Mononuclear cobalt complex based on naphthyl ligand as well as preparation method and application of mononuclear cobalt complex

Abstract

The invention discloses a mononuclear cobalt complex based on a naphthyl ligand and a preparation method and application thereof. The preparation method of the mononuclear cobalt complex comprises the following steps: putting a ligand N, N '-bis (naphthyl subunit)-4-nitro-1, 2-phenylenediamine, a divalent cobalt salt and a mixed solvent into a reaction container, sealing the reaction container or adjusting the pH value of the system to be alkaline, then sealing the reaction container, reacting under a heating condition, cooling the reactant, and collecting the precipitate, thereby obtaining the mononuclear cobalt complex. The mixed solvent is a composition composed of methanol and DMF (Dimethyl Formamide) or acetonitrile according to a volume ratio of 1: 1, and when the mixed solvent is the composition of methanol and acetonitrile, the system is firstly adjusted to be alkaline and then the reaction container is sealed. The test result of the applicant shows that 2.22 [mu] mol of CO and 0.08 [mu] mol of H2 can be generated when 0.05 [mu] M of the cobalt complex crystal form is used for photocatalytic reduction of CO2, the CO selectivity reaches up to 97%, and the TONCO value reaches 8868.

Description

Technical Field

[0001] The 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 growth of energy demand and the increasingly urgent need for environmentally friendly energy, the application of photocatalytic technology in the energy field has become increasingly important. Since the structure of CO2 is very stable, it is difficult to be reduced to other high-value carbon-containing compounds, and the reduction process is complex, and the generated products are diverse. Therefore, the development of catalysts with high activity and selectivity has become the key to the development of photocatalytic CO2 reduction. The use of organic ligands and non-precious metal ions to synthesize efficient and low-cost complex catalysts has become a hot spot in the study of homogeneous photocatalytic CO2 reduction in recent years.

[0003] There have been many studies on the application of cobalt metal complexes in photocatalytic CO2 reduction. For example, the invention applications with publication numbers CN115872897A and CN116874390A previously applied by the inventor team of this application disclosed two mononuclear cobalt complexes with similar structures, namely Schiff base cobalt complex and mononuclear cobalt complex based on Salen ligand, wherein the structure of Schiff base cobalt complex is shown in the following formula (1), and the structure of mononuclear cobalt complex based on 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 Salen ligand and the above-mentioned Schiff base cobalt complex is that the substituent at the 4th position on the benzene ring of phenylenediamine is replaced by fluorine instead of nitric acid. According to the experimental results, under the same conditions (the selection of catalyst concentration, photosensitizer, sacrificial agent and light source are the same), the amount of CO produced by the catalytic reduction of CO2 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 the change of the substituent groups on the main structure will have a great influence on the catalytic activity of the complex, but this influence is not something that can be reasonably expected by technicians in this field based on existing knowledge. Therefore, how to select and regulate the ligand and then synthesize it with cobalt metal to form a photocatalyst with high catalytic activity requires creative work. At present, there is no report on the cobalt metal complex synthesized with N,N'-bis(naphthalene)-4-nitro-1,2-phenylenediamine as an organic ligand having significant catalytic activity when used 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 naphthyl ligands 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(naphthalene)-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 naphthyl ligand, comprising: placing a ligand of the structure shown in the following formula (II), a divalent cobalt salt and a mixed solvent in a reaction container, sealing the reaction container or adjusting the pH value of the system to alkaline and then sealing the reaction container, placing the sealed reaction container under heating conditions for reaction, cooling the reactants, precipitating, and collecting the precipitate, which is 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, and 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 container;

[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)-4-nitro-1,2-phenylenediamine. The central Co(Ⅱ) ion coordinates 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, then sealing the reaction vessel and placing it under heating conditions for reaction, cooling the reactants, and crystals precipitating, collecting the crystals, which are the target product; wherein the volume ratio of methanol to acetonitrile is 3:1;

[0013]

[0014] The applicant found in the experiment that when other conditions are constant, the composition and ratio of the mixed solvent will affect the properties of the target product prepared. In the above-mentioned method for preparing the mononuclear cobalt complex, the mixed solvent is a composition of methanol and N,N-dimethylformamide or acetonitrile in a volume ratio of 1:1, and the target product prepared by this method is a powder; in this method, when the mixed solvent is a composition of methanol and N,N-dimethylformamide, there is no need to use alkali to adjust the pH of the system. In the above-mentioned method for preparing the crystal form, methanol and acetonitrile are also used as solvents, and a crystalline target product can be prepared when the volume ratio of methanol to acetonitrile is 3:1.

