Mechanistic studies of Si-CN and C-NC bond activation of silylnitriles and alkyl isonitriles by rhodium porphyrin radical: Novel cyanide transfer

Article abstract of DOI:10.1021/om0605276

The Si-CN and C-NC bonds of silylnitriles and alkyl isonitriles were activated by (tetramesitylpor-phyrinato)rhodium(II), Rh(tmp), to give rhodium porphyrin silyls or alkyls and rhodium porphyrin cyanide, respectively. Pyridine and triphenylphosphine promoted the rates and yields of the reactions with silylnitriles, but inhibited the rates and yields of the reactions with isonitriles. The reaction of Rh(tmp) with Me₃SiCN exhibited second-order kinetics (rate = k_obs[Rh(tmp)][Me₃SiCN]) at a sufficiently high concentration of pyridine. The reaction with BuNC showed fourth-order kinetics, second-order in each of the reactants; rate = k _obs[Rh(tmp)]²[BuNC]². A novel cyanide transfer rate-determining step was proposed to account for the reaction mechanism.

Full text of DOI:10.1021/om0605276

Organometallics 2006, 25, 5381-5389
5381
Mechanistic Studies of Si-CN and C-NC Bond Activation of
Silylnitriles and Alkyl Isonitriles by Rhodium Porphyrin Radical:
Novel Cyanide Transfer
Lirong Zhang, Chun Wah Fung, and Kin Shing Chan*
Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin, New Territories, Hong Kong,
People’s Republic of China
ReceiVed June 16, 2006
The Si-CN and C-NC bonds of silylnitriles and alkyl isonitriles were activated by (tetramesitylpor-
phyrinato)rhodium(II), Rh(tmp), to give rhodium porphyrin silyls or alkyls and rhodium porphyrin cyanide,
respectively. Pyridine and triphenylphosphine promoted the rates and yields of the reactions with
silylnitriles, but inhibited the rates and yields of the reactions with isonitriles. The reaction of Rh(tmp)
with Me3SiCN exhibited second-order kinetics (rate ) kobs[Rh(tmp)][Me3SiCN]) at a sufficiently high
concentration of pyridine. The reaction with BuNC showed fourth-order kinetics, second-order in each
of the reactants; rate ) kobs[Rh(tmp)]2[BuNC]2. A novel cyanide transfer rate-determining step was proposed
to account for the reaction mechanism.
Introduction
Rhodium(II) tetramesitylporphyrin, Rh(tmp) (Figure 1), is a
metalloradical and exhibits rich chemistry. Wayland and others
have reported extensively on the bond activation and spectros-
copy of Rh(tmp) metalloradical. Some typical bond activations
by Rh(tmp) include activation of the carbon-hydrogen bond
of hydrocarbon H-CH2R,1,2 abstraction of the allylic hydrogen
atom from methyl methacrylate,3 halogen atom abstraction from
a wide variety of organic halides,4 and activation of carbon-
isonitrile bonds.5 RhII(por) (por ) oep (octaethylporphyrin), tpp
Figure 1. Structure of Rh(tmp).
(tetraphenylporphyrin), tmp) also react with H2/CO mixtures
to yield rhodium formyl derivatives.6 The metal-metal bonded
dimers of Rh(II) porphyrins also exhibit wide activities, such
as carbon-hydrogen bond activation and abstraction of a
hydrogen atom.7,8
ESR.9,10 Some of the complexes are unstable. Wayland has
suggested that the disproportionation of certain RhII(por) to RhI-
(por) and RhIII(por) is driven by the favorable thermody-
namics associated with binding an axial ligand to the
Rh(III) product to account for the instability.11,12 A rhodium to
porphyrin electron transfer mechanism has also been proposed
by Collman.10
In our continuing interest in the rich chemistry of rhodium
porphyrins in bond activations, we have recently reported that
Rh(tmp)2a activates unstrained aliphatic carbon-carbon bonds
of nitroxides to yield rhodium porphyrin alkyls.13,14 Expanding
the scope of substrates, we have found that Rh(tmp) undergoes
a formal silicon-nitrogen bond activation15 with silylnitriles.16-18
Rh(tmp) exhibits unique coordination chemistry. The coor-
dination complexes Rh(tmp)L (L ) NEt3, NHEt2, py, 2,6-Me2-
py, PEt3, PPh3, AsPh3, CNR) have been stabilized at 90 K in
toluene or methyl cyclohexane and have been characterized by
* Corresponding author. E-mail: ksc@cuhk.edu.hk.
(1) Sherry, A. E.; Wayland, B. B. J. Am. Chem. Soc. 1990, 112, 1259-
1261.
(2) (a) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991,
113, 5305-5311. (b) Del Rossi, K. J.; Zhang, X. X.; Wayland, B. B. J.
Organomet. Chem. 1995, 504, 47-56. (c) Del, Rossi, K. J.; Wayland, B.
B. J. Chem. Soc., Chem. Commun. 1986, 1653-1655.
(3) Wayland, B. B.; Poszmik, G.; Fryd, M. Organometallics 1992, 11,
3534-3542.
(4) Anderson, J. E.; Yao, C.-L.; Kadish, K. M. Inorg. Chem. 1986, 25,
718-719.
(5) Poszmik, G.; Carroll, P. J.; Wayland, B. B. Organometallics 1993,
12, 3410-3417.
(6) (a) Farnos, M. D.; Woods, B. A.; Wayland, B. B. J. Am. Chem. Soc.
1986, 108, 3659-3663. (b) Wayland, B. B.; Van Voorhees, S. L.; Wilker,
C. Inorg. Chem. 1986, 25, 4039-4024. (c) Van Voorhees, S. L.; Wayland,
B. B. Organometallics 1985, 4, 1887-1888.
(9) Wayland, B. B.; Sherry, A. E. Bunn, A. G. J. Am. Chem. Soc. 1993,
115, 7675-7684.
(10) Collman, J. P.; Boulvtov, R. J. Am. Chem. Soc. 2000, 122, 11812-
11821.
(11) Ni, Y.; Fitzgerald, J. P.; Carroll, P.; Wayland, B. B. Inorg. Chem.
1994, 33, 2029-2035.
(12) Wayland, B. B.; Balkus, K. J., Jr.; Farnos, M. D. Organometallics
1989, 8, 950-955.
(13) Tse, M. K.; Chan, K. S. Dalton 2001, 510-511.
(14) Mak, K. W.; Yeung, S. K.; Chan, K. S. Organometallics 2002, 21,
2362-2364.
(7) Farnos, M. D.; Woods, B. A.; Wayland, B. B. J. Am. Chem. Soc.
1986, 108, 3659-3663.
(8) (a) Jones, W. D.; Feher, F. J. Acc. Chem. Res. 1989, 22, 2, 91-100.
(b) Selmeczy, A. D.; Jones, W. D.; Osman, R.; Perutz, R. N. Organome-
tallics 1995, 14, 5677-5685.
(15) Sakkai, S.; Ieki, M. J. Am. Chem. Soc. 1993, 115, 2373-2381.
(16) (a) Churchill, D.; Shin, J. H.; Hascall, T.; Hahn, J. M.; Bridgewater,
B. M.; Parkin, G. Organometallics 1999, 18, 2403-2406. (b) Taw, F. L.;
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4192-4193.
