Crucial role of N...Si interactions in the solid-state coloration of disilylazobenzenes

Article abstract of DOI:10.1021/jo801334a

(Chemical Equation Presented) 2,2′-Bis(fluorodiphenylsilyl) azobenzenes bearing two methyl and butyl groups at the 4- and 4′-positions were synthesized, and their X-ray crystallographic analyses revealed that they have tetracoordinated and pentacoordinated silicon atoms, respectively. Although the two compounds showed almost the same spectra in solution, the reflectance spectra of the solid revealed that the pentacoordinated state had a weaker absorbance in the long-wavelength region compared with that of the tetracoordinated state, which is ascribed to the shift of the n-π* transition. This difference in the absorptions due to the n-π* transition drastically affects the color of the azobenzenes: the color of the pentacoordinated state becomes apparently paler than that of the tetracoordinated state.

Full text of DOI:10.1021/jo801334a

Crucial Role of N · · ·Si Interactions in the Solid-State Coloration of
Disilylazobenzenes
Masaki Yamamura,† Naokazu Kano,† Takayuki Kawashima,*,† Tomonari Matsumoto,‡
Jun Harada,‡ and Keiichiro Ogawa*,‡
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-0033, Japan, and Department of Basic Science, Graduate School of Arts and Sciences,
The UniVersity of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan
takayuki@chem.s.u-tokyo.ac.jp; ogawa@ramie.c.u-tokyo.ac.jp
ReceiVed June 24, 2008
2,2′-Bis(fluorodiphenylsilyl)azobenzenes bearing two methyl and butyl groups at the 4- and 4′-positions
were synthesized, and their X-ray crystallographic analyses revealed that they have tetracoordinated and
pentacoordinated silicon atoms, respectively. Although the two compounds showed almost the same spectra
in solution, the reflectance spectra of the solid revealed that the pentacoordinated state had a weaker
absorbance in the long-wavelength region compared with that of the tetracoordinated state, which is
ascribed to the shift of the n-π* transition. This difference in the absorptions due to the n-π* transition
drastically affects the color of the azobenzenes: the color of the pentacoordinated state becomes apparently
paler than that of the tetracoordinated state.
Introduction
dinated states will be useful for change of several properties.
However, there is almost no report of change in the optical,
electronic, and magnetic properties of the organosilicon com-
pounds caused by the N · · ·Si dative bond.4 We previously
One of the main topics in silicon chemistry is the highly
coordinated organosilicon compounds,1 whose specific dative
bonding and high reactivity have attracted many researchers.2
Many pentacoordinated organosilicon compounds with an
intramolecular dative N · · ·Si bond3 equilibrate with the tetra-
coordinated species in solution because the dative bond is weak
and easily dissociated. Since the dative bond also changes the
structure around the silicon atom, formation of the pentacoor-
(2) Recent reports: (a) Wagler, J.; Doert, T.; Roewer, G. Angew. Chem., Int.
Ed. 2004, 43, 2441–2444. (b) Wagler, J.; Bo¨hme, U.; Brendler, E.; Roewer, G.
Organometallics 2004, 23, 6066–6069. (c) Gostevskii, B.; Kalikhman, I.; Tessier,
C. A.; Panzner, M. J.; Youngs, W. J.; Kost, D. Organometallics 2005, 24, 2913–
2920. (d) Gostevskii, B.; Kalikhman, I.; Tessier, C. A.; Panzner, M. J.; Youngs,
W. J.; Kost, D. Organometallics 2005, 24, 5786–5788. (e) Gerlach, D.; Brendler,
E.; Heine, T.; Wagler, J. Organometallics 2007, 26, 234–240. (f) Kalikhman,
I.; Gostevskii, B.; Kertsnus, E.; Botoshansky, M.; Tessier, C. A.; Youngs, W. J.;
Deuerlein, S.; Stalke, D.; Kost, D. Organometallics 2007, 26, 2652–2658. (g)
Wagler, J.; Hill, A. F. Organometallics 2007, 26, 3630–3632.
(3) (a) Helmer, B. J.; West, R.; Corriu, R. J. P.; Poirier, M.; Royo, G.; de
Saxce´, A. J. Organomet. Chem. 1983, 251, 295–298. (b) Wagler, J.; Bo¨hme,
U.; Brendler, E.; Roewer, G. Organometallics 2005, 24, 1348–1350. (c)
Gostevskii, B.; Silbert, G.; Adear, K.; Sivaramakrishna, A.; Stalke, D.; Deuerlein,
S.; Kocher, N.; Voronkov, M. G.; Kalikhman, I.; Kost, D. Organometallics 2005,
24, 2913–2920. (d) Kalikhman, I.; Gostevskii, B.; Botoshansky, M.; Kaftory,
M.; Tessier, C. A.; Panzner, M. J.; Youngs, W. J.; Kost, D. Organometallics
2006, 25, 1252–1258. (e) Gostevskii, B.; Zamstein, N.; Korlyukov, A. A.;
Baukov, Y. I.; Botoshansky, M.; Kaftory, M.; Kocher, N.; Stalke, D.; Kalikhman,
I.; Kost, D. Organometallics 2006, 25, 5416–5423.
