Extended TTF-type donors fused with pyrazine units; synthesis and characterization Sandra Rabaça a,⇑ , Sandrina Oliveira a , Isabel C. Santos a,b , Manuel Almeida a,⇑ a UCQR, IST/ITN, Instituto Superior Técnico/CFMCUL, Estrada Nacional 10, P-2686-953 Sacavém, Portugal b Centro de Química Estrutural, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal article info Article history: Received 12 July 2013 Revised 24 September 2013 Accepted 27 September 2013 Available online 3 October 2013 Keywords: Extended tetrathiafulvalene donors Pyrazine Synthesis X-ray structure Cyclic voltammetry abstract Pyrazinedicyanoethylthiotetrathiafulvalene, (pzdc-TTF) (1a), an extended TTF fused with a pyrazine moi- ety and also a dithiolene ligand precursor, was synthesized through a cross-coupling with triethyl phos- phite between pyrazine-1,3-dithiole-2-thione (I) and 4,5-bis(2-cyanoethylthio)-1,3-dithiole-2-one (III). This reaction also yields to dipyrazine TTF derivative (1b) and 2,3,6,7-Tetrakis(2-cyanoethylthio)TTF (1c), resulting from the self-coupling reactions of the thione (I) and ketone (III). The crystal structure of 1a is composed by pairs of head to head donor stacks of pzdc-TTF molecules along b in opposite ori- entations. Single crystals of 1b revealed a new polymorph with a face-to-face p-stacking motif. The elec- trochemical properties of 1a studied by cyclic voltammetry (CV) in DMF, shows two single electron oxidation processes typical of TTF-based donors; one pair of asymmetric redox waves centered at 627 mV versus Ag/Ag + is ascribed to the couple [pzdc-TTF] + /[pzdc-TTF] 2+ , and one pair of quasi-reversible redox waves centered at 430 mV is ascribed to the couple [pzdc-TTF] 0 /[pzdc-TTF] + . Ó 2013 Elsevier Ltd. All rights reserved. The tetrathiafulvalene molecule (TTF) and its many derivatives, due to its unique p-donor properties, have been at the basis of the large majority of organic metals and superconductors known so far. 1 This success as a building block for conducting materials is largely associated with the large possibilities of extending the delocalized p-system and incorporation of additional sulfur or other heteroatoms in the molecular periphery. The extension of the p-system in general renders more accessible the different oxidation states, and maximizes the intermolecular interactions between planar molecules that tend to be organized as stacks in the solid state. The presence of additional sulfur and other hetero- atoms in the molecular periphery promotes the side intermolecu- lar interactions along the molecular plane. Some extended TTF derivatives have been also used in the last decade as dithiolene ligands, in the preparation of extended transi- tion metal bisdithiolene complexes 2 that deserved special atten- tion because in their neutral state were often found to lead a metallic behavior, the so called single component molecular metals. 3 More recently there has been an increasing interest in TTF derivatives containing N atoms in their periphery which retaining the electroactive behavior of TTF could in addition be able to coordinate transition metals. 4 Aiming at enlarging this type of complexes we decided to explore new extended dithiolene ligands that incorporate not only TTF, but also pyrazine units. The extra pyrazine units are expected to enhance the degree of p-delocaliza- tion over the ligand extremity, while the presence of the nitrogen atom can favor the side to side intermolecular interaction at the li- gand periphery, and coordinate to other metals as found in other simple pyrazine dithiolate type ligands. 4b,5 In this Letter, we report the synthesis and characterization of a new extended TTF-type donor fused with pyrazine moieties the pyrazinedicyanoethylthiotetrathiafulvalene, (pzdc-TTF) (1a), which is also a precursor for the preparation of new extended pyrazine-TTF fused dithiolene ligands. The key compound for the synthesis of the pyrazine substituted compounds (ligand precursors and donors) is thione I (Pyrazine- 1,3-dithiole-2-thione). 6 Although this compound was previously mentioned, 7 and could be obtained from the reaction of 2,3-dichlo- ropyrazine with potassium trithiocarbonate, 8 in this work we report modifications in the reaction treatment and a full characterization of this compound. Ketone II (Pyrazine-1,3-dithiole-2-one), which can be also a key starting compound, was obtained from a regular S to O exchange reaction of I. 9 The preparation of the novel TTF- based donor, pzdc-TTF (1a), 10 was achieved following a synthetic route first used by Underhill and co-workers 2 involving the coupling with triethyl phosphite between thione I and ketone III (Scheme 1). This coupling reaction leading to the compound (1a) also gives rise to other by-products, the dipyrazine TTF derivative (1b) and 0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2013.09.131 ⇑ Corresponding authors. Tel.: +351 21 994 6203; fax: +351 21 955 0117 (S. Rabaça); tel.: +351 21 994 6171; fax: +351 21 955 0117 (M. Almeida). E-mail addresses: sandrar@ctn.ist.utl.pt (S. Rabaça), malmeida@ctn.ist.utl.pt (M. Almeida). Tetrahedron Letters 54 (2013) 6635–6639 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet