Anion lanthanide complexation

One of the more reactive and selective catalysts of this type involves a bifunctional catalyst containing an alkali metal cation and an anionic lanthanide complex resulting from addition of excess binolate with lanthanide halides. Such catalysts have been used in asymmetric nitroaidol (Henry) reactions of ketones. Heterobimetallic Li-La alkoxo complexes (Figure 4.15) catalyzed these reactions with particularly high enantioselectivity. ... 

At the simplest, an anionic lanthanide complex can interact with a cation to form a labile heterometallic complex in solution. Such complexes have been known for many years, though they have not always been recognised as such. For instance, Bryden et al. [14] showed that Na shifts were observed for sodium chloride solutions containing lanthanide complexes of DOTA. Fig. 9.2 shows the variation in observed Na chemical shift between... 

The separation of basic precipitates of hydrous Th02 from the lanthanides in monazite sands has been outlined in Fig. 30.1 (p. 1230). These precipitates may then be dissolved in nitric acid and the thorium extracted into tributyl phosphate, (Bu"0)3PO, diluted with kerosene. In the case of Canadian production, the uranium ores are leached with sulfuric acid and the anionic sulfato complex of U preferentially absorbed onto an anion exchange resin. The Th is separated from Fe, A1 and other metals in the liquor by solvent extraction. 

As a result of their low redox potentials [173], bis(phthalocyaninato) lanthanide complexes are often inadvertently reduced or oxidized, and they are also very sensitive to acids and bases. In order to solve these problems, Veciana et al. achieved certain success on designing a series of novel compounds with characteristics that would give them improved redox stability. Electroactive ligands based on phthalo-cyaninato tetra dicarboximide [175] or perfluorinated phthalocyanine [176] were used to assemble the double-decker lanthanide complexes, with the effect of stabilizing the negative charge of the anionic state of the compounds, which resulted in a strong shift of 0.7 V of their first oxidation potentials. 

Kretschmer et al. have described the formation of a lanthanide complex, [Cp6Yb6Cl13] (Cp = cydopentadienyl), which conforms to a truncated octahedron. [36] The anion contains six ytterbium ions, located at the corners of an octahedron, and 12 bridging choride ions. A single chloride ion occupies the center of the shell. 

Vicentini and Dunstan (227) have obtained tetrakis-DDPA complexes with lanthanide perchlorates in which the perchlorate groups are shown to be coordinated to the metal ion. DDPA also yields complexes with lanthanide isothiocyanates (228) and nitrates (229). All the anions in these complexes are coordinated. DPPM behaves more or less like DDPA which is reflected in the stoichiometry of the complexes of DPPM with lanthanide perchlorates (230), nitrates, and isothiocyanates (231). Hexakis-DMMP complexes of lanthanide perchlorates were recently reported by Mikulski et al. (210). One of the perchlorate groups is coordinated to the metal ion in the lighter lanthanide complexes, and in the heavier ones all the perchlorate groups are ionic. 

The variety of stoichiometries that one comes across in the case of lanthanide complexes is much more than what one usually encounters in the case of cf-block transition metal complexes. This is due to the wider range of coordination numbers that are possible for the lanthanides. The stoichiometry of a lanthanide complex depends on the size of the metal ion, the size of the ligand, the nature of the anion and the synthetic procedure used. 

Vibrational spectra, especially infrared spectra, have been obtained for a wide variety of lanthanide complexes. The main conclusions that emerge from infrared spectral studies concern the site of coordination of the ligand, the nature of anion coordination and the relative strength of metal-ligand interactions. 

Electric conductance measurements have been widely used in the study of lanthanide complexes to determine the nature of the anions in the complexes and hence to indicate the possible coordination number of the lanthanide ion. Water is a strong donor toward the lanthanides and is seldom used for the purpose of measuring electric conductance, since the complex is completely dissociated on dissolution in water. The complete dissociation of lanthanide complexes in water has been shown by molecular weight determinations in water as in the case of the complexes of DMSO (246,249, 250), PyO (147,157,158), and DMF (41, 43). Most useful data are obtained in non-aqueous solvents like nitromethane, acetonitrile, nitrobenzene, and acetone (317). 

As the size of the chelating ligand increases, a maximum in stability is normally obtained for 5 or 6 membered rings. For lanthanide complexes, oxalate forms a 5-membered ring and is more stable than the malonate complexes with 6-membered rings. In turn, the latter are more stable than the 7-membered chelate rings formed by succinate anions. 

