Preparation, properties, and reduction of heteroaromatic quinoids with 1,4-diazacyclopentadien-2-ylidene terminals

Article abstract of DOI:10.1021/ol0476927

(Chemical Equation Presented) New n-type heteroaromatic quinoids with 1,4-diazacyclopentadien-2-ylidene terminals were prepared. These quinoids possess strong electron-accepting properties, comparable to dicyanomethylene analogues. Both the radical anions

Full text of DOI:10.1021/ol0476927

ORGANIC
LETTERS
2005
Vol. 7, No. 1
115-118
Preparation, Properties, and Reduction
of Heteroaromatic Quinoids with
1,4-Diazacyclopentadien-2-ylidene
Terminals
Masatoshi Kozaki, Asuka Isoyama, Kogen Akita, and Keiji Okada*
Department of Chemistry, Graduate School of Science, Osaka City UniVersity,
3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
okadak@sci.osaka-cu.ac.jp
Received November 10, 2004
ABSTRACT
New n-type heteroaromatic quinoids with 1,4-diazacyclopentadien-2-ylidene terminals were prepared. These quinoids possess strong electron-
accepting properties, comparable to dicyanomethylene analogues. Both the radical anions and dianions were generated by metal reduction
using 3% Na−Hg in THF and were spectroscopically characterized.
Considerable work has been done on charged species of
conjugated oligomers and dendrimers to obtain insights on
doped and conducting species.1 Many spectroscopic and
crystallographic methods have been used to study cationic
species of thiophene-based oligomers, which are used as
models for polythiophene.2,3 In contrast to p-type materials,
there are only a limited number of n-type molecular as-
semblies, and only a few anionic species of those have been
characterized.4-6 Quinoidal oligothiophene with dicyano-
methylene terminals is one such compound and its reduced
species has been spectroscopically characterized.6
In the course of our studies of charged and spin systems
of molecular assemblies3,7 we have been interested in
development of new redox systems applicable to molecular
assemblies. 1,4-Diazacyclopentadienylidene (DACP) seems
to be an attractive functional group to form an aromatic 6π
(5) For some selected papers for n-dopable oligothiophenes, see: (a)
Takahashi, K.; Fujita, S.; Akiyama, K.; Miki, M.; Yanagi, K. Angew. Chem.
Int. Ed. 1998, 37, 2484-2487. (b) Sakamoto, Y.; Komatsu, S.; Suzuki, T.
J. Am. Chem. Soc. 2001, 123, 4643-4644. (c) Yonezawa, Y.; Kozaki, M.;
Okada, K. Synth. Met. 2001, 119, 99-100. (d) Ohnishi, H.; Kozaki, M.;
Okada, K. Synth. Met. 2003, 135, 85-86. (e) Tarutani, S.; Takahashi, K.
Bull. Chem. Soc. Jpn. 2004, 77, 463-475.
(6) (a) Gronowitz, S.; Uppstro¨m, B. Acta Chem. Scand. 1974, B28, 981-
985. (b) Yui, K.; Aso, T.; Otsubo, T.; Ogura, F. Bull. Chem. Soc. Jpn.
1989, 62, 1539-1546. (c) Pappenfus, T. M.; Chesterfield, R. J.; Frisbie, C.
D.; Mann, K. R.; Casado, J.; Raff, J. D.; Miller, L. L. J. Am. Chem. Soc.
2002, 124, 4184-4185. (d) Pappenfus, T. M.; Raff, J. D.; Hukkanen, E. J.;
Burney, J. R.; Casado, J.; Drew, S. M.; Miller, L. L.; Mann, K. R. J. Org.
Chem. 2002, 67, 6015-6024. (e) Chesterfield, R. J.; Newmann, C. R.;
Pappenfus, T. M.; Ewbank, P. C.; Haukaas, M. H.; Mann, K. R.; Miller, L.
L.; Frisbie, C. D. AdV. Mater. 2003, 15, 1278-1282.
(7) (a) Okada, K. In Molecular Magnetism; Itoh, K., Kinoshita, M., Eds.;
Kodansha-Gordon and Breach: Tokyo, 2000; pp 264-277. (b) Okamoto,
T.; Kozaki, M.; Yamashita, Y.; Okada, K. Tetrahedron Lett. 2001, 42,
7591-7594. (c) Hiraoka, S.; Okamoto, T.; Kozaki, M.; Shiomi, D.; Sato,
K.; Takui, T.; Okada, K. J. Am. Chem. Soc. 2004, 126, 58-59. (d) Okamoto,
T.; Kuratsu, M.; Kozaki, M.; Hirotsu, K.; Ichimura, A.; Matsushita, T.;
Okada, K. Org. Lett. 2004, 6, 3493-3496.
(1) (a) Mullen, K.; Wegner, G. Electronic Materials: The Oligomer
Approach; Wiley-VCH: Weinheim, Germany, 1998. (b) Newcome, G. R.;
Moorefield, C. N.; Vo¨gtle, F. Dendrimers and Dendrons: Concepts,
Syntheses, Applications; Wiley-VCH: Weinheim, Germany, 2001.
(2) (a) Miller, L. L.; Mann, K. R. Acc. Chem. Res. 1996, 29, 417-423.
(b) van Haare, J. A. E. H.; Havinga, E. E.; van Dongen, J. L. J.; Janssen,
R. A. J.; Cornil, J.; Bre´das, J.-L. Chem. Eur. J. 1998, 4, 1509-1522. (c)
Aso, Y.; Takimiya, K.; Otsubo, T. J. Synth. Org. Chem., Jpn. (Yuki Gousei
Kagaku Kyokaishi) 2002, 60, 52-61. (d) Nishinaga, T.; Wakamiya, A.;
Yamazaki, D.; Komatsu, K. J. Am. Chem. Soc. 2004, 126, 3163-3174.
(3) Kozaki, M.; Yonezawa Y.; Okada, K. Org. Lett. 2002, 4, 4535-
4538.
(4) Nishida, J.; Naraso; Murai, S.; Fujiwara, E.; Tada, H.; Tomura, M.;
Yamashita, Y. Org. Lett. 2004, 6, 2007-2010 and references therein.
10.1021/ol0476927 CCC: $30.25
© 2005 American Chemical Society
Published on Web 12/14/2004

