. Mol. Struct. 2017, 1134, 706-714.), or can be designed and synthesized independently. The divalent cobalt salt involved can be CoCl2·6H2O or Co(NO3)2·6H2O, or a combination of CoCl2·6H2O and Co(NO3)2·6H2O. The molar ratio of the ligand of the structure represented by formula (II) to the divalent cobalt salt is a stoichiometric ratio. The amount of the mixed solvent used in the method for preparing the mononuclear cobalt complex can be determined as needed. Specifically, it can be calculated based on 0.05 mmol of the ligand, and the total amount of the mixed solvent used for all raw materials is controlled to be 2 to 15 mL. Similarly, for the preparation method of the crystalline form, the total amount of methanol and acetonitrile can also be calculated based on 0.05 mmol of the ligand, and the total amount of the mixed solvent used for all raw materials is controlled to be 2 to 15 mL.

[0018] The present invention also includes the use of the aforementioned mononuclear cobalt complex based on a naphthyl ligand or a crystal form of the aforementioned mononuclear cobalt complex based on a naphthyl ligand as a catalyst in photocatalytic carbon dioxide reduction. In specific applications, the photocatalytic system includes a photosensitizer, a catalyst, a sacrificial agent, and a solvent, wherein the catalyst is the aforementioned mononuclear cobalt complex or crystal form. The selection of the photosensitizer, sacrificial agent, and solvent is the same as in the prior art. Specifically, the photosensitizer can be [Ru(phen)3](PF6)2, [Ru(phen)3]Cl2, or [Ru(bpy)3]Cl2, preferably [Ru(phen)3](PF6)2; the sacrificial agent is preferably triethanolamine (TEOA) and / or triethylamine (TEA); and the solvent is preferably a mixed solution comprising water and acetonitrile, wherein the volume ratio of water to acetonitrile is preferably 1:4. 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 0.30-0.35 M.

[0019] The present invention also provides a catalyst, which contains the mononuclear cobalt complex based on naphthyl ligand or the crystal form of the mononuclear cobalt complex based on naphthyl ligand.

[0020] Compared to the prior art, the present invention provides a novel mononuclear cobalt complex based on a naphthyl ligand and its crystal form for use as a homogeneous molecular catalyst in photocatalytic CO2 reduction, as well as methods for preparing the same. The applicant's experimental results show that when the mononuclear cobalt complex or crystal form of the present invention is used as a catalyst at a concentration of 0.05 μM, the photocatalytic reduction of CO2 produces 2.22 μmol of CO and 0.08 μmol of H2, with a selectivity for CO of up to 97%. CO The value reached 8868, significantly higher than that of similar mononuclear cobalt complexes reported previously. This indicates that the mononuclear cobalt complex or crystalline form prepared by the present invention exhibits excellent catalytic activity and selectivity as a homogeneous molecular catalyst in photocatalytic CO2 reduction, and can efficiently photocatalyze CO2 reduction to selectively generate CO. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the mass spectrum of the ligand N,N'-bis(naphthaleneylidene)-4-nitro-1,2-phenylenediamine.

[0022] Figure 2 This is the infrared spectrum of the ligand N,N'-bis(naphthaleneylidene)-4-nitro-1,2-phenylenediamine.

[0023] Figure 3 The mass spectrum of the final product obtained in Example 1 of the present invention.

[0024] Figure 4 This is the mass spectrum fitting diagram of the final product obtained in Example 1 of the present invention.

[0025] Figure 5 This is a crystal structure diagram of the final product obtained in Example 1 of the present invention.

[0026] Figure 6 The mass spectrum of the final product obtained in Example 3 of the present invention.

[0027] Figure 7 This is the infrared spectrum of the final product obtained in Example 3 of the present invention.

[0028] Figure 8 This is a graph showing the CO production generated by two photocatalytic CO2 reduction cycles of the final product obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0029] To better explain the technical solution of the present invention, the present invention is further described in detail below with reference to the examples, but the embodiments of the present invention are not limited thereto. Unless otherwise specified, the technical features used in the examples may be replaced with other technical features known in the art having equivalent or similar functions or effects without departing from the inventive concept.

