Transfer of a disilenyl moiety to aromatic substrates and lateral functional group transformation in aryl disilenes

Article abstract of DOI:10.1021/ja107547s

The reaction of 1 equiv of the disilenide Tip₂Si=Si(Tip)Li (5; Tip = 2,4,6-ⁱPr₃C₆H₂) with para-substituted phenyl iodides, 4-X-PhI, transfers the Tip₂Si= Si(Tip) moiety with elimination of

Full text of DOI:10.1021/ja107547s

Published on Web 11/11/2010
Transfer of a Disilenyl Moiety to Aromatic Substrates and
Lateral Functional Group Transformation in Aryl Disilenes
Jonathan Jeck,† Iulia Bejan,† Andrew J. P. White,† Dominik Nied,‡ Frank Breher,*,‡
and David Scheschkewitz*,†
Department of Chemistry, Imperial College, London SW7 2AZ, United Kingdom, and Institute of
Inorganic Chemistry, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany
Received September 1, 2010; E-mail: breher@kit.edu; d.scheschkewitz@imperial.ac.uk
Abstract: The reaction of 1 equiv of the disilenide Tip2SidSi(Tip)Li (5; Tip ) 2,4,6-iPr3C6H2) with para-
substituted phenyl iodides, 4-X-PhI, transfers the Tip2SidSi(Tip) moiety with elimination of lithium iodide to
yield the laterally functionalized disilenes Tip2SidSi(Tip)(4-X-Ph) [X ) H (6a), F (6b), Cl (6c), Br (6d), I
(6e)]. The UV-vis absorptions of 6a-d suggest a linear correlation with electronic Hammett parameters.
In addition, X-ray structural analyses of 6a-d verified the theoretically predicted linear dependence of the
SidSi bond length and trans-bent angles. The p-bromophenyl-substituted disilene 6d undergoes a
metal-halogen exchange reaction to give 6f (X ) Li), which was trapped with Me3SiCl to afford 6g (X )
SiMe3). In the case of simple phenyl halides PhX without additional functionality, the reaction with 5
proceeded smoothly for X ) Br, but phenyl chlorides and fluorides did not react at room temperature even
after one week, hinting at an SN2-type aromatic substitution mechanism. Reactions of p- and m-
diiodobenzene with 5 afford the corresponding phenylene-bridged tetrasiladienes p-7 and m-7. While red
p-7 (λmax ) 508 nm) exhibits efficient conjugation of the two SidSi bonds with the phenylene linker, the
conjugation in yellow m-7 (λmax ) 449 nm) is much less effective. Electrochemical studies of m-7 and p-7
as well as density functional theory calculations and electron paramagnetic resonance studies of their
respective radical anions provided further support for the notion of conjugation.
Chart 1. Stable Disilenes Illustrating the Conformational Flexibility
of the SidSi Bonda
1. Introduction
Compounds with SidSi double bonds have continued to
attract much interest ever since the seminal report by West, Fink,
and Michl on the isolation of the first stable example, disilene
1 (Chart 1).1,2 In comparison with alkenes, disilenes are
distinguished by considerably smaller HOMO-LUMO gaps,
which is a manifestation of the relatively weak π bonding
between heavier main-group elements.2 In addition, the com-
paratively high polarizability of silicon relative to carbon results
in shallow energy profiles for distortions of the SidSi bond
that diminish the orbital overlap and thus the HOMO-LUMO
gap of disilenes.
a In 1, Mes ) 2,4,6-Me3C6H2. In cis/trans-3, R ) SiMe3. In 4a, R ) R′
) Tip ) 2,4,6-iPr3C6H2. In 4b, R ) SitBu2Me, R′ ) Mes. In 4c, R )
SiMe3.
For example, steric repulsion between the substituents of the
blue disilene 2a gives rise to strongly twisted trigonal-planar
coordination environments of the doubly bonded silicon atoms
(the twist angle, τ, is 54°),3a while the sterically less encumbered
yellow disilene 2b is almost perfectly planar.3b Similarly, the
more pronounced ring strain in red trans-3 causes much larger
trans bending and twisting in comparison with the yellow isomer
cis-3, which explains the greater red shift of the longest-
wavelength UV-vis absorption of trans-3.4 Even energetically
weak parameters such as crystal packing forces affect the solid-
state structures: disilene 1 crystallizes in a planar, slightly
twisted, or trans-bent conformation of the SidSi bond, depend-
ing on the presence and nature of the cocrystallized solvent.5
In order to exploit the markedly different chemical and
physical properties of disilenes relative to alkenes, the incor-
† Imperial College.
‡ Karlsruhe Institute of Technology.
(1) West, R.; Fink, M. J.; Michl, J. Science 1981, 214, 1343.
(2) Recent reviews: (a) Scheschkewitz, D. Chem.sEur. J. 2009, 15, 2476.
(b) Kira, M.; Iwamoto, T. AdV. Organomet. Chem. 2006, 54, 73. (c)
Lee, V. Y.; Sekiguchi, A. Organometallics 2004, 23, 2822. (d)
Weidenbruch, M. Angew. Chem. 2003, 115, 2322; Angew. Chem., Int.
Ed. 2003, 42, 2222. (e) Sekiguchi, A.; Lee, V. Y. Chem. ReV. 2003,
103, 1429. (f) West, R. Polyhedron 2002, 21, 467. (g) Weidenbruch,
M. J. Organomet. Chem. 2002, 646, 39. (h) Kira, M. Pure Appl. Chem.
2000, 72, 2333.
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2004, 126, 9626. (b) Kira, M.; Maruyama, T.; Kabuto, C.; Ebata, K.;
Sakurai, H. Angew. Chem. 1994, 106, 1575; Angew. Chem., Int. Ed.
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Angew. Chem., Int. Ed. 2006, 45, 6371.
9
17306 J. AM. CHEM. SOC. 2010, 132, 17306–17315
10.1021/ja107547s  2010 American Chemical Society

SidSi Transfer to Aromatics
A R T I C L E S
Scheme 1. Syntheses of Para-Functionalized Phenyldisilenes
6a-e and Phenylene-Bridged Tetrasiladienes p-7 and m-7a
poration of SidSi moieties into π-conjugated systems is
particularly interesting. In this context, a few tetrasilabutadienes,
4a-c, have been synthesized and used to demonstrate that the
well-known red shift associated with an extension of the
conjugation path length in carbon systems is significant in all-
silicon systems as well.6 In light of the importance of semi-
conducting π-conjugated organic polymers, in which a variety
of supplementary linking units alternate with the CdC double
bonds,7 we decided to investigate the effects of conjugation
between such carbon-based linking units and SidSi bonds. Such
endeavors seemed particularly worthwhile in view of the fact
that the groups of Gates, Protasiewicz, and more recently Ott
have reported substantial progress with such an approach in the
case of PdC and PdP bonds, including the synthesis of
polymeric systems.8,9
In comparison with phosphorus-containing double bonds,
however, the synthetic options for the generation of SidSi bonds
are limited,2 and functionalized derivatives have been unknown
until recently, except for a few cases mentioned in passing in
the literature.6a,10 Consequently, in the past few years, consider-
able effort has been devoted to the synthesis of disilenes with
nucleophilic11,6b or electrophilic12 functionality. In all cases, the
functional group is directly attached to the silicon atom(s) of
the SidSi bond. In reactions with aryl halides, disilenides (i.e.,
disila analogues of vinyllithiums) are able to transfer the SidSi
moiety to aromatic substrates.13a,11f p-Phenylene-bridged tet-
rasiladienes were obtained in nearly quantitative spectroscopic
yield using this method,13a while a more conventional approach
via reductive coupling of halosilanes produced complicated
product mixtures from which the desired phenylene-bridged
a Abbreviations: dme ) 1,2-dimethoxyethane, Tip ) 2,4,6-iPr3C6H2).
t
Conditions: (a) -80 °C, 2 BuLi; (b) -80 °C, Me3SiCl.
tetrasiladienes were isolated by chromatographic techniques in
comparatively low yield.13b
The present study concerns the elucidation of the scope of
the SidSi transfer to aryl halides effected by nucleophilic
disilenides with respect to (1) different halide leaving groups,
(2) the compatibility with residual lateral functionalities and
functional group transformations at the periphery of the SidSi
bond, and (3) the multiple transfer of SidSi moieties and the
electronic communication mediated by the different phenylene
linkers.
