Reversible photoswitching of the coordination numbers of silicon in organosilicon compounds bearing a 2-(phenylazo)phenyl group

Article abstract of DOI:10.1021/ja060926s

Several organosilicon compounds bearing a 2-(phenylazo)phenyl group were synthesized from the corresponding chlorosilanes and 2-lithioazobenzene prepared by halogen-lithium transmetalation of 2-iodoazobenzene. Their structures were determined by ¹

Full text of DOI:10.1021/ja060926s

Published on Web 04/29/2006
Reversible Photoswitching of the Coordination Numbers of
Silicon in Organosilicon Compounds Bearing a
2-(Phenylazo)phenyl Group
Naokazu Kano, Fuminori Komatsu, Masaki Yamamura, and Takayuki Kawashima*
Contribution from the Department of Chemistry, Graduate School of Science,
The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received February 8, 2006; E-mail: takayuki@chem.s.u-tokyo.ac.jp
Abstract: Several organosilicon compounds bearing a 2-(phenylazo)phenyl group were synthesized from
the corresponding chlorosilanes and 2-lithioazobenzene prepared by halogen-lithium transmetalation of
2-iodoazobenzene. Their structures were determined by 1H, 13C, 19F, and 29Si NMR spectra, UV-vis spectra,
and X-ray crystallographic analyses. In the UV-vis spectra, silyl groups caused red shifts of both the n-π*
and π-π* transitions of the azo group compared with the transitions of the unsubstituted azobenzene. The
E-isomers of the fluorosilanes showed an intramolecular interaction between a nitrogen atom of the azo
group and the silicon atom, leading their intermediate structures between a distorted trigonal bipyramidal
structure and a tetrahedral structure around the silicon atoms, which were revealed by the X-ray
crystallographic analyses and the NMR spectra. On the other hand, silanes without fluorine atoms showed
tetrahedral structures in the absence of such an interaction. The photoirradiation of the E-isomers of the
fluorosilanes afforded reversibly the corresponding Z-isomers in good yields. The silicon atoms of the
Z-isomers were found to be tetracoordinate in the absence of Si-N interactions by the 29Si NMR spectra.
The coordination numbers of the silicon atom of the fluorosilanes were reversibly switched between four
and five by photoirradiation. These properties were compared to those of a tetrafluoro[2-(phenylazo)phenyl]-
silicate.
Scheme 1
Introduction
Pentacoordinate organosilicon compounds have been studied
very much because of their interesting structures, properties,
and reactivities, some of which are different from those of
tetracoordinate ones.1 For example, some substituents on a
pentacoordinate silicon atom are more nucleophilically activated
than those on a tetracoordinate one.1c,2 Reversible change from
a tetracoordinated and tetrahedral structure to a pentacoordinated
and trigonal bipyramidal (TBP) structure by addition and
elimination of a fluoride anion has been reported to be useful
for the construction and decomposition of a conjugation system.3
If the tetracoordination state of a silicon atom in a neutral
tetravalent organosilicon compound could be reversibly changed
to a pentacoordination state in response to an external stimulus
without addition of any external reagents at the same temper-
ature, its structure and various properties would be controlled
in company with the coordination number of its silicon atom
(Scheme 1).
Stable neutral pentacoordinate organosilanes usually have a
heteroatom tether such as nitrogen and oxygen in the vicinity
of the silicon atom, so that the silicon atom shows a pentaco-
ordination state with an intramolecular interaction between the
heteroatom and the silicon atom. The coordination states can
easily be changed depending on the conditions such as tem-
perature.4 Thus, it is difficult to control the coordination numbers
of neutral organosilicon compounds between four and five under
certain conditions in the case where the coordination site is fixed
rigidly near the silicon atom or loosely contacted to the silicon
atom.5 There are some reports on the control of equilibrium
between the pentacoordinate and hexacoordinate organosilicon
(1) (a) Chuit, C.; Corriu, R. J. P.; Reye, C.; Young, J. C. In The Chemistry of
Organic Silicon Compounds, Part 1; Patai, S., Rappoport, Z., Eds.; Wiley:
Chichester, 1989; pp 1241-1288. (b) Holmes, R. R. Chem. ReV. 1990, 90,
17. (c) Chuit, C.; Corriu, R. J. P.; Reye, C.; Young, J. C. Chem. ReV. 1993,
93, 1371. (d) Wong, C. Y.; Woolins, J. D. Coord. Chem. ReV. 1994, 130,
175. (e) Holmes, R. R. Chem. ReV. 1996, 96, 927. (f) Chuit, C.; Corriu, R.
J. P.; Reye, C. In Chemistry of HyperValent Compounds; Akiba, K.-y.,
Ed.; Wiley-VCH: New York, 1999; pp 81-146. (g) Kira, M.; Zhang, L.-
C. In Chemistry of HyperValent Compounds; Akiba, K.-y., Ed.; Wiley-
VCH: New York, 1999; pp 147-169. (h) Kost, D.; Kalikhman, I. In The
Chemistry of Organic Silicon Compounds, Volume 2, Part 2; Patai, S.,
Apeloig, Y., Eds.; John Wiley & Sons: Chichester, 1998; pp 1339-1445.
(2) Corriu, R. J. P.; Lanneau, G. F.; Perrot, M. Tetrahedron Lett. 1988, 29,
1271.
(4) (a) Helmer, B. J.; West, R.; Corriu, R. J. P.; Poirier, M.; Royo, G.; de
Saxce, A. J. Organomet. Chem. 1983, 251, 295. (b) Brelie`re, C.; Carre´, F.;
Corriu, R. J. P.; Douglas, W. E.; Poirier, M.; Royo, G.; Wong Chi Man,
M. Organometallics 1992, 11, 1586.
(5) Brelie`re, C.; Carre´, F.; Corriu, R. J. P.; Poirier, M.; Royo, G. Organome-
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6793.
9
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J. AM. CHEM. SOC. 2006, 128, 7097-7109
7097

A R T I C L E S
Kano et al.
species by changing temperature, solvents, and counterions.6
However, the process of the coordination-number change should
be carried out under constant reaction conditions in order to
achieve reactivity control by the coordination-number change
alone without any additive and without changing the temperature
and the solvent.
Scheme 2
An azobenzene, which is expected to work as a chromophore
and a structural changing unit, is well utilized for the photo-
switching of various functions and properties.7 An azobenzene
framework has been used as a bidentate ligand in some transition
metal complexes.8 In the (E)-form of a 2-silyl substituted
azobenzene, the nitrogen atom is expected to coordinate to the
silicon atom and form a five-membered ring, which should
suitably stabilize the pentacoordination state of the silicon. We
previously reported photoswitching of the coordination numbers
of the silicon between five and six in anionic silicon compounds
bearing a 2-(phenylazo)phenyl group.9 The intramolecular
coordination of an azo unit would be also applicable to
photoswitching of the coordination numbers of the silicon
between usual and high coordination states, that is, between
tetracoordinate and pentacoordinate structures. Although there
have been some methods for the syntheses of azobenzenes
bearing a silicon substituent under basic or acidic conditions,
these reaction conditions are unsuitable for the syntheses of the
fluorosilanes, in which a pentacoordination state is expected to
be stabilized.10,11 The 2-(phenylazo)phenyl group is expected
to work both as a chromophore and a coordination site toward
the silicon atom. Actually, we have applied this method to the
syntheses of a disiloxane and an allylsilane bearing an azoben-
zene moiety.12,13
Scheme 3
ethylazobenzene.14 Treatment of 2-bromoazobenzene ((E)-1a)
with n-BuLi (1.05 equiv) and chlorotrimethylsilane (1.05 equiv)
successively in THF at -105 °C gave the corresponding 2-tri-
methylsilylazobenzene ((E)-2a) in 25% yield. The low yield of
(E)-2a is ascribed to the formation of both 1-(2-bromophenyl)-
1-butyl-2-phenylhydrazine and 1-(2-bromophenyl)-2-butyl-2-
phenylhydrazine, which were confirmed by GC-MS (m/z 318)
as byproducts. Lithiation of 2-iodoazobenzene ((E)-1b) instead
of (E)-1a generated 2-lithioazobenzene more efficiently, and
successive treatment of chlorotrimethylsilane afforded (E)-2a
in a satisfactory yield (80%). Keeping the reaction temperature
at -105 °C during the addition of n-BuLi is important to avoid
its attack on the azo group; otherwise the yield of (E)-2a be-
comes low. Several 2-silylazobenzene derivatives were synthe-
sized by this method (Scheme 2). Dimethylphenylsilyl derivative
(E)-2b was synthesized in a good yield (92%), while methyl-
diphenylsilyl derivative (E)-2c was obtained in a low yield
(29%). Triphenylsilyl derivative was not obtained in the reaction
with chlorotriphenylsilane probably due to the steric hindrance.
Triethoxysilane (E)-2d, monohydrosilanes (E)-2e and (E)-2f,
and diethoxysilane (E)-2g were also synthesized in moderate
yields, 75, 63, 79, and 49% yields, respectively. Use of 0.52
equiv of dichlorodimethylsilane, dichlorodiphenylsilane, and di-
chlorophenylsilane as electrophiles in the reactions of the aryl-
lithium gave the corresponding bis[2-(phenylazo)phenyl]silanes,
(E,E)-3a (89%), (E,E)-3b (70%), and (E,E)-3c (56%), respec-
tively. Fluorination of triethoxysilane (E)-2d and diethoxysilane
(E)-2g with BF3‚OEt2 proceeded smoothly to give trifluorosilane
(E)-4d (49%) and difluorosilane (E)-4g (67%), respectively
(Scheme 3). Further fluorination of (E)-4d with KF (1.0 equiv)
in the presence of 18-crown-6 gave tetrafluorosilicate (E)-5
We report here full details of the synthetic methods, structures,
and spectral properties of several neutral organosilanes bearing
one or two azobenzene moieties and the tetrafluorosilicate. We
also report reversible photocontrol of the coordination numbers
of the silicon atom between four and five in neutral fluorosilanes.
Synthesis
Lithium-halogen transmetalation of 2-haloazobenzene was
carried out in a manner similar to lithiation of 4-bromo-4′-
(6) (a) Kalikhman, I.; Girshberg, O.; Lameyer, L.; Stalke, D.; Kost, D. J. Am.
Chem. Soc. 2001, 123, 4709. (b) Kingston, V.; Gostevskii, B.; Kalikhman,
I.; Kost, D. Chem. Commun. 2001, 1272. (c) Kost, D.; Kalikhman, I.;
Kingston, V.; Gostevskii, B. J. Phys. Org. Chem. 2002, 15, 831. (d) Kost,
D.; Kingston, V.; Gostevskii, B.; Ellern, A.; Stalke, D.; Walfort, B.;
Kalikhman, I. Organometallics 2002, 21, 2293. (e) Kalikhman, I.; Gos-
tevskii, B.; Girshberg, O.; Krivonos, S.; Kost, D. Organometallics 2002,
21, 2551. (f) Kalikhman, I.; Gostevskii, B.; Girshberg, O.; Sivaramakrishna,
A.; Kocher, N.; Stalke, D.; Kost, D. J. Organomet. Chem. 2003, 686, 202.
(g) Gostevskii, B.; Adear, K.; Sivaramakrishna, A.; Silbert, G.; Stalke, D.;
Kocher, N.; Kalikhman, I.; Kost D. Chem. Commun. 2004, 1644. (h)
Wagler, J.; Bo¨hme, U.; Brendler, E.; Roewer, G. Z. Naturforsch. 2004,
59b, 1348. (i) Kost, D.; Kalikhman, I. AdV. Organomet. Chem. 2004, 50,
1. (j) Nakash, M.; Goldvaser, M. J. Am. Chem. Soc. 2004, 126, 3436. (k)
Wagler, J.; Bo¨hme, U.; Brendler, E.; Roewer, G. Organometallics 2005,
24, 1348. (k) Gostevskii, B.; Silbert, G.; Adear, K.; Sivaramakrishna, A.;
Stalke, D.; Deuerlein, S.; Kocher, N.; Voronkov, M. G.; Kalikhman, I.;
Kost, D. Organometallics 2005, 24, 2913.
(7) (a) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim, 2001.
(b) Molecular DeVices and Machines; Balzani, V., Venturi, M., Credi, A.,
Eds.; Wiley-VCH: Weinheim, 2003.
(8) (a) Fahey, D. R. J. Chem. Soc., Chem. Commun. 1970, 417. (b) Fahey, D.
R. J. Organomet. Chem. 1971, 27, 283. (c) Bruce, M. I.; Goodall, B. L. In
The Chemistry of the Hydrazo, Azo and Azoxy Groups, Part 1; Patai, S.,
Ed.; John Wiley & Sons: New York, 1975; pp 259-311. (d) Omae, I.
Chem. ReV. 1979, 79, 287.
(9) Kano, N.; Komatsu, F.; Kawashima, T. J. Am. Chem. Soc. 2001, 123, 10778.
(10) Sunthankar, S. V.; Gilman, H. J. Org. Chem. 1950, 15, 1200.
(11) Kano, N.; Komatsu, F.; Kawashima, T. Chem. Lett. 2001, 338.
(12) Kano, N.; Yamamura, M.; Komatsu, F.; Kawashima, T. J. Organomet.
Chem. 2003, 686, 192.
(13) Kano, N.; Yamamura, M.; Kawashima, T. J. Am. Chem. Soc. 2004, 126,
6250.
(14) Kozlecki, T.; Syper, L.; Wilk, K. A. Synthesis 1997, 681.
9
7098 J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006

