Synthetic precursors for TCNQF42a- compounds: Synthesis, characterization, and electrochemical studies of (Pr 4N)2TCNQF4 and Li2TCNQF4

Article abstract of DOI:10.1021/jo301403v

Careful control of the reaction stoichiometry and conditions enables the synthesis of both LiTCNQF₄ and Li₂TCNQF₄ to be achieved. Reaction of LiI with TCNQF₄, in a 4:1 molar ratio, in boiling acetonitrile yields Li₂TCNQF₄. However, deviation from this ratio or the reaction temperature gives either LiTCNQF₄ or a mixture of Li₂TCNQF₄ and LiTCNQF₄. This is the first report of the large-scale chemical synthesis of Li ₂TCNQF₄. Attempts to prepare a single crystal of Li ₂TCNQF₄ have been unsuccessful, although air-stable (Pr₄N)₂TCNQF₄ was obtained by mixing Pr ₄NBr with Li₂TCNQF₄ in aqueous solution. Pr₄NTCNQF₄ was also obtained by reaction of LiTCNQF ₄ with Pr₄NBr in water. Li₂TCNQF₄, (Pr₄N)₂TCNQF₄, and Pr₄NTCNQF ₄ have been characterized by UVa-vis, FT-IR, Raman, and NMR spectroscopy, high resolution electrospray ionization mass spectrometry, and electrochemistry. The structures of single crystals of (Pr₄N) ₂TCNQF₄ and Pr₄NTCNQF₄ have been determined by X-ray crystallography. These TCNQF₄^2a- salts will provide useful precursors for the synthesis of derivatives of the dianions.

Full text of DOI:10.1021/jo301403v

Article
pubs.acs.org/joc
2−
Synthetic Precursors for TCNQF4 Compounds: Synthesis,
Characterization, and Electrochemical Studies of (Pr4N)2TCNQF4 and
Li2TCNQF4
Jinzhen Lu,† Thanh Hai Le,† Daouda A. K. Traore,‡ Matthew Wilce,‡ Alan M. Bond,*,†
and Lisandra L. Martin*,†
†School of Chemistry, Monash University, Wellington Road, Clayton, Victoria 3800, Australia
‡Department of Biochemistry and Molecular Biology, Monash University, Wellington Road, Clayton, Victoria 3800, Australia
S
* Supporting Information
ABSTRACT: Careful control of the reaction stoichiometry and conditions enables the synthesis of both LiTCNQF4 and
Li2TCNQF4 to be achieved. Reaction of LiI with TCNQF4, in a 4:1 molar ratio, in boiling acetonitrile yields Li2TCNQF4.
However, deviation from this ratio or the reaction temperature gives either LiTCNQF4 or a mixture of Li2TCNQF4 and
LiTCNQF4. This is the first report of the large-scale chemical synthesis of Li2TCNQF4. Attempts to prepare a single crystal of
Li2TCNQF4 have been unsuccessful, although air-stable (Pr4N)2TCNQF4 was obtained by mixing Pr4NBr with Li2TCNQF4 in
aqueous solution. Pr4NTCNQF4 was also obtained by reaction of LiTCNQF4 with Pr4NBr in water. Li2TCNQF4,
(Pr4N)2TCNQF4, and Pr4NTCNQF4 have been characterized by UV−vis, FT-IR, Raman, and NMR spectroscopy, high
resolution electrospray ionization mass spectrometry, and electrochemistry. The structures of single crystals of (Pr4N)2TCNQF4
and Pr4NTCNQF4 have been determined by X-ray crystallography. These TCNQF42− salts will provide useful precursors for the
synthesis of derivatives of the dianions.
resulted from either disproportionation of LiTCNQ/
LiTCNQF4 or direct two-electron reduction of TCNQ/
TCNQF4 by reactive metallocenes (Fe and Co). The reason
for the disproportionation of TCNQ•−/TCNQF4•− to dianions
of TCNQ/TCNQF4 is not yet fully understood.
1. INTRODUCTION
7,7,8,8-Tetracyanoquinodimethane (TCNQ) has been a mole-
cule of great of interest, as it is a strong electron acceptor.1,2
However, the tetrafluorinated TCNQ derivative 2,3,5,6-
tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4) is an
even stronger electron acceptor than TCNQ with a higher
electron affinity (∼3.3 eV3−5 compared with ∼2.9 eV for
TCNQ5,6). Despite this, to date, TCNQF4-based charge transfer
(CT) complexes have been much less studied than their TCNQ
counterparts. The radical monoanions of TCNQ and TCNQF4
are stable in air at least for several hours and can be synthesized
by chemical or electrochemical reduction of TCNQ or
TCNQF4. Thus, metal and organic derivatives based on either
TCNQ•− or TCNQF4•− have been prepared, and their physical
properties, such as magnetism,7−10 electronic conductiv-
ity,3,11−14 and electric switching as bulk materials,15,16 have
been widely studied. In contrast, studies of materials derived
from TCNQ2− as well as TCNQF42− dianions are limited.
There have been several reports of X-ray crystal structures of
TCNQ2−-based complexes.17−24 In comparison, only a few
In solution, the TCNQ2− dianion is sensitive to air and rapidly
decomposes into α,α-dicyano-p-toluoyl cyanide (DCTC−).27,28
As a result, syntheses of complexes based on the TCNQ2−
dianion are much more difficult than those for the radical
monoanion. However, recently, Robson and co-workers have
successfully used the air-stable H2TCNQ as a starting material
to produce both metal- and organic-based TCNQ2− com-
plexes.21−24 In an organic solution in the presence of a weak
base, H2TCNQ undergoes sequential deprotonations to
TCNQ2−. The generated dianion then combines with metal
or organic cations in situ to form new materials. These metal−
TCNQ2−-based coordination polymers have an ability to
establish 3D networks which intercalate redox-active species
into the structural units and which may lead to these solid
materials having interesting electronic and magnetic proper-
ties.24 Although the TCNQ2− complexes also have attractive
2−
coordination polymers containing TCNQF4 have been fully
characterized by X-ray crystallography: Mn2(TCNQF42−)-
25
(solvent)(TCNQF4
)
and [(Me5Cp)2M]2TCNQF4 (MIII:
•−
Received: July 23, 2012
Published: November 15, 2012
2
Fe, Co).26 In these studies, the TCNQ2−/TCNQF4 dianions
2−
© 2012 American Chemical Society
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a
Scheme 1. Summary of Reaction Pathways Used for Synthesis of Li2TCNQF4, LiTCNQF4, (Pr4N)2TCNQF4, and Pr4NTCNQF4
a
−
*In practice, 4 mol equiv of LiI is needed to obtain complete conversion to Li2TCNQF4 due to the I3 /I2 equilibrium.
features, they differ significantly from the better known
TCNQ•−-based materials. For example, TCNQ2− can bind
strongly to the metal ion21,23 and is free from π−π contacts.23,24
Consequently, new types of coordination polymers can be
formed with intercalation of oxidants or reductants into the
structure, which may change the electric conductivity of these
materials.21,23 Here, we report the one step, direct chemical
synthesis as well as the characterization and stability of two new
stirred for 1 h at 50−60 °C under N2. The suspension was
cooled to room temperature before filtration to collect the solid.
The crude product was washed with diethyl ether (dried and
degassed prior to use) until no triiodide was detected in the
solution. A creamy white solid (134 mg, 90%) was obtained after
drying under vacuum over P2O5 for 3 h. The solid was then
characterized using FT-IR, Raman, and UV−vis spectroscopy
and electrochemistry (see below). These data indicate that
Li2TCNQF4 is free of LiTCNQF4. High resolution electrospray
2−
complexes based on the TCNQF4 dianion, i.e., Li2TCNQF4,
(Pr4N)2TCNQF4, which can be used as starting reagents to
synthesize both metal− and organic−TCNQF42−-based materi-
als. To the best our knowledge, this is the first report of the
chemical synthesis, with a high yield of Li2TCNQF4 and
(Pr4N)2TCNQF4. In addition, single crystals of both
(Pr4N)2TCNQF4 and the radical monoanion derivative
Pr4NTCNQF4 were structurally characterized by X-ray
diffraction methods.
ionization mass spectrometry (ESI-MS) also confirms that the
2−
charge on the TCNQF4 moiety is −2 (calculated for C12F4N4
:
m/z 138.0030 (100%); found: m/z 138.0026 (100%)). The
elemental analysis is consistent with the formula
Li2TCNQF4·H2O; however, as seen later, there is no water
detected in the infrared spectrum. Li2TCNQF4 is unstable in air,
even in the solid state, and over time is gradually converted to
LiTCNQF4 (see Figure S1, Supporting Information). Efforts to
grow a single crystal of Li2TCNQF4 have proved unsuccessful.
The same reaction conditions, but using a 3:1 molar ratio of LiI
and TCNQF4, yielded a mixture of Li2TCNQF4 and
LiTCNQF4. The mixture was quantitatively analyzed using
UV−vis spectroscopy and electrochemistry as described in
Supporting Information. Li2TCNQF4 was found to be produced
under these conditions at a level of 90−95% (molar %) with the
remainder of the sample being LiTCNQF4 (Figures S2 and S3,
Supporting Information).
2. RESULTS AND DISCUSSION
2.1. Synthesis. As noted already, LiTCNQF4 is an
•−
important starting reagent for the synthesis of TCNQF4
-
based materials. Our initial attempts to synthesize LiTCNQF4
was based on the literature procedure described for the
preparation of LiTCNQ.2 However, it was found that these
reaction conditions produced a mixture of LiTCNQF4 and
Li2TCNQF4 as proved by UV−vis spectroscopy and electro-
chemistry (see below). Subsequently, it was shown that the
synthetic conditions could be optimized to favor the formation
of Li2TCNQF4, which can be an important starting salt for the
•−
To favor the reduction of TCNQF4 to TCNQF4 rather
than to TCNQF42−, the reaction conditions used for the
synthesis of Li2TCNQF4 needed to be modified. Thus, the
molar ratio of LiI and TCNQF4 was changed to 3:2, and the
reaction was carried out with cold solutions (pathway 2, Scheme
1). Three milliliters of a cold (0 °C) acetonitrile solution
containing LiI (75.4 mg, 0.57 mmol) was added dropwise into 8
mL of a cold acetonitrile solution of TCNQF4 (104 mg, 0.38
mmol). The resulting mixture was placed in an ice bath and
stirred under N2 for 30 min. The precipitate that formed was
isolated rapidly by vacuum filtration, followed by washing with
acetonitrile (3 × 3 mL) and diethyl ether (3 × 5 mL), and dried
over P2O5 under vacuum overnight. The purple solid obtained
(89.5 mg, 84%) was studied using a wide range of spectroscopic
techniques and electrochemistry. These data (see below)
confirmed that LiTCNQF4 was the only product produced in
2−
synthesis of TCNQF4 dianion coordination polymers. This
optimization study has demonstrated that the ratio of
Li2TCNQF4 and LiTCNQF4 generated depends on the reaction
temperature, duration of the reaction, and the molar ratio of the
reactants used in the synthesis.
A straightforward method for the synthesis of Li2TCNQF4 is
summarized in pathway 1 (Scheme 1) as the overall balanced
equation. However, in practice, 4 mol equiv of LiI is needed for
complete conversion to Li2TCNQF4. Without an excess LiI, a
small amount of LiTCNQF4 is obtained. Thus, a boiling
solution of LiI (273 mg, 2.04 mmol) in acetonitrile (15 mL) was
added dropwise to a boiling solution of TCNQF4 (142 mg, 0.51
mmol) in acetonitrile (15 mL). The resulting mixture was
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Figure 1. X-ray single crystal structures: (a) asymmetric unit of (Pr4N)2TCNQF4; (b) packing diagram of (Pr4N)2TCNQF4 viewed along the a axis;
(c) hydrogen-bonding interactions in (Pr4N)2TCNQF4; (d) packing diagram of TCNQF42− anions in (Pr4N)2TCNQF4; (e) the asymmetric unit of
Pr4NTCNQF4; (f) the packing diagram of Pr4NTCNQF4 viewed along the b axis.
2−
Table 1. Some Examples of Intramolecular Bond Distances Reported for TCNQF4, TCNQF4•−, and TCNQF4
a/Å
b/Å
c/Å
d/Å
e/Å
ρ
ref
0
TCNQF4
1.334
1.360
1.357
1.373
1.377
1.437
1.420
1.419
1.398
1.403
1.372
1.429
1.415
1.457
1.456
1.437
1.435
1.424
1.403
1.406
1.140
1.140
1.152
1.154
1.161
0.00
−1.00
−0.94
−2.00
−1.92
26
30
(n-Bu4N)TCNQF4
Pr4NTCNQF4
this work
26
[(Me5Cp)2Fe]2TCNQF4
(Pr4N)2TCNQF4
this work
pathway 2. Clearly, the reduction of TCNQF4 is thermodynami-
cally controlled, so an excess of LiI gives rise to the further
reduction of TCNQF4•− to TCNQF42− as illustrated in pathway
1 of Scheme 1.
Both Li2TCNQF4 and LiTCNQF4 directly reacted with
Pr4NBr in water (pathways 3 and 4, Scheme 1) to yield colorless
solids of (Pr4N)2TCNQF4 and a blue powder for
Pr4NTCNQF4. Single crystals of (Pr4N)2TCNQF4 and
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Pr4NTCNQF4 suitable for X-ray crystallographic analysis were
obtained by either diffusion of n-pentane into an acetone
solution of (Pr4N)2TCNQF4 or diffusion of diethyl ether into a
methanol solution of Pr4NTCNQF4. The polycrystalline solids
of both (Pr4N)2TCNQF4 and Pr4NTCNQF4 are stable in air
and characterized by 19F NMR, FT-IR, Raman, and UV−vis
spectroscopy and electrochemistry. All spectroscopic data are
consistent with the single crystal structures.
Table 2. Some Characteristic FT-IR Bands for TCNQF4,
LiTCNQF4, Li2TCNQF4, (Pr4N)2TCNQF4, and
Pr4NTCNQF4
compound
TCNQF4
ν(CN)/cm−1
δ(C−F)/cm−1
2228
1192
1204
1204
1250
1222
LiTCNQF4
2219, 2188
Pr4NTCNQF4
Li2TCNQF4
2200, 2182
2184, 2151, 2125
2164, 2130, 2092
2.2. X-ray Structural Characterization of
(Pr4N)2TCNQF4 and Pr4NTCNQF4. Colorless diamond-like
(Pr4N)2TCNQF4 crystallized in the monoclinic space group
P21/n. The asymmetric unit contains one Pr4N+ cation and a
half TCNQF42− anion. Figure 1a shows the cation and the whole
anion positions in this structure. The structure of
(Pr4N)2TCNQF4 is composed of a 3D layered network
supported by hydrogen bonding (Figure 1b). The charge (ρ)
on the TCNQF4 moieties in (Pr4N)2TCNQF4 has been
estimated by the mean bond lengths of TCNQF4 (Table 1)
using the Kistenmacher relationship, ρ = A[c/(b + d)] + B (A =
−46.729 and B = 22.308). A and B were determined from
(Pr4N)2TCNQF4
sensitive to the increased charge on the TCNQF4 moiety,
0
shifting to higher energies across the series TCNQF4
→
•−
TCNQF4 → TCNQF42− as shown in Table 2.
