A mechanistic investigation of the carbon-carbon bond cleavage of aryl and alkyl cyanides using a cationic Rh(III) silyl complex

Article abstract of DOI:10.1021/ja034468o

Cationic Rh(III) complex [Cp*(PMe₃)Rh(SiPh ₃)(CH₂Cl₂)]BAr4′ (1) activates the carbon-carbon bond of aryl and alkyl cyanides (R-CN, where R = Ph, (4-(CF ₃)C₆H₄), (4-(OMe)C₆H₄), Me, ⁱPr, ^tBu) to produce complexes of the general formula [Cp*(PMe₃)Rh(R)(CNSiPh₃)]BAr⁴′. With the exception of the ^tBuCN case, every reaction proceeds at room temperature (t_1/2 < 1 h for aryl cyanides, t_1,2 < 14 h for alkyl cyanides). A general mechanism is presented on the basis of (1) an X-ray crystal structure determination of an intermediate isolated from the reaction involving 4-methoxybenzonitrile and (2) kinetic studies performed on the C-C bond cleavage of para-substituted aryl cyanides. Initial formation of an η¹-nitrile species is observed, followed by conversion to an η²-iminoacyl intermediate, which was observed to undergo migration of R (aryl or alkyl) to rhodium to form the product [Cp* (PMe₃)Rh(R)(CNSiPh₃)]BAr₄′.

Full text of DOI:10.1021/ja034468o

Published on Web 07/19/2003
A Mechanistic Investigation of the Carbon-Carbon Bond
Cleavage of Aryl and Alkyl Cyanides Using a Cationic Rh(III)
Silyl Complex
Felicia L. Taw,† Alexander H. Mueller,† Robert G. Bergman,*,‡ and
Maurice Brookhart*,†
Contribution from the 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 February 3, 2003; E-mail: mbrookhart@unc.edu
Abstract: Cationic Rh(III) complex [Cp*(PMe3)Rh(SiPh3)(CH2Cl2)]BAr4′ (1) activates the carbon-carbon
i
t
bond of aryl and alkyl cyanides (R-CN, where R ) Ph, (4-(CF3)C6H4), (4-(OMe)C6H4), Me, Pr, Bu) to
produce complexes of the general formula [Cp*(PMe3)Rh(R)(CNSiPh3)]BAr4′. With the exception of the
tBuCN case, every reaction proceeds at room temperature (t1/2 < 1 h for aryl cyanides, t1/2 < 14 h for alkyl
cyanides). A general mechanism is presented on the basis of (1) an X-ray crystal structure determination
of an intermediate isolated from the reaction involving 4-methoxybenzonitrile and (2) kinetic studies performed
on the C-C bond cleavage of para-substituted aryl cyanides. Initial formation of an η1-nitrile species is
observed, followed by conversion to an η2-iminoacyl intermediate, which was observed to undergo migration
of R (aryl or alkyl) to rhodium to form the product [Cp*(PMe3)Rh(R)(CNSiPh3)]BAr4′.
undergoes oxidative addition to form (dippe)Ni(Ph)(CN).4 Both
species were isolable and characterized by X-ray crystal-
Introduction
The activation of carbon-carbon bonds by transition metal
complexes in homogeneous media remains a challenge in the
field of organometallic chemistry. Success has primarily been
limited to systems in which strain relief or aromatization is a
driving force, or where the C-C bond activation is promoted
by chelation assistance or the presence of activating groups.1
However, there are a handful of examples in which the
unstrained C-C bonds of alkyl and aryl cyanides are cleaved
by organometallic complexes. Parkin has shown that photolysis
of an ansa molybdenocene, [Me2Si(C5Me4)2]MoH2, in the
presence of acetonitrile results in the reductive loss of H2 and
oxidative addition of the C-C bond of acetonitrile to form [Me2-
Si(C5Me4)2]Mo(Me)(CN).2 Examples of the C-C cleavage of
aryl cyanides are more common.3 A recent example from Jones
showed that reaction of [(dippe)NiH]2 with benzonitrile leads
to initial formation of an η2-nitrile complex which then
lography.
