(η3-α-Silabenzyl)tungsten complexes: An isolable intermediate for interconversion between a silylene complex and a silyl complex through 1,2-aryl migration

Article abstract of DOI:10.1021/om4000105

η³-α-Silabenzyl complexes Cp W(CO)₂{η ³(Si,C,C)-Si(p-Tol)₂R} (1a, R = Me; 1b, R = p-Tol; Cp* = η⁵-C₅Me₅, p-Tol = p-tolyl), the first silicon analogues of η³-benzyl complexes, were synthesized by abstraction of DMAP (4-(dimethylamino)pyridine) with BPh₃ either from (aryl)(DMAP·silylene)tungsten complexes 2a,b at room temperature or from (arylsilyl)(DMAP)tungsten complexes 3a,b under irradiation. Complex 1 was demonstrated to be a key intermediate for the interconversion between 2 and 3 and to serve as a synthetic equivalent for both base-free silylene complexes and coordinatively unsaturated silyl complexes.

Full text of DOI:10.1021/om4000105

Communication
pubs.acs.org/Organometallics
(η3‑α-Silabenzyl)tungsten Complexes: An Isolable Intermediate for
Interconversion between a Silylene Complex and a Silyl Complex
through 1,2-Aryl Migration
Eiji Suzuki, Takashi Komuro, Yuto Kanno, and Hiromi Tobita*
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
S
* Supporting Information
ABSTRACT: η3 -α-Silabenzyl complexes Cp*W-
(CO)2{η3(Si,C,C)-Si(p-Tol)2R} (1a, R = Me; 1b, R = p-Tol;
Cp* = η5-C5Me5, p-Tol = p-tolyl), the first silicon analogues of
η3-benzyl complexes, were synthesized by abstraction of
DMAP (4-(dimethylamino)pyridine) with BPh3 either from
(aryl)(DMAP·silylene)tungsten complexes 2a,b at room
temperature or from (arylsilyl)(DMAP)tungsten complexes
3a,b under irradiation. Complex 1 was demonstrated to be a
key intermediate for the interconversion between 2 and 3 and
to serve as a synthetic equivalent for both base-free silylene complexes and coordinatively unsaturated silyl complexes.
1,2-Group migration between transition-metal and silicon
atoms in metal−silicon complexes is considered to play
important roles in metal-mediated scrambling of substituents
on organosilanes.1,2 A typical example is the interconversion
between silyl complexes and silylene complexes. Thus, 1,2-
migration of a group on the silicon atom to the metal center in
a silyl complex affords a silylene complex and vice versa. In
particular, 1,2-migration of a hydrocarbyl group between the
metal center and the silicon atom is attractive because the
reaction involves intramolecular activation or formation of a
chemically robust Si−C bond3 and therefore can be applied to
the synthesis of organosilicon compounds. However, this type
of 1,2-migration is still limited,2 and its detailed mechanism is
largely uncertain. To elucidate the mechanism and develop new
reactions involving the 1,2-migration, the most effective
approach is the isolation and structure determination of the
reaction intermediate. We now report the isolation and
characterization of an intermediate in the 1,2-migration of an
aryl group.
(CO)2(DMAP)(SiR2Ar) (3a, R2 = Me(p-Tol), Ar = p-Tol; 3b,
R = Ar = p-Tol; 3c, R = Me, Ar = Ph; 3d, R = Me, Ar = p-Tol)
through 1,2-migration of an aryl group from tungsten to silicon.
On the other hand, photoirradiation to silyl complexes 3a−d
induces the reverse reaction to reproduce silylene complexes
2a−d.4b Kinetic study on the thermal isomerization of 2a−d to
3a−d revealed its large positive entropy of activation, implying
that the rate-determining step involves dissociation of DMAP
from 2a−d.4b Therefore, to generate and isolate the DMAP-
free intermediate in the interconversion, we examined the
abstraction of DMAP from 2a,b with BPh3, which is known as
an effective Lewis acid for abstraction of pyridine.5
Reaction of silylene complexes 2a,b with BPh3 in toluene at
room temperature led to abstraction of DMAP in 2a,b to give
the first η3-α-silabenzyl complexes Cp*W(CO)2{η3(Si,C,C)-
Si(p-Tol)2R} (1a, R = Me; 1b, R = p-Tol) and DMAP·BPh3
(Scheme 1). The byproduct DMAP·BPh3 was removed by
precipitation in hexane. Product 1a was obtained as a brown oil
(crude yield: 89%), while 1b was isolated as reddish brown
crystals in 43% yield.
