Department of Chemistry, IISER Bhopal , Bhopal 462066, India.
Department of Chemistry, Texas A&M University , College Station, Texas 77843-3255, United States.
Acc Chem Res. 2016 Jun 21;49(6):1093-103. doi: 10.1021/acs.accounts.5b00531. Epub 2016 May 6.
Transition metal based high nuclearity molecular magnetic cages are a very important class of compounds owing to their potential applications in fabricating new generation molecular magnets such as single molecular magnets, magnetic refrigerants, etc. Most of the reported polynuclear cages contain carboxylates or alkoxides as ligands. However, the binding ability of phosphonates with transition metal ions is stronger than the carboxylates or alkoxides. The presence of three oxygen donor sites enables phosphonates to bridge up to nine metal centers simultaneously. But very few phosphonate based transition metal cages were reported in the literature until recently, mainly because of synthetic difficulties, propensity to result in layered compounds, and also their poor crystalline properties. Accordingly, various synthetic strategies have been followed by several groups in order to overcome such synthetic difficulties. These strategies mainly include use of small preformed metal precursors, proper choice of coligands along with the phosphonate ligands, and use of sterically hindered bulky phosphonate ligands. Currently, the phosphonate system offers a library of high nuclearity transition metal and mixed metal (3d-4f) cages with aesthetically pleasing structures and interesting magnetic properties. This Account is in the form of a research landscape on our efforts to synthesize and characterize new types of phosphonate based high nuclearity paramagnetic transition metal cages. We quite often experienced synthetic difficulties with such versatile systems in assembling high nuclearity metal cages. Few methods have been emphasized for the self-assembly of phosphonate systems with suitable transition metal ions in achieving high nuclearity. We highlighted our journey from 2005 until today for phosphonate based high nuclearity transition metal cages with V(IV/V), Mn(II/III), Fe(III), Co(II), Ni(II), and Cu(II) metal ions and their magnetic properties. We observed that slight changes in stoichiometry, reaction conditions, and presence or absence of coligand played crucial roles in determining the final structure of these complexes. Most of the complexes included are regular in geometry with a dense arrangement of the above-mentioned metal centers in a confined space, and a few of them also resemble regular polygonal solids (Archimedean and Platonic). Since there needs to be a historical approach for a comparative study, significant research output reported by other groups is also compared in brief to ensure the potential of phosphonate ligands in synthesizing high nuclearity magnetic cages.
基于过渡金属的高核分子磁性笼合物是一类非常重要的化合物，因为它们有可能用于制造新一代分子磁体，如单分子磁体、磁性制冷剂等。大多数报道的多核笼合物都含有羧酸酯或烷氧基作为配体。然而，膦酸酯与过渡金属离子的结合能力强于羧酸酯或烷氧基。三个氧供体位点的存在使膦酸酯能够同时桥接多达九个金属中心。但直到最近，文献中才报道了很少的基于膦酸酯的过渡金属笼合物，主要是因为合成困难、倾向于生成层状化合物，以及它们较差的晶体性质。因此，为了克服这些合成困难，许多研究小组采用了各种合成策略。这些策略主要包括使用小的预成型金属前体、选择合适的共配体与膦酸酯配体一起使用，以及使用空间位阻大的膦酸酯配体。目前，膦酸酯体系提供了一系列具有美学结构和有趣磁性的高核过渡金属和混合金属（3d-4f）笼合物。本报告以我们合成和表征新型膦酸酯基高核顺磁过渡金属笼合物的研究概况的形式呈现。我们在组装高核金属笼合物时经常遇到这种多功能体系的合成困难。强调了几种方法用于在实现高核的情况下，用合适的过渡金属离子自组装膦酸酯体系。突出了我们从 2005 年到今天在基于 V(IV/V)、Mn(II/III)、Fe(III)、Co(II)、Ni(II)和 Cu(II)金属离子及其磁性的膦酸酯基高核过渡金属笼合物方面的研究历程。我们观察到，化学计量、反应条件和共配体的存在与否的微小变化在决定这些配合物的最终结构方面起着至关重要的作用。大多数配合物的几何形状规则，上述金属中心在有限的空间中排列紧密，其中一些还类似于规则的多面固体（阿基米德和柏拉图）。由于需要进行历史方法的比较研究，简要比较了其他小组报告的重要研究成果，以确保膦酸酯配体在合成高核磁性笼合物方面的潜力。
