10 The Hydrogenation of 1-Hexene Catalyzed by Solutions of Hydridotetrakis(Triphenylphosphine)Rhodium(I) JES HJORTKJAER Technical University of Denmark; Bygning 227; Dk 2800 Lyngby; Denmark
The hydrogenation of 1-hexene catalyzed by solutions of HRh(PPh ) in benzene was found to follow the rate ex pression: 3
r = k
4
a
·
C
A
·
C
S
·
C
H2
+
k
·
b
C
Β
· C
s
·
C
H2
where k and k are rate constants, C and C are the con centrations of HRh(PPh ) and HRh(PPh ) , C is the alkene concentration, and C is the hydrogen concentration. A mechanism for the catalytic reaction is proposed and related to the rate expression. The temperature dependence of the reaction rate is referred to a shift in the equilibrium between the catalytically active complexes. The hydrogenation of 1-hexene was accompanied by isomerization to 2- and 3-hexenes, and the high selectivity for the hydrogenation of the terminal alkene is attributed to a difference in complexity constants. a
b
A
3 2
B
3 3
s
H2
Based on several kinetic investigations on hydrogénations catalyzed by transition metal complexes conducted over the last few years, certain general requirements must be fulfilled if a complex is to form an effective homogeneous catalyst in solution (see Réf. I ) . One condition is that the catalytically active complex must be coordinatively unsatu rated; another that M - H or M - C bonds must be present in the complex. Some notable examples of these rules are tris ( triphenylphosphine )halogenorhodium(I) studied by Osborn et al. (2) and hydridocarbonyltris(triphenylphosphine)rhodium(I) reported by O'Connor et al. (3). A common feature of these two complexes is that the coordinative unsaturation is caused by dissociation of phosphines and that M - H bonds 133
134
HOMOGENEOUS
CATALYSIS
II
are formed by oxidative addition of hydrogen. The kinetic investigations indicate that the oxidative addition of hydrogen is the rate determining step i n the catalytic cycle. It might be possible to make a more active catalyst ( per mole com plex dissolved per unit volume of solvent ) either by increasing the tend ency towards the formation of coordinatively unsaturated complexes in solution or by increasing the rate constant of the rate determining step, i.e., the oxidative addition of hydrogen. Collman (4) has reported that both these effects may be obtained by increasing the electron density around the central metal atom. Another possibility is to change the steric factors. Thus, Shaw et al. (5) have shown that, with respect to transition metal complexes, there is a relationship between the tendency toward hydride formation and the bulkiness of the phosphine ligands so that larger phosphines increase the tendency toward the formation of hydride. This may be explained by the fact that the hydride anion is the smallest conceivable ligand, and thus for purely steric reasons hydride formation is facilitated when the other ligands are very large. In rhodium-phosphine complexes an increase of the electron density is possible either by para substitution with electron-donor substituents on the aryl groups of triphenylphosphine or by replacing phenyl-phosphine by alkyl-phosphine. However, investigations made by Montelatici et al. (6) on the activity of rhodium complexes with alkyl-phosphines instead of phenyl-phosphines, which have a higher electron density, show an activity decrease. This is explained by the greater basicity of a l k y l phosphines promoting reassociation i n the presence of hydrogen and consequently a decrease of the concentration of coordinatively unsatu rated complexes. Para substitution in phenylphosphines with - O C H leads to an increase in activity of approximately 50%. Another pos sibility is to replace C O by P P h . In H R h ( C O ) ( P P h ) the electron density is reduced by ?r-back-donation to the carbonyl group. Accord ingly, replacing C O by the less demanding ττ-acid ligand P P h increases the electron density. The steric effect also contributes to the desired increase since P P h is more bulky than C O . This makes it easy to predict that solutions of H R h ( P P h ) are more active catalysts than solutions of H R h ( C O ) ( P P h ) , either because the concentration of coordinatively unsaturated complexes increases or be cause the rate constant of the oxidative addition of hydrogen increases. A kinetic investigation of the hydrogénation of hexene catalyzed by solu tions of hydridotetrakis(triphenylphosphine)rhodium(I) is reported here. 3
3
3
3
3
3
3
3
3
4
10.
