Direct Conversion of a Si-C(aryl) Bond to Si-Heteroatom Bonds in the Reactions of η3-α-Silabenzyl Molybdenum and Tungsten Complexes with 2-Substituted Pyridines

Article abstract of DOI:10.1021/acs.organomet.5b00335

η³-α-Silabenzyl complexes CpM(CO)₂-{η³(Si,C,C)-Si(p-Tol)₃} (M = Mo (1-Mo), W (1-W)) reacted with 2-substituted pyridines NC₅H₄(2-EH_n) (E = O, S (n = 1); N (n = 2)) under mild conditions to give M-Si-E-C-N (E = O, N), W-Si-N-C-S, and M-E-C-N (E = S, N) metallacycles depending on the metal M or heteroatom E. These three kinds of metallacycles were characterized by spectroscopy, elemental analysis, and X-ray crystallography. The first two silametallacycles take on some silylene complex character and are considered to form via (aryl)silylene complex intermediates generated by cleavage of the Si-C(aryl) bond in the η³-α-silabenzyl ligand of 1-Mo and 1-W. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00335

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Direct Conversion of a Si−C(aryl) Bond to Si−Heteroatom Bonds
in the Reactions of η3‑α-Silabenzyl Molybdenum and Tungsten
Complexes with 2‑Substituted Pyridines
Yuto Kanno, Takashi Komuro, 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*M(CO)2-
{η3(Si,C,C)-Si(p-Tol)3} (M = Mo (1-Mo), W (1-W)) reacted
with 2-substituted pyridines NC5H4(2-EHn) (E = O, S (n = 1);
N (n = 2)) under mild conditions to give M−Si−E−C−N
(E = O, N), W−Si−N−C−S, and M−E−C−N (E = S, N)
metallacycles depending on the metal M or heteroatom E. These
three kinds of metallacycles were characterized by spectroscopy,
elemental analysis, and X-ray crystallography. The first two
silametallacycles take on some silylene complex character and
are considered to form via (aryl)silylene complex intermediates
generated by cleavage of the Si−C(aryl) bond in the η3-α-
silabenzyl ligand of 1-Mo and 1-W.
Scheme 1. 1,2-Aryl Migration in the Reaction of
INTRODUCTION
■
(η3-α-Silabenzyl)tungsten Complexes with DMAP
Activation of robust Si−C(aryl) bonds induced by transition-
metal complexes is of considerable interest because this can be
applied to the direct functionalization of arylsilanes.1 Many
catalytic and stoichiometric reactions via Si−C(aryl) bond activa-
tion have been developed using late-transition-metal complexes.1,2
In these reactions, oxidative addition of Si−C(aryl) bonds to
low-valent metal centers plays a crucial role.2 In contrast, the
counterparts using early-transition-metal complexes have received
less attention.
In addition to the oxidative addition, 1,2-migration of aryl
substituents on silicon to metal in coordinatively unsaturated
arylsilyl complexes producing (aryl)silylene complexes has
been proposed as a crucial process for a few Si−C(aryl) bond
activation reactions.2b,3 We have recently found a reversible
1,2-aryl migration in tungsten−silicon complexes4 and finally
succeeded in isolation of key intermediates of this migration re-
action, i.e. η3-α-silabenzyl complexes Cp*W(CO)2{η3(Si,C,C)-
Si(p-Tol)2R} (R = p-Tol (1-W), Me), the first silicon analogues
of η3-benzyl complexes.5a We also found that the reaction of
these complexes with 4-(dimethylamino)pyridine (DMAP) gave
DMAP-stabilized (aryl)silylene complexes Cp*W(CO)2(p-
Tol){Si(p-Tol)R·DMAP} via Si−C(aryl) bond cleavage
(1,2-aryl migration) (Scheme 1). This result prompted us to
examine the reactions of the η3-α-silabenzyl tungsten complex
1-W and its molybdenum analogue 1-Mo5b with 2-substituted
pyridines NC5H4(2-EHn) (E = O, S (n = 1); E = N (n = 2)).
These substrates bear multiple nucleophilic heteroatoms
(N and E) that can attack the electrophilic silicon atom as well as
protic hydrogen(s) on E that can protonate the metal center. As a
result, we developed unique conversion reactions via Si−C(aryl)
bond cleavage and arene elimination to give silametallacycles.
We report here the direct conversion of a Si−C(aryl) bond to
Si−heteroatom bonds starting from 1-W and 1-Mo.
RESULTS AND DISCUSSION
■
Reactions of η3-α-Silabenzyl Complexes 1-Mo and 1-W
with 2-Substituted Pyridines. 1-Mo and 1-W reacted with
2-substituted pyridines (2-hydroxypyridine, 2-mercaptopyridine,
and 2-aminopyridine) to give three kinds of metallacycles 2−6
depending on the metal and the 2-EHn substituent (routes
A−C in Scheme 2). Complexes 1-Mo and 1-W reacted with
2-hydroxypyridine at room temperature in C6D6 to give the
M−Si−O−C−N five-membered-ring complexes Cp*M-
(CO)2{κ2(Si,N)-Si(p-Tol)2(OC5H4N)} (2-Mo, M = Mo; 2-W,
M = W) in 76% and 57% NMR yields, respectively (route A in
Scheme 2). 2-Mo and 2-W were isolated in 58% and 24% yields,
respectively, by the same reactions conducted in toluene.