Organosilicon Compds Bearing 2-(Phenylazo)phenyl Group
A R T I C L E S
Scheme 4
Table 1. 29Si NMR Data (δSi) and the λmax in the UV-vis Spectra
of Organosilicon Compounds Bearing One or Two
2-(Phenylazo)phenyl Group(s), the 29Si NMR Data (δSi′) of the
Corresponding Phenyl-Substituted Compounds, and the
Differences (∆δSi) of the 29Si Chemical Shifts
λ
max/nm (ꢀ b
)
a
nos.
δSi
π−π
*
n−π
*
references
δSi′
∆δSi
(E)-2a
(E)-2b
(E)-2c
(E)-2d
(E)-2e
(E)-2f
-4.0 325 (16 000) 450 (460) Me3PhSi
-7.5 325 (24 000) 453 (770) Me2Ph2Si
-10.3 326 (24 000) 455 (500) MePh3Si
-58.1 325 (17 000) 450 (600) (EtO)3PhSi
-17.0 325 (15 000) 448 (410) Me2PhHSi
-19.9 327 (21 000) 451 (570) Ph3HSi
-4.1c +0.1
-8.6d +1.1
-12.1d +1.8
-57.8c -0.3
-17.6e +0.6
-17.8e -2.1
(74%) (Scheme 4). Hydrosilanes (E)-2e, (E)-2f, and (E,E)-3c
were further converted to the corresponding fluorosilanes (E)-
4e (84%), (E)-4f (75%), and (E,E)-3d (87%), respectively, by
treatment with silver fluoride in THF (Schemes 2 and 3).
(E)-2g
(E,E)-3a
-32.6 325 (19 000) 450 (520) Ph2(EtO)2Si -32.4d -0.2
-7.8 326 (39 000) 456 (960) Me2Ph2Si
-8.6d +0.8
-14.0d +0.2
-17.8e -5.4
-4.7e -12.6
-73.7e -17.5
19.8e -11.5
(E,E)-3b -13.8 326 (38 000) 457 (1000) Ph4Si
(E,E)-3c -23.2 327 (33 000) 454 (770) Ph3HSi
(E,E)-3d -17.3 326 (37 000) 441 (1000) Ph3FSi
NMR Spectral Properties
(E)-4d
(E)-4e
(E)-4f
(E)-4g
(Z)-4d
(Z)-4e
(Z)-4f
(Z)-4g
(E)-5
-91.2 345 (24 000)
PhF3Si
The 1H, 13C, 19F, and 29Si NMR spectra of (E)-2a-g, (E,E)-
3a-d, (E)-4d-g, and (E)-5 were measured to clarify their
structures and coordination states of their silicon atoms. The
29Si NMR chemical shifts (δSi) are summarized in Table 1 and
compared with those of the corresponding phenyl-substituted
references (δSi′).5,15,16 In the 29Si NMR spectra, each chemical
shift of (E)-2a-g and (E,E)-3a,b is near those of the corre-
sponding phenyl-substituted derivatives. The respective differ-
ences of the chemical shifts (∆δSi ) δSi - δSi′) between (E)-
2a-g and the corresponding phenyl derivatives are less than
3 ppm. Considering the small ∆δSi values, (E)-2a-g and (E,E)-
3a,b possess tetracoordinate silicon atoms in the absence of
Si‚‚‚N interaction. Steric repulsion between the silyl groups and
a PhN moiety should prevent the Si‚‚‚N interaction.
8.3 328 (19 000) 434 (640) Me2PhFSi
-16.3 336 (20 000) 436 (700) Ph3FSi
-4.7e -11.6
-47.8 349 (20 000)
-70.6
Ph2F2Si
-30.5e -17.3
336 (2000)f PhF3Si
-73.7e
3.1
1.3
21.0 294 (7100) 444 (1900) Me2PhFSi
-7.3 288 (8000) 445 (2000) Ph3FSi
-29.8 286 (8000) 445 (2000) Ph2F2Si
19.8e
-4.7e -2.6
-30.5e
0.7
-150.9 334 (19 000)
PhF4Si-, K+ -125.8g -25.1
3.0
(Z)-5
-122.8 277 (6500) 433 (1800) PhF4Si-, K+ -125.8g
a In CDCl3. b In CH2Cl2. c Reference 5. d Reference 15. e Reference 16.
f In THF. g Reference 17.
The upfield shifts in the 29Si NMR were also observed in
tetrafluorosilicate (E)-5. In the 29Si NMR spectra, (E)-5 showed
a quintet (1JSiF ) 193 Hz) at δ -150.9 by coupling with four
equivalent fluorine nuclei at rt in CDCl3. The signal split to a
doublet of double triplet at δ -153.7 coupled with three kinds
of nuclei (1JSiF ) 149, 186, and 214 Hz, respectively) at -90
°C in CD2Cl2. The chemical shift is observed in a much higher
field than that of potassium tetrafluorophenylsilicate (δ -125.8)17
and one similar to those of a hexacoordinate tetrafluorosilicate
bearing a 2-(N,N-dimethylaminomethyl)phenyl group,4b indicat-
ing the hexacoordination state of the silicon atom due to the
coordination of the nitrogen atom of the azo group.
Compound (E,E)-3c showed spectral behavior slightly dif-
ferent from these silanes. In the 29Si NMR spectra, the signal
due to (E,E)-3c (δSi -23.2) was observed in a higher field than
that of triphenylsilane (δSi -17.8) by about 5 ppm. Considering
that the electronic perturbation through the C-Si bond in the
hydrosilane is as little as those in (E)-2a-g and (E,E)-3a,b,
the differences of the chemical shifts should be attributed to
the contribution of a pentacoordination state of the silicon atom
with a Si‚‚‚N interaction in (E,E)-3c in the solution state.
Differences in the 29Si NMR chemical shifts between 2-(phen-
ylazo)phenyl derivatives and the corresponding phenyl deriva-
tives are notable in fluorosilanes. In the 29Si NMR spectra,
trifluorosilane (E)-4d and difluorosilane (E)-4g showed a quartet
and a triplet by coupling with fluorine nuclei at δSi -91.2 (1JSiF
) 231.0 Hz) and -47.8 (1JSiF ) 264.5 Hz), respectively. Each
of the monofluorosilanes (E)-4e, (E)-4f, and (E,E)-3d showed
The pentacoordination states of some of these fluorosilanes
were confirmed by VT-19F NMR spectra. In the 19F NMR
spectra of (E)-4g in CDCl3, a singlet at δF -141.55 with satellite
peaks (1JSiF ) 268.4 Hz) at rt was split into two broad peaks at
δF -148.66 (1F) and -133.43 (1F) at -70 °C. Similarly, in
the 19F NMR spectra of (E)-4d, a broad singlet at δF -140.58
with satellite peaks (1JSiF ) 234.9 Hz) in CD2Cl2 at rt was split
to a doublet at δF -140.11 (2JFF ) 52.0 Hz) and a triplet at δF
-136.79 (2JFF ) 52.0 Hz) in the ratio of 2:1 at -90 °C.
Similarly, a broad singlet of (E)-5 at δ -127.4 in CDCl3 at rt
was split to two broad signals (δ -122.5 and -147.2) at -30
°C in CD2Cl2. They were further split to a pair of doublets at
δ -121.6 (2JFF ) 21.3 Hz) and -123.2 (2JFF ) 24.4 Hz) and
a doublet by coupling with a fluorine nucleus at δF 8.3 (1JSiF
)
)
267.7 Hz), δF -16.3 (1JSiF ) 273.6 Hz), and δF -17.3 (1JSiF
273.2 Hz), respectively. Fluorosilanes (E)-4d, (E)-4g, (E,E)-
3d, (E)-4e, and (E)-4f showed upfield shifts of about 12 ppm
compared to the corresponding trifluorophenylsilane (δSi -73.7),
difluorodiphenylsilane (δSi -30.5), fluorodimethylphenylsilane
(δSi -19.8), and fluorotriphenylsilane (δSi -4.7), respectively,
so that they are considered to have pentacoordination states in
the solution states.16 The ∆δSi values increase in the order of
numbers of fluorine atoms on the silicon atom. The electron-
withdrawing fluorine atoms raise the Lewis acidity of the silicon
atom, and the Si‚‚‚N interactions become stronger in accordance
with the number of the fluorine atoms.
one double-triplet at δ -147.2 (2JFF ) 21.3 Hz, JFF ) 24.4
Hz) at -90 °C, respectively, in CD2Cl2.
2
Such behaviors of the chemical shifts and the coupling
patterns indicate the nonequivalency of fluorine atoms due to
the coordination of a nitrogen atom of the azo group furnishing
TBP geometry around the pentacoordinate silicon atom in (E)-
4d and (E)-4g and octahedral geometry around the hexacoor-
dinate silicon atom in (E)-5.5,18 Fast pseudorotation of the
(15) Cragg, R. H.; Lane, R. D. J. Organomet. Chem. 1984, 277, 199.
(16) Williams, E. A.; Cargioli, J. D. In Annual Reports on NMR Spectroscopy;
Webb, G. A., Ed.; Academic Press: New York, 1979; Vol. 9.
(17) Swamy, K. C. K.; Chandrasekhar, V.; Harland, J. J.; Holmes, J. M.; Day,
R. O.; Holmes, R. R. J. Am. Chem. Soc. 1990, 112, 2341.
9
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A R T I C L E S
Kano et al.
ligands compared to the time scale of NMR spectroscopy would
result in observation of a singlet in the 19F NMR spectra at rt.
