Unusual locking of silicon chains into all-transoid conformation by pentacoordinate silicon atoms

Full text of DOI:10.1021/ja0040621

J. Am. Chem. Soc. 2001, 123, 3597-3598
3597
Unusual Locking of Silicon Chains into all-transoid
Conformation by Pentacoordinate Silicon Atoms
Ibrahim El-Sayed, Yasuo Hatanaka,* Shun-ya Onozawa, and
Masato Tanaka*
National Institute of Materials and Chemical Research
Tsukuba, Ibaraki 305-8565, Japan
ReceiVed NoVember 27, 2000
Conformation of flexible linear chains are of fundamental
importance in determining the physical properties of organic and
inorganic polymers.1 Over the past decade, a great deal of effort
has been made for conformational control of silicon chains2
because electronic and optical properties of σ-conjugated silicon
polymers such as polysilanes and polycarbosilanes are highly
sensitive to subtle changes in the backbone conformation.3 In this
regard, conformational locking of the silicon chains into an all-
transoid form is essential for realizing the useful physical
properties of silicon compounds,4 since all-transoid conformation
allows effective σ-conjugation along the silicon chain.2g,3b How-
ever, conformational control of a silicon chain is extremely
difficult, since the rotational barriers about Si-Si single bonds
are so small that essentially free rotation occurs at room
temperature. For example, the energy barrier to the rotation about
the Si2-Si3 bond of Si4Me10 is predicted to be only 3.5 kcal/
mol, which is significantly smaller than that of the corresponding
C3-C4 bond of octamethylhexane (>20 kcal/mol), revealing the
greater flexibility of silicon chains compared with carbon chains.5
We report herein unusual locking of silicon chains into all-
transoid conformation by pentacoordinate silicon atoms. Thus,
the internal rotation about the Si-Si single bonds of pentacoor-
dinate pentasilane 1 was found to be nearly completely inhibited
even in a room-temperature solution, although there are no
significant steric interactions between the substituents (Figure 1).
Pentacoordinate pentasilane 1 was prepared by the reaction of
N-methyl-N-trimethylsilylacetamide with {Me3SiSi(CH2Cl)-
Cl}2SiMe2 (1:1 mixture of diastereomers) in hexane at room
temperature.6 The reaction stereoselectively gave the dl-isomer
of 1, which was recrystallized from hexane/benzene to give
analytically pure 1 in 81% yield as colorless, benzene-containing
crystals.
Figure 1. X-ray structure of 1. All hydrogen atoms and crystalline solvent
are omitted for clarity. Selected bond distances (Å), bond angles (deg),
and dihedral angle (deg): Si1-Si2 2.3480(9), Si2-Si3 2.3353(8), Si2-
O1 1.990(2), Si2-Cl1 2.3499(7); C4-Si2-Cl1 88.22(7), Si1-Si2-Cl1
97.24(3), Si3-Si2-Cl1 90.96(3), Si1-Si2-C4 122.06(7), Si1-Si2-Si3
113.50(3), Si3-Si2-C4 124.08(7), O1-Si2-Cl1 170.70(5), Si2-Si3-
Si2* 111.66(4); Si1-Si2-Si3-Si2* 163.61(3).
Figure 2. UV spectra of (a) a thin film of 1, (b) 1 in ether solution, (c)
a thin film of 2, and (d) 1 in acetonitrile solution.
the almost undistorted trigonal bipyramidal (TBP) structure of 1
as indicated by the high %TBPa and %TBPe values for the
pentacoordinate silicon atoms (89 and 99%, respectively).7 The
most remarkable feature of the crystal structure of 1 is the all-
transoid conformation of the silicon backbone. Thus, pentaco-
ordinate pentasilane 1 has a stretched silicon chain; the Si-Si
bond lengths (2.3353(8), 2.3480(9) Å), Si-Si-Si bond angles
(113.50(3), 111.66(4)°), and Si-Si-Si-Si dihedral angle (163.61-
(3)°) meet expectations for the all-transoid conformation.