2. Results and Discussion
As part of our investigations into the reactivity of disilenide
5 toward organic substrates,14 we recently reported that the
reactions of 5 with iodobenzene and 1,4-diiodobenzene furnish
the phenyl-substituted disilene 6a (X ) H) and the p-phenylene-
bridged tetrasiladiene p-7, respectively.13a Here we show that
the same methodology can be used for the preparation of the
m-phenylene-bridged derivative m-7 (Scheme 1). In addition,
we have found that 6a can alternatively be prepared from 5
and bromobenzene (in place of the more expensive iodobenzene)
but not from fluoro- or chlorobenzene. Taking advantage of the
strongly differing relative reaction rates, we have also synthe-
sized phenyl-substituted disilenes 6b-d featuring residual
halogen atoms X (6b, X ) F; 6c, X ) Cl; 6d, X ) Br) as
lateral functionalities at the para positions of the newly
introduced phenyl groups (Scheme 1). All of these compounds
were completely characterized by means of multinuclear NMR
and UV-vis spectroscopy as well as mass spectrometry.
Additionally, single-crystal X-ray diffraction studies allowed
us to establish a direct correlation between the trans-bent angles
and the SidSi bond length, which are theoretically interdepen-
(5) (a) Fink, M. J.; Michalczyk, M. J.; Haller, K. J.; Michl, J.; West, R.
Organometallics 1984, 3, 793. (b) Wind, M.; Powell, D. R.; West, R.
Organometallics 1996, 15, 5772.
(6) (a) Weidenbruch, M.; Willms, S.; Saak, W.; Henkel, G. Angew. Chem.
1997, 109, 2612; Angew. Chem., Int. Ed. Engl. 1997, 36, 2503. (b)
Ichinohe, M.; Sanuki, K.; Inoue, S.; Sekiguchi, A. Organometallics
2004, 23, 3088. (c) Uchiyama, K.; Nagendran, S.; Ishida, S.; Iwamoto,
T.; Kira, M. J. Am. Chem. Soc. 2007, 129, 10638.
(7) A recent review: Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.;
Olivier, Y.; Silbey, R.; Bre´das, J.-L. Chem. ReV. 2007, 107, 926.
(8) (a) Wright, V. A.; Gates, D. P. Angew. Chem. 2002, 114, 2495; Angew.
Chem., Int. Ed. 2002, 41, 2389. (b) Smith, R. C.; Protasiewicz, J. D.
Eur. J. Inorg. Chem. 2004, 998. (c) Wright, V. A.; Patrick, B. O.;
Schneider, C.; Gates, D. P. J. Am. Chem. Soc. 2006, 128, 8836. (d)
Geng, X.-L.; Ott, S. Chem. Commun. 2009, 7206. (e) Geng, X.-L.;
Hu, Q.; Scha¨fer, B.; Ott, S. Org. Lett. 2010, 12, 692. (f) Washington,
M. P.; Gudimetla, V. B.; Laughlin, F. L.; Deligonul, N.; He, S.; Payton,
J. L.; Simpson, M. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 2010,
132, 4566.
(9) Smith, R. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 2004, 126, 2268.
(10) (a) Wiberg, N.; Niedermayer, W.; Fischer, G.; No¨th, H.; Suter, M.
Eur. J. Inorg. Chem. 2002, 1066. (b) Wiberg, N.; Vasisht, S. K.;
Fischer, G.; Mayer, P. Z. Anorg. Allg. Chem. 2004, 630, 1823.
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Int. Ed. 2004, 43, 2965. (b) Inoue, S.; Ichinohe, M.; Sekiguchi, A.
Chem. Lett. 2005, 34, 1564. (c) Abersfelder, K.; Gu¨clu¨, D.; Schesch-
kewitz, D. Angew. Chem. 2006, 118, 1673; Angew. Chem., Int. Ed.
2006, 45, 1643. (d) Kinjo, R.; Ichinohe, M.; Sekiguchi, A. J. Am.
Chem. Soc. 2007, 129, 26. (e) Ichinohe, M.; Sanuki, K.; Inoue, S.;
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M.; Uchiyama, K.; Sasaki, S.; Nagendran, S.; Isobe, H.; Kira, M. J. Am.
Chem. Soc. 2009, 131, 3156.
t
dent. Finally, the reaction of 6d with BuLi to afford para-
lithiated 6f serves as a proof of principle that functional group
transformation on the aromatic periphery of disilenes is possible
despite the high reactivity of the SidSi moiety.
2.1. Treatment of Disilenide 5 with Halobenzenes PhX (X
) F, Cl, Br, I). As we recently reported, disilenide 5 reacts with
iodobenzene to afford phenyldisilene 6a in an essentially
quantitative manner.13a This formal nucleophilic aromatic
substitution in the absence of a transition-metal catalyst proceeds
with surprising ease and selectivity. The possibly best known
reaction mechanism of nucleophilic aromatic substitution reac-
tions is referred to as SNAr and involves the anionic Meisen-
(12) (a) Sasamori, T.; Hironaka, K.; Sugiyama, Y.; Takagi, N.; Nagase,
S.; Hosoi, Y.; Furukawa, Y.; Tokitoh, N. J. Am. Chem. Soc. 2008,
130, 13856. (b) Hartmann, M.; Haji-Abdi, A.; Abersfelder, K.;
Haycock, P. R.; White, A. J. P.; Scheschkewitz, D. Dalton Trans.
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(13) (a) Bejan, I.; Scheschkewitz, D. Angew. Chem. 2007, 119, 5885;
Angew. Chem., Int. Ed. 2007, 46, 5783. (b) Fukazawa, A.; Li, Y.;
Yamaguchi, S.; Tsuji, H.; Tamao, K. J. Am. Chem. Soc. 2007, 129,
14164.
(14) (a) Bejan, I.; Gu¨clu¨, D.; Inoue, S.; Ichinohe, M.; Sekiguchi, A.;
Scheschkewitz, D. Angew. Chem. 2007, 119, 3413; Angew. Chem.,
Int. Ed. 2007, 46, 3349. (b) Bejan, I.; Inoue, S.; Ichinohe, M.;
Sekiguchi, A.; Scheschkewitz, D. Chem.sEur. J. 2008, 14, 7119.
9
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A R T I C L E S
Jeck et al.
Scheme 2. Reactivity of Halobenzenes PhX toward Disilenide 5 (X
) F, Cl, Br, I; Tip ) 2,4,6-iPr3C6H2)
Scheme 3. Synthesis of Para-Functionalized Phenyldisilenes 6a-e
(Tip ) 2,4,6-iPr3C6H2)
heimer complex as a reactive intermediate.15 Because the
stability of this complex is rate-determining in SNAr reactions,
better leaving groups (which decrease the stability of the
complex) slow down the reaction. Consequently, the rate of
SNAr reactions increases with decreasing leaving tendency of
the halogen substituent: I < Br , Cl < F.15 In order to gather
mechanistic information on the SidSi transfer reaction in this
regard and to explore its preparative potential, we treated
samples of disilenide 5 in C6D6 with different halobenzenes at
room temperature (Scheme 2).
Indeed, we found that bromobenzene reacts with 5 as cleanly
as iodobenzene whether used stoichiometrically or in excess.
The smooth reactions of both bromo- and iodobenzene with 5
and the absence of detectable traces of the homocoupling
products [tetrasilabutadiene (4a) and biphenyl] renders stepwise
mechanistic options (i.e., initial metal-halogen exchange or
single-electron redox processes) unlikely in the present case.
Conversely, Weidenbruch et al.6a had explained the formation
of 4a upon treatment of in situ-generated 5 with 0.5 equiv of
the more encumbered MesBr (Mes ) 2,4,6-Me3C6H2) in 1,2-
dimethoxyethane in terms of an initial metal-halogen exchange
followed by salt elimination between the thus-formed bromo-
disilene and residual disilenide 5. Despite the potential prepara-
tive problems that the competing formation of 4a may thus cause
with sterically more demanding substrates, the larger number
and lower price of commercially available bromoarenes sig-
nificantly extends the potential scope of the SidSi transfer
reaction using 5.
In contrast to the cases of bromo- and iodobenzene, no
reaction at all was observed after one week at room temperature
using either fluoro- or chlorobenzene, even in the presence of
a 10-fold excess of the halobenzene; not even trace amounts of
phenyldisilene 6a were detected in either experiment. The
stability of 5 in the presence of both of these reagents excludes
the SNAr mechanism for the SidSi transfer reaction. If the SNAr
mechanism were active, the reaction with aryl fluorides should
be the most rapid. On this basis, we favor an SN2-like aromatic
substitution mechanism for the reaction of disilenide 5 with
simple, sterically uncluttered aryl halides. The alternative of a
radical chain mechanism of the SRNAr type appears to be
unlikely in view of the reactivity of 5 with dihalobenzenes (see
below).
2.2. Reactions of Disilenide 5 with 1-Iodo-4-halobenzenes
4-XPhI (X ) F, Cl, Br, I). In our earlier communication, we
described the reaction of 2 equiv of disilenide 5 with 1,4-
diiodobenzene, which quantitatively affords the p-phenylene-
bridged compound p-7.13a In an attempt to obtain a disilene
with a para-functionalized phenyl substituent and in order to
elucidate the relative reaction rates of the first and second
substitution steps, we reacted 1,4-diiodobenzene with just 1
equiv of 5 (Scheme 3). Despite the use of a precise 1:1
stoichiometry, the anticipated monosubstitution product 6e could
be obtained only with ∼10% p-7 and 10% unreacted 1,4-
diiodobenzene.
