Synthesis and redox properties of π-conjugated 4,5-diazafluorene derivatives incorporating 9-cyanomethylene moiety as an electron acceptor

Article abstract of DOI:10.1016/j.tetlet.2011.08.164

We have synthesized p-conjugated acceptor-type molecules 3a-d containing a cyanomethylene unit as the electron acceptor site and a 4,5-diazafluorene ligand for metal complexation. In the crystal, the planar 3a molecules stack along the b axis in the head-to-tail fashion. Compound 3a shows distinctive electrochromism and its three differently colored redox states (dianion (3 ^2-), anion radical (3^.-), neutral (3)) exhibit remarkable stability.

Full text of DOI:10.1016/j.tetlet.2011.08.164

Tetrahedron Letters 52 (2011) 5865–5868
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Tetrahedron Letters
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Synthesis and redox properties of
p
-conjugated 4,5-diazafluorene
derivatives incorporating 9-cyanomethylene moiety as an electron acceptor
Katsuya Sako a, , Yasufumi Mugishima a, Tetsuo Iwanaga b, Shinji Toyota b, Hiroyuki Takemura c,
⇑
Motonori Watanabe d, Teruo Shinmyozu d, Michito Shiotsuka a, Hitoshi Tatemitsu a
a Nagoya Institute of Technology, Gokisocho, Showa-ku, Nagoya 466-8555, Japan
b Department of Chemistry, Faculty of Science, Okayama University of Science, 1-1 Ridaicho, Kita-ku, Okayama 700-0005, Japan
c Department of Chemical and Biological Science, Faculty of Science, Japan Women’s University, Mejirodai 2-8-1, Bunkyou-ku, Tokyo 112-8681, Japan
d Institute for Materials Chemistry and Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 July 2011
Revised 23 August 2011
Accepted 29 August 2011
Available online 7 September 2011
We have synthesized p-conjugated acceptor-type molecules 3a–d containing a cyanomethylene unit as
the electron acceptor site and a 4,5-diazafluorene ligand for metal complexation. In the crystal, the planar
3a molecules stack along the b axis in the head-to-tail fashion. Compound 3a shows distinctive electro-
chromism and its three differently colored redox states (dianion (32À), anion radical (3ÅÀ), neutral (3))
exhibit remarkable stability.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
4,5-Diazafluorene
Acceptor
Knoevenagel condensation
Microwave assisted synthesis
Electrochromism
Various 4,5-diazafluorene derivatives have been studied because
of their potential applications in organic devices. Recently, many
4,5-diazafluorene ligands were found to act as powerful chelating
units with various transition metals to afford new functional hybrid
materials possessing electron-transfer, conductive, magnetic, and
photoreceptor properties.1 This versatility makes them interesting
building blocks for the construction of extended molecular architec-
tures. Conjugated donor ligands such as 1 and 2 (Chart 1) bearing the
4,5-diazafluorene unit as the coordinating site have been reported.2
as bases. Route B was a thermal reaction using ammonium acetate
as a base in refluxing benzene-acetic acid for 12–18 h. The results
of routes A and B are outlined in Table 1.6 By route A, 3a and 3b
were obtained in higher yields than by route B. Compounds 3c
and 3d were not obtained by route B at all, but were obtained in
10–14% yields by route A. In general, reactions under microwave
irradiation were very fast (5–30 min) and clean. In contrast, reac-
tions under conventional heating (80 or 110 °C) required much
longer periods (12–18 h, Table 1). In very active methylene com-
pound, the reaction under ultrasonic irradiation also gave similar
results as the reactions under microwave irradiation.
The molecular and crystal structures of 3a were determined by
X-ray diffractional analysis.7 A general view of the molecular struc-
ture of 3a, together with the crystallographic atom-numbering
scheme, is shown in Figure 1. The diazafluorene and dicyanometh-
ylene moiety plane (C12–C13–N3–C14–N4 and C26–C27–N7–
C28–N8) of the molecule is almost planar. The C–C bond lengths
of the five-membered ring (C9–C10–C11–C7–C8 and C23–C24–
C25–C21–C22) in the diazafluorene moiety of 3a are almost equal
(0.141–0.148 nm) and the bond angles are 106–108°. All of the C–C
bonds of the five-membered ring are longer by 0.003–0.009 nm
than those of the cyclopentadienyl ring in ferrocene (0.139 nm)8
and the C@C bond of cyclopentadiene (ca. 0.137 nm).9 They are
almost equal in length to the C–C bond of cyclopentadiene in ful-
valene derivatives (0.145–0.147 nm). These facts suggest that the
Instead of p-conjugated donors, we designed new p-conjugated 4,5-
diazafluorene ligand 3 incorporating the 9-cyanomethylene moiety
as an electron acceptor. 3a was found in a report and patents in
which xerographic photoreceptor properties were described, how-
ever, synthesis and characterization of 3a were not done.3 In this
article, as the first step toward charge separation subunits for photo-
diode devices,4 we report the improved synthesis, electrochemical
properties, and X-ray crystal structures of 9-cyanomethylene-4,
5-diazafluorene acceptors (3: DAF-CN).
