Conformational Tuning of the Intramolecular Electronic Coupling in Molecular-Wire Biruthenium Complexes Bridged by Biphenyl Derivatives

Article abstract of DOI:10.1002/chem.201500509

The synthesis and characterization of a series of biphenyl-derived binuclear ruthenium complexes with terminal {RuCl(CO)(PMe₃)₃} moieties and different structural arrangements of the phenyl rings are reported. Electrochemical studies

Full text of DOI:10.1002/chem.201500509

DOI: 10.1002/chem.201500509
Full Paper
&
Molecular Wires
Conformational Tuning of the Intramolecular Electronic Coupling
in Molecular-Wire Biruthenium Complexes Bridged by Biphenyl
Derivatives
Dan-Dan Kong, Lu-Sha Xue, Rui Jang, Bin Liu, Xiang-Gao Meng, Shan Jin,* Ya-Ping Ou,
[
a]
Xiao Hao, and Sheng-Hua Liu*
Abstract: The synthesis and characterization of a series of
biphenyl-derived binuclear ruthenium complexes with termi-
nal {RuCl(CO)(PMe ) } moieties and different structural ar-
electrochemical analysis clearly showed that: 1) the intramo-
lecular electronic couplings in the binuclear ruthenium com-
plexes could be modulated by changing f; 2) the electronic
ground state of the mixed-valent cations changes from delo-
calized to localized through the biphenyl bridge with in-
creasing torsion angle f, that is, the redox processes of
these complexes change from significant involvement of the
bridging ligand to an oxidation behavior with less participa-
tion of the bridge.
3
3
rangements of the phenyl rings are reported. Electrochemi-
cal studies revealed that the two metal centers of the binu-
clear ruthenium complexes interact with each other through
the biphenyl bridge, and the redox splittings DE1 show
/2
2
a strong linear correlation with cos f, where f is the torsion
angle between the two phenyl rings. A combination of elec-
trochemical, UV/Vis/NIR, and in situ IR differential spectro-
Introduction
bridging ligand, has been carried out by using simple electro-
[6–20]
chemical and spectroscopic techniques.
If one of the metal
The miniaturization of integrated circuits is set to reach its in-
herent limitation in the foreseeable future due to certain physi-
cal and financial constraints. A bottom-up approach that
begins at the molecular level with discrete molecules and
allows further reduction of the size of the active electronic
centers is oxidized or reduced, the complex can then generate
a mixed-valent (MV) state. In the MV state, the reduced metal
site acts as a donor and the oxidized one as an acceptor.
Studying the MV state provides useful information on basic
electron-transfer (ET) and charge-transfer (CT) processes and
how the molecular structure can influence them. The large
number of compounds that has been synthesized allows the
study of various factors, including distance, solvent effect, mo-
lecular topology, metal complex, the nature of the bridge, and
tuning the degree of charge delocalization between the termi-
[
1]
components has become highly desirable. In this approach,
molecules with desired electronic functions are designed, syn-
thesized, evaluated, and then integrated into functioning mo-
lecular circuits to build a better molecular electronic device.
Since molecular wires are the key component of molecular cir-
cuits, it is crucial to understand and control electron transport
through molecular wires.
[21–33]
nal redox sites in the MV state.
Since the bridging ligands play an important role in tuning
the ET properties of MV complexes, ranging from localization
to delocalization, the synthesis of such compounds with vari-
Directly addressing the molecular conductivity often involves
fabricating the molecules on some microscopic system, which
[
2–5]
[19]
is still fraught with problems.
direct approach with an open-shell bimetallic complex
ML (bridge)ML ], in which two redox-active metal centers M,
In contrast, a more feasible in-
ous bridging ligands has attracted considerable interest. Re-
cently, an important aspect of developing the [ML (bridge)ML ]
n
n
[
system is controlling the electronic delocalization in the
ground state of the MV form and the metal-versus-ligand char-
acter of the redox processes to yield a metal-centered or
n
n
supported by auxiliary ligands, L, are linked by a conjugated
[34–37]
[a] D.-D. Kong, L.-S. Xue, R. Jang, Dr. B. Liu, Dr. X.-G. Meng, Prof. Dr. S. Jin,
bridge-centered process.
In a three-state model, intramo-
Y.-P. Ou, X. Hao, Prof. Dr. S.-H. Liu
lecular CT between two metal centers may occur by the super-
Key Laboratory of Pesticide and Chemical Biology
Ministry of Education, College of Chemistry
Central China Normal University
[
36,38–42]
exchange mechanism, the hopping mechanism,
or
In this respect, bi-
nuclear ruthenium MV complexes in which two {RuCl
CO)(PR ) L} (L=neutral two-electron donor or free coordina-
[34,42]
bridge-localized states over the bridge.
1
52 Luoyu Road, Wuhan, Hubei 430079 (P. R. China)
E-mail: jinshan@mail.ccnu.edu.cn
chshliu@mail.ccnu.edu.cn
(
3
2
tion site) redox-active termini are connected by a carbon-rich
unsaturated spacer are suitable molecular-wire candidates to
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/em.2.201500509. Crystallographic details and UV/
Vis/NIR spectra of compounds 2a–e, 3a–e, and 4a–e.
[34–36]
study the properties of the bridges.
Studies on the effect
Chem. Eur. J. 2015, 21, 9895 – 9904
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of various organic bridge ligands on the redox processes of bi-
metallic complexes will not only help to mimic the intramolec-
ular charge displacement, but are also crucially required for de-
veloping molecular electronic devices.
4,4’-diiodo biphenyls 1a–e, which proceeded in 58–74% over-
all yield: [(PPh ) PdCl ]/CuI-catalyzed coupling with trimethyl-
3
2
2
silylacetylene affording 4,4’-bis(trimethylsilylethynyl) biphenyls
2a–e, and KOH/MeOH-mediated cleavage of the protecting
groups. Diethynyl biphenyls 3a–e were treated with the ruthe-
nium hydride complex [RuHCl(CO)(PPh ) ] to give the corre-
Biphenyl linkers, which consist of two aromatic rings con-
nected by a CÀC single bond, have recently attracted consider-
able interest as bridges spanning terminally appended redox-
3
3
sponding insertion products [{(PPh ) Cl(CO)Ru} (m-CH=CHÀ
3
2
2
[
43–45]
[46–53]
active moieties
or anchoring groups.