The Raman spectrum of the neutral TCNQF4 (Figure S2,
Supporting Information) is dominated by three bands at 2226,
1665, and 1457 cm−1 which respectively correspond to CN,
CC ring, C−CN wing stretches.3,31 These Raman vibration
•−
modes exhibit shifts consistent with reduced TCNQF4
(Figure 2a,b) and TCNQF42−(Figure 2c,d), respectively.
neutral TCNQF4 (ρ = 0)29 and TCNQF4 monoanion in n-
•−
Bu4NTCNQF4 (ρ = −1).30 The calculated ρ value (−1.92)
supports the assignment of TCNQF42− dianions. Moreover, the
structural parameters in (Pr4N)2TCNQF4 are consistent with
26
2−
those for TCNQF4 species reported by Miller et al. (Table
1). The TCNQF4 moieties in (Pr4N)2TCNQF4 are not planar
because the C(CN) groups lie away from the phenyl plane with
a diangle of 17°. Each TCNQF4 moiety interacts with seven
Pr4N+ cations via hydrogen bond interactions between the CN/
F groups of the TCNQF4 moieties and the CH groups from the
Pr4N+ cations (Figure 1c; Table S1, Supporting Information).
2−
Moreover, the adjacent TCNQF4 layers are separated by
Pr4N+ cation layers and oriented with an angle between the
respective phenyl planes of about 59.2° (Figure 1d). In addition,
the Pr4N+ cations and the TCNQF42− anions are connected by
hydrogen bonding to form a 3D network (Figure 1b; Table S1,
Supporting Information).
Dark-blue needles of Pr4NTCNQF4 crystallize in the
monoclinic space group P21/n, and the 3D H-bonded network
structure consists of (TCNQF4•−)2 dimers and Pr4N+ cations
(Figure 1f; Table S2, Supporting Information). The asymmetric
unit of Pr4NTCNQF4 is shown in Figure 1e. Each TCNQF4
anion is almost planar with a slight bowing of the CN groups
Figure 2. Raman spectra of solid samples of (a) LiTCNQF4, (b)
Pr4NTCNQF4, (c) Li2TCNQF4, and (d) (Pr4N)2TCNQF4.
2.4. Characterization in Solution. UV−vis spectra for
L i T C N Q F 4 , P r 4 N T C N Q F 4 , L i 2 T C N Q F 4 , a n d
(Pr4N)2TCNQF4 dissolved in acetonitrile are shown in Figure
3. These spectra are dominated by two intense charge transfer
bands with λmax at 411 and 752 nm for the LiTCNQF4 and
•−
away from the phenyl plane. The two neighboring TCNQF4
anions eclipse to form a face-to-face π-stacked dimer with an
interplanar distance of 3.147 Å, a remarkably short spacing
compared with that of normal π−π stacked TCNQ or TCNQF4
dimers (ca. 3.30 Å).7,8,11,30 The overlap of the dimer is ring over
bond mode. The degree of charge transfer of TCNQF4 species
in Pr4NTCNQF4 has been determined using the same method
applied to (Pr4N)2TCNQF4 and found to be −0.94. Therefore,
the TCNQF4 moieties in Pr4NTCNQF4 can be described as the
4
•−
Pr4NTCNQF4 salts which are characteristic of the TCNQF4
.
The dissolved Li2TCNQF4 and (Pr4N)2TCNQF4 salts also
exhibit two intense absorption bands with λmax at 333 and 218
nm assigned to the TCNQF42− dianion.4 The molar extinction
coefficients (ε) at 411 nm (TCNQF4•−) and at 333 nm
(TCNQF42−) were calculated using electrochemically reduced
•−
TCNQF4 radical monoanion. The structural parameters for
Pr4NTCNQF4 also agree well with those expected for
TCNQF4•− (Table 1) and found in related complexes reported
by us and others.7,8,30,32,34
samples, ε(λ
= 5.08
0.17 × 104 M−1 cm−1 and
411 nm)
max
ε
(λmax
333 nm) = 4.06 0.17 × 104 M−1 cm−1 and these values were
2.3. FT-IR and Raman Spectroscopy. Infrared and Raman
spectroscopy have been used widely to determine the redox
state of TCNQF4 in previously synthesized materials.25,31,32 The
IR bands in the solid state are shown in Table 2 and compared
with the neutral TCNQF4. The CN stretch is diagnostic and
shifts to lower energy with the increased state of reduc-
tion.7,26,30,32−34 The C−F out-of-plane bending mode is also
used to determine the concentrations for the LiTCNQF4,
Pr4NTCNQF4, Li2TCNQF4, and (Pr4N)2TCNQF4 solutions.
