Unsymmetrically substituted disilyne Dsi2iPrSi- Si≡Si-SiNpDsi2 (Np = CH2tBu): Synthesis and characterization

Article abstract of DOI:10.1021/ja1091744

The unsymmetrically substituted disilyne, Dsi₂ ⁱPrSi-Si≡Si-SiNpDsi₂ (Np = CH₂ ^tBu) 2, was synthesized and characterized by X-ray crystallography to show a trans-bent structure with a silicon-silicon t

Full text of DOI:10.1021/ja1091744

Published on Web 11/05/2010
i
Unsymmetrically Substituted Disilyne Dsi2 PrSisSitSisSiNpDsi2
t
(Np ) CH2 Bu): Synthesis and Characterization
Yoshitaka Murata, Masaaki Ichinohe, and Akira Sekiguchi*
Department of Chemistry, Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba,
Tsukuba, Ibaraki 305-8571, Japan
Received October 12, 2010; E-mail: sekiguch@chem.tsukuba.ac.ip
trifluoromethanesulfonic acid, giving silyltriflate, which was reacted
i
Abstract: The unsymmetrically substituted disilyne, Dsi2 PrSis
with the silyl potassium Dsi2iPrSiK to produce tetrahydrodisilane 4.16
Tetrachloro-precursor 5 for the unsymmetrically substituted disilyne
was synthesized by the chlorination of 4 with 4 equiv of Cl2 in
CH2Cl2.17 Finally, the reductive dechlorination of 5 using 4 equiv of
KC8 in THF gave the first stable unsymmetrically substituted disilyne,
Dsi2iPrSisSitSisSiNpDsi2 (2), as air- and moisture-sensitive emerald
green crystals in 32% yield.15
t
SitSisSiNpDsi2 (Np ) CH2 Bu) 2, was synthesized and char-
acterized by X-ray crystallography to show a trans-bent structure
with a silicon-silicon triple bond length of 2.0569(12) Å. The 29Si
chemical shifts of the triply bonded silicon atoms of 2 are quite
different, being observed at 62.6 ppm for the Dsi2 PrSi side and
106.3 ppm for the Dsi2NpSi side, indicating different hybridizations
on the triply bonded silicon atoms at each site.
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Scheme 1. Synthesis of the Unsymmetrically Substituted Disilyne
2a
The chemistry of multiply bonded compounds containing heavier
group 14 elements has been developed since the isolation of the
stable distannene Dsi2SndSnDsi2 (Dsi ) CH(SiMe3)2) by Lappert
in 1973,1 the tetramesityldisilene Mes2SidSiMes2 (Mes ) 2,4,6-
trimethylphenyl), and the silene (Me3Si)2SidC(OSiMe3)Ad (Ad )
1-adamantyl) in 1981.2-4 Despite extensive experimental efforts
directed toward the synthesis of triply bonded compounds of heavier
group 14 elements, heavier analogues of alkynes (sEtEs; E )
Si, Ge, Sn, and Pb) remained unknown until recently.5 In 2000,
Power reported the synthesis and structural characterization of a
lead-alkyne analogue, ArPbPbAr (Ar ) 2,6-Tip2C6H3, Tip ) C6H2-
2,4,6-iPr3).6 Subsequently, tin7 and germanium8 analogues were also
synthesized by the same group in 2002 using bulky terphenyl
ligands (2,6-diarylphenyl groups) and by Tokitoh’s group using the
Bbt substituent (C6H2-2,6-[CH(SiMe3)2]2-4-C(SiMe3)3) for the
germanium analogue.9 Recently, this laboratory10 and Wiberg et
al.11 independently succeeded in synthesizing silicon-silicon triply
bonded species, disilynes, which in our case was isolated as emerald
a (a) (1) Ph(R2N)2SiLi (R2N ) (CH2CH2)2N), THF, 0 °C; (2) dry HCl,
i
Et2O, RT; (3) LiAlH4, Et2O, RT. (b) (1) CF3SO3H, toluene, RT; (2) Dsi2 PrK/
HMPA/THF/DME, -50 °C. (c) Cl2, CH2Cl2, -78 °C. (d) KC8, THF, -78
°C.
