Silicon-hydrogen bond activation and formation of silane complexes using a cationic rhodium(III) complex

Article abstract of DOI:10.1021/om0341303

Addition of triphenylsilane, trimethylsilane, or triethylsilane to [Cp*(PMe₃)Rh(Me)(CH₂-Cl₂)] BAr′₄ (4), a cationic Rh(III) complex, resulted in Si-H bond activation and release of methane below -80°C. The rare, nonclassical silane complexes [Cp*(PMe₃)Rh((C₆H₄) (η²-HSiPh₂))]BAr′₄ (6), [Cp*(PMe₃)Rh(SiMe₃)(η²- HSiMe₃)]BAr′₄ (9), and [Cp*(PMe₃) Rh(SiEt₃)-(η²-HSiEt₃)] BAr′₄ (11) have been generated and characterized by NMR spectroscopy. Of note was the presence of ²⁹Si satellites of the hydride resonances of each of these compounds and large ¹J_Si-H coupling constants (56-84 Hz), diagnostic of the presence of η²-silane ligands.

Full text of DOI:10.1021/om0341303

886
Organometallics 2004, 23, 886-890
Silicon -Hyd r ogen Bon d Activa tion a n d F or m a tion of
Sila n e Com p lexes Usin g a Ca tion ic Rh od iu m (III)
Com p lex
Felicia L. Taw,† Robert G. Bergman,*,‡ and Maurice Brookhart*,†
Department of Chemistry, University of North Carolina at Chapel Hill, CB #3290,
Chapel Hill, North Carolina 27599-3290, and Department of Chemistry,
University of California, and Division of Chemical Sciences,
Lawrence Berkeley National Laboratory, Berkeley, California 94720-1460
Received August 25, 2003
Addition of triphenylsilane, trimethylsilane, or triethylsilane to [Cp*(PMe3)Rh(Me)(CH2-
Cl2)]BAr′4 (4), a cationic Rh(III) complex, resulted in Si-H bond activation and release of
methane below -80 °C. The rare, nonclassical silane complexes [Cp*(PMe3)Rh((C6H4)(η2-
HSiPh2))]BAr′4 (6), [Cp*(PMe3)Rh(SiMe3)(η2-HSiMe3)]BAr′4 (9), and [Cp*(PMe3)Rh(SiEt3)-
(η2-HSiEt3)]BAr′4 (11) have been generated and characterized by NMR spectroscopy. Of note
was the presence of 29Si satellites of the hydride resonances of each of these compounds and
1
large J Si-H coupling constants (56-84 Hz), diagnostic of the presence of η2-silane ligands.
Sch em e 1
In tr od u ction
The activation of an Si-H bond is one of the key steps
in hydrosilation and other catalytic reactions.1 Nearly
all of the transition metals have been shown to undergo
reactions with silanes, by either activating an Si-H
bond via oxidative addition to form hydrido silyl com-
plexes or coordinating the silane in an η2 fashion to form
σ complexes.2 Numerous examples of neutral transition-
metal η2-silane complexes can be found in the literature,
but cationic analogues are quite rare. Only a few
examples of stable cationic silane complexes have been
reported.3 The paucity of these types of compounds has
been attributed to their propensity to undergo hetero-
lytic cleavage of the Si-H bond.4
While there are copious examples of classical rhodium
hydrido silyl complexes, nonclassical rhodium silane
compounds are rare.2b In an early report of such a
system, Perutz and co-workers characterized CpRh-
(SiMe3)2(η2-HSiEt3) (1) as an η2-silane complex based
on a J Si-H coupling constant of 24.3 Hz.5 Typically,
nonclassical silane complexes exhibit J Si-H coupling
constants of 20 Hz to an upper limit of 200 Hz, in
contrast to classical silyl hydride complexes, which
generally exhibit J Si-H values of <10 Hz.2e As further
support for the structural assignment of 1, a value of
17.9 Hz was measured for J Rh-Si for the SiEt3 moiety
versus a value of 26.6 Hz for J Rh-Si of the SiMe3 ligands,
indicating a stronger Rh-Si interaction with the trim-
ethylsilyl groups.5
Bergman and co-workers have reported the Si-H
activation of triphenylsilane using [Cp*(PMe3)Ir(Me)(CH2-
Cl2)]BAr′4 (2; Ar′ ) 3,5-(CF3)2C6H3).6 Treatment of a
dichloromethane solution of 2 with triphenylsilane
resulted in formation of a four-membered Ir(V) metal-
lacycle (3), via Si-H activation, release of methane, and
subsequent intramolecular C-H activation of an aryl
ring (Scheme 1). Assignment of complex 3 as an Ir(V)
species was based upon NMR data and X-ray crystal
structure studies.
† University of North Carolina at Chapel Hill.
‡ University of California and Lawrence Berkeley National Labora-
tory.
(1) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysts: The
Applications and Chemistry of Catalysis by Soluble Transition Metal
Complexes; Wiley: New York, 1992.
(2) For recent reviews, see: (a) Lin, Z. Chem. Soc. Rev. 2002, 31,
239. (b) Corey, J . Y.; Braddock-Wilking, J . Chem. Rev. 1999, 99, 175
(and references therein). (c) Schneider, J . J . Angew. Chem., Int. Ed.
