ELSEVIER

|¥flTH|TI( m|TRLS

Synthetic Metals 66 (1994) 275-283

Conjugated tris-tetrathiafulvalenes

M a r t i n A d a m a, E g o n F a n g h ~ i n e l b'*, K l a u s M i i l l e n a'*, Y o n g - J i a S h e n a'l,

R o b b y W e g n e r b aMax-Planck-lnstitut fiir Polymerforschung~ Postfach 3148, D-55021 Mainz, Germany

bInstitut fiir Organische Chemie der Martin-Luther-Universitiit, Halle-Wittenber~ 06217 Merseburg, Germany

R e c e i v e d 25 May 1994; accep t ed 16 June 1994

A b s t r a c t

The synthesis of oligomeric homologs of B E D T - T r F and TTCn-TTF possessing three q"rF subunits in full rr-conjugation was accomplished starting from soluble dithiones. Cyclic voltammetric investigation revealed the formation of a hexacation upon oxidation. Details are discussed in comparison to suitable model compounds. Charge-transfer complexes and radical- cation salts derived from the new donors were investigated by electrical conductivity measurements.

Keywor~ls: T e t r a t h i a f u l v a l e n e s

I. Introduction

The electrical properties of charge-transfer (CT) complexes and binary radical-cation salts derived from tetrathiafulvalenes (TITs) are particularly sensitive to the relative arrangement of the molecules in the crys- talline state [1]. Investigations of a large number of radical-cation salts derived from BEDT-TTF 1 [2] have been helpful in establishing the importance of a high dimensionality for stabilizing the 'metallic' state and for achieving superconductivity at low temperatures. There is evidence that the dimensionality of the charge- transport process might also be increased [3] by a chemical linkage of T r F moieties [4].

The number of known molecules with one or two redox-active fulvalenic moieties is increasing rapidly, while the number of molecules containing three or more T I T centers is still rather small [5]. Recently, we synthesized two of such donors (2a and 2b) which can be envisioned as extended oligomers of dibenzo- TTF a [6].

Usually, the extension of rigid ,r-systems results in a decreased solubility which enormously complicates their characterization and limits their tractability. This trend can be demonstrated when comparing 3 and 2. The solubility of 2 can be increased by attaching flexible side chains not only at its central benzene rings, but

*Cor re spond ing authors .

t P r e sen t address : Ins t i tu te of F ine Chemica l s , Eas t Ch ina Univers i ty

of Chemica l Techno logy , 130 Mei l ing R o a d , Shangha i 200237, China.

, 1 R', R' = S-CH2-CH2-S 2t R', R'= CH=CH-CH=CH li R' = S-n-alkyl

R' R R R'

R,-y -s s - y - s s - y - s ~'-~'R, R' R R R'

2a R = OC2H5, R' = H 2b R = OC6HI3, R' = H 2e R = S-i-CsHll, R' = SC2H 5

R R

R R

~tlR = S-i-pent, R'= C6H13

~ R = OC6H~3, R' = Call 9 4c R = OC6H13, R',R' = CHzCH 2

also at the terminal ones. We have chosen thioalkyl substituents because they lead not only to an increased solubility, but also are supposed to enhance the po- larizability of the molecule. Accordingly, donor 2c con- tains 12 thioether substituents. The solubility of the tris-tetrathiafulvalenes can be also increased through a replacement of the terminal benzene units by S-alkyl chains. This will result in donors 4a and 4b which are

0379-6779/94/$07.00 © 1994 Elsev ie r Science S.A. All rights r e se rved

SSDI 0 3 7 9 - 6 7 7 9 ( 9 4 ) 0 2 1 9 3 - 3

276 M. Adam et al. / Synthetic Metals 66 (1994) 275-283

'homologs' of TFC,-TTF 5 [7], while 4c can be imagined as an extended BEDT-TI'F system. It appears that a major advantage of these title systems is that they allow for a detailed cyclic voltammetric investigation of the extended donors.

2. Results and discussion

In principle, the synthesis of 'trimeric' donors can be achieved via two routes. Both (Scheme 1) start from the substituted dithione 6 as a key intermediate, but differ significantly in the sequence of the formation of the TI'F units. A central step of route A is the synthesis of dithione 7, a molecule containing the central T IT moiety of the trimers as a capped hexathioorthooxalate. Dithione 7 can be obtained by a reductive coupling reaction of the thiolium salt 8, which, in turn, is available by a monoalkylation of the dithione 6 using dime- thoxycarbonium tetrafluoroborate. We thought to in- troduce the two terminal TTF centers in an anionic coupling reaction, treating 7 with two equivalents of the lithiated trithioorthoformate (derived by deproton- ation of 9 with methyl lithium). Trishexathioorthofor- mate 10 should result after quenching with an excess of iodomethane. The thermal extrusion of three equiv- alents of dimethyl disulfide should complete the for- mation of 2 and 4, respectively [8].

In route B, the central T IT redox unit of the trimeric donors is expected to be formed within the last reaction step by a phosphite-induced coupling reaction of 11. Donor 11 should be available starting from dithione 6 as well. The anionic coupling reaction between 6 and the lithiated trithioorthoformate as described in route A (this time in a 1:1 ratio) should result in the formation of the hexathioorthooxalate 12, which is then converted into 11 by extrusion of one equivalent of dimethyl disulfide.

Unexpectedly, attempts at obtaining 4c via route A failed, because a reaction between 7 and the lithiated trithioorthoformate did not occur. Unreacted dithione 7 was recovered even after prolonged reaction at tem- peratures above - 7 8 °C, which is the temperature typically used for this reaction. Instead, the methylated orthoformate 13 was obtained after the work-up pro- cedure. An alternative procedure using the TIT-di- thione 14 instead of 7 also failed. Obviously, slight changes in the electronic structure of 7 and 14 in comparison to 6 result in a totally different chemical behavior.