We have previously communicated results concerning the
C-C bond activation of aryl and alkyl cyanides (R-CN, where
i
t
R ) Ph, (4-(CF3)C6H4), (4-(OMe)C6H4), Me, Pr, Bu) using a
cationic Rh(III) silyl complex, [Cp*(PMe3)Rh(SiPh3)(CH2Cl2)]-
BAr4′ (1).5 Herein, kinetic investigations of the cleavage of aryl
cyanides are described, and an overall reaction mechanism is
proposed. As further support for the proposed mechanism, the
structure of an intermediate isolated from the reaction involving
4-methoxybenzonitrile has been characterized by X-ray crystal-
lography.
Results and Discussion
Generation of [Cp*(PMe3)Rh(SiPh3)(CH2Cl2)]BAr4′ (1;
Ar′ ) 3,5-(CF3)2C6H3). Rhodium silyl complex 1 was generated
by addition of 1 equiv of triphenylsilane to a dichloromethane
solution of [Cp*(PMe3)Rh(Me)(CH2Cl2)]BAr4′ (2).6 This Si-H
activation reaction occurred quantitatively as assessed by NMR
spectroscopy and was complete within seconds at -80 °C.7
Complex 1 was difficult to isolate as decomposition occurred
upon removal of solvent even at low temperatures. Thus, to
perform the C-C bond activation reactions described below, 1
was generated in-situ.
† University of North Carolina at Chapel Hill.
‡ University of California and Lawrence Berkeley National Laboratory.
(1) For examples, see: (a) Jun, C. H.; Moon, C. W.; Lee, D. Y. Chem.-Eur.
J. 2002, 8, 2422. (b) Mu¨ller, C.; Lachicotte, R. J.; Jones, W. D.
Organometallics 2002, 21, 1975. For general reviews, see: (c) Rybtchinski,
B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870. (d) Murakami, M.;
Ito, Y. In Topics in Organometallic Chemistry; Murai, S., Ed.; Springer-
Verlag: New York, 1999. (e) Crabtree, R. H. Chem. ReV. 1985, 85, 245.
(2) Churchill, D.; Shin, J. H.; Hascall, T.; Hahn, J. M.; Bridgewater, B. M.;
Parkin, G. Organometallics 1999, 18, 2403.
(4) (a) Garcia, J. J.; Jones, W. D. Organometallics 2000, 19, 5544. (b) Garcia,
J. J.; Brunkan, N. M.; Jones, W. D. J. Am. Chem. Soc. 2002, 124, 9547.
(5) Taw, F. L.; White, P. S.; Bergman, R. G.; Brookhart, M. J. Am. Chem.
Soc. 2002, 124, 4192.
(6) 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.
(7) Taw, F. L.; Bergman, R. G.; Brookhart, M., unpublished results.
(3) For examples, see: (a) Miller, J. A. Tetrahedron Lett. 2001, 6991. (b) Abla,
M.; Yamamoto, T. J. Organomet. Chem. 1997, 532, 267. (c) Favero, G.;
Morvillo, A.; Turco, A. J. Organomet. Chem. 1983, 241, 251. (d) Parshall,
G. W. J. Am. Chem. Soc. 1974, 96, 2360. (e) Gerlach, D. H.; Kane, A. R.;
Parshall, G. W.; Jesson, J. P.; Muetterties, E. L. J. Am. Chem. Soc. 1971,
93, 3543.
9
9808
J. AM. CHEM. SOC. 2003, 125, 9808-9813
10.1021/ja034468o CCC: $25.00 © 2003 American Chemical Society

C−C Bond Cleavage of Aryl and Alkyl Cyanides
A R T I C L E S
Scheme 1
Figure 1. Possible structures for intermediate species.
Scheme 2
Possible structures for this intermediate are shown in Figure
1. The first possibility, A, is a Rh(V) species formed by
oxidative addition of R-CN. Migration of the silyl group to
nitrogen would result in the C-C activation product. Complex
B is a Rh(III) η2-nitrile complex which can then undergo
oxidative addition of R-CN with subsequent or concerted silyl
migration to form the product. Intermediate B is analogous to
the previously reported nickel η2-nitrile complex,4 which was
observed to undergo reversible cleavage of the C-C bond of
benzonitrile. The last possibility, C, is a Rh(III) η2-iminoacyl
complex which can lead to the final product by migration of
the R group to the Rh center.
