Novel Rh complex, catalyst for acetic acid synthesis containing said Rh complex, and method for synthesizing acetic acid

Abstract

To provide a novel noble metal complex that can allow an acetic acid synthesis reaction or the like involving carbonylation of methanol to proceed even when carbon dioxide is adopted as a constituent of the reaction system.SOLUTION: Provided is an Rh complex in which a quarter pyridine ligand and two halogens A and B are coordinated to Rh. The Rh complex can accept electrons in water and be reduced to form an electron-donating intermediate complex, and further can impart new activity to the reaction system constituent substance. This enables acetic acid synthesis by carbonylation of carbon dioxide.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a novel Rh complex. The present invention also relates to a catalyst for carbonylating methanol with carbon dioxide to produce acetic acid, to which the Rh complex is applied, and a method for synthesizing acetic acid. [Background technology]

[0002] The Monsanto process is a well-known method for synthesizing acetic acid (CH3COOH). The Monsanto process is a method in which methanol (CH3OH) is reacted with carbon monoxide (CO) in the presence of a catalyst to carbonylate methanol and synthesize acetic acid. The catalyst used in this synthesis reaction is the noble metal rhodium (Rh) catalyst, and the Rh complex in which a carbonyl ligand is coordinated to rhodium ([Rh(CO)2I2] - ) is known.

[0003] Since its discovery, the Monsanto process has undergone improvements in reaction conditions (Patent Document 1, etc.) and has become one of the mainstream industrial synthesis processes for acetic acid, but the fact that carbon monoxide is essential for the carbonylation of methanol remains unchanged. In this synthesis process, an equal or greater amount of carbon monoxide is required for the amount of acetic acid produced. Therefore, it is necessary to install carbon monoxide generating equipment in addition to carbonylation equipment in an acetic acid production plant. In addition, carbon monoxide is widely known to be toxic, and it is necessary to manage it with the utmost care.

[0004] Therefore, some studies have been conducted on the use of carbon dioxide (CO2) for the carbonylation of methanol. Carbon dioxide is less toxic than carbon monoxide, and the management burden is small. In addition, chemical plants and the like often have equipment installed to capture carbon dioxide from exhaust gases from boilers and other equipment used for heat and power generation. If this carbon dioxide capture equipment could be used as a source of carbon dioxide for carbonylation, it would lead to cost reductions in acetic acid production facilities.

[0005] As in the above example of acetic acid synthesis, promoting the use of carbon dioxide in the field of organic synthesis can also help solve recent environmental problems. That is, the greenhouse effect of carbon dioxide in exhaust gases from the combustion of fossil fuels has long been pointed out. To address this issue, measures to reduce greenhouse gas emissions have been taken worldwide. As part of these measures, for example, efforts have been made in recent years to capture and store carbon dioxide (CCS: Carbon dioxide Capture and Sequestration). If the carbon dioxide stored in this way can be widely used in organic synthesis, a cycle of carbon dioxide emission, capture, and consumption can be formed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 7-25814 Summary of the Invention [Problem to be solved by the invention]

[0007] However, it is not easy to change the composition of a reaction system that has been established so far, such as the carbonylation of methanol in the Monsanto process. Regarding the synthesis reaction of acetic acid by the carbonylation of methanol shown above, the above-mentioned Rh complex ([Rh(CO)2I2] - ), the synthesis reaction can proceed only when there is carbon monoxide in the reaction system, and this catalyst cannot cause a reaction with carbon dioxide to proceed.

[0008] The present invention has been made under the above-mentioned background, and aims to provide a new noble metal complex that can proceed with the synthesis reaction of acetic acid, including the carbonylation of methanol, even when carbon dioxide is used as a component of the reaction system, and to present the possibility of expanding the scope of use of carbon dioxide in the field of organic synthesis, which has become clear from the investigation. [Means for solving the problem]

[0009] The present inventors have conducted extensive research and found that a novel Rh complex having Rh as the central metal and quaterpyridine ((2,2':6',2'':6'',2''''-quaterpyridine (qpy)) as the ligand is a new precious metal complex that can exhibit activity toward carbon dioxide. They then investigated the catalytic action of this Rh complex and arrived at the present invention.

[0010] The present invention, which solves the above-mentioned problems, provides an Rh complex represented by the following structural formula, in which quaterpyridine or a derivative thereof and two halogens A and B are coordinated to Rh.

