1,2-Bis(ferrocenyl)disilene: A multistep redox system with an Si=Si double bond

Article abstract of DOI:10.1021/om8003543

A new type of disilene substituted by ferrocenyl groups, 1,2-bis(ferrocenyl)disilene, was synthesized as a good candidate for a novel d-π electron system, and the unique properties of the Si=Si π bond of this new disilene were revealed. On the basis of the results of the electrochemical analyses, 1,2-Tip₂-1,2-Fc₂-disilene (Tip = 2,4,6-triisopropylphenyl, Fc = ferrocenyl) was found to be a stable five-electron redox system with four steps.

Full text of DOI:10.1021/om8003543

Organometallics 2008, 27, 3325–3327
3325
1,2-Bis(ferrocenyl)disilene: A Multistep Redox System with an SidSi
Double Bond
Takahiro Sasamori,† Akihiro Yuasa,† Yoshinobu Hosoi,‡ Yukio Furukawa,‡ and
Norihiro Tokitoh*,†
Institute of Chemical Research, Kyoto UniVersity, Gokasho, Uji, Kyoto 611-0011, Japan, and Department
of Chemistry, School of Science and Engineering, Waseda UniVersity, 3-4-1 Okubo, Shinjuku-ku,
Tokyo, Japan
ReceiVed April 21, 2008
standpoint of material science.7 In the case of phosphorus, which
is the adjacent element of silicon in the periodic table,
Protasiewicz and co-workers opened the door for the chemistry
of π-conjugated systems containing PdP moieties, and this
chemistry has attracted much interest from the viewpoint of their
photochemical and electrochemical properties.8 Recently, we
have reported the synthesis of stable ferrocenyl- and anthryl-
substituted diphosphenes together with their unique physical
properties as novel conjugated systems containing a PdP
unit.9,10 In the case of silicon, however, the redox behavior of
disilenes has been less explored and the stable disilenes reported
so far showed only irreversible reduction and oxidation waves
in the cyclic voltammograms.5,11 These situations prompted us
to design and synthesize a new type of disilene substituted by
ferrocenyl groups, i.e., 1,2-bis(ferrocenyl)disilene, as a good
candidate for a novel conjugated system, which should be an
attractive target compound to reveal the unique properties of
the bridging SidSi π bond. The ferrocenyl moieties should make
it possible to construct a stable multistep redox system through
electronic conjugation.
Summary: A new type of disilene substituted by ferrocenyl
groups, 1,2-bis(ferrocenyl)disilene, was synthesized as a good
candidate for a noVel d-π electron system, and the unique
properties of the SidSi π bond of this new disilene were
reVealed. On the basis of the results of the electrochemical
analyses, 1,2-Tip2-1,2-Fc2-disilene (Tip ) 2,4,6-triisopropy-
lphenyl, Fc ) ferrocenyl) was found to be a stable fiVe-electron
redox system with four steps.
The d-π-conjugated systems, multinuclear transition-metal
complexes bridged by organic π-conjugated systems, have
attracted considerable attention from the viewpoint of their
electrochemical properties.1 Particularly, stable multistep redox
systems should be good model systems for the elucidation of
the properties of mixed-valence states.1 Therefore, there has been
much interest in ferrocene oligomers bearing a π-conjugated
spacer, e.g., FcCt CFc2 and FcNdNFc (Fc ) ferrocenyl),1b,3
as the models of mixed-valence states, while a ferrocene is
known to show unique electrochemical properties as a stable
redox system.4 On the other hand, several kinetically stabilized
disilenes have been reported5 since the first isolation of
Mes2SidSiMes2 by West and co-workers,6 showing that dis-
ilenes generally have higher HOMO and lower LUMO levels
than those of olefins due to smaller overlapping between the
3p orbitals of Si atoms.5 Such unique properties of disilenes
have prompted many chemists to explore the chemistry of novel
extended π-conjugated systems containing SidSi units from the
We chose a 2,4,6-triisopropylphenyl group (denoted as Tip)
as a steric protection group for the reactive SidSi bond on the
basis of the recent report on the synthesis of Tip2SidSi(Ph)Tip7a
as a stable crystalline compound. Dichlorosilane 2, which was
prepared by the reaction of FcLi with TipSiCl3, was reduced
by lithium naphthalenide (2.2 equiv) in THF at -78 °C to afford
the 1,2-bis(ferrocenyl)disilene 1 as orange-red crystals in 49%
* To whom correspondence should be addressed. Tel: +81-774-38-3200.
