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

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)degrees in 1 to 180 degrees 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.