Hydrolysis lanthanide oxide/hydroxide complexes

In very acidic solutions, bismuth(III) exists in the form of the nonaaquo ion [Bi(H20)9] +, which is similar to the aquo complexes of the lanthanide ions, but partial hydrolysis of bismuth(III) salts leads to the formation of bismuth oxo clusters. The core structure of these complexes is often based upon a Bie octahedral core with oxide, hydroxide, or alkoxide functions bridging the edges and/or faces of the octahedron. The [Bi6(OH)i2] + ion (11) has been studied spectroscopically. In oxo clusters, the octahedron is face-bridged by eight oxo or alkoxide functions (12). Such core structures have been found in the hydrolysis of bismuth nitrate or perchlorate. ... 

Out of the concern of lanthanide hydrolysis to produce intractable precipitates of lanthanide oxides and/or hydroxides, the coordination of the lanthanide ions with amino acids has historically been carried out under rather acidic conditions. A large number of such complexes have been obtained under low-pH (1-4) conditions and structurally characterized. Following a summary of the unique features of lanthanide coordination chemistry and the structural and functional characteristics of amino acids, the coordination modes of the amino acid ligands and the salient stmctural features of the complexes prepared under acidic conditions are discussed. 

The Ce + ion is one of the most active catalysts for peptide hydrolysis. Its activity is much higher than that of the trivalent lanthanide ions and other transition metal ions. In particular, Ce + is far superior to other tetravalent ions like Zr" or Hf +. Yashiro et al. (1994) reported that dipeptides and tripeptides were efficiently hydrolyzed under neutral conditions by the y-cyclodextrin complex of cerium(IV). Komiyama and coworkers (Takarada et al., 2000) studied the catalytic hydrolysis of oligopeptides by cerium(IV) salts. The hydrolysis is fast, especially when the oligopeptides contain no metal-coordinating side-chains. The hydrolysis rates of the dipeptides, tripeptides and tetrapeptides is similar. The hydrolysis reaction was performed at pH 7 and 50 °C and under these conditions, the half-life of the amide bond was only a few hours. The authors found that ammonium hexanitratocerate(IV) is more active than other cerium(IV) compounds like ammonium cerium(IV) sulfate, cerium(IV) sulfate and cerium(IV) hydroxide. The lower reactivity of ammonium cerium(IV) sulfate is ascribed to the competitive inhibition by sulfate ions, while the low reactivity of cerium(IV) sulfate and cerium(IV) hydroxide can be explained by their poor solubility in water. However, in the reaction mixtures at the given reaction conditions, most of the cerium(IV) consists in a gel of cerium(IV) hydroxides. No oxidative cleavage has been observed. 

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