The Influence of Potentially Bridging Groups in Homogeneous

9 The Influence of Potentially Bridging Groups in Homogeneous Hydrogenation with Rhodium Complexes: A Study of Downloaded by EAST CAROLINA UNIV on December 15, 2017 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0132.ch009

para-Dimethylamino-Substituted Phenylphosphine Metal Complexes R. B. KING and C. R. BENNETT University of Georgia, Athens, Ga. 30602 Some studies are reported on the homogeneous hydrogena tion of cyclohexene with rhodium(I) chloride catalysts of the composition [(p-Me NC H ) P(C H ) ] RhCl (x = 1, 2, or 3; n = 1, 2, or 3) generated by room temperature reac tions of the cyclooctene complex [(C H ) RhCl] with the appropriate triarylphosphine. In some cases, the rates of these hydrogenations are inhibited by excess hydrogen con centration. This inhibitory effect of excess hydrogen concen tration increases with increasing p-dimethylamino substitu tion in the ligands. The rhodium(I) chloride catalysts containing the p-dimethylaminophenyl phosphines (p-Me ΝC Η ) Ρ(C Η ) (x = 2 and 3) had optimum activity for homogeneous hydrogenations at ligand:rhodium ratios of around 1.5 rather than 2, suggesting coordination of some of the dimethylamino nitrogen atoms as well as the trivalent phosphorus atoms to the rhodium in the catalysts. 2

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4 x

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5 3-x n

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14 2

2

2

6

4 Χ

6

5 3-x

A landmark i n the developing homogeneous hydrogénation catalysts was the discovery by Coffey (1 ) and by Osborn et al. (2) of the high activity of tris(triphenylphosphine)rhodium(I) chloride as a homoge neous hydrogénation catalyst. Since then, intensive effort (2, 3, 4, 5,6,7) has been devoted to the mechanisms of catalytic hydrogénations catalyzed by rhodium(I) complexes with the ultimate objective of improving the activity and selectivity of homogeneous catalysts based on rhodium(I) and related complexes. 124 Forster and Roth; Homogeneous Catalysis—II Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

Downloaded by EAST CAROLINA UNIV on December 15, 2017 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0132.ch009

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with Rh Complexes

125

One approach i n studying the triarylphosphine rhodium(I) deriva tives as hydrogénation catalysts has been to modify the structure of the triarylphosphine by introducing substituents into the aromatic ring (3). U n t i l now such studies have been limited to innocuous substituents such as p-methoxy and p-fluoro which could have significant electronic effects on the benzenoid ring but which would not affect the mechanism by complexing directly with the rhodium atom. Such work (3) showed that electronreleasing substituents like p-methoxy increased the catalytic activity of the triarylphosphine rhodium(I) halides whereas electron-withdrawing substituents such as p-fluoro decreased the catalytic activity of the rhodium complex. The enhancement of the catalytic activity of triarylphosphine rhod i u m ( I ) halides by introducing the electron-releasing p-methoxy group suggested the possibility of preparing extremely active hydrogénation catalysts by using the very strongly electron-releasing p-dimethylamino group as a substituent i n the triarylphosphine ligands of the rhodium(I) halide complexes. The p-dimethylamino group, however, had the pos sible complication of complexing directly with the rhodium(I) atom, thereby blocking some of the coordination sites needed to activate the hydrogen and/or the olefin to effect the desired catalysis. To evaluate whether the p-dimethylamino group would enhance the catalytic activity of triarylphosphine rhodium(I) complexes through its strong electronreleasing characteristics or inhibit the catalytic activity of these rhod i u m ( I ) complexes by coordinating with the rhodium atom, we prepared rhodium(I) complexes of the type [ ( p - M e N C H ) J P ( C H 5 ) _ ^ ] R h C l (x = 1, 2, or 3; η = 1, 2, or 3) and studied their catalytic activity for the homogeneous hydrogénation of cyclohexene. The problems asso ciated with the isolation of pure rhodium(I) complexes of this type were circumvented by generating these complexes in solution from the cyclooctene-rhodium(I) complex [ ( C H i 4 ) R h C l ] 2 and the desired quantity of the tertiary phosphine ( p - M e N C H ) JP( CeH ) . . This paper de scribes some novel effects of the hydrogen pressure, the olefin concen tration, the ligand-metal ratio, and the number of p-dimethylamino substituents on the rates of homogeneous hydrogénation of cyclohexene catalyzed by this series of rhodium(I) complexes. 2

