Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA.
Acc Chem Res. 2013 Jul 16;46(7):1548-57. doi: 10.1021/ar300301v. Epub 2013 Mar 14.
Shortly after the discovery of the carbon fullerene allotrope, C₆₀, researchers recognized that the hollow spheroidal shape could accommodate metal atoms, or clusters, which quickly led to the discovery of endohedral metallofullerenes (EMFs). In the past 2 decades, the unique features of EMFs have attracted broad interest in many fields, including inorganic chemistry, organic chemistry, materials chemistry, and biomedical chemistry. Some EMFs produce new metallic clusters that do not exist outside of a fullerene cage, and some other EMFs can boost the efficiency of magnetic resonance (MR) imaging 10-50-fold, in comparison with commercial contrast agents. In 1999, the Dorn laboratory discovered the trimetallic nitride template (TNT) EMFs, which consist of a trimetallic nitride cluster and a host fullerene cage. The TNT-EMFs (A₃N@C2n, n = 34-55, A = Sc, Y, or lanthanides) are typically formed in relatively high yields (sometimes only exceeded by empty-cage C₆₀ and C₇₀, but yields may decrease with increasing TNT cluster size), and exhibit high chemical and thermal stability. In this Account, we give an overview of TNT-EMF research, starting with the discovery of these structures and then describing their synthesis and applications. First, we describe our serendipitous discovery of the first member of this class, Sc₃N@Ih-C₈₀. Second, we discuss the methodology for the synthesis of several TNT-EMFs. These results emphasize the importance of chemically adjusting plasma temperature, energy, and reactivity (CAPTEAR) to optimize the type and yield of TNT-EMFs produced. Third, we review the approaches that are used to separate and purify pristine TNT-EMF molecules from their corresponding product mixtures. Although we used high-performance liquid chromatography (HPLC) to separate TNT-EMFs in early studies, we have more recently achieved facile separation based on the reduced chemical reactivity of the TNT-EMFs. These improved production yields and separation protocols have allowed industrial researchers to scale up the production of TNT-EMFs for commercial use. Fourth, we summarize the structural features of individual members of the TNT-EMF class, including cage structures, cluster arrangement, and dynamics. Fifth, we illustrate typical functionalization reactions of the TNT-EMFs, particularly cycloadditions and radical reactions, and describe the characterization of their derivatives. Finally, we illustrate the unique magnetic and electronic properties of specific TNT-EMFs for biomedicine and molecular device applications.
富勒烯同素异形体 C₆₀发现后不久，研究人员就意识到这种中空的球状结构可以容纳金属原子或团簇，这迅速导致了内包金属富勒烯（EMF）的发现。在过去的 20 年中，EMF 的独特特性引起了无机化学、有机化学、材料化学和生物医学化学等许多领域的广泛关注。一些 EMF 产生了新的金属团簇，这些团簇在富勒烯笼外不存在，而另一些 EMF 可以将磁共振（MR）成像的效率提高 10-50 倍，与商业对比剂相比。1999 年，多恩实验室发现了三金属氮化物模板（TNT）EMF，它由一个三金属氮化物团簇和一个主体富勒烯笼组成。TNT-EMF（A₃N@C2n，n = 34-55，A = Sc、Y 或镧系元素）通常以较高的产率形成（有时仅低于空笼 C₆₀和 C₇₀，但产率可能随 TNT 团簇尺寸的增加而降低），并表现出高的化学和热稳定性。在本综述中，我们概述了 TNT-EMF 的研究，从这些结构的发现开始，然后描述它们的合成和应用。首先，我们描述了我们偶然发现的该类别的第一个成员 Sc₃N@Ih-C₈₀。其次，我们讨论了几种 TNT-EMF 的合成方法。这些结果强调了通过化学调节等离子体温度、能量和反应性（CAPTEAR）来优化所产生的 TNT-EMF 的类型和产率的重要性。第三，我们回顾了从相应的产物混合物中分离和纯化原始 TNT-EMF 分子的方法。虽然我们在早期的研究中使用高效液相色谱（HPLC）来分离 TNT-EMF，但我们最近已经基于 TNT-EMF 的化学还原反应性实现了简便的分离。这些改进的生产产率和分离方案使工业研究人员能够扩大 TNT-EMF 的生产规模，以用于商业用途。第四，我们总结了 TNT-EMF 类别的各个成员的结构特征，包括笼结构、团簇排列和动力学。第五，我们说明了 TNT-EMF 的典型功能化反应，特别是环加成和自由基反应，并描述了它们衍生物的表征。最后，我们说明了特定 TNT-EMF 在生物医学和分子器件应用中的独特磁性和电子性质。
