Environmental Influence on the Single-Molecule Magnet Behavior of [MnIII6CrIII]3+: Molecular Symmetry versus Solid-State Effects

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• 作者：Veronika Hoeke ; Maik Heidemeier ; Erich Krickemeyer ; Anja Stammler ; Hartmut B枚gge ; J眉rgen Schnack ; Andrei Postnikov ; Thorsten Glaser
• 刊名：Inorganic Chemistry
• 出版年：2012
• 出版时间：October 15, 2012
• 年：2012
• 卷：51
• 期：20
• 页码：10929-10954
• 全文大小：1173K
• 年卷期：v.51,no.20(October 15, 2012)
• ISSN：1520-510X

文摘

The structural, spectroscopic, and magnetic properties of a series of [Mn^III₆Cr^III]³⁺ (= [{(talen^t-Bu₂)Mn^III₃}₂{Cr^III(CN)₆}]³⁺) compounds have been investigated by single-crystal X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and electronic absorption spectroscopy, elemental analysis, electro spray ionization鈥搈ass spectrometry (ESI-MS) and matrix-assisted laser desorption ionization鈥搈ass spectrometry (MALDI-MS), cyclic voltammetry, AC and DC magnetic measurements, as well as theoretical analysis. The crystal structures obtained with [Cr^III(CN)₆]^{3鈥?/sup> as a counterion exhibit (quasi-)one-dimensional (1D) chains formed by hydrogen-bonded (1) or covalently linked (2) trications and trianions. The rod-shaped anion lactate enforces a rod packing of the [MnIII₆CrIII]3+ complexes in the highly symmetric space group R3̅ (3) with a collinear arrangement of the molecular S₆ axes. Incorporation of the spherical anion BPh₄鈥?/sup> leads to less-symmetric crystal structures (4鈥?b>6) with noncollinear orientations of the [MnIII₆CrIII]3+ complexes, as evidenced by the angle between the approximate molecular C₃ axes taking no specific values in the range of 2掳鈥?9掳. AC magnetic measurements on freshly isolated crystals (1a and 3a鈥?b>6a), air-dried crystals (3b鈥?b>6b), and vacuum-dried powder samples (3c鈥?b>6c) indicate single-molecule magnet (SMM) behavior for all samples with U_eff values up to 28 K. The DC magnetic data are analyzed by a full-matrix diagonalization of the appropriate spin-Hamiltonian including isotropic exchange, zero-field splitting, and Zeeman interaction, taking into account the relative orientation of the D-tensors. Simulations for 3a鈥?b>6a and 3c鈥?b>6c indicate a weak antiferromagnetic exchange between the MnIII ions in the trinuclear subunits (J_Mn鈥揗n = 鈭?.70 to 鈭?.85 cm鈥?, 膜_ex = 鈭?鈭?sub>i<j聽J_ij艤_i路艤_j) that is overcome by the stronger antiferromagnetic interaction via the Cr鈥揅鈮鈥揗n pathway (J_Cr鈥揗n = 鈭?.00 to 鈭?.00 cm鈥?), leading to an overall ferrimagnetic coupling scheme with an S_t = 21/₂ spin ground state. The differences in U_eff, J_Mn鈥揗n, and J_Cr鈥揗n for the investigated samples are rationalized in terms of subtle variations in the molecular and crystal structures. In particular, a magnetostructural correlation between the Mn鈥揘C鈮 bond length and the J_Cr鈥揗n exchange coupling is inferred from the magnetic measurements and corroborated by DFT calculations. The results of this detailed study on [MnIII₆CrIII]3+ allow the formulation of some key recipes for a rational improvement of the SMM behavior.}