Synthesis, characterization, electrochemistry, and computational studies of ferrocenyl-substituted siloles

Article abstract of DOI:10.1021/om500072q

Ferrocenylsiloles of the type 2,5-Fc₂-3,4-Ph₂-^cC₄SiR₂ (3a, R = Me; 3b, R = Ph) have been prepared by reductive cyclization from diethynylsilanes, followed by ferrocenylation using the Negishi C,C cross-coupling protocol with the silole ring serving as either the vinyl halogenide species or as the zinc organic component and the complementary functionality introduced on the ferrocenyl moiety. The electrochemical behavior of these silacyclic-bridged bis(ferrocenyl) complexes was investigated by cyclic and square wave voltammetry, and the nature of the redox products was studied by in situ UV-vis-near-IR spectroelectrochemical measurements. 3a,b each undergo two sequential ferrocenyl-based redox processes, the separation of which (δE°′ = δE₂°′ - δE₁°′ = 300 mV (3a), 280 mV (3b)) is in the range of structural similar systems such as 2,5-diferrocenyl-1-phenyl-1H-phosphole (280 mV) and 2,5-diferrocenylfuran (290 mV). Interestingly, the more electron rich silole 3b, in comparison to 3a, shows a modestly lower redox separation between the individual ferrocenyl oxidation processes, which may be due to the capacity of this group to shield the effect of an adjacent positive charge. An intervalence charge transfer (IVCT) absorption was found in the in situ NIR spectra of [3a]⁺ and [3b]⁺, the analysis of which is consistent with a moderate electronic interaction between the iron atoms through the cis-diene-like fragment of the silole bridge, allowing their description as Robin and Day class II mixed-valence systems. These conclusions are supported by results from quantum chemical calculations, which together with NMR studies of 3b, also reveal the likely presence of a range of molecular conformations in solution. (Figure Prsented)

Full text of DOI:10.1021/om500072q

Article
pubs.acs.org/Organometallics
Synthesis, Characterization, Electrochemistry, and Computational
Studies of Ferrocenyl-Substituted Siloles
Steve W. Lehrich,† Alexander Hildebrandt,† Tobias Ruffer,† Marcus Korb,† Paul J. Low,‡,§
̈
and Heinrich Lang*,†
†Faculty of Natural Sciences, Institute of Chemistry, Inorganic Chemistry, Technische Univeristat Chemnitz, D-09107 Chemnitz,
̈
Germany
‡School of Chemistry and Biochemistry, University of Western Australia, Crawley, Perth, Western Australia, Australia
S
* Supporting Information
ABSTRACT: Ferrocenylsiloles of the type 2,5-Fc2-3,4-Ph2-cC4SiR2
(3a, R = Me; 3b, R = Ph) have been prepared by reductive
cyclization from diethynylsilanes, followed by ferrocenylation using
the Negishi C,C cross-coupling protocol with the silole ring serving
as either the vinyl halogenide species or as the zinc organic
component and the complementary functionality introduced on the
ferrocenyl moiety. The electrochemical behavior of these silacyclic-
bridged bis(ferrocenyl) complexes was investigated by cyclic and
square wave voltammetry, and the nature of the redox products was
studied by in situ UV−vis−near-IR spectroelectrochemical measure-
ments. 3a,b each undergo two sequential ferrocenyl-based redox
processes, the separation of which (ΔE°′ = ΔE2°′ − ΔE1°′ = 300
mV (3a), 280 mV (3b)) is in the range of structural similar systems such as 2,5-diferrocenyl-1-phenyl-1H-phosphole (280 mV)
and 2,5-diferrocenylfuran (290 mV). Interestingly, the more electron rich silole 3b, in comparison to 3a, shows a modestly lower
redox separation between the individual ferrocenyl oxidation processes, which may be due to the capacity of this group to shield
the effect of an adjacent positive charge. An intervalence charge transfer (IVCT) absorption was found in the in situ NIR spectra
of [3a]+ and [3b]+, the analysis of which is consistent with a moderate electronic interaction between the iron atoms through the
cis-diene-like fragment of the silole bridge, allowing their description as Robin and Day class II mixed-valence systems. These
conclusions are supported by results from quantum chemical calculations, which together with NMR studies of 3b, also reveal the
likely presence of a range of molecular conformations in solution.
In order to gain a deeper insight into the electron transfer
process that can be propagated through a single repeating unit
of such polymers, and in light of our recent research on
heterocyclopentadienes15−17 and aromatic five-membered
heterocycles,18−23 we became interested in siloles as π-
conjugated bridging units between two redox-active ferrocenyl
termini. Herein, we present the synthesis and structural
characterization of siloles of the type 2,5-Fc2-3,4-Ph2-cC4SiR2
(R = Me, Ph), together with an electrochemical study of their
redox chemistry and spectroelectrochemical investigation of the
redox products. Computational calculations were carried out to
enhance our understanding of the electronic structure of the
compound in different oxidation states.
INTRODUCTION
■
Silicon-containing metalloles (metallacyclopentadienes) are
useful structural and electronic building blocks that can be
used as monomers for the preparation of conjugated
polymers1,2 or copolymers.3,4 For example, Tamao has
presented a π-conjugated thiophene silole copolymer,5 while
polysilole (PS) or silole-containing copolymers, which are
linked through the 2,5-position, exhibit unique conductivity and
semiconducting properties attributed to the small band gap
(Eg),6−8 with computational studies indicating that PS has an
even smaller band gap than other five-membered polyhetero-
cycles such as polythiophene (PT, Eg = 2.10 eV) and
polypyrrole (PP, Eg = 2.85 eV9).7,10 These properties make
silicon-containing metalloles interesting motifs for use in the
design of organic semiconductors. In addition, due to the high
electron mobility and high photoluminescence quantum yields,
1-silacyclopentadienes have attracted much interest for a
diverse range of other materials applications such as new
display devices or in organic light-emitting diodes (OLEDs),
according to the interaction of the σ* orbital of the Si−C bond
with the π* orbital of the butadiene fragment.11−14
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Silacyclopentadienes
2a,b were synthesized by intramolecular reductive cyclization
Special Issue: Organometallic Electrochemistry
Received: January 23, 2014
Published: March 27, 2014
© 2014 American Chemical Society
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a
Scheme 1. Synthesis of 3a,b using Negishi Conditions
a
Legend: (i) tetrahydrofuran, 16 h, −80 °C; (ii) tetrahydrofuran, 2 h, room temperature; (iii) tetrahydrofuran, [Pd] = [Pd(CH2CMe2PtBu2)(μ-Cl)]2
(0.25 mol %), 80 °C, 2 days. Abbreviations: LiNp, lithium naphthalenide; Fc, Fe(η5-C5H4)(η5-C5H5).