HJORTKJAER
Hydrogénation
of 1-Hexene
135
Experimental Preparation of Catalyst. Hydridotetrakis ( triphenylphosphine ) rhod i u m ( I ) was prepared by Ahmad s et al. method ( 7 ) . The complex was identified by the melting point i n air. The kinetic measurements showed that the catalytic activity decreased with time ( b y less than 25% i n 14 days) even when the complex was kept under vacuum or nitrogen. However, all kinetic experiments reported here were made within 2-3 days after the synthesis of the complex. Kinetic Experiments. The hydrogénation of alkene was followed by measuring the pressure as a function of time i n a constant volume apparatus. The reactor was a 250 m l flask surrounded by a jacket through which thermostated water was pumped. The flask was connected to a vacuum pump, a pressure transducer, a Hg-manometer, a N -source, and a H -source via SL condenser. Limitation of the reaction rate because of transport restrictions from the gas phase to the liquid phase was avoided by magnetic stirring. Immediately above the reaction flask a small glass 2
2
1
Figure 1.
2
Rate of hydrogénation tration
3
4
vs. complex concen-
jar for the catalyst was placed. The jar could be rotated from outside so that the catalyst would fall down into the reaction flask. The total volume of the system was approximately 600 m l .
136
HOMOGENEOUS
CATALYSIS
II
Procedure. Hexene (Phillips Petroleum, pure grade, 99% ) was dried over sodium and filtered through a layer of A 1 0 ( Merck, Aktivitâtsstufe I) to remove peroxides. The alkene was dissolved in benzene (Baker analyzed) dried over sodium, and the reaction mixture was transferred to the reactor. The complex was placed i n the jar and the system was closed. To remove all air dissolved i n the liquid phase, the system was evacuated and flushed with hydrogen purified for possible 0 by passing a deoxo catalyst (palladium), under a pressure of 10 atm, and then passing a molecular sieve to remove H 0 3 to 5 times under stirring. Adjustments permitted stabilization of the pressure in 1-2 min where the reaction rate was to be measured. When no observable changes of pressure were observed (equilib r i u m ) , the catalyst was added by rotating the glass jar. The catalyst dissolved i n 10-30 sec, depending on olefin and catalyst concentrations. The pressure drop was recorded as a function of time by the pressure transducer and a recorder. The reaction rate was determined by measur ing the slope of the tangent to the curve of the pressure drop at the point corresponding to the desired H -partial pressure and the vapor pressure of the reaction mixture. The variation between two independent rate determinations at the same conditions was always less than 10% of the absolute value. Molecular weight determinations were carried out osmometrically on a Hitachi Perkin Elmer model 115 molecular weight apparatus with the complex dissolved in benzene and at 38°C (the lowest possible tem perature). The isomerization was followed by G L C analysis (Perkin Elmer model 11, Squalane column). The desired amount of the complex was placed in a reaction flask which was then flushed with N . Benzene and 1-hexene which had been degassed by refluxing over sodium in an N -atmosphere, were injected through a membrane attached to the flask. Samples were removed through the membrane and injected immediately into the gas chromatograph. 2
3
2
2
2
2
2
Results Dependence on Complex Concentrations. The rate of hydrogénation was measured as a function of complex concentration at 15°, 2 5 ° , and 30°C. The initial concentration of 1-hexene was 1.5M, and the partial pressure of hydrogen was 50 cm H g . The data are shown i n Figure 1. Figure 1 indicates that the reaction rate, as a function of complex concentration in this temperature range and a complex concentration above approximately 1.0 X 10" M, may be described by the equation: 3
r = k · CAO + Κ (D where k is a rate constant independent of temperature, C is the initial complex concentration, and Κ is a temperature dependent constant. The few measurements made at complex concentrations below 1.0 X 10" M and at 25°C indicate the rate expression: b
b
A o
3
r = k
a
where k
a
~ 24 X
k. b
· CA
0
(2)
10.
