The Tricarbonylhydridocobalt-Based Hydroformylation Reaction

5

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 9, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch005

The Tricarbonylhydridocobalt-Based Hydroformylation Reaction A Theoretical Study

Tom Ziegler and Louis Versluis Department of Chemistry, University of Calgary, Calgary, Alberta T2N 1N4, Canada

Density functional calculations have been carried out on the mechanism proposed by Heck and Breslow for the hydroformylation process based on tricarbonylhydridocobalt [HCo(CO) ]. Geometries and relative energies were determined for intermediates involved in each elementary step. Reaction profiles were further traced by an approximate linear transit procedure. 3

THE

O X O O R H Y D R O F O R M Y L A T I O N R E A C T I O N , discovered in 1938

by Roe-

len, is used on a large industrial scale (1-3) to convert olefins and synthesis gas into aldehydes. The process employs homogeneous catalysts based on cobalt (1-3) or rhodium (4). The most commonly used precatalyst is H C o ( C O ) , which is generated in situ from the hydrogénation of C o ( C O ) by H . A mechanism for the cobalt-based hydroformylation process was first proposed by Heck and Breslow (5) in 1961 (Scheme I). The catalytic cycle in Scheme I consists of a number of elementary reaction steps (a-e) that we will discuss. The emphasis of the investigation lies on the identification of the equilibrium geometries and the relative energies of stable intermediates in Scheme I. The energy profile of the reaction paths connecting the intermediates will be modeled by an approximate linear transit procedure (6, 7). 4

2

2

0065-2393/92/0230-0()75$06.00/0 © 1992 American Chemical Society

In Homogeneous Transition Metal Catalyzed Reactions; Moser, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

8

76

H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D REACTIONS

HCo(CO)


4

Scheme I.

Computational Details The calculations were all based on the Hartree-Fock-Slater (HFS) model as implemented by Baerends and co-workers (8, 9). With it we used the latest version of the fully vectorized H F S - L C A O - S T O program developed by Ravenek (10). The bonding energies were calculated by the generalized transition-state method (II, 12) in conjunction with Becke's (13) nonlocal exchange corrections, as well as corrections to allow for correlations between electrons of different spins (14). The numerical integration scheme employed in this work was formulated by Becke (15). A n uncontracted triple-ζ Slater-type orbitals (STO) basis set (16) was used for the 3s, 3p, 3d, 4s, and 4p shells of cobalt. The 2s and 2p shells of C and Ο and the Is shell of Η were described by a double-ζ S T O basis set (16, 17), which was extended with one polarization function (2p on Η and 3d on C and O). The electrons in the lower energy shells on C o , C , and Ο were considered as core electrons and treated by the frozen-core approxi mation method according to Baerends and co-workers (8, 9). All molecular structures were optimized within the C symmetry group. The geometry optimizations were carried out according to the algorithm developed by Versluis and Ziegler (18). s


5.

ZiEGLER & VERSLUIS

The Hydroformylation

77

Reaction

Application of Density Functional Methods to Organometallic Substances Calculations on metal carbonyls (19), binuclear metal complexes (20), alkyl and hydride complexes (21-23), and complexes containing metal-ligand bonds for a number of different ligands (24) have shown that the approximate density functional method employed here affords metal-ligand and metal-metal bond energies of nearly chemical accuracy ( ± 5 kcal mol" ). Approximate density functional methods have also been tested in connection with vibrational frequencies (25), conformational energies (6, 7), trip let-singlet separations (26), and transition-state structures (27). More than 50 molecular structures optimized by approximate density functional theory have been compared with experiment (18). The agreement between exper iment and approximate density functional theory is excellent in most cases. The present method has also been applied to a study of C - H activation by late transition metals (28), as well as organosilane polymerization (29) and halogen abstraction by metal carbonyls (30).