10.1021/om0605276 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 09/22/2006

5382 Organometallics, Vol. 25, No. 22, 2006
Table 1. Reaction of Silylnitriles with Rh(tmp)
Zhang et al.
entry
substratea
ligand
temp/°C
time
product (% yield)d
1
2
3
4
5
6
7
8
9
Me3SiCN
Me3SiCN
Me3SiCN
Me3SiCN
none
pyb
110
70
90
110
130
110
110
130
110
8 h
3 h
1 h
1 h
1 h
3 h
1 d
1 d
1 d
Rh(tmp)SiMe3 (16) Rh(tmp)CN (14)
Rh(tmp)SiMe3 (71)
Rh(tmp)SiMe3 (73)
Rh(tmp)SiMe3 (88)
Rh(tmp)SiMe3 (82)
Rh(tmp)SiMe3 (79)
Rh(tmp)SitBuMe2 (18)
Rh(tmp)SitBuMe2 (22)
Rh(tmp)SitBuMe2 (2)
pyRh(tmp)CN (72)
pyRh(tmp)CN (70)
pyRh(tmp)CN (81)
pyRh(tmp)CN (84)
Rh(tmp)CN (78)
pyb
pyb
Me3SiCN
pyb
Me3SiCN
Ph3Pc
pyb
tBuMe2SiCN
tBuMe2SiCN
tBuMe2SiCN
pyRh(tmp)CN (85)
pyRh(tmp)CN (80)
pyb
Ph3Pb
a 10 equiv of silyl cyanide. b 2 equiv of pyridine. c 1 equiv of Ph3P. d Isolated yield.
Mechanistic studies of the reaction of Me3SiCN reveal a novel
cyanide group transfer as the key mechanistic step in the bond
activation.18 In examining the generality of the cyanide transfer
process, we have carried out further mechanistic studies of the
activation of alkyl isonitriles with Rh(tmp) and now disclose
the full results.
Results and Discussion
t
Reactions with Me3SiCN. Initially, when the reaction of -
BuCN with Rh(tmp) in benzene was examined, no reaction was
observed at 70 °C in 5 days. We reasoned that silylnitriles might
be more reactive substrates since they are sterically less hindered
Figure 2. Molecular structure of pyRh(tmp)CN, showing the
atomic labeling scheme and 30% probability displacement el-
lipsoids.
with longer Si-CN bond lengths (Me3SiCN: 1.84 Å;19 BDESi-CN
) 85 kcal mol-1 20 than C-CN bonds (CH3CN: 1.46 Å;21
)
BDEMe-CN ) 117 kcal mol-1).22 To our delight, a solution of
Rh(tmp)2a,13 in benzene reacted with 10 equiv of Me3SiCN at
change of product yields (Table 1, entry 6). Rh(tmp)CN was
also obtained without any Ph3P coordinated, and it dissolved
poorly in solvent, making isolation difficult. Pyridine is therefore
a more effective ligand promoter.
23
110 °C for 8 h to give Rh(tmp)SiMe3 and Rh(tmp)CN3 in
16% and 14% yield, respectively (eq 1; Table 1, entry 1). The
silicon-CN bond was activated, but the yields were rather poor.
Encouraged by this successful formal Si-CN activation, we
sought to optimize the yields.
t
Reactions of BuMe2SiCN. When the sterically more hin-
t
dered BuMe2SiCN was reacted with Rh(tmp), silicon-nitrile
bond activation occurred at 130 °C in the presence of added
pyridine. A lower yield of Rh(tmp)SitBuMe2 of 22% was
observed, while pyRh(tmp)CN was still obtained in a high yield
of 80% (Table 1, entries 7 and 8). Reaction at 110 °C gave
similar reaction rates and product yields. When triphenylphos-
phine was used, only a small amount of Rh(tmp)SitBuMe2 was
produced (Table 1, entry 9). To understand why the yield of
Rh(tmp)SitBuMe2 was so low, the thermal stability of Rh(tmp)-
SitBuMe2 was examined. Since Rh(tmp)SitBuMe2 in benzene
with or without excess pyridine or triphenylphosphine added
was thermally stable at 110 °C for at least 1 day, the poor yield
of Rh(tmp)SitBuMe2 is ascribed to the higher kinetic barrier in
its formation presumably due to steric hindrance.
X-ray Structure of pyRh(tmp)CN. pyRh(tmp)CN was
characterized by single-crystal X-ray analysis. The structure
shows that pyridine is trans to the cyanide group. The angles
of N(3)-Rh-N(1) and N(4)-Rh-N(2) are 178.9° and 179.8°,
respectively (Figure 2). Also the distances of Rh with four
nitrogen atoms in the pyrroles are nearly the same at 2.036 Å.
Therefore, the Rh is in the center of the pyrrolic nitrogen ring.
The bond length between the metal Rh and cyanide group is
1.983 Å, which is shorter than the bond length of Rh and
pyridine (2.157 Å). The cyanide group and pyridine ligand are
in an almost linear form (N(6)-Rh(1)-C(7) ) 178 (4)°).
The porphyrin ring is a plane with a small twist. The Rh
atom is displaced 0.025 Å from the porphyrin-core atom defined
by the corresponding 24-atom least-squares plane toward
pyridine. The largest deviation relative to the mean plane of
the pyrrole ring is at C20 (0.1194 Å), whereas N3 lies ap-
proximately above the mean plane by about 0.0703 Å. The
dihedral angles between mesitylphenyl plane and the mean
ligand
2Rh(tmp) + RMe2SiCN
8 Rh(tmp)SiRMe2 +
C6H6, N2
LRh(tmp)CN (1)
Ligands are known to enhance the reactivity of Rh(por).9,10
Indeed, when pyridine was added, both the rates and yields of
reaction were increased, with Rh(tmp)SiMe3 and trans-pyRh-
(tmp)CN isolated in 88% and 81% yields, respectively (Table
1, entries 2-5). Rh(tmp)SiMe3 did not show any change of
proton chemical shift in benzene-d6 with excess pyridine added.
Rh(tmp)SiMe3, therefore, did not coordinate with pyridine either
in solution or in the solid state after isolation, presumably due
to the strong trans effect of the silyl group. The rates of reactions
and yields of the products increased slightly from 70 to 130 °C
(Table 1, entries 2-5).
When triphenylphosphine was used, the rate and yield were
enhanced. However, compared with the reaction with pyridine
added, a longer reaction time of 3 h was required with little
(17) (a) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. Organometallics
1999, 18, 4660-4668. (b) Huang, D.; Heyn, R. H.; Bollinger, J. C.; Caulton,
K. G. Organometallics 1997, 16, 292-293.
(18) Chan, K. S.; Zhang, L.; Fung, C. W. Organometallics 2004, 23,
6097-6098.
(19) Zanchini, C. Chem. Phys. 2000, 254, 187-202.
(20) Corey. J. Y. In The Chemistry of Organic Silicon Compounds Part
1; Patai, S., Rappoport, Z., Ed.; John Wiley & Sons Ltd.: New York, 1989;
pp 5-6.
(21) Tables of Interatomic Distances and Configuration in Molecules
and Ions; The Chemical Society: London, 1958.
(22) Luo, Y.-R. Handbook of Bond Dissociation Energies in Organic
Compounds; CRC Press: Boca Raton, FL, 2003.
(23) Tse, A. K.-S.; Wu, B.-M.; Mak, T. C. W.; Chan, K. S. J. Organomet.
Chem. 1998, 568, 257-261.

Si-CN and C-NC Bond ActiVation of Silylnitriles
Organometallics, Vol. 25, No. 22, 2006 5383
Figure 4. van’t Hoff plot of the binding of Rh(tmp) with pyridine.