HjORTKjAER
Hydrogénation
of 1-Hexene
137
These results may be interpreted by assuming that actually there are two catalytically active complexes. A t low concentrations and/or relative high temperature, the catalyst is H R h ( P P h ) , presupposing that the equilibria 3
2
—PPh —PPh HRh(PPh )4^=^HRh(PPh )3; ^HRh(PPh ) 3
3
3
3
3
(3)
2
lies to the right, but at higher complex concentrations and/or lower temperatures the equilibria shift toward H R h ( P P h ) so that the catalytic activity is primarily from H R h ( P P h ) . 3
3
Table I.
3
3
Complex Molecular Weights in Benzene at 3 8 °C
Complex Concentration, 10 M
Molecular Weight
0.5 1.0 3.0 10.0
374 400 420 430
S
Rate
of
hydrogénation
Phosphine
/mol
concentration^ 7
Figure 2.
Rate of hydrogénation
1Q
I
3\ /
8
^
vs. phosphine concentration
These assumptions are confirmed by molecular weight determina tions (Table I) which show that extensive dissociation of the complex occurs in benzene. The data imply that at 38 °C the dissociation to H R h ( P P h ) is almost complete, even at a complex concentration of 10" A^. 3
2
2
138
HOMOGENEOUS
CATALYSIS
II
The rate of hydrogénation as a function of the concentration of P P h supports these assumptions. The rate was measured at a fixed complex concentration of 10~ M, and the results i n Figure 2 indicate that Reaction 3 shifts from H R h ( P P h ) toward the less active H R h ( P P h ) when the concentration of P P h is increased. The rate then is expressed as 3
3
3
2
3
3
3
r = k lHRh(PPh,) ] + k [HRh(PPh ) ] (4) where k and k are rate constants which are independent of tempera ture, the temperature dependence of the catalytic reaction must arise from a shift in the equilibrium between the catalytically active complexes. Dependence on Hydrogen Pressure. The dependence on hydrogen pressure is linear (Figure 3). The rate of hydrogénation was measured a
a
2
b
3
3
b
10
Figure 3.
20
30
40
Rate of hydrogénation
50
60
70
vs. hydrogen pressure
at 15° and 25 °C with a complex concentration of 10" M and an initial 1-hexene concentration of 1.5M. Assuming that the concentration of hydrogen i n the liquid phase is proportional to the partial pressure of hydrogen, the rate expression becomes: r = k · C (5) 3
H 2
10.
HJORTKJAER
Hydrogénation
of 1-Hexene
139
60
40
20
1
Figure 4.
2
3
Rate of hydrogénation vs. olefin concentration
Dependence on Olefin Concentration. A t 15° and 25 °C, a complex concentration of 10" M and a hydrogen pressure of 50 cm H g , the de pendence of hydrogénation rate on the olefin concentration was also linear (Figure 4 ) , implying the rate expression: 3
r = k · Cs
(6)
Isomerization. The hydrogénation is accompanied by substantial isomerization; indeed at higher catalyst concentrations the isomerization is rapid. After an almost complete hydrogénation with a complex con centration of 4 X 10~ M and 1.5M as initial 1-hexene concentration, a G L C analysis of the reaction mixture indicates a composition of 1-hexene, 0.8; hexane, 48.0; cis- and fmrw-2-hexene, 42.8; cis- and trans-3-hexene, 8.4%. The measurements of isomerization alone (Figure 5) indicate that the reaction rate is strongly correlated with the complex concentration and less so with the initial 1-hexene concentration. Selectivity. The hydrogénation of 2-hexene was very slow, making it difficult to obtain reproducible measurements of the rate at a complex concentration of 10" M and an initial 2-hexene concentration of 1.5M, but the rate was less than 2 % of that for 1-hexene under the same conditions. This rate of reaction is of the same order of magnitude as the rate ex pected following the isomerization of 2-hexene to 1-hexene since the 3
3
140
HOMOGENEOUS Concentration
of
II
1-hexene
Ccat.