8

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4

6

3

n

2

2

6

4

5 3 x

Experimental Preparation of Ligands. Tris ( p-dimethylaminophenyl ) phosphine (abbreviated as L ) and diphenyl(p-dimethylaminophenyl)phosphine (abbreviated as L ) were prepared by Konig's method (8). To prepare tris (p-dimethylaminophenyl) phosphine, a mixture of 30 m l of pyridine, 15 m l of phosphorus trichloride, and 30 m l of N,IV-dimethylaniline was boiled under reflux for 8 hrs. The resulting green solution was acidified, filtered, and excess starting materials were removed by steam distillation. 3

1

Forster and Roth; Homogeneous Catalysis—II Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

126

HOMOGENEOUS

CATALYSIS

II

The resulting dark solid mass was dissolved i n ethanol, filtered, and recrystallized several times from ethanol to give 7.03 grams (22% yield) of white tris (p-dimethylaminophenyl) phosphine, mp 305 °C (lit. (8) m p 3 0 8 ° - 3 1 0 ° C ) . The ligand diphenyl(p-dimethylaminophenyl)phosphine, mp 1 4 7 ° - 1 4 9 ° C (lit. (9) 1 5 2 ° - 1 5 3 ° C ) was prepared i n 10% yield by a similar procedure using diphenylchlorophosphine instead of phosphorus trichloride. This method was not suitable to prepare phenylbis ( p-dimethyl aminophenyl ) phosphine (abbreviated as L ) . This ligand was prepared in 23% yield by the method of Venanzi and co-workers (10) using the reaction of the lithium derivative from p-bromo-N,A/-dimethylaniline with phenyldichlorophosphine. Preparation of the Cyclooctene Complex [ ( C H i ) R h C l ] (11). A nitrogen-saturated mixture of 1.0 gram (3.8 mmoles) of rhodium tri chloride trihydrate, 3.0 m l (23 mmoles) of cyclooctene, and 30 m l of absolute ethanol was allowed to stand at room temperature for several days. The precipitate of [ ( C H i 4 ) R h C l ] 2 which gradually separated was removed by filtration, washed several times with small portions of ethanol, and dried. After drying at 5 0 ° - 6 0 ° C at 0.1 mm H g for two days, a sample was analyzed and calculated for C i H 9 C L R h : C , 53.6; H , 7.8%; found: C , 52.6; H , 7.9%. Kinetic Measurements. The hydrogénation experiments were run i n a water-jacketed two-neck reaction vessel attached to a standard atmos pheric pressure hydrogénation apparatus equipped with a gas buret. The temperature of the reaction mixture was held constant by circulating water from a constant temperature bath (controlled to ± 0 . 4 ° ) through the jacket of the reaction vessel. The neck of the reaction vessel not con nected to the hydrogénation apparatus was used to introduce substances into the reaction mixture against a counterflow of hydrogen. The reaction mixture was agitated with a magnetic stirrer at maximum speed which in the cylindrical reaction vessel produced a vortex that vigorously mixed the gas and solvent. Rate was followed by volumetric consumption of hydrogen at constant pressure. The required amount of the tertiary phosphine ligand and 100 m l of benzene was introduced into the cylindrical reaction vessel. The solution was then saturated with hydrogen by repeating four times, with constant stirring—a sequence consisting of evacuation and refill with hydrogen. The [ ( C H i ) R h C l ] was then added through the free neck i n a countercurrent of hydrogen. The mixture was then stirred for 30 to 90 m i n to allow time to generate the tertiary phosphine rhodium(I) catalyst and to hydrogenate any liberated cyclooctene. Decomposition was not ob served at this time. After this equilibration period, 10 m l of substrate solution was injected through a septum cap placed on the free neck. The uptake of hydrogen was then followed volumetrically. To ensure con stant volumes in each reaction, the substrate solution of cyclohexene ( redistilled over calcium hydride ) and hexane was held to 10 ml. Kinetic measurements were made before 5 % of the total cyclohexene had been consumed. A useful experimental technique arising from this work permits us to follow changes i n the hydrogénation rate with decreases i n the hydrogen concentration. W h e n the gas buret has been emptied by reaction of the


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Hydrogénation

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127

hydrogen with the cyclohexene, the pressure i n the apparatus can be lowered by lowering the mercury leveling bulb of the buret. The reaction is then monitored at a constant lower total pressure. Since the conditions are nearly identical for any particular reaction mixture, comparable rates can be obtained with this technique. The experimental results were put into a least squares program directly. There was seldom more than 1% variation from linearity for each run. The slope, which corresponds to the rate i n m l / m i n , was then converted as necessary for further interpretation. T h e linearity of these initial rate determinations show that there is no apparent product inhibi tion over this narrow range. Table I.