We synthesized 3a–d by Knoevenagel condensation of 4,
5-diazafluoren-9-one and active methylene compounds in two dif-
ferent ways (Scheme 1).5 Route A employed microwave assisted
condensation with piperidine or ammonium acetate-piperidine
⇑
Corresponding author. Tel./fax: +81 52 735 5167.
E-mail address: sako@nitech.ac.jp (K. Sako).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.164

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K. Sako et al. / Tetrahedron Letters 52 (2011) 5865–5868
five-membered ring in the diazafluorene moiety of 3a is an almost
ideal pentagon and the C–C bonds have some double bond
character.
N
N
N
S
S
S
S
S
S
R
R
SR
S
S
As shown in Figure 2, the molecules of 3a show p–pstacking in an
SR
N
‘orthogonal head-to-tail’ manner along the b axis (0.341 nm: diaza-
fluorene ring is regarded as the head moiety). In the same planar col-
umn, the distances of nitrogens (N1 and N2) and hydrogen (H8) in
the diazafluorene moiety are 0.248 and 0.269 nm, respectively.
Furthermore, distances of nitrogen atoms (N3 and N4) of the dicya-
nomethylene moiety and hydrogen atoms (H11 and H10) of the
diazafluorene are 0.264 and 0.272 nm, respectively. These short con-
tacts indicate that NÁ Á ÁH hydrogen bonds are formed.
2a : R = H
2c : R = SCH3
2d : R = -OCH2CH2O-
1a : R = CH3
1c : R = C4H9
2b : R =CH3
1b :
1d :
R = -CH2CH2-
R =C2H5
Chart 1. Conjugated donor ligands 1 and 2.
The redox properties of the p-conjugated acceptor ligands 3a–d
were determined by cyclic voltammetry measurements (Fig. 3).10
The data obtained are collected in Table 2 together with the redox
potentials of the related compounds. The cyclic voltammograms of
3a–d show two single-electron reduction waves. The first reduc-
tion step (Ered11/2 3a: À0.98 V; 3b: À1.13 V vs Fc0/Fc+) correspond-
ing to the formation of the radical anionic species is reversible,
whereas the second reduction leading to the dianionic species
N
N
N
CN
Y
CN
Y
O
3a : Y=CN
N
3b : Y=COOEt
3c : Y=COPh
3a-d
3d :
Y=
COOMe
(Ered2 3a: À1.62 V; 3b: À1.56 V vs Fc0/Fc+) appears to be quasi-
1/2
Scheme 1. Synthesis of the compounds 3a–d.
reversible. The first reduction potential of 3a was observed as a po-
sitive shift than that of 3b because the dicyanomethylene moiety
of 3a is a stronger electron acceptor. The first reduction potential
Table 1
Preparation of 3 with different solvents and bases
Entry
Y
Method
Base
Solvent
Irradiation power
Time (min)
Yield
1
2
3
4
5
6
7
8
CN
CN
CN
CN
A
A
A
B
A
A
A
B
A
A
B
A
A
B
C
A
A
C
D
A
—
200
200
200
—
300
300
300
—
300
300
300
100
10
10
5
1200
30
30
30
2400
30
30
20
83
99
97
51
73
31
92
73
13
14
10
63
Benzene/AcOH
EtOH
Benzene/AcOH
Toluene/AcOH
EtOH
Toluene/AcOH
Benzene/AcOH
Toluene/AcOH
Toluene/AcOH
Toluene/MeOH
Benzene/AcOH
COOEt
COOEt
COOEt
COOEt
COPh
COPh
Ph-p-COOMe
CN
9
A
A
A
10
11
12
Ultra sonic
40
a
b
c
Method A: microwave irradiation, B: oil bath heating under refluxing (benzene: 80 or toluene:110 °C).
