- Email: [email protected]
Synthesis and redox properties of π-conjugated 4,5-diazafluorene derivatives incorporating 9-cyanomethylene moiety as an electron acceptor
Tetrahedron Letters 52 (2011) 5865–5868
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
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 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 b
a r t i c l e
i n f o
Article history: Received 5 July 2011 Revised 23 August 2011 Accepted 29 August 2011 Available online 7 September 2011 Keywords: 4,5-Diazafluorene Acceptor Knoevenagel condensation Microwave assisted synthesis Electrochromism
a b s t r a c t 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 (32 ), anion radical (3 ), neutral (3)) exhibit remarkable stability. Ó 2011 Elsevier Ltd. All rights reserved.
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 architectures. Conjugated donor ligands such as 1 and 2 (Chart 1) bearing the 4,5-diazafluorene unit as the coordinating site have been reported.2 Instead of p-conjugated donors, we designed new p-conjugated 4,5diazafluorene 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, however, synthesis and characterization of 3a were not done.3 In this article, as the first step toward charge separation subunits for photodiode 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 different 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: [email protected] (K. Sako). 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.08.164
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, reactions under conventional heating (80 or 110 °C) required much longer periods (12–18 h, Table 1). In very active methylene compound, 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 structure of 3a, together with the crystallographic atom-numbering scheme, is shown in Figure 1. The diazafluorene and dicyanomethylene 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 fulvalene derivatives (0.145–0.147 nm). These facts suggest that the
5866
K. Sako et al. / Tetrahedron Letters 52 (2011) 5865–5868
N N 1a : R = CH3 1b : R =C2H5
S
SR
N
S
SR
N
1c : R = C4H9 1d : R = -CH2CH2-
S
S
S
R
S
S
S
R
2a : R = H 2b : R =CH3
2c : R = SCH3 2d : R = -OCH2CH2O-
Chart 1. Conjugated donor ligands 1 and 2.
N
CN
N
CN
N
Y
O Y
N
3a-d
3a : Y=CN 3b : Y=COOEt 3c : Y=COPh 3d : Y=
COOMe
Scheme 1. Synthesis of the compounds 3a–d.
five-membered ring in the diazafluorene moiety of 3a is an almost ideal pentagon and the C–C bonds have some double bond character. As shown in Figure 2, the molecules of 3a show p–p stacking in an ‘orthogonal head-to-tail’ manner along the b axis (0.341 nm: diazafluorene ring is regarded as the head moiety). In the same planar column, 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 dicyanomethylene moiety and hydrogen atoms (H11 and H10) of the diazafluorene are 0.264 and 0.272 nm, respectively. These short contacts indicate that N H hydrogen bonds are formed. 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 reduction step (Ered11/2 3a: 0.98 V; 3b: 1.13 V vs Fc0/Fc+) corresponding to the formation of the radical anionic species is reversible, whereas the second reduction leading to the dianionic species (Ered21/2 3a: 1.62 V; 3b: 1.56 V vs Fc0/Fc+) appears to be quasireversible. The first reduction potential of 3a was observed as a positive 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
a b c
Entry
Y
Method
Base
Solvent
Irradiation power
Time (min)
Yield
1 2 3 4 5 6 7 8 9 10 11 12
CN CN CN CN COOEt COOEt COOEt COOEt COPh COPh Ph-p-COOMe CN
A A A B A A A B A A A Ultra sonic
A A B A A B C A A C D A
— Benzene/AcOH EtOH Benzene/AcOH Toluene/AcOH EtOH Toluene/AcOH Benzene/AcOH Toluene/AcOH Toluene/AcOH Toluene/MeOH Benzene/AcOH
200 200 200 — 300 300 300 — 300 300 300 100
10 10 5 1200 30 30 30 2400 30 30 20 40
83 99 97 51 73 31 92 73 13 14 10 63
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.
5867
K. Sako et al. / Tetrahedron Letters 52 (2011) 5865–5868
Figure 2. The molecular packing manners of 3a.
+60
2.5
+40
2.0
0
Absorbance / a.u.
I (µA)
+20
-20 -40
(3a) 0V -1.0 V (3a ) 2-1.7 V (3a )
1.5
1.0
-60
0.5 -80 -2.5
-2
-1.5
-1
-0.5
0
+
Potential (V vs. Fc/Fc )
0 300
Figure 3. Cyclic voltammogram of 3a in dichloromethane.
500
600
700
Wavelength / nm Figure 4. Spectroelectrochemical absorption of 3a in CH2Cl2 solution; black line: 0 V (3a), red line: 1 V (3a ), blue line: 1.7 V (3a2 ).
