Aluminum-salen complexes and their use as catalysts

Abstract

The present invention relates to an optionally substituted aluminum-salen complex having a 3-pyridinium bridging group, and further describes a catalyst system comprising said complex, which is optionally supported, and which contains a bromine salt or an iodine salt as co-catalyst. The catalyst system is used in a process for preparing cyclic carbonates by reacting an epoxy compound with carbon dioxide at appropriate pressure.

Description

Technical Field

[0001] The present invention relates to an optionally substituted aluminum-salen complex and the use of the complex as a catalyst component in the reaction of an epoxy compound with carbon dioxide. Background Art

[0002] Substituted aluminum-salen complexes are known as components of catalytic systems for the conversion of epoxides with carbon dioxide. This reaction can be used to prepare cyclic carbonates or as a copolymerization process for the preparation of aliphatic polycarbonates or polyesters.

[0003] WO2010 / 106324 describes a process for preparing cyclic carbonates from carbon dioxide and epoxides using oxo-dimers of substituted aluminum-salen complexes as a catalyst system. The two nitrogen atoms in the salen structure are connected by a bridging group. The bridging group includes phenylene, a C5-heterocycle, and ethylene. In the presence of tetrabutylammonium bromide (TBAB) as a co-catalyst, these dimeric aluminum-salen complexes are highly active homogeneous catalysts for the reaction of epoxides with carbon dioxide to produce cyclic carbonates. The reaction can be initiated at room temperature and pressure, with a short reaction time and catalyst dosage that meets commercial requirements.

[0004] WO2009109765 discloses a dimeric aluminum (salen) complex similar to WO2010 / 106324, which is immobilized on a solid support via an anchoring group containing a quaternary ammonium cation-bromide ion pair. The anchoring group acts as the aforementioned co-catalyst. The heterogeneous catalyst also exhibits high activity in the reaction of epoxy compounds with carbon dioxide to form cyclic carbonates at room temperature and pressure.

[0005] EP3728213B1 discloses a process for preparing cyclic carbonates from carbon dioxide and an epoxide in a heterogeneous catalyst suspension. The heterogeneous catalyst is an oxy-dimer of a substituted aluminum-salen ligand, as described in WO2010 / 106324. Benzyl bromide is added to the solution to maintain catalyst activity.

[0006] The catalysts disclosed in WO2009109765 and EP3728213B1 offer the advantage of catalytically preparing cyclic carbonates from carbon dioxide and epoxy compounds at room temperature and pressure. However, the preparation of the supported aluminum-salen ligand is complex, requiring at least nine synthetic steps. The yields of some steps are low, resulting in an overall yield of less than 10%, and the synthesis requires the use of large amounts of solvent.

[0007] Antonello Decortes et al.'s article, "Ambient Fixation of Carbon Dioxide using a Zn(ll) salphen Catalyst," Chemcatchem, John Wiley & Sons, Inc., Hoboken, USA, part 3, nr. 5, 2 March 2011 (2011-03-02), pages 831-834, XP072430479, ISSN: 1867-3880, DOI: 10.1002 / CCTC.201100031, describes a process for preparing cyclic carbonates from oxirane and carbon dioxide in the presence of 1 MPa CO2 in the inert solvent dichloromethane (DCM), using Zn(ll) salphen as a catalyst. The article compares the catalytic activities of different metal complexes. The bridging group connecting the nitrogen atoms of the complex, the "en" bridge, is a phenylene group. The present disclosure also compares the catalytic activity of zinc complexes with different bridging groups under 1 MPa CO2 pressure. The complex with a methyl alkylated pyridinium bridging group showed no catalytic activity.

[0008] Martin C. et al., in their article "Easily accessible bifunctional Zn(salpyr) catalysts for the formation of organic carbonates," CATALYSIS SCIENCE & TECHNOLOGY, part 4, nr. 6, 23 January 2014 (2014-01-23), pages 1615-1621, XP093114624, ISSN: 2044-4753, DOI: 10.1039 / C3CY01043K, discloses a bifunctional Zn(salpyr) complex with a methylalkylpyridinium bridging group as a component of a catalytic system for the preparation of cyclic carbonates from ethylene oxide and carbon dioxide at 1 MPa CO2 pressure. The experiments were conducted in pure ethylene oxide. Summary of the Invention

[0009] The object of the present invention is to provide a complex which can be used as a component of a catalyst for preparing cyclic carbonates from epoxy compounds and carbon dioxide under low pressure.

[0010] This object is achieved by the following optionally substituted aluminum-salen complexes, which have the following general formula (1):

[0011] Wherein, B is a halogen, an alkoxy group or an alkyl group, and the phenyl group of the aluminum-salen complex can be selectively substituted, wherein R is a hydrocarbon group and X is a halogen anion.

[0012] Applicants have discovered that this novel complex exhibits catalytic activity at the expected low pressures. Furthermore, compared to prior art substituted aluminum-salen ligand oxydimers, the aluminum-salen complex can be prepared in a simpler manner, requiring fewer synthetic steps and yielding significantly higher yields. It has been discovered that aluminum complexes with pyridine bridges can be alkylated with various alkyl bromides, thereby functionalizing and curing the complexes. The quaternary ammonium nitrogen ion of the pyridinium bridging group acts as a co-catalyst in the catalyst system. This optionally substituted complex with a pyridine bridge is a novel intermediate in the preparation of corresponding complexes with optionally substituted pyridinium bridges. This optionally substituted complex with a pyridine bridge can also be used as a component of a homogeneous or heterogeneous catalyst for the reaction of epoxides with carbon dioxide to produce cyclic carbonates.

