Synthesis and characterization of redox-active charge-transfer complexes with 2,3,5,6-tetracyanopyridine (TCNPy) for the photogeneration of pyridinium radicals

Article abstract of DOI:10.1002/chem.201201915

The heteroaromatic polynitrile compound tetracyanopyridine (TCNPy) is introduced as a new electron acceptor for the formation of deeply colored charge-transfer complexes. In MeCN, TCNPy is characterized by a quasireversible one-electron-reduction process at -0.51 V (versus SCE). The tetracyanopyridine radical anion undergoes a secondary chemical reaction, which is assigned to a protonation step. TCNPy has been demonstrated to generate 1:1 complexes with various electron donors, including tetrathiafulvalene (TTF) and dihydroxybenzene derivatives, such as p-hydroquinone and catechol. Visible- or NIR-light-induced excitation of the intense charge-transfer bands of these compounds leads to a direct optical electron-transfer process for the formation of the corresponding radical-ion pairs. The presence of available electron donors that contain protic groups in close proximity to the TCNPy acceptor site opens up a new strategy for the photocontrolled generation of pyridinium radicals in a stepwise proton-coupled electron-transfer (PCET) sequence. Copyright

Full text of DOI:10.1002/chem.201201915

FULL PAPER
DOI: 10.1002/chem.201201915
Synthesis and Characterization of Redox-Active Charge-Transfer Complexes
with 2,3,5,6-Tetracyanopyridine (TCNPy) for the Photogeneration of
Pyridinium Radicals
Eva Wçß, Uwe Monkowius, and Gꢀnther Knçr*[a]
Abstract: The heteroaromatic poly-
nitrile compound tetracyanopyridine
strated to generate 1:1 complexes with
various electron donors, including tet-
rathiafulvalene (TTF) and dihydroxy-
benzene derivatives, such as p-hydro-
quinone and catechol. Visible- or NIR-
light-induced excitation of the intense
charge-transfer bands of these com-
pounds leads to a direct optical elec-
tron-transfer process for the formation
of the corresponding radical-ion pairs.
The presence of available electron
donors that contain protic groups in
close proximity to the TCNPy acceptor
site opens up a new strategy for the
photocontrolled generation of pyridini-
um radicals in a stepwise proton-cou-
pled electron-transfer (PCET) se-
quence.
ACHTUNGTRENNUNG
(TCNPy) is introduced as a new elec-
tron acceptor for the formation of
deeply colored charge-transfer com-
plexes. In MeCN, TCNPy is character-
ized by a quasireversible one-electron-
reduction process at À0.51 V (versus
SCE). The tetracyanopyridine radical
anion undergoes a secondary chemical
reaction, which is assigned to a proton-
Keywords: charge transfer · donor–
acceptor systems · photochemistry ·
proton transfer · radicals
ACHTUNGTRENNUNGation step. TCNPy has been demon-
Introduction
either found to function as neutral p acceptors, as stable
monoanionic radicals, or as dianions and, thus, can play an
essential role as electron-transfer mediators in inorganic and
organometallic chemistry.[5,6,7,8] In addition, the charge-
transfer interactions of TCNX acceptor systems with various
kinds of electron donors can lead to a wide variety of supra-
Unsaturated polynitrile compounds, such as tetracyanoeth-
ACHTUNGTRENNUNGylene, 1,2,4,5-tetracyanobenzene, and related p-acceptor sys-
tems (TCNX), are well-established non-innocent ligands
that result in redox-active compounds with a variety of in-
teresting structural and physical characteristics, including
magnetic behavior, conductivity, and long-wavelength opti-
cal absorption.[1,2] The much-less-studied title compound,
2,3,5,6-tetracyanopyridine (TCNPy), shares major structural
similarities with these p-conjugated tetranitrile molecules.
However, to the best of our knowledge, there are only two
reports in the literature mentioning an unpublished synthe-
sis and describing some preliminary characterization data on
TCNPy.[3,4]
Besides their important function as electron acceptors in
charge-transfer (CT) systems,[4] TCNX ligands display vari-
ous modes of participation in metal-complex chemistry.
