Stepwise Topotactic Transformations (1D to 3D) in Copper Carboxyphosphonate Materials: Structural Correlations

In this report, we demonstrate a topotactic transformation of a [Cu(HPAA)(H2O)2]·H2O one-dimensional (1D) hybrid (HPAA = HO3PCH(OH)CO2, hydroxyphosphonoacetate dianion) to an anhydrous Cu(HPAA) three-dimensional (3D) framework, via the 1D intermediate Cu(HPAA)(H2O)2. Removal of lattice water from 1D [Cu(HPAA)(H2O)2]·H2O yields 1D Cu(HPAA)(H2O)2 and results in alterations in hydrogen bonding interactions (chain···H2O···chain), and, in turn, compaction of the chains as well as “slipping”. These changes are accompanied by creation of a new hydrogen bonding scheme (chain···chain) involving the phosphonate groups and the hydroxyl/carboxy moieties. The initial zigzag 1D chain is retained throughout the dehydration process. In 3D Cu(HPAA) the resulting vacant sites on Cu (generated by removal of both Cu-bound waters) are occupied by phosphonate oxygens from neighboring chains. All three materials have been characterized by X-ray diffraction and a variety of other techniques and their structures have been determined.