Organometallics 1995,14,2297-2305
2297
Competitive Reductive Elimination of C-H and Si-H Bonds from IrH(SiHPh2)(mes)(CO)(dppe). Comparison of the Kinetic Activation Parameters for Reductive Eliminations in IrlI1(dppe)Complexes Brian P. Cleary, Rajeev Mehta, and Richard Eisenberg" Department of Chemistry, University of Rochester, Rochester, New York 14627 Received January 13,1995@ The complex IrH(SiHPhz)(mes)(CO)(dppe)(1;mes = 2,4,6-trimethylphenyl; dppe = bis(dipheny1phosphino)ethane)undergoes competing irreversible mesitylene and reversible diphenylsilane reductive eliminations in benzene, leading to the known Ir complexes Ir(mes)(CO)(dppe) and IrH(SiHPh~)z(CO)(dppe)(2). The kinetics for the C-H vs Si-H competitive reductive eliminations have been studied and indicate that both reactions follow simple first-order kinetics. For mesitylene reductive elimination, AH$= 21.8 0.3 kcal mol-', AS$ = -5.0 f 1.1eu, and AG*298 = 23.3 f 0.4 kcal mol-l, while for diphenylsilane reductive elimination, AH$= 15.6 f 0.5 kcal mol-l, AS* = -25.2 f 1.1eu, and AG'Zg8 = 23.1 f 0.6 kcal mol-l. A striking similarity exists among the kinetic parameters for diphenylsilane reductive elimination from 1 and for Hz reductive eliminations from IrH2( 5 )and IrHz(C(O)C2Hs)(CO)(dppe)(6). To determine if a kinetic similarity (C~Hs)(CO)(dppe) for silane and H2 reductive eliminations exists among 1r1I1(dppe) complexes, the kinetic parameters for triethylsilane reductive elimination from IrH2(Si(C2H5)3)(CO)(dppe)(7) and H2 reductive elimination from IrHz(SiHPh2)(CO)(dppe)( 8 ) were determined. For triethylsilane reductive elimination from 7 , AH $ = 29.2 f 0.3 kcal mol-', A S = 12.8 f 0.9 eu, and hG*298 = 25.4 f 0.4 kcal mol-l. For H2 reductive elimination from 8 , AH $ = 25.2 f 0.7 kcal mol-l, AS* = -4.2 f 2.1 eu, and AG'298 = 26.4 f 0.9 kcal mol-l. The negative entropies of activation for Si-H and H-H reductive eliminations are rationalized in terms of a-complex formation prior to dissociation. Platinum-group transition-metal complexes are known t o catalyze the hydrosilation of olefins, acetylenes, aldehydes and ketones eff~ciently.l-~Several mechanisms have been postulated to describe catalytic hydrosilation. In the Chalk-Harrod mechanism: hydrosilation occurs by oxidative addition of a silane Si-H bond t o a metal-alkene complex followed by olefin insertion into the metal hydride bond (eq 1);reductive elimination then generates the organic product by Si-C bond formation as in eq 2. In an alternative mechanism, R'
L,M-I(
+RsSi-H
-
SiRl
Si-H oxidative addition to the metal-olefin complex is followed by olefin insertion into the metal-silyl bond, @Abstractpublished in Advance ACS Abstracts, April 15, 1995. (l)Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987. (2)Speier, J.L.In Advances in Organometallic Chemistry: Catalysis and Organic Syntheses Stone, F. G. A,, West, R., Eds.; Academic Press: New York, 1979;Vol. 17,p 407.
