Department of Inorganic, Physical and Materials Chemistry, NIS Centre of Excellence, University of Turin, Via Quarello 11, 10135 Torino, Italy.
Chem Soc Rev. 2010 Dec;39(12):4885-927. doi: 10.1039/c0cs00082e. Epub 2010 Oct 29.
The large unit cells, the enormous flexibility and variation in structural motifs of MOFs represent a big challenge in the characterization of MOF materials, particularly in cases where single crystal diffraction data are not available. In this critical review it is shown that in cases where only powder diffraction data are available additional structural information, particularly regarding local coordination within the inorganic cluster, are often mandatory in order to solve the structure. There are also cases where the inorganic cluster does not follow the symmetry of the overall structure. In such cases diffraction techniques will just "see" an average structure, missing the local structure: a lack that may be critical for understanding the specific properties of the material. In both cases, EXAFS spectroscopy is the tool that provides complementary structural information on the inorganic cluster and the way it binds to the ligand. Selected examples will show how EXAFS will be relevant in: (i) confirming the structure obtained from diffraction refinements; (ii) highlighting that the inorganic cornerstone has a lower symmetry with respect to that of the organic framework; (iii) obtaining the local structure of the inorganic cluster in the desolvated material when desolvation causes a partial loss of long range order; (iv) obtaining the local structure of the inorganic cluster in the desolvated material after coordination of a probe (or reactant) molecule, including cluster deformation upon molecule coordination and metal-molecule binding distance; (v) evidencing the presence of impurities in the form of amorphous extra-phases (339 references).
大的晶胞、结构基元的巨大灵活性和多样性，这给 MOF 材料的特性描述带来了巨大挑战，特别是在单晶衍射数据不可用的情况下。在这篇评论中，我们将表明，在仅可获得粉末衍射数据的情况下，通常需要额外的结构信息，特别是关于无机团簇内的局部配位，以便解析结构。还有一些情况下，无机团簇并不遵循整个结构的对称性。在这种情况下，衍射技术只能“看到”一个平均结构，而丢失了局部结构：这种缺失对于理解材料的特定性质可能是至关重要的。在这两种情况下，扩展 X 射线吸收精细结构（EXAFS）光谱技术是提供关于无机团簇及其与配体结合方式的补充结构信息的工具。选择的示例将展示 EXAFS 将如何在以下方面发挥作用：（i）确认从衍射精修中获得的结构；（ii）突出指出，与有机骨架相比，无机基石的对称性较低；（iii）当去溶剂化导致长程有序部分损失时，获得去溶剂化材料中无机团簇的局部结构；（iv）在配位探针（或反应物）分子后，获得去溶剂化材料中无机团簇的局部结构，包括分子配位时团簇的变形和金属-分子键合距离；（v）以无定形杂相的形式证明杂质的存在（引用 339 篇文献）。