They can coordinate in a p fashion or may also act as bridg-
ing ligands, thereby connecting up to four metal centers
through s bonds.[2,5,6] In this context, TCNX ligands are
À
molecular binding motifs, including p p stacking and the as-
sembly of ionic layer structures.[2,9] These resulting materials
frequently display very attractive functional properties, such
as electrical conductivity, single-molecule magnetism, low-
energy charge-transfer transitions, and non-linear optical ac-
tivity.[2,5,10,11]
Whilst these various features of TCNX systems are inter-
esting in their own right, the aza-substituted tetracyanoben-
zene derivative TCNPy (7), which carries a central pyridine
moiety (Scheme 1), could also open new routes for applica-
tions in redox catalysis.
Recently, pyridinium derivatives and pyridinyl species
have been discussed as potential redox mediators for the ac-
celeration of net multielectron substrate conversions.[12,13]
Protonation of the pyridine nitrogen atom and electrochemi-
cal one-electron reduction to form the corresponding pyridi-
nium radical can lead to an efficient generation of hydrogen
through disproportionation.[12] Moreover, in the presence of
carbon dioxide, pyridinium radicals—or coupling products
thereof—have been postulated to participate in the stepwise
electrocatalytic reduction of CO2 into MeOH under mild re-
action conditions.[12,13] Plausible reaction mechanisms of this
interesting proton-coupled redox process are currently
under debate.
[a] E. Wçß, Dr. U. Monkowius, Prof. Dr. G. Knçr
Institute of Inorganic Chemistry
Johannes Kepler University Linz (JKU)
Altenbergerstr. 69, A-4040 Linz, Austria
Fax : (+43)732-2468- 9681
E-mail: guenther.knoer@jku.at
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201915.
Re-use of this article is permitted in accordance with the Terms and
Conditions set out at http://onlinelibrary.wiley.com/journal/10.1002/AHCTUNG-
TRENNUNG(ISSN)1521-3765/homepage/2111_onlineopen.html.
Herein, we decided to study the chemistry of 2,3,5,6-tetra-
cyanopyridine (TCNPy; Scheme 1) with the aim of combin-
Chem. Eur. J. 2013, 19, 1489 – 1495
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Figure 2. Cyclic voltammograms of 0.5 mm TCNPy in MeCN at room
temperature with 0.1m Bu4NPF6 supporting electrolyte and a Pt-disk
working electrode; scan rates of 20, 50, 100, 150, 200, and 250 mVsÀ1 are
shown.
Scheme 1. Synthesis of the heteroaromatic p-acceptor 2,3,5,6-tetracyano-
pyridine (7).
ing the excellent p-acceptor properties of aromatic TCNX-
type polynitrile compounds with the intrinsic possibility of
pyridinium-radical generation by exploiting suitable charge-
transfer interactions in protic media.
electron-reduction step occurs at a formal potential of Eo =
A
À0.27 V versus NHE or À4.3 eV versus the vacuum
level).[15] The separation between the cathodic and anodic
peak potentials (DEp) increases with scan rate from 80 mV
at 20 mVsÀ1 to 150 mV at 250 mVsÀ1. At the same time, a
shift of the cathodic peak potentials (Epc) from À0.55 to
À0.58 V is observed. The ratio between the cathodic peak
current (Ipc) and the square root of the scan rate (u) initially
decreases with increasing scan rate in a nonlinear fashion.
Moreover, the ratio of anodic/cathodic peak current is less
than unity and gradually increases to approximate Ipa/Ipc =1
at higher u values. These diagnostic criteria indicate an elec-
tron-transfer process with a coupled homogeneous chemi-
cal-reaction step (EC mechanism)[16,17] that follows first-
order kinetics (À30 mV shift of Epc with Dlogu=1 and n=1
at 298 K).
Furthermore, the set of cyclic voltammograms in Figure 2
is characterized by the presence of an isopoint at À0.56 V
versus SCE, which allows us to estimate the electrochemi-
cal-transfer coefficient for the quasireversible process at low
scan rates as a=0.55.[18] Assuming a Stokes–Einstein radius
of 5.5 ꢁ for TCNPy and a kinematic viscosity of h=0.341 cP
in MeCN at 298 K,[19] the diffusion coefficient of compound
7 can be estimated as D=1.2ꢂ10À5 cm2 sÀ1. A standard het-
erogeneous electron-transfer rate constant of ko =5ꢂ
10À3 cmsÀ1 was obtained according to the method of Nichol-
son,[20] which indicated a substantial contribution of the
inner-sphere-reorganization energy (li) to the electron-trans-
fer process, as is frequently the case in organic redox cou-
ples,[21] including pyridine derivatives that contain strongly
electron-withdrawing groups.[22] Digital simulation of the
CVs at different scan rates is consistent with these general
assignments (see the Supporting Information).
