Hydrothermal generation, structural versatility and properties of metal(ii)-organic architectures driven by a pyridine-tricarboxylic acid

Abstract

An aromatic tricarboxylic acid, 4-(6-carboxy-pyridin-3-yl)-isophthalic acid (H₃cpia), was applied as a building block for the hydrothermal syntheses of a new series of twelve metal(II) (M = Mn, Co, Ni, Zn, Cd, Pb) coordination compounds, namely [Mn(H₂cpia)₂(H₂O)₂] (1), [M(Hcpia)(phen)(H₂O)₂] (M = Co, 2; Ni, 3; Zn, 4), [Zn(Hcpia)(2,2′-bipy)(H₂O)₂]₅·4H₂O (5), [Zn₂(μ-Hcpia)₂(2,2′-bipy)₂] (6), [M(μ-Hcpia)(phen)(H₂O)]_n (M = Co, 7; Cd, 8), {[Pb(μ₄-Hcpia)]·2H₂O}_n (9), [Cd₄(μ₃-cpia)₂Cl₂(phen)₆(H₂O)₂]·10H₂O (10), {[Zn₃(μ₃-cpia)₂(phen)₃]·10H₂O}_n (11), and {[Zn₄(μ₄-cpia)₂(μ-OH)₂(μ-4,4′-bipy)₂]·4,4′-bipy·2H₂O}_n (12). These products were assembled from aqueous mixtures containing metal(II) chlorides, H₃cpia as a main tricarboxylic acid block, sodium hydroxide and an optional N-donor crystallization mediator (i.e., 1,10-phenanthroline, phen; 2,2′-bipyridine, 2,2′-bipy; or 4,4′-bipyridine, 4,4′-bipy). Compounds 1–12 were fully characterized by standard solid-state methods (IR spectroscopy, elemental analysis, TGA, PXRD, and single-crystal X-ray diffraction). Their structures range from discrete 0D monomers (1–5), dimer (6) or tetramer (10) to 1D coordination polymers (7, 8, and 11) and 2D metal–organic layers (9 and 12). Structural and topological features of H-bonded and metal–organic architectures were highlighted, showing that the structural diversity of 1–12 is influenced by the type of metal(II) node, the level of deprotonation of H₃cpia, reaction temperature, and presence of crystallization mediator. Thermal behavior, magnetic, luminescent and photocatalytic properties of selected compounds were investigated and discussed. In fact, cobalt(II) coordination polymer 7 acts as a stable and recycable photocatalyst for the oxidative UV-light-assisted degradation of an organic dye in aqueous medium; methylene blue was used as a model dye pollutant in waste water. Finally, the obtained products 2 and 4–12 represent the first examples of Co, Zn, Cd, and Pb coordination compounds derived from H₃cpia, thus introducing its application as a multifunctional picolinate-isophthalate building block for the generation of metal–organic architectures.