Methyl- and methoxy-substituted di[1,4]benzodithiio[2,3-b:2,3-e]-pyridines as new electron donor compounds: Synthesis, molecular structure, electrochemical properties, and EPR studies

Article abstract of DOI:10.1039/a900148d

Two new derivatives of di[1,4]benzodithiino[2,3-b:2,3-e]pyridine (5) tetrasubstituted with methyl (7) and methoxy (8) groups at the 2,3,9 and 10 positions have been prepared from 2,3,5,6-tetrachloropyridine, by cyclization reaction with the bidentate nucleophiles, 4,5-dimethyl-and 4,5-dimethoxy-benzene-1,2-dithiol. Cyclic voltammograms for the oxidation of both polyheterocyclic compounds 7 and 8 in CH₂Cl₂ exhibit two consecutive redox couples. The first pairs are due to the equilibria between the initial compounds and their radical cations, while in the second couples, the electrogenerated radical cations are in equilibrium with the corresponding dications. Radical cations of these molecules have been generated in fluid solution, by oxidation of the parent compounds with thallium(III) trifluoroacetate in trifluoroacetic acid in the case of 7, and by irradiation of a CH₂Cl₂ solution containing trifluoroacetic acid (10%) in the case of 8. Both species were analyzed by electron paramagnetic resonance (EPR). X-Ray analysis of the molecular structures of both 7 and 8 shows a stable chair-shaped conformation with interplanar angles between the phenyl rings and the pyridine ring of 139.9 and 141.4° for 7 and 133.7° for 8.

Full text of DOI:10.1039/a900148d

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Methyl- and methoxy-substituted di[1,4]benzodithiio[2,3-b:2,3-e]-
pyridines as new electron donor compounds: synthesis, molecular
structure, electrochemical properties, and EPR studies
B. Bueno,a B. Esteve,b J. Irurre,b E. Brillas,c X. Torrelles,d J. Rius,d A. Alvarez-Larena,e
J. F. Piniella,e C. Alemánf and L. Juliá*a
a Departament de Química Orgànica Biològica, Centre d’Investigació i Desenvolupament
(CSIC), Jordi Girona 18–26, 08034 Barcelona, Spain
b Departament de Química Orgànica, CETS Institut Químic de Sarrià,
Universitat Ramon Llull, 08017 Barcelona, Spain
c Departament de Química Física, Universitat de Barcelona, Diagonal 647, 08028 Barcelona,
Spain
d Institut de Ciència de Materials (CSIC), Campus de la U.A.B., 08193 Bellaterra, Spain
e Unitat de Cristallografia, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
f Departament d’Enginyeria Química, ETSEIB, Universitat Politècnica de Catalunya (UPC),
Diagonal 647, 08028, Spain
Received (in Cambridge) 5th January 1999, Accepted 19th May 1999
Two new derivatives of di[1,4]benzodithiino[2,3-b:2,3-e]pyridine (5) tetrasubstituted with methyl (7) and methoxy
(8) groups at the 2,3,9 and 10 positions have been prepared from 2,3,5,6-tetrachloropyridine, by cyclization reaction
with the bidentate nucleophiles, 4,5-dimethyl- and 4,5-dimethoxy-benzene-1,2-dithiol. Cyclic voltammograms for
the oxidation of both polyheterocyclic compounds 7 and 8 in CH2Cl2 exhibit two consecutive redox couples. The
first pairs are due to the equilibria between the initial compounds and their radical cations, while in the second
couples, the electrogenerated radical cations are in equilibrium with the corresponding dications. Radical cations
of these molecules have been generated in fluid solution, by oxidation of the parent compounds with thallium()
trifluoroacetate in trifluoroacetic acid in the case of 7, and by irradiation of a CH2Cl2 solution containing
trifluoroacetic acid (10%) in the case of 8. Both species were analyzed by electron paramagnetic resonance (EPR).
X-Ray analysis of the molecular structures of both 7 and 8 shows a stable chair-shaped conformation with
interplanar angles between the phenyl rings and the pyridine ring of 139.9 and 141.4Њ for 7 and 133.7Њ for 8.
2,3,7,8-tetramethoxythianthrene is folded along the S ؒ ؒ ؒ S axis,
Introduction
its oxidized radical cation species is planar in the solid salt
There is currently a great deal of interest in the design and
ϩ
Ϫ 11
[(CH3O)2H2C6S–SC6H2(OCH3)2] ؒSbCl6
.
᝽
synthesis of novel organic redox substances.1,2 The incorpor-
ation of polarizable heteroatoms, such as sulfur, within the
π-conjugation donor framework is regarded as a good strategy,
because they usually show lower ionization potentials than the
corresponding hydrocarbons, and they enhance intermolecular
interaction.3–5
On the other hand, it seems possible to use nonplanar
molecules as components of organic conductors if good
intermolecular interactions are achieved. Thus 2,3,6,9,10,13-
hexamethoxy[1,4]benzodithiino[2,3-b]thianthrene (3), which
OCH3
Recently, thianthrenes have gained new interest as com-
ponents of conducting organic materials, e.g. charge transfer
salts.6–10 Thus, the electron-rich derivative 2,3,7,8-tetramethoxy-
thianthrene (2) forms 1:1 charge transfer complexes with dif-
OCH3
OCH3
CH3O
CH3O
S
S
S
S
OCH3
3
S
S
R
R
R
R
can be viewed as two fused thianthrene units and crystallizes in
a trans-conformation (chain form), acts as a bifunctional donor
with 2,3-dichloro-5,6-dicyano-1,4-quinone (DDQ) to form a
charge transfer complex with 1:2 stoichiometry, each unit
being in contact with a DDQ molecule, the donor showing a
cis-conformation (boat form).10
Recently, we have become interested in the synthesis and
properties of these tetrathiapolycycles with a pyridine as the
central ring, taking advantage in their preparation of the good
electrophilic reactivity of 2,3,5,6-tetrachloropyridine (4).12 In
this way, we have prepared the novel di[1,4]benzodithiino-
[2,3-b:2,3-e]pyridine (5)12 by reaction of tetrachloropyridine 4
with benzene-1,2-dithiol, and the novel 16-butyldinaphtho-
[2Ј,3Ј:5,6][1,4]dithiino[2,3-b:2,3-e]pyridine (6)13 by reaction of
4-butyl-2,3,5,6-tetrachloropyridine with naphthalene-2,3-
1 R = H
2 R = OCH3
ferent acceptors,6–8,10 and one-electron oxidation of thianthrene
(1) under aprotic conditions with AlCl3–CH2Cl2 yields a stable
violet radical cation salt.9
Planarization of the donor π-system is an important condi-
tion for electrical conductivity as it usually facilitates the over-
lapping of the molecular orbitals. In this context, the molecular
structure of thianthrene presents a butterfly shape with a
dihedral angle between phenyl groups of 128Њ in its neutral
form, but it is considerably flattened upon removal of one elec-
tron to form the stable radical cation.9 Similarly, while neutral
J. Chem. Soc., Perkin Trans. 2, 1999, 1503–1512
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Cl
Cl
Cl
Cl
If equimolecular mixtures of pyridine 4 and dithiol 9 react
under similar conditions, 2,3-dichloro-7,8-dimethyl[1,4]benzo-
dithiino[2,3-b]pyridine (11) is obtained in good yield along with
a moderate yield of 2,2Ј-(4,5-dimethyl-1,2-phenylenedithio)-
bis(3,5,6-trichloropyridine) (13) as a by-product. Similarly,
nucleophilic cyclization of pyridine 4 with dithiol 10 under
lower temperature conditions yielded 2,3-dichloro-7,8-di-
methoxy[1,4]benzodithiino[2,3-b]pyridine (12) in good yield,
and a small amount of byproduct 14. Compounds 11 and 12, as
intermediates, react further with more of the sodium salt of
thiols 9 and 10 to give 7 and 8, respectively. Starting from
2,3,5,6-tetrachloropyridine, the isolation of the byproducts 13
and 14 supports the fact that the initial attack on the pyridine 4
takes place at the most reactive 2-position, as was reported for
the compound 5. Dithiols 9 and 10 were prepared according to
well-known procedures, starting from o-xylene and catechol,
respectively, as outlined in Scheme 2.