Reaction of molybdenum complex 1-Mo with 2-aminopyridine
in toluene at 40 °C also gave the similar Mo−Si−N−C−N
Received: April 9, 2015
© XXXX American Chemical Society
A
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Scheme 2. Reactions of η3-α-Silabenzyl Complexes 1-Mo and
1-W with 2-Substituted Pyridines
Figure 1. Crystal structure of 2-W. Selected bond distances (Å) and
angles (deg): W−Si 2.5072(10), W−N 2.241(4), Si−O3 1.742(3),
O3−C3 1.321(6), N−C3 1.363(6); Si−W−N 71.32(9), W−Si−O3
99.32(10), W−Si−C8 125.06(14), W−Si−C15 119.45(13), C8−Si−C15
105.21(18), W−N−C3 121.3(3), O3−C3−N 118.8(4).
and Figure S2 (Supporting Information), respectively), revealing
that these complexes have a five-membered chelate ring
formed by κ2(Si,N) coordination of a 2-pyridyloxysilyl or
2-pyridylaminosilyl ligand. The Mo−Si bond distances of 2-Mo
(2.5038(10) Å) and 3 (2.5243(15) Å)7 and the W−Si bond
distance of 2-W (2.5072(10) Å) are much shorter than the
corresponding M−Si bond distances of silyl complexes
Cp*M(CO)2(DMAP){Si(p-Tol)3} (2.6218(10) Å (M = Mo)5b
and 2.6110(13) Å (M = W)4c). On the other hand, the M−Si
distances of 2 and 3 are slightly longer than or are in the range
of those of usual base-stabilized silylene complexes (Mo−Si,
2.4439(14)−2.5008(9) Å; W−Si, 2.445(2)−2.573(4) Å).8
The sums of the bond angles among the three bonds except
for the Si−O or Si−N bond around the silicon atoms of 2-Mo,
2-W, and 3 (350.0(4), 349.7(4), and 344.9(5)°, respectively) are
nearly intermediate between the tetrahedral (329°) and trigonal
(360°) angles. These features of bond distances and angles
indicate that the silicon moieties in 2 and 3 take on some silylene
ligand character.
five-membered-ring complex Cp*Mo(CO)2[κ2(Si,N)-
Si(p-Tol)2{N(H)C5H4N}] (3) in 50% isolated yield (route A
in Scheme 2).
1
Monitoring of these reactions by H NMR spectroscopy
showed the formation of toluene together with 2 or 3 (see the
Experimental Section). This indicates that the η2-coordinated
p-tolyl moiety in the η3-α-silabenzyl ligand of 1 is eliminated as
a toluene molecule via Si−C(p-Tol) bond cleavage followed by
C−H reductive elimination from the metal center. Interestingly,
in contrast to these reactions resulting in the formation of
molybdenum complexes 2-Mo and 3 via Si−C(aryl) bond
cleavage, we have recently reported that the reaction of 1-Mo
with DMAP afforded the DMAP-coordinated silylmolybdenum
complex Cp*Mo(CO)2(DMAP){Si(p-Tol)3} exclusively via
dissociation of the coordinated aromatic carbons in 1-Mo.5b
Reaction of tungsten complex 1-W with 2-mercaptopyridine
at room temperature in toluene afforded a different type of
silametallacycle, i.e. the W−Si−N−C−S five-membered-ring
complex Cp*W(CO)2{κ2(S,Si)-(SC5H4N)Si(p-Tol)2} (4), in
38% isolated yield via Si−C(p-Tol) bond cleavage (route B in
Scheme 2). NMR monitoring of the same reaction in C6D6
showed that 4 and toluene were formed as the main products in
54% and 63% NMR yields, respectively.6
On the other hand, reaction of molybdenum complex 1-Mo
with 2-mercaptopyridine at room temperature in toluene
gave the Mo−S−C−N four-membered-ring complex Cp*Mo-
(CO)2{κ2(S,N)-SC5H4N} (5) in 76% isolated yield (route C in
Scheme 2). NMR monitoring of the same reaction in C6D6
revealed that hydrosilane HSi(p-Tol)3 was formed quantita-
tively together with 5 (91% NMR yield). This clearly shows
that the η3-α-silabenzyl ligand of 1-Mo and the SH hydrogen of
the 2-mercaptopyridine were eliminated as HSi(p-Tol)3 via
Mo−Si bond cleavage. A similar four-membered-ring complex
was obtained by the reaction of tungsten complex 1-W with
2-aminopyridine at room temperature in C6D6: the W−N−C−N
four-membered-ring complex Cp*W(CO)2{κ2(N,N)-
NC5H4N(H)} (6) and HSi(p-Tol)3 were formed in 63% and
62% NMR yields, respectively, via W−Si bond cleavage (route C
in Scheme 2). Complex 6 was isolated in ca. 31% yield as a crude
product from a mixture of the same reaction in toluene.
The 29Si{1H} NMR spectra of molybdenum complexes 2-Mo
and 3 and tungsten complex 2-W show signals at 95.5, 65.4, and
92.3 ppm, respectively. These signals are shifted substantially
downfield in comparison with the corresponding signals of
silyl complexes Cp*M(CO)2(DMAP){Si(p-Tol)3} (34.7 ppm
(M = Mo)5b and 24.3 ppm (M = W)4c) and are shifted slightly
upfield in comparison with those of donor-stabilized bis(silylene)
molybdenum and tungsten complexes CpM(CO)2{(SiMe2)···
Do···(SiMe2)} (M = Mo, 117.6 (Do = OMe) and 80.5
(Do = NEt2) ppm; M = W, 99.3 (Do = OMe) ppm).9 These
observations also support the silylene complex character of
complexes 2 and 3. Thus, each of the structures of complexes 2
and 3 can be drawn as a resonance of two canonical forms:
(pyridine)silyl complex form A and base-stabilized (amido)-
silylene complex form B (Scheme 3). Similar resonance forms
Scheme 3. Two Possible Canonical Forms for Five-
Membered-Ring Complexes 2 and 3
Characterization of Metallacycles 2−6. The molecular
structures of 2-Mo, 2-W, and 3 were determined by X-ray
crystallography (Figure S1 (Supporting Information), Figure 1,
B
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have been proposed for several previously reported M−Si−O−
C−N and M−Si−N−C−N five-membered-ring complexes.10
On the other hand, X-ray crystal structure analysis revealed
that complex 4 has an unprecedented W−Si−N−C−S
five-membered chelate ring where the nitrogen of the
2-pyridinethiolato moiety is bound to the silicon atom (Figure 2).
1H NMR and IR spectroscopic data with those of the sample
synthesized by an alternative method: i.e., the reaction of
methyl(pyridine) tungsten complex Cp*W(CO)2(py)Me13 with
2-aminopyridine. Complex 6 prepared by the latter reaction was
1
characterized by H and 13C NMR, IR, and mass spectroscopy
and elemental analysis (see the Experimental Section).
