Watkins Daniel, Roseveare Thomas M, Warren Mark R, Thompson Stephen P, Fletcher Ashleigh J, Brammer Lee
Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, UK.
Faraday Discuss. 2021 Feb 1;225:133-151. doi: 10.1039/d0fd00089b. Epub 2020 Nov 12.
The metal-organic framework (MeNH)[Cd(NOBDC)] (SHF-81) comprises flattened tetrahedral Cd(OCR) nodes, in which Cd(ii) centres are linked via NOBDC ligands (2-nitrobenzene-1,4-dicarboxylate) to give a doubly interpenetrated anionic network, with charge balanced by two MeNH cations per Cd centre resident in the pores. The study establishes that this is a twinned α-quartz-type structure (trigonal, space group P321, x = 1 or 2), although very close to the higher symmetry β-quartz arrangement (hexagonal, P622, x = 2 or 4) in its as-synthesised solvated form [Cd(NOBDC)]·2DMF·0.5HO (SHF-81-DMF). The activated MOF exhibits very little N uptake at 77 K, but shows significant CO uptake at 273-298 K with an isosteric enthalpy of adsorption (ΔH) at zero coverage of -27.4 kJ mol determined for the MOF directly activated from SHF-81-DMF. A series of in situ diffraction experiments, both single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD), reveal that the MOF is flexible and exhibits breathing behaviour with observed changes as large as 12% in the a- and b-axes (|Δa|, |Δb| < 1.8 Å) and 5.5% in the c-axis (|Δc| < 0.7 Å). Both the solvated SHF-81-DMF and activated/desolvated SHF-81 forms of the MOF exhibit linear negative thermal expansion (NTE), in which pores that run parallel to the c-axis expand in diameter (a- and b-axis) while contracting in length (c-axis) upon increasing temperature. Adsorption of CO gas at 298 K also results in linear negative expansion (Δa, Δb > 0; Δc < 0; ΔV > 0). The largest change in dimensions is observed during activation/desolvation from SHF-81-DMF to SHF-81 (Δa, Δb < 0; Δc > 0; ΔV < 0). Collectively the nine in situ diffraction experiments conducted suggest the breathing behaviour is continuous, although individual desolvation and adsorption experiments do not rule out the possibility of a gating or step at intermediate geometries that is coupled with continuous dynamic behaviour towards the extremities of the breathing amplitude.
金属有机框架（MeNH）[Cd(NOBDC)]（SHF - 81）由扁平的四面体Cd(OCR)节点组成，其中Cd(ii)中心通过NOBDC配体（2 - 硝基苯 - 1,4 - 二羧酸酯）相连，形成一个双穿插的阴离子网络，每个Cd中心的电荷由两个位于孔中的MeNH阳离子平衡。该研究表明，这是一种孪晶α - 石英型结构（三方晶系，空间群P321，x = 1或2），尽管其合成的溶剂化形式[Cd(NOBDC)]·2DMF·0.5H₂O（SHF - 81 - DMF）非常接近更高对称性的β - 石英排列（六方晶系，P622，x = 2或4）。活化后的金属有机框架在77 K时对N的吸附很少，但在273 - 298 K时对CO有显著吸附，从SHF - 81 - DMF直接活化得到的金属有机框架在零覆盖度下的吸附等容热（ΔH）为 - 27.4 kJ/mol。一系列原位衍射实验，包括单晶X射线衍射（SCXRD）和粉末X射线衍射（PXRD），表明该金属有机框架具有柔性并表现出呼吸行为，观察到a轴和b轴的变化高达12%（|Δa|，|Δb| < 1.8 Å），c轴变化为5.5%（|Δc| < 0.7 Å）。金属有机框架的溶剂化形式SHF - 81 - DMF和活化/去溶剂化形式SHF - 81均表现出线性负热膨胀（NTE），其中平行于c轴的孔在温度升高时直径（a轴和b轴）膨胀而长度（c轴）收缩。在298 K下吸附CO气体也会导致线性负膨胀（Δa，Δb > 0；Δc < 0；ΔV > 0）。从SHF - 81 - DMF到SHF - 81的活化/去溶剂化过程中观察到尺寸变化最大（Δa，Δb < 0；Δc > 0；ΔV < 0）。总共进行的九次原位衍射实验表明呼吸行为是连续的，尽管单独的去溶剂化和吸附实验并不排除在中间几何结构处存在门控或台阶的可能性，这种门控或台阶与向呼吸幅度极值的连续动态行为相关。
