Bimetallic ruthenium complexes bridged by divinylphenylene bearing oligo(ethylene glycol)methylether: Synthesis, (spectro)electrochemistry and the lithium cation effect

Article abstract of DOI:10.1039/c3dt52677a

A series of 1,4-disubstituted ruthenium-vinyl complexes, (E,E)-[{(PMe ₃)₃(CO)ClRu}₂(μ-HCCH-Ar-CHCH)], in which the 1,4-diethenylphenylene bridge bears two oligo(ethylene glycol)methyl ether side chains at different positions (2,5- and 2,3-positions), were prepared. The respective products were characterized by elemental analyses and NMR spectroscopy. The structures of complexes 1b and 1e were established by X-ray crystallography. The electronic properties of the complexes were investigated by cyclic voltammetry, and IR and UV-vis/NIR spectroscopies. Electrochemical studies showed that the 2,5-substituents better stabilized the mixed-valence states; the electrochemical behavior was greatly affected by lithium cations, especially complex 1g with 2,3-substituents, which was further supported by IR and UV-vis/NIR spectra changes. Spectroelectrochemical studies showed that the redox chemistry was dominated by the non-innocent character of the bridging fragment.

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Bimetallic ruthenium complexes bridged by
divinylphenylene bearing oligo(ethylene glycol)-
methylether: synthesis, (spectro)electrochemistry
and the lithium cation effect†
Cite this: Dalton Trans., 2014, 43,
4093
Li Yan Tian,a Yuan Mei Liu,a Guang-Xuan Tian,a Xiang Hua Wu,*a Zhen Li,a
Jun-Feng Kou,a Ya-Ping Ou,b Sheng Hua Liub and Wen-Fu Fu*a
A
series of 1,4-disubstituted ruthenium–vinyl complexes, (E,E)-[{(PMe3)3(CO)ClRu}2(μ-HCvCH–Ar–
CHvCH)], in which the 1,4-diethenylphenylene bridge bears two oligo(ethylene glycol)methyl ether side
chains at different positions (2,5- and 2,3-positions), were prepared. The respective products were
characterized by elemental analyses and NMR spectroscopy. The structures of complexes 1b and 1e were
established by X-ray crystallography. The electronic properties of the complexes were investigated by
cyclic voltammetry, and IR and UV-vis/NIR spectroscopies. Electrochemical studies showed that the 2,5-
substituents better stabilized the mixed-valence states; the electrochemical behavior was greatly affected
by lithium cations, especially complex 1g with 2,3-substituents, which was further supported by IR and
UV-vis/NIR spectra changes. Spectroelectrochemical studies showed that the redox chemistry was domi-
nated by the non-innocent character of the bridging fragment.
Received 27th September 2013,
Accepted 23rd December 2013
DOI: 10.1039/c3dt52677a
www.rsc.org/dalton
bridging ligands play an important role in tuning the elec-
tronic properties of these bimetallic ruthenium complexes, as
Introduction
Linear conjugated systems combined with two redox-active a result of their contributions to the electronic interactions of
transition-metal moieties have been extensively investigated as the metal centers.
model systems for molecular wires.1 The frontier molecular
Since they were discovered by Pedersen in 1967,7,8 crown
orbitals of these complexes generally have both metal and ethers have proved to be very popular and extremely useful
π-conjugated bridge characteristics, the relative amounts of ligands (hosts) for a wide range of metal ions and neutral or
which depend on the nature of the end groups and the bridge. ionic organic species.9 The concept of redox-sensors, contain-
Based on the structure of the bridging ligand, it is possible to ing redox-responsive receptors and crown ethers, has been
classify bridging ligands into three main groups: all-carbon sp- used in studies of electrochemical sensors because redox-
bridged,2 carbon-based sp2-bridged,3 and carbon-based sp– active groups are mostly responsive to electrochemical
sp2-bridged4 systems. Recently, carbon-based sp2-bridged signals.10 Although research in this area remains active, few
[{LnM}(μ-CHvCH–Ar–CHvCH){MLn}] systems, which include literature surveys report the activities of crown ethers in binuc-
the five-coordinated divinylphenylene-bridged diruthenium com- lear molecular wires.
plexes
and the related six-coordinated PMe3-containing ruthenium– two ruthenium centers could be fine-tuned by modification of
vinyl analogs (E,E)-[{(PMe3)3(CO)ClRu}2(μ-HCvCH–Ar– a 1,4-diethenylphenylene bridging ligand, and that electron-
(E,E)-[{(PiPr3)2(CO)ClRu}2(μ-HCvCHC6H4CHvCH-1,4)]
Previously, we reported that electronic coupling between
CHvCH-1,4)], have been studied by Winter and Liu, respect- releasing substituents could better stabilize mixed-valence
ively.5,6 It has been well established that divinylphenylene species.6a Oligo(ethylene glycol)methyl ethers, like crown
ethers, not only act as electron-donating substituents, but also
recognize metal ions. In attempts to fine tune the electronic
aCollege of Chemistry and Chemical Enginering, Yunnan Normal University,
Kunming 650500, P.R. China. E-mail: chxhwu@sina.com, fwf@mail.ipc.ac.cn
bKey Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of
properties of ruthenium-based (E,E)-[{(PMe3)3(CO)ClRu}2-
(μ-HCvCH–Ar–CHvCH-1,4)] systems further, we have refo-
cused our attention on the bridging ligand by introducing
oligo(ethylene glycol)methyl ether groups. In continuation of
previous work, a series of 1,4-disubstituted ruthenium–vinyl
complexes [RuCl(CO)(PMe3)3]2(μ-CHvCH–Ar–CHvCH), in
Chemistry, Central China Normal University, Wuhan 430079, P.R. China
†Electronic supplementary information (ESI) available: Electrochemistry,
UV-Vis/near-IR. CCDC 956627 and 956628 for 1b and 1e. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3dt52677a
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which the 1,4-diethenylphenylene bridge bears two oligo(ethyl-
ene glycol)methyl ether side chains at different positions (the
2,5-, and 2,3-positions), were developed to probe the charge-
transfer efficiency and investigate the stability of mixed-
valence species in the presence of lithium cations.