Engl. 1996, 35, 1068. (d) Braunstein, P.; Knorr, M. J . Organomet.
Chem. 1995, 500, 21. (e) Schubert, U. Adv. Organomet. Chem. 1990,
30, 151.
(3) (a) Huhmann-Vincent, J .; Scott, B. L.; Kubas, G. J . Inorg. Chim.
Acta 1999, 294, 240. (b) Fang, X.; Huhmann-Vincent, J .; Scott, B. L.;
Kubas, G. J . J . Organomet. Chem. 2000, 609, 95. (c) Fang, X.; Scott,
B. L.; Watkin, J . G.; Kubas, G. J . Organometallics 2000, 19, 4193. (d)
Fang, X.; Scott, B. L.; J ohn, K. D.; Kubas, G. J . Organometallics 2000,
19, 4141. (e) Freeman, S. T. N.; Lemke, F. R.; Brammer, L. Organo-
metallics 2002, 21, 2030.
Herein, we report the Si-H bond activation of triaryl-
and trialkylsilanes using the previously reported com-
plex [Cp*(PMe3)Rh(Me)(CH2Cl2)]BAr′4 (4).7 The prod-
ucts of these reactions, cationic Rh silyl and silane
complexes, have been characterized by NMR spectros-
(5) Duckett, S. B.; Perutz, R. N. J . Chem. Soc., Chem. Commun.
1991, 28.
(6) Klei, S. R.; Tilley, T. D.; Bergman, R. G. J . Am. Chem. Soc. 2000,
122, 1816.
(4) (a) Luo, X. L.; Crabtree, R. H. J . Am. Chem. Soc. 1989, 111, 2527.
(b) Scharrer, E.; Chang, S.; Brookhart, M. Organometallics 1995, 14,
5686.
(7) Taw, F. L.; Mellows, H.; White, P. S.; Hollander, F. J .; Bergman,
R. G.; Brookhart, M.; Heinekey, D. M. J . Am. Chem. Soc. 2002, 124,
5100.
10.1021/om0341303 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 01/20/2004

Formation of Silane Complexes Using Rh(III)
Organometallics, Vol. 23, No. 4, 2004 887
Sch em e 2
F igu r e 1. Hydride region of a 1H NMR spectrum (400
MHz, CD2Cl2, 25 °C) of 6.
F igu r e 2. Possible structures for “[Cp*(PMe3)Rh(H)2-
(SiPh3)]+” (7).
copy and have been shown to adopt formal +3 oxidation
states at rhodium, in contrast to the Ir(V) system
(complex 3) shown in Scheme 1.
Sch em e 3
Resu lts a n d Discu ssion
Activa tion of Tr ip h en ylsila n e. When 1 equiv of
triphenylsilane was added to a CD2Cl2 solution of Rh
methyl complex 4, Si-H activation and release of
methane occurred. Monitoring this reaction by low-
temperature NMR spectroscopy revealed that activation
of the silane transpired below -80 °C, to give [Cp*-
(PMe3)Rh(SiPh3)(CD2Cl2)]BAr′4 (5; Scheme 2), which
was the only observed species in the range of -80 to
-40 °C.8 Upon warming to -30 °C, a new product
appeared which we have formulated as a Rh aryl silane
complex, [Cp*(PMe3)Rh((C6H4)(η2-HSiPh2))]BAr′4 (6;
Scheme 2).
3). An alternative mechanism which cannot be excluded
is the metathesis of C-H and Rh-Si bonds via a four-
center transition state.
Addition of excess triphenylsilane (5.0 equiv) to a CD2-
Cl2 solution of rhodium methyl complex 4 at room
temperature again resulted in Si-H activation and
release of methane. However, in this case a new product
was observed, which can be formulated as [Cp*(PMe3)-
At room temperature, complex 6 is formed in ∼90%
1
1
yield (t1/2 of ∼15 min), as assessed by integrating its H
Rh(H)2(SiPh3)]BAr′4 on the basis of its H NMR spec-
NMR resonances against the invariant BAr′4 peaks. The
1H NMR spectrum of 6 revealed a hydride resonance
at δ -8.95 ppm, which appeared as a doublet of doublets
trum. The hydride region exhibited a signal at δ -10.04
1
ppm (2J P-H ) 18.0 Hz, J Rh-H ) 26.8 Hz), which was
integrated to two hydrogens relative to the Cp* and
PMe3 ligands. Since the two hydrides appear equivalent
by NMR and no 29Si satellites were detected, this
product may be characterized as either a classical Rh-
(V) dihydridosilyl complex, [Cp*(PMe3)Rh(H)2(SiPh3)]-
BAr′4 (7a ), or a nonclassical fluxional Rh(III) hydrido
silane complex, [Cp*(PMe3)Rh(H)(η2-HSiPh3)]BAr′4 (7b/
7b′; Figure 2).
