Synthesis and properties of η6-silabenzene-M(CO)3 complexes (M = Cr, Mo)

Article abstract of DOI:10.1021/om0501778

Synthesis of the first neutral η⁶-silabenzene complexes [M(η⁶-C₅H₅SiTbt)(CO)₃] (M = Cr (2), Mo (3); Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl) was achieved by the ligand exchange reactions of [M(CH₃CN)₃(CO) ₃] (M = Cr, Mo) with the kinetically stabilized silabenzene 1. X-ray crystallographic analysis of 2 revealed that the -coordinated silabenzene rings of the complexes have almost planar geometries with delocalized η-electron structures. The structures of η-silabenzene complexes 2 and 3 were fully characterized by NMR, UV/vis, and IR spectroscopic analysis. All the ¹H, ¹³C, and ²⁹Si NMR chemical shifts of the SiC₅H₅ rings of 2 and 3 were shifted to upper field relative to those for silabenzene 1, due to the π-coordination of the silabenzene ring to the metal center. The carbonyl stretching frequencies of 2 and 3 were observed in a region of lower wavenumber as compared to those of the corresponding group 6 metal complexes of benzene. In the UV/vis spectra of 2 and 3, the absorption maxima were slightly red-shifted compared to those for the corresponding benzene complexes. The formation of η⁶-silabenzene complexes 2 and 3 is very important from the viewpoints of the novel chemical reactivity of silaaromatics. Silabenzene complexes 2 and 3 were thermally stable under an inert atmosphere but air and moisture sensitive. These compounds underwent ready addition reactions with water exclusively at their 1,2-positions to give the corresponding hydroxysilane 8a. The high regioselectivity observed for the addition reactions to 2 and 3 is in a sharp contrast to the competitive 1,2- and 1,4-addition reactions of free silabenzene 1 with water.

Full text of DOI:10.1021/om0501778

Organometallics 2005, 24, 6141-6146
6141
Synthesis and Properties of η6-Silabenzene-M(CO)3
Complexes (M ) Cr, Mo)
Akihiro Shinohara, Nobuhiro Takeda, Takahiro Sasamori,
Takeshi Matsumoto, and Norihiro Tokitoh*
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
Received March 10, 2005
Synthesis of the first neutral η6-silabenzene complexes [M(η6-C5H5SiTbt)(CO)3] (M ) Cr
(2), Mo (3); Tbt ) 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl) was achieved by the ligand
exchange reactions of [M(CH3CN)3(CO)3] (M ) Cr, Mo) with the kinetically stabilized
silabenzene 1. X-ray crystallographic analysis of 2 revealed that the η6-coordinated
silabenzene rings of the complexes have almost planar geometries with delocalized π-electron
structures. The structures of η6-silabenzene complexes 2 and 3 were fully characterized by
1
NMR, UV/vis, and IR spectroscopic analysis. All the H, 13C, and 29Si NMR chemical shifts
of the SiC5H5 rings of 2 and 3 were shifted to upper field relative to those for silabenzene 1,
due to the π-coordination of the silabenzene ring to the metal center. The carbonyl stretching
frequencies of 2 and 3 were observed in a region of lower wavenumber as compared to those
of the corresponding group 6 metal complexes of benzene. In the UV/vis spectra of 2 and 3,
the absorption maxima were slightly red-shifted compared to those for the corresponding
benzene complexes. The formation of η6-silabenzene complexes 2 and 3 is very important
from the viewpoints of the novel chemical reactivity of silaaromatics. Silabenzene complexes
2 and 3 were thermally stable under an inert atmosphere but air and moisture sensitive.
These compounds underwent ready addition reactions with water exclusively at their 1,2-
positions to give the corresponding hydroxysilane 8a. The high regioselectivity observed for
the addition reactions to 2 and 3 is in a sharp contrast to the competitive 1,2- and 1,4-
addition reactions of free silabenzene 1 with water.
Chart 1
Introduction
Silaaromatic compounds have been of great interest
as heavier element analogues of aromatic compounds.
However, such species have been elusive until recently,
despite the theoretical predictions suggesting their
significant aromatic character.1 Meanwhile, we have
succeeded in the synthesis of a series of stable sila-
aromatic species, such as silabenzene, 1- and 2-sila-
naphthalenes, and 9-silaanthracene, by taking advan-
tage of kinetic stabilization using an extremely bulky
substituent, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl
(Tbt group; Chart 1).2,3 As judged by the results of
spectroscopic analysis, we revealed that the silaaromatic
compounds possess fully delocalized π-electron ring
systems.
compounds. Ma¨rkl and co-workers have reported the
observation of an Fe complex of silabenzene, [Fe{4-Cy-
(C5H4SiMe)}]•+ (Cy ) cyclohexyl), in the mass spec-
trum.5 Tilley and co-workers succeeded in the synthesis
of transition-metal complexes with a η5-coordinated
(2) (a) Tokitoh, N.; Wakita, K.; Okazaki, R.; Nagase, S.; Schleyer,
P. v. R.; Jiao. H. J. Am. Chem. Soc. 1997, 119, 6951. (b) Wakita, K.;
Tokitoh, N.; Okazaki, R.; Nagase, S.; Schleyer, P. v. R.; Jiao, H. J.
