Bonding and Electronic Properties of Linear Diethynyl Oligothienoacene-Bridged Diruthenium Complexes and Their Oxidized Forms

Article abstract of DOI:10.1021/acs.inorgchem.7b01433

A series of five diruthenium diethynyl complexes based on α,β-fused oligothienoacenes in the core of the bridging ligands [{Ru(dppe)Cp?}₂(μ-C=C-L-C=C)] [dppe = 1,2-bis(diphenylphosphino)ethane, Cp? = η⁵-C₅Me₅; L = thieno[3,2-b]thiophene (4), thieno[2,3-b]thiophene (5), 3,4-dimethylthieno[2,3-b]thiophene (6), dithieno[3,2-b:2′,3′-d]thiophene (7), and thieno[3,2-b]thieno[2′,3′:4,5]thieno[2,3-d]thiophene (8)] have been synthesized and fully characterized electrochemically and spectroscopically. Elongation of the redox noninnocent oligothienoacene bridge core causes a smaller potential difference between the initial two anodic steps, not seen for free dialkyl oligothienoacenes, and increased positive charge delocalization over the conjugated bridge backbone. The highest occupied molecular orbital of the parent complexes resides predominantly on the oligothienoacene core, with strong participation of the ethynyl linkers and slightly smaller contribution from the metallic termini. This bonding character makes the initial one-electron oxidation symmetrical, as revealed by combined voltammetric and spectroscopic (IR, UV-vis-near-IR, and electron paramagnetic resonance) methods as well as density functional theory (DFT) and time-dependent DFT calculations of truncated and selected nontruncated models of the studied series. The remarkable gradual appearance of two C=C stretching absorptions in the IR spectra of the monocationic diethynyl complexes is ascribed to increasing vibronic coupling of the IR-forbidden ν_s(C=C) mode of the oxidized -[C=C-core-C=C]⁺- bridge with a low-lying π-π?(intrabridge)/metal-to-ligand charge-transfer electronic transition in the near-to-mid-IR spectral region.

Full text of DOI:10.1021/acs.inorgchem.7b01433

Article
pubs.acs.org/IC
Bonding and Electronic Properties of Linear Diethynyl
Oligothienoacene-Bridged Diruthenium Complexes and Their
Oxidized Forms
Ya-Ping Ou,†,‡,§ Jing Zhang,†,§ Ming-Xing Zhang,† Fuxing Zhang,‡,⊥ Daizhi Kuang,‡,⊥ Frantisek Hartl,*,∥
̌
and Sheng Hua Liu*,†
†Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University
(CCNU), Wuhan 430079, P. R. China
‡College of Chemistry and Material Science and ⊥Key Laboratory of Functional Organometallic Materials of Hunan Province College,
Hengyang Normal University, Hengyang, Hunan 421008, P. R. China
∥Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, U.K.
S
* Supporting Information
ABSTRACT: A series of five diruthenium diethynyl com-
plexes based on α,β-fused oligothienoacenes in the core of the
bridging ligands [{Ru(dppe)Cp*}2(μ-CC−L−CC)]
[dppe = 1,2-bis(diphenylphosphino)ethane, Cp* = η5-
C5Me5; L = thieno[3,2-b]thiophene (4), thieno[2,3-b]-
thiophene (5), 3,4-dimethylthieno[2,3-b]thiophene (6),
dithieno[3,2-b:2′,3′-d]thiophene (7), and thieno[3,2-b]thieno-
[2′,3′:4,5]thieno[2,3-d]thiophene (8)] have been synthesized
and fully characterized electrochemically and spectroscopically.
Elongation of the redox noninnocent oligothienoacene bridge
core causes a smaller potential difference between the initial
two anodic steps, not seen for free dialkyl oligothienoacenes,
and increased positive charge delocalization over the conjugated bridge backbone. The highest occupied molecular orbital of the
parent complexes resides predominantly on the oligothienoacene core, with strong participation of the ethynyl linkers and
slightly smaller contribution from the metallic termini. This bonding character makes the initial one-electron oxidation
symmetrical, as revealed by combined voltammetric and spectroscopic (IR, UV−vis−near-IR, and electron paramagnetic
resonance) methods as well as density functional theory (DFT) and time-dependent DFT calculations of truncated and selected
nontruncated models of the studied series. The remarkable gradual appearance of two CC stretching absorptions in the IR
spectra of the monocationic diethynyl complexes is ascribed to increasing vibronic coupling of the IR-forbidden νs(CC) mode
of the oxidized −[CC−core−CC]+− bridge with a low-lying π−π*(intrabridge)/metal-to-ligand charge-transfer electronic
transition in the near-to-mid-IR spectral region.
electron paramagentic resonance (EPR), etc.] and theoretical
calculations.4−11
INTRODUCTION
■
Numerous redox-active di- and oligonuclear transition-metal
complexes exhibit a wide range of important electronic,
magnetic, and optical properties1 that can be varied reversibly
along readily accessible multiple redox states. Certain physical
properties of bimetallic complexes, such as luminescence and
nonlinear-optical activity, may display larger variability
compared to the monometallic congeners.2 Specifically, in
molecular electronics, carefully designed symmetric bimetallic
complexes with conjugated bridging ligands and two or more
redox centers featuring a variable degree of electronic
communication are of central importance to study the mixed-
valence (MV) electronic coupling3 and develop a variety of
functional materials. The ability of the molecular bridge to
mediate electronic communication has been investigated by a
wide range of methods including voltammetric techniques and
spectroelectrochemistry [UV−vis−near-IR (NIR)−IR, Raman,
Diruthenium complexes based on a few classical types of
redox-active metallic termini supported by ancillary ligands are
often explored to test diverse types of carbometalated bridging
ligands in symmetric MV systems.12 The electronic coupling
between the metal centers strongly depends on the key
properties of the bridging ligand, such the degree of
conjugation, coplanarity, and length. In the earlier literature,
researchers have mainly focused on bridging conjugated
polyaromatic hydrocarbons,13 such as oligophenylene.14 Thio-
phene-based heteroacenes have emerged as excellent building
blocks in the synthesis of a variety of optoelectronic materials.
These organic semiconductor materials exhibit considerable
Received: June 5, 2017
© XXXX American Chemical Society
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Chart 1. Previously Published11a Diethynyl Oligothiophene-Bridged Diruthenium Complexes 1−3 and the Target New Series of
a
Diethynyl Oligothienoacene-Bridged Diruthenium Complexes 4−8
a
Complex 1 links both groups. Diethynyl dithienoacene isomers 4−6 form a separate subgroup.
potential for application as photoswitches,15 dye-sensitized
solar cells,16 and organic field-effect transistors17 because of
their good conjugation. In the field of organometallic chemistry,
α,β-fused oligothienoacene moieties have been applied as
organic bridging units and introduced to gold- and platinum-
containing ethynyl complexes; their physical and luminescent
properties were investigated.18 Recently, Lapinte and co-
workers19 have described magnetic communication between
two [Fe(dppe)Cp*] units mediated by the 2,5-diethynylth-
iophene spacer. Chen and co-workers20 have also reported
thiophene-based bimetallic ruthenium complexes and studied
their electron-transfer properties. Subsequently, Liu and co-
workers investigated the electronic coupling properties of
oligothiophene-bridged binuclear ruthenium complexes (Chart
1) and their charge-transfer ability.11a Recent studies21
document that the charge-transport ability across a molecular
wire decreases and even drops exponentially with an increase in
their length. In overall consideration,22 the fused molecular
framework of thienoacenes represents one of the most
attractive candidates for low-resistance molecular-scale wires
featuring favorable electrical conductance characteristics.
