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Homogeneous catalytic hydrogenation of olefins using RhH2(Ph2N3)(PPh3)2 in tetrahydrofuran
Journul
of Molecular
Catalysis,
75 (1992)
15
15-20
M2884
Homogeneous catalytic hydrogenation of olefins using RhH2(Ph2N3)(PPh& in tetrahydrofuran Noriyuki Kameda*
and Reiko Igarashi
Department of Chemistry, Fbw.bashi 274 (Japan) (Received
September
College of Science
16, 1991; accepted
and Technology,
Nihon
University,
February 26, 1992)
Abstract The catalytic hydrogenation activity of RhHz(Ph,N,)(PPh,), in tetrahydrofuranwith different unsaturated compounds (ally1 alcohol, cinnamic alcohol, cinnamaldehyde, acrylic acid, acrolein, cinnamic acid, acrylonitrile, cinnamonitrile and styrene) has been studied. The results obtained with acrolein, acrylic acid and acrylonitrile showed reduction, although other substrates did not. The highest activity was observed with acrylonitrile. A possible mechanism for this reaction is suggested.
Introduction Rhodium hydride complexes are of great interest because of their potential as homogeneous catalysts for hydrogenation and other reactions of organic substrates [I, 21. We have found that RhHz(PhZN,)(PPh& is an effective catalyst for the homogeneous hydrogenation of olefins in THF under mild conditions. This paper describes the results of the hydrogenation of different types of olelins. The substrates studied were styrene and olefins, including those with functional groups (unsaturated nitrile, aldehyde, alcohol and carboxylic acid). Of the latter, functional groups such as cyano, aldehyde or carboxylic acid were not reduced. Kinetic studies of the hydrogenation of acrylonitrile were also carried out.
Experimental RhHs(PhsN,)(PPh& was prepared according to the literature [3]. All solvents were of Dotite Spectrosol grade, purchased from Wako Chemicals, and were freshly distilled under nitrogen (tetrahydrofuran (THF), toluene and benzene from Na, dimethyl sulfoxide (DMSO) andN,N-dimethylformamide (DMF) from CaH,, acetone from 4A molecular sieves). Acrylonitrile (AN) *Author to whom correspondence
0304-5102/92/$5.00
should be addressed.
0 1992 - Elsevier
Sequoia. All rights reserved
16
and styrene were purified by the usual methods under nitrogen and kept at - 20 “C before use. Acrylic acid and acrolein were distilled under nitrogen before use. Cinnamic acid was recrystallized from ethanol. Other substrates were of commercial origin. GLC analysis indicated that they were > 97% pure. Extra pure grade hydrogen (Nippon Oxygen Co., 99.99999% purity) was used without further purification. Measurements Infrared spectra were obtained with either a Hitachi 270-50 spectrophotometer or a JASCO FlYIR-8000 FI’-IR spectrophotometer. ‘H NMR spectrawere recorded in CDClawith a JEOL JNM-GX 400 Fl’ NMR spectrometer using tetramethylsilane as an internal standard. HPLC analysis was performed on a Waters ALC/GPC 201A liquid chromatograph system. GLC analyses were performed on either a Shimadzu 14A or 15A gas chromatograph system. Hydrogenation
procedure
In a 100 ml reaction flask was placed a Teflon-coated stirring bar; the air in the reaction flask was purged by filling the flask with nitrogen. To this was added catalyst, solvent and substrate. Immediately, the reaction flask was degassed using three freeze-pump-thaw cycles at lop3 torr on a vacuum line in a liquid nitrogen bath. The reaction flask was then immersed in a thermostatted water bath maintained at a constant temperature. Hydrogen was then admitted into the reaction flask to a pressure of 1 atm, which was maintained during the reaction. The dependence of the hydrogenation of AN on the hydrogen concentration was measured in mixed gases of hydrogen and nitrogen under 1 atm pressure. Characterixati4m
The yields of reaction products from the hydrogenation of cinnamic acid were determined by HPLC attached to a p-Bondasphere Cl&100 A column. The eluate solution contained 50 vol.% methanol and 50 mm01 H3P04. The other yields of reaction products were determined by GLC. Analyses were accomplished using a 3 mm X 3.1 m glass column packed with either SBS200 20% on Shimalite W (AW-DMCS) or Thermon25% on Shimalite W (AW-DMCS). Column temperature and gas flow rates were varied until good resolution of the components was obtained.