Am. Chem. Soc. 1999, 121, 11336. (c) Wakita, K.; Tokitoh, N.; Okazaki,
R. Bull. Chem. Soc. Jpn. 2000, 73, 2157. (d) Wakita, K.; Tokitoh, N.;
Okazaki, R.; Nagase, S. Angew. Chem., Int. Ed. 2000, 39, 634. (e)
Wakita, K.; Tokitoh, N.; Okazaki, R.; Takagi, N.; Nagase, S. J. Am.
Chem. Soc. 2000, 122, 5648. (f) Takeda, N.; Shinohara, A.; Tokitoh,
N. Organometallics 2002, 21, 256. (g) Takeda, N.; Shinohara, A.;
Tokitoh, N. Organometallics 2002, 21, 4024. (h) Shinohara, A.; Takeda,
N.; Sasamori, T.; Tokitoh, N. Bull. Chem. Soc. Jpn., in press.
(3) For recent reviews on sila- and germaaromatic compounds
kinetically stabilized with a Tbt group, see: (a) Tokitoh, N. Acc. Chem.
Res. 2004, 37, 86. (b) Tokitoh, N. Bull. Chem. Soc. Jpn. 2004, 77, 429.
(4) For reviews on π-arene complexes, see: (a) Morris, M. J. In
Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F.
G. A., Wilkinson, G., Eds.; Pergamon: Oxford, U.K., 1995; Vol. 5, p
471. (b) Semmelhack, M. F. In Comprehensive Organometallic Chem-
istry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon:
Oxford, 1995; Vol. 12, pp 979-1070.
On the other hand, complexation with transition
metals is one of the most popular reactions of aromatic
compounds,4 and it may be a useful method for the
thermodynamic stabilization of reactive silaaromatic
* To whom correspondence should be addressed. E-mail: tokitoh@
boc.kuicr.kyoto-u.ac.jp. Tel: +81-774-38-3200. Fax: +81-774-38-3209.
(1) For reviews, see: (a) Raabe, G.; Michl, J. Chem. Rev. 1985, 85,
419. (b) Raabe, G.; Michl, J. In The Chemistry of Organic Silicon
Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989;
pp 1102-1108. (c) Apeloig, Y. In The Chemistry of Organic Silicon
Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989;
pp 151-166. (d) Brook, A. G.; Brook, M. A. Adv. Organomet. Chem.
1996, 39, 71. (e) Apeloig, Y.; Karni, M. In The Chemistry of Organic
Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York,
1998; Vol. 2.
(5) Ma¨rkl, G.; Sopper, C.; Hofmeister, P.; Baier, H. J. Organomet.
Chem. 1979, 174, 305.
10.1021/om0501778 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 10/29/2005

6142 Organometallics, Vol. 24, No. 25, 2005
Shinohara et al.
O, and LANL2DZ for Cr and Mo were used as basis sets. GIAO
calculations were operated at B3LYP/6-311G(d) for C, H, and
O, 6-311G(3d) for Si, TVZ (17s10p6d)[6s3p3d] for Cr, and TVZ
(19s14p9d)[8s6p5d] for Mo levels.13
silolyl anion, which have the conjugated six-π-electron
type ligand.6 Recently, they reported the characteriza-
tion of the first cationic silabenzene-Ru complex,
[Ru{C5H5Si(t-Bu)}Cp*][BH(C6F5)3] (Cp* ) C5Me5),7 in
which the bonding of the silabenzene moiety could be
described as an η5:η1 interaction with limited delocal-
ization of the silabenzene π-electrons.
We have already succeeded in the synthesis of the
first neutral η6-germabenzene-group 6 transition metal
complexes by the ligand exchange reactions of [M(CH3-
CN)3(CO)3] (M ) Cr, Mo, W) with the kinetically
stabilized germabenzene TbtGeC5H5.8,9 These experi-
mental results are of great importance as a new finding
on the chemical reactivity of the germaaromatic com-
pound. As an extension of our chemistry on metalla-
aromatic species, we became interested in the synthesis
of transition-metal complexes of silaaromatic com-
pounds. We describe here the synthesis and properties
of the first neutral η6-silabenzene complexes coordinated
with a group 6 metal center having three carbonyl
ligands.
Synthesis of [Cr(η6-C5H5SiTbt)(CO)3] (2). Silabenzene 1
(70.8 mg, 0.109 mmol) and [Cr(CH3CN)3(CO)3] (32.3 mg, 0.125
mmol) were dissolved in benzene (2 mL). After the mixture
was stirred for 2 days at room temperature, the solvent was
removed in vacuo. The residue was recrystallized from hexane
at -40 °C to give 2 (42.2 mg, 50%). 2: yellow powder, mp 238
°C dec; 1H NMR (300 MHz, C6D6) δ 0.12 (s, 18H), 0.18 (s, 18H),
0.20 (s, 18H), 1.49 (s, 1H), 2.75 (br s, 1H), 2.93 (br s, 1H), 3.23
(d, J ) 10.8 Hz, 2H), 4.83-4.89 (m, 3H), 6.61 (br s, 1H), 6.69
(br s, 1H); 13C NMR (75 MHz, C6D6) δ 0.66 (q), 0.77 (q), 0.97
(q), 31.83 (d), 35.35 (d), 35.75 (d), 74.05 (d), 85.03 (d), 101.59
(d), 116.74 (s), 122.16 (d), 127.14 (d), 149.52 (s), 154.13 (s),
154.50 (s), 234.50 (s); 29Si NMR (59 MHz, C6D6) δ 2.39, 2.81,
52.62; IR (Nujol) 1958, 1897, 1863 cm-1 (CtO stretching); UV/
vis (hexane) λmax 228 (ꢀ ) 5.9 × 104), 306 (8.7 × 103), 340 nm
(7.3 × 103); high-resolution FAB-MS m/z calcd for C35H65O3-
CrSi7 ([M + H]+) 781.2724, found 781.2759. Anal. Calcd for
C35H64O3CrSi7: C, 53.79; H, 8.25. Found: C, 53.62; H, 8.15.