Results and Discussion
The synthesis of 2,3,5,6-tetracyanopyridine (7) was achieved
in 7 steps starting from ethyl acetoacetate, formaldehyde,
and ammonia (Hantzsch pyridine synthesis[14]), as summa-
ACHTUNGTRENNUNGrized in Scheme 1. After column chromatography on silica
gel, analytically pure TCNPy was obtained in satisfactory
yield as a colorless crystalline material, which was fully char-
acterized by elemental analysis, FTIR and NMR spectrosco-
py. The molecular structure of compound 7 was determined
by single-crystal X-ray diffraction (Figure 1).
The electron-accepting properties of TCNPy were studied
by cyclic voltammetry (Figure 2). A quasireversible one-
Figure 1. ORTEP of TCNPy (7); ellipsoids are set at 50% probability.
Only one orientation of the TCNPy moiety is shown.
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Redox-Active Charge-Transfer Complexes
FULL PAPER
During the course of the electrochemical reduction of
TCNPy under thin-layer conditions in an OTTLE cell, the
initially colorless solution of compound 7 rapidly turned
orange. The corresponding spectroscopic variations that oc-
curred at a controlled potential of À0.75 V versus SCE are
shown in Figure 3.
Figure 4. Chemical reduction of TCNPy with an excess of sodium am-
ACHUTNGRENmNUG ineACHTUNGTERNpNUGN entacyanoferrate(II) in deionized water.
were also obtained for this chemically induced redox process
(Figure 4).
At room temperature, the reaction mixture gradually
changed color from yellowish orange to green and finally
dark blue. Whereas the initial signature of the pyridine radi-
cal anion at around 410 nm completely disappeared during
the course of the reaction, other absorption maxima grew at
360, 468, and 626 nm (Figure 4). Similar to the electrochemi-
cal reaction in MeCN, this transition from 410 to 468 nm
Figure 3. Changes in the UV/Vis absorption spectra of TCNPy in MeCN
with 0.1m Bu4NPF6 supporting electrolyte during the electrochemical re-
duction of TCNPy at À750 mV versus SCE.
CÀ
The new absorption bands, which were observed within
the first few seconds of the electrolysis, displayed a maxi-
mum at around 412 nm, which is within the typical range for
a bathochromically shifted radical-anion spectrum of a pyri-
dine derivative that contains electron-withdrawing groups
(the radical anion of unsubstituted pyridine absorbs at
340 nm).[23] At longer reaction times, the maximum at
412 nm disappears and new spectroscopic features arise,
with peaks at 365, 470, and 707 nm (Figure 3). This behavior
is consistent with a coupling of the radical-anion-generation
step to a chemical secondary process (EC mechanism), as
discussed above. The formation of dimerization products
should be less favorable, according to the substitution pat-
tern in compound 7.[23] This type of reaction would also be
inconsistent with the observed first-order kinetics of the
process. As an alternative mechanism, a pseudo-first-order
might indicate a protonation of the TCNPy radical anion.
However, owing to the presence of an excess of ferrous
metal salt in solution, other possibilities, such as the involve-
ment of metal-to-ligand charge-transfer (MLCT) transitions,
also have to be considered, because further strong absorp-
tion bands were clearly overlapping with the spectroscopic
features occuring in the corresponding electrochemical re-
duction of TCNPy. After a while, colloidal particles could
be observed in the reaction mixture and, finally, a blue pre-
cipitate was obtained. This behavior strongly indicates the
gradual formation of insoluble mixed-valence iron species,
such as Prussian blue derivatives[25] or related solid materials
that involve the TCNX ligand,[26] which should give rise to
characteristic broad intervalence-charge-transfer (IVCT) or
metal-to-metal charge-transfer (MMCT) absorption bands
in the region 600–800 nm. However, any attempts to isolate
and further characterize these presumably formed mixed-va-
lence compounds, were beyond the scope of this work.
We also decided to study the electron-donor–acceptor in-
teractions of compound 7 with a series of organic reductants,
such as dihydroxybenzene derivatives and other diamagnetic
compounds with low ionization energies. A red hydroqui-
none adduct (8) and an orange catechol complex (9) were
readily obtained as solid crystalline materials. The molecular
structure of the corresponding TCNPy complex with 1,4-di-
hydroxybenzene (8) is shown in Figure 5. Details on selected
bond lengths and crystallographic data of compounds 7 and
8 are reported in Table 1 and Table 2.
