Title of Invention

A PROCESS FOR THE PREPARATION OF NOVEL RHODIUM CARBONYL COMPLEXES

Abstract

The present invention relates to a process for the preparation of novel rhodium carbonyl complexes. The present invention particularly relates to a process for the preparation of novel rhodium carbonyl complexes containing pyridine and nitrone ligands. The process steps are: novel rhodium carbonyl complexes of formula {Rh (CO) 2C1 L} wherein L is a pyridine based nitrogen donor ligand or nitrone based N-oxide donor ligand as herein described which comprises reacting chlorobridged dimeric complex [Rh(CO)2Cl]2 in a chlorinated solvent with a ligand under inert atmosphere at a temperature ranging between 20-40°C for a period ranging between of 10-30 minutes under stirring condition, removing solvent under vacuum to get complex followed by washing with a non - polar solvent as herein described to get the said rhodium carbonyl complex.

Full Text

The present invention relates to a process for the preparation of novel rhodium carbonyl complexes. The present invention perticularly relates to a process for the preparation of novel rhodium carbonyl complexes containing pyridine and nitrone ligands. The present invention particularly relates to a process for the preparation of new and novel rhodium(I) carbonyl complexes containing halogen and pyridine based nitrogen donor ligands or nitrone based N-oxide donor ligands . Such rhodium metal complexes prepared by the process of present invention are useful as catalyst precursors for carbonylation of alcohol to carboxylic acid and/or ester. Denise et. al. have reported (J. Organomet. Chem, 63, 1973, 423) a few pyridine or substituted pyridine complexes of rhodium metal of the type [Rh(COD)ClL] ( COD = 1,5-cyclooctadiene; L = pyridine, 2-vinyl pyridine, 2-methyl pyridine, 4-methyl pyridine), but they did not report any carbonyl complexes and also they did not disclose any catalytic application. It may be mentioned that Lawson & Wilkinson have reported (J. Chem. Soc. A, 1965, 1900) preparation of few rhodium(I) carbonyl complexes of pyridine or chloropyridine but no mono substituted methyl-pyridine containing complex was reported and also they did not disclose any catalytic application. It is expected from electronic point of view that methyl substituted pyridines will show higher activity compared to simple pyridine as well as chloro substituted pyridines. Sivasubramanian et. al. (Trans. Met. Chem., 1982, 7, 346) have reported the first metal-nitrone complex where metals are used from the 1st row transition metal series. Other few reports of metal nitrone complex appeared in literature e. g. Frederick et. al. (Inorg Chem. 37, 1998, 1446, J. Chem. Soc., Dalton Trans., 1998, 4055), Thirumalaikumar et. al. (Indian J. Chem, 3 8 A, 1999, 720), but no rhodium complex has so far been reported. Since the first report of catalysis by [Rh(CO)2l2]~ (F. E. Paulik and J. F. Roth, J. Chem. Soc. Chem. Comm., 1968, 1578), there has been little improvement on the intrinsic activity of the catalyst. Attempt to develop new catalysis species have been hampered by the relatively harsh condition under which the reaction is conducted commercially (150-200 °C, 25-45 atm., in the presence of I") because under such conditions, virtually any source of rhodium will be converted to [Rh(CO)2l2]~ (D. Forster, J. Am. Chem. Soc., 1976, 98, 846). Reference may be made to U.S. Pat. No. 3,769,327 issued to F. E. Paulik et. al. wherein methanol is carbonylated with carbon monoxide gas at 175°C and 1000 psig pressure to acetic acid using the catalyst [Rh(CO)2l2]~ • The process is similar to existing industrial condition for conducting the reaction. U.S. Pat. 4,990,654 issued to Wegman et.al. disclose production of acetate ester from alcohol using rhodium complex catalysts. The process comprises catalytic reaction of a mixture of methanol and ethanol and carbon monoxide in contact with a homogeneous catalyst of rhodium complex containing the ligands like Ph2P(CH2)nP(O)Ph2 (Ph = phenyl, n = 1-4) or Ph2P(CH2)nC(O)R