The predominant formation of 6e nonetheless provides
additional support for an SN2-like aromatic substitution mech-
anism, although the second substitution indeed takes place with
a competitive rate even in the presence of undoubtedly increas-
ing steric congestion. As demonstrated by Bunnett,16 SRNAr-
type reactions of diiodobenzenes with diethylphosphite tend to
afford the disubstitution product exclusively. However, we
cannot completely rule out the formation of an initial encounter
complex that may undergo electron transfer to form a solvent-
cage-stabilized radical pair prior to product formation.17
From a more preparative viewpoint, the strongly differing
reactivity of different halobenzenes offers a possible solution
to the above-mentioned selectivity problem. We therefore
decided to investigate the reactions of 5 with dihalobenzenes
featuring two differing halogens. Indeed, the reactions of 5 with
1-iodo-4-halobenzenes, 4-XPhI, in benzene at room temperature
proceeded cleanly to afford exclusively the products 6b-d
corresponding to substitution at the iodo position (Scheme 3).
Except for 6e, the p-halophenyl-substituted disilenes were stable
at room temperature in the absence of air and moisture. Despite
the essentially quantitative conversions to 6b-d as determined
by NMR spectroscopy, the isolated yields of the products
suffered from the increasing solubility in hydrocarbons in going
from 6b to 6d (6b, 86%; 6c, 47%; 6d, 23%). All of the isolated
p-halophenyl-substituted disilenes 6b-d (6e could not be
isolated) were completely characterized by multinuclear NMR
spectroscopy and single-crystal X-ray diffraction. Table 1
summarizes and compares selected spectroscopic and structural
parameters.
The 29Si NMR resonances of 6b-e (Table 1) are very similar
to those of 6a, indicating a rather weak perturbation of the SidSi
π bonding by the halogen atom at the para position of the phenyl
substituent. As pointed out by Strohmann, Kaupp, and co-
workers,18 the 29Si NMR shifts in multiply bonded silicon
species are very sensitive indicators of electronegativity differ-
ences in the substituents. Even though the longest-wavelength
absorptions in the UV-vis spectra of 6a-d obtained in hexane
(Table 1) are also only slightly perturbed, as evidenced by the
small red shift from X ) F to X ) Br, a positive linear
correlation with the modified Hammett parameter σP+ (as
introduced by Brown and Okamoto19) is evident (Figure 1). As
it is strictly empirical in nature, such a Hammett equation does
not lend itself to ready interpretation but might be useful for
the prediction of the HOMO-LUMO gap of derivatives yet to
be synthesized. It should be kept in mind, however, that it is
common practice to apply different equations for different
subsets of functional groups with similar electronic properties.20
(16) Review: Bunnett, J. F. Acc. Chem. Res. 1978, 11, 413.
(17) For example, hydrometalation of conjugated dienes with complex iron
hydrides is known to proceed via a radical-pair mechanism. See:
Shackleton, T. A.; Baird, M. C. Organometallics 1989, 8, 2225.
(18) (a) Auer, D.; Strohmann, C.; Arbuznikov, A. V.; Kaupp, M. Orga-
nometallics 2003, 22, 2442. (b) Auer, D.; Kaupp, M.; Strohmann, C.
Organometallics 2005, 24, 6331.
(19) Brown, H. C.; Okamoto, Y. J. Am. Chem. Soc. 1958, 80, 4979.
ˇ
(20) Usˇc´umlic´, G. S.; Mijin, D. Z.; Valentic´, N. V.; Vajs, V. V.; Susˇic´,
(15) Review: Terrier, F. Chem. ReV. 1982, 82, 77.
B. M. Chem. Phys. Lett. 2004, 397, 148.
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SidSi Transfer to Aromatics
A R T I C L E S
Table 1. NMR and UV-Vis Spectroscopic Parameters and Structural Parameters of Para-Functionalized Phenyldisilenes 6a-e and
Phenylene-Bridged Tetrasiladienes p-7 and m-7
6aa
6b
6c
6d
6e
p-7a
m-7
δ29Si1 (ppm)b
δ29Si2 (ppm)c
1J(Si1,Si2) (Hz)
d(Si1,Si2) (Å)
θSi1 (deg)d
71.8
55.2
70.6
69.3
69.2
69.7d
70.7
72.9
54.6
55.7
57.3
57.4
57.9d
56.8
150.0
2.1674(8)
16.5
19.3
3.4
f
f
f
f
151.4
2.175(1)f
22.8g
22.0g
5.0g
148.4
2.189(1)g
20.3g
26.7g
10.6g
450
2.147(1)f
5.5g
2.1735(4)
15.8
23.8
10.8
445
2.1707(5)
16.2
24.5
10.6
447
-
-
-
-
-
-
θSi2 (deg)d
5.9g
τ (deg)e
5.4g
λmax (nm)
439
19
437
16.8
508
27
ε (103 M-1 cm-1
)
19.2
16.2
39
a Data from ref 13a. b SiTip. c SiTip2. d Trans-bent angle [θSiX ) 90° - (angle between the Si1-Si2 vector and the normal to the plane defined by
SiX and the pendant substituents)]. e Twist angle (τ is the angle between the normals to two planes, each of which is defined by one of the silicon atoms
and the pendant substituents). f Not observed. g Average value.
conformational flexibility of the SidSi bond,2 the four inde-
pendent molecules of 6b vary significantly in their pertinent
structural features, namely, the trans-bent angles (θSi1
)
2.8-8.1°), the twist angles (τ ) 2.6-8.3°), and the Si-Si bond
lengths [d(Si1,Si2) ) 2.1417(1)-2.1536(1) Å]. Nevertheless,
unlike in the case of 6a, where considerable trans bending was
observed,13a 6b appears to have a preference for more planar
conformations, since three of the four independent molecules
exhibit rather small trans-bent angles close to planarity (Table
1).
The p-chloro- and p-bromophenyl derivatives 6c and 6d,
respectively, crystallize in isomorphous and hence very similar
structures. Both exhibit relatively high trans-bent angles and
long SidSi distances (Table 1). The finding that p-fluorophenyl-
substituted disilene 6b appears to prefer a more planar SidSi
bond conformation seemingly contradicts the bonding model
for multiple bonding in heavier alkene homologues developed
by Carter, Goddard, Malrieu, and Trinquier (the CGMT model):
the p-fluorophenyl substituent should stabilize the singlet state
of the corresponding silylene fragment through inductive and
mesomeric effects. As had been pointed out by Power and co-
workers22 in the context of distannynes, however, the shallow-
ness of the relevant potential energy surface must be kept in
mind in any discussions of substituent effects. Nonetheless, some
general conclusions with respect to the interrelation of bond
length and trans bending are warranted. The CGMT model
predicts a weakening of the SidSi bond as the extent of trans
bending increases.23,24 In a more practical approach, Karni and
Apeloig25 computationally established linear relationships be-
tween the three observable parameters (the SidSi bond length
Figure 1. Linear correlation (Hammett equation) between λmax for the
longest-wavelength absorptions of 6a-d and the electronic Hammett
parameter σP+ for the para substituent X.
In order to clarify whether the halogen atom at the para
position of 6b-d affects the solid-state structure as well, we
carried out X-ray diffraction studies on single crystals of the
entire series [Table 1 and Figure 2; crystallographic data are
summarized in Table 2 (in the Experimental Section) as well
as in the Supporting Information]. The structure of 6a was
reported in our previous communication13a and is included for
comparison. All of the phenyl-substituted disilenes crystallized
in monoclinic space groups (6a and 6b, P21; 6c and 6d, P21/c).
As previously observed for the chlorosilyl-substituted disilene
Ph2ClSi(Tip)SidSiTip2 (8),21 the p-fluorophenyl derivative 6b
crystallizes with four crystallographically independent molecules
in the asymmetric unit. As a consequence of the usual
Figure 2. Molecular structures of (left) one of the four crystallographically independent molecules in the crystal structure of p-fluorophenyldisilene 6b and
(right) p-bromophenyldisilene 6d. Hydrogen atoms and alternative positions of disordered isopropyl groups have been omitted for clarity. The ball-and-stick
representations at the center show simplified side views of the molecules.
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A R T I C L E S
Jeck et al.
