A stable 1,2-disilacyclohexene and its 14-electron palladium(0) complex

Article abstract of DOI:10.1021/ja101654b

A novel stable cyclic disilene, 1,2-diphenyl-3,3,6,6- tetrakis(trimethylsilyl)-1,2-disilacyclohexene (1), was synthesized via the reaction of the corresponding 1,4-dilithiobutane with phenylchlorosilane. This new strategy would be applicable to the synthesis of stable cyclic disilenes having rather small residual substituents on unsaturated silicon atoms. A stable tricoordinate Y-shaped disilene palladium complex, 2, that was synthesized by the reaction of 1 with bis(tricyclohexylphosphine)palladium was found to have the strongest ?-complex character among known disilene palladium complexes. 2010 American Chemical Society.

Full text of DOI:10.1021/ja101654b

Published on Web 03/19/2010
A Stable 1,2-Disilacyclohexene and Its 14-Electron Palladium(0) Complex
Takashi Abe, Takeaki Iwamoto,* and Mitsuo Kira*
Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Aoba-ku, Sendai 980-8578, Japan
Received February 25, 2010; E-mail: iwamoto@m.tains.tohoku.ac.jp; mkira@mail.tains.tohoku.ac.jp
Although various types of stable disilenes have been studied
extensively since the first isolation of tetramesityldisilene in 1981,1
cyclic disilenes are still very limited.2,3 Herein we report the
synthesis of a novel type of stable six-membered cyclic disilene,
the 1,2-disilacyclohexene 1 (Chart 1), using a unique reagent for
introducing sterically protecting groups. The reaction of disilene 1
with bis(tricyclohexylphosphine)palladium [(Cy3P)2Pd] was found
to afford the stable 14-electron tricoordinate disilene-palladium
complex 2 having an unprecedented Y-shaped structure.
respectively. Because the geometry around the SidSi double bond
is trans-bent as expected,9 the six-membered ring of 1 adopts a
conformation between an ideal chair and an ideal half-chair, which
are known as the most stable conformations in all-carbon cyclo-
hexane and cyclohexene, respectively.
The 29Si resonance for the unsaturated silicon nuclei of 1
appeared at +100.9 ppm, which is close to those expected for
tetraalkyl- and tetraaryldisilenes.2 Two kinds of singlet signals due
1
to the Me3Si groups were observed in the H, 13C, and 29Si NMR
spectra, indicating that the ring inversion accompanying the
pyramidal inversion at the unsaturated silicon atoms is slow on the
NMR time scales, probably because of the significant steric
repulsion between the vicinal trimethylsilyl and phenyl groups
during the inversion.
Chart 1
The synthesis of 1 was achieved in four steps from 1,1-
bis(trimethylsilyl)ethylene in a rather good yield, as shown in
Scheme 1. The reaction of PhSiH2Cl with 1,4-dilithio-1,1,4,4-
tetrakis(trimethylsilyl)butane 3,4 which was prepared by the reaction
of 1,1-bis(trimethylsilyl)ethylene with lithium in tetrahydrofuran
(THF) at room temperature, gave the corresponding 1,4-bis(phe-
nylsilyl)butane 4 in 66% yield; 4 was converted to the corresponding
tetrabromo derivative 5 in 90% yield by reaction with bromine.
Cyclic disilene 15,6 was synthesized in 72% yield as yellow crystals
by the reduction of 5 with potassium graphite (KC8) in THF. Details
of the synthesis are given in the Supporting Information. The
structure of 1 was characterized by 1H, 13C, and 29Si NMR
spectroscopies, mass spectrometry (MS), and X-ray diffraction
(XRD). Whereas 1,4-dilithiobutane 3 has been utilized for the
synthesis of an isolable dialkylsilylene,7 the present study indicates
that 3 is also an effective reagent for kinetic stabilization of a six-
membered cyclic disilene having relatively small residual substit-
uents such as phenyl groups.
Figure 1. Molecular structure of 1,2-disilacyclohexene 1: (a) top view;
(b) side view. Thermal ellipsoids are drawn at the 50% probability level.
Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å)
and angles (deg): Si1-Si2, 2.1595(9); Si1-C1, 1.889(2); Si1-C5, 1.878(2);
C1-C2, 1.581(3); C2-C3, 1.548(3); C3-C4, 1.586(3); Si2-C4, 1.890(2);
Si2-C6, 1.874(2); Si2-Si1-C1, 113.50(6); Si2-Si1-C5, 121.14(7);
C1-Si1-C5, 123.71(9); Si1-Si2-C4, 113.05(7); Si1-Si2-C6, 119.73(7);
C4-Si2-C6, 123.88(9).
