Synthesis of a tridentate hydrosilane and its reaction with palladium(0) complexes

Article abstract of DOI:10.1246/cl.2001.1096

A new tridentate hydrosilarte (2-SiH₃C₆H₄)₂SiH₂ reacts with a mixture of Pd(PEt₃)₄ and R₂PCH₂CH₂PR₂ (R = Me or Et) to give dinuclear (

Full text of DOI:10.1246/cl.2001.1096

1096
Chemistry Letters 2001
Synthesis of a Tridentate Hydrosilane and Its Reaction with Palladium(0) Complexes
Wanzhi Chen, Shigeru Shimada,* Teruyuki Hayashi, and Masato Tanaka*
National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565
(Received August 7, 2001; CL-010758)
A new tridentate hydrosilane (2-SiH3C6H4)2SiH2 reacts
with a mixture of Pd(PEt3)4 and R2PCH2CH2PR2 (R = Me or
Et) to give dinuclear (silyl)(µ-silylene)palladium(II) complexes,
while the reaction with a mixture of Pd(PEt3)4 and Cy2PCH2-
CH2PCy2 (Cy = cyclohexyl) afforded a mononuclear (silyl)-
(disilanyl)palladium(II) complex, the Si–Si bond of which was
formed by the intramolecular dehydrocoupling. The structures
of the new complexes were determined by X-ray diffraction.
The chemistry of silyl transition metal complexes is rapidly
growing due to its importance for understanding and develop-
ing transition metal catalyzed reactions of silicon compounds.1
Recently we have succeeded in the isolation of mononuclear
and dinuclear silyl group 10 metal complexes with high formal
oxidation states by using bidentate hydrosilane, 1,2-disilylben-
zene (1).2 These results prompted us to investigate the reactivi-
ty of a tridentate hydrosilane, bis(2-silylphenyl)silane (2),
which has analogous structure to 1. Here we report the synthe-
sis of 2 and its reaction with palladium(0) complexes, resulting
in the isolation of dinuclear (silyl)(µ-silylene)palladium(II)
complexes as well as a mononuclear (silyl)(disilanyl)palladi-
um(II) complex.
1)6 gave a mixture of two complexes consisting of yellow dinu-
clear complex 5a7,8 (21% isolated yield) and a colorless complex.
1
Although the structure of the latter has not been established, H
NMR analysis showed that it is neither tris(silyl)(hydrido)-
palladium(IV) complex 6a (no Pd–H signal was observed),
which we expected to be formed, nor (silyl)(disilanyl)-
palladium(II) complex 7a, whose analog 7b was obtained in the
reaction of 2 with a mixture of Pd(PEt3)4 and 1,2-bis(dicyclo-
hexylphosphino)ethane (dcpe) (vide infra). The yield of 5a
increased to 81% when 2 equiv of Pd(PEt3)4 and dmpe were
used. Similar results were obtained for the reaction of 2 with a
mixture of Pd(PEt3)4 and 1,2-bis(diethylphosphino)ethane
(depe);6 dinuclear complex 5b7 was isolated in 27% yield for 2 :
Pd : depe = 1 : 1 : 1 and in 82% yield for 2 : Pd : depe = 1 : 2 : 2.
The structure of 5a was unambiguously determined by X-ray
structure analysis (Figure 1).9 A dinuclear palladium complex 8
with a tridentate silicon ligand has been reported by Ito.8a
Silane 2 was prepared by the LiAlH4 reduction of the known
compound, bis[2-(trichlorosilyl)phenyl]dichlorosilane (3),3 or
new alkoxysilane 4a or 4b. The compounds 4a and 4b were pre-
pared in shorter steps than 3 based on Tamao’s method.3,4
Although 4a is more stable to moisture than 4b, purification of 2
after the LiAlH4 reduction was easier for 4b than for 4a, because
complete removal of byproducts (probably isopropoxyaluminum
species) from 2 was difficult in the case of 4a.5
In contrast, the reaction of 2 with a mixture of Pd(PEt3)4
and dcpe (2:Pd:dcpe = 1:1:1)6 gave a mononuclear (silyl)(di-
silanyl)palladium(II) complex 7b in 86% yield.7 No dinuclear
complex similar to 5a and 5b was observed, probably because
The reaction of 2 with a mixture of Pd(PEt3)4 and 1,2-
bis(dimethylphosphino)ethane (dmpe) (2 : Pd : dmpe = 1 : 1 :
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1097
the steric hindrance of cyclohexyl groups on the phosphorus
atoms prevented the formation of the dinuclear structure. The
structure of 7b determined by X-ray structure analysis shows
that intramolecular dehydrocoupling reaction has taken place to
form a Si–Si bond (Figure 2).10 Although the mechanism of the
formation of 7b is not clear at the moment, an assumption of
the intermediacy of tris(silyl)(hidrido)palladium(IV) 6b can
explain the formation of 7b.11 Scheme 1 shows two plausible
pathways of the formation of 7b from 6b. Reductive elimina-
tion on 6b to form a Si–Si bond provides 9 and successive
