Homogeneous Transition Metal Catalyzed Reactions - ACS Publications

39 Model Systems for Catalytic Reactions Peter M. Maitlis, Futai Ma, Jesus Martinez, Peter K. Byers, Isabel Saez, and Glenn J. Sunley

Downloaded by UNIV OF LEEDS on September 15, 2016 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch039

Department of Chemistry, The University, Sheffield S3 7HF, England

Models for various C - C , C - C - C , and C-C-C-C coupling reac tions have been developed, based on results from labeling stud ies of thermal or oxidative decomposition of the di- and mono -alkyldi-μ-methylenedirhodium complexes [C Me Rh(μ-CH )R] and {[C Me Rh(μ-CH )] R(L)} . Decomposition of the vinyl complex [C Me Rh(μ-CH )(CH=CH ]2 leads to very facile methylene-vinyl coupling. These results have led to a proposal for a new mechanism for promoted Fischer-Tropsch polymerization over surfaces, in which the chain carriers are surface alkenyls, not alkyls. The reaction starts at a surface vinyl, formed from a methylene and a methyne. Evidence is presented supporting this mechanism for reactions of CO-H over rhodium-ceria catalysts. When C H [from labeled Si(*C H ) ] is added to a CO-H gas stream overRh-CeO2-SiO ,there is significant incorporation of C , and very little of C , into the C and C products. Thus vinyl can be efficiently incorporated into, and can therefore be a key participant in, Fischer-Tropsch polymerization. 5

5

5

5

5

2

5

2

2

+

2

2

2

2

13

2

3

2

2

13

1

3

D E V E L O P M E N T O F H I G H - Y I E L D S Y N T H E T I C R O U T E S to the 5

2

2

+

4

2

13

2

enedirhodium complexes 1 [ C M e R h ^ - C H ) R ] CH )] R(L)}

3

5

2

2

(3), and 3 { [ C M e R h ^ - C H ) ] ( L ) } 5

5

2

2

2

4

di^-methyl-

(I, 2), 2 { [ C M e R m > 5

2+

5

(34) (R is alkyl; L is

C O , M e C N , etc.) has ahuwed an exploration of the organic chemistry of dinuclear complexes. We have been especially interested in the reactivity of alkyls and bridging methylenes and their relevance to catalytic C - C cou pling and related processes.

Organic Chemistry of Di^-methyhnedirhodium

Complexes

When 1 (R is Me) is decomposed, either thermally (300-350 °C) or oxidatively [Ir(IV) or Fe(III), 20-50 ° C ] , the main organic products are propene and 0065-2393/92/0230-0565S06.00/0 © 1992 American Chemical Society

Moser and Slocum; Homogeneous Transition Metal Catalyzed Reactions Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

566

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

methane. Labeling experiments using 1 (R is C H ) showed that the propene was derived from the coupling of two methylenes and one Rh-methyl from one molecule. The oxidative decomposition of 1 (R is C D ) led to the formation of CD =CHCDH (>75%). Other D-labeling experiments showed that the methane derives largely from Rh-methyl and μ-methylene hydrogen. This information led to the proposal of a very detailed mechanism for these processes (Scheme I) (5, 6). A key feature is intermediate A , which results from a two-electron oxidation of 1 and the simultaneous loss of methane. Internal migration of C D onto μ - C H , followed by a β-elimination of D to Rh, accompanies the formation of Rh-o-vinyl, which couples with μ - Ο Η . This reaction gives Rh-o-allyl, which reductively eliminates with R h - D to produce C D = C H C D H . 1 3

3

3

2

2


3

2

2

CD

2

CH

3

"^Rh^

CH

C Me« 5

-

C Me

2

5

CD

5

CH

3

5

5

2

-CD H

2e

3

CsMes

2 +

-2e~

(C Me )Rh^ 5

CsMes

C Me

2

^Rh*

^Rh(C Me )

s

5

CH

CH2

5

2

(A)

CD

CD, (C Me )Rh^ 5

.•'UH ? (C Me )Rh j ^RhiCsMes)

Rh(C Me )

5

s

CH

5

5

CH

2 [C Me Rh(solv) ] 5

3

2 +

T

4

T

5

2

s

2

+ CD =CHCDH 2

2

2

Scheme I. Proposed mechanism for the formation of propene-d from complex 1 (R is CD ) or 3 (R is OC,). 3

3


39.