Table 2 Redox potentialsa of 3 and related compounds
3a 3b 3c 3d TCNE TCNQ
400
Ered11/2
Ered21/2
0.98 1.13 1.07 1.39 0.25 0.28
1.62 1.56 1.37 1.60 0.79 0.86
DE 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 electrodes. Potentials were measured against an Ag/Ag+ electrode. E in V versus Fc/Fc+.
of 3d was smaller than those of 3a–c by stabilizing effect of aromatic 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 compound 3a are shown in Figure 4.11 Upon increasing the potential, as reduction of the system proceeds, we see the concomitant emergence of the low energy absorption band of non-interacting dicyanomethylene anion radicals at kmax 485 nm ( Fig. 4, spectrum at 1.0 V). In the cases of 9-alkylfluorenide and TCNQ, the typical
5868
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 9alkylfluorenide 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-withdrawing ability than that of TCNQ. The formation of the dianion species while scanning down to 1.7 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). 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 decomposition. We believe that compound 3a is a very promising electrochromic material by virtue of having three redox states (3a, 3a , 3a2 ) each with distinctive visible absorption spectra. In summary, we have prepared diazafluorene functionalized ligands (3a–d, DAF-CN). Compound 3a shows distinctive electrochromism; the remarkable stability of three differently colored redox states of 3a (3a, 3a , 3a2 ) makes it a promising electrochromic material for the visible spectral region as the first step toward charge separation. Acknowledgments This research was partially supported by a Grant-in-Aid for Exploratory Research (No.20550038) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. This work was performed under the Cooperative Research Program of ‘Network Joint Research Center for Materials and Devices (Institute for Materials Chemistry and Engineering, Kyushu University)’. Supplementary data Supplementary data (the experimental procedure and additional characterization data for all compounds along with their 1 H NMR and 13C NMR spectra, absorption spectra, cyclic voltammograms) associated with this article can be found, in the online version, at doi:10.1016/j.tetlet.2011.08.164. References and notes 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.
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 cm 1; 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 cm 1; 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]+. Compound 3c: Yellow powder; mp 156–157 °C; IR(KBr): 2208, 1669 cm 1; 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 cm 1; 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) 13 C 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 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. (#2), 7. Crystal data for 3a: 2(C14H6N4), M = 460.46, triclinic, space group P1 a = 9.4619(4), b = 10.4095(5), c = 11.7455(9) Å, a = 83.718(3), b = 81.778(4), c = 66.207(3)°, V = 1054.94(10) Å3, Z = 2, Dc = 1.462 g cm 3, l(Mo Ka) = 0.093 mm 1, T = 123(2) K, 7386 reflections, R1 = 0.0361 (I > 2.0r(I)), 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 [email protected]). 8. Seiler, P.; Dunitz, J. D. Acta Cryst. 1979, B35, 1068. 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 (3 ), dianion (32 ) show two reversible reduction peaks. 11. Absorption spectroelectrochemistry of 3a (4 10 4 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. 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.
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
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 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 b
a r t i c l e
i n f o
Article history: Received 5 July 2011 Revised 23 August 2011 Accepted 29 August 2011 Available online 7 September 2011 Keywords: 4,5-Diazafluorene Acceptor Knoevenagel condensation Microwave assisted synthesis Electrochromism
a b s t r a c t 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 (32 ), anion radical (3 ), neutral (3)) exhibit remarkable stability. Ó 2011 Elsevier Ltd. All rights reserved.
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 architectures. Conjugated donor ligands such as 1 and 2 (Chart 1) bearing the 4,5-diazafluorene unit as the coordinating site have been reported.2 Instead of p-conjugated donors, we designed new p-conjugated 4,5diazafluorene 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, however, synthesis and characterization of 3a were not done.3 In this article, as the first step toward charge separation subunits for photodiode 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 different 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: [email protected] (K. Sako). 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.08.164
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, reactions under conventional heating (80 or 110 °C) required much longer periods (12–18 h, Table 1). In very active methylene compound, 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 structure of 3a, together with the crystallographic atom-numbering scheme, is shown in Figure 1. The diazafluorene and dicyanomethylene 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 fulvalene derivatives (0.145–0.147 nm). These facts suggest that the
5866
K. Sako et al. / Tetrahedron Letters 52 (2011) 5865–5868
N N 1a : R = CH3 1b : R =C2H5
S
SR
N
S
SR
N
1c : R = C4H9 1d : R = -CH2CH2-
S
S
S
R
S
S
S
R
2a : R = H 2b : R =CH3
2c : R = SCH3 2d : R = -OCH2CH2O-
Chart 1. Conjugated donor ligands 1 and 2.