Results and discussion
Syntheses and characterization
Substituted 1,4-diethynylbenzene derivatives 3e–3g were syn-
thesized using a sequence of alkylation, coupling, and depro-
tection reactions (Scheme 1). Alkylation of oligo(ethylene
glycol)tosylates with 3,6-diiodobenzene-1,2-diol gave the
general open-chain crown ether derivatives 5. The respective
bis(trimethylsilyl acetylene) derivatives 6a–c were obtained in
moderate yields by palladium-catalyzed coupling of these di-
iodoaryls with trimethylsilylethyne.11 The ligand precursors
were smoothly converted to the corresponding diterminal
alkynes 3e–g by removal of the trimethylsilyl protecting groups
with K2CO3 in MeOH–CH2Cl2, in 74–90% yields. The immedi-
ate precursors were characterized using 1H and 13C NMR spec-
troscopies. The general synthetic route for the preparation of
binuclear ruthenium–vinyl complexes is outlined in Scheme 2.
Diethynylbenzene derivatives 3a–g were reacted with the ruthe-
nium hydride complex [RuHCl(CO)(PPh3)3] to give the inser-
tion products [{(PPh3)2(CO)ClRu}2(μ-HCvCH–Ar–CHvCH)],
which were not isolated because they are air sensitive,
especially in solution. PMe3 was then added to give the stable
six-coordinated complexes 1a–g. These complexes were charac-
terized using NMR spectroscopy. Although the 1,4-diethenyl-
phenylene bridge bears two oligo(ethylene glycol)methyl ether
side chains at different positions, the resonance signals such
as those of the CHvCH protons in the 1H NMR spectra and of
PMe3 in the 31P NMR spectra, with chemical shifts similar to
those found in [RuCl(CO)(PMe3)3]2(μ-(CHvCH)n),4d,6a,c
Scheme 2 Preparation of bimetallic ruthenium–vinyl complexes 1a–
1g.
displayed no obvious differences, suggesting that their struc-
tures are similar.
Single crystals of complexes 1b and 1e suitable for X-ray
analysis were obtained by slow diffusion of hexane into solu-
tions in CH2Cl2. The single-crystal X-ray structures of com-
plexes 1b and 1e are shown in Fig.
1 and 2. The
crystallographic details, and selected bond lengths and angles
for 1b and 1e are presented in the ESI, Tables S1 and S2,†
respectively. The compounds contain two ruthenium centers
linked by
a linear μ-CHvCH–Ar–CHvCH bridge [Ar =
C6H2(OCH2CH2OCH3)2-2,5 (1b), C6H2(OCH3)2-2,3 (1e)], as
shown in Fig. 1 and 2. In the molecular structure of 1b there is
Fig. 1 ORTEP drawing of complex 1b (thermal ellipsoids are plotted at
10%).
Fig. 2 ORTEP drawing of complex 1e·H2O (thermal ellipsoids are
Scheme 1 Preparation of 1,4-diethynylbenzene derivatives 3e–3g.
4094 | Dalton Trans., 2014, 43, 4093–4101
plotted at 10%). The solvent molecule H2O was omitted for clarity.
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one crystallographically unique ruthenium centre with the
second generated by the presence of an inversion centre. In 1e,
the two ruthenium centres are crystallographically unique. The
distances between the two ruthenium centers in 1e is
11.633(2) Å, this is slightly shorter than that in 1b (11.852(3)
Å). The two CHvCH units and the benzene ring unit are
nearly coplanar, with dihedral angles between the core
benzene ring and the two vinyl groups of 5.9(3)° in complex
1b; however, the dihedral angles are 4.5(7)° and 2.7(7)° in 1e.
The two double bonds are in a trans configuration. The three
complexes have different crystal systems and space groups, as
can be seen in Table S1 (ESI†). This is probably attributable to
the two side chains at different positions (the 2,3- and 2,5-
positions).
Electrochemical properties
Cyclic voltammetry (CV) and square-wave voltammetry (SWV)
techniques were used to study the redox properties of com-
plexes 1a–1g. The CV and SWV measurements were performed
at scan rates of 100 mV s−1 with 0.05 M [NnBu4][B(C6F5)4] as
the supporting electrolyte in dry CH2Cl2. The electrochemical
data are summarized in Table 1. Plots of the cyclic voltammo-
grams and square-wave voltammograms for complexes 1a–1g
are shown in Fig. 3, S1–S3.† The complexes 1a–1g undergo two
successive single-electron oxidation processes, giving rise to
redox waves A and B (Table 1) in the potential region
0.100–0.596 V. Additional electrochemical processes were
observed at higher potentials but have not been examined
further. In the case of 1, the first wave is strictly electrochemi-
cally quasi-reversible while the second oxidation process is
electrochemically irreversible in each case. The wave separ-
ation or potential difference ΔE1/2 [ΔE1/2 = E1/2(B) − E1/2(A)]
and the comproportionation constant Kc (Kc = eΔEF/(RT)) are
critical parameters for evaluating the thermodynamic
Fig. 3 CVs of complexes 1c, 1e, 1f, and 1g in CH2Cl2/[NnBu4][B(C6F5)4]
at scan rates of 100 mV s−1 (left). SWVs of complexes 1c, 1e, 1f, and 1g
(scan rate: 100 mV s−1, f = 10 Hz, right).