CÀ
protonation of the generated TCNPy radical anion, which
yields the corresponding pyridinium radical species,
C
TCNPyH , is tentatively suggested to account for the red-
shifted spectrum of the secondary product, with a new peak
maximum at 470 nm.
CÀ
The reduction of compound 7 to generate TCNPy could
also be chemically induced in aqueous solution (Figure 4).
For this purpose, we applied an excess of a pentacyanoferra-
te(II)–ammine complex, which acted as a one-electron
donor (E8=0.34 V versus NHE, lmax =362 nm).[24] Quite
similar initial spectroscopic variations to those observed
during the electrochemical reduction of TCNPy (Figure 3)
Chem. Eur. J. 2013, 19, 1489 – 1495
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1491

G. Knçr et al.
Upon the formation of a complex between compound 7
and hydroquinone, only minor structural changes in the
TCNPy moiety were observed. Whereas the average CN
bond length in the nitrile substituents remained essentially
constant (1.14 ꢁ), the perimeter of the planar aromatic core
of TCNPy expanded slightly (<0.1 ꢁ) when the adduct (8)
was formed (Table 1). This lack of major bond-length alter-
nations[3] or other structural reorganization indicates a 1:1
charge-transfer interaction between the weakly coupled
donor (D) and acceptor (A) systems in compound 8, with-
out a significant contribution of the antibonding p*-orbital
character of the TCNPy subunit in the ground state.
In Figure 6, the electronic spectra of charge-transfer com-
plexes 8 and 9 are shown, together with the optical proper-
ties of the corresponding TCNPy adduct with tetrathiafulva-
À
Figure 5. Molecular structure of a TCNPy hydroquinone complex (8);
symmetry code (ii): Àx, Ày+1, Àz. Only one orientation of the distorted
TCNPy moiety is shown (ORTEP, 50% probability).
Table 1. Selected bond lengths in compounds 7 and 8.
Bond length [ꢁ]
7
8
ii
À
À
N1b C1/N1 C3
À
C1 C2
1.365(3)
1.393(3)
1.349(3)
1.348(3)
1.403(3)
1.363(3)
8.221
1.370(3)
1.365(3)
1.387 (4)
1.370(3)
1.387 (4)
1.365(3)
8.244
À
À
C2 C3a/C2 C3
ii
À
À
C3a C4/C1 C3
ii
ii
À
À
C4 C5/C2 C3
ii
ii
À
À
C5 N1b/C2 N1
perimeter of the pyridine core[a]
[a] Sum of the bond lengths in the pyridine ring.
Table 2. Crystal data, data collection, and structure refinement for com-
Figure 6. Diffuse reflectance spectra of compounds 8 (b), 9 (···), and 10
(c) at 298 K in a 5 wt.% mixture with BaSO4; the data are plotted as
absorption spectra with baseline correction (BaSO4).
pounds 7 and 8.
7
8
formula
C9HN5
179.15
C15H7O2N5
289.26
Mw [gmolÀ1
]
crystal size [mm]
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
0.23ꢂ0.31ꢂ0.48
orthorhombic
Pbca
10.145(1)
12.253(2)
13.952(2)
–
0.09ꢂ0.25ꢂ0.35
monoclinic
P21/n
6.646(2)
6.563(1)
15.190(4)
101.63(1)
649.0(3)
1.480
lene (TTF) as an electron donor (10), which was obtained as
a solid blue material. The chromophoric charge-transfer
À
(CT) transitions of the TCNPy dihydroxybenzene deriva-
tives (8 and 9) appear as unresolved bands with absorption
maxima in the range 500–600 nm. The deep-blue-colored
b [8]
À
TCNPy TTF system (10) is characterized by a very broad
V [ꢁ3]
1734.3(4)
1.372
absorption band with a maximum in the NIR spectroscopic
1calcd [mgcmÀ3
]
region (lCT =1050 nm). Interestingly, there is no indication
Z
8
2
+
m [mmÀ1
T [K]
]
0.093
205
0.105
200
C
of the presence of a TTF radical cation in the electronic
ground state of compound 10, which could otherwise be
easily identified by a characteristic spectroscopic pattern at
450 nm and 580 nm.[27,28] Quite similar spectroscopic features
have been reported before at low temperatures in solution
q range [8]
l [ꢁ]
total reflns
unique reflns
2.9-25.1
0.71073
9941
1539 (Rint =0.064)
1137
2.7-25.0
0.71073
3938
1142 (Rint =0.068)
880
observed reflns [I>2s(I)]
refined parameters/restraints
absorption correction
Tmin, Tmax
À
for other TTF-containing donor–acceptor, [D A], sys-
128/0
101/0
tems.[28] From the spectroscopic signatures shown in
Figure 6, we conclude that, in compound 10, a weakly cou-
pled donor–acceptor charge-transfer state remains the domi-
nant species, even at room temperature. TTF is a rather
strong electron donor (E8=0.37 V versus SCE)[27] and the
driving force for an electron-transfer process to form the
multiscan
0.96, 0.98
0.17/À0.17
0.049
0.110
882755
0.95, 0.99
0.18/À0.20
0.048
0.118
882756
sfin (max/min) [eꢁÀ3
R1 [Iꢀ2s(I)]
wR2
]
CCDC no.