-1.0
Ccat.-
• 10-3
mo!
2,0-10-3
S»L
Time 10
Figure 5.
CATALYSIS
(min.
30
20
Concentration of 1-hexene vs. time
equilibrium mixture at 25°C consists of 1-2% 1-hexene (8). Thus, no direct hydrogénation of 2-hexene seems to occur. Furthermore, the hydro génation of 1-hexene was not influenced by adding 2-hexene since no decrease or increase in reaction rate could be detected even upon change in reaction medium by adding 0.5, 1.0, and 1.5M 2-hexene to the initial benzene mixture with a concentration of 1.5M of 1-hexene. HRh(PPh ) 3
4
-PPh
-CH —CH —Rh(III)(H )(PPh )n 2
2
2
H
3
*
H- alkane > HRh(PPh )n -CH = CH
2
- C H — C H — R h ( I ) (PPh )n 2
3
t
(7)
3
—CH-j— CH 2
3
K
2
II
2
HRh(PPh )n 3
2
10.
Hydrogénation
HjORTKjAER
of 1-Hexene
141
Kinetic Isotope Effect. The reaction rate under standard conditions (complex concentration 10~ M, C = 1.5M 1-hexene, P = 50 cm H g , Τ = 2 5 ° C ) with deuterium was 33.3 X 10" M/sec and with hydrogen it was 37.4 X 10" M/sec, producing a positive isotope effect of J C ' H A ' D = 1.12. 3
s
D 2
5
5
Discussion Kinetics. Since the hydrogénation of 1-hexene is accompanied by isomerization to the internal olefin, the catalytic cycle involves an alkyl intermediate which must be formed by inserting the coordinated 1-alkene. Reaction 7 proposes a mechanism for the hydrogénation:
Here η is 2 or 3 with the two hydrogénation cycles running i n parallel. In this cycle the oxidative addition of hydrogen is the most probable rate determining step which suggests a rate expression: _
k · Κ ι · K · Cs · C H 2 · C A 1 + Kt . Cs + K K · Cs + Calk
^
2
X
2
where k is the rate constant for the rate determining step, K i , K , and fC are equilibrium constants, C is the alkene concentration, C is the hydrogen concentration, C is the complex concentration, and C ik is the concentration of alkane. Assuming that K is large and K i is small (but finite), Equation 8 reduces to: 2
s
3
H 2
A
a
s
r = k · Κ,Κ
2
· Cs · C
H 2
· C ,
(9)
A
which agrees with the experimental results. Another possible mechanism involving an alkyl intermediate is seen at the top of the next page. Maintaining the assumption that the oxidative addition of hydrogen is rate determining, this mechanism leads to the rate expression: r
_ k · K\K Kz 2
* Cs * C H 2 · C A Calk
/
which does not agree with the experimental results. If the coordination of the olefin is assumed to be the rate determining step, the rate expression becomes : r =
k · Ki · C H · Cs · C A γ, ^— 2
Λ2Λ3
A3
/
X
(12) 1
0
142
HOMOGENEOUS CATALYSIS
HRh(PPh ) • 3
II
4
-HPPha -i-alkane —CH —CH —Rh(H )(PPh )rc 2
2
2
*
3
>
HRh(PPh )n
(10)
3
Ki H
K
2
2
k -CH2*j-CH2
-CH = CH
H Rh(PPh )n 3
3
£
2
H Rh(III)(PPh )n 3
3
Ki
which can be brought into agreement with the experimental results if K i is small and K is large. This mechanism involves a trihydrido complex which cannot be coordinatively unsaturated unless η is 2. Thus, it must be concluded that the former mechanism of Reaction 7 is the most probable and that the oxidative addition of hydrogen is rate determining. Selectivity. The insertion of the coordinated 1-alkene creating an alkyl intermediate may proceed either by Markownikoff or anti-Markownikoff addition, but only Markownikoff addition can lead to isomeriza tion. A mechanism which describes both hydrogénation and isomerization may thus be expressed: 3
HRh(PPh ) 3
4
-PPh
3
* Qjll^^j^g -CH —CH —Rh(III)(H,)(PPh,)n — 2
2
» HRh(PPh )n 3
1-alkene
CH.-J-CH2 -CH —CH —Rh(I)(PPh )n 2
2
3
«-
HRh(PPh )n 3
(13)
10.