Synthesis of [Mo(η6-C5H5SiTbt)(CO)3] (3). The Mo com-
plex 3 was synthesized by the same route as for 2 in 58% yield
(51.5 mg) from silabenzene 1 (72.4 mg, 0.112 mmol) and
[Mo(CH3CN)3(CO)3] (34.3 mg, 0.112 mmol). 3: yellow powder,
mp 240 °C dec; 1H NMR (300 MHz, C6D6) δ 0.08 (s, 18H), 0.15
(s, 18H), 0.19 (s, 18H), 1.48 (s, 1H), 2.67 (br s, 1H), 2.90 (br s,
1H), 3.34 (d, J ) 12 Hz, 2H), 4.77 (t, J ) 7.3 Hz, 1H), 5.09
(dd, J ) 12, 7.3 Hz, 2H), 6.59 (s, 1H), 6.66 (br s, 1H); 13C NMR
(75 MHz, C6D6) δ 0.63 (q), 0.79 (q), 0.94 (q), 31.77 (d), 35.85
(d), 35.97 (d), 72.08 (d), 83.06 (d), 104.16 (d), 117.22 (s), 122.11
(d), 127.19 (d), 149.39 (s), 154.14 (s), 154.18 (s), 221.48 (s); 29Si
NMR (59 MHz, C6D6) δ 2.39, 2.89, 52.47; IR (Nujol) 1966, 1898,
1870 cm-1 (CtO stretching); UV/vis (hexane) λmax 230 (ꢀ ) 6.1
× 104), 306 (1.0 × 104), 339 nm (1.5 × 104); high-resolution
FAB-MS m/z calcd for C35H64O398MoSi7 ([M]+) 826.2294, found
826.2266. Although we have performed the elemental analysis
of 3 several times, the results have not been commensurate
with the calculated values for 3, due to its highly moisture
sensitive properties.
Experimental Section
All manipulations were performed under an argon atmo-
sphere unless otherwise noted. THF, hexane, and benzene
were dried over a K mirror and distilled by trap-to-trap
1
distillation. The H NMR (300 MHz) and 13C NMR (75 MHz)
spectra were measured in CDCl3 or C6D6 with a JEOL AL-
300 spectrometer using CHCl3 (7.25 ppm) or C6D5H (7.15 ppm)
for 1H NMR and CDCl3 (77.0 ppm) or C6D6 (128.0 ppm) for
13C NMR as an internal standard. The multiplicity of signals
in 13C NMR spectra was determined by the DEPT technique.
The 29Si NMR (59 MHz) spectra were measured in CDCl3 or
C6D6 with a JEOL AL-300 spectrometer using tetramethyl-
silane as an external standard. High-resolution mass spectral
data were obtained on a JEOL JMS-700 spectrometer. Pre-
parative gel permeation liquid chromatography (GPLC) was
performed on an LC-908 instrument (Japan Analytical Indus-
try Co., Ltd.) equipped with JAIGEL 1H and 2H columns
(eluent CHCl3). Preparative thin-layer chromatography (PTLC)
was performed with Merck Kieselgel 60 PF254. The electronic
spectra were recorded on a JASCO V-570 instrument. IR
spectra were measured at room temperature on a JASCO FT/
IR-460 plus spectrometer. Melting points were measured by
a Yanaco Micro melting point apparatus and are uncorrected.
Elemental analyses were carried out at the Microanalytical
Laboratory of the Institute for Chemical Research, Kyoto
University. Silabenzene 12,10 and [M(CH3CN)3(CO)3] (M ) Cr,
Mo)11 were prepared according to the reported procedure.
Reaction of Silabenzene 1 with H2O. In a glovebox filled
with argon, the silabenzene 1 (50.7 mg, 0.0782 mmol) was
dissolved in THF (2 mL), and then H2O (0.5 mL) was added
to the solution. The mixture was stirred for 2 h, and the solvent
was evaporated. Separation of the mixture by PTLC (THF:
hexane ) 1:10) and GPLC (CHCl3) afforded 1-{2,4,6-tris[bis-
(trimethylsilyl)methyl]phenyl}-1-silacyclohexa-2,4-diene-1-ol
(6a; 15.6 mg, 30%) and 1-{2,4,6-tris[bis(trimethylsilyl)methyl]-
phenyl}-1-silacyclohexa-2,5-diene-1-ol (6b; 28.1 mg, 54%).