Figure 1. ORTEP diagram (50% probability level) of the molecular structures of 3a (left) and 3b (right) with the atom-numbering scheme. The sign
∠ indicates interplanar angles between calculated mean planes of atoms adjoining differently colored areas. All hydrogen atoms have been omitted
for clarity. Selected bond distances (Å) and angles (deg), and torsion angles (deg): 3a, Si1−C1 = 1.8787(16), C1−C2 = 1.359(2), C1−C7 =
1.463(2), Si1−C4 = 1.8817(16), Si1−C5 = 1.8710(17), Si1−C6 = 1.8692(17), C2−C3 = 1.498(2), C2−C27 = 1.494(2), C3−C33 = 1.494(2), C3−
C4 = 1.360(2), C4−C17 = 1.462(2), average D−Fe = 1.649, C2−C1−Si1 = 107.31(11), C1−Si1−C4 = 92.26(7), average D−Fe−D = 177.43; 3b,
Si−C1 = 1.871(4). C1−C2 = 1.353(5), C1−C7 = 1.463(5), Si1−C5 = 1.870(3), C2−C2A = 1.507(6), C2−C27 = 1.491(5), average D−Fe = 1.648,
C2−C1−Si1 = 107.0(2), C1−Si1−C1A = 93.0(2), average D−Fe−D = 177.4. D denotes the centroid of C5H4 or C5H5. Symmetry operation for
1
5
generating equivalent atoms: (′) −x + /2, y, −z + /2.
from dimethylbis(phenylethynyl)silane (1a) and diphenylbis-
(phenylethynyl)silane (1b) with lithium naphthalenide fol-
lowed by bromination with elemental bromine, forming
dibromide 2a, or the reaction with [ZnCl2·2thf] giving the
organozinc species 2b (Scheme 1). Applying Negishi-
ferrocenylation conditions, the reaction of 2a with FcZnCl
(Fc = Fe(η5-C5H4)(η5-C5H5)) as the ferrocenyl source and
[Pd(CH2CMe2PtBu2)(μ-Cl)]2 as the precatalyst gave silole 3a.
The analogous coupling of 2,5-Br2-3,4-Ph2-cC4SiPh2 with
ferrocenylzinc chloride did not result in the formation of the
desired 3b. The synthesis of 3b was realized by a Negishi C,C
cross-coupling reaction using iodoferrocene as the ferrocenyl
source, while the application of bromoferrocene was
unsuccessful. After the appropriate workup (Experimental
Section), molecules 3a,b were obtained in moderate (3a) to
low (3b) yield as dark red solids.
examined by cyclic voltammetry (CV), square wave voltam-
metry (SWV), and in situ UV−vis−near-IR spectroelectro-
chemistry. Furthermore, DFT calculations were carried out to
support the conclusions drawn from the spectroscopic
measurements and enhance the understanding of the under-
lying electronic structures of [3a,b]n+ (n = 0−2).
The 1H NMR spectra of 3a,b show the characteristic pattern
for the two equivalent ferrocenyl groups with one singlet
(C5H5) and two pseudotriplets (C5H4) with J = 1.90 Hz, the
latter being characteristic for AA′XX′ spin systems.24 The signal
of the C5H5 group is found at 4.06 ppm for 3a and 3.52 ppm
for 3b. The 13C{1H} and 29Si{1H} NMR spectra show typical
resonances corresponding to the heterocyclic core and the
methyl and the phenyl groups (Experimental Section).25 High-
resolution mass spectrometry (HRMS) displays an anticipated
m/z peak of 630.1298 for 3a and 754.1425 for 3b.
Siloles 3a,b are stable toward air, light, and moisture in the
solid state and in solution and were characterized by elemental
analysis, UV−vis, IR, and NMR (1H, 13C{1H}, 29Si{1H})
spectroscopy, and mass spectrometry. The molecular structures
of 3a,b in the solid state were determined by single-crystal X-
ray structure analysis. The electrochemical behavior of 3a,b was
Single crystals of 3a and of 3b in the form 3b·2CH2Cl2
suitable for X-ray diffraction analysis could be obtained by
diffusion of n-hexane into a dichloromethane solution
containing either 3a or 3b at ambient temperature. The
molecular structures of 3a,b in the solid state together with
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selected bond distances (Å) and bond angles (deg) are shown
in Figure 1.
In 3a,b the bond distances within the central SiC4
heterocyclic silole cores each display the same short−long
alternation, consistent with the valence bond description of the
silacyclopentadiene (Scheme 1, Figure 1), while the distances
between the silacycle and the ferrocenyl ipso carbon atoms are
indistinguishable (3a, 1.436(2), 1.432(2) Å; 3b, 1.436(5) Å).
Furthermore, the bond distances of the silole core of 3a,b are
identical with those reported for 1,1,2,3,4,5-hexaphenylsi-
lole28,29 and 1,1-dimethyl-2,3,4,5-tetraphenylsilole,30 respec-
tively. These observations might indicate that the substitution
of the 2,3,4,5-bonded phenyl groups by ferrocenyl moieties or
the Si-bonded phenyl groups by methyl groups have a
negligible influence on structural parameters.
The ferrocenyl ligands are almost coplanar with the
heterocyclic core (torsion angles of the C5H4 plane to the
silacyclopentadiene unit: 3a, 2.07(3)° for Fe1, 2.88(3)° for Fe2;
3b, 3.5° for Fe1 and Fe1′). In 3a,b the phenyls in 3,4-positions
are almost orthogonal to the heterocyclic core (torsion angles
of the phenyl plane to the silacyclopentadiene unit: 3a,
88.49(6), 82.47(6)°; 3b, 81.8°). The conformations of the
cyclopentadienyls in all ferrocenyl ligands are almost eclipsed
(3a, −7.64(11)° for Fe1 and 0.80(10)° for Fe2; 3b, −2.30° for
Fe1). In both 3a and 3b the central SiC4 cores are planar, with
the silicon atom possessing as expected a tetrahedral environ-
ment.31
Molecule 3a crystallizes in the orthorhombic space group
Pbca with C1 symmetry and 3b in the monoclinic space group
P2/n with two dichloromethane molecules. In contrast to 3a,
silole 3b possesses a C2 symmetry axis going through the Si1
atom and the middle of the C2−C2′ bond (Figure 1). For 3a,
the ferrocenyl groups are oriented in a syn fashion and are
located on the same side of the silacycle, while in 3b an anti
conformation is observed, similar to the structure determined
for other bis(ferrocenyl)metallacycles.15 In the case of 3b, the
anti configuration is apparently stabilized by intramolecular π
interactions26,27 between the ferrocenyl moieties and the
phenyl groups pendant to the silicon atom (Figure 2). These
π interactions also seem to cause the high-field shift of the
C5H5 protons of 3b, in comparison with 3a (3a, 4.06 ppm; 3b,
3.53 ppm).
