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 Cu^II and two Er^III) and two types of bridging anionic ligands 3,5‐dicarboxylatopyrazolate(3?) (ptc^3?) and oxalate]. The Cu^II atom is four‐coordinated in a square geometry. The Er^III 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 pdc^3? anions act as quinquedentate ligands not only chelating the Cu^II and the triangular dodecahedral Er^III centres in a bis‐bidentate bridging mode, but also connecting to Er^III 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.