Results and discussion Catalytic
hydrogenation
of various
unsaturated
compounds
The homogeneous hydrogenation of various unsaturated compounds containing an oleflnic bond using RhHa(PhzN3)(PPh3)2 in THF was carried out at 30 “C under a constant Hz pressure of 1 atm for 1 h. The results are shown in Table 1. Terminal olefins such as acrylonitrile (AN), acrolein and acrylic acid were reduced, although styrene and ally1 alcohol were not.
17 TABLE
1
Hydrogenation
of unsaturated compounds
Substrate
using RhH,(Ph,N,)(PPh&
Product
in THF Yield (%)
acrylonitrile acrolein acrylic acid Conditions: RhH,(Ph2N,)(PPh& 1 atm, 1 h.
propionitrile propionaldehyde propionic acid
100 74 53
0.01 mmol, THF 20 cm3, substrate 1 mmol; 30 “C, Hz pressure
No hydrogenation of internal oletis such as cinnamaldehyde, cinnamic acid and cinnamonitrile was observed. Functional groups such as cyano, aldehyde or carboxylic acid were not reduced under these conditions. It is of interest to note that the hydrogenation of ally1 alcohol is not observed, in contrast to its rapid hydrogenation by RhCl(PPh& [4] and RhH(CO)(PPh,), [ 2 1. The highest activity is observed with AN, where 100% conversion is achieved. Consequently, RhHz(PhsNs)(PPh& in THF is found to be an exceedingly effective catalyst for the highly selective homogeneous hydrogenation of AN. Eflect of solvent The homogeneous hydrogenation of AN catalyzed with RhHz(Ph,NJ(PPh& in several solvents such as DMSO, DMF, THF, acetone, toluene and benzene was examined at 30 “C under a constant Hz pressure of 1 atm for 1 h. The results are shown in Fig. 1. RhHz(PhzN3)(PPh& dissolved in solvents such as DMF and acetone was efficient as a catalyst for the hydrogenation of AN, but was less efficient in aromatic solvents such as benzene and toluene. Kinetics of AN hydrogenation As shown in Fig. 1, the highest rate of hydrogenation was observed in THF, which was thus used as a solvent in this kinetic study. The yield of propionitrile (PN) was proportional to reaction time up to 55% conversion, as shown in Fig. 1. The rate of hydrogenation was derived from the linear part of the curve. Unless otherwise noted, hydrogenation was carried out under standard condition in which RhHz(PhaN,)(PPh,), (5.0 X lop4 mol dne3), AN (5.0 X lop2 mol dme3) and THF (20 cm3) were heated at 30 “C under a constant H2 pressure of 1 atm. Dependence on the comxntratim of catalyst, hydrogen and AN From the plots in Figs. 2-4, it can be seen that the rate of hydrogenation of AN increased linearly with increasing catalyst and hydrogen concentrations, but was independent of AN concentration.
18
Fig. 1. Effect of solvent on the hydrogenation of acrylonitrile. Conditions: RhHa(PhaN,)(PPh,), 0.01 mmol, solvent 20 cm3, acrylonitrile 1 mmol; 30 “C, Ha pressure 1 atm. (0) THF, (A) DMF, (0) acetone, (0) DMSO, (A) benzene, (M) toluene. Fig. 2. Dependence of the rate of hydrogenation of acrylonitrile on the catalyst concentration in THF at 30 “C, with acrylonitrile (5.0 X lo-’ mol dmm3) and THF (20 cm3), under a constant Ha pressure of 1 atm.
E ._
:c 5.
E 3 0.4 E D
20-
? E n 5 E to-
Q
“0 x aI 5 cc 15 [H$x103/moi
dm3
OO
2.5
7.5
50
IANlx102/mol
10
dni3
3. Dependence of the rate of hydrogenation of acrylonitrile on the hydrogen concentration in THF at 30 “C, with RhH,(PhaN3)(PPh& (5.0X low4 mol dmm3), acrylonitrile (5.0X 10-a mol dme3) and THF (20 cm3). Fig.