MAITLIS ET AL.

567

Model Systems for Catalytic Reactions

A rather similar and even more facile reaction occurs with the cationic complex 2 (R is M e ; L is M e C N ) , where virtually the only organic product is propene (7). The alkyl homologs of 1 and 2 (R is ethyl, propyl, etc.) behave similarly (7)f Divinyl complex 4, prepared as illustrated in Scheme II from 1 via dichloride 3, underwent coupling between vinyl and methylene very easily and cleanly. Thermal decomposition gave propene (88%) and methane (8%). Even reaction with HC1 (or H Br) at low temperatures in toluene gave pro pene (8, 9). Silver salts (e.g., A g B F ) oxidatively coupled methylene and 4

vinyl, easily and essentially quantitatively, to give T| -allyl complex 5 (9).


3

A further interesting transformation is provided by the reaction of the dicarbonyl dication [ { C M e R h ^ - C H ) ( C O ) } ] 5

5

2

ter [ { C M e R h ^ - C H ) ( C 0 M e ) } ] ) . 5

5

2

2

2

2

(or its equivalent, the dies-

2+

In methanol in the presence of Fe(III)

they yield methyl acrylate, C H = C H C 0 M e (10). Again a C - C - C coupling 2

2

has taken place, but this time the product is an oxygenate.

A New Model for Fischer-Tropsch

Reactions

These results led to the development of a new model for the Fischer-Tropsch polymerization (Scheme III) (11, 12). The first steps, the formation of surface carbide, surface methyne (CH), and surface methylene ( C H ) , are generally 2

accepted features of most mechanisms (13-16). However, our model breaks new ground in its treatment of the subsequent C - C bond-formation reac tions. The first step is postulated to be a combination of a surface C H and a surface C H

2

to give a surface vinyl ( - C H = C H ) . Surface vinyls then react 2

with surface methylenes to give allyls, which isomerize to surface alkenyls, which can then add further methylene, etc. The sequence is terminated by the reaction of a surface alkenyl with surface H to give the olefin product. Organometallic reactions on clusters give models for the first stage ( C H plus C H to vinyl) (17). Although vinylic species have not yet been unam 2

biguously identified on metal surfaces (18,

19), kinetic evidence for the

intermediacy of a surface vinyl during the dehydrogenation of chemisorbed ethylene to surface ethylidyne on P t ( l l l ) has been reported (20). CHD=CD

2

(ads) - » (surface)-CD=CD

2

(surface)C-CD

3

If, as we suggest, surface vinyls (and surface alkenyls) are very reactive intermediates, it is not surprising that they are difficult to detect. This scheme allows ready explanations for • the formation of α-olefins as the primary products, • the fact that the amounts of C

2

products formed are often

anomalous by comparison with the higher hydrocarbons (they arise, in this scheme, by different routes), and


568

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

R

CH

CsMe^

X

CH

C Me

2

5

5

CH

l-CsMei

CH,

CsMe^*

2

+

R

2

R

H /L

L

(2)


(1)

(i)

2HC1

.

C

1

\

/

\

>^"X

CsMes

C

H

s

β****

CH,

CH,=CHMgBr

Cl

(3)

CH5=CH CsMe^

CH

C Me

2

5

^CH,^

5

CH=CH,

(4)

CH, (4) + 2 AgBF /MeCN • 4

VCH 2 [CsMesRh^) I ]BF I CH, MeCN

4

+ 2 Ag»

(5) Scheme 11. Routes used for the synthesis of the dirhodium-di^-methylene complexes 1-4 and the decomposition of 4 to 5.


MAITLIS ET AL.

c

c

HHHHH

ο

+"

MMMMMMMMM

C Downloaded by UNIV OF LEEDS on September 15, 2016 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch039

569


~ "

+

/l\

1

,

C

+

M^M\IIHMM

2

1Î»

c

HMHHMHHH

U

2

—

'

MMMMMMM

H C

,

2

? ?

2

MMMMMM

„

^CH , H Ç ^

2

MMMMMM

2

MMMMMMM

CH CH

2

+

"f^"

•

2

»

MMMMMMM MMMMMMMM

MeC. 2

2

.