[0013] The present invention also relates to the following process for preparing optionally substituted aluminum-salen complexes having a pyridinium bridging group.

[0014] A process for preparing an optionally substituted aluminum-salen complex having a pyridinium bridging group, wherein the starting material is the corresponding optionally substituted nickel-salen or zinc-salen complex having a pyridinium bridging group, and the following steps are performed: (a) demetallizing the corresponding optionally substituted nickel-salen or zinc-salen complex to obtain a demetallized salen ligand; (b) contacting the demetallated salen ligand obtained in step (a) with an aluminum compound to obtain an optionally substituted aluminum-salen intermediate complex having a 3-pyridine bridge; and (c) alkylating the optionally substituted aluminum-salen intermediate complex having a 3-pyridinium bridge to obtain the corresponding optionally substituted aluminum-salen complex having a 3-pyridinium bridge group.

[0015] Further advantages will be discussed when the invention is described in detail.

[0016] The pyridine bridging group is a 3-pyridyl bridging group. The pyridine bridging group is a 3-pyridine group.

[0017] The alkoxy group B in formula (1) can be a methoxy group, an ethoxy group, and / or an isopropoxy group. The alkyl group B can have 1 to 5 carbon atoms, for example, a methyl group or an ethyl group. B can also be chlorine, bromine, or iodine. The choice of B can depend on the starting aluminum compound available for synthesizing the complex. For this reason, B is preferably chlorine.

[0018] The anion X in formula (1) is suitably a halogen anion, preferably an iodide anion or a bromide anion, more preferably a bromide anion. When used as a catalyst system component, a complex based on iodine or, more preferably, bromine is preferred. The selected halogen anion X will be referred to as the catalyst system halogen.

[0019] The phenyl group in formula (1) is preferably substituted at its 3-position. The phenyl group in formula (1) may be substituted by halogen, optionally substituted C1-20 alkyl, optionally substituted C5-20 aryl, optionally substituted C3-20 heterocyclic, ether, ammonium, and nitro. It is preferably substituted at its 3-position and optionally substituted at its 5-position. For examples, see EP2146977. Preferably, the phenyl group is substituted at its 3-position by a butyl group, more preferably by a tert-butyl group, and optionally substituted at its 5-position.

[0020] The group R in formula (1) can be any group, such as an optionally substituted C1-20 alkyl group, an optionally substituted C5-20 aryl group, or an optionally substituted C3-20 heterocyclic group. Examples of optional groups are benzyl and C3-C10 alkyl groups, such as propyl, butyl, pentyl, hexyl, heptyl, octyl, and nonyl.

[0021] Preferably, R in formula (1) is an anchoring group for immobilizing the aluminum-salen complex on a support to obtain a supported catalyst. Thus, R can be an anchoring group for covalently anchoring the optionally substituted aluminum-salen ligand to a heterogeneous support as a support. Suitable anchoring groups are groups that can bind the ligand to the heterogeneous support and will depend on the type of heterogeneous support. When the support contains a silanol group (Si-OH) or an aluminum alcohol group (Al-OH), such as a silica support or an alumina support, the anchoring group is preferably an alkyl (trialkoxy) silane or an alkyl trichlorosilane that can form a covalent bond with the silanol group. Examples of suitable alkyl (trialkoxy) silane or alkyl trichlorosilane groups include propyl (trimethoxy) silane, propyl (triethoxy) silane, propyl trichlorosilane, propyl (alkyl) dichlorosilane and propyl (dialkyl) chlorosilane. Other possible anchor groups are styryl(trialkyl)silane, propylbis(methallyl)silane, propyltri(methallyl)silane, propylphosphonic acid, propyl(dialkyl)phosphonate and alkyl-1,1-bisphosphonic acid.

[0022] The heterogeneous support can be a relatively large particle, which can be present in the reactor as a packed bed. The supported catalyst can also be used as a suspension in the reaction mixture. In this case, the heterogeneous support exists in the form of a powder, the size of which is small enough to form a high active catalytic surface area per unit mass of the heterogeneous support, and the size of which is large enough to be easily separated from the cyclic carbonate inside or outside the reactor. Preferably, at least 90 wt% of the total particle size of the heterogeneous support powder particles is greater than 10 μm and less than 2000 μm. The particle size is determined by Malvern ® Mastersizer ® 2000 determinations.

