Reversible 1,2-migration of hydrogen between platinum and silicon via intermediate silylene complexes

Full text of DOI:10.1021/ja9808346

J. Am. Chem. Soc. 1998, 120, 7635-7636
7635
Communications to the Editor
Scheme 1
Reversible 1,2-Migration of Hydrogen between
Platinum and Silicon via Intermediate Silylene
Complexes
Gregory P. Mitchell and T. Don Tilley*
Department of Chemistry
UniVersity of California at Berkeley
Berkeley, California 94720-1460
ReceiVed March 13, 1998
A number of transition metal-catalyzed reactions involving
organosilanes are thought to involve silylene complexes as
intermediates.1 In addition, given the many and varied roles that
carbene complexes play in useful chemical transformations,2 it
seems that analogous silicon compounds of the type LnMdSiR2
might provide powerful synthetic intermediates. As a result of
these interests, synthetic routes to silylene complexes have
recently been developed,3 and their characteristic reactivity
patterns are beginning to emerge.4 In related work, it is becoming
increasingly clear that intramolecular migrations in metal silyl
complexes may involve intermediate silylene ligands, which might
arise via 1,2-migrations between silicon and the metal center (eq
1).5,6 However, this kind of migration reaction has never been
generation of a silylene complex in a catalytic cycle. Here we
report the formation of a platinum hydrido(silylene) complex,
which reacts via reversible 1,2-migration of hydrogen between
platinum and silicon.
We have previously demonstrated that cationic silylene com-
plexes may be formed by exchange of the triflate in LnM-SiR2-
-
- 3c,d
(OTf) complexes with BPh4 or B(C6F5)4
.
Following this
approach, we obtained the triflato-silyl complex cis-(PEt3)2Pt-
(H)Si(StBu)2OTf (1) in 58% yield by reaction of Me3SiOTf with
cis-(PEt3)2Pt(H)Si(StBu)3 in dichloromethane. The 29Si NMR
spectrum of 1 consists of a resonance at δ 52.22, which is
somewhat downfield shifted from that for cis-(PEt3)2Pt(H)Si-
(StBu)3 (δ 16.26). This value is consistent with the presence of
a covalently bound triflate group, since transition metal silylene
complexes typically possess 29Si NMR chemical shifts near 300
ppm.3
R′
M
M
SiR′R2
(1)
SiR2
In an attempt to generate and observe the silylene complex
[cis-(PEt3)2(H)PtdSi(StBu)2][B(C6F5)4], we monitored the reaction
of 1 with excess (3 equiv) (Et2O)LiB(C6F5)4 in dichloromethane-
d2 at -80 °C, by 1H and 31P NMR spectroscopy. At this
temperature, formation of an intermediate (2) is indicated by
appearance of a new hydride resonance in the 1H NMR spectrum
at δ -5.35, which displays coupling to inequivalent phosphorus
atoms. The 29Si{1H} NMR resonance for this species (δ 88.62)
is too far upfield to be consistent with a base-free silylene
complex, but does lie in the region expected for a base-stabilized
silylene complex (Scheme 1).3 Further evidence for the charac-
terization of this species as an ether adduct is seen in the 1H NMR
spectrum, which contains two resonances (at δ 3.41 and 4.55)
for -OCH2- groups, assigned to bound and free (or lithium-
bound) diethyl ether, respectively. Variable-temperature spectra
revealed a coalescence of these resonances at -50 °C, which
directly observed. Clearly, a better understanding of transition
metal-silicon chemistry will result from experimental studies on
such migrations, which seem like the most likely pathway for
(1) (a) Tilley, T. D. In The Silicon-Heteroatom Bond; Patai, S., Rappoport,
Z., Eds.; Wiley: New York, 1991; Chapters 9 and 10, pp 245 and 309. (b)
Tilley, T. D. In The Chemistry of Organic Silicon Compounds; Patai, S.,
Rappoport, Z., Eds.; Wiley: New York, 1989; Chapter 24, p 1415. (c) Tilley,
T. D. Comments Inorg. Chem. 1990, 10, 37. (d) Lickiss, P. D. Chem. Soc.
ReV. 1992, 271. (e) Corey, J. In AdVances in Silicon Chemistry; Larson, G.,
Ed.; JAI Press: Greenwich, CT, 1991; Vol. 1, p 327. (f) Pannell, K. H.;
Sharma, H. K. Chem. ReV. 1995, 95, 1351. (g) Zybill, C. Top. Curr. Chem.
