Abstract
Single-molecule magnets (SMMs) have potential applications in high-density data storage, but magnetic relaxation times at elevated temperatures must be increased to make them practically useful. Bis-cyclopentadienyl lanthanide sandwich complexes have emerged as the leading candidates for SMMs that show magnetic memory at liquid nitrogen temperatures, but the relaxation mechanisms mediated by aromatic C5 rings have not been fully established. Here we synthesize a bis-monophospholyl dysprosium SMM [Dy(Dtp)2][Al{OC(CF3)3}4] (1, Dtp = {P(CtBuCMe)2}) by the treatment of in-situ-prepared “[Dy(Dtp)2(C3H5)]” with [HNEt3][Al{OC(CF3)3}4]. SQUID magnetometry reveals that 1 has an effective barrier to magnetization reversal of 1760 K (1223 cm–1) and magnetic hysteresis up to 48 K. Ab initio calculation of the spin dynamics reveals that transitions out of the ground state are slower in 1 than in the first reported dysprosocenium SMM, [Dy(Cpttt)2][B(C6F5)4] (Cpttt = C5H2tBu3-1,2,4); however, relaxation is faster in 1 overall due to the compression of electronic energies and to vibrational modes being brought on-resonance by the chemical and structural changes introduced by the bis-Dtp framework. With the preparation and analysis of 1, we are thus able to further refine our understanding of relaxation processes operating in bis-C5/C4P sandwich lanthanide SMMs, which is the necessary first step toward rationally achieving higher magnetic blocking temperatures in these systems in the future.
Highlights
The potential for high-density data storage devices based on single-molecule magnets (SMMs) is reliant upon increasing spin relaxation times toward practically useful time scales at relatively high temperatures, away from expensive liquid helium regimes to that of cheap and abundant liquid nitrogen.[1]Lanthanide (Ln) based SMMs have been at the forefront of research in this area for the past 15 years,[2] and design principles popularized by Rinehart and Long in 20113 directed the community toward longer relaxation times by means of massive increases in the energy barrier to magnetic reversal (Ueff).[4]
These large increases in Ueff did not lead to corresponding increases in magnetic remanance temperatures[5] until the dysprosocenium cation [Dy(Cpttt)2]+ (Cpttt = C5H2tBu3-1,2,4) was shown to exhibit magnetic hysteresis at TH = 60 K in 2017.6 We attributed the high-temperature magnetic remanance in this bis-Cpttt system to the combination of a Dy3+ center with rigid, charge-dense πaromatic rings; we predicted that removal of C−H groups from the C5 ring could increase hysteresis temperatures further.6a This has been proven correct, with hysteresis temperatures up to TH = 80 K observed for peralkylated biscyclopentadienyl Ln complexes reported in the past two years.[7]
Synthesis of 1 with C3H5MgCl, while [NEt3H][Al{OC(CF3)3}4] was isolated from the reaction of Li[Al{OC(CF3)3}4]12 with NEt3HCl by adapting procedures used for the synthesis of [NEt3H][B(C6F5)4]
Summary
The potential for high-density data storage devices based on single-molecule magnets (SMMs) is reliant upon increasing spin relaxation times toward practically useful time scales at relatively high temperatures, away from expensive liquid helium regimes to that of cheap and abundant liquid nitrogen.[1]. The Ueff value for 1 is identical to that previously seen for [Dy(Cpttt)2][B(C6F5)4] (1760 K),6a and smaller than the current record-holder [Dy(C5iPr5)(C5Me5)][B(C6F5)4] (2217 K).7b To obtain relaxation times at lower temperatures, we performed magnetization decay experiments and fitted the data with stretched exponentials (Figure S14 and Table S3). 20 Oe/s around the important zero-field region where quantum tunneling of the magnetization (QTM) dominates for Ln SMMs.[4] The value of TH for 1 is lower than the majority of isolated dysprosocenium cations reported to date, which have shown TH values of 60−80 K,6,7 except for one example, [Dy(C5iPr4H)2][B(C6F5)] (TH = 32 K),7a which contains ring C−H protons that have been postulated to enhance magnetic relaxation mechanisms.6a Despite the lack of ring protons in 1, it shows open hysteresis to a maximum temperature that is 12 K lower than that previously seen for [Dy(Cpttt)2][B(C6F5)4] (TH = 60 K).[6]. Chemical alteration of the aromatic rings has made the initial steps in magnetic relaxation slower, confirming our hypothesis,6a magnetic relaxation in the Orbach regime in 1 is more efficient than for [Dy(Cpttt)2][B(C6F5)4] due to faster relaxation in the upper energy states of the manifold (Table S10)
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