stabilities of the oxidized states (or mixed-valence
species).12–15 As shown in Table 1, for complexes 1a–1d, in
which the two oligo(ethylene glycol)methyl ether side chains
were attached at the 2,5-positions of the 1,4-diethenylphenyl-
ene bridge, the ΔE1/2 and Kc values are undifferentiated
(within experimental error), with greater increases for longer
(OCH2CH2)n moieties. However, in a comparison of the ΔE1/2
values of the complexes with 2,3-substituents, i.e., 1e–1g
(Table 1), a gradual decrease in ΔE1/2 from 0.364 V (1e) to
0.348 V (1f) to 0.324 V (1g) was observed, with E1/2 increases
for longer (OCH2CH2)n moieties (Fig. 4). This means that the
stabilities of these mono-oxidized states decrease as the oligo-
Table 1 Electrochemical data for complexes 1a–1g,a and complexes
1c–g after adding 1.0 equiv. of Li+ b
ΔE c (mV)
Kc
d
Complex
E1/2(A) (V)
E1/2(B) (V)
1a
1b
1c
1d
1e
1f
1g
0.044
0.024
0.100
0.080
0.232
0.176
0.188
0.132
0.188
0.248
0.200
0.428
0.444
0.420
0.500
0.480
0.596
0.524
0.512
0.452
0.476
0.592
0.488
—
400
396
400
400
364
348
324
320
288
344
288
—
5.77 × 106
4.94 × 106
5.77 × 106
5.77 × 106
1.42 × 106
7.63 × 105
2.99 × 105
2.56 × 105
7.38 × 104
6.53 × 105
7.38 × 104
—
1c + 1.0 eq. Li+
1d + 1.0 eq. Li+
1e + 1.0 eq. Li+
1f + 1.0 eq. Li+
1g + 1.0 eq. Li+
a Potential data of 1.0 mmol L−1 solutions of 1a–1g in dry
dichloromethane containing 0.05 mol L−1 of [NnBu4][B(C6F5)4] as a
supporting electrolyte at 298 K. The potential of the [FcH]/[FcH]+
couple is +0.21 V under these conditions. b Potential data are recorded
after adding 1.0 eq. [LiB(C6F5)4]. c ΔE = E1/2(B) − E1/2(A) denotes the
Fig. 4 (Left) CVs of complexes 1e and 1g after adding 1.0 equiv. of [LiB-
(C6F5)4] in CH2Cl2/[NnBu4][B(C6F5)4], at scan rates of 100 mV s−1. (right)
SWVs of complexes 1e and 1g after adding 1.0 equiv. [LiB(C6F5)4] (scan
rate: 100 mV s−1, f = 10 Hz).
potential difference between redox processes
A
and B. d The
comproportionation constants, Kc, were calculated by the formula Kc =
exp(ΔE/25.69) at 298 K.12c
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(ethylene glycol)methyl ether lengthens from OCH3 to (OCH2- spectroelectrochemical studies were carried out on complexes
CH2)2OCH3. The ΔE1/2 values display sufficient stability of the 1c, 1e, 1g, and 1g.
monocations with respect to disproportionation to the neutral
and dicationic forms.
To investigate the stability of mixed-valence species in the
presence of lithium cations, the electrochemical properties of
IR spectroelectrochemistry
IR spectroscopy was used to clarify the electronic structures
and influence of the bridging ligands on the electronic pro-
perties. IR spectroelectrochemical studies were carried out on
complexes 1c, 1e, and 1g, using the ν(CO) band as an indicator
of the metal oxidation state. The characteristic vibrational fre-
quencies of different redox states are listed in Table 2. The
complexes 1c–1g were investigated by CV and SWV in CH2Cl2
containing 0.05 M [NnBu4][B(C6F5)4]. The E1/2 and ΔE1/2 values
seem to keep changing after gradually adding [LiB(C6F5)4].
However, the electrochemical behavior of complex 1g has
greatly changed, in which one redox event was observed, in the
presence of 1.0 equiv. of Li+. For this reason, the electrochemi-
ν(CO) band of the neutral complexes 1c, 1e, and 1g appeared
at 1921, 1922, and 1923 cm−1, respectively. A new strong band
cal properties of 1c–1g in the presence of metal ions with 1.0
at 1936, 1932, and 1928 cm−1, respectively, appeared after the
first oxidation. Similar observations have been reported for the
radical cations of the symmetrical binuclear ruthenium–vinyl
equiv. of Li+ were investigated by CV and SWV. The electroche-
mical data are recorded in Table 1. Plots of the cyclic voltam-
mograms and square-wave voltammograms for complexes 1e
and 1g are shown in Fig. 4. Complexes 1c–1f could be oxidized
separately, showing two redox events, after adding 1.0 equiv. of
Li+. As shown in Table 1, the ΔE1/2 values are 0.32 V (1c), 0.288
V (1d), 0.344 V (1e), and 0.288 V (1f). However, complex 1g had
no detectable ΔE1/2 under the same conditions. The ΔE1/2
differences after addition of 1.0 equiv. of Li+ are 0.080 V (1c),
0.112 V (1d), 0.020 V (1e), and 0.036 V (1f). It can be seen that
changes in the ΔE1/2 values occurred in the presence of
lithium cations, which may be attributable to interactions
between the oligo(ethylene glycol)methyl ether side chains
and the lithium cations, resulting from decreased electronic
delocalization through the bridges. Similar to the findings in
our previous studies,6a the –(OCH2CH2)nOCH3 units with
lithium cations can be regarded as electron-withdrawing sub-
stituents, which decrease the electron density at the metal
centers in the neutral species, making oxidation more
difficult. The monocationic species are destabilized, which
causes a cathodic shift of the second oxidation process. As a
result, the ΔE1/2 values become smaller, as shown in Table 1.
It is also found that the change in the ΔE1/2 difference after
adding 1.0 equiv. of Li+ is greater for the complex with 2,3-sub-
stituents, 1g, than for the corresponding complex 1c with 2,5-
substituents, and the longer-chain complex 1f. For the
complex with 2,3-substituents, a gradual increase in the ΔE1/2
difference was observed with increases in the length of the
OCH2CH2 moieties. The largest change in the ΔE1/2 difference
compared with that of the OCH3-bearing complex 1e is that of
the (OCH2CH2)2OCH3-bearing complex 1g, as shown in
Table 1 and Fig. 3. This means that the electrochemical behav-
ior of complex 1g has been obviously affected in the presence
of 1.0 eq. lithium cations. This may be because the two
adjoined open (OCH2CH2)2OCH3 chains in 1g can better bind
with lithium cations. This explains why the 2,3-position has a
greater effect than the 2,5-position. These results indicate that
the stability of the mixed-valence states is also highly depen-
dent on the bridging ligands.
complexes
(E,E)-[{(PMe3)3(CO)ClRu}2(μ-HCvCH–C6H4–
CHvCH-1,4)]6b and (E,E)-[{(PMe3)3(CO)ClRu}2(μ-HCvCH–
dithia[3,3]paracyclophane–CHvCH)],6c as shown in Table 2
and Fig. 5. It is found that shifts in the positions of the ν(CO)
bands for the bimetal complexes 1c, 1e, and 1g on sequential
oxidation from neutral to monocationic are 15, 10, and
5 cm−1, respectively. These small shifts are evidence of strong
participation of the bridge in the first oxidation process.
