NC-(CF2)(4)-CNSSN center dot containing 1,2,3,5-dithiadiazolyl radical dimer exhibiting triplet excited states at low temperature and thermal hysteresis on melting-solidification: structural, spectroscopic, and magnetic characterization

A high yield, one-pot synthesis of the 1,2,3,5-dithiadiazolyl radical NC-(CF2)(4)-CNSSN center dot by reduction of the corresponding 1,3,2,4-dithiadiazolium salt is reported. In the solid state, the title compound is dimerized in trans-cofacial fashion with intra-dimeric S delta+center dot center dot center dot N delta(-) interactions of ca. 3.2 angstrom, and the dimeric units are linked by electrostatic -C equivalent to N delta-center dot center dot center dot S delta+ interactions forming an infinite chain. Magnetic susceptibility measurements performed on the solid state sample indicate a magnetic moment of 1.8 mu B per dimer (1.3 mu B per monomer) at 300 K and a good fit to the Bleaney-Bowers model in the temperature range 2-300 K with 2J = -1500+/-50 cm(-1), g = 2.02(5), rho = 0.90(3)%, and TIP = 1.25(4) x 10(-3) emu mol(-1). The [NC-(CF2)(4)-CNSSN center dot](2) dimer is the second example of a 1,2,3,5-dithiadiazolyl radical dimer with an experimentally detected triplet excited state as probed by solid-state EPR [2J = -1730 +/- 100 cm(-1), vertical bar D vertical bar = 0.0278(5) cm(-1), vertical bar E vertical bar = 0.0047( 5) cm-1]. The value of the singlet-triplet gap has enabled us to estimate the in situ dimerization energy of the radical dimer as ca. -10 kJ mol(-1). The diradical character of the dimer was calculated [CASSCF(6,6)/6-31G*] as 35%. The title radical shows magnetic bistability in the temperature range of 305-335 K as probed by the solid-state EPR presumably arising from the presence of a metastable paramagnetic supercooled phase. Bistability is accompanied by thermochromic behavior with a color change from dark green (dimeric solid) to dark brown ( paramagnetic liquid).