Fax: +81-774-38-3209. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp.
† Kyoto University.
‡ Waseda University.
(7) (a) Bejan, I.; Scheschkewitz, D. Angew. Chem., Int. Ed. 2007, 46,
5783. (b) Fukazawa, A.; Li, Y. M.; Yamaguchi, S.; Tsuji, H.; Tamao, K.
J. Am. Chem. Soc. 2007, 129, 14164. (Eind ) 1,1,3,3,5,5,7,7-octaethyl-s-
hydrindacen-4-yl). (c) Weidenbruch, M. Organometallics 2003, 22, 4348.
(d) Nguyen, T.-l.; Scheschkewitz, D. J. Am. Chem. Soc. 2005, 127, 10174.
(8) For examples, see: (a) Smith, R. C.; Chen, X. F.; Protasiewicz, J. D.
Inorg. Chem. 2003, 42, 5468. (b) Smith, R. C.; Chen, X. F.; Protasiewicz,
J. D. Inorg. Chem. 2003, 42, 5468. (c) Smith, R. C.; Protasiewicz, J. D.
Eur. J. Inorg. Chem. 2004, 998. For a review, see: (d) Baumgartner, T.;
Re´au, R. Chem. ReV. 2006, 106, 4681.
(1) For recent reviews, see: (a) Kaim, W.; Lahiri, G. K. Angew. Chem.,
Int. Ed. 2007, 46, 1778. (b) Kurihara, M.; Nishihara, H. Coord. Chem. ReV.
2002, 226, 125. (c) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J. Chem.
ReV. 2001, 101, 2655. (d) Distefano, A. J.; Wishart, J. F.; Isied, S. S. Coord.
Chem. ReV. 2005, 249, 507.
(2) (a) Mcmanis, G. E.; Gochev, A.; Nielson, R. M.; Weaver, M. J. J.
Phys. Chem. 1989, 93, 7733. (b) Blackbourn, R. L.; Hupp, J. T. J. Phys.
Chem. 1990, 94, 1788. (c) Powers, M. J.; Meyer, T. J. J. Am. Chem. Soc.
1978, 100, 4393.
(3) (a) Kurosawa, M.; Nankawa, T.; Matsuda, T.; Kubo, K.; Kurihara,
M.; Nishihara, H. Inorg. Chem. 1999, 38, 5113. (b) Kurihara, M.; Matsuda,
T.; Hirooka, A.; Yutaka, T.; Nishihara, H. J. Am. Chem. Soc. 2000, 122,
12373. (c) Nishihara, H. Coord. Chem. ReV. 2005, 249, 1468.
(4) (a) Ferrocenes; Togni, A., Hayashi, T., Eds.; VCH: Weinheim,
Germany, 1995,and references cited therein. (b) Yamaguchi, Y.; Ding, W.;
Sanderson, C. T.; Borden, M. L.; Morgan, M. J.; Kutal, C. Coord. Chem.
ReV. 2007, 251, 515.
(5) For recent reviews, see: (a) Kira, M.; Iwamoto, T. AdV. Organomet.
Chem. 2006, 54, 73. (b) Sasamori, T.; Tokitoh, N. In Encyclopedia of
Inorganic Chemistry, 2nd ed.; King, R. B., Ed.; Wiley: Chichester, U.K.,
2005; p 1698. (c) Weidenbruch, M. In The Chemistry of Organic Silicon
Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K.,
2001; Vol. 3, p 391.
