Chemical and Electrochemical Investigations on the Powerful π-Electron Donor Dithiadiazafulvalene: Isolation, Spectroscopic Characterization and Charge-Transfer Complexation

Abstract

A representative of the title system has been accessible by three methods, namely the electrochemical reduction of the corresponding 2-(phenylthio)thiazolium salt, the chemical coupling of the thiazoline-2-selone in the presence of triethyl phosphite and a basic treatment of the 2-unsubstituted thiazolium salt. The last two procedures allow the isolation in a preparative scale of nearly pure dithiadiazafulvalene, which can be characterized by cyclic voltammetry and usual spectroscopic methods. The crystal structure of a 1:2 charge-transfer complex between this excellent π-donor molecule and tetracyanoquinodimethane as acceptor establishes that the dithiadiazafulvalene is in the dicationic state in the complex.

thiazolium salts - heterogeneous deprotonation - heterocycle - oxidation - complex

References

1 See the special issue on Molecular Conductors, Chem. Rev. 2004, 104, 4887-5782 and the article therein: Lorcy D. Bellec N. Chem. Rev.  2004,  104:  5185 ; see also ref.2 for a more general review about azafulvalenes

2 Beckert R. Adv. Heterocycl. Chem.  2000,  77:  115 

3 Baldwin JE. Branz SE. Walker JA. J. Org. Chem.  1977,  42:  4142 

4 Bordwell FG. Satish AV. J. Am. Chem. Soc.  1991,  113:  985 

5 Koizumi T. Bashir N. Kennedy AR. Murphy JA. J. Chem. Soc., Perkin Trans. 1  1999,  3637 

6a Tormos GV. Neilands OJ. Cava MP. J. Org. Chem.  1992,  57:  1008 

6b Tormos GV. Bakker MG. Wang P. Lakshmikantham MV. Cava MP. Metzger RM. J. Am. Chem. Soc.  1995,  117:  8528 

7 Doughty MB. Risinger GE. Bioorg. Chem.  1987,  15:  1 

8 Bellec N. Guérin D. Lorcy D. Robert A. Carlier R. Tallec A. Acta Chem. Scand.  1999,  53:  861 

9 Guérin D. Carlier R. Lorcy D. J. Org. Chem.  2000,  65:  6069 

10 Guérin D. Carlier R. Guerro M. Lorcy D. Tetrahedron  2003,  59:  5273 ; and references therein

11 Arduengo AJ. Goerlich JR. Marshall WJ. Liebigs Ann./Recl.  1997,  365 

12 Touimi Benjelloun A. Morel G. Sulfur Lett.  1996,  20:  107 

13 Morel G. Synlett  2003,  2167 

14

The reduction of compound 1 on a platinium electrode at -1.3 V vs. SCE was performed in a degassed CH₂Cl₂ solution containing Bu₄NPF₆ as the conducting salt (1 M). On the first cathodic scan, an intense reduction peak was found at -1.13 V. On the reverse scan, two oxidation peaks were measured at -0.34 V and -0.23 V vs. SCE.

15

Experimental Procedure.
NaBH₄ (0.29 g, 7.6 mmol) was added portionwise over 10 min to a suspension of black selenium powder (0.45 g, 5.6 mmol) in degassed (N₂) EtOH (25 mL). The colorless solution of NaHSe thus obtained was maintained under inert atmosphere during the addition of salt 1 (1.15 g, 2.5 mmol), which had been previously dissolved in degassed CH₂Cl₂ (10 mL). After 45 min at r.t., the mixture was poured into 60 mL of H₂O and extracted with CH₂Cl₂ (2 × 10 mL). The combined CH₂Cl₂ layers were dried over Na₂SO₄ and concentrated in vacuo to give a yellowish solid which was recrystallized from EtOH (4, 0.60 g, 80%); mp 125 °C. ¹H NMR (200 MHz, CDCl₃): δ = 2.28 (s, 3 H), 3.57 (s, 3 H), 7.30 (m, 2 H), 7.57 (m, 3 H). ¹³C NMR (50.5 MHz): δ = 20.8 (q, ¹ J = 141 Hz), 39.9 (q, ¹ J = 143 Hz), 123.5 (q, ³ J = 5.5 Hz), 129.6, 129.9, 130.4, 130.8 (arom. C), 147.4 (m), 181.8 (q, ³ J = 5 Hz). Anal. Calcd for C₁₁H₁₁NS₂Se: C, 43.85; H, 3.65; N, 4.65. Found C, 43.73; H, 3.59; N, 4.62.