due to coupling to both 103Rh and 31P nuclei (2J P-H
)
1
11.5 Hz, J Rh-H ) 27.8 Hz; Figure 1). In addition, 29Si
1
satellites were detected, which provided a J Si-H value
of 84 Hz. This value is typical of η2-silane complexes2e
and strongly suggests a σ(Si-H) interaction with Rh-
(III) in 6, as shown in Scheme 2. Integration of these
satellite signals (4.8% relative to the parent doublet of
doublets) supports their assignment as 29Si (4.7% natu-
Although no satellite peaks due to coupling to 29Si
were observed for the hydride resonance at -10.04 ppm,
structure 7b/7b′ is not necessarily precluded. Due to
rapid site exchange, the observed splitting by 29Si would
1
ral abundance) satellites.9 In addition to the H NMR
data, a 13C{1H} NMR spectrum of 6 revealed a doublet
1
of doublets at δ 154.0 ppm (2J P-C ) 18.3 Hz, J Rh-C
)
2
28.1 Hz), which is diagnostic of the ipso carbon of an
aryl ring that is bound directly to rhodium.
be an average of J Si-H(terminal) with J Si-H(η ) (e.g., J Si-H
a
in 7b with J Si-H in 7b′). Since J Si-H(terminal) will be ca.
a
0 Hz, the observed splitting by 29Si would be half the
The behavior of 4 with triphenylsilane provides an
interesting contrast to that of Bergman’s iridium ana-
logue 2. As with 2, activation of a C-H bond at an ortho
site of one of the aryl rings occurs, but in the ultimate
product 6 the formal +3 oxidation state is retained by
formation of a σ complex. A possible mechanistic path-
way for conversion of 5 to 6 is shown in Scheme 3 and
entails oxidative addition of the C-H aryl bond to form
a Rh(V) intermediate followed by reductive elimination
of the silyl hydride to form an η2-Si-H bond (Scheme
1
2
2
value of J Si-H(η ). Thus, values of J Si-H(η ) in the range
of 20-30 Hz (consequently, J observed ≈ 10-15 Hz) would
bring the satellites into the wings of the broad hydride
resonance (ν1/2 ≈ 50 Hz) and render them difficult to
detect.
Addition of 1 equiv of acetonitrile to a CD2Cl2 solution
of complex 7 at room temperature resulted in immediate
loss of silane and generation of [Cp*(PMe3)Rh(H)-
(NCMe)]BAr′4 (8), a complex which we have prepared
by an independent route (see Experimental Section).
Presumably, 7b/7b′ must be an energetically accessible
species to allow facile displacement of a silane by the
nitrile, which leads to rapid formation of 8.
(8) See Experimental Section for 1H, 31P{1H}, and 13C{1H} NMR data
for complex 5.
(9) Friebolin, H. Basic One- and Two-Dimensional NMR Spectros-
copy; VCH: Weinheim, Germany, 1993.

888 Organometallics, Vol. 23, No. 4, 2004
Taw et al.
Sch em e 4
Sch em e 5
position products, even at -80 °C. Repeating this
experiment using excess (3-5 equiv) trimethylsilane
allowed observation of an η2-silane complex, [Cp*-
(PMe3)Rh(SiMe3)(η2-HSiMe3)]BAr′4 (9/9′), in the 1H
NMR spectrum at -60 °C (Scheme 5). The hydride
resonance for 9/9′ exhibited 29Si satellites (δ -12.02
ppm; 2J P-H ) 7.5 Hz, 1J Rh-H ) 36.6 Hz, 1J Si-H(observed)
)
28.5 Hz), lending support to assignment as a silyl η2-
silane complex. The Si-H coupling constant associated
with the η2 Si-H interaction was calculated from the
1
2
observed Si-H coupling constant: J Si-H(η ) ) 2 ×
1J Si-H(observed) ) 2 × 28.5 Hz ) 57 Hz. Consistent with
the rapidly fluxional nature of 9 is the observation in
the 1H NMR spectrum of a single resonance at 0.50 ppm
for the SiMe3 groups. Over time, another product grew
in, which was formulated as [Cp*(PMe3)Rh(H)2(SiMe3)]-
BAr′4 (Scheme 5). As in the case with triphenylsilane,
this complex may have the structure [Cp*(PMe3)Rh(H)2-
(SiMe3)]BAr′4 (10a ) or [Cp*(PMe3)Rh(H)(η2-HSiMe3)]-
BAr′4 (10b/10b′). The hydride region of the 1H NMR
spectrum of 10 exhibited a resonance (δ -11.16 ppm;
A proposed mechanism for the formation of complex
7 is shown in Scheme 4. Initial Si-H activation of 1
equiv of silane and release of methane forms the
transient Rh silyl complex A. Excess silane present in
the reaction mixture allows the cationic Rh silyl species
to be trapped by a second 1 equiv of silane, resulting in
a species formulated as [Cp*(PMe3)Rh(H)(SiPh3)2]+ (B
or B′; not observed by NMR spectroscopy).10 To produce
7, elimination of (Ph3Si)2 must occur, presumably via
B′′, to generate the transient Rh hydrido cation C, which
is trapped by a third 1 equiv of silane to form 7a or 7b/
7b′.
