A 3D-Coordination Polymer Assembled from Copper Propionate Paddlewheels and Potassium Propionate 1D-Polymeric Rods Possessing a Temperature-Driven Single-Crystal-to-Single-Crystal Phase Transition

An unusual strategy for the design of coordination polymers based on the combination of large s cations and rigid d metal based secondary building units has been applied to the synthesis of the novel heterometallic potassium dicopper propionate [HKCu₂Prop₆] (Prop = propionate). The crystal structure of [HKCu₂Prop₆] is a dense 3D framework, which is constructed of alternating 0D [Cu₂Prop₄] SBUs and 1D-polymeric [K₂(Prop···H···Prop)₂]_∞ “guide rods”. Temperature-dependent X-ray diffraction experiments have revealed the existence of a reversible single-crystal-to-single-crystal phase transition at ca. 195–200 K, which is followed by the temperature-driven ordering/disordering of flexible ethyl groups, minor rearrangements of the [Cu₂Prop₄] SBUs, and small distortions of the overall supramolecular motif. The low-temperature phase exhibits a large superstructural unit cell, whereas the high-temperature phase possesses a half unit cell. This transition, being first order, is additionally preceded by preliminary structural changes in the temperature range 180–195 K. Studies of magnetic properties have shown that the magnetic exchange between copper atoms in [HKCu₂Prop₆] remains unchanged upon the structural transformation. Similar behavior has been also revealed for the previously reported copper propionate hydrate [Cu₂(H₂O)₂Prop₄].