Gradual spin crossover behaviour in a linear trinuclear Fe-II complex

The synthesis, structures and magnetic properties of a new trinuclear spin crossover complex, [Fe-3(II)(npt)(6)(EtOH)(4)(H2O)(2)](ptol)(6)center dot 4EtOH (1), and of its Co-II (2 and 3) and Ni-II (4) analogues, are reported here. The complexes were synthesized by reacting a 1,2,4-triazole-based ligand, 4-(4'-nitrophenyl)-1,2,4-triazole (npt), with the p-tolylsulfonate (ptol) metal salts in methanol or ethanol. Structural analyses revealed that all complexes are iomorphous and consist of a linear trinuclear core where metal centres are bridged by triazole groups. For 1, dc susceptibility measurements exhibit gradual spin transition with T-1/2 = 148 K which corresponds to HS -> LS crossover for the triazole bridged central Fe-II ion. This spin transition was confirmed by Single-Crystal X-Ray Diffraction data of 1 at 100 K and 181 K, where the low temperature measurement revealed a decrease in volume for the central Fe-II ion, which is in agreement with a HS -> LS transition.