Kinetic study on the interconversion between (silylacetylene)- and (β-silylvinylidene)rhodium(I) complexes

Article abstract of DOI:10.1021/om9603575

Kinetic studies on the interconversion between trans-RhCl(FcC≡CSiMe₃)(PPrⁱ₃)₂ (2a) and trans-RhCl-{=C=C(Fc)(SiMe₃)}(PPrⁱ₃)₂ (Ba) (Fc = ferrocenyl group) provided the following activation parameters for the forward and the backward reactions: ΔH^? = 19.8 ± 0.2 kcal mol^-1 and ΔS^? = -4.8 ± 0.7 eu for 2a to 3a and ΔH^? = 22.7 ± 0.1 kcal mol^-1 and ΔS^? = -8.6 ± 0.4 eu for 3a to 2a. The values are consistent with the isomerization mechanism via a sigmatropic 1,2-migration of the SiMe₃ group.

Full text of DOI:10.1021/om9603575

4642
Organometallics 1996, 15, 4642-4645
Kin etic Stu d y on th e In ter con ver sion betw een
(Silyla cetylen e)- a n d (â-Silylvin ylid en e)r h od iu m (I)
Com p lexes
Hiroyuki Katayama, Kiyotaka Onitsuka,† and Fumiyuki Ozawa*
Department of Applied Chemistry, Faculty of Engineering, Osaka City University,
Sumiyoshi-ku, Osaka 558, J apan
Received May 13, 1996X
Summary: Kinetic studies on the interconversion between
employed the Werner system using RhCl(PPri ) . The
combination of 1a and the Werner complex provided a
convenient system to perform kinetic studies on the
interconversion.
3 2
trans-RhCl(FcCtCSiMe3)(PPri ) (2a ) and trans-RhCl-
3 2
{dCdC(Fc)(SiMe3)}(PPri ) (3a ) (Fc ) ferrocenyl group)
3 2
provided the following activation parameters for the
forward and the backward reactions: ∆Hq ) 19.8 ( 0.2
kcal mol-1 and ∆Sq ) -4.8 ( 0.7 eu for 2a to 3a and
∆Hq ) 22.7 ( 0.1 kcal mol-1 and ∆Sq ) -8.6 ( 0.4 eu
for 3a to 2a . The values are consistent with the
isomerization mechanism via a sigmatropic 1,2-migra-
tion of the SiMe3 group.
Resu lts a n d Discu ssion
Isom er ization of tr a n s-Rh Cl(FcCtCSiMe )(P P r i )
3 2
3
(2a ) to tr a n s-Rh Cl{dCdC(F c)(SiMe3)}(P P r i )2 (3a ).
3
Treatment of RhCl(PPri ) with an equimolar amount
3 2
of 1a in toluene-d8 at -20 °C instantly gave the
In tr od u ction
acetylene complex trans-RhCl(FcCtCSiMe3)(PPri ) (2a )
3 2
1
Recently, we have reported the cyclocarbonylation of
1,1′-bis((trimethylsilyl)ethynyl)ferrocene catalyzed by
Ru3(CO)12.1 The structure of the reaction product
strongly suggested a catalytic process involving a (â-
silylvinylidene)ruthenium intermediate which is gener-
ated from a silylferrocenylacetylene group bound to
ruthenium. In connection with this novel catalytic
reaction, we have been interested in the mechanism of
interconversion of silylacetylenes to the corresponding
vinylidene complexes.
in quantitative yield, as confirmed by H, 13C, and 31P
NMR analysis of the reaction solution. Since complex
2a was highly liable to isomerize to the corresponding
vinylidene complex trans-RhCl{dCdC(Fc)(SiMe3)}-
(PPri ) (3a ), further investigations were carried out
3 2
without isolation of 2a .