A single-crystal X-ray analysis revealed that 1b adopts a
three-legged piano-stool structure having an unprecedented η3-
α-silabenzyl ligand that is a silicon analogue of the η3-benzyl
ligand (Figure 1). The η3-α-silabenzyl ligand in 1b takes an exo
geometry. The W−Si bond distance (2.5257(12) Å) lies
between those of silylene complex 2a (2.505(4) Å)4b and silyl
complex 3b (2.6110(13) Å)4b and is slightly shorter than that
of a related (η3-1-silaallyl)tungsten complex Cp*W-
(CO)2{η3(Si,C,C)-Me2SiCHCMe2} (2.571(1) Å).6 Two W−
C(aryl) distances (W−C3 = 2.416(4) Å and W−C4 = 2.564(4)
Å) are similar to the corresponding distances of the (η3-
We recently found the interconversion between (aryl)-
(base·silylene)tungsten complexes and (arylsilyl)(base)tung-
sten complexes via reversible 1,2-aryl migration (eq 1).4 Thus,
the DMAP-stabilized aryl silylene complexes Cp*W(CO)2(Ar)-
(SiR2·DMAP) (2a, R2 = Me(p-Tol), Ar = p-Tol; 2b, R = Ar
= p-Tol; 2c, R = Me, Ar = Ph; 2d, R = Me, Ar = p-Tol)
thermally isomerize to arylsilyl DMAP complexes Cp*W-
Received: January 8, 2013
Published: January 24, 2013
© 2013 American Chemical Society
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Communication
Scheme 1. Formation and Reactions of (η3-α-
Silabenzyl)tungsten Complexes 1a,b
bond character at the Si−C3 bond. The sum of the bond angles
around the silicon atom (341.8(3)°) is similar to that of the η3-
1-silaallyl complex Cp*W(CO)2{η3(Si,C,C)-Me2SiCHCMe2}
(346.3°)6 and lies midway between the tetrahedral (329°)
and trigonal (360°) valence angles, implying some contribution
of sp2 hybridization at the silicon atom. On the basis of these
observations, complex 1b can be drawn as a resonance of two
canonical forms: the η2-arene silyl form A and the aryl η2-silene
form B (Scheme 2).
Scheme 2. Two Canonical Forms of 1b
The NMR spectroscopic data for 1a,b are consistent with
their molecular structures having the η3-α-silabenzyl ligand. The
1H NMR spectrum of 1b at room temperature shows two sets
of two doublet signals for ortho and meta aromatic protons of
the p-tolyl groups with 1:2 intensity ratio at 6.73 (ortho), 6.44
(meta) ppm and 7.80 (ortho), 7.03 (meta) ppm.11 The first
two signals are shifted upfield in comparison with the last two
signals and with those of silyl complex 3b (7.05−8.26 ppm).4b
This upfield shift is attributable to the η2 coordination of the
aryl ring to the metal. A similar upfield shift of signals of
aromatic protons was observed in the (η3-benzyl)tungsten
complex Cp*W(CO)2{η3(C,C,C)-C(p-Tol)H(BEt2)} (5.49−
6.76 ppm).7 Each of the 13C{1H} NMR spectra of 1a,b
shows one characteristic signal assignable to the carbon bound
to silicon in the coordinating p-tolyl group of the silabenzyl
ligand at 70.2 (1a) and 69.6 ppm (1b). The signal is shifted
upfield in comparison with the corresponding carbon of the
(η3-benzyl)tungsten complex CpW(CO)2{η3(C,C,C)-C(p-
Tol)H(BEt2)} (84.8 ppm).7 The 29Si{1H} NMR spectra of
1a,b exhibit signals at 8.8 (1a) and 9.5 ppm (1b) with 183W
satellites (1JWSi = 18 (1a) and 21 Hz (1b)). The chemical shifts
of these signals lie between those of silyl complexes 3a,b (17.8
(3a) and 24.3 ppm (3b))4b and that of the (η2-silene)tungsten
complex Cp2W{η2(Si,C)-Me2Si=CH2} (−15.7 ppm).10a This is
attributable to some contribution of the aryl η2-silene form B of
1b and also the corresponding form of 1a. Similar 29Si chemical
shifts have been reported for the η3-1-silaallyl complexes
Cp*W(CO)2{η3(Si,C,C)-Me2SiCHCR2} (13.8 (R = H) and 8.8
ppm (R = Me)).6
Figure 1. Molecular structure of 1b (50% probability ellipsoids).
