Intermolecular Spin-Crossover-like Phenomenon Sensitive to Applied External Pressure in Heterospin Crystals

The interaction of Cu(acF6)₂ with the stable imino nitroxide 2-(pyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl (ImN-mPy) at a ratio of 2:1 led to the formation of a new complex [{Cu(acF6)₂}₂(ImN-mPy)], which can exist in two polymorphic modifications, 1 and 2. The solid phases of both polymorphs are formed by binuclear molecules, in which the paramagnetic ligand performs the ditopic function due to the coordination of the imine N atoms of the imidazoline and pyridine heterocycles; the >N-·O fragment is not involved in coordination. The intramolecular exchange interactions in the {Cu²⁺-N=C-N-·O} exchange clusters were found to be ferromagnetic (J_Cu-R > +100 cm^-1). When the cooling-heating cycles are repeated, the χT(T) dependence for 2 acquires a hysteresis with T = 37 K and T = 46 K due to the appearance of a strong antiferromagnetic intermolecular exchange between the >N-·O fragments of adjacent molecules. It was impossible to trace the structural changes in the solid compound at T < 50 K because the crystals cracked and lost their quality during the phase transitions. Coating the crystals with epoxide resin prevented their decomposition, but completely suppressed the spin transition. Immersion of the polymorph 2 crystals in a pressure-transmitting medium Daphne7373, which froze together with the crystals of the experimental sample at T < 200 K, also suppressed the transition to the low-spin state. An increase in the external pressure to 1.8 kbar led to stabilization of the strongly coupled low-spin state in the {>N-·O···O·-N <} intermolecular exchange clusters already at room temperature.