Addition of HCl to a base-stabilized hydrido(silylene)tungsten complex and reactivity of the adduct toward LiAlH4

Article abstract of DOI:10.1246/cl.2001.1078

The reaction of cis-Cp*(CO)₂HW=SiPh₂·Py (1, Py = pyridine) with HCl gave an adduct Cp*(CO)₂H₂WSiPh₂Cl (2). Treatment of 2 with excess LiAlH₄ afforded an anionic complex Li[cis-Cp*(CO)₂

Full text of DOI:10.1246/cl.2001.1078

1078
Chemistry Letters 2001
Addition of HCl to a Base-Stabilized Hydrido(silylene)tungsten Complex
and Reactivity of the Adduct toward LiAlH4
Hiroyuki Sakaba,* Takeshi Hirata, Chizuko Kabuto, and Hiroshi Horino
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578
(Received July 31, 2001; CL-010730)
The reaction of cis-Cp*(CO)2HW=SiPh2·Py (1, Py = pyridine)
hydrides in 2, Cp*(CO)2HDWSiPh2Cl (2-d) was synthesized by the
reaction of 1 with DCl (ca. 2 equiv), where 2 was also formed (2-
d:2 ≈ 1:0.6).7 The hydride signal of 2-d (–6.45 ppm) was observed
as a sharp singlet (ω1/2 ≈ 5 Hz) without HD coupling along with that
with HCl gave an adduct Cp*(CO)2H2WSiPh2Cl (2). Treatment of
2 with excess LiAlH4 afforded an anionic complex Li[cis-
Cp*(CO)2HWSiHPh2] (4), and the same product was obtained by
the reaction of 1 with LiAlH4.
1
of 2 (–6.42 ppm) in the H NMR spectrum at –60 °C, indicating
that 2 is not a η2-H2 complex.8
Recent extensive studies on silylene complexes have revealed
their high reactivities toward the addition of polar molecules such as
alcohols and water.1 However, the addition of hydrogen halide to a
silylene complex, one of fundamental reactions expected for the
polarized metal–silicon double bond, has not been reported so far.
We have recently reported the synthesis of the base-stabilized
hydrido(silylene) complex cis-Cp*(CO)2HW=SiR2·Py and its novel
reactivity toward a secondary silane leading to a silylene exchange
reaction.2 Here we report for the first time the addition of hydrogen
chloride to the base-stabilized silylene complex to afford a stable
adduct. The reactivities of the HCl adduct and the silylene complex
toward LiAlH4 are also described.
The X-ray crystal structure of 2 is shown in Figure 1,9 which
reveals a distorted octahedral structure with two CO ligands
arranged almost symmetrically with respect to the plane defined by
Cp*(centroid)–W1–Si1. Interesting structural characteristic is the
tilting of the W–Si bond toward two hydride ligands as shown by
the Cp*(centroid)–W1–Si1 angle of 153.8°. This tilting might be
2
due to the weak Si…H interaction suggested by the JSiWH value,
although the repulsive interaction between the bulky silyl group and
the carbonyl ligands is also conceivable. Recent studies on
chlorosilyl(hydrido) complexes by Nikonov have revealed an inter-
esting interaction between a metal hydride and a chlorosilyl group,
interligand hypervalent interaction (IHI), where the overlap of a
M–H bonding orbital and a Si–Cl antibonding orbital causes a short
Si–H contact associated with short M–Si and long Si–Cl bonds.10
This interaction requires the trans orientation of the M–H bond to
the Si–Cl bond, and interestingly, such an orientation is seen for the
W1–H1 and Si1–Cl1 bonds in 2. It is found, however, that 2 is
devoid of this type of interaction, judging from the W1–Si1
(2.5604(9) Å) and Si1–Cl1 (2.135(1) Å) bond lengths, which are
both within the respective normal ranges for W–Si (2.533–2.633
Å)11 and Si–Cl (2.094–2.148 Å)10b bond lengths in non-interacting
systems. Considering the finding that a more electron donating
The silylene complex cis-Cp*(CO)2HW=SiPh2·Py (1)2 reacted
rapidly with HCl (ca. 2 equiv) in toluene to afford the adduct
Cp*(CO)2H2WSiPh2Cl (2),3 which was isolated in 72% yield as an
air-stable pale yellow solid after filtration of the mixture to remove
pyridinium chloride and concentration of the filtrate. The ori-
entation of the addition is in accordance with the polarization of
Mδ–=Siδ+ shown by theoretical studies.4
1
The H NMR spectrum of 2 exhibited a broad WH signal at
–6.35 ppm at room temperature, and on cooling to –40 °C, it
became sharp enough to detect tungsten satellites with 1JWH = 58.6
Hz and silicon satellites with 2JSiWH = 18.3 Hz.5 This 2JSiWH value
is close to the lower limit of the known JSiH range (22–98 Hz) for
η2-Si–H bonds,6 and might suggest a weak Si…H interaction in 2.
