Coordination polymer flexibility leads to polymorphism and enables a crystalline solid-vapour reaction : a multi-technique mechanistic study

Vitõrica-Yrezábal, Iñigo J. and Libri, Stefano and Loader, Jason R. and Mínguez Espallargas, Guillermo and Hippler, Michael and Fletcher, Ashleigh J. and Thompson, Stephen P. and Warren, John E. and Musumeci, Daniele and Ward, Michael D. and Brammer, Lee (2015) Coordination polymer flexibility leads to polymorphism and enables a crystalline solid-vapour reaction : a multi-technique mechanistic study. Chemistry - A European Journal, 21 (24). pp. 8799-8811. ISSN 0947-6539

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    Abstract

    Despite an absence of conventional porosity, the 1D coordination polymer [Ag4(O2C(CF2)2CF3)4(TMP)3] ( 1 ; TMP=tetramethylpyrazine) can absorb small alcohols from the vapour phase, which insert into AgO bonds to yield coordination polymers [Ag4(O2C(CF2)2CF3)4(TMP)3(ROH)2] ( 1-ROH ; R=Me, Et, iPr). The reactions are reversible single-crystal-to-single-crystal transformations. Vapour-solid equilibria have been examined by gas-phase IR spectroscopy (K=5.68(9)×10−5 (MeOH), 9.5(3)×10−6 (EtOH), 6.14(5)×10−5 (iPrOH) at 295 K, 1 bar). Thermal analyses (TGA, DSC) have enabled quantitative comparison of two-step reactions 1-ROH 1 2 , in which 2 is the 2D coordination polymer [Ag4(O2C(CF2)2CF3)4(TMP)2] formed by loss of TMP ligands exclusively from singly-bridging sites. Four polymorphic forms of 1 ( 1-ALT , 1-AHT , 1-BLT and 1-BHT ; HT=high temperature, LT=low temperature) have been identified crystallographically. In situ powder X-ray diffraction (PXRD) studies of the 1-ROH 1 2 transformations indicate the role of the HT polymorphs in these reactions. The structural relationship between polymorphs, involving changes in conformation of perfluoroalkyl chains and a change in orientation of entire polymers (A versus B forms), suggests a mechanism for the observed reactions and a pathway for guest transport within the fluorous layers. Consistent with this pathway, optical microscopy and AFM studies on single crystals of 1-MeOH / 1-AHT show that cracks parallel to the layers of interdigitated perfluoroalkyl chains develop during the MeOH release/uptake process.