Figure 3. UV titration and analysis curves of Rh(tmp) with
pyridine at 20 °C.
observed kinetics were pseudo-first-order, in accord with eq 5.
Pseudo-first-order rate constants k′obs were plotted against the
concentration of Me3SiCN to yield linear plots, with a typical
one shown in Figure 6. The rates were also measured from 60
to 90 °C. The rate constants kobs, derived from the slopes of
these plots, are summarized in Table 3.
porphyrin plane are 78.1°, 86.2°, 87.8°, and 81.8°, respectively.
The dihedral angles between NC4 pyrroles and the mean plane
are 2.9°, 1.3°, 0.6°, and 3.3°, respectively.
UV Titration of [Rh(tmp)] and Pyridine. The stoichiometry
and binding constants of Rh(tmp) and pyridine were measured
spectrophotometrically at 536 nm from 20 to 40 °C (eq 2). The
titration curves of Rh(tmp) with pyridine at 20.0 °C are shown
in Figure 3. Analyses of the data confirmed a 1:1 adduct and
yielded the binding constants at 30.0 °C: K1 () k1/k-1 ) 1.87
× 104 ( 1.10 M-1). The enthalpy and entropy of the pyridine
coordination were estimated, by plotting ln K1 against 1/T
(Figure 4), to be ∆H1 ) -10 ( 1 kcal mol-1 and ∆S1 ) -12
( 2 cal mol-1 K-1, respectively. The negative entropy was due
to the loss of rotational and translational freedom of the ligand
and the rhodium porphyrin upon coordination.
Rate ) k′obs[Rh(tmp)]0[Me3SiCN]
) kobs[Rh(tmp)]0
(4)
(5)
Proposed mechanism:
Rh(tmp) + py yk1z pyRh(tmp)
(6)
k-1
k2 (rds)
pyRh(tmp) + Me3SiCN y
z pyRh(tmp)CN + SiMe3
k-2
(7)
Rh(tmp) + py yk1z pyRh(tmp)
(2)
k3
k-1
pyRh(tmp) + SiMe3 98 Rh(tmp)SiMe3 + py
(8)
Table 2 shows the results of the rates (k1) measured from 25
to 70 °C and demonstrated that the rates were fast.
Rate ) d[Rh(tmp)]/2dt )
d[Rh(tmp)SiMe3]/dt ) k3[pyRh(tmp)][SiMe3] (9)
Mechanistic Studies of the Reaction of Rh(tmp) and
Me3SiCN with Pyridine in Benzene. The reaction between Rh-
(tmp) and Me3SiCN with added pyridine was clean and high-
yielding. Therefore, kinetic studies were carried out to evaluate
the reaction orders, rate constants, and activation parameters
for mechanistic understandings.
d[SiMe3]/dt ) 0 (under steady-state conditions) (10)
k2[pyRh(tmp)][Me3SiCN] )
k-2[pyRh(tmp)CN][SiMe3 + k3[pyRh(tmp)][SiMe3] (11)
So
2Rh(tmp) + py + Me3SiCN f
Rh(tmp)SiMe3 + py-Rh(tmp)CN (3)
k2[pyRh(tmp)][Me3SiCN]
[SiMe3] )
Rate )
(12)
(13)
k-2[pyRh(tmp)CN] + k3[pyRh(tmp)]
k3k2[pyRh(tmp)]2[Me3SiCN]
Kinetic studies of reaction 3 were performed spectrally at
538 nm under the following conditions: 70.0 °C, initial
concentrations (3.28-9.06) × 10-5 M Rh(tmp), (0.61-6.21)
× 10-2 M Me3SiCN, and (0.01-4.80) × 10-3 M pyridine. All
reactions were monitored for at least 4 half-lives and yielded
straight lines of pseudo-first-order kinetic plots (Figure 5).
Therefore the reaction is first-order in [Rh(tmp)]. Values of k′obs
were derived from the slopes of such plots. Kinetic measure-
ments yielded the rate law shown in eq 4. The initial [Rh(tmp)]
concentration was varied over at least a 3-fold range to establish
the independence of k′obs from [Rh(tmp)] (Table 3, entries 2
and 3). Under the conditions of the measurements with Me3-
SiCN always in at least 10-fold excess over Rh(tmp), the
k-2[pyRh(tmp)CN] + k3[pyRh(tmp)]
When
k3[pyRh(tmp)] . k-2[pyRh(tmp)CN]
K1[py][Rh(tmp)]
(14)
(15)
Rate )
0 k2[Me3SiCN]
1 + K1[py]
At high [py] so that K1[py] .1, rate is independent of [py].

5384 Organometallics, Vol. 25, No. 22, 2006
Zhang et al.
Table 2. Rate Constants of Reaction 2
entry
temp/°C
[Rh(tmp)] × 105/M
[py] × 103/M
k′obs (2) × 10/s-1
kobs × 10-1/M-1 s-1
1
2
3
4
25.0
45.0
60.0
70.0
9.61
9.61
9.61
9.61
1.03
1.03
1.03
1.03
0.19 ( 0.002
0.28 ( 0.007
0.59 ( 0.011
1.08 ( 0.018
1.85 ( 0.02
2.72 ( 0.07
5.73 ( 0.11
10.49 ( 0.18
to yield [pyRhIII(tmp)]+ and [pyRhI(tmp)]-,9,25a which may react
in an electrophilic or a nucleophilic manner, respectively, with
Me3SiCN. The pathways involving [pyRhIII(tmp)]+ and [pyRhI-
(tmp)]- existing in preequilibrium or being involved in the rate-
controlling step are ruled out, since neither [pyRhIII(tmp)]+ nor
[pyRhI(tmp)]- reacted with Me3SiCN to yield Rh(tmp)SiMe3.
Therefore, the pyRh(tmp) radical is the reacting species. Another
possible mechanism involves an outer-sphere electron transfer
to the cyanide group from a six-coordinate Rh(II) complex in
a prior association step and is depicted by eqs 17-19.25b
However, our results are not consistent with this mechanism.
This outer-sphere electron transfer would suggest the third-order
rate law, k′[pyRh(tmp)][Me3SiCN][py], expected for this mech-
anism and the binding of Rh(tmp) and pyridine in the ratio of
1:2. Therefore, the outer-sphere mechanism is ruled out.
Figure 5. Pseudo-first-order rate plot for reaction 3.
pyRh(tmp) + py h Rh(tmp)py2
(17)
The rate was independent of pyridine at sufficiently high
concentration (g5.0 × 10-4 M) and exhibited saturation kinetics
in pyridine (see Figure S1, Table S1). The rate of pyridine
coordination (k1) was measured to be faster than the bond
activation.
All the above results are most readily accommodated by the
proposed mechanism depicted in eqs 6-8, with reaction 6
attaining a fast preequilibrium and the formal cyanide group
transfer step to rhodium in reaction 7 being the rate-determining
irreversible step.
A Me3Si radical is also formed in steady-state concentration
and reacts with pyRh(tmp) very rapidly. Therefore, the steady-
state approximation of the trimethylsilyl radical was introduced.
The rate law could be deduced to be eq 13.