Possible Reaction Pathways for the Formation of
Metallacycles 2−6. Possible formation mechanisms of 2−6
by the reactions of η3-α-silabenzyl complexes 1-Mo and 1-W
with 2-substituted pyridines are illustrated in Scheme 4.
Scheme 4. Possible Formation Mechanisms of Complexes
2−6
Figure 2. Crystal structure of 4. Selected bond distances (Å) and
angles (deg): W−S 2.4807(13), W−Si 2.4928(12), S−C3 1.728(5),
Si−N 1.886(4), N−C3 1.363(6); S−W−Si 75.07(4), W−S−C3
112.02(16), W−Si−C8 121.19(14), W−Si−C15 119.02(15), C8−
Si−C15 103.98(19), W−Si−N 107.37(12), S−C3−N 119.8(4).
The W−Si bond distance (2.4928(12) Å), which is slightly
shorter than that of 2-W (2.5072(10) Å), is comparable
with that of the related base-stabilized silylene complex
Cp*W(CO)2{κ2(Si,C)-SiMe2(NMe2C6H4)} (C) bearing a
W−Si−N−C−C five-membered ring (2.487(3) Å).4a The sum
of the bond angles among the three bonds except for the Si−N
bond around the silicon atom of 4 (344.2(4)°) is nearly at the
middle of the tetrahedral (329°) and trigonal (360°) angles.
The Si−N(py) bond distance of 4 (1.886(4) Å) is longer than
normal Si−N single-bond distances (1.71−1.75 Å)8 and is close
to the Si−N dative bond distance for the DMAP-stabilized
silylene tungsten complex Cp*W(CO)2(p-Tol){Si(p-Tol)-
Me·DMAP} (1.898(11) Å).4c The S−C3 bond distance of 4
(1.728(5) Å) is located at the longer end of the range of normal
SC double-bond distances (1.60−1.72 Å)8 while being nearly
at the shorter end of the range of normal S−C(sp2) single-bond
distances (1.71−1.76 Å).8 The W−S bond distance of 4
(2.4807(13) Å) is slightly shorter than that of the thiolato
tungsten complex CpW(CO)3{S(benzothiazolyl)} (2.500(3) Å).11
The 29Si{1H} NMR spectrum of 4 in C6D6 appears as a singlet at
103.0 ppm with 183W satellites (JWSi = 101 Hz), which is shifted
downfield in comparison with that of 2-W (92.3 ppm). The δSi
and JWSi values are close to those of base-stabilized silylene tungsten
complex C (115.0 ppm and JWSi = 103 Hz).4a According to the
aforementioned discussion on the crystal structure and the
29Si NMR data, 4 can be best described as a base-stabilized
silylene(thiolato) complex.
In the reactions of 1-Mo and 1-W with 2-hydroxypyridine
and of 1-Mo with 2-aminopyridine (route A in Scheme 2), the
oxygen atom of the pyridone tautomer of 2-hydroxypyridine
or the nitrogen atom of the pyridonimine tautomer of
2-aminopyridine attacks the sterically encumbered silicon atom
of 1 to give base-stabilized silylene complex intermediates D.14
A possible driving force of this nucleophilic attack is the high
affinity of electronegative oxygen and nitrogen for silicon.
Proton transfer from nitrogen to metal in D and C−H reductive
elimination of toluene then takes place to produce M−Si−E−
C−N five-membered-ring complexes 2 and 3 (route A in
Scheme 4).
In the reaction of tungsten complex 1-W with 2-mercapto-
pyridine (route B in Scheme 2), the nitrogen of 2-mercapto-
pyridine attacks the silicon atom of 1-W to give silylene
complex intermediate E. Alternative nucleophilic attack of the
sulfur atom of the thiopyridone tautomer14b of 2-mercaptopyr-
idine instead of the pyridine nitrogen is unlikely because of the
much lower affinity of sulfur to silicon in comparison with that
of nitrogen.15 Intermediate E is subsequently converted into
the W−Si−N−C−S five-membered-ring complex 4 via proton
transfer from sulfur to tungsten in E followed by toluene
elimination (route B in Scheme 4).
X-ray crystal structure analysis revealed that complex 5 adopts
a four-legged piano-stool geometry bearing a Mo−S−C−N
four-membered ring (Figure S3 in the Supporting Information).
The Mo−S (2.514(2) Å) and Mo−N (2.182(7) Å) bond
distances of 5 are similar to the corresponding bond distances
of the known Mo−S−C−N four-membered-ring complex
CpMo(CO)2{κ2(S,N)-SC5H4N} (2.5227(9) and 2.183(2) Å,
respectively).12 Complex 6 was identified by comparing the
On the other hand, in the reaction of molybdenum complex
1-Mo with 2-mercaptopyridine, the corresponding silylene
complex intermediate E (M = Mo) is thought to be less stable
than silyl complex intermediate F (M = Mo, E = S). This low
C
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Spectroscopic Measurements. 1H, 13C{1H}, and 29Si{1H} NMR
spectra were recorded on a Bruker AVANCE 300 or Bruker AVANCE
III 400 Fourier transform spectrometer. Chemical shifts are reported
in parts per million. Coupling constants (J) and line widths at
half-height (Δν1/2) are given in Hz. 29Si{1H} NMR measurements
were performed using the DEPT pulse sequence. The residual proton
(7.15 ppm) and the carbon resonances (128.0 ppm) of benzene-d6
were used as internal references for 1H and 13C NMR chemical shifts,
respectively. Aromatic proton and carbon are abbreviated as ArH and
ArC, respectively. Pyridyl proton and carbon are abbreviated as pyH
and pyC, respectively. 29Si{1H} NMR chemical shifts were referenced
to SiMe4 (0 ppm) as an external standard. All NMR data were
collected at room temperature unless otherwise indicated. Infrared
spectra were measured for samples included in KBr pellets using a
Horiba FT-730 or FT-720 spectrometer. High-resolution mass spectra
(HRMS) and mass spectra were recorded on a Bruker Daltonics
solariX 9.4T spectrometer operating in the electrospray ionization
(ESI) mode or atmospheric pressure chemical ionization (APCI)
mode or on a Shimadzu GC-MS QP-2010 SE spectrometer operating
in the electron impact (EI) mode. Elemental analyses were carried out
using a J-Science Lab JM11 microanalyzer. Measurements of some
mass spectra and elemental analyses were performed at the Research
and Analytical Center for Giant Molecules, Tohoku University.