Table 1. Approximate t1/2 Values for Activation of Aryl and Alkyl
Cyanides
R−CN
t1/2 (at 25 °C)
(4-(CF3)C6H4)-CN
Ph-CN
<5 min
10 min
1 h
3 h
14 h
(4-(OMe)C6H4)-CN
Me-CN
iPr-CN
tBu-CN
[50% after 3 d @ 50 °C]
Carbon-Carbon Bond Activation of Alkyl and Aryl
Cyanides. When 1.0 equiv of an aryl or alkyl cyanide (R-CN,
i
t
where R ) Ph, (4-(CF3)C6H4), (4-(OMe)C6H4), Me, Pr, Bu)
was added to a solution of complex 1, C-C bond activation
occurred to form the products shown in Scheme 1.8 With the
Determination of the Structure of the Intermediate Spe-
cies. Reactions involving aryl cyanides exhibited (as observed
by variable-temperature NMR spectroscopy) initial formation
of an η1-nitrile complex at low temperatures (<-40 °C). Upon
warming, significant build-up of the transient intermediate
species was observed before complete conversion to product
occurred. Thus, the η1-nitrile complex and the intermediate could
be generated at low temperatures and characterized by NMR
spectroscopy. For example, addition of 1.0 equiv of 4-meth-
oxybenzonitrile to a dichloromethane solution of 1 at -40 °C
initially led to exclusive formation of the η1-nitrile complex
[Cp*(PMe3)Rh(SiPh3)(NC(4-(OMe)C6H4))]BAr4′ (10; Scheme
3).
t
exception of the BuCN case (see below), all of the reactions
proceeded quantitatively at room temperature, as was monitored
by NMR spectroscopy. As a qualitative comparison of the
relative rates of reaction among the nitrile substrates studied,
approximate t1/2 values are listed in Table 1. An X-ray crystal
structure of complex 7, the product of C-C activation of
isopropylcyanide, has been previously reported.5
Addition of 1.0 equiv of tBuCN to a solution of 1 resulted in
the formation of the η1-nitrile adduct, [Cp*(PMe3)Rh(SiPh3)-
(NCtBu)]BAr4′ (9), which was relatively stable at room tem-
perature (trace amounts of C-C activation product 8 were
observed after ∼5 d). Heating a solution of 9 to 50 °C for 3
days resulted in approximately 50% conversion to [Cp*-
(PMe3)Rh(tBu)(CNSiPh3)]BAr4′ (8). However, conversion to 8
was incomplete, and only a mixture of decomposition products
was formed after prolonged heating. The sluggish reactivity
observed in this case may be due to steric hindrance imposed
by the bulky tert-butyl group of the substrate.
General Reaction Scheme for C-C Activation Reactions.
In the C-C bond activation reactions discussed above, initial
formation of an η1-nitrile complex at low temperatures (<-40
°C) was observed by 1H and 31P{1H} NMR spectroscopy.9 Upon
warming, variable amounts (depending on the nitrile substrate
used) of a transient intermediate grew in as the reaction
progressed and disappeared upon quantitative formation of
product (Scheme 2).10
A
29Si NMR spectrum of 10 (acquired at -20 °C) revealed
a resonance at δ 17.52 ppm (t, JRh-Si ) JP-Si ) 20 Hz), as
shown in Table 2. Because the Si atom is bound directly to the
Rh center, coupling to both 103Rh and 31P nuclei was observed.
A spectrum of the product (5) revealed a silyl resonance at δ
-19.54 ppm (s). Because the Si atom is three bonds removed
from Rh, no coupling to either 103Rh or 31P was observed. If a
solution of the η1-nitrile complex was allowed to warm to
(9) An η1 (versus η2) coordination mode has been assigned to the nitrile in the
[Cp*(PMe3)Rh(SiPh3)(NCR)]BAr4′ complexes based upon the following
observations: (a) we have obtained an X-ray crystal structure of [Cp*-
(PMe3)Rh(Me)(NCMe)]BAr4′, which clearly shows that the nitrile is bound
in an η1 fashion (Taw, F. L.; Bergman, R. G.; Brookhart, M., unpublished
1
3
results); (b) JRh-P values (146-152 Hz) and JP-C(nitrile) values (6.5-7.0
Hz) for the silyl nitrile complexes are similar to the values obtained for
[Cp*(PMe3)Rh(Me)(η1-NCMe)]BAr4′, 1JRh-P ) 151.1 Hz, 3JP-C ) 6.9 Hz.
