Synthesis, structure, and both cathodic and anodic reversible redox reactions of benzochalcogenophenes containing ferrocene units

Article abstract of DOI:10.1016/j.tetlet.2006.02.122

2,3-Diferrocenylbenzo[b]thiophene and 1,3-diferrocenylbenzo[c]thiophene have been systematically and selectively synthesized from benzo[b]thiophene and phthaloyl dichloride, respectively. Characterization of the molecules was performed by physical and spe

Full text of DOI:10.1016/j.tetlet.2006.02.122

Tetrahedron Letters 47 (2006) 2887–2891
Synthesis, structure, and both cathodic and anodic
reversible redox reactions of benzochalcogenophenes
containing ferrocene units
Satoshi Ogawa,* Kenji Kikuta, Hiroki Muraoka, Fumihito Saito and Ryu Sato*
Department of Chemical Engineering, Faculty of Engineering, Iwate University, Morioka 020-8551, Japan
Received 2 February 2006; revised 17 February 2006; accepted 21 February 2006
Available online 9 March 2006
Abstract—2,3-Diferrocenylbenzo[b]thiophene and 1,3-diferrocenylbenzo[c]thiophene have been systematically and selectively syn-
thesized from benzo[b]thiophene and phthaloyl dichloride, respectively. Characterization of the molecules was performed by physi-
cal and spectroscopic means and X-ray crystallographic analyses. The cyclic voltammograms of the novel thiophene derivatives
containing ferrocene fragments showed a well-defined reversible cathodic step derived from the unusually stable thiophene radical
anions and two distinct reversible anodic steps derived from ferrocenium cations separated from each other by a thiophene hetero-
cycle. 1,3-Diferrocenylbenzo[c]selenophene was also synthesized in a similar manner for formation of 1,3-diferrocenylbenzo[c]thio-
phene by the use of bis(dimethylaluminum) selenide as a selenating reagent.
ꢀ 2006 Elsevier Ltd. All rights reserved.
In recent years, molecules comprising multiple reduc-
tion–oxidation (redox) centers have been attracting
attention in the field of material science due to the
potentiality of new organic semi-conducting materials
with application.1 Moreover, this kind of molecule hav-
ing more than two redox-active metal centers is a funda-
mentally attractive target for the study of multi-electron
transfer processes via the mixed valence state derived
from these multi-metallic systems.2 On the other hand,
interest in the design of novel redox-active organic cen-
ters by the use of a 7p electron framework3 containing
group 16 elements has led us to explore the synthesis
of new five-membered heterocycles containing sulfur
and/or selenium atom(s). This time our studies are
aimed at the design of reversible multi-steps redox sys-
tems using simple molecules with both organic and orga-
nometallic electron transfer fragments. Although the
synthesis and characterization of substituted benzochal-
cogenophenes have been reported,4 there is no report
concerning benzannulated thiophene and selenophene
containing a ferrocene fragment on the five-membered
heterocyclic unit, which are of structural and redox
characteristic interest, in contrast to benzannulated
chalcogenophene, namely, 1,3-dithienylbenzo[c]thio-
phene and -selenophene.5 Recently, we reported a new
type of multi-steps reversible redox systems using
organic–organometallic hybrid molecules, 1-ferrocenyl-
and 1,9-diferrocenyl-thianthrenes.6 Therefore, we have
designed 2,3-diferrocenylbenzo[b]thiophene, 1,3-diferro-
cenylbenzo[c]chalcogenophenes as both cathodic and
anodic multiple-redox active organic–organometallic
hybrid molecules. In this letter, we provide the details
on the synthesis, structural characterization, and elec-
trochemical properties of 2,3-diferrocenylbenzo[b]thio-
phene, 1,3-diferrocenylbenzo[c]thiophene, and 1,3-
diferrocenylbenzo[c]selenophene.
The synthesis of 2,3-diferrocenylbenzo[b]thiophene (1)
was achieved through the transition metal-catalyzed
cross-coupling reaction7 of 2,3-dibromobenzo[b]thio-
phene, which was prepared by general bromination with
bromine,8 with ferrocenylzinc chloride in the presence of
a
catalytic amount of bis(triphenylphophine)palla-
dium(II) dichloride, (PPh3)2PdCl2, in tetrahydrofuran
(THF) under reflux condition (Scheme 1). On the other
hand, we employed phthaloyl dichloride as a starting
material for the synthesis of 1,3-diferrocenylbenzo[c]thio-
phene (2). However, two acid chlorides in the ortho po-
sition are too reactive toward nucleophilic substitution
by the use of Grignard or organolithium reagents even
Keywords: Thiophene; Selenophene; Ferrocene; Cyclic voltammetry;
Redox reaction.