1991, 160, 1.
(2) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; University Science:
Mill Valley, CA, 1987, Chapter 16.
(3) Silylene complexes with sp2 silicon atoms: (a) Straus, D. A.; Grumbine,
S. D.; Tilley, T. D. J. Am. Chem. Soc. 1990, 112, 7801. (b) Grumbine, S. D.;
Tilley, T. D.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 358. (c)
Grumbine, S. D.; Tilley, T. D.; Arnold, F. P.; Rheingold, A. L. J. Am. Chem.
Soc. 1993, 115, 7884. (d) Grumbine, S. K.; Tilley, T. D.; Arnold, F. P.;
Rheingold, A. L. J. Am. Chem. Soc. 1994, 116, 5495. (e) Denk, M.; Hayashi,
R. K.; West, R. J. Chem. Soc., Chem. Commun. 1994, 33.
corresponds to a barrier for exchange of 10.0 ( 0.3 kcal mol-1
.
Intermediate 2 is thermally unstable, and above -40 °C it is
observed to convert quantitatively to a new compound (3), as
indicated by replacement of the hydride resonance of 2 (with JPtH
(4) (a) Mitchell, G. P.; Tilley, T. D. J. Am. Chem. Soc. 1997, 119, 11236.
(b) Grumbine, S. K.; Straus, D. A.; Tilley, T. D.; Rheingold, A. L. Polyhedron
1995, 14, 127. (c) Grumbine, S. K.; Tilley, T. D. J. Am. Chem. Soc. 1994,
116, 6951.
(5) (a) Mitchell, G. P.; Tilley, T. D.; Yap, G. P. A.; Rheingold, A. L.
Organometallics 1995, 14, 5472. (b) Mitchell, G. P.; Tilley, T. D. Organo-
metallics 1996, 15, 3477.
(6) (a) Pannell, K. H.; Cervantes, J.; Hernandez, C.; Cassias, J.; Vincenti,
S. Organometallics 1986, 5, 1056. (b) Pannell, K. H.; Rozell, J. M, Jr.;
Hernandez, C. J. Am. Chem. Soc. 1989, 111, 4482. (c) Pannell, K. H.; Wang,
L.-J.; Rozell, J. M. Organometallics 1989, 8, 550. (d) Pannell, K. H.; Sharma,
H. Organometallics 1991, 10, 954. (e) Tobita, H.; Ueno, K.; Ogino, H. Bull
Chem. Soc. Jpn. 1988, 61, 2797. (f) Ueno, K.; Tobita, H.; Ogino, H. Chem.
Lett. 1990, 369. (g) Haynes, A.; George, M. W.; Haward, M. T.; Poliakoff,
M.; Turner, J. J.; Boag, N. M.; Green, M. J. Am. Chem. Soc. 1991, 113, 2011.
(h) Nlate, S.; Herdtweck, E.; Fischer, R. A. Angew. Chem., Int. Ed. Engl.
1996, 35, 186. (i) Pestana, D. C.; Koloski, T. S.; Berry, D. H. Organometallics
1994, 13, 4173. (j) Sekiguchi, A.; Sato, T.; Ando, W. Organometallics 1987,
6, 2337.
