Synthesis and reactivity of silylated tetrathiafulvalenes

Article abstract of DOI:10.1039/b803947j

Novel organosilylated tetrathiafulvalenes (TTFs) possessing Si-H or Si-Si bonds have been synthesised. The crystal structures of several derivatives have been determined by X-ray diffraction, including that of dimeric (Si ₂Me₄)(TTF)₂ (11) incorporating a diatomic SiMe₂-SiMe₂ linker. Cyclic voltammetry measurements in all cases show two oxidation waves. DFT calculations were performed to rationalize the absence of an electronic communication between the two TTF moieties of 11 through the disilanyl spacer. The reactivity of the Si-H bond has been exploited to prepare the dinuclear complex [{Ru(CO)₄}₂{μ- (Me₂Si)₄TTF}] (14), starting from Ru₃(CO) ₁₂ and TTF(SiMe₂H)₄ (1). Treatment of 14 with 2 equiv. of PPh₃ or dppm results in selective substitution of a CO ligand trans to a SiMe₂ group to afford mer-[{Ru(PPh ₃)(CO)₃}₂{μ-(Me₂Si) ₄TTF}] (13) and mer-[{Ru(CO)₃}₂(η ¹-dppm){μ-(Me₂Si)₄TTF}] (16). Attempts to transform the Si-H bonds of some TTF(SiMe₂H)_n (n = 1, 2) into Si-O functions using stoichiometric amounts of water in the presence of tris(dibenzylideneacetone)dipalladium(0) were unsuccessful. Quantitative cleavage of the C_TTF-Si bond was observed instead of formation of TTF-based-siloxanes. Essays of catalytic bis-silylation of phenylacetylene with 11 and TTF(SiMe₂-SiMe₃) (9) in the presence of Pd(OAc)₂/1,1,3,3-tetramethylbutylisocyanide failed. Again, cleavage of the C_TTF-Si bond was noticed.

Full text of DOI:10.1039/b803947j

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Synthesis and reactivity of silylated tetrathiafulvalenes†
Aure´lien Hameau,a Fabrice Guyon,*a Michael Knorr,a Christian Da¨schlein,b Carsten Strohmann*b and
Narcis Avarvaric
Received 6th March 2008, Accepted 2nd June 2008
First published as an Advance Article on the web 18th July 2008
DOI: 10.1039/b803947j
Novel organosilylated tetrathiafulvalenes (TTFs) possessing Si–H or Si–Si bonds have been
synthesised. The crystal structures of several derivatives have been determined by X-ray diffraction,
including that of dimeric (Si2Me4)(TTF)2 (11) incorporating a diatomic SiMe2–SiMe2 linker. Cyclic
voltammetry measurements in all cases show two oxidation waves. DFT calculations were performed to
rationalize the absence of an electronic communication between the two TTF moieties of 11 through
the disilanyl spacer. The reactivity of the Si–H bond has been exploited to prepare the dinuclear
complex [{Ru(CO)4}2{l-(Me2Si)4TTF}] (14), starting from Ru3(CO)12 and TTF(SiMe2H)4 (1).
Treatment of 14 with 2 equiv. of PPh3 or dppm results in selective substitution of a CO ligand trans to a
1
SiMe2 group to afford mer-[{Ru(PPh3)(CO)3}2{l-(Me2Si)4TTF}] (13) and mer-[{Ru(CO)3}2(g -
dppm){l-(Me2Si)4TTF}] (16). Attempts to transform the Si–H bonds of some TTF(SiMe2H)n (n = 1,
2) into Si–O functions using stoichiometric amounts of water in the presence of
tris(dibenzylideneacetone)dipalladium(0) were unsuccessful. Quantitative cleavage of the CTTF–Si bond
was observed instead of formation of TTF-based-siloxanes. Essays of catalytic bis-silylation of
phenylacetylene with 11 and TTF(SiMe2-SiMe3) (9) in the presence of Pd(OAc)2/1,1,3,3-
tetramethylbutylisocyanide failed. Again, cleavage of the CTTF–Si bond was noticed.
were the introduction of chalcogens in a peripheral position and
the synthesis of TTF derivatives containing extended p-electron
Introduction
conjugation.6 The incorporation of silyl groups has also been
explored, but to a lesser extend compared to thiolate or thioether.
The synthesis of flexible7 or rigid bis(tetrathiafulvalene)8 con-
taining dimethylsilyl linkers has been reported, the tetrasilylated
derivative TTF(SiMe3)4 has been obtained under high-pressure
conditions by cycloaddition of bis(trimethylsilylacetylene) with
CS2,9 while mono-silylated derivatives have been isolated starting
from mono-lithiated TTF.10 Recently we described the synthesis of
functionalized TTFs with SiMe2–H moieties11 and detailed studies
on a series of functionalized sulfur systems.12 The reactivity of the
Si–H bond for oxidative addition reactions has been exploited to
link covalently a Pt(0) centre with a TTF moiety.11 In the last
decade, there has been an increasing interest in functionalized
TTFs with substituents allowing coordination to transition metal.
Besides the quest for materials of high dimensionality, such
molecules are particularly attractive for the elaboration of hybrid
organic/inorganic multifunctional materials.13 The first TTF tran-
sition metal complexes were obtained by the covalent coordination
of metal fragments to tetrathiolate tetrathiafulvalene (TTFS44−).14
Since that time, in addition to the above-mentioned synthesis
of TTF-based silyl complexes, new strategies consisting of the
synthesis of TTF incorporating a potentially coordinating func-
tion such as thioether,15 phosphane,16 pyridine,17 acetylacetone18
or oxazoline19 have been developed.