A similar mechanism was invoked to explain the
conversion of a rhodium hydrido disilyl species to a
dihydrido silyl complex by Slough and co-workers.11
Reductive elimination of hexaethyldisilane, (Et3Si)2,
from (Ph3P)2Rh(H)(SiEt3)2 and subsequent oxidative
addition of triethylsilane resulted in production of
(Ph3P)2Rh(H)2(SiEt3). Additionally, the reaction in which
elimination of H2 from the dihydrido silyl complex and
oxidative addition of triethylsilane to re-form the hy-
drido disilyl complex was also reported.
1
2J P-H ) 20.7 Hz, J Rh-H ) 27.9 Hz) which was inte-
grated to two equivalent protons. Once again, no 29Si
satellites were observed for the hydride peak, making
assignment of the correct structure difficult.
The iridium analogue of complex 10, [Cp*(PMe3)-
Ir(H)2(SiMe3)]+[MeB(C6F5)3]-, has been reported.12 This
complex was generated in situ by addition of 1 equiv of
trimethylsilane to a solution of the cationic iridium
hydrido complex [Cp*(PMe3)Ir(H)(ClCD2Cl)]+[MeB-
(C6F5)3]-. Assignment as an Ir(V) species was based
upon NMR spectroscopic data.
By analogy to other rhodium/iridium systems in
which the iridium compound prefers a formal +5
oxidation state and the rhodium compound prefers a
lower +3 oxidation state, it is likely that η2-silane
complex 10b/10b′ is the correct structure. For example,
iridium complex 3 was assigned as an Ir(V) species,
while the Rh analogue 6 exhibited 29Si satellites (with
Complexes 6 and 7 have resisted all efforts at isola-
tion. This is not entirely surprising, as no examples of
isolated rhodium η2-silane complexes have been re-
ported. Removal of volatile materials in vacuo from
solutions of these compounds resulted in decomposition
to unidentified products. Standard crystallization at-
tempts using a variety of solvents were also unsuccess-
ful.
1
a large J Si-H coupling of 84 Hz) in the hydride region
1
of the H NMR spectrum, indicating a Rh(III) complex
with an η2-silane ligand. This trend is also mirrored
with hydride ligands; an Ir(V) classical trihydride
structure was established for [Cp*(PMe3)Ir(H)3]+,13
while the analogous Rh(III) complex is the nonclassical
hydride/dihydrogen complex [Cp*(PMe3)Rh(H)(H2)]+.7
Activa tion of Tr im eth ylsila n e. Addition of 1 equiv
of trimethylsilane to a CD2Cl2 solution of Rh methyl
complex 4 led to rapid formation of unidentified decom-
(10) This species ([Cp*(PMe3)Rh(H)(SiPh3)2]+ (B, B′, or B′′)) was not
detected either at room temperature or when the experiment was
repeated at low temperature (down to -70 °C) and the reaction mixture
observed by variable-temperature NMR spectroscopy.
(11) Sun, C.; Tu, A.; Slough, G. A. J . Organomet. Chem. 1999, 582,
235.
(12) Golden, J . T.; Andersen, R. A.; Bergman, R. G. J . Am. Chem.
Soc. 2001, 123, 5837.
(13) (a) Gilbert, T. M.; Bergman, R. G. J . Am. Chem. Soc. 1985, 107,
3502. (b) Heinekey, D. M.; Hinkle, A. S.; Close, J . D. J . Am. Chem.
Soc. 1996, 118, 5353.

Formation of Silane Complexes Using Rh(III)
Organometallics, Vol. 23, No. 4, 2004 889
Activa tion of Tr ieth ylsila n e. As with trimethylsi-
lane, addition of 1 equiv of triethylsilane to a CD2Cl2
solution of 4 led to rapid formation of unidentified
decomposition products, even at -80 °C. Addition of 5.0
equiv of triethylsilane resulted in initial formation of a
Rh silyl η2-silane complex, [Cp*(PMe3)Rh(SiEt3)(η2-
oxidation states. While [Cp*(PMe3)Rh((C6H4)(η2-HSiPh2))]-
BAr′4 (6) and [Cp*(PMe3)Rh(H)(H2)]+ are both Rh(III)
compounds,7 the corresponding Ir compounds [Cp*-
(PMe3)Ir(η2-SiPh2(C6H4))(H)]+ (3) and [Cp*(PMe3)Ir-
(H)3]+ are Ir(V) species.6,13 By analogy, since [Cp*(PMe3)-
Ir(H)2(SiMe3)]+ was reported to be an Ir(V) complex,12
it is likely that the corresponding rhodium complexes
7, 10, and 12 are fluxional hydrido silane species [Cp*-
(PMe3)Rh(H)(η2-HSiR3)]+ with a formal oxidation state
of +3.