Fig. 4. Dependence of the rate of hydrogenation of acrylonitrile on acrylonitrile concentration in THF at 30 “C, with RhHa(PhaN3)(PPh,), (5.0X lo-* mol drn3) and THF (20 cm3), under a constant Ha pressure of 1 atm.
Effect of For addition genation reaction genation
added PPh3 RhCl(PPh& (51 and RhH(CO)(PPh,), [2], it is known that the of excess triphenylphosphine (PPhJ inhibits or suppresses hydroof the oleti. The effect of the addition of PPh3 to the present system was investigated. The relation between the rate of hydroand the concentration of the added PPh3 is shown in Fig. 5. The
19
OO
10 [PPhJxlOL/mol
20 dmm3
Fig. 5. Dependence of the rate of hydrogenation of acrylonitrile on the added triphenylphosphine concentration in THF at 30 “C, with RhHa(Ph,N,)(PPh,), (5.0 X 10m4 mol dme3), acrylonitrile (5.0~ 10-a mol dmm3) and THF (20 cm3), under a constant Ha pressure of 1 atm. Fig. 6. Arrhenius plot of the hydrogenation of acrylonitrile in THF at 20, 30, 40 and 50 “C, with RhHz(Ph,N3)(PPh,), (5.0 X 10m4mol dm-a), acrylonitrile (5.0X 10-a mol dmm3) and THF (20 cm3), under a constant Ha pressure of 1 atm.
rate of hydrogenation is not decreased at all by the addition of PPha over a molar ratio of [PPh,]/[RhH,(Ph,N,)(PPh,),] ranging from 1 to 5. This indicates that the dissociation of PPh, from RhHz(Ph,N,)(PPh& either did not occur during the hydrogenation or was not involved in the rate-determining step. Dependence on reacticm temperature The rate of hydrogenation was measured at 20, 30, 40 and 50 “C, and a plot of log rate VS. l/T is shown in Fig. 6. A value of 42.7 kJ mol-’ for the activation energy is obtained from the slope of the line in Fig. 6. Reaction of RhH, (PhzN3_l(PPh3_12with AN and mechanism of AN hydrogenation Treatment of RhHz(PhzN,)(PPh,), (0.066 g) with excess AN (1 cm3) in THF (20 cm3) solution under 1 atm of nitrogen at 30 “C for 1 h gave a homogeneous solution found by GLC analysis after partial concentration of the product solution to contain PN. The addition of n-hexane to the solution yielded a precipitate. It was washed twice with n-hexane and a brown solid (36%) was obtained. This brown solid showed IR bands (KBr) at 1192, 1282, 159 1 cm- ’ assignable to the presence of a monodentate 1,3_diphenyltriazenido ligand [6], and a band at 2201 cm-’ assignable to V(C=N), which is observed to shift by ca. 40 cm-’ to lower frequency from the v(C=N) band of free AN, analogous to the observation for Pt(PPh,),(AN) [7]. The brown solid showed no IR band ascribable to u(Rh-H). The ‘H NMR spectra of this brown solid showed peaks at 7.1 to 7.8 ppm and broad peaks at 1.84 ppm respectively, in a ratio of 11:l. The peaks at 7.1 to 7.8 ppm are assignable to an overlap of the 1,3-diphenyltriazenido ligand with PPh3, and the one at 1.84 ppm is assignable to olefinic protons of AN coordinated to rhodium, which is shifted to higher field than free AN, analogous to the observation for Pt(PPh,),(AN) [7]. The ratio of the peaks suggests the formula
20
Rh(Ph2N&PPh&&4N), where Ph2N3 is the monodentate azenido ligand. Hence, the reaction is: RhHa(PhaNa)(PPh&
+ 2AN =
Rh(Ph,N,)(PPh&(AN)
+ PN
1,3-diphenyltri(1)
Recently, it was reported that the complex containing a dinitrogen ligand (N,N’-diaryl-diiminoacenaphthene)Pd(alkene) is an effective catalyst for the hydrogenation of unsaturated substrates F31. Accordingly, Rh(Ph,N,)(PPh,),(AN) is considered as a reaction intermediate. The kinetic data, i.e. zero order with respect to the concentration of AN and first order with respect to the concentrations of hydrogen and RhH2(Ph2N3)(PPh&, indicate that eqns. (1) and (3) are rapid steps and eqn. (2) is the rate-determining step: Rh(Ph,N,)(PPh,),(AN) Rh(Ph,N,)(PPh,),
+ H2 -
+AN -
Rh(Ph,N,)(PPh,),
Rh(Ph,N,)(PPh&(AN)
+ PN
(2) (3)
Although the nature of Rh(Ph,N,)(PPh,), is not clear, the foregoing equations can be obtained from the experimental results. Further studies on the catalytic reaction using RhHz(Ph2N3)(PPh& are required for the elucidation of the reaction mechanism.