Synthesis of Cp*Mo(CO)2{κ2(Si,N)-Si(p-Tol)2(OC5H4N)} (2-Mo).
In a Schlenk tube, 1-Mo (31 mg, 0.053 mmol) and 2-hydroxypyridine
(7 mg, 0.07 mmol) were dissolved in toluene (3 mL). The mixture was
stirred at room temperature for 10 min. The resulting orange reaction
mixture was concentrated, and then hexane (2 mL) was slowly layered
on it. After crystals were precipitated, the mother liquor was removed
by a syringe. The crystals were washed with hexane (0.2 mL × 2) and
then dried under vacuum to give 2-Mo (18 mg, 0.030 mmol) in 58%
stability of E (M = Mo) is probably caused by less effective
π-type overlap of the vacant 3p orbital of silicon with a
4d orbital of molybdenum in comparison with that with a larger
5d orbital of tungsten in E (M = W).5b Thus, intermediate F
(M = Mo, E = S) is formed by coordination of the sulfur atom
on the metal center.16 Protonation of the metal center by a
coordinated SH hydrogen followed by reductive elimination
of hydrosilane then occurs to give the Mo−S−C−N four-
membered-ring complex 5 (route C in Scheme 4).
In the reactions of 1 with 2-aminopyridine, it is possible that
both silylene complex D (E = N) and silyl complex F (E = N)
are generated. In the reaction of molybdenum complex 1-Mo,
protonation of the metal center by coordinated 2-aminopyridine
in F (M = Mo, E = N) is unlikely to occur, due to the weak
acidity of the NH2 group of 2-aminopyridine.17 Instead, pro-
tonation of the metal center by the silicon-bound imino tautomer
in intermediate D (M = Mo, E = N) occurs predominantly
because the NH hydrogen of the pyridonimine tautomer
coordinated to the electropositive silylene silicon is considered
to be sufficiently acidic for this protonation. As a result, five-
membered-ring complex 3 is formed similarly to the reaction of 1
with 2-hydroxypyridine (route A in Scheme 4). The much slower
rate of this reaction (reaction conditions: 40 °C, 72 h) in
comparison with those of other reactions of 1 with 2-substituted
pyridines (reaction conditions: room temperature, 10 min to
1.5 h) is possibly caused by the lower stability of the silylene
complex intermediate D (M = Mo, E = N) in comparison with
the silyl complex intermediate F (M = Mo, E = N), leading to an
equilibrium shift from D to F in the reaction system. In the case
of tungsten complex 1-W, protonation of the tungsten center of
F (M = W, E = N) is likely to occur possibly because of the higher
basicity of tungsten in comparison with that of molybdenum,
which leads to the formation of four-membered-ring complex
6 via reductive elimination of hydrosilane (route C in Scheme 4).
By comparison, generation of silylene complex intermediate
D (M = W, E = N) is considered to be unfavorable because the
proportion of the imine tautomer of 2-aminopyridine is very low
in the tautomeric equilibrium.14a
1
yield as orange crystals. H NMR (300 MHz, C6D6): δ 1.48 (s, 15H,
Cp*), 1.94 (s, 3H, C6H4Me), 2.21 (s, 3H, C6H4Me), 5.89 (ddd, 3JHH
=
4
3
7.0, 6.0 Hz, JHH = 1.4 Hz, 1H, pyH), 6.39 (br d, JHH = 8.1 Hz, 1H,
3
4
pyH), 6.57 (ddd, JHH = 8.1, 7.0 Hz, JHH = 1.8 Hz, 1H, pyH), 6.96
3
3
(d, JHH = 7.6 Hz, 2H, ArH), 7.25 (d, JHH = 7.6 Hz, 2H, ArH), 7.83
(d, 3JHH = 7.9 Hz, 2H, ArH), 8.15 (dd, 3JHH = 6.0 Hz, 4JHH = 1.8 Hz, 1H,
pyH), 8.20 (d, JHH = 7.9 Hz, 2H, ArH). 13C{1H} NMR (75.5 MHz,
3
C6D6): δ 10.5 (C5Me5), 21.2, 21.6 (C6H4Me), 103.4 (C5Me5), 112.6,
116.0, 128.79, 128.82, 134.4, 135.7, 137.2, 138.7, 138.8, 139.7, 140.1,
155.4, 168.1 (ArC), 244.8, 248.5 (CO). 29Si{1H} NMR (59.6 MHz,
C6D6): δ 95.5. IR (KBr pellet, cm−1) 1898 (s, νCOsym), 1820 (s,
νCOasym). HRMS (ESI positive, additive: NaI): m/z calcd for
[C31H33NO3SiMo + Na]+ 616.1181, found 616.1179. Anal. Calcd
for C31H33NO3SiMo: C, 62.93; H, 5.62; N, 2.37. Found: C, 63.15;
H, 5.68; N, 2.50.
CONCLUSION
■
We developed a method for direct conversion of a Si−C(aryl)
bond to Si−E bonds (E = O, N) by stoichiometric reactions
of η3-α-silabenzyl complexes 1-Mo and 1-W with 2-substituted
pyridines under mild conditions. In these reactions, Si−C(aryl)
bond cleavage and toluene elimination occur to give silametalla-
cycles 2−4 via generation of silylene complex intermediates.
Further study aiming at application of these reactions to the
catalytic functionalization of arylsilanes is currently under way.
Synthesis of Cp*W(CO)2{κ2(Si,N)-Si(p-Tol)2(OC5H4N)} (2-W).
1-W (50 mg, 0.074 mmol) and 2-hydroxypyridine (8 mg, 0.08 mmol)
were dissolved in toluene (5 mL) in a Schlenk tube. After the mixture
was stirred at room temperature for 10 min, the resulting orange
solution was evaporated under vacuum. The residual dark orange oil
was purified by silica gel column chromatography using silica gel
60N (spherical, neutral, 100−210 μm, Kanto Chemical Co., Inc.) and
toluene as an eluent. An orange fraction was collected, and the solvent
was evaporated under vacuum. The residue was recrystallized from
toluene/hexane at room temperature. 2-W (12 mg, 0.018 mmol) was
obtained as orange crystals in 24% yield. 1H NMR (400 MHz, C6D6):
δ 1.55 (s, 15H, Cp*), 1.95 (s, 3H, C6H4Me), 2.22 (s, 3H, C6H4Me),
EXPERIMENTAL SECTION
General Procedures. All manipulations were carried out under
dry argon or nitrogen in a glovebox.
