Thermally Induced Magnetic Anomalies in Solvates of the Bis(hexafluoroacetylacetonate)copper(II) Complex with Pyrazolyl-Substituted Nitronyl Nitroxide

We succeeded in synthesizing of a whole family of isostructural solvates of the copper(II) hexafluoroacetylacetonate complex with pyrazolyl-substituted nitronyl nitroxide (L): Cu(hfac)₂L·0.5Solv. The main feature inherent in nature of Cu(hfac)₂L·0.5Solv single crystals is their incredible mechanical stability and ability to undergo reversible structural rearrangements with temperature variation, accompanied by anomalies on the μ_eff(T) dependence. Structural investigation of the complexes over a wide temperature range before and after the structural transition and the ensuing magnetic phase transition showed that the spatial peculiarities of the solvent molecules incorporated into the solid govern the character of the μ_eff(T) dependence and the temperature region of the magnetic anomaly. Thus, doping of crystals with definite solvent molecules could be used as an efficient method of control over the magnetic anomaly temperature (T_a). The investigation of this special series of crystals has revealed the relationship between the chemical step and the magnetic properties. It was shown that “mild” modification of T_a for Cu(hfac)₂L·0.5Solv required a much smaller structural step than the typical change of one −CH₂− fragment in a homologous series in organic chemistry. Quantum-chemical calculations with the use of X-ray diffraction data allowed us to trace the character of changes in the exchange interaction parameters in the range of the phase transition. In the temperature range of the phase transition, the exchange parameter changes substantially. The gradual decrease in the magnetic moment, observed in most experiments during sample cooling to T_a, is the result of the gradual increase in the fraction of the low-temperature phase in the high-temperature phase.