Given the stabilization of the ferrocenyl orientations by π
interactions with the phenyl substituents at the silicon atom,
the free rotation around the silole−ferrocenyl carbon−carbon
bond could be reduced by cooling NMR samples of 3b to −100
°C. The exchange rates of those protons can be determined by
line-shape fitting of the NMR spectra at the appropriate
temperatures (Figure 3). The activation parameters of the
rotation process could be quantified from the exchange rates in
graphical analysis according to Eyring32,33 to ΔH⧧ = 38.6
( 1.5) kJ mol−1 and ΔS⧧= 18.2 ( 7) J mol−1 K−1 (Supporting
Information, Figure SI16). Comparison with similar rotation
energy barriers in 2,3,4,5-tetraferrocenyl-N-phenyl-1-H-pyrrole
(ΔH⧧ = 26.8 ( 1.2) kJ mol−1 and ΔS⧧ = −94.1 ( 4.5) J mol−1
K−1)19 showed that the activation enthalpy of 3b is ca. 12 kJ
mol−1 higher, which in part may be attributed to the T-shaped
π interaction (Figure 2). Sherrill and co-workers showed in
2002 that such aromatic T-shaped interactions can reach
Figure 2. Ball-and-stick model of the molecular structure of 3b
displaying intramolecular π interactions between the aromatic C5H5
and PhSi functionalities. All hydrogen atoms are omitted for clarity.
The sign ∠ refers to the calculated interplanar angle and D to the
distance of the geometrical centroids of π interacting aromatic units.
1
Symmetry operation for generating equivalent atoms: (′) −x + /2, y,
5
−z + /2.
Figure 3. (left) Experimental 1H NMR spectra of 3b between 2.36 and 4.05 ppm in dichloromethane-d2 at various temperatures. (right) Simulated
1H NMR spectra of 3b with different exchange rates. Protons in α,α′-positions of the the C5H4 ring are marked with asterisks.
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stabilization energies of 9−12 kJ mol−1 for the benzene dimer.26
Furthermore, in contrast to the case for 2,3,4,5-tetraferrocenyl-
N-phenyl-1-H-pyrrole19 the activation entropy of the ferrocenyl
rotation in 3b is positive. This is generally the case when the
transition state exhibits more degrees of freedom than the
ground state. The π interaction not only hinders the rotation of
the ferrocenyls but also limits the freedom of rotation for the
C5H5 rings and the phenyl groups. Therefore, the ground states
of the rotation around the silole−ferrocenyl carbon−carbon
bond could be considered entropically unfavorable, hence
resulting in a positive activation entropy. The observations of
dynamic NMR studies not only support the crystallographically
determined π interaction, but also showed that even in solution,
especially at low temperatures, a rotation barrier of the
ferrocenyls is present and should be taken into consideration.
Please note that, due to the absence of a π interaction in 3a, no
rotation barrier could be observed at temperatures down to
−100 °C (Supporting Information, Figure SI17).
Electrochemistry. The electrochemical properties of 3a,b
were investigated by cyclic voltammetry (CV) and square wave
voltammetry (SWV) (Figure 4), and the nature of the redox
Figure 5. UV−vis−near-IR spectra of [3a]n+ (n = 0−2) in
dichloromethane solution (2.0 mmol L−1) at increasing potentials
(vs Ag/AgCl): (top) −200 to 300 mV; (bottom) 300 to 700 mV at 25
°C. The supporting electrolyte was 0.1 mol L−1 of [NnBu4][B(C6F5)4].
Arrows indicates an increase or decrease in the absorptions.
Table 1. Cyclic Voltammetry Data of 3a,b and 4−8 for
Comparison
Figure 4. Cyclic voltammograms (solid lines) and square wave
voltammograms (dotted lines) of 3a,b in dichloromethane solutions
(1.0 mmol L−1) at 25 °C (scan rate 100 mV s−1; supporting electrolyte
0.1 mol L−1 of [NnBu4][B(C6F5)4]).
a
b
E1°′ (mV) (ΔEp
E2°′ (mV) (ΔEp
ΔE°′
d
c
c
compd
(mV))
(mV))
(mV)
e
3a
−145 (72)
−170 (64)
−35
−94 (65)
−110 (72)
−152 (60)
−206 (65)
155 (74)
110 (68)
90
300
280
225
260
280
290
410
e
3b
products was explored in more detail by in situ UV−vis−near-
IR spectroelectrochemistry (Figure 5). The voltammetric
measurements were carried out in dry dichloromethane
solutions of [NnBu4][B(C6F5)4] (0.1 mol L−1), the latter
being chosen to minimize ion pairing effects34−36 (for examples
of the application of [NnBu4][B(C6F5)4] as supporting
electrolyte within electrochemical measurements see refs 15,
19, 22, and 37−41). Cyclic voltammetry studies were
performed at 25 °C with a scan rate of 100 mV s−1. All
potentials are referenced to the FcH/FcH+ (FcH = Fe(η5-
C5H5)2) redox couple (E°′ = 0.0 mV).42
f
4 ,
43
e
e
e
e
44
15
44
44
,
,
,
,
5
6
7
8
166 (65)
170 (80)
138 (63)
204 (65)
a
b
E°′ = formal potential of first redox process. E1°′ = formal potential
c
of second redox process. ΔEp = difference between oxidation and
d
reduction potentials. ΔE°′ = potential difference between the two
e
ferrocenyl-related redox processes. Conditions: potentials vs FcH/
FcH+, scan rate 100 mV s−1 at a glassy-carbon electrode of 1.0 mmol
L−1 solutions in dry dichloromethane containing 0.1 mol L−1 of
f
[NnBu4][B(C6F5)4] as supporting electrolyte at 25 °C. Conditions:
The ferrocenyl substituents in 3a,b are oxidized separately,
showing two reversible redox events with redox separations
(ΔE°′ = ΔE2°′ − ΔE1°′) of 300 mV (3a) and 280 mV (3b),
respectively, which indicates some through-bond or through-
space electronic interactions between the ferrocenyl/ferroce-
nium termini (Table 1, Figure 4). While for 1,1-dimethyl-
2,3,4,5-tetraphenylsilacyclopentadiene irreversible oxidation
(Epa = 1482 mV) and reduction (Epc = −2174 mV) processes
have been observed by Tracy,31 compounds 3a,b showed no
such redox events within the measured potential range (−2500
to 1800 mV). Within the series of bis(ferrocenyl) complexes
featuring five-membered aromatic heterocyclic bridges, such as
5,36 7,35 and 819 (Chart 1), it could be shown that the
measured in [NnBu4][B(3,5-C6H3(CF3)2] (0.1 M) in dichloromethane
with a Pt working electrode and Ag/AgI reference electrode,
referenced to FcH/FcH+, at a scan rate 50 mV/s.