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Redox-Active Charge-Transfer Complexes
FULL PAPER
CÀ
TCNPy radical anion can be readily estimated (from our
time in a detailed pulse-radiolysis study.[30] It was found that
+
C
electrochemical data) as an endergonic process with DGET
=
these metastable donor radical cations (DH ) decayed into
F(E8oxÀE8red)=84 kJmÀ1.
their corresponding neutral semiquinone radicals (D ) with
C
In contrast to some TTF-based systems with almost iser-
gonic DGET values, which have been studied in detail by
Kochi and co-workers,[28,29] in the electronic ground state of
complex 10 there is no experimental evidence for more-
lifetimes of several hundreds of nanoseconds in nonpolar
media. However, when polar proton-accepting compounds
were present, the deprotonation step turned into a much
more rapid process, thereby reaching the diffusion-control-
led limit.
As shown in Scheme 2b and Equation (1), the pre-requi-
sites for such a sequential proton-coupled electron-transfer
(PCET) cascade are fulfilled in donor–acceptor systems
such as compound 8, which is a charge-transfer complex
that combines a proton-carrying donor (DH) and an elec-
tron acceptor (A) that can be protonated.
dÀ
strongly coupled [Dd+
]
charge-transfer interactions
À
A
with a significant degree of electronic delocalization, or
+
+
C
C
even for the formation of a fully charge-separated [D ,A ]
complex, which consists of a radical ion pair. At room tem-
perature, the same situation should be true for the corre-
À
sponding TCNPy dihydroxybenzene systems, such as com-
À
pound 8, where the presence of a weakly coupled [D A]
charge-transfer (CT) state rather than an electron-transfer
(ET) state with radical-ion-pair character has been assigned,
owing to a lack of significant bond-length variations in the
electron acceptor (see above; Table 1).
However, according to the Mulliken theory, the direct op-
tical excitation of the charge-transfer bands of such weakly
coupled donor–acceptor pairs should immediately generate
Conclusion
+
[29]
C
CÀ
their corresponding electron-transfer states, [D ,A ]*.
Herein, we have reported an investigation into the synthesis,
structural features, and extended characterization of the aro-
matic polynitrile compound 2,3,5,6-tetracyanopyridine
(TCNPy). The electro- and spectroelectrochemical behavior,
chemical reactivity, and electron-acceptor properties of
TCNPy were studied in some detail. In MeCN, TCNPy is re-
duced into its corresponding radical anion at À0.51 V versus
SCE (À4.3 eV versus the vacuum level), which most proba-
bly undergoes subsequent protonation into the neutral
This non-adiabatic process for the formation of the donor
radical cation and the acceptor radical anion can be exploit-
ed for the formation of permanent secondary redox prod-
ucts, when further chemical steps are successfully coupled
within the lifetime of the charge-separated species. By using
the heteroaromatic polynitrile system TCNPy as the elec-
tron acceptor (A), in combination with suitable proton-car-
rying electron donors (DH), such as dihydroxybenzene de-
rivatives, this approach could offer interesting new routes
for the visible- or NIR-light-controlled generation of pyridi-
nium radical species, as outlined in Scheme 2.