[0030] The ligand (N,N'-bis(naphthaleneylidene)-4-nitro-1,2-phenylenediamine) of the structure shown in formula (II) involved in the following examples was prepared according to the following method:

[0031]

[0032] Weigh 2-hydroxy-1-naphthaldehyde (2.927 g, 17 mmol) and 4-nitro-o-phenylenediamine (1.302 g, 8.5 mmol) and dissolve them in 50 mL of methanol, stir until completely dissolved, and then mix them to obtain a brown solution. Stir at room temperature for 8 hours, then wash with ether and filter to collect an orange precipitate. Yield: 5.12 g (72%). The orange precipitate is the ligand N,N'-bis(naphthalene)-4-nitro-1,2-phenylenediamine. Mass spectrum as shown Figure 1 As shown, the infrared spectrum Figure 2 shown.

[0033] Example 1

[0034] The ligand N,N'-bis(naphthalene-1-ylidene)-4-nitro-1,2-phenylenediamine (0.0231 g, 0.05 mmol) and Co(NO₃)₂·6H₂O (0.0146 g, 0.05 mmol) were weighed into a thick-walled glass tube. 1.5 mL of methanol and 0.5 mL of acetonitrile were added. The pH of the system was adjusted to 8.0 with triethylamine (10 μL). The tube was then evacuated and sealed by fusion. The sealed tube was placed in an 80°C oven and allowed to react for 72 h. After terminating the reaction, the tube was removed and slowly cooled to room temperature. Diamond-shaped black crystals were observed precipitating at the bottom of the tube. The crystals were collected and dried. The yield was approximately 32% (0.0082 g, based on the amount of Co).

[0035] The product obtained in this example was characterized:

[0036] (1) Mass spectrometry, the spectrum of which is as follows Figure 3 shown.

[0037] According to high-resolution mass spectrometry, the complex showed a group of main peaks ( Figure 2 ), the peak 518.05 is composed of {[(C 28 H 17 CoN3O4)+H] +} contains the complex framework and a proton.

[0038] (2) Mass spectrum fitting diagram Figure 4 shown.

[0039] The target peak was fitted and found to have the same molecular weight as the target complex.

[0040] (3) Crystal structure analysis:

[0041] A diamond-shaped black crystal of appropriate size was selected and placed on an Agilent SuperNova single crystal diffractometer. α 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 below. The crystal structure diagram of the obtained rhombus black crystals is shown in Figure 5 As shown, the obtained rhombus black crystals were determined to be the target product of the present invention.

[0042] Table 1 Crystallographic data of mononuclear cobalt complexes

[0043]

[0044]

[0045] Table 2 Partial bond length and bond angle data of the cobalt complexes of the present invention

[0046]

[0047]

[0048] Comparative Example 1-1

[0049] Example 1 was repeated, except that the amount of methanol added was 1 mL and the amount of acetonitrile added was 1 mL.

[0050] After stopping the reaction, the temperature was slowly lowered to room temperature. The glass tube contained a clear liquid with no crystals or precipitates at the bottom. The liquid in the glass tube was further concentrated under reduced pressure, but no crystals or precipitates were observed.

[0051] Example 2

[0052] Example 1 was repeated, except that the pH of the system was adjusted to 9.5 and the reaction was carried out at 70°C.

[0053] As a result, rhombus-shaped black crystals were obtained with a yield of 40% (0.0103 g, calculated based on the amount of Co).

[0054] The product obtained in this example was subjected to high-resolution mass spectrometry analysis and single crystal diffraction analysis, and it was determined that the obtained rhombus-shaped black crystals were the target product of the present invention.

[0055] Example 3

[0056] Weigh the ligand N,N'-bis(naphthalene)-4-nitro-1,2-phenylenediamine (0.0231 g, 0.05 mmol) and Co(NO3)2·6H2O (0.0146 g, 0.05 mmol) in a thick-walled glass tube, add 1 mL of methanol and 1 mL of acetonitrile, and then adjust the pH of the system to 8.0 with triethylamine. Then, evacuate and seal the tube. Place the sealed thick-walled glass tube in an oven at 80°C, react for 72 hours, take it out, stop the reaction, and slowly cool it to room temperature. A black precipitate can be observed at the bottom of the glass tube. Collect the precipitate and dry it. The yield is about 35% (0.0089 g, calculated based on the amount of Co). The mass spectrum and infrared spectrum of the obtained product are shown as follows: Figure 6 and Figure 7 The target peak was fitted and found to have the same molecular weight as the target complex. The black precipitate obtained in this example was determined to be the target product of the present invention.