[0015] In the above-mentioned method for preparing the mononuclear cobalt complex or the method for preparing the crystal form, 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 a triamine. Preferably, the pH of the system is adjusted to ≥8.0, and more preferably, the pH of the system is adjusted to 8.5-10.0. The reaction is usually carried out at ≥50°C, preferably at 60-85°C. When the reaction is carried out at 60-85°C, the reaction time is usually controlled at 48-96h.

[0016] In the above-mentioned method for preparing the mononuclear cobalt complex or the method for preparing the crystal form, the reaction vessel is a sealed container, such as a thick-walled glass tube, or a high-pressure reactor with an inner liner. Before sealing, vacuum is usually first applied.

[0017] In the preparation method of the mononuclear cobalt complex or the preparation method of the crystal form of the present invention, the ligand of the structure shown in 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 by itself. 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 shown in the 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 the raw materials is controlled to be 2 to 15 mL. Similarly, for the preparation method of the crystal 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 the raw materials is controlled to be 2 to 15 mL.

[0018] The present invention also includes the use of the above-mentioned mononuclear cobalt complex based on naphthyl ligands or the above-mentioned crystal form of mononuclear cobalt complex based on naphthyl ligands as a catalyst in photocatalytic carbon dioxide reduction. In a specific application, the photocatalytic system includes a photosensitizer, a catalyst, a sacrificial agent and a solvent, wherein the catalyst is the above-mentioned mononuclear cobalt complex or crystal form, and the selection of the photosensitizer, the sacrificial agent and the solvent is the same as 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); the solvent is preferably a mixed solution containing 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 0.30-0.35M.

[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 with the prior art, the present invention provides a novel mononuclear cobalt complex based on naphthyl ligands and its crystal form used as a homogeneous molecular catalyst in photocatalytic CO2 reduction, as well as their preparation methods. The applicant's test 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 yield of CO generated by photocatalytic CO2 reduction is 2.22 μmol, the amount of H2 generated is 0.08 μmol, the selectivity for CO is as high as 97%, and TON is 1.3%. CO The value reaches 8868, which is significantly higher than similar mononuclear cobalt complexes reported previously. This indicates that the mononuclear cobalt complex or crystal 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 and 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 of CO production generated by two photocatalytic CO2 reduction cycles of the final product obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0029] In order to better explain the technical solution of the present invention, the present invention is further described in detail below in conjunction with the embodiments, but the embodiments of the present invention are not limited thereto. If not specifically indicated, the technical features used in the embodiments can be replaced by other technical features known in the art that have equivalent or similar functions or effects without departing from the inventive concept.

[0030] The ligand (N,N'-bis(naphthalene)-4-nitro-1,2-phenylenediamine) of the structure shown in formula (II) involved in the following embodiments 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. The yield is 5.12 g (72%). The orange precipitate is the ligand N,N'-bis(naphthalene)-4-nitro-1,2-phenylenediamine. The mass spectrum is as follows Figure 1 As shown, the infrared spectrum is Figure 2 shown.