The remarkable properties of the “venerable” tetrathiafulvalene
(TTF, Scheme 1) (reversible redox processes at relatively low
potential, stacking in the solid state) continue to focus interest
not only for the synthesis of conducting organic metals1 but also
for alternative applications2 such as sensors3 or molecular motors.4
Scheme 1
Until the 1990’s, development of TTF chemistry concerned
mainly the modification of the skeleton with the aim of increasing
the dimensionality of the charge transfer materials and conse-
quently their electrical conducting properties.5 Among the variety
of chemical modifications realized, the most exploited strategies
aInstitut UTINAM, UMR CNRS 6213, Universite´ de Franche-Comte´,
Faculte´ des Sciences et des Techniques, 16, Route de Gray, 25030, Besanc¸on,
France. E-mail: fabrice.guyon@univ-fcomte.fr
bInstitut fu¨r Anorganische Chemie der Universita¨t Wu¨rzburg, Am Hubland,
D-97074, Wu¨rzburg, Germany. E-mail: c.strohmann@mail.uni-wuerzburg.
de
cLaboratoire Chimie et Inge´nierie Mole´culaire (CIMA), UMR 6200, 2
boulevard Lavoisier, 49045, Angers, France
The reactivity of Si–H bond being not restricted only to
platinum,20 an extension to other metal centres appeared as a
logical continuation to our previous studies. The presence of
one or more Si–H bonds on silyl substituted TTFs seemed also
promising for the preparation of TTF-based siloxanes since it can
1
† Electronic supplementary information (ESI) available: 29Si{ H} and 2D-
HMQC spectra of 8 and details on computational studies performed on
compounds 11 and 12. CCDC reference numbers 671046, 671047, 680221
and 680222. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/b803947j
4866 | Dalton Trans., 2008, 4866–4876
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©

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be easily transformed into a Si–O function under mild conditions.21
Moreover, the hydrosilylation reaction across unsaturated organic
substrates may allow the formation of other silylated TTF
derivatives.22 Besides the intrinsic photophysical properties of the
Si–Si bond,23 functionalization of TTFs with these moieties is
particularly of interest since it may be an alternative strategy
for the synthesis of original silylated TTFs. Indeed, the catalytic
activation of the Si–Si bond of disilanes by transition metals
is an elegant route for the bis-silylation of unsaturated organic
substrates such as alkynes, olefins, isocyanides and aldehydes.24
The synthetic potential of these above-mentioned reactions has
motivated us to continue our previous studies on silylated TTFs.
We report in this paper on the preparation and characterization
of novel compounds with Si–H or Si–Si bonds and the study of
their reactivity.
Results and discussion
Synthesis of silylated TTF derivatives
Scheme 2
We recently described the synthesis of tetra-silylated tetrathiaful-
valene TTF(SiR2H)4 (R = Me: 1, Ph: 2)11b by a classical one-
pot method used for the preparation of substituted TTF, which
takes advantage of the acidity of the TTF hydrogen atoms.25
The silylation was achieved by reaction at −78 ◦C between the
appropriate chlorosilane and TTFLi4 generated by deprotonation
of TTF with a slight excess of LDA (lithium diisopropylamide, 5
equiv.). Using a similar methodology, mono-substituted TTFs (3–
5) and di-substituted diphenyltetrathiafulvalene with a functional
group attached to the 5-membered TTF core (6–7) have been
prepared in order to gain insight into the potentialities of TTF
functionalized by SiR2H groups (Scheme 2). All these compounds
were isolated as orange-red crystalline powders which are air-
stable for prolonged periods. Note that compounds 6 and 7
were obtained using as starting material commercially available
(Aldrich) diphenyltetrathiafulvalene which is considered to be a
mixture of cis- and trans-isomers. Previous studies have shown that
the trans product is predominant26 and an X-ray crystallography
study on 7 reveals a trans structure (see below). The IR spectra
of all these compounds present a broadened m(Si–H) vibration
in the range between 2150–2180 cm−1. TTF derivatives 8 and
9 possessing Si–Si bonds were respectively prepared by reaction
of mono-lithiated TTF and di-lithiated dipenyltetrathiafulvalene
with chloropentamethyldisilane.
isolated after purification was identified by elemental and GC/MS
analysis to be a mixture of cis/trans bis(disilanyl)tetrathiafulvalene
(10a and 10b in Scheme 3). A 75 : 25 ratio has been determined,
but the spectroscopic data do not allow unambiguous assign-
ment to the corresponding isomer (see Experimental section).
The bis(TTF) compound 11, linked by a Si–Si bond, was
obtained by treating 2 equiv. of mono-lithiated TTF with 1,2-
dichlorotetramethyldisilane in 55% yield in the form of a yellow
crystalline solid. As expected, the 29Si NMR spectrum of 11
displays a single singlet resonance at d −22.8, whereas two
resonances are observed in the spectra of 8–10 due to the presence
of two non-equivalent silicon atoms.
Crystal structures of some silyl- and disilany-TTFs
The suggested molecular structures of compounds 2 (Fig. 1), 7
(Fig. 2), 9 (Fig. 3) and 11 (Fig. 4) have also been confirmed by X-
ray diffraction studies. The TTF derivatives 2, 7 and 9 present an
=
inversion centre located at the middle of the central C C bond of
the TTF core. The TTF moieties in these molecules deviate only
slightly from planarity giving rise to a chair-like conformation.
In contrast, the TTF moieties adopt a boat-like conformation in
compound 11, the midpoint of the bridging Si–Si bond being an
inversion centre. Except for this conformational view, the nature of
the substituents on the TTF core has no significant influence on the
Attempts to prepare the tetrakis-disilanyl derivative starting
from tetralithiated tetrathiafulvalene failed. The yellow powder
Scheme 3
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Fig. 3 Molecular structure of 9. Hydrogen atoms are omitted for clarity.
◦
˚
Selected bond lengths [A] and angles ( ): C(1)–C(1)# 1.331(4), C(1)–S(1)
1.7548(17), C(1)–S(2) 1.7506(19), S(2)–C(3) 1.7606(17), S(1)–C(2)
1.7542(18), C(2)–C(3) 1.345(2), C(2)–Si(1) 1.8891(18), Si(1)–Si(2)
2.3361(8); C(1)#–C(1)–S(1) 122.63 (19), C(1)#–C(1)–S(2) 123.83(18),
S(1)–C(1)–S(2) 113.54(10), C(1)–S(1)–C(2) 96.96(8), C(1)–S(2)–C(3)
95.58(8), S(1)–C(2)–C(3) 115.41(13), S(2)–C(3)–C(2) 118.24(14), S(1)–
C(2)–Si(1) 114.30(9), C(2)–Si(1)–Si(2) 109.66(6). Symmetry operation used
to generate equivalent atoms: #: −x + 2, −y, −z + 1.
Fig.