structure with N2 and F1 atoms at apical positions. One reason
for the deviation from an ideal TBP structure is rigidity of the
2-(phenylazo)phenyl group which contains benzo and azo units
in a five-membered ring framework and hence forces the
nitrogen atom to coordinate from the slanted backside of the
Si-F bond. Similarly, the structure of (E)-5 is interpreted as a
distorted octahedral configuration suggested by the bond angles
(76.41(6)°-99.47(6)°) around the Si1 atom. A calculation of
the extent that the Si1‚‚‚N2 distance [2.2498(13) Å] is displaced
from the sum of the van der Waals radii of 3.65 Å to the sum
of the covalent radii of 1.87 Å for silicon and nitrogen provides
an estimate of the octahedral character of (E)-5 of 79%.22
We tried to estimate the structural deviation of these
fluorosilanes from an ideal TBP structure. The pentacoordination
characters, %TBPa and %TBPe, have been used as indicators
of the structure about the central element in terms of the
percentage TBP geometry along a tetrahedronfTBP reaction
coordinate based on the apical-element-equatorial bond angles
and equatorial-element-equatorial bond angles, respectively.23,24
The pentacoordination characters, %TBPa and %TBPe, are
defined by the following eqs 1 and 2, where θn and æn are the
angles Lap-Si-Leq and Leq-Si-Leq, respectively. For the
pentacoordination characters of the Si1 atom, Lap is the F1 atom
in (E)-4d, (E)-4f, and (E)-4g. For the pentacoordination
characters of the Si1 atom, Leq are C1, F2, and F3 atoms in
(E)-4d, Leq are C1, C13, and C19 atoms in (E)-4f, and Leq are
C1, C13, and F2 atoms in (E)-4g, respectively.
X-ray Crystallographic Analyses of (E)-Isomers
The crystal structures of (E)-2c, (E)-2f, (E,E)-3a, (E)-4d, (E)-
4f, (E)-4g, and (E)-5 were determined by X-ray crystallographic
analyses. Their crystallographic data and selected bond lengths
(Å) and angles (deg) are summarized in Tables 2 and 3,
respectively. Their ORTEP drawings are shown in Figures 1-7.
In (E)-2c, (E)-2f, and (E,E)-3a, the silyl group and N2 atom
are arranged in a trans fashion with regard to the C6-N1 bond
(Figures 1-3). Steric repulsion between the bulky silyl group
and a PhN moiety overcomes attractive electrostatic intramo-
lecular Si‚‚‚N interaction. The intramolecular interaction cannot
defeat the steric repulsion even in hydrosilane (E)-2f bearing a
sterically less bulky silyl group.11 Even though (E,E)-3a has
two 2-(phenylazo)phenyl groups, one of which might coordinate
to Si1 atom, the other two methyl groups do not help the
coordination and there is no Si‚‚‚N interaction in it. Apparently,
(E)-2c, (E)-2f, and (E,E)-3a should be regarded as tetrahedral
structures around their silicon atoms.
In (E)-4d, (E)-4f, and (E)-4g, the N2 atoms are directed to
the Si1 atoms despite the steric repulsion (Figures 4-6). The
intramolecular N2‚‚‚Si1 distance of (E)-4d (2.3030(14) Å) is
almost the same as those of [8-(N,N-dimethylamino)naphthyl]-
trifluorosilane (2.287(4) and 2.318(6) Å for two independent
molecules).19 The intramolecular N2‚‚‚Si1 distance of difluo-
rosilane (E)-4g (2.419(2) Å) is slightly longer than that of [2-
(N,N-dimethylaminomethyl)phenyl]difluoromethylsilane (2.346(3)
Å).20 The intramolecular N2‚‚‚Si1 distance of monofluorosilane
(E)-4f (2.605(2) Å) is apparently shorter than that of 1-fluoro-
1,2-dimethyl-1-[8-(N,N-dimethylamino)naphthyl]-2,2-diphenyl-
disilane (2.852(2) Å).21 They are much shorter than the sum of
the van der Waals radii of Si and N (3.65 Å). Taking into
consideration that a hypervalent bond is generally longer than
the sum of the covalent radii and that the lone pair of the N2
atom heads for the σ* orbital of the Si1-F1 bond, intramolecular
N2‚‚‚Si1 interactions evidently exist in (E)-4d, (E)-4f, and (E)-
4g. Their N2‚‚‚Si1 interactions furnish them with pentacoordi-
nation states of the silicon atoms in the crystalline states. A
similar Si-N dative bond was observed in tetrafluorosilicate
(E)-5 (Figure 7).9 The intramolecular N2‚‚‚Si1 distance in (E)-5
is 2.2498(13) Å. The differences in their interatomic Si‚‚‚N
distances in order of (E)-5 > (E)-4d > (E)-4g > (E)-4f can be
rationalized by the electron deficiency of the silicon atoms and
the steric accessibility to the silicon atoms in this order, whereas
the silicon atom of (E)-5 is more sterically congested than that
of (E)-4d. Accordingly, N2‚‚‚Si1-F1 bond angles [(E)-4d,
176.62(6)°; (E)-5, 175.65(5)°; (E)-4g, 172.78(5)°; (E)-4f, 170.27-
(5)°] are in the order of the interatomic Si‚‚‚N distances except
(E)-5. The structures of (E)-4d, (E)-4f, and (E)-4g are interpreted
as the intermediate between the tetrahedral structure and TBP
3
1
%TBPa ) 109.5° -
θn /(109.5° - 90°) × 100 (1)
∑
{
}
)
(
3
n)1
3
1
%TBPe )
æn - 109.5° /(120° - 109.5°) × 100 (2)
∑
{
}
(
)
3
n)1
The pentacoordination characters, %TBPa and %TBPe, were
calculated to be 53% and 76% for (E)-4d, 53% and 76% for
(E)-4f, and 48% and 71% for (E)-4g, respectively. These values
suggest that the configuration around the silicon atoms are
intermediate structures between tetrahedral and TBP structures.
They are comparable with the pentacoordination characters of
previously reported pentacoordinate fluorosilanes such as [2-
(N,N-dimethylaminomethyl)phenyl]difluoromethylsilane (%TBPa,
61% and %TBPe, 83%)20 and 1-fluoro-1,2-dimethyl-1-[8-(N,N-
dimethylamino)naphthyl]-2,2-diphenyldisilane (%TBPa, 57%
and %TBPe, 80%).21,25 Although the Si‚‚‚N distances are in the
order of the number of fluorine atoms on the silicon atom for
(E)-4d, (E)-4f, and (E)-4g, the %TBP characters are not in this
order. Since the %TBP values are most effective in the case of
the intramolecular coordinating ligands which coordinate from
right behind the Si-F bond, they are not good for estimating
the TBP characters of (E)-4d, (E)-4f, and (E)-4g, in which N2
atoms coordinate to Si1 atoms obliquely from behind.
The N1-N2 bond lengths of (E)-2c, (E)-2f, and (E,E)-3a
[1.2515(16)-1.2580(18) Å] are almost the same as the average
dimensions (1.25 Å) of previously reported (E)-azobenzenes,26,27
(18) (a) Breliere, C.; Carre, F.; Corriu, R. J. P.; de Saxce, A.; Poirier, M.; Royo,
G. J. Organomet. Chem. 1981, 205, C1. (b) Boyer, J.; Brelie`re, C.; Carre´,
F.; Corriu, R. J. P.; Kpoton, A.; Poirier, M.; Royo, G.; Young, J. C. J.
Chem. Soc., Dalton Trans. 1989, 43. (c) Corriu, R. J. P.; Kpoton, A.; Poirier,
M.; Royo, G.; de Saxce´, A.; Young, J. C. J. Organomet. Chem. 1990, 395,
1.
(22) Timosheva, N. V.; Chandrasekaran, A.; Day, R. O.; Holmes, R. R. J. Am.
Chem. Soc. 2002, 124, 7035.
(23) Tamao, K.; Hayashi, T.; Ito, Y.; Shiro, M. Organometallics 1992, 11, 2099.
(24) Kano, N.; Kikuchi, A.; Kawashima, T. Chem. Commun. 2001, 2096.
(25) Toshimitsu, A.; Hirao, S.; Saeki, T.; Asahara, M.; Tamao, K. Heteroat.
Chem. 2001, 12, 392.
(19) Carre´, F.; Corriu, R. J. P.; Kpoton, A.; Poirier, M.; Royo, G.; Young, J.
C.; Belin, C. J. Organomet. Chem. 1994, 470, 43.
(20) Klebe, G. J. Organomet. Chem. 1987, 332, 35.
(21) Tamao, K.; Asahara, M.; Saeki, T.; Feng, S.; Kawachi, A.; Toshimitsu, A.
Chem. Lett. 2000, 660.
9
7100 J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006