† Department of Chemistry, The University of Tokyo.
‡ Department of Basic Science, The University of Tokyo.
(1) (a) Corriu, R. J. P.; Young, J. C. In The Chemistry of Organic Silicon
Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Chapter
20, p 1241. (b) Holmes, R. R. Chem. ReV. 1990, 90, 17–31. (c) Chuit, C.; Corriu,
R. J. P.; Reye, C.; Young, J. C. Chem. ReV. 1993, 93, 1371–1448. (d) Holmes,
R. R. Chem. ReV. 1996, 96, 927–950. (e) Kost, D.; Kalikhman, I. In The
Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley:
New York, 1998; Vol. 2, p 1339. (f) Chuit, C.; Corriu, R. J. P.; Reye, C.; Young,
J. C. In Chemistry of HyperValent Compounds; Akiba, K.-y., Ed.; Wiley-VCH:
New York, 1999; p 81. (g) Kira, M.; Zhang, L.-C. In Chemistry of HyperValent
Compounds; Akiba, K.-y., Ed.; Wiley-VCH: New York, 1999; p 147. (h) Abele,
E. Main Group Met. Chem. 2005, 28, 45.
8244 J. Org. Chem. 2008, 73, 8244–8249
10.1021/jo801334a CCC: $40.75 2008 American Chemical Society
Published on Web 10/01/2008

Coloration of Disilylazobenzenes
SCHEME 1
FIGURE 1. ORTEP drawing of 4a (50% probability). Hydrogen
atoms were omitted for clarity. Selected bond lengths [Å] and angles
[deg] and a torsion angle [deg]: Si1-F1, 1.609(1); Si1-C1, 1.877(2);
Si1-C7,1.863(2);Si1-C13,1.862(2);N1-N1*,1.257(3);F1-Si1-C1,
103.73(7); F1-Si1-C7, 107.57(8); F1-Si1-C13, 104.56(8);
C1-Si1-C7, 114.74(8); C1-Si1-C13, 114.23(8); C7-Si1-C13,
110.98(8); N1*-N1-C6-C1, 167.0(2).
reported the syntheses and properties of azobenzenes bearing a
silicon atom at the 2-position and switched their coordination
states by using their E-Z photoisomerization.5 Some of the
organosilicon compounds bearing an azobenzene moiety could
form a pentacoordinated state with a dative bond from a nitrogen
atom of the azo group to the silicon atom, which showed yellow
color in solution in contrast to reddish orange in usual
azobenzene.5d Azobenzene analogues strongly absorb visible
light, and their colored character has been used as dyes, i.e.,
azo dye.6 Their color originates with their n-π* and/or π-π*
absorption bands. One of the most useful methods to change
the color of azobenzene derivatives is to perturb the frontier
orbitals such as n, π, and π* orbitals by substitution on the two
benzene rings with electron-donating or electron-withdrawing
groups. Another conceivable method is utilization of some
interactions between a nitrogen atom and another atom nearby.
The difference in the color between azobenzene and the
2-silylazobenzenes was apparently ascribed to the perturbation
by the N· · ·Si coordination and/or the substituent effect by the
silyl group through π-bonds. However, it is not clear how they
affect the color of azobenzenes because both the tetra- and
pentacoordinated states that may have different π-electron
systems exist in solution. Therefore, the effect of the N· · ·Si
coordination is not distinguishable from the substituent effect
in solution. If the two isomers of an azobenzene bearing the
same silyl group with and without the interaction are separated,
the difference in their color will be elucidated. In this paper,
we report the isolation of 2,2′-bis(fluorodiphenylsilyl)azoben-
zenes bearing two methyl or butyl groups at the 4- and
4′-positions as the tetracoordinated form or the pentacoordinated
form, respectively. We show how the colors of the disilyl-
azobenzenes are perturbed by the N · · ·Si interactions on the
basis of the study of their structures and the colors in the solid
state, where there is no interconversion between the pentaco-
ordinated and the tetracoordinated states.
Results and Discussion
Similarly to the synthetic method for 2-silylazobenzenes,5d
2,2′-bis(fluorodiphenylsilyl)azobenzenes 4a and 4b were syn-
thesized from the corresponding 2,2′-diiodoazobenzenes (Scheme
1). The crystal structures of 4a and 4b were determined by X-ray
crystallographic analyses (Figures 1 and 2, respectively).7
In single crystals, the N1* atom is directed to the Si1 atom
in 4b within a short interatomic N1 · · ·Si1 distance (2.467(2)
Å)8 like 2-(fluorodiphenylsilyl)azobenzene (5),5d but that is not
the case in 4a. The crystal structures clearly indicate the absence
and presence of the N · · ·Si coordination in 4a and 4b,
respectively. The bond angles (F1-Si1-C, 98.47(6)-100.19(6)°;
C-Si1-C, 111.41(7)-120.32(7)°) suggest a distorted trigonal
bipyramidal structure around the silicon atom of (E)-4b, whereas
those around the silicon atom (103.73(7)-114.74(8)°) in (E)-
4a suggest that (E)-4a should be regarded as tetrahedral structure
around their silicon atom. The slightly longer Si1-F1 bond
length of (E)-4b (1.637(1) Å) than that of (E)-4a (1.609(1) Å)
is consistent with common tendency of apical bonds composed
by 3-center-4-electron bond in trigonal bipyramidal structure.