[0057] Example 4

[0058] The ligand N,N'-bis(naphthalene-1-ylidene)-4-nitro-1,2-phenylenediamine (0.0231 g, 0.05 mmol) and Co(NO₃)₂·6H₂O (0.0146 g, 0.05 mmol) were weighed into a thick-walled glass tube. 1 mL of methanol and 1 mL of DMF were added. The tube was then evacuated and sealed with a heat seal. The sealed tube was placed in an 80°C oven and allowed to react for 72 hours. After terminating the reaction, the tube was removed and slowly cooled to room temperature. A black precipitate was observed at the bottom of the tube. The precipitate was collected and dried. The yield was approximately 34% (0.0087 g, based on the amount of Co).

[0059] High-resolution mass spectrometry analysis of the product obtained in this example and fitting of the target peak revealed that it had the same molecular weight as the target complex, confirming that the black precipitate obtained in this example was the target product of the present invention.

[0060] Comparative Example 4-1

[0061] Example 4 was repeated, except that the amount of methanol added was 0.5 mL and the amount of DMF added was 1 mL.

[0062] After stopping the reaction, the temperature was slowly lowered to room temperature. The glass tube contained a clear liquid with no precipitation at the bottom. The liquid in the glass tube was further concentrated under reduced pressure, but no precipitation was observed.

[0063] Experimental Example 1: The mononuclear cobalt complex crystal form of the present invention was used as a homogeneous molecular catalyst to test its photocatalytic CO2 reduction performance in an aqueous mixed solvent system.

[0064] (1) Materials used:

[0065] Photosensitizer: [Ru(phen)3](PF6)2, catalyst: a crystalline form of a mononuclear cobalt complex based on a naphthyl ligand prepared according to Example 1 of the present invention (hereinafter referred to as complex 1), sacrificial agent: TEOA, solvent: a mixture of acetonitrile and water in a volume ratio of 4:1, LED light source (wavelength 450nm, light intensity 100mW·cm -2 , the irradiation area is 0.8cm 2 ), a 15-20 mL quartz reactor, CO2 gas, rubber tubing, analytical balance, stirrer and gas chromatograph.

[0066] (2) Specific experimental steps:

[0067] The photosensitizer [Ru(phen)3](PF6)2 (2 mg, 0.4 mM), the sacrificial agent TEOA (200 μL, 0.3 M) and the mononuclear cobalt complex 1 (5 μL, 1 μM) were weighed in sequence into a quartz glass tube, 4 mL of ultra-dry acetonitrile and 1 mL of water were added, and the tube was sealed tightly with a rubber tube. CO2 gas was introduced. After 10 to 20 minutes, the tube was irradiated with the LED light source for 10 hours at a constant temperature of 25°C while stirring. After completing the above operations, a gas sample was taken and injected into a gas chromatograph for detection of the CO2 reduction product CO. The test results are shown in Table 3 below.

[0068] Table 3 Experimental data of the mononuclear cobalt complex crystal form of the present invention as a catalyst for photocatalytic CO2 reduction

[0069]

[0070] Reaction conditions: At a constant temperature of 25°C, use LED light (wavelength 450nm, 100mW·cm -2 , irradiation area 0.8cm 2 ) irradiated the lower part of the quartz tube for 10 h. No. 1: complex 1 (1 μM); No. 2: no catalyst; No. 3: with CO2 and N2; No. 4: complex 1 (0.05 μM).

[0071] As shown in Table 3, in the photocatalytic system constructed by the present invention, when the concentration of complex 1 is 1 μM as catalyst, the photocatalytic reduction of CO2 generates 8.64 μmol CO and 0.52 μmol H2. COThe value is 1728, and the selectivity is 95%. When the concentration of complex 1 is 0.05 μM, the photocatalytic reduction of CO2 generates 2.22 μmol CO and 0.08 μmol H2. CO The value increased to 8868, with a selectivity of 97%. It is worth noting that the photocatalytic CO2 reduction activity of complex 1 with large ligand conjugation and strong substituent electron-withdrawing ability is higher than that of similar complexes reported (see patent CN202211685222.2) (TON CO =5260) was increased by 69%, which was higher than the catalytic activity (TON) of another complex with similar structure (CN202310262412.1). CO =3380) increased by 162%, and the selectivity for CO also increased significantly. However, when complex 1 was not used as a catalyst or CO2 was not present in the system, no CO was generated in the reaction system, indicating that the CO produced in this reaction was indeed derived from the photocatalytic CO2 reduction of complex 1.

