Hysteretic Room-Temperature Magnetic Bistability of the Crystalline 4,7-Difluoro-1,3,2-Benzodithiazolyl Radical

The title radical R & sdot;, synthesized by reduction of the corresponding cation R+, is thermally stable up to similar to 380 K in the crystalline state under anaerobic conditions. With SQUID magnetometry, single-crystal and powder XRD, solid-state EPR and TG-DSC, reversible spin-Peierls transition between diamagnetic and paramagnetic states featuring similar to 10 K hysteretic loop is observed for R & sdot; in the temperature range similar to 310-325 K; Delta H=similar to 2.03 kJ mol(-1) and Delta S=similar to 6.23 J mol(-1) K-1. The transition is accompanied by mechanical movement of the crystals, i. e., by thermosalient behavior. The low-temperature diamagnetic P-1 polymorph of R & sdot; consists of R & sdot;(2) pi-dimers arranged in (& mldr;R & sdot;(2)& mldr;)(n) pi-stacks; whereas the high-temperature paramagnetic P2(1)/c polymorph, of uniform (& mldr;R & sdot;& mldr;)(n) pi-stacks. With the XRD geometries, CASSCF and broken-symmetry DFT jointly suggest strong antiferromagnetic (AF) interactions within R & sdot;(2) and weak between R & sdot;(2) for the (& mldr;R & sdot;(2)& mldr;)(n) stacks; and moderate AF interactions between R & sdot; for the (& mldr;R & sdot;& mldr;)(n) stacks. The fully hydrocarbon archetype of R & sdot; does not reveal the aforementioned properties. Thus, the fluorinated 1,3,2-benzodithiazolyls pave a new pathway in the design and synthesis of metal-less magnetically-bistable materials.