Influence of structure on exchange strength and relaxation barrier in a series of FeIIReIV(CN)2 single-chain magnets

Abstract

Cyano-bridged single-chain magnets of the type L₄FeReCl₄(CN)₂, where L = diethylformamide (DEF) (1), dibutylformamide (DBF) (2), dimethylformamide (DMF) (3), dimethylbutyramide (DMB) (4), dimethylpropionamide (DMP) (5), and diethylacetamide (DEA) (6), have been synthesized to enable a systematic study of the influence of structural perturbations on magnetic exchange and relaxation barrier. Across the series, varying the amide ligand leads to Fe–N–C bond angles ranging from 154.703(7)° in 1 to 180° in 6. Variable-temperature dc magnetic susceptibility data indicate ferromagnetic exchange coupling in all compounds, with the strength of exchange increasing linearly, from J = +4.2(2) cm⁻¹ to +7.2(3) cm⁻¹, with increasing Fe–N–C bond angle. Ac magnetic susceptibility data collected as a function of frequency reveal that the relaxation barrier of the chain compounds rises steeply with increasing exchange strength, from 45 cm⁻¹ to 93 cm⁻¹. This examination demonstrates that subtle tuning of orbital overlap, and thus exchange strength, can engender dramatic changes in the relaxation barrier. Indeed, the perfectly linear Fe–N–C bond angle in 6 leads to one of the highest barriers and coercive fields yet observed for a single-chain magnet.