Base A: NH4OAc, B: piperidine, C: NH4OAc/piperidine, D: Cs2CO3.
Isolated yield.
Figure 1. The molecular structure of 3a with labeling scheme.

K. Sako et al. / Tetrahedron Letters 52 (2011) 5865–5868
5867
Figure 2. The molecular packing manners of 3a.
+60
+40
2.5
2.0
1.5
1.0
0.5
(3a)
(3a
+20
0
0 V
-1.0 V
-1.7 V
)
)
(3a2-
-20
-40
-60
-80
-2.5
-2
-1.5
-1
-0.5
0
Potential (V vs. Fc/Fc+)
0
300
400
500
600
700
Figure 3. Cyclic voltammogram of 3a in dichloromethane.
Wavelength / nm
Figure 4. Spectroelectrochemical absorption of 3a in CH2Cl2 solution; black line:
0 V (3a), red line: À1 V (3aÅÀ), blue line: À1.7 V (3a2À).
Table 2
Redox potentialsa of 3 and related compounds
of 3d was smaller than those of 3a–c by stabilizing effect of aro-
matic ring in cyanomethylene unit. The first redox potentials of
3a–d were ca. 0.70–0.85 V smaller than those of TCNE and TCNQ.
The results of spectroelectrochemical measurements of com-
pound 3a are shown in Figure 4.11 Upon increasing the potential,
as reduction of the system proceeds, we see the concomitant emer-
gence of the low energy absorption band of non-interacting dicya-
nomethylene anion radicals at kmax 485 nm (Fig. 4, spectrum at
À1.0 V). In the cases of 9-alkylfluorenide and TCNQ, the typical
Ered1
Ered2
D
E
1/2
1/2
3a
3b
3c
3d
TCNE
TCNQ
À0.98
À1.13
À1.07
À1.39
À0.25
À0.28
À1.62
À1.56
À1.37
À1.60
À0.79
À0.86
0.64
0.43
0.30
0.21
0.54
0.58
a
Conditions: 0.1 M n-Bu4NClO4, CH2Cl2, 25 °C, Pt working and counter elec-
trodes. Potentials were measured against an Ag/Ag+ electrode. E in V versus Fc/Fc+.

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K. Sako et al. / Tetrahedron Letters 52 (2011) 5865–5868
signature bands of 9-alkylfluorenide carbanion12a and TCNQ anion
radical12b were observed at kmax 460 and 842 nm in the absorption
spectrum, respectively. 3a shifted to longer wavelengths than 9-
alkylfluorenide carbanion, however, shifted to shorter wavelengths
than TCNQ anion radical. 3a has more electron-withdrawing ability
than that of 9-alkylfluorene, although 3a has less electron-with-
drawing ability than that of TCNQ. The formation of the dianion
species while scanning down to À1.7 V was monitored by loss of
the characteristic vibronic band at 485 nm, the appearance of
two new bands at kmax 245 and 340 nm (shoulder).
3. (a) Ling-Chung, S. K.; Runciman, P. J. I.; Sales, K. D.; Utley, J. H. P. J. Electroanal.
Chem. 1985, 250, 373; (b) Sekido, K.; Nagasaka, H.; Sekiya, M. JP 2007-052063.;
(c) Sekido, K.; Nagasaka, H.; Sekiya, M.; Takagi, S. JP 2009-288621.