Route B was carried out successfully for the synthesis of 2a and 2b. Two slight disadvantages of this pathway are the need of a chromatographic removal of excess dithione 6 after the anionic coupling reaction, and the rather low yield (22%) of the phosphite-induced step. Therefore, we have carried out the anionic coupling reaction under modified conditions and added a solution

1 " 3 : ' / " R'' ~S SCH 3

9

1. MeLi 2. CH3I

R

S, SCH3 S ~ R' S

IS

I A, -H3CSSCH 3

R

R

LIAR'= SCrH13, R = S-i-Cs~ l i b R' = SC4I-I 9, R = OC6H 1 l i e R', R' = S-CH2-CH2-S, R = OC61-Ii3 l l d R', R' = HsCzS, R = S-i-C5H9

H ~:2S ~ , , .

H5C2 S" T " HsC~S

Scheme 1.

R

S ~ " ~ ~'S

g 6

route B route A

OR

(H3CO)2CH3+BF4.) ==~ S + s

OR

R ~Zn R

.sc.

s-T-Ws-

P(OR)3~

] 1.2, THF, -78°C R " ~ 2. CH3I R

/

R'S. -S SCH~ ~ A-. _~ SCH~ ~ ~ _,~ SCI~ ~._ ..SR' trimer = A,-H3CSSCH~ ~ ~ _ _ _ _ ~ O ' ~ ' - ~ > ~ _ . . ~ T O ' ~ ' - ~ . ] ( ~ ""

s s-- " r " - s s - ' - ' - ,U - s s-- 'sR' J / J

R R 19.

S . Sx /SCH 3

13

M. Adam et al. / Synthetic Metals 66 (1994) 275-283 277

of the nucleophile to a solution of 6 dropwise (both in THF at - 7 8 °C, argon). The yield of the coupling reaction was increased up to 50% in the case of 15c.

The extrusion of dimethyl disulfide as the next reaction step can be carried out in high yields ( l l a (58%), l i d (77%), l l c (89%)) at 70 °C by exposing a solution of 11 in 1,1,2,2-tetrachloroethane to ultrasound 2. These donor molecules containing a 1,3-dithiol-2-thione unit allow for the formation of the extended T r F systems by a phosphite-induced coupling reaction [9,10]. At 130 °C, the conversion of the thiones l l a and 11d, containing thioalkyl moieties at the indacene subunit, resulted in significantly higher yields than that of the oxo analogs ( l i b and l lc) . While we obtained 91% of 2c and 58% of 4a, the coupling of l l c and l i b yielded only 22% and 15%, respectively. The yield of 4c, however, can be increased by a factor of two if the reaction is carried out under a pressure of 7.5 kbar at 110 °C for 24 h.

To elucidate the redox properties of the new donors, cyclic voltammetric measurements were carried out. Unexpectedly low first oxidation potentials were mea- sured for donors 2b and 4c (Table 1) as compared to their 'parent' compounds 1 and 3. In principle, this might be due to an oxidation of the central 'dibenzo- TTF' subunit (indicated by bold lines) which contains eight electron-donating substituents (four sulfur and four oxygen atoms).

For investigating this assumption, we have synthesized donor 16 as a model compound which has a similar

T a b l e 1 Cyclic v o l t a m m e t r i c po t en t i a l s of the t i t le c o m p o u n d s 2" a n d 4 b vs.

SCE in 1 ,1 ,2- t r ichloroethane , T B A P F r , v = 100 m V s -~

C o m p o u n d EII/2 E2I/2 E31/.2 E41r2 ESla Et'ln Ref.

(mV) ( m V ) (mV) (mV) ( m V ) (mV)

1 36o 78o [8b l 2b 310 610 d 810 1010 1280 [8a l

2c ~ 440 610 810 1010 1280 this w o r k 3 c 450 940 [12]

4a b 340 440 690 780 1100 1330 this work

4h 340 450 780 c 1070 1310 this w o r k

4c 280 420 680 87ff 1080 this w o r k

14 660 1030 this work

16 410 820 this work

17a 360 530 860 960 [17]

17e 380 590 900 1020 this work

B8]

aT= 363 K.

bT= 343 K. ¢CH2C12, TBAPFr , v = 1 0 0 m V s - t , 273 K.

aPoorly sepa ra ted . ~Two-elect ron t r ans fe r s tep. fAnodic p e a k po ten t i a l .

215b was o b t a i n e d as a ye l low oil and cou ld not be pur i f ied

comple te ly . The overa l l y ie ld for the two r eac t ion s teps 6b to l i b was 20%.

substitution pattern to the central 'dibenzo-TTF' subunit of the donors 2b and 4e. Molecule 16 is easily accessible by treating dithione 14 with borane-dimethylsulfide complex (Scheme 2) [11]. Although 16 exhibits a lower oxidation potential than the unsubstituted dibenzo-T-FF 3, the effect of the electron-donating substituents is not significant. Furthermore, the first oxidation potential of the model compound 16 is higher than that observed for 2b and 4e, indicating that the first electron of 2b and 4c is not removed from the inner T-I'F subunit. Increasing the potential up to 1.6 V produced strong adsorption phenomena which inhibit a detailed inves- tigation of the redox behavior of 2b and 4c. In contrast to these donors, the electrochemical behavior of 4a and 4b can be investigated in detail due to their increased solubility.

Both molecules exhibit a lower oxidation potential in comparison to that of 2c due to the benzo-substitution of the latter. Donors possessing three redox-active moieties should be oxidizable up to a hexacation, because each T]?F redox center can contribute two electrons. In 1,1,2-trichloroethane, the oxidation of 2e up to a pentacation can be accomplished successfully at a po- tential of 1280 mV. The oxidation of the solvent at higher potentials inhibits the detection of the hexacation. As shown in Fig. 1, such highly charged species can be generated, however, from donors 4a and 4b. The first redox potential for these two 'trimers' is similar to that of BEDT-TTF (1). One is, therefore, led to conclude that the first oxidation occurs on the terminal TTF subunit of the donors. In comparison with 17a,

OR OR

OR OR

L4

I BH3.Me2S

OR OR

OR OR

16 R = C6H l

Scheme 2.

