Synthesis and redox behavior of benzo-1,3-dithiafulvalenes incorporating ferrocene moiety as redox active ligands

Synthetic Metals 140 (2004) 95–100

Synthesis and redox behavior of benzo-1,3-dithiafulvalenes incorporating ferrocene moiety as redox active ligands Abd El-Wareth Sarhan*, Taeko Izumi Department of Chemistry and Chemical Engineering, Faculty of Engineering, Yamagata University, 3-16 Jonan 4-Chome, Yonezawa 992-8510, Japan Received 4 April 2003; accepted 25 April 2003

Abstract A simple route for the synthesis of several new benzo-1,3-dithiafulvalenes incorporated with ferrocene (Fc) moiety is reported. The benzo1,3-dithiafulvalenes (6a–f) were synthesized by introducing a ferrocene coordination function as new donor compounds based on the Wittig– Horner olefenation method. The redox behavior of the new donor compounds was investigated in comparison with the ferrocene and established 1,4-dithiafulvalenes by cyclic voltammetry. Two couples of redox peaks were observed for both ferrocene and extended dithiafulvalene (DTF). # 2003 Elsevier B.V. All rights reserved. Keywords: Fc-DTF; Synthesis; Organic conducting materials; Electrochemical properties

Since the discovery of the conducting properties of the tetrathiafulvalene (TTF) and its derivatives, it has become a subject of great interest for electrochemical studies [1]. Chemical modifications of the TTF’s framework have been added to improve their electrical conductance properties. Several tetrathiafulvalenes were previously prepared via one-step reaction with different yields [2] (Fig. 1). To the best of our knowledge, introducing ferrocene (Fc) as a donor moiety into TTF has not yet been intensively investigated and a few articles have appeared recently describing the solution redox behaviors of the TTF-incorporated ferrocene units [3–6]. The preparation of modified TTF’s mainly focuses on TTF with conjugated spacer like anthraquinodimethane with ferrocene units has been described [7]. Intensive studies on Fc-TTF derivatives having different substituents such as electron-donating and/or electron-withdrawing groups as well as heterocyclic moieties are also studied [8]. Tetrathifulvalenes substituted with two electron-withdrawing ester groups are therefore considered as potential ligands, which can be involved in the formation of organic–inorganic materials [9]. Recently, we have successfully reported the synthesis of TTF incorporated ferrocene moiety as a spacer unit between the two benzo-1,3-dithiole-2-ylidene rings compounds 3a–f that showed higher donor ability and formed CT-complexes [10] (Fig. 2). * Corresponding author. E-mail address: [email protected] (A. El-Wareth Sarhan).

0379-6779/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0379-6779(03)00383-7

In this context, we successfully introduced a ferrocene donor instead of the 1,3-dithiole ring, which considered as a part of the tetrathiafulvalene system. The new class of donor compounds is belongs to the ferrocene-dithiafulvalene (Fc-DTF) family. To the best of our knowledge, the synthesis, redox behavior, electron-donating ability and electrical conductivity of their salts and CT’s complexes have not been investigated intensively. Herein, we present the synthesis of new donor systems comprising covalently linked

S

S

S

S

S

S

S

S

TTF (1)

TTF (2) Fig. 1.

R

S S

Fe

S

S

R 3a-f R = H (a), CH3 (b), 2-Thienyl (c), 2-Furyl (d), C6H4CH3-m (e), C6H5-C≡C (f)

Fig. 2.