system upon one-electron reduction.8 However, until now,
there has been no report on the reduction of DACP-quinoidal
compounds. Here, we report on the preparation, properties,
and reduction of a series of heteroaromatic qunoids with
DACP-terminals 1.
Table 1. Reduction Potentialsa of 1, TCNQ, and 4
(V vs SCE)
E1
E2
∆E (V)
1/2
1/2
The heteroaromatic quinoids 1a and 1b were synthesized
by applying the method of synthesis for the known 1c
(Scheme 1).9,10 Condensation of 2 with benzil in the presence
1a
1b
1c
TCNQ
4b
-0.04
-0.25
0.01
0.27
0.07
-0.14
-0.45
-0.09
-0.45
-0.54
0.10
0.20
0.10
0.72
0.61
Scheme 1
a In CH2Cl2 in the presence of 0.1 M (n-C4H9)4NClO4 at 50 mV/s with
a SCE reference using a glassy carbon working electrode and a Pt-counter
electrode. b Reference 6b.
summarized in Table 1, along with those of TCNQ and 4.
Compounds 1a showed two reversible one-electron reduction
waves at close potentials (-0.04 and -0.14 V vs SCE)
(Figure 2). The reduction potential shifts to a lower value
of ammonium acetate in acetic acid gave compounds 3 (3a;
54%, 3b; 90% yields). Oxidation of 3 using K3Fe(CN)6 in
benzene-aqueous KOH solution afforded compounds 1 (1a;
70%, 1b; 74% yields) as deeply colored stable solids (λmax
,
nm (log ꢀ) in CH2Cl2, 1a; 632 (5.01), 1b; 669 (5.16)).
X-ray crystallographic analysis of 1a showed a highly
planar quinoid structure except for four terminal phenyl
groups (Figure 1).11 The bond lengths and angles are
consistent with the reported thienoquinoids.6d,12
Figure 2. Cyclic voltammogram of 1a measured in CH2Cl2
containing 0.1 M (n-C4H9)4NClO4.
as the central ring of the quinoid becomes electron-rich. The
reduction potential of 1c is a little higher than that of 1a,
whereas the difference is much larger between TCNQ and
4. The difference between the first and second reduction
potentials (∆E, 0.1-0.2 V) observed for 1 is significantly
smaller than those of TCNQ and 4. The smaller ∆E values
(8) (a) Dedik, S. G.; Orlov, V. D.; EÄ dzhinya, A. S.; Khodorkovskii, V.
Yu.; Neiland, O. Ya. Khim. Geterotsikl. Soedin. 1989, 10, 1421. (b)
Prasanna, S.; Radhakrishnan, T. P. Synth. Met. 1996, 78, 127-130. (c) Aqad,
E.; Leriche, P.; Mabon, G.; Gorgues, A.; Khodorkovsky, V. Tetrahedron
Lett. 2001, 42, 2813-2815.
(9) (a) Rohr, W.; Staab, H. A. Angew. Chem., Int. Ed. Engl. 1965, 4,
1073-1074. (b) Mayer, D.-C. U.; Baumuga¨rtel, H. Angew. Chem., Int. Ed.
Engl. 1966, 5, 311. (c) Mayer, D.-C. U.; Baumuga¨rtel, H.; Zimmermann,
H. Tetrahedron Lett. 1966, 42, 5221-5223.
(10) (a) Krieg, B.; Manecke, G. Z. Naturforsch., Teil B 1967, 22, 132-
141. (b) Krieg, B.; Manecke, G. Makromol. Chem. 1967, 108, 210-217.
(c) Okada, K.; Imamura, K.; Oda, M.; Kozaki, M.; Isino, K.; Tashiro, K.
Chem. Lett. 1998, 891-892.
(11) Crystallographic data for 1a: C41H36N4O2S, monoclinic, space group
C2/c (#15), a ) 38.54(2) Å, b ) 8.216(7) Å, c ) 26.46(2) Å, V ) 7212.0-
(68) Å3, Z ) 8, Fcalcd ) 1.191 g/cm3, T ) 296 K, R ) 0.075, Rw ) 0.082.
(12) (a) Hieber, G.; Hanack, M.; Wurst, K.; Stra¨hle, J. Chem. Ber. 1991,
124, 1597-1605. (b) Takahashi, K.; Suzuki, T.; Akiyama, K.; Ikegami,
Y.; Fukazawa, Y. J. Am. Chem. Soc. 1991, 113, 4576-4583. (c) Suzuki,
K.; Tomura, M.; Yamashita, T. J. Mater. Chem. 1998, 8, 1117-1119.
Figure 1. Molecular structures (ORTEP) of 1a: (a) overview; (b)
side view and labeling. Selected bond lengths (Å): S1-C1,
1.757(8); S1-C4, 1.742(7); N1-C8, 1.377(9); N1-C10, 1.305-
(9); N2-C8, 1.383(9); N2-C9, 1.313(9); N3-C5, 1.373(9);
N3-C7, 1.311(9); N4-C5, 1.371(9); N4-C6, 1.328(9); C1-C2,
1.441(10); C1-C5, 1.377(10); C2-C3, 1.365(10); C3-C4, 1.410-
(10); C4-C8, 1.382(10); C6-C7, 1.469(10); C9-C10, 1.483(10).
The electron-accepting ability of 1 was estimated by means
of cyclic voltamogram. The half-wave potentials of 1 are
116
Org. Lett., Vol. 7, No. 1, 2005

