Synthesis of kinetically stabilized 1,2-dihydrodisilenes

Article abstract of DOI:10.1021/ja300694p

Kinetically stabilized 1,2-dihydrodisilenes were successfully synthesized and isolated by the introduction of sterically protecting bulky aryl groups. These 1,2-dihydrodisilenes exhibit distinct Si=Si double-bond character in both solution and the solid s

Full text of DOI:10.1021/ja300694p

Communication
pubs.acs.org/JACS
Synthesis of Kinetically Stabilized 1,2-Dihydrodisilenes
Tomohiro Agou,†,‡ Yusuke Sugiyama,† Takahiro Sasamori,*,† Heisuke Sakai,§ Yukio Furukawa,§
Nozomi Takagi,∥ Jing-Dong Guo,∥ Shigeru Nagase,∥ Daisuke Hashizume,⊥ and Norihiro Tokitoh*,†
†Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
‡Kyoto University Pioneering Research Unit for Next Generation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
§Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo,
Shinjuku-ku, Tokyo 169-8555, Japan
∥Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Myodaiji, Okazaki 444-8585,
Japan
⊥Advanced Technology Support Division, RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
S
* Supporting Information
synthetic challenges involved, 1,2-dihydrodisilenes are desirable
ABSTRACT: Kinetically stabilized 1,2-dihydrodisilenes
targets for two important reasons: (i) the H−SiSi moiety is
were successfully synthesized and isolated by the
introduction of sterically protecting bulky aryl groups.
These 1,2-dihydrodisilenes exhibit distinct SiSi double-
bond character in both solution and the solid state. The
Si−H bonds in these 1,2-dihydrodisilenes exhibit higher s
character than those of typical σ4,λ4-hydrosilanes. Moder-
ate heating of these 1,2-dihydrodisilenes in solution
resulted in their isomerization to the corresponding
trihydrodisilanes, with an intramolecular hydrogen migra-
tion as the rate-determining step.
expected to exhibit characteristics distinctly different from those
of σ4,λ4-hydrodisilanes [H−Si(R)2−SiR3], and (ii) the H−Si
Si−H moiety represents a powerful model for the parent
disilene, H2SiSiH2, allowing fundamental insights into the
intrinsic nature of the bonding situation in H2SiSiH2. The
successful application of the bulky aryl groups Bbp and Bbt in
the kinetic stabilization of a variety of compounds containing
low-coordinated heavier main-group elements7 prompted us to
investigate their potential for the steric protection of the labile
H−SiSi−H moiety. Herein we report the synthesis,
structure, spectroscopic properties, and thermal isomerization
of kinetically stabilized 1,2-dihydrodisilenes 1a and 1b.
he high reactivity of the Si−H bond in σ4,λ4-hydrosilanes
T
(R3Si−H) and its versatility in organic and organometallic
transformations have made these compounds one of the most
attractive classes of chemical feed stock among organosilicon
compounds.1 However, the structurally closely related σ3,λ4-
hydrosilanes RnESi(R)−H (E = main-group element) remain
an exotic and underdeveloped class of hydrosilanes. The
severely changed coordination environment around the silicon
atom in σ3,λ4-hydrosilanes relative to that in σ4,λ4-hydrosilanes
is expected to result in significant differences in the properties
and reactivity of σ3,λ4-hydrosilanes. Only a few examples of
σ3,λ4-hydrosilanes, such as mono- and dihydrosilenes2 as well as
monohydrodisilenes [e.g., lithiodisilene R1(H)SiSi(Li)R1 (I)
(R1  Si(i-Pr)Dis2, Dis = CH(SiMe3)2)] have been reported
to date.3 Despite the recent progress in the chemistry of
kinetically stabilized multiply bonded silicon compounds, stable
disilenes bearing more than one hydrogen substituent [e.g., 1,2-
dihydrodisilenes R(H)SiSi(H)R]4 still remain elusive.5
Synthetically, the key challenge for the successful isolation of
such multiply hydrogen-substituted disilenes lies in the effective
stabilization of the sterically exposed H−SiSi−H moiety.