Further shifts were found for the dications [1c]2+ (1970 cm−1),
[1e]2+ (1967 cm−1), and [1g]2+ (1972 cm−1) from the second oxi-
dation of complexes 1c, 1e, and 1g. A comparison of these data
shows that the shifts in the position of the ν(CO) bands are
Table 2 Summary of IR spectra of [1c]n+, [1e]n+, [1g]n+, and [1g]n+ with
1.0 equiv. of Li+ (n = 0, 1, 2), recorded by in situ oxidation in spectroelec-
trochemical cell (OTTLE; CH2Cl2–0.05 M nBu4N(C6F5)4)
Complex
Freq
n = 0
n = 1
n = 2
[1c]n+
ν(CO)
1921(vs)
1922(vs)
1923(vs)
1923(s)
1936(vs)
1932(s)
1928(s)
1964(s), 1925(s)
1970(s)
1967(s)
1972(s)
1968(vs)
[1e]n+
[1g]n+
[1g]n+ + 1.0 eq. Li+
The radical species [1a]+–[1g]+ were stable enough for
spectroscopic characterization, as
substantial comproportionation constants (Kc, see Table 1). To
clarify the characteristics of these redox processes, electrochemical cell (0.05 M nBu4N(C6F5)4/CH2Cl2, room temperature).
a
result of their
Fig. 5 ν(CO) spectra of (a) [1c]n+, (b) [1e]n+, (c) [1f]n+, and (d) [1g]n+
+
1.0 equiv. of Li+ (n = 0–2), recorded during in situ oxidation in a spectro-
4096 | Dalton Trans., 2014, 43, 4093–4101
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obviously not affected by the different positions and lengths of
the open oligo(ethylene glycol)methyl ether side chains.
To better investigate the effects of lithium cations on
changes in the redox properties, IR spectroelectrochemical
studies were carried out on complex 1g with 1.0 equiv. of Li+.
It could be understood that Li+ is acting as the oxidizing agent.
The ν(CO) band of complex 1g shifted from 1923 cm−1 (1g +
1.0 eq. Li+) to 1964, 1925 cm−1 (Δν(CO) = 39 cm−1) ([1g]+
+
1.0 eq. Li+), and 1968 cm−1 ([1g]2+ + 1.0 eq. Li+). Compared
with complex 1g, the mono-oxidized species of the complex,
[1g]+, with 1.0 equiv. of Li+ displayed two ν(CO) bands, in
which the positions of the two ν(CO) bands were almost the
same as those in the neutral complex and the dicationic
complex, respectively, as shown in Table 2 and Fig. 5. It can be
seen that the ν(CO) band of complex 1g was affected by
lithium cations. This result illustrates that the electrons are
not fully delocalized over the whole framework, which is
attributable to interaction between the two adjoined open
(OCH2CH2)2OCH3 chains and lithium cations. These results
are associated with the electrochemical properties described
above.
Fig. 6 Changes in UV-vis/NIR spectra of complexes [1e]n+ and [1e]n+
with 1.0 equiv. of LiB(C6F5)4 (n = 0–2) in CH2Cl2 (2.0 mmol L−1), col-
lected during in situ oxidation in a spectroelectrochemical cell (0.05 M
nBu4N(C6F5)4–CH2Cl2).
UV-vis/near-IR spectroelectrochemistry
In order to obtain further insights into the oxidation processes
for the series of complexes 1a–1g, UV-vis/NIR spectroelectro-
chemical studies of complexes 1c, 1e, and 1g were performed
in an OTTLE (optically transparent thin-layer electrochemical)
cell, using a 0.05 mol L−1 CH2Cl2 solution of [NnBu4][B(C6F5)4]
as the supporting electrolyte. The changes in the UV-vis/NIR
absorption spectra are summarized in Table 3 and Fig. 6, 7,
and S4.† During the measurements, oxidation of the neutral
complexes to mixed-valence monocationic and, finally, dicatio-
nic species was observed. As expected, the neutral and dicatio-
nic complexes show no absorption bands in the NIR range, as
Fig. 7 Changes in UV-vis/NIR spectra of complexes [1g]n+ and [1g]n+
with 1.0 equiv. of LiB(C6F5)4 (n = 0–2) in CH2Cl2 (2.0 mmol L−1), col-
lected during in situ oxidation in a spectroelectrochemical cell (0.05 M
nBu4N(C6F5)4/CH2Cl2).
Table 3 Electronic absorption spectroscopic data of binuclear ruthe-
nium complexes 1c, 1e, and 1g, and complexes 1e and 1g, after adding
1.0 equiv. of Li+, in various oxidation states (0, +1, +2)
UV/vis/NIR absorption λmax (nm) (10−4 εmax
Complex
(dm3 mol−1 cm−1))
shown in Table 3 and Fig. 6. On oxidation of these complexes,
mixed-valence species [1c]+, [1e]+, and [1g]+ are formed, pre-
senting a broad band between 1000 and 1500 nm, similar to
those observed for the related radical complex (E,E)-
[{(PMe3)3(CO)ClRu}2(μ-HCvCH–C6H4–CHvCH-1,4)], in which
the bridging ligands are intimately involved in supporting
unpaired electrons/holes.6b Further oxidation results in these
absorptions gradually decrease until they completely dis-
appear. Compared with the absorption spectrum of complex
[1c]+ with 2,5-substituents, the absorption maxima of com-
plexes [1e]+ and [1g]+ with 2,3-substituents show clear batho-
chromic shifts, and the intensities of the bands are lower. The
above results are supported by a comparison of the absorption
spectra of the 2,3-substituent complexes [1e]+ and [1g]+
bearing different oligo(ethylene glycol)methyl ether side
chains.