(10) For MeCN, ca. 1% of the intermediate species was observed in the 31P-
{1H} NMR spectrum (δ -2.0 ppm, JRh-P ) 176.4 Hz); for iPrCN, ca. 5%
of the intermediate species was observed (δ -2.9 ppm, JRh-P ) 176.9 Hz);
for tBuCN, ca. 30% of the intermediate species was observed (δ -3.6 ppm,
JRh-P ) 177.2 Hz).
(8) Complete spectroscopic data have been reported for complexes 1-8 in the
Supporting Information of ref 5.
9
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A R T I C L E S
Taw et al.
Scheme 3
Table 2. 29Si NMR Data for Complexes 5, 10, and 11
complex
10: η1-nitrile
11: intermediate species
5: product
29Si NMR data
δ 17.52 (t, JRh-Si ) JP-Si ) 20 Hz)
δ -21.76 (s)
δ -19.54 (s)
15 °C for 20 min, a mixture of the η1-nitrile complex (10), the
intermediate (11), and the product (5) was observed in an
approximate ratio of 5:90:5 (by NMR spectroscopy; Scheme
3). Cooling this reaction to -20 °C to prevent further product
formation and acquiring a 29Si NMR spectrum allowed char-
acterization of the intermediate. A singlet corresponding to the
intermediate was observed at δ -21.76 ppm, indicating that
the Si atom in the intermediate is not directly bound to the Rh
center (Table 2). Thus, C is the only plausible choice among
the three proposed intermediates. Additionally, a 13C{1H} NMR
spectrum of the intermediate exhibited a resonance at 196.8 ppm
Figure 2. ORTEP diagram of [Cp*(PMe3)Rh(η2-C(4-(OMe)C6H4)d
N(SiPh3))]+ (11, BAr4′- counterion omitted for clarity).
Table 3. Selected Bond Distances and Bond Angles for Complex
11
bond distance
(Å)
bond angle
(deg)
Rh(1)-C(32)
Rh(1)-P(1)
Rh(1)-C(2)
Rh(1)-N(1)
C(3)-C(2)
C(2)-N(1)
2.163(12)
2.285(3)
1.963(10)
2.128(8)
1.465(15)
1.255(14)
P(1)-Rh(1)-C(2)
P(1)-Rh(1)-N(1)
Rh(1)-C(2)-C(3)
Rh(1)-N(1)-Si(1)
C(3)-C(2)-N(1)
C(2)-N(1)-Si(1)
90.3(3)
92.2(2)
142.1(8)
142.6(5)
137.3(1)
151.9(8)
1
2
(dd, JRh-C ) 12.8 Hz, JP-C ) 18.6 Hz), diagnostic of the
η2-iminoacyl carbon.
X-ray Crystal Structure of [Cp*(PMe3)Rh(η2-C(4-(OMe)-
C6H4)dN(SiPh3))]BAr4′ (11). X-ray quality crystals of the
intermediate species, [Cp*(PMe3)Rh(η2-C(4-(OMe)C6H4)d
N(SiPh3))]BAr4′ (11), were obtained from the reaction involving
4-methoxybenzonitrile. The ORTEP diagram of 11 is shown in
Figure 2, with selected bond distances and bond angles listed
in Table 3. The Rh(1)-C(2) bond distance of 1.963 Å and the
Rh(1)-N(1) bond distance of 2.128 Å confirm the η2-iminoacyl
structure. η2-Iminoacyl complexes have been previously re-
ported;11 however, none of these complexes exhibit reactivities
similar to those of the systems described here.
Scheme 4
silyl complex 1 at -40 °C. Upon warming to 0 °C, slow
conversion to η2-iminoacyl intermediate Y followed by C-C
bond cleavage and formation of Z can be observed by NMR
spectroscopy. The rate laws for this series of consecutive first-
order reactions are as follows:
Kinetic Studies of Aryl Cyanide Activation. On the basis
of the evidence presented above, a general reaction mechanism
may be proposed for the cleavage of aryl cyanides, as shown
in Scheme 4.12 Species X is produced by the addition of 1.0
equiv of aryl cyanide to a dichloromethane solution of rhodium
d[X]
dt
) -k1[X]
(1)
(2)
(3)
d[Y]
dt
(11) For a few representative examples, see: (a) Shin, J. H.; Savage, W.; Murphy,
V. J.; Bonanno, J. B.; Churchill, D. G.; Parkin, G. J. Chem. Soc., Dalton
Trans. 2001, 11, 1732. (b) Sa´nchez-Nieves, J.; Royo, P.; Pellinghelli, M.
