Reversible photocontrol of the coordination number of silicon in a tetrafluorosilicate bearing a 2-(phenylazo)phenyl group [21]

Full text of DOI:10.1021/ja0165739

10778
J. Am. Chem. Soc. 2001, 123, 10778-10779
Scheme 1
Reversible Photocontrol of the Coordination Number
of Silicon in a Tetrafluorosilicate Bearing a
2-(Phenylazo)phenyl Group
Naokazu Kano, Fuminori Komatsu, and
Takayuki Kawashima*
Department of Chemistry
Graduate School of Science
The UniVersity of Tokyo, 7-3-1 Hongo
Bunkyo-ku, Tokyo 113-0033, Japan
ReceiVed July 10, 2001
Highly coordinate organosilicon compounds constitute an
important class of species in organosilicon chemistry and have
been extensively studied in view of their structures, reaction
mechanism, and synthetic application.1 The coordination number
of the silicon atom considerably influences not only the confor-
mation around silicon but also the properties and reactivity of
organosilicon compounds, especially in high coordination state.
If the coordination number could be changed by external
stimulations such as light, electricity, and magnetism, the structure
and property of the compounds would be controlled without
addition of an external reagent.2 There has been no report on such
a control of the coordination number despite its promising
usefulness as a novel type of switch for the structure and property.3
We recently reported versatile synthetic methods for azobenzenes
with a silyl, germyl, or stannyl group at the 2-position, respec-
tively.4 A 2-(phenylazo)phenyl group is considered to act as a
potential coordination site judging from the structural resemblance
with the van Koten ligand, 2-(Me2NCH2)C6H4 group5 and from
many examples of transition metal complexes which show
intramolecular coordination of an azo group.6 Therefore, photo-
and thermal isomerization of an azo unit which has been widely
utilized for the switch of properties promoted by its geometrical
change of the compound are expected to be effective for the
control of the coordination number of the central atom.7 We report
here synthesis, structure, and reversible photoswitching of the
coordination number of a silicon atom of potassium, 18-crown-
6, tetrafluorosilicate bearing a 2-(phenylazo)phenyl group.
2-Iodoazobenzene (1) was allowed to react successively with
n-BuLi (1.03 equiv) and chlorotriethoxysilane (1.05 equiv) in THF
at -105 °C and further at 0 °C to give 2-triethoxysilyl derivative
2 (75%) (Scheme 1).4 Fluorination of 2 with BF3‚Et2O (1.10
equiv) at room temperature in ether afforded the trifluoro[2-
(phenylazo)phenyl]silane (3) (49%).8 Treatment of 3 with KF
(1.00 equiv) in the presence of 18-crown-6 (1.00 equiv) in toluene
gave potassium, 18-crown-6, tetrafluoro[(E)-2-(phenylazo)phenyl]-
silicate (4) (74%) as yellow solids.
In the 29Si NMR spectra, (E)-4 showed a quintet (1JSi-F ) 193
Hz) at δ -150.9 by coupling with four equivalent fluorine nuclei
at room temperature in CDCl3 and a doublet of double triplet at
δ -153.7 coupled with three kinds of nuclei (1JSi-F ) 149, 186,
and 214 Hz, respectively) at -90 °C, respectively, in CD2Cl2.9
In the 19F NMR spectra, a broad singlet at δ -127.4 in CDCl3 at
room temperature was split to two broad signals (δ -122.5 and
-147.2) at -30 °C in CD2Cl2. They were further split to a pair
(1) (a) Holmes, R. R. Chem. ReV. 1996, 96, 927. (b) 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; pp 81-146. (c) Kira, M.; Zhang,
L.-C. In Chemistry of HyperValent Compounds; Akiba, K.-Y., Ed.; Wiley-
VCH: New York, 1999; pp 147-169. (d) Kost, D.; Kalikhman, I. In The
Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.;
John Wiley & Sons: Chichester, 1998; Volume 2, Part 2, Chapter 23, pp
1339-1445.
(2) The changes of photophysical properties caused by hypercoordination
of organosilicon compounds with addition/elimination of an external fluoride
ion have been reported. See: Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am.
