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

Daniel Watkins, Thomas M. Roseveare, Mark R. Warren, Stephen P. Thompson, Ashleigh J. Fletcher, Lee Brammer

Research output: Contribution to journalArticle

Abstract

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 P3x21, x = 1 or 2), although very close to the higher symmetry β-quartz arrangement (hexagonal, P6x22,
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 COuptake at 273–298 K with an isosteric enthalpy of adsorption (ΔHads) 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 °A) and 5.5% in the c-axis (|Δc| < 0.7 °A). Both the solvated SHF-81-DMF and activated/desolvated SHF-81forms 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.
Original languageEnglish
Number of pages19
JournalFaraday Discussions
Early online date12 Nov 2020
DOIs
Publication statusE-pub ahead of print - 12 Nov 2020

Keywords

  • metal–organic frameworks
  • modular design
  • drug delivery

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