[0023] The present invention is also directed specifically to the following specific complexes: [Al(3,4-Py-Salphen(N-propyl(trimethoxy)silane))(Cl)]Br, [Al(3,4-Py-Salphen(N-propyl(triethoxy)silane))(Cl)]Br, [Al(3,4-Py-Salphen(N-propyl(triethoxy)silane))(OEt)]Br, [Al(3,4-Py-Salphen(N-propyl(triethoxy)silane))(Cl)]I, [Al(3,4-Py-Salphen(N-propyl(diethoxyphosphonate))(Cl)]Br, [Al(3,4-Py-Salphen(N-propyl(phosphonic acid))(Cl)]Br, [Al(3,4-Py-Salphen(N-butyl(diethylphosphonate))(OEt)]Br, and [Al(3,4-Py-Salphen(N-pentyl(phosphonic acid))(Cl)]Br, Wherein, for the above complex, a substituent group may optionally be present at the 3-position of the phenyl group of the salphen structure, and preferably the 3-position is a tert-butyl group, and optionally the 5-position is also a tert-butyl group. Et in the above description represents an ethyl group.

[0024] Figure 1 Shown are examples of demetallated salen ligands that, upon remetallation, can yield the complexes of the present invention.

[0025] The present invention also discloses a supported aluminum complex comprising the complex of the present invention and a heterogeneous support. The support is preferably silica, alumina, or a silica-alumina support. The complex can be immobilized via a substituent on the phenyl group, such as the diethylaminomethyl group described in EP2257559. Preferably, the complex is immobilized on the support surface via the group R in formula (1) serving as an anchoring group.

[0026] The starting materials of the optionally substituted nickel-salen or zinc-salen complexes having a pyridine bridging group can be obtained by synthetic routes well known in the art, such as those described in Catal. Sci. Technol., 2014, 4, 1615 and ChemCatChem 2011, 3, 831-834.

[0027] In step (a), the corresponding optionally substituted nickel-salen or zinc-salen complex is demetallated to obtain the demetallated salen ligand. Demetallation can be performed using known demetallation techniques, such as those described in Organometallics 2002, 21, 14, 2950–2957. Demetallation is preferably performed in acetonitrile or toluene, preferably toluene, in an anhydrous demetallation process with the presence of imidazole. This procedure provides high yields.

[0028] In step (b), the demetallated salen ligand obtained in step (a) is contacted with an aluminum compound. The aluminum compound comprises a precursor of the group B used in this specification to illustrate preferred complexes. When B is chlorine, the aluminum compound is preferably diethylaluminum chloride (Al(Et)2Cl). When B is an alkoxy group, such as ethoxy, the aluminum compound is preferably triethoxyaluminum. When B is an alkyl group, the aluminum compound is preferably a trialkylaluminum compound, preferably, the alkyl group has 1-5 carbon atoms, such as trimethylaluminum or triethylaluminum.

[0029] When the process is performed on a larger scale, to enhance reagent contact, the aluminum compound is preferably added gradually to the demetallated salen ligand, which is dissolved or slurried in a suitable solvent. It has been found that if the aluminum compound is not added gradually, the reaction mass solidifies, preventing effective mixing of the reagents. The demetallated salen ligand is preferably dissolved or slurried in a suitable solvent, such as toluene or THF (tetrahydrofuran). The gradual addition is preferably continued for at least 1 hour, more preferably at least 1.5 hours. The addition time is preferably no longer than 3 hours. The temperature is preferably between 20°C and 90°C, more preferably between 50°C and 80°C. A C5-C10 alkane solvent, preferably heptane, is preferably added to reduce the solubility of the resulting optionally substituted aluminum-salen complex. The solidified complex can be separated from the solvent by filtration.

[0030] The corresponding optionally substituted aluminum-salen complex having a pyridinium bridge group as shown in formula (1) is prepared in step (c). In step (c), the optionally substituted aluminum-salen complex having a pyridinium bridge obtained in step (b) is subjected to an alkylation reaction to obtain the corresponding optionally substituted aluminum-salen complex having a pyridinium bridge group. The alkylation is preferably carried out by contacting the optionally substituted aluminum-salen complex having a pyridinium bridge with a halogen-R compound in a suitable solvent. The halogen can be a system halogen or a different halogen. Preferably, halogen-R is a bromide Br-R, where R is as described above. Suitable solvents are tetrahydrofuran (THF), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or a mixture of dimethyl sulfoxide (DMSO) and acetonitrile. It has been found that starting the alkylation with the optionally substituted aluminum-salen complex is more advantageous than starting the alkylation with the optionally substituted zinc-salen complex having a pyridinium bridge group. It has been found that the optionally substituted zinc-salen complex cannot be alkylated with the bromide Br-R. Starting from an aluminum complex, such an alkylation can be performed with Br-R, thereby preparing the complex of the present invention, wherein X is a bromide ion. The complex having a pyridinium bridge group can be considered a novel and inventive intermediate compound in the preparation of the complex having a pyridinium bridge group.

[0031] Therefore, the present invention also relates to an optionally substituted aluminum-salen intermediate complex: , Wherein, B is halogen, alkoxy or alkyl, and the phenyl group of the aluminum-salen complex may be selectively substituted as described above.

[0032] The present invention also discloses a process for preparing the above-mentioned optionally substituted aluminum-salen intermediate complex, wherein the starting material is the corresponding optionally substituted nickel-salen or zinc-salen complex having a pyridine bridging group, and the following steps are performed: (a) demetallizing the corresponding optionally substituted nickel-salen or zinc-salen complex to obtain a demetallized salen ligand; (b) contacting the demetallated salen ligand obtained in step (a) with an aluminum compound to obtain the optionally substituted aluminum-salen intermediate complex. Steps (a) and (b) are preferably carried out in the manner described above.