Chem. Soc. 2000, 122, 6793.
(3) Equilibrium between neutral hexacoordinate silicon complexes and ionic
silicenium chlorides, which is temperature-, solvent-, counterion-, ligand-, and
substituent-dependent, has been reported. See: Kingston, V.; Gostevskii, B.;
Kalikhman, I.; Kost, D. Chem. Commun. 2001, 1272.
(4) Kano, N.; Komatsu, F.; Kawashima, T. Chem. Lett. 2001, 338.
(5) A bidentate ligand, -C6H4-2-CH2NMe2, was developed by van
Koten: (a) van Koten, G.; Schaap, C. A.; Noltes, J. G. J. Organomet. Chem.
1975, 99, 157. For hypervalent silicon compounds with the van Koten ligand,
see: (b) Brelie`re, C.; Carre´, F.; Corriu, R. J. P.; Saxce, A.; Poirier, M.; Royo,
G. J. Organomet. Chem. 1981, 205, C1. (c) Weinmann, M.; Gehrig, A.;
Schiemenz, B.; Huttner, G.; Nuber, B.; Rheinwald, G.; Lang, H. J. Organomet.
Chem. 1998, 563, 61. (d) Belzner, J.; Dehnert, U.; Ihmels, H.; Hu¨bner, M.;
Mu¨ller, P.; Uso´n, I. Chem. Eur. J. 1998, 4, 852.
of doublets at δ -121.6 (2JF-F ) 21.3 Hz) and -123.2 (2JF-F
)
24.4 Hz) and one double triplet at δ -147.2 (2JF-F ) 21.3 Hz,
2JF-F ) 24.4 Hz) at -90 °C, respectively, in CD2Cl2. Such
coupling patterns and chemical shifts, which are similar to those
of a hexacoordinate tetrafluorosilicate with the van Koten
ligand,5b-d indicate that nonequivalency of fluorine atoms in (E)-
form due to the coordination of nitrogen atom of the azo group.
Fast stereomutation of the ligands compared to the time scale of
19F NMR spectroscopy would result in observation of a singlet
at room temperature.
In UV/vis spectra of (E)-4, an absorption maximum due to
π-π* transition of the azo group was observed at 334 nm in
CH2Cl2. The color of (E)-4 is yellow in sharp contrast to the red
color of previously reported 2-silylazobenzenes, suggesting the
perturbation of the electronic structure of the azo unit.4 Isomer-
ization of (E)-4 could easily be done by irradiation (λ ) 360
nm) in CH2Cl2 for 40 min to give orange-colored (Z)-4. (Z)-4
showed its absorption maximum at 440 nm assignable to n-π*
transition. Irradiation (λ ) 431 nm) of the new absorption
(6) Bruce, M. I.; Goodall, B. L. In The Chemistry of the Hydrazo, Azo and
Azoxy Groups; Patai, S., Ed.; John Wiley & Sons: London, 1975; Chapter 9,
p 259.
(7) (a) Archut, A.; Vo¨gtle, F.; De Cola, L.; Azzellini, G. C.; Balzani, V.;
Ramanujam, P. S.; Berg, R. H. Chem. Eur. J. 1998, 4, 699. (b) Balzani, V.;
Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2000, 39,
3349. (c) Feringa, B. L.; van Delden, R. A.; Koumura, N.; Geertsema, E. M.
Chem. ReV. 2000, 100, 1789.
(8) 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.
(9) (a) Brelie`re, C.; Carre´, F.; Corriu, R. J. P.; Douglas, W. E.; Poirier,
M.; Royo, G.; Man, M. W. C. Organometallics 1992, 11, 1586. (b) Carre´, F.;
Chuit, C.; Corriu, R. J. P.; Fanta, A.; Mehdi, A.; Reye´, C. Organometallics
1995, 14, 194.