Effect of Hydrogen Pressure on the Rate of Hydrogénation

Ligand" L°

Temperature, °C 37.2

L

1

36.5

L

2

40.1

L

3

36.1

Hydrogen Pressure, mm 402 502 602 349 399 449 499 549 599 454 494 545 595 348 400 450 500 550 600

Rate in Moles/ Liter-Sec X 10~ 0.15 0.25 0.35 0.038 0.039 0.043 0.044 0.044 0.035 0.0095 0.011 0.012 0.011 0.088 0.13 0.15 0.13 0.079 0.041

b

See thé text for an explanation of these abbreviations. These experiments were all conducted at 0.714M cyclohexene concentration and with a 2:1 ligand : rhodium ratio. α

Results To clarify the mechanism of the homogeneous hydrogénation of cy clohexene with tertiary phosphine rhodium(I) complexes, the effects of the following variables on the rate of hydrogénation were examined: (1) hydrogen pressure ( Table I ) ; ( 2 ) cyclohexene concentration ( Table II ) ; and (3) ligand-rhodium ratio (Table I I I ) . In addition, determining the effect of p-dimethylamino substitution i n the triarylphosphines on the rate of hydrogénation allows evaluation of the relative importance of the electron-releasing and coordination position-blocking tendencies of the p-dimethylamino group. Forster and Roth; Homogeneous Catalysis—II Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

128

HOMOGENEOUS


Table II.

Temperature °C

L°

27.2 27.1 26.2 27.1 28.3 27.8 27.0 26.6 30.9 31.0 30.8 31.0 38.4 39.4 37.9 42.8 42.7 42.9 43.6 43.1 27.6 28.0 27.0 27.1 31.4 31.5 30.8 30.9 36.0 36.1 35.9

1

L

1

L

1

L

2

L

3

L

3

L

3

II

Effect of Cyclohexene Concentration on the Rate of Hydrogénation

Ligand"

L

CATALYSIS

Cyclohexene Rate in Moles/ Concentration, M Liter-Sec X 10~

6

0.532 0.714 0.893 1.06 0.532 0.80 1.06 1.332 0.621 0.714 0.80 0.893 0.444 0.621 0.80 0.267 0.319 0.444 0.532 0.621 0.267 0.80 1.06 1.33 0.532 0.621 0.714 0.80 0.621 0.80 0.893

0.043 0.048 0.047 0.045 0.0090 0.0079 0.010 0.0090 0.010 0.013 0.016 0.019 0.17 0.19 0.17 0.29 0.28 0.34 0.32 0.32 0.018 0.034 0.037 0.043 0.009 0.012 0.015 0.017 0.012 0.016 0.020

See the text for an explanation of these abbreviations. The experiments were all conducted at 600mm hydrogen pressure and with a 2:1 ligand : rhodium ratio. a

Wilkinson and co-workers (3) showed that the maximum activity of the tertiary phosphine rhodium(I) chloride catalysts occurred at a ligand:rhodium ratio of about 2. This ratio was used i n the systems studied for the effects of hydrogen pressure ( Table I ). In the triphenyl phosphine system (abbreviated as L ° ) , the rate of hydrogénation i n creased with pressure i n the accessible pressure range, i n accord with previous observations (2) by Wilkinson and co-workers. However, with the p-dimethylamino substituted tertiary phosphines L and L the rates of hydrogénation were essentially independent of the hydrogen pressure within the experimental errors. F o r tris ( p-dimethylaminophenyl )phos1

2



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Hydrogénation

129

with Rh Complexes

phine, the rate of hydrogénation decreased when the hydrogen pressure increased above 500 mm. These observations are the first indication that excess hydrogen pressure can inhibit the rhodium(I) catalyzed hydro génation of olefins. The inhibitory effect of excess hydrogen pressure seems to increase with increasing p-dimethylamino substitution of the triarylphosphine ligand. To measure the effect of cyclohexene concentration on the rate of the rhodium(I) catalyzed hydrogénation, a separate reaction mixture had to be prepared for the rate determination at each cyclohexene con centration. Reproducibility of the data in Table II was not determined. In some cases, the reaction rate increased with cyclohexene concentration as was found by Wilkinson and co-workers ( 2 ) . However, in a few cases the reaction rate was nearly independent of the cyclohexene concentra tion. This could arise from superimposition of inhibitory behavior of excess cyclohexene with the increase demonstrated for other cases. The measurements of the effect of the ligand-rhodium ratio on the rate of hydrogénation listed i n Table III show that for the p-dimethyl aminophenyl phosphines the optimum ligand-rhodium ratio is not 2, as with the triarylphosphines studied by Wilkinson and co-workers ( 3 ) . W i t h the p-dimethylamino substituted tertiary phosphines L and L the 2

Table III.