This is done
BiphÀCH=CH)], which were not isolated because these five-co-
ordinate complexes were air-sensitive, especially in solution.
[21]
with the hope that modulating or even controlling the degree
of torsion of the two phenyl rings might allow one to exert
control over ET between the terminal redox sites or in a single-
molecular junction. McLendon et al. reported the correlation
between the torsion angle f and the through-bond ET rates in
terminally porphyrin-functionalized ortho-substituted biphenyl
Hence, PMe was added directly to give the corresponding six-
3
1
coordinate complexes 4a–e, which were characterized by H,
13
31
1
C and P NMR spectroscopy. The H NMR spectra (in CDCl )
3
of 4a–e featured RuÀCH signals at about 8.1 ppm, and the
signal for b-CH of the vinyl group was at about 6.6 ppm. These
chemical shifts are close to those found in complexes [RuCl
[
54]
systems. Benniston et al. attempted to control the biphenyl
conformation by complexation with cations. The magnitude of
the electronic coupling between the terminal chromophores
shows a precise dependence on the dihedral angle around
a bridging biphenyl group, controlled by selecting the bound
[58,59]
(CO)(PMe ) ] (CH=CH) .
The two vinylic protons are trans
3
3 2
n
to each other and the acetylene underwent cis insertion into
the RuÀH bond, as confirmed by the X-ray structures of 4b–d.
[
55–57]
[46]
cations.
Recently, Venkataraman et al. and Wandlowski
[
47–52]
X-ray structures of 4b–4d
et al.
reported the influence of f on changes in the single-
molecule conductivity in substituted biphenyls or conforma-
tionally constrained biphenyls in which the biphenyl rings are
Single crystals of 4b–d suitable for X-ray analysis were ob-
tained by slow diffusion of hexane into a solution of the re-
spective complexes in dichloromethane, which enabled the de-
termination of the solid-state interplanar torsion angles f of
connected by a (CH ) link at the 2- and 2’-positions. On twist-
2
n
ing of the biphenyl system from coplanar (f=08) to perpen-
dicular (f=908), the conduc-
tance decreased by a factor of
[48]
30.
However, reports of sys-
tematic studies on the influence
of f on the electronic coupling
and redox processes of bimetal-
lic complexes [ML (bridge)ML ]
n
n
[
43,44]
are still scarce.
Herein, we
report the synthesis and electro-
chemical and spectroscopic char-
acterization of a series of bi-
phenyl-bridged binuclear ruthe-
nium complexes 4a–e with the
aim of tuning their electronic
coupling and redox processes
through variation of the torsion
angle between the two phenyl
groups. The electronic properties
of these complexes and their
redox behavior were investigat-
ed by electrochemistry and IR
and UV/Vis/NIR spectroscopy.
Results and Discussion
Synthesis and characterization
The general synthetic route to
binuclear ruthenium vinyl com-
plexes 4a–e is outlined in
Scheme 1. The synthesis of
3
a–e required two steps from Scheme 1. Biphenyl-based bimetallic ruthenium complexes 4a–e.
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ruthenium units are located on the same side of the biphenyl
moieties. Comparable intramolecular Ru···Ru distances were
obtained in the series of structurally characterized compounds,
with values between 1.583 nm for 4d and 1.612 nm for 4b.
This indicates that the effects of the various alkyl substituents
on the length of the m-CH=CHÀBiphÀCH=CH backbone are
minimal. As shown in Tables S2–S4 in the Supporting Informa-
tion, the bond lengths between the two phenyl rings (C16À
C16a for 4b, C16ÀC19 for 4c and 4d) are also comparable:
1
48.8(6) pm for 4b, 148.8(13) pm for 4c, and 147.4(12) pm for
2
2
4
d, which are very close to the length of the C(sp )ÀC(sp )
[60]
bond in unsubstituted biphenyl (148 pm).
As shown in Table 1, the torsion angles of these binuclear
ruthenium complexes are very similar to those of organic mo-
[44,46,47,50]
lecular wires containing a biphenyl moiety.
The ortho-
C2-bridged biphenyl compound 4b showed an almost copla-
nar arrangement of the two phenyl rings with f=0.798. In the
core-unsubstituted biphenyl compound 4c, the torsion angle
of the two phenyl rings is 34.28. Apparently, the steric repul-
sion caused by the two methyl groups at the 2-position of bi-
phenyl derivative 4d resulted in an almost perpendicular ar-
rangement of the planes of the two phenyl rings with a torsion
angle of 85.08. Even though these molecules are conformation-
ally flexible in solution, the f values measured in the solid
state remain the best approximation of the values in solution.
Electrochemistry
Figure 1. Molecular structures: a) 4b; b) 4c; c) 4d.
The redox behavior of binuclear complexes 4a–e (1 mm in
CH Cl ) was investigated by cyclic voltammetry and square-
2
2
the two phenyl rings. The molecular structures of 4b–d are
shown in Figure 1. The crystallographic details are given in
Table S1 (see the Supporting Information). Selected bond
lengths and angles for 4b–4d are listed in Tables S2–S4 in the
Supporting Information, respectively. The intramolecular
Ru···Ru distances and angles f are listed in Table 1.
wave voltammetry (SWV) with 0.1m nBu NPF as supporting
4 6
electrolyte. The potentials from SWV are compiled in Table 1.
Mononuclear complex [RuCl(CO)(PMe ) (CH=CHC H )] (5) is in-
3
3
6
5
cluded for reference purposes. Cyclic voltammograms (CVs) of
complexes 4a–e are shown in Figure 2.
Complexes 4a–c exhibited two reversible one-electron
redox waves due to stepwise one-electron redox processes.
