Coordination polymers with pyridine-2,4,6-tricarboxylic acid and alkaline-earth/lanthanide/transition metals: synthesis and X-ray structures

Abstract

Pyridine-2,4,6-tricarboxylic acid (ptcH₃) reacts with Cd(II), Mn(II), Ni(II), Mg(II), Ca(II), Sr(II), Ba(II), Dy(III) salts forming different products depending on the reaction conditions. In the presence of pyridine at room temperature the acetate, chloride or nitrate salt of Cd(II) breaks the ligand to form an open framework structure with the empirical formula, {[Cd(Ox)(H₂O)₂]H₂O}_n (Ox = oxalate), 1. In the absence of pyridine, no crystalline compound could be isolated at room temperature (RT). However, under hydrothermal conditions and in the absence of pyridine, a discrete tetrameric complex with the formula, {[Cd₂(cda)₂(H₂O)₄](H₂O)₃}₂ (cdaH₂ = 4-hydroxypyridine-2,6-dicarboxylic acid), 2, is formed where the carboxylate group at the 4-position of the ligand is reduced to a hydroxyl group. When Ni(II), Mn(II), Mg(II), Ca(II), Sr(II), Ba(II), Dy(III) salts are used in place of Cd(II), no crystalline product could be isolated at RT. But under hydrothermal conditions, coordination polymers ({[Ni_1.5(ptc)(pip)_0.5(H₂O)₄]·H₂O}_n, (pip = piperazine), 3; {Mn_1.5(ptc)·2H₂O}_n, 4; {Mg₃(ptc)₂·8H₂O}_n, 5; {[Mg(ptc)(H₂O)₂]·1/2[Mg(H₂O)₆]·H₂O}_n, 6; {Ca_1.5(ptc)·2H₂O}_n, 7; {Sr_1.5(ptc)·5H₂O}_n, 8; {[Ba(ptc)(H₂O)][Ba(ptcH₂)H₂O]}_n, 9; {[Dy(ptc)·3H₂O]·H₂O}_n, 10) are formed. The structures exhibit different dimensionality depending on the nature of the metal ions. In 1 a discrete acyclic water hexamer is also identified. All the compounds are characterized in the solid state by X-ray crystallography, IR and elemental analysis.