) 750 Hz) by an Si-H signal at δ 5.39 (JPtH ) 110 Hz; JSiH
)
252 Hz). The 31P NMR spectrum of 3 exhibits peaks for
2
inequivalent phosphorus atoms (δ 21.66, 22.17) and a low JPP
coupling constant of 5.7 Hz. Variable-temperature 1H NMR
studies indicate the presence of inequivalent and interconverting
-StBu groups (Tc ≈ 10 °C), and the latter process is also
manifested in coalescence of the 31P resonances at 40 °C (∆Gq
) 15.4 ( 0.4 kcal mol-1). This dynamic process might seem to
be associated with hindered rotation about the Pt-Si bond, but a
purely steric basis for this is not reasonable considering the fact
that restricted rotation in cis-(PEt3)2Pt(H)Si(StBu)3 is not observed.
We attribute these observations to the presence of a dative SfPt
interaction in the migrated product 3, as indicated by the structure
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7636 J. Am. Chem. Soc., Vol. 120, No. 30, 1998
Communications to the Editor
Scheme 2
stable toward migration even in the presence of excess acetonitrile
(5-12 equiv).
The rate of reaction between 1 and CH3CN is slow enough at
1
10 °C to be monitored easily by H NMR spectroscopy. The
initial rates of formation of 6 (<10% reaction, ca. 30 min) were
found to be independent of [CH3CN] (0.08-1.56 M). To assess
the kinetic role of hydrogen migration in this equilibrium, we
prepared the deuteride cis-(PEt3)2Pt(D)Si(StBu)2OTf (1-d)9,10 and
monitored its conversion to 6-d to obtain a kinetic isotope effect
(kH/kD) of 1.3 ( 0.1. This relatively small KIE suggests that
hydride migration does not feature prominently in the rate-limiting
step; however, it is difficult at this point to predict what the
magnitude of a primary effect for such a migration would be.
Since the thermodynamic isotope effect is also small (Keq(H)/Keq(D)
) 1.1 ( 0.1), the KIE for the reverse reaction (6 to 1) must be
small as well. We therefore propose that the rate-determining
step in the formation of 6 from 1 is dissociation of triflate to
form the silylene species [cis-(PEt3)2(H)PtdSi(StBu)2][OTf] (7)
(Scheme 2). This silylene intermediate is probably also best
viewed as a contact ion-pair, since the initial rate of reaction is
not inhibited by added [Bu4N][OTf], with [Bu4N][B(C6F5)4]
present to maintain conditions of constant ionic strength.8
Intermediate 7 is proposed to undergo rapid hydride migration
to generate 8 (analogous to 3), which is then trapped by
acetonitrile to form the observed product 6. It therefore appears
that in dichloromethane, 1 is in equilibrium with minute quantities
of 7 and 8. It is also possible that in the presence of acetonitrile,
7 is trapped to a small, undetectable degree as a base-stabilized
silylene complex.
in Scheme 1. This structure has precedent in the cationic alkyl
analogue cis-[(PPh3)2PtC(H)SCH2CH2S]BF4, which also features
donation from sulfur to platinum (as determined by X-ray
crystallography).7 Interestingly, the latter complex formed via a
1,2-hydrogen migration from platinum to the carbene carbon atom.
Compound 3 could not be isolated, since it is thermally unstable
and decomposes at room temperature in dichloromethane-d2 to a
number of products (t1/2 ≈ 1 h). However, simple derivatives of
3 have been isolated and completely characterized. Thus, reaction
of 3 with MeMgBr in diethyl ether produced the platinum methyl
complex trans-(PEt3)2Pt(Me)SiH(StBu)2 (4), isolated in 45% yield.
In addition, reaction of 3 with Bu4NBr in diethyl ether gave cis-
(PEt3)2Pt(Br)SiH(StBu)2 (5), isolated in 85% yield.
In conclusion, we have observed the facile and clean migration
of hydrogen from platinum to silicon, via the putative silylene
hydride intermediate [(PEt3)2(H)PtdSi(StBu)2]+.11 In the presence
of the noncoordinating counteranion B(C6F5)4-, this migration is
rapid and irreversible. However, with a triflate counteranion the
migration is slow (requiring triflate dissociation from silicon) and
reversible. Acetonitrile traps the migrated product but in the
resulting -SiH(StBu)2 derivative (6), hydrogen reversibly migrates
between silicon and platinum. These initial results on 1,2-
migration between a metal and silicon suggest that such processes
may be facile, and highly sensitive to reaction conditions and
substituent effects.
