Anhydrous lanthanide complexes

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. 

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. 

Metal alkoxides constitute a useful class of starting materials for the synthesis of the metal / -diketonates. The ethoxides of Nbv, Tav and Uv react with diketones. Here, only partial substitution of the ethoxy groups occurs and materials of the type M(diketonate)3(OEt)2 are formed.194,195 Similar reactions with lanthanide alkoxides, however, provide pure, unsolvated lanthanide tris(diketonates). The virtue of such syntheses lies in their ability to yield anhydrous diketonate complexes. Removal of water from the hydrates without decomposition is sometimes difficult.196,197... 

A recent development in this field involves the combination of benzamidi-nate ligands with cyclopentadienyl or cyclooctatetraenyl ligands in the coordination sphere of lanthanide ions. The first lanthanide complexes containing both benzamidinate and pentamethylcyclopentadienyl bonded to yttrium was reported by Teuben et al. Yttrium aryloxides as well as anhydrous yttrium trichloride can be used as starting materials in these preparations (Eq. 19, Scheme 5) [27] ... 

Homoleptic lanthanide complexes containing the [PhS(NSiMe3)2] ligand were prepared in a straightforward manner by treatment of anhydrous lanthanide trichlorides with three equivalents of Li[PhS(NSiMe3)2]0.5 Et20 (10) [72] ... 

An interesting class of homoleptic lanthanide complexes has become available through the use of heteroallylic diphosphinomethanide ligands. The first member of the series was the lanthanum derivative, [CH(PPh2)2]3La (59c), which was characterized by X-ray crystallography (Fig. 19) [74]. The preparation of 59c involved treatment of anhydrous lanthanum trichloride with the potassium salt of the ligand ... 

Synthesis of several lanthanide complexes requires anhydrous conditions. Hydrated lanthanide salts undergo decomposition in the process of dehydration under vacuum at elevated temperatures. Thus special procedures are required for the preparation of anhydrous salts. 

Anhydrous lanthanide adducts with dimethyl formamide (DMF) [4], dimethyl sulphox-ide [5] (DMSO) and hexamethyl phosphoramide [6] may be used for the synthesis of anhydrous complexes as long as their solvation does not interfere with the preparative reaction [7],... 

To avoid the complication of hydrolysed product or partial decomposition during dehydration, anhydrous diketonate complexes can be prepared by the reaction of diketone with lanthanide 2-propanolate [47] in benzene, with a lanthanide hydride or with metallic europium [48]. 

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 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. 

Complexes with cyanate were prepared [143] by reaction of anhydrous lanthanide chlorides with an excess of tetraethylammonium cyanate in absolute ethanol. The complex has the formula [(C2H5)4N]3Ln(NCO)6, where Ln = Eu, Gd, Dy, Ho, Er, and Yb and they are indicated to be six-coordinate and cyanate is N-bonded as evidenced by IR spectra [147]. Mixed complexes [(C2H5)4N]3[Ln(NCO)3X3], where Ln = Dy, Er and Yb and X = C1, NCS have been synthesized and they appear to be of six-coordinate [144]. Mixed complexes of the type [(C2H5)4N]3[Ln(NC0)3(N03)3], where Ln = La, Pr, Nd, Sm, Eu, Gd, Er and Yb have been synthesized and the IR spectra shows N-bonded cyanate, bidentate nitrate and the overall coordination number amounting to nine [145]. 

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. 

Treatment of anhydrous lanthanide(III) bromides with the sodium salt of N,N-diethyldithiocarbamate (dtc) in absolute ethanol yielded Ln(dtc>3 for Ln = La-Lu-Pm. Reaction of Ln(dtc)3 with N, /V-diethy ldithiocarbamate and tetraethyl ammonium bromide in ethanol yielded [(C2Hs)4N][Ln(dtc)4]. All the tetrakis complexes are isostructural while two series of isostructural tris complexes exist, one for Ln = La-Nd and the other for Ln = Sm-Lu. The tris complexes are probably 6-coordinate, and the tetrakis complexes 8-coordinate [261,262]. 

Literature covering the composition and structure of lanthanide complexes in nonaqueous media is not extensive [128,219]. Anhydrous LnCL in alcohol yielded crystalline tris-alcoholates. These compounds gave intense bands due to 3H4 3P2 transition in... 

For a bulky substituted cyclopentadienyl group, such as CsMes and CsMeaR (R = Et, Pr, SiMes), tris(cyclopentadienyl) lanthanide complexes cannot be prepared via the above metathesis reaction because of the steric hindrance. The reaction of anhydrous LnCls with three equivalents of alkali metal pentamethylcyclopentadienyl in THF (tetrahydrofuran) led to the THF ring-opened product (Equation 8.4) [8]. 

Recently, an improved one-pot method, that is, the metathesis reaction of anhydrous LnCb with three equivalents of allylMgCl, instead of three equivalents of allyllithium in a mixture of THF-l,4-dioxane was developed for the synthesis of neutral triallyl lanthanide complexes. 

Lanthanide triphenyl complexes have been known for about 40 years. These neutral triphenyl complexes [Ln(C6H5)3(THF) ] are synthesized by a transmetallation reaction of the lanthanide metal with diphenyl mercury or triphenyl bismuth in THF [34]. Recently, it has been reported that metathesis reaction of anhydrous lanthanide trichloride with aryllithium is a more convenient and reproducible method than the transmetallation reaction used previously for the synthesis of triaryl lanthanide complexes. Several lanthanide triphenyl and substituted triphenyl complexes have been synthesized (Equation 8.14) [35]. 

The metathesis reaction of anhydrous lanthanide halide with alkali metal alkoxo-functionalized NHC complex has recently proven to be another efficient and straightforward route to lanthanide NHC complexes (Equation 8.18) [69]. 

Anhydrous lanthanide trihalides, particularly the trichlorides, are important reactants for the formation of a variety of lanthanide complexes, including organometallics. Routes for the syntheses of anhydrous lanthanide trihalides generally involve high temperature procedures or dehydration of the hydrated halides.The former are inconvenient and complex for small scale laboratory syntheses, while dehydration methods may also be complex and have limitations, for example, use of thionyl chloride. - Moreover, the products from these routes may require purification by vacuum sublimation at elevated temperatures. Redox transmetalation between lanthanide metals and mercury(II) halides was initially carried out at high temperatures. However, this reaction can be carried out in tetrahydrofuran (THF, solvent) to give complexes of lanthanide trihalides with the solvent. These products are equally as suitable as reactants for synthetic purposes as the uncomplexed... 

E)etailed syntheses of four representative lanthanide trihalide-tetrahydro-furan (thf, ligand) complexes, YbCl3(thf)j, ErCljlthflj j, SmClj(thf)2, and NdCl3(thf), 5 by redox transmetalation are described. The first three compounds have previously been prepared by direct reaction of anhydrous lanthanide trihalides with but the composition of the last differs... 

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