JP2024125852A - Metal complex, method for producing metal complex, and water oxidation catalyst - Google Patents

Abstract

The present invention provides a metal complex that is stable and can be isolated, a water oxidation catalyst comprising the metal complex, and a method for producing the metal complex. The present invention provides a metal complex represented by the following formula (1), in which the N (nitrogen) of pyridine is coordinated to M3+. [Chemical formula 1] TIFF2024125852000061.tif41170 [Selected figure] None

Description

Application for application of Article 30, Paragraph 2 of the Patent Act has been made (1) Date of issue (publication date) September 5, 2022 Publication Abstracts of the 72nd Symposium of the Society of Coordination Chemistry Published by the Society of Coordination Chemistry *Open to the public via a dedicated address for participants (only available to participants) (Publication address URL: http://www.chem.okayama-u.ac.jp/▲～▼reg/jscc72/pdf/2PA-53.pdf) <Materials> Abstracts of the 72nd Symposium of the Society of Coordination Chemistry Abstracts of published research papers (2) Date of holding (publication date) September 27, 2022 (Dates: September 26-28, 2022) Name and location of the meeting 72nd Symposium of the Society of Coordination Chemistry Coordination Chemistry Society Sponsored by Kyushu University Ina Campus (744 Motooka, Nishi-ku, Fukuoka City, Fukuoka Prefecture) Poster presentation Room 2202 <Materials> The 72nd Symposium of the Society of Coordination Chemistry Event Summary <Materials> The 72nd Symposium of the Society of Coordination Chemistry Program <Materials> The 72nd Symposium of the Society of Coordination Chemistry Research Presentation Poster

The present invention relates to a metal complex, a method for producing the metal complex, and a water oxidation catalyst comprising the metal complex.

It is important to clarify the water oxidation process in order to elucidate the reaction mechanism of photosynthesis using model compounds and to develop oxidation reactions of organic matter using model compounds as catalysts. _Two types of Ru ^III -OH have been identified as intermediates in catalytic water oxidation reactions (see, for example, Non-Patent Document 1).

M. D. Karkas et al. , Inorg. Chem. 57, 10881-10895 (2018).

In order to construct an efficient catalytic reaction system, it is important to isolate the intermediate and investigate its properties, but the Ru ^III -OH _two species are generally unstable, making isolation difficult.

The present invention has been made in view of the above circumstances, and aims to provide a stable and isolable metal complex, a method for producing the metal complex, and a water oxidation catalyst comprising the metal complex.

The present invention has the following aspects.
[1] A metal complex represented by the following formula (1), in which N (nitrogen) of pyridine is coordinated to M3 ⁺ .

（上記式（１）中、Ｒ^１は下記式（２）～下記式（５）で表され、Ｍは三価の金属であり、Ｌは下記式（６）～下記式（１５）で表されるピリジンまたはピリジン置換体である。Ｒ^１に含まれる窒素の１か所でＭ^３＋に配位する。Ｒ^１が２つあるため、ピリジンの窒素と併せて計３か所の窒素でＭ^３＋に配位する。Ｍ^３＋に配位する３つの窒素はすべて同一面上にある。）

(In the above formula (1), R ¹ is represented by the following formulas (2) to (5), M is a trivalent metal, and L is a pyridine or a substituted pyridine represented by the following formulas (6) to (15). One nitrogen atom contained in R ¹ coordinates to M ³⁺ . Since there are two ^{R 1s} , a total of three nitrogen atoms, including the pyridine nitrogen atom, coordinate to M ^{3+. All three nitrogen atoms coordinate to M 3+ are} on the same plane.)

（上記式（２）中、符号αで示す窒素が上記式（１）中のＭ^３＋に配位し、＊は結合手を示す。）

(In the above formula (2), nitrogen represented by the symbol α is coordinated to ^M in the above formula (1), and * represents a bond.)

（上記式（３）中、＊は結合手を示す。）

(In the above formula (3), * indicates a bond.)

（上記式（４）中、＊は結合手を示す。）

(In the above formula (4), * indicates a bond.)

（上記式（５）中、符号αで示す窒素が上記式（１）中のＭ^３＋に配位し、＊は結合手を示す。）

(In the above formula (5), nitrogen represented by the symbol α is coordinated to ^M in the above formula (1), and * represents a bond.)

[2] A water oxidation catalyst consisting of the metal complex described in [1].

