Water de/adsorption associated with single‐crystal‐to‐single‐crystal structural transformation of a series of two‐dimensional metal–organic frameworks, [M(bipy)(C4O4)(H2O)2]·3H2O (M = Mn (1), Fe (2), and Zn (3), and bipy=4,4′‐bipyridine)

Abstract

AbstractAn assembly of three metal coordination polymers (CPs), [M(bipy)(C4O4)(H2O)2]·3H2O (M = Mn (1), Fe (2), Zn (3), and bipy = 4,4′‐bipyridine, C4O42− (squarate) = dianion of H2C4O4 (squaric acid)), was synthesized and structurally characterized. Single‐crystal X‐ray structural determination reveals that compounds 1–3 are iso‐structural, in which the M(II) ions are six‐coordinate in a distorted octahedral geometry. C4O42− and bipy both act as bridging ligands with bis‐monodentate coordination mode connecting the M(II) ions to form a two‐dimensional (2D) layered metal–organic framework (MOF). Adjacent 2D layers are then arranged in parallel and interpenetrated manners to construct their three‐dimensional (3D) supramolecular architecture. Compounds 1, 2, and 3 undergo two‐step dehydration processes with the first and second weight losses of 14.1 and 8.6% for 1, of 12.1 and 7.5% for 2, and of 11.2 and 8.1% for 3, respectively, corresponding to the weight losses of the three guest water molecules and the two coordinated water molecules, and all exhibit reversible sponge‐like water de/adsorption properties during de/rehydration processes for guest water molecules as per cyclic thermogravimetric analysis (TGA). The single‐crystal‐to‐single‐crystal (SCSC) structural transformation during the reversible de/rehydration processes of three guest water molecules was identified and monitored using exhaustive single‐crystal and powder X‐ray diffraction measurements.