96

A. El-Wareth Sarhan, T. Izumi / Synthetic Metals 140 (2004) 95–100 R S

H

S

P(OCH3)2

R

S

n-BuLi/THF

O + Fe

-78oC

S Fe

O (4a-f )

(5)

(6a-f)

Scheme 1.

ferrocene and dithiafulvalene, this system called shortly ferrocene-dithiafulvalene. Additionally, we describe the solution redox properties of the new materials and produce the preliminary data on complex formation with tetracyanop-quinodimethane (TCNQ). The goal of the present study is to synthesize dithiafulvalenes (DTF) (with appropriate coordination functions) and to study their electrochemical behavior (in comparison with ferrocene and the well-known 1,3benzodithiole-2-ylidene derivatives 7 and 8 which is reported in the literature [4,11d]. The well-known synthetic strategy for the preparation of benzo-1,3-dithiole-2-phosphonate (5) [11] was adapted for synthesis of this key compound to obtain the 1,1-bis-ferrocene-dithiafulvalenes (DTF-Fc-DTF) (3a–f) [10] or ferrocene-dithiafulvalenes in relatively higher yields. Treatment of 5 with ferrocenyl ketones 4a–f using the Wittig–Horner method in the presence of n-BuLi at
78 to 0 8C in dry THF afforded the donor compounds 6a–f in variant yields as shown in Scheme 1. Upon using the same method with 1-benzoylferrocene (4c) the target compound 6c could not be obtained and an unexpected and unknown product was isolated in pure form and a high yield.1 A slight modification was made for this method by the addition of ferrocenyl ketone 4c to the dithiolium solution at 0 8C with continuous stirring, Scheme 2. The structures of the compounds 6a–f were determined using spectral analyses and were found in agreement with the suggested structures (Table 1). 1. Electrochemical study The electrochemical redox properties of the newly synthesized Fc-DTF 6a–e based ferrocene carbonyls 4a–e were studied by cyclic voltammetry at room temperature in dry CH2Cl2 solutions, using Pt working electrode, Pt gauze as a 1 Data of the unknown product: 1 H NMR (CDCl3) d 7.90–7.88 (dd, J ¼ 3, and 1.5 Hz, 2H, aromatic-H), 7.55–7.45 (m, 3H, aromatic-H), 7.27– 7.26 (m, 2H, aromatic-H), 7.25–7.05 (m, 2H, aromatic-H), 4.91–4.90 (dd, J ¼ 2, and 3 Hz, 2H, ferrocene-H), 4.59–4.58 (dd, J ¼ 2, and 3 Hz, 2H, ferrocene-H), 4.21 (d, J ¼ 1.5 Hz, 5H, ferrocene-H), 3.97 (t, J ¼ 5.5 Hz, 2H, CH2), 2.50 (t, J ¼ 5.5 Hz, 2H, CH2), 1.83 (quientet, J ¼ 5.5 Hz, 2H, CH2), 1.65 (quienteti, J ¼ 5.5 Hz, 2H, CH2). 13 C NMR (CDCl3) d 139.83, 136.42 (thiafulvalene C=CS2), 131.49, 128.23, 128.07, 125.45, 122.76, 106.89 (aromatic-C and CH), 78.16 (ferrocene-C), 72.57, 71.54, 70.24 (ferrocene-CH), 63.85 (OCH2), 37.05 (CH2), 24.43 (CH2), 23.46 (CH2). FAB MS m/z failed [Mþ, 290 (100)].

counter electrode and Ag/AgCl as a reference electrode and tetra-n-butylammonium perchlorate (TBAP) as the supporting electrolyte. The electrochemical data of the investigated compounds were compared to that of ferrocene and summarized in Table 2. Two couples of redox waves are observed clearly in the cyclic voltammograms for all compounds in the potential

S

H

S

n-BuLi/THF

S

P(OCH3)2

(5)

O

P(OCH3)2

o

-78 C

S O + O

Ph

S

Ph

n-BuLi/THF

S

Fe

Fe 0oC - rt (6c)

(4c)

Scheme 2. Table 1 Physical data of compounds 6a–f Compound no.

R

mp (8C)

Yield (%)

Molecular formula

Mþ (%)

6a 6b 6c 6d 6e 6f

H CH3 C6H5 C4H3S C4H3O Xa

154–155 64–66 165–166 121–123 – –

71 90 12 61 71 10

C18H14FeS2 C19H16FeS2 C24H18FeS2 C22H16FeS3 C22H16FeOS2 C23H24FeOS2

350 364 426 432 416 436

a

(90) (100) (25) (96) (15) (100)

X ¼ CH2 OðCH2 Þ3 CH3 .