2
p
1
p
The peak separations of the two redox waves (DE =E ÀE )
p
Table 1. Selected structural data and properties.
were about 185, 174, and 127 mV for 4a–c, and the corre-
sponding comproportionation constants Kc were 1341, 874,
and 140, respectively. The DE and K values of 4a–c indicated
1
p
[a]
2
p
[a]
[b]
[c]
fexptl [8]
d
RuÀRu [Š]
E [V]
E [V]
DE [mV]
K
c
p
c
[
d]
4
4
4
4
4
5
a
b
c
d
e
1.1
–
0.336
0.372
0.463
0.532
0.508
0.628
0.521
0.546
0.590
0.608
0.590
–
185
174
127
76
81
–
1341
874
140
–
that a moderate degree of electronic communication is trans-
mitted through the biphenyl core. In contrast, the CV of 4d
and 4e showed only two irreversible and poorly resolved
0.79
34.19
84.98
16.12
16.04
15.83
–
[
d]
89.0
–
–
–
redox processes with DE =76 and 81 mV, respectively. Howev-
p
–
er, this does not necessarily mean that electronic communica-
[
a] From SWV in 0.1m CH
2
Cl
2
/nBu
4
NPF
6
solution at 10 Hz. Potentials E
p
are
tion is present between the two ruthenium units of 4d and 4e
+
2
1
in volts versus Ag /Ag. [b] Peak potential differences DE=E ÀE . [c] The
p
p
with small DE values. There is no direct relation between the
p
comproportionation constants K
c
were calculated according to the formu-
potential splitting DE and the strength of the electronic cou-
pling between two redox moieties. The potential splitting of
the consecutive redox processes depends on various factors,
such as the statistical, inductive, electrostatic, magnetic ex-
change, in addition to the resonance contribution caused by
la K =exp(DE/25.69) at 298 K. [d] Data from ref. [47].
c
The linear-conjugated complexes 4b–4d consist of two
[61,62]
(
PMe ) Cl(CO)Ru end groups linked by a biphenyl bridge
electronic coupling.
For a series of similar compounds
3
3
through RuÀC s bonding. In 4b and 4c, both vinylic double
bonds are in a transoid arrangement. However, both vinylic
double bonds are in a cisoid configuration in 4d, and the two
(similar metal–metal distance and bridging ligands) and the
same electrolyte, all contributing factors are similar or identical
with exception of the resonance contribution. Therefore, DE or
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Figure 3. Linear correlation of the difference between the first and second
2
half-wave potentials DE
p
with cos f (f is the interplanar torsion angle ob-
tained from the X-ray structures and ref. [47]).
with increasing torsion angle. As a consequence, the extent of
2
the ET is linearly proportional to cos f.
À1
Figure 2. CVs of 4a–e in CH
tentials are given relative to the AgjAg standard.
2
Cl
2
/Bu
4
NPF
6
+
at v=0.1 Vs . SWV at f=10 Hz. Po-
To further investigate the correlation between the electronic
delocalization of complexes 4a–e and f, the changes of their
UV/Vis/NIR and IR spectra in various oxidation states were
measured in dichloromethane with 0.1m nBu NPF as support-
Kc reflects the degree of electronic coupling between the two
ruthenium units.
4
6
ing electrolyte.
Introducing methylene or methyl groups at positions 2 (or
2
’) and 6 (or 6’) of the biphenyl resulted in an increase in the
UV/Vis/NIR spectroscopy
first oxidation potential and a decrease in the separation of
the redox events (Figure 2). The voltammetric features of binu-
clear complexes 4a–e show that the redox process shifts from
reversible to irreversible stepwise single ET reactions, which
can be ascribed to the increase of the torsion angle of the bi-
phenyl bridging ligand, as one can expect the chemical stabili-
ty to decrease with increasing oxidation state, at least on the
voltammetric timescale. These results clearly indicate that the
stability of the oxidized forms is highly dependent on the tor-
sion angle of the biphenyl bridging ligands.
To investigate the correlation between electronic absorption
properties and the interplanar torsion angle f, the UV absorp-
tion spectra of the series of biphenyl-based binuclear com-
plexes 4a–e were measured in dichloromethane. The main
electronic absorption spectroscopic data are listed in Table 2.
À5
Table 2. UV/Vis adsorption properties of 4a–e and 5 (1”10 m) in
2 2
CH Cl at 298 K.
We sought to correlate the DE values obtained from cyclic
À4
3
À1
À1
p
Complex
l
max [nm (eV)]
e”10 [dm mol cm ]
voltammetry and the torsion angle f obtained from the X-ray
structures and ref. [47] (Figure 3). Our previous study on [RuCl
4
4
4c
4d
4e
5
a
b
360 (3.46)
361 (3.45)
347 (3.59)
309 (4.03)
301 (4.13)
289 (4.31)
3.95
4.42
4.10
3.45
5.74
4.21
(
CO)(PMe ) ] (CH=CHC H CH=CH) compounds revealed that
3
3 2
6
4
each methyl substituent increases DE by about 4 mV through
p
[63,64]
modification of 1,4-diethenylphenylene bridging ligand.
Thus, from the potential shift induced only by the torsion
angle of the biphenyl core, one can estimate that the DEp
values should be about 181, 166, 127, 68, and 65 mV for com-
plexes 4a–e, respectively. According to theory, the orbital over-
The UV/Vis spectra of binuclear complexes 4a–e and their pre-
lap of adjacent p systems correlates linearly with the cosf cursors 2a–e and 3a–e are shown in Figure 4 and Figure S1 in
value, and the electron transmission is proportional to
the Supporting Information. As shown in Figure 4, the UV/Vis
spectra of binuclear complexes 4a–e show a single broad ab-
sorption band in the UV region. The characteristic peak is at
about 361–301 nm for 4a–e. For these biphenyl-bridged bi-
metallic complexes, the broad absorption in the UV region is
expected to arise from a mixed transition of ligand-centered
p–p* transitions of the biphenyl-core and of metal-based
HOMOs to an unoccupied ligand-based orbital [metal-to-ligand
CT (MLCT)], and the the latter is probably mainly delocalized
2
[65]
cos f. As shown in Figure 3, the potential differences be-
tween the first and second redox peaks DE exhibit a moder-
p
2
2
ately good linear correlation with cos f (R =0.973). An inter-
cept of about 64 mV was obtained for the situation in which
very little or no communication exists between the two ruthe-
nium units (f=908). In the series of biphenyl biruthenium
complexes 4a–e, this result implies that the electronic ground
state of the MV forms changes from delocalized to localized
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electrolyte. On one-electron oxidation of complexes 4a–e to
+
+
4
a –4e , the MLCT bands became weaker, and new bands at
+
+
longer wavelengths appeared (385–480 nm for 4a and 4b ,
75–460 nm for 4c , 345–420 nm for 4d , and 340–400 nm
+
+
3
+
for 4e ), attributable to ligand-to-metal CT (LMCT) transitions
as a result of the one-electron oxidation (see the Supporting
Information, Figure S2). Comparison of the absorption bands
of LMCT transitions revealed that, with increasing interplanar
torsion angle of the biphenyl core, the LMCT bands (Figure 6
À5
Figure 4. UV/Vis electronic absorption spectra of 4a–e and 5 (1”10 m) in
CH Cl
2
2
.