R

R 1TaR = OC6H13, R' = SC6H13 17b R = OC6FII3, R',R' = S-CH2-CH2-S 17¢R = S-i-CsHn, R'= SC6HI3

278 M. Adam et al. / Synthetic Metals 66 (1994) 275-283

<=

I . - I

-4 .500

-3 . 500

-2 .500

- i . 5 0 0

-0 . 500

0.500

i .500

2.500

I I I I I I I I

I I I I I I I I

0 . i00 0.300 0.500 0.700 0.900 i . i 0 0 l .S00 i .500

Fig. 1. Cyclic vol tammograms of 4a ( - - ) and 4b ( - - - ) .

E (Vl

no additional stabilization of the charge due to the extension of the ~--system is observed. Remarkably, a second redox step is observed in 4a and 4b at a potential significantly lower than that of 1. While two electrons reside on one redox unit in 1, they occupy two different redox units in the extended donors. The dications of 4a and 4b are formed at potentials 80-90 mV lower than that of 17a ÷ ---, 1 7 a 2 ÷ indicating a smaller coulombic repulsion in 4a and 4b. In general, 4a and 4b show nearly identical redox potentials due to their similar chemical structure. A difference in the electrochemical behavior is found upon oxidation from the + 2 to + 4 state. While 4a displays two separated one-electron transfer steps, donor 4b exhibits only one two-electron transfer. To our knowledge, 4a is the first example of a fully reversible stepwise oxidation of a donor containing three redox-active T I T subunits up to a hexacation.

Complexation of the 'trimeric' donors with acceptors (e.g. DDQ, TCNQF4) in dichloromethane affords deeply colored CT complexes. Elemental analysis of the com- plexes indicates a 1:1 stoichiometry with the exception of the TCNQF4 complexes of 2c and 4a which exhibit a 1:3 ratio. Conductivity measurements were carried out by a two-probe technique on compressed pellets at room temperature. The conductivity of the CT com- plexes is in the range of 10 - 3 to 10- ' S cm -1 (Table 2). The conductivity of the CT complexes of 4c at room temperature differ only insignificantly from that of the dimerie homolog 17b. Electrocrystallization experiments of 4c in the presence of the octahedral counterions PF6- and SbF6- resulted in the formation of dark-colored radical-cation salts. As known from Weissenberg photographs, these salts are isomorphous and possess incommensurable lattices of the donor and the counter-anion. Therefore, attempts to determine

Table 2 Room tempera ture conductivity and stoichiometry of CT complexes of the trimeric donors

Donor Acceptor Room temperature Stoichiometry b conductivity" (S cm -1)

2c TCNQF4 6 × 10 -2 1:3 4a TCNQF4 9 X 10 -2 1:3 4c D D Q 8 × 10-3 1:1 4e 12 3 X 10 -2 1:1 17b D D Q 5 × 10-3 1:1 17b 12 5 X 10 -2 1:1 17c D D Q 1.4 × 10 -3 1:3 17c TCNQF4 1.9 )< 10 -2 1:2

"Two-probe measurements of compressed pellets. bDetermined by elemental analysis.

their crystal structure have not yet been successful. A two-probe temperature-dependent conductivity mea- surement of the single crystals reveals a room tem- perature conductivity of 10 -2 S cm -1 for 4 c ' P F 6 and 2×10 -2 S cm -~ for 4c.SbF6. Upon cooling to 140 K, a semiconducting behavior was revealed. The activation energy for the conducting process was determined to be 240 and 250 meV, respectively, which is about 100 mV higher than that of a series of radical- cation salts derived from 17 (R=O-alkyl; R ' , R ' = -SCH2CH2S-).

3. Experimental

3.1. 4,8-Bis(alkyloxy)-l,3,5,7-tetrathia-s-indacene-2,6- dithione (6, R =OCnHe,+ I)

The general method for the synthesis as described in Ref. [12] has been used.

M. A dam et al. / Synthetic Metals 66 (1994) 275-283 279

3.2. 4,8-Bis(thioalkyl)- l,3,5, 7-tetrathia-s-indacene-2,6- dithione (6, R=SCnHz~+ ,)

67%) at 4 °C. Melting point: 124 °C. 1H NMR (90 MHz, acetone-d6), 6 (ppm)= 3.20 (s, 3H), 3.55 (s, 4H).

The general method for the synthesis as described in Ref. [3] has been used.

3.3. 2-Methylthio-4,5-ethylenedithio-l,3-dithiole (9; R',R' = S-CH2CH2--S)

3.3.1. 4,5-Ethylenedithio-l,3-dithiolyl-2-thione 4,5-Ethylenedithio-l,3-dithiolyl-2-thione was pre-

pared in an analogous manner to that given in Ref. [14]: acetone (1.71) was stirred and refluxed in a 3 1 three-necked flask equipped with a reflux condensor and two dropping funnels. Over 3 h, solutions of bis (tetraethylammonium)-bis(1,3-dithiolyl-2-thione-4,5- dithiolato)-zincate [15] (25 g, 35 mmol) in acetone (200 ml) and dibromoethane (13.2 g, 70 mmol) in acetone (100 ml) were added slowly. The mixture was then refluxed for 20 h, while a color change from deep red to orange occurred. After cooling to room temperature, inorganic residues were removed by filtration. The solvent was evaporated from the filtrate. After recrys- tallization of the crude product from ethanol, orange crystals were obtained (40 mmol, 57%).

3.3.2. Dimethoxycarbonium tetrafluoroborate BF3-etherate (1.76 ml, 14 mmol) (50% in diethyl

ether) was added via a syringe to dichloromethane (2 ml, freshly distilled from P205) under nitrogen. The stirred solution was cooled to - 3 0 °C and trimethyl- orthoformate (1.4 ml, 12.5 mmol) was injected over 5 min, resulting in a temperature rise. After removing the stirrer and cooling bath, the pale yellow CH2C12 phase was removed via a syringe and replaced by freshly distilled CH2C12. After cooling again to - 3 0 °C, the colorless product precipitated. The solvent was removed again and the crude product was used without any further purification. The yield was estimated at 90%.

3.3.3. 2-Methylthio-4,5-ethylenedithio- l,3-dithiolium-tetra- fluoroborate

CH2C12 (40 ml) was injected via a syringe into di- methoxycarbonium tetrafluoroborate (about 10 mmol) under nitrogen. At - 2 0 °C, the 4,5-ethylenedithio-l,3- dithiolyl-2-thione (1.5 g, 6.7 mmol) in CH2C12 (60 ml) was added dropwise into the stirred solution. The color changed from orange to red. The reaction was stirred for another 12 h during which time it was allowed to come to room temperature. The product was precip- itated as an orange-red oil by treating the solution with cold tetrachloromethane ( - 18 °C). After removing the solvent layer, the oil was dissolved in dichloro- methane and precipitated again. Repeating this pro- cedure once more yielded orange needles (6.6 mmol,

3.3.4. 2-Methylthio-4,5-ethylenedithio- l,3-dithiole (9; R',R' = S-CH2CH2-S)

At 0 °C 2-methylthio-4,5-ethylenedithio-l,3-dithiol- ium-tetrafluoroborate (997 mg, 3.05 mmol) was dissolved in acetonitrile (20 ml). The stirred red solution was treated portionwise with sodium borohydride until a color change to bright yellow occurred. A few milliliters of water were then added and the major part of the organic solvent was removed in vacuo. The reaction mixture was extracted with ether. After drying the organic layer with sodium sulfate, the solvent was removed in vacuo. The yellow oil (3.03 mmol, 99.5%) crystallized after storage at 4 °C for a longer period. 1H NMR (200 MHz, CDC13) , 6 (ppm)=2.24 (s, 3H), 3.12-3.38 (AA'BB', 4H), 5.75 (s, 1H).

3.4. 2-Methylthio-4,5-bis(n-butylthio)-l,3-dithiole (9; R' = S--C4Hg)

3.4.1. 4,5-Bis(n-butylthio)-l,3-dithiol-2-thione Bis (tetraethylammonium) -bis (1,3 - dithiolyl-2-thione-

4,5-dithiolato)-zincate (16.8 g, 23.4 mmol) was dissolved in acetone (1200 ml). 1-Bromobutane (18 ml, 170 retool) was added and the reaction was refluxed until a color change from red to yellow-green occurred. The solvent was evaporated and the residue was dissolved in di- ehloromethane. Insoluble products were removed by filtration. The filtrate was evaporated and the crude product was chromatographed (silica gel, petroleum ether:diehloromethane = 3:1) to yield a red oil (13.0 g, 89.5%). ~H NMR (CDCI3, 200 MHz), fi (ppm)=0.89 (t, 6H), 1.28-1.57 (m, 4H), 1.59-1.64 (m, 4H), 2.81 (t, 4H). a3C NMR (CDC13, 50 MHz), 6 (ppm)= 13.5, 21.6, 31.7, 36.5, 136.3, 211.2. EI-MS (70 eV), m/z=310.1 (M "+, 100%), 254.0 (M'+-C4H9, 18%).

3.4.2. 2-Methylthio-4,5-bis(n-butylthio)- l,3-dithiolium-te- trafluoroborate

Under argon at - 3 0 °C a solution of 4,5-bis(n- butylthio)-l,3-dithiolyl-2-thione (13.0 g, 41.9 mmol) in dichloromethane was added to a 1.5-molar excess of dimethoxycarbonium tetrafluoroborate over a period of 20 min. The solution was allowed to come to room temperature while stirred for another 12 h. The red solution was concentrated and the salt was precipitated by adding ether. Yellow crystals (38.8 mmol, 92.6%) were isolated after the solution was stored at - 2 0 °C. Melting point: 57-59 °C. 1H NMR (200 MHz, aceton- itrile-d3), ~ (ppm)=0.94 (t, 6H), 1.38-1.53 (m, 4H), 1.62-1.77 (m, 4H), 3.08 (s, 3H), 3.12 (t, 4H). 13C NMR (50 MHz, acetonitrile-d3), 6 (ppm)= 13.6, 22.1, 23.7, 32.0, 38.2, 147.7, 204.1.

280 M. Adam et al. / Synthetic Metals 66 (1994) 275-283

3.4.3. 2-Methylthio-4,5-bis(n-butylthio)-l,3-dithiole (9; R' = S---C4H9)

The same procedure as described above for the synthesis of 2-methylthio-4,5-ethylenedithio-l,3-dithiole was applied to 2-methylthio-4,5-bis(n-butylthio)-l,3-di- thiolium tetrafluoroborate (10.0 g, 24.3 mmol) in ace- tonitrile (30 ml) to yield (23.2 mmol, 95.4%) the formate as a red oil. 1H NMR (200 MHz, CDCI3) , ~ (ppm) = 0.89 (t, 6H), 1.32-1.72 (m, 8H), 2.21 (s, 3H), 2.61-2.98 (m, 4H), 5.69 (s, 1H). 13C NMR (50 MHz, CDC13), t~ (ppm)=13.7, 21.8, 32.0, 36.1, 57.3, 125.2. El-MS (70 eV), m/z=326 (M "+, 96%).