Table 2 Cyclic voltammetric parameters of Fc and Fc-DTF compounds at 20 mVs
1 Compound no. Fc 6a 6b 6c 6d 6e

Epc (mV)

Epa (mV)

E0 (mV)

DEp (mV)

P1

P2

P1

P2

P1

P2

P1

P2

481 435 468 459 473 517

– 928 979 1012 1009 753

554 522 539 538 556 592

– 1043 1067 1131 1138 1060

518 479 504 499 515 555

– 986 1023 1072 1074 907

73 87 71 79 83 75

115 88 119 129 307

A. El-Wareth Sarhan, T. Izumi / Synthetic Metals 140 (2004) 95–100

97

Fig. 3.

range ca. 0.30–1.2 V, Figs. 4 and 5. The first couple of redox waves in the potential range ca. 400–600 mV is due to the redox process of ferrocene/ferroceniumþ system. Whereas the second couple of redox waves in the potential range ca. 800–1200 mV is attributed to the DTF/DTFþ redox process. Compound 7 was also prepared following the method that was described in the literature in good yield [11]. The redox behavior of compound 7 showed the appearance of an oxidation peak as an irreversible at ca. Ep ¼ 791–828 mV depending on the scan rates (20–600 mV) (Table 2). These voltammetric data are in agreement with that previously reported [4] for the redox behavior of the compound 8 in which the oxidation peak was observed as an irreversible at more positive potential value (Ep ¼ 930 mV), Fig. 3. The cyclic voltammograms of the Fc-DTF 6a–e derivatives recorded in CH2Cl2 at different scan rates and between 0.0 and 1.4 V versus Ag/AgCl and represented in Figs. 4 and 5. On increasing the scan rate from 20 to 600 mV/s, the first anodic peak potential, Epa1, shifts slightly in the positive direction while the corresponding cathodic peak potential, Epc1, remains nearly unchanged. At low scan rates (20 mV/s) the CV exhibits a separation (DE) between Epa1 and Epc1 in the range ca. 70–80 mV, similar to that of the simple ferrocene (see Table 2). However at higher scan rate, n (n  50 mV/s), broadening of DEp for the second couple

Fig. 4. Cyclic voltammograms of donor compounds 6a (R ¼ H), 6b (R ¼ CH3 ) and 6d (R ¼ 2-thienyl) recorded on a Pt working electrode, Pt gauze counter electrode and Ag/AgCl reference electrode in dry CH2Cl2 at ambient temperature using TBAP 0.1 mol/l as the supporting electrolyte at scan rate 20 mV/s.

Fig. 5. Cyclic voltammograms of donor compounds 6c (R ¼ Ph) and 6e (R ¼ 2-furyl) at scan rate 20 mV/s.

peaks was observed (DEp > 300 mV for 6e), and complete distortion of the second oxidation peak was also indicated at n > 200 mV/s, indicating that the irreversibility of the electron-transfer process was maintained under this condition. This might be attributed to the onset of kinetic complications, Fig. 6. Interestingly, for 6e increasing the scan rate results in a significant increase of the EOx and decreases the ERed values, indicative of slow heterogeneous electrontransfer process [12]. Therefore, the effect of increasing the scan rate significantly increases the DE values. The anodic peak current (ipa1) varies linearly with the square root of the scan rate, as shown in Fig. 7. Furthermore, the CV shows the oxidation currents about two thirds of those of simple ferrocene at the same concentration, indicating a decrease in the diffusion coefficient, which, considering the much larger size of the investigated compounds, is to be expected. The oxidation potential values for the investigated compounds are strongly affected by the substitution rings and/or

Fig. 6. Cyclic voltammograms 6e (R ¼ 2-furyl) at scan rate 20–600 mV/s.