Hydrogénation
HjORTKjAER
of 1-Hexene
143
Since the hydrogénation of 1-alkene is accompanied by an extensive isomerization, the hindrance toward the formation of an intermediate secondary alkyl does not explain the high selectivity for the hydrogénation of the terminal olefin as it does for H R h ( C O ) ( P P h ) (3). The greater instability of the secondary alkyl intermediate toward the elimination of olefin and the formation of metal hydride compared with the instability of the n-alkyl intermediate provides a more probable explanation. It is assumed that the complexity constant for the formation of the inter mediate olefin complex is significantly smaller for the internal olefin than for the terminal olefin. This assumption is confirmed by the observation that the rate of hydrogénation of 1-hexene is not decreased by addition of 2-hexene which would lead to poisoning of the catalyst by 2-hexene blocking some of the vacant sites if the complexity constants were of the same order of magnitude. 3
1
Figure 6.
3
2
3
Rate of hydrogénation vs. olefin concentration
A c t i v i t y . Because of the linear dependence on the olefin concen tration, the hydrogénation rate is larger with H R h ( P P h ) than with H R h ( C O ) ( P P h ) at olefin concentration above about 0.5M (Figure 6 ) . The linear dependence also implies that K i for the olefin coordination is smaller for H R h ( P P h ) than for H R h ( C O ) ( P P h ) . Thus, the greater activity must be attributed to an increased rate constant for the oxidative addition of hydrogen or/and to an increased concentration of the cata3
3
3
3
4
3
3
4
144
HOMOGENEOUS CATALYSIS
II
lyrically most active complex H R h ( P P h ) . Both of these effects are achieved by substituting C O by P P h to increase the electron density. Absolute values for the concentration of the catalytically active complexes are unknown, and the rate constants for the oxidative addition of hydro gen are inseparable in the experimental rate expression making it difficult to determine the more important of the two effects. 3
2
3
Literature Cited 1. Ugo, R., Chim. Ind. Milan (1969 ) 51(12), 1319-31. 2. Osborn, J. Α., Jardine, F. H . , Young, J. F., Wilkinson, G., J. Chem. Soc. A (1966) 1711-32. 3. O'Connor, C., Wilkinson, G., J. Chem. Soc. A (1968) 2665-2671. 4. Collmann, J. P., Accounts Chem. Res. (1968) 1(5), 136-43. 5. Shaw, B. L., Cheney, A. J., Duff, J. M . , Gill, D., Maun, Β. E., Masters, C., Slade, R. M . , Stainbank, R. E., "Symposium on Chemistry of Hydroformylation and Related Reactions," Veszpren, Hungary, 1972. 6. Montelatici, A. van der Ernt, Osborn, J. Α., Wilkinson, G., J. Chem. Soc. A (1968) 1054-8. 7. Ahmad, N . , Robinson, S. D., Vitteley, M . F., J. Chem. Soc., Dalton (1972) 843. 8. Farkes, Α., "Physical Chemistry of the Hydrocarbons," Academic, New York, 1950. RECEIVED August 20, 1973.