6a: colorless powder, mp 182-185 °C; 1H NMR (300 MHz,
CDCl3) δ -0.01 (s, 18H), 0.03 (s, 36H), 1.30 (s, 1H), 1.65 (s,
1H), 1.82-1.84 (m, 2H), 2.47 (s, 2H), 5.95-6.00 (m, 1H), 6.07-
6.14 (m, 1H), 6.23 (br s, 1H), 6.25 (d, J ) 14 Hz, 1H), 6.35 (br
s, 1H), 6.76 (dd, J ) 14, 5.7 Hz, 1H); 13C NMR (75 MHz, CDCl3)
δ 0.65 (q), 0.71 (q). 0.99 (q), 1.13 (q), 19.87 (t), 27.94 (d), 28.20
(d), 30.34 (d), 122.03 (d), 125.47 (d), 126.82 (s), 127.03 (d),
129.89 (d), 130.47 (d), 140.08 (d), 144.74 (s), 151.31 (s), 151.71
Computational Details. Geometry optimization and GIAO
calculations for 4 and 5 were carried out using the Gaussian
98 program.12 Geometries were optimized with density func-
tional theory at the B3LYP level. 6-31G(d) for Si, C, H, and
(6) η5-Coordinated silolyl anion complexes: (a) Freeman, W. P.;
Tilley, T. D.; Rheingold, A. L.; Ostrander, R. L. Angew. Chem., Int.
Ed. Engl. 1993, 32, 1744. (b) Freeman, W. P.; Tilley, T. D.; Rheingold,
A. L. J. Am. Chem. Soc. 1994, 116, 8248. (c) Dysard, J. M.; Tilley, T.
D. J. Am. Chem. Soc. 1998, 120, 8245. (d) Dysard, J. M.; Tilley, T. D.
J. Am. Chem. Soc. 2000, 122, 3097.
(12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.,
Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J. A. Gaussian 98, revision A.9; Gaussian, Inc.: Pittsburgh, PA,
1998.
(7) Dysard, J. M.; Tilley, T. D.; Woo, T. K. Organometallics 2001,
20, 1195.
(8) Nakata, N.; Takeda, N.; Tokitoh, N. J. Am. Chem. Soc. 2002,
124, 6915.
(9) (a) Nakata, N.; Takeda, N.; Tokitoh, N. Angew. Chem., Int. Ed.
2003, 42, 115. (b) Nakata, N. Ph.D. Thesis, Kyoto University, Kyoto,
2003.
(10) Wakita, K. Ph.D. Thesis, The University of Tokyo, Tokyo, 1999.
(11) (a) Tate, D. P.; Knipple, W. R.; Augl, J. M. Inorg. Chem. 1962,
1, 433. (b) King, R. B. J. Organomet. Chem. 1967, 8, 139.
(13) Ahlrich, A.; May, K. Phys. Chem. Chem. Phys. 2000, 2, 943.

η6-Silabenzene Complexes
Organometallics, Vol. 24, No. 25, 2005 6143
(s); 29Si NMR (59 MHz, C6D6) δ -12.48, 1.61, 1.75, 2.15. Anal.
Calcd for C32H66OSi7: C, 57.93; H, 10.03. Found: C, 57.82; H,
10.04. 6b: colorless powder, mp 174-176 °C; 1H NMR (300
MHz, CDCl3) δ 0.00 (s, 18H), 0.02 (s, 27H), 0.03 (s, 9H), 1.29
(s, 1H), 1.65 (s, 1H, Si-OH), 2.46 (br s, 1H), 2.57 (br s, 1H),
2.87-2.98 (m, 1H), 3.01-3.13 (m, 1H), 6.23 (br s, 1H), 6.24
(dd, J ) 15, 2.1 Hz, 2H), 6.34 (br s, 1H), 6.77 (dd, J ) 15, 3.6
Hz, 2H); 13C NMR (75 MHz, CDCl3) δ 0.64 (q), 0.72 (q), 0.98
(q), 27.54 (d), 27.76 (d), 30.32 (d), 33.49 (t), 122.04 (d), 126.10
(s), 127.04 (d), 129.73 (d), 144.63 (s), 133.80 (d) 151.56 (s),
151.95 (s); 29Si NMR (59 MHz, C6D6) δ -30.99, 1.73, 1.76. Anal.
Calcd for C32H66OSi7: C, 57.93; H, 10.03. Found: C, 57.96; H,
10.13.
Reaction of Cr Complex 2 with H2O. To a solution of 2
(24.9 mg, 0.0319 mmol) in C6H6 (1 mL) was added 0.5 mL of
water at room temperature, and the solution was stirred for 1
h. After the solvent was removed, purification of the residue
by PTLC afforded 6a (15.2 mg, 72%).
Reaction of Mo Complex 3 with H2O. To a solution of 3
(22.5 mg, 0.0272 mmol) in C6H6 (1 mL) was added 1.0 mL of
water at room temperature, and the solution was stirred for 1
h. After the solvent was removed, purification of the residue
by PTLC afforded 6a (11.7 mg, 65%).
Scheme 1. Synthesis of η6-Silabenzene Complexes
Rigaku/MSC Mercury CCD diffractometer with graphite-
monochromated Mo KR radiation (λ ) 0.710 70 Å). The
structure of 2‚0.5C6H6 was solved by direct methods (SHELXS-
97)14 and refined by full-matrix least-squares methods on F2
for all reflections (SHELXL-97).14 The occupancies of the
disordered trimethylsilyl group at the para position of the Tbt
group were refined so that the sum of them is 1 (0.53:0.47).
All hydrogen atoms were placed using AFIX instructions, while
all the non-hydrogen atoms were refined anisotropically.
Crystallographic data for 2 are given in Table 2.
Results and Discussion
The ligand exchange reactions of silabenzene 1 with
[M(CH3CN)3(CO)3] (M ) Cr, Mo) at room temperature
in benzene resulted in the formation of the correspond-
ing neutral η6-silabenzene-transition metal tricarbonyl
complexes, [M(η6-C5H5SiTbt)(CO)3] (M ) Cr (2) Mo (3);
Scheme 1). These complexes were isolated as stable
yellow crystals by recrystallization from hexane in the
yields of 50% (for 2) and 58% (for 3), respectively.
