[1,2,5]Thiadiazolo[3,4-c][1,2,5]thiadiazole (1) is synthesized in 62% yield by fluoride ion-induced condensation of 3,4-difluoro-1,2,5-thiadiazole with (Me(3)SiN=)(2)S. The reversible electrochemical reduction of 1 leads to the long-lived [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazolidyl radical anion (2) and further to the dianion (3). The radical anion 2 is also obtained by the chemical reduction of the precursor 1 with t-BuOK in MeCN. The radical anion 2 is characterized by ESR spectroscopy in solution and in the crystalline state. The stable salts [K(18-crown-6)][2] and [K(18-crown-6)][2].MeCN (8 and 9, respectively) are isolated from the spontaneous decomposition of the [K(18-crown-6)][PhXNSN] (6, X = S; 7, X = Se) salts in MeCN solution followed by XRD characterization. The radical anion 2 acts as a bridging ligand in 8 and as chelating ligand in 9. The structural changes observed by XRD in going from 1 to 2 are explained by means of DFT/(U)B3LYP/6-311+G calculations.

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The series of title compounds has been prepared through both electrophilic (C6HnF5−n−N=S=N−SiMe3 + SCl2) and nucleophilic (C6HF4−S−N=S=N−SiMe3 + CsF) intramolecular ortho‐cyclisation reactions, and the former route seems to be the more effective. High regioselectivity of the ring‐closing procedures is observed in both cases. The compounds were characterised by X‐ray crystallography and multinuclear (1H, 13C, 15N and 19F) NMR spectroscopy. In accordance with GIAO calculations, 15N{1H} experiments and the effects observed on complete substitution of hydrogen by fluorine, the high‐field signal in the 15N NMR spectra can be assigned to N‐4 and the low‐field signal to N‐2. In the crystal, 5,6,7‐trifluoro‐ (5) and 5,6,8‐trifluoro‐1,3λ4δ2,2,4‐benzodithiadiazine (6) are planar, whereas the 6,8‐difluoro derivative 3 is bent along the S1···N4 line by 8.3°. According to NICS calculations the heterocycle moiety in this compound class is antiaromatic while the carbocycle is aromatic. The fluorine substituents increase the aromaticity − and in some cases (especially when a fluorine atom is present in the 8‐position) the antiaromaticity − of the corresponding rings. The ortho‐fluoro‐containing starting material C6HnF5−n−N=S=N−SiMe3 (n = 2: 10) cyclises to the fluorinated 2,1,3‐benzothiadiazole 27 upon treatment with CsF instead of SCl2. For starting compound 6‐HC6F4−S−N=S=N−SiMe3 (14) the planar (Z,E) configuration features a short intramolecular H···N contact, as evidenced by X‐ray diffraction. Both the reaction pathways mentioned are also discussed. (© Wiley‐VCH Verlag GmbH, 69451 Weinheim, Germany, 2003)

Interaction between Ph(3)P and 1,3,2,4-benzodithiadiazine (1); its 6,7-difluoro (2), 5,6,8-trifluoro (3) and 5,6,7,8-tetrafluoro (4) derivatives; and 5,6,8-trifluoro-3,1,2,4-benzothiaselenadiazine (5) proceeded via a 1:1 condensation to give Ph(3)P═N-R iminophosphoranes (1a-5a, R = corresponding 1,2,3-benzodichalcogenazol-2-yls), which are inaccessible by general approaches based on the Staudinger and Kirsanov reactions. In contrast, neither Ph(3)As nor Ph(3)Sb reacted with 1 and 4. Molecular structures of 1a-5a and 5 were confirmed by X-ray diffraction (XRD). The crystals formed by chiral molecules of 2a-5a were racemic, whereas the crystal of 1a was formed by a single enantiomer. In all of the Ph(3)P═N-R derivatives, one of the Ph rings is oriented face-to-face to the hetero ring, R. Upon heating to ∼120 °C in squalane (1a, 3a, 4a) or dissolving in chloroform at ambient temperatures (1a, 2a, 4a), the Ph(3)P═N-R derivatives generated the 1,2,3-benzodithiazolyls (1b-4b, respectively) whose identity was confirmed by electron paramagnetic resonance (EPR). 2,1,3-Benzothiaselenazolyls 5b and 6b were detected by EPR as the main paramagnetic products of solution thermolysis of 5 and its 5,6,7,8-tetrafluoro congener (6), respectively. Passing a chloroform solution of 4a through silica column unexpectedly gave 5-6-6-6 tetracyclic (9) and 6-10-6 tricyclic (10) sulfur-nitrogen compounds, which were characterized by XRD.

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(2) with PhS -anion followed by XRD and ESR characterization. These salts are thermally stable and soluble in MeCN. Lowtemperature crystallization of salt 5 from MeCN or storage of 5 under MeCN for a few days at ambient temperature gives the solvate 5·MeCN (7). The radical anion 1 acts as bridging ligand in salts 4 and 5 and as chelating ligand in salt 7. In

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