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. 2018 Oct 18;9(45):8492-8503.
doi: 10.1039/c8sc03907k. eCollection 2018 Dec 7.

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

K Randall McClain  1 , Colin A Gould  2 , Khetpakorn Chakarawet  2 , Simon J Teat  3 , Thomas J Groshens  1 , Jeffrey R Long  2   4   5 , Benjamin G Harvey  1
Affiliations

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

K Randall McClain et al. Chem Sci. .

Abstract

A series of dysprosium(iii) metallocenium salts, [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 NaCpiPr4R (R = H, iPr) and novel NaCpiPr4R (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 DyIII metallocenium single-molecule magnets.

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Figures

Scheme 1
Scheme 1. Synthesis of NaCpiPr4Me and NaCpiPr4Et.
Scheme 2
Scheme 2. Synthesis of metallocene salts 1–4 and Y1–Y4.
Fig. 1
Fig. 1. Structures of the metallocenium complexes in crystals of [Dy(CpiPr4R)2][B(C6F5)4] (R = H (1), Me (2), Et (3), iPr(4)). Green and gray spheres represent Dy and C atoms, respectively; hydrogen atoms, [B(C6F5)4]– counteranions, and positional disorder are omitted for clarity.
Fig. 2
Fig. 2. Field-cooled (red) and zero-field cooled (blue) magnetic susceptibility measurements collected at Hdc = 1000 Oe for 1–4 (left to right). Dashed lines mark Tirrev, the temperature at which the two plots diverge.
Fig. 3
Fig. 3. In-phase (χ′M, top) and out-of-phase (χ″M, bottom) components of the ac magnetic susceptibility for 2 under zero applied dc field at frequencies ranging from 0.1–1500 Hz and temperatures from 80–114 K (2 K steps). The colored lines are guides for the eye.
Fig. 4
Fig. 4. Arrhenius plots of magnetic relaxation time (τ) for 1–4 (left to right) extracted from ac susceptibility (red circles) and dc relaxation (blue circles) data. Black lines represent fits to the data as described in the main text and are each a sum of Orbach (dashed orange line), Raman (dashed purple line), and quantum tunneling (dashed green line) relaxation processes.
Fig. 5
Fig. 5. (a) Magnetic hysteresis data for 1–4 collected at 2, 10, 20 and 30 K with a sweep rate of 3.1(4) mT s–1 for H < 2 T and 13.2(2) mT s–1 for H > 2 T. (b) Expanded view of the region near zero field at high-temperatures.
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