Scheme 4. Metal-Halogen Exchange Reaction of 6d and Trapping
of p-Lithiophenyl-Substituted Disilene 6f with Me3SiCl To Form a
Mixture of 6g and 6a (Tip ) 2,4,6-iPr3C6H2)
(Tip)ZrCp2Cl (11)28 into the graph serves as proof of principle:
only the sterically and electronically comparatively innocuous
trimethylstannyldisilene 10 nicely fits the linear regression. For
8 and 9, a combination of both steric and electronic effects had
been invoked to explain the relatively long SidSi bond,21,27
whereas the SidSi bond in 11 interacts with the adjacent
zirconium and is strongly twisted, which results in a significant
perturbation.28
Figure 3. Linear correlation between the sum of the trans-bent angles θSi1
+ θSi2 and the SidSi bond distance for 6a-d and p-7 ([). Data points for
m-7 (9) were not considered for linear regression. Data for disilenes 8 (]),
9 (0), 10 (4), and 11 (O) are included for comparison.
and the two trans-bent angles θ) and the sum of the two
singlet-triplet gaps of the constitutent silylene fragments,
∑∆ES-T. Although they did not explicitly mention it, this result
also implies a linear correlation between the two dependent
observables, namely, the bond length and the trans-bent angles
θ. This has been qualitatively confirmed by experiment numer-
ous times,2 but a quantitative experimental correlation has not
been reported to date, presumably because of the various other
interfering factors that reduce comparability (e.g., differing steric
bulk). With a number of examples now in hand (i.e., 6a-d and
p-7) where these interfering factors are conceivably negligible,
such a correlation can be envisaged. Indeed the R2 value of
0.9437 for the plot of the SidSi bond length of the crystallo-
graphically independent disilene molecules against the sum of
the trans-bent angles (Figure 3) indicates a linear relationship.
Individual plots of the θ values against the SidSi distance
correlate less well, presumably because of the non-negligible
influence of packing forces on the molecular geometry, which
is apparent from the very fact that four significantly different
geometries are encountered in the solid state of 6b. The much
better correlation between d(Si1,Si2) and the sum of the angles
θSi1 + θSi2 as shown in Figure 3, illustrates that despite the
conformational freedom of disilenes, the overall interdependence
of the trans-bent angles and the SidSi distance is maintained.
Incidentally, the intercept with the ordinate at 2.1391 Å
corresponds nicely to the SidSi bond length of the essentially
planar disilene Tip2Si)SiTip2 (2.144 Å).26
2.3. Metal-Halogen Exchange Reaction of p-Bromophenyl-
disilene 6d. Reactivity studies of disilenes in the past mainly
concerned the SidSi double bond itself.2 As mentioned in the
Introduction, recent efforts have made available directly Si-
functionalized disilenes.6a,b,11,12 To the best of our knowledge,
however, functional group transformations at the periphery of
SidSi moieties have not been reported to date. With the para-
functionalized phenyldisilenes in hand, we investigated the
metal-halogen exchange reaction of the p-bromophenyl-
substituted derivative 6d.
t
Indeed, treatment of 6d with 2 equiv of BuLi at -80 °C
followed by intermittent warming to -50 °C for 1 h and
subsequent treatment with 1.05 equiv of Me3SiCl afforded a
product that on the basis of the NMR spectroscopic data was
tentatively identified as a mixture of p-trimethylsilylphenyld-
isilene 6g and the product of protonation of 6f, namely,
phenyldisilene 6a (Scheme 4). In different runs we obtained
6g/6a ratios between 8:2 and 9:1. Since all attempts to separate
these mixtures by crystallization failed, we synthesized 6g in
similar purity by the independent route of reacting disilenide 5
with 1 equiv of p-trimethylsilylbromobenzene in order to prove
its constitution. The identity of the multinuclear NMR spectro-
scopic data of the products of these two independent syntheses
unambiguously proved the formation of 6g and thus the
formation of p-lithiophenyldisilene 6f in the reaction of 6d with
tBuLi.
The amount of deviation from the empirical equation
[d(Si1,Si2) ) 0.008(θSi1 + θSi2) + 2.1391] may serve as a
convenient indicator that the presence of electronic or steric
effects affect the bond length of the SidSi bonds of the
Tip2SidSi(Tip)R type. Indeed, the inclusion of the previously
reported data for Tip2SidSi(Tip)2Cl (8),21 Tip2SidSi(Tip)-
SiPhSntBu2Cl (9),27 Tip2SidSi(Tip)SnMe3 (10),27 and Tip2SidSi-
2.4. Reactions of Disilenide 5 with Diiodobenzenes. In order
to lend further support to the hypothesis that the pronounced
red shift in the UV-vis spectrum of the p-phenylene-bridged
tetrasiladiene p-7 is truly due to conjugation effects, we chose
to investigate the corresponding meta derivative, m-7. It is well-
established that m-phenylene linking units interrupt the conjuga-
tion pathway and therefore the electronic communication in
conjugated systems.29
Reaction of 2 equiv of disilenide 5 with m-diiodobenzene
following the established protocol indeed yielded the desired
product m-7 quantitatively (Scheme 5). The absence of a
significant color change during the reaction already suggested
much less effective conjugation at this early stage of the study.
Crystallization from a minute amount of benzene afforded m-7
(21) Abersfelder, K.; Scheschkewitz, D. J. Am. Chem. Soc. 2008, 130, 4114.
(22) Fischer, R. C.; Pu, L.; Fettinger, J. C.; Brynda, M. A.; Power, P. P.
J. Am. Chem. Soc. 2006, 128, 11366.
(23) (a) Carter, E. A.; Goddard, W. A., III. J. Phys. Chem. 1986, 90, 998.
(b) Trinquier, G.; Malrieu, J.-P. J. Am. Chem. Soc. 1987, 109, 5303.
(c) Malrieu, J.-P.; Trinquier, G. J. Am. Chem. Soc. 1989, 111, 5916.
(24) Reviews of the CGMT model: (a) Driess, M.; Gru¨tzmacher, H. Angew.
Chem. 1996, 108, 900; Angew. Chem., Int. Ed. Engl. 1996, 35, 828.
(b) Gru¨tzmacher, H.; Fa¨ssler, T. F. Chem.sEur. J. 2000, 6, 2317.
(25) Karni, M.; Apeloig, Y. J. Am. Chem. Soc. 1990, 112, 8589.
(26) Watanabe, H.; Takeuchi, K.; Fukawa, N.; Kato, M.; Goto, M.; Nagai,
Y. Chem. Lett. 1987, 1341.
(28) Nguyen, T.-l.; Scheschkewitz, D. J. Am. Chem. Soc. 2005, 127, 10174.
(29) For example, see: (a) Pang, Y.; Li, J.; Hu, B.; Karasz, F. E.
Macromolecules 1999, 32, 3946. (b) Yang, J.-S.; Liau, K.-L.; Li, C.-
Y.; Chen, M.-Y. J. Am. Chem. Soc. 2007, 129, 13183.
(27) Abersfelder, K.; Nguyen, T.-l.; Scheschkewitz, D. Z. Anorg. Allg.
Chem. 2009, 635, 2093.
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SidSi Transfer to Aromatics
A R T I C L E S
Scheme 5. Syntheses of p-7 and m-7 (Tip ) 2,4,6-iPr3C6H2)
as yellow plates in two crops with a combined yield of 85%.
The m-phenylene-bridged isomer m-7 was fully characterized
by multinuclear NMR and UV-vis spectroscopy and single-
crystal X-ray diffraction.
The 29Si NMR data for m-7 are very similar to those for p-7
(Table 1). The one-bond coupling constant between the silicon
atoms [1J(Si,Si) ) 148.4 Hz] is marginally smaller than that of
p-7, suggesting a somewhat weaker SidSi bond, possibly as a
result of increased steric repulsion. The 1H and 13C data
confirmed the m-phenylene connectivity. The UV-vis spectrum
quantified the initial visual impression that m-7 exhibits a much
smaller red shift of the longest-wavelength absorption than does
p-7 with respect to phenyldisilene 6a (m-7, ∆λmax ) 11 nm;
p-7, ∆λmax ) 69 nm). The non-negligible red shift still present
may well be attributed to the structural effects of the increased
steric interactions due to the closer proximity of the disilenyl
groups.
Figure 5. Side views of simplified solid-state structures of (top) m-7 and
(bottom) p-7.
The solid-state structure of m-7 (Figure 4) indeed revealed
significantly greater trans bending of the substituents than in
p-7 (Table 1) and Si-Si double bonds with lengths of 2.1914(9)
and 2.1861(9) Å, which are thus considerably longer than the
SidSi bond in p-7. On the basis of the linear correlation derived
from phenyldisilenes 6a-d and p-7 (Figure 3), however, ∼0.01
Å of these bond lengths can be attributed to the increased steric
interaction in m-7. The greater steric strain between the two
disilenyl units in m-7 versus p-7 is also expressed in slightly
larger torsion angles of the SidSi units with respect to the
phenylene linker [Si2-Si1-C91-C96 ) -44.0(2)° and
Si4-Si3-C93-C94 ) -47.6(2) in m-7 vs Si2-Si1-C1-C3
41.0(2)° in p-7; Figure 5]. Nonetheless, the difference in dihedral
distortions is not large enough to explain the absence of
significant conjugation in m-7, and therefore, the electronic
nature of the linking unit appears to be the sole determining
factor. In order to corroborate this assertion, we carried out
cyclic voltammetry studies on both m-7 and p-7.