19F NMR spectroscopy was also used to investigate these salts
in methanol-d4. The 19F NMR spectrum for (Pr4N)2TCNQF4 in
methanol-d4 gives rise to a sharp singlet at −147.52 ppm (Figure
4a) indicating only the diamagnetic TCNQF42− species present
in solution. However, as expected, both paramagnetic
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material supports the high purity of the (Pr4N)2TCNQF4 in
terms of possible contamination by TCNQF4•−. Note that
2−
electron self-exchange between the TCNQF4•− and TCNQF4
species is likely to contribute to the line broadening when a
mixtures of these two species are present. Also, it was not
surprising that despite our best attempts to obtain the 19F NMR
spectrum for Li2TCNQF4, using drybox conditions and various
solvents (e.g., MeCN-d3 or MeOH-d4), only a complicated
pattern of paramagnetically broadened, weak signals were
observed (data not shown).
Steady-state voltammetry also was used to determine the
redox levels of TCNQF4 in the synthesized materials. Figure 6a
shows nearly a steady-state voltammogram for an acetonitrile
(0.1 M Bu4NPF6) solution containing the product formed via
pathway 1 (Scheme 1). Clearly, the product obtained in boiling
acetonitrile with a 4:1 molar ratio of LiI and TCNQF4 contains
only TCNQF42−, devoid of TCNQF4•−, as only oxidation
Figure 3. UV−vis spectra for LiTCNQF4, 0.018 mM; Pr4NTCNQF4,
0.012 mM; Li2TCNQF4, 0.02 mM; (Pr4N)2TCNQF4, 0.033 mM in
acetonitrile. The concentrations were calculated using ε(λ
=
at 411 nm)
max
5.08 0.17 × 104 M−1 cm−1 for TCNQF4•− and ε(λ at 333 nm) = 4.06
max
2−
0.17 × 104 M−1 cm−1 for TCNQF4
.
2−/•−
•−/0
current associated with the TCNQF4
and TCNQF4
processes are detected. Figure 6b shows the near steady-state
voltammogram in acetonitrile (0.1 M Bu4NPF6) after
dissolution of LiTCNQF4 solid, synthesized via pathway 2. In
this case, only TCNQF4•− is present as the zero current position
•−/2−
lies midway between the currents for the TCNQF4
reduction and the TCNQF4
•−/0
oxidation processes. The
same steady-state voltammetric behavior is observed for the
product formed via pathway 4 (Figure S4, Supporting
•−
Information), confirming that only the TCNQF4 species
2−
and no TCNQF4
is present in the original solid
Pr4NTCNQF4. The near steady-state voltammogram for
(Pr4N)2TCNQF4 (Figure 6c) in acetonitrile (0.1 M Bu4NPF6)
is similar to that for Li2TCNQF4. The result implies that only
2−
TCNQF4 is present in this synthesized solid.
3. CONCLUSION
The reduction of TCNQF4 by iodide is a thermodynamically
controlled process. Hence, either LiTCNQF4 or Li2TCNQF4
can be generated by controlling the reaction temperature and
the molar ratio of the reaction of TCNQF4 with LiI. This
appears to be the first report of the chemical synthesis of
Li2TCNQF4 in high yield. Li2TCNQF4 is slowly oxidized to
LiTCNQF4 in air. However, (Pr4N)2TCNQF4 is stable in air
and is recommended as a synthetic precursor for the synthesis of
a range of materials based on TCNQF42−. The X-ray structural
analysis of single crystals of both (Pr4N)2TCNQF4 and
Pr4NTCNQF4 shows the presence of 3D H-supported net-
works. Electrochemical data, Raman, FT-IR, and UV−vis
spectra, and elemental analysis are all consistent with the
formulations derived from the crystal structures of
(Pr4N)2TCNQF4 and Pr4NTCNQF4. 19F NMR spectra
obtained from the pure (Pr4N)2TCNQF4 and titration with
Figure 4. 19F NMR spectra in methanol-d4. (a) (Pr4N)2TCNQF4, (b)
(i) 34.5 mM (Pr4N)2TCNQF4, (ii) 34.5 mM (Pr4N)2TCNQF4 + 0.086
mM Pr4NTCNQF4, (iii) 34.5 mM (Pr4N)2TCNQF4 + 0.17 mM
Pr4NTCNQF4.
2−
Pr4NTCNQF4 in (Pr4N)2TCNQF4 confirmed that TCNQF4
is diamagnetic and that the paramagnetic TCNQF4•− broadens
the resonances.
•−
TCNQF4 salts show no clear resonances. To ascertain the
4. EXPERIMENTAL SECTION
effect of small amounts of the paramagnetic TCNQF4•− on the
spectrum of the diamagnetic (Pr4N)2TCNQF4 salt, a solution of
Pr4NTCNQF4 was titrated into the diamagnetic
(Pr4N)2TCNQF4, Figure 4b. The original sharp singlet from
(Pr4N)2TCNQF4 (Figure 4b(i)) is immediately broadened by
addition of only 0.25% mol equiv (Figure 4b(ii)), whereas
addition of 0.5% almost completely removed any signals (Figure
4b(iii)). This extreme sensitivity to traces of paramagnetic
4.1. Chemicals. Tetrapropylammonium bromide, lithium iodide, n-
pentane, methanol, acetone, and TCNQF4 were used as received.