The systematic investigation of thiophene-based hetero-
acenes with higher conjugation may reveal excellent charge-
transfer properties. With this in mind, we have synthesized and
characterized a series of diethynyl oligothienoacenes as bridging
ligands carbometalated to Ru(dppe)Cp* terminal groups, viz.,
complexes 4−8 (Chart 1) based on thieno[3,2-b]thiophene
(4), thieno[2,3-b]thiophene (5), 3,4-dimethylthieno[2,3-b]-
thiophene (6), dithieno[3,2-b:2′,3′-d]thiophene (7), and
thieno[3,2-b]thieno[2′,3′:4,5]thieno[2,3-d]thiophene (8) in
the bridge core. We aimed (i) to probe with spectroelec-
trochemistry and quantum-chemical calculations how the
elongation of the oligothienoacene bridge core in complexes
1, 4, 7, and 8 affects their redox and electronic coupling
properties, (ii) to evaluate the effect of the positional isomerism
and methyl substitution of the dithienoacene bridge core in
complexes 4−6 on their electronic properties, and (iii) to
compare the new experimental results with those previously
reported11a for closely related complexes 1−3 with oligothio-
phenes in the bridge core.
RESULTS AND DISCUSSION
■
Syntheses and Characterization. The general synthetic
route toward diethynyl oligothienoacene-bridged diruthenium
complexes 4−8 is outlined in Scheme 1. Bridge-core precursors
2,5-dibromothieno[3,2-b]thiophene (4a),23 2,5-dibromothieno-
[2,3-b]thiophene (5a),23 2,5-dibromo-3,4-dimethylthieno[2,3-
b]thiophene (6a),24 5,5′-dibromodithieno[3,2-b:2′,3′-d]-
thiophene (7a),17,23 and 2,6-dibromothieno[3,2-b]thieno-
[2′,3′:4,5]thieno[2,3-d]thiophene (8a)25 were prepared by
the literature methods. Intermediates 4b−8b were obtained
in moderate-to-high yields by palladium(0)/copper(I)-cata-
lyzed cross-coupling reactions26 of (trimethylsilyl)ethyne with
4a−8a, respectively. The low solubility of compound 8a
resulted in lower yields (31%) of complex 8b. Finally,
compounds 4b−8b were deprotected by removal of the
trimethylsilyl (TMS) group in a methanolic KF solution and
reacted with [RuCl(dppe)Cp*] for 24 h at 60 °C. Target
complexes 4−8 were collected by filtration and characterized by
conventional spectroscopic methods. However, low solubility
precluded the collection of sufficiently resolved 13C NMR
spectra of complexes 4 and 5. The studied diruthenium
1
complexes exhibit some characteristic H NMR signals. For
example, complex 7 shows the proton chemical shifts (in ppm)
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grown by layering the solution in dichloromethane with hexane.
The molecular structures of 7 and 8, including the top and side
views, are shown in Figure 1, respectively. Details of the data
collection and refinement are presented in Table S1 in the
Supporting Information (SI). Selected parameters [bond
lengths (Å) and angles (deg)] from the crystal structures of
parents 7 and 8 are collected in Tables 1 and S2 in the SI).
Both diruthenium complexes are symmetrical and exhibit a
planar rigid structure over the entire bridge (Figure 1). As
encountered in other Ru(dppe)Cp*-based complexes,13 the
ethynyl linker and the Cp* and dppe ligands at the ruthenium
center form a pseudooctahedral environment. The P1−Ru1−
P2 angles in complexes 7 and 8 reach 83.35° and 83.57°,
respectively, falling within the range of 83−84° reported for a
number of ruthenium phenylacetylide complexes.13b,27 The
bonding parameters of the planar oligothienoacene core, viz.,
thieno[3,2-b:2′,3′-d]thiophene in 7 and thieno[3,2-b]thieno-
[2′,3′:4,5]thieno[2,3-d]thiophene in 8, do not deviate signifi-
cantly from those reported for free triisopropylsilyl octathie-
noacene.28 The Ru1−C37−C38 angles in complexes 7 and 8
are 174.63° and 168.81°, respectively.
Scheme 1. Reaction Paths to Diethynyl Oligothienoacene-
Bridged Diruthenium Complexes 4−8 and Corresponding
a
Reaction Yields
Electrochemical Studies. The electronic properties of
complexes 4−8 were first studied by electrochemistry, revealing
differences in their stepwise one-electron oxidation. The
separation of two reversible redox waves (ΔE1/2) is known to
be potentially influenced by several factors such as electrostatic
interaction, solvation, ion pairing with the electrolyte, and
structural distortions caused by an electron-transfer proc-
ess.29−32 The anodic responses of the complexes were obtained
in CH2Cl2/10−1 M Bu4NPF6 with cyclic voltammetry (CV) and
square-wave voltammetry (SWV); the corresponding data are
listed in Table 2. The oxidation potentials of reference
complexes 1−3 have been reported in the literature.11a
Cyclic and square-wave voltammograms of complexes 1 and
4−8 are depicted in Figures 2 and 3 and S1−S4 in the SI. All of
the complexes readily undergo two consecutive fully reversible
one-electron oxidations. Figure 3a reveals that the E1/2(2)
potential in the oligothienoacene series 1, 4, 7, and 8 changes
only slightly with increasing length of the bridge core, while the
a
Reagents and conditions: (i) TMSA, [Pd(PPh3)4], CuI, THF/(i-
Pr)2NH; (ii) [RuCl(dppe)Cp*], KF, CH3OH/THF. TMSA =
(trimethylsilyl)acetylene, dppe = 1,2-bis(diphenylphosphino)ethane,
and Cp* = pentamethylcyclopentadiene.
for C5(CH3)5 at δ 1.55, CH2/dppe at δ 2.06 and 2.68, and
heptathienoacene-H at δ 6.42. The tetrathienoacene-H and
CH2/dppe signals of complex 4 coincide. Characteristic 13C
NMR resonances of Ru−CC− in 6−8 are observed at δ 117.
The 31P NMR spectra of complexes 4−8 display only one
signal at δ 79.9, reflecting the molecular symmetry. The
νas(CC) wavenumber in the IR spectra of the solid diethynyl
complexes 1, 4, 7, and 8 (in the Nujol mull) gradually decreases
from 2064 to 2041 cm−1 as the conjugated bridge core
elongates. This trend is in line with the increasing electron-
withdrawing power of the oligothienoacene bridge core,
reflected in the less negative oxidation potentials (see below).
X-ray Structure Determination. Single crystals of
complexes 7 and 8 suited for X-ray structural analyses were
E
1/2(1) values increase profoundly, from −0.387 V for 1 to
−0.180 V for 8. The potential difference ΔE1/2 decreases
Figure 1. X-ray structures of complexes 7 (a and b) and 8 (c and d) with thermal ellipsoids at the 50% probability level: (a and c) top view; (b and
d) side view. Hydrogen atoms and solvent molecules have been removed for clarity.
