CN113501849B - Mononuclear cobalt complex, and preparation method and application thereof - Google Patents

Abstract

The invention discloses a mononuclear cobalt complex with a chemical general formula of Co (bipyalk) and (OAc)₂Wherein bipyalk is 2,2'- ([2,2' -bipyridine ]]-6,6' -dimethyl) bis (propan-2-ol), OAc being the radical acetate. The invention also discloses a preparation method and application of the mononuclear cobalt complex. The mononuclear cobalt complex can be used for preparing oxygen by catalytic oxidation of water under the electrocatalysis condition.

Description

A kind of mononuclear cobalt complex, its preparation method and its application

technical field

The invention relates to a mononuclear cobalt complex, a preparation method and application thereof, and belongs to the field of electrocatalysts.

Background technique

In the context of artificial photosynthesis, the oxygen evolution reaction (OER, 2H ₂ O → O ₂ +4H ⁺ +4e ⁻ ) is a particularly challenging reaction for direct storage of solar energy in chemical fuels via a water splitting process Kinetic bottleneck. Since it is difficult to achieve multi-step electron/proton transfer through the formation of OO bonds, the reaction kinetics are slow; therefore, active and stable catalysts are required to lower this barrier to promote highly active and fast water oxidation reactions in aqueous media.

The use of complexes of the noble metals ruthenium and iridium for water oxidants has yielded some results, so the search for cheap and earth-rich first-row analogs as alternatives is very attractive. Metal complex catalysts generally need to operate at relatively high overpotentials in order to generate high oxidation state metals at catalytically active sites. This means that harsh conditions are required for this type of metal complex to catalyze water oxidation, and the design of the ligand must not only stabilize the high oxidation state of the metal center, but also strongly resist the degradation of the ligand.

Therefore, at present, metal complexes mainly select noble metals to reduce the requirements for ligand performance, but this undoubtedly increases the cost; or select first-row transition metals, but for this type of metals, the typical ligand selection is due to lack of sufficient electron donor power, or It cannot adapt to the oxidation conditions, cannot exist stably in the process of catalytic water oxidation, is prone to degradation, or can only play a catalytic role in an alkaline environment, which cannot meet practical needs.

SUMMARY OF THE INVENTION

In order to solve the deficiencies in the prior art, the present invention first discloses a mononuclear cobalt complex, a preparation method and application thereof. The mononuclear cobalt complex of the present application has high water solubility and is a stable high-valence complex.

In order to solve the above-mentioned technical problems, the technical scheme adopted in the present invention is as follows:

A mononuclear cobalt complex, characterized in that: its general chemical formula is Co(bipyalk)(OAc) ₂ , wherein bipyalk is 2,2'-([2,2'-bipyridine]-6,6' - Dimethyl)bis(propan-2-ol), OAc is the acetic acid radical.

The present application utilizes 2,2'-([2,2'-bipyridine]-6,6'-dimethyl)bis(propan-2-ol) to fix the central cobalt atom, the alkoxy group of pyridine alkoxy compound The proton ^{-coupled electron transfer is allowed and its strong σ- and π- donor} powers are favorable for metals to attain extremely high oxidation states. The benzyl group of the aromatic pyridine ring is protected by methyl groups, providing strong resistance to binding and degradation during catalytic turnover. Furthermore, this ligand has two additional benefits: its broad solubility in aqueous and non-aqueous solvents, and the anisotropic ligand field enhances the geometry-dependent redox properties of its complexes.

At present, most of the first-row transition metal complexes are used as water oxidants, mainly in alkaline conditions, or can only be carried out in organic solvents, some of which have poor solubility in water and organic systems, and can only be plated on FTO or organic solvents. ITO surface. The mononuclear cobalt complex can be dissolved in water and organic solvents, and can electrocatalytically oxidize water to prepare oxygen under neutral and acidic conditions, without any degradation after electrocatalysis, with high catalytic efficiency and a lower overpotential than Existing cobalt complexes.

Using the mononuclear cobalt complex in this application, the water oxidation is extended to the acidic environment, the use range of the metal complex catalyst is extended to the acidic condition, the selectable range of the oxygen evolution environment is increased, and the oxygen evolution environment is more flexible. .

The structural formula (I) of the mononuclear cobalt complex of the present application is as follows:

Secondly, the present application also discloses the preparation method of the above-mentioned mononuclear cobalt complex, specifically: dissolving cobalt acetate tetrahydrate and bipyalk in ethanol, stirring and reacting at 50-60° C. for 6-7 hours in a dark environment, and then taking out the spinner Evaporate, add n-hexane, and crystallize in a dark environment at -30 to -50 °C to obtain a mononuclear cobalt complex.

In this application bipyalk refers to 2,2'-([2,2'-bipyridyl]-6,6'-dimethyl)bis(propan-2-ol).