Acetonitrile and diethyl ether were dried before use. Bu4NPF6 was
recrystallized twice from ethanol. All aqueous solutions were prepared
from high purity water (resistivity of 18.2 MΩ·cm). All synthetic
reactions were performed under N2 using standard Schlenk techniques.
4.2. Synthesis of Li2TCNQF4. A boiling solution of LiI (273 mg,
2.04 mmol) in acetonitrile (15 mL) was added dropwise to a boiling
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Figure 6. Near steady-state voltammograms of (a) 1.0 mM Li2TCNQF4, (b) 1.0 mM LiTCNQF4, and (c) 4.5 mM (Pr4N)2TCNQF4 in acetonitrile
(0.1 M Bu4NPF6) obtained with a 12.0 μm diameter carbon fiber microelectrode at a scan rate of 20 mV s−1.
solution of TCNQF4 (142 mg, 0.51 mmol) in acetonitrile (15 mL).
The resulting mixture was stirred for 1 h at 50−60 °C under N2. The
suspension was cooled to room temperature before filtration to collect
the solid. The crude product was washed with diethyl ether (dried and
degassed prior use) until no LiI3 was detected in the solution. Thus,
pure Li2TCNQF4, was obtained as a creamy white solid after drying
under vacuum for 3 h (134 mg, 90%). FT-IR (ν/cm−1): CN: 2184 (s),
2151 (s) 2125 (s). UV−vis (λmax nm): 333 (TCNQF42−), 218
(TCNQF42−). HRMS: Calcd for (C12F4N4)2− m/z 138.0030; found:
m/z 138.0026 (100%). Anal. Calcd. for Li2TCNQF4·H2O
[Li2C12F4N4H2O]: C, 46.79, H 0.65, N, 18.19. Found: C 46.29, H
0.86, N 18.34%.
4.3. Synthesis of LiTCNQF4 (pathway 2, Scheme 1). Three
milliliters of a cold (0 °C) acetonitrile solution containing LiI (75.4 mg,
0.57 mmol) was added dropwise into 8 mL of a cold acetonitrile
solution of TCNQF4 (104 mg, 0.38 mmol). The resulting mixture was
placed in an ice bath and stirred under N2 for 30 min. The precipitate
that formed was isolated rapidly by vacuum filtration, followed by
washing with acetonitrile (3 × 3 mL) and diethyl ether (3 × 5 mL), and
dried over P2O5 under vacuum overnight. A purple solid was obtained
(89.5 mg, 84%). FT-IR (ν/cm−1): 2219 (m), 2188 (s), 1625 (m), 1532
(s), 1501 (m), 1351 (s), 1204 (m), 967 (s). High resolution MS: Calcd
for (C12F4N4)•− m/z 276.0059; found: m/z 276.0140 (100%). UV−vis
(λmax nm): 752 (TCNQF4•−), 411 (TCNQF4•−).
4.4. Synthesis of (Pr4N)2TCNQF4 (pathway 3, Scheme 1).
Pr4NBr (84.8 mg, 0.32 mmol) was dissolved in hot water (5 mL,
degassed) and added to an aqueous solution of Li2TCNQF4 (30 mg,
0.11 mmol, in 5 mL of degassed hot water). The white precipitate that
formed immediately was filtered, washed with hot water (3 × 10 mL)
and diethyl ether (3 × 15 mL), and dried over P2O5 under vacuum
overnight (62.5 mg, 93%). Colorless single crystals of
(Pr4N)2(TCNQF4) suitable for X-ray crystallographic analysis were
obtained by diffusion of n-pentane into a n-acetone solution of
(Pr4N)2TCNQF4 over 5 days. 19F NMR (376 MHz, δ/ppm, methanol-
d4): −147.52 (s). FT-IR (ν/cm−1): 2979 (w), 2949(w), 2885 (w), 2164
(s, CN), 2130 (s, CN), 2092 (w, CN) 1475 (s), 1222 (m), 1133 (m),
986 (w), 958 (m), 781 (m), 760 (w), 750 (w). UV−vis (nm): 333
(TCNQF42−), 218 (TCNQF42−). Anal. Calcd for (Pr4N)2TCNQF4
[C36H56F4N6]: C 66.64, H 8.70, N 12.95%. Found: C 66.49, H 9.01, N
13.31%.
4.5. Synthesis of Pr4NTCNQF4, (pathway 4, Scheme 1). Five
milliliters of a hot aqueous solution of Pr4NBr (51.7 mg, 0.19 mmol)
was added into 5 mL of a hot aqueous solution of LiTCNQF4 (50 mg,
0.18 mmol). The dark blue solid that formed immediately was filtered,
washed with hot water (3 × 15 mL) and diethyl ether (3 × 15 mL), and
then dried over P2O5 under vacuum overnight (43 mg, 68%). Dark-blue
needle crystals of Pr4NTCNQF4 suitable for X-ray crystallography were
obtained by diffusion of diethyl ether into a methanol solution of
Pr4NTCNQF4 over a week. FT-IR (ν/cm−1): 2975 (w) 2943 (w), 2881
(w), 2200 (s, CN), 2182 (s, CN), 1633 (w), 1603 (w), 1536 (m), 1502
(w), 1472(w), 1390 (s), 1347 (m), 1337 (m), 1204(w), 1142 (w), 967
(s), 872 (w), 756 (w). UV−vis (nm): 752 (TCNQF4•−), 411
(TCNQF4•−). Anal. Calcd for P4NTCNQF4, [C24H28F4N5]: C,
62.32, H 6.10, N, 15.14%. Found: C, 62.49, H 6.25, N, 15.31%.
