Enhanced reactivity of cationic vs neutral hafnocene complexes in stoichiometric and catalytic σ-bond metathesis reactions involving Si-H and Si-C bonds

Article abstract of DOI:10.1021/om010737c

A comparison of the cation-like complex CpCp*HfH(μ-H)B(C₆F₅)₃ with the neutral analogue CpCp*HfHCl reveals increased activity toward σ-bond metathesis for the more electrophilic, cationic complex. A catalytic transformation of PhSiH₃ to Ph₂SiH₂ and SiH₄ by CpCp*HfH(μ-H)B(C₆F₅)₃ appears to occur by σ-bond metathesis of the Si-C bond, which occurs concurrently with dehydropolymerization to highly cross-linked polysilanes.

Full text of DOI:10.1021/om010737c

Organometallics 2001, 20, 4457-4459
4457
En h a n ced Rea ctivity of Ca tion ic vs Neu tr a l Ha fn ocen e
Com p lexes in Stoich iom etr ic a n d Ca ta lytic σ-Bon d
Meta th esis Rea ction s In volvin g Si-H a n d Si-C Bon d s
Aaron D. Sadow and T. Don Tilley*
Department of Chemistry, University of California at Berkeley,
Berkeley, California 94720-1460
Received August 13, 2001
Summary: A comparison of the cation-like complex
CpCp*HfH(µ-H)B(C6F5)3 with the neutral analogue
CpCp*HfHCl reveals increased activity toward σ-bond
metathesis for the more electrophilic, cationic complex.
A catalytic transformation of PhSiH3 to Ph2SiH2 and
SiH4 by CpCp*HfH(µ-H)B(C6F5)3 appears to occur by
σ-bond metathesis of the Si-C bond, which occurs
concurrently with dehydropolymerization to highly cross-
linked polysilanes.
σ-bond metathesis reactivity at d0 metal centers. Given
similarities in the transition states for σ-bond metath-
esis and the propagation step of alkene polymerization,
as catalyzed by metallocene derivatives (both involve
four-center, four-electron, electrocyclic transition states),
it seemed that cationic complexes6 might exhibit en-
hanced reactivity in σ-bond metathesis. Indeed, a recent
theoretical study supports this view,7 and whereas
[Cp*2ZrH][HB(C6F5)3] reacts with benzene-d6 to form
[Cp*2ZrD][DB(C6F5)3],6a Cp*2ZrH2 is not deuterated by
benzene-d6.8 However, attempts to employ formally 14-
electron cationic metallocene catalysts for the dehydro-
polymerization of silanes have not led to significant
increases in polymer chain lengths, and in many cases
significant reductions in molecular weight have been
observed (implying reduced activity for the cationic
catalyst).9 Harrod and co-workers have identified a
cationic zirconocene system that is active in silane
polymerization, and interestingly they suggest that silyl
radicals may be involved in the mechanism for polysi-
lane formation.10 To better understand these reactivity
patterns, we have directly compared related cationic and
neutral d0 CpCp*Hf complexes in simple σ-bond me-
tathesis steps involving Si-H bonds. Here we report
evidence that the cationic complexes exhibit remarkably
higher reactivities.
Metal-mediated reactions involving bond cleavage and
bond formation via concerted, four-centered transition
states (σ-bond metathesis) offer considerable potential
for the development of new catalytic conversions. In
principle, such processes could be analogous to well-
known catalytic cycles based on oxidative addition and
reductive elimination but appear to be almost exclu-
sively associated with d0 transition metal centers. The
activation of hydrocarbons by σ-bond metathesis has
been well-known for over 20 years,1-3 but the develop-
ment of catalytic reactions involving this chemistry has
been hindered by the apparent restriction that carbon
cannot adopt the â-position of a four-centered transition
state.3b This restriction does not exist for silicon, and
therefore, catalytic reactions involving the σ-bond me-
tathesis of Si-H bonds (i.e., dehydropolymerization4 and
hydrosilylation5) have been developed.
The neutral dimethyl complex CpCp*HfMe2 (1) does
not react with 1 equiv of PhSiH3 over 1 week in benzene-
d6 (25 °C). In contrast, the cation-like, zwitterionic
complex CpCp*HfMe(µ-Me)B(C6F5)3 (2) reacts with 1
equiv of PhSiH3 over 3 h in benzene-d6 at room tem-
perature to quantitatively form CpCp*HfH(µ-H)B(C6F5)3
(3) and PhMe2SiH (by 1H NMR spectroscopy; eq 1).
