Hydrosilylation - Advances in Chemistry (ACS Publications)

37

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 10, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch037

Hydrosilylation A "Homogeneous" Reaction Really Catalyzed by Colloids Larry N. Lewis, Nathan Lewis , and Richard J. Uriarte 1

2

General Electric Research and Development, P.O. Box 8, Schenectady, NY 12301

The evidence for the intermediacy of colloids in the hydrosilylation reaction is reviewed. Data are presented to support the fact that reactions catalyzed by the active catalyst Pt[(vinyl)Me SiOSiMe (vinyl)] (Karstedt's catalyst) form colloids. Analytical electron microscopy (AEM) and high-resolution electron microscopy (HREM) confirm the formation of colloids. Additional evidence comes from analysis of X-ray absorption near-edge spectroscopy (XANES) measurements, which show that Pt reduction occurs upon reaction of Karstedt's catalyst with (EtO) SiH and that Pt is in a new environment. 2

2

3

TTHE

H Y D R O S I L Y L A T I O N R E A C T I O N (eq 1) has been known since the 1940s but has received considerable attention since Speier's pioneering work using H P t C l - i - P r O H "homogeneous" catalysts (1-5). Chalk and Harrod's (6, 7) mechanistic work and proposals, which became accepted in the literature (8), were based on the intermediacy of molecular compounds. 2

6

(1) In 1986 we reported (9) that strong evidence implicated colloids as important intermediates in the catalytic cycle of hydrosilylation. For example, if C O D P t C l ( C O D is 1,5-cyclooctadiene) was reacted with excess 2

'Current address: Knolls Atomic Power Laboratory, Niskayuna, NY 12309 'Current address: G E Silicone, Waterford, NY 12188

0065-2393/92/0230-0541806.00/0 © 1992 American Chemical Society

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

542

H O M O G E N E O U S TRANSITION M E T A L CATALYZED REACTIONS

( E t O ) S i H in C H C 1 , a Pt colloid formed after about 1 h with release of H (eq 2). 3

2

2

2

CODPtCl

2

+ (EtO) SiH 3

CH2CI2

[Pt(0)] + H Î colloid 1 Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 10, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch037

x

>

+ ClSi(OEt)

2

3

+ cyclo-C H 8

+ cyelo-C H

14

8

16

(2)

The colloidal product, identified by analytical electron microscopy (AEM), was more active than C O D P t C l itself. A n induction period was observed in the hydrosilylation reaction catalyzed by C O D P t C l . However, no induction period was noted when colloid 1 was used as a catalyst. The length of the induction period corresponded to the time it took to form colloid 1 from C O D P t C l . 2

2

2

This chapter addresses recent results that further support Pt colloids as intermediates in the Pt-catalyzed hydrosilylation reaction. A E M , high-resolution electron microscopy ( H R E M ) , and X-ray absorption near-edge spectroscopy (XANES) were employed to analyze the Pt colloids.

Experimental Procedures General Information. A E M analyses were performed on an analytical electron microscope (Hitachi H-600-1 instrument) operated at 100 kV and equipped with an energy-dispersive X-ray detector [ E E G Ortec Si(Li)]. H R E M was performed on an electron microscope (Philips E M 430 instrument) operated at 300 kV. The X A N E S measurements were made at Brookhaven National Laboratory National Light Source; zero energy was taken at the L edge of platinum metal (by using Pt foil) at 11,568.4 eV with electron energy of 2.5 GeV. Data were collected in the fluorescence mode, and normalized plots of the near edge were produced by using published procedures (10). N M R spectra were recorded on one of two N M R spectrometers (Varian X L 300 spectrometer, C , Si, and Pt at 75.43, 59.3, and 64.12 M H z , respectively, or on a G E QE-300 instrument, C at 75.48 MHz). U I

1 3

29

l95

l 3

Pt Catalyst. Catalyst solutions were either the complex of divinyltetramethyldisiloxane ( D V T M D S , see structure), Pt(DVTMDS) ("Karstedt" catalyst in xylene, available from Petrarch systems as PC 072); or Pt(DVTMDS) in neat D V T M D S made by following a modification of a published procedure (II, 12). H P t C l (5 g, 39.4% Pt, 10 mmol) was combined with 5 m L of E t O H and D V T M D S (50 g, 0.27 mol) and heated at 60 °C with stirring for Φ-6 h. A x

x

2

6

SiO—Si

DVTMDS


37.