[0033] The present invention also discloses a process for preparing the optionally substituted aluminum-salen intermediate complex of the present invention from the optionally substituted aluminum-salen intermediate complex of the present invention, wherein the optionally substituted aluminum-salen intermediate complex is alkylated to obtain the optionally substituted aluminum-salen intermediate complex.

[0034] The present invention also discloses a catalyst system comprising the complex or supported aluminum complex of the present invention and a co-catalyst. The co-catalyst can be any compound capable of providing bromide or iodide anions as the halogen in the catalyst system. The compound can be an organic or inorganic compound. The optionally substituted salen complex can be advantageously used as a catalyst system. Because the optionally substituted salen complex contains a pyridinium halide salt, it can be used as a catalyst without the addition of a separate halide salt. Typical separate halide salts added to such catalyst systems are bromide or iodide salts. The bromide salt can be a quaternary ammonium bromide salt or a phosphonium bromide salt, such as tetrabutylammonium bromide (TBAB); the iodide salt can be a quaternary ammonium iodide salt or a phosphonium iodide salt. When the complex of the present invention is used, the catalytic process can be carried out without the addition of a halide salt or with the addition of only a relatively small amount of a halide salt.

[0035] The catalyst system can be used as a catalyst, or as part of a catalyst, in a copolymerization process to prepare aliphatic polycarbonates using an epoxy compound and carbon dioxide as raw materials. The catalyst system can also be used to prepare polyesters via the ring-opening polymerization of lactone compounds or via the ring-opening polymerization of maleic anhydride, as described in WO2012 / 065711.

[0036] Preferably, the catalyst system is used as a catalyst or a part of a catalyst in a process for preparing cyclic carbonate by reacting an epoxy compound with carbon dioxide.

[0037] Therefore, the present invention also discloses a process for preparing cyclic carbonates, wherein an epoxy compound is reacted with carbon dioxide in the presence of the catalyst system. The catalyst system preferably includes, in addition to the complex of the present invention, a separate halogen salt, such as the above-mentioned bromide salt or iodide salt.

[0038] The molar ratio of the optionally substituted aluminum-salen ligand having a pyridinium bridge to the bromide salt is expressed as Al / Br (mol / mol), preferably 0.1-20, more preferably 1-8, most preferably 2-5.

[0039] The initial epoxide compound suitably contains from 2 to 8 carbon atoms. Suitable epoxides include ethylene oxide, propylene oxide, butylene oxide, pentylene oxide, glycidol, styrene oxide, epichlorohydrin or epifluoroethane.

[0040] The carbon dioxide is preferably contacted with the epoxide in a liquid that also contains the catalyst system. The liquid can be an inert solvent. Optional inert solvents include toluene, a cyclic carbonate, and / or preferably the cyclic carbonate product itself. A supported catalyst can also be used as a suspension in the reaction mixture. The carbon dioxide is preferably contacted with the epoxide in a suspension of the aforementioned inert solvent and the heterogeneous catalyst, preferably a liquid cyclic carbonate. The temperature and pressure conditions are set to maintain the cyclic carbonate in a liquid state. The temperature and pressure conditions are further set to allow the carbon dioxide and epoxide to be readily soluble in the liquid cyclic carbonate reaction medium. The temperature can be between 0°C and 200°C, and the pressure can be between 0 and 5.0 MPa (absolute), wherein the temperature is below the boiling point of the cyclic carbonate product at the set pressure. At the upper end of this temperature and pressure range, complex reaction vessels are required. In view of the good selectivity and yield of the target cyclic carbonate product that can be achieved at lower temperatures and pressures, the temperature is preferably 20-150°C, more preferably 40-120°C; the absolute pressure is preferably 0.1-0.5 MPa, more preferably 0.1-0.3 MPa. The carbon dioxide partial pressure will be equal to or lower than the absolute pressure. DETAILED DESCRIPTION

[0041] Example 1 In this example, [Zn(3,4-Py Salphen)] was prepared as shown in the figure below.

[0042]

[0043] To a round-bottom flask, 3,4-diaminopyridine (1.44 g, 13.2 mmol), 3-tert-butylsalicylaldehyde (4.8 mL, 28.0 mmol), zinc acetate dihydrate (3.08 g, 13.8 mmol), and triethylamine (5.0 mL, 35.9 mmol) were added in 100 mL of methanol. The mixture was stirred and heated under reflux for 24 hours to yield a bright red solid. The solid was filtered and washed with methanol (3 × 20 mL). The yield of [Zn(3,4-Py Salphen)] was 6.24 g (95.6%).

[0044] Product passed 1 Characterization by H-NMR: 1H NMR (300 MHz, DMSO-d6) δ 9.20 (s, 1H), 9.15 (s, 2H), 8.48 (d, J = 5.5 Hz, 1H), 7.88 (d, J = 5.5 Hz, 1H), 7.29 (m, 4H), 6.48 (td, J = 7.5, 3.2 Hz, 2H), 1.48 (s, 18H). NMR data indicated the formation of the desired product.