1c
300 (3.06), 360 (4.25)
346 (1.05), 522 (2.22), 574 (3.50), 1078 (2.6)
340 (1.59), 408 (1.32)
1c+
1c2+
1e
238 (4.79), 336 (3.81)
1e+
242 (3.15), 336 (1.66), 404 (0.635), 508 (0.88),
562 (0.98), 1188 (1.07)
242 (3.25), 330 (1.14), 398 (1.44)
334 (3.61)
330 (2.12), 400 (0.77), 506 (0.32), 560 (0.38),
1188 (0.42)
398 (1.15)
257 (4.01), 336 (2.12)
334 (0.79), 396 (0.49), 510 (0.39), 564 (0.44),
1194 (0.47)
396 (0.73)
1e2+
1e + 1.0 eq. Li+
1e+ + 1.0 eq. Li+
1e2+ + 1.0 eq. Li+
1g
1g+
1g2+
1g + 1.0 eq. Li+
1g+ + 1.0 eq. Li+
334 (2.17)
330 (1.55), 402 (0.31), 522 (0.15), 574 (0.15),
1252 (0.16)
408 (0.54)
1g2+ + 1.0 eq. Li+
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To further investigate the effect of lithium cations on the (PPh3)3,17 1,4-diethynyl-2,5-dimethoxybenzene (3a),18 1,4-bis(2-
oxidation processes, UV-vis/NIR spectroelectrochemical studies methoxyethoxy)-2,5-diethynylbenzene (3b),19 1,4-bis(2-(2-meth-
of complexes 1e and 1g in the presence of 1.0 eq. Li+ were per- oxyethoxy)ethoxy)-2,5-diethynylbenzene (3c),20 1,4-bis(2-(2-(2-
formed, using the same conditions as described above. The methoxyethoxy)ethoxy)ethoxy)-2,5-diethynylbenzene (3d),20 1,4-
changes in their UV-vis/NIR absorption spectra are shown in diiodo-2,3-dihydroxybenzene (4),21 1,4-diiodo-2,3-dimethoxy-
Table 3. The UV-vis/NIR absorption spectra of radical cations benzene (5a),21 and 1a6a were prepared by the procedures
[1e]+ and [1g]+ with 1.0 equiv. of Li+ featured absorption bands described in literature methods.
between 1000 and 1500 nm, which were not present in the
Synthesis of 2,3-bis(2-methoxyethoxy)-1,4-diiodobenzene
spectra of the neutral or dicationic states (Fig. 6). For [1e]+, a (5b). 1,4-Diiodo-2,3-dihydroxybenzene 4 (2.88 g, 8.0 mmol)
low-energy absorption band was still detected at 1188 nm after and 2-methoxyethyl 4-methylbenzenesulfonate (4.4 g,
adding 1.0 equiv. of Li+, but it was less intense. For [1g]+, the 19.2 mmol) were dissolved in DMF (8 mL). Then K2CO3 (4.4 g,
results are similar to those observed for [1e]+ with 1.0 equiv. of 32.0 mmol) was added to the stirred solution. The reaction
Li+. However, not only the absorption intensity decreased, but mixture was stirred at 75 °C for 48 h. The solvent was then
there was an additional band in the spectrum with an absorp- evaporated and ethyl acetate was added to the residue and
tion in the range 1194 to 1252 nm, suggesting a lower degree extracted with water. The organic phases were dried with
of electronic interaction (Fig. 7). These results are similar to Na2SO4 and the solvent removed in vacuo. The crude product
those reported for endiyne–diiron complexes, in which the was purified by column chromatography on silica gel, with pet-
electron-withdrawing substituents weaken the absorptions of roleum ether–ethyl acetate (v/v, 1 : 1) as the eluent, to give 2.2 g
1
lower-energy bands.1e,16
(57%) of a white solid. H NMR (500 MHz, CDCl3): δ = 3.45 (s,
6H, –OCH3), 3.77 (t, J = 5.0 Hz, 4H, –OCH2), 4.19 (t, J = 5.0 Hz,
4H, –OCH2), 7.24 (s, 2H, Ph–H). 13C NMR (125 MHz, CDCl3):
δ = 58.98, 71.59, 72.44, 93.09, 135.53, 152.20.
Experimental section
General materials
Synthesis of 2,3-bis(2-(2-methoxyethoxy)ethoxy)-1,4-diiodo-
benzene (5c). The synthesis is similar to 5b, with 2-methoxy-
All manipulations were carried out at room temperature under ethyl 4-methylbenzenesulfonate being replaced by 2-(2-
a nitrogen atmosphere using standard Schlenk techniques, methoxyethoxy)ethyl 4-methylbenzenesulfonate. Yellow oil,
1
unless otherwise stated. 1H, 13C, and 31P NMR spectra were yield: 2.8 g, 59%. H NMR (500 MHz, CDCl3): δ = 3.41 (s, 6H,
collected on
a Varian Mercury Plus 500 spectrometer –OCH3), 3.59 (t, J = 5.0 Hz, 4H, –OCH2), 3.74 (t, J = 5.0 Hz, 4H,
(500 MHz). 1H and 13C NMR chemical shifts are relative to –OCH2), 3.89 (t, J = 5.0 Hz, 4H, –OCH2), 4.23 (t, J = 5.0 Hz, 4H,
TMS, and 31P NMR chemical shifts are relative to 85% H3PO4. –OCH2), 7.25 (s, 2H, Ph–H). 13C NMR (125 MHz, CDCl3): δ =
Elemental analyses (C, H, N) were performed with a Vario ElIII 59.02, 70.24, 70.49, 71.93, 72.49, 93.05, 135.38, 152.06.