separation of the two ferrocenyl redox waves depends on the
electronic characteristics of the bridging moiety. The more
electron-rich the bridging group, the lower (less positive) the
first ferrocenyl oxidation (E1°′) potential process becomes, and
hence electron-rich heterocycles show higher ΔE°′ values
c
(Table 1). The C4Si bridging unit within silole 3b donates
more electron density toward the ferrocenyl moieties in
comparison with 3a, and thus those ferrocenyls are more
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Chart 1. Selected Diferrocenyl Molecules for Comparison
easily oxidized (3a, E1°′ = −145 mV; 3b, E1°′ = −170 mV);
however, 3b possesses a lower redox separation (3a, 300 mV;
3b, 280 mV), which is in contrast to the trends revealed by
other species in this five-membered heterocyclic family (Table
1). The discrepancy between the observed and expected
electrochemical behavior of 3a,b might be explained by the
influence of the cross-hyperconjugation of the SiMe2 moiety to
the butadiene system.12,46−49 Due to the involvement of the
silicon atom in the π conjugation, the electronic interactions
between the respective iron centers might be increased, leading
to some additional stabilization of the mono-oxidized redox
product [3a]+ relative to [3b]+. In order to explore this
hypothesis, the IVCT transitions within the mixed-valent
cations [3a]+ and [3b]+ have been studied using spectroelec-
trochemical methods.
Spectroelectrochemistry. Spectroelectrochemical studies
were performed in an optically transparent thin-layer electro-
chemistry (OTTLE) cell containing 2.0 mmol L−1 of 3a or 3b
and 0.1 mol L−1 of [NnBu4][B(C6F5)4] as the supporting
electrolyte, with stepwise increase of the applied potential from
−200 to 700 mV vs Ag/AgCl. The potential was increased
using varying step heights of 25, 50, and 100 mV. This
procedure allowed the sequential in situ generation of [3a,b]+
and [3a,b]2+ (Figure 5). In their charge-neutral, Fe(II/II) state,
siloles 3a,b are, as expected, transparent in the near-IR region,
while broad intense transitions (IVCT and LMCT) could be
observed as 3a,b were oxidized to [3a]+ and [3b]+ by increasing
the applied potential. Deconvolution of this absorption
envelope required four Gaussian-shaped spectral components,
consistent with an IVCT transition at 4700 cm−1 ([3a]+) or
Figure 6. Four-band Gaussian deconvolution of the near-IR
absorption envelope of [3a]+ obtained by spectroelectrochemistry in
an OTTLE cell.
to those of the diene analogue [4]+. The higher extinction
coefficient of the IVCT absorption in [3a]+ might be a
consequence of cross-hyperconjugation of the SiMe2 building
block with the butadiene unit causing involvement of the sp3-
hybridized silicon atom in the π-conjugated system, a
phenomenon that has been described in, for example, methyl-
and H-substituted siloles.12,46−49 The fact that the IVCT
transition associated with [3b]+ is less intense and broader than
that of [3a]+ demonstrates a weaker electronic coupling
between the redox centers. In support of this, we note that
although ΔE°′ depends on many factors, including ion-pairing
energies, solvation factors, magnetic effects, and metal−ligand
bonding variations in different oxidation states, in addition to
statistic and electrostatic terms,50,51 these are not expected to
be substantively different in 3a/[3a]+ vs 3b/[3b]+. Hence, the
smaller ΔE°′ value observed for 3b (see Electrochemistry) is
also consistent with a smaller contribution of the resonance
term. Comparison of the characteristics of the IVCT transition
of [3a]+ and [3b]+ with those from other aromatic five-
membered heterocycles revealed that the Δν1/2 values for
44,53,54
[5]+,21,45,52 [6]+,15 [7]+,44 and [8]+
are smaller (2300−
2400 cm−1) than those of [3a]+ and [3b]+, while the intensities
are within the same range. Phosphole [6]+,15 which is similar to
siloles 3a,b, also contains a nonaromatic five-membered
heterocycle showing IVCT absorptions with Δν1/2 values
comparable to [3a]+ and [3b]+, respectively.
4650 cm−1 ([3b]+) ([3a]+, εmax = 3150 L mol−1 cm−1, Δν1/2
=
2950 cm−1; [3b]+, εmax = 2270 L mol−1 cm−1, Δν1/2 = 3310
cm−1) and two LMCT bands at 3470 and 4000 cm−1 ([3a]+)
and 3460 and 3930 cm−1 ([3b]+) (Table 2 and Figure 6). The
fourth component was used to simulate the low-energy edge of
higher energy absorptions that protrude into the near-IR
region. Due to the sp3 character of the silicon atom, the silole
The electronic coupling parameter Hab can be calculated
according to Hush’s two-state model for a class II system as
shown in eq 1, where rab is the effective electron transfer
43
c
fragment C4Si is rather comparable to a cis-diene system.
νmaxεmaxΔν1/2
Hab = 2.06 × 10−2
However, the intensity of the IVCT absorption found in [3a]+
exceeds those found in mixed valence [6]+ and [4]+,43 while
15
rab
(1)
the IVCT characteristics (εmax, Δν1/2) of [3b]+ are very similar
a
Table 2. Near-IR Data of the Absorptions of Siloles [3a]+ and [3b]+
b
b
compd
νmax (cm−1) (ε (L mol−1 cm−1)) Δν1/2 (cm−1
)
Δν1/2(theor) (cm−1
)
Hab(syn conformation) (cm−1
)
Hab(anti conformation) (cm−1
)
3a+
IVCT
4700 (3150)
3470 (2650)
4000 (1250)
4650 (2250)
3460 (1450)
3930 (800)
2950
950
3300
542
500
I LMCT
II LMCT
IVCT
450
3b+
3300
800
3300
482
445
I LMCT
II LMCT
450
a
b
Conditions: in dry dichloromethane containing 0.1 mol L−1 of [NnBu4][B(C6F5)4] as supporting electrolyte at 25 °C. Hab was calculated according
to eq 1 with rab(syn) = 7.9438 Å and rab(anti) = 8.6074 Å.