1
Molecular structure of 2. Hydrogen atoms are omit-
ted for clarity. Selected bond lengths [A] and angles (◦): C(3)–
C(3)# 1.342(4), C(3)–S(1) 1.756(2), C(3)–S(2) 1.759(2), S(1)–C(1)
1.763(2), S(2)–C(2) 1.764(2), C(2)–C(1) 1.350(3), C(1)–Si(1) 1.880(2),
C(2)–Si(2) 1.881(2); C(3)#–C(3)–S(1) 123.1(2), C(3)#–C(3)–S(2) 123.7(2),
S(1)–C(3)–S(2) 113.23(11), C(3)–S(1)–C(1) 96.57(9), C(3)–S(2)–C(2)
96.45(10), S(2)–C(2)–C(1) 116.68(16), S(1)–C(1)–C(2) 116.71(16), S(2)–
C(2)–Si(2) 116.51(11), S(1)–C(1)–Si(1) 116.44(11), C(1)–C(2)–Si(2)
126.45(16), C(2)–C(1)–Si(1) 126.82(16). Symmetry operation used to
generate equivalent atoms: #: −x, −y, 1 − z.
˚
Fig. 4 Molecular structure of 11. Hydrogen atoms are omitted for clarity.
◦
˚
Selected bond lengths [A] and angles ( ): C(4)–C(3) 1.349(7), C(4)–S(3)
1.748(5), C(4)–S(4) 1.744(5), S(4)–C(6) 1.719(7), S(3)–C(5) 1.715(6),
C(6)–C(5) 1.312(9), C(3)–S(1) 1.754(5), C(3)–S(2) 1.741(5), S(2)–C(2)
1.738(5), S(1)–C(1) 1.758(5), C(2)–C(1) 1.346(7), C(1)–Si(1) 1.862(5),
Si(1)–Si(1)# 2.327(3); C(3)–C(4)–S(3) 123.0(4), C(3)–C(4)–S(4) 122.5(4),
S(4)–C(4)–S(3) 114.4(3), C(4)–S(4)–C(6) 94.1(3), C(4)–S(3)–C(5) 94.4(3),
S(4)–C(6)–C(5) 118.3(5), S(3)–C(5)–C(6) 118.1(6), C(4)–C(3)–S(1)
121.5(4), C(4)–C(3)–S(2) 123.9(4), S(2)–C(3)–S(1) 114.5(3), C(3)–S(1)–
C(1) 96.0(2), C(3)–S(2)–C(2) 95.1(3), S(1)–C(1)–C(2) 115.1(4),
S(2)–C(2)–C(1) 119.2(4), S(1)–C(1)–Si(1) 119.2(3), C(1)–Si(1)–Si(1)#
107.34(18). Symmetry operation used to generate equivalent atoms:
#: −x + 2, −y, −z + 1.
Fig. 2 Molecular structure of 7. Hydrogen atoms are omitted for clarity.
◦
˚
Selected bond lengths [A] and angles ( ): C(1)–C(1)# 1.326(12), C(1)–S(1)
1.743(7), C(1)–S(2) 1.768(7), S(2)–C(2) 1.750(7), S(1)–C(3) 1.762(7),
C(2)–C(3) 1.365(9), C(2)–Si(1) 1.872(7); C(1)#–C(1)–S(1) 124.6(7),
C(1)#–C(1)–S(2) 121.5(8), S(2)–C(1)–S(1) 113.9(4), C(1)–S(1)–C(3)
96.3(3), C(1)–S(2)–C(2) 96.0(3), S(2)–C(2)–C(3) 116.9(5), S(1)–C(3)–C(2)
116.5(5), S(2)–C(2)–Si(1) 115.3(4). Symmetry operation used to generate
equivalent atoms: #: 1 − x, 2 − y, 1 − z.
structural parameters of the TTF unit. For these four compounds,
˚
the C–Si distances lie in the range between 1.86–1.89 A and are
7
˚
similar to those reported for (TTF)2SiMe2 [1.868 A] and 1 [1.876
and 1.883 A]. The Si–Si bond lengths are almost identical in 9
and 11 [2.336(1) and 2.327(3) A respectively] and similar to the
11b
˚
˚
values generally observed for this bond in non-sterically-crowded
organosilanes.27 The coordination geometry around each Si atom
is essentially tetrahedral. In compound 11, the two TTF moieties
are almost parallel, with the Si–Si bond which is normal to the
TTF planes. Worth mentioning are the intermolecular interactions
observed in the solid state packing diagram of molecule 11 with
˚
short S · · · S contacts of 3.518 and 3.705 A (Fig. 5). Due to
the steric hindrance of the phenyl substituent, no significative
intermolecular interactions deserving a detailed discussion were
noticed in the case of the other derivatives.
˚
Fig. 5 View of the bc plane in 11 showing short S · · · S contacts of 3.518 A
(dashed lines).
solution at a scan rate of 100 mV s−1. The results are collected in
Table 1. It should be noted that two reversible redox processes are
observed for diphenyltetrathiafulvalene in both solvents whereas
the second wave of TTF is found to be irreversible in CH2Cl2
Electrochemistry
The redox properties of each new compound have been investi-
gated by cyclic voltammetry in dichloromethane and acetonitrile
4868 | Dalton Trans., 2008, 4866–4876
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Table 1 Electrochemical data: reference electrode AgClO4 (0.1 M in
simultaneously, followed by a further oxidation to generate the
tetracationic species (Fig. 6). This finding contrasts to what is
noticed for the (TTF)2(SiMe2) (12) incorporating a monoatomic
–SiMe2– spacer, since three oxidation steps are observed in this
case.7 Note that a recent study devoted to dinuclear ferrocenes of
type [{Fe(Cp*)(C5H4)}2X] (X = CMe2, SiMe2, GeMe2, Si2Me4;
Cp* = C5Me5) has shown that the electronic coupling is more
important with the CMe2 linker than with the SiMe2 bridge.29
This has been rationalized by the closer approach of the two
ferrocenes (Fcs) due to the shorter Fc–C–Fc separation compared
to the Fc–Si–Fc separation. The CV data presented in this
report indicate a single wave in the case of the disilanyl-bridged
compound [{Fe(Cp*)(C5H4)}2Si2Me4], ruling out a significant
interaction between the two redox centres. These findings are
somewhat contradictory with another electrochemical study of
ferrocenyloligosilane of type [{Fe(Cp)(C5H4)}2(SiMe2)n].30 In the
latter example, electrochemical communication was established
even across a trisilanyl spacer.