\

H

MMMMM

CH

+

M

^CH

'—

MMMMMMM

2

MMMMMMM MMMMMM

. MeC^

I

f

^

^CH

MeCH

ι

MMMMM

f

H C.

^CH

H (T

2

3

I

+

e

C

^ „

J^CMe r

H (T

H

V\

2

2

MMMMMM MMMMMM

I

—

MMMMM

CH R 2

H

» B.CH2—0H=CH2

MMMMMMMMMMMMM Scheme HI. New model for Fischer-Tropsch polymerization. M is monomer.


570



• the formation of some branched (methyl) hydrocarbons in ad dition to the normal straight-chain hydrocarbons. Allyls isomerize easily; chain growth can occur at either end, but is easier at an unsubstituted carbon.

Other attempts to answer some of these points have not been totally successful (21-23). For example, most previous Fischer-Tropsch mecha nisms require surface alkyls to undergo β-elimination of hydride to give alkenes. That must occur under strongly hydrogenating conditions, so such a step is unexpected, to say the least. To see whether the suggested Fischer-Tropsch mechanism has any validity, we began experiments to test the hypothesis. We therefore studied some reactions in which a C O - H mixture (1:2) was passed over supported rhodium. 2

Experimental Procedures Labeled Tetravinylsilane. Tetravinylsilane was prepared by a modified literature procedure (24). Labeled vinyl Grignard was made by reacting a 1:1 mixture of vinyl bromide ( C H B r , 0.5 g) and labeled vinyl bromide (99% C H B r , Matheson of Canada, 0.5 g) with magnesium (0.22 g) in dry tetrahydrofuran (THF) under nitrogen. The reaction was initiated with a trace of 1,2dibromoèthane. The vinyl Grignard was then treated with silicon tetrachloride (0.24 g) in pentane (3.4 cm ) and gently refluxed (24). Workup gave a solution of labeled tetravinylsilane in T H F . The pure labeled S i ( * C H ) was obtained by preparative G C . Mass spectroscopic and N M R analysis ( H and C) showed that the sample prepared contained 44% C H . 1 2

1 3

2

2

3

3

3

2

l

1 3

2

3

4

l3

3

Preparation of Rhodium and Ceria on Silica Catalyst and Its Use. The rhodium (4 wt %) and ceria (9 wt %) on silica catalyst was prepared by the incipient wetness method. Silica gel (Davisil, grade 645, 60-100 mesh) was successively impregnated with (a) an acidic aqueous solution of cerium(IV) and dried (200 °C), and (b) a methanol solution of rhodium trichloride hydrate. The catalyst was dried at 120 °C in air. It was then transferred to a reactor tube and activated by a temperature-programmed reduction in flowing hydrogen gas (4 °C min" to 400 °C). It was held at 400 °C for 4 h and then cooled in hydrogen gas. Carbon monoxide and hydrogen (1:2) were premixed and allowed to flow through the catalyst (1 g, in a fixed-bed microreactor, 6 x 350 mm) at 250 °C, 1 atm, and at aflowrate of300 c m h" , for 2 h. This procedure led to equilibrium, after which tetravinylsilane (0.5 μ!,) was injected into the gas stream via a septum. Separate experiments showed that the gases deriving from this pulse exited into a collecting*sample tube (25 cm ) after 3 min (15 cm ). The gases were collected in the sample tube and analyzed by gas chroma tography and gas chromatography-mass spectrometry (GC-MS) (Poropak Q; Carlo-Erba chromatograph; Kratos MS-25 mass spectrometer). They were also quantified by reference to authentic samples. The data are presented in Tables I and II. 1

3

3

1

3


39.

MAITLIS ET AL.

571


Table I. Products from Fischer-Tropsch Reactions over Rhodium Catalyst Rh Rh Rh Rh

CH

C2H6

C.3

c

4

c

70 60 71 48

6 16 12 31

12 12 10 10

8 9 5 9

5 3 2 2

4

(5%)-Si0 (5%)-Si(VSi(Vi) (4%)-Si02-Ce0 (4%)-SiO^Ce(VSi(Vi) 2

4

4

4

5

NOTE: All values are given in percents. In all cases, only a trace of M e C H O was found. Vi is C H .


