EDT-TTF and alkene-functionalised BEDT-TTF derivatives : synthesis and radical cation salts †

School of Science and Technology, Nott Nottingham NG11 8NS, UK. E-mail: john.wa UK National Crystallography Service, Unive Southampton, SO17 1BJ, UK. E-mail: S.J.Co Diamond Light Source Ltd, Harwell Scienc Didcot, OX1 0DE, UK. E-mail: claire.wilson@ Department of Chemistry, Graduate Scho Japan. E-mail: akutsu@chem.sci.osaka-u.ac † CCDC 1051494–1051496. For crystallog format see DOI: 10.1039/c5ra03921e Cite this: RSC Adv., 2015, 5, 31104


Introduction
Organosulfur donors such as TTF 1 and BEDT-TTF 2 have been used as substrates for preparation of conducting materials by oxidation either by electrocrystallisation or treatment with electron-decient acceptor substances, leading to conducting, semi-conducting and in some cases, superconducting materials. 1A wide range of substituted derivatives of these donors has been prepared and applications explored. 2There has been less interest in donor 3, known as VT, the fully unsaturated derivative of BEDT-TTF containing endocyclic double bonds at both ends of the molecule, though several salts have been prepared. 3,4Substituted derivatives of VT have been prepared including the tetra-methyl, -phenyl and -pyrid-4-yl substituted derivatives, for example 4, 5 as well as the dimethyl derivative 5 which has been prepared as a mixture of cis and trans isomers. 4here are a few reports of the BEDT-TTF derivative 6 where just one dithiin ring contains an extra double bond, 6,7 and some of its monosubstituted derivatives such as the methyl derivative 7. 6,8 The inclusion of one or two extra double bonds raises the oxidation potential from that of BEDT-TTF by 0.06 and 0.14 V respectively. 6We have been interested in preparing donors with alkene functionality, partly because of the potential for polymerising such systems, as well as for the donors themselves as substrates for salt formation.Here we report the preparation of three new donors 8-10, one with an exocyclic double bond attached to the BEDT-TTF framework and thus a tautomer of 7, and two with an alkene group at the end of a side chain.

Discussion
Synthesis and properties of exo-methylene-BEDT-TTF Hydroxymethyl-BEDT-TTF 12 is readily prepared in four steps from allyl acetate and trithione 11, 9 but has not been signicantly exploited synthetically.We now report that conversion to its O-tosylate 13 9 and subsequent treatment with t-butoxide eliminates tosic acid to give exo-methylene-BEDT-TTF 8 which has an exocyclic double bond connected to the BEDT-TTF system.We were unable to isomerise 8 to the donor 7 with an exocyclic methyl group and endocyclic double bond.The 1 H NMR spectrum of donor exo-methylene BEDT-TTF shows signals for the alkene hydrogens at d: 5.13 and 5.22 ppm and for the adjacent methylene group at d: 3.52 ppm.In contrast donor 7 clearly shows the methyl and vinyl hydrogen signals (d: 2.11 and 6.53 ppm) from the unsaturated ring and quite a different melting point. 6,8Furthermore, the 1 H NMR of the dimethyl-VT donor 5 also clearly shows the presence of the methyl and vinyl hydrogens (Scheme 1). 4 For the new donor 8 the consequence of conjugation of the organosulfur system with the exocyclic bond is a small increase in the rst two oxidation potentials from 0.52 and 0.94 V (relative to Ag/AgCl) for BEDT-TTF to 0.56 and 0.97 V for exomethylene-BEDT-TTF (Table 1).
Electrocrystallisation experiments on exo-(methylene)BEDT-TTF 8 in the presence of various anions were carried out, and the most successful was that with perchlorate which gave thin black crystals of a 1 : 1 radical cation salt.Furthermore, diffusion of a dichloromethane solution of iodine into a benzonitrile solution of 8 led to a 2 : 2 : 1 salt of the donor monocation with triiodide and iodine which has been shown to be a semiconductor.
The crystal structure of the 1 : 1 salt with perchlorate was measured at 100 K.The crystal system is monoclinic, in space group P2 1 /c, with one donor cation and one perchlorate anion in the asymmetric unit (Fig. 1).The crystals are twinned and there is some orientational disorder of the perchlorate ion, as well as some of the commonly observed conformational disorder in the unsubstituted dithiin ring.The donor cations are organised in centrosymmetric pairs, and these are packed in the bc plane along with the anions (Fig. 2) which isolate pairs from one another.However, there are short contacts between pairs which lie side by side in the a direction (Fig. 3).In the donor cation the effect of the exocyclic alkene is a shortening of the endocyclic bonds connecting it to its sulfur and carbon atom neighbours to 1.758(13) and 1.457(18) Å.The alkene bond length is 1.32 (2)  Å but the apparent shortness may be due to some slight positional disorder of the terminal methylene group.The conformation of the substituted dithiin ring is an envelope with the sp 3 carbon atom at the envelope's ap.The exocyclic sp 2 methylene group lies to the opposite side.Within a donor cation dimer there are four short S/S contacts between the centrosymmetrically related TTF moieties (3.375(4) and 3.504(5) Å), and there are four short S/S contacts in the range 3.460(5)-3.535(5) Å between dimers (Fig. 3), thus providing a possible route for conduction of electrons through the structure.Unfortunately, the crystals were too brittle for attachment of electrodes for conductivity measurements.The X-ray crystal structure of the salt formed from donor 8 and iodine showed that it had a composition 2 : 2 : 1 donor : I 3 : I 2 .Two probe conductivity measurements showed that it was a semiconductor with room temperature conductivity of 1.89 Â 10 À4 S cm À1 and an energy of activation of 0.373 eV.The crystal system is triclinic and the space group is P 1.The asymmetric unit is composed of one donor cation, one triiodide and half of an iodine molecule, since the iodine molecule lies on a centre of symmetry.The donor cations are organised in face to face pairs, which are almost surrounded by iodine molecules and triiodide anions (Fig. 4 and 5).However, there are side to side contacts between adjacent pairs of donor cations in the a direction (Fig. 6).The I-I bond lengths are 2.8054(13) Å in the iodine molecule and 2.8730(10) and 2.9535 (10) Å in the triiodide anion, and the two species approach each other at 85 with a I/I contact of 3.3789( 13) Å between them.Lines of triiodide anions run through the structure between the donor cation pairs, with successive short and long head to tail I/I contacts between triodides of 3.4274(11) and 3.8388(12) Å.Within the radical cation pairs there are four short S/S contacts between the S atoms of the TTF core, all of length 3.396(4) Å.In addition there are two sets of two short S/S contacts between adjacent cation pairs of length 3.462(4) and 3.543(4) Å (Fig. 6).