or Ph2P(CH2)nCOOR (R=alkyl/aryl). The reaction was carried out at a temperature up to 130°C and reaction pressure up to about 250 psig. The main drawback of the process is that methanol conversion is only 70 %. Reference may be made to Wegman et.al. (J. Chem. Soc. Chem. Comm., 1987, 1891) wherein carbonylation in presence of catalyst [Rh(CO)2Cl(Ph2P(CH2)2P(O)Ph2)] was carried out at 80°C and 50 psig CO. The turnover frequency was 400 h"1. U.S.Pat. 5,488,153 issued to Baker et.al. discloses a process for the liquid phase carbonylation of methanol in presence of CO, a halogen promoter ( e.g. CE3I), water, rhodium complexes of Ph2PCH2P(S)Ph2 or 2-(diphenylphosphino)thiophenol. The reaction was performed in the temperature range of 25 - 250°C and at a pressure in the range 10 to 200 bar. The carbonylation rate was found to be considerably higher i.e. about 6 times higher than that in absence of the ligand in the system. The main drawback of the process is that it involves higher temperature and pressure. Baker et.al. (J.Chem.Soc.Chem.Soc., 1995, 197) also disclose that a catalyst cis- RhI(CO)Ph2PCH2P(S)Ph2 is about 8 times more active than the classic Monsanto catalyst [Rh(CO)2I2]" for carbonylation of methanol at 185°C and at 70 bar pressure. In another disclosure, Dilworth et.al. (J. Chem. Soc. Chem. Comm., 1995, 1579) claimed that use of rhodium(I) complexes containing phosphinothiolate and thioether ligands resulted about 4 times higher rate in carbonylation of methanol to ethanoic acid than that of [Rh(CO)2I2]". The reaction was carried out at 185°C and at 70 bar pressure. The draw back of the process is that it involves high temperature and pressure. Reference may be made to U.S.Pat No. 5,973,197 issued to Denis et.al., wherein a method for preparing carboxylic acids by carbonylation of an alcohol in presence of rhodium complex catalyst. The carbonylation reaction was conducted at a temperature in the range 150 - 250°C and 1 - 1 0 0 bar pressures. Again the main drawback is the involvement of drastic reaction condition. The main object of the present invneiton is to provide a process for the preparation of novel rhodium carbonyl complexes containing pyridines and nitrones ligand which obviates the drawback as detailed above. Another object of the present invention is to provide a process for preparing rhodium (I) complexes containing different types of electron rich ligands containing 'Hard' donor atom facilitating high electron density on the rhodium center and thus causing facile oxidative addition reactions with different electrophiles and consequently can act as efficient catalyst precursors for carbonylation of alcohol to acid and/or ester. Accordingly, the present invention provides a process for the preparation of novel rhodium carbonyl complexes of formula {Rh (CO) 2C1 L} wherein L is a pyridine based nitrogen donor ligand or nitrone based N-oxide donor ligand as herein described which comprises reacting chlorobridged dimeric complex [Rh(CO)2Cl]2 in a chlorinated solvent with a ligand under inert atmosphere at a temperature ranging between 20-40°C for a period ranging between of 10-30 minutes under stirring condition, removing solvent under vacuum to get complex followed by washing with a non - polar solvent as herein described to get the said rhodium carbonyl complex. The process for the preparation of novel rhodium carbonyl complexes which comprises preparation of novel rhodium (I) carbonyl complexes containing chloride and pyridine based nitrogen donor ligands like pyridine (Py), 2-methylpridine (2-MePy), 3-mythylpyridine (3-MePy), 4-methylpridine (4-MePy) or nitrone based N-oxide donor ligands like α,N-diphenylnitrone, α-styryl-N-phenylnitrone, N,N-diphenyldintrone, α-(2-furyl)-N-phenylnitrone by reacting chlorobridged dimeric complex [Rh(CO)2C 1 ]2 in an a chlorinated solvent with a ligand under nitrogen atmosphere at a temperature ranging between 20-40 ° C for a period ranging between of 10-30 min. under stirring condition , removing solvent under vauum to get complex followed by washing