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Espallargas, Guillermo Mínguez
in Cooperation with on an Cooperation-Score of 37%
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article
Coordination polymer flexibility leads to polymorphism and enables a crystalline solid-vapour reaction
Abstract
<p>Despite an absence of conventional porosity, the 1D coordination polymer [Ag<sub>4</sub>(O<sub>2</sub>C(CF<sub>2</sub>)<sub>2</sub>CF<sub>3</sub>)<sub>4</sub>(TMP)<sub>3</sub>] (<b>1</b>; TMP=tetramethylpyrazine) can absorb small alcohols from the vapour phase, which insert into AgO bonds to yield coordination polymers [Ag<sub>4</sub>(O<sub>2</sub>C(CF<sub>2</sub>)<sub>2</sub>CF<sub>3</sub>)<sub>4</sub>(TMP)<sub>3</sub>(ROH)<sub>2</sub>] (<b>1-ROH</b>; R=Me, Et, <em>i</em>Pr).The reactions are reversible single-crystal-to-single-crystaltransformations. Vapour-solid equilibria have been examined by gas-phaseIR spectroscopy (<em>K</em>=5.68(9)×10<sup>−5</sup> (MeOH), 9.5(3)×10<sup>−6</sup> (EtOH), 6.14(5)×10<sup>−5</sup> (<em>i</em>PrOH) at 295 K, 1 bar). Thermal analyses (TGA, DSC) have enabled quantitative comparison of two-step reactions <b>1-ROH</b><b>1</b><b>2</b>, in which <b>2</b> is the 2D coordination polymer [Ag<sub>4</sub>(O<sub>2</sub>C(CF<sub>2</sub>)<sub>2</sub>CF<sub>3</sub>)<sub>4</sub>(TMP)<sub>2</sub>] formed by loss of TMP ligands exclusively from singly-bridging sites. Four polymorphic forms of <b>1</b> (<b>1-A<sup>LT</sup></b>, <b>1-A<sup>HT</sup></b>, <b>1-B<sup>LT</sup></b> and <b>1-B<sup>HT</sup></b>;HT=high temperature, LT=low temperature) have been identifiedcrystallographically. In situ powder X-ray diffraction (PXRD) studies ofthe <b>1-ROH</b><b>1</b><b>2</b>transformations indicate the role of the HT polymorphs in thesereactions. The structural relationship between polymorphs, involvingchanges in conformation of perfluoroalkyl chains and a change inorientation of entire polymers (A versus B forms), suggests a mechanismfor the observed reactions and a pathway for guest transport within thefluorous layers. Consistent with this pathway, optical microscopy andAFM studies on single crystals of <b>1-MeOH</b>/<b>1-A<sup>HT</sup></b> show that cracks parallel to the layers of interdigitated perfluoroalkyl chains develop during the MeOH release/uptake process.</p>