Thermodynamics and Electronic Properties of Heterometallic Multinuclear Actinide-Containing Metal–Organic Frameworks with “Structural Memory”

Thermodynamic studies of actinide-containing metal–organic frameworks (An-MOFs), reported herein for the first time, are a step toward addressing challenges related to effective nuclear waste administration. In addition to An-MOF thermochemistry, enthalpies of formation were determined for the organic linkers, 2,2′-dimethylbiphenyl-4,4′-dicarboxylic acid (H2Me2BPDC) and biphenyl-4,4′-dicarboxylic acid (H2BPDC), which are commonly used building blocks for MOF preparation. The electronic structure of the first example of An-MOF with mixed-metal AnAn′-nodes was influenced through coordination of transition metals as shown by the density of states near the Fermi edge, changes in the Tauc plot, conductivity measurements, and theoretical calculations. The “structural memory” effect (i.e., solvent-directed crystalline–amorphous–crystalline structural dynamism) was demonstrated as a function of node coordination degree, which is the number of organic linkers per metal node. Remarkable three-month water stability was reported for Th-containing frameworks herein, and the mechanism is also considered for improvement of the behavior of a U-based framework in water. Mechanistic aspects of capping linker installation were highlighted through crystallographic characterization of the intermediate, and theoretical calculations of free energies of formation (ΔGf) for U- and Th-MOFs with 10- and 12-coordinated secondary building units (SBUs) were performed to elucidate experimentally observed transformations during the installation processes. Overall, these results are the first thermochemical, electronic, and mechanistic insights for a relatively young class of actinide-containing frameworks.本文首次报道的含锕系元素金属有机框架（actinide-containing metal–organic frameworks, An-MOFs）的热力学研究，为解决高效核废料管理相关挑战迈出了关键一步。除An-MOF热化学研究外，本工作还测定了两种MOF制备中常用的有机配体——2,2'-二甲基联苯-4,4'-二羧酸（H2Me2BPDC）与联苯-4,4'-二羧酸（H2BPDC）的生成焓。首例含混合金属锕系节点的An-MOF的电子结构可通过过渡金属配位进行调控，费米边附近的态密度、塔奇图（Tauc plot）变化、电导率测试结果与理论计算均可佐证这一结论。本研究还证实了‘结构记忆效应’（即溶剂导向的晶态-非晶态-晶态结构动态转变）随节点配位数的变化规律，节点配位数指每个金属节点结合的有机配体数目。本文所报道的含钍（Th）框架材料展现出长达三个月的优异水稳定性，同时本工作还探讨了优化铀（U）基框架材料水稳定性能的相关机制。通过对封端配体安装过程中间体的晶体学表征，并针对具有10配位与12配位次级结构单元（secondary building units, SBUs）的U基与Th基MOF开展生成自由能（ΔGf）的理论计算，本研究阐明了该安装过程中实验观测到的转化机制。总体而言，本研究成果为这一相对新兴的含锕系元素框架材料家族提供了首批热化学、电子结构与机制方面的系统性认知。