S
S
S
S
N
N
4
5
C4H9
S
S
S
S
N
6
dithiol, in dimethylformamide (DMF) and in the presence of
an excess of sodium hydrogen carbonate (NaHCO3), with excel-
lent yields. The molecular structures of these neutral penta-
cyclic molecules show two folds along the S ؒ ؒ ؒ S lines of the S-
containing six-membered rings, with general chair-shaped con-
formations. The electron paramagnetic resonance spectra of the
stable oxidized species, 5 ϩ and 6 ϩ, show uniform distributions
R
R
Br
i
᝽
᝽
of the spin-density over the whole molecule, supporting the
planarity of the molecules upon one-electron removal.
The poor solubility of 5 in common organic solvents and its
moderately high oxidation potential have prompted us to intro-
duce electron donating groups as substituents in the extreme
benzo positions. Herein, we report on the synthesis and physical
properties of two examples of tetrasubstituted di[1,4]benzo-
dithiino[2,3-b:2,3-e]pyridines with methyl and methoxy groups
in the 2, 3, 9 and 10 positions, as well as preliminary results for
their oxidized species as dication salts, and the complexes
obtained in fluid solutions with the strong electron acceptor
DDQ.
Br
R
R
ii
R
R
SH
SH
R
SBu
SBu
iii
R
9 R = CH3
10 R = OCH3
Scheme 2 Reagents: i, Br2; ii, n-BuSCu, quinoline–pyridine; iii, Na,
liquid NH3.
Cyclic voltammetry (CV)
Synthesis
Compound 7. Cyclic voltammograms for the oxidation of
compound 7 (0.5 × 10Ϫ3 M) in CH2Cl2 containing tetra-n-
butylammonium perchlorate (TBAP) (~ 10Ϫ1 M) at 25 ЊC dis-
played two consecutive redox couples, O1/R1 and O2/R2, as
shown in Fig. 1(b). These two pairs were recorded at scan rates
(ν) ranging between 20 and 200 mV sϪ1. The two consecutive
oxidation peaks, O1 and O2, showed similar heights, indicating
that the same number of electrons is involved in both processes.
The preparation of the di[1,4]benzodithiino[2,3-b:2,3-e]pyrid-
ines 7 and 8 was performed by reaction of pyridine 4 with
4,5-dimethylbenzene-1,2-dithiol (9)14 and 4,5-dimethoxy-
benzene-1,2-dithiol (10),15 respectively, using the same method
reported for the synthesis of 5. Thus, pyridine 4 with two
equivalents of aromatic dithiol 9 or 10 in DMF and in the
presence of an excess of NaHCO3, to generate the sodium salts
of the thiols, gave 7 or 8, respectively, in good yield (Scheme 1).
a
The |Ip |/Ipc ratio for the O1/R1 couple (Ipa = anodic peak current
of O1 peak, Ipc = cathodic peak current of R1 peak) was always
Cl
R
S
Cl
Cl
Cl
Cl
R
SH
SH
a
c
close to 1. However, the |Ip |/Ip ratio for the O2/R2 couple was
only slightly increased from ca. 0.4 with increasing ν. In add-
+
Cl
N
S
N
R
R
a
ition, the |Ip |/ν1/2 function for both oxidation peaks gradually
4
11 R = CH3
12 R = OCH3
decreased as ν increased. Under these conditions, both couples
9 R = CH3
10 R = OCH3
a
showed a shift in the anodic peak potential (Ep ) to more
c
positive values and a shift in the cathodic peak potential (Ep ) in
the cathodic direction. This causes a progressive increase in the
c
difference (Epa Ϫ Ep ), starting from ca. 80 mV at 20 mV sϪ1
.
R
R
S
S
S
R
The standard potential EЊ for each couple was then calculated
c
using (Epa ϩ Ep )/2, remaining largely constant for all ν values
tested with a precision lower than 10 mV, indicating that
N
S
R
a
c
the shift in Ep and Ep with increasing ν is mainly due to the
quasi-reversibility of the charge transfer processes.
7 R = CH3
8 R = OCH3
These results agree with the theoretical behaviour expected
for quasi-reversible one-electron systems.16 Thus, 7 undergoes
two consecutive charge transfer reactions. The O1/R1 couple
with standard redox potential EЊ1 = 1.17 V vs. SSCE can be
Cl
Cl
Cl
Cl
Cl
ascribed to the equilibrium reaction between the neutral sub-
Cl
N
N
S
S
ϩ
strate and its radical cation: 7
7
ϩ ϩ 1e. The species 7
᝽
᝽
shows great stability in solution, at least during the CV meas-
urements, being the electroactive species of the second oxid-
ation process, where it produces its dication 72ϩ. The O2/R2
R
R
couple with standard redox potential EЊ2 = 1.37 V vs. SSCE is
ϩ
then due to the equilibrium reaction: 7
7
2ϩ ϩ 1e. The
᝽
13 R = CH3
14 R = OCH3
᝽
dication 72ϩ is not as stable as the radical cation 7 ϩ in solution
and disappears from the medium, probably via hydrolysis with
Scheme 1
1504
J. Chem. Soc., Perkin Trans. 2, 1999, 1503–1512

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behavior of this compound was investigated in CH2Cl2 using
n-tetrabutylammonium hexafluorophosphate (TBAPF6) (0.1
M) as supporting electrolyte, in the presence of an excess of
neutral alumina. This electrolyte was chosen to avoid the pos-
Ϫ
sible nucleophilic attack of ClO4 anion on 8 ϩ, while neutral
᝽
alumina was added as desiccant.19 All cyclic voltammograms
recorded for a solution of 8 (10Ϫ3 M) in this medium showed
two consecutive O1/R1 and O2/R2 redox couples, with respective
standard potentials EЊ1 = 1.03 V vs. SSCE (similar to 1.05 V
found in CH2Cl2 with 0.1 M TBAP) and EЊ2 = 1.19 V vs. SSCE.
The R1 and O2 peaks exhibited similar heights, ca. 0.4 times
lower than that of the O1 peak, independent of the scan rate
c
tested. The difference (Epa Ϫ Ep ) for both couples was always
80–90 mV. This makes it impossible to determine their corre-
sponding ks values, since the charge transfers related to the
oxidation and reduction peaks of 8 are faster than those of the
analogous peaks of 7. Thus, the O1/R1 couple for 8 can be
associated with the electrode process: 8
O2/R2 pair can be ascribed to the equilibrium reaction:
8
ϩ ϩ 1e, while its
᝽
ϩ
8
8
2ϩ ϩ 1e. The close proximity between both couples,
᝽
along with the much lower height of the R1 and O2 peaks with
respect to that of the O1 peak, even in the presence of neutral
alumina, suggests a real instability of 8 ϩ, which could undergo
᝽
a chemical disproportionation process as follows: 2 8 ϩ→8 ϩ
᝽
82ϩ. The dication 82ϩ is also unstable in the medium: its reduc-
tion peak R2 is only detected in the presence of neutral alumina,
indicating its fast hydrolysis with water impurities. In addition,
the height of the R2 peak is not much greater than that of the
O2 peak, suggesting that part of the 82ϩ formed by oxidation
ϩ
and/or disproportionation of 8 disappears via precipitation
due to its low solubility.
᝽
Compounds 2 and 5. CV results for the oxidation of com-
pounds 2 and 5 in CH2Cl2 with TBAP (0.1 M), under the same
conditions as for 7 and 8, are also reported for the sake of
comparison. Compound 2 exhibited two consecutive redox
couples. Its first pair, O1/R1, was observed at all scan rates, with
a standard potential EЊ1 = 0.87 V vs. SSCE (lit.,7 0.88 V), and
Fig. 1 Cyclic voltammograms for the oxidation of: (a) a 0.5 mM solu-
tion of 5 in CH2Cl2 with 0.1 M of TBAP on Pt, initial and final poten-
tial 0.900 V, reversal potentials (–〉–〉–) 1.400 V, (–〉–〉–〉–) 1.650 V; (b) a 0.5 mM
solution of 7 in CH2Cl2 with 0.1 M of TBAP on Pt, initial and final
potential0.800V,reversalpotential(–〉–〉–)1.300V,(–〉–〉–〉–)1.550V.Scanrate
100 mV sϪ1. T 25 ЊC.
corresponds to the equilibrium reaction: 2
2
ϩ ϩ 1e.
᝽
Although the second oxidation peak O2 had a similar height to
that of the O1 peak, its concomitant reduction peak R2 was only
detected at a scan rate of 200 mV sϪ1. This O2/R2 couple with
the trace amounts of water present17 and/or by precipitation
due to its insolubility.