Synthesis of donors with a side chain alkene group
Reaction of diallyl ether with an excess of the trithione 11 in hot toluene gave thione 14, the product of one cycloaddition, as the main product in 68% yield.A small amount of the bis(thione) 15, formed by both alkene groups reacting with a trithione molecule was also isolated in 6% yield.Increasing the amount of the trithione in the reaction did not alter the product distribution.Conversion of the thione 14 to oxo compound 16 with mercuric acetate proceeded in 83% yield, and the oxo compound was coupled with the unsubstituted thione 18 to yield the BEDT-TTF derivative with an allyloxymethyl side chain 9 in 53% yield aer separation from homo-coupled products (Scheme 2).The bis(thione) 15 decomposed slowly.Although it can be converted to the corresponding bis(oxo compound) 17, attempts to couple this product with 18 did not lead to any bis(BEDT-TTF) derivatives.The corresponding chemistry starting with diallyl sulphide proceeded in a similar way though with lower yields.Thus, reaction with trithione 11 gave the thione 19 resulting from just one cycloaddition in 31% yield.This was converted in 53% yield to the oxo compound 20, which was then coupled to the unsubstituted thione 18 to give the BEDT-TTF derivative with an allylthiomethyl side chain 10 in 31% yield (Scheme 3).Zhu et al. have reported the closely related alkyne analogue 21. 10 The oxidation potentials of donors 9 and 10 were similar to those of BEDT-TTF.Electrocrystallisation experiments with 9 have yielded a 1 : 2 (Table 1) perchlorate salt whose crystal structure has been determined.Diffusion experiments with iodine only gave very small crystals of low quality.The crystal structure of 9$2ClO 4 , measured at 100 K, is monoclinic, in space group P2 1 and there are two donors each bearing a charge of 2+ and four perchlorate anions in the unit cell.The crystal structure is shown in Fig. 7.The donor dication shows the typically very long bond between dithiole units of 1.44(2) Å, and the four central C-S bonds, shortened by the 2+ charge, lie in the range 1.676(18)-1.717(18)Å similar to those reported for the dication of BEDT-TTF. 11The monosubstituted donor is disordered between two positions, which are roughly related by a 2 fold rotation about the long axis of the molecule.The positions differ in the point of attachment of the side chain to the dithiin ring, however the side chain lies so that the two structures have a common position for the terminal allyloxy group.This is achieved by the dithiin ring in the two structures adopting an approximate envelope conformation with the side chain attached to the "ap" methine carbon and oriented in a pseudoaxial manner (Fig. 8).There is no stacking of donor dications, but there are lines of dications along the a axis organised so that the molecules have edge to edge S/S contacts in the range 3.630(7)-4.198(8)Å. Adjacent donors in the b direction are strongly slipped relative to each other so that there are only three S/S contacts (3.728(7)-3.785(6)Å) between sulfurs from the unsubstituted end of one donor with sulfurs from the substituted end of the next donor (Fig. 9).The perchlorate anions are distributed among the donor dications, and each donor has S/O contacts to four anions (Fig. 10).Although donor stacking and segregated anions are common in BEDT-TTF radical cation salts, the larger donor charge in this case leads to anion contacts with the organosulfur residue.Conductivity measurements showed that the material was an insulator as might be expected.