3.5. 2-Methylthio-4,5,6, 7-tetrakis(ethylthio)-benzo-l,3- dithiole (9; R',R' = (C(SC2H5)4)

The synthesis was described in Ref. [6c].

3.6. 2-Methylthio-4,8-bis(n-hexyloxy)-l,3-dithiolium-5, 7- dithia-s-indacene-6-thiono-tetrafluoroborate (8)

In an analogous manner to the methylation reaction described above, 4,5-bis(n-hexyloxy)-l,3,5,7-tetrathia-s- indacene-2,6-dithione (6, R = OC6H13, 2.45 g, 5 mmol) in dichloromethane (50 ml) was treated with one equiv- alent of dimethoxycarbonium tetrafluoroborate. The salt (2.1 mmol, 43%) was precipitated with cold CC14. 1H NMR (200 MHz, CD2C12) , t~ (ppm)=0.92 (t, 6H), 1.24-1.63 (m, 12H), 1.77-1.95 (m, 4H), 3.38 (s, 3H), 4.40 (t, 4H). 13C NMR (50 MHz, CD2CI2) , 8 (ppm)= 13.9, 22.5, 24.4, 25.3, 30.0, 31.4, 75.7, 133.1, 135.9, 142.0, 208.3, 210.7.

3. 7. 2,2'-Bis(methylthio)-6,6'-dithiono-4, 4',8,8'- tetrakis (n-hexyloxy) - A z2"-bis- l,3,5, 7-tetrathia-s-indacene (7)

A solution of 2-methylthio-4,8-bis(n-hexyloxy)-l,3-di- thiolium-5,7-dithia-s-indacene-6-thiono-tetrafluorobor- ate (8, 900 mg, 1.5 mmol) in THF (100 ml) was treated with zinc (200 mg, 6 mmol) and a trace amount of bromine. The reaction was stirred for 2 h at room temperature and subsequently filtered. After removing the solvent in vacuo from the filtrate, the residue was recrystallized from chloroform/methanol. 2,2'- Bis(methylthio)-6,6'-dithiono-4,4',8,8'-tetrakis(n-hexyl- oxy)-A~'2'-bis-l,3,5,7-tetrathia-s-indacene (7, 6 x 10 -5 tool, 12%) was obtained as a yellow powder. Melting point: 98 °C. ~H NMR (400 MHz, CDC13, 8 (ppm) = 0.95 (t, 12H), 1.25-1.45 (m, 24H), 1.55-1.87 (m, 8H), 2.52 (s, 6H), 3.80 (t, 8H). 13C NMR (50 MHz, CDCI3), 8 (ppm)--14.0, 18.3, 22.5, 25.4, 30.0, 31.5, 72.9, 96.6, 131.4, 132.6, 140.2, 210.5.

3.8. 6,6'-Dithiono-4, 4',8,8'-tetrakis(n-hexyloxy)-AZ2'-bis- L 3,5, 7-tetrathia-s-indacene (14)

Under argon a solution of 2,2'-bis(thiomethyl)-6,6'- dithiono-4,4',8,8'-tetrakis(n-hexyloxy)-A 2"2'-bis-l,3,5,7- tetrathia-s-indacene (7, 100 mg, 0.1 mmol) in tetra- chloroethane was treated at 70 °C with ultrasound. After 2 h the color changed to orange. The solvent was removed in vacuo. Recrystallization of the crude product from chloroform/methanol resulted in 14 (0.073 mmol, 73%). Melting point: above 245 °C. ~H NMR (400 MHz, CDCI3, ~ (ppm)=0.95 (t, 12H), 1.26-1.54 (m, 24H), 1.67-1.88 (m, 8H), 4.00 (t, 8H). 13C NMR (100 MHz, CDCI3, 8 (ppm)= 14.0, 22.6, 25.5, 30.1, 31.5, 73.4, 112.0, 130.7, 132.5, 141.1, 210.6. EI-MS (70 eV), re~z=916 (M "÷, 2.6%).

3.9. 6,6'-Bis(dihydro)-4,8-tetrakis(n-hexyloxy)-AZ2'-bis - 1,3,5, 7-tetrathia-s-indacene (16)

Under argon, 6,6'-dithiono-4,4',8,8'-tetrakis(n-hexyl- oxy)-A2"2'-bis-l,3,5,7-tetrathia-s-indacene (14, 26.5 rag, 0.029 mmol) was dissolved by stirring in toluene (1 ml). At 95 °C, BH 3" (H3C)2S (0.06 mmol, 2 M solution in toluene) was added via a syringe. The reaction was held at this temperature for another 10 min. After the reaction had cooled to room temperature, methanol (0.5 ml) was added. The solvent was evaporated in vacuo and the crude product was chromatographed (silica gel, petroleum ether:dichloromethane=2:l) to yield 16 (0.012 mmol, 40.5%) as a yellow powder. Melting point: 134 °C. 1H NMR (200 MHz, CDCI3),

(ppm) = 0.92 (t, 12H), 1.24-1.52 (m, 24H), 1.66-1.83 (m, 8H), 4.00 (t, 8H), 4.2-5.0 (broad, 4H). 13C NMR (50 MHz, CDCI3), 8 (ppm) = 14.0, 22.6, 25.5, 30.1, 31.5, 36.8, 72.5, 128.6. FD-MS, re~z=857 (M "+, 100%).