0276-7333/95/2314-2297$09.00/0
yielding a B-silylalkyl hydride intermediate (eq 3) which gives the product after C-H reductive e l i m i n a t i ~ n . ~ - ~
-
R' L,,M-[(
+&3i-H H
H
Formation of vinylsilane and alkane, common byproducts of hydrosilation catalysis, are also accounted for by this mechanism: the former via P-hydride elimination from the P-silylalkyl hydride intermediate (eq 4) and the latter from the dihydride intermediate generated in /?-elimination,which serves as a catalyst for olefin hydrogenation. Finally, a third mechanism involving two silanes has been proposed for hydrosilations catalyzed by Co(CO)&iR3 and CpRh(CzHd(SiRs)H(Cp = cy~lopentadienide).~~~ (3)Lukevics, E.; Belyakova, Z. V.; Pomerantseva, M. G.; Voronkov, M. G. In Organometallic Chemistry Reviews; Seyferth, D., Davies, A. G., Fischer, E. O., Normant, J. F., Reutov, 0. A., Eds.; Elsevier: Amsterdam, Oxford, New York, 1977;pp 1-179. (4)Chalk, A. J. J. Organomet. Chem. 1970,21,207-213. (5)Schroeder, M.A.; Wrighton, M. S. J. Organomet. Chem. 1977, 128,345-358. ( 6 )Brookhart, M.;Grant, B. E. J.Am. Chem. SOC.1993,115,21512156. (7)Wakatsuki, Y.; Yamazaki, H.; Nakano, M.; Yamamoto, Y. J. Chem. SOC.,Chem. Commun. 1991, 703-704. (8)Seitz, F.; Wrighton, M. S. Angew. Chem., Int. Ed. Engl. 1988, 27,289-291.
0 1995 American Chemical Society
Cleary et al.
2298 Organometallics, Vol. 14, No. 5, 1995
Regardless of the mechanism operating during hydrosilation, reductive eliminations involving formation of C-H, Si-H, and H-H bonds may be occurring at the metal center, leading t o product formation in the catalysis or exchange reactions.lJoJ1 Since the distribution of the organic products is influenced in part by relative rates of reductive elimination, it is important to acquire kinetic information about these processes. Toward this end, mechanistic and kinetic data for C-H,12-50H-H,51-57 and Si-H58-69 reductive eliminations have been reported for a number of transition(9) Duckett, s. B.; Perutz, R. N. Organometallics 1992, 11, 90-98. (10) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988; p 1455. (11) Stille, J. K. In The Chemistry ofthe Metal-Carbon Bond: The Nature and Cleavage of Metal-Carbon Bonds; Hartley, F. R., Patai, S., Eds.; Wiley: New York, 1985; Vol. 2, p 625. (12) Jones, W. D.; Hessell, E. T. J. A m . Chem. SOC.1992,114,60876095. (13)Jones, W. D.; Kuykendall, V. L. Inorg. Chem. 1991,30,26152622. (14)Jones, W. D.; Feher, F. J. J . A m . Chem. SOC.1985,107, 620631. (15) Jones, W. D.; Feher, F. J. J . A m . Chem. SOC.1984,106,16501663. (16)Jones, W. D.; Feher, F. J. Acc. Chem. Res. 1989,22, 91-100. (17) Hostetler, M. J.; Bergman, R. G. J . Am. Chem. SOC.1992, 114, 7629-7636. (18) Periana, R. A,; Bergman, R. G. J . Am. Chem. SOC.1986, 108,
-
7.132-7.146. . - - . - .- .