[
43,50]
over the biphenyl core.
This band shows an apparent blue-
shift with increasing dihedral angle between the phenyl rings.
On twisting the biphenyl core from coplanar (f=08) to per-
pendicular (f=908), the longest wavelength of the absorption
band shows a notable blueshift of about 60 nm from 361 to
301 nm. Compared with the mononuclear complex 5, a batho-
chromic shift of about 10–70 nm was observed for binuclear
complexes 4a–e, which can be attributed to enlargement of
the conjugated system.
The lowest transition energy as the lmax value should corre-
[
50]
late with the conformation of the biphenyl center. Thus, in
Figure 5, the longest-wavelength absorption of each spectrum,
Figure 6. UV/Vis/NIR spectra of a) 4a and b) 4b collected during in situ oxi-
dation in a spectroelectrochemical cell (0.1m nBu PF in CH Cl ).
4 6 2 2
and Supporting Information, Figure S2) not only becomes pro-
gressively bathochromically shifted and narrower, but also the
molar extinction coefficient decreases significantly. In the pres-
+
+
ent case, the two low-energy p–p* transitions (4a and 4b )
À1
observed between 20000 and 14000 cm (500–700 nm) are
[
34,35]
2
very similar to those observed for aromatic radical cations,
and this demonstrates that the charge is in part delocalized
Figure 5. HOMO–LUMO bandgap of 4a–e plotted against cos f, where f is
the interplanar torsion angle obtained from the X-ray structures and
ref. [47].
+
+
onto the biphenyl bridge in 4a and 4b . However, these
bands could not be observed for 4c –4e , which implies that
+
+
the charge may be mainly localized on the metal centers. In
+
+
which reflects the HOMO–LUMO energy gap of the compound
addition, MV species 4a and 4b (Figure 6) show characteris-
2
under investigation, is plotted against cos f. In this series of
tic low-energy absorption bands in the NIR region between
2
À1
compounds, a good linear correlation (R =0.996) was found
11100 and 5000 cm (900–2000 nm), similar to those observed
2
between the HOMO–LUMO bandgap and cos f.
for the radical complex [{(PMe ) (CO)ClRu} (CH=CHC H CH=
3
3
2
6
4
+
[34,35,66]
To gain insight into the oxidation processes for the series of
complexes 4a–e, UV/Vis/NIR spectroelectrochemical experi-
ments were performed in an optically transparent thin-layer
electrochemical (OTTLE) cell with 0.1m nBu NPF as supporting
CH)] ,
in which the hole generated on oxidation is delo-
calized across the molecular backbone. In contrast, such
a near-IR band was essentially unobserved in the one-electron-
+
+
oxidized species 4c –4e . Although low solubility and partial
4
6
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2
+
decomposition of more highly oxidized 4a were encoun-
may change for 4c and 4d. For example, 4c has similar
2
5
tered in UV/Vis/NIR spectroelectrochemistry, the MV species
molecular length and structure to [{(h -dppe)(h -C Me )Fe} (Cꢀ
5
5
2
+
4
a
shows better chemical reversibility and stability during
CC H C H CꢀC)], reported by Ghazala et al., which shows an in-
[43]
6
4
6
4
+
+
a full 4a!4a !4a!4a cycle (see the Supporting Informa-
tense IVCT band. However, the NIR bands of the MV species
+
+
tion, Figure S3).
4c and 4d could not be observed by chemical or electro-
chemical methods, which may be due to the high instability or
smaller change of the torsion angle from their neutral to MV
+
+
In CH Cl solution 4a or 4b show broad NIR bands with
moderate intensity, which can be well deconvoluted into two
2
2
À1
[70,71]
Gaussian-shaped subbands centered at 6304 and 7455 cm
state in solution at room temperature.
+
À1
+
for 4a and 6109 and 7396 cm for 4b . The broader sub-
À1
+
À1
+
band at 7455 cm for 4a and 7396 cm for 4b showing
solvent-dependent behavior (see the Supporting Information,
Figure S4) is most likely ascribable to intervalence CT (IVCT)
transition. In contrast, the solvent polarity has no influence on
IR spectroelectrochemistry
Compared with conventional IR spectra, in situ IR differential
spectra can provide higher sensitivity and resolution for moni-
toring the course of a reaction and the existence of transient
À1
+
À1
the MLCT transitions (6304 cm for 4a and 6109 cm for
+
[67,68]
[72]
4
b ).
The observed width at half-height D ~n of the IVCT
species or unstable intermediates. To better probe the char-
1
+
/2
À1
+
À1
band (2140 cm for 4a and 2340 cm for 4b ) is much nar-
acter of these redox processes, in situ IR transmission differen-
tial spectra and conventional IR spectroscopic studies were car-
ried out on complexes 4a–e and 5 by using the n(CO) band to
evaluate the contributions of metal centers and bridging li-
gands to redox processes. The characteristic n(CO) vibrational
frequencies for different redox states are listed in Table 4.
À1
+
À1
rower than that calculated (4150 cm for 4a and 4133 cm
+
1/2
for 4b ) by Hush’s theory (D ~n =(2310 ~n max) ), which implies
1/2
[
43]
partially delocalized behavior of the MV system. By compari-
son of the IVCT transitions, it was found that the IVCT band for
+
+
4
5
a
is slightly more intense (e=802) than that of 4b (e=
00). This indicates that 4a is slightly more strongly coupled
than 4b . According to Marcus–Hush theory, H can be ob-
tained from Equation (1) for a quantitative discussion of the
electronic coupling from the position ~n max of the lowest-energy
IVCT band.
+
+
ab
À1
n+
n+
Table 4. IR ~n (CO) [cm ] data for 4a –e (n=0, 1, 2).
n
4a
4b
4c
4d
4e
0
1
2
1919
1937
1976
1919
1940
1978
1920
1948
1976
1920
–
1979
1921
–
1979
À
Á
Hab ¼ ð2:06 Â 10À2=d Þ emax ~n maxDn
1=2
ð1Þ
ab
1=2
The electron-transfer distance d , which is notoriously diffi-
ab
cult to measure experimentally, is substantially smaller than
[
61,62]
Time-resolved in situ IR differential spectra and IR spectra are
depicted in Figure 7 and Figure S5 (see the Supporting Infor-
mation), respectively.
the spatial RuÀRu distance.