Wiberg et al.6 have suggested the intermediate formation of 1,2-
dihydrodisilene R2(H)SiSi(H)R2 (II) (R2  SiH[Si(t-
Bu)3]2) on the basis of NMR data of the crude reaction
mixture. However, because of its facile thermal decomposition,
II could not be isolated, and the structure of II was assigned by
analysis of the decomposition product. Regardless of the
Scheme 1. Synthesis of 1,2-Dihydrodisilenes 1a and 1b
Reduction of dibromosilanes 2a and 2b with 2 equiv of
lithium naphthalenide (LiNaph) at −110 °C in THF/Et2O/
hexane (2a) or THF/hexane (2b) afforded 1,2-dihydrodisilenes
1a and 1b as pale-yellow solids in yields of 57 and 32%,
respectively (Scheme 1). Both in the solid state and in solution
(C6D6), 1a and 1b did not show any susceptibility toward
decomposition below 25 °C.
Structural parameters of 1a in the solid state were
determined by X-ray diffraction analysis (Figure 1a).8 Disilene
1a has a crystallographic center of symmetry at the middle of
the Si−Si bond. Hydrogen atom H1 was located on the
difference Fourier maps and refined isotropically. The Si−Si
bond length in 1a [2.1708(6) Å] is contracted by 8% relative to
that in the tetrahydrodisilane Bbp(H)2Si−Si(H)2Bbp (3)
[2.3633(2) Å] and comparable to those in previously reported
Received: January 30, 2012
Published: February 22, 2012
© 2012 American Chemical Society
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are known to be observed in the range from 500 to 600 cm−1,13
indicating that 1a and 1b contain distinct SiSi double bonds
in the solid state. The IR spectra of 1a and 1b in a KBr disk
showed Si−H vibrational frequencies at νSiH = 2160 cm−1 (1a)
and 2151 cm−1 (1b). The observed values are again in good
conformity with the calculated value for 1a (2168 cm−1) and
slightly larger than those for σ4,λ4-hydrosilanes (e.g., 2122 cm−1
for Ph3SiH).2a
Moderate heating of solutions of 1a and 1b in C6D6 to 80 °C
resulted in the isomerization of these disilenes into the
corresponding disilanes 4a and 4b (Scheme 2).14 The
formation of 4a and 4b can be logically explained by a
mechanism proceeding via an initial 1,2-hydrogen migration to
form intermediate silylsilylenes 5a and 5b, followed by a
subsequent insertion of the silylene moiety into the benzylic
C−H bonds of the sterically demanding CH(SiMe3)2 groups.15
A similar mechanism was proposed for the decomposition of
transient disilene II.6,16
Figure 1. (a) Molecular structure of 1a. Thermal displacement
ellipsoids are drawn at the 50% probability level. H atoms except for
H1 and H1* have been omitted for clarity. Selected bond lengths (Å)
and angles (deg): Si1−Si1* = 2.1708(6), Si1−C1 = 1.8843(10), Si1−
H1 = 1.45(2), Si1*−Si1−C1 = 126.87(4), C1−Si1−H1 = 114.5(9),
H1−Si1−Si1* = 118.1(9), Si1*−Si1−C1−C2 = 81.85(9)°. (b)
Experimental and calculated SiSi bond lengths and trans-bent
angles (θ) in 1a.
disilenes (2.138−2.360 Å),9 indicating significant SiSi
double-bond character for 1a in the crystalline state. In
addition, 1a exhibits a trans-bent structure, with a trans-bent
angle (θ) of 6.3° around the Si1 atom. The molecular structure
of 1a was reproduced by density functional theory (DFT)
calculations,10 and the calculated Si1−Si1* bond length (2.157
Å) and trans-bent angle (2.9°) were in good agreement with
those observed experimentally.
Scheme 2. Thermolysis of 1a and 1b
The 29Si NMR spectra of 1a and 1b in C6D6 showed signals
in the low-field region (1a, δSi = 63.3 ppm; 1b, δSi = 61.8 ppm),
characteristic of disilenes with adjacent carbon substituents. In
the 1H NMR spectra of 1a and 1b (C6D6), the hydrogen nuclei
attached to the low-coordinated silicon atoms resonated at 6.04
and 6.11 ppm respectively. Both these values are low-field
shifted relative to those of σ4,λ4-hydrosilanes (e.g., δH = 4.83
ppm for 3 in C6D6), reflecting the magnetic anisotropy effect of
Kinetic studies of the thermolysis of 1b in hexane or THF by
UV−vis spectrometry revealed that the isomerization is a first-
order, unimolecular reaction. For hexane solutions, the
activation parameters ΔH⧧ and ΔS⧧ were estimated to be
20.6(5) kcal mol−1 and −15(2) cal K−1 mol−1. The negative
value of ΔS⧧ indicates a considerable conformational change of
1b during the rate-determining step. For THF solutions, the
activation parameters were determined to be ΔH⧧ = 23.6(7)
kcal mol−1 and ΔS⧧ = −6(2) cal K−1 mol−1. The small negative
value of ΔS⧧ does not support any involvement of THF
molecules during the rate-determining step. The isomerization
mechanism of 1b was further investigated using DFT
calculations.10 Figure 2 shows the computationally generated
energy profile for the thermal isomerization of 1b. The terminal
hydrogen atom H2 in 1b initially moves via transition state TS1
toward a bridging position above the SiSi π plane, resulting
in the formation of intermediate 6b. The calculated activation
parameters for this step (ΔH⧧ = 19.4 kcal mol−1, ΔS⧧ = −2.4
cal K−1 mol−1) are in close agreement with the experimentally
estimated values, suggesting that this step is rate-determining.
