“Tetramethylsilanoviologen”: Synthesis, characterization, and hydrolysis of a Silolodipyridinium ion

Article abstract of DOI:10.1016/j.poly.2017.05.040

A silicon-bridged viologen has been synthesized by methylation of 4,4′-dipyridinodimethylsilole, and represents a new addition to the growing family of heteroatom bridged viologens. The dicationic species emits at 360?nm and exhibits two chemically reversible one electron reduction waves in acetonitrile at E_1/2?(2+/1+)?=??0.829?V and E_1/2?(1+/0)?=??1.307?V (versus Fc/Fc⁺). The UV–Vis spectrum for each of the three charge states is similar to the corresponding unbridged methyl viologen species. The silanoviologen is susceptible to ring-opening hydrolysis.

Full text of DOI:10.1016/j.poly.2017.05.040

Polyhedron 133 (2017) 358–363
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
‘
‘Tetramethylsilanoviologen”: Synthesis, characterization, and hydrolysis
of a Silolodipyridinium ion
David A. Lee, Derek M. Peloquin, Edi W. Yapi, Lucas A. McMinn, Jon W. Merkert,
Bernadette T. Donovan-Merkert, Ashley Ariel Vitallo, Ashley R. Peterson, Daniel S. Jones,
⇑
Christopher Ceccarelli, Thomas A. Schmedake
University of North Carolina – Charlotte, Department of Chemistry, Charlotte, NC 28223, USA
a r t i c l e i n f o
a b s t r a c t
0
Article history:
A silicon-bridged viologen has been synthesized by methylation of 4,4 -dipyridinodimethylsilole, and
represents a new addition to the growing family of heteroatom bridged viologens. The dicationic species
Received 4 April 2017
Accepted 23 May 2017
Available online 31 May 2017
emits at 360 nm and exhibits two chemically reversible one electron reduction waves in acetonitrile at
+
E
1/2(2+/1+) = ꢀ0.829 V and E1/2(1+/0) = ꢀ1.307 V (versus Fc/Fc ). The UV–Vis spectrum for each of the
three charge states is similar to the corresponding unbridged methyl viologen species. The silanoviologen
is susceptible to ring-opening hydrolysis.
Keywords:
Hexacoordinate silicon
Bipyridine
Ó 2017 Elsevier Ltd. All rights reserved.
Coordination chemistry
Electrochemistry
Crystallography
1
. Introduction
The N-alkylated 4,4 -bipyridininium salts (viologens) have been
hand enforce planarity and are an attractive synthetic target for
tailoring the chemical, electrochemical, and optical properties of
0
the viologens. Heteroatom bridged methyl viologen analogs have
appeared in the literature with N, P, S, and Ge bridges (Scheme 1),
with various effects attributed to the heteroatom incorporation.
The 9-oxide-9H-phospholodipyridinium salts, e.g. P-MV, exhibit
significantly lowered reduction potentials thanks to the phosphine
oxide bridge [10]. This could enable sensing or catalytic applica-
tions involving electron transfer from weaker electron donors.
Phosphaviologens have also been explored as tunable elec-
widely adopted as redox active components in a diverse array of
applications such as electron mediators for hydrogen generation
[
[
1], electrochemical capacitors [2], and electrochromic displays
3]. The 4,4 -bipyridinium di-cations can be readily reduced to
0
form relatively stable radical cation species [4] with intense colors
that vary with the N-substituent. Likewise, metalloles including
siloles and dibenzosiloles constitute an important class of materi-
als with extensive academic interest and practical applications,
including luminescent devices [5], photovoltaics [6], and sensors
for explosives [7]. Siloles are electron acceptors due to contribution
of the silicon
ring, and there has been extensive interest in incorporating fused
siloles and other metalloles into polymers and extended conju-
0
trochromic materials [11]. The 2,7-dimethylthieno[2,3-c:5,4-c ]
dipyridinium ion, S-MV, was shown to act as an electron shuttle
and a photosensitizer for the photoreduction of water [12]. By
comparison, unbridged methyl viologen can only act as an electron
shuttle for the photoreduction of water in the presence of a photo-
⁄
r
orbitals to the LUMO of the sila-cyclopentadiene
2
+
3
sensitizer such as Ru(bpy) . Pyrrolodipyridinium salts such as
gated systems [8]. Ohshita and coworkers synthesized the first
N-MV were developed for potential cytostatic activity [13]. A
germanium bridged analog, Ge-MV, was recently synthesized,
and it was shown that the germanium bridge slightly increased
the electron affinity of the viologen with a slight shift of +0.08 V
relative to methyl viologen’s first reduction potential [14].
