Lehrstuhl für Anorganische Chemie I, Fakultät für Chemie, Universität Bielefeld, Universitätsstr. 25, D-33615 Bielefeld, Germany.
Inorg Chem. 2012 Oct 15;51(20):10929-54. doi: 10.1021/ic301406j. Epub 2012 Sep 27.
The structural, spectroscopic, and magnetic properties of a series of Mn(III)(6)Cr(III) (= {(talen(t-Bu(2)))Mn(III)(3)}(2){Cr(III)(CN)(6)}) 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-mass spectrometry (ESI-MS) and matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS), cyclic voltammetry, AC and DC magnetic measurements, as well as theoretical analysis. The crystal structures obtained with Cr(III)(CN)(6) 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 Mn(III)(6)Cr(III) complexes in the highly symmetric space group R3[overline] (3) with a collinear arrangement of the molecular S(6) axes. Incorporation of the spherical anion BPh(4)(-) leads to less-symmetric crystal structures (4-6) with noncollinear orientations of the Mn(III)(6)Cr(III) complexes, as evidenced by the angle between the approximate molecular C(3) axes taking no specific values in the range of 2°-69°. AC magnetic measurements on freshly isolated crystals (1a and 3a-6a), air-dried crystals (3b-6b), and vacuum-dried powder samples (3c-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-6a and 3c-6c indicate a weak antiferromagnetic exchange between the Mn(III) ions in the trinuclear subunits (J(Mn-Mn) = -0.70 to -0.85 cm(-1), Ĥ(ex) = -2∑(i<j )J(ij)Ŝ(i)·Ŝ(j)) that is overcome by the stronger antiferromagnetic interaction via the Cr-C≡N-Mn pathway (J(Cr-Mn) = -3.00 to -5.00 cm(-1)), leading to an overall ferrimagnetic coupling scheme with an S(t) = (21)/(2) spin ground state. The differences in U(eff), J(Mn-Mn), and J(Cr-Mn) 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-N(C≡N) bond length and the J(Cr-Mn) exchange coupling is inferred from the magnetic measurements and corroborated by DFT calculations. The results of this detailed study on Mn(III)(6)Cr(III) allow the formulation of some key recipes for a rational improvement of the SMM behavior.
一系列 {(talen(t-Bu(2)))Mn(III)(3)}(2){Cr(III)(CN)(6)} 型 Mn(III)(6)Cr(III) 化合物的结构、光谱和磁性性质已通过单晶 X 射线衍射 (XRD)、傅里叶变换红外 (FT-IR) 和电子吸收光谱、元素分析、电喷雾电离-质谱 (ESI-MS) 和基质辅助激光解吸电离-质谱 (MALDI-MS)、循环伏安法、交流和直流磁测量以及理论分析进行了研究。用 Cr(III)(CN)(6) 作为抗衡离子得到的晶体结构表现出由氢键 (1) 或共价连接 (2) 的三阳离子和三阴离子形成的 (准)一维 (1D) 链。棒状阴离子乳酸盐在高度对称的 R3[overline] (3) 空间群中强制 Mn(III)(6)Cr(III) 配合物的棒状堆积，分子 S(6) 轴呈共线排列。球形阴离子 BPh(4)(-) 的掺入导致晶体结构不太对称 (4-6)，Mn(III)(6)Cr(III) 配合物的取向非共线，这可以从分子 C(3) 轴的近似角度得到证明，该角度在 2°-69°的范围内没有特定值。对新鲜分离的晶体 (1a 和 3a-6a)、风干晶体 (3b-6b) 和真空干燥粉末样品 (3c-6c) 进行交流磁测量表明，所有样品均表现出单分子磁体 (SMM) 行为，有效场 U(eff) 值高达 28 K。直流磁数据通过对角化适当的自旋哈密顿量进行分析，其中包括各向同性交换、零场分裂和塞曼相互作用，同时考虑 D 张量的相对取向。对 3a-6a 和 3c-6c 的模拟表明，三核亚基中的 Mn(III) 离子之间存在较弱的反铁磁交换 (J(Mn-Mn) = -0.70 至 -0.85 cm(-1)，Ĥ(ex) = -2∑(i<j )J(ij)Ŝ(i)·Ŝ(j)))，该交换被通过 Cr-C≡N-Mn 途径的更强的反铁磁相互作用克服 (J(Cr-Mn) = -3.00 至 -5.00 cm(-1))，导致具有整体亚铁磁耦合方案和 S(t) = (21)/(2) 自旋基态。对所研究样品的 U(eff)、J(Mn-Mn) 和 J(Cr-Mn) 的差异可以根据分子和晶体结构的细微变化来解释。特别是，从磁性测量和密度泛函理论 (DFT) 计算推断出 Mn-N(C≡N) 键长与 J(Cr-Mn) 交换耦合之间的磁结构关系。对 Mn(III)(6)Cr(III) 的详细研究结果为合理改善 SMM 行为制定了一些关键的方案。