MeCN)/Ag
Compound
Solvent
E11/2/V
E21/2/V
TTF
CH2Cl2
MeCN
CH2Cl2
MeCN
CH2Cl2
MeCN
CH2Cl2
MeCN
CH2Cl2
MeCN
CH2Cl2
MeCN
CH2Cl2
MeCN
CH2Cl2
MeCN
CH2Cl2
MeCN
CH2Cl2
MeCN
CH2Cl2
CH2Cl2
+0.07
−0.01
+0.13
+0.025
+0.01
−0.05
+0.07
+0.02
−0.02
−0.10
+0.02
+0.01
+0.10
Insoluble
+0.01
−0.07
−0.02
−0.04
−0.03
−0.08
−0.08
−0.23
Irreversible
+0.36
+0.62
+0.375
+0.54
+0.33
+0.57
+0.38
+0.52
+0.28
+0.53
+0.39
+0.56
Ph2TTF
1
2
4
6
7
8
Irreversible
+0.32
9
+0.50
+0.35
11
Irreversible
+0.36
Electronic structure of silyl-bridged TTFs
14
15
+0.50
+0.41
According to the experimental CV results of the disilyl-bridged
compound 11, there is no electronic communication between the
two TTF units in this molecule. Contrary to this, Fourmigue´
et al. observed a considerable communication in (TTF)2SiMe2
(12), where both TTF units are separated by only one silyl group.7
Most recently, a communication between the two TTF units has
also been reported in the “double” mono-silyl bridged system 13.8
Thus, the two TTF units are able to communicate with each other
if there is only one silicon spacer in-line between them (12 and 13)
whereas a communication appears impossible with two spacers
in-line between the TTF units of 11 (Scheme 4). Therefore the
question arises, what are the origins for the divergent electrochem-
ical behaviour of the TTF compounds depicted in Scheme 4? In
the early 1990’s, a sandwich structure with some degree of sharing
of the p-electrons between the two TTF units was postulated to
explain the stabilization of the mono-radical cation in flexible TTF
dimers.31 However, extended Hu¨ckel calculations performed on
(TTF)2X (X = SiMe2, Hg, S, PPh) dimers interpreted the splitting
of the oxidation waves as due rather to Coulombic repulsions
between the two TTF units and not arising from through space
or bond orbital overlap.7 In this study, we have re-examinated this
problem using DFT calculations at the B3LYP/6-31+G(d) level32
on compounds 11 and 12.
solution, as already mentioned by Wudl et al.28 The silylated TTFs
present the same features as their parent compounds (Fig. 6).
Substitution of the TTF hydrogen by SiPh2H groups induces no
significant change in the potential of the two redox processes.
However, they are lowered by ca. 0.05 V per Si2Me5 substituent
introduced in the case of 8 and 9. No oxidative cleavage of the Si–
Si bond is observed for the complexes 8–11 in the potential range
explored. According to the electrochemical data, no electronic
coupling between the two redox units for disilanyl compound
11 occurs. Indeed, only two distinct oxidation waves are present,
indicating that monocationic and dicationic species are formed
Fig. 6 Cyclic voltammograms of 8 (top) and 11 (5 scans) recorded in
MeCN solution on a platinum electrode at a scan rate of 100 mV s−1
reference electrode AgClO4 (0.1 M in MeCN)/Ag.
;
Scheme 4 Electronic communication between the TTF units of 11–13.
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The starting coordinates were constructed using the solid state
structures. In addition to the conformer found in the crystal,
a second energetically comparable conformer exists for 11 and
12, respectively, where the TTF units are twisted against each
other (other imaginable conformers were neglected as the energy
of these conformers is too high). Since the second conformer
shows similar results to the dominant one (in both cases), we limit
our discussion to the one found in the crystal. Fig. 7 visualizes
the calculated frontier orbitals (LUMO, HOMO and HOMO−1)
of the investigated compounds. DFT studies on the same level
significantly separated from the one dominant in 11. To conclude,
a communication between spatial separated TTF units is possible
if they are connected over the spacer by a low lying, suitable,
unoccupied molecular orbital which is capable of accepting an
excited electron.
Activation of the Si–H bond for oxidative addition across Ru(0)
We have previously shown that 1 reacts by oxidative addition
with 2 equiv. of [Pt(C2H4)(PPh3)2] in toluene to afford the
dinuclear diplatinum complex [{Pt(PPh3)2}2{l-(Me2Si)4TTF}].11b
In order to develop further the organometallic coordination
chemistry of this novel type of silyl complexes, we have in-
vestigated the reactivity of [Ru3(CO)12] towards 1. The pio-
neering work of the research groups of Stone and Graham
has demonstrated that silanes R3SiH oxidatively add across
[Ru3(CO)12] or [Ru(CO)5] to afford first hydridosilyl complexes
of the type [Ru(H)(SiR3)(CO)4]. The latter may then evolve
photochemically or thermally in the presence of excess of R3SiH to
bis(silyl) complexes [Ru(SiR3)2(CO)4], with concomitant extrusion
of H2.33 This route has quite recently been applied to the
preparation of bis(silyl) complexes chelated by the ligand (9,9-
dimethylxanthene-4,5-diyl)bis(dimethylsilyl).34 Fink has shown
that treatment of [Ru3(CO)12] with tetrakis(dimethylsilyl)benzene
C6H2(SiMe2H)4 produces in a one-pot reaction the dinuclear com-
pound [{Ru(CO)4}2{l-(Me2Si)4C6H2}], in which the two ruthe-
nium(II) tetracarbonyl units are linked through the bis(chelating)
silylated benzene spacer.35 In a similar manner, reaction of 0.67
molar equiv. of [Ru3(CO)12] with 1 in cyclohexane at 65 ◦C provided
[{Ru(CO)4}2{l-(Me2Si)4TTF}] (14) in the form of a stable orange-
brown microcrystalline powder in 72% yield. The proposed struc-
ture of 14 was ascertained by IR spectra, NMR and elemental ana-
lysis. The presence of four distinct m(CO) vibrations (2102 m,
2044 s, 2033 vs, 2024 vs), whose positions and intensities are quite
similar to those reported for cis-[Ru(Me2SiCH2CH2SiMe2)(CO)4]
and cis-[Ru(SiMe3)2(CO)4], confirms the local C2v symmetry
around each octahedral Ru(SiR3)2(CO)4 fragment. The proton
NMR spectrum recorded in CDCl3 consists of just one singlet at
d 0.33 indicating the equivalence of the four SiMe2 groups. Upon
prolonged heating a solution of 14 in cyclohexane in the presence
of 2 equiv. of PPh3, selective substitution of a CO ligand trans
to a SiMe2 group occurred to yield mer-[{Ru(PPh3)(CO)3}2{l-
(Me2Si)4TTF}] (15) (Scheme 5). The meridional carbonyl ar-
rangement is deduced from the IR spectrum, which displays
three m(CO) vibrations at 2054 m, 2000 s and 1986 vs cm−1.36
•
also showed that the frontier orbitals of radical cations 11+
•
and 12+ are comparable to the neutral TTF derivatives. The
basic requirement for an electronic communication of spatial
separated TTF units is the presence of a low lying frontier
orbital, capable of electron transfer. For this electron transfer, the
relevant frontier orbital has to connect the different parts of the
molecule to each other. In both systems, HOMO−1 and HOMO
are analogous, principally located at the TTF units. The crucial
and important difference between both compounds concerns
the lowest unoccupied molecular orbital. In the disilyl-bridged
compound 11, the LUMO is exclusively located at the TTF units
without any part at the –SiMe2SiMe2– bridge. In contrast, the
LUMO of 12 connects both TTF units over the monosilyl-bridge
through space. Due to this decisive difference, an electron transfer
is possible from the first TTF unit along the silicon bridge to the
second TTF unit. This transfer is facilitated since the LUMO is
close in energy to the occupied orbitals (e.g. HOMO).