(19) Buchanan, J. M.; Stryker, J. M.; Bergman, R. G. J . Am. Chem. SOC.1986,108, 1537-1550. (20)Wax, M. J.; Styker, J . M.; Buchanan, J. M.; Kovac, C. A.; Bergman, R. G. J. Am. Chem. SOC.1984,106, 1121-1122. (21) Nappa, M. J.; Santi, R.; Diefenbach, S. P.; Halpern, J . J . A m . Chem. SOC.1982,104, 619-621. (22) Chan, A. S. C.; Halpern, J . J. A m . Chem. SOC.1980,102, 838. (23)Abis, L.; Sen, A,; Halpern, J. J . Am. Chem. SOC.1978, 100, 2915-2916. (24) Halpern, J. ACC. Chem. Res. 1982, 15, 332-338. (25) Bullock, R. J.; Headford, C. E. L.; Hennessy, K. M.; Kegley, S. E.; Norton, J. R. J . Am. Chem. SOC.1989, 111, 3897-3908. (26) Bullock, R. M.; Headford, C. E. L.; Kegley, S. E.; Norton, J . R. J . A m . Chem. SOG.1985,107, 727-729. (27) Okrasinski, S. J.; Norton, J. R. J . A m . Chem. SOC.1977, 99, 295-297. (28) Schwartz, J.;Cannon, J. B. J . Am. Chem. SOC. 1974,96,22762278. (29) Norton. J. R. Acc. Chem. Res. 1979. 12. 139. (30) Milstein, D. J . Am. Chem. SOC.1982, 104, 5227-5228. (31) Milstein, D. Acc. Chem. Res. 1984, 17, 221-226. (32) Gould, G. L.; Heinekey, D. M. J . A m . Chem. SOC.1989, 111, 5502-5504. (33) Michelin, R. A,; Faglia, S.; Uguagliati, P. Inorg. Chem. 1983, 22, 1831-1834. (34)McAlister, D. R.; Erwin, D. K.; Bercaw, J. E. J . Am. Chem. SOC. 1978,100, 5966-5968. (35)Pedersen, A,; Tilset, M. Organometallics 1993,12,3064-3068. (36) Basato, M.; Longato, B.; Morandini, F.; Bresadola, S. Inorg. Chem. 1984,23, 3972-3976. (37) Basato, M.; Morandini, F.; Longato, B.; Bresadola, S. Inorg. Chem. 1984,23,649-653. (38) Bianchini, C.; Masi, D.; Meli, A,; Peruzzini, M.; Zanobini, F. J . Am. Chem. SOC.1988,110,6411-6423. (39) Bianchini, C.; Barbaro, P.; Meli, A.; Peruzzini, M.; Vacca, A.; Vizza, F. Organometallics 1993, 12, 2505-2514. (40) Smith, G. M.; Carpenter, J. D.; Marks, T. J. J . Am. Chem. SOC. 1986, 108, 6805-6807. (41)Werner, H.; Gotzig, J. J . Organomet. Chem. 1985,284, 73-93. (42) Gibson, V. C.; Kee, T. P.; Carter, S. T.; Sanner, R. D.; Clegg, W. J . Organomet. Chem. 1991,418, 197-217. (43)Price, R. T.; Andersen, R. A,; Muetterties, E. L. J . Organomet. Chem. 1989,376, 407-417. (44) Roper, W. R.; Wright, L. J. J . Organomet. Chem. 1982, 234, C5-C8. (45)Thompson, J . S.; Bernard, K. A,; Rappoli, B. J.; Atwood, J . D. Organometallics 1990, 9, 2727-2731. (46)Atwood, J. D. Coord. Chem. Rev. 1988,83, 93-114. (47) Bianchini, C.; Meli, A,; Peruzzini, M.; Zanobini, F. J . Chem. SOC.,Chem. Commun. 1987, 971-973.
metal compounds. Relative rates of C-H and Si-H bond formation can be obtained by investigating either (1) reductive eliminations of R-H and R3Si-H from related alkyl hydride and silyl hydride complexes to yield the same L,M fragment, or (2)competitive reductive eliminations of R-H and R3Si-H from the same alkyl silyl hydride complex, L,MH(SiR3’)(R). The latter approach was employed by us to examine and establish the rate constants and activation parameters for the formation of H2 and ethane from IrH2(C2Hd(CO)(dppe) (5; dppe = 1,2-bis(diphenylphosphino)ethane)and the formation of H2 and propionaldehyde from IrHdC(0)CzHs)(CO)(dppe) (6) via competitive H-H and C-H reductive elimination reactions according to eq 5.70,71
*..