(9) Although Niecke et al. have succeeded in the synthesis of a
marginally stable ferrocenyldiphosphene, Mes*PdPFc (Mes* ) 2,4,6-tri-
tert-butylphenyl) as a novel conjugated system, they reported that this
diphosphene undergoes ready dimerization under ambient conditions. See:
Pietschnig, R.; Niecke, E. Organometallics 1996, 15, 891.
(10) (a) Nagahora, N.; Sasamori, T.; Takeda, N.; Tokitoh, N. Chem.
Eur. J. 2004, 10, 6146. (b) Nagahora, N.; Sasamori, T.; Tokitoh, N. Chem.
Lett. 2006, 35, 220. (c) Nagahora, N.; Sasamori, T.; Watanabe, Y.;
Furukawa, Y.; Tokitoh, N. Bull. Chem. Soc. Jpn. 2007, 80, 1884. (d)
Sasamori, T.; Tsurusaki, A.; Nagahora, N.; Matsuda, K.; Kanemitsu, Y.;
Watanabe, Y.; Furukawa, Y.; Tokitoh, N. Chem. Lett. 2006, 35, 1382.
Pietschnig et al. have independently reported the synthesis of 1,1′-
bis(diphosphenyl)ferrocenes; see: (e) Moser, C.; Nieger, M.; Pietschnig,
R. Organometallics 2006, 25, 2667.
(6) West, R.; Fink, M. J.; Michl, J. Science 1981, 214, 1343.
(11) Shepherd, B. D.; West, R. Chem. Lett. 1988, 17, 183.
10.1021/om8003543 CCC: $40.75
2008 American Chemical Society
Publication on Web 06/24/2008

3326 Organometallics, Vol. 27, No. 14, 2008
Communications
Scheme 1
yield12 (Scheme 1). After heating of the toluene-d8 solution of
1 in a sealed tube at 90 °C for 10 days, no change was observed
1
in the H NMR spectra, showing the high thermal stability of
Figure 1. Structure of 1,2-bis(ferrocenyl)disilene 1. Displacement
ellipsoids were drawn at the 30% probability level.
1 in solution.
X-ray crystallographic analysis revealed that 1 has a center
of symmetry in the middle of its SidSi bond with an SidSi
bond length of 2.1733(15) Å (Figure 1, which is slightly longer
than that of Tip2SidSiTip2 (2.144 Å)13 and close to those of
the recently reported, π(phenyl)-conjugated disilenes
7b
(2.1593(16) Å for Eind(Ph)SidSi(Ph)Eind
,
2.1754 (11) Å for
Tip2SidSi(Ph)Tip7a). The Si-C(Tip) bond length (1.880(3) Å)
is slightly longer than the Si-C(Fc) bond length (1.848(3) Å),
indicating the weak conjugation between the ferrocenyl and
SidSi moieties of 1. In addition, 1 exhibits a characteristic trans-
bent structure with a trans-bent angle (θ) of 27.9°, in contrast
13
to the almost planar structure of Tip2SidSiTip2 (ca. 1°) and
Figure 2. Kohn-Sham (KS) frontier orbitals of 1 (B3PW91/Si:6-
31+G(2d) for C, H; 6-31G(d) for Fe; DZVP).
E
ind(Ph)SidSi(Ph)Eind7b (2.7°). The character of the SidSi bond
of 1 was further examined by vibrational spectra. In the solid-
state Raman spectra, 1 exhibited an intense Raman line at 595
cm-1, which should predominantly correspond to the SidSi
vibration.14 Although only a few examples have been reported
for the measurement of vibrational spectra of stable disilenes,
the SidSi vibrational frequencies generally depend on the
substituents and are observed around 500-550 cm-1.5 This
suggests a relatively strong SidSi bond in 1.