The single crystals of each 4a and 4b were ground to powder,
and subsequently, the MAS solid-state 29Si NMR spectra of the
powder of 4a and 4b were measured to reveal the coordination
states of the silicon atoms. The chemical shift (δSi -4.9) of 4a
in the solid state was almost the same as that (δSi -4.7) of
fluorotriphenylsilane in solution,9 but that (δSi -29.0) of 4b
showed an upfield shift by over 20 ppm, which is generally
shown in pentacoordinated silicon compounds.10 These results
and the X-ray crystal structure lead to the conclusion that 4a
(7) Crystal data for 4a. C38H32N2F2Si2, M ) 610.84, triclinic, space group
P-1, Z ) 1, µ ) 0.159 cm-1, a ) 7.916(4) Å, b ) 8.961(4) Å, c ) 11.390(5)
Å, R ) 92.725(6)°, ꢀ ) 105.385(6)°, γ ) 97.941(6)°, V ) 768.5(6) Å3, R1 )
0.0369 for 4942 (I > 2σ(I)) reflections, wR2 ) 0.1092 (all data). Crystal data
for 4b. C44H44N2F2Si2, M ) 694.99, triclinic, space group P-1, Z ) 2, µ )
0.139 cm-1, a ) 10.851(4) Å, b ) 11.525(4) Å, c ) 15.430(5) Å, R ) 95.857(4)°,
ꢀ ) 100.122(4)°, γ ) 99.054(4)°, V ) 1859.1(10) Å3, R1 ) 0.0319 for 1 1999
(I > 2σ(I)) reflections, wR2 ) 0.0858 (all data). CCDC-648142 (4a) and CCDC-
648143 (4b)
(8) In single crystals of 4b, there are two independent molecules in the unit
cell, and they are almost the same structure except for the conformation of their
butyl groups.
(9) (a) Harris, R. K.; Jones, J.; Ng, S. J. Magn. Reson. 1978, 30, 521. (b)
Cella, J. A.; Cargioli, J. D.; Williams, E. A. J. Organomet. Chem. 1980, 186,
13.
(4) (a) Wagler, J.; Roewer, G. Z. Naturforsch. B 2005, 60, 709–714. (b)
Wagler, J.; Gerlach, D.; Bo¨hme, U.; Roewer, G. Organometallics 2006, 25, 2929–
2933. (c) Wagler, J.; Roewer, G. Inorg. Chim. Acta 2007, 360, 1717–1724.
(5) (a) Kano, N.; Komatsu, F.; Kawashima, T. Chem. Lett. 2001, 338–339.
(b) Kano, N.; Komatsu, F.; Kawashima, T. J. Am. Chem. Soc. 2001, 123, 10778–
10779. (c) Kano, N.; Yamamura, M.; Komatsu, F.; Kawashima, T. J. Organomet.
Chem. 2003, 686, 192–197. (d) Kano, N.; Komatsu, F.; Yamamura, M.;
Kawashima, T. J. Am. Chem. Soc. 2006, 128, 7097–7109. (e) Yamamura, M.;
Kano, N.; Kawashima, T. J. Organomet. Chem. 2007, 692, 313–325. (f)
Yamamura, M.; Kano, N.; Kawashima, T. Tetrahedron Lett. 2007, 48, 4033–
4036.
(6) Zollinger, H. Color Chemistry, 3rd ed.; Wiley-VCH: Switzerland, 2003.
J. Org. Chem. Vol. 73, No. 21, 2008 8245

Yamamura et al.
FIGURE 3. UV-vis absorption spectra of azobenzene (blue line), 4a
(red line), and 4b (green line) in dichloromethane at room temperature.
FIGURE 2. ORTEP drawing of one of the two independent
molecules of 4b (50% probability). Hydrogen atoms were omitted
for clarity. Selected bond lengths [Å] and angles [deg] and a torsion
angle [deg]: N1 · · · Si1*, 2.467(2); Si1-F1, 1.637(1); Si1-C1,
1.867(2); Si1-C7, 1.866(2); Si1-C13, 1.865(2); N1-N1*, 1.264(2);
F1-Si1-C1, 100.19(6); F1-Si1-C7, 98.47(6); F1-Si1-C13,
99.95(6); C1-Si1-C7,120.22(6); C1-Si1-C13,111.41(7); C7-Si1-
C13, 120.32(7); N1*-N1-C6-C1, 0.3(2).