Chnso instrument. UV/Vis/NIR spectra were recorded using a
Shimadzu UV-3600 UV/Vis/NIR spectrophotometer by liquid benzene (6a). To
Synthesis of 2,3-dimethoxy-1,4-bis(2-(trimethylsilyl)ethynyl)-
solution of 2,3-dimethoxy-1,4-diiodo-
a
sample cells with a path length of 200 μm. Solid-state IR benzene (0.78 g, 2 mmol), CuI (38 mg, 0.2 mmol), and
spectra were recorded using a Nicolet Avatar spectrometer Pd (PPh3)2Cl2 (70 mg, 0.1 mmol) in 50 mL of triethylamine
from Nujol mull suspended between KBr discs and liquid was added (trimethylsilyl)acetylene (0.42 g, 4.4 mmol). The
sample cells with a path length of 200 μm, respectively. Infra- mixture was stirred at 35 °C for 12 h. After removal of the
red spectra were obtained on a TENSOR 27 instrument using solvent in vacuo, the desired product was separated by column
KBr pellets. The electrochemical measurements were per- chromatography on silica gel, with petroleum ether–ethyl
formed using a CHI 660C potentiostat (CHI USA). A three-elec- acetate (v/v, 1 : 1) as the eluent, to yield 0.50 g (75%) of a yellow
1
trode one-compartment cell was used to contain the solution oil. H NMR (500 MHz, CDCl3): δ = 0.26 (s, 18H, –SiCH3), 3.94
of complexes and supporting electrolyte in dry CH2Cl2. Deaera- (s, 6H, –OCH3), 7.08 (s, 1H, Ph–H). 13C NMR (125 MHz,
tion of the solution was achieved by argon bubbling through CDCl3): δ = −0.15, 61.07, 100.37, 100.70, 118.98, 128.14,
the solution for about 10 min before measurement. The ligand 154.47.
and the electrolyte [n-Bu4N][BC6F5)4] concentrations are typi-
Synthesis of 2,3-bis(2-methoxyethoxy)-1,4-bis(2-(trimethyl-
cally 0.001 and 0.05 mol dm−3, respectively. The added lithium silyl)ethynyl)benzene (6b). The synthesis is similar to 6a, with
salt is LiB(C6F5)4. A 500 μm diameter platinum-disk working 2,3-dimethoxy-1,4-diiodobenzene being replaced by 2,3-bis(2-
electrode, a platinum-wire counter electrode, and an Ag/Ag+ methoxyethoxy)-1,4-diiodobenzene (0.95 g, 2.0 mmol). Yellow
reference electrode were used. The Ag/Ag+ reference electrode oil, yield: 0.52 g, 63%. H NMR (500 MHz, CDCl3): δ = 0.25 (s,
1
contained an internal solution of 0.01 M AgNO3 in acetonitrile. 18H, –SiCH3), 3.44 (s, 6H, –OCH3), 3.74 (t, J = 5.0 Hz, 4H,
All electrochemical experiments were carried out under –OCH2), 4.25 (t, J = 5.0 Hz, 4H, –OCH2), 7.08 (s, 2H, Ph–H).
ambient conditions. Solvents were pre-dried, distilled and 13C NMR (125 MHz, CDCl3): δ = −0.16, 58.91, 71.81, 72.69,
degassed prior to use, except those for spectroscopic measure- 100.49, 100.54, 119.30, 128.17, 153.63.
ments, which were of spectroscopic grade. The reagent ethynyl-
Synthesis of 2,3-bis(2-(2-methoxyethoxy)ethoxy)-1,4-bis(2-(tri-
trimethylsilane was purchased from Alfa Aesar. Others were methylsilyl)ethynyl)benzene (6c). Similar coupling procedures
commercially available. The starting materials RuHCl(CO)- as in 6a were adopted using 2,3-bis(2-(2-methoxyethoxy)-
4098 | Dalton Trans., 2014, 43, 4093–4101
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ethoxy)-1,4-diiodobenzene (1.19 g, 2.0 mmol) to obtain a J = 22.0 Hz, PMe3), −7.06 (d, J = 22.0 Hz, PMe3). IR (KBr/cm−1):
yellowish brown oil, yield: 0.64 g, 67%. 1H NMR (500 MHz, 1912 (CO); 1631 (CvC). Element analysis. Calcd (%) for
CDCl3): δ = 0.25 (s, 18H, –SiCH3), 3.38 (s, 6H, –OCH3), 3.55 (t, J = C36H74Cl2O6P6Ru2: C, 40.72; H, 7.02. Found: C, 40.65; H, 7.10.
5.0 Hz, 4H, –OCH2), 3.71 (t, J = 5.0 Hz, 4H, –OCH2), 3.84 (t, J =
1c. Yellow solid, 0.10 g (Yield: 17%). 1H NMR (500 MHz,
5.0 Hz, 4H, –OCH2), 4.28 (t, J = 5.0 Hz, 4H, –OCH2), 7.07 (s, 2H, CDCl3): δ = 1.39 (t, J = 5.0 Hz, 36H, PMe3), 1.47 (d, J = 10.0 Hz,
Ph–H). 13C NMR (125 MHz, CDCl3): δ = −0.15, 58.98, 70.53, 18H, PMe3), 3.38 (s, 6H, OCH3), 3.54 (t, J = 5.0 Hz, 4H, OCH2),
70.55, 71.96, 72.84, 100.48, 100.64, 119.16, 128.10, 153.61.