Figure 6. Cyclic voltammogram of p-7 at room temperature in THF vs
the Fc/Fc+ couple obtained using a scan rate 200 mV s-1 and Pt/
[nBu4N][PF6]/Ag. Both redox waves showed completely the same behavior
in a second CV cycle; only one cycle is shown here. One irreversible
oxidation process centered at Epa ) -0.35 V is not shown for clarity (see
the Supporting Information).
display irreversible oxidation processes centered at Epa ) -0.14
V for m-7 and Epa ) -0.35 V for p-7 (for full CV scans, see
the Supporting Information). In contrast to the reported cyclic
voltammograms of Tip2SidSiTip2 (12; E01/2 ) +0.56; +1.32 V
vs Fc/Fc+),30 both phenylene-bridged tetrasiladienes show only
one oxidation wave shifted significantly toward cathodic
potentials. Notably, the oxidation of m-7 is slightly less
cathodically shifted (i.e., p-7 is easier to oxidize), in agreement
with an increase in HOMO energy due to conjugation effects.
Unlike the case of 12, which exhibits only one reduction wave
at E01/2 ) -2.66 V, we observed in each case two well-separated
quasi-reversible reduction waves centered at E01/2(1) ) -2.70
and E01/2(2) ) -3.30 V for m-7 (Figure S18 in the Supporting
Information) and E10/2(1) ) -2.70 and E10/2(2) ) -3.02 V for
p-7 (Figure 6).
2.5. Electrochemical and EPR Studies of m-7 and p-7. The
redox behavior of m-7 and p-7 was studied using cyclic
voltammetry (CV) in THF under strictly anaerobic and anhy-
drous conditions. The potentials were referenced against the
ferrocene/ferrocenium (Fc/Fc+) couple. Both m-7 and p-7
The second reduction of m-7 is very close to the cathodic
limit of the redox window of the solvent and hence must be
treated with some caution. The invariance of the first reduction
wave, however, suggests that the presumably silicon-centered
reductions are almost independent of conjugative effects. The
a
c
differences in the peak potentials, ∆ꢀp ) ꢀp - ꢀp , for the
two compounds and each reduction step were revealed to be
∆ꢀp(1) ) 0.17 V and ∆ꢀp(2) ) 0.15 V for p-7 and ∆ꢀp(1) )
Figure 4. Solid-state structure of m-phenylene-bridged tetrasiladiene m-7.
Hydrogen atoms, alternative positions of disordered isopropyl groups, and
the 3.5 molecules of benzene have been omitted for clarity.
(30) Kira, M.; Ishima, T.; Iwamoto, T.; Ichinohe, M. J. Am. Chem. Soc.
2001, 123, 1676.
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A R T I C L E S
Jeck et al.
Figure 7. DFT-calculated spin densities for (a) [m-7q]•- and (b) [p-7q]•-. Color code: silicon, red; carbon, gray; hydrogen, pale-yellow. Only positive spin
density is shown.
Figure 8. Experimental (exp.) and simulated (sim.) X-band CW EPR spectra of (left) [m-7]•- [g| ) 2.0064, g⊥ ) 2.0057, A|(Si1) ) 155 G, A⊥(Si1) ) 69
G, A|(Si2) ) 145 G, A⊥(Si2) ) 65 G] and (right) [p-7]•- [g| ) 2.0062, g⊥ ) 2.0058, A|(Si1) ) 90 G, A⊥(Si1) ) 38 G, A|(Si2) ) 72 G, A⊥(Si2) ) 32 G]
in THF (frozen solution, 100 K). Insets: satellite signals due to couplings to the 29Si nuclei.
0.50 V and ∆ꢀp(2) ) 0.15 V for m-7. Because of the notably
different electron-transfer kinetics for the first reduction of m-7,
it is reasonable to assume a distinctive structural change upon
one-electron reduction of m-7.31 This suggestion was supported
by density functional theory (DFT) calculations at the BP86/
def2-TZVP level conducted on slightly modified model radical
anions [m/p-7q]•- featuring mesityl (Mes )2,4,6-Me3C6H2)
instead of Tip substituents on the Si atoms (Figure 7).
bridge in [p-7q]•- (1.853 Å) were revealed to be significantly
shorter than in [m-7q]•- (1.884 Å), indicating partial double
bond character and therefore demonstrating the delocalization
of the negative charge into the linking unit. This was further
supported by spin density calculations (Figure 7), which showed
a strong contribution of the ipso carbon atoms in the case of
[p-7q]•- but not for [m-7q]•-. Overall, however, the electron
density in both radical anions is predominantly centered on the
four silicon atoms (∼44% for both [m-7q]•- and [p-7q]•-).
A number of radical anions and one radical cation of disilenes
with one SidSi bond have been studied experimentally in the
past.3a,32 In order to support our computational findings by
experiment, we thus performed electron paramagnetic resonance
(EPR) measurements on frozen solutions of the in situ-generated
radical anions at 100 K (Figure 8). [m/p-7]•- gave strong, slightly
axial symmetric signals with small satellites due to hyperfine
coupling (hfc) to the 29Si nuclei. The experimental spectra were
The most obvious difference between the two optimized
structures is that [p-7q]•- adopts an almost coplanar arrangement
of the two SidSi units and the p-phenylene bridge (torsion
angles of 8.0 and 7.8°) whereas in [m-7q]•- the SidSi bonds
are severely twisted out of the plane of the m-phenylene bridge
(54.6 and 55.9°). In contrast, in the calculated structures of the
neutral precursors p-7q and m-7q, the corresponding torsion
angles are very similar, reiterating the experimental observations
for m/p-7 [p-7q, 19.9 and 20.0°, ∆(neutral/anion) ) 11.9 and
12.2°; m-7q, 23.4 and 24.0°, ∆(neutral/anion) ) 31.2 and 31.9°].
Obviously, the one-electron reduction of m-7 incurs a consider-
ably larger distortion of the molecular structure, nicely cor-
roborating the interpretation of the electrochemical findings. This
seems to be in line with the expected more efficient delocal-
ization of the negative charge in the p-phenylene-linked [p-7q]•-.
Consequently, the trans bending of [p-7q]•- is slightly less
pronounced than that of [m-7q]•- ([m-7q]•-, θ ) 30.5-34.5°;
[p-7q]•-, θ ) 28.4-32.6°). For both compounds, the SidSi
bonds are considerably elongated by 6.9 ([p-7q]•-) and 8.3 pm
([m-7q]•-) relative to the neutral precursors (p-7q, 2.176 Å;
m-7q, 2.173 Å). Furthermore, the Si-C bonds to the phenylene
1
successfully simulated using a system with S ) /2 and two
different silicon atoms. The spin Hamiltonian parameters used
for the simulation of [m-7]•- are g| ) 2.0064 and g⊥ ) 2.0057
with hfcs to two different 29Si atoms (I ) 1/2) of A|(Si1) ) 155
G, A⊥(Si1) ) 69 G and A|(Si2) ) 145 G, A⊥(Si2) ) 65 G. The
(31) Geiger, W. E. Organometallics 2007, 26, 5738.
(32) (a) Weidenbruch, M.; Kramer, K.; Scha¨fer, A.; Blum, J. K. Chem.
Ber. 1985, 118, 107. (b) Weidenbruch, M.; Thom, K.-L. J. Organomet.
Chem. 1986, 308, 177. (c) Kira, M.; Iwamoto, T. J. Organomet. Chem.
2000, 611, 236. (d) Wiberg, N.; Niedermayer, W.; Polborn, K. Z.
Anorg. Allg. Chem. 2002, 628, 1045. (e) Inoue, S.; Ichinohe, M.;
Sekiguchi, A. J. Am. Chem. Soc. 2008, 130, 6078.
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SidSi Transfer to Aromatics
A R T I C L E S
simulation for [p-7]•-, however, turned out to be challenging
because the satellites were poorly resolved. A simulation with
the spin Hamiltonian parameters g| ) 2.0062 and g⊥ ) 2.0058
and hfcs to two different 29Si nuclei [A|(Si1) ) 90 G, A⊥(Si1)
) 38 G and A|(Si2) ) 72 G, A⊥(Si2) ) 32 G] nicely reproduced
the line shape of the satellites of [p-7]•-. It is noteworthy that
although the intensity of the satellites was slightly smaller than
in the experimental spectra, other parameters did not furnish
improved simulations. Nonetheless, the smaller hfcs of [p-7]•-
in comparison with [m-7]•- are in line with the less pronounced
trans bending in [p-7]•- and hence a smaller s-orbital contribu-
tion within the SidSi bonds (see above). Remarkably, the hfcs
in the case of [m-7]•- are significantly larger than that of the
radical anion of disilene 1 with the sterically less demanding
mesityl groups [Aiso(29Si) ) 49.6 G],32a implying larger trans
bending despite the greater steric strain in [m/p-7]•-. Despite
these gradual differences, however, all of the hfcs observed are
consistent with a strongly predominant p character of the orbitals
hosting the unpaired electron.33 Overall, both EPR investigations
nicely support the DFT calculations and the electrochemical
findings by confirming the Si-centered nature of the reduction
processes of the phenylene-bridged tetrasiladienes.