The silicon-silicon triple bonds of all reported disilynes are
trans-bent, not linear, which results in two nondegenerate occupied
π-MOs (πin as the HOMO - 1 and πout as the HOMO) and two
unoccupied antibonding π*-MOs (π*in as the LUMO and π*out as
the LUMO + 1).10 The molecular structure of disilyne 2 as
determined by X-ray crystallographic analysis is shown in Figure
1.15 The disilyne 2 also exhibits a trans-bent structure with bending
green crystals, Dsi2 PrSisSitSisSiiPrDsi2 (1a), and its trans-bent
i
i
structure was unequivocally determined by X-ray crystallography
as well as a solid-state 29Si NMR study.12 Later, Tokitoh’s group
reported an aryl-substituted disilyne by the use of very bulky aryl
groups.13 However, these heavy alkyne analogues have the same
substituent on each skeletal atom, and no unsymmetrically substi-
tuted heavy alkynes, which will be important for elucidating the
bonding nature of alkyne analogues, have been reported.14 Herein,
we report the synthesis and structure of the first isolable unsym-
angles of 138.78(5)° for the Dsi2 PrSi side and 137.89(5)° for the
Dsi2NpSi side. The SitSi bond length of 2 (2.0569(12) Å) is
remarkably shorter than typical SidSi bond lengths (2.143-2.228
Å),18 demonstrating its triple-bond character, and is also shorter
than those of 1a (2.0622(9) Å)10 and BbtsSitSisBbt (2.108(5)
Å).13 In the 29Si NMR spectrum of 2 in C6D6, eight signals are
observed at -3.5, -0.5, -0.3, 0.0, 0.1, 22.1, 62.6, and 106.3 ppm,
the former six signals being due to the silicon atoms of the
substituents. On the basis of 2-D Si-H NMR spectroscopy, the
low magnetic field signal at 62.6 ppm can be assigned to the triply
i
metrically substituted disilyne Dsi2 PrSisSitSisSiNpDsi2 (2) (Np
t
) CH2 Bu), including its X-ray crystal structure, the unusual
difference in the 29Si chemical shifts of the triply bonded silicon
atoms, theoretical calculations, and its thermal isomerization to a
cyclotrisilene derivative.
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bonded silicon atom bearing the Dsi2 PrSi group, and the other low
magnetic field signal at 106.3 ppm can be assigned to the triply
bonded silicon atom bearing the Dsi2NpSi group (see Supporting
Information). These 29Si NMR resonances of the triply bonded
silicon atoms in 2 are remarkably shifted to higher and lower fields
compared with those of the corresponding symmetrically substituted
disilynes (e.g., Dsi2iPrSi-substituted disilyne 1a: 89.9 ppm, Dsi2NpSi-
substituted disilyne 1b: 77.1 ppm).10,15
To prepare the unsymmetrically substituted disilyne 2, we have
designed a stepwise method starting from Dsi2NpSiSiH2Cl, as depicted
in Scheme 1.15 Thus, the reaction of Dsi2NpSiSiH2Cl with
Ph(R2N)2SiLi (R2N ) pyrrolidino) produced Dsi2NpSiSiH2Si(NR2)2Ph.
Deaminative chlorination of Dsi2NpSiSiH2Si(NR2)2Ph using HCl gas,
followed by reaction with LiAlH4, gave Dsi2NpSiSiH2SiH2Ph (3). The
phenyl group of 3 was converted to the triflate by reaction with
The structural parameters determined by X-ray crystallography
are not fully explained by the different 29Si chemical shifts of the
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16768 J. AM. CHEM. SOC. 2010, 132, 16768–16770
10.1021/ja1091744  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Isomerization of the Disilyne 2 to Cyclotrisilene 6
the Si1 atom has a larger bond angle than the Si2 atom (when θ is
larger than 135°), Si1 is deshielded compared with Si2, which
supports the experimental values of the different 29Si NMR chemical
shifts of 2 (the Dsi2NpSi side has a larger bond angle and is more
i
deshielded than the Dsi2 PrSi side). Based on the calculations in
Figure 2b, we attribute the different 29Si chemical shifts of the two
triply bonded silicon atoms to the different RsSitSi bond angles
at each terminus. A more acute RsSitSi bond angle causes a
higher degree of 3s contribution to the in-plane π-orbital and as a
result a higher field 29Si NMR chemical shift (i.e., of the
Figure 1. ORTEP drawing of 2 (50% thermal ellipsoids). Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å): Si1-Si2 ) 2.0569(12),
Si1-Si3 ) 2.3598(11), Si2-Si8 ) 2.3671(11), Si3-C1 ) 1.901(3),
Si3-C8 ) 1.909(3), Si3-C15 ) 1.920(3), Si8-C18 ) 1.908(3), Si8-C25
) 1.907(3), Si8-C32 ) 1.896(3). Selected bond angles (deg): Si3-Si1-Si2
) 138.78(5), Si1-Si2-Si8 ) 137.89(5).