The solid-state UV spectrum of the thin film of 1 exhibited an
X-ray analysis of 1 revealed that strong intramolecular O f
Si coordination in an N-[(chlorosilyl)methyl]amide system led to
(1) (a) Dehong, H.; Ji, Y.; Kim, W.; Bagchi, B.; Rossky, P. J.; Barbara, P.
F. Nature 2000, 405, 1030-1033. (b) Mark, J. E.; Allcock, H. R.; West, R.
Inorganic Polymers; Prentice Hall: New Jersey, 1992.
(2) (a) Obata, K.; Kabuto, C.; Kira, M. J. Am. Chem. Soc. 1997, 119,
11345-11346. (b) Yuan, C.-H.; West, R. Macromolecules 1994, 27, 629-
630. (c) KariKari, E. K.; Greso, A. J.; Farmer, B. L.; Miller, R. D.; Rabolt,
J. F. Macromolecules 1993, 26, 3937-3945. (d) Schilling, F. C.; Lovinger,
A. J.; Davis, D. D.; Bovey, F. A.; Zeigler, J. M. Macromolecules 1993, 26,
2716-2723. (e) Harrah, L. A.; Zeigler, J. M. Macromolecules 1987, 20, 601-
608. (f) Miller, R. D.; Hofer, D.; Rabolt, J. J. Am. Chem. Soc. 1985, 107,
2172-2174. (g) Miller, R. D.; Michl, J. Chem. ReV. 1989, 89, 1359-1410
and references therein.
(3) a) Imhof, R.; Teramae, H.; Michl, J. Chem. Phys. Lett. 1997, 270, 500-
505. (b) Klingensmith, K. A.; Downing, J. W.; Miller, R. D.; Michl, J. J. Am.
Chem. Soc. 1986, 108, 7438-7439.
(4) The term “all-transoid” denotes a backbone conformation whose
dihedral angle is close to 165°. In the recent past, this conformation has been
confused with “all-trans conformation”; however, “all-trans” should refer to
a dihedral angle of 180°.
intense absorption at 257 nm, which is attributed to the σSiSi
f
σ*SiSi excitation of the silicon backbone with the all-transoid
conformation (Figure 2a).6b A weak absorption around 220 nm
is assignable to the amide chromophore. To our surprise, the UV
spectrum of 1 in ether solution was essentially similar to the solid-
state spectrum, showing an intense absorption at 257 nm (ꢀ )
22 000 M-1 cm-1) (Figure 2b). The obvious similarity between
the solid state and the solution spectra reveals that the most stable
conformation of 1 is all-transoid even in a room-temperature
solution, because UV spectra of oligosilanes are highly sensitive
to the conformational change of the silicon backbones.3a In sharp
contrast, the conformational properties of tetracoordinate oligosi-
(5) Neumann, F.; Teramae, H.; Downing, J. W.; Michl, J. J. Am. Chem.
Soc. 1998, 120, 573-582.
(6) Recently, we have reported that introduction of pentacoordinate silicon
atoms into oligosilanes leads to a drastic change in the electron transition
energies of the Si-Si bonds: (a) El-Sayed, I.; Hatanaka, Y.; Muguruma, C.;
Shimada, S.; Tanaka, M.; Koga, N.; Mikami, M. J. Am. Chem. Soc. 1999,
121, 5095-5096. (b) Muguruma, C.; Koga, N.; Hatanaka, Y.; El-Sayed, I.;
Mikami, M.; Tanaka, M. J. Phys. Chem. A 2000, 104, 4928-4935.
(7) %TBP values indicate the degree of pentacoordinate character of a
silicon atom: Tamao, K.; Hayashi, T.; Ito, Y.; Shiro, M. Organometallics
1992, 11, 2099-2114.
