High-Nuclearity Lanthanide-Containing Clusters as Potential Molecular Magnetic Coolers.
High-Nuclearity Lanthanide-Containing Clusters as Potential Molecular Magnetic Coolers.
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DOI：
10.1021/acs.accounts.7b00579
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发表时间：
2018-02
影响因子：
18.3
通讯作者：
Xiu-Ying Zheng;Xiang‐Jian Kong;Zhiping Zheng;L. Long;Lan-Sun Zheng
中科院分区：
文献类型：
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作者：
Xiu-Ying Zheng;Xiang‐Jian Kong;Zhiping Zheng;L. Long;Lan-Sun Zheng
高原子核簇合物是一类独特的化合物，其中许多具有美观的分子结构。它们有趣的物理和化学性质主要来自组分金属离子之间的电子和/或磁性相互作用。在过去的二十年中，对含镧系元素簇合物、镧系元素独占簇合物和过渡金属杂原子簇合物的研究最为引人注目。这项研究是由这些通常难以捉摸的物种的合成挑战及其有趣的磁性驱动的，这些磁性对于开发节能和环保的磁冷却技术非常有用。我们在这方面的努力一直集中在发展合理的合成方法，为高核镧系元素的集群。在这一发展阶段的前半期（1999-2006年），人们利用“配体控制水解法”合成了大量的簇合物型稀土氢氧化物配合物。本报告总结了我们自2007年以来所做的努力。这些包括（1）进一步开发合成策略以扩大配体范围和/或增加簇物种的核性（>25）和（2）与追求具有大磁热效应（MCE）的材料相关的磁性研究。具体地，希望扩大配体家族和产生以前未知结构的簇，我们在水热或溶剂热条件下测试了使用容易获得但不常用的配体来控制镧系元素水解;这样的配体，羧酸盐作为普通的例子，倾向于在任何可能的水解之前形成不溶性络合物。我们还验证了使用预先形成的过渡金属配合物作为金属配体，用于随后控制镧系元素向异金属3d-4f簇水解。此外，我们证明了使用大量的例子，小阴离子作为模板的存在下是必不可少的组装的高核含镧系元素的集群和保持低浓度的阴离子模板（S）是这样的成功的关键。已经发现，通过原位配体分解或吸收大气CO2来缓慢产生/释放这种阴离子模板在防止其镧系元素盐沉淀方面是有效的，不仅允许可控的镧系元素水解，而且允许巨型簇物种的逐渐和模块化组装。针对这种簇作为分子磁冷却器的潜在应用的磁性研究也已经进行。本说明的第二部分总结了这些结果，以建立某种磁结构关系。特别相关的是MCE（使用等温磁熵变，-ΔSM评估）和磁密度之间的可能相关性，以及集群内反铁磁交换耦合。我们也做了一些初步的尝试，在制备可加工的和实际有用的材料的单分散核-壳纳米结构的形式。我们成功地将单个纳米尺寸的异金属分子簇封装在二氧化硅的纳米壳中。发现这种钝化不仅有助于稳定团簇，而且还降低了单个团簇之间的磁相互作用。这些效应反映在核-壳复合物的-ΔSM值略高于母体未保护簇。
High-nuclearity cluster-type metal complexes are a unique class of compounds, many of which have aesthetically pleasing molecular structures. Their interesting physical and chemical properties arise primarily from the electronic and/or magnetic interplay between the component metal ions. Among the extensive studies in the past two decades, those on lanthanide-containing clusters, lanthanide-exclusive or heterometallic with transition metal elements, are most notable. The research was driven by both the synthetic challenges for these generally elusive species and their intriguing magnetic properties, which are useful for the development of energy-efficient and environmentally friendly magnetic cooling technologies. Our efforts in this vein have been concentrated on developing rational synthetic methods for high-nuclearity lanthanide-containing clusters. By means of the now widely adopted approach of "ligand-controlled hydrolysis" of lanthanide ions, a great variety of cluster-type lanthanide hydroxide complexes had been prepared in the first half of this developing period (1999-2006). In this Account, our efforts since 2007 are summarized. These include (1) further development of synthetic strategies in order to expand the ligand scope and/or to increase the nuclearity (>25) of the cluster species and (2) magnetic studies pertinent to the pursuit of materials with a large magnetocaloric effect (MCE). Specifically, with the hope of expanding the family of ligands and producing clusters of previously unknown structures, we tested under hydrothermal or solvothermal conditions the use of readily available yet not commonly used ligands for controlling lanthanide hydrolysis; such ligands, carboxylates as mundane examples, tend to form insoluble complexes prior to any possible hydrolysis. We have also validated the use of preformed transition metal complexes as metalloligands for subsequent control of lanthanide hydrolysis toward heterometallic 3d-4f clusters. Furthermore, we demonstrated using ample examples that the presence of small anions as templates is essential to the assembly of high-nuclearity lanthanide-containing clusters and that maintaining a low concentration of the anion template(s) is a key to such success. It has been found that slow production/release of such anion templates by in situ ligand decomposition or absorption of atmospheric CO2 is effective in preventing precipitation of their lanthanide salts, allowing not only controllable lanthanide hydrolysis but also gradual and modular assembly of the giant cluster species. Magnetic studies targeting potential applications of such clusters as molecular magnetic coolers have also been conducted. The results are summarized in the second portion of this Account in an effort to establish a certain magneto-structure relationship. Of particular relevance is the possible correlation between MCE (evaluated using the isothermal magnetic entropy change, -ΔSM) and magnetic density, and the intracluster antiferromagnetic exchange coupling. We have also made some preliminary attempts at preparing processable and practically useful materials in the form of a monodisperse core-shell nanostructure. We succeeded in encapsulating a single nanosized heterometallic molecular cluster in a nanoshell of silica. It was found that such passivation not only helped stabilize the cluster but also reduced the magnetic interactions between individual clusters. These effects are reflected in the slightly enhanced value of -ΔSM for the core-shell composite over the parent unprotected cluster.