FIGURE 4. Colors of 4a and 4b.
large as 20 nm in magnitude.12 The n-π* band is not observed
in the visible region. This is probably because the n-π* band
overlaps with the strong π-π* band, the tail of which extends
to 500 nm.
and 4b have the tetracoordinated and pentacoordinated silicon
states in the solid, respectively.
In CDCl3 solution, the 1H, 13C, 19F, and 29Si NMR spectra of
4a and 4b were almost the same except for the signals due to
the methyl and butyl moieties. In the 29Si NMR spectra at 20
°C, 4a and 4b showed at δSi -21.0 and -20.3, respectively,
both of which are apparently different from their chemical shifts
in the solid state, indicating the presence of the equilibrium in
solution. The equilibrium is confirmed by the significant
temperature dependence of the δSi of 4b in CD2Cl2 (δSi (-100
°C) -28.8; δSi (-40 °C) -26.7; δSi (20 °C) -21.4).11 Assuming
that the equilibrium is composed of the only two isomers, the
equilibrium constants were calculated from the observed chemi-
cal shifts as the weighted average of the penta- and tetracoor-
dinated state (see Supporting Information).3a The equilibrium
constant of 4b in CD2Cl2 is estimated to be 2.2 at 20 °C, and
therefore the pentacoordinated state exists twice as much as the
tetracoordinated state.
The colors of 4a and 4b in dichloromethane are both yellow,
which is markedly different from reddish orange of azobenzene.
This difference in color is explained by these absorption spectra.
In the absorption spectrum of azobenzene, the π-π* band is
in the UV region and the n-π* band in the visible region. The
n-π* band extends to 535 nm and is responsible for the reddish
orange.6 For 4a and 4b, the strong π-π* band, which extends
to 500 nm, is responsible for their yellow color. This means
that the colors of 4a and 4b are dominated by the π-π*
absorption, whereas the color of azobenzene is dominated by
the n-π* absorption. Although the π-π* bands of 4a and 4b
are substantially red-shifted compared with that of azobenzene,
they lie in the considerably shorter wavelength region than the
n-π* band of azobenzene. As a result, 4a and 4b are paler
than azobenzene, despite a substantial bathochromic effect of
their strongest absorption band, i.e., the π-π* band.
In contrast to the solutions, the colors of 4a and 4b in the solid
state are noticeably different from each other; 4a is orange, but 4b
is yellow (Figure 4). They are also different from the reddish
orange of azobenzene. This difference in color can be understood
in terms of the diffuse reflectance spectra of the powdered
samples (Figure 5). A rapid decrease of the reflectance of 4a
and 4b begins in the wavelength region shorter than 560 and
530 nm, respectively, whereas that of azobenzene begins at
further longer wavelength of 590 nm. Thus, the wavelength of
the absorption edge increases in the order of 4b, 4a, and
azobenzene. Their color becomes deeper in this order.
The UV-vis absorption spectra of 4a and 4b in dichlo-
romethane are almost the same (Figure 3). Each of these spectra
is of the equilibrium mixture of the tetracoordinated and the
pentacoordinated forms with an approximate molar ratio of 2:1,
as shown by the 29Si NMR spectra. There is a strong absorption
band with λmax ) 380 nm, which is assigned to π-π* transition.
This band is appreciably red-shifted compared to the corre-
sponding band of azobenzene (λmax ) 318 nm). The red shift
as large as 60 nm is ascribed to the fluorodiphenylsilyl groups
at 2- and 2′-positions of the benzene rings, because the
bathochromic effect of alkyl groups on azobenzenes is only as
(10) Williams, E. A.; Cargioli, J. D. In Annual Report on NMR Spectroscopy;
Academic Press: New York, 1979; Vol. 9.
(11) In the 29Si NMR spectral measurement of 4a, the S/N ratio was not
good owing to its low solubility. The measurement of 4b alone was sufficient
for the further discussion.
(12) (a) Suzuki, H. In Electronic Absorption Spectra and Geometry of
Organic Molecules; Academic Press: New York, 1967; p 514. (b) Grammaticakis,
P. Bull. Soc. Chim. Fr. 1951, 18, 951.
8246 J. Org. Chem. Vol. 73, No. 21, 2008

Coloration of Disilylazobenzenes
FIGURE 5. Diffuse reflectance spectra of azobenzene (blue line), 4a
(red line), and 4b (green line) at room temperature.
FIGURE 6. Kubelka-Munk spectra of azobenzene (blue line), 4a (red
line), and 4b (green line) in BaSO4 at room temperature and at the
following concentrations: azobenzene, 5 wt %; 4a, 1 wt %; 4b, 1 wt
%. Barium sulfate was used as a reference in the measurement of the
diffuse reflectance spectra. The scales on the left vertical axis correspond
to F(R) of 4a and 4b, and those of the right vertical axis correspond to
F(R) of azobenzene.
