Thermal stability of coordination polymers

Abstract

SynopsisPolymers formed between three ligands and selected metal ions were prepared and their thermal stability in air was investigated. 1,5-Dihydroxynaphthalene-2,6-dicarboxylic acid (1,5-N-2,6) formed coordination polymers with Zn, Ni, Al, and Fe+*. Zn-1,5-N-2,6 was more stable than the corresponding Fe and Ni polymers. Pyromellitic acid (PMA) formed coordination polymers of high molecular weight with Th and U+4. The Th-PMA was stable up to 405"C., while the corresponding uranium compound decomposed below 400°C. The coordination polymer of 2,3,6,7-naphthalenetetracarboxylic acid (NT) with Th was less stable (decomposition temperature: 36OOC.) than the corresponding pyromellitic polymer because the coordination of Th in Th-NT was partially accomplished by water molecules rather than carboxylic oxygens as in the Th-PMA polymer. The thermal stability of these polymers was a function of the metal involved in coordination. Higher thermal stability resulted when Zn or T h were used than if metals capable of changing their ionic valency were involved in polymer formation.Prerequisites for the thermal stability of coordination polymers can be compiled from the published All the polymers which were found to be stable at 300°C. or higher met one or several of the following criteria: (1) use of thermally stable starting materials; (2) use of aromatic rather than aliphatic ligands; (3) use of ligands capable of forming five-, six-, or seven-membered rings with a metal, resulting in chelates of high thermodynamic stability; (4) the satisfaction of all ionic valencies of the metal by the ligand; (5) high molecular weight of the resulting polymer. Rarely, however, did any one of the published coordination polymers fulfill more than two or three of the above cited prerequisites.The role of the metal in coordination polymers has also been discussed extensively, and the hypothesis was advanced that use of metal ions with only one ionic valency results in formation of more stable coordination polymers than the use of metals which possess several oxidation ~t a t e s .~J This hypothesis, however, does not seem to be applicable to all ligand~.~.'It was the aim of this investigation to synthesize polymers which would fulfill all or most of the listed prerequisites, and in particular, to study the influence of the metal on the thermal stability of the resulting polymers. RESULTS AND DISCUSSIONThe presumed thermal stability of 1,5-dihydroxynaphthalene-2,6-dicarboxylic acid (1,5-N-2,6) (m.p. 300°C.) and its two a systems made this 3745

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