[0045] Example 2 This example illustrates the process step (a) of preparing the complex according to the present invention. The [Zn(3,4-Py Salphen)] obtained in Example 1 was subjected to a demetallization treatment as shown in the figure below.

[0046]

[0047] [Zn(3,4-PySalphen)] (6.24 g, 12.65 mmol) obtained in Example 1 was added to a flask and dissolved in 60 mL of toluene. Imidazole (3.48 g, 50.6 mmol) was added to the solution and heated to 60°C for 2 hours. The solid was filtered and washed with toluene. Alternative solvents to toluene include methanol and MTBE. The solid was resuspended in methanol (60 mL) and heated to 60°C for 2 hours. The solid was filtered and dried to yield 4.27 g of an orange powder with a yield of 78.6%.

[0048] Product passed 1 Characterization by H-NMR: 1 H NMR (300 MHz, DMSO-d6) δ 13.80 (s, 1H), 13.73 (s, 1H), 9.07 (d, J = 8.7 Hz, 2H), 8.71 (s, 1H), 8.58 (d, J = 5.3 Hz,1H), 7.62 – 7.48 (m, 3H), 7.42 (td, J = 8.0, 1.7 Hz, 2H), 6.95 (td, J = 7.7,3.3 Hz, 2H), 1.38 (s, 18H). NMR data indicated the formation of the desired product.

[0049] Example 3 This example illustrates the process step (b) for preparing the intermediate complex according to the present invention. A substituted aluminum-salen complex having a pyridine bridge is prepared from the demetallated ligand obtained in Example 2 as shown in the following figure.

[0050]

[0051] To a flame-dried Schlenk tube, the 3,4-PySalphen ligand obtained in Example 2 (2.6 g, 6.0 mmol) was added and dissolved in 24 mL of toluene. The mixture was heated to 60°C. Al(Et)2Cl (1.8 M in toluene, 6.4 mL, 11.6 mmol) was added, and the reaction was stirred for 17 hours. The reaction was cooled to room temperature, and the product was filtered and washed with toluene (10 mL) and pentane (2 × 10 mL) to obtain a yellow solid (1.19 g, 40%).

[0052] Product passed 1 Characterization by H-NMR: 1 H NMR (300 MHz, DMSO-d6) δ 9.50 (s, 1H), 9.44 (d, J = 5.0 Hz, 2H), 8.65 (d, J = 5.6 Hz, 1H), 8.13 (d, J = 5.7 Hz, 1H), 7.57 (td, J = 11.8, 10.6, 4.8 Hz, 4H), 6.90 – 6.75 (m, 2H), 1.56 (d, J = 1.8 Hz,19H). NMR data indicated the formation of the desired product.

[0053] Example 3a Example 3 was repeated except that Al(OEt) 3 was used as the aluminum compound containing B and 2,3-Py-Salphen was used to obtain a 2,3-Py-Salphen complex in which B was ethoxy.

[0054] 2,3-PySalphen ligand (0.2 g) was added to a flame-dried Schlenk tube and dissolved in dry toluene (9 mL). The mixture was heated to reflux. Aluminum triethoxide (0.151 g) was added, followed by 2 mL of toluene, and the reaction was stirred for 17 hours. The reaction was cooled to room temperature, and the product was filtered and washed three times with 5 mL of toluene. The solid was dried under vacuum. The yield was 43%.

[0055] The product is characterized as follows: 1H-NMR (300 MHz, Chloroform-d) δ 9.80 (s, 1H), 8.94 (s,1H), 8.51 – 8.37 (d, J = 4.1 Hz, 1H), 8.04 (d, J = 8.1 Hz, 1H), 7.61 (t, J = 8.6 Hz, 2H), 7.47 (d, J = 7.4, 1H), 7.38 – 7.25 (m, 2H), 6.85 (q, J = 8.1 Hz,2H), 3.41 (q, J = 6.9 Hz, 2H), 1.64 (s, 18H), 0.76 (t, J = 6.9 Hz, 3H). NMR data indicated the formation of the desired product.

[0056] Example 3b Example 3 was repeated on a larger scale, with 3,4-PySalphen ligand (327.32 g, 0.76 mmol) charged to a dry 5-L autoclave under a nitrogen atmosphere, followed by the addition of 1 L of toluene and 0.5 L of heptane. The reaction mixture was stirred with an overhead stirrer and heated to 60°C. Once the 3,4-PySalphen ligand had dissolved, Al(OEt)3 (1.0 M in hexane, 800 mL, 0.8 mol) was added over 2 hours. The reaction temperature was maintained at 60°C during the addition. After the addition was complete, the reaction mixture was stirred at 60°C for an additional 18 hours before being cooled to 25°C. Heptane (600 mL) was then added, and the reaction mixture was stirred for an additional 30 minutes before the autoclave contents were drained. The precipitate was filtered, washed with heptane (3 x 500 mL), and dried to yield 346 g of product (92% yield).

[0057] In large-scale operations, it is important to add Al(OEt) slowly over a period of more than one hour, preferably 1.5 to 3 hours, to avoid a violent exothermic reaction and uncontrolled gas evolution. Furthermore, the addition of heptane facilitates product precipitation. Alternatives to heptane include alkanes, cycloalkanes, and ethers such as MTBE.