C
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Table 1. Selected Bond Lengths (Å), Angles (deg), and Interatomic Distances (Å) from the Crystal Structure of Complex 7 and
a
the DFT-Optimized Structures of Models [7-H]n+ (n = 0, 1, 2) and trans-7+
b
parameter
Ru1−C37
7
trans-7+ (BLYP35)
[7-H] (B3LYP)
[7-H]+ (B3LYP)
[7-H]2+ (B3LYP)
1.986(3)
1.913
1.910
2.011
2.280, 2.279
1.230
1.401
1.385
1.418
1.757
1.402
1.418
1.736
1.787
93.08
178.41
178.90
15.4
1.966
2.301, 2.301
1.242
1.373
1.410
1.391
1.756
1.428
1.392
1.736
1.789
92.57
178.00
178.07
15.3
1.921
2.323, 2.322
1.256
1.351
1.437
1.369
1.757
1.453
1.371
1.739
1.793
91.18
177.53
176.71
15.2
Ru1−P1,2
C37−C38
C38−C39
C39−C40
C40−C41
C41−S2
C41−C42
C42−C42′
S1−C42
2.262(7), 2.276(7)
1.219(4)
1.416(4)
1.372(4)
1.414(4)
1.745(3)
1.382(4)
1.407(4)
1.722(3)
1.757(3)
83.35(3)
174.6(2)
179.7(3)
15.2
2.281, 2.273
1.206
2.287, 2.277
1.205
1.336
1.338
1.363
1.363
1.343
1.347
1.697
1.697
1.373
1.373
1.349
1.349
1.679
1.680
S1−C39
1.719
1.719
P1−Ru1−P2
Ru1−C37−C38
C37−C38−C39
Ru···Ru′
83.35
83.09
174.99
179.53
14.9
173.80
179.50
14.9
a
b
The values are identical for the molecule halves, with the exception of slightly asymmetric trans-7+. The values for trans-7+ in the right column
correspond to the atom numbering with apostrophes in Figure 1a.
the terminal ruthenium centers (Figure S5 in the SI). The
increasing delocalization of the unpaired electron/hole over the
longer redox-noninnocent diethynyl oligothienoacene bridge
decreases the further input of energy required to place the
other hole in the monocationic system. Notably, previously
reported diethynyl oligothiophene-bridged complexes 1−311a
exhibit more pronounced changes in ΔE1/2 because of the
larger amplitude of the chain-length variation from a single
thiophene to bithiophene and linear terthiophene. The
comproportionation constants Kc for complexes 1, 4, 7, and
8 dwindle along the series from 2.25 × 105 to 191 (Table 2),
marking the gradually decreasing thermodynamic stability of
the electrochemically generated open-shell monocations.
In contrast to the studied oligothienoacene series with the
ruthenium ethynyl termini, the oxidation potentials of free
alkyl-disubstituted oligothienoacenes, R−Tn−R (n = 4−8),
strongly decrease with the chain length, and the ΔE1/2 values
do not decline below 300 mV.33 This difference in the anodic
behavior is one of the characters reflecting the involvement of
the conjugated (ruthenium−)ethynyl units in the stepwise
oxidation.
Table 2. Electrochemical Data Determined for Complexes
1−8
a
b
c
complex
E1/2(1) (V)
E1/2(2) (V)
ΔE1/2 (mV)
Kc
2.25 × 105
207
22.5
c
1
−0.387
−0.224
−0.052
−0.320
−0.296
−0.343
−0.224
−0.180
−0.067
−0.087
−0.132
−0.072
−0.026
−0.055
−0.040
−0.045
320
137
80
c
2
c,d
3
4
5
6
7
8
248
270
288
184
135
1.56 × 104
3.66 × 104
7.61 × 104
1.29 × 103
191
a
The electrode potential values versus the Fc/Fc+ standard redox
couple were recorded at 25 °C in dry dichloromethane containing
10−1 M Bu4NPF6. Additional anodic waves observed at higher
potentials, most likely due to the oligothienoacene bridge core, were
b
c
not examined further. ΔE1/2 = E1/2(2) − E1/2(1). Reference 11a. The
comproportionation constant Kc = exp(ΔE1/2/25.69) at 298 K.
d
Calculated according to ref 34 for the experimental value Ep = −0.092
mV (unresolved maxima) and a width of 154 mV (the differential-
pulse method).
The differences in the anodic potentials are marginal in the
series of diethynyl dithienoacene complexes 4−6 (Figure 3b
almost linearly from 320 to 135 mV with the number (1−4) of
α,β-fused thiophene rings (Figure 4) and the distance between
Figure 2. Anodic voltammetric responses of complex 7 in CH2Cl2/Bu4NPF6. (a) Cyclic voltammograms at different scan rates (0.05, 0.1, 0.2, 0.4,
and 0.6 V s−1). (b) Square-wave voltammogram at f = 10 Hz (tp = 25 mV). The potential scale is referenced versus Fc/Fc+.
D
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Chemical Oxidation Monitored by IR and UV−Vis−
NIR Spectroscopy. The characteristic IR CC stretching
absorption can conveniently be used to monitor structural
changes accompanying the strongly bridge-localized oxidation
of the studied series of complexes (Chart 1). The CV and SWV
studies have revealed that both the first and second oxidation
potentials of complexes 1−8 are lower compared to E1/2 of the
ferrocene standard (Table 2). Ferrocenium hexafluorophos-
phate (FcPF6) therefore served conveniently as a mild oxidizing
agent to selectively generate the corresponding mono- and
dications in both the oligothiophene-core series 1−311a and the
new oligothienoacene-core series 4−8.
The νas(CC) vibrational frequencies recorded for
complexes 1n+−8n+ (n = 0−2) in dichloromethane are
presented in Table 3. In the oligothienoacene-core series 1,
Table 3. IR ν(CC) Wavenumbers (cm−1) [av. = Average
Value] Recorded for Complexes [1−8]n+ (n = 0, 1, 2) in
a,b
Dichloromethane
complex
n = 0
n = 1
n = 2
c
1n+
2056 (w) 1961 (s) [av. 1961]
1914 (m-w)
c
2n+
2055 (w) 1982 (sh), 1933 (s) [av. 1958] 1909 (m-w)
2046 (w) 2007 (w), 1920 (m) [av. 1964] 1971 (w), 1917 (w)
2052 (w) 1974 (sh), 1948 (s) [av. 1961] 1912 (m-w)
2050 (m) 1992 (m), 1923 (s) [av. 1958] 1916 (w)
c
3n+
4n+
5n+
6n+
2047 (w) 2000(m-s), 1909 (s) [av.
1955]
1900 (vw)
Figure 3. Anodic square-wave voltammograms of (a) the series of
oligothienoacene complexes 1, 4, 7, and 8 and (b) the series of
dithienoacene complexes 4, 5, and 6 in CH2Cl2/Bu4NPF6 at f = 10 Hz
(tp = 25 mV). The potential scale is referenced versus Fc/Fc+.
7n+
8n+
2045 (w) 1981 (w), 1939 (s) [av. 1960] 1907 (m-w)
2043 (w) 1988 (w), 1933 (s) [av. 1961] 1911 (m-w)
a
Oxidation of the neutral parent complexes to corresponding mono-
and dications was carried out by the addition of exact equivalent
b
amounts of FcPF6. The low-intensity νs(CC) mode was not
detected in the experimental IR spectra of neutral parent complexes
and corresponding dications. For 7 and 7+, both νs(CC) and
νas(CC) wavenumbers were obtained by harmonic-frequency IR
c
and Raman calculations; see the DFT section. Reference 11a.