Although the transition state between 6b and 5b (TS2) could
not be located, the necessary activation barrier was estimated to
be sufficiently low to allow fast interconversion between 5b and
6b prior to the intramolecular cyclization.17,18 The final step
(5b → 4b) proceeds via TS3 with a large exothermicity (−36.2
kcal mol−1), suggesting that the isomerization from 1b to 4b
should proceed readily. The activation barrier for this step
(ΔH⧧ = 16.3 kcal mol−1) was found to be smaller than that for
the first step. Moreover, the activation entropy of this step was
the SiSi π electrons.11 The observed JSiH values of the H−
1
SiSi−H moieties in 1a (216 Hz) and 1b (210 Hz) are larger
than those in σ4,λ4-hydrosilanes such as 3 (188 Hz), suggesting
increased s character of the Si−H bonds. In contrast, previously
reported hydrogen-substituted disilenes I and II exhibited
much smaller JSiH values (I, 155 Hz;3a II, 149.8 Hz6),
1
suggesting a decrease in the s character of the Si−H bonds due
to the presence of the electropositive silyl groups.12 Similarly,
2
the observed JSiH values for the H−SiSi−H moieties in 1a
(16 Hz) and 1b (16 Hz) were larger than those in 3 (6.5 Hz)
and II (0.9 Hz), corroborating the increased s character of the
Si−H bonds in 1a and 1b. Independently, natural bond orbital
(NBO) calculations supported the greater s character of the
Si−H bonds in 1a [σSiH = 0.6591Si(sp2.50) + 0.7520H(s)]
relative to those in 3 [σSiH = 0.6557Si(sp3.12) + 0.7550H(s) and
0.6506Si(sp3.20) + 0.7594H(s)].10
The UV−vis spectra of 1a and 1b in hexane exhibited lowest-
energy absorption maxima λmax at 411 nm (ε = 2.1 × 104 M−1
cm−1) and 421 nm (ε = 2.0 × 104 M−1 cm−1) respectively,
which can be assigned to π−π* electron transitions in the Si
Si moiety. The almost identical λmax values for 1a (412 nm) and
1b (423 nm) in THF would exclude coordination of THF to
these disilenes. In the solid-state Raman spectra, 1a and 1b
exhibited Raman lines at 566 and 575 cm−1, respectively.
Theoretical calculations for 1a estimated the SiSi vibrational
frequency to be 588 cm−1,10 which is in good agreement with
the experimentally observed values. The SiSi stretching
frequencies for previously reported carbon-substituted disilenes
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Journal of the American Chemical Society
Communication
financially supported by the Nanotechnology Network of
MEXT, Japan.
REFERENCES
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Figure 2. Calculated reaction path for the isomerization of 1b. Values
beneath the energy levels represent the SCF energies relative to 1b (in
kcal mol−1); R  SiMe3. The activation parameters in the boxes were
calculated with zero-point vibrational energy corrections.
calculated to be close to zero (−0.06 cal K−1 mol−1), suggesting
that this final step, intramolecular cyclization, is not the
experimentally observed rate-determining step. In conclusion,
the computational results support the assumption that a gradual
isomerization of 1,2-dihydrodisilenes 1a and 1b via a 1,2-
hydrogen shift is what was observed in solution.
In summary, we have reported the synthesis, structure, and
spectroscopic properties of kinetically stabilized 1,2-dihydrodi-
silenes 1a and 1b. Both in the solid state and in solution, 1a and
1b retain a pronounced SiSi double bond, and their Si−H
bonds have higher s character than those of σ4,λ4-hydrosilanes.