Other techniques, such as C.D. spectral change, have been used to demonstrate the presence of octa coordination for lanthanide ion in a system containing Eu(FOD)3 and alcohols or ketones (28). However, the anionic tetrakis complexes e.g. Eu(acac)i, Eu(benzac)i, Eu(DBM)i, Eu(BTFA)4, tend to dissociate into the tris-complex and L in alcoholic solution. The degree of dissociation depends on the complex as well as the polarity of the medium. In alcohol-DMF medium the dissociation is enhanced compared to the alcoholic solutions (29). The end product of these dissociation reaction may well be an octacoordinated species. Fluorescence emission from the coordinated europium ion was also helpful in estabhshing the nature of the species in solution 29). 

Another well characterized anionic octahedral lanthanide complexes are the hexaisothiocyanato complexes, [M(NCS)6] (35). The conductivity measurements of the hexachlorides, [(C4H9)4N]3 [MCle] and the hexaisothiocyanates in nitrobenzene (Table 2) solution indicate [35,36) a 3 1 electrolyte behaviour of the complexes, showing that the species MX is not appreciably dissociated in solution of reasonably polar solvent. 

Complexes of calixarenes with bipyridyl chromophores can be stabilized by the addition of anionic side arms, such as iminodiacetate units (85). Whilst the lanthanide complexes of ligands [L51]4- and [L52]4- are not soluble in water, their photophysical properties in... 

Charbonniere, L. J. Ziessel, R. Montalti, M. Prodi, L. Zaccheroni, N. Boehme, C. Wipff, G. Luminescent lanthanide complexes of a bis-bipyridine-phosphine-oxide ligand as tools for anion detection. J. Am. Chem. Soc. 2002, 724(26), 7779-7788. 

Paramagnetic shifts may be induced in cations which associate with suitable lanthanide complexes. Thus in aqueous solution, [M(hpda)3]6, where M = Dy or Tm and H3hpda = 4-hydroxypyridine-2,6-dicarboxylic add, associates with Mg2-1, 39K+, 23Na+, 87Rb+ or 14NH4 and produces shifts of up to 25 p.p.m. which are most marked above pH 7.582 A similar effect is shown for cations (and anions and uncharged molecules) by lanthanide tetra-p-sulfonatophenyl-porphyrins in aqueous solutions.583 This is discussed in Section 39.2.6.3, as are the large shifts produced in aqueous solution by Na[Eu(dota)(H20)] (H4 dota = 23).584... 

The complexes Mnr(dtc)3 (Mni = Np, Pu) are obtained by treating the metal tribromide with Na(dtc) in anhydrous ethanol. Pu(dtc)3 is fairly stable to oxidation, but Np(dtc)3 and the even less stable U(dtc)3 are rapidly oxidized to M(dtc)4, so that neither can be isolated. However, the anionic complexes, (NEt4)[Mm(dtc)4] (M11 = Np, Pu), have been prepared and the geometry about the metal atom is a distorted dodecahedron, best regarded as a planar pentagon of five S atoms with one S atom above and two S atoms below the pentagon.23 These salts of the [M(dtc)4] ion are isostructural with the analogous lanthanide complexes, whereas Pu(dtc)3 is not isostructural with any of the lanthanide Ln(dtc)3. 

A few observations of photosubstitution in lanthanide complexes have been reported. Irradiation into the f—f bands of [Pr(thd)3], [Eu(thd)3] and [Ho(thd)3] (thd is the anion of 2,2,6,6-tetramethyl-3,5-heptanedione) results in substitution of thd by solvent.153 The proposed mechanism involves intramolecular energy transfer from an f—f excited state to a reactive IL excited state which is responsible for the observed ligand loss. Photosubstitution has also been observed upon direct excitation into the ligand absorption bands of [Tb(thd)3].154... 

The angular and distance information provided by the lanthanide induced shift has found widespread application from the determination of solution structures of Ln chelates [18,19] to gaining structural information on proteins, nucleotides and amino acids [19], More recently anion binding to coordinatively unsaturated lanthanide complexes has been effectively signalled as the observed lanthanide induced shift has been directly correlated to the nature of the donor atom in the axial position [8,20,21], It is the polarisability of the axial donor that ranks the second order crystal field coefficient, B02, and hence determines the magnitude of the observed shift. Values of the mean shift of the four most-shifted axial protons of the 12-Nq ring for [Yb.la]3+ are collated in Table 2. 

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