TABLE 1. Calculated Chromaticity Coordinates and Corresponding
Colors of the Powder of Disilylazobenzenes 4a and 4b and Azobenzene
chromaticity
coordinate (x, y)
corresponding
color
perceived
color
compound
azobenzene
4a
4b
0.478, 0.384
0.422, 0.411
0.429, 0.464
orange/pinka
yellowish orange yellowish orange
yellow yellow
reddish orange
reflectance spectrum (Figure 5), should be, therefore, responsible
for the orange color.
On the other hand, 4b exhibits a strong absorption band with
a maximum at a significantly longer wavelength (λmax ) 400
nm), which is assigned to the π-π* transition. The wavelength
of its absorption edge is shorter than that of 4a by ca. 30 nm.
This difference is in accord with the fact that the starting
wavelength (530 nm) of the rapid decrease in reflectance of 4b
(Figure 5) is significantly shorter than that of 4a (560 nm).
Accordingly, the shorter wavelength of the absorption edge of
4b corresponds to the absence of the absorption assigned to
the n-π* band of 4b in the longer wavelength region (530-560
nm). This absence of the n-π* band in 530-560 nm can be
interpreted by blue shift of the n-π* band, which should lie
under the strong π-π* band as in the solution spectra of 4a
and 4b. A very weak inflection at 440 nm might be interpreted
as a superposition of the n-π* and π-π* bands.16 The blue
shift of the n-π* band should be due to σ-π* character of the
n-π* band, which results from the formation of the N · · ·Si
dative bond. The yellow color of 4b in the solid state can be,
therefore, explained in terms of the blue shift of the n-π* band.
It is to be noted that the n-π* band plays a dominant role in
the solid-state coloration of these compounds, although it is
much weaker than the π-π* band.
Azobenzene exhibits the π-π* band at much shorter
wavelength (λmax ) 320 nm) than 4a (λmax ) 375 nm) and 4b
(λmax ) 400 nm). The red shift of the π-π* band in 4b, where
the N· · ·Si dative bond is formed, is considerably larger than
that in 4a, where the N · · ·Si dative bond is not formed. This
means that the fluorodiphenylsilyl group induces a substantial
bathochromic effect for the π-π* transition inductively and
that the formation of the N · · ·Si dative bond further induces a
significant bathochromic effect. The n-π* band of azobenzene
(λmax ) 445 nm) lies in a wavelength region slightly longer
than 4a. The blue shift of the n-π* band in 4a may be due to
a substitution effect of the fluorodiphenylsilyl group. The
absorption edge of the n-π* band of azobenzene (590 nm) is
a The coordinates lie on the boundary between the two colors.
That the diffuse reflectance spectra correctly reflect the actual
color was quantitatively confirmed by the use of the chromaticity
coordinate of the powders of the compounds, which is a
numerical representation of the color.13 Their coordinates and
corresponding color14 are listed in Table 1. The coordinates of
4a, 4b, and azobenzene lie in the region of yellowish orange,
yellow, and the boundary between orange and pink, respectively.
Each of these colors agrees well with the perceived color of
the corresponding compounds.
The above results show that difference in the diffuse
reflectance spectra of 4a and 4b can explain the difference in
their perceived colors. The deeper color of 4a is therefore due
to the fact that the reflectance of 4a is smaller than that of 4b
in the wavelength region of 500-600 nm.
To relate the difference in diffuse reflectance spectra to that
in absorption bands corresponding to electronic transitions
between molecular orbitals, the Kubelka-Munk spectra were
obtained for the samples in which the powder is diluted with
barium sulfate (Figure 6).15 The Kubelka-Munk spectra are
significantly different between 4a and 4b. Their spectra are also
different from the Kubelka-Munk spectrum of azobenzene.
Compound 4a exhibits a strong absorption band with λmax
)
375 nm, which is assigned to the π-π* transition, and an
unresolved weak absorption band appearing as an inflection at
440 nm, which is assigned to the n-π* transition. The extension
of the n-π* band of 4a to 560 nm, which corresponds to the
rapid decrease of reflectance from 560 nm in the diffuse
(13) For the application of the chromaticity coordinate in chemistry, see:
Harada, J.; Fujiwara, T.; Ogawa, K. J. Am. Chem. Soc. 2007, 129, 16216–16221.
(14) Kelly, K. L. J. Opt. Soc. Am. 1943, 33, 627–632.
(15) Diffuse reflectance spectra of powders are usually featureless because
both weak and strong absorption bands lead to similar reflectance values. To
obtain solution-like absorption spectra, the Kubelka-Munk transformation of
diffuse reflectance spectra of highly diluted samples is required. See: (a) Kortu¨m,
G. Trans. Faraday Soc. 1962, 58, 1624–1631. (b) Kortu¨m, G.; Braun, W.; Herzog,
G. Angew. Chem., Int. Ed. Engl. 1963, 2, 333–404. (c) Kortu¨m, G. In Reflectance
Spectroscopy; Springer: New York, 1969. (d) Fujiwara, T.; Harada, J.; Ogawa,
K. J. Phys. Chem. B 2004, 108, 4035–4038.
(16) It might be also interpreted as a vibrational fine structure. The very
weak inflection at 440 nm does not necessarily mean that there is a band, λmax
of which is 440 nm. Even if the inflection is interpreted as a superposition of
the n-π* and π-π* bands, the λmax of the n-π* band could be different from
440 nm.