[0011] [ka] (In the above formula, the substituents X1, X2, X3, X4, X5, X6, and X7 of the quaterpyridine are each any one of hydrogen, halogen, methyl group, methoxy group, amino group, and dimethylamino group. Furthermore, the halogens A and B are any one of chlorine, bromine, and iodine. A and B may be the same halogen or different halogens. Y - is the counter anion.)

[0012] As described later, the Rh complex according to the present invention undergoes a catalytic cycle in which the central metal Rh accepts electrons from hydrogen (H2) in water to transform into an intermediate complex (complex 2), while donating electrons to the components of the reaction system to regenerate into the initial complex. At this stage, it has been confirmed that this catalytic cycle acts on the reaction of carbonylating methanol with carbon dioxide to synthesize acetic acid. The composition of the intermediate in this reaction cycle has also been clarified. The Rh complex according to the present invention, the catalyst for acetic acid synthesis to which this Rh complex is applied, and the process of acetic acid synthesis will be described below.

[0013] (I) Rh complex according to the present invention The Rh complex according to the present invention is a rhodium (Rh 3+In the Rh complex according to the present invention, the qpy ligand is a complex in which a quaterpyridine (hereinafter, this ligand may be referred to as a qpy ligand) is coordinated to the Rh central metal (Rh 3+ ) accepts (stores) electrons from hydrogen and becomes highly active +1-valent Rh (Rh + ) can facilitate the formation of an intermediate complex. The qpy ligand, which has electron-accepting properties, disperses and stabilizes the negative charge of the Rh central metal, which has electron-storing properties. This reaction proceeds favorably by using water as a solvent. In addition, the qpy ligand has a distorted planar tetracoordinate structure as a structural feature, which can provide the Rh central metal with a reaction field with the reactants. The intermediate complex thus formed acts as an electron donor, and the desired reaction proceeds through the insertion and removal of the reactants.

[0014] The qpy ligand of the Rh complex according to the present invention is quaterpyridine or a derivative thereof. The substituents X1 to X7 of the qpy ligand in the above formula are each hydrogen, halogen, a methyl group, a methoxy group, an amine, or a dimethylamino group. The reason for being limited to these elements and substituents is that the introduction of these elements and substituents is expected to have the effect of appropriately adjusting the stability, catalytic activity, solubility in a solvent, etc. of the Rh complex.

[0015] Preferably, all of X1 to X7 in the qpy ligand are hydrogen. In another preferred embodiment, any one of X1 to X7 is a halogen, a methyl group, a methoxy group, an amino group, or a dimethylamino group, and the others are hydrogen. The reason why these embodiments are preferred is that they are relatively easy to synthesize, and can provide functionality by introducing a substituent, resulting in a complex with excellent practicality.

[0016] In addition, the Rh complex according to the present invention has halogen atoms A and B coordinated with the qpy ligand. The halogen is any one of chlorine, bromine, and iodine. In the Rh complex according to the present invention, it is desirable to adopt a halogen that is highly reactive with hydrogen and promotes the progress of the catalytic cycle, and that easily regenerates the starting material after the catalytic cycle is completed. In this case, A and B may be the same halogen or different halogens.

[0017] (II) Catalyst for acetic acid synthesis containing the Rh complex according to the present invention As described above, the Rh complex according to the present invention is useful as a catalyst for synthesizing acetic acid by carbonylating methanol with carbon dioxide. The Rh complex according to the present invention exhibits the following catalytic cycle in the process of acetic acid synthesis. Each step in this catalytic cycle will be described below.

[0018] In the following description, the counter anion of the Rh complex is omitted. In the acetic acid synthesis process, methyl iodide (CH3I) may be added as a reaction promoter together with methanol. In addition, iodine compounds or metal iodides may be added to generate methyl iodide. In the description here, it is assumed that methyl iodide is generated in-situ in the reaction system by any of the reaction promoters.

[0019] [ka]

[0020] In the above catalytic cycle, the Rh ([Rh III (qpy)Cl2] + ) is "complex 1". Complex 1 reacts with hydrogen in the solvent water, reducing rhodium (Rh 3+ →Rh +1 ) and low-valent complexes ([Rh I (qpy)] + ) (this will be called "complex 2"). In this reaction, complex 1 accepts two hydrogen electrons to form the lower valent complex 2.