[3] a first step of obtaining a compound A represented by the following formula (16);
A second step of dissolving the compound A and a trivalent metal salt in a solution containing alcohol to obtain a solution B;
a third step of adding pyridine or a pyridine substitute to the solution B to obtain a solution C, and heating the solution C;
The present invention relates to a method for producing a metal complex comprising the steps of:

（上記式（１６）中、Ｒ^１は下記式（２）～下記式（５）で表される。）

(In the above formula (16), R ¹ is represented by the following formulas (2) to (5).)

（上記式（２）中、＊は結合手を示す。）

(In the above formula (2), * indicates a bond.)

（上記式（３）中、＊は結合手を示す。）

(In the above formula (3), * indicates a bond.)

（上記式（４）中、＊は結合手を示す。）

(In the above formula (4), * indicates a bond.)

（上記式（５）中、＊は結合手を示す。）

(In the above formula (5), * indicates a bond.)

The present invention provides a stable and isolable metal complex, a method for producing the metal complex, and a water oxidation catalyst comprising the metal complex.

FIG. 2 is a diagram showing the molecular structure of a metal complex (L=pyridine) obtained in an example. FIG. 13 is a diagram showing the results of measuring the absorption spectrum of a [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex (L=pyridine) depending on the pH change. FIG. 3 is a graph showing the relationship between pH and absorbance at a wavelength of 464 nm in FIG. 2.

An embodiment of the metal complex, the method for producing the metal complex, and the water oxidation catalyst comprising the metal complex of the present invention will be described.
It should be noted that the present embodiment is specifically described to allow a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified.

[Metal complexes]
A metal complex according to one embodiment of the present invention is represented by the following formula (1). The charge of the metal complex according to this embodiment is +1. The metal ion (M) has a valence of +3, and the ligand R1 dissociates and coordinates with two hydrogen ions (H ⁺ ) when the N-H at the ¹ site coordinates with the metal, so the ligand bound to the metal formally has a valence of -2. Therefore, the metal complex has a valence of +1.

（上記式（１）中、Ｒ^１は下記式（２）～下記式（５）で表され、Ｍは三価の金属であり、Ｌは下記式（６）～下記式（１５）で表されるピリジンまたはピリジン置換体である。Ｒ^１に含まれる窒素の１か所でＭ^３＋に配位する。Ｒ^１が２つあるため、ピリジンの窒素と併せて計３か所の窒素でＭ^３＋に配位する。Ｍ^３＋に配位する３つの窒素はすべて同一面上にある。）

(In the above formula (1), R ¹ is represented by the following formulas (2) to (5), M is a trivalent metal, and L is a pyridine or a substituted pyridine represented by the following formulas (6) to (15). One nitrogen atom contained in R ¹ coordinates to M ³⁺ . Since there are two ^{R 1s} , a total of three nitrogen atoms, including the pyridine nitrogen atom, coordinate to M ^{3+. All three nitrogen atoms coordinate to M 3+ are} on the same plane.)

（上記式（２）中、符号αで示す窒素が上記式（１）中のＭ^３＋に配位し、＊は結合手を示す。）

(In the above formula (2), nitrogen represented by the symbol α is coordinated to ^M in the above formula (1), and * represents a bond.)

（上記式（３）中、＊は結合手を示す。）

(In the above formula (3), * indicates a bond.)

（上記式（４）中、＊は結合手を示す。）

(In the above formula (4), * indicates a bond.)

（上記式（５）中、符号αで示す窒素が上記式（１）中のＭ^３＋に配位し、＊は結合手を示す。）

(In the above formula (5), nitrogen represented by the symbol α is coordinated to ^M in the above formula (1), and * represents a bond.)

In the above formula (1), pyridine and two R ^1s are on the same plane. In addition, in the above formula (1), when R ¹ is a compound represented by the above formula (2) to formula (5), the structure composed of pyridine, heterocycles contained in two R ^1s , and pyridine is rigid, and the heterocycles cannot freely rotate with respect to the pyridine in R ¹ .

Examples of trivalent metals represented by M in the above formula (1) include ruthenium (Ru), iron (Fe), Os (osmium), cobalt (Co), rhodium (Rh), iridium (Ir), etc.

In the above formula (1), L is preferably a pyridine substitute such as pyridine represented by the above formula (6), 4-methylpyridine (4-picoline) represented by the above formula (7), or 4-tert-butylpyridine represented by the above formula (8).