4, 7, and 8 (Figure 5), the experimental IR spectra in the
ν(CC) region exhibit a single band of low-to-medium
intensities, with absorption maxima shifting gradually to a larger
wavenumber for the shorter bridge core, viz., from 2043 cm−1
(for 8) to 2056 cm−1 (for 1). This trend, which is also seen11a
in the diethynyl oligothiophene-bridged series 1−3 (Table 3),
indicates less π conjugation between the ethynyl linker and the
shorter bridge core. As a result, the highest occupied molecular
orbital (HOMO) energy rises in this direction and the
oxidation potential becomes more negative (vide supra). In
contrast, the corresponding dications do not display any
obvious trend in the main ethynyl stretching wavenumber
values, all lying close to 1910 cm−1 (Table 3), in line with the
conjugated symmetric {MCC}2C(core)2+ backbone.
This observation complies with the stagnant electrode
potentials for the mono/dication redox couples (Table 2).
The most interesting observations in the IR ν(CC) region
have been made for the singly oxidized cationic species divided
into the oligothiophene series 1, 4, 7, and 8 and the
dithienoacene series 4−6. Except for 1+ absorbing at 1961
cm−1, the cations with the elongated oligothienoacene core
feature two ν(CC) absorption bands with increasing
̃ ̃
separation: 4+ (Δν = 26 cm−1), 7+ (Δν = 42 cm−1), and 8+
Figure 4. Plot of the anodic potential difference ΔE1/2 for diruthenium
complexes 1, 4, 7, and 8 versus the number of α,β-fused thiophene
rings in the bridge core.
and Table 2). The isomerization of the bridge core from
thieno[3,2-b]thiophene in 4 to thieno[2,3-b]thiophene in 5
(with the sulfur atoms located on the same side of the unit)
caused a slight positive shift of the anodic potentials and a twice
as large Kc value. Even higher stability was observed for 6+ with
two methyl substituents attached to the thieno[2,3-b]thiophene
isomer in the bridge core. In contrast to the minor variability of
the anodic voltammetric behavior, the structural changes in the
dithienoacene series have profound consequences for the NIR-
to-mid-IR electronic and IR vibrational absorption of 4+−6+, as
presented in the following section.
(Δν
̃
= 55 cm−1); the average wavenumber value, however,
remains close to 1961 cm−1 (Table 3). At the same time, the
increased band separation is accompanied by the rising
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Figure 5. IR spectra in the ν(CC) region of complexes 1 and 4−8 in dichloromethane and their corresponding mono- and dications formed by
the addition of equivalent amounts of FcPF6.
intensity of the ν(CC) band at the larger wavenumber.
series where 5+ (Δν = 69 cm−1) and 6+ (Δν = 91 cm−1) largely
̃ ̃
the HOMO → lowest unoccupied molecular orbital (LUMO;
π−π*) transition characteristic for free oligothienoacenes,33
with participation of the π system of the ethynylene linkers.
The one-electron oxidation to 7+ generates two intense subgap
absorption bands at 683 and 1800 nm with a shoulder at
around 1375 nm. This new visible and asymmetric NIR
absorption is typical for radical cations of free oligothienoa-
cenes and can be ascribed to singly occupied molecular orbital
(SOMO) → LUMO and HOMO → SOMO (π−π*)
transitions, respectively.33 It is evident that the oxidation of 7
to 7+ is largely localized on the trithiophene bridge core. The
participation of the (ruthenium−)ethynyl linkers, revealed by
the IR spectral monitoring and anomalous voltammetric
responses (see above), will be discussed in greater detail in
the following time-dependent DFT (TD-DFT) section.
Continued oxidation of 7+ to 72+ led to the appearance of a
new intense absorption band at 909 nm, which also complies
with the dominantly terthiophene-localized anodic steps; in the
spectra of free oligothiophene dications, this band is attributed
to a HOMO → LUMO (π−π*) transition.33
Notably, this trend reaches the maxima in the dithienoacene
exceed the values obtained for 8+. In this case, the remarkable
IR spectral changes in the ν(CC) region cannot explain the
coexistence of different rotamers of the monocations, modeled
by density functional theory (DFT) calculations,35−37 as the
one-electron oxidation largely resides on the oligothiophene
bridge core (see below). Instead, it is important to consider the
variable energy of lowest NIR-to-mid-IR π−π* (intrabridge)
electronic absorption accompanying the conversion of the
neutral parent complexes to the monocations, as highlighted
hereafter.
The changes in the electronic UV−vis−NIR and IR
absorption recorded for complexes 1 and 4−8 upon the
gradual addition of 1 and 2 equiv of the FcPF6 oxidant to their
solutions in dichloromethane are depicted in Figures 6 and S6−
S8 in the SI. The corresponding electronic absorption data for
the neutral parent complexes and their mono- and dications are
listed in Table 4. The UV−vis−NIR spectral responses to the
two initial oxidation steps within the diruthenium diethynyl
oligothienoacene series 1 and 4−8 are very similar and will be
demonstrated in detail for complex 7 with alternating
trithienoacene in the bridge core (Figure 6). Neutral parent 7
exhibits an intense absorption at 445 nm that corresponds to
All three members of the diethynyl tetrathienoacene redox
series, 8n+ (n = 0, 1, 2), exhibit the intrabridge electronic
transitions red-shifted compared to 7n+, while the opposite blue
shift is encountered for the two shorter members 4n+ and 1n+ (n
= 0, 1, 2). The descendent trends for the elongated
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Figure 6. Red shift of the corresponding (generally intrabridge) electronic transitions in the UV−vis−NIR absorption spectra recorded for the
diruthenium diethynyl oligothienoacene redox series 1n+, 4n+, 7n+, and 8n+ (n = 0, 1, 2), with increasing length of the oligothienoacene bridge core:
(a) n = 0; (b) n = 1; (c) n = 2. Conditions: CH2Cl2, 298 K.
Figure 5), analysis of the corresponding NIR electronic
absorption provides strong support for vibronic coupling13b
of the ν(CC) modes of the oxidized −[CC−core−C
C]+− bridge to the low-lying electronic transition. The stronger
coupling, taking place upon a significant decrease in the
excitation energies, activates the νs(CC) mode of the bridge
in terms of both gained intensity and wavenumber difference
from the asymmetric stretching mode. Thus, for singly oxidized
1+, 4+, 7+, and 8+, the absorption maxima of the lowest NIR
absorption bands shift from 7195 to 6370, 5555, and 4930
cm−1, respectively, resulting in the pronounced activation of the
symmetric stretching mode of the oxidized diethynyl
oligothienoacene bridge in the same direction (Figure 5).
Strong support for this explanation is obtained from monitoring
of the oxidation of the diethynyl dithienoacene series 4−6.
Whereas the redox properties in this series are very similar (see
Table 2), the energy of the lowest electronic transition further
decreases from 6370 cm−1 for 4+ to 4000 cm−1 for 5+ and even
3825 cm−1 for 6+, which is the minimum value in the studied
diethynyl oligothienoacene series. Accordingly, the strongest
effect on the ν(CC) modes is observed for 6+ (Figure 5),
Table 4. Electronic Absorption of Complexes 1 and 4−8 in
a
Their Mono- and Dicationic Forms in CH2Cl2 at 298 K
complex
λmax, nm (10−4 εmax, dm3 mol−1 cm−1
)
1
400 (2.51)
1+
12+
4
600 (2.63), 1390 (2.11)
754 (1.73)
421 (1.15)
4+
42+
5
639 (2.81), 1569 (2.75)
812 (4.86)
336 (2.06)
5+
52+
6
341 (1.56), 767 (0.52), 2500 (0.72)
446 (1.37), 820 (0.93), 1237 (0.39)
329 (2.25), 841 (0.14)
425 (1.11), 799 (0.88), 2614 (0.68)
445 (3.12)
6+
7
7+
72+
8
683 (1.89), 1800 (2.04)
909 (3.05)
461 (2.97)
8+
82+
707 (2.06), 2028 (2.42)
1017 (3.67)
a
Oxidized species 62+ was poorly soluble under the given experimental
showing the largest energy gap Δν
̃
= 91 cm−1 between the
conditions.