LEWIS ET AL.

543

Hydrosilylation

homogeneous orange solution was obtained, to which solid N a H C 0 (1.7 g, 70.8 mmol) was added. After an additional 1 h of stirring, the mixture was filtered and the precipitate was washed with two 10-g aliquots of D V T M D S . A yellow liquid was obtained, 2.27% Pt by analysis. The xylene solution of Pt(DVTMDS) or the Pt(DVTMDS) in neat D V T M D S previously described had identical Pt N M R resonances at -6148 ppm upfield from N a P t C l . In addition, concentrates of this solution were obtained by vacuum distilling (30 °C, 0.5 mm Hg pressure) a portion of the D V T M D S . C N M R spectra of these concentrated mixtures showed the presence of bound D V T M D S , on the basis of the appearance of an upfield set of resonances from 55-59 ppm versus 131.89 and 139.62 ppm for the vinyl portion of free D V T M D S . 3

r

ia5

x

2

6


1 3

Hexene Reactions. In a typical run, 3,3'-dimethyl-l-butene (f-hexene, 1.38 mL, 10.8 mmol) was combined with a xylene solution of Karstedt catalyst (2.7 μ ί , 0.7 mmol) and decane (internal standard, 0.2 mL, 1.03 mmol). Addition of E t S i H (1.72 mL, 10.8 mmol) initiated the reaction; a yellow color was noted after about 2 min at ambient temperature. N M R data are as follows: 3

(CH CH ) SiCH CH C(CH ) a b c d e f 3

2

3

2

2

3

3

C : 37.76 (d), 31.04 (f), 28.73 (e), 7.38 (b), 5.02 (c), and 3.18 (a) ppm ^Si: 7.58 ppm, single isomer 13

Results and Discussion Colloid Synthesis and Structure.

In 1989 we described the syn

thesis and structure of platinum group metal colloids from metal halide salts and alkoxysilane [e.g., P t C l + M e ( E t O ) S i H (13)]. A E M and H R E M anal yses of these colloids showed (for Pt) that 1-5-nm diameter platinum crys tallites formed (Figure 1). 4

2

The work described and recent mechanistic work (14, 15) showed that the most active catalyst precursors for hydrosilylation were those that could most easily form colloids. Thus the most active catalysts were Pt(O) com plexes with olefin ligands. Conversely, Pt(II) complexes with strongly bind ing ligands such as phosphines were the least active. A n example of one of the most active catalyst precursors was the Karstedt catalyst (12, 16). As we (13) and others (16) showed, the Karstedt catalyst is a mixture of compounds, but the main component is the η = 0 compound. The Karstedt catalyst is referred to as "solution A " in the Chandra work (16).

Karstedt catalyst, n=0-9



544


Figure 1. HREM for the Pt colloid formed between PtCl and Me^EtOjSiH; 1-5-nm particles are evident. Also evident are diffraction fringes of the (111) planes. (Reproduced from reference 13. Copyright 1989 American Chemical Society.) 4

The reaction of an orange xylene Karstedt catalyst solution with ( E t O ) S i H resulted in instantaneous formation of a black solution (eq 3). 3

Si(OEt)

Si(OEt)

3

3

(3) H2 + (EtO) Si-Si(OEt) + 3

3

— S i — Ο

Si

+ Pt colloid

2 A E M analysis confirmed that platinum colloids formed. X A N E S was used to analyze the platinum reactant and product of eq 3. In general, the area under the near-edge curve (white line) is related to the metal oxidation state. The electronic dipole transitions at the Pt L , edge are from 2 p n

3 / 2

core

level to the empty 5d state (Δ/ = 0 ± 1, where Δ/ corresponds to the difference between the excited state and ground state; / is the total angular momentum quantum number.) Therefore the peak intensity or "white line"


37.