In view of the quasi-reversible behavior observed for the two
one-electron redox processes of 7, Nicholson’s theory18 was
applied to calculate the standard rate constants ks for each
couple, according to eqn. (1), where the ψ-parameter for a given
EЊ2 = 1.22 V vs. SSCE (lit.,7 1.27 V) is related to the electrode
ϩ
process: 2
2
2ϩ ϩ 1e. The disappearance of the R2 peak
᝽
at low scan rates is due to the instability of dication 22ϩ, which
is probably hydrolysed by water present in the medium.
A typical cyclic voltammogram recorded for 5 is presented in
Fig. 1(a). It reveals the presence of two consecutive redox pairs,
which behave as quasi-reversible one-electron systems, in a simi-
lar way to that described above for the two analogous couples
of 7. While the first couple, O1/R1 with EЊ1 = 1.27 V vs. SSCE
(lit.,11 1.07 V in acetonitrile with LiClO4), related to the process
πnFνD
ks = ψ
(1)
Ί
RT
ν was taken from the ψ vs. (Epa Ϫ Ep ) plot calculated by
c
Nicholson for a transfer coefficient α = 0.5. The diffusion co-
5
5
ϩ ϩ 1e, was recorded for all scan rates tested, the sec-
efficient D for 7 was assumed to be equal to that of 7 ϩ and 72ϩ
.
᝽
᝽
This D value is 2.3 × 10Ϫ5 cm2 sϪ1, as determined by applying
ond pair O2/R2 with EЊ2 = 1.49 V vs. SSCE, associated with the
reaction 5
ϩ
5
2ϩ ϩ 1e, was only clearly detected above
the Randles–Sevcik equation16 ( |Ip | = 2.69 × 105 n3/2AD1/2cν1/2
,
᝽
a
ν = 50 mV sϪ1, because the R2 peak was not observed at lower ν
values. Note that the O2 peak for 5 is totally irreversible below
200 mV sϪ1 when recorded in acetonitrile.12 These results in
CH2Cl2 and acetronitrile indicate the instability of dication 52ϩ
to reaction with water impurities in the medium. The standard
rate constants ks for the two couples of 5 were also calculated
using Nicholson’s theory.17 A D value of 9.4 × 10Ϫ5 cm2 sϪ1 was
determined for this compound by applying the Ranles–Sevcik
equation16 to the O1 peak at 20 mV sϪ1. Then, from eqn. (1), ks
values of (8.4 0.2) × 10Ϫ3 cm2 sϪ1 and (1.11 0.02) × 10Ϫ2
cm2 sϪ1 were calculated for the O1/R1 and O2/R2 couples of 5.
These ks values are of the same order of magnitude as those
obtained for the analogous couples of 7, although the diffusion
coefficient for 5 is about four times higher than that of 7, as can
be deduced from Fig. 1, because of its smaller size.
where A is the electrode area and n = 1) to the O1 peak at 20 mV
s
Ϫ1, where it shows a better reversibility. From eqn. (1), similar
ks values of (8.1 0.3) × 10Ϫ3 cm2 sϪ1 and (6.2 0.2) × 10Ϫ3
cm2 sϪ1 were obtained for the O1/R1 and O2/R2 couples of 7.
Compound 8. Cyclic voltammograms for the oxidation of
compound 8 (0.5 × 10Ϫ3 M) in CH2Cl2 containing TBAP
(~ 10Ϫ1 M) at 25 ЊC exhibited two consecutive and partially
overlapped oxidation peaks, O1 and O2, and a small reduction
peak R1, associated with the O1 peak. In fact, the O2 and R1
peaks have similar height and practically disappear at scan rates
ν < 50 mV sϪ1, increasing in intensities with increasing ν. This
fact suggests that the electroactive species giving rise to the O2
and R1 peaks, which should correspond to the radical cation
8
ϩ, is not stable in the medium tested. For this reason, the CV
᝽
J. Chem. Soc., Perkin Trans. 2, 1999, 1503–1512
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Table 1 Standard potentials for the two consecutive couples deter-
mined by CV for the oxidation of 5, 7 and 8, and tetramethoxy-
thianthrene 2 in CH2Cl2 with 0.1 M TBAP at 25 ЊC
Table 2 Selected bond lengths for 7
Atom 1
Atom 2
Bond length/Å
EЊ/V vs. SSCE
C11
C11
C11
C12
C12
C13
C13
C14
C14
C15
C15
C16
C17
C17
C17
C18
C18
C18
S1
H11
C12
C16
C13
C17
C14
C18
H14
C15
C16
S2
0.930(4)
1.389(4)
1.389(4)
1.404(4)
1.509(4)
1.382(4)
1.512(4)
0.930(4)
1.393(3)
1.388(3)
1.766(3)
1.769(2)
0.960(6)
0.960(6)
0.960(11)
0.960(8)
0.960(11)
0.960(11)
1.761(3)
1.759(2)
1.337(3)
1.335(3)
1.404(3)
1.377(3)
0.930(3)
1.377(3)
1.398(3)
1.762(3)
1.763(2)
1.765(3)
1.768(2)
0.930(4)
1.381(4)
1.387(3)
1.394(4)
1.509(4)
1.394(4)
1.506(4)
0.930(4)
1.382(4)
1.393(3)
0.960(11)
0.960(8)
0.960(10)
Compound
O1/R1 couple
O2/R2 couple
∆EЊ/V
5
7
8
1.27
1.17
1.05
1.03b
0.87
1.49
1.37
0.22
0.20
—
a
—
1.19b
1.22
0.16b
0.35
2
a No R2 peak was observed; b Determined in 0.1 M TBAPF6 in presence
of neutral alumina.
S1
H17A
H17B
H17C
H18A
H18B
H18C
C22
C23
C22
C26
C23
C24
H24
C25
C26
S4
S2
N21
N21
C22
C23
C24
C24
C25
C25
C26
S3
S3
C36
C35
H31
C32
C36
C33
C37
C34
C38
H34
C35
C36
H37A
H37B
H37C
S4
Fig. 2 Molecular structure of 7.
C31
C31
C31
C32
C32
C33
C33
C34
C34
C35
C37
C37
C37
Table 1 summarizes the standard potentials for the two con-
secutive redox couples of compounds 5, 7 and 8, and of the
tetramethoxythianthrene 2. An analysis of these results for 5, 7
and 8 suggests that the presence of methyl and methoxy sub-
stituents, as electron donors, increases the oxidative ability of
both the neutral compounds and their radical cations. This
effect is more pronounced in the case of the methoxy substitu-
ent than in that of the methyl substituent. The last column of
Table 1 shows the difference ∆EЊ ( = E2Њ Ϫ E1Њ) between the
standard potentials of both couples. While ∆EЊ remains prac-
tically constant, close to 0.20 V, on going from 5 to 7, it
decreases significantly in the tetramethoxy derivative 8 as a
consequence of the lower intramolecular repulsion (Coulomb
energy) between the positive charges in the dication 82ϩ. In addi-
with four molecules in the elemental cell. All bond lengths and
angles are shown in Tables 2 and 3, respectively. The asym-
metric unit is the whole molecule without any crystallographic
inversion center. The pyridine and the two benzo rings are
planar with rms deviations of 0.009, 0.011 and 0.007 Å,
respectively. The two six-membered rings containing the sulfur
atoms posses a boat conformation, and the S1, S2, S3 and S4
atoms deviate by 0.017, 0.062, Ϫ0.132 and Ϫ0.172 Å from the
pyridine mean plane, respectively. The folding of the dithiine
rings along the S ؒ ؒ ؒ S axes (dihedral angle between S1–C16–
C15–S2 and S1–C22–C23–S2 mean planes) is 133.7(1)Њ. The
pentacyclic molecule has a general chair-shaped conformation
with interplanar angles between benzo rings and pyridine of
139.9(1)Њ and 141.4(1)Њ.
+
OCH3
OCH3
CH3O
CH3O
S
S
+
22+
+
S
OCH3
OCH3
S
CH3O
CH3O
S
+
S
N
82+
Compound 8. The molecular structure of compound 8 is dis-
played in Fig. 3 with the atom numbering scheme. The molecule
crystallizes in the triclinic system, space group P1 (no. 1), with
tion, this difference for 8 (∆EЊ = 0.16 V) is much lower than for
2,3,7,8-tetramethoxythianthrene 2 (∆EЊ = 0.35 V), due to the
much shorter distance between the positive charges in dication
¯
one molecule in the elemental cell. All bond lengths and angles
are shown in Tables 4 and 5, respectively. In this case, the
asymmetric unit is half a molecule, and the molecule without a
centrosymmetric center possesses a crystallographic inversion
center in the middle of the pyridine ring. Positional disorder is
present in the molecule, placing the nitrogen in either of the two
possible positions with the same probability. The pyridine and
the two benzo rings are planar with rms deviations of 0.010 Å
22ϩ
.
