Abstract
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 Å 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.
Highlights
Spin crossover (SCO) is a representative phenomenon of molecular bistability and an inter-conversion between high-spin (HS) and low-spin (LS) states can occur by external physical perturbations [1]
The red color of the dilute ethanol solution at ambient temperature changes to green at the temperature of liquid nitrogen, where red and green are typical colors for the HS and LS FeIII complexes with the Schiff-base ligands with N4O2 donor atoms [16]
Molecular expansion and contraction associated with the spin transition can be transmitted along the chain without resistance
Summary
Spin crossover (SCO) is a representative phenomenon of molecular bistability and an inter-conversion between high-spin (HS) and low-spin (LS) states can occur by external physical perturbations [1]. We reported an abrupt SCO FeIII complex [FeIII(Him)2(hapen)]PF6 [14] and a hysteresis. SCO FeIII complex [FeIII(Him)2(hapen)]AsF6 [15], where SCO FeIII compounds, in general, exhibit gradual spin equilibrium and show no thermal hysteresis [1]. The unusual SCO property of the SCO FeIII complex is due to the molecular distortion of the complex-cation [FeIII(Him)2(hapen)]+ and the chiral one-dimensional structure constructed by the inter-cation O···HN hydrogen bond between a phenoxo oxygen and an imidazole nitrogen. Only the structure in the high-spin HS state is determined for the PF6 and AsF6 salts. We could consider the abrupt SCO profile and the shift of the spin transition temperature within the frame work of a 1D structure
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