Hydrogen atoms have been omitted for clarity. Selected bond
distances (Å) and angles (deg): W−Si = 2.5257(12), W−C1 =
1.949(6), W−C2 = 1.926(5), W−C3 = 2.416(4), W−C4 = 2.564(4),
Si−C3 = 1.834(5), Si−C10 = 1.867(5), Si−C17 = 1.867(5), C3−C4 =
1.429(6), C3−C8 = 1.451(7), C4−C5 = 1.430(7), C5−C6 =
1.359(7), C6−C7 = 1.427(7), C7−C8 = 1.355(7); Si−W−C1 =
109.54(16), Si−W−C2 = 65.25(15), Si−W−C3 = 43.50(11), Si−W−
C4 = 65.28(11), C1−W−C2 = 83.8(2), C1−W−C3 = 102.31(18),
C1−W−C4 = 70.17(19), C2−W−C3 = 106.60(18), C2−W−C4 =
110.17(18), C3−W−C4 = 33.19(15), W−Si−C3 = 65.07(14), W−
Si−C10 = 119.02(15), W−Si−C17 = 124.92(16), C3−Si−C10 =
118.9(2), C3−Si−C17 = 114.5(2), C10−Si−C17 = 108.4(2), Si−C3−
C4 = 114.0(4).
The NMR spectra also demonstrate the existence of some
dynamic behaviors of 1a,b in solution. As mentioned above,
only two doublets of the η2-coordinated aryl ring protons in 1b
1
benzyl)tungsten complex Cp*W(CO)2{η3(C,C,C)-C(p-Tol)H-
(BEt2)} (W−Cβ = 2.374(9) Å and W−Cγ = 2.552(9) Å),7
indicating the coordination of the two aromatic carbons C3 and
C4 to the tungsten center in 1b. Bond alternation is observed
for the C−C bonds in the aryl ring other than the coordinated
C3−C4 bond (C4−C5 = 1.430(7) Å, C5−C6 = 1.359(7) Å,
C6−C7 = 1.427(7) Å, C7−C8 = 1.355(7) Å). Similar bond
alternation was also reported for η3-benzyl complexes.7,8 The
Si−C3 bond distance (1.834(5) Å) is slightly shorter than the
other two Si−C bonds (1.867(5) Å) and lies between typical
Si−C single-bond distances (1.86−1.91 Å)9 and coordinated
SiC double-bond distances in η2-silene complexes (1.78(2)−
1.810(6) Å).10 This indicates the existence of some double-
were observed in the H NMR spectrum at room temperature.
This observation suggests that the fast exchange between ortho
positions and between meta positions is achieved by facile
migration of the tungsten fragment between two η2-C−C
coordination sites in the p-tolyl group of the silabenzyl ligand.
In the 13C NMR spectrum of 1b, only one signal for two
carbonyl ligands is observed at 227.3 ppm. Only one set of the
signals of two uncoordinated p-tolyl groups of 1b was also
observed in both the 1H and 13C NMR spectra. These
observations indicate that the dynamic behavior of 1b involves
exchange between the CO ligands and between the
uncoordinated p-tolyl groups. Low-temperature NMR spectro-
scopic measurements showed that this dynamic behavior of 1b
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Communication
is very fast. Thus, upon cooling of a toluene-d8 solution of 1b,
the H NMR spectra (700 MHz) exhibited broadening of the
silicon atom or by photoinduced dissociation of DMAP in 3a,b
to generate coordinatively unsaturated arylsilyl complexes D
followed by coordination of a p-tolyl group on silicon to
tungsten.
1
four ArH signals and the methyl signal of the uncoordinated p-
tolyl substituents. Especially, the o-ArH signal (6.65 ppm at
room temperature) broadened most significantly and finally
disappeared at −70 °C. Interestingly, in the 1H NMR spectrum
of 1a, four upfield-shifted doublets assignable to the aromatic
protons in the η2-coordinated p-tolyl group are observed at
6.16, 6.40, 6.50, and 6.73 ppm at room temperature. Since a fast
exchange similar to that in 1b is also expected for 1a, this
observation indicates that two ortho and two meta positions in
1a are kept inequivalent, respectively, during the fast exchange.