3.10. 4,8-Bis(isopentylthio)-2-(2'-methylthio-4',5'-bis(n- hexylthio)-l ',3'-dithiolyl)-2-methylthio- l,3, 5, 7-tetrathia-s- indacene-6-thione (15a)

A solution of 2-methylthio-4,5-bis(n-hexylthio)-l,3- dithiole (522 mg, 1.36 mmol) in dry THF (15 ml) was cooled to - 7 8 °C under argon in a dropping funnel. Methyllithium (1.2 ml, 1.3 M solution in diethyl ether) was added via a syringe and the reaction mixture was stirred for 1.5 h. The resulting solution was then added to a solution of 4,8-bis(isopentylthio)-l,3,5,7-tetrathia- s-indacene-2,6-dithione (6, R=i-SCsHll, 472 mg, 0.95 mmol) in dry THF (160 ml) at - 1 0 °C. Immediately, the reaction mixture was cooled to - 7 8 °C. After 1.5 h of additional stirring iodomethane (1.2 ml) was added. The solution was allowed to warm to room temperature and after 2 h the solvent was removed in vacuo. Column chromatography (silica gel, petroleum ether:CH2CI2--4:l) afforded 153 (0.34 mmol, 25%) as

M. Adam et al. / Synthetic Metals 66 (1994) 275-283 281

a yellow powder. Melting point: 81-83 °C. 1H NMR (200 MHz, CDC13), 6 (ppm)=0.89 (t, 6H), 0.92 (d, 12H), 1.22-1.80 (m, 22H), 2.51 (s, 3H), 2.52 (s, 3H), 2.71 (t, 2H), 2.74 (t, 2H), 2.95 (t, 4H). 13C NMR (50 MHz, CDC13), t~ (ppm)=14.5, 18.3, 18.4, 22.7, 23.1, 27.9, 30.4, 31.9, 36.7, 39.4, 92.6, 92.7, 119.3, 126.7, 145.0, 146.6, 211.6.

3.11. 4,8-Bis(n-hexyloxy)-2-(2'-methylthio-4',5'-bis(n- butylthio)-I ', 3'-dithiolyl)-2-methylthio-l,3, 5, 7-tetrathia-s- indacene-6-thione (15b)

Under argon, 2-methylthio-4,5-bis(n-butylthio)-l,3- dithiole (300 mg, 0.93 mmol) was dissolved in dry THF (20 ml) and cooled to -78 °C. MethyUithium (0.92 mmol) was then added and the reaction mixture was stirred for 20 rain. At this temperature the reaction mixture was then added dropwise over 60 min to a solution of 4,8-bis(n-hexyloxy)-l,3,5,7-tetrathia-s-inda- cene-2,6-dithione (6, R = n-OC6H16, 490 mg, 1.0 mmol) in 20 ml THF at - 78 °C. After stirring for an additional 2 h at - 7 8 °C, the reaction was terminated with iodomethane (1 ml). The cooling bath was removed and the reaction was stirred at room temperature for 12 h. After evaporation of the solvent in vacuo a yellow oil was obtained after column chromatography (neutral alumina, petroleum ether:CHzC12=5:l). Traces of pe- troleum ether were difficult to remove, but did not obstruct the subsequent pyrolysis reaction.

3.12. 4,8-Bis(n-hexyloxy)-2-(2'-methylthio-4',5'- ethylenedithio-1 ',3 '-dith iolyl )- 2-methylthio- l ,3, 5, 7- tetrathia-s-indacene-6-thione (15c)

The same procedure as described for 15b was em- ployed for 15e using 2-methylthio-4,5-ethylenedithio- 1,3-dithiole (1.9 g, 8.0 mmol in 50 ml THF), methyl- lithium (8.0 mmol), 4,8-bis(n-hexyloxy)-l,3,5,7-tetrathia- s-indacene-2,6-dithione (6, R = OC6H13 , 3.8 g, 7.8 mmo1 in 150 ml THF) and iodomethane (10 ml). After chro- matography (petroleum ether:CH2C12=2:l), 15c (3.9 mmol, 50%) was obtained as a yellow powder. Melting point: 96 °C. 1H NMR (400 MHz, CDC13), 6 (ppm) = 0.89 (t, 6H), 1.29-1.33 (m, 8H), 1.40-1.47 (m, 4H), 1.68-1.75 (m, 4H), 2.47 (s, 3H), 2.50 (s, 3H), 3.17-3.27 (m, 4H), 3.91-4.03 (m, 4H). 13C NMR (100 MHz, CDC13), 6 (ppm)= 14.0, 18.0, 18.1, 22.5, 25.5, 30.09, 30.12, 31.5, 73.1, 93.1, 95.8, 112.0, 131.6, 132.2, 140.3, 211.3. EI- MS (70 eV), re~z= 650.0 (M'+-CzH6S2, 31.12%), 564.0 (M'+-C2H6S2-C6H13, 5.5%).

3.13. 4, 8-Bis ( isopentylthio )- 2- [ 2' -methylthio-4 ', 5',6', 7' - tetrakis ( ethylthio ) benzo-1 ', 3 '-dithiotyl ]- 2-methylthio- 1,3,5, 7-tetrathia-s-indacene-6-thione (15d)

The same procedure as described for 15a was em- ployed for 15d using 2-methylthio-4,5,6,7-tetrakis(ethyl-

thio)benzo-l,3-dithiole (9, R',R' = (C(SC2H5)4), 923 mg, 2.09 mmol in 10 ml THF), methyllithium (2.1 mmol), 4,8-bis(isopentylthio)-l,3,4,7-tetrathia-s-indacene-2,6- dithione (6, R=i-SCsHal, 723 mg, 1.5 mmot in 200 ml THF) and iodomethane (1.8 ml). After chromatography (petroleum ether:CH2C12=4:l), 15d (0.48 mmol, 23%) was obtained as yellow needles. Melting point: 169 °C. 1H NMR (200 MHz, CDC13) , ~ (ppm)= 0.88 (d, 12H), 1.20 (t, 12H), 1.4-1.8 (m, 6H), 2.53 (s, 3H), 2.54 (s, 3H), 2.85 (t, 4H), 2.98 (q, 8H). 13C NMR (50 MHz, CDC13), 6 (ppm) = 14.8, 15.0, 18.4, 18.5, 22.7, 27.8, 31.2, 32.7, 33.6, 39.4, 89.7, 93.1, 119.3, 132.3, 144.7, 144.8, 147.1, 147.6, 211.5.