References 1 (a) B. R. James, Homogeneous Hydrogenation, Wiley-Interscience, New York, 1973, chap. 11, pp. 198-287; (b) B. R. James, Adv. Orgumt. Chem., 17 (1979) 319; (c) T. Okano and T. Yoshida, Yuki Gosei Kagaku Kyokaishi, 41 (1983) 359. 2 C. O’Connor and G. Wilkinson, J. Chem. Sot., A (1968) 2665. 3 K. R. Laing, S. D. Robinson and M. F. Uttley, J. Chem. Sot., Dalton Trans., (1974) 1205. 4 F. H. Jardine, J. A. Osbom and G. Wilkinson, .J. Chem. Sot., A (1967) 1574. 5 J. A. Osbom, F. H. Jardine, J. F. Young and G. Wilkinson, J. Chm. Sot., A (1966) 1711. 6 W. H. Knoth, Imrg. Chem., 12 (1973) 38. 7 S. Cenini, R. Ugo and G. La Monica, J. Cha. Sot., A (1971) 409. 8 R. Van Asselt and C. J. Elsevier, J. Mol. Cat&., 65 (1991) L13..
of Molecular
Catalysis,
75 (1992)
15
15-20
M2884
Homogeneous catalytic hydrogenation of olefins using RhH2(Ph2N3)(PPh& in tetrahydrofuran Noriyuki Kameda*
and Reiko Igarashi
Department of Chemistry, Fbw.bashi 274 (Japan) (Received
September
College of Science
16, 1991; accepted
and Technology,
Nihon
University,
February 26, 1992)
Abstract The catalytic hydrogenation activity of RhHz(Ph,N,)(PPh,), in tetrahydrofuranwith different unsaturated compounds (ally1 alcohol, cinnamic alcohol, cinnamaldehyde, acrylic acid, acrolein, cinnamic acid, acrylonitrile, cinnamonitrile and styrene) has been studied. The results obtained with acrolein, acrylic acid and acrylonitrile showed reduction, although other substrates did not. The highest activity was observed with acrylonitrile. A possible mechanism for this reaction is suggested.
Introduction Rhodium hydride complexes are of great interest because of their potential as homogeneous catalysts for hydrogenation and other reactions of organic substrates [I, 21. We have found that RhHz(PhZN,)(PPh& is an effective catalyst for the homogeneous hydrogenation of olefins in THF under mild conditions. This paper describes the results of the hydrogenation of different types of olelins. The substrates studied were styrene and olefins, including those with functional groups (unsaturated nitrile, aldehyde, alcohol and carboxylic acid). Of the latter, functional groups such as cyano, aldehyde or carboxylic acid were not reduced. Kinetic studies of the hydrogenation of acrylonitrile were also carried out.
Experimental RhHs(PhsN,)(PPh& was prepared according to the literature [3]. All solvents were of Dotite Spectrosol grade, purchased from Wako Chemicals, and were freshly distilled under nitrogen (tetrahydrofuran (THF), toluene and benzene from Na, dimethyl sulfoxide (DMSO) andN,N-dimethylformamide (DMF) from CaH,, acetone from 4A molecular sieves). Acrylonitrile (AN) *Author to whom correspondence
0304-5102/92/$5.00
should be addressed.