(5) On the basis of the set of solid-state structures of disilenes
with comparable steric congestion that is now available, the
interrelated parameters of the SidSi bond length and the trans-
bent angles were satisfactorily correlated for the first time. A
pseudolinear relationship was established that can be used as
an approximate gauge of the degree of steric or electronic
perturbation in compounds featuring the Tip2SidSi(Tip) moiety.
We have certainly barely scratched the surface of the inherent
preparative possibilities of the SidSi transfer reaction using
disilenide 5. There can be little doubt that the availability of a
general protocol for the transfer of silicon-silicon double bonds
will contribute to the firm establishment of the SidSi moiety
as a useful functional group and potent chromophore.
4. Experimental Section
General. All of the manipulations were carried out under a
protective atmosphere of argon using standard Schlenk techniques
or in a drybox. Ethereal solvents were refluxed over sodium/
benzophenone and pentane, hexane, and (deuterated) benzene over
sodium/potassium alloy; toluene was refluxed over sodium. All of
the solvents were distilled and stored under argon and degassed
prior to use. Tip2SidSi(Tip)Li (5) was prepared following our
published procedure.11a Iodo- and diiodobenzenes were purchased
from Aldrich, checked for purity by NMR analysis prior to use,
and distilled or sublimed when necessary. The NMR spectra were
recorded on a Bruker AV 500 or AV 400 FT-NMR spectrometer.
1H and 13C{1H} NMR spectra were referenced to external tetram-
ethylsilane (TMS) via the residual protons of the deuterated solvent
(1H) or the solvent itself (13C). 29Si{1H} NMR spectra were
referenced to external TMS and 19F{1H} NMR spectra to external
CFCl3. UV-vis spectra were recorded on Shimadzu UV Mini 1240
and PerkinElmer Lambda 25 spectrophotometers. Melting points
were determined in closed NMR tubes under argon and are
uncorrected.
3. Conclusion
The transfer of SidSi moieties to sterically innocent bromo-
and iodoarenes using disilenide 5 is a powerful method for the
synthesis of compounds with one or more SidSi double bonds.
The main conclusions of our study can be summarized as
follows:
(1) The inertness of chloro and fluoro functionalities toward
disilenide 5, together with the absence of homocoupling
products, favors a concerted aromatic substitution mechanism
of the SN2 type without, however, completely excluding the
intermediate occurrence of radical pairs. Moreover, the low
reactivity of 5 toward fluoro- and chlorobenzene raises the
possibility of using these highly polar aromatics as solvents for
reactions of disilenides.
(2) Residual halogen functionality at the para position is
readily tolerated by disilenide 5, opening the exciting perspective
of functional group transformations in the presence of a SidSi
double bond. With the metal-halogen exchange reaction from
6d to 6f, we provided a first proof of principle for such a
transformation.
(3) The (functionalized) phenyldisilenes 6a-d exhibit a good
(albeit not strong) correlation of the longest-wavelength UV-vis
absorption with a modified electronic Hammett parameter that
will be helpful in the deliberate tuning of HOMO-LUMO gaps
of disilenes, although the cautionary note that only para
substituents with similar electronic properties were analyzed
must be added.
General Procedure for the Preparation of Phenyldisilenes
6a-e,g and Bis(disilenyl)benzenes m-7 and p-7. Via cannula, a
solution of the required stoichiometric amount of disilenide in 20
mL of benzene was added dropwise to a solution of 1.17 mmol of
the appropriate iodobenzene (bromobenzene in the case of 6g) in
15 mL of benzene at room temperature. After the mixture was
stirred for 1 h at room temperature, the solvent was distilled off
under vacuum. Hexane (35 mL) was added and the lithium halide
separated from the solution by filtration. Crystallization at room
temperature from a minimum amount of the indicated solvent
afforded the corresponding product.
1-Phenyl-1,2,2-(2′,4′,6′-triisopropylphenyl)disilene(6a).13aBright-
yellow crystals from hexane, 0.50 g (58%, mp 156-157 °C, dec.).
1H NMR (500.1 MHz, C6D6, 298 K): δ 7.38 (d, 2H, o-Ph-H), 7.12,
7.09, 7.04 (each s, each 2H, Tip-H), 6.92 (pseudo-t, 1H, p-Ph-H),
i
6.83 (pseudo-t, 2H, m-Ph-H), 4.29, 4.24 (each hept, each 2H, Pr-
CH), 4.03 (br., 2H, iPr-CH), 2.76, 2.75, 2.70 (each hept, each 1H,
iPr-CH), 1.21, 1.19, 1.18, 1.15, 1.13, 1.11 (br.), 1.06 (each d,
altogether 54H, iPr-CH3). 13C NMR (125.1 MHz, C6D6, 298 K): δ
156.28, 155.72, 155.15, 151.72, 151.28, 150.73 (Tip-Co/p), 138.67
(Ph-Ci), 136.30 (Ph-CH), 133.41, 133.13, 130.62 (Tip-Ci), 128.71,
128.06 (Ph-CH), 122.10, 122.06, 121.70 (Tip-CH), 38.43, 38.22,
37.57, 34.93, 34.76, 34.59 (iPr-CH), 25.68, 25.02 (br.), 24.69, 24.42
(br.), 24.32, 24.17, 24.01 (iPr-CH3). 29Si NMR (99.4 MHz, C6D6,
298 K): δ 71.8 (1J(Si,Si) ) 151.4 Hz, SiTipPh), 55.2 (1J(Si,Si) )
151.4 Hz, SiTip2). UV-vis (hexane) λmax (ε): 439 nm (19000 M-1
cm-1). Exact mass (EI): found, 742.5331; calcd for C51H74Si2,
742.5329.
(4) The introduction of two SidSi bonds was demonstrated
for the m-phenylene-bridged derivative m-7 in addition to the
previously reported para isomer p-7.13a The conjugation medi-
ated by the para linkage in p-7 was corroborated by the absence
of a significant red shift relative to phenyldisilene 6a in the
case of m-7. In addition, the presence of conjugation was lent
support by electrochemical studies on m/p-7 and EPR investiga-
tions on in situ-generated radical anions [m/p-7]•-, which were
further elucidated by DFT calculations on slightly modified
model systems.
1-(p-Fluorophenyl)-1,2,2-(2′,4′,6′-triisopropylphenyl)disilene
(6b). Orange crystals from benzene, 0.77 g (86%, mp 143-145
°C, partial dec.). 1H NMR (400.2 MHz, C6D6, 298 K): δ 7.32 (dd,
2H, Ph-Ho), 7.23, 7.19, 7.15 (each s, each 2H, Tip-H), 6.62 (dd,
2H, Ph-Hm), 4.30 (hept, 4H, iPr-CH), 4.09 (hept, 2H, iPr-CH), 2.85
(33) Review: Lee, Y. Va.; Sekiguchi, A. Eur. J. Inorg. Chem. 2005, 1209.
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A R T I C L E S
Jeck et al.
(hept, 2H, iPr-CH), 2.81 (hept, 1H, iPr-CH), 1.31, 1.29, 1.27, 1.26,
1.21, 1.16 (each d, altogether 54H, iPr-CH3). 13C NMR (100.6 MHz,
C6D6, 298 K): δ 163.90 (1J(C,F) ) 248.8 Hz, Ph-Cp) 156.20,
155.73, 155.10, 151.88, 151.49, 150.84 (Tip-Co/p), 138.20 (3J(C,F)
) 7.1 Hz, Ph-Co), 134.21 (4J(C,F) ) 3.7 Hz, Ph-Ci) 133.19, 132.76,
130.45 (Tip-Ci), 122.12 (Tip-CoH), 121.75 (Tip-CpH), 115.30
(2J(C-F) ) 20.0 Hz, Ph-Cm), 38.45, 38.21, 37.61, 34.94, 34.76,
34.58 (iPr-CH), 25.64, 24.68, 24.32, 24.15, 23.99 (iPr-CH3). 19F{1H}
NMR (376.8 MHz, C6D6, 298 K): δ -111.51. 29Si NMR (79.5 MHz,
C6D6, 298 K): δ 70.6 (SiPh), 55.7 (SiTip). UV-vis (hexane) λmax
(ε): 437 nm (16800 M-1 cm-1). Exact mass (EI): found, 760.5228;
calcd for C51H73Si2F, 760.5235.