3.14. 4,8-Bis(isopentylthio)-2-[4',5'-bis(n-hexylthio)- 1 ',3'-dithiolylidene]-l,3, 5, 7-tetrathia-s-indacene-6,thione (114)

A solution of 4,8-bis(isopentylthio)-2-[2'-methylthio- 4',5'-bis(n-hexylthio)-l',3'-dithiolyl]-2-methylthio-l,3,- 5,7-tetrathia-s-indacene-6-thione (154, 200 rag, 0.22 mmol) in 1,1,2,2-tetrachloroethane (15 ml) was heated to 70 °C and treated with ultrasound for 12 h under argon. The solvent was removed in vacuo and the residue was recrystallized from CH2Cl2/methanol to yield l l a (0.13 mmol, 58%) as a red-orange powder. Melting point: 158-160 °C. 1H NMR (200 MHz, CDCI3) ,

(ppm)=0.90 (t, 6H), 0.93 (d, 12H), 1.28-1.81 (m, 22H), 2.84 (t, 4H), 2.95 (t, 4H). '3C NMR (50 MHz, CDCI3), 6 (ppm) = 14.5, 22.7, 23.0, 27.9, 28.7, 30.2, 31.8, 33.8, 36.9, 39.3, 107.2, 113.7, 120.5, 128.3, 145.8, 145.9, 211.2. FD-MS, m/z=796 (M "+, 100%).

3.15. 4, 8-Bis (n- hexyloxy) -2- (4', 5'-bis (n-butylthio) - I ', 3 '- dithiolylidene )- l,3,5, 7-tetrathia-s-indacene-6-thione (11b)

A solution of 4,8-bis(n-hexyloxy)-2-(2'-methylthio- 4',5'-bis(n-butylthio)-l',3'-dithiolyl)-2-methylthio-l,3,- 5,7-tetrathia-s-indacene-6-thione (15b, 230 mg) in C2HIC14 (40 ml, freshly distilled) was treated for 12 h at 70 °C with ultrasound. After evaporating the solvent in vacuo, the solid was dissolved in dichloromethane and filtered through silica gel. A recrystallization of the yellow solid from acetone resulted in l ib (0.19 mmol, 20% with respect to the trithioorthoformate used for the synthesis of 15b). Melting point: 41 °C. 1H NMR (200 MHz, CDCI3), t$ (ppm)=0.95 (2t, 12H), 1.23-1.48 (m, 16H), 1.50-1.84 (m, 8H), 2.85 (t, 4H), 4.02 (t, 4H). 13C NMR (50 MHz, CDC13), 6 (ppm) = 13.6, 14.0, 21.6, 22.5, 25.4, 30.1, 31.5, 31.8, 36.1, 73.3, 108.5, 127.9, 131.0, 132.2, 141.0, 210.3. FD-MS, m/z = 736 (M "÷ , lOO%).

282 M. Adam et al. / Synthetic Metals 66 (1994) 275-283

3.16. 4,8-Bis(n-hexyloxy)-2-(4',5'-ethylenedithio-l',Y- dithiolylidene)-l,3,5,7-tetrathia-s-indacene-6-thione (11c)

A solution of 4,8-bis(n-hexyloxy)-2-(2'-methylthio- 4',5'-ethylenedithio-l',3'-dithiolyl)-2-methylthio-l,3,- 5,7-tetrathia-s-indacene-6-thione (15c, 2.1 g, 2.8 mmol) in tetrachloroethane was refluxed at 130 °C for 5 h. The solution was then cooled to 50 °C and, at this temperature, methanol was added until precipitation occurred. The cooled solution was filtered. The residue was recrystallized from CH2Cl2/petroleum ether to give orange-red crystals of l lc (2.16 mmol, 77%). Melting point: 140 °C. aH NMR (200 MHz, CDC13), (ppm) = 0.91 (t, 6H), 1.35-1.55 (m, 12H), 1.77 (q, 4H), 3.30 (s, 4H), 3.97 (t, 4H). 13C NMR (40 MHz, CDC13), /~ (ppm)= 13.9, 22.5, 25.4, 30.1, 30.3, 31.5, 73.3, 111.5, 112.5, 114.0, 130.8, 132.3, 141.1, 210.7. EI-MS (70 eV), m/z= 650 (M "+, 13.0%), 76 (CS2 +, 100%).

3.17. 4,8-Bis(isopentylthio)-2-[4',5',6', 7'- tetrakis (ethylthio) benzo- 1 ', 3 '-dithiotylidene]- 1, 3, 5, 7- tetrathia-s-indacene-6-thione (11d)

The same procedure as described for l l a was em- ployed for l ld using 4,8-bis(isopentylthio)-2-[2'-meth- ylthio-4',5',6',7'-tetrakis(ethylthio)benzo-l',3'-dithio- lyl]-2-methylthio-l,3,5,7-tetrathia-s-indacene-6-thione (15d, 120 mg, 0.12 mmol in 10 ml 1,1,2,2-tetrachlo- roethane). After recrystallization from CH2C12/meth- anol, orange needles of l ld (0.11 mmol, 89%) were obtained. Melting point: 251-252 °C. aH NMR (200 MHz, CDCI3), ~ (ppm)=0.93 (d, 12H), 1.22 (t, 6H), 1.28 (t, 6H), 1.46--1.83 (m, 6H), 2.96 (t, 4H), 3.03 (q, 4H), 3.07 (q, 4H). ~3C NMR (50 MHz, CDC13), (ppm)=14.8, 15.1, 22.7, 27.9, 31.5, 32.6, 33.9, 39.3, 107.5, 112.3, 120.5, 133.9, 145.6, 145.8, 146.0, 146.6, 211.1. FD-MS, m/z=854 (M "÷, 100%).