with a non -polar solvent to get the said rhodium carbonyl complex. The process for the preparation of novel rhodium carbonyl complexes which comprises preparation of novel rhodium(I) carbonyl complexes containing chloride and pyridine based nitrogen donor ligands like pyridine(Py), 2-methylpyridine (2-MePy), 3- methylpyridine (3-MePy), 4-methylpyridine (4-MePy) or nitrone based N-oxide donor ligands like a,N-diphenylnitrone, a-styryl-N-phenylnitrone, NjN'-diphenyldinitrone, a- (2-furyl)-N-phenylnitrone by reacting chlorobridged dimeric complex [Rh(CO)aCl]2 in an organic solvent like CH2C12, CHC13 etc. with two molar equivalent (Rh : Ligand = 1 : 1 mol ratio) of the respective ligands under nitrogen atmosphere at room temperature for a period of 10-30 min. under stirring condition. In an embodiment of the present invention provides the preparation of novel rhodium(I) carbonyl complex of the type [Rh(CO)2ClL], where L is Py, 2-MePy, 3-MePy, 4-MePy, by stirring a solution of chlorobridged dimeric complex [Rh(CO)2Cl]2 in an organic solvent like CH2C12, CHC13 etc. with two molar equivalent respective ligand under nitrogen atmosphere at room temperature for 10-30 min. The IR spectra of the complexes show two almost equal intense terminal v(CO) bands in the range 2003-2090 cm"1, attributable to the cis-disposition of the carbonyl groups. The 1H NMR spectra of the 2 & 3 substituted methyl pyridine complexes exhibit a doublet in the range 8 8.28 - 8.96 ppm for α-proton, and two multipletes in the region 8 6.86 - 8.06 and 8 7.74 - 8.76 ppm for, p and y protons of the pyridine ring respectively. On the other hand, the 4- substituted methyl pyridine complex show two doublets in the range 8 8.40 - 9.04 ppm and 8 7.17 - 8.75 ppm for a and p protons of the pyridine respectively. In addition the complexes also show a singlet in the region 8 2.27 - 2.95 ppm for the substituted methyl protons. The 1H NMR values of the complexes, are in general, show a downfield shift compared to that of the free ligands. Such chemical shift is more prominent in case of a compared to the P and y analogous as they are remote from the coordinating N-donor of the pyridines. In another embodiment of the present invention provides the preparation of novel rhodium(I) carbonyl complex [Rh(CO)2ClL], α,N-diphenylnitrone, α-styryl-Nphenylnitrone, N.N-diphenyldinitrone, α-(2-furyl)-N-phenylnitrone by reacting chlorobridged dimeric complex [Rh(CO)2Cl]2 in an organic solvent like CH2C12, CHC13 etc. with two molar equivalent of the respective ligands. The IR spectra of the complexes exhibit two almost equally intense terminal v(CO) bands in the range 2000 - 2080 cm'1 indicating cis disposition of the two carbonyl groups. IR spectra also show a strong v(N- 0) stretching band in the range 1160 - 1200 cm"1, which is about 25 - 40 cm"1 lower compared to the corresponding free ligands value indicate formation of rhodium-oxygen bond. The 'H NMR spectra of all the complexes show a set of multipletes in the range 8 6.8 - 7.7 ppm assigned to the aromatic proton and other characteristic resonances for different types of-CH protons. It may be mentioned that except α-(2-furyl)-N-phenylnitrone all the nitrone complexes are very much stable in air. The complex containing α-(2-furyl)-Nphenylnitrone ligand on storing in a desiccator for about 10 days undergoes a decarbonylation reaction resulting a monocarbonyl complex [Rh(CO)Cl{a-(2-furyl)-Nphenylnitrone, indicated by a single terminal v(CO) value at 2000 cm"1. The v(COC) band occurs at 1025 cm"1 which is about 15 cm"1 lower compared to that of the parent complex indicating chelate formation through furyl oxygen atom. The chelation of the ligand is further substantiated by 'H NMR spectroscopy where the signal due to the -CH proton attached to furan oxygen atom is shifted to downfield compared to that of the nonchelated complex. Since the discovery, Monsanto catalyst [Rh(CO)2l2]" is still preferred as commercial catalyst for carbonylation of methanol to acetic acid. It is well known that a complex to act as an