Therefore, the electronic cou-
pling H is very likely underestimated by assuming the geo-
ab
+
metrical RuÀRu separation as the CT distance d . In 4a , H =
ab
ab
À1
+
À1
Figure 7 shows a series of in situ IR transmission differential
1
53 cm , whereas for 4b H =122 cm . This difference cor-
ab
À1
À1
+
+
spectra (1600–2100 cm ) simultaneously recorded with a po-
responds to 31 cm stronger coupling in 4a than in 4b
Table 3).
We note that previous work has already indicated that oxi-
tential sweep from 0!+0.50!+0.70 V with respect to an IR
spectrum obtained at 0 V as a reference for which 4a–e exist
(
II,II
as Ru2 . The downward and upward peaks correspond to the
dation induces a more coplanar arrangement of the biphenyl
bridge by decrease of the torsion angle from neutral complex
to MV cation, which may even enhance conjugation and elec-
species formed and disappearing, respectively, in comparison
with the reference potential (0 V). As shown in Figure 7A, on
À1
[
69]
mono-oxidation of 4a, the n(CO) band at 1919 cm shifted to
tronic coupling. This will have no effect on complexes 4a,
À1
1
937 cm . On the second oxidation, these two bands disap-
4
b,and 4e, in which this angle is more or less fixed, but it
À1
peared and a new n(CO) band at 1976 cm grew. Similarly,
when 4b was oxidized to the monocation, the original CO
stretching band at 1919 cm was gradually replaced by a new
À1
Table 3. Selected parameters derived from deconvolution of the NIR data
+
+
À1
envelopes of 4a and 4b (2 mm in CH
2
Cl
2
).
n(CO) abnd at 1940 cm , and during the second oxidation
step the n(CO) band of the monocation was replaced by just
+
+
4
a
4b
2
+
one CO band for the fully oxidized dication 4b , which is lo-
[
a]
d
ab [Š]
15.2
15.6
À1
cated at 1979 cm (not shown). Sequential oxidation of 4c
À1
À1
À1
~
n [cm ] (e [m cm ])
6304 (2100)
7455 (802)
2140
6109 (1100)
7396 (500)
2340
a
À1
À1
À1
first led to the replacement of the single n(CO) band of the
~
n
[cm ] (e [m cm ])
(IVCT) [cm
D ~n 1=2(theor) [cm
IVCT
À1
À1
D~n
]
neutral form at 1920 cm by two new n(CO) bands at 1948
a
À1 [b]
À1
]
4150
153
4133
122
and 1976 cm (Figure 7B). As the oxidation proceeds, the
À1 [c]
H
ab [cm
]
+
À1
weak CO band of 4c at 1948 cm gives way to much stron-
2
+
À1
[
a] Evaluated from X-ray structures. [b] The theoretical D ~n 1=2 value was
calculated by using the equation D ~n 1=2 =(2310 ~n max
coupling Hab was calculated by using Equation (1).
ger absorptions of 4c at 1976 cm . The shifts of the n(CO)
bands for complexes 4a–c on sequential oxidation from neu-
1/2
) . [c] The electronic
À1
tral to monocationic are 18, 21, and 28 cm , respectively.
Chem. Eur. J. 2015, 21, 9895 – 9904
www.chemeurj.org
9900
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
À1
2+
2+
1
920 (4d) and 1921 (4e) to 1979 cm (4d or 4e ), and
+
+
À1
monocationic 4d and 4e were not detected. The 59 cm
shift to higher wavenumbers from neutral to dication is smaller
À1
than the shift of about 100 cm expected for a metal-centered
process and indicates that an appreciable fraction of the
charge is lost from the bridging ligand during the oxidation
[35]
process.
Conclusion
We have prepared a series of ruthenium vinyl complexes
bridged by biphenyls with different torsion angles between
the two phenyl rings, which is controlled by introducing meth-
ylene or methyl substituents in the 2- and 2’-positions of the
biphenyl backbone. We found that their electronic properties
are highly dependent on the torsion angle of the biphenyl
core, as revealed by electrochemistry, IR, and UV/Vis/NIR meth-
ods.
II
III
All members of the series of formal Ru Ru MV ions contain-
ing a biphenyl core display behavior ranging from moderately
coupled class II (4a–c) to class I (or very weakly class II) MV sys-
tems (4d and 4e) with increasing torsion angle. Both electro-
chemistry and UV/Vis/NIR/IR spectroscopy are consistent with
this conclusion.
Electrochemical studies and electronic spectra indicated that
1
) the stability of these mono-oxidized states decreases as the
bridging-ligand torsion angle increases from 08 (4a) to close to
08 (4e) and 2) the two metal centers of binuclear ruthenium
9
complexes 4a–e interact with each other through the biphenyl
bridge, and the redox splittings DE1/2 and the lowest transition
energy (HOMO–LUMO bandgap) show a strong linear correla-
2
tion with cos f. A combination of electrochemical, UV/Vis/NIR,
and IR analysis clearly showed 1) the ET pattern changes from
delocalization to localization through the biphenyl bridge, and
2) the redox processes of these complexes vary from signifi-
cant involvement of the bridging ligands (a redox-noninnocent
ligand) to an oxidation behavior with less participation of the
bridge. Therefore, the intramolecular electronic couplings and
the redox processes in 4a–e could be modulated by changing
the torsion angle f between the two phenyl rings from copla-
nar to perpendicular.
Figure 7. Time-resolved IR transmission spectra of a) 4a, b) 4c, and c) 4e re-
corded sequentially during a potential sweep from 0 V!+0.50 V!+0.70 V
4 6 2 2
in a 0.1m Bu NPF /CH Cl solution at 298 K.
Experimental Section
Owing to the synergistic nature of the d–p* orbitals in the
bond between the Ru and the CO ligand the oxidation-in-
duced shift of the n(CO) band provides an ideal tool for gaug-
ing the metal contribution to the oxidation process. If oxida-
tion is more centered on the biphenyl ligand, the electron ex-
traction is weaker on Ru and therefore the n(CO) band should
remain weakly altered. With increasing interplanar torsion
angle of the biphenyl core, the CO frequency is more affected.