In the presence of 10 equiv of pyRh(tmp)CN with respect to
the Rh(tmp) concentration, the reaction rate did not vary within
experimental error (Table 3, entries 3-5). Hence, k-2[pyRh-
(tmp)CN] , k3[pyRh(tmp)] is justified and the backward
reaction rate of step 7 is not important. The rate law was
simplified as shown in eq 15.
py2Rh(tmp) + Me3SiCN f
pyRh(tmp)CN + SiMe3 + py (18)
pyRh(tmp) + SiMe3 f Rh(tmp)SiMe3 + py (19)
(2) A classical two-electron oxidative addition process can
occur with the involvement of a formal, neutral cis-silyl Rh-
(IV) cyanide intermediate, which then yields Rh(tmp)CN and
a silyl radical. This mechanism is less preferred, as a crowded
side-on cis coordination26 of silyl cyanide to rhodium is
necessary. Furthermore, this pathway requires an uncommon
highly energetic and crowded seven-coordinated Rh(IV)27
organometallic species.
(3) A one-electron oxidative addition occurs with the formal
cyanide transfer. In this mechanism, SiCN can coordinate to
Rh(tmp) in a linear, kinetically indistinguishable N- or C-bound
form. The C-bound (i.e., isonitrile) form may be the more
preferred coordination mode, as Me3SiNC is also present in Me3-
SiCN in small amounts and the proportion increases at elevated
Substitution of extrapolated values of K1 at 60 to 90 °C from
eq 6 allows the estimation of k2 in eq 15. At high concentration
of pyridine, K1[py] is much larger than 1, so that kobs is
approximately equal to k2; hence the rate law could be simplified
as eq 16.
temperature,28,29 and the sluggish reaction of BuCN with Rh-
t
(tmp) at 70 °C supports the C-bound (isonitrile) form of Me3-
SiCN in coordination (Scheme 1). Also, the preferred coordi-
nation of the isonitrile form in Me3SiCN with transition metal
complexes has been documented.30 Hertler and co-workers have
reported that tri(tert-butoxy)silyl cyanide and -isocyanide
The Eyring plot of eq 16 is shown in Figure 7 for the
determination of the activation parameters of reaction 3.
Evaluation of the activation parameters concerning k2 from 60
to 90 °C yielded ∆H2 ) 13.1 ( 1.3 kcal mol-1 and ∆S2
)
q
q
(25) (a) Collman, J. P.; Boulatov, R. J. Am. Chem. Soc. 2000, 122,
11812-11821. (b) Marzilli, L. G.; Marzilli, P. A.; Halpern, J. J. Am. Chem.
Soc. 1970, 92, 5752-5753.
(26) Mak, K. W.; Chan, K. S. J. Am. Chem. Soc. 1998, 120, 9686-
9687.
-26.3 ( 3.7 cal mol-1 K-1
.
Kinetic studies indicate that the reaction of Rh(tmp) and Me3-
SiCN with pyridine added is bimolecular with a rate that is first-
order in both Rh(tmp) and Me3SiCN. The entropy of activation
is in the range expected for a bimolecular process.24
The results of the kinetic studies identified the formal Si-
CN bond cleavage of Me3SiCN by rhodium(II) porphyrin radical
via oxidative addition. A few plausible mechanisms exist for
the formal oxidative addition of a Si-CN bond. (1) An electron
transfer mechanism via disproportionation of pyRh(tmp) occurs
(27) (a) Pestovsky, O.; Bakac, A. Inorg. Chem. 2002, 41, 3975-3982.
(b) Acharyya, R.; Dutta, S.; Basuli, F.; Peng, S.-M.; Lee, G.-H.; Falvello,
L. R.; Bhattacharya, S. Inorg. Chem. 2006, 45, 1252-1259. (c) Osakada,
K.; Koizumi, T.-a.; Yamamoto, T. Bull. Chem. Soc. Jpn. 1997, 70, 189-
195. (d) Wang, Z.-Q.; Turner, M. L.; Taylor, B. F.; Maitlis, P. M.
Polyhedron 1995, 14, 2767-2769.
(28) Bither, T. A.; Knoth, W. H.; Lindsey, R. V., Jr.; Sharkey, W. H. J.
Am. Chem. Soc. 1958, 80, 4150-4153.
(29) Ru¨chardt, C.; Meier, M.; Haaf, K.; Pakusch, J.; Wolber E. K. A.;
Mu¨ller, B. Angew. Chem. 1991, 30, 893-1050.
(24) Halpern, J. Acc. Chem. Res. 1970, 3, 386-392.
(30) Seyferth, D.; Kahlen, N. J. Am. Chem. Soc. 1960, 82, 1080-1082.

Si-CN and C-NC Bond ActiVation of Silylnitriles
Table 3. Rate Constants of Reaction 3
Organometallics, Vol. 25, No. 22, 2006 5385
[Rh(tmp)]
[Me3SiCN]
[py]
[pyRh(tmp)CN]
kobs × 102
entry
temp (°C)
× 105 (M)
× 102 (M)
× 103 (M)
× 105 (M)
k′obs × 104 (s-1
)
(M-1 s-1
)
1
2
3
4
5
6
7
60.0
70.0
9.93
9.60
3.28
3.28
3.28
9.60
9.47
1.29
1.28
1.27
1.27
1.27
1.23
1.23
4.97
4.80
4.80
4.77
4.79
9.56
9.46
4.64 ( 0.14
7.00 ( 0.15
7.37 ( 0.16
7.68 ( 0.17
7.57 ( 0.16
11.60 ( 0.24
24.70 ( 0.60
3.60 ( 0.11
5.47 ( 0.12
5.80 ( 0.13
6.05 ( 0.13
5.96 ( 0.13
9.43 ( 0.19
20.08 ( 0.49
9.60
32.80
80.0
90.0
the free Me3Si radical. The mechanism is consistent with
q
activation parameters ∆H2q ) 13.1 ( 1.3 kcal mol-1 and ∆S2
) -26.3 ( 3.7 cal mol-1 K-1, typical of a bimolecular atom
transfer reaction in cobalt(II) chemistry.24,35
To further explore the generality of the proposed cyanide
transfer, the reported reactions of Rh(tmp) with alkyl isonitriles
by Wayland5 were reexamined in more detail.
Reactions of BuNC. An immediate color change occurred
upon addition of BuNC to a benzene solution of Rh(tmp) at
room temperature (eq 18). The reaction was completed within
half an hour to result in cleavage of the C-NC bond and
quantitative formation of equal amounts of Rh(tmp)Bu(BuNC)
and Rh(tmp)CN(BuNC). Rh(tmp)Bu(BuNC) was unstable to-
ward column chromatography and was therefore quantified as
Rh(tmp)Bu (Table 4, entry 1). In solution, Rh(tmp)Bu was
observed to coordinate with a molecule of BuNC. Table 5 shows
that the chemical shifts of the butyl groups were upfield shifted
with increasing equivalents of BuNC ligand added and therefore
supported BuNC coordination.
Figure 6. Linear plot of k′obs with [Me3SiCN] for reaction 3.
