Coordination polymers of 1,4-piperazinedipropionic acid: domination by flexibility, coordination, and/or configuration?

Abstract

Three coordination polymers based on a designed flexible ligand 1,4-piperazinedipropionic acid (H₂pdp), formulated as Cd₂(H₂O)₂(pdp)(NO₃)₂ (1), Zn₂(OH)₂(pdp) (2), and Cu(pdp)·3H₂O (3), have been successfully synthesized and characterized by both single-crystal X-ray diffraction and spectroscopic methods. Compound 1 possesses a chain structure based on alternating dinuclear cadmium-carboxylate clusters and pdp²⁻ ligands, compound 2 has a layered structure constructed from linking one-dimensional chains by pdp²⁻ ligands, compound 3 features a chain-like structure of mono copper centers joined by pdp²⁻ ligands. Unlike in compounds 1 and 2, the piperazine rings of the pdp²⁻ ligands in compound 3 adopt the thermodynamically less stable boat configurations rather than the stable chair configurations. It is also of note that, in the structure of compound 3, an interesting water tape with T5(2) pattern is formed.