■
Materials. Hexane was dried using a Glass Contour alumina
column (Nikko Hansen & Co., Ltd.). Toluene was dried using an
MBRAUN Solvent Purification System, further dried over CaH2, and
vacuum-transferred. Benzene-d6 was dried over CaH2 and vacuum-
transferred. 2-Hydroxypyridine and 2-aminopyridine were recrystal-
lized from a toluene solution. 2-Mercaptopyridine was purchased
from Sigma-Aldrich and used as received. All solvents were stored
under argon over 4 Å molecular sieves in a glovebox. Cp*Mo-
(CO)2{η3(Si,C,C)-Si(p-Tol)3} (1-Mo)5b and Cp*W(CO)2(py)Me13
were prepared according to the literature methods. Cp*W(CO)2-
{η3(Si,C,C)-Si(p-Tol)3} (1-W)5a was synthesized by a new method
starting from Cp*W(CO)2(py)Me as described below.
3
4
5.78 (ddd, JHH = 6.8, 6.0 Hz, JHH = 1.4 Hz, 1H, pyH), 6.40 (ddd,
3JHH = 8.0 Hz, 4JHH = 1.4 Hz, 5JHH = 0.6 Hz, 1H, pyH), 6.50−6.58 (m,
1H, pyH), 6.97 (d, JHH = 7.5 Hz, 2H, ArH), 7.25 (d, JHH = 7.4 Hz,
3
3
3
3
2H, ArH), 7.84 (d, JHH = 7.9 Hz, 2H, ArH), 8.14 (d, JHH = 7.9 Hz,
2H, ArH), 8.21 (ddd, 3JHH = 6.0 Hz, 4JHH = 1.9 Hz, 5JHH = 0.6 Hz, 1H,
pyH).18 13C{1H} NMR (101 MHz, C6D6): δ 10.5 (C5Me5), 21.2, 21.6
(C6H4Me), 102.0 (C5Me5), 112.1, 116.3 (pyC), 128.8, 128.9, 134.3,
136.2, 138.2, 138.5, 138.8 (ArC), 140.2 (pyC), 140.7 (ArC), 157.1,
169.7 (pyC), 239.8, 242.1 (CO).18 29Si{1H} NMR (79.5 MHz, C6D6):
δ 92.3 (s with satellites, JWSi = 81 Hz). IR (KBr pellet, cm−1): 1901
(s, νCOsym), 1809 (s, νCOasym). EI-MS (70 eV): m/z 679 (M+, 54),
D
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623 (M+ − 2 CO, 100). Anal. Calcd for C31H33NO3SiW: C, 54.79; H,
4.89; N, 2.06. Found: C, 54.86; H, 4.85; N, 2.24.
dissolved in toluene (5 mL). The mixture was stirred at room
temperature for 10 min. After evaporation of volatiles under vacuum
from the resulting red solution, the residue was recrystallized from
toluene/hexane (a 1/2 mixture) at −35 °C to give complex 5 (26 mg,
0.065 mmol) in 76% yield as red crystals. 1H NMR (400 MHz, C6D6):
δ 1.52 (s, 15H, Cp*), 5.99 (ddd, 3JHH = 7.2, 5.6 Hz, 4JHH = 1.2 Hz, 1H,
Synthesis of Cp*Mo(CO)2[κ2(Si,N)-Si(p-Tol)2{N(H)C5H4N}] (3).
1-Mo (26 mg, 0.044 mmol) and 2-aminopyridine (5 mg, 0.05 mmol)
were dissolved in toluene (3 mL) in a Schlenk tube. After the solution
was stirred at 40 °C for 48 h, the resulting orange reaction mixture was
evaporated under vacuum. The residue was dissolved in a minimum
amount of toluene, and hexane was then slowly layered on the toluene
solution. After crystals were precipitated, the mother liquor was
removed using a Pasteur pipet, and the crystals were dried under
vacuum. Complex 3 (13 mg, 0.022 mmol) was obtained in 50% yield
as orange crystals. 1H NMR (400 MHz, C6D6): δ 1.50 (s, 15H, Cp*),
2.03 (s, 3H, C6H4Me), 2.27 (s, 3H, C6H4Me), 4.62 (br s, Δν1/2 = 3 Hz,
1H, NH), 5.63 (br d, 3JHH = 7.8 Hz, 1H, pyH), 5.80 (ddd, 3JHH = 6.8,
3
4
5
pyH), 6.33 (ddd, JHH = 8.4 Hz, JHH = 1.2 Hz, JHH = 1.0 Hz, 1H,
3
4
pyH), 6.49 (ddd, JHH = 8.4, 7.2 Hz, JHH = 1.6 Hz, 1H, pyH), 7.40
(ddd, 3JHH = 5.6 Hz, 4JHH = 1.6 Hz, 5JHH = 1.0 Hz, 1H, pyH). 13C{1H}
NMR (101 MHz, C6D6): δ 10.4 (C5Me5), 106.6 (C5Me5), 115.7,
127.2, 140.2, 151.2, 179.4 (pyC), 257.5, 263.7 (CO). IR (KBr
pellet, cm−1): 1928 (s, νCOsym), 1832 (s, νCOasym). EI-MS (70 eV): m/z
399 (M+, 15), 371 (M+ − CO, 4), 343 (M+ − 2 CO, 100). HRMS
(APCI): m/z calcd for [C17H19NO2SMo + H]+ 400.0263, found
400.0263. Anal. Calcd for C17H19NO2SMo: C, 51.39; H, 4.82; N, 3.53.
Found: C, 51.39; H, 4.82; N, 3.52.
