Complex oxides lanthanides

Ytterby, a village in Sweden) Discovered by Mosander in 1843. Terbium is a member of the lanthanide or "rare earth" group of elements. It is found in cerite, gadolinite, and other minerals along with other rare earths. It is recovered commercially from monazite in which it is present to the extent of 0.03%, from xenotime, and from euxenite, a complex oxide containing 1% or more of terbia. 

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. 

The synthesis of a series of chiral organophosphine oxide/sulfide-substituted binaphtholate ligands has recently been reported by Marks and Yu and their corresponding lanthanide complexes characterized. These complexes, generated in situ from Ln[N(TMS)2]3, cleanly catalysed enantioselective intramolecular hydroamination/cyclisation of 1-amino-2,2-dimethyl-4-pentene albeit with a low enantioselectivity of 7% ee (Scheme 10.82). 

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. 

Pietraszkiewicz, M., Karpiuk, J., and Rout, A.K. (1993) Lanthanide complexes of macrocyclic and macro-bicyclic N-oxides light-converting supramolecular devices. Pure Appl. Chem, 65(3), 563-566. 

Shortly after the key mechanistic papers on rhodium-catalyzed hydroboration, Marks reported a hydroboration reaction catalyzed by lanthanide complexes that proceeds by a completely different mechanism.63 Simple lanthanide salts such as Sml3 were also shown to catalyze the hydroboration of a range of olefins.64 The mechanism for this reaction was found to be complex and unknown. As in other reactions catalyzed by lanthanides, it is proposed that the entire catalytic cycle takes place without any changes in oxidation state on the central metal. 

In addition to these actinide(IV) compounds, the increasing stabihty of the - -3 oxidation state for the trans-uranium elements has recently led to the preparation of compounds of formula K[M(CgH8)2] where M=Np or Pu 31). In their chemical behavior these compounds axe similar to the corresponding lanthanide complexes vide infra) and their X-ray powder patterns suggest they have the same structure. They appear to be much more ionic than their -f4 analogues. 

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. 

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. 

As discussed in previous sections the phase relationships in the lanthanide higher oxides of Ce, Pr, and Tb are quite complex and sensitive to environmental conditions, especially the oxygen partial pressures. Unless extreme care is taken, the property measurements will be on inadequately characterized materials. 

The solid-state structure of the ethylaluminum oxide metallocene complex (Fig. 30) shows two trivalent samarocene units connected by an [(A EtsO ]2-ethylalumoxane ( EAO ) unit. The latter was described as an adduct of two molecules of triethylaluminum with two [AUfoO]- anions [116]. An unusual asymmetric fx-r q1 (side-on) ethyl coordination mode was observed, which previously has been found only in a small number of lanthanide complexes, i.e., (CsMeshYM/zy 1-Et)AlEt2(THF) [ 144], (C5Me5) 2Sm(THF) (fx-rj1 ijx-Et) AlEt3 (Fig. 9) [115] and (C5Me5)2Sm(THF) (/i-r]1 X-Et) (/z-Cl)AlEt2 [116]. 

Ans. No. Other mechanisms (radical, heterolytic, etc.) may be available with transition metal complexes. With lanthanide complexes in the highly stable 3 + oxidation state, an OA/RE-based mechanism is not possible. 

Very few calculations have so far been performed for lanthanides and not much is known about the choice of the active space. However, most lanthanide complexes have the metal in oxidation state 3+. Furthermore, are the 4/ orbitals inert and do not interact strongly with the ligands. It is therefore likely that in such complexes only the 4/ orbitals have to be active unless the process studied includes charge transfer from the ligands to the metal. In systems with the metal in a lower oxidation state, the choice of the active space would show similar problems as in the actinides, in particular because the 5d orbitals may also take part in the bonding. As an example we might mention a recent study of the SmO molecule and positive ion where 13 active orbitals where shown to produce results of good accuracy [42],... 

Highly pure lanthanide oxides (99.99% purity) that are commercially available can be used as starting materials for the preparation of lanthanide complexes after treatment at 1100°C... 

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. 

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