C
TCNPyH pyridinium radical. The interactions of TCNPy
with metal complexes and the formation of organic donor–
acceptor systems have also been investigated. Depending on
the strength of the corresponding electron donor, several of
these compounds show intense chromophoric absorption
bands in the visible- and NIR spectroscopic regions; accord-
ing to bond-length variation and diagnostic spectroscopic
features, this result is ascribed to the presence of weakly
coupled donor–acceptor charge-transfer states.
Photoexcitation of the donor–acceptor complexes by the
direct irradiation of the charge-transfer absorption bands
corresponds to an optical electron-transfer process; this
Scheme 2. a) Pyridinium radical core of the reduced and protonated
C
À
[TCNPyH] species. b) Representation of the weakly coupled [DH A]
charge-transfer ground state of compound 8, which illustrates the possi-
bility of pyridinium-radical formation by coupling a direct optical elec-
tron-transfer process and a proton-transfer step.
process can be applied to generate the charge-separated ex-
+
C
CÀ
CÀ
cited state [D ,A ]*, which contains the TCNPy radical-
anion fragment. It is expected that, upon a subsequent pro-
tonation step, similar to the reactivity that was postulated
under cyclovoltammetry conditions (EC mechanism), the
corresponding TCNPy-derived pyridinium radical can be
generated in a proton-coupled electron-transfer process, es-
pecially when pre-organized proton donors are present in
close proximity to the acceptor site. Further study of the
TCNPy derivatives and their charge-transfer complexes in
the context of pyridinium-radical-dependent applications in
electro-, photoelectro-, and photocatalysis of multielectron-
reduction processes,[12,13,31] such as dihydrogen generation or
carbon-dioxide recycling, are currently underway.
The uptake of a proton to generate a neutral radical
C
(AH ) is a typical stabilizing step of short-lived radical
CÀ
anions (A ) in solution, which has already been discussed
for the electrochemically generated TCNPy intermediate,
CÀ
thereby producing a pyridinium radical species (see above).
On the other hand, deprotonation is a very common decay
mechanism of aromatic cation radicals that contain protic
substituents. Recently, the primary proton-transfer reactions
of o- and p-dihydroxybenzene radical cations of relevance
for compounds 8 and 9 have been characterized for the first
Chem. Eur. J. 2013, 19, 1489 – 1495
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G. Knçr et al.
(22.5 mL) was added to compound 1 (106.2 g, 0.42 mol) and the mixture
was heated at 908C for 20 min. After cooling, water (400 mL) was added
and the resulting solution was neutralized with aqueous NH3. Compound
2 (gray precipitate) was filtered, washed with water, and dried. Yield:
98 g (93%); 1H NMR (200 MHz, DMSO, 208C, TMS): d=8.51 (s, 1H;
Experimental Section
General methods: All commercially available chemicals and solvents
were reagent-grade quality and used as received without any further puri-
fication. NMR spectroscopy was performed on a Bruker Digital Avance
NMR spectrometer DPX200 (1H: 200.1 MHz, 13C: 50.3 MHz; T=303 K).
Chemical shifts are given in parts per million (ppm) on the delta (d)
Ar), 4.33 (q, 3J
(H,H)=7 Hz, 6H; Et); IR (ATR): n˜ =1712 (C=O), 1220 cmÀ1 (C O).
Dipotassium-2,6-dimethylpyridine-3,5-dicarboxylate (3): Compound
(H,H)=7 Hz, 4H; Et), 2.76 (s, 6H; Me), 1.33 ppm (t, 3J-
ACHTUNGTRENNUNG
À
AHCTUNGTRENNUNG
scale. 1H and 13C NMR shifts are reported relative to Si
ACTHUNGTRNEUNG(CH3)4 and were
2
referred internally to the residual signal of the deuterated solvent. Cou-
pling constants are given in Hertz (Hz). IR spectroscopy was performed
on a Shimadzu IRAffinity-1 FTIR spectrophotometer that was equipped
with a Specac Golden GateTM single-reflection diamond ATR accessory.
Elemental analysis was performed at the Institut fꢃr Technologie Organ-
ischer Stoffe (Universitꢄt Linz).
(98 g, 0.39 mol) was dissolved in EtOH (300 mL) and a solution of KOH
(52.8 g, 1.03 mol) in EtOH (300 mL) was added dropwise, followed by
heating at reflux for 1 h. After cooling to RT, compound 3 (yellow pre-
cipitate) was filtered, washed with EtOH, and dried. Yield: 73.58 g
(69%); 1H NMR (200 MHz, DMSO, 208C, TMS): d=7.69 (s, 1H; Ar),
2.52 ppm (s, 6H; Me); IR (ATR): n˜ =1712 cmÀ1 (C=O).