J. Org. Chem. Vol. 73, No. 21, 2008 8247

Yamamura et al.
g, 2.35 mmol) at -105 °C was added n-BuLi (1.6 M in hexane,
3.1 mL, 5.0 mmol) rapidly. After the reaction solution was stirred
further at -105 °C for 10 min, chlorodiphenylsilane (1.0 mL, 8.0
mmol) was added to it, and the reaction mixture was stirred at room
temperature for 12 h. Evaporation of the solvent, separation of the
residue by alumina-gel column chromatography (eluent, hexane/
CHCl3), and recrystallization from CHCl3 after evaporation of the
eluate gave orange crystals of 2,2′-bis(diphenylsilyl)-4,4′-dimeth-
ylazobenzene (3a) (0.95 g, 70%). Similarly, 4,4′-dibutyl-2,2′-
bis(diphenylsilyl)azobenzene (3b) was synthesized in 82% yield.
3a: orange crystals, mp 227-228 °C. 1H NMR (400 MHz, CDCl3)
δ 2.30 (s, 6H), 5.69 (s, 1JHSi ) 203.6 Hz, 2H), 6.91 (d, 3JHH ) 8.0
Hz, 2H), 7.06 (dd, 3JHH ) 8.0 Hz, 3JHH ) 1.5 Hz, 2H), 7.27-7.36
(m, 14H), 7.76 (dd, 3JHH ) 8.0 Hz, 3JHH ) 1.5 Hz, 8H). 13C NMR
(126 MHz, CDCl3) δ 21.4 (CH3), 116.6 (CH), 127.8 (CH), 129.2
(CH), 131.8 (CH), 134.7 (CSi), 135.0 (CSi), 135.7 (CH), 137.9
(CH), 140.5 (CMe), 155.0 (CN). 29Si NMR (99 MHz, CDCl3) δ
-18.3 (d, 1JSiH ) 203.6 Hz). Anal. Calcd for C38H34N2Si2: C, 79.39;
H, 5.96; N, 4.87. Found: C, 79.19; H, 5.94; N, 4.72. 3b: orange
slightly longer than that of 4a (560 nm). This should be
responsible for the deeper color of azobenzene than 4a.
In summary, the difference in color of 4a, 4b, and azobenzene
in the solid state was explained by considering the absorption
due to the n-π* transition. In compound 4a, where the N· · ·Si
dative bond is not formed in the solid state, the n-π* band
appears in the longer wavelength region and results in a deep
color. In compound 4b, where the N · · ·Si dative bond is formed
in the solid state, the n-π* band with some σ-π* character
lies under the strong π-π* band in the shorter wavelength
region. As a result, the color of 4b becomes paler than that of
4a. The color of azobenzene is deeper than that of 4a, because
the n-π* band of azobenzene extends to longer wavelength
than 4a. It is thus clearly shown that the difference in color of
the powders of 4a and 4b does not result from trivial changes
in structures owing to packing effects but from essential
difference in coordination states.
1
crystals, mp 136-137 °C. H NMR (400 MHz, CDCl3) δ 0.86 (t,
Conclusion
3JHH ) 7.5 Hz, 6H), 1.27 (sextet, 3JHH ) 7.5 Hz, 4H), 1.50 (quint,
3
1
3JHH ) 7.5 Hz, 4H), 2.51 (t, JHH ) 7.5 Hz, 4H), 5.61 (s, JHSi
)
We demonstrate here that the N · · ·Si interaction plays a
crucial role in the coloration of disilylazobenzenes in the solid
state. Both tetracoordinated and pentacoordinated states of the
azobenzenes bearing two fluorodiphenylsilyl groups were suc-
cessfully isolated by changing the substituents at the 4- and 4′-
positions. Their reflectance spectra in the solid state were very
different: the pentacoordinated state had a weaker absorbance
in the long-wavelength region than the tetracoordinated state,
which is ascribed to the shift of the n-π* transition caused by
the N· · ·Si interaction. This difference in the absorptions due
to the n-π* transition drastically affects the color of the
azobenzenes; the color of the pentacoordinated state becomes
apparently paler than that of the tetracoordinated state. The
N· · ·Si interactions play a very important role in determining
the color of the disilylazobenzenes.
3
3
202.3 Hz, 2H), 6.88 (d, JHH ) 8.0 Hz, 2H), 7.02 (dd, JHH ) 8.0
3
3
Hz, JHH ) 1.5 Hz, 2H), 7.22-7.32 (m, 14H), 7.49 (dd, JHH
)
8.0 Hz, JHH ) 1.5 Hz, 8H). 13C NMR (126 MHz, CDCl3) δ 14.0
(CH3), 22.4 (CH2), 33.4 (CH2), 35.5 (CH2), 116.8 (CH), 127.8 (CH),
129.3 (CH), 131.1 (CH), 134.84 (CSi), 134.86 (CSi), 135.7 (CH),
137.5 (CH), 145.5 (CBun), 155.2 (CN). 29Si NMR (99 MHz, CDCl3)
3
1
δ -18.8 (d, JSiH ) 202.3 Hz). Anal. Calcd for C44H46N2Si2: C,
80.19; H, 7.04; N, 4.25. Found: C, 80.06; H, 7.22; N, 4.01.
