Abstract
Treatment of the new methanediide–methanide complex [Dy(SCS)(SCSH)(THF)] (1Dy, SCS = {C(PPh2S)2}2−) with alkali metal alkyls and auxillary ethers produces the bis-methanediide complexes [Dy(SCS)2][Dy(SCS)2(K(DME)2)2] (2Dy), [Dy(SCS)2][Na(DME)3] (3Dy) and [Dy(SCS)2][K(2,2,2-cryptand)] (4Dy). For further comparisons, the bis-methanediide complex [Dy(NCN)2][K(DB18C6)(THF)(toluene)] (5Dy, NCN = {C(PPh2NSiMe3)2}2−, DB18C6 = dibenzo-18-crown-6 ether) was prepared. Magnetic susceptibility experiments reveal slow relaxation of the magnetisation for 2Dy–5Dy, with open magnetic hysteresis up to 14, 12, 15, and 12 K, respectively (∼14 Oe s−1). Fitting the alternating current magnetic susceptibility data for 2Dy–5Dy gives energy barriers to magnetic relaxation (Ueff) of 1069(129)/1160(21), 1015(32), 1109(70), and 757(39) K, respectively, thus 2Dy–4Dy join a privileged group of SMMs with Ueff values of ∼1000 K and greater with magnetic hysteresis at temperatures >10 K. These structurally similar Dy-components permit systematic correlation of the effects of axial and equatorial ligand fields on single-molecule magnet performance. For 2Dy–4Dy, the Dy-components can be grouped into 2Dy–cation/4Dy and 2Dy–anion/3Dy, where the former have almost linear C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> DyC units with short average DyC distances, and the latter have more bent CDyC units with longer average DyC bonds. Both Ueff and hysteresis temperature are superior for the former pair compared to the latter pair as predicted, supporting the hypothesis that a more linear axial ligand field with shorter M–L distances produces enhanced SMM properties. Comparison with 5Dy demonstrates unusually clear-cut examples of: (i) weakening the equatorial ligand field results in enhancement of the SMM performance of a monometallic system; (ii) a positive correlation between Ueff barrier and axial linearity in structurally comparable systems.
Highlights
Lanthanide (Ln) Single Molecule Magnets (SMMs) are of burgeoning interest due to their potential applications in high density storage and quantum computing.[1]
This approach has permitted design of large barriers to magnetisation reversal, Ueff, over which magnetic relaxation occurs with an Arrhenius-like exponential temperature dependence, and larger Ueff values should lead to slower magnetic relaxation at a given temperature
In order to introduce more modular variation of the alkali metal, since this was anticipated to provide greater opportunities for the structural variations needed to underpin a magneto-structural correlation study, we adapted the alkane elimination route previously used in our NCN work to the synthesis of the bis-SCS Ln complexes reported here, Scheme 1.5c treatment of LnI3THF3.5 with three equivalents of KCH2Ph and a sub-stoichiometric amount of SCSH2 produced the heteroleptic methanediide-methanide complex [Dy(SCS)(SCSH)(THF)] (1Dy), which was isolated as colourless crystals in 90% yield with toluene as a byproduct
Summary
Lanthanide (Ln) Single Molecule Magnets (SMMs) are of burgeoning interest due to their potential applications in high density storage and quantum computing.[1] Following the discovery that single Ln-ions can function as effective SMMs,[2] there has been a huge development in the eld.[3] Ln-based SMMs have been amenable to systematic improvement by optimisation of the crystal eld (CF) generated by the coordination environment in order to best stabilise the most magnetic projections of the spin–orbit coupled total angular momentum (mJ states).[4] This approach has permitted design of large barriers to magnetisation reversal, Ueff, over which magnetic relaxation occurs with an Arrhenius-like exponential temperature dependence, and larger Ueff values should lead to slower magnetic relaxation at a given temperature This is well established for DyIII, where near-linear coordination environments stabilise the mJ 1⁄4 jÆ15/2i ground state.1b,3c,5 The preparation and computational investigation of prepared compounds has been instrumental in reinforcing and developing the theory behind SMMs, and yet very few studies have explicitly probed the correlation between axial and equatorial CF effects.5g. The effect of axial linearity on magnetic performance has only ever been modelled computationally or observed as a general qualitative trend for incomparable systems.4e
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