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Synthesis of 1,3,2-dithiazolyl radicals and their charge-transfer salts
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A number of non-fused 1,3,2-dithiazolylium salts were synthesised via the cycloaddition reaction of [SNS][SbF6] with di-substituted alkynes (R1/R2 = Ph/Ph, CH2/Cl/CH2Cl, CO2Me/H, C3H7H, Ph/H, SiMe3/SiMe3, Ph/C6H4NO2, CO2Et/CO2Et, Ph/C6F4CN, C6F5/C6F5, CO2Me/CO2Me, C1C6H4/SiMe3, Ph/CO2Et, Ph/CO2Me). Benzo-fused salts were synthesised using Less’ reagent (tBuSNa) to provide a route through which the bis(sulfanyl chloride) intermediate may be obtained. Five substituted benzo-1,3,2-dithiazolylium chloride salts are obtained with R =Me, CN, (OMe)2 and CF3 as well as the pyridyl derivative. Reduction of these salts yielded the corresponding 1,3,2-dithiazolyl radicals. EPR studies were used to estimate spin density distributions on the heterocyclic rings which were found to be in good agreement with results from DFT calculations. Four fused radicals were isolated in the solid state and their physical properties studied; [o-MBDTA]• crystallises as and eclipsed dimer which adopts a sandwich herringbone motif; [PyDTA]• is polymorphic crystallising in either of two forms depending on sublimation conditions. α-[PyDTA]• contains π-stacked arrays of eclipsed [PyDTA]• dimers but undergoes an exothermic and irreversible phase transition (ΔH = -1.2 kJ mol-1) to β-[PyDTA]• at 349 K. β-[PyDTA]• also contains eclipsed dimers but adopts an antiparallel stacking arrangement in the solid state. Whilst α-[PyDTA]• is diamagnetic at all temperatures, β-[PyDTA]• is diamagnetic at low temperature but shows a steady increase in paramagnetism on warming associated with a thermally activated unpairing of the spins in the π*-π* dimer; [o-CNBDTA]• is paramagnetic at room temperature and adopts a regular π-stacked structure but undergoes a reversible phase transition on cooling below 250 K to a diamagnetic π*-π* dimer phase; the structure of [m-CNBDTA]• was determined by XRPD and found to adopt a regular π-stacked structure at room temperature. Like [o-CNBDTA]• it too exhibits a phase transition. However, this transition occurs with thermal hysteresis ((TC↓ = 291 K, TC↑ = 304 K, ΔT = 13 K). The hysteresis associated with the phase transition is attributed to a large activation energy to interconversion which is believed to arise through interlayer interstack contacts. Both [o-CNBDTA]• and [m-CNBDTA]• were also shown to undergo a second, low temperature, transition from χD to χp phases upon cooling. Both phase transitions exhibited thermal hysteresis ([o-CNBDTA]• TC↓ = 39 K, TC↑ = 26 k; [m-CNBDTA]• TC↓ = 37 K, TC↑ = 28 K). Chapter 4 investigates the use of 1,3,2-dithiazolylium salts as potentially non-innocent planar counterions in charge-transfer salts with metal bis(dithiolene) anions.
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