shows a diminished on-site Coulombic repulsion in the
dianionic species compared with 4, due to the delocalization
of negative charges in the larger DACP rings. The ∆E value
of 0.1 V for 1a and 1c indicates that disproportionation of
two radical anions to the neutral and dianion occurs to a
small extent (Kdisp ∼ 0.02 at 25 °C). Hence, both the radical
anion and the dianion species would be stable and observable,
provided that a suitable reducing method is used.
tion bands grew at 1033 (log ꢀ ) 4.05)13 and 1211 nm
(4.39). Similar spectral changes were observed for the
reduction of 1b and 1c, giving absorptions in the NIR region
(λmax ) 892 (log ꢀ ) 3.87), 1020 (4.21) for 1b•-, λmax
)
1185 (log ꢀ ) 4.16), 1423 (4.42) nm for 1c•-). Obviously,
these species are ascribed to the formation of the anion
radical species as evidenced by the observation of the EPR
signal [g ) 2.00234 (1a•-), 2.00016 (1b•-), 2.00017 (1c•-)]
in THF-toluene (1:1).
Metal reduction of 1 using 3% Na-Hg as a reducing agent
in dry THF under degassed conditions was carried out
(Scheme 2). The reduction was monitored by measuring the
The observed NIR absorptions are characteristic of inter-
valence bands in class II or charge resonance bands in class
III compounds according to the classification of Robin and
Day.14 The observed absorptions accompany vibrational
progressions and the bandwidth for the 1211 nm (8258 cm-1)
absorption is about 720 cm-1, which is considerably smaller
than the theoretical minimum value for intervalence bands,
ν1/2(HTL) ) 4254 cm-1,15 indicating the observed absorption
is a charge resonance band rather than an intervalence band.
Theoretical calculations using the ZINDO method using the
Gaussian program package16 gave a 1149 nm absorption
(osillator strength ) 0.66 with 96% contribution of HOMOâ
f LUMOâ transition to the first excited state). The MO
diagram and the MO shapes are depicted in Figure 4. Both
Scheme 2
UV-vis-NIR spectrum. The spectrum change as a func-
tion of contact time with 3% of Na-Hg alloy can be
separated in two stages, i.e, formation of the anion radical
(Figure 3a) and the dianion formation (Figure 3b) for the
reduction of 1a.
In the first stage, the intense original absorption (λmax
)
633 nm, log ꢀ ) 4.96) of 1a disappeared and new absorp-
Figure 4. Orbital diagram and the shape of HOMO(â) and
LUMO(â) for the radical anion 1a•- calculated at the ZINDO/S
level theory.
orbitals are well delocalized in the whole molecule, compat-
ible with these anion radicals being categorized as class III
(Robin and Day) species.
Figure 3. UV-vis-NIR spectral change for 1a, plotted as a
function of contact time (t) with 3% Na-Hg in THF. Arrows
indicate the direction of changes. Reduction of 1a to the radical
anion 1a•- (a) and 1a•- to the dianion 1a2- (b).
(13) The molar absorptivities for the radical anions and dianions were
calculated by assuming clean conversion.
(14) Robin, M. B.; Day, P. AdV. Inorg. Chem., Radiochem. 1967, 10,
247-422.
(15) Hush, N. S. Coord. Chem. ReV. 1985, 64, 135-157.
Org. Lett., Vol. 7, No. 1, 2005
117