In order to improve the reaction efficiency and ensure the quality of the obtained product, the molar ratio of cobalt acetate tetrahydrate and bipyalk is 1.0-1.4:1, and excess cobalt acetate tetrahydrate helps the complete reaction of bipyalk, reduces ligand loss, and reduces production costs.

The molar ratio of ethanol and bipyalk is 500-700:1. On the one hand, excess ethanol can ensure the full dissolution of bipyalk, and on the other hand, it utilizes the large polarity of ethanol, which is helpful to improve the reaction efficiency and is conducive to the next step to separate out monolayers. crystal.

The molar ratio of n-hexane and bipyalk is 40-60:1. The mixture of n-hexane with low polarity and ethanol with high polarity is conducive to the precipitation of single crystals. At the same time, the concentration is also to be as close to the saturation concentration as possible to reduce the difficulty of growing single crystals. , improve production efficiency.

Thirdly, the present application also discloses the application of the mononuclear cobalt complex in electrocatalytic water oxidation, and the mononuclear cobalt complex can be used as a water oxidant in an aqueous solution or an organic solution containing water for electrocatalytic oxidation of water to produce oxygen. Specifically, the mononuclear cobalt complex can be dissolved in DMF, and an organic electrolyte is used as the supporting electrolyte; the mononuclear cobalt complex can be dissolved in water, and the pH of the solution can be adjusted with phosphoric acid or boric acid, and electrocatalytic oxidation can be realized at a low overpotential. Oxygen from water. The advantage of the mononuclear cobalt complex is that it has a wide range of application systems and can adapt to different needs.

At the same time, when the mononuclear cobalt complex is used as a water oxidant in an aqueous solution, it can work under the condition of pH 3-7. At present, water oxidation under acidic conditions is not possible for many metal complexes, high concentrations of protons will destroy their structures and lead to dissociation, and because higher concentrations of protons are less conducive to water oxidation, they are not enough even if they exist stably. catalytic efficiency.

The technologies not mentioned in the present invention all refer to the prior art.

Description of drawings

Fig. 1 is the high-resolution mass spectrogram of the mononuclear cobalt complex of Example 1;

FIG. 2 is a cyclic voltammogram of the mononuclear cobalt complex of Example 1 used in DMF solution electrocatalytic oxidation of water to prepare oxygen (DMF refers to dimethylformamide).

3 is a cyclic voltammogram of the mononuclear cobalt complex of Example 1 used for electrocatalytic oxidation of water to prepare oxygen in pure water.

4 is a graph showing the oxygen output of the mononuclear cobalt complex of Example 1 used for electrocatalytic oxidation of water to prepare oxygen.

Detailed ways

In order to better understand the present invention, the content of the present invention is further illustrated below in conjunction with the embodiments, but the content of the present invention is not limited to the following embodiments.

Example 1

0.74 mmol of cobalt acetate tetrahydrate (183 mg) and 0.74 mmol of bipyalk (200 mg) (molar ratio of 1:1) were dissolved in 30 mL of ethanol, heated to 50 ° C in a dark environment and stirred for 6 hours under reflux, and the reaction ended. After that, take out rotary evaporation to the remaining 3 mL, add 5 mL of n-hexane, and crystallize in a dark environment at -40° C. to obtain a mononuclear cobalt complex with a yield of 82%.

The mononuclear cobalt complex was detected by high-resolution mass spectrometry (ESI-MS (CH ₃ OH) is shown in Figure 1): m/z 330.07663[M-2OAc] ²⁺ .

Example 2

1.09 mmol of cobalt acetate tetrahydrate (272 mg) and 0.90 mmol of bipyalk (246 mg) (molar ratio of 11:9) were dissolved in 30 mL of ethanol, heated to 50 °C in a dark environment and stirred for 6 hours under reflux, and the reaction ended. Then, take out the rotary evaporation to the remaining 4 mL, add 6 mL of n-hexane, and crystallize in a dark environment at -45°C to obtain a mononuclear cobalt complex with a yield of 95%. The mass spectrum of the obtained product is completely consistent with Example 1.

Application Test 1

The mononuclear cobalt complex prepared in the above Example 1 was used for electrocatalytic oxidation of water to prepare oxygen. The test method was as follows: 25 μmol of mononuclear cobalt complex was added to 0.1M Bu ₄ NPF ₆ (DMF solution) (50 mL), The electrocatalytic water oxidation test (cyclic voltammetry test) was carried out with an electrochemical workstation in a three-electrode system (platinum electrode as the counter electrode, silver-silver chloride electrode as the reference electrode, and glassy carbon electrode as the working electrode). The CV test results are shown in Figure ₂ , _Bu4NPF6 is tetrabutylammonium hexafluorophosphate and DMF is dimethylformamide.

It can be seen from Figure 2 that after adding water, the catalytic current is significantly enhanced, and the strength is enhanced by 10-20 times. It can be seen that the mononuclear cobalt complex has electrocatalytic water oxidation and oxygen production activity.

