HETEROCYCLES

Communication | Special issue | Vol. 80, No. 2, 2010, pp. 909-915
Received, 31st August, 2009, Accepted, 13th October, 2009, Published online, 14th October, 2009.
DOI: 10.3987/COM-09-S(S)122

■ Synthesis and π-Amphoteric Properties of Tris(tetrathiafulvaleno)hexadehydro[12]annulene

Kenji Hara, Masashi Hasegawa, Yoshiyuki Kuwatani, Hideo Enozawa, and Masahiko Iyoda*

Graduate School of Science, Tokyo Metropolitan University, 1-1, Minami-ohsawa, Hachioji, Tokyo 192-0364, Japan

Abstract

The Sonogashira coupling reaction of the diiodide 6 of 1,2-[4,5-bis(butylthio)tetrathiafulvalenyl]ethyne with 4,5-bis(ethynyl)-4’,5’-bis(butylthio)tetrathiafulvanene 5 produced the corresponding tris(tetrathiafulvaleno)hexadehydro[12]annulene 1 in moderate yield. The [12]annulene 1 exhibits multi-redox behavior and solvatochromism in the neutral state.

INTRODUCTION
Hexadehydro[12]annulene has received considerable attention, because its tribenzo-analogue is regarded as a structural unit of graphyne,1 and because various unique transition-metal complexes have been constructed using the [12]annulene frame.2,3 Furthermore, tribenzohexadehydro[12]annulene (TBA) has been employed as a starting material for the synthesis of cage molecules and polyethers.4,5 Recently, we have reported the synthesis and π-amphoteric properties of bis(tetrathiafulvaleno)hexadehydro[12]- annulene 2 and related compounds based on the tetrathiafulvalene (TTF) and [12]annulene moieties.6,7 The annulene 2 exhibited multi-redox potentials, solvatochromism, and the formation of a large sandwich complex. Based on these results, we next synthesized tris(tetrathiafulvaleno)hexadehydro[12]annulene 1. We report here the synthesis, unique redox behavior, and solvatochromic properties of 1.

RESULTS AND DISCUSSION
The synthesis of 1 is summarized in Scheme 1. Although various synthetic methods of accessing hexadehydro[12]annulenes have been reported to date,8,9 we employed the Sonogashira coupling of the bis(ethynyl)-TTF 5 with the diiodo-bi-TTF 6 similar to our previously reported procedure6 owing to the instability of 1 to light, atmospheric oxygen, and acidic condition. Thus, the reaction of the diiodo-TTF 3 with trimethylsilylacetylene (4 equiv) in the presence of Pd(PPh3)4 (15 mol%), CuI (30 mol%), and Et3N

in benzene at 50 °C for 12 h produced the bis(trimethylsilylethynyl)-TTF 4 in 74% yield. The treatment of 4 with KOH (excess) in THF-methanol (1:1) at room temperature for 3 min yielded 5 to remove the trimethylsilyl groups. Since 5 was unstable and readily polymerized after removal of the solvent, a solution of 5 in benzene was employed for the following reaction without further purification. The Sonogashira coupling of 6 with 5 (1.65 equiv based on 100% conversion of 4) in the presence of Pd(PPh3)4 (50 mol%) and CuI (100 mol%) in benzene-triethylamine (10:3) at room temperature for 5 h produced the desired 1 in 36% yield based on 6.10 For the synthesis of 1, almost stoichiometric amounts of Pd(PPh3)4 and CuI were required to complete the reaction.

Interestingly, the tris(TTF)annulene 1 shows solvatochromism, and a solution of 1 is deep green in CS2 but bright green in CH2Cl2. As shown in Figure 1, the UV-Vis-NIR spectrum of 1 shows strong (322 nm, ε = 90,000) and weak (623-656 nm, ε = 3500-4000) absorptions. The strong absorption is unchanged with the type of solvent, whereas the weak absorption varies with the type of solvent used [λmax (CS2) 656 nm and λmax (CH2Cl2) 623 nm]. Since the longest absorption is assigned to the charge-transfer (CT) band from the TTF (π-donor) to [12]annulene (π-acceptor) moieties, this transition is sensitive to the polarity of the solvent.

The cyclic voltammetric (CV) analysis of 1 shows unique redox properties owing to the π-amphoteric nature of 1. As shown in Table 1, 1 and 2 indicated 4-step redox processes; namely, the formation of 12-, 1-, 13+, and 16+, or 22-, 2-, 22+, and 24+. Since tribenzohexadehydro[12]annulene (TBA) shows two reduction waves at -2.50 and -2.19 V vs Fc/Fc+ under the same conditions, the reduction potential of the [12]annulene unit increases in the order 1 > 2 > TBA, reflecting the increase in the degree of cyclic conjugation.11 In contrast, the oxidation potentials of 1 and 2 seemed to be similar. However, the first oxidation potential of 1 was split into two (Eox11/2(1) = 0.12 V; Eox11/2(2) = 0.26 V vs Fc/Fc+) when measured at a very slow rate (3 mV s-1), while the first oxidation potential of 2 showed a broad oxidation even when measured at a very low rate.12 Consequently, the oxidation potential of the TTF units decreases in the order 2 ≥ 1 > TTF, reflecting the increase in donor ability.

The CV analysis of 1 showed three oxidation potentials (Table 1). Accordingly, the chemical oxidation of 1 with Fe(ClO4)3 revealed characteristic changes in color and electronic spectra.13 As shown in Figure 2, the oxidation of 1 with 1, 2, 3, and 6 equiv of Fe(ClO4)3 in CH2Cl2‒CH3CN (v/v 4:1) resulted in the formation of 1+· (859 and ca. 2000 nm), 12+ (872 nm), 13+ (860 nm), and 16+ (694 nm), respectively. The solutions changed from green (1) to dark orange (1+·), greenish orange (12+), dark green (13+), and blue (16+). The cation radical 1+· shows a very broad absorption at approximately 2000 nm probably owing to the strong intermolecular interaction between the TTF and TTF+· units. However, the possible formation of a mixed valence dimer (123+) was ruled out, because 123+ (i.e., 11.5+ in Figure 2)14 exhibited a weak absorption at approximately 2000 nm as shown in Figure 2. Regarding 13+, no π-dimer formation was observed based on its electronic spectra, and the absorption of 13+ (860 nm) appeared almost the same as that of 1+· (859 nm).15 However, the absorption of 12+ (872 nm) showed a red shift corresponding to the intramolecular head-to-tail interaction of two TTF+· units.16 In contrast to the preferable π-dimer formation of tris(TTF)[18]annulene trications,17 the absence of the π-dimer formation of 13+ might have been due to the difficulty in stacking the [4n] π-electron system.18

In summary, the synthesis of the tris(TTF)[12]annulene 1 was successfully carried out using the nearly stoichiometric Sonogashira coupling of the diiodo-biTTF 6 with the diethynyl-TTF 5. The TTF-annulene 1 exhibits solvatochromism, electrochromism, and multi-redox behavior owing to the π-amphoteric nature of 1. Although 1 is unstable in the solid state, presumably owing to the combination of the [4n] π-electron system with π-donors, the introduction of electron-withdrawing groups into the TTF units in 1 can stabilize the molecule.

ACKNOWLEDGEMENTS
This work was supported in part by a Grant-in-Aid for Scientific Research from JSPS and CREST of JST. KH would like to acknowledge a research fellowship for young scientists from JSPS. We are grateful to Prof. Masato Yoshida (Shimane University) and Prof. Haruo Matsuyama (Muroran Institute of Technology) for their helpful discussions.

References

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