4.6. Electrochemical Procedures and Instrumentation.
Voltammetric experiments under near steady-state conditions were
conducted in acetonitrile (0.1 M Bu4NPF6) with an electrochemical
workstation using a standard three-electrode cell configuration at room
temperature (22 2 °C). The working electrode (WE) was calibrated
to be a 12.0 μm diameter carbon fiber microelectrode (diameter is 11
2 μm). A platinum wire was used as the counter electrode. A silver wire
immersed in acetonitrile solution containing 1.0 mM AgNO3 and 0.1 M
Bu4NPF6 was used as a Ag/Ag+ reference electrode (RE) (the potential
is −135 5 mV versus the ferrocene/ferrocenium couple Fc0/+). The
RE was separated from the test solution by a salt bridge containing the
relevant supporting electrolyte.
4.7. Physical Characterization Procedures. FT-IR spectra were
recorded using an ATR instrument using the neat solids. Raman spectra
were acquired from the neat solid with a spectrometer and microscope
using 633 nm laser line. 19F NMR spectra were recorded using a 400/
300 MHz spectrometer at 376/282 MHz with chemical shifts reported
relative to an external reference of CFCl3 at 0.00 ppm. Mass spectra
were acquired on an electrospray ionization mass spectrometer by
direct infusion using a syringe pump, and the spectra were run in
positive ion mode. X-ray structural data were collected at the Australian
Synchrotron using the PX1 beamline operating at 15 keV (λ = 0.7292
Å). The collection temperature was maintained at 100 K using an open-
flow N2 cryostream. Initial data processing was carried out using XDS
software.35 Both structures were solved by direct methods using
SHELXS-97.36 Least-squares refinements against F2 were carried out
using SHELXL-97, with the program X-Seed as a graphical interface.37
All hydrogen atoms were placed in idealized positions and refined using
a
riding model. Documentations CCDC-831469 (for
(Pr4N)2TCNQF4) and -831468 (for Pr4NTCNQF4) contain the
supplementary crystallographic data obtained for this report. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figure S1: Stability of Li2TCNQF4; Figure S2: UV−vis study of
a mixture of Li2TCNQF4 and LiTCNQF4; Figure S3:
electrochemical study of a mixture of Li2TCNQF4 and
LiTCNQF4; Figure S4: electrochemistry of Pr4NTCNQF4;
Figure S5: Raman spectrum of neutral TCNQF4. Tables S1 and
S2 contain additional crystallographic information for
(Pr4N)2TCNQF4 and Pr4NTCNQF4. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Alan.Bond@monash.edu; Lisa.Martin@monash.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Australian Research Council (ARC)
to A.M.B. and L.L.M. is gratefully acknowledged. T.H.L. is very
appreciative of a graduate scholarship from Danang City in
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The Journal of Organic Chemistry
Article
(30) O’Kane, S. A.; Clerac, R.; Zhao, H. H.; Xiang, O. Y.; Galan-
Mascaros, J. R.; Heintz, R.; Dunbar, K. R. J. Solid State Chem. 2000, 152,
159−173.
(31) Ouyang, C. B.; Guo, Y. B.; Liu, H. B.; Zhao, Y. J.; Li, G. X.; Li, Y.
J.; Song, Y. L.; Li, Y. L. J. Phys. Chem. C 2009, 113, 7044−7051.
(32) Le, T. H.; Nafady, A.; Lu, J.; Peleckis, G.; Bond, A. M.; Martin, L.
L. Eur. J. Inorg. Chem. 2012, 2889−2897.
(33) Wang, X. T.; LiableSands, L. M.; Manson, J. L.; Rheingold, A. L.;
Miller, J. S. Chem. Commun. 1996, 1979−1980.
(34) Le, T.; O’Mullane, A.; Martin, L.; Bond, A. J. Solid State Chem.
2011, 15, 2293−2304.
Vietnam and a stipend provided by the Monash University,
Faculty of Science Dean’s International Postgraduate Research
Scholarship. D.A.K.T. thanks the ARC Super Science Fellow-
ship. The authors also thank Professor Don McNaughton for
helpful discussions. Finally, we acknowledge the Australian
Synchrotron for beam-time.
REFERENCES
■
(1) Acker, D. S.; Harder, R. J.; Hertler, W. R.; Mahler, W.; Melby, L.
R.; Benson, R. E.; Mochel, W. E. J. Am. Chem. Soc. 1960, 82, 6408−
6409.
(35) Kabsch, W. J. Appl. Crystallogr. 1993, 26, 795−800.
