A three‐dimensional coordination polymer based on the Zn 4 (μ 3 ‐OH) 2 unit: poly[[(μ 4 ‐benzene‐1,4‐dicarboxylato)bis(μ 3 ‐benzene‐1,4‐dicarboxylato)bis(μ 3 ‐hydroxido)bis(1,10‐phenanthroline‐5,6‐dione)tetrazinc] dihydrate]

The title compound, {[Zn 4 (C 8 H 4 O 4 ) 3 (OH) 2 (C 12 H 6 N 2 O 2 ) 2 ]·2H 2 O} n , has been prepared hydrothermally by the reaction of Zn(NO 3 ) 2 ·6H 2 O with benzene‐1,4‐dicarboxylic acid (H 2 bdc) and 1,10‐phenanthroline‐5,6‐dione (pdon) in H 2 O. In the crystal structure, a tetranuclear Zn 4 (OH) 2 fragment is located on a crystallographic inversion centre which relates two subunits, each containing a [ZnN 2 O 4 ] octahedron and a [ZnO 4 ] tetrahedron bridged by a μ 3 ‐OH group. The pdon ligand chelates to zinc through its two N atoms to form part of the [ZnN 2 O 4 ] octahedron. The two crystallographically independent bdc 2− ligands are fully deprotonated and adopt μ 3 ‐κ O :κ O ′:κ O ′′ and μ 4 ‐κ O :κ O ′:κ O ′′:κ O ′′′ coordination modes, bridging three or four Zn II cations, respectively, from two Zn 4 (OH) 2 units. The Zn 4 (OH) 2 fragment connects six neighbouring tetranuclear units through four μ 3 ‐bdc 2− and two μ 4 ‐bdc 2− ligands, forming a three‐dimensional framework with uninodal 6‐connected α‐Po topology, in which the tetranuclear Zn 4 (OH) 2 units are considered as 6‐connected nodes and the bdc 2− ligands act as linkers. The uncoordinated water molecules are located on opposite sides of the Zn 4 (OH) 2 unit and are connected to it through hydrogen‐bonding interactions involving hydroxide and carboxylate groups. The structure is further stabilized by extensive π–π interactions between the pdon and μ 4 ‐bdc 2− ligands.