10.1021/ja0040621 CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/21/2001

3598 J. Am. Chem. Soc., Vol. 123, No. 15, 2001
Communications to the Editor
lanes are strikingly different from those of 1. For instance, the
UV spectrum of the thin film of analogous tetracoordinate
pentasilane Me3SiSi(Cl)MeSiMe2Si(Cl)MeSiMe3 2 (viscous liq-
uid) showed three absorption maxima at 215, 230, and 255 nm,
indicating that 2 exists as a mixture of several conformers in liquid
state (Figure 2c).8 These results imply that the pentacoordinate
silicon atoms effectively hinder the segmental motion of the
silicon chains, stabilizing the all-transoid conformation.
29Si NOE (nuclear Overhauser effect) experiments in deuterated
benzene solution have provided conclusive evidence for the
conformational rigidity of 1 in room-temperature solution. The
negative NOE η values of 29Si nucleus are correlated to the
segmental motion of the silicon chains in solution.9 The -η value
of the central Si3 atom of 1 (-24.89 ppm) comes close to the
limit, that is, 2.37 (94% DD relaxation),10 indicating that the
segmental motion of the Si2-Si3-Si2* skeleton is nearly
completely hindered. Thus, it is clear that the rotation about the
Si2-Si3 bonds is completely inhibited in NMR time scale in ether
and benzene solutions. In contrast, 29Si NOE experiment in CD3-
CN revealed a marked decrease in the -η value of the central
Si3 atom (-11.75 ppm; -η ) 1.86; 74% DD relaxation),
indicating that the rotation about the Si2-Si3 bond is allowed in
polar solvent. Consequently, the UV spectrum of 1 in acetonitrile
solution displayed a broadening of the band with a significant
Figure 3. (a) Potential energy of 1 as a function of the dihedral angle ω
(Si1-Si2-Si3-Si2*). For the rotation around the Si2-Si3 bond, change
of the ω is positive for counterclockwise rotation when viewed from the
side of the Si1 atom. (b) Change of dipole moment of 1 as a function of
the ω (Si1-Si2-Si3-Si2*).
All of these things make it clear that electrostatic interactions
between the strongly polarized Cl-Si-O bonds play a critical
role in hindering the rotation about the Si2-Si3 bonds. Thus,
the all-transoid conformer is effectively stabilized by cancellation
of the bond dipole moments, whereas other conformers would
be significantly destabilized by unfavorable alignment of the bond
dipoles.15 The conformational lability of 1 in highly polar solvents
such as acetonitrile is entirely consistent with this conclusion.16
These results can be sharply contrasted with the situation in
tetracoordinate peralkylated silicon chains, where the contribution
of electrostatic interactions to the rotational barriers are negligible.5
Thus, it is concluded that retardation of the free rotation about
Si-Si bonds by pentacoordinate silicon atoms is a unique and
most practical method for locking the conformations of silicon
chains. Although conformational control of oligosilanes by steric
interactions between the substituents17 or conformationally rigid
cyclic systems8a,18 have been recently reported, this is the first
example where hypervalent silicon atoms exert a strong influence
on the backbone conformation. This strategy will stimulate the
development of useful organic materials with unique electronic
and optical properties which cannot be realized by tetracoordinate
silicon compounds.19
decrease of the intensity (λmax ) 253 nm, ꢀ ) 7500 M-1 cm-1
)
(Figure 2d), apparently due to the conformational lability of 1 in
acetonitrile.
The origin of the conformational locking by the pentacoordinate
silicon atoms is profoundly interesting. The relatively short Si2-
Si3 bond length of 1 (2.3353(8) Å), which is comparable to that
of Me3Si-SiMe3 (2.340(9) Å),11 suggests that there is no
appreciable steric interaction between the Si2 and Si3 atoms.
Therefore, the conformational rigidity of 1 cannot be explained
by the steric interference alone. To gain an insight into the nature
of the rotational barriers about the Si2-Si3 bonds, the potential
energy profile for the rotation was calculated by freezing the
Si1*-Si2*-Si3-Si2 dihedral angle (ω) at 160° and optimizing
all other coordinates (Figure 3).12 The energy curves calculated
by semiempirical PM3 method13 are essentially consistent with
the experimental observations: for example, the presence of the
all-transoid minimum at 200° [160° as ω(Si1-Si2-Si3-Si2*)]
and two energy maxima at 100° and 340°, which are 14.9 and
16.8 kcal/mol higher in energy than the all-transoid minimum,
respectively.
