Abstract
A metal–organic framework (MOF) with one-dimensional channels of approximately hexagonal cross-section [Ag2(O2CCF2CF2CO2)(TMP)] 1 (TMP =2,3,5,6-tetramethylpyrazine) has been synthesized with MeOH filling the channels in its as-synthesized form as [Ag2(O2CCF2CF2CO2)(TMP)]·n(MeOH) 1-MeOH (n = 1.625 by X-ray crystallography). The two types of ligand connect columns of Ag(I) centres in an alternating manner, both around the channels and along their length, leading to an alternating arrangement of hydrocarbon (C–H) and fluorocarbon (C–F) groups lining the channel walls, with the former groups projecting further into the channel than the latter. MeOH solvent in the channels can be exchanged for a variety of arene guests, ranging from xylenes to tetrafluorobenzene, as confirmed by gas chromatography, 1H nuclear magnetic resonance (NMR) spectroscopy, thermogravimetric analysis and 13C cross-polarization magic angle spinning NMR spectroscopy. Alkane and perfluoroalkane guests, however, do not enter the channels. Although exhibiting some stability under a nitrogen atmosphere, sufficient to enable crystal structure determination, the evacuated MOF 1 is unstable for periods of more than minutes under ambient conditions or upon heating, whereupon it undergoes an irreversible solid-state transformation to a non-porous polymorph 2, which comprises Ag2(O2CCF2CF2CO2) coordination layers that are pillared by TMP ligands. This transformation has been followed in situ by powder X-ray diffraction and shown to proceed via a crystalline intermediate.This article is part of the themed issue ‘Coordination polymers and metal–organic frameworks: materials by design’.
Highlights
Coordination polymers, and metal–organic frameworks (MOFs), which are the focus of this theme issue, have become an intensely studied field of research over the past 20–25 years [1,2,3]
We further show that the MeOH molecules can be removed to reveal an empty channel material, which upon heating can be collapsed to a new condensed phase, the transition to which has been followed in situ by powder X-ray diffraction (PXRD)
We have previously established a class of crystalline materials based on Ag(I) centres linked via perfluoroalkylcarboxylates and diimine linkers that exhibit flexibility in coordination number and geometry at the Ag(I) centres and/or in the fluoralkyl groups, each enabling these materials to undergo uptake and release of small molecule guests and changes in structure and/or composition that involve coordination bond formation or breaking [16,17,18,19,20,21,22]
Summary
Coordination polymers, and metal–organic frameworks (MOFs), which are the focus of this theme issue, have become an intensely studied field of research over the past 20–25 years [1,2,3]. The diffraction patterns were merged, and the combined pattern was indexed and fitted using the TOPAS Academic program [34], by Pawley refinement [35] for data with dmin ≤ 1.22 Å, using a starting unit cell model from the single-crystal structure determination (figure 1). Five initial scans were collected at a scan speed of 6° min−1 in the range −2.5 ≤ 2θ ≤ 12°, during which the capillary was spinning (note: a larger range of 2θ is accessed due to the angular range of the nine-channel detector) These patterns were merged and used to check the phase purity of the starting material, establishing by Pawley fitting that both compound 1-MeOH and 1 are present and that the sample contains a very small amount of Ag2CO3 starting material as an impurity (figure 2a) [36]. The gas chromatograms can be found in the electronic supplementary material, figures S15–S22
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