Silicon bridged viologens would be an attractive addition to this
family combining benefits of siloles and viologens, and the silicon
bridge could also enforce planarity while providing a useful site for
subsequent functionalization or immobilization. Immobilization of
viologens [15] on surfaces has proven beneficial, for example, in
0
4
,4 -dipyridinosilole and explored its solid state luminescence
[
9], but N-alkylation to make the silicon bridged viologen has not
yet been reported.
Synthetic modification at the C3 position of 4,4 -bipyridinium
salts is usually undesirable since it prevents planarity and destabi-
0
0
lizes the reduced species. C3-C3 bridging substituents on the other
⇑
Corresponding author.
E-mail address: tschmeda@uncc.edu (T.A. Schmedake).
http://dx.doi.org/10.1016/j.poly.2017.05.040
277-5387/Ó 2017 Elsevier Ltd. All rights reserved.
0

D.A. Lee et al. / Polyhedron 133 (2017) 358–363
359
Scheme 1. Previously reported heteroatom bridged methyl viologen analogs,
P-MV, S-MV, N-MV, and Ge-MV.
electrochromic device applications [16] and renewable carbohy-
drate fuel cells [17]. In this manuscript, we report the synthesis
and structural characterization of a silanoviologen and explore its
electrochemical, electrochromic, and hydrolytic stability.
2
. Results and discussion
0
The precursor, 4,4 -dipyridinodimethylsilole, 1, was synthesized
Fig. 1. Crystal structure of 1. Selected bond distances (Å) and angles (°): Si–C1
1
.8801(18), C1–C5 1.408(2), C5–C6 1.485(2), C6–C10 1.411(2), C10–Si 1.8858(18),
C1–Si–C10 90.09(8), Si–C1–C5 110.27(13), C1–C5–C6 114.78(15), C5–C6–C10
14.71(15), C6–C10–Si 110.04(13). Torsion angles (°): C1–C5–C6–C10 0.2(2), Si–
2
by allowing dichlorodimethylsilane to react with Li bipy at 85 °C
following the method of Ohshita (Scheme 2). Subsequent chro-
matography and recrystallization from hexane provided a colorless
crystalline material in 49% yield. Single crystals suitable for X-ray
analysis were obtained via slow evaporation from hexane. The
crystal structure of 1 is shown in Fig. 1.
1
C1–C2–N1 ꢀ178.12(15).
Crystals of 1 were subsequently dissolved in acetonitrile and
methylated with methyl triflate to produce the tetramethylsi-
lanoviologen, 2. Colorless crystals of 2 were recovered from the
reaction mixture in 61% yield by diffusion of ether into the solu-
tion. The crystal structure of 2 is shown in Fig. 2 with anions and
solvent omitted for clarity. Both structures exhibit a nearly planar
bipyridinium framework with a small value for the C1–Si–C10
bond angle of 90.09(8)° for 1 and 88.55(11)° for 2.
The dimethylsilylene bridge has a subtle effect on the spectro-
scopic properties of the methyl viologen analog. A deoxygenated
acetonitrile solution of compound 2 absorbs in the UV with a
kmax = 270 nm and emits with a broad emission band centered at
3
60 nm (Fig. 3). Both values are only slightly lower in energy than
those reported for methyl viologen 260 nm [18] and 350 nm [19],
respectively.
