A novel three-dimensional hetero­metallic coordination polymer: poly[[hexa­aqua­bis[μ3-3,5-dicarboxyl­ato­pyrazol­ato-κ5O3,N2:N1,O5:O5′](μ2-oxalato-κ4O1,O2:O1′,O2′)copper(II)dierbium(III)] trihydrate]

The title novel heterometallic 3d–4f coord­ination polymer, {[CuEr2(C5HN2O4)2(C2O4)(H2O)6]·3H2O}n, has a three-dim­ensional metal–organic framework composed of two types of metal atoms (one CuII and two ErIII) and two types of bridging anionic ligands [3,5-dicarboxyl­ato­pyrazol­ate(3−) (ptc3−) and oxalate]. The CuII atom is four-coord­inated in a square geometry. The ErIII atoms are both eight-coordinated, but the geometries at the two atoms appear different, viz. triangular dodeca­hedral and bicapped trigonal prismatic. One of the oxalate anions is located on a twofold axis and the other lies about an inversion centre. Both oxalate anions act as bis-bidentate ligands bridging the latter type of Er atoms in parallel zigzag chains. The pdc3− anions act as quin­quedentate ligands not only chelating the CuII and the triangular dodeca­hedral ErIII centres in a bis­-bidentate bridg­ing mode, but also connecting to ErIII centres of both types in a monodentate bridging mode. Thus, a three-dim­ensional metal–organic framework is generated, and hydrogen bonds link the metal–organic framework with the uncoordinated water mol­ecules. This study describes the first example of a three-dimensional 3d–4f coordination polymer based on pyrazole-3,5-dicarboxylate and oxalate, and therefore demonstrates further the usefulness of pyrazoledicarboxylate as a versatile multidentate ligand for constructing heterometallic 3d–4f coordination polymers with interesting architectures.