Organosilicon Compds Bearing 2-(Phenylazo)phenyl Group
A R T I C L E S
b
Z
b
E
Z
E
E
Z
E
E
Z
E
E
E
E
E
a
E
E
E
E
E
E
2.
Tbale
9
J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006 7101

A R T I C L E S
Kano et al.
Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) for (E)-2c, (E)-2f, (E,E)-3a, (E)-4d, (E)-4f, (E)-4g, (Z)-4f, (E)-5, and (Z)-5
(E)-2c
Si1-C1
1.8899(18)
1.8651(19)
1.8786(18)
1.8803(17)
1.8854(19)
Si2-C38
Si2-C39
Si2-C45
N1-N2
1.8688(19)
1.8771(18)
1.8821(17)
1.2564(18)
1.2580(18)
C1-Si1-C13
C1-Si1-C14
C1-Si1-C20
C13-Si1-C14
C13-Si1-C20
C14-Si1-C20
111.97(7)
110.46(7)
106.47(7)
110.50(7)
108.71(7)
108.58(8)
C26-Si2-C38
C26-Si2-C39
C26-Si2-C45
C38-Si2-C39
C38-Si2-C45
C39-Si2-C45
112.09(7)
110.64(7)
107.66(7)
109.70(7)
107.87(8)
108.77(7)
Si1-C13
Si1-C14
Si1-C20
Si2-C26
N3-N4
(E)-2fa
Si1-C1
Si1-C13
Si1-C19
1.8757(13)
1.8712(13)
1.8741(13)
Si1-H1
N1-N2
1.397(13)
1.2546(15)
C1-Si1-C13
C1-Si1-C19
C13-Si1-C19
110.93(6)
108.94(6)
109.83(6)
C1-Si1-H1
C13-Si1-H1
C19-Si1-H1
111.4(6)
110.2(6)
105.5(6)
(E,E)-3a
Si1-C1
Si1-C13
Si1-C25
1.8899(14)
1.8906(14)
1.8671(13)
Si1-C26
N1-N2
N3-N4
1.8649(14)
1.2515(16)
1.2548(16)
C1-Si1-C13
C1-Si1-C25
C1-Si1-C26
108.16(6)
111.47(6)
109.64(6)
C13-Si1-C25
C13-Si1-C26
C25-Si1-C26
110.45(6)
106.47(7)
110.50(6)
(E)-4d
Si1-C1
Si1-F1
Si1-F2
1.8433(18)
1.5991(12)
1.5750(13)
Si1-F3
N1-N2
Si1‚‚‚N2
1.5720(12)
1.2589(18)
2.3030(14)
C1-Si1-F1
C1-Si1-F2
C1-Si1-F3
F1-Si1-F2
F1-Si1-F3
100.62(7)
120.19(7)
120.47(8)
98.59(7)
98.58(7)
F2-Si1-F3
C1-Si1‚‚‚N2
F1-Si1‚‚‚N2
F2-Si1‚‚‚N2
F3-Si1‚‚‚N2
111.61(7)
76.21(6)
176.62(6)
82.17(6)
84.15(6)
(E)-4f
Si1-C1
Si1-C13
Si1-C19
1.871(2)
1.857(2)
1.860(2)
Si1-F1
N1-N2
Si1‚‚‚N2
1.6343(17)
1.263(2)
2.605(2)
C1-Si1-C13
C1-Si1-C19
C13-Si1-C19
C1-Si1-F1
116.36(9)
118.87(9)
117.13(8)
99.10(7)
99.21(6)
C19-Si1-F1
C1-Si1‚‚‚N2
C13-Si1‚‚‚N2
C19-Si1‚‚‚N2
F1-Si1‚‚‚N2
99.49(7)
71.21(6)
84.77(6)
86.40(7)
170.27(5)
C13-Si1-F1
(E)-4g
Si1-C1
Si1-C13
Si1-F1
1.854(2)
1.850(2)
1.6201(14)
Si1-F2
N1-N2
Si1‚‚‚N2
1.5931(12)
1.2594(19)
2.419(2)
C1-Si1-C13
C1-Si1-F1
C1-Si1-F2
C13-Si1-F1
C13-Si1-F2
117.16(7)
99.65(6)
120.49(7)
103.14(7)
113.29(8)
F1-Si1-F2
C1-Si1‚‚‚N2
C13-Si1‚‚‚N2
F1-Si1‚‚‚N2
F2-Si1‚‚‚N2
97.65(6)
73.99(6)
83.07(6)
172.78(5)
83.03(6)
(Z)-4f
Si1-C1
Si1-C13
Si1-C19
1.8698(17)
1.8597(18)
1.8600(17)
Si1-F1
N1-N2
1.6190(12)
1.255(2)
C1-Si1-C13
C1-Si1-C19
C13-Si1-C19
120.40(7)
108.70(8)
114.29(7)
C1-Si1-F1
C13-Si1-F1
C19-Si1-F1
102.28(7)
103.45(7)
105.80(6)
(E)-5b
Si1-C1
Si1-F1
Si1-F2
Si1-F3
1.9120(15)
1.6428(10)
1.6661(10)
1.6923(9)
Si1-F4
N1-N2
Si1‚‚‚N2
1.6597(9)
1.2581(17)
2.2498(13)
C1-Si1-F1
C1-Si1-F2
C1-Si1-F3
C1-Si1-F4
F1-Si1-F2
F1-Si1-F3
F1-Si1-F4
F2-Si1-F3
99.47(6)
90.62(6)
90.10(6)
164.73(6)
96.93(5)
94.12(5)
95.76(5)
168.65(5)
F2-Si1-F4
F3-Si1-F4
C1-Si1‚‚‚N2
F1-Si1‚‚‚N2
F2-Si1‚‚‚N2
F3-Si1‚‚‚N2
F4-Si1‚‚‚N2
88.69(5)
87.63(5)
76.41(6)
175.65(5)
84.58(5)
84.57(5)
88.34(5)
(Z)-5b
Si1-C1
Si1-F1
Si1-F2
Si1-F3
Si1-F4
N1-N2
1.890(5)
1.617(3)
1.668(3)
1.697(3)
1.611(3)
1.261(5)
Si2-C25
Si2-F5
Si2-F6
Si2-F7
Si2-F8
N3-N4
1.872(4)
1.607(3)
1.676(3)
1.681(3)
1.622(3)
1.250(5)
C1-Si1-F1
C1-Si1-F2
C1-Si1-F3
C1-Si1-F4
F1-Si1-F2
F1-Si1-F3
F1-Si1-F4
F2-Si1-F3
F2-Si1-F4
F3-Si1-F4
117.77(19)
C25-Si2-F5
C25-Si2-F6
C25-Si2-F7
C25-Si2-F8
F5-Si2-F6
F5-Si2-F7
F5-Si2-F8
F6-Si2-F7
F6-Si2-F8
F7-Si2-F8
117.73(19)
91.28(17)
91.08(17)
122.51(19)
90.24(19)
89.93(17)
119.75(17)
177.23(16)
89.25(16)
88.25(15)
91.16(17)
90.53(17)
126.67(19)
90.29(16)
88.88(15)
115.53(16)
178.31(16)
90.26(16)
88.77(16)
a Reference 11. b Reference 9.
while those of the fluorosilanes (E)-4d, (E)-4f, and (E)-4g
[1.2589(18)-1.263(2)Å] tend to be slightly longer than those
of the (E)-azobenzenes which have no Si‚‚‚N interaction.
Elongation of these NdN bonds indicates slight decrease of its
double-bond character caused by the coordination of the lone
pairs to the silicon atoms.
n-π* transitions are summarized in Table 1. The absorption
maxima due to the 2-silylazobenzenes were expected to shift
from that of the unsubstituted azobenzene because the electro-
positive silyl substituents should perturb both π-π* and n-π*
transitions. Actually, 2-silylazobenzenes (E)-2a-g and (E,E)-
3a-d showed their absorption maxima of π-π* and n-π*
transitions at 325-327 and 441-456 nm, respectively, in CH2-
UV-vis Spectral Properties
(26) Allman, R. In The Chemistry of the Hydrazo, Azo and Azoxy Groups, Part
1; Patai, S., Ed.; John Wiley & Sons: New York, 1975; pp 41-48.
(27) Harada, J.; Ogawa, K.; Tomoda, S. Acta Crystallogr., Sect. B 1997, 53,
662.
The absorption maxima in the UV-vis spectra of (E)-2a-g,
(E,E)-3a-d, and (E)-4d-g in CH2Cl2 due to their π-π* and
9
7102 J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006

Organosilicon Compds Bearing 2-(Phenylazo)phenyl Group
A R T I C L E S
Figure 3. ORTEP drawing of (E,E)-3a (50% probability).
Figure 1. ORTEP drawing of (E)-2c (50% probability). One of two
independent molecules in the crystal is shown.
Figure 4. ORTEP drawing of (E)-4d (30% probability).
Figure 2. ORTEP drawing of (E)-2f (50% probability).
Cl2. Both the π-π* and n-π* transitions showed slight red-
shifts from those of the unsubstituted azobenzene (π-π*
transition, 318 nm; n-π* transition, 441 nm). The n and π
orbitals should be destabilized by the inductive effect of the
electron-donating silyl groups. The color of the solutions is red
similarly to the unsubstituted azobenzene, indicating a relatively
small perturbation caused by the silyl groups.
By contrast, the color of fluorosilanes (E)-4d-g is yellow,
suggesting a considerable perturbation of the electronic structure
of the azo unit. In the UV-vis spectra in CH2Cl2, they showed
a red shift of the π-π* transition and a blue shift of the n-π*
transition from those of the unsubstituted azobenzene. The red-
shifted π-π* transitions of the difluorosilane (E)-4g and
trifluorosilane (E)-4d overlapped with n-π* transitions, which
could not be identified. The wavelengths of their n-π*
transitions can be estimated to be shorter than those of
monofluorosilanes (E)-4e and (E)-4f. The wavelengths of
absorption maxima of the π-π* transitions of (E)-4g and (E)-
Figure 5. ORTEP drawing of (E)-4f (50% probability).
4d are longer than those of monofluorosilanes (E)-4e and (E)-
4f, while those of their n-π* transitions are in the reverse order,
considering their overlap as described above. These spectral
features are explained by the Si‚‚‚N interaction. The n orbital
(lone pair of the nitrogen atom) is stabilized significantly by
its donation to the silicon atom. The π and π* orbitals are less
stabilized by the Si-N interaction than the n orbital; what is
9
J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006 7103

A R T I C L E S
Kano et al.
Scheme 5
used to limit the wavelengths, for 1 h to give (Z)-4f (Scheme
5). When the photoisomerization of (E)-4f was monitored by
the UV-vis spectra, the absorption maximum at 336 nm of the
π-π* transition of the azo group decreased, and a new
absorption maximum at 445 nm assignable to the n-π*
transition of (Z)-4f increased (Figure 8). The absorption
maximum is almost the same as that of the (Z)-isomer of
unsubstituted azobenzene. Irradiation (λ ) 431 nm) of (Z)-4f
caused regeneration of (E)-4f.
Figure 6. ORTEP drawing of (E)-4g (50% probability).
Figure 8. UV/vis spectra of the CH2Cl2 solution of (E)-4f irradiated with
high pressure Hg lamp (λmax ) 360 nm). The spectra were recorded every
30 s.
Figure 7. ORTEP drawing of (E)-5 (50% probability).
The photoisomerization reactions of (E)- and (Z)-isomers of
more, the π orbital is less stabilized than the π* orbital. The
greater the number of the fluorine atoms, the more these orbitals
are stabilized and the longer the wavelength of the absorption
maxima shift.
In the UV-vis spectra of (E)-5, an absorption maximum due
to π-π* transition of the azo group was observed at 334 nm
in CH2Cl2. The red shift of the absorption maximum is explained
by the above discussion, considering its overlap with the n-π*
transition.
1
these 2-silylazobenzenes were monitored by the H, 19F, and
29Si NMR spectra. In the 19F NMR spectra in CDCl3, the signal
of (E)-4f (δF -152.1) decreased and a new singlet (δF -170.6),
which was assigned to (Z)-4f, emerged and increased during
irradiation of (E)-4f. The ratio of (E)-4f/(Z)-4f was estimated
as 2:98 based on the integral of the 19F NMR spectra of the
reaction solution at the photostationary state. Similarly, in the
1H NMR spectra in CDCl3 the peaks of (E)-4f decreased and
new peaks of its (Z)-isomers increased during the irradiation.
Irradiation (λ ) 431 nm) of (Z)-4f caused isomerization to (E)-
4f in the ratio (E)-4f/(Z)-4f ) 83:17 based on the integral of
the 19F NMR spectra. Such changes of the signals are in
accordance with the behavior in the UV-vis spectra. In the
29Si NMR spectra, a doublet of (Z)-4f resonated at δSi -7.3,
which is almost the same chemical shift as that of fluorotriphen-
ylsilane (δSi -4.7) and is much lower than that of pentacoor-
dinate (E)-4f (δSi -16.3), suggesting a tetracoordinate structure
around the silicon atom of (Z)-4f in the absence of coordination
of nitrogen to silicon. That is, the coordination numbers of
Isomerization of Azobenzenes Bearing a Fluorosilyl
Substituent
In the (Z)-isomers of the fluorosilanes bearing an azobenzene
unit, the silicon atom is expected to have a tetracoordination
state without the Si-N interaction. To achieve the photoswitch-
ing of the coordination numbers of the silicon atom, photo-
isomerization of the azobenzene moiety of the pentacoordinate
fluorosilanes was carried out. A CH2Cl2 solution of (E)-4f (λmax
) 336 nm in CH2Cl2) was irradiated with a high-pressure Hg-
lamp (λ ) 360 nm) fitted with a colored-glass filter, which was
9
7104 J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006

