Chemical and electrochemical investigations on the powerful π-electron donor dithiadiazafulvalene: Isolation, spectroscopic characterization and charge-transfer complexation

Article abstract of DOI:10.1055/s-2005-865204

A representative of the title system has been accessible by three methods, namely the electrochemical reduction of the corresponding 2-(phenylthio) thiazolium salt, the chemical coupling of the thiazoline-2-selone in the presence of triethyl phosphite and

Full text of DOI:10.1055/s-2005-865204

LETTER
1117
Chemical and Electrochemical Investigations on the Powerful p-Electron
Donor Dithiadiazafulvalene: Isolation, Spectroscopic Characterization and
Charge-Transfer Complexation
Investigations on
t
e
he Powerfu
o
l
p
-Electron
r
D
onor
D
g
ithiadiazaful
evalene s Morel,* Grégory Gachot, Dominique Lorcy
Synthèse et Electrosynthèse Organiques, UMR 6510, Université de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France
Fax +33(223)236738; E-mail: Georges.Morel@univ-rennes1.fr
Received 18 January 2005
representative examples because of the simplicity of the
Abstract: A representative of the title system has been accessible
NMR spectra in this series.
by three methods, namely the electrochemical reduction of the cor-
responding 2-(phenylthio)thiazolium salt, the chemical coupling of
the thiazoline-2-selone in the presence of triethyl phosphite and a
basic treatment of the 2-unsubstituted thiazolium salt. The last two
procedures allow the isolation in a preparative scale of nearly pure
dithiadiazafulvalene, which can be characterized by cyclic voltam-
metry and usual spectroscopic methods. The crystal structure of a
1:2 charge-transfer complex between this excellent p-donor mole-
cule and tetracyanoquinodimethane as acceptor establishes that the
dithiadiazafulvalene is in the dicationic state in the complex.
Initially, we studied the electrochemical reduction of 2-
(phenylthio)thiazolium iodide 1 (Scheme 1, upper se-
quence). The method was shown to offer an interesting
pathway toward DTDAFs.1 The formation of the antici-
pated fulvalene 3 was detected immediately by cyclic vol-
tammetry. Two monoelectronic reversible oxidation
waves were observed at very low potentials,14 proving the
high donating properties of dimer 3. However, such elec-
trosynthesis did not allow the isolation of product 3 or
even the determination of its proportion in the reaction
mixture.
Key words: thiazolium salts, heterogeneous deprotonation, hetero-
cycle, oxidation, complex
Thiazoline-2-selones represent alternative starting com-
pounds for the selective synthesis of dithiadiazaful-
valenes.1 Conversion of the 2-(phenylthio)thiazolium
iodide 1 to the selone 4 (Scheme 1, lower sequence)15 and
coupling of the latter under treatment with triethyl
phosphite16 were carried out by analogy with procedures
described by Cava and co-workers6 and explored in our
laboratory.8,10
The synthesis and redox properties of 2,2¢-bithiazol-
inylidenes (dithiadiazafulvalenes, DTDAFs) and elabora-
tion of their charge-transfer salts constitute an interesting
aspect in molecular conductors research.1 However, the
study of DTDAFs has been limited owing to the facility of
air oxidation, making handling of solutions rather diffi-
cult. The question of very easy oxidation is of consider-
able importance in the chemistry of such compounds.
Isolated dithiadiazafulvalenes are represented in the liter-
ature mainly by benzo-annulated systems.2–5 A few un-
condensed substrates bearing electron-withdrawing
groups on carbon atoms have also been purified.5–7 On the
other hand, alkyl- and aryl-substituted DTDAFs have only
been identified in situ by cyclic voltammetry investigation
carried out on the medium where they were formed.4,8–10
Notable exceptions are DTDAFs bearing a bulky group on
the nitrogen atom and the hexamethyl derivative which
have been isolated and characterized under rigorously
controlled conditions.11
The dimer 3 was thus obtained as a crystalline material
with satisfying yield in admixture with very slight quanti-
ties of two oxidation derivatives. It was characterized by
cyclic voltammetry, mass spectrometry and NMR spec-
troscopy with particular regard to the chemical shifts for
the two equivalent central carbons C2 and C2¢ (syn/anti
mixture).16 The resonances observed at d = 117.3 and
119.7 ppm are in excellent conformity with values record-
ed by Arduengo et al.11 and also noted by others.17 Unfor-
tunately, we failed to obtain 3 in a pure state due to its
rapid oxidation during recrystallization from CH2Cl2/
Et2O (see the basic treatment of the salt 2 for comments on
the relative stability of DTDAF 3 and identification of
secondary oxidation products 5, 6). However, in our opin-
ion, this first chemical approach presents distinct dis-
advantages concerning the preparation and use of the air
oxidizable hydrogen selenide anion to form the selone 4;
the coupling reaction of this selone in refluxing degassed
toluene and the need for repeated washing in order to re-
move the by-product (triethyl selenophosphate) from the
DTDAF 3.