1

Dissociation of CO from HCo(CO) HCo(CO)

4

To Form the Catalyst

3

The initial key step (a of Scheme I) in the hydroformylation process is rep resented by the dissociation of a C O ligand from H C o ( C O ) . The dissociation process results in the formation of the catalytically active species tricarbonylhydridocobalt [HCo(CO) ] (eq 1). 4

3

HCo(CO)

4

HCo(CO)

3

+ CO

(1)

The dissociation is assumed to take place prior to, or in concert with, the complexation of an olefin leading to the olefinic ττ complex H C o ( C O ) ( η olefin) (step b of Scheme I). The coordinatively unsaturated 16-electron species, H C o ( C O ) , has been identified by matrix isolation techniques (31), but its structure is unknown. We explored (6, 7) possible structures for H C o ( C O ) . O u r investigation was confined to singlet states of H C o ( C O ) because the carbonyl dissociation process of eq 1 probably takes place on the singlet surface. The precatalyst H C o ( C O ) has (6, 7) a trigonal bipyramidal (TBP) groundstate conformation with the hydride in an axial position. Dissociation of either an equatorial or axial C O ligand from H C o ( C O ) will thus give rise to l a and l b , respectively. 3

2

3

3

3

4

4

In our calculations both l a and l b constitute local minima on the singlet surface of H C o ( C O ) . We calculate H C o ( C O ) with the butterfly shape (la) 3

3



78


1b

to be 38 kj mol lower in energy than the trigonal-shaped H C o ( C O ) species, l b . The equatorial ( H C o ( C O ) - * la) and axial [ H C o ( C O ) lb] C O dissociation energies were found to be 186 and 224 kj mol" , respectively. The C O dissociation energies for the d complex H C o ( C O ) are not known experimentally. However, our calculated values compare well with an experimental (32) (singlet) dissociation energy of 183 kj mol" in the d complex F e ( C O ) . The energy required to convert the precatalyst H C o ( C O ) into the active coordinatively unsaturated 16-electron species H C o ( C O ) of structure l a (186 kj m o l ) is substantial. We shall show that step a is the most energetically demanding of the steps in Scheme I. 1

3

4

4

1

8

4

1

8

5

4

3

1

Some experimental evidence for the existence of H C o ( C O ) in different configurations was given by Sweany and Russell (33, 34), who inferred on the basis of results from the photolysis of H C o ( C O ) in an argon matrix that H C o ( C O ) forms two isomers consistent with structures l a and l b . 3

4

3


5.

Z I E G L E R & VERSLUIS


79

Reaction

Olefin Insertion into the Co-Η Bond The active catalyst H C o ( C O ) of conformation l a combines in step b of Scheme I with olefin to generate a ir-olefin complex where C H is coor dinated to the vacant equatorial site and the C - C bond is placed either in the equatorial plane (2a) or parallel to the C o - Η bond (2b). Conformation 2a was, as one might expect (35), calculated (7, 36) to be more stable than 2b. However, the difference is only 20 kj mol , and the C o - C H bond energy in 2a was estimated to be 70 kj mol" . 3

2

l

4

2

4


1

According to step c of Scheme I, the olefin will undergo a migratory insertion into the C o - H bond after its complexation, and thus form an ethyl complex. HCo(CO) en -C H ) - » Co(CO) C H 3

2

2

4

3

2

5

(2)

Only 2b has the proper relative orientation of ethylene and the hydride for the insertion. Conformation 2a must, as a consequence, rearrange to 2b before the process in eq 2 can take place. The reaction profile for the insertion is shown in Figure 1. The insertion process 2b —» 3a is exothermic by 8 kj mol and has a small activation barrier of 5 kj m o l . The calculated exothermicity and modest activation for the process is in agreement with the experimental observation. Thus, the migration of a hydride to a coordinated olefin group is observed experimentally to be very facile (37). In fact, the hydride-olefin insertion reaction has, with a few exceptions (38), rarely been directly observed. As a consequence, metal complexes containing both hy dride and olefin are scarce. 1

1



80


Reaction Coordinate Figure

1. Energy

profile

of the hydride

HCo(CO)^f\ -C H ). 2

2

migration

to the ethylene

The energy zero refers to structure

4

group

in

3a.

