A New Family of 1D Exchange Biased Heterometal Single-Molecule Magnets: Observation of Pronounced Quantum Tunneling Steps in the Hysteresis Loops of Quasi-Linear {Mn2Ni3} Cluster

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• 作者：Animesh Das ; Klaus Gieb ; Yulia Krupskaya ; Serhiy Demeshko ; Sebastian Dechert ; R眉diger Klingeler ; Vladislav Kataev ; Bernd B眉chner ; Paul M眉ller ; Franc Meyer
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：March 16, 2011
• 年：2011
• 卷：133
• 期：10
• 页码：3433-3443
• 全文大小：1203K
• 年卷期：v.133,no.10(March 16, 2011)
• ISSN：1520-5126

文摘

First members of a new family of heterometallic Mn/Ni complexes [Mn₂Ni₃X₂L₄(LH)₂(H₂O)₂] (X = Cl: 1; X = Br: 2) with the new ligand 2-{3-(2-hydroxyphenyl)-1H-pyrazol-1-yl}ethanol (H₂L) have been synthesized, and single crystals obtained from CH₂Cl₂ solutions have been characterized crystallographically. The molecular structures feature a quasi-linear Mn^III鈭扤i^II鈭扤i^II鈭扤i^II鈭扢n^III core with six-coordinate metal ions, where elongated axes of all the distorted octahedral coordination polyhedra are aligned parallel and are fixed with respect to each other by intramolecular hydrogen bonds. 1 and 2 exhibit quite strong ferromagnetic exchange interactions throughout (J_Mn鈭扤i 鈮?40 K (1) or 42 K (2); J_Ni鈭扤i 鈮?22 K (1) or 18 K (2)) that lead to an S_tot = 7 ground state, and a sizable uniaxial magnetoanisotropy with D_mol values 鈭?.55 K (1) and 鈭?.45 K (2). These values are directly derived also from frequency- and temperature-dependent high-field EPR spectra. Slow relaxation of the magnetization at low temperatures and single-molecule magnet (SMM) behavior are evident from frequency-dependent peaks in the out-of-phase ac susceptibilities and magnetization versus dc field measurements, with significant energy barriers to spin reversal U_eff = 27 K (1) and 22 K (2). Pronounced quantum tunnelling steps are observed in the hysteresis loops of the temperature- and scan rate-dependent magnetization data, but with the first relaxation step shifted above (1) or below (2) the zero crossing of the magnetic field, despite the very similar molecular structures. The different behavior of 1 and 2 is interpreted in terms of antiferromagnetic (1) or ferromagnetic (2) intermolecular interactions, which are discussed in view of the subtle differences of intermolecular contacts within the crystal lattice.