Ligand" L°

L

l

L

2

3

Effect of Ligand-Rhodium Ratio on the Rate of Hydrogénation

Temperature, °C 36.6 36.5 37.2 36.6 36.7 36.9 36.6 36.8 36.5 36.4 36.9 36.6 36.4 36.1 37.0 36.7 36.6 36.8 36.2

LigandRate in Moles/ Hydrogen Pressure, mm Rhodium Ratio Liter-Sec X 10~ 0.042 1 549 545 1.5 0.095 552 2 0.15 539 2.4 0.06 ~545 2.6 ~0.04 ~550 3 ~0.023 1 0.062 607 598 1.5 0.006 599 2 0.035 600 2.5 0.064 604 3 0.01 1 0.037 603 599 1.5 0.042 598 2 0.004 598 2.5 0.0036 594 1 0.05 599 1.5 0.094 603 2 0.008 598 2.5 0.008

&

See the text for an explanation of these abbreviations. These experiments were all conducted at 0.714M cyclohexene concentration. a


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rates of hydrogénation in systems with a ligand-rhodium ratio of 2 are only about 10% of the rates of hydrogénation in equivalent systems with a ligand-rhodium ratio of 1.5. This suggests that the coordination of the nitrogen atoms of the dimethylamino substituent to the rhodium atom is significant in these systems. Addition of sufficient ligand L or L to give what appears from the triphenylphosphine experiments to be the optimum phosphorus-rhodium ratio of 2 introduces sufficient p-dimethyl amino substituents to inhibit the reaction drastically by blocking needed sites on the catalyst by coordination with the rhodium atom. The most obvious conclusion from this work is that the introduction of p-dimethylamino substituents into the triarylphosphine ligands in the rhodium(I) halide complexes does not result in an unusually active homogeneous hydrogénation catalyst although the catalytic activity of the triarylphosphine-rhodium(I) chloride system is maintained. W e have not yet evaluated whether the p-dimethylamino substituents modify the selectivity of the rhodium(I) halide hydrogénation catalysts in any way. It is apparent, however, that the mechanistic complexities introduced into the system by the presence of a potentially strongly coordinating p-dimethylamino group preclude meaningful detailed quantitative treat ment of the data obtained in this work. Nevertheless, the results permit discussion of some of the more probable mechanisms of the homogeneous hydrogénation of olefins catalyzed by the triarylphosphine-rhodium ( I ) complexes. 2


II

3

Discussion The homogeneous hydrogénation systems discussed in this paper may be treated as analogues of enzyme systems with the rhodium catalyst as the enzyme ( E ) , hydrogen (Si) and cyclohexene (S ) as the substrates, and excess ligand or other donor site as the inhibitor ( I ) . The wellestablished mathematical operations of enzyme kinetics (12) can then be used to derive rate equations for various possible mechanisms. The following equations represent the essential features of the mech anism used by Wilkinson and co-workers (2, 3) with the added explicit expression of inhibition by excess tertiary phosphine ligand: 2

1 L RhCl + H — L RhH Cl -1 (E) (Si) (Ci) 2

2

2

2

2 L R h C l + olefin — L RhCl(olefin) -2 (E) (S ) (C ) 2

2

2

2


(1)

(2)

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KING AND BENNETT

Hydrogénation

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131

3 L RhCl + L — L RhCl -3 2

(3)

3

(Ε)

(I)

(C)

4 L R h H C l + olefin -> olefin H + L R h C l (Ci) (S ) (Ρ) (E) Equation 5 is then obtained for this mechanism: 2

2

2

(4)

2


2

. _ ^P _ 7 η π _ À: iflSiS Ef rate - ^ - h ^ , - γ ψ ^ Κ + Κ ^ Τ κ α 4

.-ν

2

{ d )