The isomerization of 2a to 3a obeyed a first-order rate
law in the concentration of 2a over 3 half-lives, giving
a quantitative yield of 3a , which was isolated as a violet
crystalline solid after recrystallization from a toluene/
hexane solution (67%) and characterized by NMR
spectroscopy and elemental analysis (eq 1). First-order
There have been numerous reports concerning the
isomerization of terminal acetylenes to vinylidene com-
plexes.2 On the other hand, the studies of formation of
vinylidene complexes from silylacetylenes are still lim-
ited.3 With regard to the reaction mechanism, Werner
et al. suggested a concerted process involving a 1,2-
SiMe3 group migration of silylacetylenes (RCtCSiMe3)
coordinated to RhCl(PPri ) , while no direct evidence
3 2
was presented.3a Connelly et al. carried out kinetic
studies on the redox-induced interconversion between
Cr(CO)2(Me3SiCtCSiMe3)(C6Me6) and Cr(CO)2{dCdC-
(SiMe3)2}(C6Me6) and proposed a similar reaction pro-
cess involving a sigmatropic 1,2-shift of the SiMe3
group.3b
In this paper we examined the mechanism of inter-
conversion of ((trimethylsilyl)ethynyl)ferrocene (FcCtC-
SiMe3; 1a ) to a (trimethylsilyl)ferrocenylvinylidene
species. This study was initially undertaken with
ruthenium systems related to the catalytic cyclocarbo-
nylation. However, no appropriate systems for mecha-
nistic investigations have been found. Therefore, we
rate constants measured at four different temperatures
are listed in Table 1. The reaction rate was independent
of the initial concentration of 2a (entries 4 and 5) and
was little affected by addition of free 1a to the system
(entries 4 and 6), indicating an intramolecular process
for the isomerization. The Eyring plot of the data in
entries 1-4 was linear and led to the following activa-
tion parameters: ∆Hq ) 19.8 ( 0.2 kcal mol-1, ∆Sq )
-4.8 ( 0.7 eu.
It has been shown that the stability of acetylene
complexes and the ease of formation of vinylidene
complexes are dependent mainly upon the electronic
properties of the R groups of silylacetylenes (RCtC-
SiMe3).3a For example, Me3SiCtCSiMe3 (1b) forms no
detectable amount of rhodium acetylene complex with
† Present address: The Institute of Scientific and Industrial Re-
search, Osaka University, Ibaraki, Osaka 567, J apan.
X Abstract published in Advance ACS Abstracts, September 15, 1996.
(1) Onitsuka, K.; Katayama, H.; Sonogashira, K.; Ozawa, F. J .
Chem. Soc., Chem. Commun. 1995, 2267.
RhCl(PPri ) , while the formation of the corresponding
3 2
(2) Bruce, M. I. Chem. Rev. 1991, 91, 197.
vinylidene complex trans-RhCl{dCdC(SiMe3)2}(PPri )
3 2
(3) (a) Werner, H.; Baum, M.; Schneider, D.; Windmu¨ller, B.
Organometallics 1994, 13, 1089. (b) Connelly, N. G.; Geiger, W. E.;
Lagunas, M. C.; Metz, B.; Rieger, A. L.; Rieger, P. H.; Shaw, M. J . J .
Am. Chem. Soc. 1995, 117, 12202. (c) Sakurai, H.; Fujii, T.; Sakamoto,
K. Chem. Lett. 1992, 339. (d) Naka, A.; Okazaki, S.; Hayashi, M.;
Ishikawa, M. J . Organomet. Chem. 1995, 499, 35.