16

Procedure for Preparation of DTDAF 3 from Selone 4.
A solution of selone (0.17 g, 0.56 mmol) in freshly distilled toluene (15 mL) was degassed with nitrogen and heated to reflux for 5-6 min. After addition of P(OEt)₃ (0.11 g, 0.65 mmol), the solution was stirred 10 min under N₂ in boiling toluene and the mixture was cooled to r.t. and concentrated under reduced pressure. Trituration of the residue with anhyd Et₂O produced an orange-colored solid, which was washed several times with Et₂O, absolute EtOH and then dried in a vacuum desiccator. This crude material was used for cyclic voltammetry and spectroscopic experiments without further purification (0.10 g, about 80% yield). ¹H NMR analysis (500 MHz, degassed CDCl₃) confirmed that the fulvalene 3 was the major isolated product along with oxidation derivatives 5, 6 (£10%). The system showed a ten proton multiplet centered at δ = 7.41 ppm and four broad singlets, indicating a mixture of syn/anti isomers and the spectrum close to coalescence at r.t. (rel. int. 1:4, twelve protons on the totality); major isomer: δ = 2.22, 2.81; minor isomer: δ = 2.24, 2.84. The assignments of the ¹³C NMR signals (125.8 MHz, degassed CDCl₃) were made under proton decoupling and HMBC; major isomer: δ = 20.3 (CH₃S), 38.6 (CH₃N), 108.3 (C5), 119.7 (C2), 128.3, 128.8, 129.9, 131.3 (arom. C), 145.7 (C4); minor isomer: δ = 20.0, 36.6, 109.8, 117.3, 128.3, 129.6, 129.9, 131.5, 146.2. MS (EI): m/z calcd for C₂₂H₂₂N₂S₄: 442.06659 [M]⁺; found: 442.06454. Cyclic voltammetry [MeCN, Bu₄NPF₆ (1 M)]: one oxidation peak at -0.31 V vs. SCE.

For ¹³C NMR spectra of crude non-isolated DTDAFs, see:

17a Chen Y.-T. Jordan F. J. Org. Chem.  1991,  56:  5029 

17b López-Calahorra F. Castells J. Domingo L. Martí J. Bofill JM. Heterocycles  1994,  37:  1579 

18a Morel G. Seux R. Foucaud A. Bull. Soc. Chim. Fr.  1975,  1865 

18b Marchand E. Morel G. Foucaud A. Synthesis  1978,  360 

18c Morel G. Marchand E. Foucaud A. Synthesis  1980,  918 

19

Procedure for Preparation of DTDAF 3 from Thiazolium Iodide 2.
To a suspension of salt 2 (0.35 g, 1 mmol), Na₂SO₄ (0.4 g) and CH₂Cl₂ (10 mL) was added finely ground KOH (0.45 g as a commercial product containing about 15% H₂O). After shaking at r.t. for 6 min, the mixture was rapidly filtered and the filtrate was concentrated in vacuo. The crude 3 (0.35 g) was isolated in the way described in ref.16; analysis by ¹H NMR spectroscopy allowed assessment of the efficiency of the method (the 3, 5, 6 distribution was estimated at roughly 85:10:5). For ¹H NMR and ¹³ C NMR see ref.16. Cyclic voltammetry [CH₂Cl₂, Bu₄NPF₆ (1 M)]: -0.34 and -0.19 V vs. SCE.