ν(CC) absorption maxima and comparable band intensities.
These characteristics nicely correspond with a very similar
situation reported for another singly oxidized ruthenium
diethynyl complex, viz., [{Ru(dppe)Cp*}2(μ-CC−L−C
C)]+, where L = benzo[1,2-b;4,3-b′]-dithiophene (bent).35 In
the latter case, the lowest electronic absorption (featuring a
mixed ML(bridge)-CT and π−π*(benzodithiophene) charac-
oligothienoacene bridge core in each oxidation state are
visualized in parts a (n = 0), b (n = 1), and c (n = 2) in
Figure 6.
Focusing on the remarkable spectral changes observed in the
IR ν(CC) region of the monocationic (n = 1) series (see
G
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ter) lies at 3200 cm−1 and the separation of the two ν(CC)
̃
absorption maxima of comparable intensities reaches Δν = 86
cm−1. The substitution with L = benzo[1,2-b;4,5-b′]-
dithiophene (linear) caused a blue shift of the NIR absorption
to 4540 cm−1, accompanied by a significant decrease of the
ν(CC) separation to Δν
̃
= 41 cm−1 and a reduced relative
intensity of the symmetric stretching mode. The closely related
diethynyl oligothiophene series11a 1+−3+ represents another
example of the vibronic coupling affecting the νs(CC)
vibrations of the singly oxidized bridge.
DFT and TD-DFT Calculations. In order to assist analysis
of the molecular and electronic structures in the diethynyl
oligothienoacene series [1]n+, [4]n+, [7]n+, and [8]n+ (n = 0, 1,
2), truncated model complexes [1-H]n+, [4-H]n+, [7-H]n+, and
[8-H]n+ were selected for DFT calculations at the B3LYP/6-
31G* level. The extension “-H” indicates the replacement of
the Cp* and dppe ligands in the parent complexes by Cp and
two PH3 ligands, respectively. Representative nontruncated
model complex 7+ has been selected for DFT calculations based
on the global hybrid BLYP35 functional and the 6-31G* basis
set, following examples in the literature.13,35−38 Key frontier
orbitals of [4-H]n+−[8-H]n+ (n = 0, 1, 2) with electron density
distribution are shown in Figures S9−S13 in the SI. The lists of
frontier molecular orbital energies and compositions resulting
from Mulliken analysis are provided in Tables S3−S17 in the
SI.
DFT (G09-B3LYP)-optimized truncated model structures
[7-H]n+ and [8-H]n+ (n = 0, 1, 2) reveal significant changes in
the bond lengths accompanying the sequential one-electron
oxidation of the conjugated bridging ligands (see below), i.e.,
gradual lengthening of the CC and CC bonds and
shortening of the C−C bonds (Chart 1). The bridge core in the
dications obtains a markedly quinoid structure, much like the
crystalline dication of free triisopropylsilyl octathienoacene;28
the Ru−CC−C(core) moiety converted to delocalized
{RuCC}2C(core)2+. More accurate bond lengths and
angles in singly oxidized 7+ have been obtained with the G09-
BLYP35 DFT method (Table 1). The HOMOs of complexes
[4-H]−[8-H] are indeed largely delocalized over the diethynyl
oligothienoacene bridge, with contributions rising from 76% to
84% upon elongation of the bridge core; the metallic termini
become less involved in the same direction.
The calculated spin-density distribution in monocations [1-
H]+, [4-H]+, [7-H]+, and [8-H]+ reveals, in line with the parent
HOMO character, that the oxidation is increasingly localized on
the elongated diethynyl oligothienoacene bridge, viz., from 72%
in [1-H]+ to 80% in [8-H]+. However, participation of the
ethynyl linkers decreases along this series from 36% in [1-H]+
to 24% in [8-H]+, whereas involvement of the oligothienoacene
core rises from 36% in [1-H]+ to 56% in [8-H]+ (Figure 7).
These changes have a strong implication for the ν(CC)
wavenumbers in the IR spectra (see below). For the dicationic
species [1-H]2+, [4-H]2+, [7-H]2+, and [8-H]2+, the HOMO
and LUMO are also largely delocalized over the ethynyl-core
backbone with some participation of the ruthenium centers,
having a composition very similar to those of the HOMO−1
and HOMO of the neutral parents (Figures S9−S13 in the SI).
The LUMO+1 and LUMO are different, except those of [8-
H]2+ and [8-H], respectively.
Figure 7. Spin-density distributions in oligothienoacene (Tn)-bridged
diruthenium monocations [1-H]+, [4-H]+, [7-H]+, and [8-H]+ and
trans-7+ and cis-7+ (Ru/CHCH/Tn/CHCH/Ru) with the
corresponding compositions. Contour values: 0.0004 e/bohr.3
cm−1 (n = 2), [7-H]n+, 2111 cm−1 (n = 0) and 1982 cm−1 (n =
2), and [8-H]2+, 2110 cm−1 (n = 0) and 1986 cm−1 (n = 2)
(scaled by 0.9614; see the Computational Details section). The
large Δν
̃
(CC) values of 143, 137, 134, and 129 cm−1 along
the series, which are very close to the experimental values of
142, 140, 138, and 132 cm−1, respectively (Table 3), reveal
significant weakening of the CC bond upon two-electron
oxidation. The smaller Δν
̃
(CC) values reflect the declining
involvement of the ethynyl linkers in the oxidation that
becomes increasingly localized on the elongated, strongly
conjugated oligothienoacene bridge core. The same descending
trend applies for the corresponding cationic series [1-H]+, [4-
H]+, [7-H]+, and [8-H]+. The calculated Δν
̃
(CC) values of
91, 87, 85, and 82 cm−1, respectively, are consistent with the
changing spin-density distribution in the model cationic
complexes, as shown in Figure 7.
