Abrupt Spin Transition and Chiral Hydrogen-Bonded One-Dimensional Structure of Iron(III) Complex [FeIII(Him)2(hapen)]SbF6 (Him = imidazole, H2hapen = N,N′-bis(2-hydroxyacetophenylidene)ethylenediamine)

Solvent-free spin crossover (SCO) iron(III) complex, [FeIII(Him)2(hapen)]SbF6 (Him = imidazole, H2hapen = N,N′-bis(2-hydroxyacetophenylidene)ethylenediamine), is synthesized. The FeIII ion has an octahedral coordination geometry, with N2O2 donor atoms of hapen and N2 atoms of two imidazoles at the axial positions. The saturated five-membered chelate ring of hapen moiety assumes a gauche-type δ- or λ-conformation to give chiral species of δ-[FeIII(Him)2(hapen)]+ or λ-[FeIII(Him)2(hapen)]+. One imidazole is hydrogen-bonded to phenoxo oxygen atom of hapen of the adjacent unit to give a hydrogen-bonded chiral one-dimensional structure, {δ-[FeIII(Him)2(hapen)]+}1∞ or {λ-[FeIII(Him)2(hapen)]+}1∞. The adjacent chains with the opposite chiralities are arrayed alternately. The temperature dependences of the magnetic susceptibilities revealed an abrupt one-step spin transition between high-spin (S = 5/2) and low-spin (S = 1/2) states at the spin transition temperature of T1/2 = 105 K. The crystal structures were determined at 296 and 100 K, where the populations of HS:LS of high- and low-spin ratio are evaluated to be 1:0 and 0.3:0.7, respectively, based on magnetic measurements. During the spin transition from 296 K to 100 K, the average Fe–N distance and O–Fe–O angle decrease to a regular octahedron by 0.16 A and 13.4°, respectively. The structural change in the coordination environment is transmitted to the adjacent spin crossover (SCO) sites along the chiral 1D chain through hydrogen-bonds. The abrupt SCO profile and the spin transition temperature for the isomorphous compounds [FeIII(Him)2(hapen)]Y (Y = PF6, AsF6, SbF6) are ascribed to the chiral hydrogen-bonded 1D structure and chain-anion interaction.