XY -\
stable Ir(ll1) products
In the course of continuing investigations on indium complexes containing the chelating ligand dppe, and in particular the Ir(1) dppe complex containing a mesityl (mes) ligand, Ir(mes)(CO)(dppe),we observed oxidative addition of SiHsPhz t o generate the six-coordinate Ir(111)complex IrH(SiHPhz)(mes)(CO)(dppe)(11, as shown (48) Cooper, N. J.; Green, M. L.; Mahtab, R. J . Chem. SOC.,Dalton Trans. 1979, 1557-1562. (49) Hartwig, J. F.; Andersen, R. A.; Bergman, R. G. Organometallics 1991,10, 1710-1719. (50) McFarland, J. M.; Churchill, M. R.; See, R. F.; Lake, C. H.; Atwood, J. D. Organometallics 1991, 10, 3530-3537. (51) Evans, J.; Norton, J . R. J . Am. Chem. SOC.1974, 96, 75777578. (52) Packett, D. L.; Trogler, W. C. Inorg. Chem. 1988, 27, 17681775. (53) Duggan, T. P.; Golden, M. J.; Keister, J . B. Organometallics 1990,9, 1656-1665. (54) Nevinger, L. R.; Keister, J. B.; Maher, J. Organometallics 1990, 9, 1900-1905. ( 5 5 )Strohmeier, W.: Muller. F. J. 2. Naturforsch. 1969.24B, 931932. (56) Bitterwolf, T. E.; Raghuveer, K. S. Inorg. Chim. Acta 1990,172, 59-64. (57) Collman, J. P.; Hutchison, J . E.; Wagenknecht, P. S.; Lewis, N. S.; Lopez, M. A.; Guilard, R. J . A m . Chem. SOC.1990, 112, 82068208. (58) Harrod, J. F.; Smith, C. A.; Than, K. A. J. Am. Chem. SOC.1972, 94, 8321-8325. (59) Harrod, J . F.; Smith, C. A. J . A m . Chem. SOC.1970, 92, 26992701. (60) Zhang, S.;Dobson, G. R.; Brown, T. L. J . Am. Chem. SOC.1991, 113, 6908-6916. (61)Aizenberg, M.; Milstein, D. Angew. Chem. Int. Ed. Engl. 1994, .?.? - - , 217-319 (62) Hill, R. H.; Wrighton, M. S. Organometallics 1987,6,632-638. (63) Hester, D. M.; Sun, J.;Harper, A. W.; Yang, G. K. J. Am. Chem. SOC.1992,114, 5234-5240. (64) Hostetler, M. J.; Butts, M. D.; Bergman, R. G. Organometallics 1993,12,65-75. (65) Rappoli, B. J.; McFarland, J . M.; Thompson, J. S.; Atwood, J. D. J . Coord. Chem. 1990,21, 147-154. (66) Schubert, U.Angew. Chem., Int. Ed. Engl. 1994,33,419-421. (67) Schubert, U.; Scholz, G.; Muller, J.; Ackermann, K.; Worle, B. J . Organomet. Chem. 1986,306, 303-326. (68) Schubert, U . In Advances in Organometallic Chemistry; Academic Press: New York, 1990; Vol. 30, pp 151-187. (69) Kraft, G.; Kalbas, C.; Schubert, U . J . Organomet. Chem. 1985, 289, 247-256. (70)Deutsch, P. P.; Eisenberg, R. J. Am. Chem. SOC.1990,112,714721. (71) Deutsch, P. P.; Eisenberg, R. Organometallics 1990, 9, 709718.
Reductive Eliminations in Ir"I1(dppe)Complexes
Organometallics, Vol. 14, No. 5, 1995 2299
Pathway A H
2
0
L
3 Pathway B Figure 1. Kinetic pathways possible for the reductive elimination of diphenylsilane and mesitylene from IrH(SiHPhz)(mes)(CO)(dppe), 1. in eq 6. Complex 1 was found to be unstable in toluene petitive reductive elimination, two unimolecular reaction pathways exist and the kinetic parameters for each can be determined. The two specific routes of elimination observed during this study are illustrated in Figure 1,while the overall rate equation describing the disappearance of 1 is shown in eq 8. Four-coordinate Ir(1)
-d[llldt = (hcH
+ hsiH)[l] -
hsiH~SiH2Ph2~[Ir(mes)(~~)(dppe)l (8) O
1
and benzene solutions a t room temperature, leading to regeneration of the original Ir(1) complex as well as formation of IrH(SiHPhz)z(CO)(dppe)(2) and mesitylene after several days, (eq 7). The observation of eq 7 r
H
indicated that 1 was undergoing facile reaction chemistry that proceeded via reductive elimination. Moreover, the fact that the two complexes Ir(mes)(CO)(dppe) and IrH(SiHPhz)z(CO)(dppe)were formed as the metalcontaining products established that two different reductive eliminations were occurring competitively. Complex 1thus provided a rare opportunity t o investigate by kinetic methods competitive reductive elimination of aryl C-H and Si-H bonds. Herein we describe the results of that study and compare the kinetic and activation parameters of this competitive reductive elimination with those reported previously by us for H-H and C-H bonds. Additionally, we describe kinetic studies that compare Si-H reductive elimination with that of H2 in related 1r"I (dppe) systems. Results Kinetics of Competitive Reductive Elimination of Mesitylene and Diphenylsilane from 1. In com-