4
3
6.2 Hz, JHH = 1.2 Hz, 1H, pyH), 6.55 (ddd, JHH = 8.4, 6.8 Hz,
4JHH = 2.0 Hz, 1H, pyH), 7.02 (d, JHH = 7.2 Hz, 2H, ArH), 7.26
(d, JHH = 7.2 Hz, 2H, ArH), 7.75 (d, JHH = 7.6 Hz, 2H, ArH), 7.89
3
Synthesis of Cp*W(CO)2{κ2(N,N)-NC5H4NH} (6). 1-W (50 mg,
0.074 mmol) and 2-aminopyridine (8 mg, 0.08 mmol) were dissolved
in toluene (5 mL) in a Schlenk tube. After the mixture was stirred
at room temperature for 10 min, the resulting orange solution was
evaporated under vacuum. The residual dark orange oil was washed
with hexane and then dried under vacuum to give complex 6 as a crude
product, which contained an unidentified impurity bearing a Cp*
ligand,19 in ca. 31% yield (11 mg, 0.023 mmol). Complex 6 in the
crude product was identified by comparing the IR and 1H NMR
spectra with those of the authentic sample synthesized by the method
described below.
3
3
3
3
4
(d, JHH = 7.6 Hz, 2H, ArH), 8.10 (dd, JHH = 6.2 Hz, JHH = 2.0 Hz,
1H, pyH).18 13C{1H} NMR (101 MHz, C6D6): δ 10.5 (C5Me5),
21.3, 21.6 (C6H4Me), 103.1 (C5Me5), 110.4, 112.2 (pyC), 128.6,
128.8, 134.8, 135.4 (ArC), 137.5 (pyC), 137.6, 138.0, 138.3, 142.1
(ArC), 155.8, 166.4 (pyC), 246.1, 249.1 (CO).18 29Si{1H} NMR
(79.5 MHz, C6D6): δ 65.4. IR (KBr pellet, cm−1): 3379 (w, νNH), 1890
(s, νCOsym), 1805 (s, νCOasym). EI-MS (70 eV): m/z 592 (M+, 13), 564
(M+ − CO, 26), 536 (M+ − 2 CO, 100). Anal. Calcd for
C31H34N2O2SiMo: C, 63.04; H, 5.80; N, 4.74. Found: C, 63.03; H,
5.72; N, 4.71.
Synthesis of Cp*W(CO)2{κ2(S,Si)-(SC5H4N)Si(p-Tol)2} (4). 1-W
(25 mg, 0.037 mmol) and 2-mercaptopyridine (4 mg, 0.04 mmol)
were dissolved in toluene (3 mL). After the mixture was stirred at
room temperature for 1.5 h, the solution became red. The solution was
concentrated, and then hexane (2 mL) was slowly layered on it. After
crystals were precipitated, the mother liquor was removed using a
Pasteur pipet, and the crystals were washed with hexane (0.2 mL × 2).
Complex 4 (10 mg, 0.014 mmol) was obtained as dark red crystals in
38% yield. 1H NMR (400 MHz, C6D6): δ 1.69 (s, 15H, Cp*), 1.98 (s,
Alternative Synthesis of Complex 6 by a Reaction of
Cp*W(CO)2(py)Me13 with 2-Aminopyridine. Cp*W(CO)2(py)-
Me (50 mg, 0.11 mmol) and 2-aminopyridine (10 mg, 0.11 mmol)
were dissolved in toluene (3 mL) in a Schlenk tube. After the mixture
was stirred at room temperature for 1 h, the resulting reddish orange
solution was evaporated under vacuum. The residue was washed
with hexane and dried under vacuum to give complex 6 in 83% yield
1
(43 mg, 0.092 mmol) as an orange-red powder. H NMR (400 MHz,
C6D6): δ 1.65 (s, 15H, Cp*), 3.50 (br s, Δν1/2 = 18 Hz, 1H, NH), 5.19
3
4
5
(ddd, JHH = 8.8 Hz, JHH = 1.2 Hz, JHH = 1.0 Hz, 1H, pyH), 5.79
3
3H, C6H4Me), 2.22 (s, 3H, C6H4Me), 5.71 (ddd, JHH = 6.8, 6.2 Hz,
3
4
3
(ddd, JHH = 6.8, 5.6 Hz, JHH = 1.0 Hz, 1H, pyH), 6.67 (ddd, JHH
=
4JHH = 1.2 Hz, 1H, pyH), 6.06 (ddd, 3JHH = 8.4, 6.8 Hz, 4JHH = 1.6 Hz,
1H, pyH), 7.01 (d, 3JHH = 8.0 Hz, 2H, ArH), 7.13−7.18 (m, 2H, ArH),
7.19 (ddd, 3JHH = 8.4 Hz, 4JHH = 1.2 Hz, 5JHH = 0.8 Hz, 1H, pyH), 7.73
8.2, 6.8 Hz, 4JHH = 1.6 Hz, 1H, pyH), 7.31 (ddd, 3JHH = 5.6 Hz, 4JHH
=
1.6 Hz, JHH = 1.2 Hz, 1H, pyH). 13C{1H} NMR (101 MHz, C6D6):
δ 10.7 (C5Me5), 105.5 (C5Me5), 107.3, 110.0, 134.3, 147.7, 169.5
(pyC), 256.7, 261.9 (CO). IR (KBr pellet, cm−1): 3435 (w, νNH), 1909
(s, νCOsym), 1801 (s, νCOasym). EI-MS (70 eV): m/z 468 (M+, 13), 440
(M+ − CO, 4), 410 (100). Anal. Calcd for C17H20N2O2W: C, 43.61;
H, 4.31; N, 5.98. Found: C, 43.60; H, 4.33; N, 6.00.
5
3
3
(d, JH3H = 8.0 Hz, 2H, ArH), 7.82 (d, JHH = 8.0 Hz, 2H, ArH), 7.95
4
5
(ddd, JHH = 6.2 Hz, JHH = 1.6 Hz, JHH = 0.8 Hz, 1H, pyH). The
multiplet ArH signal at 7.13−7.18 ppm was overlapped with the signal
of C6D5H.18 13C{1H} NMR (101 MHz, C6D6): δ 10.7 (C5Me5), 21.2,
21.4 (C6H4Me), 102.3 (C5Me5), 116.0, 128.6 (pyC), 128.9, 129.0,
135.3 (ArC), 135.4 (pyC), 136.5, 138.3, 138.5, 138.8, 140.1 (ArC),
142.9, 178.6 (pyC), 234.7, 238.1 (CO).18 29Si{1H} NMR (79.5 MHz,
C6D6): δ 103.0 (s with satellites, JWSi = 101 Hz). IR (KBr pellet,
cm−1): 1900 (s, νCOsym), 1817 (s, νCOasym). EI-MS (70 eV): m/z 695
(M+, 50), 667 (M+ − CO, 8), 639 (M+ − 2 CO, 98), 560 (100). Anal.