Unfortunately, the reaction of silabenzene 1 with [W(CH3-
CN)3(CO)3] afforded a complicated mixture.
Although the complexes 2 and 3 were highly air and
moisturesensitive, they are thermally stable even at 120
°C in solution under an inert atmosphere. The struc-
tures of 2 and 3 were confirmed by their spectroscopic
analyses, and the η6-coordinated geometries were de-
termined by X-ray crystallographic analysis (Figure 1).15
In the structure of the silabenzene ring of 2, the silicon
atom adopts a nearly planar geometry, where the sum
of the bond angles around the central Si atom and the
sum of the interior bond angles of the silabenzene ring
are 359.8 and 719.8°, respectively. The bond lengths of
the silabenzene ring of 2 (1.781(4), 1.797(4) Å for Si-C
and 1.396(6)-1.414(5) Å for C-C) were longer than
those of the free silabenzene 1 (1.770(4), 1.765(4) Å for
Si-C and 1.381(6)-1.399(6) Å for C-C).2e The Si-Cr
distance (2.4864(13) Å) of 2 was somewhat longer than
that of the base-stabilized silylene complex HMPA(t-
BuO)2SiCr(CO)5 (2.431 Å).16,17 The distance between the
center of the silabenzene ring of 2 and the chromium
atom was 1.692(2) Å, which was longer than those
reported for some arene complexes.18 The dihedral angle
between the silabenzene core and the benzene ring of
the Tbt group was 37.8(3)°. A projection of three
carbonyl groups and the chromium atom onto the
silabenzene ring revealed deviations from idealized
Reaction of Silabenzene 1 with Phenylacetylene. In a
glovebox filled with argon, silabenzene 1 (32.1 mg, 0.0495
mmol) was dissolved in C6D6 (0.6 mL) and the solution was
put into a 5 mm i.d. NMR tube. After phenylacetylene (0.5
mL, distilled from CaH2 under Ar prior to use) was added to
the solution, the NMR tube was degassed and sealed. The
NMR signals of silabenzene 1 disappeared after the mix-
ture stood for 2 days. Then, the tube was opened and the
solvent was evaporated. Purification of the residue by PTLC
afforded 3-phenyl-1-{2,4,6-tris[bis(trimethylsilyl)methyl]phenyl}-
1-silabicyclo[2.2.2]octa-2,5,7-triene (7a; 20.6 mg, 55%) and
1-(phenylethynyl)-1-{2,4,6-tris[bis(trimethylsilyl)methyl]phenyl}-
1-silacyclohexa-2,4-diene (7b; 6.2 mg, 17%). 7a: colorless
powder, mp 151-153.0 °C; 1H NMR (300 MHz, CDCl3) δ 0.05
(s, 36H), 0.08 (s, 18H), 1.38 (s.1H), 2.35 (br s, 2H), 5.30 (m,
1H), 6.36 (br s, 1H), 6.49 (br s, 1H), 6.71 (d, J ) 2.0 Hz, 1H),
6.81 (d, J ) 10.6 Hz, 2H), 7.15-7.43 (m, 5H), 7.49 (dd, J )
10.6, 6.9 Hz, 2H); 13C NMR (75 MHz, CDCl3) δ 0.77 (q), 1.03
(q), 28.97 (d), 29.13 (d), 30.62 (d), 51.76 (d), 121.55 (s), 121.95
(d), 124.89 (d), 126.86 (d), 127.29 (d), 128.39 (d), 131.49 (d),
137.32 (d), 141.96 (s), 145.50 (s), 151.07 (d), 152.44 (s), 152.79
(s), 162.56 (s); 29Si NMR (59 MHz, C6D6) δ 1.49, 1.87, 2.06.
Anal. Calcd for C40H70Si7: C, 64.26; H, 9.44. Found: C, 63.99;
H, 9.55. 7b: colorless powder, mp 59-61 °C; 1H NMR (300
MHz, CDCl3) δ 0.01 (s, 9H), 0.03 (s, 9H), 0.04 (s, 18H), 0.05 (s,
9H), 0.06 (s, 9H), 1.30 (s, 1H), 1.91 (ddd, J ) 19.2, 5.7, 1.5 Hz,
1H), 2.09 (ddd, J ) 19.2, 4.5, 2.1 Hz, 1H), 2.65 (br s, 1H), 2.67
(br s, 1H), 5.91-5.96 (m, 1H), 6.00-6.05 (m, 1H), 6.247 (br s,
1H), 6.252 (d, J ) 13.8 Hz, 1H), 6.36 (br s, 1H), 6.67 (dd, J )
13.8, 6.0 Hz, 1H), 7.26-7.28 (m, 3H), 7.41 (m, 2H); 13C NMR
(75 MHz, CDCl3) δ 0.76 (q), 0.77 (q), 0.83 (q), 1.13 (q), 1.19
(q), 17.31 (t), 27.96 (d), 28.03 (d), 30.40 (d), 95.52 (s), 109.33
(s), 122.17 (d), 123.62 (s), 123.97 (s), 125.93 (d), 127.22 (d),
127.55 (d), 128.11 (d), 128.35 (d), 128.43 (d), 131.77 (d), 138.69
(d), 144.87 (s), 152.03 (s), 152.38 (s); 29Si NMR (59 MHz, C6D6)
δ -47.05, 1.80, 1.92, 2.29; high-resolution FAB-MS m/z calcd
for C40H70Si7 ([M]+) 746.3862, found 746.3871.