These CO-band shifts provide one piece of evidence for partici-
pation of the bridging ligand in the oxidation process, and
suggest that the radical cation is partly localized on the bi-
phenyl core. However, stepwise oxidation of bimetallic com-
plexes 4d and 4e caused the n(CO) band to directly shift from
General materials
All manipulations were carried out at room temperature under a ni-
trogen atmosphere by using standard Schlenk techniques, unless
otherwise stated. Solvents were predried, distilled, and degassed
prior to use, except those for spectroscopic measurements, which
were of spectroscopic grade. The reagents ethynyltrimethylsilane
and 4,4’-diiodobiphenyl (1c) were purchased from Alfa Aesar.
Others were commercially available. The starting materials [RuHCl
[
73]
[74]
(
CO)(PPh ) ],
2,7-diiodofluorene (1a),
2,7-diiodo-9,10-dihydro-
3
3
[60]
[60]
phenanthrene (1b),
,4’-diiodo-2,2’,6,6’-tetramethylbiphenyl (1e),
Me ) (CH=CHC H )] (5)
4,4’-diiodo-2,2’-dimethylbiphenyl (1d),
[75]
4
and [RuCl(CO)(P-
[76]
were prepared by the procedures de-
3
3
6
5
scribed in the literature.
9901
Chem. Eur. J. 2015, 21, 9895 – 9904
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Synthesis of bis-trimethylsilyl diphenyl derivatives
,7-Bis(trimethylsilylethynyl)fluorene (2a): Trimethylsilylacetylene
potassium hydroxide (0.44 g, 7.8 mmol). The mixture was purified
by chromatography (petroleum ether/dichloromethane 10/1).
Yield: 0.21 g (81%) of a light brown solid. H NMR (400 MHz,
2
1
(
0.59 g, 6 mmol) was added to a stirred solution of 1a (0.418 g,
CDCl ): d=3.14 (s, 2H), 7.55 ppm (m, 8H).
3
1
0
mmol), CuI (0.021 g, 0.11 mmol), and [Pd(PPh ) Cl ] (0.042 g,
3 2 2
.06 mmol) in triethylamine (2 mL) and THF (6 mL) under an argon
4
,4’-Diethynyl-2,2’-dimethylbiphenyl (3d): The procedure for 3d
was similar to that for 3a: 2d (0.37 g 1 mmol), THF (8 mL), metha-
nol (8 mL), potassium hydroxide (0.34 g, 6 mmol). The crude prod-
atmosphere, and the mixture was heated to reflux for 10 h at
58C. The cold solution was filtered through a bed of Celite. The
5
uct was purified by chromatography (petroleum ether). Yield
filtrate was evaporated under reduced pressure and the residue
purified by silica-gel column chromatography (petroleum ether/di-
1
0
.21 g (90%) of a light brown solid. H NMR (400 MHz, CDCl ): d=
3
2
.02 (s, 6H), 3.09 (s, 2H), 7.03 (d, J=8.0 Hz, 2H), 7.36 (d, J=7.6 Hz,
chloromethane 5/1) to give a yellow brown solid (0.28 g, 79%).
1
2H), 7.42 ppm (s, 2H).
,4’-Diethynyl-2,2’,6,6’-tetramethylbiphenyl (3e): The procedure
for 3e was similar to that for 3a: 3e (0.60 g, 1.5 mmol), THF
12 mL), (8 mL), potassium hydroxide (0.51 g, 9 mmol). The crude
H NMR (400 MHz, CDCl ): d=0.26 (s, 18H), 3.84 (s, 2H), 7.49 (d, J=
3
4
5
.2 Hz, 2H), 7.64 (s, 2H), 7.68 ppm (d, J=5.2 Hz, 2H).
,7-Bis(trimethylsilylethynyl)-9,10-dihydrophenanthrene
The procedure of 2b was similar to that for 2a: 1b (1.23 g,
2
(2b):
(
product was purified by chromatography (petroleum ether/di-
3
0
mmol), CuI (0.062 g 0.32 mmol), [Pd(PPh ) Cl ] (0.124 g,
3 2 2
.17 mmol), triethylamine (6 mL), THF (18 mL), and trimethylsilyl-
chloromethane 10/1). Yield 0.31 g (79%) of a light brown solid.
1
H NMR (400 MHz, CDCl ): d=1.86 (s, 12H), 3.05 (s, 2H), 7.28 ppm
3
acetylene (2.94 g, 30 mmol). Yield: 0.90 g (80%) of brown solid.
1
(s, 4H).
H NMR (400 MHz, CDCl ): d=0.26 (s, 18H), 2.84 (s, 4H), 7.37 (s,
3
2
H), 7.43 (d, J=5.2 Hz, 2H), 7.66 ppm (d, J=5.6 Hz 2H).
,
General synthesis of binuclear ruthenium complexes
4
,4’-Bis(trimethylsilylethynyl)biphenyl (2c): The procedure of 2c
was silimar to that for 2a: 1c (0.812 g, 2 mmol), CuI (0.041 g,
.22 mmol), [Pd(PPh ) Cl ] (0.081 g, 0.11 mmol), triethylamine
A solution of the corresponding diethynyl biphenyl derivative
(0.18 mmol) in CH Cl (5 mL) was slowly added to a suspension of
0
3
2
2
2
2
(
4 mL), THF (12 mL), and trimethylsilylacetylene (1.960 g, 20 mmol).
[
RuHCl(CO)(PPh ) ] (0.31 g, 0.33 mmol) in CH Cl (20 mL). The reac-
3
3
2
2
1
Yield: 0.29 g (85%) of white solid. H NMR (400 MHz, CDCl ): d=
3
tion mixture was stirred for 1 h to give a red solution. Then a 1m
0
.26 (s, 18H), 7.53 ppm (s, 8H).
THF solution of PMe (1.8 mL, 1.8 mmol) was added to the red so-
3
4
,4’-Bis(trimethylsilylethynyl)-2,2’-dimethylbiphenyl (2d): The
lution. The mixture was stirred for another 20 h. The solution was
filtered through a column of Celite. The volume of the filtrate was
reduced to about 2 mL under vacuum. Addition of hexane (30 mL)
to the residue produced a solid, which was collected by filtration,
procedure for 2d was similar to that for 2a: 1d (0.434 g, 1 mmol),
CuI (0.023 g, 0.12 mmol), Pd(PPh ) Cl (0.046 g, 0.06 mmol), triethyl-
amine (2 mL), THF (6 mL), and trimethylsilylacetylene (1.960 g,
3
2
2
1
2
0 mmol). Yield: 0.28 g (82%) of a light yellow solid. H NMR
washed with hexane, and dried under vacuum.