2

3

Table II. Labeling Found in Cases from CO-Hr-Si( C2H3)n( C2H )4-n 13

Gas

C

12

n

12

3

' C,_ C

C^ C

12

n

l3

2

l

2

Rh-ceria-silica at 250 °C 55 5 80 5 85 5 70 5 Rh-silica at 250 °C 50 10 95 3 97 3 95 4

Ethane Propane Propene Butene 0

Ethane Propane Propene Butene

i3

;

40 15 10 20 40 2 1

NOTE: All values are given in percents. °5% C , C H was found. l 2

, 3

3

8

First Tests of the Alkenyl Chain-Growth Reaction Mechanism Although rhodium is generally better at promoting methanation than Fischer-Tropsch reactions, in the presence of various oxide promoters (for example, C e , L a , M o , T h , T i , and V) its activity toward the formation of higher hydrocarbons is substantially enhanced. In addition, such catalysts also show some selectivity toward oxygenates (especially ethanol) (25-30). In these preliminary studies we used two catalysts, one with a 5% loading of rhodium on silica (Rochester-McQuire). The other, also made by the incipient wetness method, contained Rh (4%) and C e 0 (9%) on silica. Table 2

I shows the products identified over these catalysts. To test the hypothesis, tetravinylsilane, Si(CH = C H ) , was chosen as 2

4

a source of vinyl; silicon-containing byproducts should have little effect on the reaction. In some experiments reported in the literature (for example, references 31-33), ethylene (or propene) added to Fischer-Tropsch reaction streams gave somewhat ambiguous results; that is, only small changes in Fischer-Tropsch products were usually seen, and hydroformylation products often dominated. To avoid ambiguity, we used double

1 3

C labeling in our

experiments.


572


Baseline experiments were carried out both with and without the ad dition of unlabeled tetravinylsilane. Addition of the pulse of Si(CH = C H ) slightly deactivated the catalyst and increased the amount of ethane formed, but appeared to have no other effect on the course of the reaction. To make the required material, C H M g B r , diluted with an equal amount of normal C H M g B r , was reacted with S i C l . The product, labeled S i ( * C H ) , contained a statistical mixture of S i ( C H ) ( C H ) . (n = 0-4). Pulses of 0.5 μ ι of this S i ( * C H ) were introduced into a Fischer-Tropsch gas stream passing through the catalyst (1 atm, 250 °C). The product gases were collected and analyzed by G C - M S for isotopic content. 2

1 3

1 2

2

3

2

2

3

3

4

1 3

4

2


4

3

2

3

n

1 2

2

3

4

n

4

The data from the reactions with labeled tetravinylsilane are presented in Table II. The values for ethane reflect the hydrogénation of S i ( * C H ) and show approximately the amount of doubly labeled C H expected. The labels in the propene, propane, and butene from the reaction of R h - S i 0 are not significantly different from natural abundance. This fact indicates only small incorporation of vinyl into these products over that catalyst. 2

1 3

2

3

4

6

2

A dramatic difference is shown in the results from the Rh-ceria-silica experiment. The amount of C present in the C and C products is close to that expected for natural abundance. However, the amount of C for propene, propane, and butene is very much higher (by ca. 4 orders of magnitude) than natural abundance would predict. This result indicates that labeled vinyl has been incorporated directly into these products (34). 1 3

3

4

1 3

2

Blank tests showed that tetraethylsilane (SiEt ), injected under similar conditions, had no effect on the reaction at all. It passed straight through the catalyst without reacting, in both the presence and the absence of H - C O . Thus the reaction we find with vinyl [from Si(CH = C H ) ] does 4

2

2

4

not occur with ethyl. At this admittedly early stage of the work, we draw the conclusion that, at least in some cases, vinyl can be efficiently incorporated into, and can therefore participate in, Fischer-Tropsch polymerization. Vinyl incorporation has so far only been found in the R h - C e 0 experiments. Thus it seems that this type of oligomerization is promoted by a mild cooxidizing site. This result mirrors the model experiments described. The coupling reactions of the ligands in dinuclear complexes 1-4 are very significantly improved with a cooxidant. Work on other organometallic models, both dinuclear and mononuclear (35-37), and detailed analysis of Fischer-Tropsch kinetics (38) is providing further support for the ideas presented here. The term "Fischer-Tropsch" refers to a group of processes, not just a single reaction. The products obtained from C O - Η reactions under different conditions could arise by various routes. For example, the hydrocarbon and oxygenate products from a Fischer-Tropsch reaction over K-promoted iron catalysts have been shown to arise by separate paths (39). 2

2


39.