Reaction of Silabenzene Complexes 2 with Phenyl-
acetylene. In a glovebox filled with argon, the silabenzene
Cr complex 2 (10.0 mg, 0.0128 mmol) was dissolved in C6D6
(0.6 mL) and the solution was put into a 5 mm i.d. NMR tube.
After phenylacetylene (0.5 mL, distilled from CaH2 under Ar
prior to use) was added to the solution, the NMR tube was
degassed and sealed. No reaction occurred even on standing
for 2 days.
(14) Sheldrick, G. M. University of Go¨ttingen, Go¨ttingen, Germany,
1997.
(15) The molecular geometry of 3 was also determined by X-ray
crystallographic analysis. Although the crystal data of 3 were rather
poor, the geometry of 3 was almost analogous to that of 2.
(16) Zybill, C.; Mu¨ller, G. Organometallics 1988, 7, 1368.
(17) For reviews on complexes containing bonds between silicon
atoms and transition metals, see: Corey, J. Y.; Braddock-Wiking, J.
Chem. Rev. 1999, 99, 175.
X-ray Data Collection. Single crystals of 2‚0.5C6H6 were
grown at room temperature from the corresponding saturated
solution in C6H6. The intensity data were collected on a
(18) Hunter, A. D.; Shilliday, L.; Furey, W. S.; Zaworotko, M. J.
Organometallics 1992, 11, 1550.

6144 Organometallics, Vol. 24, No. 25, 2005
Shinohara et al.
Figure 1. ORTEP drawing of [Cr(η6-C5H5SiTbt)(CO)3] (2; 50% probability ellipsoids). The hydrogen atoms and the fragment
of solvated benzene were omitted for clarity. Selected bond lengths (Å) and angles (deg): Si-C1 ) 1.781(4), C1-C2 )
1.414(5), C2-C3 ) 1.396(6), C3-C4 ) 1.404(6), C4-C5 ) 1.397(6), C5-Si ) 1.797(4), Si-Cr ) 2.4864(13), Cr-C6 ) 1.839(4),
Cr-C7 ) 1.841(4), Cr-C8 ) 1.840(4), C6-O9 ) 1.156(5), C7-O10 ) 1.157(4), C8-O11 ) 1.158(4); C1-Si-C5 ) 102.56(18),
C5-Si-C12 ) 126.89(17), C1-Si-C12 ) 130.36(16).
axis, as judged by the NMR spectra.20 The protons on
the silabenzene ring of 2 resonated at 3.23 (H1) and
4.83-4.89 ppm (H2 and H3), and those of 3 were
observed at 3.34 (H1), 4.77 (H3), and 5.09 ppm (H2).
The 13C NMR spectra showed peaks at 74.05 (C1),
101.59 (C2), and 85.03 ppm (C3) for 2 and at 72.08 (C1),
104.16 (C2), and 83.06 ppm (C3) for 3 (Table 1). All of
Figure 2. Distance (Dcent) between the silabenzene ring
and the Cr center (left) and the projection of Cr(CO)3 onto
the silabenzene ring plane (right).
1
the H and 13C NMR signals for 2 and 3 were clearly
shifted to upper field relative to those for silabenzene 1
(6.71, 7.11, and 8.00 ppm for 1H NMR and 116.1, 122.2,
and 143.4 ppm for 13C NMR, respectively).2e The 13C
NMR signals of carbonyl carbons were assignable to
234.50 ppm for 2 and 221.48 ppm for 3. These signals
appeared in a region quite similar to those for [M(η6-
mesitylene)(CO)3] (M ) Cr (δ 235.1 ppm), Mo (δ 223.7
ppm)).
geometry, and the angle of Si-Cr-C is about 26.1° for
2 (Figure 2). Since the theoretically optimized structure
of silabenzene-Cr complex 4, [Cr(η6-C5H5SiH)(CO)3],
showed completely syn-eclipsed geometry,12 the stag-
gered structure of 2 was most likely explained by the
steric repulsion between the Tbt group and the three
CO ligands.4,19,20
Arene tricarbonyl complexes generally showed up-
field-shifted chemical shifts in the NMR spectra com-
pared with those for the free aromatic compounds, due
to the reduction of π-electron density in the arene ring
moiety.21 In solution, the silabenzene complexes 2 and
3 were found to have a Cs symmetry with a mirror plane
vertical to the silabenzene rings containing the Si-C3
Tilley and co-workers reported that the interaction
between the silabenzene moiety and the ruthenium
center in [Ru{C5H5Si(t-Bu)}Cp*][BH(C6F5)3] may be
described as an η5:η1 interaction with limited delocal-
ization in the silabenzene ligand. The 13C NMR spectra
showed the C1 atom for the Ru complex to be at higher
field at 39.8 ppm than C2 (92.5 ppm) and C3 (83.3
ppm).7 Taking into account the differences in13C NMR
chemical shifts between our complexes (2 and 3) and
Tilley’s Ru complex, it can be considered that the
bonding between the central metals (Cr, Mo) and the
silabenzene rings for 2 and 3 can be described as fully
delocalized η6 coordination.