ο

3a A hydrogen atom bound to carbon can interact weakly with a metal center. We shall in the following refer to such an interaction as agostic. The optimized structure, 3a, for the resulting ethyl complex exhibits a clear agostic interaction between a β-hydrogen and the vacant metal center. However, under catalytic conditions, with P = 200-300 atm, the coordinatively unsaturated complex 3a will coordinate a C O ligand to form the saturated complex, C H C o ( C O ) , with the ethyl group in the equatorial position (3b). The coordinatively saturated ethyl complex of conformation 3b can subsequently rearrange to the more stable (6) conformation 3c, in which the ethyl group is in the axial position, by a Berry pseudorotation for which the activation energy is predicted (39) to be low. ( : o

2

5

4



5.

ZiECLER & VERSLUIS

The Hydroformylation Reaction

3b

81

3c

We might conclude that the olefin insertion into the C o - Η bond (steps b and c of Scheme I) is very facile. Thus the olefin insertion should not constitute a bottleneck in the hydroformylation process.

Migratory Insertion of Alkyl into the Co-CO Bond We modeled (6, 7) the migratory insertion process (step d of Scheme I) with C H C o ( C O ) (4) rather than C H C o ( C O ) . 3

4

2

5

4

RCo(CO) -> RC(0)Co(CO) 4

3

101


(3)

82

H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D

REACTIONS

The process in eq 3 could in principle proceed by an insertion of a cis-CO into the C o - C H bond. This insertion would produce the coordinatively unsaturated complex, 5a, with the acyl group in an axial position. Alternatively, the methyl group might migrate to a cis-earbonyl and thus form complex 5b with the acyl group in an equatorial position. Perhaps not surprisingly, we find that the energy profile for the C O insertion, 4 —> 5a, into


3

5a

5b


5.


The Hydroformyfotion

83

Reaction

the C 0 - C H 3 bond (Figure 2a) has a prohibitively high activation barrier of 200 kj mol" . C O insertion, as a consequence, cannot be a viable mechanism for the process in eq 3. 1

The migration of C H to the cis~CO ligand, 4 —> 5b, was calculated (Figure 2b) to have an endothermicity, Δ Η , of 71 kj mol" and a very modest activation barrier, Δ £ * , of only 9 kj mol" . Thus the C H migration, 4 —» 5b, seems to be favored as the mechanism for the process in eq 3. The calculated reaction enthalpy and activation barrier for 4 —» 5b compare well with an earlier study (40) on the C H - » C O migration in C H M n ( C O ) . In that study we found ΔΗ to be 75 kj mol" and Δ £ * to be 11 kj mol" . O u r findings are also in agreement with a recent kinetic study by Roe (41), who found the rate constant of the methyl back migration of C H C ( 0 ) C o ( C O ) to be considerably larger than the rate constant for the corresponding forward reaction. The structures in Figure 2a illustrate how the methyl group can slide almost parallel along the cis C - C o bond onto the cis carbonyl carbon while the remaining C o ( C O ) framework stays almost unchanged. The 9 kj mol calculated for Δ £ * in the present study is an upper bound (6, 7) to the actual value. We can thus conclude that the methyl migration, 4 —» 5b, should proceed with a rather modest activation barrier. 3