4. Fujihira, M.; Nishiyama, N.; Yamada, H. Thin Solid Films 1985, 132, 77.
5. Compound 3a: Yellow powder; mp 279–280 °C; IR(KBr): 2224 cmÀ1
;
1H
NMR(CDCl3, 300.4 MHz):
d 7.41 (dd, J = 5.0, 8.2 Hz, 2H), 8.71 (dd, J = 1.4,
8.2 Hz, 2H), 8.82 (dd, J = 1.4, 5.0 Hz, 2H); 13C NMR (CDCl3, 75.6 MHz): d 112.2,
124.5, 129.0, 133.5, 155.2, 156.5, 159.6; HRMS (FAB), calcd for C14H6N4 m/z
231.0671, found: m/z 231.0666 [M]+. Compound 3b: Yellow powder; mp 131–
132 °C; IR(KBr): 2212, 1731 cmÀ1 1H NMR(CDCl3, 300.4 MHz): d 1.47 t, .3 Hz,
;
3H), 4.50 (q, J = 7.3 Hz, 2H), 7.29 (dd, J = 4.9, 7.6 Hz, 1H), 7.38 (dd, J = 4.9, 7.6 Hz,
1H), 8.64 (dd, J = 1.4, 8.3 Hz, 1H), 8.72 (dd, J = 1.4, 3.7 Hz, 1H), 8.77 (dd, J = 1.4,
3.7 Hz, 1H), 8.92 (dd, J = 1.4, 8.3 Hz, 1H); 13C NMR (CDCl3, 75.6 MHz): d 14.0,
63.6, 103.7, 115.7, 124.1, 129.6, 130.6, 133.6, 136.4, 150.8, 153.8, 159.4, 161.4;
HRMS(FAB), calcd for C16H11O2N3 m/z 278.0930, found: m/z 278.0932 [M]+.
The two reduction processes were completely reversible for
compound 3a, that is the initial spectra returned after re-setting
the potential to 0 V and the system can be cycled between these
two redox states for at least a few hours. The intensities of the
bands belonging to the ionic states did not change, however, which
indicate that the acceptor moieties were not the sites of decompo-
sition. We believe that compound 3a is a very promising electro-
Compound 3c: Yellow powder; mp 156–157 °C; IR(KBr): 2208, 1669 cmÀ1 1H
;
NMR(CDCl3, 300.4 MHz): d 7.10 (dd, J = 4.9, 7.9 Hz, 1H), 7.45 (dd, J = 5.0, 8.1 Hz,
1H), 7.57–7.75 (m, 4H), 8.11–8.14 (m, 2H), 8.68 (dd, J = 1.4, 4.9 Hz, 1H), 8.82
(dd, J = 1.4, 5.3 Hz, 1H), 8.89 (dd, J = 1.5, 8.1 Hz, 1H); 13C NMR (CDCl3,
75.6 MHz): d 110.0, 115.3, 123.6, 124.1, 129.5, 130.1, 132.6, 133.4, 133.9,
135.9, 138.1, 146.6, 153.3, 158.9, 187.9; MS(FAB), m/z 310 [M+H]+. Compound
3d: Yellow powder; mp 166–167 °C; IR(KBr): 2206, 1727 cmÀ1 1H NMR(CDCl3,
;
300.4 MHz): d 4.01 (s, 3H), 6.92 (dd, J = 1.8, 8.0 Hz, 1H), 6.97 (dd, J = 4.5, 8.1 Hz,
1H), 7.44 (dd, J = 4.9, 8.1 Hz, 1H), 7.65–7.68 (m, 1H), 8.23–8.27 (m, 1H), 8.64
(dd, J = 1.4, 4.9 Hz, 1H), 8.80 (dd, J = 1.5, 5.0 Hz, 1H), 8.93 (dd, J = 1.4, 7.9 Hz, 1H)
13C NMR (CDCl3, 75.6 MHz): d 52.6, 111.3, 117.7, 123.1, 124.8, 129.5, 130.5,
131.5, 132.1, 137.4, 143.3, 152.7, 158.7, 166.0; MS(FAB), m/z 340 [M+H]+.
6. Typical experimental procedure (route A and B) is as follows (Table 1, entry 1
and entry 2): Route A: Ammonium acetate (1.3 mmol) was added into a
mixture of 4,5-diazafluoren-9-one (0.50 mmol), malononitrile (0.60 mmol) in a
mixed solvent (benzene/AcOH (10/0.15(v/v) 5.3 mL). The mixture was kept in a
chromic material by virtue of having three redox states (3a, 3aÅÀ
3a2À) each with distinctive visible absorption spectra.
,
In summary, we have prepared diazafluorene functionalized li-
gands (3a–d, DAF-CN). Compound 3a shows distinctive electro-
chromism; the remarkable stability of three differently colored
redox states of 3a (3a, 3aÅÀ, 3a2À) makes it a promising electrochro-
mic material for the visible spectral region as the first step toward
charge separation.
microwave reactor operated (Shikoku Keisoku Kogyo Corp.,
l Reactor Ex,
200 watts) and was irradiated for 10 min with stirring. The reaction mixture
was then removed from the oven and cooled to room temperature. Aqueous
NaHCO3 solution was added to the reaction mixture and the organic layer was
separated. The organic layer was washed with water and brine, and dried over
anhydrous magnesium sulfate. After evaporation of the solvent, the crude
product was purified by column chromatography on silica gel with ethyl
acetate to give 3a in 99% yield. Route B: A mixture of ammonium acetate
(13.0 mmol), 4,5-diazafluoren-9-one (5.0 mmol) and malononitrile (7.5 mmol)
in a mixed solvent (benzene/AcOH(10/0.15(v/v) 46 mL) was refluxed for 20 h,
with the water formed during the reaction was removed azeotropically by
Dean–Stark trap. After similar work-up, 3a was obtained in 51% yield.
