High-field EPR of copper(ii)–nitroxide compound exhibiting three-step phase transition: structural insights from the field-induced sample orientation

Abstract

Copper(II)–nitroxide based Cu(hfac)₂L^R compounds exhibit unusual magnetic behavior that can be induced by various stimuli. In many aspects, the magnetic phenomena observed in Cu(hfac)₂L^R are similar to classical spin-crossover behavior. However, these phenomena originate from polynuclear exchange-coupled spin clusters Cu²⁺–O˙–N< or >N–˙O–Cu²⁺–O˙–N<. Such peculiarities may result in additional multifunctionality of Cu(hfac)₂L^R compounds, making them promising materials for spintronic applications. Herein, we investigate the Cu(hfac)₂L^MeMe material, which demonstrates a three-step temperature-induced magnetostructural transition between high-temperature, low-temperature, and intermediate states, as revealed by magnetometry. Two main steps were resolved using variable-temperature Fourier-transform infrared and Q-band electron paramagnetic resonance (EPR) spectroscopies. The intermediate-temperature states (∼40–90 K) are characterized by the coexistence of two types of copper(II)–nitroxide clusters, corresponding to the low-temperature and high-temperature phases. High-field EPR experiments revealed the effect of partial alignment of Cu(hfac)₂L^MeMe microcrystals in a strong (>20 T) magnetic field. This effect was used to unveil the structural features of the low-temperature phase of Cu(hfac)₂L^MeMe, which were inaccessible using single-crystal X-ray diffraction (XRD) technique. In particular, high-field EPR allowed us to determine the relative direction of the Jahn–Teller axes in CuO₆ and CuO₄N₂ units.