Crystalline 4,7-difluoro-1,3,2-benzodithiazolyl, the first isolated and structurally defined representative of ring-fluorinated Wolmershäuser radicals, reveals room-temperature magnetic bistability (spin-Peierls transition) with the hysteretic loop of 10 K.

Abstract

The title radical R⋅, synthesized by reduction of the corresponding cation R ⁺ , is thermally stable up 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 ~10 K hysteretic loop is observed for R⋅ in the temperature range ~310–325 K; ΔH=~2.03 kJ mol⁻¹ and ΔS=~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⋅ consists of R⋅₂ π-dimers arranged in (…R⋅₂…)_n π-stacks; whereas the high-temperature paramagnetic P2₁/c polymorph, of uniform (…R⋅…)_n π-stacks. With the XRD geometries, CASSCF and broken-symmetry DFT jointly suggest strong antiferromagnetic (AF) interactions within R⋅₂ and weak between R⋅₂ for the (…R⋅₂…)_n stacks; and moderate AF interactions between R⋅ for the (…R⋅…)_n stacks. The fully hydrocarbon archetype of R⋅ 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.