Chemical transformations of a crystalline coordination polymer: a multi-stage solid–vapour reaction manifold

Abstract

In its crystal structure the one-dimensional coordination polymer [Ag₄(O₂C(CF₂)₂CF₃)₄(TMP)₃]_n (1) (TMP = 2,3,5,6-tetramethylpyrazine) adopts a zig-zag arrangement in which pairs of silver(I) centres bridged by two fluorocarboxylate ligands are linked alternately via one or two neutral TMP ligands. This material can reversibly absorb/desorb small alcohols (ROH) in single-crystal-to-single-crystal transformations, despite the lack of porosity in the crystals, to yield a related material of formula [Ag₄(O₂C(CF₂)₂CF₃)₄(TMP)₃(ROH)₂]_n (1-ROH). The absorption process includes coordination of the alcohol to silver(I) centres and, in the process, insertion of the alcohol into one-quarter of the Ag–O bonds of coordination polymer. When in place, the alcohol molecule is also supported by formation of an O–H⋯O hydrogen bond to the now partially dissociated carboxylate group. The reverse process leading to desorption of the alcohol takes place upon mild heating to regenerate 1. Upon further heating, 1 can release molecules of TMP into the vapour phase resulting in a separate chemical and structural transformation to yield a two-dimensional layered material of composition [Ag₄(O₂C(CF₂)₂CF₃)₄(TMP)₂]_n (2). This new transformation occurs via dissociation of Ag–N bonds upon ligand release and formation of new Ag–O bonds. The whole series of transformations has been followed in situ by single-crystal and/or powder X-ray diffraction and studied by thermogravimetric analysis. As a mechanistic probe to explore transport within formally nonporous 1, gravimetric CO₂ gas sorption/desorption has been conducted. It is proposed that transport of small molecules occurs through the fluorous layers in the crystal.