The reaction of disilene 1 with (Cy3P)2Pd was investigated, with
expectation that the geometry and electronic nature of the disilene
palladium complex thus obtained may be significantly different from
those of other complexes previously reported by us.10,11 A new
disilene palladium complex, 2, was obtained as purple crystals in
56% yield together with free PCy3 (eq 1):12
Scheme 1
1. (Cy P) Pd, toluene, rt8 2. (56 %)
(1)
3
2
-PCy3
The structure of 2 was determined by H, 13C, and 29Si NMR
spectroscopies, MS, and XRD. The resonance of the unsaturated
29Si nuclei was observed at +40.2 ppm as a doublet with J(29Si-31P)
) 17 Hz.
The molecular structure of 2 is shown in Figure 2 together with
selected bond lengths and bond angles.13 The central palladium
atom of 2 is tricoordinated by one phosphine P atom and the two
Si atoms of disilene 1, similarly to the 14-electron disilene complex
(Cy3P)(R2SidSiR2)Pd (6; R ) SiMe2Bu-t).10a The geometry around
Pd in 2 is slightly pyramidalized, probably to avoid steric hindrance
1
The molecular structure of 1,2-disilacyclohexene 1 was deter-
mined by single crystal XRD analysis (Figure 1).8 The Si1-Si2
bond length is 2.1595(9) Å, which is in the range of typical SidSi
double-bond lengths (2.14-2.29 Å).2 The Si1 and Si2 atoms are
slightly pyramidalized, with bond-angle sums of 358.35(8) and
356.66(8)° around the unsaturated silicon atoms Si1 and Si2,
9
5008 J. AM. CHEM. SOC. 2010, 132, 5008–5009
10.1021/ja101654b  2010 American Chemical Society

C O M M U N I C A T I O N S
between the bulky trimethylsilyl groups and tricyclohexylphosphine;
the sum of the bond angles around Pd in 2 is 349.46(17)°, as
opposed to 357.60(3)° in 6. However, the location of the phosphine
is very different in 2 relative to 6. Complex 2 is a Y-shaped
tricoordinate complex with symmetric coordination of the phos-
phine, while 6 is a T-shaped complex with unsymmetric coordina-
tion of the phosphine;10a the two P-Pd-Si angles in 2 are close
to each other [142.66(2) and 151.48(2)°], while those in 6 are
significantly different [128.94(3) and 171.83(3)°]. The Si1-Si2
bond length in 2 [2.2009(7) Å] is only 0.0411 Å longer than that
in free disilene 1.
References
(1) West, R.; Fink, M. J.; Michl, J. Science 1981, 214, 1343.
(2) For recent comprehensive reviews of disilenes, see: (a) Okazaki, R.; West,
R. AdV. Organomet. Chem. 1996, 39, 231. (b) Weidenbruch, M. In The
Chemistry of Organic Silicon Compounds Volume 3; Rappoport, Z.,
Apeloig, Y., Eds.; Wiley: Chichester, U.K., 2001; p 391. (c) West, R.
Polyhedron 2002, 21, 467. (d) Kira, M.; Iwamoto, T. AdV. Organomet.
Chem. 2006, 54, 73 .
(3) Three-membered cyclic disilenes: (a) Iwamoto, T.; Kabuto, C.; Kira, M.
J. Am. Chem. Soc. 1999, 121, 886. (b) Ichinohe, M.; Matsuno, T.; Sekiguchi,
A. Angew. Chem., Int. Ed. 1999, 38, 2194. (c) Lee, V. Y.; Ichinohe, M.;
Sekiguchi, A.; Takagi, N.; Nagase, S. J. Am. Chem. Soc. 2000, 122, 9034.
Four-membered cyclic disilenes: (d) Kira, M.; Iwamoto, T.; Kabuto, C.
J. Am. Chem. Soc. 1996, 118, 10303. (e) Wiberg, N.; Niedermayer, W.;
Noeth, H.; Warchhold, M. Z. Anorg. Allg. Chem. 2001, 627, 1717. (f)
Wiberg, N.; Auer, H.; Noeth, H.; Knizek, J.; Polborn, K. Angew. Chem.,
Int. Ed. 1998, 37, 2869. (g) Sekiguchi, A.; Matsuno, T.; Ichinohe, M. J. Am.