Acknowledgments
This research was partially supported by a Grant-in-Aid for
Exploratory Research (No.20550038) from the Ministry of Educa-
tion, Culture, Sports, Science, and Technology, Japan. This work
was performed under the Cooperative Research Program of ‘Net-
work Joint Research Center for Materials and Devices (Institute
for Materials Chemistry and Engineering, Kyushu University)’.
ꢀ
7. Crystal data for 3a: 2(C14H6N4), M = 460.46, triclinic, space group P1 (#2),
a = 9.4619(4), b = 10.4095(5), c = 11.7455(9) Å,
a
= 83.718(3), b = 81.778(4),
Dc = 1.462 g cmÀ3
(Mo
T = 123(2) K, 7386 reflections, R1 = 0.0361 (I > 2.0 (I)),
c
= 66.207(3)°,
a
V = 1054.94(10)
,
Å3,
Z = 2,
,
l
r
K
) = 0.093 mmÀ1
Supplementary data
wR2 = 0.0941. GOF 1.044. Crystallographic data reported in this manuscript
have been deposited with Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC-818293. Copies of the data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge, CB2 1EZ, UK; fax: +44 1223 336033; or deposit@ccdc.cam.ac.uk).
8. Seiler, P.; Dunitz, J. D. Acta Cryst. 1979, B35, 1068.
Supplementary data (the experimental procedure and addi-
tional characterization data for all compounds along with their
1H NMR and 13C NMR spectra, absorption spectra, cyclic voltam-
mograms) associated with this article can be found, in the online
version, at doi:10.1016/j.tetlet.2011.08.164.
9. Aqad, E.; Leriche, P.; Mabon, G.; Gorges, A.; Khodorkovsky, V. Tetrahedron Lett.
2001, 42, 2813.
10. Cyclic voltammograms of neutral (3) to anion radical (3ÅÀ), dianion (32À) show
two reversible reduction peaks.
References and notes
11. Absorption spectroelectrochemistry of 3a (4 Â 10À4 M) in Figure
2 was
measured in dichloromethane containing n-BuNPF6 (0.1 M) as a supporting
electrolyte. A 2 mm quartz cell with Pt mesh as working, Pt wire as counter,
and Ag/Ag+ as reference electrodes, were used. Using an optically transparent
thin-layer electrolytic (OTTLE) cell, we investigated the one reversible
transition during stepwise one-electron reduction.
1. (a) Hintermaier, F.; Beck, W. Polyhedron 1998, 17, 483; (b) Su, H.-C.; Chen, H.-F.;
Wu, C.-C.; Wong, K.-T. Chem. Asian J. 2008, 3, 1922; (c) Valore, A.; Balordi, M.;
Colombo, A.; Dragonetti, C.; Righetto, S.; Roberto, D.; Ugo, R.; Benincori, T.;
Rampinini, G.; Sannicol‘o, F.; Demartin, F. Dalton Trans. 2010, 39, 10314; (d)
Pelerin, O.; Olivier, C.; Roisnel, T.; Touchard, D.; Rigaut, S. J. Organomet. Chem.
2008, 693, 2153; (e) Ono, K.; Saito, K. Heterocyles 2008, 75, 2381.
2. (a) Sako, K.; Kusakabe, M.; Tatemitsu, H. Mol. Cryst. Liq. Cryst. 1996, 285, 101;
(b) Sako, K.; Misaki, Y.; Fujiwara, M.; Maitani, T.; Tanaka, K.; Tatemitsu, H.
Chem. Lett. 2002, 592.
12. (a) Chan, L. L.; Smid, J. J. Am. Chem. Soc. 1968, 90, 4634; (b) Melby, L. R.; Harder,
R. J.; Hertler, W. R.; Mahler, W.; Benzon, R. E.; Mochel, W. E. J. Am. Chem. Soc.
1962, 84, 3374.