Porous lanthanide coordination polymers built with a podand and its decomposition product oxalate: identification of discrete water clusters of different nuclearity

Abstract

A tripodal ligand, tris-(4-carboxy-2-phenoxy-ethyl-)amine [ptaH₃], bearing one carboxylate group at each terminal reacts hydrothermally with Dy(III), Er(III) and Ho(III) nitrates to afford porous co-ordination polymers, {[Dy(ptaH)(½ox) · H₂O]₅H₂O} _n , (1), {[Er(ptaH)(½ox) · H₂O]₅H₂O} _n , (2) and {[Ho₂(ptaH)(ox)₂ · H₂O]₄H₂O} _n (3) [ox=oxalate]. The oxalate group is formed as the decomposition product of ptaH3 under hydrothermal condition. The structure of 1 and 2 consists of an array of interlinked metallocycles with oval shaped cavity. Compound 3 however, forms a 2D sheet structure built with Ho(III) and oxalate and the podand units bind to this sheet to extend the porous structure in the third dimension. Supramolecularly assembled water clusters with different shapes and sizes occupy the voids in the MOFs. In 1 and 2, discrete (H₂O)₁₂ clusters as open-cube octamers buttressed on two sides by dimers have been identified. In 3, both (H₂O)₄ and (H₂O)₆ clusters could be found. All the three compounds are characterized by X-ray crystallography.