[0021] Complex 2 reacts with methyl iodide generated in situ in the reaction system to give the methyl iodide complex ([Rh III (qpy)(CH3)(I)] + ) is produced (hereinafter referred to as "complex 3"). This reaction means that the two electrons that complex I received from water (hydrogen) are transferred to methyl iodide via Rh. Note that complex 2 and complex 3 are novel Rh complexes, just like complex 1.

[0022] Then, through an insertion reaction with complex 3, carbon dioxide is inserted between the rhodium and methyl group (Rh-C bond) of ligand 3. Furthermore, by supplying hydrochloric acid, CH3COOI is eliminated from the complex and protonated to generate acetic acid (CH3COOH). At the same time, complex 3 is regenerated into complex 1.

[0023] Through the above catalytic cycle, methanol (methyl iodide produced from methanol) is carbonylated with carbon dioxide to synthesize acetic acid. This series of catalytic cycle reactions is a reaction in which the Rh complex according to the present invention (complex 1) transfers electrons absorbed and stored from hydrogen in water to methyl iodide and carbon dioxide.

[0024] The catalyst according to the present invention is in a solid state at room temperature and pressure, and is water-soluble. The catalyst according to the present invention acts as a homogeneous catalyst in a reaction system using water as a solvent. This is a common characteristic as long as the substituents X1 to X7 of the qpy ligand of the Rh complex are within the above-mentioned range. - ) is not particularly limited, but chloride ions (Cl) are preferred because of their small size and chemical stability. - ), bromide ion (Br - ), iodine ion (I - ), nitrate ion (NO3 - ) is preferred.

[0025] The catalyst according to the present invention is not limited in shape (powder, granules, pellets, etc.) or particle size. In addition, since the catalyst is water-soluble as described above, it may be dissolved in a suitable solvent such as water. In this case, the concentration of the complex in the solution is not particularly limited.

[0026] (III) Method for synthesizing acetic acid using the catalyst according to the present invention In the method for synthesizing acetic acid using the catalyst according to the present invention, the carbonylation reaction of methanol is carried out with carbon dioxide. As described above, the catalyst according to the present invention is a water-soluble homogeneous catalyst, and reacts with water to generate an intermediate complex (complex 2), which activates methyl iodide.

[0027] That is, the method for synthesizing acetic acid according to the present invention is a method for synthesizing acetic acid, which comprises a step of carbonylating methanol with carbon dioxide in a reaction system containing methanol, carbon dioxide, and water, and the reaction proceeds in the presence of the catalyst for synthesizing acetic acid according to the present invention.

[0028] As described above, in the method for synthesizing acetic acid according to the present invention, water is used as a solvent, to which methanol, carbon dioxide and a catalyst are added to form a reaction system. In the conventional Monsanto process, an organic solvent containing a carbonyl group, such as acetic acid, carboxylic acid such as methyl acetate, or carboxylate, is used. In the present invention, the synthesis reaction can be carried out using a solvent mainly composed of water, and acetic acid can be synthesized in a clean environment. However, the solvent may be water that partially contains the organic solvent.

[0029] It is also preferable to add hydrogen to the reaction system. Since the Rh complex of the present invention reacts with hydrogen in water to form a low-valent complex (complex 2), it is preferable to supply hydrogen to the reaction system in order to efficiently proceed with the reaction.

[0030] Furthermore, in the present invention, in order to smoothly start the above-mentioned catalytic cycle, it is preferable to add an iodine compound such as an alkyl iodide (e.g., methyl iodide, ethyl iodide, propyl iodide, etc.) or a metal iodide (e.g., lithium iodide, etc.) to the reaction system as a reaction accelerator.

[0031] The reaction system of the present invention mainly contains a liquid phase formed by mixing water (solvent) and methanol, and further contains a catalyst (Rh) complex and a reaction promoter dissolved therein. The ratio of methanol in the liquid phase before the start of the reaction (charging stage) is preferably 1% by mass or more and 50% by mass or less. Methanol is quickly converted to methyl iodide when the synthesis reaction is started by the reaction promoter. If a large amount of methanol is added to the reaction system, an excessive amount of methyl iodide will be present in the reaction vessel, and there is a concern that iodide will be mixed into acetic acid. In addition, the amount of the Rh complex, which is the catalyst, added is preferably 100 ppm or more and 10,000 ppm or less in terms of rhodium concentration. In addition, the amount of the reaction promoter added is preferably 0.1% by mass or more and 20% by mass or less based on the mass of the compound added relative to the liquid phase mass.