intermediates in the reaction sequence such as Ir(C0)(mes)(dppe) and [Ir(SiHPhz)(CO)(dppe)lwere not detected, since an excess of silane trapping agent (relative to 1) was used during the kinetic runs. The silane trapping agent was either SiH2Ph2, when experiments , SiHsPh, when the sum were run to determine ~ C H or of kc^ & kSiHf was the kinetics objective (vide infra). Since Ir(mes)(CO)(dppe) is not detected when SiHzPhz is present in large excess, it is possible under these conditions to apply the steady-state approximation t o [Ir(CO)(mes)(dppe)l,leading to [Ir(CO)(mes)(dppe)l = k~i~tIlYksi~,[SiH2Phz]. After substitution of the steadystate approximation into eq 8, the simple first-order integrated rate law for the determination of C-H reductive elimination shown in eq 9 is obtained. The (9)
rate constant for the elimination of mesitylene ( ~ c H )is thus determined by monitoring the disappearance of 1 in the presence of excess diphenylsilane (SiHzPhz). Under these conditions diphenylsilane reductive elimination from 1is "transparent" because any Ir(CO)(mes)(dppe)generated reacts rapidly with SiHzPhz to re-form 1. Therefore, only mesitylene elimination contributes t o the decay of signal intensity due to 1. The reductive elimination of mesitylene was followed by lH NMR spectroscopy between the temperatures of 295 and 323 K. For each experimental run,the decrease in signal intensity for the hydride resonance of 1 followed first-order kinetics through at least 3 half-lives. Changes in the concentration of either 1 or diphenylsilane (10-80 equiv relative t o 1) did not affect the rate of mesitylene elimination. Plots of -ln([lY[llo) vs time were linear and are shown in Figure 2, while rate constants for mesitylene reductive elimination are listed in Table 1.
Cleary et al.
2300 Organometallics, Vol. 14, No. 5, 1995 3.5
'T
+
2.5
5
2.5 2
Y
A
-
1.5 1
0.5
0.5 0
0
0
20000
40000
6oooO
80000
10oooO
n
5000
Time (s)
2
~
m30000
Figure 3. Plot of -ln([l]/[llo) vs time for the reductive elimination of mesitylene diphenylsilane from 1: (B) 295 K (e)303 K (A)313 K ( 0 )323.K.
+
Table 1. Rate Constants for Mesitylene and Diphenylsilane Reductive Eliminations from IrH(SiHPhd(mes)(CO)(dppe),(1) 3.43 i 0.03 9.41 f 0.03 31.0 i 0.3 94.6 + 0.1
2oooo
Time (s)
Figure 2. Plot of -ln([l]/[l]o) vs time for the reductive elimination of mesitylene from 1: (B) 295 K, (*) 303 K (A)313 K ( 0 )323 K.
295 303 313 323
io000 is000
5.26 f 0.09 10.9 + 0.4 25.8 0.7 57.9 + 0.4
*
-12
T
8.69 + 0.08 20.3 f 0.4 56.8 f 0.6 152.5 f 0.4
a Reaction conditions: [ l ] = 0.027 M in C&j; 0.270 M diphenylsilane. Reaction conditions: [ l ] = 0.027 M in C&; 0.540 M phenylsilane. e Values obtained from a least-squares fit of lines from plots of - l n [ l l / [ l l ~vs time. Values calculated from ~ s =~
H
~
kobs - hCH.
The overall rate constant (hobs) for the disappearance of 1 is equal to the sum of the rate constants for both reductive-elimination processes G o b s = ~ C Hf kSiHf). Monitoring the reaction in the presence of excess phenylsilane (SiH3Ph),which reacts more rapidly with IrY(CO)(dppe) (Y = mes and SiHPhd than does SiHzPhz allows for the determination of hobs. Under these conditions, regeneration of 1via the back-reaction of Ir(CO)(mes)(dppe)with diphenylsilane is eliminated because any Ir(CO)(mes)(dppe)generated quickly undergoes oxidative addition with phenylsilane (k~i~,[SiHzPhzl