In the IR spectrum of 2, a weak absorption assigned to νWH band
was observed at 1848 cm–1. The assignment is supported by the
disappearance of the absorption and the appearance of a new
absorption at 1323 cm–1 in the IR spectrum of 2-d2, which was syn-
thesized by the reaction of the pyridine complex 32 with Ph2SiD2
followed by DCl addition to the resulting deuteride complex 1-d.
The ratio of the absorption frequencies observed for 2 and 2-d2 is
very close to the calculated value.
In order to get information on the interaction between the two
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1079
hydrosilyl complex Cp*(PMe3)2RuSiHPh2.12 In order to clarify a
potential reactivity of the neutral silylene complex C toward
hydride described above, the reaction of C with LiAlH4 was exam-
ined. When LiAlH4 was added to a THF-d8 solution of C, the clean
formation of 4 was shown by 1H NMR spectroscopy. Similar reac-
tion of 1 with LiAlH4 also afforded 4, and it was isolated as an air-
sensitive white solid in 56% yield based on 5 from the photolysis of
5 and Ph2SiH2 in THF followed by treatment of the resulting sily-
lene complex C with LiAlH4 in ether. We are currently exploring
the reactivity of 4 as a precursor to a variety of hydrido(hydrosilyl)
tungsten complexes.
phosphine favors IHI in complexes of the type Cp*(PR3)RuH2Si-
R2Cl,10b the existence of π-acidic CO ligands probably disfavors
IHI in 2.
When 2 was treated with excess LiAlH4 in THF-d8 to reduce
the Si–Cl bond, a rapid reaction took place to give the anionic com-
plex Li[cis-Cp*(CO)2HWSiHPh2] (4)3 (Scheme 1), showing WH
and SiH signals at –7.16 (1JWH = 73.6 Hz) and 5.35 (1JSiH = 160.1
Hz) ppm, respectively, in the 1H NMR spectrum. Its anionic char-
acter is supported by the 183 cm–1 difference in the average CO
stretching frequencies between 4 (1867 and 1717 cm–1, νav = 1792
cm–1) and the neutral complex 2 (2006 and 1944 cm–1, νav = 1975
cm–1).
This work was supported by Grants-in-Aid for Scientific
Research (No. 10640536) from the Ministry of Education, Science,
Sports, and Culture of Japan.
Dedicated to Prof. Hideki Sakurai on the occasion of his 70th
birthday.
References and Notes
1
2
3
H. Wada, H. Tobita, and H. Ogino, Organometallics, 16, 2200 (1997)
and references therein.
H. Sakaba, M. Tsukamoto, T. Hirata, C. Kabuto, and H. Horino, J. Am.
Chem. Soc., 122, 11511 (2000).
1
Selected data: 2: H NMR (400 MHz, C7D8) δ 7.92–7.90 (m, 4H, Ph),
7.25–7.21 (m, 4H, Ph), 7.14–7.10 (m, 2H, Ph), 1.75 (s, 15H, Cp*),
–6.35 (br s, 2H, WH); 29Si NMR (C7D8, –60 °C) δ 23.0; IR (toluene)
νCO = 2006 (s), 1944 (s) cm–1; Anal. Calcd for C24H27ClO2SiW: C,
1
48.46; H, 4.57%. Found: C, 48.54; H, 4.77%. 4: H NMR (400 MHz,
THF-d8) δ 7.65 (dd, J = 8.1, 1.6 Hz, 4H, Ph), 7.01 (t, J = 7.5 Hz, 4H,
Ph), 6.94 (tt, J = 7.5, 1.6 Hz, 2H, Ph), 5.35 (d, J = 1.1 Hz, 1JSiH = 160.1
Hz, 1H, SiH), 1.92 (s, 15H, Cp*), –7.16 (s, 1JWH = 73.6 Hz, 1H, WH);
29Si NMR (THF-d8) δ 29.5; IR (toluene) = 1867 (s), 1717 (s) cm–1. A:
1H NMR (400 MHz, THF-d8, –60 °C) δ 7.87 (s, 2H, Ph), 7.55 (s, 2H,
Ph), 7.14 (s, 2H, Ph), 7.00 (s, 4H, Ph), 1.89 (s, 15H, Cp*), –8.03 (s,
1JWH = 68.2 Hz, 1H, WH); 29Si NMR (THF-d8, –60 °C) δ 69.5. C: 1H
NMR (400 MHz, THF-d8) δ 7.39 (dd, J = 8.1, 1.6 Hz, 4H, Ph),
Possible mechanisms for the formation of 4 are shown in
Scheme 1. Path a is a sequence of deprotonation of the tungsten
hydride of 2 and chloride/hydride exchange in the resulting anionic
intermediate A, and path b is the reversed combination of these two
processes via B. The third pathway is E2 elimination to produce
silylene complex C followed by hydride attack to the electrophilic
silylene center of C. To get mechanistic information, the reaction
1
7.20–7.13 (m, 6H, Ph), 1.84 (s, 15H, Cp*), –9.31 (s, JWH = 68.7 Hz,
1H, WH); 29Si NMR (THF-d8) δ 119.3; IR (THF) νCO = 1896 (s), 1809
(m) cm–1. In each 13C NMR spectrum of 4 and A, two CO signals are
observed, indicating their cis configurations.