During the course of this reaction the intermediate
PhMeSiH2 was observed, and use of 0.5 equiv of PhSiH3
in the reaction resulted in formation of a 1:1 mixture of
Clearly, new metal complexes that are more active
in σ-bond metathesis could greatly expand the set of
useful catalytic reactions associated with this chemistry.
The design of such systems will require a better
understanding of the electronic factors that influence
(1) (a) Watson, P. J .; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51.
(b) Rothwell, I. P. Polyhedron 1985, 4, 177. (c) Gell, K. I.; Posin, B.;
Schwartz, J .; Williams, G. M. J . Am. Chem. Soc. 1982, 104, 1846.
(2) (a) Lin, Z.; Marks, T. J . J . Am. Chem. Soc. 1987, 109, 7979. (b)
J eske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T. J . J .
Am. Chem. Soc. 1985, 107, 8111. (c) Fagan, P. J .; Manriquez, J . M.;
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(3) (a) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J .;
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Chem. Soc. 1987, 109, 203. (b) Piers, W. E.; Shapiro, P. J .; Bunel, E.
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(4) (a) Tilley, T. D. Acc. Chem. Res. 1993, 26, 22. (b) Woo, H.-G.;
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E. J . Organomet. Chem. 1985, 279, C11. (g) Chang, L. S.; Corey, J . Y.
Organometallics 1989, 8, 1885.
(5) See for example: (a) Fu, P.; Brard, L.; Li, Y.; Marks, T. J . J .
Am. Chem. Soc. 1995, 117, 7157. (b) Molander, G. A.; Corrette, C. P.
Tetrahedron Lett. 1998, 39, 5011. (c) Gountchev, T. I.; Tilley, T. D.
Organometallics 1999, 18, 5661.
(6) (a) Xang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1994,
116, 10015. (b) J ordan, R. F.; Bajgur, C. S.; Dasher, W. E.; Rheingold,
A. L. Organometallics 1987, 6, 1041. (c) J ordan, R. F.; LaPointe, R.
E.; Bradley, P. K.; Baenziger, N. Organometallics 1989, 8, 2892. (d)
J ordan, R. F.; Taylor, D. F.; Baenziger, N. C. Organometallics 1990,
9, 1546.
(7) Ustynyuk, Yu. A.; Ustynyuk, L. Yu.; Laikov, D. N.; Lunin, V. V.
J . Organomet. Chem. 2000, 597, 182.
(8) (a) Manriquez, J . M.; McAlister, D. R.; Sanner, R. D.; Bercaw,
J . E. J . Am. Chem. Soc. 1978, 100, 2716. (b) Lee, H.; Desrosiers, P. J .;
Guzei, I.; Rheingold, A. L.; Parkin, G. J . Am. Chem. Soc. 1998, 120,
3255.
(9) Imori, T.; Tilley, T. D. Polyhedron 1994, 13, 2231.
(10) (a) Dioumaev, V. K.; Harrod, J . F. J . Organomet. Chem. 1996,
521, 133. (b) Dioumaev, V. K.; Harrod, J . F. Organometallics 1994,
13, 1548. (c) Dioumaev, V. K.; Harrod, J . F. Organometallics 1996,
15, 3859.
10.1021/om010737c CCC: $20.00 © 2001 American Chemical Society
Publication on Web 09/27/2001

4458 Organometallics, Vol. 20, No. 22, 2001
Communications
Sch em e 1
C6D6
might be expected to result in higher activity toward
the dehydropolymerization of hydrosilanes, relative to
similar neutral complexes,9 there is little difference in
the molecular weight (Mn ≈ 2000) of polysilane produced
by the dehydropolymerization of neat PhSiH3 with 3 or
CpCp*HfHCl (4) as the catalyst (2 mol %, 2 days). This
result prompted further investigation of the mecha-
nisms of these dehydropolymerizations.
The neutral catalyst 4, which is known to react with
silanes via a four-centered transition state,4 reacts with
excess PhSiH3 in benzene-d6 (50 °C) to give oligomeric
polysilanes and H2. The disappearance of PhSiH3 obeys
second-order kinetics (over 3 half-lives), as demon-
strated by plots of 1/[PhSiH3] vs time, and the reaction
also displays a first-order dependence on catalyst con-
centration. This kinetic behavior is consistent with a
metal-catalyzed condensation polymerization in which
the first step of the catalytic cycle is reversible (Scheme
1, pathway A), and mass balance requires 2 equiv of
PhSiH3 for a single turnover of the cycle. The overall
rate law for this mechanism is rate ) kobs[4][PhSiH3]2,
where kobs ) KAeqkA2/[H2] (kobs ) 3.3 × 10-3 s-1 M-2 at
51.2 °C), and as expected the reaction rate is inhibited
by H2. Although this kinetic behavior describes only
monomer consumption, we assume that further steps
in the polymerization (involving dehydrocoupling of
polysilanes) proceed in an analogous fashion.