(3b) proceeds readily even at low temperature. On the
other hand, trans-RhCl(PhCtCSiMe3)(PPri ) (2c) is
3 2
stable enough to be isolated at room temperature and
its conversion to trans-RhCl{dCdC(Ph)(SiMe3)}(PPri )
3 2
S0276-7333(96)00357-3 CCC: $12.00 © 1996 American Chemical Society

Notes
Ta ble 1. Ra tes for th e Isom er iza tion of 2a to 3a a
Organometallics, Vol. 15, No. 21, 1996 4643
Ta ble 2. Ra tes for th e Rea ction s of 3a w ith
Acetylen esa
entry no.
temp (°C)
104kobsd (s-1
)
rb
entry
no.
temp 104kobsd
1
2
3
-5.0
5.0
10.0
15.0
15.0
15.0
0.368
1.26
2.34
5.55
5.55
5.17
1.000
0.999
0.999
0.999
0.999
0.998
acetylene [amt (equiv)]
(°C)
(s-1
)
rb
1
2
3
4
5
6
7
8
PhCtCPh (1d ) [10]
PhCtCPh (1d ) [20]
MeO2CCtCCO2Me (1e) [10]
PhCtCH (1f) [10]
PhCtCSiMe3 (1c) [10]
PhCtCPh (1d ) [10]
PhCtCPh (1d ) [10]
PhCtCPh (1d ) [10]
60.0
60.0
60.0
60.0
60.0
55.0
65.0
70.0
1.27
1.26
1.86
1.27
1.08
0.660
2.08
3.18
0.997
0.999
1.000
0.998
0.998
0.999
0.999
0.998
4
5c
6d
a
All reactions were carried out in toluene. The initial concen-
b
tration of 2a was 0.016 M, unless otherwise noted. Correlation
coefficient for the first-order plot. c The initial concentration of 2a
d
was 0.032 M. The reaction was examined in the presence of 0.048
a
M of free FcCtCSiMe3 (1a ).
All reactions were carried out in benzene-d6. The initial
b
concentration of 3a was 0.016 M. Correlation coefficient for the
(3c) requires heating.4 Silylferrocenylacetylene (1a )
examined in this study possesses properties intermedi-
ate between those of Me3SiCtCSiMe3 (1b ) and
PhCtCSiMe3 (1c).
first-order plot.
Sch em e 1
Isom er iza tion of tr a n s-Rh Cl{dCdC(F c)(SiMe3)}-
(P P r i
)
(3a ) t o tr a n s-R h Cl(F cCtCSiMe3)(P P r i
)
3
2
3
2
(2a ). Complex 3a was found to release the vinylidene
ligand as the original acetylene 1a . This reaction
occurred instantly in benzene-d6 at room temperature
under a CO atmosphere.5 On the other hand, in the
presence of free acetylenes, the reaction proceeded at
elevated temperatures (eqs 2 and 3). The reactions of
The rates of the formation of 1a in the presence of a
variety of acetylenes are summarized in Table 2. Good
first-order kinetics was observed for all the reactions
examined. The reaction rate was little dependent upon
the concentration and the sort of acetylenes (entries
1-5), indicating the reaction process initiated by the
rate-determining isomerization of 3a to 2a , followed by
a rapid ligand displacement of the resulting 2a with
added acetylenes (Scheme 1). The Eyring plot of the
rate constants for the reactions with PhCtCPh (Table
2, entries 1 and 6-8) provided the following activation
parameters for the interconversion of 3a to 2a : ∆Hq )
22.7 ( 0.1 kcal mol-1, ∆Sq ) -8.6 ( 0.4 eu.
Relative stability of the three (â-silylvinylidene)-
rhodium complexes (3a -c) was examined by crossover
reactions of these complexes with the parent silylacety-
lenes (1a -c). Complex 3a (0.0113 mmol) and acetylene
1b (0.0154 mmol) were heated in benzene-d6 at 60 °C
for 1 day. 1H NMR analysis of the reaction solution
revealed the presence of 3a , 3b, 1a , and 1b in the ratio
0.10/3.48/3.71/1.54: K(3b/3a ) ) [3b][1a ]/[3a ][1b] ) 83.8
(∆G°333 ) -2.9 kcal mol-1) (eq 4). Almost the same
equilibrium constant was obtained by starting from 3b
and 1a in place of 3a and 1b, respectively. Similar sets
3a with 10 equiv of internal acetylenes (PhCtCPh (1d )
and MeO2CCtCCO2Me (1e)) in benzene-d6 at 60 °C
provided 1a and the corresponding acetylene complexes
(2d and 2e, respectively) in quantitative yields (eq 2).