1


1

3

3

3

1

5

1

3

3

3

1

The 1,2 shift reaction of an alkyl group in which a metal-alky 1 system is converted into a metal-acyl complex, 4 —> 5b, is well documented for a variety of alkyl complexes. The corresponding 1,2 shift reaction, 6a —» 6b, involving Η rather than alkyl, has proven to be rather elusive. The 1,2 hydride shift reaction was inferred in earlier work (42-44) as an elementary reaction step. In spite of considerable efforts it has been detected with certainty only in a few cases (45-47). It is now widely accepted that the hydride migration, in contrast to the alkyl migration, is thermodynamically unfavorable, at least for middle to late transition metals. We studied (6, 7) the 1,2 shift reaction 6a —> 6b, which represents a 1,2 shift of a hydride in H C o ( C O ) with C symmetry. The formyl structures 6b do not represent a local energy minima on the H F S energy surface. Thus, any attempt to optimize 6b resulted in a back migration of the formyl hy drogen to the parent hydrido metal complexes 6a. Our findings indicate that the formyl complex 6b is kinetically unstable with respect to the parent hydrido complex 6a. That is, the decarbonylation reaction (6b —» 6a) should have at most a minimal activation barrier. Our findings can be reconciled with the experimental observation that most neutral metal formyl complexes decarbonylate readily to the corresponding hydrido complexes (48-51). The decomposition is believed to occur by a back migration of the formyl hy drogen to the metal center under loss of a ligand of the coordinatively saturated formyl complex. We calculate (6, 7) the process in eq 4 to have an exothermicity of AH = -69 kj mol" . 4

3 v

1

H C ( 0 ) C o ( C O ) -> H - C o ( C O ) 4

4

+ CO


(4)

84

H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D REACTION

J


260

Reaction Coordinate Figure 2. a: Energy profile for the insertion of CO into the C0-CH3 bond, 4 —• 5a. b: Energy profile for the migration of CH to C O , 4 -> 5b. The zero point refers to 4 in both plots. 3


5.


The Hydroformylation Reaction

H

H

CO

OC-Ci

CO

C ο

ο


6a

85

CO CO

C ο

6b

Acyl complexes are, in contrast to their formyl counterparts, well-known molecules. We calculate the corresponding decarbonylation process of C H C ( 0 ) C o ( C O ) to be endothermic with Δ Η - 20 kj mol" . Thus the acyl complex is thermodynamically stable, although the formyl analog is unstable and decomposes to H C o ( C O ) according to eq 4. The higher exothermicity of the decarbonylation process for the formyl compared to the acyl can largely be ascribed to the higher bond strength (21, 22, 52) of C o - Η compared to C o - C H . For middle to late transition metals D ( M - H ) is - 2 4 0 kj m o l whereas the corresponding D ( M - C H ) bond strength is only ~160 kj m o l . The isoelectronic d formyl complex HC(0)Fe(CO) ~ has been found (53) to decompose slowly to H F e ( C O ) " . The kinetic stability can probably be as cribed to the stabilization of the M - C O bond through increased back-bond ing interactions of the carbonyl ligands in the charged species. However, the overall reaction is thermodynamically favorable with a reported exo thermicity of Δ Η = -43 ± 30 kj mol" (54). 3

1

4

4

1

3

-1

3

8

4

4

1

H 'lnduced Aldehyde Elimination 2

The last step in the catalytic cycle of the hydroformylation process, e of Scheme I, is the reaction of the acyl intermediate with H . This reaction results in the formation of the desired aldehyde molecule and the regen eration of the catalyst H C o ( C O ) . 2

3

The aldehyde product can be formed from the acyl intermediate by several possible routes. Heck and Breslow (5) proposed a mechanism in which the coordinatively unsaturated acyl complex undergoes first an oxi dative addition of H to afford a dihydro acyl compound (eq 5a) followed by an irreversible reductive elimination of an aldehyde molecule (eq 5b). 2

R(0)CCo(CO)

3

+ H -> R(0)CCo(CO) (H) 2

R(0)CCo(CO) (H) 3

2

3

(5a)

2

R C H O + HCo(CO)

3

(5b)

This type of process has been inferred for numerous catalytic and stoichio metric systems (37). As a consequence, these reactions have been studied


86


extensively by both experimental (55-65) and theoretical (66-75) techniques. However, in the cobalt-based hydroformylation process it has not been es tablished that the product formation proceeds in fact via the oxidative ad dition-reductive elimination mechanism. Some experimental observations indicate (76-79) that the acyl complex might react with H C o ( C O ) , and thereby form an aldehyde molecule and a binuclear cobalt compound (eq 6a).