Rh(tmp) + RNC ligand (L)8 Rh(tmp)R +
rt
C6H6, N2
LRh(tmp)CN + Rh(tmp)CN(CNR) (20)
When pyridine was added, both reaction rates and yields
decreased significantly. In this case, pyRh(tmp)CN was pro-
duced instead of Rh(tmp)CN(BuNC), probably due to the
stronger coordination of pyridine (Table 4, entry 2). Likely,
coordination with pyridine competed with BuNC and hindered
the rhodium center from accessing RNC; therefore, sluggish
C-NC bond cleavage resulted. Triphenylphosphine was also
not an effective promoter of rate and yield (Table 4, entry 3).
t
t
Reactions of BuNC with Rh(tmp). When BuNC was
reacted with Rh(tmp), Rh(tmp)CN(tBuNC) was formed in 57%
yield. No alkyl derivative was observed presumably due to the
steric hindrance of the tert-butyl group in forming Rh(tmp)tBu
in a manner similar to the reaction of tBuMe2SiCN. Rh(tmp)tBu
is likely very unstable. Indeed, the synthesis of an analogous,
sterically hindered (DMG)2RhtBu (DMG ) dimethylglyoxi-
mato) was not successful due to its thermal lability.36 Similar
to the case of BuNC, pyridine slowed the reaction rate and
lowered the product yields (Table 4, entries 4-6).
Figure 7. Eyring plot of reaction 16.
Rate ) k2[Rh(tmp)]0[Me3SiCN]
[Rh(tmp)]0 ) [Rh(tmp)]total
(16)
equilibrated at room temperature very slowly.31 The equilibration
of cyanide and isocyanide forms in tBuMe2SiCN may also exist.
A Si-NC bond activation occurred, and this cyanide group
transfer mechanism occurs in a manner analogous to the halogen
abstraction reactions of Co(II) and Cr(II) with organic halides,
with the generation of an alkyl radical intermediate and the
formation of the more highly stabilized six-coordinate cobalt-
(III) derivative.32-34 Likewise, the formation of a six-coordinate
pyRh(tmp)CN is very favorable. The other co-intermediate is
Reactions of Me3SiCH2NC with Rh(tmp). The reactions
between Me3SiCH2NC and Rh(tmp) also proceeded fast at room
temperature. However, Rh(tmp)Me was unexpectedly formed
in 19% yield. Rh(tmp)CN with and without a coordinated Me3-
SiCH2NC ligand were both produced (Table 4, entries 7-9).
No Rh(tmp)CH2SiMe3 was observed. Presumably the Me3SiCH2
radical is too bulky to react. Alternatively, C-Si bond activation
proceeded to yield 12% of Rh(tmp)CH3.
(31) Hertler, W. R.; Dixon, D. A.; Matthews, E. W.; Davidson, F.; Kitson,
F. G. J. Am. Chem. Soc. 1987, 109, 6532-6533.
(32) Schneider, P. W.; Phelan, P. F.; Halpern, J. J. Am. Chem. Soc. 1969,
91, 77-81.
(33) Castro, C. E.; Kray, W. C., Jr. J. Am. Chem. Soc. 1963, 85, 2768-
2773.
(35) Halpern, J.; Phelan, P. F. J. Am. Chem. Soc. 1972, 94, 1881-1886.
(36) (a) Anderson, J. E.; Yao, C.; Kadish, K. M. J. Am. Chem. Soc. 1987,
109, 1106-1111. (b) Giese, B.; Hartung, J.; Kesselheim, C.; Lindner, H.
J.; Svoboda, I. Chem. Ber. 1993, 126, 1193-1200.
(34) Halpern, J.; Maher, J. P. J. Am. Chem. Soc. 1965, 87, 5361-5366.

5386 Organometallics, Vol. 25, No. 22, 2006
Scheme 1. Transition State of the Reaction of Rh(tmp) with Silyl Isonitriles
Zhang et al.
Table 4. Reactions of RNC with Rh(tmp)
entry
substratea
BuNC
ligand (L)
time/h
product (% yield)e
1
2
3
4
5
6
7
8
9
none
pyb
0.5
3
0.5
0.5
1
0.5
0.5
1
Rh(tmp)Bu (45)f
Rh(tmp)Bu (6)f
Rh(tmp)Bu (31)f
Rh(tmp)CN(CNBu) (47)
BuNC
pyRh(tmp)CN (17)
pyRh(tmp)CN (35)
BuNC
Ph3Pd
none
pyb
Rh(tmp)CN(CNBu) (33)
tBuNC
Rh(tmp)CN(CNtBu) (57)
tBuNC
Rh(tmp)CN(CNtBu) (trace)
tBuNC
Ph3Pd
none
pyc
Rh(tmp)CN(CNtBu) (37)
Me3SiCH2NC
Me3SiCH2NC
Me3SiCH2NC
Rh(tmp)Me(19)
Rh(tmp)Me(12)
Rh(tmp)Me(23)
Rh(tmp)CN (45)
pyRh(tmp)CN (30)
Rh(tmp)CN (41)
Rh(tmp)CN(CNCH2SiMe3) (21)
Rh(tmp)CN(CNCH2SiMe3) (15)
Rh(tmp)CN(CNCH2SiMe3) (29)
Ph3Pd
0.5
a 10 equiv. b 25 equiv. c 2 equiv. d 1 equiv. e Isolated yield. f In solution as Rh(tmp)Bu(BuNC).
C6H6, N2
2Rh(tmp) + 3BuNC
8 Rh(tmp)Bu(BuNC) + Rh
(tmp)CN(BuNC) (21)
Rate ) kobsd [Rh(tmp)]2[BuNC]2
Proposed mechanism:
(22)
Rh(tmp) + BuNC yk1′z Rh(tmp)(BuNC)
(23)
k-1′
Rh(tmp) (BuNC) + Rh(tmp)(BuNC) k ′(rds)8 Rh(tmp)Bu
2
(BuNC) + Rh(tmp)CN (24)
k3′
Rh(tmp)CN + BuNC y\z Rh(tmp)CN(BuNC) (25)
Figure 8. Relation between chemical shift of Rh(tmp)Bu and
equivalents of BuNC.
Rate ) d[Rh(tmp)]/2dt ) -d[Rh(tmp)Bu(BuNC)]/dt
) k2′[Rh(tmp)(BuNC)]2
Table 5. 1HNMR Chemical Shifts of the Reaction of
Rh(tmp)Bu and BuNC
) k2′K1′[Rh(tmp)]2[BuNC]2
(26)
where K1′ ) k1′/k-1′
chemical shift (ppm)
The proposed mechanism consists of a fast precoordination
of Rh(tmp) with BuNC.37,38 Subsequently, the rate-determining
step involves the C-NC bond cleavage assisted by a second
molecule of Rh(tmp)CNBu in the transition state to simulta-
neously form Rh-Bu and Rh-CN bonds. Finally, BuNC ligand
coordination between the cyanide complex gives Rh(tmp)CN-
(CNBu). The transition state involves four molecules, two in
Rh(tmp) and two in BuNC, fully consistent with the observed
rate law.A possible alternative mechanism that merits consid-
eration is that involving ligand-induced disproportionation of
the Rh(tmp) complex according to eq 27, followed by nucleo-
philic attack of the anionic rhodium(I) fragment at the carbon
atom of the alkyl group of isonitrile.31 The latter mechanism
seems unlikely in the cleavage of the alkyl-NC bond by the
rhodium(II) porphyrin radical for the following reason. Isonitrile
ligands are electronically related to carbon monoxide and their
coordination chemistry is closely related and well-known.5
π-Acidic ligands, such as CO or RNC, have much lower affinity
entry
equiv of BuNC
a
b
c
d
1
2
3
4
5
0
2
10
50
100
-0.67
-0.53
-0.53
-0.55
-0.55
-1.14
-0.94
-0.94
-0.95
-0.95
-3.94
-3.91
-3.91
-3.97
-4.02
-4.45
-5.13
-5.21
-5.28
-5.32
Mechanistic Studies of Rh(tmp) and BuNC in Benzene.
