Coordination polymers and hydrogen-bonded assemblies of 2,2′-[2,5-bis-(carboxymethoxy)-1,4-phenylene]-diacetic acid with ammonium, lanthanum and zinc cations

We report the synthesis of the 2,2′-[2,5-bis-(carboxymethoxy)-1,4- phenylene]diacetic acid (TALH₄) ligand and the structures of its adducts with ammonium, namely diammonium 2,2′-[2,5-bis-(carboxymethyl)-1, 4-phenylenedi-oxy]diacetate, 2NH₄⁺·-C ₁₄H₁₂O₁₀^2-, (I), lanthanum, namely poly[[aquabis-[μ₄-2,2′-(2-carboxylatomethyl-5-carboxymethyl- 1,4-phenylenedioxy)diacetato]dilanthanum(III)] monohydrate], {[La ₂(C₁₄H₁₁O₁₀)₂(H ₂O)]·H₂O}_n, (II), and zinc cations, namely poly[[{μ₄-2,2′-[2,5-bis-(carboxymethyl)-1,4- phenylenedi-oxy]diacetato}-zinc(II)] trihydrate], {[Zn(C₁₄H ₁₂O₁₀)]·3H₂O}_n , (III), and poly[[diaqua(μ₂-4,4′-bipyrid-yl){μ₄-2, 2′-[2,5-bis-(carboxymethyl)-1,4-phenylenedioxy]diacetato}dizinc(II)] dihydrate], {[Zn₂(C₁₄H₁₀O₁₀)(C ₁₀H₈N₂)(H₂O)2]·2H ₂O}_n , (IV), the formation of all four being associated with deprotonation of TALH₄. Adduct (I) is a diammonium salt of TALH₂^2-, with the ions located on centres of crystallographic inversion. Its crystal structure reveals a three-dimensional hydrogen-bonded assembly of the component species. Reaction of TALH₄ with lanthanum trinitrate hexa-hydrate yielded a two-dimensional double-layer coordination polymer, (II), in which the La^III cations are nine-coordinate. With zinc dinitrate hexa-hydrate, TALH₄ forms 1:1 two-dimensional coordination polymers, in which every Zn^II cation is linked to four neighbouring TALH₂^2- anions and each unit of the organic ligand is coordinated to four different tetra-hedral Zn ^II cation connectors. The crystal structure of this compound accommodates molecules of disordered water at the inter-face between adjacent polymeric layers to give (III), and it has been determined with low precision. Another polymer assembly, (IV), was obtained when zinc dinitrate hexa-hydrate was reacted with TALH4 in the presence of an additional 4,4′-bipyridyl ligand. In the crystal structure of (IV), the bipyridyl and TAL^4- entities are located on two different inversion centres. The ternary coordination polymers form layered arrays with corrugated surfaces, with the Zn^II cation connectors revealing a tetra-hedral coordination environment. The two-dimensional polymers in (II)-(IV) are inter-connected with each other by hydrogen bonds involving the metal-coordinated and noncoordinated molecules of water. TALH₄ is doubly deprotonated, TALH ₂^2-, in (I) and (III), triply deprotonated, viz. TALH ^3-, in (II), and quadruply deprotonated, viz. TAL^4-, in (IV). This report provides the first structural characterization of TALH ₄ (in deprotonated form) and its various supra-molecular adducts. It also confirms the potential utility of this tetra-acid ligand in the formulation of coordination polymers with metal cations.