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A. El-Wareth Sarhan, T. Izumi / Synthetic Metals 140 (2004) 95–100

As can be recognized also in Table 2 all the DE values of compounds 6a–e are more positive than that of ferrocene itself, this might be indicates the existence of intramolecular electronic interactions between the ferrocene and 1,3-benzodithiole moieties in the charged states [13]. While compound 6b compared with Fc the DEp almost the same for Fc (DE ¼ 73 mV), while for 6b (DEp1 ¼ 71 mV) this attributed to the electrondonating properties of the methyl substitution to the center CH3C=C in the Fc-DTF system.

30 25

ipa ( A)

20 15 10 5 0 0

2

4

6

8

10

SQR (V)

2. Conclusion

Fig. 7. Variation of peak current in case of 6e vs. SQR (n1/2).

groups to the central C=C bond of the DTF’s. In this context, the replacement of the hydrogen atom in compound 6a by methyl (CH3), phenyl (C6H5) and thiophene (C4H3S) would be responsible for the positive shift of the oxidation potential for the two couples of the redox peaks in the case of compounds 6b, 6c and 6d, respectively (see Table 2). On the other hand, the substitution by furan ring (C4H3O) as in case of compound 6e leads to a positive shift of the oxidation potential of the first couple of the redox peaks while the second couple remains nearly unchanged. The results show that, a slight change in chemical structure of DTF compounds causes significant changes in the electrochemical behaviour and consequently in their application. An interesting feature for compound 6c (R ¼ Ph) is at scan rate higher than 50 mV by means at scan rates of 100–600 mV 2 the Epc is shifted to the negative and appeared as in Fig. 8 and Table 3.

In conclusion the new donor compounds 6a–f were synthesized successfully by using the Wittig–Horner reaction and the structures were confirmed by spectral and microanalyses. The electrochemical properties of these compounds were studied using cyclic voltammetry at ambient temperature on a Pt working electrode, using TBAP as the supporting electrolyte. The CV exhibited good donor properties, showing a two-electron quazireversible oxidation wave to the dication. The oxidation potential values, and in a larger extent the reduction potential values processes, are strongly influenced by the scan rate and the substitution of the DTF system. A two-electron redox behavior was observed as a two waves. The advantage of introducing ferrocene into the donor is that ferrocene has only a single one-electron redox process. Considering the fact that these donors contain two different types of donor moieties, it was important to determine which is easier to oxidize via different scan rates. Thereby, a double two-electron oxidation process encountered quasireversible process in CH2Cl2 as a solvent and the peak-to-peak separation could be recognized.

25

3. Experimental

20 Current A

15 10 5 0 -5 -10 -15 0

200

400

600

800

1000

1200

1400

1600

E (mV)

Fig. 8. Cyclic voltammogram of 6c (R ¼ Ph), scan rate 600 mV, current range 50 mA. Table 3 Cyclic voltammetric parameters (mV) of 6c at scan rate 600 mV Peak

Epc

Epa

E0

DEp

P1 P2

471 817

613 1212

542 1015

142 395

Melting points were recorded on a Gallencamp melting point apparatus and are uncorrected. Infrared spectra (IR) were measured on a Hitachi 260-10 spectrometer. 1 H NMR and 13 C NMR spectra were recorded at room temperature on INOVA-Varian Nuclear Magnetic Resonance Spectrometer (500 MHz). Chemical shifts are denoted in d units (ppm), relative to tetramethylsilane (TMS) as internal standard, J values are given in Hz. CDCl3 is used as a deuterated solvent unless otherwise stated. MS and FAB-MS spectra were obtained using a JEOL JMSAX505HA. Cyclic voltammetry was measured on a cyclic voltammogram CS-1090/Model CS-1087. Column chromatography was performed on silica gel 60 (230–400 Mesh ASTM). Solvents were distilled before use. Ferrocenyl ketones 4a–f were prepared and purified by column chromatography using silica gel and chloroform/hexane as eluent [12].

