The recently reported cation, [Dy(Cpttt)2]+ (Cpttt = 1,2,4-tri(tert-butyl)cyclopentadienyl), notable for large improvements in magnetic hysteresis and relaxation rates over previous efforts, is the first example of a new class of Dy(III) single-molecule magnets of general formula [Cpx2Dy]+. These complexes are attractive as single-molecule magnets due to their synthetic accessibility and highly axial ligand field. Additionally, through-barrier magnetic relaxation is limited by the lack of equatorial coordination and more constrained metal-ligand vibrational modes. Through careful selection and subtle variation of the cyclopentadienyl ligands, further improvements in the operating temperature of [Cpx2Dy]+ can be realized. To this end, a series of Dy(III) metallocenium cations, [Dy(CpiPr4R)2][B(C6F5)4] (R = H (1), Me (2), Et (3), iPr (4)), was synthesized by reaction of DyI3 with the corresponding known tetra- and pentaisopropylcyclopentadienyl salts, NaCpiPr4R (R = H, iPr), as well as the new methyl and ethyl substituted tetraisopropylcyclopentadienyl salts, NaCpiPr4R (R = Me, Et), at high temperature, followed by halide abstraction with [H(SiEt3)2][B(C6F5)4]. The incremental variation of substituents results in substantial changes to Dy-Cp distances and Cp–Dy–Cp angles across the series. Concomitant changes in the magnetic relaxation behavior and operating temperature of each compound are also observed, as evidenced by a 45 K increase in the 100-s blocking temperature across the series. In general, magnetic relaxation slows and magnetic hysteresis increases as the Dy–Cp distance decreases and the Cp–Dy–Cp angle increases, with the Dy-Cp distance becoming increasingly important at higher angles. Largely by achieving a balance between a relatively high Cp–Dy–Cp angle and small Dy–Cp distance, complex 2 is observed to be superlative among the series with a 100-s blocking temperature of 62 K and maximum hysteresis temperature of 72 K. Ligand variation is discussed with the goal of guiding the synthesis of future high operating temperature Dy(III) single-molecule magnets.

Recent advances in dysprosium single-molecule magnet design: high-temperature magnetic blocking and magneto-structural correlations

2019 Spring National Meeting

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