This is attributable to the existence of two different substituents
on the silicon atom and the absence of rotation around the
coordinated Si−C bond.
Complexes 1a,b are considered to serve as synthetic
equivalents for both reactive silyl and silylene complexes. A
tentative study of the reaction of 1b with acetonitrile gave the
N-silyliminoacyl complex Cp*W(CO)2{η2(C,N)-C(Me)NSi-
(p-Tol)3} (4)12 in 55% yield (Scheme 1). This reaction
apparently proceeds through a nitrile silyl complex via
dissociation of the η2-coordinated p-tolyl group. This is in
contrast with the aforementioned reaction of 1a,b with DMAP,
where silylene complexes 2a,b were quantitatively produced.
This is probably attributable to the difference in the Lewis
basicity and coordinating ability of the substrates and also due
to the smaller steric hindrance around the silicon atom in
comparison with the metal center in 1a,b. Thus, the DMAP
molecule, which is a stronger Lewis base and is bulkier than
acetonitrile, is likely to attack at the silylene silicon atom. These
results demonstrate the potential utility of η3-α-silabenzyl
complexes in general as the synthetic equivalents for reactive
transition-metal complexes.
On the basis of the aforementioned observations, we suggest
that a possible mechanism of the dynamic behavior involves
ring slippage of the coordinated p-tolyl group on the
Cp*W(CO)2 moiety without rotation around the coordinated
Si−C bond.
Reaction of η3-α-silabenzyl complexes 1a,b with DMAP in
toluene produced silylene complexes 2a,b almost quantitatively
(Scheme 1). Since 2a,b are converted into silyl complexes 3a,b
by thermal reactions (eq 1),4 2a,b and 3a,b can be regarded as
the kinetically controlled products and the thermodynamically
controlled products, respectively, of the reactions of 1a,b with
DMAP. On the other hand, photoirradiation of a mixture of
silyl complexes 3a,b and BPh3 in C6D6 led to the formation of
silabenzyl complexes 1a,b as the main product (Scheme 1).
This reaction possibly proceeds through (1) dissociation of the
DMAP molecule in 3a,b by photoirradiation, (2) reaction of
DMAP with BPh3 to give DMAP·BPh3, and (3) coordination of
a p-tolyl group on silicon to tungsten. These results strongly
support that 1a,b are important and isolable intermediates in
the interconversion between 2a,b and 3a,b via 1,2-aryl
migration. Thus, as illustrated in Scheme 3, intermediates
1a,b in this interconversion are formed by thermal dissociation
of DMAP in 2a,b to generate base-free aryl silylene complexes
C followed by 1,2-migration of the p-tolyl ligand to the silylene
In conclusion, we newly synthesized and characterized (η3-α-
silabenzyl)tungsten complexes 1a,b with an unusual η3(Si,C,C)
coordination of the silabenzyl ligand. Complexes 1a,b were
demonstrated to be key intermediates in the interconversion
between DMAP-stabilized aryl silylene complexes 2a,b and
arylsilyl DMAP complexes 3a,b through 1,2-aryl migration. The
study of the reactions of complexes 1a,b with nitrogen-
containing substrates, i.e., DMAP and MeCN, revealed that
1a,b serve as synthetic equivalents for both base-free silylene
complexes and coordinatively unsaturated silyl complexes.
Further investigations into the reactivity of 1a,b toward other
organic molecules are in progress.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text and figures giving synthetic procedures and character-
ization data for 1a,b and a table and a CIF file giving X-ray
crystallographic data for 1b. This material is available free of
charge via the Internet at http://pubs.acs.org.
Scheme 3. Probable Mechanism of the Interconversion
between Aryl Silylene Complexes 2a,b and Arylsilyl DMAP
Complexes 3a,b through η3-α-Silabenzyl Complexes 1a,b
AUTHOR INFORMATION
Corresponding Author
*E-mail: tobita@m.tohoku.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grants-in-Aid for Scientific
Research (Nos. 22350024, 20750040, and 23750053) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy of Japan. We are grateful to Mr. Shinichiro Yoshida
(Tohoku University) for his help with the low-temperature
NMR spectroscopic measurements of 1b. We also acknowledge
the Research and Analytical Center for Giant Molecules,
Tohoku University, for spectroscopic measurements and
elemental analysis.
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■
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1
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