A facile, clean, and high-yielding reaction occurred with Rh-
(tmp) and BuNC. To elucidate the mechanism, kinetic measure-
ments of reaction 20 were therefore performed spectrophoto-
metrically by monitoring the absorbance changes accompanying
the reaction at 522 nm at 25.0 °C, with initial concentrations of
(1.11-0.56) × 10-4 M for Rh(tmp) and (0.18-2.15) × 10-2
M for BuNC for at least 4 half-lives. Kinetic measurements
yielded the rate law shown in eq 22. Pseudo-second-order rate
constants kobs′, determined from the slopes of second-order rate
plots (Figure 9), were plotted against [BuNC]2 to yield the linear
plot (Figure 10). The proposed mechanistic scheme depicted in
eqs 23-25 conforms to the rate law (eq 22). Incorporating the
binding constant (K1), the rate law was derived as eq 26.
(37) Wayland, B. B.; Sherry, A. E.; Poszmik, G.: Bunn, A. G. J. Am.
Chem. Soc. 1992, 114, 1673-1681.
(38) Paonessa, R. S.; Thomas, N. C.; Halpern, J. J. Am. Chem. Soc. 1985,
107, 4333-4335.

Si-CN and C-NC Bond ActiVation of Silylnitriles
Organometallics, Vol. 25, No. 22, 2006 5387
Scheme 2. Transition State of the Reaction of Rh(tmp) with
BuNC
hindrance of the Me3Si group, an alternate mode of cleavage
of the Si-NC bond is required and took place at a high
temperature of 70 °C. Furthermore, the more electron-rich pyRh-
(tmp) is formed in order to facilitate oxidative addition. For
BuNC, backside attack by Rh(tmp) is possible for the stercially
less hindered primary Bu group, and the reaction occurred even
at room temperature with the assistance of two Rh(tmp)
molecules. As the σ-donating series of ligands follow the order
py > RCN > Ph3P > RNC,39 addition of pyridine or Ph3P
would displace the weaker BuNC from Rh(tmp)BuNC and
decrease either the rates or the yields of reactions, presumably
partially by disproportionation of Rh(tmp)L (L ) py or
Ph3P).9-12
Conclusion
In summary, we have discovered the novel bond activation
of silylnitriles by rhodium(II) porphyrin. The reactions were
promoted by pyridine and Ph3P. The mechanism of oxidative
addition Si-CN bond activation of Me3SiCN has been identified
to be a bimolecular reaction with the cyanide group transfer to
rhodium being the rate-controlling step. The C-NC bonds of
alkyl isonitriles have been successfully activated by Rh(tmp).
Pyridine and Ph3P inhibited the bond activation. The rate law
of reaction of BuNC with Rh(tmp) was second-order in both
Rh(tmp) and BuNC. The cleavage of C-NC of coordinated
BuNC was assisted by two Rh(tmp).
Figure 9. Representative pseudo-second-order rate plots for
reaction 20.
Experimental Section
General Procedures. All materials were obtained from com-
mercial suppliers and used without further purification unless
otherwise specified. Benzene was distilled from sodium. Hexane
was freshly distilled from calcium chloride. Benzene-d6 was vacuum
distilled from sodium, degassed three times by the freeze-thaw-
pump method, and stored in a Teflon screwhead stoppered flask.
Pyridine was distilled over KOH under N2. Triphenylphosphine was
recrystallized from EtOH. Chromatographic purification of products
was carried out in air using silica gel. Samples for elemental analysis
were prepared by vacuum-drying (0.005 mmHg) at 50-60 °C for
2 days.
Reaction of Rh(tmp) and Me3SiCN with Pyridine Added. To
a Teflon screwhead stoppered flask containing Rh(tmp) (0.0088
mmol) dissolved in C6H6 (4.0 mL) under nitrogen at rt, prepared
from the photolysis of Rh(tmp)Me (10.0 mg) in C6H6 (4.0 mL) in
80% yield,13 were added distilled pyridine (2.0 µL, 0.02 mmol)
and degassed trimethylsilylnitrile (0.01 mL, 0.08 mmol), respec-
tively. The resultant reaction mixture was heated at 110 °C for 1 h
under nitrogen in the absence of light. Rh(tmp) was completely
consumed, as no Rh(tmp)CH2Cl formed from the reaction of
residual Rh(tmp) with CH2Cl2 by TLC analysis. The crude product
was purified by chromatography on silica gel eluting with a solvent
mixture of hexane/CH2Cl2 (10:1) to hexane/CH2Cl2 (5:1) to give
an orange solid of Rh(tmp)SiMe3 (3.8 mg, 4.0 µmol, 88%), Rf )
Figure 10. Representative plots of k′obsd vs [BuNC]2 for reaction
20.
to RhIII than RhII and stabilize the +2 state, and thus suppress
disproportionation.25a Therefore, disproportionation is less likely
for reactions with alkyl isonitriles.
2Rh(tmp)CNR h Rh(tmp)CNR+ + Rh(tmp)CNR-
(27)
Comparison of Mechanisms of Si-CN and C-NC Acti-
vations. The unique mechanisms of the activations of the Si-
CN bond in Me3SiCN or Me3SiNC and the C-NC bond in
BuNC deserve some comments. The activations of Si-CN and
C-NC all require precoordination.
1. Coordination Mode. The coordination of both the
isocyanide and cyanide form in Me3SiCN is possible.19 The
C-bound form is productive for the cyanide transfer, while the
N-bound form is not. For BuNC, only the C-bound form is
possible.
2. Ligand Effect. The rates and yields of products were
increased by the addition of pyridine or Ph3P for the reaction
with Me3SiCN but were suppressed for the reaction with BuNC.
For Me3SiCN, since the activation of the Si-CN bond by two
Rh(tmp) is unlikely to occur by backside attack due to the steric
1
0.55 (hexane/CH2Cl2, 5:1); H NMR (C6D6, 300 Hz) δ -3.07 (s,
9 H), 1.63 (s, 12 H), 2.41 (s, 24 H), 6.84 (s, 4 H), 7.41 (s, 4 H)
8.69 (s, 8 H); 1H NMR (CDCl3, 300 Hz) δ -3.47 (s, 9 H), 1.52 (s,
12 H), 2.52 (s, 12 H), 2.60 (s, 12 H), 7.17 (s, 4 H), 7.30 (s, 4 H),
8.43 (s, 8 H); 13C NMR (C6D6, 75.5 Hz) δ 1.4, 21.4, 21.5, 23.0,
30.2, 127.1, 131.0, 137.7, 138.5, 140.1, 143.5; 13C NMR (CDCl3,
75.5 Hz) δ 1.0, 21.3, 21.4, 22.7, 29.7, 127.8, 130.4, 138.3, 138.4,
142.9; HRMS (FABMS) calcd for (C59H61N4SiRh)+ m/z 956.3715,
found m/z 956.3729. Anal. Calcd for C59H61N4SiRh: C, 74.04; H,
6.42; N, 5.85. Found: C, 74.18; H, 6.61; N, 5.64. A deep red solid
(39) Huheey, J. E. Inorganic Chemistry, 2nd ed.; Harper & Row: New
York, 1978; p 460.