Notably, the 29Si NMR resonances in the silabenzene
rings of 2 (δ 52.6) and 3 (δ 52.5) lay in an upfield region
as compared with that of 1 (δ 93.6). These upfield shifts
observed for 2 and 3 strongly suggested the π-coordina-
tion of the silabenzene ring to the Cr and Mo center,
since the 29Si NMR chemical shifts of 2 and 3 were
comparable to those of the Hf (δ 49.7) and Zr (δ 48.9)
complexes of the η5-silolyl anion.6 In addition, 29Si NMR
(19) It was known that disubstituted π-complexes normally adopted
a fully staggered structure due to the steric repulsion. See: (a) Albright,
T. A.; Hofmann, P.; Hoffmann, R. J. Am. Chem. Soc. 1977, 99, 7546.
(b) Heppert, J. A.; Morgenstern, M. A.; Scherubel, D. M.; Takusagawa,
F.; Shaker, M. R. Organometallics 1988, 7, 1715.
(20) We measured the VT-NMR spectra of the toluene-d8 solution
of the complex 2 in the range of -100 to +25 °C. As a result, neither
obvious changes nor peak separations were observed even at -100 °C,
though the ortho protons of the silabenzene ring slightly shifted to
the upper field region (δ 4.88 (+25 °C), δ 4.63 (-100 °C)). Although
the reason for the slight upper-field shift is not clear at present, it
was indicated that the rotation barrier of the Cr(CO)3 unit should be
quite low. We are grateful to a reviewer for a kind comment concerning
this matter.
(21) (a) Mcfarlane, W.; Grim, S. O. J. Organomet. Chem. 1966, 5,
147. (b) Emanel, R. V.; Randall, E. W. J. Chem. Soc. A 1969, 3002. (c)
Mann, B. E. J. Chem. Soc., Chem. Commun. 1971, 976. (d) Mann, B.
E. J. Chem. Soc., Dalton Trans. 1973, 2012.

η6-Silabenzene Complexes
Organometallics, Vol. 24, No. 25, 2005 6145
Table 1. Observed and Calculated NMR Chemical
Shifts for Silabenzene and Silabenzene
Complexesa
Scheme 2. Comparisons of Reactivities of
Silabenzene Complexes (2 and 3) with Those of
Silabenzene 1
compd
Si
C1
C2
C3
Observed
silabenzene 1b
93.6 122.2 143.6 116.1
52.6 74.1 101.6 85.0
52.5 72.1 104.2 83.1
[Cr(η6-C5H5SiTbt)(CO)3] (2)
[Mo(η6-C5H5SiTbt)(CO)3] (3)
[Ru{C5H5Si(t-Bu)}Cp*][BH(C6F5)3]c -23.1 39.8 92.5 83.3
Calculated
[Cr(η6-C5H5SiH)(CO)3] (4)d
[Mo(η6-C5H5SiH)(CO)3] (5)d
52.0 74.3 115.7 84.8
46.2 76.5 118.1 85.8
a C1 is the carbon ortho to silicon in a ring, C2 is in the meta
position, and C3 is para to silicon. b See ref 6e. c See ref 5.
d B3LYP/6-311G(d) (C, H, O), 6-311G(3d) (Si), TVZ (17s10p6d)-
[6s3p3d] (Cr), and TVZ (19s14p9d)[8s6p5d] (Mo)//B3LYP/6-31G(d)
(C, H, O, Si) and LANL2DZ (Cr, Mo).13
Table 2. Crystallographic Data for 2
formula
C35H64O3CrSi7‚0.5C6H6
formula wt
820.55
cryst size (mm)
cryst syst, space group
0.20 × 0.20 × 0.20
monoclinic, P21/n (No. 14)
12.635(5)
32.737(13)
12.691(5)
116.605(4)
4693.3(3)
4
1.161
1.89-25.00
0.454
1764
40 504
8249 (Rint ) 0.051)
620
a (Å)
b (Å)
c (Å)
â (deg)
V (Å3)
Z
1926 cm-1 (E ) As) and 1990, 1919 cm-1 (E ) Sb).23
The IR spectra for 2 and 3 demonstrate that silabenzene
might be a weaker π-acceptor than benzene and hetero-
benzenes for π-back-bonding from the Cr and Mo metal
centers.
Dcalcd (g cm-3
θ range (deg)
)
The UV/vis spectra of 2 and 3 in hexane showed three
absorption maxima (2, λmax 228 (ꢀ, 5.9 × 104), 306 (8.7
× 103), 340 nm (7.3 × 103); 3, λmax 230 (ꢀ, 6.1 × 104),
306 (1.0 × 104), 339 nm (1.5 × 104)). These values are
slightly red-shifted compared to those for the corre-
sponding benzene complexes: 217 (ꢀ, 23 700), 263
(6720), and 315 nm (9450) for [Cr(η6-C6H6)(CO)3]; 220
(ꢀ, 17 900), 240 (10 560), 282-287 (3980), and 323 nm
(18 400) for [Mo(η6-C6H6)(CO)3].24 The prominent bands
of 2 and 3 at around 340 nm can be assigned to the
metal-to-ligand charge transfer (MLCT) bands, in ac-
cordance with the calculated and experimental elec-
tronic spectra of [Cr(η6-C6H6)(CO)3].25
µ (mm-1
F(000)
)
no. of rflns measd
no. of unique rflns
no. of params
R1 (I > 2σ(I))
0.057
0.149
0.405 and -0.509
1.24
wR2 (all data)
largest diff peak and hole (e Å-3
goodness of fit
)
chemical shifts of 2 and 3 were reasonably supported
by the comparison of the experimentally observed
chemical shifts with those calculated for the model
molecules, hydrogen-substituted silabenzene complexes
4 (Cr) and 5 (Mo) (Table 1).13 The JSiC coupling constants
of η6-silabenzene complexes 2 (67 Hz) and 3 (65 Hz) were
smaller than that of silabenzene 1 (83 Hz). The results
of the NMR study for 2 and 3 imply a decrease of the
π-bonding characters in the silabenzene ring upon
coordination with the transition-metal unit.