[CpCp*HfMe][µ-MeB(C6F5)3] + PhSiH3
8
2
[CpCp*HfH][µ-HB(C6F5)3] + PhMe2SiH (1)
3
2:3, rather than a mixed hafnium hydride/methyl borate
complex. While it has been observed that some reactions
of early-transition-metal compounds with silanes are
accelerated by light,3,10,11 this transformation proceeds
identically in the presence or absence of ambient light.
As might be expected from what is known about steric
influences on the rates of σ-bond metathesis reactions,4b
bulkier silanes react more slowly with 2 (relative
rates: CySiH3 > MesSiH3 (Mes ) 2,4,6-Me3C6H2), Ph2-
SiH2 > Mes2SiH2). These results suggest that zwitter-
ionic 2 reacts as an associated ion pair with increased
electrophilicity due to CpCp*HfMe+ character and is
considerably more reactive than neutral 1 toward
σ-bond metathesis with Si-H bonds.
Compound 3 was isolated from the reaction of 2 with
1.5 equiv of PhSiH3 in pentane, and it may also be
prepared by the reaction of 1 with hydrogen (1 atm).
Unlike [Cp*2ZrH][HB(C6F5)3],6a which features a ter-
minal B-H bond and coordination of the anion to Zr
via Zr‚‚‚F-C interactions, 3 appears to possess a Hf-
H-B-bonded structure. Thus, no terminal B-H stretch
was observed in the infrared spectrum of 3, but a
stretching vibration at 1950 cm-1 may be attributed to
The cation-like hafnium hydride 3 catalyzes a more
complex reaction of PhSiH3, in that both dehydrocou-
pling and redistribution products are observed (eq 2;
benzene-d6, 5 mol % 3). Analysis of the reaction products
M-H-B bonding.12 In addition, the observed J BH
coupling constant (50 Hz) reflects the presence of a
1
bridging hydrogen.13
In contrast to [Cp*2ZrH][HB(C6F5)3],6a which reacts
with benzene-d6 via H/D exchange involving the Zr-H
and the B-H positions, no deuteration of 3 is observed
in benzene-d6 before thermal decomposition at 100 °C
after 4 days. Though the “cationic” nature of hydride 3
nPhSiH3 5 mol % 38
C6D6
H-(PhSiH)x-H + (n - 1)H2 + Ph2SiH2 + SiH4 (2)
by NMR spectroscopy revealed that ca. 75% of the
starting PhSiH3 reacted via the redistribution pathway
and that SiH4 is incorporated into the polymer chain,
yielding cross-linked oligomers. A 29Si{1H} DEPT 90°
NMR spectrum of the polymer contained resonances for
linear poly(phenylsilane) segments,14 and a group of
resonances in the range δ -95 to -120 was ascribed to
(11) Casty, G. L.; Lugmair, C. G.; Radu, N. S.; Tilley, T. D.; Walzer,
J . F.; Zargarian, D. Organometallics 1997, 16, 8.
(12) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds, Part A: Theory and Applications in Inor-
ganic Chemistry, 5th ed.; Wiley: New York, 1997; Part A, p 247.
(13) Marsella, J . A.; Caulton, K. G. J . Am. Chem. Soc. 1982, 104,
2361.

Communications
Organometallics, Vol. 20, No. 22, 2001 4459
HSi(Si)3 branching sites. Though the molecular weights
of polysilanes produced by the neutral and cationic
hydride catalysts are similar, the polysilane backbone
structure produced by 3 differs significantly from that
derived from 4, in that cross-linking is not observed with
the neutral catalyst (see Supporting Information). Thus,
these reaction products reveal an important limitation
in the effectiveness of 3 in producing high-molecular-
weight polysilanes, in that significant side reactions
compete with polymerization.15 In addition, the observa-
tion of Si-C bond cleavage under such mild conditions
implicates the presence of highly active catalytic species,
since Si-C bonds are generally unreactive in σ-bond
metathesis transformations.
of PhSiH3 with formation of SiH4 and a cationic hafnium
phenyl complex. Evidence in support of the phenyl-
transfer steps in the redistribution cycle was observed
in the reaction of CpCp*HfPh(µ-Me)B(C6F5)3 (5) with
PhSiH3 (in benzene-d6), to give Ph2SiH2. Given the
mechanism of Scheme 1, it should be possible to influ-
ence the extent of dehydrocoupling vs redistribution by
controlling the H2 and SiH4 pressures (eq 3). Such an
effect was observed in the reaction of 3 with 1 equiv of
PhSiH3 under 1 atm of H2. As expected, added H2
inhibits the dehydrocoupling pathway to favor redistri-
bution by a factor of 4:1. In summary, we strongly favor
the mechanistic proposal of Scheme 1, and other pos-
sibilities (e.g., involving radicals or purely Lewis acidic
species) seem less likely.16 Additional experiments are
underway to investigate this issue.