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3.1. 1-[(1,3-Benzodithiol-2-ylidene)alkyl/heteroaryl/ phenyl-methyl]ferrocene (6a–f)

C22H16FeS3 (432.3954), Calcd: C; 61.11, H; 3.73, S; 22.24. Found: C; 60.69, H; 3.80; S; 22.68.

3.1.1. General A sample of the 1,3-benzdithiole-2-phosphonate (3) (0.786 g, 3 mmol) was stirred in dry THF (50 ml) under a stream of nitrogen at
78 8C. A solution of n-BuLi (2.3 ml, 2.6 M/l) was added portion wise and the mixture was stirred for 15 min. A solution of ferrocenyl ketones 4a–f (3 mmol) in dry THF (50 ml) was added portion wise within 15 min. The temperature of the reaction was raised to room temperature and the reaction mixture was kept over night with stirring. The tetrahydrofuran (THF) was removed under vacuum, the residue was washed with water and extracted with chloroform and dried over sodium sulfate. The crude oil product was chromatographed on silica gel using chloroform/hexane mixture (1:1) to give the corresponding Fc-DTF 6a–f in variant yields.

3.4. 1-[(Benzo-1,3-dithiol-2-ylidene)fur-2ylmethyl]ferrocene (6e)

3.2. 1-[(Benzo-1,3-dithiol-2-ylidene)phenylmethyl]ferrocene (6c) This compound was obtained as red crystals according to the general method, while the addition of 1-benzoylketone in dry THF was achieved at 0 8C. Yield 12%, mp 165– 166 8C. IR (KBr) n ¼ 3098m, 3055s, 2390m, 1598s, 1560s, 1480m, 1450s, 1440s, 1270m, 1250m, 1130s, 1105s, 1050s, 1000s, 810s, 730s, 710s cm
1. 1 H NMR (CDCl3) d 7.49–7.46 (m, 2H, aromatic-H), 7.39–7.27 (m, 4H, aromatic-H), 7.07–7.03 (m, 3H, aromatic-H), 4.34 (t, J ¼ 1:5 Hz, 2H, ferrocene-H), 4.22 (t, J ¼ 1:5 Hz, 2H, ferrocene-H), 4.18 (s, 5H, ferrocene-H). 13 C NMR (CDCl3) d 142.72 (aromatic-C), 137.14, 135.81 (thiafulvalene C=CS2), 129.37, 129.02, 127.75, 127.02, 125.53, 125.27, 124.33, 121.42, 120.74 (aromatic-C and CH), 86.19 (ferrocene-C), 69.13, 68.30, 67.91, 67.63 (ferrocene-CH). FAB MS m/z (%) [Mþ, 426 (42)]. Anal. Calcd. for C24H18FeS2 (426.3732), Calcd: C; 67.61, H; 4.26, S; 15.04. Found: C; 67.49, H; 3.98; S; 15.11. 3.3. 1-[(Benzo-1,3-dithiol-2-ylidene)thien-2ylmethyl]ferrocene (6d) Yield 0.7 g, 61%, mp 121–123 8C. IR (KBr) n ¼ 3095w, 3058w, 1654m, 1612m, 1569s, 1448s, 1272m, 1220m, 1105s, 1049m, 1000s, 819s, 742s, 698s cm
1. 1 H NMR (CDCl3) d 7.43 (dd, J ¼ 1, and 5 Hz, 1H, thiophene), 7.28–7.27 (dd, J ¼ 1:5, and 8 Hz, 1H, thiophene-H), 7.12 (m, 1H, thiophene-H). 7.11–7.02 (m, 4H, aromatic-H), 4.44 (t, J ¼ 2 Hz, 2H, ferrocene-H), 4.24 (t, J ¼ 2 Hz, 2H, ferrocene-H), 4.19 (s, 5H, ferrocene-H). 13 C NMR (CDCl3) d 143.15 (thiophene, C-2), 136.87, 135.70 (thiafulvalene C=C), 131.91, 127.43, 126.88 (thiophene-CH), 125.95, 125.68, 125.40, 121.54, 120.85, 116.05 (aromatic-C and CH), 86.52 (ferrocene-C), 70.40, 69.30, 68.29, 67.71 (ferrocene-CH). FAB MS m/z [Mþ, 432 ( )]. Anal. Calcd. for