Acknowledgment. We are grateful to Professor Yoshio Kabe
(Tsukuba University) and Dr. Tadafumi Uchimaru (NIMC) for helpful
discussion and to the Japan Science and Technology Corporation (JST)
for financial support through the CREST (Core Research for Evolution
Science and Technology) program and for a postdoctoral fellowship to
I.E.-S.
In Figure 3, the calculated dipole moments of pentasilane 1 as
a function of the dihedral angle ω are also shown. It is obvious
that the change of the dipole moments of 1 is closely similar to
that of the potential energy curve. Moreover, the hypervalent Cl-
Si-O bonds should have a large dipole moment as a result of
donor-acceptor O f Si interaction involving charge transfer.14
Supporting Information Available: Experimental details and char-
acterization data for all new compounds, including the results of NOE
experiments and X-ray experimental details (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(8) Silicon chains adopting all-transoid conformation usually exhibit a
single absorption band: (a) Mazieres, S.; Raymond, M. K.; Raabe, G.; Prodi,
A.; Michl, J. J. Am. Chem. Soc. 1997, 119, 6682-6683. (b) Plitt, H. S.; Balaji,
V.; Michl, J. Chem. Phys. Lett. 1993, 213, 158-162.
(9) The silicon atoms with limited mobility relax predominantly via the
dipole-dipole mechanism as shown by the large -η values (DD relaxation),
while more mobile silicon atoms tend to relax via a spin-rotation mechanism
as indicated by smaller -η values (SR relaxation): (a) Levy, G. C.; Cargioli,
J. D.; Juliano, P. C.; Mitchell, T. D. J. Am. Chem. Soc. 1973, 95, 3445-
3454. (b) Pannell, K. H.; Bassindale, A. R. J. Organomet. Chem. 1982, 229,
1-9.
JA0040621
(14) Tandula, S. N.; Voronkov, M. G.; Alekseev, N. V. Top. Curr. Chem.
1986, 131, 99-189. Ab initio calculations (B3LYP/6-31G**) on model
compound, pentacoordinate (O-Si) chelate (H3Si)2Si(Cl)CH2NHCHO indi-
cated that the pentacoordinate silicon moiety has a dipole moment of 5.75 D.
(15) The PM3 calculations indicate the charge distributions for Cl (-0.665),
Si2 (0.549), and O (-0.310) atoms.
(16) Wong, M. W.; Frisch, M. J.; Wiberg, K. B. J. Am. Chem. Soc. 1991,
113, 4776-4782.
(10) NOE η value of -2.52 corresponds to 100% DD relaxation. The
contribution of DD relaxation mechanism to spin-lattice (T1) ralaxation is
indicated by % DD relaxation value defined as -η/2.52 × 100.9b
(11) Beagley, B.; Monaghan, J. J.; Hewitt, T. G. J. Mol. Struct. 1971, 8,
401-411.
(17) Tanaka, R.; Unno, M.; Matsumoto, H. Chem. Lett. 1999, 595-596.
(18) Tamao, K.; Tsuji, H.; Terada, M.; Asahara, M.; Yamaguchi, S.;
Toshimitsu, A. Angew. Chem., Int. Ed. 2000, 39, 3287-3290.
(19) For example, the emission property of 1 is vastly different from those
of tetracoordinate oligosilanes. Thus, 1 exhibited fluorescence emission at
283.4 nm when excited at 260.0 nm in isooctane at room temperature. The
observed Stokes shift (3175 cm-1) is markedly small, compared to that of
tetracoordinate oligosilanes such as Me12Si5 which shows the emission at 370
nm with a large Stokes shift (13 000 cm-1).8b
(12) Although this method does not yield all conformational minima of 1,
it is sufficient for quantitative understanding of the nature of the rotational
barriers.
(13) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 221-264.