Interestingly, addition of 2 equiv of [Bu4N][OTf] to a solution
of 3 does not form a complex analogous to 5, but results instead
in complete regeneration of 1, demonstrating facile reversibility
of this hydride migration. This reversibility is also indicated by
the reaction of 1 with a large excess of MeCN (ca. 300 equiv;
dichloromethane-d2), which induced quantitative hydride migra-
tion from Pt to Si to form the migrated complex [cis-(PEt3)2Pt-
(NCMe)SiH(StBu)2][OTf] (6, Scheme 2), characterized in solution
by NMR spectroscopy. Like 3, 6 displays fluxional behavior, as
observed by 31P NMR spectroscopy. At -20 °C, fully resolved
signals were observed for the inequivalent PEt3 ligands, and the
barrier to phosphine interconversion (15.0 ( 0.5 kcal mol-1) is
similar to that for the analogous complex 3. Attempts to isolate
6 were unsuccessful, presumably due to the lability of the MeCN
ligand.
Acknowledgment is made to the National Science Foundation for their
generous support of this work. We also thank Professors Robert Bergman
and Andrew Streitwieser for valuable discussions on isotope effects.
Interestingly, compounds 1 and 6 exist in equilibrium, as
demonstrated by monitoring the [6]/[1] ratio as a function of
[MeCN]. This equilibrium is unaffected by the presence of
[Bu4N][OTf], suggesting that 6 is best described as a contact ion-
pair.8 Thus, the equilibrium constant expression, [6]/[1][CH3-
CN], was determined to have a value of 6.0 ( 0.1 M-1 at 298 K.
Note that in nonpolar solvents such as benzene and toluene, 1 is
Supporting Information Available: Synthetic details and charac-
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JA9808346
(10) Selected data for cis-(PEt3)2Pt(D)Si(StBu)3: 31P{1H} NMR (dichlo-
2
romethane-d2, 161.98 MHz): δ 14.44 (dt with 195Pt satellites, JPP ) 17.00
Hz, 2JPD ) 23.00 Hz, 1JPPt ) 2219 Hz), 18.22 (d with 195Pt satellites, 1JPPt
)
(7) Michelin, R. A.; Bertani, R.; Mozzon, M.; Bombieri, G.; Benetollo,
F.; Guedes da Silva, M. F. C.; Pombeiro, A. J. L. Organometallics 1993, 12,
2372.
1749 Hz). IR (KBr pellet): 1508 s (PtD). 1-d: 31P{1H} NMR (121.5 MHz):
δ 14.37 (q with 195Pt satellites, 2JPP ) 19.4 Hz, 2JPD ) 19.4 Hz, 1JPPt ) 2260
Hz), 19.85 (d with 195Pt satellites, 1JPPt ) 1959 Hz). IR (KBr pellet): 1483 m
(PtD).
(8) Ions and Ion Pairs in Organic Reactions; Szwarc, M., Ed.; Wiley-
Interscience: New York, NY, 1972-1974; Vols. 1-2.
(11) A related migration involving Pt and Ge may occur, see: (a) Litz, K.
E.; Henderson, K.; Gourley, R. W.; Banaszak Holl, M. M. Organometallics
1995, 14, 5008. (b) Litz, K. E.; Bender, J. E., IV; Kampf, J. W.; Banaszak
Holl, M. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 496.
(9) DSiCl3, which was used to synthesize DSi(StBu)3, was prepared in situ
via reduction of SiCl4 with Bu3SnD by using a modified procedure: Pa¨tzold,
U.; Roewer, G.; Herzog, U. J. Organomet. Chem. 1996, 508, 147.