The effect of the dimethylsilylene bridge on the reduction
potentials is also subtle. The standard reduction potentials of 2
were determined via cyclic voltammetry in acetonitrile solution
with tetra-n-butylammonium hexafluorophosphate (0.10 M) as
the electrolyte (Fig. 4). A platinum disk working electrode was used
along with a Ag/AgCl reference electrode and the ferrocene/fer-
rocenium couple was used as an internal standard. The silanoviolo-
Fig. 2. Crystal structure of 2. Counterions (two triflate ions) and solvent molecules
omitted for clarity. Selected bond distances (Å) and angles (°): Si–C1 1.889(2), C1–
C5 1.402(4), C5–C6 1.484(4), C6–C10 1.406(4), C10–Si 1.889(2), C1–Si–C10 88.55
(
11), Si–C1–C5 111.30(18), C1–C5–C6 114.5(2), C5–C6–C10 113.8(2), C6–C10–Si
11.74(19). Torsion angles (°):C1–C5–C6–C10 ꢀ1.2(3), Si–C1–C2–N1 ꢀ179.74(18).
1
gen dication exhibits two chemically reversible one electron
reduction waves at 1/2(1+/0) =
1/2(2+/1+) = ꢀ0.829 V and
1.307 V (versus Fc/Fc ), very similar to values reported for
E
E
+
ꢀ
Ge-MV of E1/2(2+/1+) = ꢀ0.80 V and E1/2(1+/0) = ꢀ1.25 [14].
The UV–Vis spectra of the 1 + and neutral reduced states of 2
were obtained using an argon flushed spectroelectrochemical cell
with a 1.0 mm pathlength and a printed gold honeycomb working
electrode (Pine Instruments), a gold counter electrode, and a Ag/
AgCl reference electrode standardized to the ferrocene/ferroce-
nium couple. Dilute samples of 2 were prepared in anhydrous ace-
tonitrile with tetra-n-butylammonium hexafluorophosphate
(0.1 M). Figs. 5 and 6 contain the spectra of the 2 + species (red),
Scheme 2. Synthesis of 1 and 2.

3
60
D.A. Lee et al. / Polyhedron 133 (2017) 358–363
Fig. 3. UV–Vis (solid line) and fluorescence (dashed line) spectra of 2 in degassed
CH CN solution.
Fig. 5. UV–Vis spectra of 2n+ states generated electrochemically at ꢀ0.695 V (red
line, n = 2), ꢀ1.095 mV (blue line, n = 1), and at 50 mV intervals (grey). Simulated
3
spectra of 22 and 2 are also included (insert). (Color online.)
+
1+
Fig. 6. UV–Vis spectra of 2n+ states generated electrochemically at ꢀ1.095 V (blue
Fig. 4. CV of scan of 2 in acetonitrile with N(n-Bu)
electrolyte. Scan rate = 200 mV/s with iR compensation.
4 6
PF (0.10 M) as supporting
1+
line, n = 1), ꢀ1.445 V (green line, n = 0). Simulated spectra of 2 and neutral 2
(
insert). (Color online.)
1
+ species (blue), and neutral species (green) obtained at working
electrode potentials of ꢀ0.695 V, ꢀ1.095 V, and ꢀ1.445 V respec-
tively versus Fc/Fc+. Additional spectra were taken at 0.050 V inter-
vals (grey) to indicate isosbestic points for each transition. The
observed spectra match up reasonably well with simulated spectra
calculated using time dependent density functional theory (Figs. 5
and 6 inserts) and are very similar to the corresponding unbridged
methyl viologen spectra. Details of spectra simulations are
included in the ESI.
The spectroscopic properties of 2 are more consistent with the
properties of a methyl viologen than the properties of a silole.