3.18. 4, 4',8,8'- Tetrakis(isopentylthio)-6,6'-bis(4',5',6', 7'- tetrakis(ethylthio)benzo-l ',3'-dithio~ylidene-2,2'-bis- 1,3, 5, 7-tetrathia-s-indacenylidene (2c)

A suspension of 4,8-bis(isopentylthio)-2-[4',5',6',7'- tetrakis(ethylthio)benzo-l',3'-dithiolylidene]-l,3,5,7-te- trathia-s-indacene-6-thione (lid, !40 mg, 0.16 mmol) in triethylphosphite (8 ml) was refluxed for 23 h under argon. The mixture was then cooled and methanol was added. The precipitated orange solid (0.15 mmol, 91%) was filtered off, washed with methanol and dried. Melt- ing point: above 360 °C. XH NMR (200 MHz, CDCI3) , 8 (ppm)=0.95 (d, 24H), 1.21 (t, 12H), 1.28 (t, 12H), 1.48-1.82 (m, 12H), 2.98 (t, 8H), 3.02 (q, 8H), 3.06 (q, 8H). FD-MS, m/z = 1645 (M "÷ + 1).

3.19. 4, 4 ', 8, 8'- Tetrakis (isopentylthio)-6, 6 '-bis[4 ', 5'-bis (n- hexylthio) benzo- l ',3'-dithiolylidene]-2,2'-bis-1, 3,5, 7- tetrathia-s-indacenylidene (44)

A solution of 4,8-bis(isopentylthio)-2-[4',5'-bis(n- hexylthio)-l',3'-dithiolylidene]-l,3,5,7-tetrathia-s-inda- cene-6-thione (lla, 137 mg, 0.17 mmol) in triethyl- phosphite (10 ml) was refluxed for 48 h under argon. After cooling, the precipitation was completed with methanol and the orange powder (0.1 mmol, 58%) filtered off. Melting point: 320 °C. ~H NMR (200 MHz, CDCI3), ~ (ppm) = 0.90 (t, 12H), 0.93 (d, 24H), 1.27-1.82 (m, 44H), 2.83 (t, 8H), 2.96 (t, 8H). FD-MS, m/z = 1528 (M'+).

3.20. 4, 4',8,8'- Tetrakis(n-hexyloxy)-6,6'-bis[4',5'-bis(n- butylthio)- l ',3'-dithiolylidene]-2,2'-bis- L 3,5, 7-tetrathia-s- indacenylidene (4b)

4,8- Bis (n-hexyloxy)-2-[4',5'-bis (n-butylthio) 1',3'- di- thiolylidene]-l,3,5,7-tetrathia-s-indacene-6-thione (lib, 110 mg, 0.15 mmol) was dissolved in triethylphosphite (1.5 ml). The solution was kept at a pressure of 7.5 kbar for 12 h at 110 °C. After cooling, the phosphite was removed in vacuo. Column chromatographic work- up yielded 4b (0.011 mmol, 15%). Melting point: 125 °C. 1H NMR (500 MHz, CDCI3), ~ (ppm)=0.89 (t, 12H), 0.91 (t, 12H), 1.13-1.49 (m, 32H), 1.58 (q, 8H), 1.72 (q, 8H), 2.79 (t, 8H), 3.94 (q, 8H). ~3C NMR (125 MHz, CDC13), t$ (ppm)=13.6, 14.1, 21.7, 22.7, 25.4, 30.1, 31.5, 31.8, 36.0, 72.6, 110.2, 111.9, 112.1, 127.9, 128.9, 129.1, 142.2. FD-MS, re~z= 1410.9 (M "÷, 100%), 705 (M 2÷, 90%).

3.21. 4, 4',8,8'- Tetrakis(n-hexyloxy)6,6'-bis[4',5'- ethylenedithio-l',3'-dithiolylidene]-2,2'-bis-l,3,5, 7- tetrathia-s-indacenylidene (4c)

4,8-Bis(n-hexyloxy)-2-(4',5'-ethylenedithio-l',3'-di- thiolylidene)-l,3,5,7-tetrathia-s-indacene-6-thione (llc, 83.5 mg, 0.12 mmol) was subjected to the same conditions as l ib for 24 h. Work-up was carried out as above to yield 4c (0.028 mmol, 45%). Melting point: above 300 °C. 1H NMR (400 MHz, CDC13), t$ (ppm)=0.93 (t, 12H), 1.23-1.42 (m, 16), 1.46-1.55 (m, 8H), 1.77 (quint, 8H), 3.28 (s, 8H), 3.98 (t, 8H). 13C NMR (50 MI-Iz, CDC13) , t$ (ppm)=14.1, 22.6, 25.5, 30.1, 30.2, 31.5, 72.7, 110.2, 114.1, 128.9, 129.0, 142.3. FD-MS, re~z= 1236.3 (M "+, 100%), 618 (M 2+, 23%).

4. Conclusions

We have shown that suitably solubilized 'trimeric' tetrathiafulvalenes can be oxidized up to a hexacation. Within the dieationic state, the coulombic interaction

M. Adam et al. / Synthetic Metals 66 (1994) 275-283 283

is smaller than that within the 'dimeric' parent com- pounds. The synthesis and investigation of further ex- tended homologs will show whether this effect can be enhanced. Radical-cation salts and CT complexes of the 'trimeric' donors are available by chemical and electrochemical doping. Temperature-dependent mea- surements of the salts reveal a semiconducting behavior. Radical-cation salts with a crystal structure different from that of the salts with octahedral anions might result from the introduction of other acceptors or counterions. The use of planar and polymeric coun- terions might result in CT complexes and radical-cation salts possessing more attractive electrical properties [16].

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