Structural, thermal and photomagnetic properties of spin crossover [Fe(bpp)2]2+ salts bearing [Cr(L)(ox)2]− anions

Abstract

This paper is divided into two parts: in the first part, the influence of solvate molecules on the magnetic properties of spin crossover salts of [Fe(bpp)₂][Cr(L)(ox)₂]ClO₄·nS (bpp = 2,6-bis(pyrazol-3yl)pyridine; L = 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen); ox = oxalate dianion; S = solvent) is analyzed. The second part is devoted to the photomagnetic properties of the previously reported [Fe(bpp)₂][Cr(L)(ox)₂]₂ family of compounds. The study describes the crystal structure, differential scanning calorimetry (DSC) and magnetic properties of [Fe(bpp)₂][Cr(bpy)(ox)₂]ClO₄·EtOH·4H₂O (1) and [Fe(bpp)₂][Cr(phen)(ox)₂]ClO₄·1.5EtOH·4H₂O (2). Both salts are high-spin (HS) compounds. Desolvation of 1 yields a material exhibiting a gradual spin crossover that involves 50% of the Fe²⁺ cations. Rehydration of this desolvated salt induces a significant increase in the low-spin (LS) population. Desolvation of 2 affords a material showing a more abrupt spin crossover with thermal hysteresis (T_1/2↑ = 286 K and T_1/2↓ = 273 K). This material is not very sensitive to rehydration. The anhydrous compounds [Fe(bpp)₂][Cr(bpy)(ox)₂]₂ (3) and [Fe(bpp)₂][Cr(phen)(ox)₂]₂ (4) display some quantitative photomagnetic conversion with T(LIESST) values of 41 and 51 K, respectively. Kinetic parameters governing the photo-induced HS–LS relaxation process have been determined and used to reproduce the T(LIESST) curves.