Neodymium isopropoxide

Hydrogen transfer reactions are highly selective and usually no side products are formed. However, a major problem is that such reactions are in redox equilibrium and high TOFs can often only be reached when the equilibria involved are shifted towards the product side. As stated above, this can be achieved by adding an excess of the hydrogen donor. (For a comparison, see Table 20.2, entry 8 and Table 20.7, entry 3, in which a 10-fold increase in TOF, from 6 to 60, can be observed for the reaction catalyzed by neodymium isopropoxide upon changing the amount of hydrogen donor from an equimolar amount to a solvent. Removal of the oxidation product by distillation also increases the reaction rate. When formic acid (49) is employed, the reduction is a truly irreversible reaction [82]. This acid is mainly used for the reduction of C-C double bonds. As the proton and the hydride are removed from the acid, carbon dioxide is formed, which leaves the reaction mixture. Typically, the reaction is performed in an azeotropic mixture of formic acid and triethylamine in the molar ratio 5 2 [83],... 

As an example, Table 20.2 lists the rate of the racemization of 61 via an MPVO procedure utilizing the catalyst neodymium(III) isopropoxide (62) as a function of the solvent. In this case, an equimolar amount of acetone was applied as the oxidant. The best results were obtained with hydrocarbons such as hexane (entry 7) and heptane (entry 8) as solvents, while the reaction rates in dioxane (entry 2) and acetonitrile (entry 1) were much lower due to inactivation of the catalyst by coordination of the solvent to the metallic center (Table 20.2) [84]. 

The most important result of the structure studies was undoubtedly the establishment of the fact that the crystalline isopropoxides of all rare earths are not the homoleptic Ln(OPr )3 complexes but oxoalkoxides of Ln50(0Pri)13 composition, where Ln = Sc, Y, Er, Yb (see also Fig. 4.9 a). They appear to be desolvation products of the very unstable [Ln(OPri)3( PrOH)]2 solvates (perfectly soluble and rather reactive) the complex of such composition has been isolated and characterized only for neodymium, but the IR spectroscopic evidence for the existence of such solvates was obtained also for Pr and Er. Desolvation of Ln(OBu )3 2L (Ln = Y, La L = BuOH, THF, Py) leads also to the formation of oxocomplexes the ions corresponding to the fragmentation of the homoleptic species are absent in their mass-spectra (except for [Y3(OBu,)9(tBuOH)2], where the Y3(OR)8+ ion was found along with Y30(0R)6+). The same kind transformations have been observed also for... 

The neodymium atoms are placed in three different environments as depicted in Fig. 37. However, all neodymium atoms are 7-coordinate adopting a distorted mono capped prismatic geometry. Most interesting structural feature are the presence of terminal ethyl and isopropoxide ligands at neodymium. More simple molecular arrangments were found in ligand related complexes Ln[(jt-... 

Poncelet, O. and Hubert-Pfalzgraf, L.G. (1989) Reactivity of neodymium(lll) isopropoxide derivatives synthesis, characterization and crystal structure of [Nd4(p,3-OH)2(p.2,M-i-acac)6(acac)4]. Polyhedron, 8, 2183-2188. 

There is an example of a chlorine ligand with coordination number six, in the compound Nd6(OPr 2)nCl. The chlorine atom lies at the centre of a trigonal prism, formed by the six neodymium atoms which are held together by bridging isopropoxide groups. TTie Nd—Cl distance of 3.05 A is compatible with an ionic formulation, and the stereochemistry does not appear significant. 

On the basis of mass spectral data, Mazdiyasni et al. reported tetrameric natnre for neodymium, erbium, and lutetium isopropoxides. The tetrameric structure of [Ln(OCH2Bu )3]4 (Ln = La, Nd) has recently been confirmed by X-ray crystallography," whereas the t-butoxide is trinuclear [La3(OBu )9(Bu OH)2]." ... 

Similarly, preparation of anhydrous and/or unsolvated lanthanide tris-acetylacetonates remained a challenge till 1965, as the efforts to remove ligated water from Ln()S-dik>3.2H2O (obtained from aqueous media) led to hydrolyzed products. However, pure tris-j6-diketonates of lanthanum, praseodymium, and neodymium could be easily synthesized by the reactions of their alkoxides (methoxides/isopropoxides) with 3 equivalents of acetylacetone (acacH) ... 

As already mentioned the tervalent melal isopropoxides were shown to be pentanuclear oxo-isopropoxides [M5(/Li5-0)(/r3-OPr )4(OPr )5] with the square pyramidal structure for the M5O14 core (Fig. 5.5). Structures have been reported for M = Y, Pr, Yb, and In, and also for the heterometallic compound in which the two metals were distributed randomly over the five sites [Y4PrO(OPr )i3]. Although single crystals of [Er5(/u,5-0)(/r3-OPr )4(/u,-OPr )4(OPr )5] could not be obtained, the X-ray powder diffraction data showed that it was isomorphous and isostructural with Pr, Yb, and In compounds which are all monoclinic P2i/n (cf. the yttrium compound which is orthorhombic). Interestingly neodymium formed the pentanuclear complex [Nd5(/i5-0)(/r,3-OPr )2(/i-OPr )6(OPr )5(I OH)2] in which the fi5-oxo ligand is at the centre of a trigonal bipyramid of Nd atoms. ... 

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