The quantitative formation of 1a was also observed
when complex 3a was treated with 10 equiv of silyl and
terminal acetylenes (PhCtCSiMe3 (1c) and PhCtCH
(1f)) at 60 °C (eq 3). These reactions afforded the
vinylidene complexes trans-RhCl{dCdC(Ph)(SiMe3)}-
(PPri ) (3c) and trans-RhCl{dCdC(H)(Ph)}(PPri ) (3f),
3 2
3 2
respectively.
(4) We confirmed the first-order rate constant for the conversion of
2c to 3c to be 3.13 × 10-4 s-1 (∆Gq ) 24.9 kcal mol-1) at 60 °C in
toluene.
(5) The CO-induced formation of acetylene from the corresponding
vinylidene complex has been reported: (a) Werner, H.; Stark, A.;
Steinert, P.; Gru¨nwald, C.; Wolf, J . Chem. Ber. 1995, 128, 49. (b)
Rappert, T.; Nu¨rnberg, O.; Mahr, N.; Wolf, J .; Werner, H. Organome-
tallics 1992, 11, 4156.

4644 Organometallics, Vol. 15, No. 21, 1996
Notes
Sch em e 2
of experiments were performed between 3a and 3c, and
between 3b and 3c, and the following equilibrium
constants were estimated: K(3a /3c) ) 2.1 (∆G°333
)
-0.49 kcal mol-1), K(3b/3c) ) 142 (∆G°333 ) -3.3 kcal
mol-1). Clearly, the thermodynamic stability of vinyl-
idene complexes increases in the order 3c < 3a < 3b.
Isom er iza tion Mech a n ism . We could measure the
activation parameters for both the forward and back-
ward reactions between silylacetylene and â-silylvi-
nylidene complexes (2a and 3a , respectively). Scheme
2 illustrates the reaction coordinate. The simple kinetic
features and the magnitude of activation entropies (-4.8
and -8.6 eu) observed in this study are in good accord
with an isomerization mechanism with least motion;
namely, the 1,2-SiMe3 group shifts in a concerted
fashion.6 This type of mechanism has been proposed
independently by Werner and Connelly3a,b and also
predicted by Hoffmann using the EHMO calculations.7
It has been noted that the stability of (â-silylvi-
nylidene)rhodium(I) complexes, trans-RhCl{dCdC(R)-
{dCdC(R)(SiMe3)}(PPri )2 (R ) SiMe3 (3b), Ph (3c))3a were
prepared according to the literature. All other compounds
used in this study were obtained from commercial sources and
used without further purification.
3
P r ep a r a tion of tr a n s-Rh Cl(F cCtCSiMe3)(P P r i3)2 (2a ).
To a solution of [RhCl(PPri )2]2 (30 mg, 33 mmol) in toluene-
3
(SiMe3)}(PPri ) , increases as the R substituent becomes
3 2
d8 (1 mL) was added FcCtCSiMe3 (1a ; 19 mg, 67 mmol) at
-40 °C. A part of the solution (ca. 0.6 mL) was transferred
into an NMR sample tube by means of a cannula at the same
temperature. NMR analysis of the solution at -20 °C revealed
the quantitative formation of 2a . 1H NMR (C6D5CD3, -20
°C): δ 4.40 (apparent triplet, J ) 2.0 Hz, 2H, two H of C5H4),
3.98 (s, 5H, C5H5), 3.78 (apparent triplet, J ) 2.0 Hz, 2H, two
H of C5H4), 2.38 (m, 6H, PCHCH3), 1.28 (doublet of virtual
more electron-donating (R ) SiMe3 > Fc > Ph). Since
it is generally assumed that the transition metal-
vinylidene bond is stabilized by π-back-donation from
the filled d orbital to the empty p orbital on the
R-vinylidene carbon,2,7 the stability order observed in
this study is the reverse of the general assumption.