b
1
Table 2. Selected H NMR Dataa of Dianion 12- (δ/ppm)
H (central rings)
H (Ph-o)
H (Ph-m)
H (Ph-p)
1a
8.40
7.16
8.04
6.28
8.58
7.88
7.74-7.82
7.52
7.78, 7.74
7.64
7.40-7.51
1a2-
1b
7.08
6.93
7.38-7.46
1b2-
1c
1c2-
7.09
7.45
7.11
6.90
7.51
6.95
7.83
7.59
a 400 MHz, in THF-d8. b Obtained by the reduction with Na mirror
(1a2- and 1c2-) and K mirror (1b2-).
Supporting Information Available: Detailed synthetic
procedure for 1a and 1b, crystallographic data (CIF), and
spectroscopic data (UV-vis-NIR including line-shape
analysis, EPR, and NMR). These materials are available free
of charge via the Internet at http://pubs.acs.org.
Figure 5. 1H NMR spectra of neutral 1a (top) and dianion 1a2-
(bottom) measured in THF-d8.
OL0476927
(16) The geometrical optimization (UB3LYP/6-31G*) was carried out
using the X-ray structure of 1a as an initial input. In the ZINDO/S-CI
calculation, 20 highest occupied and 20 lowest vacant orbitals were
considered. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E. Yazyev, O.; Austin, A.; Cammi, R.; Pomelli,
C.; Ochterski, J.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain,
M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski,
J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.01;
Gaussian Inc.: Pittsburgh, PA, 2003.
Further reduction resulted in a decrease of the radical
anion bands and growth of a new band assignable to di-
anion at λmax ) 400 nm (log ꢀ ) 4.59) for 1a2- (Figure 3b),
and 342 (4.41) and 382 nm (4.70) for 1b2- and 1c2-,
respectively.
1
Figure 5 shows H NMR spectra for the neutral 1a and
the dianion 1a2-.17 All the protons appear at higher fields in
the dianion; the protons on the central ring received the
largest upfield shift to the normal region of the heteroaro-
matic ring. There are obviously two kinds of phenyl groups
in 1a, whereas they are equivalent in 1a2-. A similar spectral
change was also observed for the reduction of 1b and 1c
(Table 2).
A study on oligomeric heteroaromatic-quinoids with
DACP-terminals is in progress.
(17) Dianions for the 1H NMR specta were generated with Na mirror
for 1a2- and 1c2- and K mirror for 1b2-
.
118
Org. Lett., Vol. 7, No. 1, 2005