Organosilicon Compds Bearing 2-(Phenylazo)phenyl Group
A R T I C L E S
Figure 10. UV/vis spectra of the CH2Cl2 solution of (E)-5 irradiated with
high pressure Hg lamp (λmax ) 360 nm). The spectra were recorded every
30 s.
Figure 9. ORTEP drawing of (Z)-4f (50% probability).
silicon are five in (E)-4f and four in (Z)-4f. Since (E)- and (Z)-
4f are converted reversibly by photoirradiation, it can be said
that the coordination number of silicon in the fluorosilane is
switched reversibly by photoirradiation.
178.05(13)°. The Si1-F1 bond length (1.6169(11) Å) is
comparable to that of fluorotris(2-methylphenyl)silane (Si-F:
1.601(1) Å).29 Structural changes between (E)-4f and (Z)-4f were
found on the widening of the Cn-Si1-F1 (n ) 1, 13, 19) bond
angles (102.26(6)°-105.84(6)°) in (Z)-4f compared to those
(99.10(7)°-99.49(7)°) in (E)-4f and shortening of the Si1-F1
bond length (1.6169(11) Å) in (Z)-4f compared to that (1.6343-
(17) Å) in (E)-4f. In (Z)-4f, the N1-N2 bond length (1.252(2)
Å) is almost the same as that of unsubstituted (Z)-azobenzene
(1.253 Å).30 In the crystalline state, the Si1 atom evidently lacks
the interaction with the N2 atom and adopted a tetracoordination
state with a distorted tetrahedral structure. The pentacoordination
character values of (Z)-4f (%TBPa, 29% and %TBPe, 47%) are
much lower than those of (E)-4f (%TBPa, 53% and %TBPe,
76%), respectively. The configuration of the aryl groups around
the silicon atom of 4f in the crystalline state has appreciably
been changed from a distorted TBP structure to a tetrahedral
structure by the photoisomerization.
The coordination number of the silicon atom of tetrafluoro-
silicate (E)-5 can be converted from six to five by elimination
of a fluoride ion induced by addition of a Lewis acid. Treatment
of (E)-5 with an excess amount of BF3‚OEt2 gave the corre-
sponding trifluorosilane (E)-4d and a tetrafluoroborate anion
quantitatively within 5 min (Scheme 4). Furthermore, the
coordination number of the silicon atom can also be changed
by photoirradiation. Isomerization of (E)-5 could easily be done
by irradiation (λ ) 360 nm) in CH2Cl2 for 40 min to give
orange-colored (Z)-5 (Figure 10).9 (Z)-5 showed its absorption
maximum at 433 nm assignable to the n-π* transition.
Irradiation (λ ) 431 nm) of the new absorption maximum led
to the recovery of (E)-5 with a decrease of its absorption. In
variable-temperature 19F NMR spectra, the signal of (Z)-5 was
observed at δSi -114.7 as a broad singlet and did not split even
at -70 °C in contrast to that of (E)-5. The ratio of (E)-5/(Z)-5
is estimated as 2:98 by the integral of 19F NMR spectra of the
reaction solution after irradiation for 40 min. Furthermore, the
higher concentration of the solution of (Z)-5 was, the broader
the signal became, suggesting the intermolecular fluorine
exchange. The 29Si NMR spectra of (Z)-5 at -65 °C showed a
quintet at δSi -122.8, which is lower field by 28 ppm than that
Photoisomerization (λ ) 360 nm) of (E)-4d, (E)-4e, and (E)-
4g occurred similarly to give the corresponding (Z)-isomers
(Scheme 5). The 29Si NMR chemical shifts of the (Z)-isomers
are summarized in Table 1. Irradiation (λ ) 431 nm) of the
absorption maxima of the (Z)-isomers caused a decrease of the
absorption maxima of the n-π* transitions and an increase of
the π-π* transitions of the azo group of the (E)-isomers.
Namely, the (E)-isomers were regenerated by the photoisomer-
ization of the (Z)-isomers. The reversibility of the isomerization
and the changes in the 29Si chemical shifts show that the
coordination number of the silicon atom in the fluorosilanes is
switched reversibly by photoirradiation. Interestingly, the
coordination state can visually be recognized with ease by the
change of the color from yellow to red because the azo moiety
works both as a chromophore and as a coordination site in the
(E)-form. Photoirradiation of (E,E)-3d, which has two azoben-
zene moieties, resulted in the mixture of three isomers in the
ratio (E,E)-3d/(E,Z)-3d/(Z,Z)-3d ) 13:15:72 as judged by the
integral of 19F NMR spectra. Unfortunately, the signal due to
neither (E,Z)-3d nor (Z,Z)-3d was observed in the 29Si NMR
spectrum.
In addition, in the 19F NMR spectra at -70 °C, the peak of
(Z)-4d was maintained as a singlet and did not split in contrast
to that of (E)-4d. Furthermore, the higher the concentration of
the solution of (Z)-4d was, the broader the signal became. These
results suggest that the silicon atom has no interaction with the
nitrogen atom and that the fluorine nuclei exchange both inter-
and intramolecularly faster than the time scale of 19F NMR
spectroscopy even at -70 °C. Compound (Z)-4d is thermally
more labile than (Z)-4e-g, and its complete thermal isomer-
ization to the (E)-4d within 6 h upon standing at rt in the dark
was confirmed by the UV-vis spectra.
A single crystal of (Z)-4f was obtained by recrystallization
from hexane. The crystal structure of (Z)-4f was determined by
the X-ray crystallographic analysis. Its crystallographic data and
selected bond lengths (Å) and angles (deg) are summarized in
Tables 2 and 3, respectively. The ORTEP drawing of (Z)-4f is
shown in Figure 9. In (Z)-4f, the azobenzene moiety clearly
shows a (Z)-form with a small C6-N1-N2-C7 torsion angle
(5.4(2)°), while the corresponding torsion angle in (E)-4f is
(28) Birnbaum, P. P.; Linford, J. H.; Style, D. W. G. Trans. Faraday Soc. 1953,
49, 735.
(29) Dell, S.; Ho, D. M.; Pascal, R. A., Jr. J. Org. Chem. 1999, 64, 5626.
(30) Mostad, A.; Rømming, C. Acta Chem. Scand. 1971, 25, 3561.
9
J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006 7105

A R T I C L E S
Kano et al.
silicon atom. Structural changes between (E)-5 and (Z)-5 were
found on the widening of the bond angles of C1-Si1-F4
(164.73(6)°) and the narrowing of F1-Si1-F4 (95.76(5)°) and
C1-Si1-F1 (99.47(6)°) in (E)-5 compared to those of (Z)-5
(126.67(19)°, 115.53(16)°, and 117.77(19)°, respectively). The
changes in the bond angles between (E)-5 and (Z)-5 are more
remarkable than the changes between (E)-4f and (Z)-4f. Con-
figuration around the silicon atom in 5 has completely been
changed between distorted octahedral and TBP structures by
photoirradiation. Such a change around the silicon atom by
irradiation has never been reported as far as we know.
In the previous reports of the photoswitching by taking
advantage of the photoisomerization of azobenzenes, the
azobenzenes were substituted at the 4- and 4′-positions or the
3- and 3′-positions.7 The differences in the directions of two
C(ipso)-N(azo) bonds and the relative location of the sub-
stituents between the (E)- and (Z)-isomers were utilized for the
photoswitching. In contrast with the previous reports, location
and direction of the lone pair of the nitrogen atom that
coordinates to the silicon atom are important for our photo-
switching of the coordination numbers of the silicon. The
switching is achieved by change of the relative location of the
lone pair of the nitrogen and the silicon atom. If the silicon
atom is located on the other side of the lone pair, such a
photoswitching cannot be achieved. The configuration of the
lone pair of the nitrogen atom in the (Z)-isomers is too far away
from the silicon to coordinate, while the lone pair is close
enough to the silicon atom in the (E)-form.
Concluding Remarks
Figure 11. ORTEP drawings of two independent isomers of (Z)-5 (50%
probability).
We have synthesized several neutral pentacoordinate organo-
silicon compounds bearing one or two 2-(phenylazo)phenyl
groups. They have been characterized by 1H, 13C, 19F, and 29Si
NMR spectroscopy, UV-vis spectroscopy, and X-ray crystal-
lographic analyses. The spectral properties depending on the
coordination numbers are exposed in the chemical shifts and
the coupling patterns in the 19F and 29Si NMR spectra. The
silicon atom of the silanes bearing no fluorine atom has a
tetracoordinate structure, while that of the (E)-isomers of the
fluorosilanes bearing at least one fluorine atom has a pentaco-
ordinate structure in both the solution state and the crystalline
state. In the tetracoordinate silanes, the silyl group causes the
red shifts of the n-π* and π-π* transitions of the azo groups
compared to the unsubstituted azobenzene in the UV-vis
spectra. The (E)-isomers of the fluorosilanes show the intramo-
lecular electron-pair donation from a nitrogen atom of the azo
group to the silicon atom and exhibited distorted TBP geometry
around the pentacoordinate silicon atom. The photoirradiation
of the (E)-isomers of fluorosilanes reversibly gives the corre-
sponding (Z)-isomers in good yields. The (Z)-isomers have been
found to have a tetracoordinate silicon atom in the absence of
the Si‚‚‚N interaction. We have achieved reversible photocontrol
of the coordination numbers of the silicon atom of the
fluorosilanes between four and five as well as that of the silicate
between five and six.
of (E)-5. These results suggest that fluorine nuclei exchange
faster than the time scale of 19F NMR spectroscopy even at
-65 °C both intra- and intermolecularly and that (Z)-5 bears a
pentacoordinate structure around a silicon atom in the absence
of coordination of the nitrogen atom. Addition of BF3‚OEt2 to
(Z)-5 caused both elimination of a fluoride ion and isomerization
of the azo moiety to give the corresponding trifluorosilane (E)-
4d quantitatively within 5 min.
The (Z)-5 is stable under argon at ambient temperature for a
few days in the solid state, and it gradually isomerizes to (E)-5
at room temperature in solution state even in the dark. The
crystal structure of (Z)-5 was finally determined by X-ray
crystallographic analysis (Figure 11).9 There are two nonidenti-
cal molecules (molecules 1 and 2) of (Z)-5 in the unit cell, and
they have a similar conformation around the silicon atom. In
(Z)-5, the silicon atom evidently lacked the Si1-N2 interaction
and adopted a pentacoordinate state of a TBP structure with F2
and F3 atoms at apical positions and with F1, F4, and C1 atoms
at equatorial positions. The pentacoordination characters, %TBPa
and %TBPe, are 97% and 100%, respectively. The deviation
from an ideal TBP structure to a square pyramidal structure,
TBPfSP, was calculated to be 22%.31 The bond lengths of
Si1-F2 (1.668(3) Å) and Si1-F3 (1.697(3) Å) bonds are
significantly longer than those of Si1-F1 (1.617(3) Å) and Si1-
F4 (1.611(3) Å) bonds. Such elongation of apical bonds is
indeed characteristic of TBP structure around a pentacoordinate
Experimental Section
General Procedure. Solvents were purified before use by
reported methods. All reactions were carried out under argon
1
(31) Holmes, R. R.; Deiters, J. A. J. Am. Chem. Soc. 1977, 99, 3318.
atmosphere unless otherwise noted. The H NMR (500 MHz),
9
7106 J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006