A.; Tiripicchio, A. Organometallics 2000, 19, 3161. (c) Wu, Z.; Diminnie,
J. B.; Xue, Z. Organometallics 1999, 18, 1002. (d) Daff, P. J.; Monge, A.;
Palma, P.; Poveda, M. L.; Ruiz, C.; Valerga, P.; Carmona, E. Organome-
tallics 1997, 16, 2263. (e) Bochmann, M.; Wilson, L. M.; Hursthouse, M.
B.; Motevalli, M. Organometallics 1988, 7, 1148. (f) Chetcuti, P. A.;
Knobler, C. B.; Hawthorne, M. F. Organometallics 1988, 7, 650. (g)
Campion, B. K.; Falk, J.; Tilley, T. D. J. Am. Chem. Soc. 1987, 109, 2049.
(12) Kinetic analyses assume the first step is essentially irreversible (k-1 < k1).
This is clearly the case for 4-methoxybenzonitrile because the intermediate
(complex 11) builds up to over 90%. We assume this assumption is valid
for the other aryl nitriles.
) k1[X] - k2[Y]
d[Z]
dt
) k2[Y]
The concentrations of species X, Y, and Z as the reaction
progresses may be monitored by integration of their respective
signals in the 31P{1H} NMR spectra (Table 4). The concentration
versus time plots of each of these species for the activation
9
9810 J. AM. CHEM. SOC. VOL. 125, NO. 32, 2003

C−C Bond Cleavage of Aryl and Alkyl Cyanides
A R T I C L E S
Figure 3. Concentration versus time plots for activation of aryl cyanides (at 0 °C).
Table 4. 31P{1H} NMR Data for Species X, Y, and Z Associated
with Each Aryl Cyanide Studied
The following expressions13 may be obtained by integration
of the rate laws shown in eqs 1-3:
X
Y
Z
[X] ) [X]oe-k t
(4)
(5)
1
δ -6.4 ppm
δ -2.3 ppm
δ 5.3 ppm
4-trifluoromethyl-
benzonitrile
1
1
1JRh-P ) 147.0 Hz JRh-P ) 174.2 Hz JRh-P ) 129.9 Hz
δ -6.3 ppm
1JRh-P ) 146.3 Hz JRh-P ) 175.2 Hz JRh-P ) 132.6 Hz
δ -6.1 ppm δ -1.7 ppm δ 5.9 ppm
1JRh-P ) 146.6 Hz JRh-P ) 176.0 Hz JRh-P ) 132.7 Hz
δ -2.0 ppm
δ 5.7 ppm
benzonitrile
1
1
k1
[e-k t - e-k t
]
2
1
[Y] ) [X]
o k1 - k2
4-methoxy-
benzonitrile
1
1
[X]o
[Z] ) [X]o +
[k2e-k t - k1e-k t
]
(6)
1
2
reactions using 4-trifluoromethylbenzonitrile, benzonitrile, and
4-methoxybenzonitrile are shown in Figure 3. Analysis of these
data as described below yields values of k1 and k2.
k1 - k2
A plot of -ln[X] versus time reveals a line whose slope is equal
9
J. AM. CHEM. SOC. VOL. 125, NO. 32, 2003 9811

A R T I C L E S
Taw et al.
Figure 4. Concentration versus time plot comparing experimental and calculated curves for [Y].
Table 5. Values of k1 for Activation of Aryl Cyanides
case, which in turn is an order of magnitude faster than when
an electron-donating para-methoxy group is on the aryl ring.
Destabilization of the Rh-N interaction due to the presence of
an electron-withdrawing group in intermediate species Y as well
as the formation of a stronger Rh-Ar bond in the transition
state may account for enhanced rates of conversion to product.