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distance between the two redox-active sites, which is
notoriously hard to determine experimentally.51,55−58 There-
fore, the crystallographic Fe−Fe distances were used to
estimate Hab. Since the ferrocenyl substituents within 3a,b
can adopt both syn and anti conformations, Hab was calculated
using crystallographic data derived from 3a representing the syn
orientation (rab(syn) = 7.9438(18) Å) and anti-oriented 3b
(rab(anti) = 8.6074(9) Å). Please note that the effective
electron transfer distance is expected to be shorter than the
geometric Fe−Fe distances and hence Hab might be under-
estimated.54 As expected, due to the lower rab value, Hab is
higher for the syn conformation for both molecules [3a]+ and
[3b]+, in comparison with the respective anti conformer (Table
2). Silole [3a]+ exhibits higher Hab values in comparison with
[3b]+; nevertheless, the determined values are quite similar,
showing that there are only minor differences in the electronic
coupling caused by the different substitution on the silicon
atom.
length alternation in the silacyclopentadiene ring and significant
cis-diene-like character in the bridging moiety linking the two
ferrocenyl fragments, in a manner entirely analogous to the
crystallographically determined stuctures of 3a,b and other
heterocyclic bridged bis(ferrocenyl) complexes. In both the
computational models and the crystallographically determined
structures, the C1−C15, silacyclopentadiene, and C6−C65
rings are essentially coplanar, while the geometry at Si is
distorted from idealized tetrahedral by the inclusion within the
C4Si ring. The HOMO of syn-3a′ features a substantial (ca.
42%) character from the cis-butadiene-like portion of the
silacycle, admixed with contributions from the ferrocenyl
substituents (ca. 28% each) (Figure 8). There is negligible
contribution from both the SiMe2 fragment and the phenyl
groups, the latter being oriented approximately perpendicularly
to the plane of the silacycle. The electronic structure varies little
as a function of the relative disposition of the ferrocenyl
moieties across the silacyclic ring, and the composition and
energy of the frontier orbitals of the anti conformer anti-3a are
essentially the same as those described for the syn form
(Supporting Information).
The unexpectedly strong IVCT interactions allowed
classification of siloles [3a]+ and [3b]+ as moderately and
moderately to weakly coupled class II systems, respectively,
according to the classification system introduced by Robin and
Day.58
The monocation syn-[3a′]+ displays a number of structural
and orbital features that are entirely consistent with the
description of this species as an Fe(II/III) mixed-valence (MV)
complex. The C1−C2 bond is rather shorter in the monocation
(1.445 Å) than in the neutral species (1.467 Å), although the
C5−C6 bond lengths are more consistent between the two
oxidation states (3a′, 1.467 Å; [3a′]+, 1.458 Å), while within
the silacyclopentadiene ring the bond length alternation is
modestly less pronounced (Table 3), giving rise to a valence
bond description with more cumulenic character in the diene-
like backbone. There is also a substantial elongation of the Fe−
Cp1 distance in [3a′]+ in comparison with 3a′, consistent with
the oxidation of this site (Table 3). However, there is little
variation in the local geometry at the silicon center. The
composition of the molecular orbitals also supports the MV
description of [3a]+, with the β-LUSO in [3a′]+ essentially
localized (80%) on one ferrocenyl center and a small
contribution (14%) from the diene-like backbone (Figure 9),
while the β-HOSO has more character derived from the other
ferrocenyl moiety (58%) and the diene (30%) with 10% arising
from the formally oxidized ferrocenyl center (Figure 10).
Interestingly, anti-[3a′]+ is essentially isoenergetic with the
syn isomer, lying barely 0.65 kJ/mol lower in energy. This,
together with the virtually barrierless rotation determined from
the dynamic NMR studies, indicates that solutions of [3a]+
contain more than one conformer. The key bond lengths and
angles of anti-[3a′]+ are for all intents identical with those of
the syn isomer, and there would also therefore appear to be
little by way of net reorganization energy in the interconversion
of the two conformers. The composition of the β-LUSO and β-
HOSO in anti-[3a′]+ is, perhaps surprisingly, almost indis-
tinguishable from that of the syn conformer, with the β-LUSO
being largely localized on one ferrocenyl moiety (81%) and the
diene (14%) with the β-HOSO again exhibiting more diene-like
character (Fc 59%, diene 30%, Fc+ 10%) and being somewhat
more delocalized.
Further oxidation of [3a]+ and [3b]+ to the dications [3a]2+
and [3b]2+ leads to a disappearance of the IVCT bands as
homovalent Fe(III/III) species are formed. In association with
this, two MLCT transitions emerge at ca. 1000 nm (10000
cm−1). The LMCT bands that are determined for [3a]+ and
[3b]+ can also be found in dications [3a]2+ and [3b]2+,
exhibiting a decreased intensity (ca. 200 L mol−1 cm−1)
(Supporting Information).
Computational Studies. To explore the electronic
structures of [3a,b]n+ (n = 0−2) in more detail, the
computational model systems syn-[3a′]n+, anti-[3a′]n+, and
anti-[3b′]+ (where the prime (′) nomenclature is introduced to
distinguish the computational from the real system) were
analyzed (B3LYP using Dunning’s all-electron valence double-ζ
(D95 V) for C and H atoms and the Los Alamos ECP/double-
ζ valence basis set on Fe and Si (LANL2DZ) and a CPCM-
dichloromethane solvent model). Tables giving orbital energies
and compositions are available in the Supporting Information,
together with the results from benchmarking studies carried out
on 3a′ using 3-21G* (all atoms) and 6-31G** (H, C, Si)/
LANL2DZ (Fe) basis sets, which reveal no structural or
electronic differences of any significance to the results reported
in the main body of the paper.
The optimized geometry of syn-3a′ compares very well with
that determined crystallographically for 3a (Figure 7 and Table
3) and differs little from that of anti-3a′, which lies only +0.37
kJ/mol lower in energy. The optimized geometries of the syn
and anti conformers of 3a′ both exhibit pronounced bond-
In order to probe the possible conformational and chemical
differences of the mixed-valence complexes [2,5-Fc2-3,4-
Ph2-cC4SiR2]+ further, the complex anti-[3b′]+ was also
modeled. The presence of the phenyl substituents made
essentially no change to the core geometry of the cis-diene-like
Fc+CCPhCPhCFc fragment in comparison with syn- or
Figure 7. Atom-labeling scheme of 3a.