We have previously developed efficient preparations of
5-(methylthio)-2-(phenylthio)thiazolium iodides12 and
their corresponding 2-reduced salts.13 As an extension of
this work and in view of the significant properties of poly-
heterofulvalenes,1,2 we now describe our investigations
into the utility of available thiazolium salts for the elabo-
ration of DTDAFs and the crucial problem of their isola-
tion. The N-methyl derivatives 1 and 2 were chosen as
The base-catalyzed deprotonation of 2-unsubstituted
thiazolium salts has also received great attention for the
generation of dimeric DTDAFs.1,2 In the late 1970s, we
SYNLETT 2005, No. 7, pp 1117–1120
Advanced online publication: 14.04.2005
DOI: 10.1055/s-2005-865204; Art ID: D01405ST
© Georg Thieme Verlag Stuttgart · New York
0
2.
0
5.
2
0
0
5

1118
G. Morel et al.
LETTER
Me
selenide21 to afford the oxamide 7 (several isomers,
Figure 1).22 On the other hand, hydrolysis of salt 1 by
NaOH in ethanol solution is the clear formation of 2-thia-
zolone 6.
PhS
Ph
SMe
Ph
N
S
S
N
4 e– / – 2 PhS–
CH2Cl2
S
I
2
N
SMe
Me
MeS
Me
Ph
1
3
P(OEt)3
Me
N
O
Ph
O
toluene
Ph
S
– SeP(OEt)3
MeS
MeS
N
SMe
N
H
Se
S
O
Me
Ph
S
S
2
Me
2 NaHSe / – 2 PhSH
EtOH
N
SMe
Me
7
9
Ph
Figure 1
4
Scheme 1
A few 1,2,5,8-dithiadiazecine-6,7-diones have been
reported in the literature resulting from oxidation of
benzoannelated
DTDAFs5,23
and
noncondensed
DTDAFs.23,24 Such conversion was suggested to proceed
via an unstable dioxetane which undergoes cleavage of
O-O and C-S bonds.5
reported that powdered potassium hydroxide allowed the
convenient formation of diverse carbanions18 and present-
ed the advantage of being easily removed from heteroge-
neous mixtures by filtration. Consequently, we have
developed a simple experimental protocol using KOH in
a two-phase system (solid–liquid) for the ionization of the
thiazolium iodide 2. The procedure promoted the rapid
preparation of the expected fulvalene 3 which was collect-
ed by filtration in 75% yield or better in the presence of
minor oxidation products 5, 6 (Scheme 2).19
In the course of our study, we also examined the efficien-
cy of other heterogeneous and homogeneous reagents for
the deprotonation of salt 2. When the less basic potassium
fluoride on alumina was used instead of the KOH in
CDCl3 suspension, a mixture of thiazolium iodides 2 and
1
8 was observed by H NMR spectroscopy. Substantial
quantities of monoprotonated dimer 8 was found after ten
or twelve hours at room temperature and 13C NMR spec-
tral data allowed its identification.25 On standing under
the same conditions for two days, the solution generated a
small quantity of DTDAF 3. The main products were
oxidation species 5, 6 and formamide 9 (two isomers,
Figure 1)26 which presumably results from the hydrolysis
of salt 2. In accordance with previous observations,4,11,17a
evidence of intermediate 8 supports the nucleophilic addi-
tion of carbene 10 to the C2 position of starting cation 2
for the generation of DTDAF 3 (see Scheme 2).
