High-temperature magnetic blocking and magneto-structural correlations in a series of dysprosium(iii) metallocenium...

A series of dysprosium(iii) metallocenium salts, [Dy(Cp^iPr4R)₂][B(C₆F₅)₄] (R = H (1), Me (2), Et (3), iPr (4)), was synthesized by reaction of DyI₃ with the corresponding known NaCp^iPr4R (R = H, iPr) and novel NaCp^iPr4R (R = Me, Et) salts at high temperature, followed by iodide abstraction with [H(SiEt₃)₂][B(C₆F₅)₄]. Variation of the substituents in this series results in substantial changes in molecular structure, with more sterically-encumbering cyclopentadienyl ligands promoting longer Dy-C distances and larger Cp-Dy-Cp angles. Dc and ac magnetic susceptibility data reveal that these structural changes have a considerable impact on the magnetic relaxation behavior and operating temperature of each compound. In particular, the magnetic relaxation barrier increases as the Dy-C distance decreases and the Cp-Dy-Cp angle increases. An overall 45 K increase in the magnetic blocking temperature is observed across the series, with compounds 2-4 exhibiting the highest 100 s blocking temperatures yet reported for a single-molecule magnet. Compound 2 possesses the highest operating temperature of the series with a 100 s blocking temperature of 62 K. Concomitant increases in the effective relaxation barrier and the maximum magnetic hysteresis temperature are observed, with 2 displaying a barrier of 1468 cm^-1 and open magnetic hysteresis as high as 72 K at a sweep rate of 3.1 mT s^-1. Magneto-structural correlations are discussed with the goal of guiding the synthesis of future high operating temperature Dy^III metallocenium single-molecule magnets.