A specific example of the metal complex represented by the above formula (1) is the [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex represented by the following formula (17). In the [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex, two H ⁺ are eliminated from the H ₂ pip represented by the following formula (18), and the three nitrogens of the pip (the nitrogen of the pyridine in the above formula (1), the two nitrogens on the pyridine side of R ¹ in the above formula (1)) are bonded to the trivalent metal Ru, and the two nitrogens of the pip (the two nitrogens located away from the pyridine in the above formula (1)) are hydrogen-bonded to the water (H ₂ O) bonded to Ru. This maintains the Ru(III)-OH ₂ bond. Note that when R ¹ is the above formula (3), oxygen is hydrogen-bonded to the water (H 2 O) bonded to Ru, and when R ¹ is the above formula (4), sulfur is hydrogen-bonded to the water (H ₂ O) bonded to Ru. This maintains the Ru(III)-OH ₂ bond.

The metal complex of this embodiment can provide a stable metal complex that can be isolated.

[Water oxidation catalyst]
A water oxidation catalyst according to one embodiment of the present invention comprises a metal complex according to one embodiment of the present invention.

The water oxidation catalyst of this embodiment allows for efficient water oxidation.

[Metal Complex Production Method]
A method for producing a metal complex according to one embodiment of the present invention is a method for producing a metal complex represented by the above formula (1).
The method for producing a metal complex of the present embodiment includes a first step of obtaining a compound A represented by the following formula (16), a second step of dissolving the compound A and a trivalent metal salt in a solution containing an alcohol to obtain a solution B, and a third step of adding pyridine or a substituted pyridine to the solution B to obtain a solution C, and heating the solution C.

（上記式（１６）中、Ｒ^１は上記式（２）～上記式（５）で表される。）

(In the above formula (16), R ¹ is represented by the above formulas (2) to (5).)

(First step)
In the first step, compound A represented by the above formula (16) is obtained.
When R ¹ is represented by any one of the above formulas (2) to (5), the compound A can be synthesized, for example, through a synthetic route of 8-hydrazinoquinoline → 2,6-diacetylpyridine-bis(8-quinolinylhydrazone) → 2,6-bis(pyrido[3,2-di]ind-2′-yl)pyridine (=compound A when R ¹ is any one of the above formulas (2) to (5)).
Compound A can be synthesized by, for example, the method described in R. A. Taylor et al., Polyhedron 131 34-39 (2017), V. Hegde et al., J. Am. Chem. Soc. 115 872-878 (1993), S. F. Liu et al., J. Am. Chem. soc. 122 3671-3678 (2000), W. L. Jia et al., Organometallics 22 4070-4078 (2003), but is not limited thereto.

(Second step)
In the second step, compound B is reacted with a trivalent metal. First, a metal salt is dissolved in alcohol to obtain solution B. In this case, the method for dissolving the metal salt in alcohol is not particularly limited as long as the metal salt can be dissolved, and for example, heating under reflux can be performed.

Examples of metal salts include halides and nitrates, and specific examples include _RuCl3 , _CoF3 , FeCl3, _{FeBr3 ,} Fe( _NO3 ) ₃ , _OsCl3 , _RhCl3 , Rh( _NO3 ) ₃ , _IrCl3 , and _IrBr3 .

Examples of alcohols include methanol and ethanol.

The concentration of solution B is preferably 0.005 mol/L to 0.05 mol/L.

Next, compound A synthesized in the first step is dissolved in solvent A containing alcohol to obtain solution A.

Solvent A may contain alcohol, but it is more preferable to mix dimethylformamide from the viewpoint of improving yield. When mixing dimethylformamide, the mixing ratio of dimethylformamide to alcohol is preferably 2/10 or more and 8/10 or less by volume.

It is preferable to use the same alcohol as the solvent for solution B.

The concentration of solution A is preferably 0.0015 mol/L to 0.02 mol/L.

Next, solution A and solution B are mixed to obtain solution C. The method and time for mixing solution A and solution B are not particularly limited as long as solution A and solution B become uniform, but heating to reflux is preferable. When heating to reflux, the heating to reflux time is also not particularly limited as long as solution A and solution B can be mixed uniformly, but heating to reflux for about 3 to 12 hours is preferable. The mixing ratio of solution A and solution B should be such that the amount of metal salt contained in solution A and the amount of compound B contained in solution B are approximately the same in molar terms, but preferably the difference between the amount of metal salt and the amount of compound B is within 20%.