A number of earlier reported singly oxidized diruthenium
diethynyl complexes with a planar aromatic bridge core show
multiple ν(CC) absorption bands assigned to different
rotamers with a variable degree of the MV state.13,28 Within the
studied diethynyl oligothienoacene series, three principal
conformations, viz., trans, cis, and perp, were modeled for
representative nontruncated cation 7+, having employed the
expedient global hybrid functional BLYP35 introduced recently
by Kaupp, Low, and co-workers.35−38 The basis set 6-31G*
(Lanl2dz for the ruthenium atom) was used in combination
with the conductor polarizable continuum model (CPCM) in
CH2Cl2. Notably, the DFT method has only afforded potential
minima for slightly asymmetrical trans-7+ (Table 1) and
symmetrical cis-7+ conformations, while several optimization
programs failed to obtain a stable perpendicular conformation,
perp-7+. In this regard, 7+ resembles the electronically closely
related diethynyl benzodithiophene-bridged diruthenium com-
plexes.36 The DFT results obtained for nontruncated isomers
trans-7+ and cis-7+ within the harmonic approach show strong
νas(CC) absorption at 2231 cm−1; in addition, a very weak
νs(CC) band at 2239 cm−1 was obtained for trans-7+ (scaled
by 0.95; see the Computational Details section). In addition,
the calculated Raman wavenumbers of the symmetric and
The B3LYP DFT calculations yielded single IR ν(CC)
wavenumbers for both neutral [1-H], [4-H], [7-H], and [8-H]
and the corresponding dications [1-H]n+, 2118 cm−1 (n = 0)
and 1980 cm−1 (n = 2), [4-H]n+, 2113 cm−1 (n = 0) and 1981
H
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Table 5. Major electronic excitations of trans- and cis-7+ determined by TD-DFT methods
rotamer
λ/nm [cm−1
]
oscillator strength (f)
major contributions
assignment
trans-7+
1419 [7047]
568 [17605]
481 [20790]
1413 [7077]
575 [17391]
480 [20833]
1.1214
0.8393
0.1787
1.1727
0.8892
0.1975
β-HOSO → β-LUSO (98%)
α-HOSO → α-LUSO (76%)
β-HOSO−6 → β-LUSO (88%)
β-HOSO → β-LUSO (98%)
α-HOSO → α-LUSO (83%)
β-HOSO−6 → β-LUSO (92%)
π−π*(diethynyl trithienoacene) ML(bridge)CT
π−π*(diethynyl trithienoacene) ML(bridge)CT
L(Cp*,dppe)MCT
π−π*(diethynyl trithienoacene) ML(bridge)CT
π−π*(diethynyl trithienoacene) ML(bridge)CT
L(Cp*,dppe)MCT
cis-7+
asymmetric CC stretching modes for trans-7+ are identical
with the IR results; their absorption intensities are inversed. For
comparison, harmonic frequency IR and Raman calculations for
parent 7 also yielded νas(CC) and νs(CC) absorptions at
cationic complexes therefore exhibit certain organic radical
character.39 Importantly, at 150 K, the EPR spectra of the
cations displayed broad signals showing only small g-tensor
anisotropy. The EPR spectra are close to those reported for
cationic oligothiophene complexes 1+−3+.11a The total g-tensor
anisotropy (Δg) values for complexes 1+, 4+, 7+, and 8+ are
0.042, 0.040, 0.030, and 0.033, respectively, falling apparently
out of the range of Δg values for paramagnetic organometallic
complexes with ruthenium(III) centers (0.3−0.6).40−43 The
small giso and Δg values in this series of complexes therefore
comply with the dominant participation of the diethynyl
oligothienoacene bridge in the one-electron oxidation, which is
fully consistent with the results of the DFT calculations (vide
supra).
2294 and 2298 cm−1, respectively. The Δν
̃
(CC) value
calculated for parent 7 and corresponding trans-7+ is smaller
than the experimental value of 106 cm−1 for νas(CC) (Table
3). Both trans-7+ and cis-7+ isomers show an almost symmetric
distribution of the spin density over the bridge core and ethynyl
linkers, viz., 70% for trans-7+ and 72% for cis-7+ (Figure 7),
thereby closely resembling truncated [7-H]+. This outcome
supports the assumption that the proximity of the electronic
absorption in the NIR region is the main factor responsible for
the appearance of the two significantly separated IR-active
ν(CC) modes of 7+, as discussed in the preceding
spectroscopic section. The origin of the low-energy electronic
absorption has been unraveled with TD-DFT (BLYP35)
calculations carried out on the structurally fully optimized
nontruncated models of 7+ (Figure S14 in the SI). The major
electronic excitations in trans-7+ and cis-7+ are listed in Table 5.
The corresponding isosurface plots of molecular orbitals
involved in the major electronic excitations are displayed in
Figure S15 in the SI. Both cationic isomers of 7+ exhibit an
intense absorption band near 7000 cm−1 and two smaller bands
between 17000 and 21000 cm−1 (trans-7+, 7047, 17605, and
20790 cm−1; cis-7+, 7077, 17391, and 20833 cm−1; Table 5),
thereby reproducing well the absorption maxima in the NIR
and visible regions of the experimental electronic absorption
spectrum (Figure 6 and Table 4). Thus, the strong NIR
absorption corresponds to the β-HOSO → β-LUSO excitation
having a dominant π−π*(diethynyl trithienoacene) character
mixed with some ML(bridge)CT. The direct involvement of
the ethynyl linkers in the low-energy electronic excitation is not
surprising, having a big impact on their vibrational behavior
(Figure S15 in the SI). The visible region features an intense
absorption band below 600 nm, largely due to α-HOSO → α-
LUSO displaying very similar π−π*(intrabridge)/MLCT
characteristics (Figure S15 in the SI). Finally, the fairly weak
electronic absorption of the cationic complexes in the blue
spectral region, 400−500 nm (Figure 6b), belongs mainly to
L(Cp*,dppe)MCT, also directed to the β-LUSO.
CONCLUSIONS
■
In this work, we describe the successful syntheses and full
characterization of five redox-responsive diruthenium com-
plexes, 4−8, having thieno[3,2-b]thiophene (4), thieno[2,3-
b]thiophene (5), 3,4-dimethylthieno[2,3-b]thiophene (6),
dithieno[3,2-b:2′,3′-d]thiophene (7), and thieno[3,2-b]thieno-
[2′,3′:4,5]thieno[2,3-d]thiophene (8) in the core of the
diethynyl oligothienoacene bridge. Different from free
oligothienoacenes, elongation of the bridge core leads to
positive potential shifts and a reduced potential difference,
ΔE1/2, between the two anodic steps. This behavior reflects the
involvement of ethynyl linkers, and to a small extent even the
ruthenium centers, in line with the presented DFT calculations
and EPR spectra. In the monocationic series, the spin density is
localized symmetrically on the bridge, with an increasing
contribution of the elongated oligothienoacene core. This trend
for 4+, 7+, and 8+ cannot explain the appearance of two IR
ν(CC) absorption bands with increasing wavenumber
difference along this series and varied intensity ratio. According
to the frequency DFT calculations, the IR spectrum of
nontruncated model trans-7+, electronically very close to the
cis isomer but showing small asymmetry, exhibits one intense
νas(CC) band, while the νs(CC) absorption is vanishingly
low. Other theoretical rotamers on the potential-energy
landscape with asymmetric localization of the spin density
along the molecular backbone have not been obtained. This
behavior can be explained by considering a strong vibronic
coupling of the νs(CC) mode of the oxidized −[CC−
core−CC]+− bridge to a low-lying π−π*(intrabridge)/
MLCT electronic transition observed in the NIR−IR spectral
region. Indeed, the significant red shift of the NIR−IR
absorption maxima is accompanied by the conspicuous
appearance of the dual ν(CC) absorption. Strong support
for this plausible explanation comes from the diethynyl
dithienoacene series 4+−6+ showing otherwise very similar
redox properties. The likely general nature of this phenomenon
is proposed, based on the literature data for other dinuclear
diethynyl complexes with a readily oxidized conjugated bridge
EPR Studies. The contribution of the terminal metal centers
and bridging ligands to the one-electron anodic processes was
further explored by the EPR spectroscopy for monocations 1+,
4+, 7+, and 8+ in CH2Cl2 at 298 and 150 K. The EPR signals
were recorded after 1 equiv of the FcPF6 oxidant was added
(see Figures S16−S19 in the SI). Detailed data are summarized
in Table S18 in the SI. The solutions of the monocations
exhibit at 298 K isotropic singlets with no apparent hyperfine
coupling to 31P nuclei of the ancillary dppe ligands. All
determined giso values in Table S18 in the SI are close to 2.04.