1
HSiEt3)]BAr′4 (11), observed in the H NMR spectrum
at -60 °C. Characterization of 11 as a σ complex is
supported by the presence of 29Si satellites of the
2
hydride resonance (δ -12.39 ppm; J P-H ) 6.8 Hz,
1
1
1
2
J Rh-H ) 36.0 Hz, J Si-H(observed) ) 27.8 Hz, J Si-H(η )
)
56 Hz).14
Exp er im en ta l Section
When the temperature was increased, a broadening
of resonances associated with both free and bound silane
was observed in the H NMR spectrum. For example,
Gen er a l Con sid er a tion s. Unless otherwise noted, all
reactions and manipulations were performed using standard
high-vacuum, Schlenk, or drybox techniques. Argon and
nitrogen were purified by passage through columns of BASF
1
at 10 °C, the hydride signal at δ -12.39 ppm as well as
the Et3Si-H signal at δ 3.49 ppm have broadened into
the baseline. When the temperature was lowered back
to -60 °C, all resonances sharpened up again, with the
exception of the methylene protons of bound silane,
which are diastereotopic and exhibit a broad resonance
throughout the entire temperature range. These effects
can clearly be attributed to rapid intermolecular ex-
change of the bound silane in complex 11 with the
excess free silane present in solution. In addition to this
exchange of silane occurring, raising the temperature
also resulted in the appearance of a species formulated
as [Cp*(PMe3)Rh(H)2(SiEt3)]BAr′4 (12). When the tem-
perature was increased further to 20 °C, unidentified
side products appeared and significant decomposition
ensued. Complex 12 exhibited a hydride resonance in
1
R3-11 catalyst (Chemalog) and 4 Å molecular sieves. H and
1
13C NMR chemical shifts were referenced to residual H and
13C NMR signals of the deuterated solvents, respectively. 31P
NMR chemical shifts were referenced to an 85% H3PO4 sample
used as an external standard, and 29Si NMR chemical shifts
were referenced to TMS used as an external standard.
Elemental analyses were performed by Atlantic Microlab Inc.
of Norcross, GA.
Ma ter ia ls. All solvents were deoxygenated and dried via
passage over a column of activated alumina.16 Deuterated
solvents (Cambridge Isotope Laboratories) were purified by
vacuum transfer from CaH2 and stored over 4 Å molecular
sieves. Unless otherwise noted, all chemicals were purchased
from Aldrich and used without further purification. The
synthesis and full characterization of complex 4 has been
previously reported.7
Sp ectr a l Da ta for BAr ′4-. The 1H and 13C NMR resonances
of the BAr′4 (Ar′ ) 3,5-C6H3(CF3)2) counteranion in CD2Cl2
were essentially invariant for all cationic complexes discussed
here. Therefore, spectroscopic data for this anion are not
1
the H NMR spectrum at δ -11.38 ppm (2J P-H ) 20.0
1
Hz, J Rh-H ) 27.2 Hz), which was integrated to two
equivalent protons. As in the previous examples, no
29Si satellites were observed and thus definitive assign-
ment as a Rh(III) or Rh(V) species was not possible.
1
repeated for each compound. H NMR (400 MHz, CD2Cl2): δ
7.73 (s, 8 H, Ho), 7.57 (s, 4 H, Hp). 13C{1H} NMR (101 MHz,
CD2Cl2): δ 161.9 (q, 1J C-B ) 49.8 Hz, Cipso), 135.0 (s, Co), 129.0
2
1
(q, J C-F ) 31.4, Cm), 124.7 (q, J C-F ) 272.6 Hz, CF3), 117.7
(s, Cp).
Su m m a r y a n d Con clu sion s
[Cp *(P Me3)Rh (H)(NCMe)]BAr ′4 (8). Complex 8 was gen-
erated by addition of 1 equiv of acetonitrile to a dichlo-
romethane solution of complex 7 at room temperature. Alter-
nately, a Schlenk flask was charged with 43 mg (0.035 mmol)
of 8b (see below) and 5 mL of chlorobenzene. The contents of
the flask were subjected to three freeze-pump-thaw cycles.
The flask was back-filled with 1 atm of H2, and the contents
were stirred vigorously for 10 min at room temperature. All
volatile materials were removed in vacuo to produce a yellow
solid in >95% yield. When this reaction is monitored by NMR
spectroscopy, quantitative yields are observed. This complex
decomposed in dichloromethane but was stable in chloroben-
We have shown that [Cp*(PMe3)Rh(Me)(CH2Cl2)]-
BAr′4 (4), a cationic rhodium(III) complex, will activate
the Si-H bonds of triaryl- and trialkylsilanes at low
temperatures (below -80 °C).15 Silane complexes [Cp*-
(PMe3)Rh((C6H4)(η2-HSiPh2))]BAr′4 (6), [Cp*(PMe3)Rh-
(SiMe3)(η2-HSiMe3)]BAr′4 (9), and [Cp*(PMe3)Rh(SiEt3)-
(η2-HSiEt3)]BAr′4 (11) have been generated and char-
acterized by NMR spectroscopy. Of note was the pres-
ence of 29Si satellites of the hydride resonances of each
1
of these compounds and large J Si-H coupling constants
(56-84 Hz), signifying the presence of η2-silane ligands.
