Lanthanide Hydroxide Complexes

The central point in this consideration is the Ln-OH moiety, the preferred formation of which is considered to be a dilemma in organolanthanide chemistry. Organolanthanide compounds containing Ln-X a-bonds such as alkyls, amides and alkoxides readily hydrolyze when exposed to moist air, with the formation of the hydroxides. Lanthanide complexes with Ln-C linkages are considered to be oversensitive compounds [89]. Even ligands with lower pKa values than water, as exemplified by substituted phenol ligands, tend to hydrolyze in organic solvents because the insoluble hydroxides formed act as... 

The most thermodynamically stable and kinetically inert complexes of the trivalent lanthanides are those of the ligand DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) (42, 43). Our search for lanthanide macrocyclic complexes that would remain intact for longer time periods led us to examine derivatives of DOTA. There are two potential difficulties with the use of DOTA complexes of the trivalent lanthanides for RNA cleavage. First, the overall negative charge on the complex is not conducive to anion binding for example, Gd(DOTA)-does not bind hydroxide well (44). Second, DOTA complexes of the middle lanthanides Eu(III) and Gd(III) have only one available coordination site for catalysis. The previous lanthanide complexes that we used, e.g., Eu(L1)3+, were good catalysts and had at least two available coordination sites. 

The complexes may also be prepared by the addition of a solution of carboxylic ligand to an equivalent amount of (i) a lanthanide carbonate [28], (ii) hydroxide [29] or (iii) oxide [30] with a slight excess of the latter. The insoluble part is filtered and the filtrate evaporated to obtain crystalline complex. Anhydrous lanthanide complexes of small chain carboxylic acids may be prepared by (i) the dissolution of lanthanide carbonate in excess of the carboxylic acid, followed by heating to obtain complete dissolution of the suspension and partial evaporation of the solution to obtain the crystals [31], (ii) anhydrous lanthanide is converted into the corresponding monochloroacetate by the addition of an excess of monochloroacetic acid, followed by heating under reflux at reduced pressure for 2 h. Then ether is added to precipitate the salt [32], (iii) the addition of dimethyl formamide and benzene to lanthanide acetates and distillation of the water azeotropes to obtain anhydrous complexes. The last procedure yielded lighter lanthanide complexes solvated with dimethyl formamide [33], The DMF may be removed by heating in a vacuum at 120°C. 

Lanthanide complexes of amines are prepared in an anhydrous medium to avoid precipitation of lanthanide hydroxides. In one of the procedures, a solution of ethylenediamine in acetonitrile is mixed with a solution of anhydrous lanthanide nitrate or perchlorate [13, 62,63]. 

In aqueous solutions it is needless to state that any complexing ion or ligand has to compete with water molecules for coordination to the metal ion and also should not be so basic in nature that lanthanide hydroxide/hydroxo species are precipitated. With respect to non-aqueous solutions, the limitation is the solubility of ligands in organic solvents in the formation of lanthanide complexes in non-aqueous solutions. Some typical rare earth complexes in solution are given in Table 4.3. 

The impetus for the development of synthesis and characterization of complexes of lanthanides with organic nitrogen donor ligands is due to the search for more efficient luminescent rare earth compounds. One of the difficulties is the risk of precipitating lanthanide hydroxides in the process of synthesis of lanthanide complexes with organic amines. In the early stages, lanthanide complexes of heterocyclic bases of low basicities were prepared in aqueous alcoholic media [224], In the synthesis section it was appropriately pointed out the need for the anhydrous conditions and involved procedures for the preparation of lanthanide complexes of ligands of non-ionizable nature. Some representative complexes of both aliphatic and aromatic amines are listed in Table 4.19. 

The most successful synthetic approach to structurally well defined lanthanide hydroxide complexes is the ligand-controlled hydrolysis approach [69-71]. The essence of this methodology is schematically shown in Figure 6.25. It makes use of the high propensity of lanthanide ions toward hydrolysis, but controlled and limited by certain supporting ligands. The scheme starts with a lanthanide complex whose coordination sphere constitutes both organic and aqua... 

For the synthesis of a REE complex connection, it is a matter of principle to have a pH-medium since the type of a lanthanides complex formation depends on it, and so does the character of a complex formation in water and in the water-organic mediums. When carrying out reaction in water with REE salts it is required that pH of a reaction mass doesn t exceed value at which ytteibium hydroxide is formed. Value of size pH sedimentation of hydroxide of ytterbium and its control in the course of synthesis are necessary conditions when receiving complexes of ytterbium. 

Lanthanides Luminescence Applications Lanthanides in Living Systems Lanthanide Oxide/Hydroxide Complexes Lanthanides Coordination Chemistry Sustainability of Rare Earth Resources The Electronic Structure of the Lanthanides Variable Valency. 

General Comments about Lanthanide Oxide/Hydroxide Complexes 184... 

GENERAL COMMENTS ABOUT LANTHANIDE OXIDE/HYDROXIDE COMPLEXES... 

Rational Synthesis of Lanthanide Oxide/Hydroxide Complexes... 

Lanthanide Oxide/Hydroxide Complexes of Different Nuclearities... 

Figure 2 Hydrolysis of aqua lanthanide complexes via olation of the intermediate Ln-OH species, affording lanthanide hydroxide complexes...

Lanthanide oxide/hydroxide complexes featuring discrete and recognizable tetranuclear Ln O/OH units are most abundant among all reported lanthanide hydroxides. Three distinct core motifs, the distorted cubane, the rhombus, and the square, have been found, with the cubane-like structure being dominant. 

Lanthanide oxide/hydroxide complexes are a class of intriguing lanthanide-containing compounds, not just because... 

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