*
Corresponding authors. Tel./fax: +81 19 621 6934; e-mail: ogawa@
iwate-u.ac.jp
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.02.122

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S. Ogawa et al. / Tetrahedron Letters 47 (2006) 2887–2891
Scheme 1.
Scheme 2.
at low temperature. Therefore, we synthesized 1,2-di[S-
(2-pyridinyl)]benzenedithioate which has two functional
groups with lower reactivity as compared with phthaloyl
dichloride by the modified method previously reported.9
Since we obtained 1,2-di[S-(2-pyridinyl)]benzenedithio-
ate, the next step consisted of performing the reaction
with a ferrocenyllithium reagent. The ferrocenyllithium,
which was prepared from ferrocene with t-butyllithium,
was slowly added at ꢀ60 ꢁC to 1,2-di[S-(2-pyridin-
yl)]benzenedithioate in a solution of THF. The mixture
was stirred at ꢀ60 ꢁC for 30 min and finally quenched
by addition of 2 mol dmꢀ3 HCl aqueous solution. After
usual work-up, ortho diferrocenoylbenzene was ob-
tained in moderate yield. Finally, the isothianaphthene
core was formed through ring closure of ortho diferroce-
noylbenzene by means of Lawesson’s reagent to give
1,3-diferrocenylbenzo[c]thiophene (2) in moderate yield
(Scheme 2). In the case of the synthesis of 1,3-
diferrocenylbenzo[c]selenophene (3), the ring-closure
reaction was performed by the use of bis(dimethylalumi-
num) selenide developed by Segi and Zingaro10 as a
selenating reagent.
Figure 1. ORTEP drawing of compound 1. Dichloromethane molecule
is omitted for clarity. Thermal ellipsoids are drawn at 50% probability.
˚
Selected bond distances (A), bond angles (ꢁ), and dihedral angles (ꢁ):
S1–C2 1.760(4), C2–C3 1.355(4), C3–C4 1.451(5), C4–C5 1.404(5), C5–
S1 1.735(3), C2–C6 1.458(4), C3–C8 1.490(5), S1–C2–C3 112.5(3), C2–
C3–C4 113.0(3), C3–C4–C5 111.3(2), C4–C5–S1 112.2(2), C5–S1–C2
91.0(2), C8–C3–C2 122.3(3), C6–C2–C3 130.3(3), C7–C6–C2–C3
34.3(5), C2–C3–C8–C9 42.3(4), C6–C2–C3–C8 4.8(5).
Structural characterization of new benzothiophenes 1, 2,
and benzoselenophene 3 was performed by physical and
spectroscopic means.11 In addition, single crystals of 1–3
were successfully obtained by slow crystallization from
effectual organic solvents at room temperature. The
crystal structures of 112 (Fig. 1), 213 (Fig. 2), and 314
(Fig. 3) were determined by X-ray crystallographic anal-
yses and revealed that two ferrocene fragments were
located in anti-conformation, respectively, having the
dihedral angles of 34.3(5) and 42.3(4)ꢁ for 1, ꢀ23.7(2)
and +30.0(2)ꢁ for 2, ꢀ21.9(5) and +30.1(5)ꢁ for 3. The
C–C distances between the ferrocenyl carbon and the
carbon(s) next to thienyl sulfur suggest a double bond
˚
character because the lengths of 1.458(4) A for 1,
˚
˚
1.461(2) and 1.461(2) A for 2, 1.451(6) and 1.447(5) A

S. Ogawa et al. / Tetrahedron Letters 47 (2006) 2887–2891
2889
The redox properties of chalcogenophene–ferrocene sys-
tems have been furnished by electrochemical measure-
ments; the data are collected in Table 1 and cyclic
voltammograms (CV) of compounds 1–3 are shown in
Figure 4. The dominant feature of CV scans of 1–3 at
concentrations 2.0 mmol dmꢀ3 in THF/0.1 mol dmꢀ3
nBu4NPF6 is three one-electron redox couples, E1/2
=
ꢀ2.87, +0.22, +0.41 V for 1; ꢀ2.38, +0.14, +0.34 V
for 2; ꢀ2.31, +0.15, +0.39 V for 3; versus Ag/Ag+,
respectively, that is well-defined reversibility. At nega-
tive scan, the reduction of 1–3 is electrochemically
reversible and a cathodic product wave appears, as-
signed to formation of the corresponding radical anion.