[0033] Example 1

[0034] 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 1.5 mL of methanol and 0.5 mL of acetonitrile, and then adjust the pH of the system to 8.0 with triethylamine (10 μL), then evacuate and seal, put the sealed thick-walled glass tube into an oven at 80°C, react for 72 hours, take it out, stop the reaction and slowly cool it to room temperature, and diamond-shaped black crystals can be observed at the bottom of the glass tube, collect the crystals and dry them. The yield is about 32% (0.0082 g, calculated 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 has 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, some bond length and bond angle data are shown in Table 2 below, and the crystal structure diagram of the obtained rhombus black crystal is shown in Figure 5 As shown, the obtained rhombus black crystals are 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 complex 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 the reaction was stopped, the temperature was slowly lowered to room temperature, and the glass tube contained a clear liquid with no crystals or precipitation at the bottom. The liquid in the glass tube was further concentrated under reduced pressure, but still no crystals or precipitation 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 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. Put the sealed thick-walled glass tube into an oven at 80°C, take it out after reacting for 72 hours, 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 As shown. The target peak was fitted and found to be the same as the molecular weight of 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] 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 DMF, then evacuate and seal, put the sealed thick-walled glass tube into an oven at 80°C, react for 72 hours, take it out, stop the reaction and slowly cool it to room temperature, black precipitate can be observed at the bottom of the glass tube, collect the precipitate and dry it. The yield is about 34% (0.0087 g, calculated based on the amount of Co).

[0059] The product obtained in this example was subjected to high-resolution mass spectrometry analysis, and the target peak was fitted, and it was found that it had 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.

[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 the reaction was stopped, the temperature was slowly lowered to room temperature, and 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: the crystal form of the mononuclear cobalt complex based on 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 tube, analytical balance, stirrer and gas chromatograph.

[0066] (2) Specific experimental steps:

[0067] The photosensitizer [Ru(phen)3](PF6)2 (2 mg, 0.4 mM) was weighed in turn, the sacrificial agent TEOA (200 μL, 0.3 M) and the mononuclear cobalt complex 1 (5 μL, 1 μM) were transferred into a quartz glass tube, 4 mL of ultra-dry acetonitrile and 1 mL of water were added, and the tube was tightly sealed 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, and stirred during irradiation. After completing the above operations, a gas sample was taken and injected into a gas chromatograph for detection of CO2 reduction product CO. The detection results are shown in Table 3 below.

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

[0069]

[0070] Reaction conditions: At 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, the generated H2 is 0.52 μmol, and TON COThe value is 1728, and the selectivity is 95%. When the concentration of complex 1 is 0.05 μM as catalyst, the photocatalytic reduction of CO2 generates 2.22 μmol CO and 0.08 μmol H2. CO The value was 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 catalytic activity (TON) higher than that of another complex with similar structure (CN202310262412.1). CO =3380) increased by 162%, and the selectivity for CO was also significantly improved. However, when there was no complex 1 as a catalyst or no CO2 in the system, no CO was generated in the reaction system, indicating that the CO produced in this reaction did come from the photocatalytic CO2 reduction of complex 1.

[0072] As shown in Table 3, in the photocatalytic system constructed by 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 by this patent effectively improves the photocatalytic CO2 reduction performance of the catalyst.

[0073] Depend on Figure 8 It can be seen that the mononuclear cobalt complex of the present invention was subjected to a continuous experiment, and it was recorded that after 10 hours of photocatalysis, the reaction stopped and the amount of CO generated was 8.64 μmol; when a fresh photosensitizer was added again, the light was continued, and the reaction was resumed. After continuing to light for 10 hours, 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 indicates that the reason for the cessation of the catalytic reaction is the degradation of the photosensitizer. The above 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):

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 shown in the following formula (II), a divalent cobalt salt and a mixed solvent are placed in a reaction container, the reaction container is sealed or the pH value of the system is adjusted to alkaline and then the reaction container is sealed, the sealed reaction container 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 container; 3. A crystal form of a mononuclear cobalt complex based on a naphthyl ligand, which belongs to the monoclinic system, P21 / c space group, and the unit cell parameters are: α=90.00°, β=94.080(5)°, γ=90.00°.

4. The method for preparing the crystal form according to claim 3, characterized in that: A ligand of the structure shown in the following formula (II) and a divalent cobalt salt are placed in a reaction container, methanol and acetonitrile are added, and the pH value of the system is adjusted to alkaline, and then the reaction container is sealed and placed under heating conditions for reaction, the reactants are cooled, crystals are precipitated, and the crystals are collected to obtain the target product; wherein the volume ratio of methanol to acetonitrile is 3:1; 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 naphthyl ligands according to claim 1 or the crystalline form of the mononuclear cobalt complex based on naphthyl ligands according to claim 3 as a catalyst in photocatalytic carbon dioxide reduction.

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

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