Organosilicon Compds Bearing 2-(Phenylazo)phenyl Group
A R T I C L E S
13C NMR (125 MHz), and 29Si NMR (99 MHz) spectra were
measured with a JEOL A500 spectrometer using tetramethyl-
silane (TMS) as an internal standard. The 19F NMR (254 MHz)
and 19F NMR (376 MHz) spectra were measured with a JEOL
EXcalibur270 spectrometer and a JEOL AL400 spectrometer,
respectively, using Fleon as an external standard. All melting
points are uncorrected. FAB-Mass and EI-Mass spectral data
were obtained on a JEOL JMS-SX102 spectrometer. Preparative
gel permeation liquid chromatography was performed by LC-
918 with JAIGEL H1 + H2 columns (Japan Analytical Industry)
with chloroform as the solvent. Elemental analyses were
performed by the Microanalytical Laboratory of Department of
Chemistry, Faculty of Science, The University of Tokyo.
2-Bromoazobenzene and 2-iodoazobenzene were prepared ac-
cording to literature.32
Preparation of 2-(Phenylazo)phenyllithium. To a THF
solution (12.5 mL) of 2-iodoazobenzene (1b) (500 mg, 1.62
mmol) at -105 °C, n-BuLi (1.50 M in hexane, 1.11 mL, 1.67
mmol) was added rapidly. After the reaction solution was stirred
further at -105 °C for 5 min, the black solution of 2-lithio-
azobenzene was used at -105 °C for the syntheses of (E)-2a-g
and (E,E)-3a-c.
A General Procedure for the Synthesis of [2-((E)-Phenyl-
azo)phenyl]silanes ((E)-2a-g) and Bis[2-((E)-phenylazo)-
phenyl]silanes (E,E)-3a-c. To a THF solution (12.5 mL) of
2-lithioazobenzene (1.70 mmol) at -105 °C was added a
chlorosilane (1.79 mmol) for the synthesis of (E)-2 or a
dichlorosilane (0.88 mmol) for the synthesis of (E,E)-3. The
reaction mixture was stirred at 0 °C for 3 h and quenched with
aq. NaHCO3. After separation of the organic layer, extraction
of the aqueous layer with CHCl3, drying of the combined
solution with MgSO4, and evaporation of the solvent, the residue
was separated with alumina-gel column chromatography. A red-
colored fraction was collected, and the eluent was evaporated.
For oil compounds, the products were used as such for the next
reactions, and further purification with gel permeation liquid
chromatography gave analytically pure products. For crystalline
compounds, recrystallization gave pure products. The yields of
(E)-2a-g and (E,E)-3a-c are summarized in Scheme 2.
Spectral and Physical Data of (E)-2a-g. Trimethyl[2-((E)-
phenylazo)phenyl]silane ((E)-2a): Red oil. 1H NMR (500
(s). 29Si NMR (99 MHz, CDCl3) δ -7.53 (s). Anal. Calcd for
C20H20N2Si: C, 75.90; H, 6.37; N, 8.85. Found: C, 75.67; H,
6.39; N, 8.95%.
Methyldiphenyl[2-((E)-phenylazo)phenyl]silane ((E)-2c):
Orange needles (ethanol), mp 115-116 °C. 1H NMR (500 MHz,
3
CDCl3) δ 0.88 (s, 3H), 7.27-7.36 (m, 11H), 7.38 (dt, JHH
)
)
4
3
4
7.5 Hz, JHH ) 1.5 Hz, 1H), 7.43 (dd, JHH ) 7.5 Hz, JHH
1.5 Hz, 1H), 7.51-7.54 (m, 5H), 7.82 (dd, 3JHH ) 7.5 Hz, 4JHH
) 1.5 Hz, 1H). 13C NMR (126 MHz, CDCl3) δ -1.45 (q),
115.08 (d), 123.11 (d), 127.69 (d), 128.76 (d), 128.95 (d), 130.24
(d), 130.65 (d), 130.68 (d), 135.13 (d), 137.33 (s), 137.35 (d),
138.14 (s), 152.49 (s), 157.18 (s). 29Si NMR (53 MHz, CDCl3)
δ -10.29 (s). Anal. Calcd for C25H22N2Si: C, 79.32; H, 5.86;
N, 7.40. Found: C, 79.17; H, 5.98; N, 7.53%.
Triethoxy[2-((E)-phenylazo)phenyl]silane ((E)-2d): Red oil.
3
1H NMR (500 MHz, CDCl3) δ 1.18 (t, JHH ) 7.0 Hz, 9H),
3.86 (q, 3JHH ) 7.0 Hz, 6H), 7.45-7.56 (m, 5H), 7.81 (d, 3JHH
) 8.0 Hz, 1H), 8.01-8.03 (m, 3H). 13C NMR (126 MHz,
CDCl3) δ 18.13 (q), 58.62 (t), 114.92 (d), 123.35 (d), 129.03
(d), 130.24 (d), 130.99 (d), 131.31 (d), 132.75 (s), 137.45 (d),
152.62 (s), 157.45 (s). 29Si NMR (53 MHz, CDCl3) δ -58.08
(s). Anal. Calcd for C18H24N2O3Si: C, 62.76; H, 7.02; N, 8.13.
Found: C, 62.65; H, 6.98; N, 8.18%.
Dimethyl[2-((E)-phenylazo)phenyl]silane ((E)-2e): Red oil.
3
1H NMR (500 MHz, CDCl3) δ 0.44 (d, JHH ) 3.8 Hz, 6H),
3
3
4
4.69 (sept, JHH ) 3.8 Hz, 1H), 7.45 (dt, JHH ) 7.5 Hz, JHH
3
) 1.5 Hz, 1H), 7.48-7.55 (m, 4H), 7.76 (dd, JHH ) 7.5 Hz,
4JHH ) 1.5 Hz, 1H), 7.83 (d, 3JHH ) 7.5 Hz, 1H), 7.94 (d, 3JHH
) 7.5 Hz, 2H). 13C NMR (125 MHz, CDCl3) δ -2.24 (q),
116.74 (d), 123.07 (d), 129.15 (d), 130.30 (d), 130.35 (d), 130.99
(d), 135.83 (d), 138.51 (s), 152.48 (s), 157.18 (s). 29Si{1H} NMR
(53 MHz, CDCl3) δ -16.95 (s). Anal. Calcd for C14H16N2Si:
C, 69.95; H, 6.71; N, 11.65. Found: C, 69.73; H, 6.76; N,
11.39%.
Diphenyl[2-((E)-phenylazo)phenyl]silane ((E)-2f): Red crys-
1
tals (hexane), mp 74-75 °C. H NMR (500 MHz, CDCl3) δ
1
5.71(s, JSiH ) 207.5 Hz, 1H), 7.28-7.37 (m, 9H), 7.42-7.46
3
4
(m, 3H), 7.54 (dd, JHH ) 7.5 Hz, JHH ) 1.5 Hz, 4H), 7.57-
3
4
7.60 (m, 2H), 7.94 (dd, JHH ) 7.5 Hz, JHH ) 1.5 Hz, 1H).
13C NMR (126 MHz, CDCl3) δ 118.91 (d), 122.95 (d), 127.83
(d), 128.79 (d), 129.29 (d), 130.56 (d), 130.85 (d), 131.11 (d),
133.11 (s), 134.82 (s), 135.67 (d), 138.03 (d), 152.04 (s), 157.02
(s). 29Si{1H} NMR (53 MHz, CDCl3) δ -19.90 (s). Anal. Calcd
for C24H20N2Si: C, 79.08; H, 5.53; N, 7.69. Found: C, 79.06;
H, 5.60; N, 7.89%.
3
4
MHz, CDCl3) δ 0.38 (s, 9H), 7.43 (dt, JHH ) 7.5 Hz, JHH
)
3
1.5 Hz, 1H), 7.45-7.49 (m, 2H), 7.53 (t, JHH ) 7.5 Hz, 2H),
7.69 (dd, 3JHH ) 7.5 Hz, 4JHH ) 1.5 Hz, 1H), 7.75 (dd, 3JHH
)
7.5 Hz, JHH ) 1.5 Hz, 1H), 7.92 (d, JHH ) 7.5 Hz, 2H). 13C
NMR (125 MHz, CDCl3) δ 0.52 (q), 114.84 (d), 123.18 (d),
129.15 (d), 129.94 (d), 130.06 (d), 130.89 (d), 134.73 (d), 141.77
(s), 152.59 (s), 157.42 (s). 29Si NMR (53 MHz, CDCl3) δ -3.98
(s). HRMS (EI, 70 eV) m/z Calcd for C15H18N2Si: 254.1239.
Found: 254.1243. Anal. Calcd for C15H18N2Si: C, 70.82; H,
7.13; N, 11.01. Found: C, 70.67; H, 6.97; N, 10.98%.
4
3
Diethoxyphenyl[2-((E)-phenylazo)phenyl]silane ((E)-2g):
1
Red crystals (hexane), mp 55-56 °C. H NMR (500 MHz,
CDCl3) δ 1.18 (t, 3JHH ) 7.5 Hz, 6H), 3.82 (q, 3JHH ) 7.5 Hz,
3
4H), 7.25-7.32 (m, 3H), 7.37-7.39 (m, 3H), 7.51 (t, JHH
)
3
7.5 Hz, 1H), 7.55 (t, JHH ) 7.5 Hz, 1H), 7.60-7.61 (m, 2H),
3
4
3
7.64 (dd, JHH ) 7.5 Hz, JHH ) 1.5 Hz, 2H), 7.77 (d, JHH
)
Dimethylphenyl[2-((E)-phenylazo)phenyl]silane ((E)-2b):
Orange needles (ethanol), mp 110-111 °C. 1H NMR (500 MHz,
CDCl3) δ 0.63 (s, 6H), 7.29-7.35 (m, 3H), 7.39-7.46 (m, 4H),
7.5 Hz, 1H), 8.10 (d, 3JHH ) 7.5 Hz, 1H). 13C NMR (126 MHz,
CDCl3) δ 18.30 (q), 58.94 (t), 115.32 (d), 127.59 (d), 128.84
(d), 129.65 (d), 130.37 (d), 130.86 (d), 131.35 (d), 134.44 (s),
134.49 (d), 134.88 (s), 137.26 (d), 152.45 (s), 157.38 (s). 29Si
NMR (99 MHz, CDCl3) δ -32.64 (s). Anal. Calcd for
C22H24N2O2Si: C, 70.18; H, 6.42; N, 7.44. Found: C, 70.12;
H, 6.40; N, 7.27%.
3
4
7.49 (dt, JHH ) 7.5 Hz, JHH ) 1.5 Hz, 1H), 7.54-7.58 (m,
3H), 7.66-7.68 (m, 2H), 7.76 (d, 3JHH ) 7.5 Hz, 1H). 13C NMR
(126 MHz, CDCl3) δ -0.56 (q), 114.94 (d), 123.15 (d), 127.65
(d), 128.75 (d), 128.97 (d), 130.13 (d), 130.33 (d), 130.80 (d),
134.17 (d), 135.94 (d), 139.22 (s), 139.84 (s), 152.54 (s), 157.34
Dimethylbis[2-((E)-phenylazo)phenyl]silane ((E,E)-3a): Red
crystals (hexane), mp 122-123 °C. 1H NMR (500 MHz, CDCl3)
(32) Badger, G. M.; Drewer, R. J.; Lewis, G. E. Aust. J. Chem. 1964, 17, 1036.
9
J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006 7107