On the other hand, at positive scan, the oxidation of
1–3 is also electrochemically reversible and two anodic
product waves appear, assigned to stepwise formation
of the corresponding mono- and bis-(ferrocenium
cation).
Figure 2. ORTEP drawing of compound 2. Thermal ellipsoids are
drawn at 50% probability. Selected bond distances (A), bond angles
˚
(ꢁ), and dihedral angles (ꢁ): S1–C2 1.720(2), C2–C3 1.393(2), C3–C4
1.443(2), C4–C5 1.390(2), C5–S1 1.714(2), C2–C6 1.461(2), C5–C8
1.461(2), S1–C2–C3 110.2(1), C2–C3–C4 112.4(2), C3–C4–C5 112.9(1),
C4–C5–S1 110.3(1), C5–S1–C2 94.16(8), S1–C2–C6–C7 ꢀ23.7(2), C9–
C8–C5–S1 30.0(2).
Figure 3. ORTEP drawing of compound 3. Thermal ellipsoids are
˚
drawn at 50% probability. Selected bond distances (A), bond angles
(ꢁ), and dihedral angles (ꢁ): Se1–C2 1.867(4), C2–C3 1.381(5), C3–C4
1.456(5), C4–C5 1.386(5), C5–Se1 1.862(4), C2–C6 1.447(5), C5–C8
1.451(6), C5–Se1–C2 89.8(4), Se1–C2–C3 109.9(3), C2–C3–C4
115.3(3), C3–C4–C5 115.0(3), C4–C5–Se1 110.0(3), C9–C8–C5–Se1
ꢀ21.9(5), Se1–C2–C6–C7 30.1(5).
for 3 are significantly shorter than that of the sp2–sp2
15
˚
Figure 4. Cyclic voltammograms of 1 (a), 2 (b), and 3 (c).
single bond (1.516 A).
Table 1. Redox potentials (V vs Ag/Ag+)a of diferrocenylbenzochalcogenophene 1, 2, and 3
1
2
3
First
Second
Third
First
Second
Third
First
Second
Third
Epa
Epc
E1/2
ꢀ2.71
ꢀ3.02
ꢀ2.87
+0.38
+0.06
+0.22
+0.56
+0.25
+0.41
ꢀ2.31
ꢀ2.45
ꢀ2.38
+0.21
+0.07
+0.14
+0.41
+0.27
+0.34
ꢀ2.19
ꢀ2.42
ꢀ2.31
+0.27
+0.02
+0.15
+0.51
+0.26
+0.39
a Conditions: concentration, 2 mmol dmꢀ3 sample in 0.1 mol dmꢀ3 [Bu4N]+[PF6]ꢀ/THF solution; temperature, 223 K for 1, 293 K for 2, 233 K for 3;
working electrode, glassy-carbon; reference electrode, Ag/0.01 mol dmꢀ3 AgNO3 in 0.1 mol dmꢀ3 [Bu4N]+[PF6]ꢀ/CH3CN solution; counter elec-
trode, Pt; scan rate, 200 mV sꢀ1 for 1, and 100 mV sꢀ1 for 2 and 3.

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S. Ogawa et al. / Tetrahedron Letters 47 (2006) 2887–2891
Figure 5. Cyclic (top) and differential pulse (bottom) voltammograms of 1 (a), 2 (b), and 3 (c).
By CV and differential pulse voltammograms (DPV)
measurements of 1–3 employing dichloromethane
(CH2Cl2) as a solvent, the oxidation waves derived from
the ferrocene fragments showed good separated and
well-defined reversible one-electron redox couples
(Fig. 5).2a The differences between the first and second
half-potentials ðDE1=2 ¼ E11=2 ꢀ E12=2Þ for the Fe(II)–
Fe(III) and Fe(III)–Fe(III) couples of 1, 2, and 3 were
192, 280, and 305 mV, respectively (Table 2). These
results suggest that the structural difference between
diferrocenylbenzothiophenes 1 and 2 based on the annu-
lation mode and the substituent position should cause
the differences in their redox behavior and thermo-
dynamic stability in mixed-valence intermediates. The
comproportionation constants (Kc)2a for the Fe(II)–
Fe(III) mixed-valence state estimated from DE1/2 were
1.1 · 105 for 2 and 3.0 · 105 for 3. Therefore, the elec-
tronic interaction between the metal center of ferrocene
and the ferrocenium cation in the mixed-valence state of
2 and 3 can be expected to be strong because of potential
through-bond and/or through-space interactions. In the
case of compound 1, it is difficult to discuss the compro-
portionation constant Kc, because it has essentially dif-
ferent oxidation potentials based on the position of
ferrocenes.
tive-charged oxidation products (mono- and bis-ferro-
cenium cations). Therefore, we succeeded in establishing
a new type of multi-steps reversible redox systems using
neutral organic–organometallic hybrid molecules.
