Molecules at the Quantum–Classical Nanoparticle Interface: Giant Mn70 Single-Molecule Magnets of ∿ nm Diameter

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• 作者：Alina Vinslava ; Anastasios J. Tasiopoulos ; Wolfgang Wernsdorfer ; Khalil A. Abboud ; George Christou
• 刊名：Inorganic Chemistry
• 出版年：2016
• 出版时间：April 4, 2016
• 年：2016
• 卷：55
• 期：7
• 页码：3419-3430
• 全文大小：839K
• ISSN：1520-510X

文摘

Two Mn₇₀ torus-like molecules have been obtained from the alcoholysis in EtOH and 2-ClC₂H₄OH of [Mn₁₂O₁₂(O₂CMe)₁₆(H₂O)₄]·4H₂O·2MeCO₂H (1) in the presence of NBuⁿ₄MnO₄ and an excess of MeCO₂H. The reaction in EtOH afforded [Mn₇₀O₆₀(O₂CMe)₇₀(OEt)₂₀(EtOH)₁₆(H₂O)₂₂] (2), whereas the reaction in ClC₂H₄OH gave [Mn₇₀O₆₀(O₂CMe)₇₀(OC₂H₄Cl)₂₀(ClC₂H₄OH)₁₈(H₂O)₂₂] (3). The complexes are nearly isostructural, each possessing a Mn₇₀ torus structure consisting of alternating near-linear [Mn₃(μ₃-O)₄] and cubic [Mn₄(μ₃-O)₂(μ₃-OR)₂] (R = OEt, 2; R = OC₂H₄Cl, 3) subunits, linked together via syn,syn-μ-bridging MeCO₂^– and μ₃-bridging O^2– groups. 2 and 3 have an overall diameter of ∼4 nm and crystallize as highly ordered supramolecular nanotubes. Alternating current (ac) magnetic susceptibility measurements, performed on microcrystalline samples in the 1.8–10 K range and a 3.5 G ac field with oscillation frequencies in the 5–1500 Hz range, revealed frequency-dependent out-of-phase signals below ∼2.4 K for both molecules indicative of the slow magnetization relaxation of single-molecule magnets (SMMs). Single-crystal, magnetization vs field studies on both complexes revealed hysteresis loops below 1.5 K, thus confirming 2 and 3 to be new SMMs. The hysteresis loops do not show the steps that are characteristic of quantum tunneling of magnetization (QTM). However, low-temperature studies revealed temperature-independent relaxation rates below ∼0.2 K for both compounds, the signature of ground state QTM. Fitting of relaxation data to the Arrhenius equation gave effective barriers for magnetization reversal (U_eff) of 23 and 18 K for 2 and 3, respectively. Because the Mn₇₀ molecule is close to the classical limit, it was also studied using a method based on the Néel–Brown model of thermally activated magnetization reversal in a classical single-domain magnetic nanoparticle. The field and sweep-rate dependence of the coercive field was investigated and yielded the energy barrier, the spin, the Arrhenius pre-exponential, and the cross-over temperature from the classical to the quantum regime. The validity of this approach emphasizes that large SMMs can be considered as being at or near the quantum–classical nanoparticle interface.