5388 Organometallics, Vol. 25, No. 22, 2006
Zhang et al.
of pyRh(tmp)CN (2.8 mg, 2.8 µmol, 81%) was also obtained. Rf
) 0.30 (100% CH2Cl2); 1H NMR (CDCl3, 300 MHz) δ 1.26 (d, 2
H, J ) 6.0 Hz), 1.59 (s, 12 H), 1.95 (s, 12 H), 2.60 (s, 12 H), 4.97
(t, 2 H, J ) 6.9 Hz), 6.00 (t, 1 H, J ) 7.8 Hz), 7.19 (s, 4 H), 7.28
(s, 4 H), 8.60 (s, 8 H); 13C NMR (CDCl3, 75 MHz) 21.36, 21.60,
22.08, 29.85, 118.47, 127.43, 128.09, 131.46, 134.86, 138.47,
140.26, 141.75, 146.22, 158.80; HRMS (FABMS) calcd for
Reaction of Rh(tmp) and BuNC.5 BuNC (0.01 mL, 0.08 mmol),
which was degassed by the freeze-pump-thaw method (3 cycles),
was added via a microsyringe to a benzene solution of Rh(tmp)
(0.0088 mmol), and the reaction mixture was stirred at rt for 0.5 h
under N2 in the absence of light. The crude product was purified
by chromatography on silica gel using a solvent mixture of hexane/
CH2Cl2 (10:1) to hexane/CH2Cl2 (5:1) as the gradient eluent. An
orange solid of Rh(tmp)Bu (3.7 mg, 3.9 µmol, 45%) was obtained,
(C62H57N6Rh)+ m/z 989.3773, found m/z 989.3771; IR (KBr, cm-1
ν 2305, 2365. The single crystal was grown from THF.
)
1
Rf ) 0.61 (hexane/CH2Cl2, 5:1); H NMR (C6D6, 300 MHz) δ
2
Reaction of Rh(tmp) and tBuMe2SiCN with Pyridine Added.
Distilled and degassed pyridine (2 µL, 0.02 mmol) was added to a
solution of Rh(tmp) (0.0088 mmol) at rt. Then degassed tert-
butyldimethylsilylnitrile (0.10 mL, 0.08 mmol, 0.88M in benzene)
was added to the above solution and then heated at 110 °C for 1
day under N2 in the absence of light. The crude product was purified
by chromatography on silica gel eluting with a solvent mixture of
hexane/CH2Cl2 (10:1) to hexane/CH2Cl2 (5:1) to give an orange
solid of Rh(tmp)SitBuMe2 (0.8 mg, 0.8 µmol, 18%), Rf ) 0.64
-4.45 (td, 2 H, JRh-H ) 3 Hz, J ) 7.5 Hz), -3.94 to -3.83 (m,
2 H), -1.14 to -1.04 (m, 2 H), -0.67 (t, 3 H, J ) 7.5 Hz), 1.91
(s, 12 H), 2.18 (s, 12 H), 2.44 (s, 12 H), 7.12 (s, 4 H), 7.29 (s, 4
H), 8.75 (s, 8 H); HRMS (FABMS) calcd for (C60H61N4Rh)+ m/z
940.3946, found m/z 940.3971. A deep red solid of Rh(tmp)CN-
(CNBu) (4.1 mg, 4.1 µmol, 47%) was obtained, Rf ) 0.55 (hexane/
1
CH2Cl2, 5:1); H NMR (C6D6, 300 MHz) δ -0.93 to -0.92 (m 4
H), -0.29 (t, 3 H, J ) 6.9 Hz), 0.15 (t, 2 H, J ) 6 Hz), 1.82 (s, 12
H), 2.05 (s, 12 H), 2.44 (s, 12 H), 7.04 (s, 4 H), 7.21 (s, 4 H), 8.91
(s, 8 H); HRMS (FABMS) calcd for (C62H61N6Rh)+ m/z 993.4086,
found m/z 993.4066.
1
(hexane/CH2Cl2, 5:1); H NMR (C6D6, 300 MHz) δ -2.88 (s, 6
H), -1.25 (s, 9 H), 1.91 (s, 12 H), 2.41 (s, 12 H), 2.56 (s, 12 H),
6.91 (s, 4 H), 7.42 (s, 4 H), 8.66 (s, 8 H); HRMS (FABMS) calcd
for (C62H67N4SiRh)+ m/z 998.4185, found m/z 998.4179. A deep
red solid of pyRh(tmp)CN (3.7 mg, 3.7 µmol, 85%), Rf ) 0.30
(CH2Cl2), was also obtained.
Reaction of Rh(tmp) and BuNC with Pyridine Added.
Distilled and degassed pyridine (2 µL, 0.02 mmol) was added to a
solution of Rh(tmp) (0.0088 mmol) at rt. Then degassed BuNC
(0.01 mL, 0.08 mmol) was added to the above solution, and the
mixture was then stirred at rt for 3 h under N2 in the absence of
light. The crude product was purified by chromatography on silica
gel eluting with a solvent mixture of hexane/CH2Cl2 (10:1) to
hexane/CH2Cl2 (5:1) to give an orange solid of Rh(tmp)Bu (0.5
mg, 0.5 µmol, 6%); red deep solids of pyRh(tmp)CN (1.5 mg, 1.5
µmol, 17%) were also obtained.
Reaction of Rh(tmp) and Me3SiCN with PPh3 Added. A
triphenylphosphine solution (0.1 mL, 0.01 mmol) was added to a
solution of [Rh(tmp)] at rt. Degassed trimethylsilylcyanide (0.01
mL, 0.08 mmol) was added to the above solution. After heating at
110 °C for 3 h under N2 in the absence of light, the crude product
was purified by chromatography on silica gel eluting with hexane/
CH2Cl2 (10:1) to hexane/CH2Cl2 (5:1) to give an orange solid of
Rh(tmp)SiMe3 (3.3 mg, 3.5 µmol, 79%) with Rf ) 0.55 (hexane/
CH2Cl2, 5:1) as well as a red solid, Rh(tmp)CN (3.1 mg, 3.4 µmol,
78%): 1H NMR (C6D6, 300 MHz) δ 1.92 (s, 12 H), 1.98 (s, 12 H),
2.42 (s, 12 H), 8.87 (s, 8 H).
Reaction of Rh(tmp) and Me3SiCH2NC with Pyridine Added.
Distilled and degassed pyridine (2 µL, 0.02 mmol) was added to a
solution of Rh(tmp) (0.0088 mmol) at rt. Then degassed Me3SiCH2-
NC (0.01 mL, 0.08 mmol) was added to the above solution, and
the mixture was then stirred at rt for 1 h under N2 in the absence
of light. The crude product was purified by chromatography on
silica gel eluting with a solvent mixture of hexane/CH2Cl2 (10:1)
to hexane/CH2Cl2 (5:1) to give an orange solid of Rh(tmp)Me (1.0
mg, 1.1 µmol, 12%) with Rf ) 0.57 (hexane/CH2Cl2, 5:1); 1H NMR
(C6D6, 300 MHz) δ -5.26 (d, 3 H 2JRhH ) 2.7 Hz), 1.72 (s, 12 H),
2.25 (s, 12 H), 2.43(s, 12 H), 7.07 (s, 4 H), 7.20 (s, 4 H), 8.75 (s,
8 H); a red deep solid of pyRh(tmp)CN (2.6 mg, 2.6 µmol, 30%)
was also obtained and deep red solids of Rh(tmp)CN(CNCH2SiMe3)
t
Reaction of Rh(tmp) and BuMe2SiCN with PPh3 Added. A
triphenylphosphine solution (0.1 mL, 0.01 mmol) was added to a
solution of [Rh(tmp)] at rt. Degassed tert-butyldimethylsilyl cyanide
(0.10 mL, 0.08 mmol, 0.88 M in benzene) was added, and the
solution was heated at 110 °C for 1 day under N2 in the absence of
light. The crude product was purified by chromatography on silica
gel to give an orange solid of Rh(tmp)SitBuMe2 (0.2 mg, 0.2 µmol,
2%).