(
2
400 MHz, CDCl ): =0.26 (s,18H), 1.98 (s, 6H,), 7.0 (d, J=8.0 Hz,
31
3
4a: Yield 0.11 g, 63%; P NMR (160 MHz, CDCl ): d=18.9 (t, J=
3
H), 7.33 ppm (d, J=7.6 Hz,2H), 7.39 (s, 2H).
1
,
22.7 Hz), À7.2 ppm (d, J=22.6 Hz); H NMR (400 MHz, CDCl ): d=
3
4
,4’-[(trimethylsily)ethynyl]-2,2’,6,6’-tetramethylbiphenyl
(2e):
1.41 (t, J(P,H)=3.2 Hz, 36H, PMe ), 1.49 (d, J(P,H)=6.8, 18H, PMe ),
3
3
The procedure of 2e was similar to that for 2a: 1e (0.460 g,
3.82 (s, 2H, CH ), 6.60–6.66 (m, 2H, ArÀCH=), 7.28 (d, J=8.0 Hz, 2H,
2
1
0
mmol), CuI (0.023 g, 0.12 mmol), [Pd(PPh ) Cl ] (0.046 g,
.06 mmol), triethylamine (2 mL), THF (6 mL), and trimethylsilylace-
Ar), 7.50 (s, 2H, Ar), 7.58 (d, J=8.0 Hz, 2H, Ar), 8.08–8.14 ppm (m,
3
2
2
1
3
2H, RuÀCH=); C NMR (100 MHz, CDCl ): d=16.85 (t, J(P,C)=
3
tylene (0.980 g, 10 mmol). Yield: 0.29 g (73%) of a yellow solid.
20.00 Hz, PMe ), 20.34 (d, J(P,C)=26.20 Hz, PMe ), 37.00 (CH ),
3
3
2
1
H NMR (400 MHz, CDCl ): d=0.26 (s, 18H), 1.84 (s, 12H), 7.25 ppm
119.25, 120.74, 123.49, 135.35, 138.77, 139.83, 143.90 (BiphÀCH=),
164.18, 165.10 (RuÀCH=), 202.57, 202.73 ppm (CO); Elemental anal-
ysis calcd (%) for C H Cl O P Ru : C 44.36, H 6.64; found: C 44.42,
3
(
s, 4H).
37
66
2
2
6
2
H 6.58.
Synthesis of diethynyl biphenyl derivatives
,7-Bis(ethynyl)fluorene (3a): 2,7-Bis(trimethylsilylethynyl)fluorene
31
4
b: Yield 0.11 g 57%. P NMR (160 MHz, CDCl ): d=19.0 (t, J=
3
1
2
22.56 Hz), À7.3 ppm (d, J=22.56 Hz); H NMR (400 MHz, CDCl3):
(2a, 0.36 g, 1 mmol) was dissolved in a mixture of THF (6 mL) and
d=1.41 (t, J(P,H)=3.2 Hz, 36H, PMe ), 1.56 (d, J(P,H)=8.0, 18H,
3
methanol (6 mL). Powdered potassium hydroxide (0.34 g, 6 mmol)
was added, and the reaction mixture was stirred at room tempera-
ture for 15 h. The reaction mixture was diluted with dichlorome-
thane and washed with brine. The organic layer was dried over
PMe ), 2.84 (s, 4H, CH ), 6.55–6.61 (m, 2H, ArÀCH=), 7.19 (s, 2H,
3
2
Ar), 7.22 (d, J=8.4 Hz, 2H, Ar), 7.60 (d, J=8.0 Hz, 2H, Ar), 8.11–
13
8.17 ppm (m, 2H, RuÀCH=); C NMR (100 MHz, CDCl ): d=16.66
3
(PMe ), 20.10 (PMe ), 29.43 (CH ), 122.97, 123.29, 123.72, 131.15,
3
3
2
NaSO , and the solvent removed in vacuo. The crude product was
134.72, 136.94, 139.77 (BiphCH=), 165.07, 165.57 (RuÀCH=), 202.46,
4
purified by chromatography (petroleum ether/dichloromethane 8/
202.54 ppm (CO); elemental analysis calcd (%) for C H Cl O P Ru :
38
68
2
2
6
2
1
1
). Yield: 0.16 g (75%) of a brown solid. H NMR (400 MHz, CDCl ):
C 44.93, H 6.75; found: C 44.77, H 6.91.
3
d=3.13 (s, 2H), 3.89 (s, 2H), 7.53 (d, J=4.8 Hz, 2H), 7.67 (s, 2H),
.72 ppm (d, J=4.8 Hz, 2H).
31
4
2
c: Yield 0.10 g, 53%. P NMR (160 MHz, CDCl ): d=18.9 (t, J=
3
7
1
2.72 Hz), À7.2 ppm (d, J=22.56 Hz); H NMR (400 MHz, CDCl3):
2
3
,7-Diethynyl-9,10-dihydrophenanthrene (3b): The procedure for
b was similar to that for 3a: 2b (0.83 g, 2.2 mmol), THF (16 mL),
d=1.41 (t, J(P,H)=3.2 Hz, 36H, PMe ), 1.48 (d, J(P,H)=6.8, 18H,
3
PMe ), 6.58–6.64 (m, 2H, ArÀCH=), 7.37 (d, J=8.0 Hz, 4H, Ar), 7.50
3
1
3
methanol (16 mL), potassium hydroxide (0.75 g, 13.3 mmol). The
crude product was purified by chromatography (petroleum ether/
(d, J=8.4 Hz, 4H, Ar), 8.13–8.20 ppm (m, 2H, RuÀCH=); C NMR
(100 MHz, CDCl ): d=16.56 (t, J=15.2 Hz, PMe ), 20.07 (d, J=
3
3
1
dichloromethane 8/1). Yield: 0.35 g (70%) of a brown solid. H NMR
21.0 Hz, PMe ), 124.56, 126.59, 134.22, 136.98, 139.78 (BiphCH=),
3
(
400 MHz, CDCl ): d=2.85 (s, 4H), 3.12 (s, 2H), 7.37 (s, 2H), 7.43 (d,
165.64 (RuÀCH=), 202.45 ppm (CO); elemental analysis calcd (%)
3
J=5.2 Hz, 2H), 7.67 ppm (d, J=5.6 Hz, 2H).