4

CO

R(0)CCo(CO)

3

+ HCo(CO)

Co (CO) 2

8

> R C H O + Co (CO)

4

+ H

2

2

> 2HCo(CO)

8

4

(6a) (6b)

In a subsequent reaction C o ( C O ) is then (according to eq 6b) transformed back to the mononuclear hydrido-cobalt complex. Experimental studies (5) of the stoichiometric reactions (eqs 6a and 6b) revealed that the process is very facile. However, under catalytic conditions the overall concentration of cobalt species is low in comparison to the reactants. That is, the probability for reaction between two cobalt complexes is small (77). For electron-poor systems such as early transition metal complexes (80-83), the hydrogenolysis of the M - C bond proceeds via a mechanism in which an incoming H molecule initially forms a η adduct with the metal complex. This reaction is followed by the concerted cleavage of the hydrogen bond and the formation of H - M and H - C bonds by way of a four-center intermediary structure such as that illustrated by structure 7. 2

8

2

2

Η

Η

/

\/°

M

C

7

This reaction mode thus omits the oxidative addition-reductive elimi nation mechanism. Such a process could also be envisioned for the cobalt system studied here and was speculated upon by earlier workers (2, 48). In this section we will concentrate our efforts on the study of those cobalt complexes that can result from an interaction of the acyl intermediates with an incoming H molecule. Furthermore, attention will be given to the ox idative addition process of H to the metal fragment as well as to the hy drogenolysis reaction through a four-center intermediary structure. 2

2


5.



87

Reaction

The two most stable products from the interaction between an acyl complex and H are the dihydrogen complexes 8a and 8b with H in the 2

2

equatorial position. Both represent energy minima on the H F S energy sur face; 8b is about 19 kj mol" above 8a in energy.


1

8a

8b

The mechanism by Heck and Breslow (5) suggests that H adds oxidatively to the unsaturated acyl complex to form a dihydride, 8c. The dihydride complex of configuration 8c was found to be 25 kj m o l higher in energy than the η adduct 8a. Thus the H complex is more stable than the product of the oxidative addition. Complex 8c is also 6 kj mol" higher in energy than the η compound 8b. This result is somewhat surprising because a number of d complexes containing phosphine ligands are known to add H readily; this addition results in the formation of dihydrides (66-74). Ther modynamically stable η - Η complexes have been prepared (85-91). T h e first of these complexes was the d compound W[P(i-Pr) ] (CO) (H ) (85), for which the H - H distance was found to be 0.75 ± 0.16 Â. 2

1

2

2

1

2

8

2

2

2

6

3

2

3

2

A theoretical study (92) on the related model systems W ( P H ) ( C O ) ( H ) and W ( P H ) ( H ) revealed that the η - Η complex is stabilized by the iracceptor C O ligands, which lower the energy levels of the metal d orbitals 3

3

5

2

2

2

2


3

2


88


8c w i t h respect to the c o r r e s p o n d i n g orbitals i n W ( P H ) ( H ) . A s a conse q u e n c e , the c a p a b i l i t y of the m e t a l fragment to donate electrons into the a n t i b o n d i n g σ * o r b i t a l of the H m o l e c u l e has d i m i n i s h e d . I n t u r n , this r e s t r i c t i o n prevents the system from u n d e r g o i n g oxidative a d d i t i o n . It has also b e e n f o u n d i n e x p e r i m e n t a l studies i n v o l v i n g d a n d d complexes that the oxidative a d d i t i o n of Η is facilitated b y electron-releasing ligands such as p h o s p h i n e s a n d r e t a r d e d b y IT acceptors s u c h as c a r b o n m o n o x i d e (37). 3

