Metal carboxylated groups, stability

It should he noted that the complication of the spatial organization of metal carboxylated groups leads to an enhancement of their thermal stability. The increasing of the portion of metal carboxylated groups in a polymer chain reveals the same effect. It was found that the end and intrachain metal carboxylic groups of metallopolymer and copolymer formed during thermal transformations also differ in their thermal stability. 

These monomers provide a means for introducing carboxyl groups into copolymers. In copolymers these acids can improve adhesion properties, improve freeze-thaw and mechanical stability of polymer dispersions, provide stability in alkalies (including ammonia), increase resistance to attack by oils, and provide reactive centers for cross-linking by divalent metal ions, diamines, or epoxides. 

The Reformatsky reaction is a classical reaction in which metallic zinc, an a-haloester, and a carbonyl compound react to give a (i-hydroxyester.162 The zinc and a-haloester react to form an organozinc reagent. Because the carboxylate group can stabilize the carbanionic center, the product is essentially the zinc enolate of the dehalogenated ester.163 The enolate effects nucleophilic attack on the carbonyl group. 

Addition of benzene to a solution of / -CD8 causes a two-fold enhancement of Eu3+ luminescence intensity. The stability constant of the complex between / -CD8 and benzene ( 200) is comparable to that of the complex with the native /i-CD. In contrast, benzoic and naphthoic acids show much stronger associations with / -CD8 than with native / -CD because association is assisted by the interaction between the carboxylic groups and the metal ion. Moreover, the enhancement factor of the luminescence is larger. 

Transition metal ions cause a dramatic increase in the rate of hydrolysis of /Madam antibiotics [75][133][134], For example, copper(II) and zinc(II) ions increase the rate of alkaline hydrolysis ca. 108-fold and 104-fold, respectively [76], It has been suggested that the metal ion coordinates with both the carboxylate group and the /3-lactam N-atom of penicillins (A, Fig. 5.20). This complex stabilizes the tetrahedral intermediate and, thus, facilitates cleavage of the C-N bond catalyzed by the HO ion [74] [75], Such a model appears applicable also to clavulanic acid, imipenem, and monobactams, but it re-... 

Fig. 5.20. Modes of coordination of transition metal ions with /3-lactam antibiotics. Complex A In penicillins, the metal ion coordinates with the carboxylate group and the /3-lactam N-atom. This complex stabilizes the tetrahedral intermediate and facilitates the attack of HO-ions from the bulk solution. Complex B In benzylpenicillin Cu11 binds to the deprotonated N-atom of the amide side chain. The hydrolysis involves an intramolecular attack by a Cu-coordinated HO- species on the carbonyl group. Complex C In cephalosporins, coordination of the metal ion is by the carbonyl O-atom and the carboxylate group. Because the transition state is less stabilized than in A, the acceleration factor of metal ions for the hydrolysis of cephalosporins is lower than for penicillins. Complex D /3-Lactams with a basic side chain bind the metal ion to the carbonyl and the amino group in their side chain. This binding mode does not stabilize the tetrahedral transition complex and, therefore, does not affect the rate of... 

Catalytic decarboxylation processes occur in aliphatic keto acids in which the keto group is in an a-position to one carboxyl group and in a P-relationship to another. Thus, the normal decarboxylation of a p-keto acid is facilitated by metal coordination to the a-keto acid moiety. The most-studied example is oxaloacetic acid and it has been shown that its decarboxylation is catalyzed by many metals following the general order Ca2+ < Mn2+ < Co2+ < Zn2+ < Ni2+ < Cu2+ < Fe3+ < Al3"1".66 67 The overall rate constants can be correlated with the stability constants of 1 1 complexes of oxalic acid rather than oxaloacetic acid, as the uncoordinated carboxylate anion is essential for the decarboxylation. The generally accepted mechanism is shown in Scheme 15. Catalysis can be increased by the introduction of x-bonding ligands, which not only increase the... 

Both monensin (24) and nigericin (25) complex Na+ and K+ strongly but not selectively. The crystal structures of the Na+, Tl+ and Ag+ complexes all show the metal ion to be in an O-rich cavity. The carboxylate group is not involved however.97 With the antibiotics (26), (27) and (28) the thermodynamic stabilities (Table 9) are greater for the divalent than for the monovalent metal ions.98 The conformations adopted in these complexes axe very solvent dependent, and the implication of these to the biological transportation of the cations has been discussed.99... 

Interactions of ofloxacin and levofloxacin with Al3+, Fe3+, and Mg2+ ions were investigated by capillary zone electrophoresis (07CHR489). A fluorescence enhancement phenomenon in the europium-ofloxacin-sodium dodecyl benzenesulfonate fluorescence system was observed when Gd3+ was added (07JFL105). A quantitative relation was found between the relative fluorescences of Zr, Mo, and V chelates of ofloxacin and the ionization of the carboxylic group and the calculated stability constants of the formed chelates (06MI34). Factors that significantly influence the stability of ofloxacin- and levofloxacin-metal (Al3+, Fe3+, Cu2+,... 

---