环形镧系分子簇合物的组装与单分子磁体性质

Cyclic Lanthanide-based Molecular Clusters: Assembly and Single Molecule Magnet Behavior

由于稀土离子具有很强的磁各向异性,近年来基于单核或多核稀土化合物的单分子磁体研究得到了人们广泛的关注.环形簇合物是一类特殊的簇合物,也称分子轮或金属冠醚.设计合成环形稀土簇合物不仅可以获得新的稀土单分子磁体,而且环形簇合物中稀土离子磁偶极的涡旋分布还可以产生环形磁矩,因而在量子计算、信息存储、自旋分子器件等方面具有潜在的应用.鉴于环形稀土簇合物特别是含奇数核的环形簇合物的合成依然充满挑战,本综述将着重阐述已报道的环形稀土簇合物的组装规律、结构特点及磁性研究.

Lanthanide-based single molecule magnets have received tremendous attentions in recent years owing to the strong magnetic anisotropies of the lanthanide ions arising from the strong spin-orbital couplings.Cyclic metal clusters,also called molecular wheels or metallacrown ether,are a subclass of metal clusters.From the magnetic point of view,cyclic transition metal clusters can be devided into three types,e.g.ferromagnetically coupled cyclic clusters which favor single molecule magnet behavior,and antiferromagnetically coupled even-or odd-numbered cyclic clusters with S=0 or S=1/2 ground state.The magnetic properties of lanthanide-based cyclic clusters are more complicated because the magnetic interactions between the lanthanide ions are extremely weak.The overall magnetic behavior is largely dominated by the single ion anisotropy and the dipole-dipole interactions between the metal ions.When the anisotropy axes of the lanthanide ions in the cyclic clusters are arranged in a toroidal manner,single-molecule toroics could be achieved.Therefore,the design and synthesis of cyclic lanthanide-based clusters can provide not only new materials with architectural beauty and single molecule magnet behavior,but also single-molecule toroics with vortex distribution of the magnetic dipoles of lanthanide ions,which would have potential applications in information storage,quantum computing,spintronic devices and multiferroic materials.Noting that lanthanide-based single-molecule toroics have been described detailly in several reviews,this article will summarize the current status of the cyclic lanthanide clusters with the focus on the design and assembly strategies,the structural characteristics and magnetic studies.Most work have been concentrated on the Ln3,Ln4 and Ln6 cyclic clusters,including those containing oxygen centers.Examples of even-numbered cyclic clusters Lnx(x≥8)are much less,and those of odd-numbered cyclic clusters Lnx(x≥5)are rare.As the cyclic clusters are frequently distorted to different extent,many of them exhibit single molecule magnet behavior,and only few of them show toroic magnetization.It remains future challenges to design and synthesize new lanthanide-based cyclic clusters with regular and flat geometries and toroically arranged magnetic moments,and to achieve the multifunctions in the same molecular composite.