Conclusion
The small size of the substituent on exo-methylene-BEDT-TTF 8 provides minimal steric hindrance to crystal packing and may offer access to a wide range of new radical cation salts, as observed for BEDT-TTF.The ready preparation of donors 9 and 10 with a terminal alkene substituent provide donors with the potential for incorporation into polymers.Both of these aspects are now under investigation in our laboratory.

Experimental
General Solution NMR spectra were measured on a JEOL Eclipse 400 spectrometer at 400 MHz for 1 H and at 100.6 MHz for 13 C using CDCl 3 as solvent and tetramethylsilane (TMS) as standard unless otherwise stated, and measured in p.p.m. downeld from TMS with coupling constants reported in Hz.IR spectra were recorded on a Perkin Elmer Spectrum 100 FT-IR Spectrometer using Attenuated Total Reection sampling unless    otherwise stated, and are reported in cm À1 .Mass spectra were recorded at the EPSRC UK National Mass Spectrometry Facility at the University of Swansea.Chemical analysis data were obtained from Mr Stephen Boyer, London Metropolitan University.Flash chromatography was performed on 40-63 silica gel.Cyclic voltammetry was conducted on a Metrohm m-Autolab Type III.

exo-Methylene-BEDT-TTF, 8
To a solution of tosyloxymethyl-BEDT-TTF 13 9 (0.103 g, 0.18 mmol) in dry THF (5 ml) under nitrogen was added sodium tertbutoxide (27 mg, 0.28 mmol).The solution was stirred at room temperature for 3 h.solvent was evaporated, DCM added and the mixture washed with water.The organic phase was collected and dried over Na

Radical cation salts of exo-methylene-BEDT-TTF, 8
Perchlorate salt.To the anodic side of a H-shaped electrochemical cell tted with a glass frit was placed donor 8 (5 mg, 0.0126 mmol), and to the cathodic side of the cell was added a 0.0089 M solution of tetrabutylammonium perchlorate in chlorobenzene (40 ml).The level of solution in each compartment was allowed to equilibrate, and into each side was inserted a platinum-tipped electrode.A constant current of 2.0 mA was applied across the cell for 3 days to give black needleshaped crystals of 8$ClO 4 .

Radical cation salts of allyloxymethyl-BEDT-TTF, 9
Perchlorate salt.To the anodic side of a H-shaped electrochemical cell tted with a glass frit was placed donor 9 (10 mg, 0.022 mmol), and to the cathodic side of the cell was added a 0.0089 M solution of tetrabutylammonium perchlorate in chlorobenzene (40 mL).The level of solution in each compartment was allowed to equilibrate, and into each side was inserted a platinum-tipped electrode.A constant current of 0.5 mA was applied across the cell for 4 weeks to give black needle shaped crystals of 9$(ClO 4 ) 2 .The structure was solved with SHELXS97, 14 and rened with SHELX2013 15 within OLEX2. 16Data were collected from a twinned crystal.ROTAX 17 was used to identify the twin law, corresponding to a 2-fold rotation about the 1 0 0 direction: twin law [1.0000.000 0.000] [0.000 À1.000 0.000] [À0.498 0.000 À1.000] and the twin component fraction rened to 0.71.Disorder in the ClO 4 anion was modelled with two of the oxygen sites split over two sites with the occupancy rened competitively, 0.63/0.37(3),and isotropic atomic displacement parameters for partially occupied atoms.Distance restraints were applied to the Cl-O distances.

X-ray crystallography
Crystal data for 8$I 3 $(I 2 ) 0.  , all data) ¼ 0.15.Data were collected at room temperature using a Rigaku Mercury sII CCD congured with the Rigaku MicroMax-007HF generator and VariMax confocal mirror using Crystal Clear soware 13 for data collection and reduction.The structure was solved by SIR92 18 and rened with SHELX2013 15 using the Rigaku CrystalStructure® soware package.

Conductivity Measurements
Two-probe DC transport measurements were made on several crystals of 8$I 3 $0.5I 2 using a HUSO HECS 994C multi-channel conductometer, and on crystals of 9$(ClO 4 ) 2 using an Oxford Instruments Optistat and temperature controller.Gold wires were attached to the crystal, and the attached wires were connected to an four-pin integrated circuit plug with conductive cement.

Fig. 2
Fig. 2 Crystal packing of the salt 8$ClO 4 viewed down the a axis.

Fig. 4 A
Fig. 4 A face to face radical cation pair surrounded by triiodide anions and iodine molecules in the crystal structure of 8$I 3 $0.5I 2 .

Fig. 7
Fig. 7 Crystal structure of 9$2ClO 4 viewed down the a axis, showing the integration of dicationic donors with anions.

Fig. 8
Fig. 8 View of the donor dication of 9 showing the two disordered orientations of the side chain and how they overlap.

Fig. 9
Fig. 9 Partial crystal packing diagram of 9$2ClO 4 showing the relation between donor dications in the a and b directions.Only one conformation of the disordered donor is shown and anions are omitted for clarity.

Table 1
Cyclic voltammetry of donors 8, 9 and 10 Molecular structure of the salt 8$ClO 4 with anisotropic displacement parameters drawn at the 50% level.