Electrochemical measurements: Cyclic voltammetry (CV) experiments
were performed on an Eco Autolab potentiostat at scan rates in the
range 20–250 mVsÀ1. A standard three-electrode arrangement was used
with a Pt-disk working electrode (BAS, surface area (A): 0.020 cm2), a
Pt-wire counter electrode, and a Ag-wire reference electrode in MeCN
with 0.1m Bu4NPF6 supporting electrolyte. Ferrocene was used as an in-
ternal standard for potential referencing and the potentials were subse-
quently referenced versus the SCE. Digital simulations of the CVs were
carried out with the BAS DigiSimꢅ 3.03 software package. Spectroelec-
trochemistry was performed in an optically transparent thin-layer elec-
trode (OTTLE cell)[32] with a Pt working electrode, a Pt-wire counter
electrode, and a Ag reference electrode. The corresponding absorption
spectra were recorded on a Jasco V670 UV/Vis/NIR spectrophotometer.
Pyridine-2,3,5,6-tetracarboxylic acid (4): Compound 3 (15 g, 55 mmol)
was heated at reflux with KMnO4 (71.4 g, 452 mmol) in water (500 mL)
for 90 min. Excess KMnO4 was treated with EtOH and the hot mixture
was filtered. Concd HCl was added to the filtrate until it reached pH 1,
followed by evaporation of the solution to afford a mixture of compound
4 and KCl. 1H NMR (200 MHz, D2O, 20 8C, TMS): d=8.84 ppm (s, 1H;
Ar); IR (ATR): n˜ =1693 cmÀ1 (C=O).
Tetramethylpyridine-2,3,5,6-tetracarboxylate (5): Concd H2SO4 (15 mL)
was added to a cooled suspension (08C) of compound 4 (14 g, 55 mmol)
in MeOH (160 mL), followed by heating at reflux for 12 h. After cooling
to RT, the solvent was evaporated and a saturated aqueous solution of
NaHCO3 was added until it reached pH 8. Compound 5 was extracted
from the mixture with EtOAc. Yield: 3.26 g (19%); 1H NMR (200 MHz,
DMSO, 208C, TMS): d=8.75 (s, 1H; Ar), 3.92 ppm (s, 12H; Me); IR
Electronic and diffuse reflectance spectroscopy: Electronic absorption
spectra were recorded in a 1 cm quartz cell on a Varian Cary 300 Bio
UV/Vis spectrophotometer. Diffuse reflectance spectra were recorded on
a Jasco V670 UV/Vis/NIR spectrometer that was equipped with an ISN-
723 integrating-sphere attachment (inside diameter of the integrating
sphere: 60 mm). BaSO4 was used for baseline correction and the samples
were prepared as a 5 wt.% mixture of the compound in BaSO4. The re-
sulting data were converted into absorption spectra by using Spectra
Managerꢅ.
(ATR): n˜ =1724 (C=O), 1240 cmÀ1 (C O).
À
Pyridine-2,3,5,6-tetracarboxamid (6): Compound 5 (3 g, 10 mmol) and
NH4Cl (0.18 g, 3.5 mol) were dissolved in aqueous NH3 (28 mL) and stir-
red for 12 h at RT. Compound 6 was removed by filtration and dried.
Yield: 2.29 g (94%); 1H NMR (200 MHz, D2O/TFA 100:1, 208C, TMS):
d=7.50 ppm (s, 1H; Ar); IR (ATR): n˜ =1651 (C=O), 1581 cmÀ1 (C N).