[0058] Example 4 This example illustrates the process step (c) for preparing the complex according to the present invention.

[0059]

[0060] 400 mg of the [Al(3,4-Py-Salphen(Cl)] complex (0.93 mmol), prepared similarly to Example 1-3, was suspended in 5 mL of toluene and 1 mL of dry DMF, and 800 μL of octyl bromide (4.6 mmol) was added. Under a nitrogen atmosphere, the solution was heated to 80°C, completely dissolving the complex and changing from orange to a red slurry. After reacting overnight, the conversion rate reached 37%. 2 mL of octyl bromide was added to the reaction mixture, and heating was continued until the initial starting material was completely converted. The complex was isolated as a DMF complex with an overall molecular formula of [Al(3,4-Py-Salphen(N-Oct))(Cl)]Br(DMF)2. The yield was 480 mg, or 63%.

[0061] The product is characterized as follows: 1 H-NMR (300 MHz, DMSO-d6) δ 9.92 (s, 1H), 9.63 (s, 1H), 9.56 (s, 1H), 9.09 (d, J = 6.9 Hz, 1H), 8.69 (d, J = 7.0 Hz, 1H), 7.96 (s,3H), 7.71 – 7.51 (m, 4H), 6.89 (t, J = 7.6 Hz, 2H), 4.61 (t, J = 7.3 Hz, 2H), 2.08 (br. 2H), 1.33 (br., 12H), 0.97 – 0.81 (m, 3H). NMR data indicated the formation of the desired product.

[0062] Examples 1-4 complete the synthesis of aluminum-salen complexes in only four steps with high overall yield.

[0063] Example 5 Example 4 was repeated except that benzyl bromide was used instead of octane bromide to obtain a catalyst complex containing a benzyl group, the complex being [Al(3,4-Py-Salphen(N-benzyl))(Cl)]Br. The product was characterized as follows: 1H-NMR (300 MHz, DMSO-d6) δ 10.18 (s, 1H), 9.60 (s, 2H), 9.16 (d, J = 6.9 Hz, 1H), 8.69 (d, J = 7.0 Hz, 1H), 7.72 (s, 1H), 7.85 – 7.35 (m, 9H), 6.89 (td, J = 7.7, 3.2 Hz, 2H), 5.84 (s, 2H), 1.55 (d, J = 4.6 Hz, 18H). NMR data indicated the formation of the desired product.

[0064] Example 5a To a flame-dried Schlenk tube, [Al(3,4-Py-Salphen(Cl)] complex (7.36 g, 15 mmol) and dry DMF (16.8 mL) were added. The reaction was heated to 80 °C until all solids dissolved, followed by the addition of 3-bromopropyltrimethoxysilane (11.3 mL, 60 mmol). The mixture was stirred at 80 °C for 3 days and then cooled. The resulting precipitate was filtered, washed with Et2O (3 × 20 mL), and dried to afford 8.54 g (57% yield) of an orange solid. The product was characterized as follows: 1 H-NMR (300 MHz, DMSO-d6) δ 10.04 (s, 1H), 9.68 (s, 1H), 9.64 (s, 1H), 9.09 (d, J = 6.9 Hz, 1H), 8.74 (d, J = 7.0 Hz, 1H), 7.82 – 7.46 (m, 4H), 6.89 (t, J = 7.6 Hz, 2H), 4.58 (t, J = 7.5 Hz, 2H), 3.47 (s, 9H), 2.13 (p, J = 7.9 Hz, 2H), 0.77 – 0.53(m, 2H).

[0065] Example 6 This example tests the catalytic activity of the complex obtained in Example 4 in the preparation of styrene carbonate (4-phenyl-1,3-dioxolan-2-one) by the reaction of styrene oxide with carbon dioxide in the presence of tetrabutylammonium bromide.

[0066]

[0067] To a Schlenk reaction flask, add 57.98 mg of [Al(3,4-Py-Salphen(N-Oct))(Cl)]Br and 22.59 mg of tetrabutylammonium bromide (TBAB), heat to 100°C, and evacuate. The flask is then backfilled with CO2 gas, followed by the addition of propylene carbonate (3.5 mL) and styrene oxide (1.0 mL). Purge the flask with CO2 and begin stirring. The CO2 partial and total pressures at the start of the experiment are 0.106 MPa. Samples are collected and analyzed at regular intervals.

[0068] Substrate loading = 2.0 M Catalyst loading = 0.8 mol% Tetrabutylammonium bromide loading = 0.8 mol% The conversion of chlorostyrene and the amount of styrene carbonate (4-phenyl-1,3-dioxolan-2-one) produced are listed in Table 1.

[0069] Example 7 This example tested the catalytic activity of the complex obtained in Example 4 in the reaction of styrene oxide with carbon dioxide to prepare styrene carbonate in the absence of exogenous tetrabutylammonium bromide. Example 6 was repeated except that tetrabutylammonium bromide was omitted. The conversion of styrene chloride and the amount of styrene carbonate (4-phenyl-1,3-dioxolan-2-one) produced are listed in Table 1.