3.80 (t, J = 5.0 Hz, 4H, OCH2), 3.89 (t, J = 5.0 Hz, 4H, OCH2),
Synthesis of 1,4-diethynyl-2,3-dimethoxybenzene (3e). 6a 4.11 (t, J = 5.0 Hz, 4H, OCH2), 6.92 (m, 2H, Ph–CHv), 7.02 (s,
(0.86 g, 2.63 mmol) was dissolved in a mixture of dichloro- 2H, Ph–H), 7.88 (m, 2H, Ru–CHv). 13C NMR (125 MHz,
methane and methanol (150 mL, 1 : 1, v/v). Powdered potass- CDCl3): δ = 16.65 (t, J = 15.0 Hz, PMe3), 20.17 (d, J = 20.0 Hz,
ium carbonate (1.27 g, 8.51 mmol) was added, and the PMe3), 30.84, 58.91, 70.03, 70.20, 70.95, 72.03, 109.90, 128.03,
reaction mixture was stirred at room temperature for 12 h. The 128.50, 149.02 (s), 206.87 (CO). 31P NMR (200 MHz, CDCl3): δ =
reaction mixture was diluted with dichloromethane and −18.95 (t, J = 22.0 Hz, PMe3), −7.10 (d, J = 22.0 Hz, PMe3).
washed with brine. The organic layer was dried over Na2SO4 IR (KBr/cm−1): 1917 (CO); 1631 (CvC). Element analysis.
and the solvent removed in vacuo. The crude product was puri- Calcd (%) for C40H82Cl2O8P6Ru2: C, 41.78; H, 7.19. Found: C,
fied by chromatography (petroleum ether–ethyl acetate, V/V, 41.82; H, 7.13.
1 : 1). Yield: 0.44 g (91%) of yellow oil. 1H NMR (500 MHz,
1d. Yellow solid, 0.24 g (Yield: 39%). 1H NMR (500 MHz,
CDCl3): δ = 3.37 (s, 2H, uCH), 3.97 (s, 6H, –OCH3), 7.13 (s, 2H, CDCl3): δ = 1.39 (t, J = 5.0 Hz, 36H, PMe3), 1.47 (d, J = 5.0 Hz,
Ph–H). 13C NMR (125 MHz, CDCl3): δ = 61.22, 79.06, 82.87, 18H, PMe3), 3.37 (s, 6H, OCH3), 3.55 (t, J = 5.0 Hz, 4H, OCH2),
118.34, 128.38, 154.65.
3.65 (t, J = 5.0 Hz, 8H, OCH2), 3.80 (t, J = 5.0 Hz, 4H, OCH2),
Synthesis of 2,3-bis(2-methoxyethoxy)-1,4-diethynylbenzene 3.87 (t, J = 5.0 Hz, 4H, OCH2), 4.10 (t, J = 5.0 Hz, 4H, OCH2),
(3f). Similar desilylation procedures as in 3e were adopted 6.92 (m, 2H, Ph–CHv), 7.01 (s, 2H, Ph–H), 7.87 (m, 2H, Ru–
using 6b (1.09 g, 2.63 mmol) to obtain a yellowish brown oil in CHv). 13C NMR (125 MHz, CDCl3): δ = 16.61 (t, J = 15.0 Hz,
1
80% yield. H NMR (500 MHz, CDCl3): δ = 3.35 (s, 2H, uCH), PMe3), 20.20 (d, J = 21.3 Hz, PMe3), 30.86, 58.95, 70.03, 70.19,
3.43 (s, 6H, –OCH3), 3.73 (t, J = 5.0 Hz, 4H, –OCH2), 4.28 (t, J = 70.40, 70.72, 71.06, 71.90, 110.04, 128.12, 128.58, 149.08,
5.0 Hz, 4H, –OCH2), 7.14 (s, 2H, Ph–H). 13C NMR (125 MHz, 206.91 (CO). 31P NMR (200 MHz, CDCl3): δ = −18.96 (t, J = 22.0
CDCl3): δ = 58.88, 71.65, 72.82, 79.15, 82.89, 118.73, 128.42, Hz, PMe3), −7.12 (d, J = 22.0 Hz, PMe3). IR (KBr/cm−1): 1919
153.83.
Synthesis of 2,3-bis(2-(2-methoxyethoxy)ethoxy)-1,4-diethynyl- C44H90Cl2O10P6Ru2: C, 42.69; H, 7.33. Found: C, 42.61; H, 7.27.
benzene (3g). Similar desilylation procedures as in 3e were
1e Yellow solid, 0.37 g (Yield: 76%). 1H NMR (500 MHz,
(CO); 1631 (CvC). Element analysis. Calcd (%) for
adopted using 6c (1.41 g, 2.63 mmol) to obtain a yellowish CDCl3): δ = 1.40 (t, J = 5.0 Hz, 36H, PMe3), 1.47 (d, J = 5.0 Hz,
brown oil in 74% yield. 1H NMR (500 MHz, CDCl3): δ = 3.35 (s, 18H, PMe3), 3.85 (s, 6H, –OCH3), 6.87 (m, 2H, Ar–CHv), 7.27
2H, uCH), 3.39 (s, 6H, –OCH3), 3.56 (t, J = 5.0 Hz, 4H, –OCH2), (s, 2H, Ph–H), 8.00 (m, 2H, Ru–CHv). 13C NMR (125 MHz,
3.71 (t, J = 5.0 Hz, 4H, –OCH2), 3.83 (t, J = 5.0 Hz, 4H, –OCH2), CDCl3): δ = 16.67 (t, J = 15.0 Hz, PMe3), 20.18 (d, J = 20.0 Hz,
4.31 (t, J = 5.0 Hz, 4H, –OCH2), 7.13 (s, 2H, Ph–H). 13C NMR PMe3), 60.95, 127.99, 131.69, 133.58, 133.78, 148.30, 202.33
(125 MHz, CDCl3): δ = 58.99, 70.34, 70.46, 71.93, 72.92, 79.26, (CO). 31P NMR (200 MHz, CDCl3): δ = −18.89 (t, J = 22.0 Hz
82.91, 118.61, 128.31, 153.78.
General synthesis of binuclear ruthenium complexes 1. To a 1632
suspension of RuHCl(CO)(PPh3)3 (0.86 g, 0.9 mmol) in CH2Cl2 C32H66Cl2O4P6Ru2: C, 39.47; H, 6.83. Found: C, 39.54; H, 6.76.