À
2,3,5,6-Tetracyanopyridine (7): Compound 6 (2.2 g, 9.1 mmol) was dis-
solved in DMF (30 mL) at 08C under a N2 atmosphere. SOCl2 (3.0 mL,
42 mmol) was added dropwise, followed by stirring for 2 h at 08C and
12 h at RT. The reaction was quenched with water and crude TCNPy was
extracted with EtOAc. After evaporation of the solvent, the reddish resi-
due was purified by column chromatography on silica gel (CH2Cl2/Et2O,
1:1). TCNPy was afforded as a white powder. Yield: 1.17 g (72%);
1H NMR (200 MHz, DMSO, 208C, TMS): d=9.53 ppm (s, 1H; Ar);
13C NMR (50 MHz, DMSO, 208C, TMS) d=149.95 (CH), 141.43
(NCCN), 120.25 (NCCC), 116.81 (CN), 116.17 ppm (CN); IR (ATR): n˜ =
Crystal structures: Single-crystal structure analysis was carried out on a
Bruker Smart X2S diffractometer that was operating with MoKa radiation
(l=0.71073 ꢁ). Further crystallographic and refinement data can be
found in Table 2. The structures were solved by using direct methods
(SHELXS-97[33]
)
and refined by full-matrix least-squares on F2
(SHELXL-97[34]). The H atoms were calculated geometrically and a
riding model was applied during the refinement process. In both com-
pounds, the TCNPy moieties were found to be disordered and the pyri-
dine N atom and the para-C atoms were occupied by both N and C
atoms. Therefore, the structures were refined with an equal occupation of
N and C atoms at each of the two positions by using the EXYZ and
EADP commands.
2249 cmÀ1 (C N); elemental analysis calcd (%) for C9HN5: C 60.34,
ꢁ
H 0.56, N 39.09; found: C 60.08, H 0.61, N 38.86.
CCDC-882755 (7) and CCDC-882756 (8) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
À
TCNPy hydroquinone (8): Compound 7 (89.5 mg, 0.50 mmol) and hy-
droquinone (55 mg, 0.50 mmol) were stirred at RT in water (20 mL).
Within 12 h, red crystals had formed and were removed by filtration. IR
(ATR): n˜ =2247, 1514, 1456, 1417, 1361, 1188, 1097, 1010, 920, 829,
756 cmÀ1; elemental analysis calcd (%) for C15H7N5O2: C 62.29, H 2.44,
N 24.21; found: C 62.62, H 2.69, N 24.56.
Synthesis: TCNPy was prepared in analogy to similar reaction steps re-
ported in the literature.[35,36,37]
Diethyl-1,2-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate (1): Ethyl
acetoacetate (204 mL, 1.6 mol), aqueous formaldehyde (37%, 60 mL, 0.8
mol), and diethyl amine (1.2 mL, 0.01 mol) were stirred for 6 h at 08C
and for 40 h at RT. The organic layer was separated and dried and EtOH
(200 mL) was added. At 08C, NH3 gas was passed through this solution
until saturation occurred and the mixture was stirred for 40 h at RT.
Compound 1 (yellow precipitate) was filtered, washed with EtOH, and
dried. Yield: 121.5 g (63%); 1H NMR (200 MHz, DMSO, 208C, TMS):
À
TCNPy catechol (9): Compound 7 (89.5 mg, 0.50 mmol) and catechol
(55 mg, 0.50 mmol) were stirred at RT in water (20 mL). Within 12 h,
orange crystals had formed and were removed by filtration. IR (ATR):
n˜ =2247, 1558, 1458, 1417, 1354, 1282, 1222, 1097, 1037, 931, 798 cmÀ1; el-
emental analysis calcd (%) for C15H7N5O2: C 62.29, H 2.44, N 24.21;
found: C 62.47, H 2.71, N 24.19.
À
TCNPy tetrathiafulvalene (10): Compound 7 (89.5 mg, 0.5 mmol) and
d=8.25 (s, 1H; NH), 4.06 (q, 3J
A
tetrathiafulvalene (102 mg, 0.5 mmol) were stirred at RT in MeCN
(5 mL). Within 12 h, dark-blue crystals formed and were filtered. IR
(ATR): n˜ =2237, 1558, 1541, 1417, 1338, 1165, 1097, 925, 867, 792,
665 cmÀ1; elemental analysis calcd (%) for C15H7N5O2: C 46.98, H 1.31,
N 18.26, S 33.45; found: C 46.51, H 1.33, N 17.98, S 33.18.
(s, 6H; Me), 1.19 ppm (t, 3J
ACHTUNGTRENNUNG
(C=O), 1209 cmÀ1 (C O).
À
Diethyl-2,6-dimethylpyridine-3,5-dicarboxylate (2):
A cooled solution
(5 8C) of water (150 mL), concd HNO3 (27 mL), and concd H2SO4
1494
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 1489 – 1495

Redox-Active Charge-Transfer Complexes
FULL PAPER
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Received: May 31, 2012
Revised: October 21, 2012
Published online: December 11, 2012
Chem. Eur. J. 2013, 19, 1489 – 1495
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1495