These substituent effects are consistent with previous reports
that the migratory insertion in aryl carbonyl complexes is
accelerated by electron-donating groups on the migrating aryl
group.14
When the concentration of η1-nitrile species X becomes zero,
the value of k2 may be determined directly from the first-order
logarithmic decay of [Y]. As confirmation of the values of k2
derived above, we calculated k2 from a plot of -ln[Y] versus
time (measured when [X] ) 0). For the reaction involving
benzonitrile, a value of 5.2 × 10-5 s-1 was determined for k2,
and for the reaction involving 4-methoxybenzonitrile, a value
of 8.3 × 10-6 s-1 was determined. Accurate values for the case
of 4-trifluoromethylbenzonitrile could not be obtained using this
particular technique, as the concentration of species Y was less
than 10% when species X was depleted from the reaction
mixture.
aryl cyanide
k1 (at 0 °C)
4-trifluoromethylbenzonitrile
benzonitrile
4-methoxybenzonitrile
5.6(3) × 10-4 s-1
3.1(2) × 10-4 s-1
3.0(2) × 10-4 s-1
Table 6. Values of k2 for Activation of Aryl Cyanides
aryl cyanide
k2 (at 0 °C)
4-trifluoromethylbenzonitrile
benzonitrile
4-methoxybenzonitrile
3.8(3) × 10-4 s-1
5.6(3) × 10-5 s-1
8.5(1) × 10-6 s-1
to k1 (as derived from eq 4). Table 5 lists values of k1 determined
from these plots for each aryl cyanide substrate. For the first
step in which an η1-nitrile species undergoes first-order
logarithmic decay to an η2-iminoacyl complex, the rate is
slightly enhanced by the presence of an electron-withdrawing
group (CF3) in the para position of the aryl ring. Presumably,
an electron-withdrawing substituent will render the nitrile carbon
more electrophilic and thus encourage migration. However, there
is essentially no difference in rate between benzonitrile and
4-methoxybenzonitrile; thus the sensitivity of k1 to electronic
effects is small.
Summary and Conclusions
Values of k2 may be determined by using eq 5 or 6. Because
the only unknown is k2 for either of these equations, any standard
mathematical software (such as MathCad) may be used to
optimize for values of k2 that provide the best theoretical fit for
experimental curves. For example, for the reaction involving
benzonitrile, a concentration versus time plot for experimental
values of [Y] and calculated values of [Y] using an optimized
value of k2 is shown in Figure 4. Clearly, there is a good fit
between the experimental and calculated curves, indicating a
valid k2 value.
Table 6 lists the values of k2 associated with conversion of
the η2-iminoacyl complex to the final C-C activation product,
for each aryl cyanide substrate studied. Unlike the first step of
the activation mechanism, the substituent in the para position
of the aryl ring has a large effect on the overall rate. The
presence of the electron-withdrawing CF3 group results in an
increase in rate by an order of magnitude over the benzonitrile
We have shown that [Cp*(PMe3)Rh(SiPh3)(CH2Cl2)]BAr4′
(1), a cationic Rh(III) silyl complex, will activate the C-C bonds
of a wide range of aryl and alkyl cyanides, including cases
involving bond cleavage of a secondary or tertiary alkyl cyanide.
With the exception of tert-butyl cyanide, facile cleavage of the
C-CN bond occurred quantitatively at 25 °C. Activation of the
aryl cyanides was facile, as evidenced by the short reaction times
required for the reaction at room temperature. For the alkyl
cyanides, an increase in the steric bulk of the R substituent led
to increased reaction times.
From spectroscopic and kinetic data, a general reaction
mechanism for the C-C bond cleavage of aryl and alkyl
cyanides has been proposed (Scheme 5). An η2-iminoacyl
intermediate, [Cp*(PMe3)Rh(η2-C(4-(OMe)C6H4)dN(SiPh3))]-
(14) (a) Axe, F. U.; Marynick, D. S. J. Am. Chem. Soc. 1988, 110, 3728. (b)
Cotton, J. D.; Markwell, R. D. J. Organomet. Chem. 1990, 388, 123. (c)
Clark, G. R.; Roper, W. R.; Wright, L. J.; Yap, V. P. D. Organometallics
1997, 16, 5135.
(13) Connors, K. A. Chemical Kinetics; VCH Publishers: New York, 1990.