The explanation for this can be seen in the DFT calculated LUMOs
of three heteroatom bridged viologens, 2, P-MV, and Ge-MV
(1.3 eV higher in energy than the LUMO, see ESI). Furthermore,
the calculated LUMO energies of the three heteroatom bridged vio-
logens correlate strongly with the experimentally observed reduc-
tion potentials (Fig. 8).
During our initial attempts to methylate 1, methyl iodide was
used as the methylating agent. Addition of methyl iodide to a solu-
3
tion of 1 in CD CN resulted in a red precipitate and an NMR spec-
trum consistent with the desired silanoviologen. The intense color
of the compound was attributed to charge transfer with the iodide
counter ion. Slow evaporation of the supernatant in air afforded
crystals suitable for X-ray analysis. The structure was found to be
that of the hydrolyzed compound (Fig. 9), indicating a significant
increase in hydrolytic susceptibility upon alkylation. The increased
hydrolytic susceptibility of the silanoviologen was unexpected
since 1 is not observed to undergo hydrolysis in the presence of
(
Fig. 7). The LUMO of 2 is localized on the viologen framework,
⁄
while the silole-like contribution of the exocyclic silicon
r
orbitals
to the sila-cyclopentadiene ring is relegated to the LUMO + 1

D.A. Lee et al. / Polyhedron 133 (2017) 358–363
361
Fig. 7. LUMOs of heteroatom bridged methyl viologens calculated using density functional theory using B3LYP/6-31G(D) and Conductor-Like Polarizable Continuum Model
+
14
10
(
CPCM, dielectric = 37.22) along with first reduction potentials reported in volts vs. Fc /Fc for 2, Ge-MV, and P-MV.
Fig. 9. Crystal structure of 3. Selected bond distances (Å) and angles (°): Si–C1 1.922
2), C1–C5 1.405(3), C5–C6 1.486(3), C6–C10 1.389(3), O–Si–C1 103.18(11), Si–C1–
(
C5 125.77(17), C1–C5–C6 121.5(2), C5–C6–C10 120.6(2), C6–C10–Si 111.74(19).
Torsion angle (°):C1–C5–C6–C10 ꢀ54.5(3).
Fig. 8. Graph of first reduction potential E1/2(2+/1+) of heteroatom bridged methyl
viologens vs. calculated LUMO level.
water. However, the increased reactivity is rationalized by assum-
ing the cationic N-methylpyridinium makes a much better leaving
group than the neutral pyridine. To further explore the hydrolysis
reaction, a solution of 2 (initial concentration of 38 mM) in D
2
O
Scheme 3. Hydrolsyis of 2.
1
was maintained at 23 °C and monitored by H NMR at 20 min
intervals. A clean conversion of 2–3 (Scheme 3) was observed with
nanoparticle, or other substrate. However, the silanoviologen is
susceptible to ring opening hydrolysis, which is likely to prevent
its use in aqueous based applications such as hydrogen generation.
It is possible that bulkier substituents on silicon could kinetically
stabilize the silanoviologen with respect to hydrolysis, which will
be the subject of further study.
ꢀ
4
ꢀ1 ꢀ1
a rate constant of 2.3 * 10
M
s
, which corresponds to a half-
O (Fig. 10).
