The metal–organic framework (Me₂NH₂)₂[Cd(NO₂BDC)₂] (SHF-81) comprises flattened tetrahedral Cd(O₂CR)₄²⁻ nodes, in which Cd(II) centres are linked via NO₂BDC²⁻ ligands (2-nitrobenzene-1,4-dicarboxylate) to give a doubly interpenetrated anionic network, with charge balanced by two Me₂NH₂⁺ cations per Cd centre resident in the pores. The study establishes that this is a twinned α-quartz-type structure (trigonal, space group P3_x21, x = 1 or 2), although very close to the higher symmetry β-quartz arrangement (hexagonal, P6_x22, x = 2 or 4) in its as-synthesised solvated form [Cd(NO₂BDC)₂]·2DMF·0.5H₂O (SHF-81-DMF). The activated MOF exhibits very little N₂ uptake at 77 K, but shows significant CO₂ uptake at 273–298 K with an isosteric enthalpy of adsorption (ΔH_ads) at zero coverage of −27.4 kJ mol⁻¹ 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 CO₂ 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.

中文翻译：

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呼吸M（O2CR）4节点MOF的多刺激线性负膨胀

金属有机骨架（Me ₂ NH ₂）₂ [Cd（NO ₂ BDC）₂ ]（SHF-81）包括扁平的四面体Cd（O ₂ CR）₄ ^2-节点，其中Cd（II）中心通过NO ₂ BDC ^2-配体（2-硝基苯-1,4-二羧酸酯）形成双互穿的阴离子网络，每个孔中的Cd中心电荷由两个Me ₂ NH ₂ ⁺阳离子平衡。研究表明，这是孪生的α石英型结构（三角形，空间群P 3_X 21，X= 1或2），尽管非常接近更高的对称性β石英布置（六角形，P6 _X 22，X= 2或4）在其合成后原样的溶剂化形式[CD（NO ₂ BDC） ₂ ]·2DMF·0.5H ₂ O（SHF-81-DMF）。活化的MOF在77 K时几乎没有N ₂吸收，但是在273–298 K时却显示出显着的CO ₂吸收，并且等渗吸附焓（ΔH _ads）在零覆盖下为-27.4 kJ mol ^-1直接为MOF从SHF-81-DMF激活。一系列原位单晶X射线衍射（SCXRD）和粉末X射线衍射（PXRD）的衍射实验表明，MOF具有柔韧性，并表现出呼吸行为，观察到a和b轴的变化高达12％（|Δ一个|，|Δ b | <1.8），并在5.5％ç轴（|Δ ç | <0.7埃）。MOF的溶剂化SHF-81-DMF和活化/去溶剂化的SHF-81均表现出线性负热膨胀（NTE），其中平行于c轴的孔的直径（a和b轴）扩展长度收缩时（c-轴）。CO的吸附₂在298K气体也导致线性负膨胀（Δ一个，Δ b > 0;Δ ç <0;Δ V > 0）。在尺寸最大变化是激活/去溶剂化过程中观察到的从SHF-81-DMF到SHF-81（Δ一个，Δ b <0;Δ Ç > 0;Δ V <0）。总共九个原地 进行的衍射实验表明，呼吸行为是连续的，尽管单独的去溶剂化和吸附实验并未排除在中间几何形状上进行门控或台阶操作的可能性，而这种可能性是伴随着朝向呼吸幅度末端的连续动态行为。