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Table 3. Selected Bond Lengths (Å) and Bond and Torsion Angles (deg) from the Crystallographically Determined Structure of
3a and the Optimized Geometry of Models [3a′]n+ (n = 0, 1, 2)
3a
syn-3a′
anti-3a′
syn-[3a′]+
anti-[3a′]+
anti-[3b′]+
syn-T-[3a′]2+
anti-T-[3a′]2+
a
b
Fe−Cp1
Fe−Cp6
C1−C2
C2−C3
C3−C4
C4−C5
C5−C6
C2−Si
1.647
1.648
1.462
1.360
1.498
1.359
1.463
1.882
1.879
127.12
116.33
116.66
126.84
92.26
109.07
−1.68
0.11
1.730
1.730
1.467
1.380
1.507
1.380
1.467
1.904
1.904
127.97
116.43
116.43
127.97
91.85
111.11
−0.88
0.84
1.730
1.732
1.467
1.380
1.507
1.380
1.467
1.905
1.905
127.88
116.43
116.43
127.88
91.86
110.89
1.24
1.800
1.731
1.445
1.395
1.487
1.390
1.458
1.902
1.909
126.50
116.70
115.93
127.80
91.16
111.68
4.55
1.799
1.731
1.445
1.394
1.487
1.390
1.458
1.902
1.909
126.71
116.67
115.97
127.67
91.16
111.62
2.68
1.795
1.730
1.447
1.391
1.490
1.387
1.457
1.901
1.910
126.94
116.74
116.06
127.83
91.30
1.798
1.796
1.457
1.386
1.497
1.386
1.457
1.906
1.906
126.47
116.00
115.98
126.70
90.73
112.42
2.41
1.798
1.798
1.458
1.386
1.498
1.386
1.458
1.906
1.906
126.44
115.99
115.99
126.44
90.76
112.09
2.90
C5−Si
C1−C2−C3
C2−C3−C4
C3−C4−C5
C4−C5−C6
C2−Si−C5
c
CH3−Si−CH3
C12−C1−C2−Si
C62−C6−C5−Si
111.71
1.83
1.14
1.25
2.40
0.88
4.39
2.91
a
b
c
Cp1 is the midpoint of the C1−C15 cyclopentadienyl ring. Cp2 is the midpoint of the C6−C65 cyclopentadienyl ring. (ipso-C)−Si−(ipso-C).
Figure 8. Plot of the HOMO of 3a′ (isocontour value 0.04 (e/
bohr3)1/2).
Figure 10. Plot of the β-HOSO of [3a′]+ (isocontour value 0.04 (e/
bohr3)1/2).
Fc 4%), the diene contribution to the β-HOSO in anti-[3b′]+ is
somewhat smaller (Fc 65%, diene 25%, Fc+ 8%). Given that the
IVCT transition is approximated in terms of the β-HOSO → β-
LUSO transition, the lower overlap between these orbitals may
be responsible for the modestly weaker coupling. Such
suggestions must be tempered against the relatively high
barriers to rotation about the C1−C2 and C5−C6 bonds
determined by dynamic NMR arising from the T-shaped π−π
interactions, taken in light of the likely conformational
distribution in solution. The notion that the SiPh2 moiety
presents a steric barrier to the optimal coplanar arrangement of
the ferrocenyl and silacyclic moieties would be consistent with
the modestly weaker coupling observed in [3b]+ in comparison
to that in [3a]+.
As an aside, we note that at the level of theory employed, a
local minimum with a symmetric (i.e., delocalized or class III)
electronic structure was also found for anti-[3a′]+, which differs
only slightly in individual bond lengths and is only 14 kJ/mol
higher in energy than the class II form described above
(Supporting Information). Given the well-known overestima-
tion of delocalized electronic structures from the B3LYP
functional, it is not appropriate to base any substantive
comment on the apparently low energy difference between
the class II and class III forms of anti-[3a′]+. However, the
Figure 9. Plot of the β-LUSO of [3a′]+ (isocontour value 0.04 (e/
bohr3)1/2).
anti-[3a′]+ (Table 3), although the ferrocenyls are slightly
distorted from coplanarity (FeCp1···FeCp2: anti-[3b′]+,
−172.00°; anti-[3a′]+, −175.25° Me2). This minor geometric
variation and the impact on the electronic structure are
potentially interesting with regards to the suggestion of a
degree of attenuation of the coupling between the ferrocenyl
moieties through the 1,1,3,4-tetraphenylsilacyclopentadiene
scaffold of [3b]+ vs [3a]+ made on the basis of the
electrochemical and near-IR data. While the β-LUSO of this
lowest energy geometry of anti-[3b′]+ exhibits the same
composition as in syn- and anti-[3a′]+ (Fc+ 80%, diene 15%,
4842
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Fourier transform mode; the 13C{1H} NMR spectra were recorded at
125.800 MHz. Chemical shifts are reported in ppm downfield from
tetramethylsilane with the solvent as reference signal (1H NMR,
δ(CHCl3) 7.26 ppm; 13C{1H} NMR, δ(CHCl3) = δ(CDCl3) 77.16
ppm; 29Si{1H} NMR, δ(TMS) 0.00 ppm). The melting points were
determined using a Gallenkamp MFB 595 010 M melting point
apparatus. Elemental analyses were performed with a Thermo FlashEA
1112 Series instrument. High-resolution mass spectra were recorded
using a micrOTOF QII Bruker Daltonite workstation. UV−vis spectra
were recorded with a THERMO Genesys 6 spectrometer.
X-ray Diffraction. Data were collected with an Oxford Gemini S
diffractometer at 110 K using Mo Kα (λ = 0.71073 Å) radiation. The
structures were solved by direct methods and refined by full-matrix
least-squares procedures on F2.65,66 All non-hydrogen atoms were
refined anisotropically, and a riding model was employed in the
treatment of the hydrogen atom positions.
point to note is not that the B3LYP hybrid functional identifies
a structure with a delocalized electronic structure but rather
that there are only very subtle changes in bond length required
to pass between class II and class III in these complexes (a
point that is implicit in the thermal barrier to electron transfer
associated with class II complexes).
The dication syn-[3a′]2+ proved to be substantially more
stable in the triplet (or high-spin, HS) ground state (ΔET‑S
:
−118.6 kJ/mol [3′]2+), and consequently the discussions here
are restricted to this state. The molecular geometry of HS-
[3a′]2+ reflects the homovalent nature of the two iron centers,
with essentially identical geometric parameters observed in each
half of the dication, and a restoration of the more pronounced
bond-length alternation in the cis-diene backbone (Table 3).
The dication is therefore well described as two ferrocenium
moieties pendant to the silacycle.
Electrochemistry. Electrochemical measurements on 1.0 mmol
L−1 solutions of the analytes in dry, air-free dichloromethane
containing 0.1 mol L−1 of [NnBu4][B(C6F5)4] as supporting
electrolyte were conducted under a blanket of purified argon at 25
°C utilizing a Radiometer Voltalab PGZ 100 electrochemical
workstation interfaced with a personal computer. A three-electrode
cell, which utilized a Pt auxiliary electrode, a glassy-carbon working
electrode (surface area 0.031 cm2), and an Ag/Ag+ (0.01 mol·L−1
AgNO3) reference electrode mounted on a Luggin capillary, was used.