Yield 0.44 g, 70.5%, mp 165–166 oC,. IR (KBr) n ¼ 3093m, 3050w, 1658m, 1569s, 1538s, 1450s, 1286m, 1218m, 1147s, 1105s, 1000s, 917s, 817s, 759s, 738s cm
1. 1 H NMR (CDCl3) d 7.48 (dd, J ¼ 1, and 0 Hz, 1H, furan-H), 7.25–7.24 (m, 2H, aromatic-H), 7.19–7.17 (m, 1H, aromatic-H). 7.10–7.05 (m, 2H, aromatic-H), 6.55 (dd, J ¼ 0:5, and 3.5 Hz, 1H, furan-H), 6.51 (dd, J ¼ 2, and 3.5 Hz, 1H, furan-H), 4.54 (t, J ¼ 2 Hz, 2H, ferrocene-H), 4.27 (t, J ¼ 2 Hz, 2H, ferrocene-H), 4.11 (s, 5H, ferroceneH). 13 C NMR (CDCl3) d 153.38 (furan, C-2), 140.69 (aromatic-C), 136.87, 135.70 (thiafulvalene C=CS2), 132.99, 125.62, 121.26, 120.94, 113.80 (aromatic-C and CH), 110.58, 109.06 (furan-CH), 85.32 (ferrocene-C), 69.29, 69.34, 67.85 (ferrocene-CH). FAB MS m/z (%) [Mþ, 416 (7)]. Anal. Calcd. for C22H16FeOS2 (416.3304), Calcd: C; 63.47, H; 3.87, S; 15.40. Found: C; 63.37, H; 3.70; S; 15.13. 3.5. 1-[(Benzo-1,3-dithiol-2ylidene)butyloxymethyl]ferrocene (6f) A sample of the 1,3-benzdithiole-2-phosphonate (3) (0.524 g, 3 mmol) was stirred in dry THF (50 ml) under a stream of nitrogen at
78 8C. A solution of n-BuLi (2.3 ml, 2.6 M/l) was added portion wise and the mixture was stirred for 15 min. A solution of 1-chloroacetylferrocene 4f (0.525 g, 2 mmol) in dry THF (50 ml) was added portion wise within 15 min. The reaction was worked up as in general procedures to give beside 6f (10%) yield, several complex products which could not be isolated in pure form. 1 H NMR (CDCl3) d 7.25–7.22 (m, 2H, aromatic-H), 7.08–7.06 (m, 2H, aromatic-H), 4.55 (t, J ¼ 1:5 Hz, 2H, ferrocene-H), 4.34 (s, 2H, OCH2), 4.26 (t, J ¼ 1:5 Hz, 2H, ferrocene-H), 4.19 (s, 5H, ferrocene-H), 3.53 (t, J ¼ 6:5 Hz, 2H, OCH2), 1.65 (quentit, J ¼ 6:5 Hz, 2H, CH2CH2CH3), 1.45 (sixtet, J ¼ 7:5 Hz, 2H, CH2CH2CH3), 0.94 (t, J ¼ 7:5 Hz, 3H, CH2CH2CH3). 13 C NMR (CDCl3) d 136.42, 136.04 (thiafulvalene C=CS2), 131.54, 125.40, 125.38, 121.26, 120.99, 119.09 (aromatic-C and CH), 86.51 (ferrocene-C), 73.45 (OCH2) 69.79, 69.02, 68.02 (ferrocene-CH), 67.29 (OCH2), 31.96 (CH2), 29.70 (CH2), 19.56 (CH3). FAB MS m/z (%) [Mþ, 436 (100)].

Acknowledgements This work was partially supported by a Grant-Aid for Scientific Research No. 12650844 from the Ministry of Education, Sciences, Sports and Culture (Japan).

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