efficient catalyst, it should possess high electron density on the metal center which can be achieved by introducing different types of electron donor ligands in the metal complex species. It is anticipated that a 'Hard' donor like nitrogen or oxygen atom would increase the electron density on the central rhodium atom causing the complexes to behave as an efficient catalyst. In the present invention, the high electron density on the metal in the complexes [Rh(CO)2ClL], (where L is a pyridine or nitrone) is due to the presence of nitrogen and oxygen donor atom which normally do not have or very low 7t-accepting capacity. More over, some of the nitrone complexes are extra stable due to bidentate nature (Chelate formation) of the nitrone ligand to form a chelate complex. The following examples are given by way of illustration of the present invention and therefore should not be construed to limit the scope of the present invention. EXAMPLE -1 To a solution of of [Rh(CO)2Cl]2 ( 0.25 mmol) in CH2C12 (10 ml) pyridine (0.5 mmol) was added. The reaction mixture was stirred for 10 min. at 250c temperature under nitrogen atmosphere. The resulting solution was dried under vacuum and the solid compound obtained was washed with petroleum ether. Yield : 98 % . Anal. Calc. for C7H5ClO2NRh : C, 30.8 %; H, 1.83 %; N, 5.12 %. Found : C, 31.6 %; H, 1.86 %; N, 5.16. IR (KBr / cm'1) : 2086, 2015 v(CO). 'H NMR (CDC13 / 6 in ppm): 8.68 d (aH); 7.42-7.86 m ((3H); 7.42-7.86 m (λ.H). EXAMPLE-2 To a solution of [Rh(CO)2Cl]2 (0.25 mmol) in CH2C12 (10 ml) 2-methyl pyridine (0.5 mmol) was added. The reaction mixture was stirred for 10 min. at:- 280c cutemperature under nitrogen atmosphere. The resulting solution was dried in vacuum to get a solid mass which was washed with petroleum ether. Yield : 98 % . Anal. Calc. for C8H7ClO2NRh : C, 33.4 %; H, 2.4 %; N, 4.9, Cl, 12.2. Found : C, 34.2 %; H, 2.5 % N, 4.7 %, Cl, 11.9 %. IR (KBr / cm'1) : 2081 , 2009 v(CO). H NMR (CDC13 / 8 in ppm): 8.63 d (aH), 7.26-7.31 m ((3H), 7.74-7.80 m (AH), 2.95 s (Me). EXAMPLE-3 To a solution of of [Rh(CO)2Cl]2 (0.25 mmol) in CHC12 (10 ml), 3-methyl pyridine (0. 5 mmol) was added. The reaction mixture was stirred for 10 minutes at 30oc temperature under nitrogen. The solvent was removed under reduced pressure to get a solid mass which was washed with petroleum ether and kept in a desiccator. Yield : 98 % . Anal. Calc. for C8H7ClO2NRh: C, 33.4 %; H, 2.4 %; N, 4.9, Cl, 12.2. Found : C, 32.8 %; H, 2.3 % N, 5.0 %, Cl, 12.1 %. IR (KBr / cm'1): 2080 , 2003 v(CO). 'H NMR (CDC13 / 8 in ppm): 8.28 d (aH), 6.86-7.24 m (pH), 7.91-8.18 m (λH), 2.27 s (Me). EXAMPLE-4 To a solution of of [Rh(CO)2Cl]2 (0.25 mmol) in CHC12 (10 ml), 4-methyl pyridine (0. 5 mmol) was added. The reaction mixture was stirred for 10 min. at r£.£y> temperature under nitrogen. The solvent was removed under reduced pressure to get a solid mass which was washed with petroleum ether and kept in a desiccator. Yield : 98 % . Anal. Calc. for C8H7ClO2NRh: C, 33.4 %; H, 2.4 %; N, 4.9, Cl, 12.2. Found : C, 34.7 %; H, 2.5 % N, 5.1 %, Cl, 12.0 %. IR (KBr / cm'1): 2082 , 2013 v(CO). 'H NMR (CDC13 8 in ppm): 8.40 d (aH), 7.17 d ((3H), 2.40 s (Me). EXAMPLE - 5 [RhCO)2Cl]2 (0.05 mmol) in 15 ml CH2C12 was treated a,N-diphenylnitrone (0.10 mmol) and stirred for about 30 min. at room temperature under nitrogen. The solvent was then evaporated under reduced pressure to produce a solid compound which was washed with petroleum ether - hexane mixture. Yield : 93 % . Anal. Calc. for Ci5HnClNO3Rh : C, 46.03 %; H, 2.81 % N, 3.58 %. Found : C, 45.93 %; H, 2.76 % N, 3.49 %. IR (KBr / cm'1) : 2070, 2000 v(CO); 1170 v(NO). 'H NMR (CDC13 / 5 in ppm) : 7.0-7.6 (m,10H,C6H5), 8.2 (s,lH,CH). EXAMPLE - 6 [RhCO)2Cl]2 (0.05 mmol) in 15 ml CH2C12 was treated α-styryl-N-phenylnitrone (0.10 mmol) and stirred for about 30 min. at room temperature under nitrogen. The solvent was then evaporated under reduced pressure to produce a solid compound which was washed with petroleum ether - hexane mixture. Yield : 95 % . Anal. Calc. for Ci7Hi3ClNO3Rh : C, 48.92 %; H, 3.11 % N, 3.35 %. Found : C, 48.76 %; H, 3.21 % N, 3.44 %. IR (KBr / cm'1): 2070, 2005 v(CO); 1165 v(NO). 