In this equation, Zc is the rate constant for Reaction 4 (product forma tion) above, K K , and K are the equilibrium constants for the corre sponding equilibria given above, and E is the total rhodium concentration. This rate equation does not account for our observations of inhibition by excess hydrogen nor possibly excess olefin since the powers of the Si and S terms in the denominator are less than or equal to the powers of Si and S in the numerator. To accommodate these features, we prefer the mechanism summarized below: 4

u

2

3

t

2

2

[LoRhCl] + 2 H 2

(E)

1 ^ [L RhCl] H -1

2

2

(Si)

2

(6)

4

(Ci)

2 [ L R h C l ] + H + olefin ^ [ L R h C l ] H (olefin) -2 2

2

2

(E)

2

(Si)

2

(S )

(C )

2

2

3 [ L R h C l ] + 2 olefin ^ [ L R h C l ] (olefin) -3 (E) (S ) (C.) 4 [L RhCl] + 2 L ^ [L,RhCl] -4 2

(7)

2

2

2

2

(8)

2

2

2

2

(Ε)

(9)

2

(I)

(C ) 4

5 [ L R h C l ] H (olefin) > (olefin)H + [ L R h C l ] (10) (C ) (Ρ) (E) In this mechanism, the catalytically active rhodium(I) derivative is a bimetallic compound thereby providing active sites on two adjacent rhodium atoms. The methods of enzyme kinetics (12) yield the following rate equation for this bimetallic mechanism: , _ dP τ, ρ __ A;5UL SiS Ef .ν ~ dt ~ ^ ~ 1 + ATxSi + KSS, + # S + K,V ' 2

2

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m

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2


{

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HOMOGENEOUS CATALYSIS

II

The quadratic terms i n the denominator are consistent with our observa tion of inhibition by excess hydrogen and possibly excess cyclohexene. The amount of inhibition by excesses of hydrogen and cyclohexene will depend on the magnitudes of K i and K , respectively. A further advantage of the bimetallic mechanism suggested above is that a bimetallic complex of the type [ L R h C l ] , which is proposed as the active catalyst, has been isolated in the case of triphenylphosphine (i.e. L = ( C H r , ) P ) from the decomposition of [ ( C e H s ^ P L R h C l . The ob served (2) reactivity of [ L R h C l ] ( L = ( C H ) P ) toward oxygen and carbon monoxide makes plausible the postulated reactivity of [ L R h C l ] toward hydrogen and olefins which is an essential part of the proposed bimetallic mechanism. Despite the work presented, no definitive mechanism for the homo geneous hydrogénation of olefins with triarylphosphinerhodium ( I ) ha lide catalysts is postulated here. However the experimental observations of inhibition by excess hydrogen and possibly excess olefin are incon sistent with the previously postulated mechanisms (2, 3) involving monometallic active species. The analysis presented suggests that a mechanism involving bimetallic intermediates with the metal sites in reasonable proximity can account for all presently available experimental observations. 3

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2

Acknowledgment W e acknowledge many helpful discussions with J. F . Garst. Literature Cited 1. Coffey, R. S., British Patent 1,121,642 (February 18, 1965). 2. Osborn, J. Α., Jardine, F. H., Young, J. F., Wilkinson, G., J. Chem. Soc. A (1966) 1711. 3. Montelatici, S., van der Ent, Α., Osborn, J. Α., Wilkinson, G., J. Chem. Soc. A (1968) 1054. 4. O'Connor, C., Wilkinson, G., Tetrahedron Lett. (1969) 1375. 5. Candlin, J. P., Oldham, A. R., Discuss. Faraday Soc. (1968) 60. 6. Augustine, R. L., van Peppen, M. F., Chem. Commun. (1970) 571. 7. van Bekkum, H., van Rantwijk, F., van de Putte, T., Tetrahedron Lett. (1969) 1. 8. König, E., Friedrich, H . , Ann. (1934) 509, 138. 9. Trippett, S., Walker, D. M . , J. Chem. Soc. (1961) 2130. 10. Fritz, H. P., Gorden, I. R., Schwarzhans, K. E., Venanzi, L. M.,J.Chem. Soc. (1965) 5210. 11. Porri, L., Lionetti, Α., Allegra, G., Immirzi, Α., Chem. Commun. (1965) 336. 12. Reiner, J. M., "Behavior of Enzyme Systems," 2nd ed., van Nostrand Reinhold, New York, 1969. RECEIVED July 30, 1973. This work was supported by the Air Force Office of Scientific Research under Grants AF-AFOSR-68-1435 and AF-AFOSR-71-2000. Abstracted in part from the doctoral dissertation of C. R. Bennett, University of Georgia, 1973.


The Influence of Potentially Bridging Groups in Homogeneous

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