2D-coordination polymers based on rare earth propionates of layered topology demonstrate polytypism and controllable single-crystal-to-single-crystal phase transitions

A series of rare-earth propionate based complexes [Ln(2)(H2O)(2)Prop(6)] (Ln = Y, Ho-Lu) were synthesized in the form of two polymorphs (alpha and beta), both being layered 2D-coordination polymers with identical geometries of the single layers. The polymorphs differ from each other in the layer arrangement, and therefore they are considered the first example of polytypes among aliphatic rare-earth carboxylates. Strategies toward the controllable crystallization of each polytype have been developed. The beta polytype undergoes an irreversible single-crystal-to-single-crystal (SCSC) phase transition to the alpha polytype upon cooling, whereas a can be artificially converted into beta via mechanical treatment. The performed DFT calculations reveal the alpha polytype to be more energetically favorable by 13 kJ per 1 mol of unit cells that is determined predominantly by the difference between energies of weak interlayer van der Waals interactions for the two polytypes. A low interlayer interaction energy makes it possible to use mechanical exfoliation to obtain a crystal surface of atomic smoothness suitable for mechanical decoration.