9
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C−C Bond Cleavage of Aryl and Alkyl Cyanides
A R T I C L E S
2
) 2.7 Hz, 15H, Cp*), 1.11 (d, JP-H ) 10.0 Hz, 9H, PMe3). 31P{1H}
Scheme 5
1
NMR (162 MHz, CD2Cl2, -40 °C): δ -6.0 (d, JRh-P ) 145.9 Hz,
PMe3). 13C{1H} NMR (101 MHz, CD2Cl2, -40 °C): δ 164.5 (s, Ar of
nitrile), 136.6 (s, SiPh3), 135.8 (s, Ar of nitrile), 135.6 (s, Ar of nitrile),
134.5 (s, SiPh3), 128.9 (s, SiPh3), 128.1 (s, Ar of nitrile), 127.5 (s,
1
SiPh3), 103.2 (s, Cp*-Ar), 55.99 (s, OMe), 17.36 (d, JP-C ) 31.7
Hz, PMe3), 9.81 (s, Cp*-Me) [the nitrile carbon was not located due
to overlap with broad aryl resonances]. 29Si NMR (119 MHz, CD2Cl2,
1
2
-20 °C): δ 17.52 (t, JRh-Si ) JP-Si ) 20 Hz, SiPh3).
[Cp*(PMe3)Rh(η2-C(4-(OMe)C6H4)dN(SiPh3))]BAr4′ (11). Let-
ting an NMR tube containing a dichloromethane solution of 10 (see
above procedures) warm to 15 °C for 20 min generated a 5:90:5 mixture
of 10, 11, and 5, respectively. To grow X-ray quality crystals of 11,
this solution was layered with pentane and placed in a freezer (-30
°C) in the glovebox. After ∼4 weeks, orange crystals of 11 were isolated
1
in 30% yield. H NMR (400 MHz, CD2Cl2, -20 °C): δ 7.50 (br m,
15H, SiPh3), 7.50 (m, 2H, Ar-OMe, difficult to assign-overlaps with
3
BAr4′ (11), isolated from the activation reaction involving
4-methoxybenzonitrile, has been characterized by NMR spec-
troscopy and X-ray diffraction. Additionally, mechanistic studies
of the activation of 4-trifluoromethylbenzonitrile, benzonitrile,
and 4-methoxybenzonitrile revealed that the presence of electron-
withdrawing substituents enhanced the overall rate and that the
second step, consisting of the conversion of the η2-iminoacyl
species to the C-C activation product, exhibited a greater
sensitivity to substituent effects, with acceleration by electron-
withdrawing groups.
SiPh3 resonance), 6.86 (d, JH-H ) 8.5 Hz, 2H, Ar-OMe), 3.82 (s,
4
2
3H, OMe), 1.58 (d, JP-H ) 2.6 Hz, 15H, Cp*), 0.98 (d, JP-H ) 9.8
Hz, 9H, PMe3). 31P{1H} NMR (162 MHz, CD2Cl2, -20 °C): δ -1.3
(d, 1JRh-P ) 175.4 Hz, PMe3). 13C{1H} NMR (101 MHz, CD2Cl2, -20
°C): δ 196.8 (dd, 2JP-C ) 18.6 Hz, 1JRh-C ) 12.8 Hz, η2-C([4-OMe]-
Ph)dN(SiPh3)), 165.4 (s, Ar-OMe), 135.9 (s, SiPh3), 131.8 (s, SiPh3),
130.9 (s, Ar-OMe), 129.0 (s, Ar-OMe), 128.8 (s, SiPh3), 115.0 (s,
2
1
Ar-OMe), 99.04 (dd, JP-C ) 5.1 Hz, JRh-C ) 2.8 Hz, Cp*-Ar),
1
54.19 (s, OMe), 15.89 (d, JP-C ) 29.8 Hz, PMe3), 10.00 (s, Cp*-
Me). 29Si NMR (119 MHz, CD2Cl2, -20 °C): δ -21.76 (s, SiPh3).