1
The H NMR spectrum gives rise to two SiMe2 resonances at
4
d 0.13 and 0.61. Due to a trans-coupling JP,H of 2.4 Hz, the
signal at d 0.13 is split into a doublet. Addition of 2 equiv.
Fig. 7 Presentation of the frontier orbitals (HOMO−1, HOMO and
LUMO) of the two di-TTF derivates 11 (left) and 12 (right).
1
of dppm afforded, as the main compound, mer-[{Ru(CO)3(g -
1
dppm)}2{l-(Me2Si)4TTF}] (16), possessing two pendant g -dppm
To corroborate these results, we compared our theoreti-
cal findings with the one very recently performed on the
rigid (o-DMTTF)2(SiMe2)2 (o-DMTTF = ortho-dimethyltetra-
thiafulvalene) using DFT calculations with the B-P86 exchange–
correlation functional.8 In this case, only one main conformer
exists since the two TTF units are fixed by two –SiMe2– bridges.
The TTF units are able to communicate with each other (like in 12)
as the LUMO of 13 is spread over both –SiMe2– units. Therefore,
the electronic situation of 12 and 13 can be compared and
ligands (Scheme 5). The crude product was contaminated by small
amounts (ca. 15%) of another uncharacterized complex displaying
1
a singlet resonance at d −5.3 in the 31P{ H} spectrum. Analytically
pure 16 was separated from this byproduct by recrystallisation.
The 1H NMR spectrum of 16 is similar to that of 15 with two sets
1
of SiMe2 groups. The 31P{ H} spectrum proves unambiguously
the coordination of a phosphorus atom to ruthenium at d 21.4 Hz,
whereas the dangling phosphorus gives rise to a doublet at d =
−25.0 Hz, a region typical for non-coordinated dppm phosphorus.
4870 | Dalton Trans., 2008, 4866–4876
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Scheme 5
Attempts to form siloxanes by conversion of Si–H to Si–O
Both doublets are coupled through a 2JP–P coupling of 90 Hz. These
1
data also match well with those reported for [Ru(CO)4(g -dppm)]
Kawakami et al. have recently reported that the catalytic
cross-dehydrocoupling polymerization of 1,4-bis(dimethylsilane)-
benzene under mild reaction conditions leads to a poly(carbo-
siloxane) chain (Scheme 7).21 In light of this result, it seemed
promising to probe this reaction with our silylated TTFs.
Compounds 3, 4, 6 and 7 were reacted with stoichiometric
amounts of water in the presence of tris(dibenzylideneacetone)-
dipalladium(0)–chloroform adduct (Pd2(dba)3) in THF. In all
cases, IR spectroscopy confirms the disappearance of the Si–
(d 39.8 and −25.7; 2JP–P = 91 Hz).37
CV experiments performed in CH2Cl2 at
a scan rate
of 100 mV s−1 on compounds 14 and 15 evidence the
same electrochemical behaviour as previously encountered for
[{Pt(PPh3)2}2{l-(Me2Si)4TTF}]. For both systems, two reversible
oxidation waves are observed. No additional oxidation involving
the RuII centres is noticed below 1.2 V (vs. Ag+/Ag). However,
the potential depends strongly on the nature of the metallic
fragment as already observed with carbonyl complexes chelated
with tetrakis(diphenylphosphino)tetrathiafulvalene.16 Changing
the metallic fragment ligated on the [TTF(Me2Si)4]4− entity from
[Pt(PPh3)2]2+ to [Ru(CO)4]2+ species induces a shift of 0.3 V towards
anodic potential. Moreover, this study evidences that the half-wave
potential can be fine-tuned by substituting a carbonyl by a more
electron-donating phosphane ligand. Indeed, there is a significant
shift of the anodic waves from 14 to 15 (Table 1).
1
H bond. However, melting point and a H NMR study of the
resulting materials reveal the quantitative cleavage of the CTTF
–
Si bond and the formation of unsilylated TTFs according to a
protodesilylation reaction (Scheme 7).
Attempts to activate the Si–Si bond for double silylation reactions
The activation of the silicon–silicon bond has been studied
extensively in the past and among the numerous reactions which
can be performed, the bis-silylation of alkynes has attracted
much attention (Scheme 6). We have recently reported on the
formation of bis-silylated olefins by catalytic cleavage of the Si–
Si bond of 1,1,2,2-tetramethyl-1,2-bis(phenylthiomethyl)disilane
in the presence of phenylacetylene, diethynylbenzene or 4-
ethynyl[2.2]paracyclophane.38 Using the same methodology, we
reacted compounds 8 and 11 with phenylacetylene in the presence
of Pd(OAc)2/1,1,3,3-tetramethylbutylisocyanide as catalyst. To
our disappointment, monitoring the reactions by 29Si and 1H
NMR evidences no formation of bis-silylated olefin but a partial
cleavage of the CTTF–Si bond (Scheme 6). Note that there is a
precedent in the literature where C–Si cleavage is preferred over
Si–Si rupture.39
Scheme 7
Conclusion
We have shown that TFFs can be easily functionalized with several
SiR2H groups and have prepared the first TTF-representatives
bearing disilanyl units. This synthetic part is completed by some
crystallographic and electrochemical studies. The absence of an
electrochemical communication between the two TTF units in the
disilanyl-bridged compound 11 has been rationalized by means of
DFT calculations. Unfortunately, the a priori promising concept
to combine TTF chemistry with the literature-known reactivity
of the Si–Si bond suffers from a serious drawback. Instead of a
transition metal-mediated activation of the Si–Si bond, homolytic
cleavage of the CTTF–Si bond is noticed. Similarly, several attempts
to convert the Si–H function of SiMe2H bearing TTFs to siloxane
Si–O–Si failed. Again, rupture of the CTTF–Si bond is evidenced.
