Switching on thermal and light-induced spin crossover by desolvation of [Fe(3-bpp)2](XO4)2·solvent (X = Cl, Re) compounds

Abstract

Thermal desolvation is a very attractive method for the post-synthetic modification of the physicochemical properties of switchable materials. In this field of research, special attention has been paid to the possibility of modifying the thermo- and photo-induced spin crossover (SCO) properties of metal complexes as they can act as solvent sensors. Two new [Fe(3-bpp)₂](ClO₄)₂·2.5H₂O·MeOH (1·sol) and [Fe(3-bpp)₂](ReO₄)₂·3H₂O (2·sol) compounds, where 3-bpp is 2,6-di-(1H-pyrazol-3-yl)pyridine, were prepared and structurally characterized, and their solvated and desolvated phases were additionally investigated spectroscopically, magnetically and photomagnetically. Single-crystal X-ray structures of 1·sol and 2·sol consist of similar [Fe(3-bpp)₂]²⁺ units arranged in π–π stacked layers separated by H-bond-stabilised layers consisting of solvent molecules and anions. Moreover, both materials show desolvation-assisted SCO from a low (LS, S = 0) to high-spin state (HS, S = 2) at around 340 K, followed by a reversible gradual spin conversion with T_1/2 ≈ 210 K and 304 K for 1 and 2, respectively. Photomagnetic studies of 1·sol and 1 confirmed the efficiency of the light-induced excited spin-state trapping (LIESST) phenomenon with relaxation temperatures T(LIESST) = 82 K and 66 K for the solvated and desolvated phases, respectively. In the case of 2·sol, no LIESST effect was observed while the desolvated phase 2 exhibits a LIESST behaviour at T(LIESST) ≈ 50 K.