Molecular structures
Compound 7. The molecular structure of compound 7 is
shown in Fig. 2 with the atom numbering scheme. The mole-
cule crystallizes in the monoclinic system, space group P21/n,
1506
J. Chem. Soc., Perkin Trans. 2, 1999, 1503–1512

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Table 3 Selected bond angles for 7
Table 4 Selected bond lengths for 8
Atom 1
Atom 2
Atom 3
Bond angle (Њ)
Atom 1
Atom 2
Bond length/Å
C12
C11
C11
C13
C12
C12
C14
C13
C14
C14
C16
C11
C15
C11
C16
C15
S1
C11
C12
C12
C12
C13
C13
C13
C14
C15
C15
C15
C16
C16
C16
S1
C16
C17
C13
C17
C18
C14
C18
C15
S2
C16
S2
C15
S1
121.7(3)
119.9(2)
118.7(2)
121.3(2)
120.6(2)
119.2(2)
120.2(2)
121.8(2)
118.7(2)
118.9(2)
122.3(2)
119.6(2)
122.4(2)
117.9(2)
102.2(1)
101.9(1)
115.3(2)
122.5(2)
122.2(2)
122.2(2)
118.2(2)
119.5(2)
119.8(2)
119.3(2)
118.4(2)
122.1(2)
122.6(2)
122.2(2)
115.1(2)
102.4(1)
102.0(1)
121.7(2)
119.85(2)
119.3(2)
120.9(2)
121.6(2)
119.0(2)
119.4(2)
121.4(2)
118.5(2)
119.5(2)
122.0(2)
119.0(2)
122.3(2)
118.6(2)
C11
C11
C11
C12
C12
C13
C13
C14
C14
C15
C15
C16
C17
C17
C17
C17
C18
C18
C18
C18
S1
H11
C12
C16
C13
O1
C14
O2
H14
C15
C16
S2
S1
O1
H17A
H17B
H17C
O2
H18A
H18B
H18C
C22
C23
C22
C26
C23
C24
H24
C25
0.970(2)
1.385(3)
1.395(3)
1.411(3)
1.356(3)
1.377(3)
1.362(2)
1.010(3)
1.400(3)
1.382(3)
1.765(2)
1.768(2)
1.423(3)
0.960
S1
C22
C23
N21
C23
C23
C22
C24
C24
C25
S4
C26
S4
C25
S3
0.960
0.960
1.422(3)
0.960
0.960
S2
C22
C22
C22
C23
C23
C23
C24
C25
C25
C25
C26
C26
C26
S3
N21
S1
S2
C22
S2
0.960
1.761(2)
1.763(2)
1.310(3)
1.420(2)
1.394(3)
1.260(3)
0.890(6)
1.440(4)
S2
C23
C24
C24
C26
N21
C25
N21
C26
C25
C32
C31
C31
C33
C32
C32
C34
C33
S4
N21
N21
C22
C23
C24
C24
S3
C36
C35
C36
C37
C33
C37
C38
C34
C38
C35
C34
C36
C36
C35
C35
C31
Table 5 Selected bond angles for 8
S4
C31
C32
C32
C32
C33
C33
C33
C34
C35
C35
C35
C36
C36
C36
Atom 1
Atom 2
Atom 3
Bond angle (Њ)
C23
C22
C12
C13
C22
C23
C12
C13
O2
S2
S1
O1
O2
C15
C16
C17
C18
C23
C22
C16
C15
C14
C12
C12
C14
S2
100.93(10)
100.63(9)
117.5(29)
117.7(2)
118.2(18)
120.6(28)
120.1(2)
120.3(29)
125.0(2)
115.1(2)
120.0(2)
119.8(2)
121.7(2)
118.5(2)
120.3(2)
121.3(2)
118.3(2)
125.2(2)
115.3(2)
119.5(2)
122.2(19)
119.8(12)
116.7(19)
121.0(2)
116.2(10)
121.7(2)
N21
C24
C11
C14
C13
C13
C13
C15
C15
C15
C16
C16
C16
C12
C12
C12
C23
C22
C23
C23
C22
C22
C34
S4
C31
S3
O2
S3
C14
C16
C16
C14
C15
C15
C11
O1
S2
C11
S1
S1
C11
C13
C13
C22
N21
S2
S2
S1
S1
O1
C11
C24
C23
C24
C22
N21
C23
pyridine mean plane, respectively. The pentacyclic molecule has
a general chair-shaped conformation with the same value of
133.7Њ for the interplanar angles between the benzo rings and
pyridine.
Hexachloroantimonate salts of 72؉ and 82؉
Fig. 3 Molecular structure of 8.
Compounds 7 and 8 yielded stable dication salts upon oxid-
ation with antimony pentachloride (SbCl5). SbCl5 was added
slowly to slightly yellow solutions of substrates in freshly dis-
tilled CH2Cl2, which were deoxygenated by bubbling argon
through them for 5 min. The solutions instantaneously dark-
ened and solids precipitated in excellent yield. From 7, a green
for the two benzo rings and 0.004 Å for the pyridine. The fold-
ing of the dithiine rings along the S ؒ ؒ ؒ S axes, as defined above,
is 129.1(1)Њ. As in the case of 7, the two six-membered rings
containing the sulfur atoms posses a boat conformation, and
the S1 and S2 atoms deviate 0.068 and 0.083 Å from the
J. Chem. Soc., Perkin Trans. 2, 1999, 1503–1512
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Fig. 4 (a) EPR spectrum of radical cation 7 ϩ generated from a solu-
ؒ
tion of 7 in trifluoroacetic acid containing thallium() trifluoroacetate
ϩ
ؒ
Fig. 5 (a) EPR spectrum of radical cation 8 generated from an
irradiated solution of 8 in CH2Cl2 containing trifluoroacetic acid (10%)
at rt. (b) Computer simulation using the data given in the text.
at rt. (b) Computer simulation using the data given in the text.
solid was separated and characterized as the dication salt
72ϩؒ2SbCl6Ϫ, and from 8, a dark brown solid was separated and
characterized as the dication salt 82؉ؒ2SbCl6Ϫ. Both salts are
active under EPR analysis displaying under conditions of
high amplification single, broad peaks (∆Hpp ~ 16.5 G for
and the last value to the four hydrogens in positions 1, 4, 8 and
11. Under conditions of high amplification, the hfs values due
to the coupling of the electron with naturally abundant 33S
(0.76%) were observed as two multiplet satellite lines [a(33S) ~
4.7 G] which should correspond to the MS = 3/2 groups of the
quartet [I(33S) = 3/2)], the two MS = 1/2 groups being lost in the
envelope of the very intense central multiplet.
Ϫ
72؉ؒ2SbCl6 and ∆Hpp ~ 16.2 G for 82؉ؒ2SbCl6Ϫ) without
any structure, which should correspond to the residual radical
cations present at the samples.