This value is close to the g values for radical cations of free
oligothienoacenes33b,c and a free electron (ge = 2.0023); the
I
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h. The crude product was collected by filtration, washed with methanol
and hexane. The solid was dissolved in dichloromethane, precipitated
by slow diffusion of hexane, then filtered and dried to give 4 as a
yellow powder. Yield: 250 mg, 60%. Note: the compound is poorly
soluble in many deuterated solvents including CDCl3, and its 13C
NMR spectrum could not be collected. 1H NMR (400 MHz, CDCl3):
δ 1.57 (s, 30H, 2C5(CH3)5), 2.04 (br, 4H, CH2/dppe), 2.66 (br, 4H,
CH2/dppe), 7.21−7.37 (m, 32H+2H, HAr/dppe+Hthiophene), 7.74 (br, 8H,
HAr/dppe). 31P NMR (160 MHz, CDCl3): δ 79.9 (s, dppe). IR (Nujol/
cm−1): ν(CC) 2052 (s). Anal. Calcd for C82H80P4Ru2S2: C, 67.66;
H, 5.54. Found: C, 67.71; H, 5.49.
core. Our study provides further helpful information for
potential applications of redox-responsive conjugated oligo-
thienoacene systems as components of molecular electronic
devices.
EXPERIMENTAL SECTION
■
General Materials. All manipulations were carried out at room
temperature under a dry nitrogen atmosphere using standard Schlenk
techniques, unless otherwise stated. Solvents were predried, distilled,
and degassed prior to use. The main reagents 3-bromothiophene,
thieno[2,3-b]thiophene, acetylacetone, [Pd(PPh3)4], and TMSA were
commercially available. The starting materials [RuCl(dppe)Cp*],44
2,5-dibromothieno[3,2-b]thiophene (4a),23 2,5-dibromothieno[2,3-b]-
thiophene (5a),23 2,5-dibromo-3,4-dimethylthieno[2,3-b]thiophene
(6a),24 5,5′-dibromodithieno[3,2-b:2′,3′-d]thiophene (7a),17,23 and
2,6-dibromothieno[3,2-b]thieno[2′,3′:4,5]thieno[2,3-d]thiophene
(8a)25 were prepared by literature methods.
Preparation of 5. The synthetic procedure applied to 5 mirrored
that for 4, having used [RuCl(dppe)Cp*] (381 mg, 0.57 mmol), 5b
(90 mg, 0.27 mmol), KF (188 mg, 3.24 mmol), CH3OH (20 mL),
THF (5 mL). Yield: 221 mg (55%) of a yellow solid. Note: the
compound is poorly soluble in many deuterated solvents including
1
CDCl3, and its 13C NMR spectrum could not be collected. H NMR
(400 MHz, CDCl3): δ 1.54 (s, 30H, 2C5 (CH3)5), 2.05 (br s, 4H,
CH2/dppe), 2.67 (br s, 4H, CH2/dppe), 6.27 (s, 2H, Hthiophene), 7.21−7.37
(m, 43H, HAr/dppe), 7.74 (br s, 8H, HAr/dppe). 31P NMR (160 MHz,
CDCl3): δ 79.1 (s, dppe). IR (Nujol/cm−1): ν(CC) 2050 (s). Anal.
Calcd for C82H80P4Ru2S2: C, 67.66; H, 5.54. Found: C, 67.59; H, 5.56.
Preparation of 6. The synthetic procedure applied to 6 mirrored
that for 4, having used [RuCl(dppe)Cp*] (195 mg, 0.29 mmol), 6b
(50 mg, 0.14 mmol), KF (97 mg, 1.67 mmol), CH3OH (20 mL), THF
Syntheses of Bis[(trimethylsilyl)ethynyl]thienoacenes.26 2,5-
Bis[(trimethylsilyl)ethynyl]thieno[3,2-b]thiophene (4b).
(Trimethylsilyl)acetylene (TMSA; 392 mg, 4.00 mmol) was added
to a stirred solution of 4a (300 mg, 1.0 mmol), CuI (19 mg, 0.10
mmol), and [Pd(PPh3)4] (116 mg, 0.10 mmol) in (i-Pr)2NH (10 mL)
and tetrahydrofuran (THF; 10 mL) under an argon atmosphere; the
mixture was heated at 60 °C for 24 h. The solution was then cooled
and filtered through a bed of Celite. The filtrate was evaporated under
reduced pressure and purified by silica gel column chromatography
(petroleum ether) to give of a light-yellow powder. Yield: 251 mg,
1
(5 mL). Yield: 97 mg (44%) of a yellow solid. H NMR (400 MHz,
CDCl3): δ 1.55(s, 30H, 2C5(CH3)5), 2.01(s, 6H, CH3), 2.11 (br, 4H,
CH2/dppe), 2.73(br, 4H, CH2/dppe), 7.17−7.33 (m, 32H, HAr/dppe), 7.74
(br, 8H, HAr/dppe). 13C NMR (100 MHz, CDCl3): δ 10.1 (CH3), 13.6
(thiophene-CH3), 29.4 (t, J = 23.00 Hz, CH2/dppe), 92.8 (CH/C5Me5),
102.3 (thiophene-CCH), 117.2 (Ru−CCH), 126.5, 127.4, 128.8,
128.8, 133.2, 133.7, 136.7, 137.2, 138.7, 139.1. 31P NMR (160 MHz,
CDCl3): δ (ppm) 79.9 (s, dppe). IR (Nujol/cm−1): ν(CC) 2049
(s). Anal. Calcd for C84H86P4Ru2S2: C, 67.91; H, 5.83. Found: C,
67.87; H, 5.88.
1
75%. H NMR (400 MHz, CDCl3): δ 0.26 (s, 18H, SiCH3), 7.29 (s,
2H, thiophene H), as reported in ref 45.
2,5-Bis[(trimethylsilyl)ethynyl]thieno[2,3-b]thiophene (5b). This
compound was synthesized by the same method as 4b, having used 5a
(600 mg, 2.00 mmol), CuI (38 mg, 0.20 mmol), [Pd(PPh3)4] (524
mg, 0.46 mmol), (i-Pr)2NH (30 mL), THF (30 mL), and TMSA
(1772 mg, 18.08 mmol). Yield: 535 mg (80%) of a light-yellow
1
Preparation of 7. The synthetic procedure applied to 7 mirrored
that for 4, having used [RuCl(dppe)Cp*] (289 mg, 0.43 mmol), 7b
(80 mg, 0.21 mmol), KF (143 mg, 2.47 mmol), CH3OH (20 mL),
powder. H NMR (400 MHz, CDCl3): δ 0.27 (s, 18H, SiCH3), 7.16
(s, 2H, thiophene H). ESI-MS: m/z 332.11 ([M]+). Note: the
compound was thermally unstable and hence used directly in the next
reaction step after characterization and a purity check with NMR and
MS.
1
THF (5 mL). Yield: 155 mg (47%) of a brown solid. H NMR (400
MHz, CDCl3): δ 1.55 (s, 30H, 2C5(CH3)5), 2.06 (br, 4H, CH2/dppe),
2.68 (br, 4H, CH2/dppe), 6.42 (s, 2H, thiophene), 7.22−7.40 (m, 32H,
HAr/dppe), 7.75 (br, 8H, HAr/dppe). 13C NMR (100 MHz, CDCl3): δ
10.0 (CH3), 29.4 (t, J = 22.80 Hz, CH2/dppe), 92.8 (CH/C5Me5), 102.9
(thiophene-CCH), 117.1 (Ru−CCH), 126.5, 127.4, 129.0, 133.2,
133.5, 136.3, 136.8, 138.3, 138.6, 140.2. 31P NMR (160 MHz, CDCl3):
δ 79.9 (s, dppe). IR (Nujol/cm−1): ν(CC) 2044 (w). Anal. Calcd
for C84H80P4Ru2S3: C, 66.74; H, 5.33. Found: C, 66.69; H, 5.41.