20

Compound 5: pale yellow needles from EtOH, mp 154 °C. ¹H NMR (300 MHz, CDCl₃): δ = 2.18 (s, 6 H), 2.95 (s, 6 H), 7.40 (m, 6 H), 7.80 (m, 4 H). ¹³C NMR (75.5 MHz): δ = 18.7, 34.4 (2 × q, ¹ J = 141 Hz), 128.6, 129.7, 130.5, 134.6 (arom. C), 134.9 (q, ³ J = 4.4 Hz), 145.6 (m), 165.1 (q, ³ J = 3 Hz). MS (EI): m/z calcd for C₂₂H₂₂N₂O₂S₄: 474.0564 [M]⁺; found: 474.0578. MS: m/z = 265, 237, 222, 165, 162, 118. Anal. Calcd: C, 55.67; H, 4.67; N, 5.90. Found: C, 55.56; H, 4.60; N, 5.87.
Compound 6: colorless solid from petroleum ether-Et₂O, mp 98 °C. ¹H NMR (300 MHz, CDCl₃): δ = 2.22 (s, 3 H), 3.10 (s, 3 H), 7.31 (m, 2 H), 7.49 (m, 3 H). ¹³C NMR (75.5 MHz): δ = 20.7 (q, ¹ J = 141 Hz), 32.0 (q, ¹ J = 142 Hz), 107.9 (q, ³ J = 5.3 Hz), 128.7, 129.6, 129.8, 129.9 (arom. C), 140.8 (m), 171.9 (q, ³ J = 3.3 Hz). MS (EI): m/z calcd for C₁₁H₁₁NOS₂: 237.02821 [M]⁺; found: 237.02820.

21 For similar reduction of acyclic disulfides to corresponding thiols by NaHSe/EtOH, see: Woods TS. Klayman DL. J. Org. Chem.  1974,  39:  3716 

22

Compound 7, main isomer: ¹H NMR (300 MHz, CDCl₃): δ = 2.66 (s, 6 H), 2.87 (s, 6 H), 6.74 (s, 2 H), 7.26 (s, 4 H), 7.37 (s, 6 H). ¹³C NMR (75.5 MHz): δ = 19.8 (q, ¹ J = 142 Hz), 33.4 (qd, ¹ J = 140 Hz, ³ J = 3.5 Hz), 71.7 (br d, ¹ J = 142 Hz), 128.8, 130.1, 130.2, 135.0 (arom. C), 165.2 (m), 234.7 (m). MS (EI, oily mixture): m/z calcd for C₂₀H₂₁N₂O₂S₂: 385.10445 [M - CS₂CH₃]⁺; found: 385.10460.

23 Itoh T. Nagata K. Okada M. Ohsawa A. Tetrahedron  1993,  49:  4859 

24 Bssaibis M. Robert A. Lemaguerès P. Ouahab L. Carlier R. Tallec A. J. Chem. Soc., Chem. Commun.  1993,  601 

25

Thiazolium salt 8: ¹H NMR (300 MHz, CDCl₃): δ = 2.26 (s, 3 H), 2.49 (s, 3 H), 3.04 (s, 3 H), 3.99 (s, 3 H), 7.03 (s, 1 H), 7.26-7.67 (m, 10 H). The assignments of ¹³C NMR signals (75.5 MHz, see numbering in Scheme [2] ) were made after proton decoupling, HMBC and HMQC: δ = 19.6, 20.8 (CH₃S), 40.3, 41.0 (CH₃N), 68.5 (C2′-sp³), 108.6 (C5′), 126.0, 130.2 (quat. arom. C), 135.3 (C5), 144.5 (C4′), 150.7 (C4), 178.9 (C2-sp²).

26

Formamide 9 (two isomers): ¹H NMR (300 MHz, CDCl₃): δ = 2.67, 2.72, 2.85, 2.88 (4 s), 5.85, 6.65 (2 s), 7.35-7.80 (m), 8.23 (s). ¹³C NMR (75.5 MHz): δ = 19.8, 20.0, 30.4, 33.1, 70.1, 77.0, 163.2, 163.9, 233.9, 234.1 and eight aromatic carbons.

27

A sat. MeCN solution of TCNQ was added to a degassed CH₂Cl₂ solution of DTDAF 3 (crude solid, 0.05 g). The mixture immediately turned from orange to black. After a couple of days at r.t. with protection from light, black shiny single crystals produced were filtered and washed with Et₂O: mp 226 °C. FTIR (KBr diffuse reflectance spectra): ν_CN = 2180 cm^-1.

28

Full details of the X-ray structure determination have been deposited at the Cambridge Crystallographic Data Centre (deposition number CCDC 259963). The crystallographic data can be obtained free of charge from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; e-mail: [e-mail].

29 Kistenmacher TJ. Emge TJ. Bloch AN. Cowan DO. Acta Crystallogr., Sect. B: Struct. Sci.  1982,  38:  1193 

30 Chappel JS. Bloch AN. Bryden WA. Maxfield M. Poehler TO. Cowan DO. J. Am. Chem. Soc.  1981,  103:  2442