5

2

2

8

1 0

2

F i g u r e 3 displays the e n e r g y profile for the oxidative a d d i t i o n reaction 8a —» 8c. W e calculated an activation energy Δ Ε * of 77 k j m o l " . T h i s v a l u e is m a r k e d l y larger than the activation energies that have b e e n d e t e r m i n e d theoretically for the oxidative a d d i t i o n of H to transition m e t a l complexes c o n t a i n i n g p h o s p h i n e ligands (49). T h i s difference can b e a t t r i b u t e d to the stabilization of the η - Η adduct b y the ττ-acceptor C O ligands. T h e e n e r g y c u r v e m o d e l e d i n F i g u r e 3 ascends steeply d u r i n g the early stages of the reaction. T h e slope indicates that the activation e n e r g y arises largely f r o m the i n i t i a l elongation o f the H - H b o n d distance. 1

2

2

2

T h e fact that the oxidative a d d i t i o n process, 8a —» 8c, exhibits a sizable activation b a r r i e r l e d us to consider an alternative m e c h a n i s m for the last p r o d u c t - f o r m i n g step (e of S c h e m e I) i n the h y d r o f o r m y l a t i o n c y c l e . T h e


5.


The Hydroformyhtion

89

Reaction


100 H

04


3. Energy

profile for

the oxidative

addition

of H to

CH C(0)Co(CO) .

2

3

3

alternative mechanism assumes that one hydrogen atom of the η - Η com plex shifts directly toward the acyl group and thereby forms an aldehyde molecule and regenerates the catalyst H C o ( C O ) . This type of reaction mech anism, in which a direct oxidative addition of Η is avoided, has, to our knowledge, never been investigated theoretically for the cobalt-based hy droformylation process. 2

2

3

2

The approximate profile for the reaction 9a —> 9b —• 9c is given in Figure 4. The first part of the profile connecting 9a and 9b was obtained by changing the internal coordinates of 9a into those of 9b in a linear and stepwise fashion. A total of six steps was used in the transit. We find the reaction proceeds with a minimal activation barrier (Figure 4). The activation energy for the first part, 9a —» 9b, is therefore essentially equivalent to the reaction enthalpy Δ£ for the reaction 9a —* 9b, which was calculated to be 83 kj mol" . We have also traced the energy surface for the reaction 9b —> 9c by using a similar five-step linear transit procedure. The step 9b —• 9c, which is exo thermic with a reaction enthalpy of Δ£ = -26 kj mol" , has a negligible activation energy. Finally, the system, 9c, is lowered in energy by 69 kj m o l as the adduct 9c breaks up into acetaldehyde and H C o ( C O ) in its ground-state conformation la. 1

1

1

3

C H

r £

Ο

1

C

ο 9a

c^ I r c ο 9b

3

_

C

Co—Η

9c


90



100+


Concluding

4. Energy

profile

for

the reaction

9a - » 9b —» 9c.

Rental

We carried out calculations on the elementary steps of the hydroformylation process. Our calculations indicate that the last product-forming step (e of Scheme I, in which aldehyde elimination is induced) has the highest acti vation energy with Δ Η * ~ 80-90 kj mol" . Step d was calculated to have a similar activation energy (ΔΗ* ~ 70-80 kj mol" ). The insertion of olefin into the C o - Η bond (steps b-c) was, on the other hand, estimated to have a negligible activation energy of ΔΗ* ~ 10-20 kj mol" . We finally found that the formation of the active catalyst H C o ( C O ) from H C o ( C O ) by C O dissociation (step a) requires 185 kj mol" . More detailed studies of steps a (6), b (93), c (6), d (94), and e (94) of Scheme I can be found elsewhere. 1

1

1

3

4

1

Acknowledgment This investigation was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). We also acknowledge access to the Cyber-205 installations at the University of Calgary. We are thankful to E . J . Baerends and W. Ravenek from the Free University of Amsterdam for a copy of their latest vectorized version of the H F S - L C A O program system. We would also like to thank Pieter Vernooijs for help with the installation of the program system.

References 1. Heck, R. F. Adv. Organomet. Chem. 1966, 4, 243. 2. Orchin, M.; Rupilius, W. Catal. Rev. 1972, 6, 85. 3. Orchin, M. Acc. Chem. Res. 1981, 14, 25.



5.

ZIEGLER & VERSLUIS The Hydroformyhtion Reaction

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