Heating 1a and 1b resulted in isomerizations via intramolecular
hydrogen migrations, resembling the hydrogen-shift equili-
brium suggested for the parent disilene H2SiSiH2.18
(6) Wiberg, N.; Niedermayer, W.; Noth, H.; Warchhold, M. Z. Anorg.
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(8) The Si−Si bond length of 1b was estimated to be in the range of
2.16−2.18 Å on the basis of a preliminary X-ray crystallographic
analysis.
(9) The range of the SiSi bond lengths of disilenes was taken from
the Cambridge Crystallographic Data Centre database (82 examples).
(10) These calculations were performed at the B3PW91/6-
ASSOCIATED CONTENT
■
311+G(2df)[Si]:6-31G(d)[C,H] level of theory.
S
* Supporting Information
1
(11) The calculated 29Si and H chemical shifts of 1a and 1b (δSi
=
Experimental procedures, analytical data for new compounds,
computational results, and X-ray crystallographic data (CIF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
72.6 ppm, δH = 6.06 ppm for 1a; δSi = 66.7, 68.7 ppm, δH = 6.27, 6.28
ppm for 1b) were comparable to the experimentally observed values
(HCTH407/6-311+G(2df)[Si]:6-31G(d)[C,H]). For Gauge-Inde-
pendent Atomic Orbital (GIAO) calculations on disilenes, see:
Karni, M.; Apeloig, Y.; Takagi, N.; Nagase, S. Organometallics 2005,
24, 6319.
AUTHOR INFORMATION
■
(12) Bent, H. A. Chem. Rev. 1961, 61, 275.
Corresponding Author
(13) (a) Leites, L. A.; Bukalov, S. S.; Mangette, J. E.; Schmedake, T.
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2009, 82, 793.
(14) Thermolysis of 1a afforded small amount of unidentified
byproducts, resulting in a moderate yield of 4a, while disilene 1b
isomerized quantitatively to 4b.
(15) A transient silylene Tbt(Mes)Si: (Tbt = 2,4,6-tris[bis(trime-
thylsilyl)methyl]phenyl) undergoes facile cyclization in a fashion
similar to that of 5b. See: Suzuki, H.; Tokitoh, N.; Okazaki, R.;
Harada, J.; Ogawa, K.; Tomoda, S.; Goto, M. Organometallics 1995, 14,
1016.
(16) Although a direct pathway from disilene 1 to cyclic disilane 4 via
a [1,5]-H shift of the benzylic proton along with intramolecular
cyclization (as in the case of Mes2SiSiMes2) can be proposed for the
isomerization of 1, the mechanism shown here is considered to be the
most reasonable because the results of the theoretical calculations
reproduced well those obtained in the experimental kinetic study. See:
(a) Nguyen, T.-L.; Scheschkewitz, D. J. Am. Chem. Soc. 2005, 127,
sasamori@boc.kuicr.kyoto-u.ac.jp; tokitoh@boc.kuicr.kyoto-u.
ac.jp
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by Grants-in-Aid for
Scientific Research (B) (22350017), Young Scientists (A)
(23685010), and Young Scientists (B) (21750037) and the
Global COE Program B09 and Specially Promoted Research
(No. 22000009) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan. Computa-
tional time was provided by the Super Computer System,
Institute for Chemical Research, Kyoto University. This work
was partly performed within the Nanotechnology Support
Project in Central Japan (Institute for Molecular Science),
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Communication
10174. (b) Fink, M. J.; De Young, D. J.; West, R.; Michl, J. J. Am.
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(17) As for the isomerization of model disilenes R(H)SiSi(H)R (R
 H, Me, t-Bu), transition states corresponding to TS2 were found in
the DFT calculations, and the activation barrier was estimated to be <1
kcal mol−1. See the Supporting Information for details.
(18) The isomerization among 1b, 6b, and 5b resembles the
hydrogen-shift equilibrium of the parent disilene H2SiSiH2. See:
(a) Trinquier, G. J. Am. Chem. Soc. 1990, 112, 2130. (b) Dolgonos, G.
Chem. Phys. Lett. 2008, 466, 11. (c) Sari, L.; McCarthy, M. C.;
Schaefer, H. F. III; Thaddeus, P. J. Am. Chem. Soc. 2003, 125, 11409.
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