Hydrothermal preparation and characterization of a rare example of sodium tetracarboxylate polymer with bridging water

A 3D coordination polymer, Na₂(μ₈-btaH₂)(μ₂-H₂O)_2n (1) (btaH₄ = benzene-1,2,4,5-tetracarboxylic acid) was hydrothermally synthesized and characterized by elemental analysis and X­ray diffraction studies. Compound 1 crystallizes in the triclinic P–1 space group. The Na⁺ ions are linked together with μ₈-btaH₂²⁻ and μ₂-H₂O bridging ligands. The btaH₂²⁻ units are bound to eight Na⁺ ions. The Na atoms are seven-coordinate with three oxygen atoms of three individual btaH₂²⁻ ions, two oxygen atoms form one bidentate btaH₂²⁻ ligand, and two oxygen atoms from bridging water molecules. An interesting intramolecular hydrogen bond is present between protonated and deprotonated carboxylate groups (O4–H···O2). Its length is 2.3991(17) Å with 172(2)° angle and the proton is located near the center of the bond (O4–H 1.11(3) Å and H···O 1.29(3) Å).