Although we do not have an exact explanation for this
unexpected observation, the particularly high stability
of the bis(trimethylsilyl)vinylidene complex 3b may be
attributed in part to the great stabilization effect of the
SiMe3 groups via (σ-p)π-hyperconjugation.8
3
triplets, J HH ) 6.6 Hz, J ) 6.8 Hz,12 18H, PCHCH3), 1.20
3
(doublet of virtual triplets, J HH ) 6.6 Hz, J ) 6.5 Hz,12 18H,
PCHCH3), 0.41 (s, 9H, SiCH3). 13C{1H} NMR (C6D5CD3, -20
°C): δ 105.1 (dt, 1J RhC ) 18 Hz, 2J PC ) 3 Hz, Me3SiCtC), 76.1
1
(d, J RhC ) 14 Hz, FcCtC), 72.0 and 70.7 (both s, C5H4), 69.8
(s, C5H5), 68.2 (s, ipso C of C5H4), 22.2 (virtual triplet, J ) 9
Hz,12 PCHCH3), 21.0 (s, PCHCH3), 19.5 (s, PCHCH3), 2.5 (s,
Exp er im en ta l Section
1
SiCH3). 31P{1H} NMR (C6D5CD3, -20 °C): δ 31.6 (d, J RhP
)
Gen er a l P r oced u r e a n d Ma ter ia ls. All manipulations
were carried out under an argon atmosphere using conven-
tional Schlenk techniques. Argon gas was dried by passage
through P2O5 (Merck, SICAPENT). NMR spectra were re-
corded on a J EOL J NM-A400 spectrometer (1H NMR, 399.65
MHz; 13C NMR, 100.40 MHz; 31P NMR, 161.70 MHz). Chemi-
cal shifts are reported in δ (ppm) referenced to an internal
SiMe4 standard for 1H and 13C NMR and to an external 85%
H3PO4 standard for 31P NMR.
119 Hz).
P r ep a r a tion of tr a n s-Rh Cl{dCdC(F c)(SiMe3)}(P P r i
)
3
2
(3a ). The complex [RhCl(PPri )2]2 (328 mg, 367 mmol) was
3
placed in a Schlenk tube and dissolved in toluene (20 mL) at
room temperature. FcCtCSiMe3 (1a ; 214 mg, 758 mmol) was
added at -20 °C, and the solution was stirred at room
temperature for 2 h and then concentrated to dryness under
reduced pressure. The resulting solid was dissolved in toluene
(ca. 5 mL) at room temperature; this solution was diluted with
hexane (ca. 7 mL) and allowed to stand at -70 °C overnight
to give violet crystals of 3a (381 mg, 67%). 1H NMR (C6D6,
room temp): δ 4.12 (apparent triplet, J ) 2.0 Hz, 2H, two H
of C5H4), 4.04 (s, 5H, C5H5), 3.97 (apparent triplet, J ) 2.0
Hz, 2H, two H of C5H4), 2.79-2.71 (m, 6H, PCHCH3), 1.36
Toluene, benzene, and hexane were dried over sodium
benzophenone ketyl and distilled just before using. Benzene-
d6 and toluene-d8 were dried over LiAlH4, vacuum-transferred,
and stored under an argon atmosphere. Triisopropylphos-
phine,9 [RhCl(PPri )2]2,10 FcCtCSiMe3 (1a ),11 and trans-RhCl-
3
3
(doublet of virtual triplets, J HH ) 6.6 Hz, J ) 7.1 Hz,12 18H,
(6) The values of activation entropy are in the typical range for 1,2-
sigmatropic shifts of silyl groups in pure organic systems (∆Sq ) -9.2
to -3.8 eu): Spangler, C. W. Chem. Rev. 1976, 76, 187.
(7) Silvestre, J .; Hoffmann, R. Helv. Chim. Acta 1985, 68, 1461.