0 1992 - Elsevier
Sequoia. All rights reserved
16
and styrene were purified by the usual methods under nitrogen and kept at - 20 “C before use. Acrylic acid and acrolein were distilled under nitrogen before use. Cinnamic acid was recrystallized from ethanol. Other substrates were of commercial origin. GLC analysis indicated that they were > 97% pure. Extra pure grade hydrogen (Nippon Oxygen Co., 99.99999% purity) was used without further purification. Measurements Infrared spectra were obtained with either a Hitachi 270-50 spectrophotometer or a JASCO FlYIR-8000 FI’-IR spectrophotometer. ‘H NMR spectrawere recorded in CDClawith a JEOL JNM-GX 400 Fl’ NMR spectrometer using tetramethylsilane as an internal standard. HPLC analysis was performed on a Waters ALC/GPC 201A liquid chromatograph system. GLC analyses were performed on either a Shimadzu 14A or 15A gas chromatograph system. Hydrogenation
procedure
In a 100 ml reaction flask was placed a Teflon-coated stirring bar; the air in the reaction flask was purged by filling the flask with nitrogen. To this was added catalyst, solvent and substrate. Immediately, the reaction flask was degassed using three freeze-pump-thaw cycles at lop3 torr on a vacuum line in a liquid nitrogen bath. The reaction flask was then immersed in a thermostatted water bath maintained at a constant temperature. Hydrogen was then admitted into the reaction flask to a pressure of 1 atm, which was maintained during the reaction. The dependence of the hydrogenation of AN on the hydrogen concentration was measured in mixed gases of hydrogen and nitrogen under 1 atm pressure. Characterixati4m
The yields of reaction products from the hydrogenation of cinnamic acid were determined by HPLC attached to a p-Bondasphere Cl&100 A column. The eluate solution contained 50 vol.% methanol and 50 mm01 H3P04. The other yields of reaction products were determined by GLC. Analyses were accomplished using a 3 mm X 3.1 m glass column packed with either SBS200 20% on Shimalite W (AW-DMCS) or Thermon25% on Shimalite W (AW-DMCS). Column temperature and gas flow rates were varied until good resolution of the components was obtained.
Results and discussion Catalytic
hydrogenation
of various
unsaturated
compounds
The homogeneous hydrogenation of various unsaturated compounds containing an oleflnic bond using RhHa(PhzN3)(PPh3)2 in THF was carried out at 30 “C under a constant Hz pressure of 1 atm for 1 h. The results are shown in Table 1. Terminal olefins such as acrylonitrile (AN), acrolein and acrylic acid were reduced, although styrene and ally1 alcohol were not.
17 TABLE
1
Hydrogenation
of unsaturated compounds
Substrate
using RhH,(Ph,N,)(PPh&
Product
in THF Yield (%)
acrylonitrile acrolein acrylic acid Conditions: RhH,(Ph2N,)(PPh& 1 atm, 1 h.
propionitrile propionaldehyde propionic acid
100 74 53
0.01 mmol, THF 20 cm3, substrate 1 mmol; 30 “C, Hz pressure
No hydrogenation of internal oletis such as cinnamaldehyde, cinnamic acid and cinnamonitrile was observed. Functional groups such as cyano, aldehyde or carboxylic acid were not reduced under these conditions. It is of interest to note that the hydrogenation of ally1 alcohol is not observed, in contrast to its rapid hydrogenation by RhCl(PPh& [4] and RhH(CO)(PPh,), [ 2 1. The highest activity is observed with AN, where 100% conversion is achieved. Consequently, RhHz(PhsNs)(PPh& in THF is found to be an exceedingly effective catalyst for the highly selective homogeneous hydrogenation of AN. Eflect of solvent The homogeneous hydrogenation of AN catalyzed with RhHz(Ph,NJ(PPh& in several solvents such as DMSO, DMF, THF, acetone, toluene and benzene was examined at 30 “C under a constant Hz pressure of 1 atm for 1 h. The results are shown in Fig. 1. RhHz(PhzN3)(PPh& dissolved in solvents such as DMF and acetone was efficient as a catalyst for the hydrogenation of AN, but was less efficient in aromatic solvents such as benzene and toluene. Kinetics of AN hydrogenation As shown in Fig. 1, the highest rate of hydrogenation was observed in THF, which was thus used as a solvent in this kinetic study. The yield of propionitrile (PN) was proportional to reaction time up to 55% conversion, as shown in Fig. 1. The rate of hydrogenation was derived from the linear part of the curve. Unless otherwise noted, hydrogenation was carried out under standard condition in which RhHz(PhaN,)(PPh,), (5.0 X lop4 mol dne3), AN (5.0 X lop2 mol dme3) and THF (20 cm3) were heated at 30 “C under a constant H2 pressure of 1 atm. Dependence on the comxntratim of catalyst, hydrogen and AN From the plots in Figs. 2-4, it can be seen that the rate of hydrogenation of AN increased linearly with increasing catalyst and hydrogen concentrations, but was independent of AN concentration.