CH), 38.46, 37.99, 37.54, 34.92, 34.77, 34.48 (iPr-CH), 26.12,
25.85, 24.55, 24.22, 24.17, 23.97 (iPr-CH3). 29Si NMR (99.4 MHz,
C6D6, 298 K): δ 72.9 (1J(Si,Si) ) 148.4 Hz, SiTipPh), 54.6 (1J(Si,Si)
) 148.4 Hz, SiTip2). UV-vis (hexane) λmax (ε): 450 nm (39000
M-1 cm-1). MS (EI) m/z (%): 1407.9 (M+, 10), 1267.7 (10), 1194.8
(20), 1176.8 (30), 1062.5 (85), 1021.5 (55), 1007.6 (25), 989.6 (10),
859.3 (10), 817.3 (10), 451.3 (100), 433.3 (80). Anal. Calcd for
C96H142Si4: C, 81.86; H, 10.16. Found: C, 79.21; H, 9.85. Deviation
likely due to the high oxygen sensitivity of m-7 (calcd for
C96H142Si4O3: C, 79.16; H, 9.83).
1,4-Bis[1′,2′,2′-tris(2′′,4′′,6′′-triisopropylphenyl)disilenyl]benzene
(p-7).13a Red crystals from pentane, 1.20 g (72%, mp 190 °C, dec.).
1H NMR (500.1 MHz, C6D6, 298 K): δ 7.11, 7.06, 7.03 (each s,
each 4H, Tip-H), 6.84 (s, 4H, -C6H4-), 4.18, 4.13, 3.94, 2.76 (each
1-(p-Chlorophenyl)-1,2,2-(2′,4′,6′-triisopropylphenyl)disilene (6c).
Orange crystals from hexane, 0.43 g (47%, mp 145-147 °C, partial
dec.). 1H NMR (500.1 MHz, C6D6, 298 K): δ 7.25 (d, 2H, Ph-Hm),
7.22, 7.18, 7.14 (each s, each 2H, Tip-H), 6.92 (d, 2H, Ph-Ho),
i
i
hept, each 4H, Pr-CH), 2.68 (hept, 2H, Pr-CH), 1.24, 1.22, 1.18,
1.14 (br.), 1.12, 1.04 (each d, altogether 108H, iPr-CH3). 13C NMR
(125.1 MHz, C6D6, 298 K): δ 156.31, 155.54, 155.09, 151.48,
151.31, 150.58 (Tip-Co/p), 139.14 (C6H4-Ci), 134.75 (Ph-CH),
133.59, 133.37, 130.16 (Tip-Ci), 128.59 (C6H6), 122.15, 122.06,
121.60 (Tip-CH), 38.29, 38.10, 37.38, 34.94, 34.81, 34.53 (iPr-
CH), 25.95, 25.19 (br.), 24.71, 24.41, 24.29, 24.19, 23.98 (iPr-CH3).
29Si NMR (99.36 MHz, C6D6, 298 K): δ 70.7 (1J(Si,Si) ) 150.0
Hz, SiTip), 56.8 (1J(Si,Si) ) 150.0 Hz, SiTip2). UV-vis (hexane)
i
i
4.27 (hept, 4H, Pr-CH), 4.07 (hept, 2H, Pr-CH), 2.85 (hept, 2H,
iPr-CH), 2.80 (hept, 1H, iPr-CH), 1.31, 1.30, 1.29, 1.24, 1.23, 1.20,
1.15 (each d, altogether 54H, Pr-CH3). 13C NMR (125.8 MHz,
i
C6D6, 298 K): δ 156.28, 155.72, 155.15, 151.94, 151.62, 150.92
(Tip-Co/p), 137.45 (Ph-Cp/i/Tip-Ci), 137.37 (Ph-Co), 134.97, 133.04,
132.66, 130.10 (Ph-Cp/i/Tip-Ci), 128.35 (Ph-Cm), 122.15, 121.77
(Tip-Cm), 38.46, 38.29, 37.58, 34.94, 34.77, 34.58 (iPr-CH), 25.67,
24.69, 24.31, 24.15, 23.98 (iPr-CH3). 29Si NMR (99.4 MHz, C6D6,
298 K): δ 69.3 (SiPh), 57.3 (SiTip). UV-vis (hexane) λmax (ε):
445 nm (19200 M-1 cm-1). Exact mass (EI): found, 776.4935; calcd
for C51H73Si2Cl, 776.4939.
λ
max (ε): 508 nm (27000 M-1 cm-1). MS (EI) m/z (%): 1407.9 (M+,
15), 1176.8 (100), 1062.5 (40), 1021.5 (20), 945.6 (15), 492.2 (20),
451.2 (15), 433.3 (70). A satisfactory elemental analysis could not
be obtained, presumably because of the high oxygen sensitivity.
t
Lithiation of 6d with BuLi To Afford 6f. A pentane solution
1-(p-Bromophenyl)-1,2,2-(2′,4′,6′-triisopropylphenyl)disilene
(6d). Orange crystals from hexane, 0.22 g (23%, mp 143-145 °C,
t
of BuLi (0.71 mL, 1.7M) was added dropwise via syringe within
1
10 min to a stirred solution containing 0.50 g (0.61 mmol) of 6d
in 15 mL of Et2O at -75 °C. The mixture was stirred for 2 h at
-75 °C, allowed to warm to -50 °C within 1 h, kept at this
temperature for 10 min, and then cooled again to -75 °C. Me3SiCl
was added dropwise over the course of 5 min via syringe, after
which the reaction mixture was very slowly (10 °C/h) brought back
to room temperature. Solvents and volatiles were removed under
vacuum, after which hexane was added and the lithium salts filtered
off. Removal of hexane under vacuum afforded a mixture of 6g
and 6a according to NMR spectroscopic data (in different runs,
6g/6a ratios between 8:2 and 9:1 were obtained).
dec.). H NMR (500.1 MHz, C6D6, 298 K): δ 7.22, 7.19 (each s,
each 2H, Tip-H), 7.18 (d, 2H, Ph-Hm), 7.14 (s, 2H, Tip-H), 7.07
i
i
(d, 2H, Ph-Ho), 4.28 (hept, 4H, Pr-CH), 4.08 (hept, 2H, Pr-CH),
2.85 (hept, 2H, iPr-CH), 2.80 (hept, 1H, iPr-CH), 1.31, 1.29, 1.28,
1.23, 1.23, 1.20, 1.16 (each d, altogether 54H, iPr-CH3). 13C NMR
(125.8 MHz, C6D6, 298 K): δ 156.29, 155.72, 155.15, 151.97,
151.66, 150.94 (Tip-Co/p), 137.89 (Ph-Cp), 137.63 (Ph-Co), 133.01,
132.64 (Ph-Ci/Tip-Ci), 131.21 (Ph-Cm), 130.02, 123.39 (Ph-Ci/Tip-
Ci), 122.16, 121.78 (Tip-Cm), 38.47, 38.30, 37.58, 34.92, 34.75,
34.57 (iPr-CH), 25.67, 24.68, 24.29, 24.15, 24.13, 23.96 (iPr-CH3).
29Si NMR (99.4 MHz, C6D6, 298 K): δ 69.2 (SiPh), 57.4 (SiTip2).
UV-vis (hexane) λmax (ε): 447 nm (16200 M-1 cm-1). Exact mass
(EI): found, 820.4425; calcd for C51H73Si2Br, 820.4434.
Crystallography. Single crystals of 6b-d and m-7 were coated
with perfluorinated polyether and transferred to the cold nitrogen
stream at the goniometer. Data were collected on a Bruker Smart
Apex II (m-7), Oxford Diffraction Xcalibur PX Ultra (6b and 6c),
or Oxford Diffraction Xcalibur 3 (6d) diffractometer. The cells were
determined accurately on a representative set of frames. Data
reduction was carried out with SAINT+ version 6.22 (m-7),
CrysAlis Pro version 1.171.33.48 (6b and 6c), or CrysAlis Pro
version 1.171.33.41 (6d). Semiempirical absorption corrections were
applied to the data for m-7 using SADABS, while analytical
absorption corrections were applied to the data for 6b and 6c using
CrysAlis Pro version 1.171.33.48 and to the data for 6d using
CrysAlis Pro version 1.171.33.41. Refinement proceeded smoothly
to yield the crystallographic parameters summarized in Table 2.