oxidative addition of Si–H bond and elimination of H2 from 10
give 7b. Another possibility is the concerted mechanism;
simultaneous elimination of H2 and Si migration from Pd to
another Si via transition state 11 gives 7b.
Dedicated to Professor Hideki Sakurai on the occasion of
his 70th birthday.
References and Notes
1
2
For recent reviews, see: M. S. Eisen, in “The Chemistry of
Organic Silicon Compounds,” ed. by Z. Rappoport and Y.
Apeloig, Wiley, New York (1998), Vol. 2, Part 3, Chap. 35, p
2037; H. Ogino and H. Tobita, Adv. Organomet. Chem., 42, 223
(1998); J. Y. Corey and J. Braddock-Wilking, Chem. Rev., 99,
175 (1999).
S. Shimada, M. Tanaka, and K. Honda, J. Am. Chem. Soc., 117,
8289 (1995); S. Shimada, M. Tanaka, and M. Shiro, Angew.
Chem., Int. Ed. Engl., 35, 1856 (1996); S. Shimada, M. L. N.
Rao, and M. Tanaka, Organometallics, 18, 291 (1999); S.
Shimada, M. L. N. Rao, T. Hayashi, and M. Tanaka, Angew.
Chem. Int. Ed., 40, 213 (2001).
3
4
G.-R. Sun, Ph. D. Thesis, Kyoto University, Kyoto, Japan, 1995;
private communications from K. Tamao.
K. Tamao, H. Yao, Y. Tsutsumi, H. Abe, T. Hayashi, and Y. Ito,
Tetrahedron Lett., 31, 2925 (1990); K. Tamao, T. Hayashi, Y.
Ito, and M. Shiro, Organometallics, 11, 2099 (1992).
t
5
Preparation of 2 via 4b: A pentane solution of BuLi (1.54 M,
137 mL, 0.21 mol) was added to PhSi(NMe2)2-
(NMeCH2CH2NMe2)3 (12) (47.7 g, 0.162 mol) in 200 mL of
hexane at 0 °C, and the mixture was stirred overnight at room
temperature. To the resulting solution cooled to –40 °C was
added SiHCl3 (13.0 g, 0.096 mol), and the solution was allowed
to warm to room temperature. Successive addition of dry MeOH
(100 mL) and SiCl4 (30 mL, as a source of HCl) at 0 °C, filtra-
tion of the resulting white precipitates, and removal of volatiles
under vacuum gave crude 4b as a pale-yellow waxy solid (29Si
1
NMR (C6D6, δ) –13.52 (d, J(H,Si) = 219, Si–H)). LiAlH4 (21
g, 0.55 mol) was added to an ether solution (500 mL) of crude
4b at 0 °C, and the mixture was refluxed overnight. Filtration of
insoluble materials, evaporation of the solvent, and bulb-to-bulb
distillation afforded 2 as a colorless liquid (4.3 g, 22.0% based
on 12). 1H NMR (C6D6, δ) 4.32 (s, 6H, SiH3), 5.31 (s, 2H, SiH2),
1
7.04 (m, 4H), 7.47 (m, 4H); 29Si (C6D6, δ) –61.1(qd, J(H,Si) =
2
1
2
201, J(H,Si) = 5.9, SiH3 ), –38.4 (tt, J(H,Si) = 200, J(H,Si) =
5.0, SiH2); Anal. Calcd for C12H16Si3: C, 58.95; H, 6.60%.
Found: C, 58.79; H, 6.57%.
6
7
8
To a toluene solution of Pd(PEt3)4 and dmpe, depe, or dcpe was
added 2 at 0 °C. Then, the mixture was stirred at room tempera-
ture for 20 min. The products were isolated by recrystallization
from toluene (for 5a and 7b) or Al2O3 column chromatography
(for 5b).
5a: 29Si{1H} NMR: (C4D8O, δ) –23.8 (tt, J(Si,P) = 11, 79), 79.8
(tt, J(Si,P) = 11, 128). 5b: 29Si{1H} NMR: (C7D8, δ) –22.8 (tt,
J(Si,P) = 10, 76), 82.4 (tt, J(Si,P) = 11, 125). 7b: 29Si NMR
2
(C6D6, δ) 23.8 (dd, J(P–Si) = 13, 143 Hz, CSiHC), –31.4 (dd,
2J(P–Si) = 13, 142 Hz, PdSiHSiH2), –61.8 (s, SiH2SiH).
Examples of µ-silylene-bridged dinuclear palladium complexes,
see: a) M. Suginome, Y. Kato, N. Takeda, H. Oike, and Y. Ito,
Organometallics, 17, 495 (1998). b) Y.-J. Kim, S.-C. Lee, J.-I.
Park, K. Osakada, J.-C. Choi, and T. Yamamoto,
Organometallics, 17, 4329 (1998). c) Y.-J. Kim, S.-C. Lee, J.-I.
Park, K. Osakada, J.-C. Choi, and T. Yamamoto, J. Chem. Soc.,
Dalton Trans., 2000, 417.
9
Crystal Data for 5a·toluene: C31H52P4Pd2Si3, fw = 845.70, a =
18.759(1), b = 15.071(2), c = 14.609(1) Å, β = 113.008(5)°, V =
3801.7(5) Å3, monoclinic, space group C2/c, Z = 4, Dcalc = 1.477
g·cm–3, R1 = 0.040 (for 3894 reflections with I > 2σ(I)), wR2 =
0.140 (for all data (4366 reflections)).
10 Crystal Data for 7b: C38H60P2PdSi3, fw = 769.50, a = 20.080(2),
b = 9.696(3), c = 20.275(2) Å, V = 3947(1) Å3, orthorhombic,
space group Pna21, Z = 4, Dcalc = 1.295 g·cm–3, R1 = 0.047 (for
3164 reflections with I > 2σ(I)), wR2 = 0.135 (for all data (4650
reflections)).
11 Tilley has reported that a tris(silyl)(hidorido)platinum(IV) com-
plex can be an intermediate for the dehydrocoupling reaction of
phenylsilane: R. H. Heyn and T. D. Tilley, J. Am. Chem. Soc.,
114, 1917 (1992).
We are grateful to the Japan Science and Technology
Corporation (JST) for financial support through the CREST
(Core Research for Evolutional Science and Technology) pro-
gram and for a postdoctoral fellowship to W.C.