18
Fig. 1. Effect of solvent on the hydrogenation of acrylonitrile. Conditions: RhHa(PhaN,)(PPh,), 0.01 mmol, solvent 20 cm3, acrylonitrile 1 mmol; 30 “C, Ha pressure 1 atm. (0) THF, (A) DMF, (0) acetone, (0) DMSO, (A) benzene, (M) toluene. Fig. 2. Dependence of the rate of hydrogenation of acrylonitrile on the catalyst concentration in THF at 30 “C, with acrylonitrile (5.0 X lo-’ mol dmm3) and THF (20 cm3), under a constant Ha pressure of 1 atm.
E ._
:c 5.
E 3 0.4 E D
20-
? E n 5 E to-
Q
“0 x aI 5 cc 15 [H$x103/moi
dm3
OO
2.5
7.5
50
IANlx102/mol
10
dni3
3. Dependence of the rate of hydrogenation of acrylonitrile on the hydrogen concentration in THF at 30 “C, with RhH,(PhaN3)(PPh& (5.0X low4 mol dmm3), acrylonitrile (5.0X 10-a mol dme3) and THF (20 cm3). Fig.
Fig. 4. Dependence of the rate of hydrogenation of acrylonitrile on acrylonitrile concentration in THF at 30 “C, with RhHa(PhaN3)(PPh,), (5.0X lo-* mol drn3) and THF (20 cm3), under a constant Ha pressure of 1 atm.
Effect of For addition genation reaction genation
added PPh3 RhCl(PPh& (51 and RhH(CO)(PPh,), [2], it is known that the of excess triphenylphosphine (PPhJ inhibits or suppresses hydroof the oleti. The effect of the addition of PPh3 to the present system was investigated. The relation between the rate of hydroand the concentration of the added PPh3 is shown in Fig. 5. The
19
OO
10 [PPhJxlOL/mol
20 dmm3
Fig. 5. Dependence of the rate of hydrogenation of acrylonitrile on the added triphenylphosphine concentration in THF at 30 “C, with RhHa(Ph,N,)(PPh,), (5.0 X 10m4 mol dme3), acrylonitrile (5.0~ 10-a mol dmm3) and THF (20 cm3), under a constant Ha pressure of 1 atm. Fig. 6. Arrhenius plot of the hydrogenation of acrylonitrile in THF at 20, 30, 40 and 50 “C, with RhHz(Ph,N3)(PPh,), (5.0 X 10m4mol dm-a), acrylonitrile (5.0X 10-a mol dmm3) and THF (20 cm3), under a constant Ha pressure of 1 atm.
