Preparation and structures of novel silamacrocyclic compounds: Silacalix[4]quinone and silacalix[4]hydroquinone octamethyl ether

Article abstract of DOI:10.1039/b307048d

X-Ray crystallographic analysis revealed that novel silamacrocyclic compounds, 2,8,14,20-tetrasilacalix[4]quinone and 2,8,14,20-tetrasilacalix[4]hydroquinone octamethyl ether, adopted 1,3-alternate structures in the solid state.

Full text of DOI:10.1039/b307048d

Preparation and structures of novel silamacrocyclic compounds:
silacalix[4]quinone and silacalix[4]hydroquinone octamethyl ether†‡
Shinobu Tsutsui*a and Kenkichi Sakamoto*ab
a Photodynamics Research Center, The Institute of Physical and Chemical Research (RIKEN), 519-1399
Aoba, Aramaki, Aoba-ku, Sendai 980-0845, Japan. E-mail: stsutsui@postman.riken.go.jp
b Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578,
Japan. E-mail: sakamoto@si.chem.tohoku.ac.jp
Received (in Cambridge, UK) 19th June 2003, Accepted 25th July 2003
First published as an Advance Article on the web 5th August 2003
1
X-Ray crystallographic analysis revealed that novel silama-
crocyclic compounds, 2,8,14,20-tetrasilacalix[4]quinone and
2,8,14,20-tetrasilacalix[4]hydroquinone octamethyl ether,
adopted 1,3-alternate structures in the solid state.
characterized by H, 13C, and 29Si NMR and mass spectrome-
try.§¶ Finally, their structures were unequivocally determined
by X-ray crystallography as described below.∑** To the best of
our knowledge, 4 is the first example of a heteroatom-bridged
calix[4]quinone derivative.
1,4-Benzoquinone (quinone) is one of the most interesting
fundamental p-electron systems because its high electron
affinity and photoreactivity.1–3 Calixquinones4,5 (e.g. 1) and
calixhydroquinones4–7 (e.g. 2 and 3), which are members of the
calixarenes,8 have received intense interest due to their
electrochemical reactivities, conformational behavior in solu-
tion, structures in the solid states, and incorporation of metal
ions (Chart 1). However, silicon-bridged calixquinones and
calixhydroquinones have not been reported.9,10 We report the
first successful syntheses of tetrasilacalix[4]quinone 4 and
tetrasilacalix[4]hydroquinone octamethyl ether 5.
Compounds 4 and 5 were prepared as shown in Scheme 1.
Monolithiation of 7 and reaction with Me2SiCl2 yielded 8 as
colorless crystals in 64% yield. Dilithiation of 8 followed by the
addition of Me2SiCl2 gave 5 as colorless crystals in 9% yield.
Oxidation of 5 by cerium ammonium nitrate3 gave 4 as orange
crystals in 16% yield. The structures of 4 and 5 were
A single crystal of 5 suitable for X-ray crystallographic
analysis was obtained from a hexane solution. The crystal lattice
of 5 included no solvent molecule. The observed molecular
structure of 5 is shown in Fig. 1. Compounds 610 and 37 (Chart
1) have a 1,2-alternate conformation and an acutely pinched
cone structure, respectively, in the solid state. However, a
1,3-alternate structure of 5 was revealed using X-ray analysis.
The two pairs of opposing benzene rings are nearly parallel.
Interatomic distances between Si(1)…Si(3) and Si(2)…Si(4),
corresponding to the diameter of an internal 16-membered ring
of 5, were 8.35 Å and 8.30 Å, respectively. The angles of
C(arene)–Si–C(arene) in 5 were 111.2–114.9° (av. 112.9°),
which are slightly wider than those in 6 (av. 109.7°). The bond
lengths of C(arene)–Si in 5 were 1.894–1.918 Å (av. 1.909 Å),
which are slightly longer than those in 6 (av. 1.873 Å). The
slightly wider C(arene)–Si–C(arene) angles and longer
C(arene)–Si bonds in the structure of 5 are indicative of its
strained structure.
A single crystal of 4 suitable for X-ray analysis was obtained
from a p-xylene solution and contained solvent molecules in the
lattice at a 4 : p-xylene ratio of 2 : 3. Although the corresponding
carbon-bridged calix[4]quinone 1 has a partial cone conforma-
tion,4 4 had a 1,3-alternate structure as shown in Fig. 2. The
interatomic distance between Si(1)…Si(2)*, corresponding to
the diameter of an internal 16-membered ring of 4, was 8.17 Å.
The closest interatomic distance between C(4)…C(25) was
3.351 Å, which is slightly shorter than the sum of the van der
Waals radii (3.4 Å), indicating that hydrophobic interactions
between 4 and p-xylene would exist. The angles of C(quinone)–
Si–C(quinone) in 4 were 108.52 and 108.54°, which are close to
the ideal angle in an sp3 silicon atom (109.5°). Neither an