[0032] As reaction conditions in the present invention, the hydrogen partial pressure is preferably 0.1 MPa or more and 10 MPa or less. Also, the carbon dioxide partial pressure is preferably 0.1 MPa or more and 10 MPa or less. The total pressure in the entire reaction system (inside the reactor) is preferably 1 MPa or more and 10 MPa or less. In the present invention, the synthesis reaction can be carried out under relatively low pressure conditions compared to the conventional Monsanto process.

[0033] The reaction temperature is preferably 20°C or higher and 100°C or lower. In the conventional Monsanto process, carbon monoxide carbonylation requires a reaction temperature of more than 150°C. The carbonylation reaction with carbon dioxide in the presence of the catalyst of the present invention can proceed at a lower temperature. Considering this reaction temperature and the above pressure conditions, the method for synthesizing acetic acid according to the present invention has mild reaction conditions at low temperature and low pressure, making it possible to design a low-energy, low-load device. Effect of the Invention

[0034] As described above, the Rh complex coordinated with the quaterpyridine ligand (qpy ligand) according to the present invention has electron accepting properties based on the properties of the ligand and the central metal, and functions as an electron storage body. This allows new activity to be imparted to carbon dioxide and the like. The Rh complex of the present invention also enables the synthesis of acetic acid via the carbonylation of methanol with carbon dioxide, something that has not been clearly reported so far. The present invention is a technology that can promote the use of carbon dioxide in organic synthesis, suggesting a new method for solving environmental problems and economic benefits. [Brief description of the drawings]

[0035] [Figure 1] FIG. 1 is an ORTEP diagram of the Rh complex ([RhIII(qpy)Cl2]Cl: Complex 1) synthesized in this embodiment. [Diagram 2] 1H NMR spectrum of the Rh complex ([RhIII(qpy)Cl2]Cl: Complex 1) synthesized in this embodiment. [Diagram 3] FIG. 2 is an ORTEP diagram of the Rh complex ([RhI(qpy)]Cl: complex 2) synthesized in this embodiment. [Figure 4] 1 shows XPS spectra of the Rh3d region of the Rh complex ([RhI(qpy)]Cl: complex 2) and complex 1 synthesized in this embodiment. [Diagram 5] FIG. 1 is an ORTEP diagram of the methyl iodine complex ([RhIII(qpy)(CH3)(I)]Cl: Complex 3) synthesized in this embodiment. [Figure 6] 1H NMR spectrum of the methyl iodine complex ([RhIII(qpy)(CH3)(I)]Cl: Complex 3) synthesized in this embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Hereinafter, an embodiment of the present invention will be described. In this embodiment, first, a Rh complex (complex 1) was synthesized. Then, the above-mentioned catalytic steps were traced, and the presence of complexes (complex 2, complex 3) formed at each step was confirmed. Then, a synthesis test of acetic acid using carbon dioxide with the catalyst (Rh complex) according to the present invention was carried out. In addition, in the synthesis of complexes 1 to 3, electrospray ionization mass spectrometry (ESI-MS: JEOL Ltd. JMS-T100LC AccuTOF), nuclear magnetic resonance spectrometry ( 1 H NMR: Bruker Vision Avance III 600 FT-NMR spectrometer), X-ray photoelectron spectroscopy (XPS: ULVAC-PHI PHI 5000 VersaProbe II), and organic elemental analysis (Yanaco Technical Science Co., Ltd. Carbon, Hydrogen, and Nitrogen Simultaneous Quantitative Analysis CHN Coder MT-5) were performed as appropriate.

[0037] [Synthesis of Rh complex (complex 1)] 67 mg (0.32 mmol) of rhodium chloride (RhCl3) and 100 mg (0.32 mmol) of qpy (2,2':6',2'':6'',2''-quaterpyridine) were dissolved in ethanol. The ethanol solution was then refluxed for 5 hours to allow the reaction to occur. After the resulting solution was cooled to room temperature, the light brown solid was collected by filtration, washed with diethyl ether and dried. The above procedure produced the Rh complex ([Rh III (qpy)Cl2]Cl: complex 1) was obtained. The yield of this Rh complex based on rhodium chloride was 62%. 1 H NMR (600 MHz, in DMSO-d6, referenced to TMS): δ8.09 (t, 2H, CH), 8.54 (t, 2H, CH), 8.70 (t, 2H, CH), 8.90 (d, 2H, CH), 8.94 (dd, 4H, CH), 9.66 (d, 2H, CH). Anal. Calcd for [Rh III (qpy)Cl2](Cl)·3.5H2O: C 20 H 21Cl3N4O 3.5 Rh: C, 41.23; H, 3.63; N, 9.62%. Found: C, 41.26; H, 3.38; N, 9.61%

[0038] The ORTEP diagram obtained by X-ray crystal structure analysis of the synthesized Rh complex is shown in Figure 1 (counter anion is omitted). 1 The 1 H NMR spectrum shows that the peaks assigned to the qpy ligand were observed in the aromatic region. These results confirmed that the complex 1 with the desired structure had been synthesized.