Reaction of Rh(tmp) and tBuNC. tBuNC (0.01 mL, 0.08 mmol),
which was degassed by the freeze-pump-thaw method (3 cycles),
was added via a microsyringe to a benzene solution of Rh(tmp)
(0.0088 mmol), and the reaction mixture was stirred at rt for 0.5 h
under N2 in the absence of light. The crude product was purified
by chromatography on silica gel using a solvent mixture of CH2-
Cl2 as the gradient eluent. A deep red solid of Rh(tmp)CN(CNtBu)
(5.0 mg, 5.0 µmol, 57%) was obtained, Rf ) 0.27 (100% CH2Cl2);
1H NMR (C6D6, 300 MHz) δ -0.99 (s, 9 H), 1.94 (s, 24 H), 2.44
(s, 12 H), 7.07 (s, 4 H), 7.19 (s, 4 H), 8.89 (s, 8 H); 13C NMR
(CDCl3, 75 MHz) δ 21.99, 22.17, 22.78, 28.77, 118.86, 127.89,
128.64, 131.71, 138.18, 138.96, 139.28, 140.60, 142.44; HRMS-
(FABMS) calcd for (C62H61N6Rh)+ m/z 992.4007, found m/z
992.4037. Anal. Calcd for C62H61N6Rh: C, 74.98; H, 6.19; N, 8.46.
Found: C, 74.59; H, 6.13; N, 8.06.
1
(2.8 mg, 2.8 µmol, 15%) with Rf ) 0.50 (CH2Cl2/EA, 100:3); H
NMR (C6D6, 300 MHz) δ -1.62 (s, 9 H), -0.22 (s, 2 H), 1.36 (s,
12 H), 1.83 (s, 12 H), 2.43 (s, 12 H), 6.89 (s, 4 H), 7.23 (s, 4 H),
8.90 (s, 8 H); 13C NMR (CDCl3, 75 MHz) δ -4.50, 22.16, 22.32,
22.65, 118.86, 127.87, 128.64, 131.76, 138.15, 138.91, 139.00,
140.97, 142.33; HRMS (FABMS) calcd for (C62H63N6SiRh)+ m/z
1022.3965, found m/z 1002.3933. Anal. Calcd for C62H63N6SiRh:
C, 72.78; H, 6.21; N, 8.21. Found: C, 72.46; H, 6.33; N, 8.00.
Kinetic Studies on Reaction of Rh(tmp) and Me3SiCN with
Pyridine Added. Kinetic studies were carried out in the temperature
range (60-90) ( 0.2 °C maintained by a constant-temperature
circulating bath. Rh(tmp)Me (5 mg) was dissolved in benzene (4
mL) in a Rotaflo flask and degassed, and after photolysis a stock
solution of Rh(tmp) (2.225 × 10-3 M) was produced. Pyridine (1.95
× 10-1 M), substrate Me3SiCN (1.661 M), and pyRh(tmp)CN
(2.225 × 10-3 M) were prepared and degassed. A measured amount
of the stock solution of Rh(tmp) and pyridine were added with
benzene in a curvette-Schlenck UV cell, and the mixture was
thermally equilibrated for 30 min. Then the Me3SiCN solution in
benzene was added to the mixture quickly, and absorbance was
measured at 538 nm.
Reaction of Rh(tmp) and tBuNC with Pyridine Added.
Distilled and degassed pyridine (2 µL, 0.02 mmol) was added to a
t
solution of Rh(tmp) (0.0088 mmol) at rt. Then degassed BuNC
(0.01 mL, 0.08 mmol) was added to the above solution, and the
mixture was then stirred at rt for 1 h under N2 in the absence of
light. The crude product was purified by chromatography on silica
gel eluting with a solvent mixture of hexane/CH2Cl2 (10:1) to
hexane/CH2Cl2 (5:1) to give red deep solids of pyRh(tmp)CN (1.2
mg, 1.2 µmol, 35%).
UV Titration of Rh(tmp) with Pyridine. The UV titrations were
carried out on a UV-vis spectrometer equipped with a temperature
controller. The titrations were carried out at temperatures of 20.0,

Si-CN and C-NC Bond ActiVation of Silylnitriles
Organometallics, Vol. 25, No. 22, 2006 5389
25.0, 30.0, 35.0, and 40.0 ((0.2) °C. The temperatures of the
solutions were measured by a thermocouple wire placed in a UV
cell that was placed inside the sample compartment. Benzene was
freshly distilled over sodium under N2, and pyridine was distilled
over NaOH under N2. Stock solutions of Rh(tmp) in benzene (∼5.45
× 10-6 M) and pyridine in benzene (∼1.0 × 10-1 M) were
prepared. The solutions of Rh(tmp) (3.00 mL) was transferred to a
Teflon-stoppered Schlenck UV cell with a gastight syringe under
N2. Pyridine solution was then titrated into the Rh(tmp) solutions
via a gastight microsyringe at 2.0 µL steps up to a total of 20.0 µL
and then at 4.0 µL steps up to a total of 60.0 µL of pyridine solution
added. Finally 100 µL of neat pyridine was added to the sample
solution to obtain the estimated absorbance for the Rh(tmp)-
pyridine complex. The UV spectra were scanned at 600 nm/min
between 350 and 700 nm. The experimental absorbance was
measured at 536 nm. The binding constants and the number of
pyridine ligands coordinated to each Rh(tmp) complex were
calculated by the equation40
the UV sample, Ae ) experimental absorbance measured (volume
correction made), Am ) absorbance of the Rh(tmp) without pyridine,
An ) theoretical absorbance obtained for the Rh(tmp)-pyridine
complex (obtained from the absorbance measured for the infinite
point with volume correction).
The measured (Ae) were corrected for volume change due to the
addition of ligand solution and the thermal expansion/contraction
by Vt ) V0(1 + ât) and â ) 0.00372 (for benzene).41
Kinetic Studies on Reaction of Rh(tmp) and BuNC. Kinetic
studies were carried out at the temperature 25.0 ( 0.2 °C maintained
by a constant-temperature circulating bath. After photolysis the stock
solution of Rh(tmp) (1.11 × 10-3 M) was produced. Substrate
BuNC (0.535 M) was prepared and degassed. A measured amount
of the stock solution of Rh(tmp) was added with benzene in a
curvette-Schlenck UV cell and was thermally equilibrated for 30
min. Then the BuNC solution in benzene was added to the mixture
quickly, and absorbance was measured at 522 nm.
Ae - A
An - Ae
Acknowledgment. We thank the Research Grants Council
of Hong Kong of the SAR of China for financial support (No.
400203).
log Kn ) log
m - n log[L]
where n ) number of pyridine ligands coordinated to each Rh-
(tmp), Kn ) binding constant, [L] ) concentration of pyridine in
Supporting Information Available: Kinetic data, NMR spectra,
and X-ray structural data (cif). This material is available free of
charge via the Internet at http://pubs.acs.org.
(40) Perkampus H. H. UV-Vis Spectroscopy and Its Applications;
Springer-Verlag: Berlin, 1992.
(41) Atkins. P. Physical Chemistry, 6th ed.; Oxford, 1998.
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