(Third process)
In the third step, pyridine or a substituted pyridine is further reacted to obtain a metal complex represented by the above formula (1). First, pyridine or a substituted pyridine is dissolved in water to obtain a solution D. The mixing ratio of pyridine or a substituted pyridine to water is preferably 0.1/8 or more and 1/8 or less in volume ratio.

Examples of pyridine substitution compounds include compounds represented by the above formulas (7) to (15).

Next, solution D is added to solution C to obtain solution E, and solution E is heated to reflux. The mixing ratio of solutions C and D should be such that the amount of pyridine or pyridine substitute contained in solution D is approximately twice the amount of metal contained in solution C, but it is preferable that the amount of pyridine or pyridine substitute is 1.5 to 3 times the amount of metal in molar terms.

The time for refluxing solution E is not particularly limited, but is, for example, 12 hours or more and 40 hours or less.

The metal complex represented by the above formula (1) is obtained by the above steps 1 to 3.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples.

[Example]
H ₂ pip represented by the above formula (18) was synthesized by the following synthesis method (first step).
Hydrazine monohydrate (24 mL) and 8-quinolinol (3.06 g, 21.1 mmol) were added to a three-neck flask protected from light and under a nitrogen atmosphere. The mixture was heated to reflux at 130° C. for 5 days. After cooling, the mixture was left in a refrigerator overnight. The precipitated solid was filtered by suction, washed with a small amount of cold water, and dried overnight in a desiccator to obtain 2.70 g (16.9 mmol) of 8-hydrazinoquinoline as a yellow needle-like solid. The yield was 81%.
Next, in a three-neck flask under a nitrogen atmosphere, 2,6-diacetylpyridine (219.2 mg, 2.72 mmol) and 8-hydrazinoquinoline (427.1 mg, 2.69 mmol) were dissolved in ethanol (25 ml). The mixture was heated to reflux at 90°C for 2 hours. After cooling, the mixture was left in a refrigerator overnight. The precipitated solid was filtered by suction, washed with ethanol, and dried under reduced pressure in a desiccator overnight to obtain 442.0 mg (0.99 mmol) of 2,6-diacetylpyridine-bis(8-quinolinylhydrazone) as a yellow solid. The yield was 37%.
Next, 2,6-diacetylpyridine-bis(8-quinolinylhydrazone) (202.7 mg, 0.455 mmol) and polyphosphoric acid (3.0 g) were added to a beaker, and the mixture was heated and stirred at 160° C. for 12 hours using a hot stirrer. After cooling, an aqueous sodium hydroxide solution was added dropwise while stirring until the pH reached 7. The mixture was extracted three times with dichloromethane (150 mL), washed three times with water (75 mL), and dehydrated with sodium sulfate (anhydrous). The mixture was dried in a rotary evaporator and dried under reduced pressure in a desiccator to obtain 154.3 mg (0.376 mmol) of 2,6-bis(pyrido[3,2-di]indo-2'-yl)pyridine as a yellow solid. The yield was 83%. The yellow solid obtained was confirmed to be 2,6-bis(pyrido[3,2-di]indo-2'-yl)pyridine (H ₂ pip) by TOF-MS (time-of-flight mass spectrometer ESI-TOF/microOTOF (manufactured by Bruker Daltonics)) and ¹ H-NMR (NMR: nuclear magnetic resonance method, AL300 type nuclear magnetic resonance apparatus (manufactured by JEOL)).

In the second step, H ₂ pip was reacted with Ru ³⁺ as follows.
Ruthenium chloride trihydrate (31.4 mg, 0.109 mmol) and ethanol (6 mL) were added to a flask, and the mixture was heated to reflux at 100° C. to obtain solution B.
Solution A was prepared by dissolving H ₂ pip (42.6 mg, 0.104 mmol) in ethanol (10 mL) and dimethylformamide (5 mL).

Solution A was added dropwise to solution B, and the mixture was heated under reflux for 6 hours to obtain solution C.

In the third step, the product was further reacted with pyridine as follows.
Pyridine (0.2 mL) was dissolved in water (4 mL) to obtain solution D.
Solution D was added to solution C to obtain solution E, which was then heated to reflux for 24 hours.