Chem. Soc. 2001, 123, 1436. (h) Lee, V. Y.; Takanashi, K.; Matsuno, T.;
Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2004, 126, 4758. (i) Wiberg,
N.; Vaisht, S. K.; Fischer, G.; Mayer, P. Z. Anorg. Allg. Chem. 2004, 630,
1823. Five-membered cyclic disilenes: (j) Grybat, A.; Boomgaarden, S.;
Saak, W.; Marsmann, H.; Weidenbruch, M. Angew. Chem., Int. Ed. 1999,
38, 2010. Six-membered cyclic disilenes: (k) Wiberg, N.; Vasisht, S. K.;
Fischer, G.; Mayer, P. Z. Anorg. Allg. Chem. 2004, 630, 1823. (l) Tanaka,
R.; Iwamoto, T.; Kira, M. Angew. Chem., Int. Ed. 2006, 45, 6371. (m)
Kinjo, R.; Ichinohe, M.; Sekiguchi, A.; Takagi, N.; Sumimoto, M.; Nagase,
S. J. Am. Chem. Soc. 2007, 129, 7766.
(4) Kira, M.; Hino, T.; Kubota, Y.; Matsuyama, N.; Sakurai, H. Tetrahedron
Lett. 1988, 29, 6939.
(5) Analysis data for 1: Yellow crystals; mp > 100 °C (decomp.). 1H NMR
(400 MHz, C6D6): δ 0.16 (s, 18H, SiMe3), 0.46 (s, 18H, SiMe3), 2.40-
2.50 (m, 4H, CH2), 6.96-7.76 (m, 10H, aryl). 13C NMR (100 MHz, C6D6):
δ 1.1 (SiMe3), 4.0 (SiMe3), 32.2 (CH2), 127.5, 128.3, 129.1, 136.3. 29Si
NMR (79 MHz, C6D6): δ 2.3 (SiMe3), 2.9 (SiMe3), 100.9 (SidSi). MS
(EI, 70 eV) m/z (%): 540 (10, [M+-15]), 477 (35), 73 (100). UV-vis
(hexane) λmax/nm (ε): 427 (8400).
(6) A 1,2-disilacyclohexene has been generated as a transient species by the
photolysis of 1,4-bis[phenylbis(trimethylsilyl)silyl]butane. See: Sakurai, H.;
Iimura, T.; Matsumoto, S.; Sanji, T. Chem. Lett. 2002, 31, 22.
(7) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc. 1999,
121, 9722. For a recent review of the chemistry of the silylene, see: Kira,
M.; Iwamoto, T.; Ishida, S. Bull. Chem. Soc. Jpn. 2007, 80, 258.
(8) Crystal data for 1 (173 K): C28H50Si6, fw 555.22; monoclinic; space group
P21/a; a ) 17.071(6) Å, b ) 9.127(3) Å, c ) 22.864(8) Å, ꢀ )
111.7874(11)°; V ) 3307.9(19) Å3; Z ) 4; Dcalcd ) 1.115 Mg/m3; R )
0.0534 [I > 2σ(I)], wR2 ) 0.1144 (all data); GOF ) 1.227. For details, see
the Supporting Information.
Figure 2. Molecular structure of disilene palladium complex 2: (a) top
view; (b) side view. Thermal ellipsoids are drawn at the 50% probability
level. Hydrogen atoms have been omitted for clarity. Selected bond lengths
(Å) and angles (deg): Pd1-Si1, 2.3916(6); Pd1-Si2, 2.3488(6); Pd1-P1,
2.4284(6); Si1-Si2, 2.2009(7); P1-Pd1-Si1, 142.66(2); P1-Pd1-Si2,
151.48(2); Si1-Pd1-Si2, 55.32(2).
We have recently shown10 that a disilene complex has a
character between a π complex and a metallacycle such as an
alkene complex.14 The π-complex character is estimated using
the bent back angle around the SidSi double bond (θ), the SidSi
bond elongation (∆r/r0), and the extent of high-field shift of the
unsaturated 29Si nuclei (∆δSi);15 the smaller these values are,
the stronger the π-complex character. On the basis of these
criteria, the 14-electron T-shaped complex 6 with θ ) 7.0° (the
average of 4.41 and 9.65°), ∆r/r0 ) 3.2%, and ∆δSi ) 76.8 ppm
is characterized as a π complex, while the related 16-electron
complex (Me3P)2(R2SidSiR2)Pd (7; R ) SiMe2Bu-t) is a typical
metallacycle.10a However, theoretical calculations for the model
14-electron disilene complex (Me3P)(R2SidSiR2)Pd(0) (6′; R )
SiH3)10a showed that the π-complex character is enhanced when
the complex has a symmetric Y-shaped structure, although that
structure is a transition state only 2.9 kcal mol-1 higher in energy
than the T-shaped structure. The θ, ∆r/r0, and ∆δSi values for
complex 2 are 6.9° (the average of 6.70 and 7.15°), 1.9%, and
60.7 ppm, respectively, which are even smaller than those for
6. The results indicate that the Y-shaped complex 2 has the
strongest π-complex character among known disilene palladium
complexes, in accord with the theoretical prediction.