Acknowledgements
This work was supported by Grant-in-Aid for Scientific
Research (No. 15550023) from the Ministry of Educa-
tion, Culture, Sports, Science, and Technology. We
thank Ms. Shiduko Nakajo (Division of Elemental
Analysis, Iwate University), for elemental analyses.
References and notes
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In conclusion, we have synthesized and characterized
novel benzochalcogenophenes containing ferrocene
units. The electrochemical properties showed reversible
multi-electron transfer phenomena assigned to chalco-
genophene (organic) and ferrocene (organometallic)
fragments owing to good stability of the negative-
charged reduction products (radical anions) and posi-
Table 2. Redox potentials (V vs Ag/Ag+)a of 1, 2, and 3
b
E11=2 (V)
E21=2 (V)
DE1/2 (mV)
1
2
3
+0.21
+0.13
+0.12
+0.40
+0.41
+0.42
+192
+280
+305
a Conditions: concentration, 1 mmol dmꢀ3 sample in 0.1 mol dmꢀ3
[Bu4N]+[PF6]ꢀ/CH2Cl2 solution; temperature, 281 K; working elec-
trode, glassy-carbon; reference electrode, Ag/0.01 mol dmꢀ3 AgNO3
in 0.1 mol dmꢀ3 [Bu4N]+[PF6]ꢀ/CH3CN solution; counter electrode,
Pt; scan rate, 100 mV sꢀ1
.
b DE1=2 ¼ E21=2 ꢀ E11=2
.

S. Ogawa et al. / Tetrahedron Letters 47 (2006) 2887–2891
2891
97, 173–205; Muller, E.; Thomas, R.; Sauerbier, M.;
6.9 Hz, 2H, ArH); 13C NMR (101 MHz, CDCl3): d 68.4,
68.6, 69.9, 80.2, 121.9, 123.0, 130.7, 135.2; IR (KBr) m
3093, 2927, 1741, 1644, 1464, 804, 740 cmꢀ1; MS (70 eV)
m/z 502 (M+). Anal. Calcd for C28H22Fe2S: C, 66.96, H,
4.42. Found: C, 66.80, H, 4.47. Spectral and physical data
¨
Langer, E.; Streichfuss, D. Tetrahedron Lett. 1971, 12,
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K.; Talham, D. R.; Poehler, T. O.; Bloch, A. N. Mol.
Cryst. Liq. Cryst. 1982, 86, 1–4.
1
for 3: purple crystals; mp 196.0 ꢁC (decomp.); H NMR
5. Lorcy, D.; Cava, M. P. Adv. Mater. 1992, 4, 562–564;
Bauerle, P.; Gotz, G.; Emerle, P.; Port, H. Adv. Mater.
1992, 4, 564–568; Musinanni, S.; Ferraris, J. P. J. Chem.
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Lett. 2005, 46, 7201–7204.
(400 MHz, CDCl3): d 4.19 (s, 10H, free-Cp), 4.39 (t,
J = 1.8 Hz, 4H, ArH), 4.69 (t, J = 1.8 Hz, 4H, ArH), 6.86
(dd, J = 3.1, 7.1 Hz, 2H, ArH), 7.67 (dd, J = 3.1, 7.1 Hz,
2H, ArH); 13C NMR (101 MHz, CDCl3): d 68.7, 69.2,
70.2, 82.4, 122.5, 122.7, 137.6, 139.6; 77Se NMR (76 MHz,
CDCl3): d 687.8; IR (KBr) m 3090, 2346, 1655, 1410, 1314,
1104, 1000, 838, 819, 741, 478 cmꢀ1; MS (70 eV) m/z 550
(M+). Anal. Calcd for C28H22Fe2Se: C, 61.24; H, 4.04.
Found: C, 60.99, H, 4.34.
6. Ogawa, S.; Muraoka, H.; Sato R. Tetrahedron Lett. 2006,
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Okabe, T.; Katada, M.; Kuwatani, Y. J. Organomet.
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9. Araki, M. Bull. Chem. Soc. Jpn. 1974, 47, 1777–1780;
Kiebooms, R. H. L.; Adriaensens, P. J. A.; Vanderzande,
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1480.