A series of complexes formulated as dihydrido silyl
species (7, 10, and 12) have also been generated and
characterized by NMR spectroscopy. It is likely that 7,
10, and 12 are all Rh(III) compounds. As discussed
above, when rhodium complexes are compared with the
analogous iridium complexes, the former tend to adopt
formal +3 oxidation states, while the latter prefer +5
1
zene. H NMR (400 MHz, C6D5Cl, room temperature): δ 1.71
2
(s, 3 H, NCMe), 1.62 (s, 15 H, Cp*), 1.21 (d, J P-H ) 10.7 Hz,
1
2
9 H, PMe3), -11.12 (dd, J Rh-H ) 46.4 Hz, J P-H ) 18.8 Hz, 1
H, hydride). 31P{1H} NMR (162 MHz, C6D5Cl, room temper-
ature): δ 3.7 (d, 1J Rh-P ) 136.4 Hz, PMe3). 13C{1H} NMR (101
MHz, C6D5Cl, room temperature): δ 123.1 (d, 2J Rh-C ) 7.0 Hz,
1
NCMe), 100.6 (s, Cp*-Ar), 17.66 (d, J P-C ) 33.3 Hz, PMe3),
10.03 (s, Cp*-Me), 2.33 (s, NCMe). Anal. Calcd for C47H40
NBF24PRh: C, 46.29; H, 3.31. Found: C, 46.11; H, 3.23.
-
1
1
2
(14) As in the previous case, J Si-H(η ) 2 × J Si-H(observed). A value
[Cp *(P Me3)Rh (Me)(NCMe)]BAr ′4 (8b). A Schlenk flask
was charged with 150 mg (0.12 mmol) of 4 and 5 mL of
dichloromethane. Acetonitrile (1.0 equiv, 6.1 µL, 0.12 mmol)
)
1
1
2
of 27.8 Hz was measured for J Si-H(observed); thus, J Si-H(η ) 56 Hz.
)
(15) Similar experiments involving triisopropylsilane led to transient
formation of the Si-H activation product [Cp*(PMe3)Rh(SiiPr3)(CH2-
Cl2)]BAr′4 (13) at -40 °C. In contrast to the previously discussed
examples, activation did not take place at temperatures below -40 °C
and was likely hindered by the presence of bulky alkyl groups. Rapid
decomposition of the reaction mixture occurred, and no other discern-
ible Rh species were observed in solution.
(16) (a) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R.
K.; Timmers, F. J . Organometallics 1996, 15, 1518. (b) Alaimo, P. J .;
Peters, D. W.; Arnold, J .; Bergman, R. G. J . Chem. Educ. 2001, 78(1),
64.

890 Organometallics, Vol. 23, No. 4, 2004
Taw et al.
was added, and all volatile materials were removed in vacuo
to produce a yellow-orange solid (138 mg, 0.11 mmol) in 95%
1.24 (d, 2J P-H ) 11.2 Hz, 9 H, PMe3), -10.04 (dd, 2J P-H ) 18.0
1
Hz, J Rh-H ) 26.8 Hz, 2 H, H). 31P{1H} NMR (162 MHz, CD2-
1
1
yield. H NMR (400 MHz, CD2Cl2, room temperature): δ 2.27
Cl2, room temperature): δ 4.4 (d, J Rh-P ) 107.7 Hz, PMe3).
4
(s, 3 H, NCMe), 1.63 (d, J P-H ) 2.8 Hz, 15 H, Cp*), 1.37 (dd,
13C{1H} NMR (101 MHz, CD2Cl2, room temperature): δ 136.3
3
3
2J P-H ) 10.2 Hz, J Rh-H ) 0.6 Hz, 9 H, PMe3), 0.52 (dd, J P-H
(s, SiPh3), 133.5 (s, SiPh3), 131.2 (s, SiPh3), 129.4 (s, SiPh3),
) 6.9 Hz, J Rh-H ) 2.1 Hz, 3 H, Rh-Me). 31P{1H} NMR (162
106.6 (s, Cp*-Ar), 20.00 (d, J P-C ) 36.9 Hz, PMe3), 10.01 (s,
2
1
MHz, CD2Cl2, room temperature): δ 5.2 (d, 1J Rh-P ) 151.1 Hz,
PMe3). 13C{1H} NMR (101 MHz, CD2Cl2, room temperature):
Cp*-Me).
[Cp *(P Me3)Rh (SiMe3)(η2-HSiMe3)]BAr ′4 (9/9′). Complex
9/9′ was generated by addition (at -78 °C) of 5.0 equiv of
trimethylsilane to a CD2Cl2 solution of 4. After ∼10 min at
-60 °C, complex 9/9′ was the predominant species (∼90%) in
solution. Small amounts of 10 (5%) and other unidentified side
3
2
δ 122.6 (d, J P-C ) 6.9 Hz, NCMe), 117.9 (dd, J P-C ) 6.8 Hz,
1J Rh-C ) 3.2 Hz, Cp*-Ar), 14.82 (d, J P-C ) 31.9 Hz, PMe3),
1
2
9.24 (s, Cp*-Me), 3.89 (s, NCCH3), -0.42 (dd, J P-C ) 22.7
1
Hz, J Rh-C ) 13.5 Hz, Rh-Me). Anal. Calcd for C48H42NBF24
-
1
PRh: C, 46.74; H, 3.43. Found: C, 46.51; H, 3.37.