(9) The bent angles, which are defined as the angles between the C-Si1(Si2)-
C planes and the Si1-Si2 axis, are 13.7 and 19.1°. The twist angle, which
is defined as the angle between the two axes bisecting the C1-Si1-C5
and C4-Si2-C6 angles in a Newman projection view along the Si1-Si2
bond, is 4.76°.
(10) (a) Kira, M.; Sekiguchi, Y.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc.
2004, 126, 12778. (b) Hashimoto, H.; Sekiguchi, Y.; Sekiguchi, Y.;
Iwamoto, T.; Kabuto, C.; Kira, M. Can. J. Chem. 2003, 81, 1241. (c)
Iwamoto, T.; Sekiguchi, Y.; Yoshida, N.; Kabuto, C.; Kira, M. Dalton
Trans. 2006, 177.
(11) For other disilene transition-metal complexes, see: (a) Pham, E. K.; West,
R. Organometallics 1990, 9, 1517. (b) Pham, E. K.; West, R. J. Am. Chem.
Soc. 1989, 111, 7667. (c) Pham, E. K.; West, R. J. Am. Chem. Soc. 1996,
118, 7871. (d) Berry, D. H.; Chey, J. H.; Zipin, H. S.; Carroll, P. J. J. Am.
Chem. Soc. 1990, 112, 452. (e) Berry, D. H.; Chey, J.; Zipin, H. S.; Carroll,
P. J. Polyhedron 1991, 10, 1189. (f) Hong, P.; Damrauer, N. H.; Carroll,
P. J.; Berry, D. H. Organometallics 1993, 12, 3698. (g) Fischer, R.; Zirngast,
M.; Flock, M.; Baumgartner, J.; Marschner, C. J. Am. Chem. Soc. 2005,
127, 70. (h) Zirngast, M.; Flock, M.; Baumgartner, J.; Marschner, C. J. Am.
Chem. Soc. 2009, 131, 15952. (i) Nguyen, T.-l.; Scheschkewitz, D. J. Am.
Chem. Soc. 2005, 127, 10174.
(12) Analysis data for 2: Purple crystals; mp 175 °C (decomp.). 1H NMR (400
MHz, C6D6): δ 0.60 (s, 18H, SiMe3), 0.69 (s, 18H, SiMe3), 1.53-1.93 (m,
33H, cyclohexyl), 2.20-2.40 (m, 4H, CH2), 6.80-8.10 (m, 10H, aryl).
13C NMR (100 MHz, C6D6): δ 4.2 (SiMe3), 4.8 (SiMe3), 23.1 (C), 27.0
(CH2), 26.5 (PCHCH2CH2CH2), 27.9 (d, 1J(P-C) ) 11 Hz, PCH), 31.7
(d, 3J(P-C) ) 6 Hz, PCHCH2CH2), 33.5 (d, 2J(P-C) ) 7 Hz, PCHCH2),
125.7, 126.9, 129.3, 136.9. 29Si NMR (79 MHz, C6D6): δ 2.7 (SiMe3), 3.1
(SiMe3), 40.2 (d, 2J(P-Si) ) 17 Hz). 31P NMR (161 MHz, C6D6): δ 23.0.
(13) Crystal data for 2 (173 K): C46H83PPd Si6; fw 942.03; monoclinic; space
group P21/n; a ) 11.231(2) Å, b ) 20.760(4) Å, c ) 22.617(5) Å, ꢀ )
99.409(2)°; V ) 5202.6(18) Å3; Z ) 4; Dcalcd ) 1.203 Mg/m3, R ) 0.0258
[I > 2σ(I)], wR2 ) 0.0701 (all data); GOF ) 1.058. For details, see the
Supporting Information.
(14) For recent reviews of olefin complexes, see: (a) Frenking, G.; Fro¨hlich, N.
Chem. ReV. 2000, 100, 717. (b) Hartly, F. R. In ComprehensiVe Organo-
metallic Chemistry; Wilkinson, G., Ed.; Pergamon Press: Oxford, U.K.,
1982; Vol. 6.
(15) For a complex with an R2SidSiR2 ligand, θ is defined as the angle between
an R2Si plane and the SidSi bond, ∆r/r0 ) (r-r0)/r0, where r0 is the SidSi
bond length in the corresponding free disilene, and-∆δSi ) δSi(coordinated
disilene)-δSi(free disilene).
Acknowledgment. This work was supported by the Ministry
of Education, Culture, Sports, Science, and Technology of Japan
[Specially Promoted Research (17002005 to M.K. and T.I.)].
Supporting Information Available: Synthetic details for 1 and 2
and their X-ray crystallographic data (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
JA101654B
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J. AM. CHEM. SOC. VOL. 132, NO. 14, 2010 5009