N
CN
N
CN
N
Y
O Y
N
3a-d
3a : Y=CN 3b : Y=COOEt 3c : Y=COPh 3d : Y=
COOMe
Scheme 1. Synthesis of the compounds 3a–d.
five-membered ring in the diazafluorene moiety of 3a is an almost ideal pentagon and the C–C bonds have some double bond character. As shown in Figure 2, the molecules of 3a show p–p stacking in an ‘orthogonal head-to-tail’ manner along the b axis (0.341 nm: diazafluorene ring is regarded as the head moiety). In the same planar column, 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 dicyanomethylene moiety and hydrogen atoms (H11 and H10) of the diazafluorene are 0.264 and 0.272 nm, respectively. These short contacts indicate that N H hydrogen bonds are formed. 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 reduction step (Ered11/2 3a: 0.98 V; 3b: 1.13 V vs Fc0/Fc+) corresponding to the formation of the radical anionic species is reversible, whereas the second reduction leading to the dianionic species (Ered21/2 3a: 1.62 V; 3b: 1.56 V vs Fc0/Fc+) appears to be quasireversible. The first reduction potential of 3a was observed as a positive 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
a b c
Entry
Y
Method
Base
Solvent
Irradiation power
Time (min)
Yield
1 2 3 4 5 6 7 8 9 10 11 12
CN CN CN CN COOEt COOEt COOEt COOEt COPh COPh Ph-p-COOMe CN
A A A B A A A B A A A Ultra sonic
A A B A A B C A A C D A
— Benzene/AcOH EtOH Benzene/AcOH Toluene/AcOH EtOH Toluene/AcOH Benzene/AcOH Toluene/AcOH Toluene/AcOH Toluene/MeOH Benzene/AcOH
200 200 200 — 300 300 300 — 300 300 300 100
10 10 5 1200 30 30 30 2400 30 30 20 40
83 99 97 51 73 31 92 73 13 14 10 63
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.
5867
K. Sako et al. / Tetrahedron Letters 52 (2011) 5865–5868
Figure 2. The molecular packing manners of 3a.
+60
2.5
+40
2.0
0
Absorbance / a.u.
I (µA)
+20
-20 -40
(3a) 0V -1.0 V (3a ) 2-1.7 V (3a )
1.5
1.0
-60
0.5 -80 -2.5
-2
-1.5
-1
-0.5
0
+
Potential (V vs. Fc/Fc )
0 300
Figure 3. Cyclic voltammogram of 3a in dichloromethane.
500
600
700
Wavelength / nm Figure 4. Spectroelectrochemical absorption of 3a in CH2Cl2 solution; black line: 0 V (3a), red line: 1 V (3a ), blue line: 1.7 V (3a2 ).
Table 2 Redox potentialsa of 3 and related compounds
3a 3b 3c 3d TCNE TCNQ
400
Ered11/2
Ered21/2
0.98 1.13 1.07 1.39 0.25 0.28
1.62 1.56 1.37 1.60 0.79 0.86
DE 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 electrodes. Potentials were measured against an Ag/Ag+ electrode. E in V versus Fc/Fc+.
of 3d was smaller than those of 3a–c by stabilizing effect of aromatic 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 compound 3a are shown in Figure 4.11 Upon increasing the potential, as reduction of the system proceeds, we see the concomitant emergence of the low energy absorption band of non-interacting dicyanomethylene anion radicals at kmax 485 nm ( Fig. 4, spectrum at 1.0 V). In the cases of 9-alkylfluorenide and TCNQ, the typical
5868
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 9alkylfluorenide 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-withdrawing ability than that of TCNQ. The formation of the dianion species while scanning down to 1.7 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). 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 decomposition. We believe that compound 3a is a very promising electrochromic material by virtue of having three redox states (3a, 3a , 3a2 ) each with distinctive visible absorption spectra. In summary, we have prepared diazafluorene functionalized ligands (3a–d, DAF-CN). Compound 3a shows distinctive electrochromism; the remarkable stability of three differently colored redox states of 3a (3a, 3a , 3a2 ) makes it a promising electrochromic material for the visible spectral region as the first step toward charge separation. Acknowledgments This research was partially supported by a Grant-in-Aid for Exploratory Research (No.20550038) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. This work was performed under the Cooperative Research Program of ‘Network Joint Research Center for Materials and Devices (Institute for Materials Chemistry and Engineering, Kyushu University)’. Supplementary data Supplementary data (the experimental procedure and additional characterization data for all compounds along with their 1 H NMR and 13C NMR spectra, absorption spectra, cyclic voltammograms) associated with this article can be found, in the online version, at doi:10.1016/j.tetlet.2011.08.164. References and notes 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.
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 cm 1; 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 cm 1; 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]+. Compound 3c: Yellow powder; mp 156–157 °C; IR(KBr): 2208, 1669 cm 1; 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 cm 1; 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) 13 C 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 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. (#2), 7. Crystal data for 3a: 2(C14H6N4), M = 460.46, triclinic, space group P1 a = 9.4619(4), b = 10.4095(5), c = 11.7455(9) Å, a = 83.718(3), b = 81.778(4), c = 66.207(3)°, V = 1054.94(10) Å3, Z = 2, Dc = 1.462 g cm 3, l(Mo Ka) = 0.093 mm 1, T = 123(2) K, 7386 reflections, R1 = 0.0361 (I > 2.0r(I)), 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 [email protected]). 8. Seiler, P.; Dunitz, J. D. Acta Cryst. 1979, B35, 1068. 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 (3 ), dianion (32 ) show two reversible reduction peaks. 11. Absorption spectroelectrochemistry of 3a (4 10 4 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. 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.