[0072] As shown in Table 3, in the photocatalytic system constructed in the present invention, by simultaneously improving the conjugation of the ligand and the electron-withdrawing ability of the substituent, the photocatalytic CO2 reduction performance of the synthesized complex 1 is significantly improved compared with the performance of the reported complexes, indicating that the strategy adopted in this patent effectively improves the photocatalytic CO2 reduction performance of the catalyst.

[0073] Depend on Figure 8 As shown, a continuous experiment with the mononuclear cobalt complex described in the present invention recorded that after 10 hours of photocatalysis, the reaction ceased, with the amount of CO generated being 8.64 μmol. After the addition of a fresh portion of photosensitizer and continued illumination, the reaction resumed, and after another 10 hours of illumination, the total amount of CO generated was 17.21 μmol, indicating that the amount of CO generated in the second catalytic cycle was almost equal to that in the first catalytic cycle. This suggests that the cessation of the catalytic reaction was due to degradation of the photosensitizer. These experimental results confirm that the mononuclear cobalt complex is a stable molecular catalyst.

[0074] The above experimental results show that the bulk homogeneous molecular catalyst of the mononuclear cobalt complex of the present invention has very good photocatalytic CO2 reduction activity, selectivity and stability in an aqueous system.

Claims

1. A mononuclear cobalt complex based on a naphthyl ligand, the chemical formula of which is C 28 H 17 CoN3O4, the structural formula is shown in the following formula (I): (I)。 2. The method for preparing the mononuclear cobalt complex based on naphthyl ligand according to claim 1, characterized in that: A ligand having a structure represented by the following formula (II), a divalent cobalt salt, and a mixed solvent are placed in a reaction vessel, the reaction vessel is sealed or the pH value of the system is adjusted to alkaline and then the reaction vessel is sealed, the sealed reaction vessel is placed under heating conditions for reaction, the reactants are cooled, a precipitate is precipitated, and the precipitate is collected to obtain the target product; wherein the mixed solvent is a composition composed of methanol and N,N-dimethylformamide or acetonitrile in a volume ratio of 1:

1. When the mixed solvent is a composition composed of methanol and acetonitrile in a volume ratio of 1:1, it is necessary to first adjust the pH value of the system to alkaline and then seal the reaction vessel; (II)。 3. A crystal form of a mononuclear cobalt complex based on a naphthyl ligand, wherein the planar structural formula of the mononuclear cobalt complex based on the naphthyl ligand is shown in the following formula (I), and the crystal form belongs to the monoclinic system. P2 1 / c Space group, unit cell parameters are: a =13.1678(8)Å, b =7.5908(7)Å, c =21.6282(12)Å, α =90.00 o , β= 94.080(5) o , γ= 90.00 o ; (I)。 4. The method for preparing the crystal form according to claim 3, characterized in that: A ligand of the structure represented by the following formula (II) and a divalent cobalt salt are placed in a reaction vessel, methanol and acetonitrile are added, and the pH value of the system is adjusted to alkaline. The reaction vessel is then sealed and placed under heating conditions for reaction. The reactants are cooled, and crystals precipitate. The crystals are collected to obtain the target product; wherein the volume ratio of methanol to acetonitrile is 3:1; (II)。 5. The preparation method according to claim 2 or 4, characterized in that: Adjust the pH of the system to ≥8.

0.

6. The preparation method according to claim 2 or 4, characterized in that: Adjust the pH of the system to 8.5~10.

0.

7. The preparation method according to claim 2 or 4, characterized in that: The reaction was carried out at ≥50°C.

8. The preparation method according to claim 2 or 4, characterized in that: The divalent cobalt salt is CoCl2·6H2O or Co(NO3)2·6H2O, or a combination of CoCl2·6H2O and Co(NO3)2·6H2O.

9. Use of the mononuclear cobalt complex based on a naphthyl ligand according to claim 1 or the crystal form of the mononuclear cobalt complex based on a naphthyl ligand according to claim 3 as a catalyst in photocatalytic carbon dioxide reduction.

10. A photocatalyst comprising the mononuclear cobalt complex based on a naphthyl ligand according to claim 1 or the crystal form of the mononuclear cobalt complex based on a naphthyl ligand according to claim 3.

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