Department of Chemistry & Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States.
Department of Chemistry, Columbia University, New York, New York 10027, United States.
J Am Chem Soc. 2021 Feb 24;143(7):2801-2811. doi: 10.1021/jacs.0c11718. Epub 2021 Feb 11.
Molten phases of metal-organic networks offer exciting opportunities for using coordination chemistry principles to access liquids and glasses with unique and tunable structures and properties. Here, we discuss general thermodynamic strategies to provide an increased enthalpic and entropic driving force for reversible, low-temperature melting transitions in extended coordination solids and illustrate this approach through a systematic study of a series of bis(acetamide)-based networks with record-low melting temperatures. The low melting temperatures of these compounds are the result of weak coordination bonds, conformationally flexible bridging ligands, and weak electrostatic interactions between spatially separated cations and anions, which collectively reduce the enthalpy and increase the entropy of fusion. Through a combination of crystallography, spectroscopy, and calorimetry, enthalpic trends are found to be dictated by the strength of coordination bonds and hydrogen bonds within each compound, while entropic trends are strongly influenced by the degree to which residual motion and positional disorder are restricted in the crystalline state. Extended X-ray absorption fine structure (EXAFS) and pair distribution function (PDF) analysis of Co(bba)[CoCl] [bba = ,-1,4-butylenebis(acetamide)], which features a record-low melting temperature for a three-dimensional metal-organic network of 124 °C, provide direct evidence of metal-ligand coordination in the liquid phase, as well as intermediate- and extended-range order that support its network-forming nature. In addition, rheological measurements are used to rationalize differences in glass-forming ability and relaxation dynamics. These results provide new insights into the structural and chemical factors that influence the thermodynamics of melting transitions of extended coordination solids, as well as the structure and properties of coordination network-forming liquids.
金属有机网络的熔融相为利用配位化学原理来获得具有独特和可调结构和性质的液体和玻璃提供了令人兴奋的机会。在这里，我们讨论了一般热力学策略，以提供扩展配位固体中低温熔融转变的增加焓和熵驱动力，并通过对一系列具有创纪录低熔融温度的双（乙酰胺）基网络的系统研究说明了这种方法。这些化合物的低熔点是弱配位键、构象柔性桥联配体以及空间分离的阳离子和阴离子之间的弱静电相互作用的结果，这些共同降低了熔融焓并增加了熔融熵。通过晶体学、光谱学和量热法的结合，发现焓趋势由每个化合物中配位键和氢键的强度决定，而熵趋势则强烈受到晶体状态下残余运动和位置无序受限程度的影响。具有创纪录低的 124°C 三维金属有机网络熔点的 Co(bba)[CoCl] [bba =,-1,4-丁二烯双（乙酰胺）]的扩展 X 射线吸收精细结构 (EXAFS) 和配分函数 (PDF) 分析提供了液相中金属-配体配位的直接证据，以及支持其网络形成性质的中程和扩展程有序。此外，流变学测量用于合理地解释玻璃形成能力和弛豫动力学的差异。这些结果为影响扩展配位固体熔融转变热力学的结构和化学因素以及配位网络形成液体的结构和性质提供了新的见解。
