Thermally Induced Structural Transformation in a Two-Dimensional Conductive Metal-Organic Framework

A long-standing structural question in triphenylene-based conductive metal-organic frameworks is why closely related materials adopt either fully extended honeycomb lattices or intercalated structures containing alternating extended layers and discrete metal–ligand clusters. Ni(II)–HHTP and Co(II)–HHTP (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) have both been reported to have the latter “intercalated structure”, but here we discover that thermal activation and dehydration convert both frameworks to the fully extended structures. Focusing on Ni(II)–HHTP, we show that this structural transformation is reversible, with roomtemperature hydration restoring the intercalated structure. Variable-temperature synchrotron powder X-ray diffraction supports the intercalated-to-fully-extended structural transformation, while solid-state nuclear magnetic resonance spectroscopy, thermal analysis, and elemental analysis provide evidence for the accompanying dehydration. Rehydration of Ni(II)–HHTP restores the intercalated structure, establishing reversible structural switching through removal and reincorporation of water. The resulting change in interlayer architecture alters electrical conductivity, gas response and adsorption properties in Ni(II)–HHTP. Importantly, common sample pre-treatment and testing conditions can themselves change the type of framework structure, meaning that measured properties may not correspond to the initially assigned structure. Because thermal and hydration history determine structure and function, careful structural verification under measurement conditions is, therefore, essential for establishing reliable structure–property relationships. Ultimately our findings resolve a persistent structural ambiguity in this family of materials and allow access to new fully extended electrically conductive framework materials.