(36) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
2008, 64, 112−122.
(2) Melby, L. R.; Mahler, W.; Mochel, W. E.; Harder, R. J.; Hertler, W.
R.; Benson, R. E. J. Am. Chem. Soc. 1962, 84, 3374−3387.
(3) Xiao, K.; Rondinone, A. J.; Puretzky, A. A.; Ivanov, I. N.; Retterer,
S. T.; Geohegan, D. B. Chem. Mater. 2009, 21, 4275−4281.
(4) Le, T. H.; Nafady, A.; Qu, X.; Martin, L. L.; Bond, A. M. Anal.
Chem. 2011, 83, 6731−6737.
(37) Barbour, L. J. J. Supramol. Chem. 2001, 1, 189−191.
(5) Sobczyk, M.; Skurski, P.; Simons, J. J. Phys. Chem. A 2003, 107,
7084−7091.
(6) Jerome, D. Chem. Rev. 2004, 104, 5565−5591.
(7) Lopez, N.; Zhao, H. H.; Prosvirin, A. V.; Wernsdorfer, W.;
Dunbar, K. R. Dalton Trans. 2010, 39, 4341−4352.
(8) Lopez, N.; Prosvirin, A. V.; Zhao, H. H.; Wernsdorfer, W.;
Dunbar, K. R. Chem.Eur. J. 2009, 15, 11390−11400.
(9) Jain, R.; Kabir, K.; Gilroy, J. B.; Mitchell, K. A. R.; Wong, K. C.;
Hicks, R. G. Nature 2007, 445, 291−294.
(10) Nakamura, Y.; Iwamura, H. Bull. Chem. Soc. Jpn. 1993, 66, 3724−
3728.
(11) Qu, X. H.; Lu, J. Z.; Zhao, C. A.; Boas, J. F.; Moubaraki, B.;
Murray, K. S.; Siriwardana, A.; Bond, A. M.; Martin, L. L. Angew. Chem.,
Int. Ed. 2011, 50, 1589−1592.
(12) Ishiguro, T.; Yamaji, K.; Saito, G. Organic Superconductors, 2nd
ed.; Springer-Verlag: New York, 1998.
(13) Chen, W.; Chen, S.; Qi, D. C.; Gao, X. Y.; Wee, A. T. S. J. Am.
Chem. Soc. 2007, 129, 10418−10422.
(14) Yan, J. M. Solid State Commun. 1992, 84, 895−899.
(15) Potember, R. S.; Poehler, T. O.; Rappa, A.; Cowan, D. O.; Bloch,
A. N. Synth. Met. 1982, 4, 371−380.
(16) Kotsiliou, A. M.; Risen, W. M. Solid State Commun. 1988, 68,
503−505.
(17) Shimomura, S.; Matsuda, R.; Tsujino, T.; Kawamura, T.;
Kitagawa, S. J. Am. Chem. Soc. 2006, 128, 16416−16417.
(18) Miller, J. S.; Zhang, J. H.; Reiff, W. M.; Dixon, D. A.; Preston, L.
D.; Reis, A. H.; Gebert, E.; Extine, M.; Troup, J.; Epstein, A. J.; Ward,
M. D. J. Phys. Chem. 1987, 91, 4344−4360.
(19) Choukroun, R.; Lorber, C.; de Caro, D.; Vendier, L.
Organometallics 2006, 25, 4243−4246.
(20) Oshio, H.; Ino, E.; Ito, T.; Maeda, Y. Bull. Chem. Soc. Jpn. 1995,
68, 889−897.
(21) Abrahams, B. F.; Hudson, T. A.; Robson, R. Cryst. Growth Des.
2008, 8, 1123−1125.
(22) Hudson, T. A.; Robson, R. Cryst. Growth Des. 2009, 9, 1658−
1662.
(23) Abrahams, B. F.; Elliott, R. W.; Hudson, T. A.; Robson, R. Cryst.
Growth Des. 2010, 10, 2860−2862.
(24) Abrahams, B. F.; Elliott, R. W.; Hudson, T. A.; Robson, R.
CrystEngComm 2012, 14, 351−354.
(25) Lopez, N.; Zhao, H. H.; Prosvirin, A. V.; Chouai, A.; Shatruk, M.;
Dunbar, K. R. Chem. Commun. 2007, 4611−4613.
(26) Dixon, D. A.; Calabrese, J. C.; Miller, J. S. J. Phys. Chem. 1989, 93,
2284−2291.
(27) Grossel, M. C.; Duke, A. J.; Hibbert, D. B.; Lewis, I. K.; Seddon,
E. A.; Horton, P. N.; Weston, S. C. Chem. Mater. 2000, 12, 2319−2323.
(28) Suchanski, M. R.; Vanduyne, R. P. J. Am. Chem. Soc. 1976, 98,
250−252.
(29) Emge, T. J.; Maxfield, M.; Cowan, D. O.; Kistenmacher, T. J. Mol.
Cryst. Liq. Cryst. 1981, 65, 161−178.
10574
dx.doi.org/10.1021/jo301403v | J. Org. Chem. 2012, 77, 10568−10574