Hydroxycarboxylate, stability

Stability constants for calcium complexes of a selection of hydroxycarboxylate ligands are listed in Table VII (239,246,272-274). For tartrate, malate, and citrate stabilities decrease in the expected order Ca2+> Ba2+> Ra2+ (231,275). The stability constant for the complex of pyruvate (logiOifi 0.8 (273)) is similar to that for acetate calcium complexes of a-ketoglutarate and of oxaloacetate are somewhat more stable (logio-Ki = 1.3, 1.6 respectively (273)). The sequence logio-Ki = 3.0, 1.4, 1.1, 0.6 for the dicarboxylate ligands oxalate, malonate, succinate,... 

The enthalpies and entropies of formation of mono-mandelato-complexes have been determined and, in comparison with other hydroxycarboxylic acid complexes, the enthalpy order of stabilization is lactate > a-hydroxyiso-butyrate mandelate > glycolate, whereas the entropy order of stabilization is glycolate > a-hydroxyisobutyrate > mandelate > lactate. The stability constants and enthalpy of formation of mono- and di-malonate complexes have also been measured.The mono-1,1-cyclopentanedicarboxylato-complexes are less stable than the corresponding malonate species. 

Formation constants for the species [M(OAc) ]<2-")+ (M = Zn or Cd n = 1-3) have been determined in addition, zinc forms a tetraacetato complex.692,693 Stability constants for complexation of Zn2+ and Cd2+ by other carboxylic694,695 and hydroxycarboxylic adds696 699 have been reported. 

Portanova, R. Lajunen, L.H.J. Tolazzi, M. Piispanen, J. Critical evaluation of stability constants for alpha-hydroxycarboxylic acid complexes with protons and metal ions and the accompanying enthalpy changes. Part II. Aliphatic 2-hydroxycarboxylic acids. Pure Appl. Chem. 2003, 75 (4), 495-540. 

Analogously, Van Pelt et al. (2009) have realized a one-pot bienzymatic cascade combining the (5 )-selective oxynitrilase from M. esculenta and a purified nitrile hydratase from Nitriliruptor alkaliphilus for the synthesis of aliphatic S)-a-hydroxycarboxylic amides (Table 17.6) both enzymes were immobilized as CLEAs to enhance their stability (Van Pelt et al., 2009). 

Hydroxycarboxylic acids, which include citric acid, malic acid, lactic acid, etc., are benign to the environment and very convenient for the solution processing. Moreover, since these reagents can form stable complexes with other cations, they rarely yield a precipitate. For several complexes single crystals of well-defined composition suitable for the X-ray structural analysis were isolated. Thus, these water-soluble titanium complexes of hydroxycarboxylic acids are promising precursors for the synthesis of ceramics from an aqueous solution and their industrial utilization is expected in the future. In this chapter we decribe the method of synthesis, structural analysis, and stability of these complexes. The examples of multicomponent oxide materials preparation using these compounds are presented. 

The hydroxycarboxylates are known in a range of crystalline binary Mn hydrates,but the only ternary compounds are a SNOj-salicylate, which is probably Na2[MnL2(H20)2], and the Na and K tartrates M,MnC4H20eBoth the aliphatic and aromatic (only salicylates) hydroxycarboxylates form 1 1 and 1 2 species in aqueous solution, which are of relatively low stability, and much of the interest to date has been in their oxidations to Mn " and Mn —such oxidations being readily achieved in alkaline solutions, especially as the number of hydroxy groups on the ligand is increased. 

The same concept was used for sensing a-hydroxycarboxylic acids and diols using chiral boronate 56 (Fig. 17a) containing a displaceable fluorescent group [144]. Formation of the boronate resulted in an increased fluorescence of a couma-rin derived fluorophore, while displacement by the hydroxy acid restored the weaker emission of the free fluorophore. The enantioselective fluorescent response was sufficient for the determination of the enantiomeric excess of phenyllactic acid with a 0.13 (13%) maximum deviation from the actual values. A mathematical algorithm has recently been proposed for the calculation of both stability constants and enantiomeric excess in these indicator-displacement assays [145]. 

Quite stable complexes of arise from a-hydroxycarboxylic acids, even up to 5-M HsO". The stabilities of the 1 1 complexes are in the order Zr>Hf (as with the sulfates), and lactate > citrate > glycolate > malate > tartrate. Various 1 2, 1 3 and 1 4 complexes also are formed. 

Examination of Table 8.9 reveals that aminopolycarboxylic adds complex Am(iii) more strongly than do either hydroxycarboxylic or aminoalkylpolyphos-phoric adds (e.g. ethyIenediaminebis(methyIene)phosphonic add). Keller [3] observes that in the series of a-hydroxycarboxylic adds (e.g. glycolic and lactic) the stability of the ameridum complex decreases with increasing number of carbon atoms. The logarithm of the stability constant of the complexes of Am with aminopolycarboxylic adds increases linearly (Fig. 8.9) with the number of bound donor atoms of the ligand. 

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