Single-molecule magnetism in lanthanide organometallics

具有磁记忆的分子被称为单分子磁体(SMM)。就其大小和组成而言，SMM的尺寸为几纳米，它们由一个或多个金属原子组成，这些原子与一组被称为配体的非金属原子相连。相邻分子之间的相互作用非常弱，这意味着SMM的磁性真正来自于单个分子。与SMM形成鲜明对比的是，日常家用电器中使用的传统条形磁体是纯无机材料，如金属氧化物或简单的磁性元素。它们应用的一个特别重要的例子是在计算机硬盘驱动器中。在尺寸和组成方面，传统的磁体不是由分子组成，而是具有更大的磁域。由于SMM与传统磁体的主要区别之一与尺寸有关，因此SMM可能代表了磁信息存储的极限尺寸。SMM的特性有朝一日可能会让它们被开发出来用于量子计算机。然而，SMM的一个问题是，它们的磁记忆功能在大约-250摄氏度的温度下工作，只有用液氦冷却才能达到这一温度，因此是不切实际的。此外，SMM的磁化松弛机制(即磁信息的丢失或擦除)尚不清楚，但了解这些过程很可能会导致在更高温度下性能的提高。我们提出了一类新的基于稀土元素的SMM(Ln-SMM)。由于这些元素具有特别吸引人的磁性，因此这些元素为发展SMM提供了相当大的潜力。最终，我们的Ln-SMM将具有在-196oC以上可观察到的磁记忆效应，这将是一个重大的进步，因为可以通过液氮冷却达到这个温度，液氮是一种比液氦便宜得多的制冷剂，而且更容易使用。我们将通过使用我们作为合成化学家可用的分子设计工具来实现我们的目标：我们可以显著改变我们选择的稀土元素与配体相互作用的方式。这一点很重要，因为用于与稀土元素相互作用的非金属原子，以及配体在稀土元素周围排列的对称性，允许我们影响磁性。我们设计Ln-SMM的一个独特方面是，我们的合成方法提供了非常广泛的化学环境。现有的常规Ln-SMM几乎完全局限于氧或氮与稀土元素相互作用的配体，然而我们可以使用碳、氧、硫、硒、碲、氮、磷、砷、锑或卤素来影响Ln-SMM的磁性。通过了解不同的化学环境对分子磁性的影响方式，我们将能够确定在前所未有的高温下生产Ln-SMM的最佳条件。

Molecules that have a magnetic memory are called single-molecule magnets (SMMs). In terms of their size and composition, SMMs have dimensions of a few nanometres and they consist of one or more metal atoms bonded to a group of non-metal atoms called a ligand. The interactions between neighbouring molecules are very weak, meaning that the magnetic properties of an SMM genuinely arise from within individual molecules.In stark contrast to SMMs, traditional 'bar' magnets used in everyday appliances are purely inorganic materials such as metal oxides or simply magnetic elements. A particularly important example of their application is in computer hard disk drives. In terms of their size and composition, rather than consisting of molecules traditional magnets feature much larger magnetic domains.Because one of the main differences between SMMs and traditional magnets relates to size, it is possible that SMMs represent the ultimate size limit for magnetic information storage. The properties of SMMs may one day allow them to be developed for use in quantum computers. A problem with SMMs is, however, that their magnetic memories function at temperatures of about -250oC, which can only be reached by cooling with liquid helium and is therefore impractical. Furthermore, the mechanisms by which SMMs relax their magnetization (i.e. the magnetic information is lost or 'wiped') are not clear, but it is likely that gaining an understanding of these processes will lead to enhanced performance at higher temperatures.We propose a new family of SMMs based on the lanthanide elements (Ln-SMMs). The lanthanides offer considerable potential for developing SMMs because these elements have particularly appealing magnetic properties. Ultimately, our Ln-SMMs will have magnetic memory effects observable above -196oC, which will be a major advance because this temperature can be reached by cooling with liquid nitrogen, a cryogen that is much cheaper than liquid helium, and easier to use.We will achieve our aims by using a molecular design tool available to us as synthetic chemists: we can make significant changes to the ways in which our ligands interact with our choice of lanthanide. This is important because the non-metal atoms used to interact with the lanthanides, and the symmetry with which the ligands are arranged around the lanthanides, allow us to influence the magnetism.A unique aspect of our approach to the design of Ln-SMMs is that our synthetic method gives access to an extremely broad range of chemical environments. Existing, conventional Ln-SMMs are almost entirely limited to ligands in which oxygen or nitrogen interacts with the lanthanide, however we can influence the magnetism using carbon, oxygen, sulphur, selenium, tellurium, nitrogen, phosphorus, arsenic, antimony or the halogens.By understanding the ways in which the different chemical environments influence the molecular magnetism we will be able to identify the optimum conditions for producing Ln-SMMs that function at unprecedentedly high temperatures.