Strong hydrogen bond in the crystal structure design of CuNbOF5·4H2O

Fluoride metal-organic frameworks (MOFs) based on pillared [NbOF5]2– anion have been recently received research attention as ultramicroporous materials for gas storage and separation. The trans-directing property of the NbOF5 octahedron plays a significant role in designing mixed metal oxyfluoride compounds composed of the alternating transition metal polyhedra, which are linked via oxide and fluoride ligands. Mixed metal CuNbOF5·4H2O is the only unique example when the trans-directing properties of [NbOF5]2– are realized in the formation of a strong O–H⋯F hydrogen bond (HB). Well-shaped single crystals of this compound were grown and their structure was refined by X-ray diffraction. Under quasi-isobaric conditions, a phase transition at 88 ​°C takes place, which is associated with the weakening of HBs and formation of ordinary chains through conventional trans-(O,F) bridges. The character of hydrogen bonding in CuNbOF5·4H2O affects its thermal behavior described by the dehydration and pyrohydrolysis processes. Single crystals of mixed metal CuNbOF5·4H2O, the structure which consists of alternating Cu and Nb octahedra joint by the F atoms into infinite bent chains. [Display omitted] •High quality single crystals of the unique mixed metal CuNbOF5·4H2O are grown.•Trans-coordinating nature of the [NbOF5]2– anion is realized in the strong O–H⋯F bond.•The alternation of Cu and Nb octahedra occurs through the F atoms, not via O/F bridges.•Unusual phase transition under quasi-isobaric conditions with trans-direction change.•Strong O–H⋯F hydrogen bond ensures pyrohydrolysis of the compound.