However, oxidative addition of the Si–H function across low-
valent transition metals works straightforwardly and allows the
preparation of stable dinuclear platinum(II) and ruthenium(II)
Scheme 6
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bis(silyl) chelate complexes. The later octahedral complex readily
loses one meridional carbonyl ligand in the presence of PR3. No-
(472.86): C, 55.88; H, 5.12; S, 27.12. Found: C, 55.81; H, 5.09;
1
S, 27.53%. H NMR (298 K, CDCl3): d 0.09 (d, 12H, SiCH3,
1
tably the complex mer-[{Ru(CO)3(g -dppm)}2{l-(Me2Si)4TTF}]
3JH,H = 3.92 Hz), 4.34 (hept, 2H, SiH, 3JH,H = 3.92 Hz), 6.95–7.55
(16), which may be considered as a metalloligand,40 is a very
promising precursor to assemble further metal centres for the
construction of polymetallic TTFSi4-bridged arrays. Although
the aim of the present work rather concerns organometallic
chemistry than materials science, future electrocrystallization
studies will reveal whether disilanyl-funtionalized TTFs may find
an application as precursors in this domain.
(m, 10H, Har). mp 115–117 ◦C.
Ph2TTF(SiPh2H)2 (7). Prepared in an analogous manner to
6 starting from Ph2TTF and chlorodiphenylsilane except the
crystallisation which was performed using dichloromethane (50%
yield). Anal. calcd for C42H32S4Si2 (721.10): C, 69.96; H, 4.47; S,
1
17.79. Found: C, 69.23; H, 4.40; S, 18.59%. H NMR (298 K,
1
C6D6): d 5.31 (m, 2H, SiH, JSi,H= 215 Hz), 7.95–7.35 (m, 30H,
◦
1
Harom). 29Si-{ H} NMR (C6D6): d −19.16. mp 231–233 C.
Experimental section
TTF(Si2Me5) (8). LDA (5.8 mmol) was added dropwise to a
solution of TTF (1.08 g, 5.28 mmol) in diethyl ether (50 mL) at
−78 ◦C. After stirring for 1.5 h, 1.5 equiv. of pentamethylchloro-
disilane (1.52 mL, 7.92 mmol) were added, then the temperature
was allowed to rise slowly overnight up to room temperature.
After filtration and evaporation of the solvent, the crude product
was then warmed under reduced pressure (40 ◦C under 12 mbar)
to eliminate the excess of pentamethylchlorodisilane. The yellow
powder was purified by a first crystallisation from hexane (10 mL)
at 0 ◦C to recuperate 130 mg of TTF; a second crystallisation
from heptane (15 mL) at −30 ◦C permitted the isolation of the
target compound (970 mg, 55% yield). Anal. calcd for C11H18S4Si2
(334.70): C, 39.47; H, 5.42; S, 38.32. Found: C, 39.61; H, 5.30; S,
Preparation of compounds
All reactions were performed in Schlenk-tube flasks under purified
nitrogen. Solvents were dried and distilled prior to use by standard
procedures. LDA (1.8 M or 2.0 M in THF–heptane–ethylbenzene),
TTF and Ph2TTF were purchased from Aldrich while Me3TTF
was prepared according to a published procedure.41 Chlorosilanes
have been purchased from Wacker Chemie. Syntheses of com-
pounds 1 and 2 have been already reported elsewhere.11b
TTF(SiMe2H) (3). LDA (0.85 mmol) was added dropwise
to a solution of TTF (160 mg, 0.8 mmol) in diethyl ether
(15 mL) at −78 ◦C. After stirring for 1.5 h, 1.05 equiv. of freshly
distilled chlorodimethylsilane (100 lL, 0.85 mmol) were added
and the reaction was allowed to warm slowly to room temperature
overnight. Filtration and evaporation under reduced pressure of
the solvent gave a red oil which was purified by precipitation from
hexane at −60 ◦C (139 mg, 62% yield). Anal. calcd for C8H10S4Si
(262.94): C, 36.60; H, 3.84; S, 48.86. Found: C, 36.58; H, 3.81; S,
1
38.14%. H NMR (298 K, C6D6): d 0.06 (s, 9H, SiCH3), 0.12 (s,
6H, SiCH3), 5.43 (AB system, 2H, Ha, Hb, 3JAB = 6.5 Hz), 5.72 (s,
1
1H). 29Si-{ H} NMR (C6D6): d −19.16 (SiCH3), −22.37 (SiCH3).
13C NMR (C6D6): d 136.29, 124.10, 119.21, 119.06, 113.39, 109.13,
−2.34, −3.80. mp 61–62 ◦C.
Ph2TTF(Si2Me5)2 (9). To a solution of Ph2TTF (1.02 g,
2.8 mmol) in diethyl ether (60 mL) at −78 ◦C were added 2.2 equiv.
of LDA. The reaction temperature was allowed to rise slowly to
1
3
47.98%. H NMR (298 K, C6D6): d 0.54 (d, 6H, SiCH3, JH,H
=
3
3.7 Hz), 4.48 (hept, 1H, SiH, JH,H = 3.7 Hz), 5.39 (AB system,
2H, 3JAB = 6.3 Hz), 5.68 (s, 1H).
◦
−15 C and the mixture was stirred at this temperature for 4 h.