The DTDAF 3 exhibited satisfying stability in the solid
state secure from aerial contact and could be stored for
four days at room temperature inside a vacuum desiccator.
On the contrary, it gradually oxidized on standing under
atmospheric oxygen and much faster in CDCl3 solution.
Evidence of alteration was noticeable one hour after dis-
solution and oxidation was complete after about thirty
hours to give a mixture of cyclodithiadiazecinedione 5
and thiazolinone 6. These compounds were easily separat-
ed by flash chromatography on a silica gel column.
Structures of heterocycles 5, 6 were established by their
analytical and spectral data20 and confirmed by the fol-
lowing chemical evidence. Reduction of the ten-mem-
bered bis-sulfide 5 took place readily at ambient
temperature in the presence of sodium hydrogen
Use of standard organic bases in homogeneous media
(Et3N, DABCO, DBN) led to complex mixtures with very
little detectable DTDAF 3. Salt 8, oxidation derivative 6
and formamide 9 were obtained instead.
Me
Ph
–
SMe
4'
5'
5
4
S
N
S
I–
SMe
S
+
S
2'
2
base
I–
N
N
SMe
2
N
Me
Me
MeS
Ph
Ph
Ph
10
Me
2
8
O
Me
Me
Ph
O
N
SMe
S
Ph
SMe
Ph
N
N
S
S
O2
base
Ph
+
S
SMe
Me
S
N
MeS
N
SMe
Ph
Me
Me
O
3
5
6
Scheme 2
Synlett 2005, No. 7, 1117–1120 © Thieme Stuttgart · New York

LETTER
Investigations on the Powerful p-Electron Donor Dithiadiazafulvalene
1119
The sequence of dimerization described in Scheme 2 was deprotonation of the corresponding 2-unsubstituted
shown to be reversible by the addition of a small amount thiazolium salt represents a good alternative to standard
of aqueous hydrogen chloride to a CDCl3 solution of methods for the construction of such systems. The
DTDAF 3. Thiazolium chlorides like 2 and 8 were regen- DTDAF 3 has been studied with respect to its remarkable
erated in admixture with the hydrolysis compound 9. Such redox properties and potentialities to serve as precursor
a reversibility has already been reported in the for organic materials.
literature.7,17a
Interest in compounds such as 3 in the organic material
field is demonstrated by the formation of a crystalline
charge transfer salt with tetracyanoquinodimethane
(TCNQ) as acceptor.27 The X-ray structure analysis of this
complex reveals a stoichiometry of one donor 3 for two
acceptors (Figure 2).28 The TCNQ molecules form isolat-
ed dimers with an interplanar separation of 3.25 Å. Within
this salt, the fulvalene core itself is planar but the two phe-
nyl groups are located in planes forming an angle of 47.7°
with the fulvalene core. Comparison of the central C–C
bond length [1.454(4) Å] of 3 in (DTDAF 3)(TCNQ)2
with the central C–C bond length observed in other
DTDAF salts favors a dicationic state for this donor.1 By
using the empirical formula of Kistenmacher29 which cor-
relates the bonds lengths to the formal charge of the
TCNQ in various charge transfer salts, we derive a value
of –0.91 for the charge of TCNQ in such a complex. This
is supported by a strong nitrile absorption band at 2180
cm–1 in the IR spectrum, which is close to that observed
for the TCNQ anion (2183 cm–1) in the Na+TCNQ– salt.30
Therefore, the total charge transfer of approximately +2
between 3 and the two TCNQs reveals that the donor is in
its dicationic form. This is in accordance with the redox
potentials of 3 (–0.34 V and –0.19 V in CH2Cl2 vs. SCE),
which oxidizes far below the reduction potential of TCNQ
(+0.18 V).
Acknowledgment
We thank Thierry Roisnel from the Centre de Diffractométrie X,
Université de Rennes 1, for X-ray structural determination.
References
(1) See the special issue on Molecular Conductors, Chem. Rev.
2004, 104, 4887-5782 and the article therein: Lorcy, D.;
Bellec, N. Chem. Rev. 2004, 104, 5185; see also ref.2 for a
more general review about azafulvalenes.
(2) Beckert, R. Adv. Heterocycl. Chem. 2000, 77, 115.
