Volume 225, 2021

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

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 CO2 uptake 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 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-81a, 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.

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

Associated articles

Supplementary files

Article information

Article type
Paper
Submitted
30 jun. 2020
Accepted
17 ago. 2020
First published
12 nov. 2020
This article is Open Access
Creative Commons BY-NC license

Faraday Discuss., 2021,225, 133-151

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

D. Watkins, T. M. Roseveare, M. R. Warren, S. P. Thompson, A. J. Fletcher and L. Brammer, Faraday Discuss., 2021, 225, 133 DOI: 10.1039/D0FD00089B

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