The working electrode was pretreated by polishing on a Buehler
CONCLUSION
■
The synthesis of 2,5-diferrocenyl-3,4-diphenyl siloles bearing
methyl (3a) or phenyl (3b) groups at the silicon atom are
realized by a reductive cyclization reaction of dimethyl- or
diphenylbis(phenylethynyl)silane with lithium naphthalenide.
The ferrocenyl substituents are introduced to the silole ring by
Negishi C,C cross-coupling reactions, whereas the silole ring
either was applied as a vinyl halogenide or as a zinc organic
component. Electrochemical investigations showed that the
ferrocenyl units of both synthesized siloles can be oxidized
separately. In contrast to findings among the series of aromatic
diferrocenyl five-membered heterocycles,15,18,19,44,45 the more
electron rich siloles showed a lower redox separation between
the individual ferrocenyl oxidation processes. Spectroelectro-
chemical investigations confirmed electron charge transfer
interactions between the individual ferrocenyl termini across
the silole linking unit, which are comparable to those found in
diferrocenyl phosphole15 or diferrocenyl cis-butadiene sys-
tems,43 arguing that the C4 chain at the molecule’s backbone
mediates the electronic coupling. Molecules [3a,b]+ can be
characterized as moderate or moderate to weakly coupled class
II species according to Robin and Day. Despite being more
electron rich, 3b+ showed less intense IVCT absorptions than
the dimethylsilole [3a]+, which may be due to steric
interactions. DFT calculations are consistent with the class II
mixed valence description of [3a,b]+, with the apparent low
differences in energy between syn and anti conformations
suggesting a distribution of molecular geometries, and hence
variation in electronic coupling, in solution.
1
microcloth first with a 1 μm and then a /4 μm diamond paste. The
reference electrode was constructed from a silver wire inserted into a
solution of 0.01 mol L−1 [AgNO3] and 0.1 mol L−1 [NnBu4][B-
(C6F5)4] in acetonitrile, in a Luggin capillary with a Vycor tip. This
Luggin capillary was inserted into a second Luggin capillary with a
Vycor tip filled with a 0.1 mol L−1 [NnBu4][B(C6F5)4] solution in
dichloromethane.36 Successive experiments under the same exper-
imental conditions showed that all formal reduction and oxidation
potentials were reproducible within 5 mV. Experimentally, potentials
were referenced against an Ag/Ag+ reference electrode, but results are
presented referenced to ferrocene as an internal standard, as required
by IUPAC.42 When decamethylferrocene was used as an internal
standard, the experimentally measured potential was converted into E
vs FcH/FcH+ by addition of −619 mV.67 Data were then manipulated
on a Microsoft Excel worksheet to set the formal reduction potentials
of the FcH/FcH+ couple to ΔE°′ = 0.0 V. Ferrocene itself showed a
redox potential of 220 mV vs Ag/Ag+ (ΔEp = 61 mV) within our
measurements.68,69 The cyclic voltammograms, which are depicted
(Figure 4), were taken after typically two scans and are considered to
be steady-state cyclic voltammograms in which the signal pattern does
not differ from the initial sweep.
Spectroelectrochemistry. UV−vis−near-IR measurements were
carried out in an OTTLE (optically thin layer electrochemistry) cell
with quartz windows similar to that described previously70 in dry
dichloromethane solutions containing 2.0 mmol L−1 analyte and 0.1
mol L−1 of [NnBu4][B(C6F5)4] as supporting electrolyte using a Varian
Cary 5000 spectrophotometer at 25 °C. The Pt-mesh working
electrode, the AgCl-coated Ag wire reference electrode, and the Pt-
mesh auxiliary electrode were melt-sealed into a polyethylene spacer.
The values obtained by deconvolution could be reproduced within
εmax = 100 L mol−1 cm−1, νmax = 50 cm−1, and Δν1/2 = 50 cm−1.
Between the spectroscopic measurements the applied potentials were
increased in a stepwise fashion using a step height of 25, 50, or 100
mV. At the end of the measurements the analyte was reduced at −400
mV for 30 min and an additional spectrum was recorded to prove the
reversibility of the oxidations.
EXPERIMENTAL SECTION
■
General Conditions. All reactions were carried out under an
atmosphere of argon using standard Schlenk techniques. Tetrahy-
drofuran was purified by distillation from sodium/benzophenone ketyl.
For column chromatography, alumina with a particle size of 90 μm
(standard, Merck KGaA) was used.
Reagents. 2,5-Dibromo-1,1-dimethyl-3,4-diphenyl-1H-silole59 and
diphenylbis(phenylethynyl)silane60 were prepared in analogy to
published procedures. [N(nBu)4][B(C6F5)4] was prepared by metha-
thesis of lithium tetrakis(pentafluorophenyl)borate etherate (Boulder
Computational Chemistry. Calculations were carried out with
the Gaussian 0971 package, with GaussView 5.0.872 and Gauss-
Sum3.073 used to further analyze the results. All geometries were
optimized with the B3LYP functional, using the LAN2DZ basis set on
Fe and 6-31G** on all other atoms, with a CPCM (dichloromethane)
solvent model. All optimized geometries were confirmed as true
minima by the absence of imaginary frequencies.
Scientific) with tetra-n-butylammonium bromide according to ref 35.
61,62
[P(tC4H9)2C(CH3)2CH2Pd(μ-Cl)]2
and iodoferrocene63,64 were
prepared according to published procedures. All other chemicals were
purchased from commercial suppliers and were used as received.
Instrumentation. Infrared spectra were recorded using a FT-
Nicolet IR 200 equipment. The 1H NMR spectra were recorded with a
Bruker Avance III 500 spectrometer operating at 500.303 MHz in the
4843
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Synthesis of 2,5-Diferrocenyl-1,1-dimethyl-3,4-diphenyl-1H-
silole (3a). To a solution of 9.52 mmol of ferrocenyl zinc chloride in
40 mL of tetrahydrofuran, prepared by monolithiation of ferrocene
(1.95 g, 10.47 mmol)74 followed by addition of [ZnCl2·2thf] (4.23 g,
15.08 mmol) at −80 °C, was added 2,5-dibromo-1,1-dimethyl-3,4-
diphenylsilole (2.0 g, 10.47 mmol) in a single portion at 25 °C. To the
solution was added 16.3 mg (24 μmol) of [P(tC4H9)2C(CH3)2CH2Pd-
(μ-Cl)]2, and the mixture was stirred for 2 days at 80 °C. After
evaporation of all volatiles, the precipitate was dissolved in 70 mL of
diethyl ether and washed three times with 60 mL portions of water.