[0070] Example 8 Example 7 was repeated, except that the benzyl-containing catalyst [Al(3,4-Py-Salphen(N-Benzyl))(Cl)]Br, obtained in Example 5, was used. The conversion of chlorostyrene and the amount of styrene carbonate (4-phenyl-1,3-dioxolan-2-one) produced are listed in Table 1.

[0071] Example 9 Example 7 was repeated, except that the benzyl-containing catalyst [Al(3,4-Py-Salphen(N-Benzyl))(Cl)]Br, obtained in Example 5, was used. The conversion of chlorostyrene and the amount of styrene carbonate (4-phenyl-1,3-dioxolan-2-one) produced are listed in Table 1.

[0072] Comparative Test A Example 6 was repeated using a complex similar to that used in Example 6, except that the bridging group was a phenylene bridging group used in the prior art. The conversion of chlorostyrene and the amount of styrene carbonate (4-phenyl-1,3-dioxolan-2-one) produced are listed in Table 1.

[0073] Comparative Test B Example 7 was repeated using a complex similar to that used in Example 7, except that the bridging group was a phenylene bridging group used in the prior art. The conversion of chlorostyrene and the amount of styrene carbonate (4-phenyl-1,3-dioxolan-2-one) produced are listed in Table 1.

[0074] Table 1

[0075] Examples 6-9 demonstrate that the catalyst system according to the present invention can be advantageously used as a catalyst, or a component of a catalyst, for the reaction of preparing cyclic carbonates from epoxy compounds and carbon dioxide. The results in Table 1 show that when the reaction is carried out in the presence of tetrabutylammonium bromide, the conversion is higher. The results also show that the complexes with the pyridinium bridging group still exhibit catalytic activity in the absence of tetrabutylammonium bromide. It is believed that the bromide ions associated with the pyridinium nitrogen act as a co-catalyst. The results also show that the activity of the complexes with the pyridinium bridging group is significantly higher than that of the prior art complexes with phenylene as the bridging group.

[0076] Example 10 0.020 mmol of catalyst and 4.0 mmol of maleic anhydride (MA) were placed in a flame-dried Schlenk flask equipped with a stirrer. An appropriate amount of solvent (1.0 mL of toluene) was added, followed by 4.0 mmol of styrene oxide (SO). The flask was sealed with a Teflon-lined lid and placed in an aluminum heating block preheated to the set temperature (65°C-80°C). When the reaction became viscous, the flask was removed from the heating block and a small sample was taken for 1H NMR analysis to determine monomer conversion. The viscous reaction mixture was then dissolved with a minimal amount of dichloromethane and precipitated into excess hexane. The resulting solid was isolated and dried under vacuum. 1 H-NMR confirmed the formation of an unsaturated polyester. 1H NMR (300 MHz, DMSO-d6) δ 6.46 (m, 2H), 5.12 (m, 1H), 4.21 (m, 2H), 1.23 (d, J = 6.6 Hz, 3H).

[0077] This example was repeated at different catalyst loadings and temperatures. The results are listed in Table 2, where conv (%) represents the monomer conversion, MW is the molecular weight of the polyester, and Mn / Mw represents the molar mass distribution / dispersity of the polyester.

[0078] Table 2

[0079] Comparative Test C Alkylation of the zinc complex shown below was attempted with octane bromide.

[0080]

[0081] Runs C1-C10 were conducted on the alkylation of [Zn(3,4-Py-Salphen)] with octyl bromide, testing various solvent combinations, listed in Table 3. It was found that the alkylation of [Zn(3,4-Py-Salphen)] with octyl bromide did not result in reactivity or decomposition of [Zn(3,4-Py-Salphen)]. Therefore, alkylation of [Zn(3,4-Py-Salphen)] with octyl bromide could not be performed in a manner similar to [Al(3,4-Py-Salphen)(Cl)] in Example 4.

[0082] Table 3

[0083] Example 11 This example illustrates the process step (c) for preparing a complex according to the present invention starting from [Al(3,4-Py-Salphen)(Cl)] using iodomethane as the halogen-R compound, as shown below:

[0084] The [Al(3,4-Py-Salphen)(Cl)] complex (200 mg, 0.41 mmol, 1.0 equiv) was dissolved in DMF (4 mL) to yield a red solution. Methyl iodide (1 mL, 3.08 mmol, 7.5 equiv) was added to this solution, and the reaction mixture was heated at 40°C for 20 hours. After the reaction time, the reaction mixture was added to isopropyl acetate (40 mL), producing a red precipitate that was isolated. The solid was washed with isopropyl acetate (3 × 5 mL) and dried in a vacuum oven (40°C, 5 mbar, 16 hours). The product was isolated as a red powder. 1 H-NMR confirmed the formation of the product. 1H NMR (300 MHz, DMSO-d6) δ 9.80 (d, J = 1.4 Hz, 1H), 9.60 (s, 1H), 9.48 (s, 1H), 8.99 (dd, J= 7.0, 1.3 Hz, 1H), 8.66 (d, J = 7.1 Hz, 1H), 7.65-7.54 (m, 2H), 6.89 (t, J =7.6 Hz, 1H), 4.38 (s, 2H), 1.55 (d, J = 3.9 Hz, 18H).