Spectral properties of mixtures of 7 and 8 with 2,3-dichloro-5,6-
dicyano-1,4-quinone (DDQ)
Similarly, a single, broad line (g = 2.0066; ∆Hpp = 3.4 G) of a
persistent radical cation was detected from dilute sulfuric acid
solutions of 8. A highly resolved spectrum (g = 2.0072; ∆Hpp
=
Preliminary results obtained when the strong electron acceptor
2,3-dichloro-5,6-dicyano-1,4-quinone (DDQ) was added into
diluted solutions (~ 10Ϫ2 M) of 7 and 8 in CHCl3 indicated the
existence of charge transfer complexes. Thus, both substrates in
the presence of DDQ developed in their electronic spectra a
weak and very broad band with maxima at λ = 749 nm and
λ = 807 nm for 7 and 8, respectively. The DDQ charge transfer
complex with the unsubstituted species 5 also gave rise to a
broad and less intense absorption band, hypsochromically
shifted at λ = 705 nm. Only in the case of 8, when solutions in
0.08 G) was achieved at room temperature after prolonged UV
irradiation of degassed solutions of 8 in distilled CH2Cl2 con-
taining trifluoroacetic acid (10%) [Fig. 5(a)]. The resolution of
the spectrum was not sensitive to lowering of the temperature,
and it was analyzed via a computer simulation [Fig. 5(b)] using
the following hyperfine data: a(1H) = 0.42 G; a(1N) = 0.08 G;
a(12H) = 0.21 G; a(4H) = 0.63G. This spectrum was attributed
ϩ
to the radical cation 8 and the assignment of the hfs con-
᝽
stants to sets of equivalent nuclei is straightforward: the one
proton coupling constant value corresponds to the hydrogen
in position 13 and the values for four hydrogens to those in
positions 1, 4, 8 and 11. Under conditions of over-modulation
and high amplification, the hfs due to coupling with 33S
nuclei appeared as two broad satellite lines [a(33S) ~ 4.2 G],
corresponding to the two MS = 3/2 groups of the quartet, as
CH2Cl2 were examined by EPR spectroscopy, a weak signal
ϩ
(g = 2.0072) due to the radical cation 8 appeared together
᝽
with the stronger signal (g = 2.0050) of the DDQ radical anion.
However, all attempts to isolate solid complexes have been
unsuccessful so far, only crystalline 8 being separated from
those solutions.
ϩ
in 7 ϩ. In both radical cations 7 and 8 ϩ, the magnitude
᝽
᝽
᝽
of the splitting constant a(33S) is consistent with the
Electron paramagnetic resonance (EPR) spectroscopy
ϩ
value observed for the unsubstituted radical cation 5
,
᝽
A single, broad line (g = 2.0075; peak to peak linewidth,
∆Hpp = 5.75 G) of a stable radical cation was detected by EPR
from a dilute sulfuric acid solution of 7. A highly resolved EPR
spectrum (g = 2.0077; ∆Hpp = 0.08 G) at room temperature was
obtained when degassed solutions of 7 in trifluoroacetic acid
were treated with thallium() trifluoroacetate. This spectrum
a(33S) = 4.7 G.12
When the radical cation of the parent compound 5 was
obtained and characterized by EPR for the first time, there was
an inconsistency in the simulated spectrum, as was pointed
out.12 A thorough and more accurate analysis has prompted
us to review and improve its spectral parameters. In Fig. 6, the
was undoubtedly due to 7 ϩ [Fig. 4(a)] and was simulated [Fig.
observed and simulated spectra of 5 are displayed. The fol-
ϩ
᝽
᝽
4(b)] using the following hyperfine splitting (hfs) constant
values: a(1H) = 0.50 G; a(1N) = 0.09 G; a(12H) = 0.80 G;
a(4H) = 0.11 G. The one proton coupling constant value should
be assigned to the hydrogen in position 13 in the pyridine ring,
lowing values have been used in the simulation: ∆Hpp = 0.18 G;
a(1H) = 1.0 G (hydrogen in position 13); a(1N) = 0.67 G;
a(4H) = 0.58 G (hydrogens in positions 2, 3, 9, 10); a(4H) = 0.13
G (hydrogens in positions 1, 4, 8, 11).
1508
J. Chem. Soc., Perkin Trans. 2, 1999, 1503–1512

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Table 6 MNDO optimized interplanar angles between benzo rings
Table 7 MNDO optimized bond lengths in the neutral 7 and oxidized
ϩ
ϩ
and pyridine in the neutral 7 and oxidized 7 and 72ϩ. Experimental
7
and 72ϩ. Experimental X-ray data for the neutral molecule are in
parentheses
ؒ
ؒ
X-ray data for the neutral molecule are in parentheses
Angle (Њ)
Bond length/Å
ϩ
ϩ
72ϩ
Bonds
7
7
72ϩ
ؒ
ؒ
Ring
7
7
Benz 1-Py
Benz 2-Py
160.1 (141.4)
159.8 (139.9)
176.8
178.3
176.8
178.4
C15–C16
C15–S2
S2–C23
C22–C23
C22–S1
C25–C26
C25–S4
S4–C35
C35–C36
S3–C36
1.413 (1.388)
1.691 (1.766)
1.685 (1.759)
1.419 (1.404)
1.691 (1.761)
1.418 (1.398)
1.685 (1.762)
1.690 (1.768)
1.414 (1.393)
1.694 (1.765)
1.421
1.682
1.657
1.447
1.664
1.447
1.659
1.681
1.420
1.690
1.426
1.677
1.645
1.462
1.651
1.463
1.646
1.674
1.428
1.679
Fig. 6 (a) EPR spectrum of radical cation 5 ϩ. (b) Computer
ؒ
simulation using the data given in the text.
Fig. 8 Minimal energy conformations of the MNDO equilibrium
geometries calculated for (a) and (b) 7, (c) 7 ϩ and (d) 72ϩ
.
ؒ
HOMO. Thus, the most important effect is the planarization
ϩ
of the molecule which tends to stabilize the radical cation 7
᝽
Fig. 7 Electronic structure of the HOMO of 7.
[Fig. 8(c)] and the dication 72ϩ [Fig. 8(d)] by extending
the conjugation along the whole molecule. In Table 6, the
Theoretical calculations
displayed values of the interplanar angles in the oxidized
ϩ
species 7 and 72ϩ are practically 180Њ. This planarization
In order to rationalize the experimental results, the geometrical
and electronic structures of 7 were investigated by performing
᝽
process is made possible by the lengthening and shortening of
those bonds in the S-containing rings which correspond in
the HOMO to bonding and antibonding character, respect-
ively. The variation of these bond lengths is shown in Table 7,
where the values for the dithiine rings of the neutral and
oxidized species are collected together with those from the
solid state analysis of the neutral molecule. Note that the
C–C and C–S bond lengths computed at the MNDO level for
the neutral compound are slightly over- and under-estimated,
respectively, with respect to the experimental values. However,
there is a good correlation between predicted and experimental
bond lengths.
molecular orbital calculations, using the semiempirical MNDO
method.20 In the same way, geometries of the radical cation 7
ϩ
᝽
and the dication 72ϩ were also fully optimized by the MNDO
method to observe the effects of the oxidation. Fig. 7 shows the
atomic coefficients of 7 in the HOMO, indicating that this
orbital is mainly located on the two S-containing rings.
Theoretical calculations predict that the minimum energy
geometries obtained for the neutral molecule correspond to
those in which the molecule is folded in the opposite direction
[Fig. 8(a)] or in same direction [Fig. 8(b)] along the S ؒ ؒ ؒ S
axes in each dithiine ring, the second conformation being
higher in energy (0.4 kcal molϪ1) than the first. This result con-
firms that obtained by X-ray analysis. In Table 6, the theoretical
data for the interplanar angles between the benzo rings and
pyridine in the neutral molecule and oxidized species are com-
pared with those obtained from the crystalline structure.
The oxidation process in the molecule mainly affects the
dithiine rings, which is in accord with the localization of the
Summary and conclusions
We have prepared two polyheterocyclic organic compounds 7
and 8 with π-conjugated electronic structures, derived from
the novel di[1,4]benzodithiino[2,3-b:2,3-e]pyridine reported
by us recently,12 as a new class of molecular donors. In both
J. Chem. Soc., Perkin Trans. 2, 1999, 1503–1512
1509

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derivatives 7 and 8, the solubility in common organic solvents is
improved with respect to that of the parent compound. The
bidentate nucleophiles benzene-1,2-dithiol 9 and 10 reacted
with 2,3,5,6-tetrachloropyridine to give the unsymmetrically
substituted species 11 and 12, respectively, or the symmetrical
species 7 and 8, respectively, depending on the stoichiometries
of the substitution reactions.
The X-ray structures of both molecules 7 and 8 show chair-
shaped conformations with the two dithiine rings folded in
opposite directions along the S ؒ ؒ ؒ S line. Theoretical calcu-
lations performed on 7 predict that the completely planar con-
formation is about 3.4 kcal molϪ1 more energetic than the
chair-shaped conformation, but when the molecule is oxidized
by removal of one or two electrons, the two dithiines are con-
siderably flattened to stabilize the charged species by extending
the π-conjugation.