Pentamethylchlorodisilane (3 equiv., 1.65 mL, 8.58 mmol) was
added after renewed cooling at −78 ◦C, then the temperature was
allowed to rise slowly overnight up to room temperature. Filtration
and evaporation of all volatiles under reduce pressure (45 ◦C under
10 mbar) yielded 9 as a red powder which was crystallized from a
mixture of heptane–diethyl ether (V: 20/15) at −30 ◦C (967 mg,
56% yield). Anal. calcd for C28H40S4Si4 (617.2): C, 54.49; H, 6.53;
Me3TTF(SiMe2H) (4). Prepared in an analogous manner to
3 starting from Me3TTF (36% yield). Anal. calcd for C11H16S4Si
(304.59): C, 43.38; H, 5.29; S, 42.11. Found: C, 43.49; H, 5.26; S,
42.06%. 1H NMR (298 K, C6D6): d 0.54 (d, 6H, SiMe2, 3JH,H = 3.9
Hz), 2.30 (s, 3H, CH3) 2.31 (s, 3H, CH3), 2.35 (s, 3H, CH3), 4.40
(hept, 1H, SiH, 3JH,H = 3.9 Hz, 1JH,Si = 196 Hz).
1
Me3TTF(SiPh2H) (5). Prepared in an analogous manner to
3 starting from Me3TTF and chlorodiphenylsilane (41% yield).
Anal. calcd for C21H20S4Si (304.59): C, 58.83; H, 4.70; S, 29.92.
Found: C, 58.85; H, 4.70; S, 30.07%. 1H NMR (298 K, CDCl3): d
2.15 (s, 3H, CH3) 2.17 (s, 3H, CH3), 2.33 (s, 3H, CH3), 5.40 (s, 1H,
S, 20.78. Found: C, 54.53; H, 6.38; S, 20.59%. H NMR (298 K,
C6D6): d 0.05 (s, 12H, SiCH3), 0.07 (s, 18H, SiCH3), 6.96–7.32 (m,
1
10H, Har). 29Si-{ H} NMR (C6D6): d −18.02 (SiCH3), −22.77
(SiCH3). 13C NMR (C6D6): d 131.80, 131.78, 130.86, 130.45,
130.40, 130.19, 128.33, 0.01, −0.29. mp 158–159 ◦C.
1
1
SiH, JH,Si = 208 Hz), 7.53–7.94 (m, 10H, Har). 29Si-{ H} NMR
TTF(Si2Me5)2 (10a/b). To
a solution of TTF (0.5 g,
(C6D6): d −19.35.
2.44 mmol) in diethyl ether (30 mL) at −78 ◦C were added 4.5
equiv. of LDA. The reaction temperature was allowed to rise
slowly to −15 ◦C and the mixture was stirred at this temperature
for 4 h. Pentamethylchlorodisilane (6 equiv., 2.82 mL, 14.6 mmol)
was added after renewed cooling at −78 ◦C, then the temperature
was allowed to rise slowly overnight up to room temperature.
Filtration and evaporation of all volatiles under reduce pressure
(45 ◦C under 10 mbar) yielded 1.45 g of a red oil which was purified
by precipitation from pentane at −80 ◦C to afford a mixture of cis
and trans isomers (535 mg, 47% yield). Anal. calcd for C16H32S4Si4
(465.03): C, 41.32; H, 6.94; S, 27.58. Found: C, 41.23; H, 6.99; S,
Ph2TTF(SiMe2H)2 (6).
A solution of Ph2TTF (0.3 g,
0.84 mmol) in THF (10 mL) was treated at −78 ◦C with 2.2 equiv.
of LDA. The reaction temperature was allowed to rise slowly to
◦
−15 C and the mixture was stirred at this temperature for 4 h.
Chlorodimethylsilane (3 equiv. 280 lL, 2.52 mmol) was added after
renewed cooling at −78 ◦C, then the temperature was allowed to
rise slowly overnight up to room temperature. After evaporation
of the solvent and the excess of chlorodimethylsilane, the target
product was extracted with diethyl ether and crystallized in hexane
at −20 ◦C (278 mg, 70% yield). Anal. calcd for C22H24S4Si2
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1
26.98%. 1H NMR (298 K, C6D6): d major isomer: 0.07 (s, SiCH3),
0.13 (s, SiCH2), 5.72 (s, 2H); minor isomer 0.05 (s, SiCH3), 0.12
53.48; H, 4.36; S, 8.16 Found: C, 53.41; H, 4.10; S, 7.84%. H
NMR: d 0.13 (d, 12H, SiCH3, 4JP,H = 2.2 Hz), 0.62 (s, 12H, SiCH3,
4JP,H not resolved), 3.33 (d br, 4H, CH2, 2JP,H = 7.6 Hz), 7.39–7.48
1
(s, SiCH2), 5.74 (s, 2H), 29Si-{ H} NMR (C6D6): d major isomer
1
2
−19.17 (SiCH3), −22.48 (SiCH2); minor isomer, −19.17 (SiCH3),
(4 m, 40H, Ph). 31P-{ H} NMR: d 21.4 0 (d, JP,P = 90.0 Hz),
−25.0 (d, 2JP,P ) = 90.0 Hz). IR (C6H12) mCO: 2053 m, 1998 s, 1983
vs cm−1.
−22.48 (SiCH2); −22.51 (SiCH2). GC/MS m/z: 464.
(TTF)2(Si2Me4) (11). LDA (7.32 mmol) was added dropwise
to a solution of TTF (1.36 g, 6.65 mmol) in diethyl ether
(60 mL) at −78 ◦C. After stirring for 1.5 h, 1.5 equiv. of
1,2-dichloro-1,1,2,2-tetraamethyldisilane (0.62 mL, 3.33 mmol)
were added, then the temperature was allowed to rise slowly
overnight up to room temperature. After filtration, the precipitate
was washed with pentane and extracted with hot toluene. Slow
evaporation of toluene afforded 11 as yellow plates suitable for X-
ray measurement (700 mg, 55% yield). Anal. calcd for C16H18S8Si2
(522.96): C, 36.74; H, 3.47; S, 49.05. Found: C, 36.61; H, 4.39; S,
49.18%. 1H NMR (298 K, C6D6): d 0.12 (s, 12H, SiCH3), 5.43 (AB
Electrochemical set-up
Voltammetric analyses were carried out in a standard three-
electrode cell with a Radiometer PGP 201 potentiostat at ambient
temperature. The electrolyte consisted of a 0.2 M nBu4PF6 solution
in CH2Cl2 or MeCN. The working electrode was a platinum
disk electrode and the auxiliary electrode was a platinum wire.
