Zhang Xian-Ming, Hou Juan-Juan, Guo Cai-Hong, Li Chun-Fang
School of Chemistry & Material Science, Shanxi Normal University , Linfen 041004, P. R. China.
Inorg Chem. 2015 Jan 20;54(2):554-9. doi: 10.1021/ic5024168. Epub 2014 Dec 29.
Although a variety of functional metal-organic frameworks (MOFs) have been synthesized, post-modified, and applied in various areas, there is little knowledge about how molecular cluster building units are stepwise evolved into MOFs via intermediates. Coordination bonds are generally stronger than hydrogen bonds, and thus equivalent replacement of X-H···Y hydrogen bonds by X-M-Y coordination bonds can transform hydrogen bond networks into MOFs. In this work, solvothermal in situ reduction reactions of CuBr2 and 1,4-diazoniabicyclo[2,2,2]octane (DABCO) generated a myriad of tunable photoluminescent cuprous body-centered cubic bromide cluster-based networks with the general formula Cu4+xH4-xBr6(DABCO)42·S (x = 0, 0.56, 0.81, 1.27, 1.39, 2.56, 2.78, and 4 for compounds 1-8, respectively). All of these compounds crystallize in the cubic space group with the largest volume difference being only 5.2%, but they belong to three remarkably different kinds of crystals. Complex 1 is a molecular crystal and consists of tetrahedral Cu4Br6(HDABCO)4 clusters with monodentate HDABCO groups that are supported via N-H···Br synthons in the hydrogen bond network. Compound 8 is a Cu8Br6 cube cluster-based MOF with bridged DABCO ligands. Complexes 2-7 are seemingly impossible Cu/H-substituted solid solutions of 1 and 8. The CuBr framework components in 1-8 are Cu4Br6, Cu4.56Br6, Cu4.81Br6, Cu5.27Br6, Cu5.39Br6, Cu6.56Br6, Cu6.78Br6, and Cu8Br6, respectively. Crystallization kinetics studies revealed that the Cu4Br6(HDABCO)4 cluster-based hydrogen bond network (1) was initially formed such that N-H···Br hydrogen bonds could be stepwise replaced by N-Cu-Br coordination bonds to form the Cu8Br6 cube cluster-based MOF (8) via solid solutions. These observations directly reveal the equivalence and transformation between the N-H···Br hydrogen bond and the N-Cu-Br coordination bond and the evolutionary mechanism of a molecular crystal to a MOF via solid solutions, which is of fundamental importance in materials but has never before been revealed. DFT calculations suggest that equivalent replacement of a N-H···Br hydrogen bond by a N-Cu-Br coordination bond is exothermic and exergonic, which also supports the transformation from molecule 1 to MOF 8.
尽管已经合成、后修饰了多种功能性金属有机框架（MOF）并将其应用于各个领域，但对于分子簇构建单元如何通过中间体逐步演变成MOF却知之甚少。配位键通常比氢键更强，因此用X-M-Y配位键等效取代X-H···Y氢键可以将氢键网络转变为MOF。在这项工作中，CuBr₂和1,4 - 二氮杂双环[2,2,2]辛烷（DABCO）的溶剂热原位还原反应生成了无数可调谐的基于光致发光的体心立方溴化亚铜簇网络，通式为Cu₄₊ₓH₄₋ₓBr₆(DABCO)₄₂·S（化合物1 - 8的x分别为0、0.56、0.81、1.27、1.39、2.56、2.78和4）。所有这些化合物都在立方空间群中结晶，最大体积差异仅为5.2%，但它们属于三种截然不同的晶体类型。配合物1是分子晶体，由四面体Cu₄Br₆(HDABCO)₄簇组成，单齿HDABCO基团通过氢键网络中的N-H···Br合成子得到支撑。化合物8是基于Cu₈Br₆立方簇的MOF，带有桥连的DABCO配体。配合物2 - 7似乎是1和8的看似不可能的Cu/H取代固溶体。1 - 8中的CuBr框架组分分别是Cu₄Br₆、Cu₄.56Br₆、Cu₄.81Br₆、Cu₅.27Br₆、Cu₅.39Br₆、Cu₆.56Br₆、Cu₆.78Br₆和Cu₈Br₆。结晶动力学研究表明，基于Cu₄Br₆(HDABCO)₄簇的氢键网络（1）最初形成，使得N-H···Br氢键可以逐步被N-Cu-Br配位键取代，通过固溶体形成基于Cu₈Br₆立方簇的MOF（8）。这些观察结果直接揭示了N-H···Br氢键和N-Cu-Br配位键之间的等效性和转变以及分子晶体通过固溶体向MOF的演化机制，这在材料领域具有根本重要性，但此前从未被揭示过。密度泛函理论（DFT）计算表明，用N-Cu-Br配位键等效取代N-H···Br氢键是放热且放能的，这也支持了从分子1到MOF 8的转变。