,4’-Diethynylbiphenyl (3c): The procedure for 3c was similar to
that for 3a: 2c (0.45 g 1.3 mmol), THF (11 mL), methanol (11 mL),
for C H Cl O P Ru : C 43.68, H 6.72; found: C 43.56, H 6.87.
36
66
2
2
6
2
31
4
4d: Yield 0.07 g, 77%. P NMR (160 MHz, CDCl ): d=18.9 (t, J=
3
1
22.56 Hz), À7.2 ppm (d, J=21.12 Hz); H NMR (400 MHz, CDCl3):
Chem. Eur. J. 2015, 21, 9895 – 9904
www.chemeurj.org
9902
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
d=1.42 (t, J(P,H)=3.2 Hz, 36H, PMe ), 1.49 (d, J(P,H)=6.4, 18H,
the electrode potential. The working electrode was a glassy carbon
electrode of 5 mm in diameter, and the counter electrode was
a platinum foil. The reference electrode (Ag/Ag ) was separated
3
PMe ), 2.08 (s, 6H, ÀCH ), 6.56–6.59 (m, 2H, ArÀCH=), 7.01 (d, J=
3
3
+
7
8
.6 Hz, 2H, Ar), 7.16 (d, J=7.6 Hz, 2H, Ar), 7.20 (s, 2H, Ar) 8.04–
13
.09 ppm (m, 2H, RuÀCH=); C NMR (100 MHz, CDCl ): d=16.62 (t,
from the bulk of the solution by a fritted-glass bridge of low poros-
ity, which contained the solvent/supporting electrolyte mixture.
3
J(P,C)=15.3 Hz, PMe ), 20.14 (d, J(P,C)=16.0 Hz, PMe3), 30.08
3
À1
(
ÀCH ), 121.39, 125.57, 129.65, 134.64, 135.65, 137.82, 140.07
The IR spectra were collected in single beam mode at 2 cm , and
3
(
BiphCH=), 163.74, 164.36 (RuÀCH=), 202.51 ppm (CO); elemental
differential absorbance spectra were presented against the refer-
ence spectrum recorded immediately prior to the application of
the potential. All electrochemical experiments were carried out
under ambient conditions.
analysis calcd (%) for C H Cl O P Ru : C 44.84, H 6.93; found: C
3
8
70
2
2
6
2
4
4.71, H 7.05.
3
1
4
2
e: Yield 0.09 g, 69%. P NMR (160 MHz, CDCl ): d=19.0 (t, J=
3
1
1.12 Hz), À7.0 ppm (d, J=22.72 Hz); H NMR (400 MHz, CDCl3):
d=1.42 (t, J(P,H)=3.2 Hz, 36H, PMe ), 1.49 (d, J(P,H)=6.4, 18H,
3
PMe ), 1.89 (s, 12H, ÀCH ), 6.50–6.56 (m, 2H, ArÀCH=), 7.04 (s, 4H, Acknowledgements
3
3
13
Ar) 7.94–8.00 ppm (m, 2H, RuÀCH=); C NMR (100 MHz, CDCl ):
3
d=16.62 (t, J(P,C)=15.3 Hz, PMe ), 20.14 (d, J(P,C)=16.4 Hz, PMe ),
3
3
The authors acknowledge financial support from National Nat-
ural Science Foundation of China (nos. 20803027, 21173094,
3
1
0.83 (ÀCH ), 123.25, 135.03, 135.30, 136.53, 139.80 (BiphCH=),
3
62.36, 163.13 (RuÀCH,), 202.43 ppm (CO); Elemental analysis calcd
21272088, and 21202054), and self-determined research funds
(
%) for C H Cl O P Ru : C 45.93, H 7.13; found: C 45.76, H 7.32.
40 74 2 2 6 2
of CCNU from the college’s basic research for financial support.
The authors thank Prof. Z. Y. Zhou (Xiamen Univ.) for the help
with in situ FTIR experiments.
Crystallographic details
Crystals suitable for X-ray diffraction were grown from dichlorome-
thane solutions of 4b–4d layered with hexane. A crystal with ap-
proximate dimensions of 0.23”0.20”0.15 mm for 4b, 0.20”0.10”
Keywords: bridging ligands · electrochemistry · mixed-valent
compounds · redox chemistry · ruthenium
0
.10 mm for 4c, and 0.15”0.12”0.10 mm for 4d was mounted on
a glass fiber. Intensity data were collected on a Nonius Kappa CCD
diffractometer with MoKa radiation (0.71073 Š) at room tempera-
ture. The structures were solved by a combination of direct meth-
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[2] B. Q. Xu, N. J. Tao, Science 2003, 301, 1221–1223.
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3] W. Hong, D. Z. Manrique, P. Moreno-García, M. Gulcur, A. Mishchenko,
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C. J. Lambert, M. R. Bryce, T. Wandlowski, J. Am. Chem. Soc. 2012, 134,
by full-matrix least squares methods (SHELXL-97).
All non-H
2292–2304.
atoms were refined anisotropically. The hydrogen atoms were
placed in the ideal positions and refined as riding atoms. Further
crystal data and details of the data collection are summarized in
Table S1 in the Supporting Information. Selected bond lengths and
angles are given in Tables S2–S4 (see the Supporting Information).
[
4] H.-M. Wen, Y. Yang, X.-S. Zhou, J.-Y. Liu, D.-B. Zhang, Z.-B. Chen, J.-Y.
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[
[
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Physical measurements
1
Elemental analyses were performed with a Vario EL III CHNSO. H,
1
3
1
31
1
[9] M. I. Bruce, K. Costuas, T. Davin, B. G. Ellis, J. F. Halet, C. Lapinte, P. J.
C{ H}, and P{ H} NMR spectra were collected on a Varian MERCU-
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2
005, 24, 3864–3881.
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13
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Chem. Eur. J. 2015, 21, 9895 – 9904
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9903
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Received: February 7, 2015
Published online on May 27, 2015
Chem. Eur. J. 2015, 21, 9895 – 9904
www.chemeurj.org
9904
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim