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

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).2H(2)O (I) is evaluated in light of its calorimetric properties, which yielded the thermodynamic values ADelta(trs)H = 5.81 kJ mol(-1) and Delta(trs)S = 39.5 J mol(-1) K-1. The light-induced excited spin-state trapping effect is performed on [Fe(Hpt)(3)](BF4)(2).2H(2)O (I) and [Fe(HPt)(3)](SO4)(0.4)(BF4)(1.2).3H(2)O (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. (C) 2003 Elsevier Science Ltd. All rights reserved.