10.1021/ja0165739 CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/06/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 43, 2001 10779
Figure 1. ORTEP drawing of (E)-4 with thermal ellipsoid plot (30%
probability). Hydrogen atoms are omitted for clarity. Selected bond lengths
(Å) and bond angles (deg): Si1-F1, 1.672(2); Si1-F2, 1.687(2); Si1-
F3, 1.647(2); Si1-F4, 1.661(2); Si1-C1, 1.909(4); N1-N2, 1.272(4);
Si1-N2, 2.260(3); F1-Si1-F2, 168.4(1); F1-Si1-F3, 97.1(1); F1-
Si1-F4, 88.9(1); F2-Si1-F3, 94.2(1); F2-Si1-F4, 87.2(1); F3-Si1-
F4, 95.8(1); C1-Si1-F1, 90.7(1); C1-Si1-F2, 90.1(1); C1-Si1-F3,
99.6(2); C1-Si1-F4, 164.5(2); N2-Si1-F1, 84.4(1); N2-Si1-F2, 84.5-
(1); N2-Si1-F3, 175.9(1); N2-Si1-F4, 88.0(1); N2-Si1-C1, 76.6(1).
Figure 2. ORTEP drawing of (Z)-4 with thermal ellipsoid plot (30%
probability). Hydrogen atoms are omitted for clarity. Selected bond lengths
(Å) and bond angles (deg): Si1-F1, 1.670(3); Si1-F2, 1.698(3); Si1-
F3, 1.617(3); Si1-F4, 1.611(3); Si1-C1, 1.888(4); N1-N2, 1.259(5);
F1-Si1-F2, 178.2(1); F1-Si1-F3, 90.4(1); F1-Si1-F4, 90.4(1); F2-
Si1-F3, 88.8(1); F2-Si1-F4, 88.5(1); F3-Si1-F4, 115.3(1); C1-Si1-
F1, 91.3(2); C1-Si1-F2, 90.5(2); C1-Si1-F3, 117.8(2); C1-Si1-F4,
126.8(2).
In (Z)-4, the silicon atom evidently lacked Si1-N2 interaction
and adopted a pentacoordinate state with a trigonal bipyramidal
structure with F1 and F2 atoms at apical positions and with F3,
F4, and C1 atoms at equatorial positions, respectively, (TBP f
SP 22.3%) (Figure 2).12 The bond lengths of Si1-F1 (1.670(3)
Å) and Si1-F2 (1.698(3) Å) bonds are significantly longer than
those of Si1-F3 (1.617(3) Å) and Si-F4 (1.611(3) Å) bonds.
Such elongation of apical bonds is indeed characteristic of TBP
structure around a pentacoordinate silicon atom. Structural changes
between (E)-4 and (Z)-4 were found on the widening of the bond
angles of C1-Si1-F4 (164.5(2)°) and the narrowing of F3-Si1-
F4 (95.8(1)°) and C1-Si1-F3 (99.6(2)°) in (E)-4 compared to
those of (Z)-4 (126.8(2)°, 115.3(1)°, and 117.8(2)°, respectively).
Thus, configuration around the silicon atom in 4 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 this
case, the N1-N2 bond length (1.272(4) Å) of the (E)-isomer is
almost same as that of azobenzenes without donation of the
nitrogen atom, indicating little structural perturbation of the
azobenzene unit regardless of the coordination of N2 to Si1.4,13
Interestingly, the coordination state can visually be recognized
with ease by the change of the colors from yellow to red because
the azo unit in 4 works as a coordination site in the (E)-form as
well as a chromophore. Furthermore, irradiation of the silicate 4
caused changes of their stabilities to the humidity. Although the
(E)-4 isomer easily handled without any special care in the air,
the (Z)-4 isomer is easily hydrolyzed in the aerial humidity. We
believe that this new strategy, that is, the photoswitching of the
coordination number of highly coordinate main-group element
compounds promising the control of the structures, spectral
properties, and reactivities, would be useful for the construction
of molecular devices.
maximum led to the recovery of (E)-4 with a decrease of its
absorption.