products (5%) were present. H NMR (300 MHz, CD2Cl2, -60
°C): δ 1.74 (d, 4J P-H ) 2.6 Hz, 15 H, Cp*), 1.39 (d, 2J P-H ) 9.8
Typ ica l P r oced u r es for Gen er a tion of Si-H Activa tion
P r od u cts. The complexes listed below have been generated
in situ and characterized by NMR spectroscopy. Typical
procedures for preparing NMR-tube reactions for spectroscopic
study are as follows: an NMR tube was charged with 20 mg
(0.016 mmol) of 4; ∼0.3 mL of CD2Cl2 was added to dissolve 4,
and the NMR tube was capped with a rubber septum; the tube
was then placed in a -78 °C dry ice/acetone bath, and a CD2-
Cl2 solution (∼0.3 mL) of the appropriate silane was admin-
istered to the NMR tube contents via syringe. Attempts at
isolation of products by removal of all volatiles in vacuo led to
rapid decomposition of rhodium compounds. Standard recrys-
tallization techniques also proved fruitless.
2
Hz, 9 H, PMe3), 0.50 (s, 18 H, SiMe3), -12.02 (dd, J P-H ) 7.5
1
1
1
2
Hz, J Rh-H ) 36.6 Hz, J Si-H(observed) ) 28.5 Hz, J Si-H(η ) ) 57
Hz, 1 H, Si-H). 31P{1H} NMR (121 MHz, CD2Cl2, -60 °C): δ
-10.8 (d, J Rh-P ) 155.5 Hz, PMe3). 13C{1H} NMR (75 MHz,
1
1
CD2Cl2, -60 °C): δ 105.3 (s, Cp*-Ar), 21.32 (d, J P-C ) 33.5
Hz, PMe3), 11.43 (s, Cp*-Me), 3.26 (s, SiMe3).
[Cp *(P Me3)Rh (H)2(SiMe3)]BAr ′4 (10a ) or [Cp *(P Me3)-
Rh (H)(η2-HSiMe3)]BAr ′4 (10b/10b′). When a CD2Cl2 solution
of complex 9/9′ (generated as described above) was allowed to
stand at -60 °C, this complex grew in slowly (as observed by
NMR spectroscopy). After ∼1 h, 10 comprised ∼10% of the
reaction mixture, with the rest composed of 9/9′ and unidenti-
fied side products. Upon warming to -20 °C, the amount of
10 increased to ∼50% of the reaction mixture. Complex 10 is
not stable, and decomposition occurred over the course of
several hours at 0 °C. 1H NMR (300 MHz, CD2Cl2, -20 °C): δ
[Cp *(P Me3)Rh (SiP h 3)(CD2Cl2)]BAr ′4 (5). This complex
was generated by addition (at -78 °C) of 1.0 equiv of triph-
enylsilane to a CD2Cl2 solution of 4. At -40 °C, complex 5 was
the only observable rhodium species in solution. 1H NMR (300
MHz, CD2Cl2, -40 °C): δ 7.53 (br m, 3 H, SiPh3), 7.45 (br m,
4
2
1.94 (d, J P-H ) 3.1 Hz, 15 H, Cp*), 1.58 (d, J P-H ) 11.2 Hz,
2
4
9 H, PMe3), 0.58 (s, 9 H, SiMe3), -11.16 (dd, J P-H ) 20.7 Hz,
6 H, SiPh3), 7.34 (br m, 6 H, SiPh3), 1.37 (d, J P-H ) 1.8 Hz,
1J Rh-H ) 27.9 Hz, 2 H, H). 31P{1H} NMR (121 MHz, CD2Cl2,
15 H, Cp*), 1.08 (d, 2J P-H ) 9.6 Hz, 9 H, PMe3). 31P{1H} NMR
-20 °C): δ 4.2 (d, J Rh-P ) 109.9 Hz, PMe3). 13C{1H} NMR
1
1
(121 MHz, CD2Cl2, -40 °C): δ -6.8 (d, J Rh-P ) 170.6 Hz,
(75 MHz, CD2Cl2, -20 °C): δ 103.4 (s, Cp*-Ar), 20.74 (d, 1J P-C
) 32.1 Hz, PMe3), 10.49 (s, Cp*-Me), 1.93 (s, SiMe3).
PMe3). 13C{1H} NMR (75 MHz, CD2Cl2, -40 °C): δ 135.6 (s,
SiPh3), 131.0 (s, SiPh3), 130.8 (s, SiPh3), 129.5 (s, SiPh3), 102.7
[Cp *(P Me3)Rh (SiEt3)(η2-HSiEt3)]BAr ′4 (11). Complex 11
was generated by addition (at -78 °C) of 5.0 equiv of trieth-
ylsilane to a CD2Cl2 solution of 4. When the temperature was
raised to -60 °C, this complex was present in >90% yield, as
determined by NMR spectroscopy. A 13C{1H} NMR spectrum
could not be obtained, due to solubility problems. 1H NMR (400
1
(s, Cp*-Ar), 17.93 (d, J P-C ) 31.5 Hz, PMe3), 9.96 (s, Cp*-
Me).
