Lanthanide hydrolysis

It should be noted that most structurally characterized dinuclear complexes are products of adventitious lanthanide hydrolysis, and are not reproducible. With the ligand-controlled hydrolytic approach, they can now be rationally synthesized starting from a mononuclear aqua complex. Shown in Figure 6.30 is the synthesis of [(EDTA)Er (ti-OH)2Er(EDTA)] (EDTA=ethylenediaminetetraacetate) the dinuclear core doubly bridged by the fi-OH groups is encapsulated by two hexadentate EDTA ligands, one on each metal atom [69],... 

The coordination of lanthanide elements with amino acids has been studied extensively, and almost all studies prior to the late 1990s have been conducted at low pH. This practice probably originated from the willingness to avoid lanthanide hydrolysis that gives generally intractable... 

Finally, little systematic study has been devoted to lanthanide hydrolysis. Speci-ation in these systems is uncertain, reflecting diflSculties in analysis of the types of measurements normally used. The development of techniques with better detection limits, such as fluorescence and photoacoustic spectroscopy, as well as studies at tracer levels (below concentrations for precipitation and polymerization) should result in more definitive data on the nature of the hydrolytic species present in solution. 

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 synthetic approach is schematically shown in Figure 7. The essence of this methodology is to limit the extent of lanthanide hydrolysis by using amino acid ligands (not shown) to pre-occupy part of the metal s coordination... 

Mohapatra, P.K. and Khopkar, P.K. (1989) Hydrolysis of actinides and lanthanides hydrolysis of some trivalent actinide and lanthanide ions studied by extraction with thenoyltrifluoroacetone. Polyhedron, 16, 2071-2076. 

Thorough compilations of lanthanide hydrolysis data are given by Rizkalla and Choppin (1991), and Baes and Mesmer (1976). Lanthanide hydrolysis constants are potentially the most controversial constants used in inorganic lanthanide speciation calculations. The hydrolysis constant estimates shown in table 6 are based on the assessment by Lee and Byrne (1992) and were derived from the recommended constants of Baes and Mesmer (1976). The hydrolysis constants of Lee and Byrne (1992) at 25°C and zero ionic strength are larger than the log)3° estimates of Millero (1992) by about 0.2 units, and are in... 

Hydroxides. Thorium (TV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides, eg, U, Np, and Pu. Many of the thorium(IV) hydrolysis studies indicate stepwise hydrolysis to yield monomeric products of formula Th(OH) , where n is integral between 1 and 4, in addition to a number of polymeric species (40—43). More recent potentiometric titration studies indicate that only two of the monomeric species, Th(OH) " and thorium hydroxide [13825-36-0], Th(OH)4, are important in dilute (<10 M Th) solutions (43). However, in a Th02 [1314-20-1] solubiUty study, the best fit to the experimental data required inclusion of the species. Th(OH) 2 (44). In more concentrated (>10 Af) solutions, polynuclear species have been shown to exist. Eor example, a more recent model includes the dimers Th2(OH) " 2 the tetramers Th4(OH) " g and Th4(OH) 2 two hexamers, Th2(OH) " 4 and Th2(OH) " 2 (43). 

The coordination chemistry of the large, electropositive Ln ions is complicated, especially in solution, by ill-defined stereochemistries and uncertain coordination numbers. This is well illustrated by the aquo ions themselves.These are known for all the lanthanides, providing the solutions are moderately acidic to prevent hydrolysis, with hydration numbers probably about 8 or 9 but with reported values depending on the methods used to measure them. It is likely that the primary hydration number decreases as the cationic radius falls across the series. However, confusion arises because the polarization of the H2O molecules attached directly to the cation facilitates hydrogen bonding to other H2O molecules. As this tendency will be the greater, the smaller the cation, it is quite reasonable that the secondary hydration number increases across the series. 

Hydrothermal hydrolysis of metal ions is useful in producing crystalline phases which contain metals in a state of partial hydrolysis, i.e., a state intermediate between that of the hydrated metal ion and that of the hydrous hydroxide. Such reactions have been used to produce numerous crystalline phases of actinides (1-4), Group IV metal ions (5-14) and lanthanides (15-21). 

Previous studies of the hydrothermal hydrolysis of tetravalent Th, U and Np (1-4) have shown a remarkable similarity in the behavior of these elements. In each case compounds of stoichiometry M(0H)2S0i, represent the major product. In order to extend our knowledge of the hydrolytic behavior of the actinides and to elucidate similarities and differences among this group of elements, we have investigated the behavior of tetravalent plutonium under similar conditions. The relationships between the major product of the hydrothermal hydrolysis of Pu(IV), Pu2(OH)2(SO.,)3 (H20) t, (I)> and other tetravalent actinide, lanthanide and Group IVB hydroxysulfates are the subject of this re-... 

Group IVB, actinide, and lanthanide, hydrothermal hydrolysis spectroscopic studies. 58-62... 

Group IVB, actinide and lanthanide hydrothermal hydrolysis (continued)... 

The simple hydrocarbon substrates included ethene, 1,2-propa-diene, propene and cyclopropane (22). Their reactivity with Sm, Yb and Er was surveyed. In contrast to the reactions discussed above, lanthanide metal vapor reactions with these smaller hydrocarbons did not provide soluble products (with the exception of the erbium propene product, Er(C H ) ). Information on reaction pathways had to be obtained primarily by analyzing the products of hydrolysis of the metal vapor reaction product. 

In a different approach three different structurally defined aza-crown ethers were treated with 10 different metal salts in a spatially addressable format in a 96-well microtiter plate, producing 40 catalysts, which were tested in the hydrolysis of /xnitrophenol esters.32 A plate reader was used to assess catalyst activity. A cobalt complex turned out to be the best catalyst. Higher diversity is potentially possible, but this would require an efficient synthetic strategy. This research was extended to include lanthanide-based catalysts in the hydrolysis of phospho-esters of DNA.33... 

When not complexed, lanthanide ions have a high affinity for bone in vivo because they act as calcium ion mimics. Because the lanthanides undergo hydrolysis above a pH of 4, they readily form radiocolloids when not complexed, and are then taken up by the liver. This bone and liver uptake results in non-specific radiation doses to non-target (normal) tissues and organs and is undesirable.91 The polyaminocarboxylate class of ligands are considered to be the optimal choice for the basis of BFCAs for the+3 metal cations, including the lanthanides. It is essential that the... 

If the excess of lanthanide is sufficiently great, overloading of the transport system occurs and colloidal aggregates of large size are formed by hydrolysis. The interstitial or intracavitary formation of immobilized lanthanide colloids labeled with relatively short-lived radioisotopes was the basis for the attempted use of radioactive lanthanides as internal sources of therapeutic radiation (Kyker, 1962a, 1962b). 

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