[0039] [Reproduction test of catalytic cycle)] (i) Complex 1 → Complex 2 The Rh complex synthesized above ([Rh III 20 mg (38.5 μmol) of [Rh(qpy)Cl2]Cl: complex 1) was dissolved in 6 mL of water to prepare an aqueous suspension. This aqueous suspension was then stirred at room temperature in a hydrogen atmosphere (0.6 MPa) for 12 hours to obtain complex 2 ([Rh I (qpy)]Cl) was synthesized. An aqueous solution of NaCF3SO3 (9.9 mg, 57.8 μmol) was added to the reaction solution to replace the counter anion from chlorine to CF3SO3. The precipitate formed was collected by filtration, washed with water, and dried in vacuum to recover the black solid complex 2. The yield of complex 2 based on complex 1 was 72%. ESI-MS (in methanol): m / z 413.0 {[Rh I (qpy)] + , I = 100% in the mass range 200 to 2000}. Anal. Calcd for [Rh I (qpy)](CF3SO3) + 0.2NaCF3SO3: C 21.2 H 14 F 3.6 N4Na 0.2 O 3.6 RhS 1.2 : C, 42.67; H, 2.36; N, 9.39%. Found: C, 42.56; H, 2.40; N, 9.35%.

[0040] Figure 3 shows the ORTEP diagram obtained by X-ray crystal structure analysis of the synthesized Rh complex 2 (counter anion is omitted). Figure 4 shows the XPS spectra in the Rh3d region measured for complexes 1 and 2. The Rh peaks located at 309.2 eV and 313.9 eV in complex 1 shift to 306.8 eV and 311.5 eV in complex 2. This is because the formation of complex 2 from complex 1 causes the central metal Rh to become trivalent (Rh 3+ ) to 1(Rh + ) These results confirmed that the complex 2 with the desired structure was synthesized.

[0041] (ii) Complex 2 → Complex 3 Complex 2 ([Rh I Methyl iodide (12 μL, 193 μmol) was added to 3 mL (6.4 mmol) of an aqueous solution of Rh(qpy)]Cl) under dark conditions. The resulting mixture was stirred for 6 hours, and the resulting yellow precipitate was collected by filtration, washed with water, and dried in vacuum. The collected yellow powder was dissolved in an acetonitrile / DMF mixture (1 / 4), and insoluble matter was removed by filtration. After that, orange crystals formed by recrystallization with diethyl ether were collected by filtration and dried in vacuum to obtain complex 3, which is a methyl iodide complex ([Rh(qpy)]Cl). III (qpy)(CH3)(I)]Cl) was obtained. The yield of this complex 3 based on complex 2 was 32%. 1 H NMR (600 MHz, in DMSO-d6, referenced to TMS): δ0.79 (d, 3H, Rh-CH3), 7.96 (t, 2H, CH), 8.43 (t, 2H, CH), 8.54 (t, 2H, CH), 8.77-8.80 (m, 6H, CH), 9.43 (d, 2H, CH). ESI-MS (in methanol): m / z 554.9 {Rh III (qpy)(CH3) + , I = 100% in the mass range 200 to 2000}. Anal. Calcd for [Rh III(qpy)(CH3)(I)·0.5(CH3)2NCHO: C 22.5 H 20.5 I2N 4.5 O 0.5 Rh: C, 37.60; H, 2.88; N, 8.77%. Found: C, 37.73; H, 2.72; N, 8.53%.

[0042] The ORTEP diagram obtained by X-ray crystal structure analysis of the synthesized Rh complex 3 is shown in Figure 5 (counter anion is omitted). 1 The H NMR spectrum shows a peak at around 0.8 ppm that is attributable to the methyl group bonded to Rh. These results confirmed that the complex 3 with the desired structure had been synthesized.