An EPR analysis of the stable radical cations of both mol-
ecules, obtained by smooth oxidation in fluid solutions,
displays high isotropic g-values that indicate a considerable
spin–orbit coupling with the sulfur atoms in the molecule. This
result is in agreement with the fact that the HOMO is mainly
located on the dithiine rings, as is predicted by the semiem-
pirical MNDO method for 7. The hyperfine structures of the
signals in the EPR demonstrate that the singly-occupied
molecular orbital (SOMO) in each species is extended over
the whole molecule. The values of the free electron coupling
with 33S nuclei are approximately half the values observed in
the [1,4]benzodithiino[2,3-b]pyridine and thianthrene radical
cations. This is why 7 and 8 can be properly viewed as two
Experimental
Melting points were obtained by using a Köfler microscope
“Reichert” and are uncorrected. The electronic spectra were
recorded with a Perkin-Elmer Lambda Array 3840 spec-
trometer coupled with a Perkin-Elmer 7300 computer, and the
IR spectra with a FT-IR “Bomem-Michalson” model MB-120
1
spectrophotometer. H NMR spectra were determined at 200
and 300 MHz with Varian Gemini 200HC and 300HC spec-
trometers, respectively, and 13C NMR spectra at 75 MHz with a
Varian Gemini 300HC spectrometer. All the NMR spectra were
recorded in CDCl3 with tetramethylsilane as standard. EI mass
spectra were obtained on a VG-AutoSpec-Q (Micromass,
Manchester) by electron impact at 70 eV; the ion source was set
to 200 ЊC and the current trap was 30 µA.
Dimethylformamide (DMF) was used as received. All the
solvents used under anhydrous conditions were dried and dis-
tilled before use. CHCl3 and CH2Cl2 were dried over calcium
chloride, and distilled. Pyridine was dried over potassium
hydroxide and distilled, and quinoline was distilled at reduced
pressure. Trifluoroacetic acid and sulfur() chloride were dis-
tilled before use.
4,5-Dimethylbenzene-1,2-dithiol (9) was prepared according
to the method of Klinsberg:14 bromination of o-xylene gave
4,5-dibromo-o-xylene which yielded 1,2-di-n-butylthio-4,5-
dimethylbenzene by reaction with copper() n-butanethiolate.
Debutylation to the dithiol 9 was accomplished with sodium in
liquid ammonia.
4,5-Dimethoxybenzene-1,2-dithiol (10) was prepared
according to the method of Garner et al.:15 1,2-dibromo-4,5-
dimethoxybenzene was produced by prior dibromination
of catechol, followed by methylation of the hydroxy groups
with dimethyl sulfate in concentrated aqueous sodium hydrox-
ide. Subsequently, 1,2-dibromo-4,5-dimethoxybenzene was
converted into 1,2-di-n-butylthio-4,5-dimethoxybenzene by
reaction with copper() n-butanethiolate which yielded dithiol
10 by cleavage of the butyl groups with sodium in liquid
ammonia.
benzodithiinopyridines sharing a common pyridine ring. The
ϩ
four hydrogens in positions 1, 4, 8 and 11 in 8 are mag-
᝽
netically equivalent in their coupling with the unpaired elec-
tron (a = 0.63 G), and the twelve hydrogens of the four
ϩ
methyl groups in 7 also display the same coupling (a = 0.80
᝽
G). These observations suggest that a pyridine ring instead of
a benzo ring does not cause any appreciable asymmetry in
the SOMO. A similar observation was made in the thian-
threne series when the benzo ring is substituted by a pyrid-
ine.12 In this case, the spectrum of its radical cation consisted of
a quartet of lines corresponding to the coupling with the
hydrogens in positions 2, 3, 7 and 8. The narrow field range
Electrochemical measurements
The cyclic voltammetric (CV) experiments were carried out in a
three-electrode cell under an argon atmosphere. A Pt disk with
an area of 0.093 cm2 was used as the working electrode and a Pt
wire as the counter electrode. The reference electrode was a
SSCE (calomel electrode saturated with an aqueous solution of
NaCl) connected to the cell through a salt bridge containing a
CH2Cl2 solution of tetrabutylammonium perchlorate (TBAP)
(0.1 M). The temperatures of test solutions and of the SSCE
were kept at 25 ЊC. Solutions of the organic substrates (5 × 10Ϫ4
M) in CH2Cl2 containing TBAP (0.1 M) as background electro-
lyte were studied by CV. The volume of all test solutions was
25 mL. CV measurements were performed with standard
equipment consisting of a PAR 175 universal programmer, an
Amel 551 potentiostat, and a Phillips 8043 X-Y recorder. Cyclic
voltammograms of all solutions were recorded at scan rates (ν)
ϩ
(∆H = 9 G for 7 and ∆H = 5 G for 8 ϩ) over which the EPR
᝽
᝽
spectra extend is characteristic of these radical cations, and
demonstrates once more that the bulk of the spin population
resides in the dithiine rings. It is also in accord with the theor-
etical calculations which locate the HOMO of the neutral
molecule mainly in the S-containing rings. If the EPR
ϩ
ϩ
parameters of 7 and 8 are compared with those of the
᝽
᝽
parent radical cation 5 ϩ, there is a shifting of the spin dens-
᝽
ity from the pyridine to the external benzo rings, due to the
electron donating property of the substituents being greater
ϩ
.
in 8 ϩ than in 7
᝽
᝽
The values of the standard potentials for the first redox
couples in 7 and 8 (Table 1) indicate that these are not par-
ticularly strong donors, although the presence of electron-
donating substituents decreases their values with respect to the
parent compound 5, particularly when the methoxy group is
the substituent. So, a difference of approximately 0.2 V is
displayed between the values of the standard potentials of 5
and 8. Both derivatives 7 and 8 form stable 1:2 ionic salts
ranging between 20 and 200 mV sϪ1
.
EPR experiments
EPR spectra were recorded with a Varian E-109 spectrometer
working in the X band, using a Varian E-257 temperature con-
troller. Solutions (~ 10Ϫ3 M) of organic substrates in trifluoro-
acetic acid (TFA) or mixtures of CH2Cl2–TFA, containing
thallium() trifluoroacetate (TTFA), or in CH2Cl2 containing
AlCl3 or SbCl5 were placed in quartz EPR tubes, degassed by
three freeze-pump-thaw cycles or deoxygenated by bubbling
argon through them for 5 min, and then inserted into the EPR
cavity. The EPR simulations were carried out with the WinSIM
program, received free from The National Institute of
Environmental Sciences.
Ϫ
with SbCl6 as the counterion. On the other hand, mixtures
of each with DDQ, a strong electron acceptor, in CHCl3
solution show well-defined absorption maxima in the elec-
tronic spectra which can be attributed to the presence of
charge transfer complexes. EPR spectra of CH2Cl2 solu-
tions of 8, but not of 7, show a weak signal for the radical
ϩ
cation 8 together with a stronger one corresponding to
᝽
the radical anion of DDQ. However, all attempts to isolate
a charge transfer complex from 8 and DDQ have failed so
far.
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J. Chem. Soc., Perkin Trans. 2, 1999, 1503–1512

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Reaction of 2,3,5,6-tetrachloropyridine (4) and 4,5-dimethyl-
benzene-1,2-dithiol (1:1)
1050 (s), 1025 (m), 920 (w), 890 (m), 845 (m), 785 (m),
740 (w), 705 (m), 665 (w), 640 (m), 620 (w). (Anal. Calc. for
C18H10Cl6N2O2S2: C, 38.4; H, 1.8; Cl, 37.8; N, 5.0; S, 11.4.
