Arene guest selectivity and pore flexibility in a metal-organic framework with semi-fluorinated channel walls

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'.