[Cp *(P Me3)Rh ((C6H4)(η2-HSiP h 2))]BAr ′4 (6). Complex 6
was generated by addition (at room temperature) of 1.0 equiv
of triphenylsilane to a CD2Cl2 solution of 4. Within 3 h, 6 was
formed in ∼90% yield, as determined by NMR spectroscopy
(integration relative to invariant BAr′4 peaks). The identities
of small amounts of side products could not be determined,
due to the low intensities of resonances exhibited in the NMR
spectra. Alternately, when a solution of complex 5 was warmed
to room temperature, 6 was formed. 1H NMR (400 MHz, CD2-
Cl2, room temperature): δ 7.6 (br m, 2 H, Rh-Ar), 7.5 (br m,
4
MHz, CD2Cl2, -60 °C): δ 1.71 (d, J P-H ) 1.9 Hz, 15 H, Cp*),
1.37 (d, 2J P-H ) 9.6 Hz, 9 H, PMe3), 1.03 (br s, 12 H, methylene
3
H’s of Et), 0.89 (t, J H-H ) 7.9 Hz, 18 H, methyl H’s of Et),
2
1
1
-12.39 (dd, J P-H ) 6.8 Hz, J Rh-H ) 36.0 Hz, J Si-H(observed)
)
27.8 Hz, J Si-H(η ) ) 56 Hz, 1 H, Si-H). 31P{1H} NMR (121
MHz, CD2Cl2, -60 °C): δ -12.5 (d, J Rh-P ) 158.0 Hz, PMe3).
1
2
1
4
[Cp *(P Me3)Rh (H)2(SiEt3)]BAr ′4 (12). When
a CD2Cl2
10 H, Ph), 7.3 (br m, 2 H, Rh-Ar), 1.54 (d, J P-H ) 3.1 Hz, 15
H, Cp*), 1.09 (d, 2J P-H ) 10.5 Hz, 9 H, PMe3), -8.95 (dd, 2J P-H
solution of 11 (generated as described above) was warmed to
-40 °C, some conversion to complex 12 occurred, as observed
by NMR spectroscopy. However, the rate of formation was slow
and significant amounts of unidentified side products were also
formed. The amount of 12 in solution never exceeded ∼15%,
and thus a satisfactory 13C{1H} NMR spectrum could not be
obtained. 1H NMR (400 MHz, CD2Cl2, -20 °C): δ 1.90 (d, 4J P-H
1
1
) 11.5 Hz, J Rh-H ) 27.8 Hz, J Si-H ) 84 Hz, 1 H, Si-H). 31P-
{1H} NMR (162 MHz, CD2Cl2, room temperature): δ 4.6 (d,
1J Rh-P ) 141.6 Hz, PMe3). 13C{1H} NMR (101 MHz, CD2Cl2,
2
1
room temperature): δ 154.0 (dd, J P-C ) 18.3 Hz, J Rh-C
)
28.1 Hz, Rh-Ar(Cipso)), 136.5 (s, Ar), 135.5 (s, Ar), 135.2 (s,
Ar), 134.7 (s, Ar), 133.8 (s, Ar), 133.7 (s, Ar), 133.4 (s, Ar), 133.1
(s, Ar), 132.6 (s, Ar), 131.9 (s, Ar), 131.5 (s, Ar), 131.3 (s, Ar),
131.2 (s, Ar), 129.6 (s, Ar), 129.3 (s, Ar), 128.5 (s, Ar), 125.6 (s,
2
) 3.0 Hz, 15 H, Cp*), 1.57 (d, J P-H ) 11.3 Hz, 9 H, PMe3)
(SiEt3 protons overlapped with those of free silane and was
2
1
1
not observed), -11.38 (dd, J P-H ) 20.0 Hz, J Rh-H ) 27.2 Hz,
2 H, H). 31P{1H} NMR (162 MHz, CD2Cl2, -20 °C): δ 3.6 (d,
1J Rh-P ) 111.8 Hz, PMe3).
Ar), 104.1 (s, Cp*-Ar), 16.21 (d, J P-C ) 34.3 Hz, PMe3), 9.70
(s, Cp*-Me).
[Cp *(P Me3)Rh (H)2(SiP h 3)]BAr ′4 (7a ) or [Cp *(P Me3)Rh -
(H)(η2-HSiP h 3)]BAr ′4 (7b/7b′). When 5.0 equiv of triphenyl-
silane was added to a CD2Cl2 solution of 4, this complex was
formed in ∼80% yield within 10 min at room temperature, as
determined by NMR spectroscopy (integration relative to
invariant BAr′4 peaks). Complex 6 accounted for 10% of the
reaction mixture, and the remaining 10% was comprised of
Ack n ow led gm en t. F.L.T. and M.B. acknowledge
the National Science Foundation (Grant No. CHE-
0107810) for support of this work, and R.G.B. acknowl-
edges support by the Director, Office of Energy Re-
search, Office of Basic Energy Sciences, Chemical
Sciences Division, of the U.S. Department of Energy
under Contract No. DE-AC03-7600098.
1
unidentified side products. H NMR (400 MHz, CD2Cl2, room
temperature): δ 7.6 (br m, difficult to integratesoverlaps with
free Ph3SiH, SiPh3), 7.4 (br m, difficult to integratesoverlaps
4
with free Ph3SiH, SiPh3), 1.65 (d, J P-H ) 2.9 Hz, 15 H, Cp*),
OM0341303