(8) There is a possibility that σ-donation from the vinylidene ligand
to the rhodium center predominates over the π-back-donation in the
present 16-electron complexes. In this case, the vinylidene complex
becomes more stable as the donating ability of the R substituent
increases, consistent with the experimental observations. We thank a
reviewer for bringing this possibility to our attention.
PCHCH3), 1.34 (doublet of virtual triplets, 3J HH ) 6.6 Hz, J )
7.1 Hz,12 18H, PCHCH3), 0.44 (s, 9H, SiCH3). 13C{1H} NMR
1
2
(C6D6, room temp): δ 284.5 (dt, J RhC ) 61 Hz, J PC ) 15 Hz,
(10) Werner, H.; Wolf, J .; Ho¨hn, A. J . Organomet. Chem. 1985, 287,
395.
(11) Doisneau, G.; Balavoine, G.; Fillebeen-Khan, T. J . Organomet.
Chem. 1992, 425, 113.
(9) Cowley, A. H.; Mills, J . L. J . Am. Chem. Soc. 1969, 91, 2915.
(12) Apparent coupling constant for the virtual triplet signal.

Notes
Organometallics, Vol. 15, No. 21, 1996 4645
2
3
CR), 98.2 (dt, J RhC ) 15 Hz, J PC ) 6 Hz, Câ), 70.4 and 69.2
(both s, C5H4), 66.6 (s, C5H5), 66.1 (s, ipso C of C5H4), 24.5
(virtual triplet, J ) 9 Hz,12 PCHCH3), 21.1 and 20.9 (both s,
PCHCH3), 1.8 (s, SiCH3). 31P{1H} NMR (C6D6, room temp):
tube, which was sealed under vacuum. The sample was placed
in an NMR sample probe controlled to 60.0 ( 0.1 °C and
examined by 1H NMR spectroscopy. The amount of FcCt
CSiMe3 (1a ) produced with time was determined by measuring
the relative peak integration of the methyl signals of 1a (δ
0.24) and MeSiPh3 (δ 0.73). After completion of the reaction
the sample solution was analyzed by 31P{1H} NMR spectros-
1
δ 42.4 (d, J RhP ) 136 Hz). Anal. Calcd for C33H60ClFeP2-
RhSi: C, 53.48; H, 8.16. Found: C, 53.56; H, 8.02.
Kin etic Stu d ies on th e Isom er iza tion of 2a to 3a .
A
solution of 2a (ca. 0.6 mL), which was prepared from [RhCl-
copy, showing the selective formation of trans-RhCl(PhCtCPh)-
1
(PPri )2]2 (30 mg, 33 µmol) and FcCtCSiMe3 (1a ; 19 mg, 67
(PPri )2 (δ 32.7; d, J RhP ) 116 Hz).13
3
3
µmol) in toluene (1 mL) at -40 °C, was placed in an NMR
sample tube. The sample was placed in an NMR sample probe
controlled to 15.0 ( 0.1 °C and examined by 31P{1H} NMR
spectroscopy. The time course of the reaction was followed
by measuring the relative peak integration of the signals of
2a (δ 31.6) and 3a (δ 42.4).
Kin etic Stu d ies on th e Rea ction s of 3a w ith Acety-
len es. A typical procedure is as follows. Complex 3a (11.7
mg, 16 µmol), PhCtCPh (28.5 mg, 0.16 mmol), and MeSiPh3
(8.3 mg, 30 µmol) were placed in a Schlenk tube and dissolved
in benzene-d6 (1 mL) at room temperature. A part of the
solution (ca. 0.6 mL) was transferred into an NMR sample
Ack n ow led gm en t. This work was supported by a
Grant-in-Aid for Scientific Research on Priority Area of
Reactive Organometallics (No. 07216261) from the
Ministry of Education, Science, Sports and Culture of
J apan. Partial financial support from Asahi Glass
Foundation is gratefully acknowledged.
OM9603575
(13) Wang, K.; Goldman, A. S.; Li, C.; Nolan, S. P. Organometallics
1995, 14, 4010.