A R T I C L E S
Kano et al.
3
δ 0.69 (s, 6H), 7.35-7.39 (m, 8H), 7.43 (dt, JHH ) 7.5 Hz,
8.0 Hz, 1H). 13C{1H} NMR (126 MHz, CDCl3) δ 118.85 (t,
2JCF ) 17.4 Hz, CSi), 123.05 (s, CH), 127.97 (s, CH), 128.96
(s, CH), 129.71 (s, CH), 130.45 (s, CH), 131.79 (s, CH), 132.47
(s, CH), 132.56 (t, 2JCF ) 20.4 Hz, CSi), 132.94 (s, CH), 133.28
(s, CH), 137.81 (t, 3JCF ) 2.4 Hz, CH), 148.72 (s, CN), 156.85
(t, 3JCF ) 3.6 Hz, CN). 19F NMR (376 MHz, CDCl3) δ -142.85
(s, 1JSiF ) 264.5 Hz); (-100 °C) δ -149.45 (br s, 1F), -131.29
(br s, 1F). 29Si{1H} NMR (99 MHz, CDCl3) δ -47.75 (t, 1JSiF
) 264.5 Hz). Anal. Calcd for C18H14F2N2Si: C, 66.64; H, 4.35;
N, 8.64. Found: C, 66.67; H, 4.59; N, 8.47%.
4JHH ) 1.5 Hz, 2H), 7.53-7.55 (m, 4H), 7.69 (dd, JHH ) 7.5
3
4
3
4
Hz, JHH ) 1.5 Hz, 2H), 7.72 (dd, JHH ) 7.5 Hz, JHH ) 1.5
Hz, 2H). 13C NMR (68 MHz, CDCl3) δ 0.91 (q), 114.93 (d),
123.10 (d), 128.83 (d), 129.86 (d), 130.02 (d), 130.65 (d), 135.60
(d), 141.18 (s), 152.41 (s), 156.74 (s). 29Si NMR (53 MHz,
CDCl3) δ -7.79 (s). Anal. Calcd for C26H24N4Si: C, 74.25; H,
5.75; N, 13.32. Found: C, 74.16; H, 5.79; N, 13.41%.
Diphenylbis[2-((E)-phenylazo)phenyl]silane ((E,E)-3b): Red
needles (hexane), mp 174.5-176 °C. 1H NMR (500 MHz,
CDCl3) δ 7.08-7.16 (m, 8H), 7.20-7.29 (m, 8H), 7.32 (t, 3JHH
Fluorophenylbis[2-((E)-phenylazo)phenyl]silane ((E,E)-
3d). To a THF solution (5 mL) of (E,E)-3c (174 mg, 0.37 mmol)
was added AgF (150 mg, 1.18 mmol) at rt, and the reaction
mixture was stirred for 2 d. After filtration of insoluble materials
through Celite and evaporation of the solvent, recrystallization
of the residue from hexane gave orange crystals of (E,E)-3d
(158 mg, 87%). (E,E)-3d: Orange crystals (hexane), mp 107-
3
4
) 7.5 Hz, 2H), 7.44 (dt, JHH ) 7.5 Hz, JHH ) 0.9 Hz, 2H),
7.56 (t, JHH ) 7.5 Hz, 6H), 7.76 (d, JHH ) 7.5 Hz, 2H). 13C
NMR (68 MHz, CDCl3) δ 115.24 (d), 123.14 (d), 127.48 (d),
128.44 (d), 128.80 (d), 130.23 (d), 130.34 (d), 130.51 (d), 136.11
(s), 136.49 (d), 137.59 (d), 137.78 (s), 152.08 (s), 156.54 (s).
29Si NMR (53 MHz, CDCl3) δ -13.76 (s). Anal. Calcd for
C36H28N4Si: C, 79.38; H, 5.18; N, 10.29. Found: C, 79.09; H,
5.42; N, 10.19%.
3
3
1
109 °C. H NMR (500 MHz, CDCl3) δ 7.20-7.27 (m, 7H),
7.31 (t, 3JHH ) 7.5 Hz, 2H), 7.32-7.38 (m, 4H), 7.48 (td, 3JHH
4
3
4
) 7.5 Hz, JHH ) 1.5 Hz, 2H), 7.57 (dd, JHH ) 7.5 Hz, JHH
Phenylbis[2-((E)-phenylazo)phenyl]silane ((E,E)-3c): Red
oil. H NMR (500 MHz, CDCl3) δ 6.06 (s, JSiH ) 219.0 Hz,
1H), 7.30-7.37 (m, 9H), 7.39 (t, JHH ) 7.5 Hz, 2H), 7.49-
7.55 (m, 6H), 7.59 (dd, JHH ) 7.5 Hz, JHH ) 1.5 Hz, 2H),
3
4
1
1
) 1.5 Hz, 2H), 7.71 (dd, JHH ) 7.5 Hz, JHH ) 1.5 Hz, 2H),
3
3
4
3
7.89 (d, JHH ) 7.5 Hz, 2H), 7.94 (dd, JHH ) 7.5 Hz, JHH
)
1.5 Hz, 2H). 13C{1H} NMR (126 MHz, CDCl3) δ 120.44 (s,
CH), 122.80 (s, CH), 127.56 (s, CH), 128.49 (s, CH), 129.59
(s, CH), 130.60 (s, CH), 130.70 (s, CH), 130.76 (s, CH), 133.36
3
4
3
4
3
7.63 (dd, JHH ) 7.5 Hz, JHH ) 1.5 Hz, 2H), 7.93 (d, JHH
)
8.0 Hz, 2H). 13C NMR (126 MHz, CDCl3) δ 118.42 (d), 122.94
(d), 127.74 (d), 128.74 (d), 128.95 (d), 130.38 (d), 130.51 (d),
130.73 (d), 135.07 (s), 135.68 (d), 136.07 (s), 137.57 (d), 152.10
(s), 156.68 (s). 29Si{1H} NMR (99 MHz, CDCl3) δ -23.22 (s).
HRMS (FAB) m/z calcd for C30H24N4Si [M]+, 468.1770; found,
468.1741.
2
2
(d, JCF ) 18.9 Hz, CSi), 134.98 (d, JCF ) 20.0 Hz, CSi),
135.00 (d, 4JCF ) 1.4 Hz, CH), 136.04 (d, 3JCF ) 3.3 Hz, CH),
151.27 (s, CN), 156.27 (s, CN). 19F NMR (254 MHz, CDCl3)
δ -151.13 (s, JSiF ) 273.2 Hz). 29Si{1H} NMR (99 MHz,
1
1
CDCl3) δ -17.27 (d, JSiF ) 273.2 Hz). Anal. Calcd for
C30H23N4SiF: C, 74.05; H, 4.76; N, 11.51. Found: C, 73.88;
H, 5.00; N, 11.27%.
Synthesis of Trifluoro[2-((E)-phenylazo)phenyl]silane ((E)-
4d). To an ethereal solution of triethoxy[2-((E)-phenylazo)-
phenyl]silane 2d (3.35 g, 9.72 mmol) was added BF3‚Et2O (1.34
mL, 10.7 mmol), and the reaction mixture was stirred for 7 h.
After the solvent was removed, distillation from the reaction
mixture (0.1 mmHg, 200 °C) and subsequent recrystallization
from hexane gave yellow crystals of (E)-4d (1.28 g, 49%). (E)-
The fluorosilanes (E)-4e and (E)-4f were synthesized similarly
from the hydrosilanes (E)-2e and (E)-2f, respectively.
Fluorodimethyl[2-((E)-phenylazo)phenyl]silane ((E)-4e):
1
3
Brown oil. H NMR (500 MHz, CDCl3) δ 0.50 (d, JFH ) 7.9
3
4
Hz, 6H), 7.51-7.57 (m, 4H), 7.63 (dt, JHH ) 7.5 Hz, JHH
)
1
4d: Yellow crystals (hexane), mp 51-52 °C. H NMR (500
1.5 Hz, 1H), 7.92-7.96 (m, 3H), 8.05 (d, 3JHH ) 7.5 Hz, 1H).
3
2
MHz, CDCl3) δ 7.54-7.60 (m, 3H), 7.68 (t, JHH ) 7.5 Hz,
13C{1H} NMR (126 MHz, CDCl3) δ 1.27 (d, JCF ) 19.6 Hz,
3
4
1H), 7.80 (dt, JHH ) 7.5 Hz, JHH ) 1.5 Hz, 1H), 8.01 (dd,
CH3), 122.83 (s, CH), 124.94 (s, CH), 129.32 (s, CH), 131.03
(s, CH), 131.24 (s, CH), 131.38 (s, CH), 131.92 (d, 2JCF ) 16.6
Hz, CSi), 135.27 (d, 3JCF ) 6.3 Hz, CH), 150.79 (s, CN), 156.29
(d, 3JCF ) 2.1 Hz, CN). 19F NMR (254 MHz, CDCl3) δ -152.61
4
3
3JHH ) 7.5 Hz, JHH ) 1.5 Hz, 1H), 8.15 (dd, JHH ) 7.5 Hz,
4JHH ) 1.5 Hz, 2H), 8.18 (d, 3JHH ) 7.5 Hz, 1H). 13C{1H} NMR
2
(126 MHz, CDCl3) δ 112.92 (q, JCF ) 19.7 Hz, CSi), 123.60
(s, CH), 129.51 (s, CH), 130.50 (s, CH), 132.90 (s, CH), 133.26
(s, CH), 134.33 (s, CH), 138.47 (s, CH), 147.43 (s, CN), 156.01
(q, JCF ) 5.2 Hz, CN). 19F NMR (254 MHz, CD2Cl2) (rt) δ
1
3
(sept, JSiF ) 267.7 Hz, JHF ) 7.9 Hz). 29Si{1H} NMR (53
1
MHz, CDCl3) δ 8.27 (d, JSiF ) 267.7 Hz). Anal. Calcd for
3
C14H15N2SiF: C, 65.08; H, 5.85; N, 10.84. Found: C, 65.26;
H, 6.02; N, 10.68%.
1
-140.53 (s, JSiF ) 234.7 Hz, 3F); (-90 °C) δ -141.11 (d,
2JFF ) 52.0 Hz, 2F), -137.13 (t, 2JFF ) 52.0 Hz, 1F). 29Si NMR
Fluorodiphenyl[2-((E)-phenylazo)phenyl]silane ((E)-4f):
Yellow crystals (hexane), mp 102-103 °C. 1H NMR (500 MHz,
CDCl3) δ 7.15-7.19 (m, 4H), 7.23-7.28 (m, 5H), 7.32 (tt, 3JHH
1
(53 MHz, CD2Cl2) (rt) δ -91.21 (q, JSiF ) 231.0 Hz); (-90
°C) δ -94.55 (q, 1JSiF ) 231.0 Hz). Anal. Calcd for C12H9F3N2-
Si: C, 54.12; H, 3.41; N, 10.52. Found: C, 54.10; H, 3.56; N,
10.62%.
4
3
4
) 7.5 Hz, JHH ) 1.2 Hz, 2H), 7.58 (dd, JHH ) 7.5 Hz, JHH
3
4
) 1.5 Hz, 4H), 7.63 (dt, JHH ) 7.5 Hz, JHH ) 1.5 Hz, 1H),
3
4
Synthesis of Difluorophenyl[2-((E)-phenylazo)phenyl]si-
lane ((E)-4g). Similarly to the synthesis of (E)-4d, (E)-4g was
synthesized from (E)-2g (312 mg, 0.827 mmol) and BF3‚Et2O
(0.20 mL, 1.6 mmol) in 67% yield. (E)-4g: Yellow crystals
7.72 (dt, JHH ) 7.5 Hz, JHH ) 1.5 Hz, 1H), 8.12-8.14 (m,
2H). 13C{1H} NMR (126 MHz, CDCl3) δ 122.77 (s, CH),
125.22 (s, CH), 127.83 (s, CH), 128.63 (s, CH), 129.90 (s, CH),
130.89 (s, CH), 131.65 (s, CH), 131.98 (s, CH), 134.42 (d, 2JCF
) 1.6 Hz, CSi), 134.56 (s, CH), 134.73 (s, CH), 137.12 (d,
2JCF ) 5.6 Hz, CSi), 150.63 (s, CN), 157.04 (s, CN). 19F NMR
(254 MHz, CDCl3) δ -152.14 (s, 1JSiF ) 273.6 Hz). 29Si NMR
(99 MHz, CDCl3) δ -16.25 (d, 1JSiF ) 273.6 Hz). Anal. Calcd
1
(hexane), mp 82-83 °C. H NMR (500 MHz, CDCl3) δ 7.22
3
3
(t, JHH ) 8.0 Hz, 3H), 7.30-7.41 (m, 4H), 7.44 (dd, JHH
)
7.5 Hz, 4JHH ) 1.5 Hz, 2H), 7.65-7.71 (m, 3H), 7.80 (d, 3JHH
3
3
) 8.0 Hz, 1H), 8.09 (d, JHH ) 7.5 Hz, 1H), 8.22 (d, JHH
)
9
7108 J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006