(3) Baldwin, J. E.; Branz, S. E.; Walker, J. A. J. Org. Chem.
1977, 42, 4142.
(4) Bordwell, F. G.; Satish, A. V. J. Am. Chem. Soc. 1991, 113,
985.
(5) Koizumi, T.; Bashir, N.; Kennedy, A. R.; Murphy, J. A. J.
Chem. Soc., Perkin Trans. 1 1999, 3637.
(6) (a) Tormos, G. V.; Neilands, O. J.; Cava, M. P. J. Org.
Chem. 1992, 57, 1008. (b) Tormos, G. V.; Bakker, M. G.;
Wang, P.; Lakshmikantham, M. V.; Cava, M. P.; Metzger,
R. M. J. Am. Chem. Soc. 1995, 117, 8528.
(7) Doughty, M. B.; Risinger, G. E. Bioorg. Chem. 1987, 15, 1.
(8) Bellec, N.; Guérin, D.; Lorcy, D.; Robert, A.; Carlier, R.;
Tallec, A. Acta Chem. Scand. 1999, 53, 861.
(9) Guérin, D.; Carlier, R.; Lorcy, D. J. Org. Chem. 2000, 65,
6069.
(10) Guérin, D.; Carlier, R.; Guerro, M.; Lorcy, D. Tetrahedron
2003, 59, 5273; and references therein.
(11) Arduengo, A. J.; Goerlich, J. R.; Marshall, W. J. Liebigs
Ann./Recl. 1997, 365.
(12) Touimi Benjelloun, A.; Morel, G. Sulfur Lett. 1996, 20, 107.
(13) Morel, G. Synlett 2003, 2167.
(14) The reduction of compound 1 on a platinium electrode at
–1.3 V vs. SCE was performed in a degassed CH2Cl2
solution containing Bu4NPF6 as the conducting salt (1 M).
On the first cathodic scan, an intense reduction peak was
found at –1.13 V. On the reverse scan, two oxidation peaks
were measured at –0.34 V and –0.23 V vs. SCE.
(15) Experimental Procedure.
NaBH4 (0.29 g, 7.6 mmol) was added portionwise over 10
min to a suspension of black selenium powder (0.45 g, 5.6
mmol) in degassed (N2) EtOH (25 mL). The colorless
solution of NaHSe thus obtained was maintained under inert
atmosphere during the addition of salt 1 (1.15 g, 2.5 mmol),
which had been previously dissolved in degassed CH2Cl2
(10 mL). After 45 min at r.t., the mixture was poured into 60
mL of H2O and extracted with CH2Cl2 (2 ꢀ 10 mL). The
combined CH2Cl2 layers were dried over Na2SO4 and
concentrated in vacuo to give a yellowish solid which was
recrystallized from EtOH (4, 0.60 g, 80%); mp 125 °C. 1H
NMR (200 MHz, CDCl3): d = 2.28 (s, 3 H), 3.57 (s, 3 H),
7.30 (m, 2 H), 7.57 (m, 3 H). 13C NMR (50.5 MHz): d = 20.8
(q, 1J = 141 Hz), 39.9 (q, 1J = 143 Hz), 123.5 (q, 3J = 5.5
Hz), 129.6, 129.9, 130.4, 130.8 (arom. C), 147.4 (m), 181.8
(q, 3J = 5 Hz). Anal. Calcd for C11H11NS2Se: C, 43.85; H,
3.65; N, 4.65. Found C, 43.73; H, 3.59; N, 4.62.
Figure 2 Molecular structure of 32+ in the TCNQ complex (DTDAF
3)(TCNQ)2
In conclusion, the novel hexasubstituted dithiadiazaful-
valene 3, a powerful electron donor, has been synthesized
and isolated in almost pure state. The KOH heterogeneous
Synlett 2005, No. 7, 1117–1120 © Thieme Stuttgart · New York

1120
G. Morel et al.
LETTER
118. Anal. Calcd: C, 55.67; H, 4.67; N, 5.90. Found: C,
(16) Procedure for Preparation of DTDAF 3 from Selone 4.