(30 mL) was slowly added a solution of diethynylaryls 3
1f. Yellow solid, 0.38 g (Yield: 71%). 1H NMR (500 MHz,
PMe3), −6.99 (d, J = 22.0 Hz, PMe3). IR (KBr/cm−1): 1916 (CO);
(CvC). Element analysis. Calcd (%) for
(0.50 mmol) in CH2Cl2 (10 mL). The reaction mixture was CDCl3): δ = 1.40 (t, J = 5.0 Hz, 36H, PMe3), 1.47 (d, J = 10.0 Hz,
stirred for 30 min to give a red solution. Then a 1 M THF solu- 18H, PMe3), 3.43 (s, 6H –OCH3), 3.77 (t, J = 5.0 Hz, 4H,
tion of PMe3 (4.0 mL, 4.5 mmol) was added to the red solu- –OCH2), 4.12 (t, J = 5.0 Hz, 4H, –OCH2), 6.93 (m, 2H, Ar–
tion. The mixture was stirred for another 20 h. The volume of CHv), 7.25 (s, 2H, Ph–H), 7.99 (m, 2H, Ru–CHv). 13C NMR
the filtrate was reduced to ca. 2 mL under vacuum. Addition of (125 MHz, CDCl3): δ = 16.66 (t, J = 15.0 Hz, PMe3), 20.16 (d, J =
hexane (30 mL) to the residue produced a yellow solid, which 21.3 Hz, PMe3), 58.83, 72.03, 72.29, 119.67, 128.10, 133.60,
was collected by filtration, washed with hexane, and dried 133.76, 147.42, 202.25 (CO). 31P NMR (200 MHz, CDCl3): δ =
under vacuum.
−18.95 (t, J = 22.0 Hz, PMe3), −6.95 (d, J = 22.0 Hz, PMe3). IR
1b. Yellow solid, 0.17 g (Yield: 32%). 1H NMR (500 MHz, (KBr/cm−1): 1917 (CO); 1631 (CvC). Element analysis. Calcd
CDCl3): δ = 1.39 (t, J = 5.0 Hz, 36H, PMe3), 1.47 (d, J = 5.0 Hz, (%) for C36H74Cl2O6P6Ru2: C, 40.72; H, 7.02. Found: C, 40.66;
18H, PMe3), 3.47 (s, 6H, OCH3), 3.77 (t, J = 5.0 Hz, 4H, OCH2), H, 6.94.
4.09 (t, J = 5.0 Hz, 4H, OCH2), 6.94 (m, 2H, Ar–CHv), 7.04 (s,
1g. Yellow solid, 0.40 g (Yield: 70%). 1H NMR (500 MHz,
2H, Ph–H), 7.89 (m, 2H, Ru–CHv). 13C NMR (125 MHz, CDCl3): δ = 1.39 (t, J = 5.0 Hz, 36H, PMe3), 1.47 (d, J = 10 Hz,
CDCl3): δ = 16.54 (t, J = 15.6 Hz, PMe3), 20.14 (d J = 22.5 Hz, 18H, PMe3), 3.38 (s, 6H, –OCH3), 3.56 (t, J = 5.0 Hz, 4H,
PMe3), 30.84, 59.34, 70.09, 71.45, 110.53, 128.46, 131.98, –OCH2), 3.72 (t, J = 5.0 Hz, 4H, –OCH2), 3.88 (t, J = 5.0 Hz, 4H,
149.21, 206.89 (CO). 31P NMR (200 MHz, CDCl3): δ = −18.91 (t, –OCH2), 4.13 (t, J = 5.0 Hz, 4H, –OCH2−), 6.88 (m, 2H, Ar–
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CHv), 7.24 (s, 2H, Ph–H), 7.97 (m, 2H, Ru–CHv). 13C NMR 21001110), Program for Changjiang Scholars and Innovative
(125 MHz, CDCl3): δ = 16.69 (t, J = 15.0 Hz, PMe3), 20.19 (d, J = Research Team in University (no. IRT0979), and the Natural
21.3 Hz, PMe3), 58.97, 70.41, 70.74, 71.98, 72.50, 119.72, Science Foundation of Yunnan Province (no. 2010ZC071). The
128.07, 128.11, 131.62, 147.34, 202.42 (CO); 31P NMR scientific research foundation of Yunnan Provincial Depart-
(200 MHz, CDCl3): δ = −18.94 (t, J = 22.0 Hz, PMe3), −6.96 (d, ment of Education (grant no. 22012Z019).
J = 22.0 Hz, PMe3). IR (KBr/cm−1): 1916 (CO); 1631 (CvC).
Element analysis. Calcd (%) for C40H82Cl2O8P6Ru2: C, 41.78;
H, 7.19. Found: C, 41.83; H, 7.25.
Notes and references
Crystallographic details
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In summary, a series of ruthenium–vinyl complexes bridged by
1,4-diethenylphenylene bearing two oligo(ethylene glycol)
methyl ether side chains at different positions (the 2,3- and
2,5-positions) were successfully prepared and characterized.
The electronic properties were examined using electrochemical
techniques and IR and UV-vis/NIR spectroscopies. The electro-
chemical studies indicated that the 2,5-position substituents
could better stabilize the mixed-valence states; the mono-oxi-
dized state of complex 1g with 2,3-substituents was obviously
destabilized by lithium cations. The IR and UV-vis/NIR spectra
studies showed that the oxidation of these systems was largely
centered on the organic bridge. Importantly, the characteristic
absorptions of the CO bonds in the IR range and in the UV-vis/
NIR spectra of the monocation [1g]+ exhibited clear changes
after addition of 1.0 equiv. of lithium cations. A combination
of the results of the electrochemical and spectroelectrochem-
ical measurements led to the conclusion that the character-
istics of these mono-oxidized radical species were significantly
affected by the bridging ligands, and the stability of mixed-
valence species of 1g greatly decreased in the presence of
lithium cations. This research on the electronic properties of
these complexes provides guidance for the design and syn-
thesis of new mixed-valence systems.
Acknowledgements
The authors acknowledge financial support from National
Natural Science Foundation of China (no. 21002086, 21071123,
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