life of 72 min at 23 °C for compound 2 in D
2
3
. Conclusion
The 2,7,9,9-tetramethylsilolo[2,3-c:5,4-c ]dipyridinium ion has
0
been synthesized and is the first reported example of a silanovio-
4. Experimental
0
logen. The incorporation of a silylene bridge at the 3,3 -position
0
of a 4,4 -bipyridinium ion has a subtle effect on the electronic
4.1. Instrumentation and general methods
and electrochromic properties of the compound, comparable to
that of a germanium bridge, and could be a potential site for
anchoring the viologen to an electrode, chromophore, catalyst,
Absorption spectra were measured with a Cary 300 UV–Vis
spectrophotometer. Spectroelectrochemical measurements were

3
62
D.A. Lee et al. / Polyhedron 133 (2017) 358–363
5
. Synthetic details
0
0
5
.1. 3,3 -dibromo-4,4 -bipyridine [10]
To a solution of 2.56 g diisopropylamine (25.3 mmol) in 50 mL
tetrahydofuran, 15.8 mL of 1.6 M n-butyl lithium (25.3 mmol)
was added dropwise at ꢀ90 °C. The solution was stirred at this
temperature for 45 min. 4.0 g 3-bromopyridine (25.3 mmol) was
2
added dropwise and allowed to stir for 1 h. 8.0 g CuCl (59.5 mmol)
was added, air was allowed into the reaction vessel, and the mix-
ture was slowly warmed to room temperature and stirred over-
night. The resulting brown solid was taken up in a mixture of
3
0 mL H
then extracted three times with chloroform. The combined organic
phases were dried with NaSO and the volatiles removed under
2 4 4
O, 15 mL 30% w/w NH OH, and 15 mL saturated NH Cl
4
vacuum. The crude product was purified by column chromatogra-
phy on silica gel (hexanes/ethyl acetate 2:1) to yield 1.23 g (31%) of
0
0
3
,3 -dibromo-4,4 -bipyridine as off-white crystals.
0
5
.2. 9,9-dimethyl-9H-silolo[2,3-c:5,4-c ]dipyridine (1) [9]
0
0
To a solution of 1.0 g 3,3 -dibromo-4,4 -bipyridine (3.18 mmol)
2
Fig. 10. Hydrolysis of 2–3 in D O. Inset shows semi-log plot of [2] over time.
in 40 mL THF was added 4.0 mL 1.6 M n-butyl lithium (6.4 mmol)
dropwise at ꢀ90 °C. The mixture was stirred at this temperature
2 2
for 1 h at 0.5 mL SiMe Cl (4 mmol) was added dropwise and the
mixture was allowed to warm to room temperature and stir over-
night. The mixture was taken up in 50 mL 1 M NaHCO and
extracted three times with diethyl ether. The combined organic
layers were dried over MgSO and evaporated under vacuum to
yield a light brown oil. The crude product was purified by column
chromatography (SiO , hexanes/ethyl acetate, 1:2) followed by
recrystallization from hexanes to yield 1 as colorless crystals
0.33 g, 49% yield). Crystals suitable for single-crystal X-ray analy-
sis were grown via slow evaporation of a filtered diethyl ether
made with an Agilent 8453 diode array UV–Vis spectrophotometer
in a Pine Research Instrumentation honeycomb spectroelectro-
chemical cell. Fluorescence measurements were conducted on a
3
1
13
4
Jobin-Yvon Fluorolog 3 fluorescence spectrometer. H and
C
NMR spectra were acquired using a JEOL 300 MHz NMR spectrom-
eter. For the [29] Si spectra a JEOL 500 MHz NMR spectrometer was
used. Combustion analysis (C, H, and N) was conducted by Atlantic
Microlab, Inc.
Cyclic voltammetry studies were performed using a Princeton
Applied Research Model 273A Potentiostat/Galvanostat employ-
ing a conventional three-electrode setup consisting of a platinum
disk or glassy carbon working electrode, a silver/silver chloride
reference electrode, and a platinum wire auxiliary electrode.
Positive feedback iR compensation was routinely used. Voltam-
mograms were obtained in 0.1 M tetrabutylammonium hexafluo-
2
(
1
solution at room temperature.
d = 8.90 (s, 2 H), 8.75 (d, 2 H), 7.76 (d, 2 H) 0.56 (s, 6 H). C{ H}
NMR (CDCl , 75 MHz): d = 153.9, 153.1, 151.7, 133.3, 116.9,
3
H NMR (CDCl , 300 MHz):
13
1
3
2
9
ꢀ
3
3.47 ppm. Si NMR (CDCl , 60 MHz): d = 3.62 ppm. MS (EI): m/
+
+
z = 212.1 (M ), 197.1 ([MꢀCH
3
] ).