The organic phase was dried over MgSO4, and all volatiles were
removed. The remaining crude solid was purified by column
chromatography (column size 20 × 4 cm, on alumina) using a 2/1
(v/v) n-hexane/dichloromethane mixture as eluent. All volatiles were
removed under reduced pressure, and the wine red solid was
crystallized from n-hexane at 68 °C. Yield: 35% (based on 2,5-
dibromo-1,1-dimethyl-3,4-diphenylsilole). Anal. Calcd for C38H34Fe2Si
(M = 630.45: C, 72.39; H, 5.44. Found: C, 71.89; H, 5.38. Mp: 275 °C
dec. IR (KBr, in cm−1): 3087 (w), 3076 (m), 3056 (m), 3022 (m),
2947 (w), 2923 (w), 1682 (w), 1442 (m), 1241 (s), 1106 (s) 1000 (s),
793 (s), 745 (s), 702 (s), 506 (s). 1H NMR (CDCl3, δ in ppm): 0.73
(s, 6H, CH3), 3.71 (pt, JHH = 1.89 Hz, 4H, C5H4), 4.06 (s, 10H,
C5H5), 4.10 (pt, JHH = 1.89 Hz, 4H, C5H4), 6.96−7.00 (m, 4H, o-
C6H5), 7.09−7.14 (m, 2H, p-C6H5) 7.18−7.22 (m, 4H, m-C6H5).
13C{1H} NMR (CDCl3, δ in ppm): −1.0 (CH3), 68.6 (C5H4), 68.7
(C5H4), 69.5 (C5H5), 83.3 (iC-C5H4), 126.4 (o-C6H5), 128.0 (m-
C6H5), 129.3 (p-C6H5), 134.8 (2,5-C4Si), 141.6 (i-C6H5), 152.6 (3,4-
C4Si). 29Si{1H} NMR (CDCl3, δ in ppm): 5.70 (s, Si). UV−vis: 391
nm (12057 L mol−1 cm−1), 507 nm (5073 L mol−1 cm−1). HRMS
(ESI-TOF, m/z): calcd for C38H34Fe2Si 630.1129, found 630.1198
[M]+. Crystal data for 3b: C38H34Fe2Si, Mr = 630.44, orthorhombic,
Pbca, λ = 0.71073 Å, a = 13.036(5) Å, b = 19.934(5) Å, c = 22.598(5)
Å, V = 5872(3) Å3, Z = 8, ρcalcd = 1.426 g m−3, μ = 1.023 mm−1, T =
110 K, θ range 3.13−26.00°, 56340 reflections collected, 5734
independent reflections (Rint = 0.0338), R1 = 0.0258, wR2 = 0.0635 (I
> 2σ(I)).
Å, b = 10.8183(4) Å, c = 14.8040(4) Å, V = 2073.56(12) Å3, Z = 2,
ρcalcd = 1.481 g m−3, μ = 1.023 mm−1, T = 110 K, θ range 2.942−
24.997°, 8460 reflections collected, 3644 independent reflections (Rint
= 0.0205), R1 = 0.0520, wR2 = 0.1284 (I > 2σ(I)).
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, tables, and .xyz and CIF files giving further
(spectro)electrochemical spectra, computational data, and
crystallographic data for 3a,b. This material is available free of
charge via the Internet at http://pubs.acs.org .Crystallographic
data for 3a,b are also available from the Cambridge Crystallo-
graphic Database as file numbers CCDC 981390 (3a) and
981391 (3b).
AUTHOR INFORMATION
Corresponding Author
*H.L.: e-mail, heinrich.lang@chemie.tu-chemnitz.de; tel, +49
(0)371-531-21210; fax, +49 (0)371-531-21219.
■
Author Contributions
§Author to whom correspondence pertaining to the calculations
should be directed.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Holm Petzold for assistance with the dynamic
NMR measurements. We are grateful to the Fonds der
Chemischen Industrie for generous financial support. M.K.
thanks the Fonds der Chemischen Industrie for a Chemiefonds
fellowship. P.J.L. holds an Australian Research Council Future
Fellowship (FT120100073) and gratefully acknowledges
financial support for this work from the ARC (DP140100855).
Synthesis of 2,5-Diferrocenyl-1,1,3,4-tetraphenyl-1H-silole
(3b). Lithium naphthalenide was prepared by stirring 84.5 mg (12.18
mmol) of lithium with 1.56 g (12.18 mmol) of naphthalene in 10 mL
of dry tetrahydrofuran at room temperature for 18 h. To this solution
was added 1.17 g (3.04 mmol) of diphenylbis(phenylethynyl)silane in
a single portion, and the mixture was stirred for 1 h. Afterward, 3.41 g
(12.18 mmol) of [ZnCl2·2thf] was added in a single portion at 0 °C,
10 mL of dry tetrahydrofuran was added, and the mixture was stirred
again for 1 h. Iodoferrocene (1.97 g (6.33 mmol)) and 12 mg (17.5
μmol) of [P(tC4H9)2C(CH3)2CH2Pd(μ-Cl)]2 were added, and the
reaction mixture was stirred for 2 days at 80 °C. After evaporation of
all volatiles, the solid material was dissolved in 40 mL of diethyl ether
and washed three times with 30 mL portions of water. The organic
phase was dried over MgSO4, and all volatiles were removed. The
remaining crude solid was purified by column chromatography
(column size 20 × 2 cm, on alumina) using a 4/1 (v/v) n-hexane/
dichloromethane mixture as eluent. All volatiles were removed under
reduced pressure, and the wine red solid was crystallized from n-
hexane at 68 °C. Yield: 12% (based on diphenylbis(phenylethynyl)-
silane). C48H38Fe2Si (M = 754.59). Mp: 280 °C dec. IR (KBr, in
cm−1): 3089 (w), 3070 (m), 3048 (m), 3021 (m), 1594 (w), 1557
(m), 1483 (m), 1429 (m), 1267 (s), 1112 (s), 1105 (s) 1000 (s), 819
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1
(s), 770 (s) 735 (s), 706 (s), 504 (s). H NMR (CDCl3, δ in ppm):
3.52 (s, 10H, C5H5), 3.57 (pt, JHH = 1.89 Hz, 4H, C5H4), 3.95 (pt, JHH
= 1.89 Hz, 4H, C5H4), 7.06−7.09 (m, 4H, o-C6H5), 7.13−7.17 (m, 2H,
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68.6 (C5H4), 69.5 (C5H5), 69.5 (C5H4), 82.8 (iC−C5H4), 126.6 (C-o-
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nm (5297 L mol−1 cm−1). HRMS (ESI-TOF, m/z): calcd for
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