[0043] (iii) Synthesis of acetic acid by reaction of complex 3 with CO2 1.5 mL of an aqueous solution containing 3.0 mg (5.1 μmol) of the above-synthesized complex 3 was reacted under CO2 atmosphere (0.8 MPa) at 80°C and pH 2.0 for 12 hours with stirring to synthesize acetic acid. After the reaction was completed and cooled to room temperature, the reaction solution was passed through a silica column to remove the complex. The yield of acetic acid obtained by quantitatively analyzing the purified reaction solution for acetic acid using high performance liquid chromatography (HPLC) was 10% based on complex 3.

[0044] From the reproduction tests of the catalytic cycle carried out in the above (i) to (iii), it was possible to confirm the structures of Complex 1 and the intermediate complexes Complex 2 and Complex 3 produced at each stage. It was also confirmed that the catalyst containing the Rh complex of the present invention can exert a series of actions to activate methyl iodide and reduce CO2, thereby synthesizing acetic acid in water.

[0045] [Acetic acid synthesis test] Based on the results of the above-mentioned catalytic cycle reproduction test, we next examined whether the Rh complex was active in a reaction system containing water, methanol, and carbon dioxide, and whether acetic acid could be synthesized.

[0046] Prepare 3.9 mL of a water / methanol mixed solution (mixing ratio 1 / 1) and add the same Rh complex 1 ([Rh III 1.0 mg (1.92 μmol) of (qpy)Cl2]Cl) and 6.7 μL (0.108 μmol) of methyl iodide were dissolved in the mixture. The mixture was stirred at 80°C for 24 hours under a mixed atmosphere of hydrogen and carbon dioxide (H2: 0.15 MPa, CO2: 0.8 MPa). After the reaction was completed, the reaction solution was cooled to room temperature, extracted with diethyl ether, and then concentrated.

[0047] The resulting extract was used for quantitative analysis of acetic acid by HPLC. The maximum catalytic turnover number (TON: number of moles of acetic acid / number of moles of Rh complex) was 1.0, confirming that acetic acid synthesis was possible. [Industrial Applicability]

[0048] The present invention is a novel Rh complex in which a quaterpyridine ligand (qpy ligand) is coordinated to the central metal rhodium. The Rh complex according to the present invention serves as an active catalyst in the synthesis reaction of acetic acid, which involves carbonylating methanol with carbon dioxide. This enables the expansion of the use of carbon dioxide, providing a new solution to environmental problems and economic benefits.

[0049] In addition, the Rh complex according to the present invention can form a suitable electron-donating intermediate complex by accepting electrons and being reduced in water, and can impart new activation to the components of the reaction system. The electron-accepting and electron-storing properties of the Rh complex according to the present invention will encourage the study of new reaction processes in the field of organic synthesis.

Claims

1. A Rh complex represented by the following structural formula, in which quaterpyridine or a derivative thereof and two halogens A and B are coordinated to Rh. 【Chemistry 1】 (In the above formula, X which is a substituent of quaterpyridine １ , X ２ , X ３ , X ４ , X ５ , X ６ , X ７ are each either hydrogen, halogen, methyl group, methoxy group, amino group, or dimethylamino group. Furthermore, halogens A and B are either chlorine, bromine, or iodine. A and B may be the same halogen or different halogens. Y － is the counter anion.)

2. X １ ~X ７ 2. The Rh complex of claim 1, wherein all of are hydrogen.

3. X １ ~X ７ 2. The Rh complex according to claim 1, wherein any one of the above is a halogen element, a methyl group, a methoxy group, an amino group, or a dimethylamino group, and the rest are hydrogen.

4. A catalyst for acetic acid synthesis, comprising the Rh complex according to any one of claims 1 to 3.

5. A method for synthesizing acetic acid, comprising a step of carbonylating methanol with carbon dioxide in a reaction system containing the methanol, carbon dioxide, and water, A method for synthesizing acetic acid, which comprises carrying out a reaction in the presence of the catalyst for synthesizing acetic acid according to claim 4.

6. The reaction system further comprises hydrogen; The hydrogen partial pressure is 0.1 MPa or more and 10 MPa or less, and the carbon dioxide partial pressure is 0.1 MPa or more and 10 MPa or less, 6. The method for synthesizing acetic acid according to claim 5, wherein the reaction is carried out at a reaction temperature of 20° C. or higher and 100° C. or lower.

7. 7. The method for synthesizing acetic acid according to claim 5 or 6, wherein the reaction system further contains an iodine compound or a metal iodide as a reaction accelerator.

Images