Synthesis of 2,2′-Bis(fluorodiphenylsilyl)azobenzenes 4. To a
THF solution (10 mL) of 2,2′-bis(diphenylsilyl)-4,4′-dimethyl-
azobenzene (3a) (204 mg, 0.36 mmol) was added AgF (140 mg,
1.01 mmol) at room temperature, and the reaction mixture was
stirred for 20 h. After insoluble materials were filtered off through
Celite, the filtrate was evaporated. Recrystallization of the residue
from CHCl3 gave orange crystals of 2,2′-bis(fluorodiphenylsilyl)-
4,4′-dimethylazobenzene (4a) (88.2 mg, 40%). Similarly, 4,4′-
dibutyl-2,2′-bis(fluorodiphenylsilyl)azobenzene (4b) was synthe-
sized in 79% yield. 4a: orange crystals, mp 247-248 °C. 1H NMR
(400 MHz, CDCl3, 20 °C) δ 2.32 (s, 6H), 6.90 (d, 3JHH ) 8.0 Hz,
2H), 7.03 (dd, 3JHH ) 8.0 Hz, 3JHH ) 1.5 Hz, 2H), 7.26-7.38 (m,
Experimental Section
Synthesis of 2,2′-Diiodoazobenzenes 2.17 To a toluene solution
(100 mL) of 2-iodo-4-methylaniline 1a (21.0 g, 90.3 mmol) was
added manganese dioxide (80 g, 0.92 mol), and the reaction mixture
was refluxed for 2.5 h. Filtration of excess manganese dioxide(IV),
evaporation of the solvent from the filtrate, separation of the residue
by silica-gel chromatography (eluent, CHCl3), and recrystallization
from ethanol gave orange crystals of 2,2′-diiodo-4,4′-dimethyl-
azobenzene (2a, 9.35 g, 45%). Similarly, 4,4′-dibutyl-2,2′-diiodo-
azobenzene (2b) was synthesized in 20% yield. 2a: orange crystals,
mp 194-196 °C. 1H NMR (270 MHz, CDCl3) δ 2.37 (s, 6H), 7.21
(d, 3JHH ) 8.0 Hz, 2H), 7.65 (d, 3JHH ) 8.0 Hz, 2H), 7.83 (s, 2H).
13C NMR (126 MHz, CDCl3) δ 21.2 (CH3), 103.8 (CI), 117.9 (CH),
130.0 (CH), 140.3 (CH), 143.6 (CMe), 149.1 (CN). Anal. Calcd
for C14H12N2I2: C, 36.39; H, 2.62; N, 6.06. Found: C, 36.24; H,
2.73; N, 5.86. 2b: orange crystals, mp 106-108 °C. 1H NMR (270
3
3
4
12H), 7.49 (dd, JHH ) 8.0 Hz, JHH ) 1.5 Hz, 8H), 7.76 (d, JHH
) 1.5 Hz, 2H). 13C{1H} NMR (126 MHz, CDCl3, 60 °C) δ 21.6
(s, CH3), 124.0 (s, CH), 127.8 (s, CH), 128.5 (d, JCF ) 15.7 Hz,
2
CSi), 129.9 (s, CH), 132.0 (s, CH), 134.3 (d, JCF ) 1.6 Hz, CH),
135.0 (d, 2JCF ) 21.5 Hz, CSi), 137.5 (d, JCF ) 5.0 Hz, CH), 141.9
(s, CMe), 153.1 (s, CN). 19F NMR (376 MHz, CDCl3) δ -149.92
(brs). 29Si{1H} NMR (99 MHz, CDCl3) δ -21.0 (d, 1JSiF ) 270.7
Hz). Anal. Calcd for C38H32N2F2Si2 ·0.5H2O: C, 73.63; H, 5.37;
N, 4.52. Found: C, 73.48; H, 5.47; N, 4.32. 4b: yellow crystals,
1
mp 136-137 °C. H NMR (400 MHz, CDCl3, 20 °C) δ 0.87 (t,
3JHH ) 7.5 Hz, 6H), 1.30 (sextet, 3JHH ) 7.5 Hz, 4H), 1.54 (quint,
3
3
3JHH ) 7.5 Hz, 4H), 2.56 (t, JHH ) 7.5 Hz, 4H), 6.89 (d, JHH
)
3
3
8.0 Hz, 2H), 7.01 (dd, JHH ) 8.0 Hz, JHH ) 1.5 Hz, 2H),
7.22-7.33 (m, 12H), 7.48 (dd, 3JHH ) 8.0 Hz, 3JHH ) 1.5 Hz, 8H),
7.75 (d, 4JHH ) 1.5 Hz, 2H). 13C{1H} NMR (126 MHz, CDCl3, 60
°C) δ 13.8 (s, CH3), 22.5 (s, CH2), 33.1 (s, CH2), 35.7 (s, CH2),
123.6 (s, CH), 127.8 (s, CH), 128.8 (d, 2JCF ) 15.7 Hz, CSi), 129.8
3
3
MHz, CDCl3) δ 0.94 (t, JHH ) 8.0 Hz, 6H), 1.37 (sextet, JHH
)
8.0 Hz, 4H), 1.51-1.66 (m, 4H), 2.63 (t, 3JHH ) 8.0 Hz, 4H), 7.23
3
4
3
(dd, JHH ) 7.5 Hz, JHH ) 1.5 Hz, 2H), 7.67 (d, JHH ) 7.5 Hz,
2H), 7.84 (d, 4JHH ) 1.5 Hz, 2H). 13C NMR (126 MHz, CDCl3) δ
13.9 (CH3), 22.3 (CH2), 33.2 (CH2), 35.0 (CH2), 103.8 (CI), 117.8
(CH), 129.3 (CH), 139.5 (CH), 148.4 (CBun), 149.0 (CN). Anal.