1H NMR (CDC13 / 6 in ppm): 6.8-7.4 (m,10H,C6H5), 7.6 (d,2H,CH), 7.8 (d,lH,CH=N). EXAMPLE - 7 [RhCO)2Cl]2 (0.05 mmol) in 15 ml CH2C12 was treated N-diphenyldinitrone (0.10 mmol) and stirred for about 30 min. at room temperature under nitrogen. The solvent was then evaporated under reduced pressure to produce a solid compound which was washed with petroleum ether - hexane mixture. Yield : 91 % . Anal. Calc. for Ci6Hi2ClN2O4Rh : C, 44.23 %; H, 2.76 % N, 6.46 %. Found : C, 44.34 %; H, 2.65 % N, 6.57 %. IR (KBr / cm'1) : 2080, 2020 v(CO); 1160 v(NO). 1H NMR (CDC13 / 8 in ppm): 7.1-7.7 (m,10H,C6H5), 8.4 (s,2H,CH). EXAMPLE-8 [RhCO)2Cl]2 (0.05 mmol) in 15 ml CH2C12 was treated oc-(2-furyl)-Nphenylnitrone (0.10 mmol) and stirred for about 30 min. at room temperature under nitrogen. The solvent was then evaporated under reduced pressure to produce a solid compound which was washed with petroleum ether - hexane mixture. Yield : 90 % . Anal. Calc. for Ci3H9ClNO4Rh : C, 40.94 %; H, 2.36 % N, 3.67 %. Found : C, 40.73 %; H, 2.45 % N, 3.71 %. IR (KBr / cm'1) : 2060, 2020 v(CO); 1200 v(NO). !H NMR (CDC13 / 6 in ppm): 6.9-7.4 (m,5H,C6H5),7.9 (s,lH,CH), 6.3 (s,lH,C4H3O), 7.6 (s,!H,C4H30), 7.7 (s, 1H,C4H30). In summary, a few rhodium metal complex of the type [Rh(CO)2ClL] where L = pyridine, 2-methylpyridine, 3-methylpyridine, 4-methylpyridine, a,N-diphenylnitrone, α-styryl-N-phenylnitrone, N.N^diphenyldinitrone and α-(2-furyl)-N-phenylnitrone is prepared by bridge splitting reaction of chlorobridged dimeric complexes [Rh(CO)2Cl]2 with the appropriate ligand at room temperature. Due to 'Hard' nature of the donor atom in the ligands, the metal atom become electron rich and thus may behave as a good carbonylation catalyst. The main advantages of the present invention are : (i) The complexes in general are very much stable in air as well as solution. (ii) The complexes are coordinatively unsaturated and easily undergo oxidative addition reaction with different electrphiles like alkyl halide, which is the key step in carbonylation of alcohol. (iii) The complexes can be used as catalyst precursors for carbonylation of alcohol. We Claim: 1. A process for the preparation of novel rhodium carbonyl complexes of formula {Rh (CO) 2C1 L} wherein L is a pyridine based nitrogen donor ligand or nitrone based N-oxide donor ligand as herein described which comprises reacting chlorobridged dimeric complex [Rh(CO)2Cl]2 in a chlorinated solvent with a ligand under inert atmosphere at a temperature ranging between 20-40°C for a period ranging between 10-30 minutes under stirring condition, removing solvent under vacuum to get complex followed by washing with a non - polar solvent as herein described to get the said rhodium carbonyl complex. 2. A process as claimed in claim 1 wherein the rhodium metal complex is [Rh (CO)2Cl(pyridine)] or [Rh(CO)2Cl(nitrone)]. 3. A process as claimed in claims 1 & 2 wherein the pyridine type ligand is selected from pyridine, 2-methyIpyridine, 3-methylpyridine, 4-methylpyridine. 4. A process as claimed in claims 1 to 3 wherein the nitrone ligand is selected from α-N-diphenylnitrone, α-styryl-N-phenylnitrone, N,N-diphenyldinitrone, α-(2-furyl)-N-phenylnitrone 5. A process as claimed in claim 1 to 4 wherein the ratio of rhodium and ligand is 1:2. 6. A process as claimed in claims 1-5 wherein the complexes can be used as catalyst for carbonization of alcohol to acid and/or ester. 7. A process as claimed in claim 1 to 6 wherein the chlorinated solvent is selected from CH2C12, CHC13. 8. A process as claimed in claims 1 to 7 wherein the non polar solvent is selected from petroleum, hexane and mixture thereof. 9. A process as claimed in claims 1 to 7 wherein the reaction is carried out under inert atmosphere. 10. A process for the preparation of novel rhodium carbonyl complexes substantially as herein described with references to the examples.

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