New spin-crossover iron(III) complexes with large hysteresis effects and time dependence of their magnetism

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Abstract
The new complexes [FeL1 2] ClO4(HL1= tridentate Schiff base formed by 1 : 1 condensation of o-aminophenol and pyridine-2-carbaldehyde) and [FeL2 2] NCS ( HL2= tridentate Schiff base formed by 1 : 1 condensation of 8-aminoquinoline and salicylaldehyde) were synthesized. The spin-crossover behaviour between S=½ and 5//2 which is dependent on temperature was observed for the solid complexes using variable-temperature 57Fe Mössbauer spectral and magnetic measurements. Both compounds show a pronounced hysteresis effect in the high spin ⇌ low spin transition. For [ FeL1 2] ClO4, sharp and strong Mössbauer absorptions were observed over the temperature range examined and the effective vibrating mass, Meff., was evaluated using the high-temperature approximation of the Debye model. The value for the low-spin isomer was larger than that for the high-spin isomer. The complex [FeL2 2]NCS when freshly recrystallized below 280 K is predominantly in a low-spin state. However, the magnetism at room temperature increases with time, spin-crossover behaviour is exhibited, and the magnetism is dependent on temperature, with a pronounced hysteresis loop of about 70 K at 286 K 10 d after preparation.
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