1
was monitored by low-temperature H NMR spectroscopy.
Interestingly, the reaction slowly proceeded around –70 °C to form
an intermediate, which is characterized as the deprotonation product
Li[cis-Cp*(CO)2HWSiClPh2] (A)3 from the observation of WH
(–8.03 ppm, 1JWH = 68.2 Hz, 1H) and Cp* (1.89 ppm, 15H) signals,
and occurrence of the deprotonation is supported by detection of a
signal at 4.56 ppm assigned to H2. The 29Si signal is observed at
69.5 ppm at –60 °C. When the temperature was raised above –45
°C, the signals due to A decreased and those of the final product 4
became predominant.
Although the observation of the WH and Cp* signals with an
intensity ratio 1:15 for the intermediate is not inconsistent with the
silylene complex C, such a possibility is ruled out by comparison of
the spectroscopic data of the intermediate with those of C synthe-
sized by photolysis of Cp*(CO)3WMe (5) and Ph2SiH2 in THF-d8,
showing characteristic 1H and 29Si signals at –9.31 ppm (W–H) and
119.3 ppm (W=Si).3 These results clearly demonstrate that 4 is pro-
duced from 2 via path a.
4
5
H. Jacobsen and T. Ziegler, Inorg. Chem., 35, 775 (1996).
No decoalescence of the hydride signal was observed on cooling down
to –80 °C. In the low-temperature NMR experiments, very weak hydride
signals of trace amounts of minor components were detected. Although
their structures cannot be characterized, the interconversion between 2
and the minor components might cause the dynamic behavior.
6
7
J. Y. Corey and J. Braddock-Wilking, Chem. Rev., 99, 175 (1999).
The formation of 2-d2 is also suggested by the relative intensity of the
Cp* signal to the combined hydride signals of 2-d and 2 (ca. 13:1), which
is larger than a 10.9:1 ratio expected for a 1:0.6 mixture of 2-d and 2 only.
Although it is suggested that the H/D scrambling might be promoted by a
catalytic amount of pyridine liberated from 1 by the fact that addition of
pyridine to a mixture of 2 and 2-d2 in toluene-d8 led to the rapid forma-
tion of 2-d, detailed studies are required to clarify the mechanism.
JHD coupling constants for η2-H2 complexes are in the range of 20–34
Hz: R. H. Crabtree, “The Organometallic Chemistry of the Transition
Metals,” 3rd ed., John Wiley & Sons, Inc., New York (2001), p 73.
Crystal data for 2: C24H27ClO2SiW, MW= 594.86, monoclinic, space
group P21/a (No. 14), a = 16.067(3), b = 8.901(2), c = 16.856(3) Å, β =
99.64(1)°, V = 2376.7(8) Å3, Z = 4, Dcalc = 1.662 g/cm3, µ(Mo Kα) =
50.45 cm–1, R = 0.021 (Rw = 0.021) for 5594 observed reflections [I >
5.00σ(I)]. All Hydrogens were found from successive D-Fourier syn-
theses, in which W–H atoms were assigned geometrically out of several
peaks around W atom, and all hydrogens were refined isotropically.
However, no detailed discussion about W–H bonds is made here
because of the low accuracy.
8
9
10 a) G. I. Nikonov, L. G. Kuzmina, S. F. Vyboishchikov, D. A.
Lemenovskii, and J. A. K. Howard, Chem. Eur. J., 5, 2947 (1999). b) S.
B. Duckett, L. G. Kuzmina, and G. I. Nikonov, Inorg. Chem. Commun.,
3, 126 (2000).
Despite of the electrophilic nature of the silylene center of a
silylene complex, a reactivity study of an isolated silylene complex
toward a hydride reagent is rare. To our knowledge, the only pre-
ceeding example is the reaction of the cationic silylene complex
Cp*(PMe3)2Ru=SiPh2(NCMe)+ with LiAlH4 to form the neutral
11 Bond lengths for complexes of the type Cp(*)(CO)2(L)WSiR3: see ref 2.
12 D. A. Straus, C. Zhang, G. E. Quimbita, S. D. Grumbine, R. H. Heyn,
T. D. Tilley, A. L. Rheingold, and S. J. Geib, J. Am. Chem. Soc., 112,
2673 (1990).