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Dsi2 PrSisSi*tSi part).
The unsymmetrically substituted disilyne 2 slowly underwent
isomerization in solution at room temperature, giving cyclotrisilene
derivative 6 as the sole product (Scheme 2).21 Although the
mechanism for the formation of cyclotrisilene 6 by the thermal
reaction of 2 is uncertain, we can suggest that the reaction pathway
would involve two steps: migration of the Dsi group followed by
cyclization of the SisSitSi unit. Interestingly, the rearrangement
and cyclization occurred only on the Dsi2NpSi side to produce 6
as the sole product, probably for steric reasons.
two triply bonded silicon atoms. Therefore, we have performed
density functional theory (DFT) calculations on Dsi2 PrSis
i
SitSisSiNpDsi2 (2) at the GIAO/B3LYP/6-31G(d)//B3LYP/6-
31G(d) level (see Supporting Information). The optimized structure
of 2 shows a large bond angle difference between the two silicon
i
termini; the RsSitSi bond angles are 134.8° for the Dsi2 PrSi side
and 141.5° for the Dsi2NpSi side, with a SitSi bond length of
2.091 Å. The calculated 29Si chemical shifts of the triply bonded
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silicon atoms of 2 were 87.6 ppm for the Dsi2 PrSi side and 152.0
ppm for the Dsi2NpSi side, supporting the experimental values of
the different chemical shifts of the triply bonded silicon atoms.
Apeloig, Nagase et al. reported that small geometry changes in
the RsSitSi bond angle cause significant changes in the 29Si
chemical shifts of triply bonded silicon atoms.19 To provide an
estimate of the relationship between the 29Si chemical shifts of triply
bonded silicon atoms and the RsSitSi bond angles, calculations
were performed on the model compound Me3SisSi1tSi2sSiMe3
(2′). In these calculations, the bond angle of SisSi2tSi1 was fixed
at 135° and θ (SisSi1tSi2) was the variable parameter, θ (θ )
90° to 180°). The potential energy curve for the bond angle of
Me3SisSi1tSi2-SiMe3 is very flat (less than 1 kcal/mol) in the
range from 120° to 150° (Figure 2a). Therefore, the difference in
the bond angle of 2 between its X-ray structure and the optimized
calculated structure is most probably due to crystal packing forces.20
The calculated 29Si chemical shifts of the two skeletal silicon atoms
as a function of the bond angle θ are plotted in Figure 2b. As the
bond angle θ increases, the calculated 29Si values of Si2 are
significantly upfield shifted, from +100 (θ ) 135°) to -25 ppm
(θ ) 180°), whereas the calculated 29Si values of Si1 are downfield
shifted, from +100 (θ ) 135°) to +200 ppm (θ ) 180°). When
Acknowledgment. We are grateful to Prof. Yitzhak Apeloig
(Technion) for helpful discussions. This work was supported by a
Grant-in-Aid for Scientific Research (Nos. 19105001, 21350023,
21108502) from the Ministry of Education, Science, Sports, and
Culture of Japan.
Supporting Information Available: The experimental procedures
and spectral data for new compounds in Scheme 1, 1b, and 6,
computational results for the compound 2 and the model compound
2′, tables of crystallographic data, including atomic positional and
thermal parameters for 2 (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
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Figure 2. Theoretical calculations on Me3SisSi1tSi2sSiMe3 2′ (the bond
angle of SisSi2tSi1 is fixed at 135° and θ (SisSi1tSi2) is the variable
parameter): (a) relative energies and (b) chemical shifts of skeletal silicon
atoms (red line for Si1 and blue line for Si2) calculated at the GIAO/B3LYP/
6-311G(d)//6-31G(d) level.
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C O M M U N I C A T I O N S
(12) Kravchenko, V.; Kinjo, R.; Sekiguchi, A.; Ichinohe, M.; West, R.; Balazs,
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(15) For the experimental procedures and spectral data for new compounds in
Scheme 1, a disilyne 1b, and 6 and crystal data for 2, see the Supporting
Information.
(16) The Dsi2NpSi group is large enough to protect the active triply bonded
silicon part against dimerization and isomerization. Disilyne
Dsi2NpSisSitSisSiNpDsi2 (1b) was synthesized by the reductive debro-
mination of the corresponding tetrabromo-precursor (see Supporting
Information).
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