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The metal-organic framework (Me 2 NH 2 ) 2 [Cd(NO 2 BDC) 2 ] (SHF-81) comprises flattened tetrahedral Cd(O 2 CR) 4  2- nodes, in which Cd(ii) centres are linked via NO 2 BDC 2- ligands (2-nitrobenzene-1,4-dicarboxylate) to give a doubly interpenetrated anionic network, with charge balanced by two Me 2 NH 2  + cations per Cd centre resident in the pores. The study establishes that this is a twinned α-quartz-type structure (trigonal, space group P3 x 21, x = 1 or 2), although very close to the higher symmetry β-quartz arrangement (hexagonal, P6 x 22, x = 2 or 4) in its as-synthesised solvated form [Cd(NO 2 BDC) 2 ]·2DMF·0.5H 2 O (SHF-81-DMF). The activated MOF exhibits very little N 2 uptake at 77 K, but shows significant CO 2 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| &lt; 1.8 Å) and 5.5% in the c-axis (|Δc| &lt; 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 2 gas at 298 K also results in linear negative expansion (Δa, Δb &gt; 0; Δc &lt; 0; ΔV &gt; 0). The largest change in dimensions is observed during activation/desolvation from SHF-81-DMF to SHF-81 (Δa, Δb &lt; 0; Δc &gt; 0; ΔV &lt; 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.

Multi-stimulus linear negative expansion of a breathing M(O2CR)4-node MOF

Multi-stimulus linear negative expansion of a breathing M(O₂CR)₄-node MOF