Coordination polymers thermal stability

The drive for the synthesis and characterization of synthetic coordination polymers was catalyzed by work supported and conducted by the U.S. Air Force in a search for materials which exhibited high thermal stabilities. Attempts to prepare highly stable, tractable coordination polymers were disappointing. Typically, only oligomeric products were formed and the monomeric versions were often more stable than the polymeric versions. 

Bailar listed a number of principles that can be considered in designing coordination polymers as follows (1) Little flexibility is imparted by the metal ion or within its immediate environment thus, flexibility must arise from the organic moiety. Flexibility increases as the covalent nature of metal-ligand bond increases. (2) Metal ions only stabilize ligands in their immediate vicinity thus, the chelates should be strong and close to the metal ions. (3) Thermal, oxidative, and hydrolytic stabilities are not directly related polymers must be designed specifically for the properties desired. (4) Metal-ligand bonds have sufficient ionic character... 

Yu T, Zhou Y, Liu K, Zhao Y, Chen E, Wang E, Wang D (2009) Improving thermal stability of biodegradable aliphatic polycarbonate by metal ion coordination. Polym Degrad Stab 94 253-258... 

SchifTs Bases. Coordination polymers of Ag(I), Cu(II), Zn(II) and Ti(IV) exhibit improved thermal stability in the order Cu < Ag < Ti < Zn [100]. Replacement of a benzyl by a phenyl group as substituent at the amino group of the ligand also enhances the stability. 

The high thermal stability of zeolites and related micro-porous solids is one of their most attractive features. Whilst it Is clear that materials with organic components cannot withstand ultra-high temperatures, quite respectable compositional stability can be achieved. Thus the [Er(TMA)] polymer mentioned above shows no weight loss in its TGA curve before 550°C. However for porous solids another key issue is that of structural stability. Many open framework coordination polymers lose their crystalline structure upon mild heating, or even evacuation, through loss of guest molecules. 

Biological significance can sometimes arise in rather unexpected ways the thermal properties of chelate polymers of 2,6-diaminopimelic acid (dap 12) and 4,4 -diamino-3,3 -dicarboxybiphenyl (bbdc 13) with Zn11 have been compared241 with those of non-polymeric divalent metal chelates with amino acids. This confirms the expected enhancement of thermal stability when coordination polymerization occurs, these results possibly being relevant to the thermal stability of certain bacterial spores which contain dap. Zn11 complexes of dap are more thermally stable than those of bbdc, possibly because the latter chelate cannot pack as well, due to the intermolecular repulsions of the biphenyl groups. 

A coordination polymer such as (65) proves to have exemplary thermal and chemical stability and it has been used as the solid phase for separating a variety of aliphatic and heterocyclic bases. Retention volumes remained constant after many hours use at 100 °C.98... 

In view of the high thermal stability of monomeric chelation compounds, coordination polymers were expected to be promising for use at high temperatures. This has not proved to be the case. Thermal stabilities arc usually lower than for low molecular weight materials. In addition, if tlie polymerization goes beyond a few monomer units, the materials tend to become insoluble and infusible so they cannot be fabricated into useful items See Chelation Compounds. 

Horowitz discusses organometallic systems of high thermal stability. Coordination polymers of bis (8-hydroxy quinoline) with Mn, Co, Ni, Cu, and Zn show outstanding thermal stability in the sequence cited. 

When 8-hydroxyquinoline and derivatives of bis(8-hydroxy-quinoline) react with metal ions, coordination complexes and polymers are formed, respectively, which exhibit improved thermal stability. This paper reviews the reaction of first-row transition metal ions with such ligands and their effect on the stabilization of these organic molecules. For the polymers containing divalent Mn, Co, Ni, Cu, or Zn the decomposition temperature is related to the periodic properties of the metal as well as the composition of the ligand to which the metal is coordinated. Trivalent chromium produces a crosslinked polymer when it reacts with bis(8-hydroxy-5-quinolyl)methane, and the thermogram for this polymer is also reported. 

The work described here supports the view that the chemical combination of metal ions with organic molecules leads to coordination complexes and polymers with enhanced stability with respect to weight loss, thermal degradation, or oxidation. Bis(8-hydroxyquinoline) derivatives were used to prepare a series of coordination polymers containing first-row transition metals, and the thermal stabilities of the polymers were evaluated. The influence of the structure of the organic molecule and the role of the metal are discussed. 

Another case in which the thermal stability of the coordination polymer is reported to be lower than that of the monomeric coordination compound is worth mentioning. Marvel and Tarkoy (18) examined the thermal stability of the nonpolymeric Zn-salicylaldehyde-o-phenylenedi-amine compound (X), which they reported was stable when heated in air at 250°C., losing less than 5% by weight after 24 hours. The related Schiff base coordination polymer (XI) was less stable and decreased in weight by 12% after only 3 hours in air at 250°C. 

The stability sequence for these divalent metal coordination polymers was reported to be Fe > Ni > Zn > Co > Cu. Thus, it is evident that different thermal stability sequences will be developed when different experimental conditions and criteria are employed. 

Table II. Thermal Stabilities of Divalent First-Row Transition Metal Coordination Complexes and Polymers...

The coordination polymers of 8-hydroxyquinoline derivatives are not only stabilized with respect to temperature, but a preliminary study (24) has disclosed that these polymers are also highly resistant to thermal oxidation. When samples were heated at 190 °C. in a sealed system in an air atmosphere, very little oxidation occurred, even after several days,... 

Several functional properties can be expected from the materials of coordination polymers owing to their extended nature and high thermal stability and the existence of cavities and channels. Here we describe some of the properties that have been explored using above-described layered structures. 

This reaction seems to be specific for monomers containing amide groups (acrylamide or methacrylamide), but once these monomers are present in the electrolytic medium, other monomers, e. g., acrylonitrile, can be polymerized. The authors attribute the polymerization initiation to the electrogenerated metal ions only, but it is possible that even the perchlorate ion plays a role in the formation of the initiating species. The polyacrylamide thus obtained has an electrical conductivity 3 to 4 times higher than that of polymers obtained by the usual methods. This is due to the presence of metallic cations coordinatively bound in the polymer bulk. The presence of these cations increases the thermal stability of the polymer by 20—40%. 

For the selective separation of organic compounds, specially prepared metal salt adsorbents (e.g., copper salts) are used, after volatile components have been eluted from them [106—108]. Good separations on these columns were obtained for isomeric ketones for example, heptanone-2, heptanone-3 and heptanone-4 were separated. Delventhal et al. [109] proposed the use of metal-containing polymers for the separation of various amines and thiophene. The advantage of these stationary phases was stated to be their high chemical and thermal stability. Nawrocki et al. [156] studied the application of coordination polymers as adsorbents and LSPs in GC. 

Coordination polymers of Zn(II) derived from bisthiopicolinamides show the best thermal stabilities and can be heated at 300°C for 6 h without changed The Zn(II) derivatives of bissalicaldimines also show the best thermal stability. The often found superior thermal stability of Zn(II) derivatives arise from its single oxidation state. Transition metals can be oxidized to higher states, catalyzing the decomposition of the polymer chains. 

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