Lehrstuhl für Anorganische Chemie I, Fakultät für Chemie, Universität Bielefeld, Universitätsstr. 25, D-33615 Bielefeld, Germany.
Chem Commun (Camb). 2011 Jan 7;47(1):116-30. doi: 10.1039/c0cc02259d. Epub 2010 Sep 22.
Since the discovery that Mn(12)OAc acts as a single-molecule magnet (SMM), an increasing number of transition metal complexes have been demonstrated to behave as SMMs. The signature of a SMM is a slow relaxation of the magnetization at low temperatures accompanied by a magnetic hysteresis. The origin of SMM behaviour is the existence of an appreciable thermal barrier U for spin-reversal called magnetic anisotropy barrier which is related to the combination of a large total spin ground state (S(t)) and an easy-axis magnetic anisotropy. The extensive research on Mn(12)OAc and other SMMs has established more prerequisites for a rational development of new SMMs besides the high-spin ground state and the magnetic anisotropy: the symmetry should be at least C(3) to minimize the quantum tunneling of the magnetization through the anisotropy barrier but lower than cubic to avoid the cancellation of the local anisotropies upon projection onto the spin ground state. Based on these prerequisites, we have designed the ligand triplesalen which combines the phloroglucinol bridging unit for high spin ground states by the spin-polarization mechanism with a salen-like ligand environment for single-site magnetic anisotropies by a strong tetragonal ligand field. The C(3) symmetric, trinuclear complexes of the triplesalen ligand (talen(t-Bu(2)))(6-) exhibit a strong ligand folding resulting in an overall bowl-shaped molecular structure. This ligand folding preorganizes the axial coordination sites of the metal salen subunits for the complementary binding of three facial nitrogen atoms of a hexacyanometallate unit. This leads to a high driving force for the formation of heptanuclear complexes M(t)(6)M(c) by the assembly of three molecular building blocks. Attractive van der Waals interactions of the tert-butyl phenyl units of two triplesalen trinuclear building blocks increase the driving force. In this respect, we have been able to synthesize the isostructural series Mn(III)(6)Cr(III), Mn(III)(6)Fe(III), and Mn(III)(6)Co(III) with Mn(III)(6)Cr(III) being a SMM. A detailed analysis and comparison of the magnetic properties of the three heptanuclear complexes and the tetranuclear half-unit Mn(III)(3)Cr(III) provides significant insight for further optimization of the SMM properties. The modular assembly of the heptanuclear complexes from three molecular building blocks allows the fine-tuning of the molecular and steric properties of each building block without losing the driving force for the formation of the heptanuclear complexes. This possibility of rational improvements of our isostructural series is the main advantage of our supramolecular approach.
自从发现 Mn(12)OAc 作为单分子磁体 (SMM) 以来，越来越多的过渡金属配合物被证明具有 SMM 行为。SMM 的特征是低温下磁化缓慢弛豫，伴随着磁滞。SMM 行为的起源是自旋反转的磁各向异性势垒（称为磁各向异性势垒）的存在，这与大的总自旋基态（S(t)）和易轴磁各向异性的组合有关。对 Mn(12)OAc 和其他 SMM 的广泛研究除了高自旋基态和磁各向异性之外，还为新的 SMM 的合理发展确立了更多的先决条件：对称性至少应为 C(3)以最小化通过各向异性势垒的磁化量子隧穿，但低于立方以避免在投影到自旋基态时局部各向异性的抵消。基于这些前提，我们设计了三联 salen 配体，它结合了间苯三酚桥联单元通过自旋极化机制实现高自旋基态，以及三联 salen 配体环境通过强四方配位场实现单分子磁各向异性。三联 salen 配体的 C(3)对称三核配合物（talen(t-Bu(2)))(6-)表现出强烈的配体折叠，导致整体呈碗状分子结构。这种配体折叠预先组织了金属 salen 亚基的轴向配位位点，以便互补结合六氰合金属单元的三个面氮原子。这导致通过三个分子构建块的组装形成七核配合物M(t)(6)M(c)的驱动力很大。两个三联 salen 三核构建块的叔丁基苯基单元的吸引力范德华相互作用增加了驱动力。在这方面，我们已经能够合成同结构系列Mn(III)(6)Cr(III)、Mn(III)(6)Fe(III)和Mn(III)(6)Co(III)，其中Mn(III)(6)Cr(III)是一个 SMM。对三个七核配合物和四核半单元Mn(III)(3)Cr(III)的磁性性质进行详细分析和比较，为进一步优化 SMM 性质提供了重要的见解。七核配合物从三个分子构建块的模块化组装允许精细调整每个构建块的分子和空间性质，而不会失去形成七核配合物的驱动力。这种对我们同结构系列进行合理改进的可能性是我们超分子方法的主要优势。