0
0
6
rophosphate (TBAPF )/acetonitrile solution using solvent that had
5.3. N,N ,Si,Si -tetramethylsilanoviologen triflate (2)
previously been purified and dried using a solvent purification
system (SPS-400, Innovative Technologies) and subsequently
purged with nitrogen. The supporting electrolyte (TBAPF ) was
6
To a solution of 1 (0.5 g, 2.4 mmol) in acetonitrile (20 mL) was
added methyl triflate (0.79 g, 4.8 mmol) dropwise at 0 °C. The
solution was stirred overnight and diethyl ether allowed to slowly
diffuse into the reaction vessel, yielding the tetramethylsilanovio-
logen, 2, as colorless needles (0.78 g, 61% yield). Crystals suitable
for single-crystal X-ray analysis were grown via slow ether
recrystallized from ethanol and dried under vacuum prior to
use. Ferrocene was used as an internal standard without further
purification.
X-ray crystallographic data was acquired with an Agilent Gem-
ini A Ultra diffractometer. Crystals of suitable size were coated
with a thin layer of paratone-N oil, mounted on the diffractometer,
and flash cooled to 100 K in the cold stream of the Cryojet XL liquid
nitrogen cooling device (Oxford Instruments) attached to the
diffractometer. The diffractometer was equipped with two
sealed-tube long fine focus X-ray sources (Mo target,
diffusion into a saturated acetonitrile solution under a nitrogen
1
atmosphere. H NMR (CDCl
3
, 300 MHz): d = 9.06 (s, 2 H), 8.83 (d,
1
3
1
2 H), 8.61 (d, 2 H), 4.36 (s, 6 H) 0.70 (s, 6 H). C{ H} NMR (CDCl
3
,
75 MHz): d = 157.2, 151.0, 148.7, 142.4, 123.9, 49.5, ꢀ4.58 ppm.
2
9
Si NMR (CDCl
SiS : C, 35.55, H, 3.36, N, 5.18. Found: C, 35.38, H,
3.19, N, 5.20%. MS (MALDI-TOF): m/z = 242.5 (M ), 227.6
3
, 60 MHz): d = 10.0 ppm. Anal. Calc. (%) for
C
16
H
18
N
2
O
6
F
6
2
+
k = 0.71073 Å; Cu target, k = 1.5418 Å),
a four-circle kappa
+
goniometer, and an Atlas CCD detector. CrysAlisPro [20] software
was used to control the diffractometer and perform data reduction
and preliminary analysis. The crystal structure was solved with
([MꢀCH
3
] ).
5.4. Ring-opening hydrolysis product of the silanoviologen (formation
SHELXS [21]. All non-hydrogen atoms appeared in the E-map of the
correct solution. Alternate cycles of model-building in Olex2 [22]
and refinement in SHELXL [21] followed. All non-hydrogen atoms
were refined anisotropically. All hydrogen atom positions were cal-
culated based on idealized geometry, and recalculated after each
cycle of least squares. During refinement, hydrogen atom – parent
atom vectors were held fixed (riding motion constraint).
of 3)
To a solution of 25 mg 1 (0.12 mmol) in 1 mL CD
34 mg CH I (0.24 mmol). The mixture was allowed to stand over-
night, resulting in a red precipitate. The supernatant was decanted
and allowed to slowly evaporate in air to yield crystals of the
hydrolyzed compound 3 which were suitable for single-crystal
3
CN was added
3

D.A. Lee et al. / Polyhedron 133 (2017) 358–363
363
X-ray analysis. NMR data (triflate salt): 1H NMR (D
d = 9.01 (m, 2 H), 8.94 (m, 2 H), 8.21 (d, 1 H), 8.02 (d, 1 H) 4.51
s, 3 H), 4.47 (s, 3 H), 2.22 (s, 1 H), 0.33 (s, 6H).
O, 300 MHz):
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Acknowledgement
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[
[
[
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[
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