By comparison with well-studied neutral dehydropo-
lymerization catalysts, the results reported here indicate
that cationic group 4 metallocenes react with silanes
via four-center, four-electron transition states. In ad-
dition, the cationic complexes exhibit increased reac-
tivities in σ-bond metathesis. This enhanced activity is
seen in increased rates of reaction and in the activation
of Si-C bonds, which appear to be generally less
reactive than Si-H bonds in σ-bond metathesis.17 Thus,
the design of more reactive catalysts can be based on
formation of cationic metal complexes; however, as is
often the case, issues of selectivity then come into play.
During the conversion of eq 2, compound 3 remained
unchanged and no identifiable intermediates (e.g., Hf-
1
Ph or Hf-SiH2Ph species) were observed by H NMR
spectroscopy. Despite the complexity of this reaction,
its kinetic behavior, in terms of loss of PhSiH3, is
surprisingly well-behaved and analogous to that ob-
served for the dehydrocoupling of PhSiH3 catalyzed by
4. Thus, a linear second-order plot of 1/[PhSiH3] vs time
and first-order dependence on catalyst concentration
yields a rate law (rate ) k′obs[3][PhSiH3]2, k′obs ) 4.5 ×
10-2 s-1 M-2) which suggests operation of only transi-
tion-metal-catalyzed coupling reactions. A mechanism
that is consistent with the kinetic data, the observed
products, and established trends for σ-bond metathesis
features two concurrent reaction pathways that employ
3 as a common catalyst (Scheme 1). This mechanism
would give rise to second-order dependence on substrate
concentration, with a rate law of the form
Ack n ow led gm en t is made to the National Science
Foundation for their generous support of this work and
to J ack Kyte (UC San Diego) for valuable discussions.
Su p p or tin g In for m a tion Ava ila ble: Text and figures
giving procedures for the synthesis and characterization of 2,
3, and 5, experimental details for the kinetic experiments,
kinetics data for the dehydropolymerization reactions, and 29Si
NMR spectra for poly(phenylsilanes). This material is available
free of charge via the Internet at http://pubs.acs.org.
KAeq kA2 KBeq kB2
rate )
+
[3][PhSiH3]2
(3)
(
)
[H2]
[SiH4]
From this expression, it can be seen that k′obs is a
combination of terms corresponding to the two reaction
pathways, the first of which results from dehydrocou-
pling (vide supra). The redistribution pathway is mani-
OM010737C
(16) For the reaction mixture involving CpCp*HfH(µ-H)B(C6F5)3 and
PhSiH3, radicals were not observed by EPR spectroscopy, after 1 h
and 1 week. To test for the activity of a Lewis acidic catalyst (without
a reactive σ-bond) in this system, PhSiH3 was added to CpCp*Hf(OTf)-
(µ-Me)B(C6F5)3, generated in solution by addition of B(C6F5)3 to
CpCp*Hf(OTf)Me. After 5 days at room temperature, no reaction of
the silane had occurred.
(17) We have recently observed related redistribution processes with
lanthanide catalysts: (a) Castillo, I.; Tilley, T. D. Organometallics 2000,
19, 4733. (b) Radu, N. S.; Tilley, T. D.; Rheingold, A. L. J . Am. Chem.
Soc. 1992, 114, 8293.
fested in a second term (KBeq kB/[SiH4]), which arises
2
from an equilibrium involving cleavage of the Si-C bond
(14) (a) Grimmond, B. J .; Corey, J . Y. Organometallics 2000, 19,
3776. (b) Grimmond, B. J .; Rath, N. P.; Corey, J . Y. Organometallics
1999, 18, 2975. (c) Dioumaev, V. K.; Rahimian, K.; Gauvin, F.; Harrod,
J . F. Organometallics 1999, 18, 2249.
(15) Odian, G. Principles of Polymerization, 3rd ed.; Wiley-Inter-
science: New York, 1991.