Preparation of 8. The synthetic procedure applied to 8 mirrored
that for 4, having used [RuCl(dppe)Cp*] (158 mg, 0.24 mmol), 8b
(50 mg, 0.11 mmol), KF (78 mg, 1.35 mmol), CH3OH (20 mL), THF
2,5-Bis[(trimethylsilyl)ethynyl]-3,4-dimethylthieno[2,3-b]-
thiophene (6b). This compound was synthesized by the same method
as 4b, having used 6a (100 mg, 0.31 mmol), CuI (5.85 mg, 0.20
mmol), [Pd(PPh3)4] (35.5 mg, 0.41 mmol), triethylamine (NEt3; 5
mL), THF (15 mL), and TMSA (196 mg, 2.00 mmol). Yield: 95 mg
(86%) of a light-yellow powder. 1H NMR (400 MHz, CDCl3): δ 0.26
(s, 18H, SiCH3), 2.51 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3): δ
0.0 (SiMe3), 14.3 (CH3), 97.4, 102.2, 121.2, 136.5, 137.2, 144.0. Anal.
Calcd for C18H24S2Si2: C, 59.94; H, 6.71. Found: C, 59.78; H, 6.65.
5,5′-Bis[(trimethylsilyl)ethynyl]dithieno[3,2-b:2′,3′-d]thiophene
(7b). This compound was synthesized by the same method as 4b,
having used 7a (800 mg, 2.26 mmol), CuI (43 mg, 0.23 mmol),
[Pd(PPh3)4] (262 mg, 0.23 mmol), (i-Pr)2NH (30 mL), THF (30
mL), and TMSA (886 mg, 9.04 mmol). Yield: 562 mg (64%) of a
1
(5 mL). Yield: 80 mg (43%) of a dark brown solid. H NMR (400
MHz, CDCl3): δ 1.55 (s, 30H, 2C5(CH3)5), 2.08 (br, 4H, CH2/dppe),
2.67 (br, 4H, CH2/dppe), 6.43 (s, 2H, thiophene), 7.31−7.39 (m, 32H,
HAr/dppe), 7.75 (br, 8H, HAr/dppe). 13C NMR (100 MHz, CDCl3): δ
10.0 (CH3), 29.6 (t, J = 22.80 Hz, CH2/dppe), 93.0 (CH/C5Me5), 103.0
(thiophene-CCH), 117.1 (Ru−CCH), 126.7, 127.4, 129.0, 131.5,
132.3, 133.2, 133.5, 136.3, 136.8, 137.7, 138.6, 142.0. 31P NMR (160
MHz, CDCl3): δ 79.9 (s, dppe). IR (Nujol/cm−1): ν(CC) 2041
(w). Anal. Calcd for C86H80P4Ru2S4: C, 65.88; H, 5.14. Found: C,
65.95; H, 5.01.
Crystallographic Details. Single crystals of complexes 7 and 8
suitable for X-ray analysis, were grown by slow diffusion of hexane into
a solution of dichloromethane. Crystals with approximate dimensions
of 0.16 × 0.12 × 0.10 mm3 for 7 and 0.12 × 0.10 × 0.10 mm3 for 8
were mounted on a glass fiber for diffraction experiments. Intensity
data were collected on a Nonius Kappa CCD diffractometer with Mo
Kα radiation (0.71073 Å) at low temperature (100 K) for 7 and room
temperature (296 K) for 8. The structures were solved by a
combination of direct methods (SHELXS-97)47 and Fourier difference
techniques and refined by full-matrix least-squares (SHELXL-97).48 All
1
light-yellow powder. H NMR (400 MHz, CDCl3): δ 0.28 (s, 18H,
SiCH3), 7.40 (s, 2H, thiophene H), as reported in ref 46.
2,6-Bis[(trimethylsilyl)ethynyl]thieno[3,2-b]thieno[2′,3′:4,5]-
thieno[2,3-d]thiophene (8b). This compound was synthesized by the
same method as 4b, having used 8a (300 mg, 0.73 mmol), CuI (14
mg, 0.073 mmol), [Pd(PPh3)4] (85 mg, 0.073 mmol), (i-Pr)2NH (30
mL), THF (30 mL), and TMSA (358 mg, 3.66 mmol). Yield: 100 mg
1
(31%) of a yellow powder. H NMR (400 MHz, CDCl3): δ 0.28 (s,
18H, SiCH3), 7.42 (s, 2H, thiophene H). Anal. Calcd for C20H20S4Si2:
C, 54.01; H, 4.53. Found: C, 53.87; H, 4.61. Note: the compound has
poor solubility in many deuterated solvents including CDCl3, and the
13C NMR spectrum could not be collected.
General Syntheses of Diruthenium Complexes.11 Prepara-
tion of 4. A solution of [RuCl(dppe)Cp*] (381 mg, 0.57 mmol), 4b
(90 mg, 0.27 mmol), and KF (188 mg, 3.24 mmol) in 20 mL CH3OH
and 5 mL THF was heated to reflux under nitrogen atmosphere for 24
J
DOI: 10.1021/acs.inorgchem.7b01433
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
non-hydrogen atoms were refined anisotropically. The hydrogen
atoms were placed in ideal positions and refined as riding atoms. The
partial solvent molecules have been omitted. Further crystal data and
details of the data collection are summarized in Table S1. Selected
bond distances and angles are given in Tables 1 and S2 (Supporting
Information), respectively.
Author Contributions
§These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
1
Physical Measurements. H, 13C, and 31P NMR spectra were
ACKNOWLEDGMENTS
collected on a Varian Mercury Plus 400 spectrometer (400 MHz). 1H
and 13C NMR chemical shifts are given relative to TMS and 31P NMR
chemical shifts to 85% H3PO4. Elemental analyses (carbon, hydrogen,
and nitrogen) were performed with a Vario ElIII Chnso instrument.
UV−vis−NIR spectra were recorded using a Shimadzu UV-3600
spectrophotometer and liquid-sample cells of 0.2 mm optical path
length. IR spectra of solid samples dispersed in Nujol between KBr
disks and solutions in 0.2 mm optical cells were obtained with a
Nicolet Avatar spectrometer. Electrochemical measurements were
conducted with a CHI 660C potentiostat. A single-compartment
electrochemical cell contained a prepolished platinum disk working
electrode (d = 0.5 mm), a platinum wire counter electrode, and a silver
wire pseudoreference electrode. Dry CH2Cl2, deaerated by bubbling
■
The authors acknowledge financial support from the National
Natural Science Foundation of China (Grants 21272088,
21472059, 21402057, and 21602049) and the Natural Science
Foundation of Hunan Province, China (Grant 2017JJ3004).
F.H. also thanks the University of Reading for continued
support of the Spectroelectrochemistry Reading laboratories
(Project D14-015) and the CCNU Wuhan for his participation
in the Global Talent Plan 111 Project (B17019).
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: f.hartl@reading.ac.uk;.
*E-mail: chshliu@mail.ccnu.edu.cn.
■
ORCID
Frantisek Hartl: 0000-0002-7013-5360
̌
K
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DOI: 10.1021/acs.inorgchem.7b01433
Inorg. Chem. XXXX, XXX, XXX−XXX