A solution of selone (0.17 g, 0.56 mmol) in freshly distilled
toluene (15 mL) was degassed with nitrogen and heated to
reflux for 5–6 min. After addition of P(OEt)3 (0.11 g, 0.65
mmol), the solution was stirred 10 min under N2 in boiling
toluene and the mixture was cooled to r.t. and concentrated
under reduced pressure. Trituration of the residue with
anhyd Et2O produced an orange-colored solid, which was
washed several times with Et2O, absolute EtOH and then
dried in a vacuum desiccator. This crude material was used
for cyclic voltammetry and spectroscopic experiments
without further purification (0.10 g, about 80% yield). 1H
NMR analysis (500 MHz, degassed CDCl3) confirmed that
the fulvalene 3 was the major isolated product along with
oxidation derivatives 5, 6 (≤10%). The system showed a ten
proton multiplet centered at d = 7.41 ppm and four broad
singlets, indicating a mixture of syn/anti isomers and the
spectrum close to coalescence at r.t. (rel. int. 1:4, twelve
protons on the totality); major isomer: d = 2.22, 2.81; minor
isomer: d = 2.24, 2.84. The assignments of the 13C NMR
signals (125.8 MHz, degassed CDCl3) were made under
proton decoupling and HMBC; major isomer: d = 20.3
(CH3S), 38.6 (CH3N), 108.3 (C5), 119.7 (C2), 128.3, 128.8,
129.9, 131.3 (arom. C), 145.7 (C4); minor isomer: d = 20.0,
36.6, 109.8, 117.3, 128.3, 129.6, 129.9, 131.5, 146.2. MS
(EI): m/z calcd for C22H22N2S4: 442.06659 [M]+; found:
442.06454. Cyclic voltammetry [MeCN, Bu4NPF6 (1 M)]:
one oxidation peak at –0.31 V vs. SCE.
55.56; H, 4.60; N, 5.87.
Compound 6: colorless solid from petroleum ether–Et2O,
mp 98 °C. 1H NMR (300 MHz, CDCl3): d = 2.22 (s, 3 H),
3.10 (s, 3 H), 7.31 (m, 2 H), 7.49 (m, 3 H). 13C NMR (75.5
MHz): d = 20.7 (q, 1J = 141 Hz), 32.0 (q, 1J = 142 Hz),
107.9 (q, 3J = 5.3 Hz), 128.7, 129.6, 129.8, 129.9 (arom. C),
140.8 (m), 171.9 (q, 3J = 3.3 Hz). MS (EI): m/z calcd for
C11H11NOS2: 237.02821 [M]+; found: 237.02820.
(21) For similar reduction of acyclic disulfides to corresponding
thiols by NaHSe/EtOH, see: Woods, T. S.; Klayman, D. L.
J. Org. Chem. 1974, 39, 3716.
(22) Compound 7, main isomer: 1H NMR (300 MHz, CDCl3):
d = 2.66 (s, 6 H), 2.87 (s, 6 H), 6.74 (s, 2 H), 7.26 (s, 4 H),
7.37 (s, 6 H). 13C NMR (75.5 MHz): d = 19.8 (q, 1J = 142
Hz), 33.4 (qd, 1J = 140 Hz, 3J = 3.5 Hz), 71.7 (br d, 1J = 142
Hz), 128.8, 130.1, 130.2, 135.0 (arom. C), 165.2 (m), 234.7
(m). MS (EI, oily mixture): m/z calcd for C20H21N2O2S2:
385.10445 [M – CS2CH3]+; found: 385.10460.
(23) Itoh, T.; Nagata, K.; Okada, M.; Ohsawa, A. Tetrahedron
1993, 49, 4859.
(24) Bssaibis, M.; Robert, A.; Lemaguerès, P.; Ouahab, L.;
Carlier, R.; Tallec, A. J. Chem. Soc., Chem. Commun. 1993,
601.
(25) Thiazolium salt 8: 1H NMR (300 MHz, CDCl3): d = 2.26 (s,
3 H), 2.49 (s, 3 H), 3.04 (s, 3 H), 3.99 (s, 3 H), 7.03 (s, 1 H),
7.26-7.67 (m, 10 H). The assignments of 13C NMR signals
(75.5 MHz, see numbering in Scheme 2) were made after
proton decoupling, HMBC and HMQC: d = 19.6, 20.8
(CH3S), 40.3, 41.0 (CH3N), 68.5 (C2¢-sp3), 108.6 (C5¢),
126.0, 130.2 (quat. arom. C), 135.3 (C5), 144.5 (C4¢), 150.7
(C4), 178.9 (C2-sp2).
