Conformational effect of a spin crossover iron(II) complex: Bis[N-(2-methylimidazol-4-yl)methylidene-2-aminoethyl]propanediamineiron(II) perchlorate

Abstract

A iron(II) complex of the linear hexadentate N6 ligand H2L2-3-2, [Fe(H2L2-3-2)](ClO4)2, was synthesized and the spin crossover properties were investigated, where H2L2-3-2 denotes bis[N-(2-methylimidazol-4-yl)methylidene-2-aminoethyl]propanediamine. The complex showed a gradual and reversible one-step spin crossover (SCO) between the high-spin (S=2) and low-spin (S=0) states at T1/2=208K without hysteresis. The crystal structures were determined at 296 and 250K (HS state), 230, 210, and 200K (intermediate between the HS and LS states) and 150 and 110K (LS state). The spin transition from 296 to 150K accompanies with the conformation change of the chelate rings at the triamine moiety and the formation of the hydrogen bond network in the same space group of orthorhombic Pbcn (no. 60). However, in the LS state at 110K, the space group changed from orthorhombic Pbcn at 150K (Pcan when the same axial setting to 110K was used) to monoclinic P21/a (no. 14) at 110K, although no spin transition and no change of assembly structure between 150 and 110K were observed. It give us an idea that the space group transformation is mainly related to the conformational thermodynamic stability of the chelate rings at the triamine moiety and is not directly correlated with the spin transition.