On the nature of the plateau in two-step dinuclear spin-crossover complexes

A remarkable feature of the spin-crossover process in several dinuclear iron(II) compounds is a plateau in the two-step transition curve. Up to now, it has not been possible to analyse the spin state of dinuclear pairs that constitute such a plateau, due to the relative high temperatures at which the transition takes place in complexes investigated so far. We solved this problem by experimentally studying a novel dinuclear spin-crossover compound [{Fe(phdia)(NCS)(2)}(2)(phdia)] (phdia: 4,7-phenanthroline-5,6-diamine). We report here on the synthesis and characterisation of this system, which exhibits a two-step spin transition at T-cl = 108 K and T-c2 = 80 K, displaying 2 K and 7 K wide thermal hysteresis loops in the upper and the lower steps, respectively. A plateau of approximately 20 K, width centred at about 90 K, which corresponds to the 50% of the spin conversion, separates the two transitions. The composition of the plateau was identified in metastable state after quenching to 4.2 K by means of Mossbauer spectroscopy in an external magnetic field. Such experiments revealed that the plateau consists mainly of [HS-LS] pairs (HS=high spin, LS = low spin) and confirmed the hypothesis that the spin conversion in dinuclear entities proceeds through [LS-LS]<---->[HS-LS]<---->[HS-HS] pairs. The results are discussed in terms of a thermodynamic model based on the regular solution theory adapted for dinuclear spin-crossover compounds.