[0085] Example 12 The [Al(3,4-Py-Salphen(N-Me))(Cl)]I complex prepared in Example 11 was tested under low CO2 partial pressure. The remaining reaction conditions were consistent with those described in the article "Ambient Fixation of Carbon Dioxide Using Zn(II) Salphen Catalysts," by Antonello DeCortes et al., CHEMCATCHEM, JOHN WILEY & SONS, INC., HOBOKEN, USA, part 3, no. 5, 2 March 2011 (2011-03-02), pages 831-834, XP072430479, ISSN: 1867-3880, DOI: 10.1002 / CCTC.201100031. The results of Experiment D in Table 4 are the same as those in Table 2 of the article.

[0086] Table 4

Claims

1. An optionally substituted aluminum-salen complex having the general formula: in, B is a halogen, an alkoxy group or an alkyl group, and the phenyl group of the aluminum-salen complex can be selectively substituted, wherein R is a hydrocarbon group and X is a halogen anion.

2. The optionally substituted aluminum-salen complex according to claim 1, characterized in that X is an iodide anion or a bromide anion.

3. The optionally substituted aluminum-salen complex according to claim 1 or 2, characterized in that R is an anchoring group used to immobilize the aluminum-salen complex on a heterogeneous support.

4. The optionally substituted aluminum-salen complex according to claim 3, characterized in that The anchoring group is an alkyl(trialkoxy)silane.

5. The aluminum-salen complex according to claim 4, characterized in that The aluminum-salen complex is [Al(3,4-Py-Salphen(N-propyl(trimethoxy)silane))(Cl)]Br, and the 3-position of the phenyl group in the Salphen structure may be selectively substituted.

6. A supported complex, characterized in that The aluminum-salen complex according to any one of claims 1 to 5 is fixed on the surface of silica, alumina and / or silica-alumina particles through the anchoring group R.

7. A catalyst system comprising the aluminum-salen complex according to any one of claims 1 to 5 or the supported aluminum-salen complex according to claim 6, and a bromide salt or an iodide salt as a co-catalyst.

8. The catalyst system according to claim 7, characterized in that The bromide salt is a quaternary ammonium bromide salt or a phosphonium bromide salt, and the iodide salt is a quaternary ammonium iodide salt or a phosphonium iodide salt.

9. An optionally substituted aluminum-salen intermediate complex: in, B is a halogen, an alkoxy group or an alkyl group, and the phenyl group of the aluminum-salen complex may be optionally substituted.

10. A process for preparing the optionally substituted aluminum-salen intermediate complex of claim 9, wherein the starting material is the corresponding optionally substituted nickel-salen or zinc-salen complex having a pyridine bridging group, and the following steps are performed: (a) demetallating the corresponding optionally substituted nickel-salen or zinc-salen complex to obtain a demetallized salen ligand; and (b) contacting the demetallated salen ligand obtained in step (a) with an aluminum compound to obtain the optionally substituted aluminum-salen intermediate complex.

11. The process according to claim 10, characterized in that In step (b), the demetallated salen ligand obtained in step (a) is contacted with diethylaluminum chloride (Al(Et)2Cl) as an aluminum compound.

12. The process according to claim 10 or 11, characterized in that In step (a), the corresponding optionally substituted nickel-salen or zinc-salen ligand is demetallated in the presence of imidazole in acetonitrile or imidazole in toluene under anhydrous conditions.

13. The process according to any one of claims 10 to 12, characterized in that The method further comprises a step (c) wherein the optionally substituted aluminum-salen intermediate complex having a 3-pyridinium bridge is alkylated to obtain the corresponding optionally substituted aluminum-salen complex having a 3-pyridinium bridging group.

14. A process for preparing the optionally substituted aluminum-salen intermediate complex according to any one of claims 1 to 5, starting from the optionally substituted aluminum-salen intermediate complex according to claim 9, characterized in that: The optionally substituted aluminum-salen intermediate complex is alkylated to obtain the optionally substituted aluminum-salen intermediate complex according to any one of claims 1 to 5.

15. Use of the catalyst system according to claim 7 or 8 as a catalyst or a part of a catalyst in a process for preparing cyclic carbonates from an epoxy compound and carbon dioxide.

16. A preparation process for preparing cyclic carbonate by reacting an epoxy compound with carbon dioxide at a pressure of 0.1-0.3 MPa in the presence of the catalyst system according to claim 7 or 8.

17. The preparation process according to claim 16, characterized in that: X is a bromide ion, and the catalyst system includes a bromide salt co-catalyst.

18. The preparation process according to claim 16 or 17, characterized in that: The carbon dioxide is contacted with the epoxide compound in a liquid which also contains the catalyst system, wherein the liquid is a cyclic carbonate.

19. The preparation process according to claim 18, characterized in that: The catalyst system is the supported aluminum complex according to claim 6, and the obtained reaction mixture is a suspension.

20. The preparation process according to any one of claims 16 to 19, characterized in that: The epoxy compound contains 2 to 8 carbon atoms.

21. The preparation process according to claim 20, characterized in that: The epoxy compound includes ethylene oxide, propylene oxide, butylene oxide, epoxypentene, glycidol, styrene oxide, epichlorohydrin or epifluoroethane.

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