In variable-temperature 19F NMR spectra, the signal of (Z)-4
was observed at δ -114.7 as a broad singlet and did not
split even at -70 °C in contrast to that of (E)-4. The ratio of
(E)-4:(Z)-4 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)-4 was, the broader
the signal became, suggesting the intermolecular fluorine ex-
change. The 29Si NMR spectra of (Z)-4 at -65 °C showed a
quintet at δ -122.8, which is lower field by 28 ppm than that of
(E)-4. 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)-4 bears a pentaco-
ordinate structure around a silicon atom in the absence of
coordination of the nitrogen atom. The (Z)-isomer of 4 thus
obtained is stable under argon at ambient temperature for a few
days in the solid state, and it gradually isomerizes to (E)-isomer
at room temperature in solution state even in the dark.
The structures of both (E)- and (Z)-stereoisomers were finally
determined by X-ray crystallographic analysis.10 In the (E)-isomer,
the N2 atom on the azobenzene unit is directed to the Si1 atom
despite the steric repulsion of a bulky silicate group (Figure 1).4
The interatomic N2-Si1 distance (2.260(3) Å), which is almost
same as those of highly coordinate organosilicon compounds with
the van Koten ligand, is much shorter than the sum of the
corresponding van der Waals radii (3.65 Å).5b-d,11 Taking into
consideration that a hypervalent bond is generally long, the
intramolecular interaction between N2 and Si1 evidently exists
in (E)-4, presenting the hexacoordinate structure of the silicon.
(E)-4 was shown to have a distorted octahedral configuration
suggested by the bond angles (76.6(1)-99.6(2)°) around Si1 atom.
Acknowledgment. We thank Shin-etsu Chemical Co., Ltd., and Tosoh
Finechem Corp. for the generous gift of chlorosilanes and alkyllithiums,
respectively.
(10) Crystallographic data. (E)-4: C24H33F4KN2O6Si, orthorhombic, P212121,
a ) 9.3340(2) Å, b ) 16.3190(8) Å, c ) 18.1480(8) Å, V ) 2764.3(2) Å3,
Z ) 4, MW ) 588.71, T ) 150 K, R ) 0.046, wR2(all data) ) 0.147, GOF
) 1.01. (Z)-4: C24H33F4KN2O6Si, triclinic, P-1, a ) 13.445(2) Å, b ) 14.836-
(2) Å, c ) 15.750(2) Å, R ) 74.097(6)°, â ) 69.804(3)°, γ ) 79.731(2)°, V
) 2823.4(7) Å3, Z ) 4, MW ) 588.71, T ) 103 K, R ) 0.066, wR2(all data)
) 0.138, GOF ) 1.28. The unit cell contains two independent molecules,
which have a similar conformation around the silicon atom in (Z)-4.
(11) (a) Klebe, G. J. Organomet. Chem. 1987, 332, 35 (b) Brelie`re, C.;
Carre´, F.; Corriu, R. J. P.; Poirier, M.; Royo, G.; Zwecker, J. Organometallics
1989, 8, 1831 (c) Probst, R.; Leis, C.; Gamper, S.; Herdtweck, E.; Zybill, C.;
Auner, N. Angew. Chem., Int. Ed. Engl. 1991, 30, 1132 (d) Handwerker, H.;
Leis, C.; Probst, R.; Bissinger, P.; Grohmann, A.; Kiprof, P.; Herdtweck, E.;
Blumel, J.; Auner, N.; Zybill, C. Organometallics 1993, 12, 2162 (e) Belzner,
J.; Schar, D.; Kneisel, B. O.; Herbst-Irmer, R. Organometallics 1995, 14, 1840.
Supporting Information Available: Synthetic procedures and spec-
tral data for 2, 3, (E)-4, and (Z)-4 (PDF). X-ray crystallographic files in
CIF format for (E)-4 and (Z)-4. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA0165739
(12) Holmes, R. R.; Deiters, J. A. J. Am. Chem. Soc. 1977, 99, 3318.
(13) (a) Allmann, R. In The Chemistry of the Hydrazo, Azo and Azoxy
Groups; Patai, S., Ed.; John Wiley & Sons: London, 1975; Chapter 2, p 43.
(b) Harada, J.; Ogawa, K.; Tomoda, S. Acta Crystallogr. 1997, B53, 662.