Found: C, 38.6; H, 1.8; Cl, 37.7; N, 4.9; S, 11.4%). (b) 2,3-
Dichloro-7,8-dimethoxy[1,4]benzodithiino[2,3-b]pyridine (12)
(68 mg; 53%), mp 251–253 ЊC (from benzene); δH 7.76 (s, 1H),
6.97 (s, 1H), 6.95 (s, 1H), 3.88 (s, 6H); δC 156.5, 149.6, 149.5,
146.8, 131.6, 128.7, 125.2, 123.1, 112.2, 111.8, 56.3; IR (KBr)
νmax/cmϪ1 3030 (w), 2960 (w), 2930 (w), 2900 (w), 2840 (w), 1580
(s), 1510 (s), 1485 (s), 1455 (s), 1440 (s), 1425 (s), 1380 (m), 1365
(s), 1350 (s), 1335 (s), 1300 (s), 1255 (s), 1205 (s), 1175 (m), 1160
(m), 1070 (m), 1030 (s), 900 (m), 880 (m), 850 (s), 785 (m), 725
A mixture of 4,5-dimethylbenzene-1,2-dithiol (0.39 g; 2.3
mmol), pyridine 4 (0.5 g; 2.3 mmol), sodium bicarbonate (0.82
g), and DMF (25 mL) was stirred in argon at 100 ЊC for 2.5 h
and then at reflux for 4 h. The mixture was cooled, poured into
water, acidified with hydrochloric acid and filtered. The precipi-
tate was chromatographed (silica gel) eluting with CCl4–CHCl3
(4:1) to give: (a) 2,2Ј-(4,5-Dimethyl-1,2-phenylenedithio)bis-
(3,5,6-trichloropyridine) (13) (0.01 g, 1.2%); δH 7.60 (s, 2H),
7.49 (s, 2H), 2.32 (s, 6 H); δC 155.8, 146.2, 139.8, 138.4, 137.8,
131.2, 127.1, 126.1, 19.6 (Found: M 527.8416. C18H10Cl6N2S2
requires Mϩ 527.8405). (b) 2,3-Dichloro-7,8-dimethyl[1,4]-
benzodithiino[2,3-b]pyridine (11) (0.54 g; 65%), mp 216 ЊC
(from hexane); δH 7.72 (s, 1H), 7.22 (s, 1H), 7.21 (s, 1H), 2.23 (s,
6H); δC 156.2, 146.6, 137.5, 137.4, 137.0, 131.3, 130.5, 130.0,
129.5, 128.5, 19.2; IR (KBr) νmax/cmϪ1 2910 (w), 1510 (m), 1470
(m), 1440 (m), 1370 (s), 1350 (s), 880 (m), 870 (m), 670 (w); UV
(CHCl3) λmax/nm (ε dm3 molϪ1 cmϪ1) 333.5 (39 000) (Anal. Calc.
for C13H9Cl2NS2: C, 49.7; H, 2.9; Cl, 22.6; N, 4.4; S, 20.4.
Found: C, 49.7; H, 2.9; Cl, 22.6; N, 4.3; S, 20.4%). (c) 2,3,9,10-
Tetramethyldi[1,4]benzodithiino[2,3-b:2,3-e]pyridine (7) (0.17
g; 20%), mp 322 ЊC (from CH2Cl2); δH 7.62 (s, 1H), 7.22 (s, 2H),
7.19 (s, 2H), 2.21 (s, 12 H); δC 156.3, 137.1, 137.0, 134.4, 131.0,
130.0, 129.6, 129.4, 19.3; IR (KBr) νmax/cmϪ1 2910 (w), 1510
(w), 1470 (m), 1440 (m), 1360 (s), 1350 (s), 890 (m), 870 (m), 860
(m); UV (CHCl3) λmax/nm (ε dm3 molϪ1 cmϪ1) 350 (49 200)
(Anal. Calc. for C21H17NS4: C, 61.2; H, 4.1; N, 3.4; S, 31.2.
Found: C, 61.2; H, 4.0; N, 3.3; S, 31.2%).
(m), 670 (m), 625 (m); UV (CHCl3) λmax/nm (ε, dm3 molϪ1 cmϪ1
)
252 (29 200) (Anal. Calc. for C13H9Cl2NO2S2: C, 45.1; H, 2.6;
Cl, 20.5; N, 4.0, S, 18.5. Found: C, 45.0; H, 2.5; Cl, 20.6, N, 4.0,
S, 18.5%).
Reaction of 2,3,5,6-tetrachloropyridine (4) and 4,5-dimethoxy-
benzene-1,2-dithiol (2.5:1)
A mixture of 4,5-dimethoxybenzene-1,2-dithiol (1.0 g; 5.0
mmol), pyridine 4 (0.43 g; 2.0 mmol), sodium bicarbonate (1.44
g) and DMF (55 mL) was stirred in argon at 100 ЊC for 1.5 h
and then at reflux for 5 h. The mixture was cooled, poured into
water, acidified with hydrochloric acid and extracted with
CHCl3. The organic solution, washed with an excess of water,
dried over Na2SO4 and distilled at reduced pressure, yielded a
residue which was chromatographed (silica gel) eluting with
CHCl3 to give 2,3,9,10-tetramethoxydi[1,4]benzodithiino[2,3-b:
2,3-e]pyridine (8) (0.56 g; 60%), mp 305–307 ЊC (from hexane);
δH 7.69 (s, 1H), 6.97 (s, 2H), 6.94 (s, 2H), 3.87 (s, 6H), 3.86 (s,
6H); δC 156.8, 149.4,149.3, 139.6, 130.0, 125.8, 123.9, 112.2,
111.7, 56.2; IR (KBr) νmax/cmϪ1 3060 (vw), 3000 (w), 2935 (w),
2900 (w), 2835 (w), 1585 (m), 1490 (s), 1460 (w), 1430 (m), 1355
(s), 1340 (s), 1295 (w), 1260 (s), 1215 (s), 1180 (m), 1150 (w),
1120 (m), 1030 (s), 925 (w), 890 (w), 850 (m), 830 (w), 790 (m),
740 (w), 680 (m); UV (CHCl3) λmax/nm (ε/dm3 molϪ1 cmϪ1) 278
(43 300), 258 (38 700) (Anal. Calc. for C21H17NO4S4: C, 53.0; H,
3.6; N, 2.9; O, 13.5; S, 26.9. Found: C, 53.3; H, 3.6; N, 2.8; O,
13.5; S, 26.2%).
Reaction of 2,3,5,6-tetrachloropyridine (4) and 4,5-dimethyl-
benzene-1,2-dithiol (1:2)
A mixture of 4,5-dimethylbenzene-1,2-dithiol (0.20 g; 1.17
mmol), pyridine 4 (0.13 g; 0.59 mmol), sodium bicarbonate
(0.41 g) and DMF (15 mL) was stirred in argon at 100 ЊC for 1.5
h and then at reflux for 5 h. The mixture was cooled, poured
into water, acidified with hydrochloric acid and filtered. The
precipitate was chromatographed (silica gel) eluting with
CHCl3 to give: (a) 2,3-Dichloro-7,8-dimethyl[1,4]benzodithiino-
[2,3-b]pyridine (11) (0.09 g; 17%). (b) 2,3,9,10-Tetramethyldi-
[1,4]benzodithiino[2,3-b:2,3-e]pyridine (7) (0.27 g; 55%).
Reaction of 2,3-dichloro-7,8-dimethoxy[1,4]benzodithiino[2,3-b]-
pyridine (12) and 4,5-dimethoxybenzene-1,2-dithiol (1:1)
Reaction of 2,3-dichloro-7,8-dimethyl[1,4]benzodithiino[2,3-b]-
pyridine (11) and 4,5-dimethylbenzene-1,2-dithiol (1:1)
A mixture of 4,5-dimethoxybenzene-1,2-dithiol (0.10 g; 0.52
mmol), 12 (0.15 g; 0.43 mmol), sodium bicarbonate (0.25 g) and
DMF (15 mL) was stirred under argon at 110 ЊC for 8 h. The
mixture was cooled, poured into water, acidified with hydro-
chloric acid and filtered. The precipitate was chromatographed
(silica gel) eluting with CHCl3 to give 2,3,9,10-tetramethoxy-
di[1,4]benzodithiino[2,3-b:2,3-e]pyridine (8) (0.06 g; 30%).
A mixture of 4,5-dimethylbenzene-1,2-dithiol (0.35 g; 2.02
mmol), 11 (0.53 g; 1.70 mmol), sodium bicarbonate (0.73 g) and
DMF (25 mL) was stirred under argon at 100 ЊC for 1.5 h and
then at reflux for 5 h. The mixture was cooled, poured into
water, acidified with hydrochloric acid and filtered. The precipi-
tate was chromatographed (silica gel) eluting with CHCl3 to
give 2,3,9,10-tetramethyldi[1,4]benzodithiino[2,3-b:2,3-e]pyrid-
ine (7) (0.41 g; 74%).