Calcd for C31H33NO2SiSW: C, 53.53; H, 4.78; N, 2.01. Found: C,
53.36; H, 4.69; N, 2.21.
General Procedure for NMR Monitoring of the Reactions of
η3-α-Silabenzyl Complexes 1 with 2-Substituted Pyridines.
These reactions were all carried out by the same procedures, and a
general procedure for them is as follows. The η3-α-silabenzyl complex
1-Mo or 1-W and Cp2Fe (internal standard, less than 1 mg) were
dissolved in benzene-d6 (ca. 0.5 mL). The solution was transferred to
1
an NMR tube with a J. Young Teflon valve (5 mm o.d.). A H NMR
spectrum of the mixture was measured to determine the ratio of
the intensities of the signals of 1-Mo or 1-W and Cp2Fe. 2-Substituted
pyridine was then added to the solution. The reaction was monitored
Synthesis of Cp*Mo(CO)2{κ2(S,N)-SC5H4N} (5). 1-Mo (50 mg,
0.085 mmol) and 2-mercaptopyridine (11 mg, 0.099 mmol) were
Table 1. Reaction Conditions and Yields of Products for the NMR Monitoring of the Reactions of 1 with 2-Substituted
Pyridines
product(s) (NMR yield(s), %)
a
entry
complex (amount)
substrate (amount)
temp, time
1
2
3
4
5
6
1-Mo (5 mg, 8 μmol)
1-W (5 mg, 8 μmol)
1-Mo (6 mg, 0.01 mmol)
1-W (5 mg, 8 μmol)
1-Mo (5 mg, 8 μmol)
1-W (6 mg, 9 μmol)
2-hydroxypyridine (1 mg, 0.01 mmol)
2-hydroxypyridine (1 mg, 0.01 mmol)
2-mercaptopyridine (2 mg, 0.02 mmol)
2-mercaptopyridine (1 mg, 9 μmol)
2-aminopyridine (1 mg, 0.01 mmol)
2-aminopyridine (1 mg, 0.01 mmol)
room temp, 10 min
room temp, 10 min
room temp, 15 min
room temp, 1.5 h
40 °C, 72 h
2-Mo (76)
2-W (57)
5 (91)
toluene (quant)
toluene (quant)
HSi(p-Tol)3 (quant)
4 (54)
7 (16)
toluene (63)
HSi(p-Tol)3 (14)
3 (77)
6 (63)
toluene (85)
room temp, 15 min
HSi(p-Tol)3 (62)
a
A period until the signals of 1-Mo or 1-W in a reaction mixture completely disappeared.
E
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Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
by H NMR spectroscopy. The signals of a five- or four-membered-
AUTHOR INFORMATION
Corresponding Author
*E-mail for H.T.: tobita@m.tohoku.ac.jp.
Notes
■
ring complex (2−6) and toluene or HSi(p-Tol)3 appeared during the
reaction. NMR yields of metallacycles 2−6 were determined by
comparison of intensities of the Cp* signals of these complexes with
that of the signal of Cp2Fe. NMR yields of toluene and HSi(p-Tol)3
were determined by comparison of intensities of the Me signals of
toluene and HSi(p-Tol)3 with that of the signal of Cp2Fe. Complexes
The authors declare no competing financial interest.
1
2−6 were identified by comparing the H NMR spectra with those
ACKNOWLEDGMENTS
■
of the corresponding authentic samples synthesized by the above-
mentioned procedures. The amounts of reactants and the NMR yields
of products are given in Table 1.
This work was supported by JSPS KAKENHI Grant Numbers
23750053 and 25410058 from the Japan Society for the
Promotion of Science (JSPS). We are grateful to the Research
and Analytical Center for Giant Molecules, Tohoku University,
for spectroscopic measurements and elemental analysis.
In the reaction of 1-W with 2-mercaptopyridine (entry 4, Table 1),
1
the H NMR spectrum of the reaction mixture showed not only the
signals of 4 and toluene but also some other signals of minor products.
Two of them were Cp*W(CO)2{κ2(S,N)-SC5H4N} (7, a putative
structure) and HSi(p-Tol)3. The structure of 7 was tentatively deduced
by 1H NMR spectroscopy: The chemical shifts of the 1H NMR signals
of 7 are close to the corresponding chemical shifts of the molybdenum
analogue 5. H NMR (400 MHz, C6D6) data for 7: δ 1.61 (s, 15H,
Cp*), 5.98 (ddd, JHH = 7.2, 6.0 Hz, JHH = 1.4 Hz, 1H, pyH), 6.14
(ddd, JHH = 8.6 Hz, JHH = 1.4 Hz, JHH = 1.0 Hz, 1H, pyH), 6.41
REFERENCES
■
(1) For selected recent reports on catalytic transformations of
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3
4
3
4
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3
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(ddd, JHH = 8.6, 7.2 Hz, JHH = 1.4 Hz, 1H, pyH), 7.45 (ddd, JHH
6.0 Hz, JHH = 1.4 Hz, JHH = 1.0 Hz, 1H, pyH).
=
4
5
Synthesis of Cp*W(CO)2{η3(Si,C,C)-Si(p-Tol)3} (1-W). 1-W was
synthesized according to the following newly developed method.20
Cp*W(CO)2(py)Me13 (200 mg, 0.426 mmol), HSi(p-Tol)3 (130 mg,
0.430 mmol), and BPh3 (104 mg, 0.430 mmol) were dissolved in
toluene (5 mL). The solution was stirred at room temperature for 1 h.