LEWIS ET AL.

545

Hydrosilylation


is proportional to the d-electron vacancies (17, 18). As shown in Figure 2, the reaction product, colloid 2, was more "reduced" than the starting Karstedt catalyst. Figure 2 shows that there is a lower white-line area for colloid 2 than for Karstedt. In addition, the change in fine structure observed after the edge in going from Karstedt to colloid 2 suggested that Pt was in a different environment in colloid 2 than in Karstedt catalyst.

-2.

0.

-1.

2.

1.

3.

ENERGY (EV)

5.

4.

6.

7.

X10

1

Figure 2. Near-edge spectra for Karstedt catalyst (solid line) and colloid 2, eq 3 (dashed line). Figure 3 compares the near-edge region for colloids 1 and 2. Although two different Pt starting materials were used to form colloids 1 and 2 ( P t C l and P t ( D V T M D S ) , respectively), colloids with equivalent white-line areas were obtained. The fine-structure region suggests a different environment around Pt in colloids 1 and 2, however. 4

A

Platinum Colloids from Actual Catalyzed Reactions.

A E M and

H R E M were used to analyze the Pt product from hydrosilylation reactions catalyzed by Karstedt catalyst. E t S i H was reacted with n-hexene (eq 4) or f-hexene (eq 5) in the presence of about 100 ppm of Pt as Karstedt catalyst. 3

CH =CH(CH ) CH 2

2

3

3

+ Et SiH

> Et Si(CH ) CH

3

3

Karstedt

l

2

Q

%

œ

h

n

5

e

s

s


3

(4)


546


C H I

-6.

-4.

-2.

0.

2.

ENERGY (EV)

X10

1

Figure 3. Comparison of the near-edge regions for colloid 1 (solid line) and colloid 2 (dashed line). CH =CHC(CH ) 2

3

3

+ Et SiH 3

Karstedt

Et SiCH CH C(CH ) 100%, colorless 3

2

2

3

3

(5)

Equations 4 and 5 show that high conversion to product occurred for r-hexene in 17 h at ambient temperature, but that only 10% conversion occurred in the n-hexene reactions under the same conditions. The morphology of the colloids formed from eqs 4 and 5 were different. A E M analysis of the reaction solution from eq 4 (Figure 4) showed that amorphous-looking agglomerates of Pt-containing material were present. The inset to Figure 4 ( H R E M ) showed that the amorphous clump contained 1.5-2.0-nm Pt crystallites. The particles exhibited fringes that corresponded to the spacing of the (111) planes of platinum (9). A E M analysis (Figure 5) of the reaction solution of eq 5 showed that there was a lower degree of agglomeration of the Pt particles but that the individual Pt crystallites ( H R E M inset) were larger, 2.2-2.8 nm, than those from eq 4. As in Figure 4, the particles in Figure 5 exhibited the (111) planes attributed to crystalline Pt. In addition, an electron-diffraction pattern obtained from this sample indexed to crystalline platinum. The larger individual particles of Figure 5 presumably gave rise to the yellow appearance in eq 5 vis-a-vis the colorless solution of eq 4.



37.

LEWIS ET AL.

547

Hydrosilylation

Figure 4. AEM image of the reaction solution from eq 4, n-hexene + Et SiH, catalyzed by Karstedt catalyst. Inset shows HREM image. The fringes correspond to the (111) planes of crystalline Pt. (Reproduced with permission from reference 18. Copyright 1991 Academic Press.) 3

Figure 5. AEM image of the reaction solution from eq 5, t-hexene + Et SiH catalyzed by Karstedt catalyst. Inset shows HREM image. (Reproduced with permission from reference 18. Copyright 1991 Academic Press.) 3