X-ray analysis of 2,3,9,10-tetramethyldi[1,4]benzodithiino-
[2,3-b:2,3-e]pyridine (7) and 2,3,9,10-tetramethoxydi[1,4]benzo-
dithiino[2,3-b:2,3-e]pyridine (8)
Reaction of 2,3,5,6-tetrachloropyridine (4) and 4,5-dimethoxy-
benzene-1,2-dithiol (1:1)
Suitable crystals of 7 were grown by slow vapor diffusion of
n-hexane into a CH2Cl2 solution of the substrate, and crystals
of 8 were obtained by slow crystallization from a benzene solu-
tion of the substrate. Crystal, experimental and refinement data
for 7 and 8 are presented in Table 8. Crystals were mounted in
an Enraf-Nonius CAD4 diffractometer with graphite mono-
chromated Mo-Kα radiation (λ = 0.71069 Å) and the ω-2θ scan
method was used. Cell parameters were determined from
refinement of 25 centered reflections. Structures were solved by
direct methods using SumF-TF21 for 8 and SHELXS-8622 for 7,
and refined on F 2 for all reflections using the SHELXL-93
program.23 Non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were placed in calculated positions and their
isotropic temperature factors were refined. CCDC 188/168. See
A mixture of 4,5-dimethoxybenzene-1,2-dithiol (0.1 g; 0.50
mmol), pyridine 4 (0.08 g; 0.37 mmol), sodium bicarbonate
(0.17 g) and DMF (10 mL) was stirred in argon at room tem-
perature for 5 h. The mixture was cooled, poured into water,
acidified with hydrochloric acid and filtered. The precipitate
was chromatographed (silica gel) eluting with CHCl3 to give: (a)
2,2Ј-(4,5-Dimethoxy-1,2-phenylenedithio)bis(3,5,6-trichloro-
pyridine) (14) (31 mg; 30%), mp 249–252 ЊC (from benzene); δH
7.62 (s, 2H), 7.24 (s, 2H), 3.94 (s, 6H); δC 156.6, 150.4, 146.3,
137.9, 127.0, 126.2, 126.1, 119.6, 56.2; IR (KBr) νmax/cmϪ1 3050
(w), 3005 (w), 2950 (w), 2920 (m), 2840 (w), 1585 (m), 1550 (m),
1500 (s), 1460 (w), 1445 (w), 1430 (m), 1370 (s), 1355 (s), 1325
(m), 1310 (m), 1260 (s), 1225 (m), 1205 (s), 1175 (w), 1145 (s),
J. Chem. Soc., Perkin Trans. 2, 1999, 1503–1512
1511

View Article Online
Table 8 Crystal data and structure refinement for 7 and 8a
Acknowledgements
Support of this research by DGICYT of MEC (Spain) through
project PB96–0836, and by the “Generalitat de Catalunya”
(Grant: GRQ93–8036) is gratefully acknowledged. J. S. thanks
the “Generalitat de Catalunya” for a doctoral scholarship. The
authors express their gratitude to the EPR service of Centre
d’Investigació i Desenvolupament (CSIC) in Barcelona.
Empirical formula
Formula weight
T/K
Crystal system
Space group
Unit cell dimensions
a/Å
b/Å
c/Å
α(Њ)
β(Њ)
C21H17NS4
411.60
293(2)
Monoclinic
P21/n
C21H17NO4S4
475.60
293(2)
Triclinic
¯
P1
8.100(1)
12.520(3)
18.733(3)
90
95.22(1)
90
1891.9(6)
4
0.507
856
4.881(1)
9.764(1)
11.582(1)
70.542(8)
88.603(8)
85.520(8)
518.86(13)
1
References
1 Introduction to Molecular Electronics, ed. M. C. Petty, M. R. Bryce
and D. Bloor, Edward Arnold, London, 1995.
2 Proceedings of the International Conference on Science and
Technology of Synthetic Metals (ICSM’94), Seoul, Korea, 1994,
published in Synth. Met., 1995, 69–71.
3 J. M. Williams, Acc. Chem. Res., 1985, 18, 261.
4 M. R. Bryce, Chem. Soc. Rev., 1991, 20, 355.
5 T. Jorgensen, T. K. Hansen and J. Becher, Chem. Soc. Rev., 1994, 23,
41.
6 W. Hinrichs, P. Berges, G. Klar, E. Sánchez-Martínez and W.
Gunsser, Synth. Met., 1987, 20, 357.
γ(Њ)
V/Å3
Z
µ/mmϪ1
0.488
246
3121
F(000)
Unique reflections
Final R indices
R(F) [I > 2σ(I)]
R(R) (all data)
Rw(F2) [I>2σ(I)]
(all data)
3325
0.0358
0.0689
0.0875
0.0913
0.0443
0.0944
0.1148
7 H. Bock, A. Rauschenbach, K. Ruppert and Z. Havlas, Angew.
Chem., Int. Ed. Engl., 1991, 30, 714.
8 S. Hünig, K. Sinzger, R. Bau, T. Metzenthin and J. Salbeck, Chem.
Ber., 1993, 126, 465.
2
2
a w = [Σσ2(Fo ) ϩ (aP)2 ϩ bP]Ϫ1; where P = [Max(Fo ,0) ϩ 2Fc2]/3.
9 H. Bock, A. Rauschenbach, Ch. Näther, M. Kleine and Z. Havlas,
Chem. Ber., 1994, 127, 2043.
10 P. Berges, V. Mansel and G. Klar, Z. Naturforsch., Teil B, 1992, 47,
211.
http://www.rsc.org/suppdata/p2/1999/1503 for crystallographic
data in .cif format.
Hexachloroantimonate salts of 72؉ and 82؉
11 G. Klar, W. Hinrichs and P. Berges, Z. Naturforsch., Teil B, 1987, 42,
169.
From 7. To a stirred solution of 7 (0.10 g; 0.24 mmol) in
anhydrous CH2Cl2 (20 mL) under an argon atmosphere at room
temperature neat SbCl5 (0.15 mL; 0.98 mmol) was slowly
added. The solution immediately turned deep green, and a
powdered green solid precipitated. The resultant mixture was
kept for 2.5 h, and the precipitate was filtered and washed thor-
oughly with an excess of CH2Cl2. The solid was dried to afford
a green solid (0.26 g; 98%) (Anal. Calc. for C21H17NS4Cl12Sb2:
C, 23.3; H, 1.6; N, 1.3; S, 11.8; Cl, 39.3. Found: C, 23.2; H, 1.7;
N, 1.3; S, 11.6; Cl, 36.9%).
12 C. Martí, J. Irurre, A. Alvarez-Larena, J. F. Piniella, E. Brillas, Ll.
Fajarí, C. Alemán and L. Juliá, J. Org. Chem., 1994, 59, 6200.
13 C. Troya, A. Vidal, E. Brillas, J. Sañé, J. Rius, J. Irurre and L. Juliá,
unpublished results.
14 E. Klinsberg, Synthesis, 1972, 29.
15 N. D. Lowe and C. D. Garner, J. Chem. Soc., Dalton Trans., 1993,
2197.
16 Fundamentals of Electrochemical Analysis, ed. Z. Galus, Horwood,
Chichester, 1976.
17 Organic Electrochemistry. An Introduction and a Guide, ed. H. Lund
and M. M. Baizer, Marcel Dekker, New York, 1991.
18 R. S. Nicholson, Anal. Chem., 1965, 37, 1351.
19 O. Hammerich and V. D. Parker, J. Am. Chem. Soc., 1974, 96, 4289.
20 M. J. S. Dewar and W. Thiel, J. Am. Chem. Soc., 1977, 99, 4899.
21 J. Rius, Acta Crystallogr., Sect. A, 1993, 49, 406.
22 G. M. Sheldrick, SHELXS-86, Crystallographic Computing 3,
ed. G. M. Sheldrick, C. Krüger and R. Goddard, Oxford University
Press, England, 1985, pp. 175–189.
From 8. To a stirred solution of 8 (0.091 g; 0.19 mmol) in
anhydrous CH2Cl2 (15 mL) under an argon atmosphere at room
temperature, neat SbCl5 (0.12 mL; 0.94 mmol) was slowly
added. The solution immediately turned deep brown, and a
powdered dark solid precipitated. The resultant mixture was
kept for 2 h, and the precipitate was filtered and washed thor-
oughly with an excess of CH2Cl2. The solid was dried to afford
a dark brown solid (0.215 g; 97%) (Anal. Calc. for C21H17NO4-
S4Cl12Sb2: C, 22.0; H, 1.5; N, 1.2; S, 11.2; Cl, 37.2. Found: C,
20.1; H, 1.5; N, 1.2; S, 9.8; Cl, 37.5%).
23 SHELXL-93, A program for the refinement of crystal structures,
ed. G. M. Sheldrick, Göttingen University, 1993.
Paper 9/00148D
1512
J. Chem. Soc., Perkin Trans. 2, 1999, 1503–1512