Hydroxy lanthanides

The ratio of the size of the metal ion and the radius of the internal cavity of the macrocyclic polyether determines the stoichiometry of these complexes. The stoichiometry of these complexes also depends on the coordinating ability of the anion associated with the lanthanide. For example, 12-crown-4 ether forms a bis complex with lanthanide perchlorate in acetonitrile while a 1 1 complex is formed when lanthanide nitrate is used in the synthesis [66]. Unusual stoichiometries of M L are observed when L = 12 crown-4 ether and M is lanthanide trifluoroacetate [67]. In the case of 18-crown-6 ligand and neodymium nitrate a 4 3 stoichiometry has been observed for M L. The composition of the complex [68] has been found to be two units of [Nd(18-crown-6)(N03)]2+ and [Nd(NCh)<--)]3. A similar situation is encountered [69] when L = 2.2.2 cryptand and one has [Eu(N03)5-H20]2- anions and [Eu(2.2.2)N03]+ cations. It is important to note that traces of moisture can lead to polynuclear macrocyclic complexes containing hydroxy lanthanide ions. Thus it is imperative that the synthesis of macrocyclic complexes be performed under anhydrous conditions. 

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

An interesting novel coupling reaction of allenes with carbonyl compounds mediated by a lanthanide metal species was reported recently [80], The samarium(II) iodide-mediated reaction of various ketones or aldehydes 153 with methoxyallene (56) afforded exclusively y-addition products 4-hydroxy-l-enol ethers 154 in moderate to good yields with low cis/trans selectivity (Scheme 14.39). 

Benzene-induced shifts of methoxy protons have proved of value in the structural elucidation of various flavonoids (68JCS(C)2477). Additionally, information on the position of hydroxy groups follows from ASIS shown by the trimethylsilyl ethers of the flavonoids (72P409). Lanthanide shift reagents also contribute usefully to structural assignments, notably with biflavones (75JCS(P1)1563). 

Triply bridged dinuclear Fe(n) complexes of a bis-N-hydroxy-pyridinone [9.82a] and of bis(bipy) ligands [9.61, 9.82b] possess triple-helical features. A triple-helical arrangement has been assigned to dinuclear Fe(lll) complexes of tripodal ligands on the basis of NMR and circular dichroism (CD) data [9.82c]. The self-assembly of well-defined triple-helical dinuclear cobalt(ll) [9.83] and lanthanide(lll) [9.84] complexes has been achieved. 

The use of aqueous chiral lanthanide complexes in the determination of the enantiomeric purity of chiral a-hydroxy acids has also been assessed by H NMR [21], Large lanthanide induced shifts, chemical shift non-equivalence and an apparent absence of kinetic resolution in complex formation is observed upon addition of racemic lactate to [Yb.3a]3+ (Figure 1). The lactate CH3 resonances are clearly resolved for the... 

Catalytic ring opening of epoxides and aziridines was also observed (Eq. 27). The acetone cyanohydrine reaction provided j8-hydroxy nitrile and / -amino nitriles, with the lanthanide isopropoxides exhibiting a higher reactivity than Et3N [233]. 

The L1 complexes of the middle lanthanides Gd(III), Eu(III), and Tb(III) decompose less rapidly at pH 7.4, 37 °C than do the L1 complexes of La(III) or Lu(III) (14). The fit of the lanthanide ion into the macrocycle may be important here. Certainly, the macrocycle fit will vary for La3+ (116 pM) compared to Lu3+ (97.7 pM) (41). A recent study using luminescence measurements suggests a greater lability of the Eu(L1)3+ complex than previously reported (28). Detection of the Eu(DPTA)-complex produced upon addition of diethylenetriaminepentaacetic acid (DTPA) to Eu(L1)3+ indicates that the complex decomposes approximately 12% in 48 h at 37 °C, pH 7.4. It is noteworthy that solutions of Eu(L1)3+ contain two different species (28). One of them, possibly a hydroxy-bridged dimer, is present in greater amounts at high concentrations of Eu(L1)3+. 

Hydrated lanthanide salts may also be obtained by the addition of an excess of lanthanide oxide to a concentrated acid solution, heating at 80°C until the pH is between 5 and 6. The residual oxide is removed by filtration, and the filtrate is subjected to rotary evaporation. This procedure may lead to the presence of oxo and hydroxy species in solution. At present hydrated lanthanide salts of 99.9% purity are available commercially. Salts of the highest purity are generally used in spectroscopic and magnetic studies. The purity of lanthanide salts can be determined by complexometric titration with ethylenediamine tetraacetic acid [1]. 

In the process of lanthanide complex formation with the porphyrins, the ligand loses two protons and yields lanthanide hydroxy porphyrin or lanthanide porphyrin X, where X = C1, Br, NOJ, etc. Many lanthanide complexes with substituted porphyrins have been prepared by heating a mixture of porphyrin and the lanthanide salt in imidazole melt in the range 210-240°C. When the complex formation is complete the solvent (i.e.) imidazole is eliminated by either sublimation [81] or by dissolution of the mixture in benzene, followed by washing with water [82]. Further purification requires column chromatography. The starting material can be anhydrous lanthanide chloride or hydrated lanthanide acetylacetonate. After purification the final product tends to be a monohydroxy lanthanide porphyrin complex. 

Carboxylic and hydroxy carboxylic acids form complexes readily with lanthanides with high stability constants (Chapter 3) and they have been widely used in the ion exchange... 

The complexes formed between lanthanide isopropoxides and 2-hydroxy-1-naphthyli-dene-n-butylamine and 2-hydroxy- 1-naphthylideneaniline in benzene show the formation of different products with different molar ratios of the reactants. In these complexes the ligand is coordinated in the anionic form to the lanthanide [275], It has been pointed out earlier in the synthesis section that it is necessary to synthesize these complexes in anhydrous media because of the weaker donor capacity of the ligands. 

More intense bands in the 4f-4f region were observed with lanthanide complexes of mandelic, salicylic, thiosalicylic, furoic, and thiophenic acids than the acetate and haloacetate analogues [235]. Both increase in intensity and nephelauxetic effect have been observed. It seems plausible that the aromatic groups in the hydroxy acids such as benzene, thiophene or furan contribute significantly to the intensity of the hypersensitive bands of the complexes. It appears that pH has an important role in determining the stoichiometry of the complex formed. Only ML species is formed up to pH 4 and both ML2+ and ML2 are formed at a pH of 6.0. 

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