Application Test 2

The mononuclear cobalt complex prepared in the above Example 1 was used for electrocatalytic oxidation of water to prepare oxygen. The test method was as follows: 25 μmol of mononuclear cobalt complex was added to water (50 mL), and phosphoric acid was used to adjust the pH value of the mixed solution so that The pH values of the solution were 3, 4, 5, 6, and 7, respectively. Electrochemical work was carried out under a three-electrode system (platinum electrode as the counter electrode, silver silver chloride electrode as the reference electrode, and glassy carbon electrode as the working electrode) with an electrochemical workstation. Catalytic water oxidation test (cyclic voltammetry test CV and controlled potential electrolysis CPE), the oxygen generated by the reaction is quantitatively analyzed by gas chromatography, and the test results are shown in Figure 3 and Figure 4.

It can be seen from Figure 3 that the electrocatalytic effect of the mononuclear cobalt complex in the pure water system is better than that of the organic system. After adding the mononuclear cobalt complex, the catalytic current can reach hundreds of times that without the complex. The potential is very low, calculated to be only 360mV at pH 6 and 390mV at pH 7. In addition, it can be seen from FIG. 4 that the oxygen production in 1 hour reaches 32 μmol, and the TON (conversion number: the number of moles of reactants converted/the number of moles of catalyst) for one hour reaches 5.

Comparative Example 1

Preparation of CoH ^βF CX using literature Dilek K. Dogutan, Robert McGuire, Jr., and Daniel G. Nocera. Electocatalytic Water Oxidation by Cobalt (III) Hangman β-OctafluoroCorroles. J. Am. Chem. Soc. 2011, 133, 9178-9180 -CO ₂ H (H ^βF CX=β-octafluorohangmancorrol), then CoH ^βF CX-CO ₂ H was plated on the FTO surface using Nafion in pure water system, pH was adjusted to 7 with phosphoric acid, at 1.4 V (relative to silver chloride) The catalytic current is only 1/4 of the mononuclear cobalt complex of the present application, and the overpotential is as high as 600mV. In this comparative example 1, when the reaction system was adjusted to be acidic, no aerobic generation was detected at 1.4V.

Comparative Example 2

Utilize literature Dong Wang and John T.Groves.Efficient water oxidationcatalyzed by homogeneous cationic cobalt porphyrins with critical roles for the buffer base.Proc.Natl.Acad.Sci USA2013,110,15579-15584, prepare Co ^II TDMImP (TDMImP is 5,10 , 15,20-tetra-(1,3-dimethylimidazol-2-yl) porphyrin), the Co ^II TDMImP is placed in the pure water system, and the pH is adjusted to 7 with phosphoric acid, and the concentration is the concentration of application example 2. Twice, at 1.4V (relative to silver silver chloride electrode), the catalytic current is only 1/8-1/9 of the mononuclear cobalt complex of the present application, and the overpotential is 580mV. In this comparative example 2, when the reaction system was adjusted to be acidic, no aerobic generation was detected at 1.4V.

Since water oxidation is a deprotonation reaction, the higher the proton concentration, the less favorable it is for water oxidation. Therefore, under acidic conditions, the difficulty of water oxidation increases, and as the acidity increases, water oxidation tends to be more difficult. The single-core cobalt complex in the application extends water oxidation to an acidic environment, extends the use range of metal complex catalysts to acidic conditions, increases the optional range of oxygen evolution environment, and makes oxygen evolution environment more flexible.

Claims (8)

1. A mononuclear cobalt complex characterized by: the chemical formula is Co (bipyalk) and (OAc)₂The structural formula is as follows:

2. the method for preparing the mononuclear cobalt complex as claimed in claim 1, wherein the mononuclear cobalt complex is obtained by dissolving cobalt acetate tetrahydrate and bipyalk in ethanol, stirring and reacting for 6-7 hours at 50-60 ℃ in a dark environment, taking out and carrying out rotary evaporation, adding n-hexane, and crystallizing at-40 to-50 ℃ in a dark environment.

3. The method of claim 2, wherein the molar ratio of cobalt acetate tetrahydrate to bipyalk is 1.0 to 1.4: 1.

4. the preparation method according to claim 2, wherein the molar ratio of ethanol to bipyalk is 500-700: 1.

5. the method according to claim 2, wherein the molar ratio of n-hexane to bipyalk is 40 to 60: 1.

6. use of the mononuclear cobalt complex according to claim 1 as an oxidation agent for water in electrocatalytic oxidation of water, characterized in that the mononuclear cobalt complex is capable of acting as an oxidation agent for water in aqueous or water-containing organic solution.

7. Use according to claim 6, wherein the aqueous solution has a pH of 3 to 7.

8. Use according to claim 7, characterized in that the pH of the aqueous solution is adjusted with phosphoric acid or boric acid.

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