rate of hydrogenation is not decreased at all by the addition of PPha over a molar ratio of [PPh,]/[RhH,(Ph,N,)(PPh,),] ranging from 1 to 5. This indicates that the dissociation of PPh, from RhHz(Ph,N,)(PPh& either did not occur during the hydrogenation or was not involved in the rate-determining step. Dependence on reacticm temperature The rate of hydrogenation was measured at 20, 30, 40 and 50 “C, and a plot of log rate VS. l/T is shown in Fig. 6. A value of 42.7 kJ mol-’ for the activation energy is obtained from the slope of the line in Fig. 6. Reaction of RhH, (PhzN3_l(PPh3_12with AN and mechanism of AN hydrogenation Treatment of RhHz(PhzN,)(PPh,), (0.066 g) with excess AN (1 cm3) in THF (20 cm3) solution under 1 atm of nitrogen at 30 “C for 1 h gave a homogeneous solution found by GLC analysis after partial concentration of the product solution to contain PN. The addition of n-hexane to the solution yielded a precipitate. It was washed twice with n-hexane and a brown solid (36%) was obtained. This brown solid showed IR bands (KBr) at 1192, 1282, 159 1 cm- ’ assignable to the presence of a monodentate 1,3_diphenyltriazenido ligand [6], and a band at 2201 cm-’ assignable to V(C=N), which is observed to shift by ca. 40 cm-’ to lower frequency from the v(C=N) band of free AN, analogous to the observation for Pt(PPh,),(AN) [7]. The brown solid showed no IR band ascribable to u(Rh-H). The ‘H NMR spectra of this brown solid showed peaks at 7.1 to 7.8 ppm and broad peaks at 1.84 ppm respectively, in a ratio of 11:l. The peaks at 7.1 to 7.8 ppm are assignable to an overlap of the 1,3-diphenyltriazenido ligand with PPh3, and the one at 1.84 ppm is assignable to olefinic protons of AN coordinated to rhodium, which is shifted to higher field than free AN, analogous to the observation for Pt(PPh,),(AN) [7]. The ratio of the peaks suggests the formula
20
Rh(Ph2N&PPh&&4N), where Ph2N3 is the monodentate azenido ligand. Hence, the reaction is: RhHa(PhaNa)(PPh&
+ 2AN =
Rh(Ph,N,)(PPh&(AN)
+ PN
1,3-diphenyltri(1)
Recently, it was reported that the complex containing a dinitrogen ligand (N,N’-diaryl-diiminoacenaphthene)Pd(alkene) is an effective catalyst for the hydrogenation of unsaturated substrates F31. Accordingly, Rh(Ph,N,)(PPh,),(AN) is considered as a reaction intermediate. The kinetic data, i.e. zero order with respect to the concentration of AN and first order with respect to the concentrations of hydrogen and RhH2(Ph2N3)(PPh&, indicate that eqns. (1) and (3) are rapid steps and eqn. (2) is the rate-determining step: Rh(Ph,N,)(PPh,),(AN) Rh(Ph,N,)(PPh,),
+ H2 -
+AN -
Rh(Ph,N,)(PPh,),
Rh(Ph,N,)(PPh&(AN)
+ PN
(2) (3)
Although the nature of Rh(Ph,N,)(PPh,), is not clear, the foregoing equations can be obtained from the experimental results. Further studies on the catalytic reaction using RhHz(Ph2N3)(PPh& are required for the elucidation of the reaction mechanism.
References 1 (a) B. R. James, Homogeneous Hydrogenation, Wiley-Interscience, New York, 1973, chap. 11, pp. 198-287; (b) B. R. James, Adv. Orgumt. Chem., 17 (1979) 319; (c) T. Okano and T. Yoshida, Yuki Gosei Kagaku Kyokaishi, 41 (1983) 359. 2 C. O’Connor and G. Wilkinson, J. Chem. Sot., A (1968) 2665. 3 K. R. Laing, S. D. Robinson and M. F. Uttley, J. Chem. Sot., Dalton Trans., (1974) 1205. 4 F. H. Jardine, J. A. Osbom and G. Wilkinson, .J. Chem. Sot., A (1967) 1574. 5 J. A. Osbom, F. H. Jardine, J. F. Young and G. Wilkinson, J. Chm. Sot., A (1966) 1711. 6 W. H. Knoth, Imrg. Chem., 12 (1973) 38. 7 S. Cenini, R. Ugo and G. La Monica, J. Cha. Sot., A (1971) 409. 8 R. Van Asselt and C. J. Elsevier, J. Mol. Cat&., 65 (1991) L13..