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

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.

Ferromagnetic Coupling in the Heterospin Bis-Catecholato-Manganese(IV) Complex with Pyridine Substituted by Nitronyl-nitroxide.

A new bis(3,6-di-tert-butyl-catecholato)manganese complex with two 4-NIT-Py ligands was synthesized and characterized [4-NIT-Py = pyridine substituted at position 4 with nitronyl-nitroxide radical, 2-(pyridin-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl]. X-ray diffraction indicated an octahedral environment of the manganese atom with a trans arrangement of ligands. Bonds lengths in the inner coordination core of the metal and in the chelate cycles that are representative of the charge distribution between the metal and ligands displayed a Mn(IV)(Cat2-)2 charge distribution. Variable-temperature magnetic susceptibility measurements detected intramolecular ferromagnetic coupling between the Mn(IV) S = 3/2 spins and spins of nitronyl-nitroxyls and intermolecular ferromagnetic interactions of spins of adjacent nitronyl-nitroxide fragments in a chain of molecules at low temperatures. The last phenomenon is revealed by short contacts between nitronyl-nitroxide radicals of adjacent complex molecules.

Theory of spin-peierls transitions in chains of exchange clusters.

A theoretical approach to the analysis of magnetostructural phase transitions of chain polymeric heterospin complexes is suggested. The approach is based on a model of the spin-Peierls transition in chains of exchange clusters. The chain elasticity parameter is found to be a main factor determining the order of phase transition.