Kinetic Studies. The following general procedures were used for
kinetic studies of the C-C bond activation reactions involving each of
the aryl cyanide substrates (4-trifluoromethylbenzonitrile, benzonitrile,
and 4-methoxybenzonitrile): An NMR tube was charged with a CD2-
Cl2 solution of 1 (0.025 mmol) and sealed with a septum. The tube
was then placed in a -78 °C bath, and a CD2Cl2 solution of 1.0 equiv
of the appropriate nitrile was added via syringe. To monitor the
activation reaction by NMR spectroscopy, the NMR tube was placed
in the NMR probe at -20 °C. The probe temperature was then set to
0 °C, and the time at which this temperature was achieved was
considered to be t ) 0. Spectra were obtained at regular intervals (5,
10, 15, or 60 min), and integration of 31P{1H} NMR signals for each
species provided their relative concentrations. See the Supporting
Information for full details on these calculations.
Experimental Section
General Considerations. 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 R3-11 catalyst (Chemalog) and 4 Å
molecular sieves. 1H and 13C NMR chemical shifts were referenced to
1
residual H and 13C NMR signals of the deuterated solvents, respec-
tively. 31P NMR chemical shifts were referenced to an 85% H3PO4
sample used as an external standard. 29Si NMR chemical shifts were
referenced to an external standard of TMS.
Materials. All solvents were deoxygenated and dried via passage
over a column of activated alumina.15 Deuterated solvents (Cambridge
Isotope Laboratories) were purified by vacuum transfer from CaH2 and
stored over 4 Å molecular sieves. NaBAr4′ was purchased from Boulder
Scientific and used without further purification. Triphenylsilane and
all nitrile substrates were purchased from Aldrich and used without
further purification. The synthesis and characterization of complexes
1,5 2,6 and 3-95 have been previously reported.
Crystallographic Studies. Crystallographic studies were performed
by Dr. Peter S. White at the University of North Carolina, Chapel Hill,
Single-Crystal X-ray Facility. For complex 11, data were collected at
-100 °C on a Bruker SMART diffractometer, using the ω scan mode.
Crystallographic data and collection parameters are reported in the
Supporting Information. All computations were performed using the
NRCVAX suite of programs.16
Spectral Data for BAr4′-. The 1H and 13C NMR resonances of the
BAr4′ (Ar′ ) 3,5-C6H3(CF3)2) counteranion in CD2Cl2 were essentially
invariant for all cationic complexes discussed here, and its spectroscopic
Acknowledgment. We thank Dr. Peter S. White for solving
the X-ray crystal structure of complex 11. F.L.T. and M.S.B.
acknowledge the National Science Foundation (CHE-0107810)
for support of this work, and R.G.B. acknowledges support by
the Director, Office of Energy Research, Office of Basic Energy
Sciences, Chemical Sciences Division, of the U. S. Department
of Energy under Contract No. DE-AC03-7600098.
1
data are not repeated for each compound. H NMR (400 MHz, CD2-
Cl2): δ 7.73 (s, 8H, Ho), 7.57 (s, 4H, Hp). 13C{1H} NMR (101 MHz,
CD2Cl2): δ 161.9 (q, 1JC-B ) 49.8, Cipso), 135.0 (s, Co), 129.0 (q, 2JC-F
1
) 31.4, Cm), 124.7 (q, JC-F ) 272.6, CF3), 117.7 (s, Cp).
[Cp*(PMe3)Rh(SiPh3)(NC(4-(OMe)C6H4))]BAr4′ (10). An NMR
tube was charged with a CD2Cl2 solution of 1 (0.025 mmol) and sealed
with a septum. The tube was then placed in a -40 °C bath, and a
CD2Cl2 solution of 1.0 equiv (3.3 mg, 0.025 mmol) of 4-methoxyben-
zonitrile was added via syringe. Complex 10 was formed within 5 s.
Supporting Information Available: Details of the kinetic
studies performed and crystallographic data for complex 11
(PDF and CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
3
1H NMR (400 MHz, CD2Cl2, -40 °C): δ 7.60 (d, JH-H ) 8.8 Hz,
2H, Ar of nitrile), 7.52 (m, 6H, SiPh3), 7.33 (m, 9H, SiPh3), 7.03 (d,
4
3JH-H ) 8.8 Hz, 2H, Ar of nitrile), 3.83 (s, 3H, OMe), 1.44 (d, JP-H
JA034468O
(15) (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, 64.
(16) Gabe, E. J.; Le Page, Y.; Charland, J.-P.; Lee, F. L.; White, P. S. J. Appl.
Crystallogr. 1989, 22, 384.
9
J. AM. CHEM. SOC. VOL. 125, NO. 32, 2003 9813