(17) For 13C NMR spectra of crude non-isolated DTDAFs, see:
(a) Chen, Y.-T.; Jordan, F. J. Org. Chem. 1991, 56, 5029.
(b) López-Calahorra, F.; Castells, J.; Domingo, L.; Martí, J.;
Bofill, J. M. Heterocycles 1994, 37, 1579.
(18) (a) Morel, G.; Seux, R.; Foucaud, A. Bull. Soc. Chim. Fr.
1975, 1865. (b) Marchand, E.; Morel, G.; Foucaud, A.
Synthesis 1978, 360. (c) Morel, G.; Marchand, E.; Foucaud,
A. Synthesis 1980, 918.
(19) Procedure for Preparation of DTDAF 3 from
Thiazolium Iodide 2.
(26) Formamide 9 (two isomers): 1H NMR (300 MHz, CDCl3):
d = 2.67, 2.72, 2.85, 2.88 (4 s), 5.85, 6.65 (2 s), 7.35–7.80
(m), 8.23 (s). 13C NMR (75.5 MHz): d = 19.8, 20.0, 30.4,
33.1, 70.1, 77.0, 163.2, 163.9, 233.9, 234.1 and eight
aromatic carbons.
(27) A sat. MeCN solution of TCNQ was added to a degassed
CH2Cl2 solution of DTDAF 3 (crude solid, 0.05 g). The
mixture immediately turned from orange to black. After a
couple of days at r.t. with protection from light, black shiny
single crystals produced were filtered and washed with Et2O:
To a suspension of salt 2 (0.35 g, 1 mmol), Na2SO4 (0.4 g)
and CH2Cl2 (10 mL) was added finely ground KOH (0.45 g
as a commercial product containing about 15% H2O). After
shaking at r.t. for 6 min, the mixture was rapidly filtered and
the filtrate was concentrated in vacuo. The crude 3 (0.35 g)
was isolated in the way described in ref.16; analysis by 1H
NMR spectroscopy allowed assessment of the efficiency of
the method (the 3, 5, 6 distribution was estimated at roughly
85:10:5). For 1H NMR and 13 C NMR see ref.16. Cyclic
voltammetry [CH2Cl2, Bu4NPF6 (1 M)]: –0.34 and –0.19 V
vs. SCE.
mp 226 °C. FTIR (KBr diffuse reflectance spectra): nCN
=
2180 cm–1.
(28) Full details of the X-ray structure determination have been
deposited at the Cambridge Crystallographic Data Centre
(deposition number CCDC 259963). The crystallographic
data can be obtained free of charge from the CCDC, 12
Union Road, Cambridge CB2 1EZ, UK; e-mail:
deposit@ccdc.cam.ac.uk.
(29) Kistenmacher, T. J.; Emge, T. J.; Bloch, A. N.; Cowan, D. O.
Acta Crystallogr., Sect. B: Struct. Sci. 1982, 38, 1193.
(30) Chappel, J. S.; Bloch, A. N.; Bryden, W. A.; Maxfield, M.;
Poehler, T. O.; Cowan, D. O. J. Am. Chem. Soc. 1981, 103,
2442.
(20) Compound 5: pale yellow needles from EtOH, mp 154 °C.
1H NMR (300 MHz, CDCl3): d = 2.18 (s, 6 H), 2.95 (s, 6 H),
7.40 (m, 6 H), 7.80 (m, 4 H). 13C NMR (75.5 MHz):
d = 18.7, 34.4 (2 ꢀ q, 1J = 141 Hz), 128.6, 129.7, 130.5,
134.6 (arom. C), 134.9 (q, 3J = 4.4 Hz), 145.6 (m), 165.1 (q,
3J = 3 Hz). MS (EI): m/z calcd for C22H22N2O2S4: 474.0564
[M]+; found: 474.0578. MS: m/z = 265, 237, 222, 165, 162,
Synlett 2005, No. 7, 1117–1120 © Thieme Stuttgart · New York