Organosilicon Compds Bearing 2-(Phenylazo)phenyl Group
A R T I C L E S
3
3
for C24H19FN2Si: C, 75.36; H, 5.01; N, 7.32. Found: C, 75.60;
H, 5.16; N, 7.35%.
7.26 (m, 3H), 7.44 (t, JHH ) 7.5 Hz, 2H), 7.51 (t, JHH ) 7.5
3
4
Hz, 1H), 7.88 (dd, JHH ) 7.5 Hz, JHH ) 1.5 Hz, 2H), 7.94
(dd, 3JHH ) 7.5 Hz, 4JHH ) 1.5 Hz, 1H). 19F NMR (254 MHz,
Synthesis of Potassium, 18-Crown-6 Tetrafluoro[2-((E)-
phenylazo)phenyl]silicate ((E)-5). To a toluene solution (2.5
mL) of (E)-4d (100 mg, 0.376 mmol) were added KF (21.8
mg, 0.376 mmol) and 18-crown-6 (99.3 mg, 0.376 mmol) at
room temperature, and the reaction mixture was stirred for 26
h. Recrystallization of the resulting yellow precipitates from
CHCl3/ether gave yellow crystals of silicate (E)-5 (164 mg,
CDCl3) δ -140.36 (s, JSiF ) 287.5 Hz). 29Si{1H} NMR (99
1
1
MHz, CDCl3) δ -29.80 (t, JSiF ) 287.5 Hz).
(Z)-5: Red crystals (THF/Et2O). 1H NMR (500 MHz, CDCl3)
3
3
δ 3.57 (s, 24H), 5.89 (d, JHH ) 7.6 Hz, 1H), 6.84 (t, JHH
)
3
7.6 Hz, 1H), 6.97-7.04 (m, 4H), 7.15 (t, JHH ) 7.8 Hz, 2H),
7.80 (d, 3JHH ) 6.7 Hz, 1H). 13C{1H} NMR (126 MHz, CDCl3,
-65 °C) δ 69.52 (s), 115.38 (s), 120.85 (s), 125.35 (s), 126.43
74%). (E)-5: Yellow crystals (CHCl3/Et2O), mp 185-186 °C.
2
3
1H NMR (500 MHz, CDCl3) δ 3.55 (s, 24H), 7.30 (t, JHH
)
(s), 127.33 (s), 127.99 (s), 134.35 (s), 135.79 (quint, JCF
35.9 Hz), 153.09 (s), 158.04 (s). 19F NMR (254 MHz, CDCl3)
δ -114.71 (s).
Photoisomerization of (E,E)-3d: Similarly, a CDCl3 solution
(5 mM, 0.5 mL) of (E,E)-3d was irradiated (λ ) 360 nm) for
12 h to give fluorophenyl[2-((E)-phenylazo)phenyl][2-((Z)-
phenylazo)phenyl]silane ((E,Z)-3d) and fluorophenylbis[2-((Z)-
phenylazo)phenyl]silane ((Z,Z)-3d) ((E,E)-3d/(E,Z)-3d/(Z,Z)-3d
) 13:15:72). (E,Z)-3d: 19F NMR (254 MHz, CDCl3) δ -151.59
(brs, 1JSiF ) 275.7 Hz). (Z,Z)-3d: 19F NMR (254 MHz, CDCl3)
)
3
7.2 Hz, 1H), 7.36-7.38 (m, 3H), 7.45 (t, JHH ) 7.2 Hz, 1H),
7.71 (d, 3JHH ) 6.7 Hz, 1H), 7.96 (d, 3JHH ) 7.3 Hz, 1H), 8.11-
8.13 (m, 2H). 13C{1H} NMR (126 MHz, CDCl3) δ 69.87 (s),
123.91 (s), 126.34 (s), 127.98 (s), 129.52 (s), 129.58 (s), 130.57
2
(s), 133.18 (s), 142.47 (quint, JCF ) 40.3 Hz), 149.99 (s),
152.24 (s). 19F NMR (254 MHz) (50 °C, CDCl3) δ -127.40 (s,
2
1JSiF ) 189.0 Hz); (-90 °C, CD2Cl2) δ -147.17 (dt, JFF
)
2
2
21.3 Hz, JFF ) 24.4 Hz, 1F), -123.18 (d, JFF ) 24.4 Hz,
2F), -121.63 (d, 2JFF ) 21.3 Hz, 1F). 29Si{1H} NMR (53 MHz)
(rt, CDCl3) δ -150.88 (quint, 1JSiF ) 192.5 Hz); (-90 °C, CD2-
Cl2) δ -153.73 (ddt, 1JSiF ) 148.9 Hz, 1JSiF ) 185.7 Hz, 1JSiF
) 213.9 Hz). Anal. Calcd for C24H33F4KN2O6Si: C, 48.96; H,
5.65; N, 4.76. Found: C, 48.69; H, 5.49; N, 4.92%.
1
δ -166.76 (s, JSiF ) 275.7 Hz).
Reactions of (E)- and (Z)-5 with BF3‚OEt2: To a CDCl3
solution (0.5 mL) of (E)-5 (5.9 mg, 0.010 mmol) was added
BF3‚Et2O (10 µL, 0.079 mmol). Quantitative formation of (E)-
4d and a tetrafluoroborate ion was observed in the H and 19F
1
Photoisomerization of (E)-4d: A CDCl3 solution (80 mM,
0.5 mL) of (E)-4d in a 5 mmφ NMR tube was irradiated with
high-pressure mercury lamp through a colored-glass filter (λ )
360 nm) for 1 h in the dark room. The color of the reaction
solution gradually changed from yellow to red during the
irradiation. In 19F NMR spectra, a broad signal at δ -140.6
decreased, and a new sharp singlet was observed at δ -138.0,
which was assigned as (Z)-4d.12 Ratio of (E)-4d/(Z)-4d was
determined to be 19:81 by integral of the 19F NMR spectra.
After the reaction solution was kept at rt for 6 h in the dark,
(Z)-4d completely disappeared and (E)-4d was recovered
quantitatively. Irradiation (λ ) 431 nm) of the mixture of (E)-
4d and (Z)-4d for 3 h changed the ratio of (E)-4d/(Z)-4d to
100:0. Photoisomerizations of (E)-4e-g (10 mM, 0.5 mL) and
(E)-5 (40 mM, 0.5 mL) to the corresponding (Z)-isomers were
carried out similarly.
NMR spectra after 5 min. The reaction of (E)-5 gave the same
results.
X-ray Crystallographic Analyses of (E)-2c, (E)-2f, (E,E)-
3a, (E)-4d, (E)-4f, (E)-4 g, (Z)-4f, (E)-5, and (Z)-5: All data
for (E)-2c, (E)-2f, (E,E)-3a, (E)-4d, (E)-4f, (E)-4g, (Z)-4f, (E)-
5, and (Z)-5 were recorded on a Rigaku Mercury CCD
diffractometer with graphite monochromated Mo KR radiation
(λ ) 0.710 70 Å). Data were collected and processed using
CrystalClear (Rigaku). The data were corrected for Lorentz and
polarization effects. The structure was solved by direct methods
(SHELX-97) and expanded using Fourier techniques.33 The non-
hydrogen atoms were refined anisotoropically. Hydrogen atoms
were refined isotropically. Crystallographic data are summarized
in Table 2.
Acknowledgment. We thank Shin-etsu Chemical Co., Ltd.,
and Tosoh Finechem Corporation for the gifts of chlorosilanes
and alkyllithiums, respectively. This work was partially sup-
ported by Grant-in-Aid for The 21st Century COE Program for
Frontiers in Fundamental Chemistry (T.K.) and for Scientific
Research Nos. 15036217 (T.K. and N.K.) and 16033216 (T.K.
and N.K.) from Ministry of Education, Culture, Sports, Science
and Technology, Japan. This work was also supported by
Dainippon Ink & Chemicals, Incorporated and by Grant-in-Aid
for Scientific Research Nos. 15105001 (T.K.), 14740395 (N.K.),
and 1611512 (M.Y.) by the Japan Society for the Promotion of
Science.
(Z)-4e: Red solution (in CDCl3). 1H NMR (500 MHz, CDCl3)
3
3
δ 0.60 (d, JFH ) 7.7 Hz, 6H), 6.11 (d, JHH ) 8.0 Hz, 1H),
6.85 (d, 3JHH ) 7.5 Hz, 2H), 7.06 (t, 3JHH ) 7.5 Hz, 1H), 7.16-
7.25 (m, 4H), 7.75 (d, 3JHH ) 7.5 Hz, 1H). 13C{1H} NMR (126
2
MHz, CDCl3) δ -0.18 (d, JCF ) 14.3 Hz, CH3), 115.75 (s,
CH), 120.32 (s, CH), 127.19 (s, CH), 127.53 (s, CH), 128.79
(s, CH), 129.87 (s, CH), 134.52 (s, CH), 135.24 (d, 2JCF ) 10.5
Hz, CSi), 153.28 (s, CN), 158.03 (s, CN). 19F NMR (376 MHz,
1
3
CDCl3) δ -163.12 (sept, JSiF ) 273.7 Hz, JFH ) 7.7 Hz);
29Si{1H} NMR (99 MHz, CDCl3) δ 21.01 (d, JSiF ) 273.7
1
Hz).
1
(Z)-4f: Red crystals (hexane). H NMR (500 MHz, CDCl3)
δ 6.11 (d, JHH ) 7.5 Hz, 1H), 6.53 (d, JHH ) 7.5 Hz, 2H),
Supporting Information Available: Details of crystallo-
graphic analyses of compounds (E)-2c, (E)-2f, (E,E)-3a, (E)-
4d, (E)-4f, (E)-4g, (Z)-4f, (E)-5 and (Z)-5 in CIF format. This
material is available free of charge via the Internet at
http://pubs.acs.org.
3
3
3
3
7.09 (t, JHH ) 7.5 Hz, 2H), 7.16 (t, JHH ) 7.5 Hz, 2H), 7.22
3
3
(t, JHH ) 7.5 Hz, 1H), 7.38-7.45 (m, 6H), 7.81 (dd, JHH
)
7.5 Hz, JHH ) 1.5 Hz, 4H), 7.94 (d, JHH ) 7.5 Hz, 1H). 19F
4
3
NMR (254 MHz, CDCl3) δ -170.58 (s, 1JSiF ) 275.7 Hz). 29Si-
{1H} NMR (99 MHz, CDCl3) δ -7.29 (d, JSiF ) 275.7 Hz).
1
JA060926S
1
3
(Z)-4g: H NMR (500 MHz, CDCl3) δ 6.16 (d, JHH ) 7.5
(33) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
Hz, 1H), 6.79 (d, 3JHH ) 7.5 Hz, 2H), 7.16-7.19 (m, 2H), 7.21-
9
J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006 7109