American Chemical Society Library 1155 IBth St., N.W. In Homogeneous Transition Metal Catalyzed Reactions; Moser, W., et al.; Advances in Chemistry;Washington, American Chemical Society: Washington, DC, 1992. D.C. 20036

548

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

Summary


The combination of X A N E S and A E M - H R E M showed that colloidal plati num formed from Karstedt catalyst reactions with the S i H functionality either directly or in a hydrosilylation reaction catalyzed by Karstedt catalyst. The colloid formed was more reduced than the starting complex. It is proposed that this reduction step is the commonly observed induction period in hy drosilylation. The morphology of the colloid formed was controlled by the reagents of the reaction. It is not known whether the differences in the two colloids of Figures 4 and 5 were due to the differences in steric or electronic factors between n-hexene and f-hexene or to both.

Acknowledgments Joe Wong, Livermore National Laboratory, assisted in the X A N E S mea surements and analyses. Joanne Smith carried out the N M R measurements. Ernie Hall carried out the H R E M measurements.

References

1. Ojima, I. In The Chemistry of Organic Silicon Compounds; Patai, S.; Rappopo Z., Eds.; Wiley: New York, 1989; p 1479. 2. Armitage, D. A. In Comprehensive Organometallic Chemistry; Wilkinson, G Stone, F. G. Α.; Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol. 2, p 117. 3. Speier, J. L. Adv. Organomet. Chem. 1979, 17, 407. 4. Lukeviks, E.; Belyakova, Ζ. V; Pomeransteva, M. G.; Voronkov, M. G. J. Organomet. Chem. Libr. 1977, 5, 1. 5. Eaborn, C.; Bott, R. W. In The Bond to Carbon; MacDiarmid, A. G., Ed.; Dekker: New York, 1968; Vol. 1. 6. Harrod, J. F.; Chalk, A. J. In Organic Synthesis via Metal Carbonyls; Wender, I.; Pino, P., Eds.; Wiley: New York, 1977; p 673. 7. Chalk, A. J.; Harrod, J. F.J.Am. Chem. Soc. 1965, 87, 16. 8. Collman, J. P.; Hegedus, L. S. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1980; pp 384ff. 9. Lewis, L. N.; Lewis, N.J.Am. Chem. Soc. 1986, 108, 7228. 10. Wong, J.; Lytle, F. W.; Messmer, R. P.; Maylotte, D. H. Phys. Rev. B: Condens. Matter 1984, 30, 5587. 11. Karstedt, B. D. U.S. Patent 3 775 452, 1973. 12. Ashby, Β. Α.; Modic, F. J. U.S. Patent 4 288 345, 1981. 13. Lewis, L. N.; Lewis, N. Chem. Mater. 1989,1,106. 14. Lewis, L. N.J.Am. Chem. Soc. 1990, 112, 5998. 15. Lewis, L. N.; Uriarte, R. J. Organometallics 1990, 9, 621. 16. This complex is referred to as "Solution A" in Chandra, G.; Lo, P. Y.; Hitchcock, P. B.; Lappert, M. F. Organometallics 1987, 6, 191.


37.

LEWIS ET AL.

Hydrosilylation

549


17. (a) Lytle, F. W.; Greegor, R. B.; Marques, E. C.; Biebesheimer, V. Α.; Sandstrom, D. R.; Horsley, J. Α.; Via, G. H.; Sinfelt, J. H. In Catalyst Character ization Science; Deviney, M. L.; Gland, J. L., Eds.; ACS Symposium Series 288; American Chemical Society: Washington, DC, 1985; p 280. (b) Lytle, F. W.; Via, G. H.; Sinfelt, J. H. In Synchrotron Radiation Research; Winick, H.; Doniach, S., Eds.; Plenum: New York, 1980; p 401. 18. Lewis, L. N.; Uriarte, R. J.; Lewis, N. J. Catal 1991, 127, 67. RECEIVED for review October 19, 1990. A C C E P T E D revised manuscript May 24, 1991.


Hydrosilylation - Advances in Chemistry (ACS Publications)

Recommend Documents