The resulting product was allowed to cool, and impurities were removed by suction filtration. Potassium hexafluorophosphate (55.2 mg, 3 equivalents) was then added. After cooling, water was added to precipitate a green solid, which was then suction filtered, washed with water and diethyl ether, and dried overnight under reduced pressure in a desiccator. The resulting crude crystals were dissolved in acetone, and purified using silica gel column chromatography to recover [Ru(pip)(py) ₂ ( _OH2 )( _PF6 )(L=pyridine) as a green solid.
A single crystal was obtained from the recovered product by a vapor diffusion method, and X-ray diffraction (XRD, Saturn-CCD diffractometer, manufactured by Rigaku Corporation) was performed.
As a result, the obtained product had the molecular structure shown in FIG. 1, and was confirmed to be a [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex (L=pyridine) represented by the above formula (17).

The yield of the product obtained was 42%.
Moreover, by using the same method but replacing pyridine with 4-picoline represented by the above formula (7), a [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex (L=4-picoline) was obtained in a yield of 50%.
Furthermore, by using the same method but replacing pyridine with 4-tert-butylpyridine represented by the above formula (8), a [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex (L=4-tert-butylpyridine) was obtained in a yield of 49%.
Furthermore, it was confirmed that the obtained products were the desired complexes by TOF-MS and FT-IR (infrared absorption analysis, Fourier transform infrared spectrophotometer FT/IR-4100 (manufactured by JASCO Corporation)).

(evaluation)
The acid dissociation constant (pK _a ) of the [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex obtained in Example 1 was measured.
The acid dissociation constant was measured by changing the pH of the solution and measuring the absorbance with a spectrophotometer. The spectrophotometer used was a UV-Visible Spectrophotometer V-560DS (manufactured by JASCO Corporation), with a sample concentration of 0.05 mmol, acetone as the solvent, and a quartz cell.
The deprotonation reaction that occurs when a base (for example, a saturated aqueous sodium hydroxide solution) is added to a [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex is shown in the following formula (19). In the following formula (19), Ru ^III -OH ₂ is a [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex.

The results of measurement of the absorbance etc. of the [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex (L=pyridine) are shown in FIG. 2 and FIG.
In the above formula (19), it can be said that the smaller the K _a is, that is, the more difficult it is for the reaction to proceed from the left side to the right side, the more stable the [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex is.
From the results shown in Figures 2 and 3, it was found that the absorbance at a wavelength of 464 nm of the [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex (L = pyridine) increases as the pH increases. It was also found that the acid dissociation constant (pK _a ) of the [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex (L = pyridine) is at least 13 or more. It was also found that the acid dissociation constant (pK _a ) when L is a pyridine substitute is approximately the same as when L is pyridine. It can be said that the larger the acid dissociation constant (pK _a ), the more stable the [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex is. For example, the acid dissociation constant (pK _a ) of Ru ^III (iba)(4-pic) ₂ (OH ₂ ), i.e., the complex represented by the following formula (20), is 5.6, and therefore it has been found that the [Ru(pip)(OH ₂ )L ₂ ] ⁺ complex is considerably more stable than conventional complexes. Thus, the metal complex of the present invention is stable and is useful in the investigation of water oxidation processes and as a water oxidation catalyst.

Claims (3)

A metal complex represented by the following formula (1), in which N (nitrogen) of pyridine is coordinated to M3 ⁺ .

(In the above formula (1), R ¹ is represented by the following formulas (2) to (5), M is a trivalent metal, and L is a pyridine or a substituted pyridine represented by the following formulas (6) to (15). One nitrogen atom contained in R ¹ coordinates to M ³⁺ . Since there are two ^{R 1s} , a total of three nitrogen atoms, including the pyridine nitrogen atom, coordinate to M ^{3+. All three nitrogen atoms coordinate to M 3+ are} on the same plane.)

(In the above formula (2), nitrogen represented by the symbol α is coordinated to ^M in the above formula (1), and * represents a bond.)

(In the above formula (3), * indicates a bond.)

(In the above formula (4), * indicates a bond.)

(In the above formula (5), nitrogen represented by the symbol α is coordinated to ^M in the above formula (1), and * represents a bond.)

A water oxidation catalyst comprising the metal complex according to claim 1. A first step of obtaining a compound A represented by the following formula (16);
A second step of dissolving the compound A and a trivalent metal salt in a solution containing alcohol to obtain a solution B;
a third step of adding pyridine or a pyridine substitute to the solution B to obtain a solution C, and heating the solution C;
The present invention relates to a method for producing a metal complex comprising the steps of:

(In the above formula (16), R ¹ is represented by the following formulas (2) to (5).)

(In the above formula (2), * indicates a bond.)

(In the above formula (3), * indicates a bond.)

(In the above formula (4), * indicates a bond.)

(In the above formula (5), * indicates a bond.)

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