Trapping of metastable high-spin species and weak cooperativity in a mononuclear Fe(II) spin-crossover system Original Research Article

The magnetic and structural properties of three gradual spin transition monomeric compounds based on the cation [Fe(Hpt)3]2+ (Hpt=3-(pyrid-2-yl)-1,2,4-triazole) are presented. The non-cooperative character of the spin-crossover in [Fe(Hpt)3](BF4)2·2H2O (I) is evaluated in light of its calorimetric properties, which yielded the thermodynamic values ΔtrsH=5.81 kJ mol−1 and ΔtrsS=39.5 J mol−1 K−1. The light-induced excited spin-state trapping effect is performed on [Fe(Hpt)3](BF4)2·2H2O (I) and [Fe(Hpt)3](SO4)0.4(BF4)1.2·3H2O (II), and the subsequent HS→LS relaxations are studied. Their merely first-order kinetics are affected by disorder in the structure of both complexes, which appears in the presence of a distribution of activation energies. HS species can also be frozen-in in I by rapid cooling. Continuous irradiation is shown to induce only apparent light-induced thermal hysteresis effect in I and II, stemming from slow kinetics of relaxation with respect to the kinetics of measurement.