Comparisons of the IR spectra between the complexes
of silabenzene and benzene can explain the features of
silabenzene 1 as an η6 ligand. The IR spectrum for 2
showed characteristic CtO stretching frequencies at
1958, 1897, and 1863 cm-1, while that for 3 showed
similar frequencies at 1966, 1898, and 1870 cm-1. In
both cases, the carbonyl stretching frequencies were
observed at a region of lower wavenumber as compared
to those of the corresponding benzene complexes [M(η6-
C6H6)(CO)3] (M ) Cr, 1982, 1915 cm-1; M ) Mo, 1983,
1913 cm-1).22 Similar spectral features were observed
for νCtO stretching of the Tbt-substituted η6-germaben-
zene complexes [M(η6-C5H5GeTbt)(CO)3].9 In the case
of the heterobenzene-Mo complexes [Mo(EC5H5)(CO)3]
(E ) As, Sb), νCtO stretchings were observed at 1996,
The reactivity of η6-silabenzene complexes was dif-
ferent from that of the free silabenzene 1 previously
reported.2d Silabenzene complexes 2 and 3 reacted with
H2O at only their 1,2-positions to give the corresponding
hydrolysis product 6a in 72 and 65% yields, respectively
(Scheme 2). The high regioselectivity observed for the
addition of water to 2 is in sharp contrast to the
competitive 1,2- (6a, 30%) and 1,4-addition reactions
(6b, 54%) of water with the free silabenzene 1.26
Although silabenzene 1 reacted with phenylacetylene
to afford the two isomers 7a (55%) and 7b (17%), the
silabenzene complex 2 did not react with phenylacet-
ylene under the same conditions.
In summation, the novel η6-silabenzene complexes 2
and 3 were successfully synthesized by the ligand
exchange reactions of silabenzene 1 with [M(CH3CN)3-
(CO)3] (M ) Cr, Mo). The structures of η6-silabenzene
complexes were experimentally determined by spectro-
(23) (a) Ashe, A. J., III; Colburn, J. C. J. Am. Chem. Soc. 1977, 99,
8099. (b) Elschenbroich, C.; Kroker, J.; Nowotny, M.; Behrendt, A.;
Metz, B.; Harms, K. Organometallics 1999, 18, 1495.
(24) Evdokimova, M. G.; Yavorskii, B. M.; Trembouler, V. N.;
Baranetskaya, N. K.; Krivyk, V. U.; Zaslavskaya, G. B. Dokl. Phys.
Chem. 1978, 19, 154.
(25) (a) Carroll, D. G.; McGlynn, S. P. Inorg. Chem. 1968, 7, 1285.
(b) Kanis, D. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 1992,
114, 10338.
(22) (a) Mahaffy, C. A. L.; Pauson, P. L. Inorg. Synth. 1979, 19, 154.
(b) Nesmeyanov, A. N.; Krivykh, V. V.; Kaganovich, V. S.; Rybinskaya,
M. I. J. Organomet. Chem. 1975, 102, 185.

6146 Organometallics, Vol. 24, No. 25, 2005
Shinohara et al.
scopic data and X-ray crystallographic analysis. These
results are the first examples showing the feature of
the silaaromatics undergoing complexation with a tran-
sition metal in the η6 form. Further investigations on
the synthesis and properties of the novel transition-
metal complexes of silaaromatic compounds are cur-
rently under way.
Acknowledgment. This work was partially sup-
ported by Grants-in-Aid for COE Research on Elements
Science (No. 12CE2005), Scientific Research on Priority
Areas (No. 14078213), Scientific Research A (No.
1424064), and the 21st Century COE Program on Kyoto
University Alliance for Chemistry (No. 14212101) from
the Ministry of Education, Culture, Sports, Science and
Technology of Japan. We thank Central Glass Co. Ltd.
for a generous gift of tetrafluorosilane.
(26) In the hydrolysis of the complex 2, the selective formation of
6a may be due to the Cr-catalyzed conversion process of 6b to 6a.
Therefore, we have performed the reaction of a mixture of the
silabenzene 1 and the Cr complex 2 with H2O. To a benzene solution
(2.5 mL) of a 1:1 mixture of silabenzene 1 (9.3 mg, 0.014 mmol) and
the Cr complex 2 (11.5 mg, 0.014 mmol) was added H2O (0.4 mL, 22
mmol). After the mixture was stirred for 2 h and the solvents were
evaporated, the reaction mixture was separated by PTLC (THF:hexane
) 1:40) to afford 6a (10.0 mg, 54%) and 6b (7.0 mg, 39%). Therefore,
it can be considered that the hydrolysis of the complex 2 selectively
afforded only the 1,2-adduct 6a and that this did not occur via the
metal-catalytic conversion process of 6b to 6a. We are grateful to a
reviewer for fruitful discussions on this matter.
Supporting Information Available: X-ray structural
data of 2, as a CIF file. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM0501778