In the 29Si NMR (C6D6) spectrum, 1 showed a signal at
72.6 ppm in the low-field region characteristic of low-
coordinated silicon atoms, supporting the symmetric structure
1
Figure 3. Cyclic voltammograms of disilene 1 (V vs FcH/FcH+):
(a) oxidation region measured in 0.1 M Bu4NB(C6F5)4 in o-
dichlorobenzene (E1/2 (FcH/FcH+) ) +0.34 V vs Ag/Ag+); (b)
reduction region measured in 0.1 M Bu4NBF4 in THF (E1/2 (FcH/
FcH+) ) +0.57 V vs Ag/Ag+). The scan rate was 0.01 V/s.
and the disilene character of 1 in solution. Although the H
NMR spectrum (C6D6) of 1 suggested that the ferrocenyl and
Tip groups should freely rotate without any steric restriction
at room temperature, two doublet signals are observed for
the methyl groups (CH3(A) and CH3(B) in Scheme 1) of
o-isopropyl groups, suggesting the trans-bent structure of 1
in solution due to the diastereotopically independent methyl
groups.
UV/vis spectra of 1 (hexane) showed two characteristic
absorption maxima at 332 nm (ꢀ 5900) and 427 nm (ꢀ
24 000). TD-DFT calculations for 114 indicated the former
weak absorption should predominantly correspond to the
d(Fe)-π*(SidSi) electron transition and the latter should be
assignable to the π-π* (SidSi) electron transition, though
it was difficult to make a detailed assignment for the observed
absorbance, since several d-π* electron transitions were
estimated to be around 400-500 nm.5 The latter λmax value
for the π-π* electron transition (SidSi) is similar to those
of previously reported stable disilenes (400-440 nm),
indicating the π conjugation between the Cp and SidSi
moieties of 1 should not be effective in the electronic spectra.
As shown in Figure 2), the KS-HOMO and KS-LUMO of 1
predominantly consist of SidSi π and π* orbitals, respec-
tively, while π and d orbitals of ferrocenyl units contribute
to the frontier orbitals to some extent.14 It should be noted
that the absorption for d(Fe)-π*(SidSi) electron transitions
was clearly observed in the visible light region.
The redox behavior of 1 was revealed by the cyclic and
differential pulse voltammetric analyses (Figure 3). In the
oxidation region, 1 exhibits two-step one-electron redox waves
as reversible couples at E1/2 ) +0.05 and +0.24 V (vs FcH/
FcH+). The difference between the two redox potentials is ∆E1/2
) 0.19 V, which is comparable to that of the corresponding
carbon analogue, (E)-Ph(Fc)CdC(Fc)Ph (∆E1/2 ) 0.18 V),15a
indicating significant coupling between the two ferrocenyl
groups of 1 through the SidSi π bond, as in the case of the
carbon analogue.16 On the other hand, two-step reversible redox
(12) Experimental procedures and chemical data for the newly obtained
compounds are given in the Supporting Information.
(13) Watanabe, H.; Takeuchi, K.; Fukawa, N.; Kato, M.; Goto, M.;
Nagai, Y. Chem. Lett. 1987, 16, 1341.
(14) Theoretical calculations for 1 were performed at the (U)B3PW91/
Si:6-31+G(2d) (C, H) and 6-31G(d) levels (Fe), with DZVP (See: Godbout,
N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Can. J. Chem. 1992, 70,
560. ). The molecular orbital coefficients of HOMO of 1 dominantly consist
of 3PZ (0.339 95), 4PZ (0.245 95), and 5S (0.276 87) orbitals of Si atoms
along with 9D-2 orbitals (0.176 02) of Fe atoms. The SidSi bond length
for the optimized structure of 1 was 2.194 Å. The characteristic vibrational
mode for νSiSi was estimated as 606 cm-1
.