elongation of the covalent bonds nor a significant distortion of
the coordination geometry was observed in the structure of 4,
indicative of its strain-free structure.
Chart 1
Scheme 1
† Dedicated to Prof. Mitsuo Kira on the occasion of his 60th birthday.
‡ Electronic supplementary information (ESI) available: experimental
procedures, spectral data and X-ray data of the products. See http://
www.rsc.org/suppdata/cc/b3/b307048d/
Fig. 1 Molecular structure of 5.** Hydrogen atoms are omitted for
clarity.
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which would originate from conjugation of the quinone
moieties through the silicon groups.
In conclusion, we have succeeded in preparing novel
silamacrocyclic compounds, tetrasilacalix[4]quinone 4 and
tetrasilacalix[4]hydroquinone octamethyl ether 5. Compound 5
was converted to 4 by cerium ammonium nitrate oxidation.
Compound 4 is the first example of a heteroatom-bridged
calix[4]tetraquinone. The 1,3-alternate structures of 4 and 5
were revealed by X-ray crystallographic analysis. The UV–Vis
spectrum of 4 indicated that a hypochromic effect appeared in
the p–p* transition band. Further investigations on the
properties of 4 and 5 are in progress.
This work was supported by the Special Postdoctoral
Researchers Program of RIKEN.
Notes and references
§ Spectral data for 4: orange crystals; mp 163–165 °C (decomp.); 1H NMR
(CDCl3, d) 0.31 (s, 24H), 6.97 (s, 8H); 13C NMR (CDCl3, d) 24.26, 144.18,
150.12, 185.15, 192.44; 29Si NMR (CDCl3, d) 211.92; FAB-HRMS m/z
found 657.1269 [M + H+], calcd for C32H33O8Si4, 657.1253; IR nCNO 1648,
1653 cm21; UV (CH2Cl2) lmax/nm (e) 249 (32300).
¶ Spectral data for 5: colorless crystals; mp 294–294.5 °C; 1H NMR
(CDCl3, d) 0.51 (s, 24H), 2.68 (s, 12H), 3.78 (s, 12H), 7.04 (s, 8H); 13C
NMR (CDCl3, d) 20.57, 55.56, 61.61, 121.82, 132.52, 154.51, 164.05; 29Si
NMR (CDCl3, d) 211.16; MS (70 eV) 777 (M+); Anal. Calcd for
C40H56O8Si4: C, 61.81; H, 7.26%. Found: C, 61.71; H, 7.14%.
∑ Crystal data for 4: 2(C32H32O8Si4)·3(C8H10), M = 1632.35, orange prism,
monoclinic, Mo-Ka (l = 0.71073 Å); space group = C2/m, a = 24.866(4)
Å, b = 13.099(2) Å, c = 16.060(3) Å, b = 122.575(3)°, V = 4407.9(13)
Å3, Z = 2, T = 123 K, Dc = 1.230 g cm23, GOF = 1.027, R = 0.0570 [I
> 2s(I)], R = 0.1872 (all data). CCDC 213340. See http://www.rsc.org/
suppdata/cc/b3/b307048d/ for crystallographic data in .cif or other elec-
tronic format.
Fig. 2 Molecular structure of 4 with the closest p-xylene.∑ Hydrogen atoms
and the other p-xylene are omitted for clarity.
The conformational behavior of 4 and 5 in solution was
investigated by 1H NMR spectroscopy. The 1H NMR spectrum
of 4 showed one methyl peak and one quinone-ring proton peak
1
in CDCl3 at room temperature. The H NMR spectrum of 5
contained two types of methoxy peaks, one methyl peak, and
one phenyl peak in CDCl3 at room temperature. We conducted
1
variable-temperature H NMR analyses of 4 and 5 in acetone-
d6, but no splitting of the peaks derived from freezing or
changing of the conformation was observed, even at 183 K, not
only for 4 but also for the slightly strained molecule 5.
Compounds 4 and 5 would adopt 1,3-alternate structures, or
ring inversion of 4 and 5 occurred rapidly on the NMR time
scale even at low temperatures. Yoshida et al. reported that 6
possesses significant conformational flexibility in solution
judging from the X-ray data and PM3 calculations.10 Thus, it is
reasonable that 4 and 5 possess significant conformational
flexibility in solution.
Electronic spectra of 4, 9 and 10 in CH2Cl2 are shown in Fig.
3. A band centered at ca. 250 nm assignable to a p–p* transition
was observed for the silylquinones. The absorption maxima of
the p–p* transition was similar to that in 1 (248 nm in CHCl3).5
The spectrum of 4 showed an apparent hypochromic effect. In
the spectra of 4 and 9, a broad band was observed at ca. 300 nm,
** Crystal data for 5: C40H56O8Si4; M = 777.21, colorless prism; Mo-Ka
(l = 0.71073 Å); monoclinic; P2(1)/n, a = 11.569(11) Å, b = 16.408(16)
Å, c = 23.63(2) Å, b = 103.093(17)°, V = 4369(8) Å3, Z = 4, Dcalc
=
1.182 g cm23, T = 120 K, GOF = 0.821, R = 0.0525 [I > 2s(I)], R =
0.1202 (all data). CCDC 213339. See http://www.rsc.org/suppdata/cc/b3/
b307048d/ for crystallographic data in .cif or other electronic format.
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Fig. 3 UV-Vis spectra of 4, 9, and 10 in CH2Cl2.
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