Calcd for C20H24N2I2: C, 43.98; H, 4.43; N, 5.13. Found: C, 43.79;
H, 4.40; N, 4.98.
2
(s, CH), 131.2 (s, CH), 134.4 (s, CH), 134.9 (d, JCF ) 20.7 Hz,
CSi), 136.7 (d, JCF ) 4.9 Hz, CH), 146.7 (s, CBun), 153.3 (s, CN).
19F NMR (376 MHz, CDCl3) δ -150.15 (brs). 29Si{1H} NMR (99
1
MHz, CDCl3) δ -20.3 (d, JSiF ) 273.3 Hz). Anal. Calcd for
C44H44N2F2Si2: C, 76.04; H, 6.38; N, 4.03. Found: C, 76.05; H,
6.32; N, 3.80.
Synthesis of 2,2′-Bis(diphenylsilyl)azobenzenes 3. To a THF
Spectral Measurements. Diffuse UV-vis reflectance spectra
were measured on a spectrophotometer equipped with an integrating
sphere accessory. For the spectra of nondiluted samples, NaCl
powderwasgroundandusedasthereference.FortheKubelka-Munk
solution (30 mL) of 2,2′-diiodo-4,4′-dimethylazobenzenes (3a) (1.00
(17) Takahashi, H.; Ishioka, T.; Koiso, Y.; Sodeoka, M.; Hashimoto, Y. Biol.
Pharm. Bull. 2000, 23, 1387.
8248 J. Org. Chem. Vol. 73, No. 21, 2008

Coloration of Disilylazobenzenes
spectra, barium sulfate was ground and used as the diluent and the
reference. The ground sample powders and the barium sulfate
powder were put in a glass vial and mixed together by rolling and
shaking the vial by hand. The mixed powder was used as the
sample. Diffuse reflectance spectra were measured and transformed
into the Kubelka-Munk spectra using the following equation:
reflectance spectrum is necessary for the calculation of the
chromaticity coordinates. By the use of diffuse reflectance spectra
(Figure 5), the chromaticity coordinates for the powder of each of
the compounds were calculated.
Acknowledgment. We are grateful to Shin-etsu Chemical
Co., Ltd., and Tosoh Finechem Corporation for the gift of
chlorosilanes and alkyllithiums, respectively. This work was
partially supported by Grants-in-Aid for The 21st Century COE
Program for Frontiers in Fundamental Chemistry and for
Scientific Research from Ministry of Education, Culture, Sports,
Science and Technology, Japan. This work was also supported
by Grant-in-Aid for Scientific and for Young Scientists by Japan
Society for the Promotion of Science. N.K. thanks Foundation
Advanced Technology Institute for the financial support.
(1 - R)2
F(R) )
(1)
2R
where R is the observed diffuse reflectance.
Calculations of Chromaticity Coordinates. The chromaticity
coordinate of a sample under a specific illuminating light can be
calculated from the diffuse reflectance spectrum of the sample, the
spectrum of the illuminating light, and the spectral sensitivity of
the human eye.6,18 Among these spectra, both the spectrum of the
illuminating light, which depends on the lamp used, and the spectral
sensitivity of the human eye are given by the Commision
International de l’Eclairage (CIE).19 Therefore, only the diffuse
Supporting Information Available: Crystallographic data
for 4a and 4b in CIF format, compound characterization data,
and detailed experiments. This material is available free of
charge via the Internet at http://pubs.acs.org.
(18) Nassau, K. In The Physics and Chemistry of Color, 2nd ed.; John Wiley
and Sons, New York, 2001.
(19) CIE Home Page, http://www.cie.co.at/cie/.
JO801334A
J. Org. Chem. Vol. 73, No. 21, 2008 8249