MAITLIS ET AL.


573

To summarize, model studies have shown that methylenes couple very easily with vinyls (alkenyls) in dirhodium complexes. Labeling studies in dicate that a related mechanism, the essential step of which is a coupling of surface methylenes with surface vinyls (or alkenyls), is also useful for un derstanding at least some Fischer-Tropsch polymerization reactions over rhodium metal. Work is in progress to define these Fischer-Tropsch reac tions further and to find out the extent to which such processes occur in other metal systems.


Acknowledgments We thank the Science and Engineering Research Council and BP Chemicals for support; Johnson Matthey for the loan of rhodium salts; C . H . Rochester and M . W. McQuire for generously providing the rhodium on silica catalyst; and B. F. Taylor, D . G . Andrews, P. Ashton, and I. Johnstone for spectro scopic and analytical measurements. We also thank S. A. R. Knox, V. C . Gibson and J. E . Bercaw for sending preprints prior to publication.

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16. Sheldon, R. A. Chemicals from Synthesis Gas; Reidel: Dordrecht, 1983. 17. Davies, D. L.; Parrott, M. J.; Sherwood, P.; Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1987, 1201. 18. Sheppard, N.; De La Cruz, C. React. Kinet. Catal. Lett. 1987, 35, 21. 19. Sheppard, N. Annu. Rev. Phys. Chem. 1988, 39, 589. 20. Zaera, F.J.Am. Chem. Soc. 1989, 111, 4240. 21. McCandlish, L. E. J. Catal. 1983, 83, 362. 22. Hoel, E. L.; Ansell, G. B.; Leta, S. Organometallics 1984, 3, 1633; 1986, 5, 585. 23. Hoel, E. L. Organometallics 1986, 5, 587. 24. Rosenberg, S. D.; Walburn, J. J.; Stankovitch, J. D.; Balint, A. E.; Ramsden, H. E. J. Org. Chem. 1957, 22, 1200. 25. Union Carbide. Belg. Patent 824 822, 1975. 26. Ichikawa, M. J. Chem. Soc., Chem. Commun. 1978, 566. 27. Ichikawa, M. Tailored Metal Catalysts; Reidel: Dordrecht, 1985; p 183. 28. Gysling, H. J.; Monnier, J.; Apai, G. J. Catal. 1987, 103, 407. 29. Yu-Hua, D.; De-An, C.; Khi-Rui, T. Appl. Catal. 1987, 35, 77. 30. Kieffer, R.; Kiennemann, Α.; Rodriguez, M.; Bernal, S.; Rodriguez-Izquierdo, J. M. Appl. Catal. 1988, 42, 77. 31. Watson, P. R.; Somorjai, G. A. J. Catal. 1981, 72, 347. 32. Thivolle-Cazat, J. Appl. Catal. 1986, 24, 211. 33. Kip, B. J.; Hermans, E. G. F.; van Wolput, J. H. M. C.; Haermans, Ν. Μ. Α.; van Grondelle, J.; Prins, R. Appl. Catal. 1987, 35, 109. 34. Ma, F.; Sunley, G. J.; Saez, I. M.; Maitlis, P. M. J. Chem. Soc., Chem. Commun. 1990, 1279. 35. Doherty, Ν. M.; Howard, J. A. K.; Knox, S. A. R.; Terrill, N. J.; Yates, M. I. J. Chem. Soc., Chem. Commun. 1989, 638. 36. Colborn, R. E.; Dyke, A. F.; Gracey, B. P.; Knox, S. A. R.; MacPherson, Κ. Α.; Mead, Κ. Α.; Orpen, A. G. J. Chem. Soc., Dalton Trans. 1990, 761. 37. Gibson, V. C.; Parkin, G.; Bercaw, J. E. Organometallics 1991, 10, 220. 38. Santilli, D. S.; Castner, D. G. J. Energy Fuels 1989, 3, 8. 39. Miller, D.; Moskovits, M. J. Am. Chem. Soc. 1989, 111, 9250. RECEIVED for review October 19, 1990. A C C E P T E D revised manuscript August 14, 1991.


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