The absolute structure of 6b was shown to be a partial racemic
1-(p-Trimethylsilylphenyl)-1,2,2-(2′,4′,6′-triisopropylphenyl)di-
1
silene (6g). H NMR (400.1 MHz, C6D6, 298 K): δ 7.40 (d, 2H,
Ph-Ho), 7.14, 7.10 (each s, each 2H, Tip-H), 7.07 (d, 2H, Ph-Hm),
i
i
7.05 (s, 2H, Tip-H), 4.32 (hept, 2H, Pr-CH), 4.22 (hept, 2H, Pr-
CH), 4.03 (hept, 2H, iPr-CH), 2.76 (hept, 2H, iPr-CH), 2.71 (hept,
1H, iPr-CH), 1.23, 1.20, 1.28, 1.18, 1.17, 1.14, 1.10, 1.07 (each d,
altogether 54H, iPr-CH3). 13C NMR (100.6 MHz, C6D6, 298 K): δ
156.29, 155.74, 155.10 (Tip-Co), 151.70, 151.28, 150.70 (Tip-Cp),
140.54 (Ph-Ci), 139.20 (Ph-Cp), 135.46 (Ph-Co), 133.55, 133.24
(Tip-Ci), 132.89 (Ph-Cm), 130.56 (Tip-Ci), 122.09, 121.65 (Tip-
Cm), 38.50, 38.16, 37.63, 34.94, 34.75, 34.59 (iPr-CH), 25.74, 24.70,
24.34, 24.17, 24.03 (iPr-CH3), -1.24 (SiMe3). 29Si NMR (79.5 MHz,
C6D6, 298 K): δ 71.5 (SiPh), 56.5 (SiTip2), -4.6 (SiMe3). Exact
mass (EI): found, 814.5718; calcd for C54H82Si3, 814.5724.
1,3-Bis[1′,2′,2′-tris(2′′,4′′,6′′-triisopropylphenyl)disilenyl]benzene
(m-7). Orange crystals from benzene, 0.76 g (39%, mp 162 °C).
From the mother liquor, up to 0.90 g (46%) of somewhat impure
additional product was obtained. 1H NMR (500.1 MHz, C6D6, 298
K): δ 7.52 (s, 1H, Ph-H(2)), 7.09 (d, 2H, Ph-H(4,6)), 7.08 (s, 4H,
+
-
twin by a combination of R-factor tests [R1 ) 0.0472, R1
)
0.0507] and by use of the Flack parameter [x+ ) +0.167(11), x-
) +0.833(11)].
Cyclic Voltammetry. Cyclic voltammetry measurements were
performed with an EG&G potentiostat (PAR model 263A) and an
electrochemical cell for sensitive compounds.34 We used a freshly
polished Pt disk working electrode, a Pt wire as the counter
electrode, and a Ag wire as the (pseudo)reference electrode;
[nBu4N][PF6] (0.1 M) was used as the electrolyte. Potentials were
calibrated against the Fc/Fc+ couple (internal standard), which has
a potential of E01/2 ) 0.35 V versus Ag/AgCl.
1
Tip-H), 6.99, 6.97 (each s, each 4H, Tip-H), 6.49 (t, 1H, J(H,H)
) 7.50 Hz, Ph-H(5)), 4.11, 4.07 (each hept, each 4H, iPr-CH), 3.83
i
i
(hept, 4H, Pr-CH), 2.75, 2.72 (each hept, each 2H, Pr-CH), 2.66
i
(hept, 2H, Pr-CH), 1.24, 1.22, 1.14, 1.10, 0.98, 0.90 (each d,
altogether 108H, Pr-CH3). 13C NMR (125.1 MHz, C6D6, 298 K):
i
δ 156.10, 155.67, 154.89 (Tip-Co), 151.32, 151.19, 150.40 (Tip-
Cp), 142.00 (Ph-C(2)H), 137.84 (Ph-Ci), 136.41 (Ph-C(4,6)H), 133.91,
133.52, 129.77 (Tip-Ci), 127.67 (Ph-C(5)H), 122.10, 121.47 (Tip-
(34) (a) Hinkelmann, K.; Heinze, J.; Schacht, H.-T.; Field, J. S.; Vahren-
kamp, H. J. Am. Chem. Soc. 1989, 111, 5078. (b) Heinze, J. Angew.
Chem. 1984, 96, 823; Angew. Chem., Int. Ed. Engl. 1984, 23, 831.
9
17314 J. AM. CHEM. SOC. VOL. 132, NO. 48, 2010

SidSi Transfer to Aromatics
A R T I C L E S
Table 2. Selected Crystallographic Details for Compounds 6b-d
and m-7
DFT Calculations. Calculations were performed with the
TURBOMOLE program package.35 The geometries were optimized
without symmetry restrictions at the (RI)-BP8636 level using def2-
TZVP basis sets (see the Supporting Information).37 The spin
densities were calculated using the pointval module of TURBO-
MOLE. The spin density quantification was done using a home-
written script. Because of the large number of atoms, analytical
frequency calculations were not performed.
6b
6c
6d
m-7·3.5C6H6
formula
T (K)
C51H73FSi2
173(2)
C51H73ClSi2 C51H73BrSi2 C117H163Si4
173(2)
173(2)
173(2)
λ (Å)
1.54184
1.54184
0.71073
0.71073
color, habit
crystal system
space group
a (Å)
yellow, block orange, block orange, block yellow, plate
monoclinic
P21
monoclinic
P21/c
monoclinic
P21/c
triclinic
j
P1
Acknowledgment. This work was supported by the DFG
(Sche906/3-1 and 3-2), the Karl Winnacker-Fonds of the Aventis
Foundation, the Gottlieb Daimler and Karl Benz Foundation, the
EPSRC (EP/H048804/1), and in part by the Fonds der Chemischen
Industrie. We thank Peter R. Haycock and Richard N. Sheppard
for the acquisition of NMR spectra and Alexander Baldes for
technical assistance.
15.38356(8) 18.55902(9) 18.5602(3)
10.2487(7)
b (Å)
16.80979(9) 12.81965(5) 12.84960(16) 21.6060(15)
c (Å)
R (deg)
(deg)
37.8732(2)
90
21.65869(9) 21.6993(3)
90 90
26.9115(2)
68.187(2)
100.2271(5) 109.6339(5) 109.5139(16) 81.340(2)
γ (deg)
V (Å3)
90
90
90
82.160(2)
9638.19(9)
8
4853.43(3)
4
4877.84(13) 5448.3(7)
4
Z
2
d (g cm-3
)
1.049
0.919
3328
72.58
73962
36582
1.064
1.386
1696
72.47
55005
9592
8423
0.0263
63
1.120
0.923
1768
31.97
43582
14784
8546
0.0228
57
1.025
0.099
1842
25.03
43080
19141
15398
0.0510
60
1326
1.107
0.0656
0.1585
µ (mm-1
F(000)
)
Supporting Information Available: Crystallographic data for
6b-d and m-7 (CIF), NMR spectra, full cyclic voltammograms,
and Cartesian coordinates of the DFT-calculated structures. This
material is available free of charge via the Internet at http://
pubs.acs.org.
θmax (deg)
total reflns
unique reflns
reflns with I > 2σ 33450
Rint
restraints
parameters
S
R1 (I > 2σ)
wR2 (all data)
0.0262
439
2024
1.045
0.0470
0.1272
514
514
JA107547S
1.086
0.0375
0.1081
0.909
0.0357
0.0895
(35) (a) TURBOMOLE, version 6.0; TURBOMOLE GmbH: Karlsruhe,
Germany, 2009. TURBOMOLE was developed by the University of
Karlsruhe (TH) and Forschungszentrum Karlsruhe GmbH from 1989-
2007; since 2007, it is available at http://www.turbomole.com. (b)
Ahlrichs, R.; Ba¨r, M.; Ha¨ser, M.; Horn, H.; Ko¨lmel, C. Chem. Phys.
Lett. 1989, 162, 165. (c) Treutler, O.; Ahlrichs, R. J. Chem. Phys.
1996, 102, 346. (d) Sierka, M.; Hogekamp, A.; Ahlrichs, R. J. Chem.
Phys. 2003, 118, 9136. (e) Ahlrichs, R. Phys. Chem. Chem. Phys.
2004, 6, 5119.
(36) (a) Slater, J. C. Phys. ReV. 1951, 81, 385. (b) Vosko, S. H.; Wilk, L.;
Nusair, M. Can. J. Phys. 1980, 58, 1200. (c) Becke, A. D. Phys. ReV.
A 1998, 38, 3098. (d) Perdew, J. P. Phys. ReV. B 1986, 33, 8822.
(37) (a) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297.
(b) Weigend, F. Phys. Chem. Chem. Phys. 2006, 8, 1057.
EPR Spectroscopy. Continuous wave (CW) EPR spectroscopy
was performed at the X-band on a Bruker EMXplus spectrometer
(microwave frequency 9.43 GHz) equipped with a liquid nitrogen
cryostat. The spectra were measured with a modulation amplitude
of 1 or 10 G and a modulation frequency of 100 kHz, and the field
was calibrated by using 2,2-diphenyl-1-picrylhydrazyl (DPPH) with
a g value of 2.0036. CW EPR simulations were calculated using
the Bruker XSophe program package (version 1.1.4). In the
simulations, we used the experimental central field and sweep width.
9
J. AM. CHEM. SOC. VOL. 132, NO. 48, 2010 17315