Communications
Organometallics, Vol. 27, No. 14, 2008 3327
couples were observed in the reduction region at E1/2 ) -2.64
(two electrons) and -3.09 V (one electron) (vs FcH/FcH+), in
contrast to the case of (E)-Ph(Fc)CdC(Fc)Ph, which was
reported to show no evident reduction wave.15a The differential
pulse voltammograms suggested that the first redox wave should
be related with two electrons and the second one should be a
one-electron-transfer process, on the basis of a comparison with
those of the FcH/FcH+ redox system. Therefore, it was
demonstrated that the ferrocenyl groups of 1 should stabilize
the corresponding anion and cation species of 1, probably due
to electronic conjugation, in contrast to the case of tetraaryl-
disilenes such as Mes2SidSiMes2 and Tip2SidSiTip2,11 which
showed only irreversible reduction wave in the cyclic voltam-
mogram. In the first step of the oxidation and reduction process
of 1, the corresponding cation radical and dianion species should
be generated, respectively. Structural optimization14 of cation
and dianion species of 1 showed the structural change of 1
through the oxidation and reduction process. The optimized
structure of [1]+ shows a slightly planar geometry (trans-bent
angle 13.3°) as compared with 1 and slightly elongated Si-Si
bond (2.259 Å), indicating the effective interaction between the
two ferrocenyl groups through the Si-Si unit in [1]+. The
second oxidation step of 1 should correspond to the elimination
of an electron from the SOMO of [1]+. On the other hand, the
highly bent structure (trans-bent angle 68.3°) and much longer
Si-Si bond length (2.487 Å) of [1]2- as compared with those
of 1 suggest that the structure of [1]2- should be interpreted in
terms of a bis silyl anion species, Tip(Fc)Si--Si-(Fc)Tip, the
HOMO of which reflects the LUMO of 1. Since the LUMO of
[1]2- should dominantly consist of π* orbitals of the Tip groups,
the second step of the reduction of 1 should occur on the π
orbitals of the Tip groups, which seem to be conjugated through
the Si-Si σ bond.17 Thus, disilene 1 was found to exhibit unique
electrochemical properties quite different from those of the
ferrocenyl-substituted olefins,15 which were reported to show
multistep redox behavior only in the oxidation region. The novel
character of disilene 1 as a stable four-step redox system with
five electrons should be interpreted in terms of the unique
electronic conjugation between the ferrocenyl and SidSi π-bond
moieties.
In summary, the 1,2-bis(ferrocenyl)disilene 1 was synthesized
and characterized as a novel d-π electron system containing
an SidSi bond. It was demonstrated that 1 exhibits unique
multistep redox behavior, indicating the possible application of
π-electron systems between heavier main-group elements toward
electrochemical materials. Further investigations on the proper-
ties of 1 are currently in progress.
Acknowledgment. This work was partially supported by
Grants-in-Aid for Scientific Research (Nos. 17GS0207,
19027024, and 18750030) and the Global COE Program
from the Ministry of Education, Culture, Sports, Science and
Technology of Japan.
Supporting Information Available: Text, tables, and figures
giving experimental procedures and spectral data for new com-
pounds and CIF files giving X-ray data for 1 and 2. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM8003543
(16) The carbon analogue, (E)-Ph(Fc)CdC(Fc)Ph, was reported to show
two reversible redox waves in the oxidation region at +0.35 and +0.53 V
(vs SCE, 0.5 × 10-3 M Bu4NB(C6F5)4, CH2Cl2),15a which should be
convertible to-0.11 and +0.07 V (vs FcH/FcH+), respectively, indicating
oxidation potentials lower than those of 1. However, it should be difficult
to draw a conclusion on the basis of these results, since the solvent effects
should not be negligible.
(15) (a) Skibar, W.; Kopacka, H.; Wurst, K.; Salzmann, C.; Ongania,
K. H.; de Biani, F. F.; Zanello, P.; Bildstein, B. Organometallics 2004, 23,
1024. (b) Bildstein, B.; Hradsky, A.; Kopacka, H.; Malleier, R.; Ongania,
K. H. J. Organomet. Chem. 1997, 540, 127. (c) Hradsky, A.; Bildstein, B.;
Schuler, N.; Schottenberger, H.; Jaitner, P.; Ongania, K. H.; Wurst, K.;
Launay, J. P. Organometallics 1997, 16, 392.
(17) The second step of the oxidation and reduction process should reflect
the HOMO and LUMO of the cation and dianion species of 1, respectively,
though the definitive conclusion of the theoretical calculations on the dication
and trianion species of 1 has not been obtained at present. Optimized
structures of the neutral, cation, and dianion species of 1 are given in the
Supporting Information.