10. Li, G. M.; Zingaro, R. A.; Segi, M.; Reibenspies, J. H.;
Nakajima, T. Organometallics 1997, 16, 756–762.
11. Spectral and physical data for 1: brown crystals; mp
175.9–176.3 ꢁC; 1H NMR (400 MHz, CDCl3): d 4.06 (s,
5H, free-Cp), 4.15 (s, 5H, free-Cp), 4.25 (t, J = 1.9 Hz, 2H,
C5H4), 4.26 (t, J = 1.9 Hz, 2H, C5H4), 4.32 (t, J = 1.9 Hz,
2H, C5H4), 4.36 (t, J = 1.9 Hz, 2H, C5H4), 7.35 (td,
J = 1.3, 8.0 Hz, 1H, ArH), 7.44 (td, J = 1.3, 8.0 Hz, 1H,
ArH), 7.78 (d, J = 8.0 Hz, 1H, ArH), 8.57 (d, J = 8.0 Hz,
1H, ArH); 13C NMR (101 MHz, CDCl3): d 68.3, 68.1,
69.2, 69.6, 70.0, 70.6, 80.5, 80.8, 121.7, 123.5, 123.8, 124.1,
128.9, 138.7, 138.9, 140.0; IR (KBr) m 3100, 2362, 1412,
1320, 1107, 1000, 820, 733, 490 cmꢀ1; MS (70 eV) m/z 502
(M+). Anal. Calcd for C28H22Fe2S: C, 66.96; H, 4.42.
Found: C, 67.28; H, 4.65. Spectral and physical data for 2:
purple crystals; mp 196.0 ꢁC (decomp.); 1H NMR
(400 MHz, CDCl3): d 4.18 (s, 10H, free-Cp), 4.39 (t,
J = 1.8 Hz, 4H, C5H4), 4.74 (t, J = 1.8 Hz, 4H, C5H4),
7.00 (dd, J = 3.1, 6.9 Hz, 2H, ArH), 7.81 (dd, J = 3.1,
12. Crystal data for 1ÆCH2Cl2: M = 587.17, C29H24Fe2SCl2,
˚
triclinic, space group P-1 (#2), a = 10.429(2) A, b =
˚
˚
˚
11.043(2) A, c = 11.191(2) A, a = 83.94(2)ꢁ, b = 68.99(2)ꢁ,
c = 84.01(2)ꢁ, V = 1193.4(5) A , Z = 2, Dcalcd = 1.634 g cmꢀ3
,
3
˚
T = 123 1 K, k(MoKa) = 0.71075 A. 10,031 reflections
measured, 4830 unique (Rint = 0.045). The final cycle of
full-matrix least-squares refinement was based on 4405
observed reflections (I > 2.00r(I)) and 403 variable para-
meters with R1 = 0.046, wR2 = 0.112 (all data) (CCDC
299170).
13. Crystal data for 2: M = 502.24, C28H22Fe2S, monoclinic,
˚
space group P21/c (#14), a = 11.9107(8) A, b =
˚
˚
14.1854(8) A, c = 12.218(1) A, b = 92.359(3)ꢁ, V =
2062.6(3) A , Z = 4, Dcalcd = 1.617 g cmꢀ3, T = 123 1 K,
3
˚
˚
k(MoKa) = 0.71075 A. 20,093 reflections measured, 4721
unique (Rint = 0.033). The final cycle of full-matrix least-
squares refinement was based on 4721 observed reflections
(I > 2.00r(I)) and 368 variable parameters with R1 =
0.025, wR2 = 0.031 (all data) (CCDC 299169).
14. Crystal data for 3: M = 549.14, C28H22Fe2Se, monoclinic,
˚
space group P21/a (#14), a = 12.139(4) A, b =
3
˚
˚
˚
14.190(5) A, c = 12.083(4) A, b = 92.01(2)ꢁ, V = 2080(1) A ,
Z = 4, Dcalcd = 1.753 g cmꢀ3, T = 123 1 K, k(MoKa) =
˚
0.71075 A. 19,106 reflections measured, 4593 unique
(Rint = 0.089). The final cycle of full-matrix least-squares
refinement was based on 3783 observed reflections
(I > 2.00r(I)) and 368 variable parameters with R1 =
0.041, wR2 = 0.114 (all data) (CCDC 299168).
15. Bryan, C. D.; Cordes, A. W.; Haddon, R. C.; Hicks, R.
G.; Oakley, R. T.; Palstra, T. T. M.; Perel, A. J. J. Chem.
Soc., Chem. Commun. 1994, 1447–1448.