After the volatiles were evaporated under vacuum, hexane (20 mL) was
added to the residue, and the mixture was stirred at room temperature
for 1.5 h. During the stirring, py·BPh3 was precipitated as a colorless
solid. The mixture was filtered to remove py·BPh3, and the filtrate was
concentrated under vacuum. When the concentrated solution was
cooled at −35 °C, a brown solid was precipitated. After removal of the
mother liquor, the residual solid was washed with hexane, and drying
the solid under vacuum gave 1-W as a brown powder in 52% yield
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1H NMR and IR spectra with those of the authentic sample.5a
(2) For selected examples of stoichiometric Si−C(aryl) bond
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X-ray Crystal Structure Determination. X-ray-quality single
crystals of 2-Mo (an orange block), 2-W (an orange-yellow block), 3
(a yellow block), 4 (a dark red block), and 5 (a red block) were
obtained from a toluene/hexane (a 1/2 mixture) solution at −35 °C
for 2-Mo or from a toluene/hexane solution at room temperature for
2-W and 3−5. Intensity data for the analysis were collected on a
Rigaku RAXIS-RAPID imaging plate diffractometer with graphite-
monochromated Mo Kα radiation (λ = 0.71069 Å) under a cold nitrogen
stream (T = 150 K). Numerical absorption corrections were applied to
the data. The structures of 2−5 were solved by direct methods using the
SHELXS-9721 program and refined by full-matrix least-squares
techniques on all F2 data with SHELXL-97.21 The asymmetric unit of
the single crystal of 3 contains two crystallographically independent
molecules, namely, 3-A and 3-B. Anisotropic refinement was applied to
all non-hydrogen atoms. All calculations were carried out using Yadokari-
XG 2009.22 CCDC reference numbers: 1404813 (2-Mo), 1404814
(2-W), 1404815 (3), 1404816 (4), and 1404817 (5). Crystallographic
data are available in the Supporting Information as a CIF file, and
selected crystallographic data are also given in Tables S1 (for 2-Mo and
2-W), S2 (for 3 and 4), and S3 (for 5) in the Supporting Information.
Crystal structures of 2-Mo, 3, and 5 are depicted in Figures S1−S3,
respectively (see the Supporting Information).
(6) Cp*W(CO)2{κ2(S,N)-SC5H4N} (7, a putative structure) and
HSi(p-Tol)3 were also formed as minor products in 16% and 14%
NMR yields, respectively (see the Experimental Section).
(7) The Mo−Si bond distance of only one of the two crystallo-
graphically independent molecules of 3 in the asymmetric unit was
described in the text because the geometric parameters of these
molecules are nearly identical.
(8) Based on a survey of the Cambridge Structural Database, CSD
version 5.35, November 2013.
(9) (a) Ueno, K.; Masuko, A.; Ogino, H. Organometallics 1999, 18,
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F.; Roper, W. R.; Wright, L. J. J. Organomet. Chem. 2004, 689, 2979−
2987.
(11) Brandenburg, K. L.; Heeg, M. J.; Abrahamson, H. B. Inorg.
ASSOCIATED CONTENT
* Supporting Information
Chem. 1987, 26, 1064−1069.
■
(12) Begum, N.; Kabir, S. E.; Hossain, G. M. G.; Rahman, A. F. M.
M.; Rosenberg, E. Organometallics 2005, 24, 266−271.
S
Crystal structures of complexes 2-Mo, 3, and 5 (Figures S1−S3,
respectively), NMR spectra of complexes 2−6 (Figures S4−S19),
and tables (Tables S1−S3) and a CIF file giving X-ray
crystallographic data for 2−5. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.organomet.5b00335.
(13) Sakaba, H.; Tsukamoto, M.; Hirata, T.; Kabuto, C.; Horino, H.
J. Am. Chem. Soc. 2000, 122, 11511−11512.
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Klingsberg, E., Ed.; Interscience: New York, 1960; Vol. 14 (Pyridine
and Its Derivatives, Part One), Chapter 1, pp 65−74. (b) The
existence of tautomerization of 2-hydroxypyridine and 2-mercaptopyr-
F
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Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
idine in nonpolar solvents (decane, cyclohexane, and chloroform) is
demonstrated. See: Beak, P.; Covington, J. B.; Smith, S. G. J. Am.
Chem. Soc. 1976, 98, 8284−8286.
(15) Reactions that proceed through different pathways depending
on the affinity of oxygen and sulfur of heterocumulenes to silicon in a
silylene complex have been reported recently. See: (a) Ochiai, M.;
Hashimoto, H.; Tobita, H. Organometallics 2012, 31, 527−530.
(b) Xie, H.; Lin, Z. Organometallics 2014, 33, 892−897.
(16) An alternative mechanism involving formation of a silyl complex
intermediate ligated by thiopyridone tautomer instead of 2-
mercaptopyridine cannot be ruled out.
(17) The pKa values of 2-substituted pyridines in DMSO solutions
have been reported: 17.0 for 2-hydroxypyridine (2-pyridone), 13.3 for
2-mercaptopyridine (2-thiopyridone), and 27.7 for 2-aminopyridine.
See: Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456−463.
(18) Assignments of the 1H and 13C NMR signals of complexes 2-W,
3, and 4 were confirmed by their 1H−13C HSQC and 1H−13C HMBC
spectra in C6D6.
1
(19) The H NMR spectrum of the crude product in C6D6 shows a
Cp* signal of the impurity at 1.55 ppm together with the signals of 6
(intensity ratio of the Cp* signals of 6 and the impurity 1:0.17).
(20) In our previous study, 1-W had been synthesized by the
following two-step method starting from a (DMAP)(methyl)tungsten
complex Cp*W(CO)2(DMAP)Me: (1) the reaction of the methyl
complex with HSi(p-Tol)3 to give the DMAP-stabilized aryl(silylene)
complex Cp*W(CO)2(p-Tol){Si(p-Tol)2·DMAP} and (2) the
reaction of the resulting silylene complex with BPh3.4c,5a
(21) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
2008, 64, 112−122.
(22) (a) Wakita, K. Yadokari-XG, Software for Crystal Structure
Analyses, 2001. (b) Release of Software (Yadokari-XG 2009) for
Crystal Structure Analyses. See: Kabuto, C.; Akine, S.; Nemoto, T.;
Kwon, E. Nippon Kessho Gakkaishi 2009, 51, 218−224.
G
DOI: 10.1021/acs.organomet.5b00335
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