The reference electrode was a silver–silver ion electrode, Ag/Ag+
(0.1 M AgClO4 in MeCN), separated from the analyzed solution
by a sintered glass disk. After each measurement the reference
was checked against the ferrocene–ferrocenium couple (+ 0.025 V
and + 0.16 V against this reference electrode in acetonitrile
solution and in dichloromethane solution respectively).
1
system, 4H, 3JAB= 6.5 Hz), 5.66 (s, 2H). 29Si-{ H} NMR (C6D6):
d −22.77 (SiCH3). 13C NMR (C6D6): d 134.55, 125.48, 119.20,
119.06, 112.92, 109.65, −3.76. mp 160 ◦C (decomp.).
[{Ru(CO)4}2{l-(Me2Si)4TTF}] (14). To
a solution of 1
(437 mg, 1.0 mmol) in cyclohexane (15 ml) was added [Ru3(CO)12]
(422 mg, 0.66 mmol). After stirring for 1 d at 65 ◦C, the red-
brown solution was filtered and concentrated to ca. 6 ml until
precipitation of 14 began. The precipitation was completed by
addition of hexane. The orange-brown solid, which is air stable for
short periods of time, was then dried in vacuo. (620 mg, 72% yield).
Anal. calcd for C22H24O8S4Si4Ru2 (859.25): C, 30.75; H, 2.82; S,
Computational studies
All calculations were performed with predetermined symmetry.
Starting coordinates were taken from the crystal structure analysis
of compounds 11 and 12 prior to energy optimization at the
B3LYP/6-31+G(d) level. Additional harmonic vibrational fre-
quency analyses (to establish the nature of the stationary point
on the potential energy surface) were performed on the same level
and showed no imaginary frequency.
1
14.93 Found: C, 30.93; H, 3.10; S, 14.31%. H NMR (298 K,
CDCl3): d 0.62 (s, SiCH3). IR (C6H12) mCO: 2102 m, 2044 s, 2033
vs, 1926 vs cm−1.
mer-[{Ru(CO)3(PPh3)}2{l-(Me2Si)4TTF}] (15). A mixture of
12 (172 mg, 0.2 mmol) and PPh3 (105 mg, 0.4 mmol) in
methylcyclohexane (15 ml) was heated for 1 d at 90 ◦C. After ca.
12 h, a small amount of 13 began to precipitate as a yellowish solid.
After almost complete consumption of 12 (IR monitoring), the
red-brown solution was concentrated to ca. 5 ml with concomitant
precipitation of 13. The precipitation was completed by addition
of hexane. The yellowish solid, which is air stable for short periods
of time, was filtered off and then dried in vacuo. (171 mg, 64%
yield). Anal. calcd for C56H54O6P2S4Si4Ru2 (1327.77): C, 50.65;
Crystal structure determinations
A suitable crystal of each complex was mounted in an inert oil
(perfluoropolyalkyl ether) and used for X-ray crystal structure de-
terminations. Data were collected on a Stoe IPDS diffractometer at
173(2) K for 9 and 11 and at 293(2) K for 2 and 7. The intensities
were determined and corrected by the program INTEGRATE
in IPDS (Stoe & Cie, 1999). Numerical absorption corrections
were employed using the FACEIT-program in IPDS (Stoe & Cie,
1999) (9 and 11), while multi-scan type absorption corrections
were applied for the compounds 2 and 7. All structures were
solved applying direct and Fourier methods, using SHELXS-
97 and SHELXL-97.42,43 The completeness of data set was only
0.91 in the case of compound 2, which is a recurrent problem
encountered with Image Plate detectors. The R(int) value for
compound 7 is high, very likely because of the shape of the
crystals which were thin needles. However the final R factor
and the esds were reasonably low. For each structure, the non-
hydrogen atoms were refined anisotropically. All of the H-atoms
were placed in geometrically calculated positions and each was
assigned a fixed isotropic displacement parameter based on a
riding-model. Refinement of the structures was carried out by full-
matrix least-squares methods based on Fo2 using SHELXL-97. All
calculations were performed using the WinGX crystallographic
software package, using the programs SHELXS-97 and SHELXL-
97. Crystallographic parameters are listed in Table 2.
1
H, 4.10. Found: C, 50.90; H, 4.32%. H NMR (CDCl3): d 0.13
4
4
(d, 12H, SiCH3, JP,H = 2.4 Hz), 0.61 (s, 12H, SiCH3, JP,H not
resolved), 7.45 (30H, Ph). 31P-{ H} NMR: d 30.4 (s). IR (C6H12):
1
mCO: 2054 m, 2000 s, 1986 vs cm−1.
mer-[{Ru(CO)3(g1-dppm)}2{l-(Me2Si)4TTF}] (16). A mixture
of 12 (172 mg, 0.2 mmol) and dppm (155 mg, 0.4 mmol) in
methylcyclohexane (15 ml) was heated for 1 d at 90 ◦C. After
almost complete consumption of 12 (IR monitoring), the red-
brown solution was concentrated to ca. 5 ml with concomitant
precipitation of 16. The precipitation was completed by addition
of hexane. The yellowish solid, which is air stable for short periods
of time, was filtered off. IR and 31P NMR examination of the crude
product revealed the presence of ca. 10% of the dppm-chelated
2
complex [{Ru(g -dppm)(CO)2}2{(Me2Si)4TTF}] as by-product.
Recrystallisation from CH2Cl2–Et2O yielded 16 with more than
95% purity, so that a satisfying elemental analysis was obtained.
(214 mg, 68%). Anal. calcd For C70H68O6P4S4Si4Ru2 (1571.96) C,
CCDC numbers: 671046 (2), 671047 (7), 680222 (9) 680221 (11).
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4874 | Dalton Trans., 2008, 4866–4876
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K. R. Dunbar, J. Organomet. Chem., 1997, 529, 343; (c) C. E. Uzelmeier,
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Acknowledgements
Financial support from the French Research Ministry (Ph.D. grant
to A. H.) is gratefully acknowledged. This work was also supported
by the Centre de Coope´ration Universitaire Franco-Bavarois, the
CNRS and the Deutsche Forschungsgemeinschaft. C. S. thanks the
Fonds der Chemischen Industrie for financial support. C. D. thanks
the Studienstiftung des deutschen Volkes for a Ph.D. scholarship.
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