Lanthanide hydrated salts

Dehydrating agents have commonly been employed in the preparation of lanthanide sulfoxide complexes from hydrated lanthanide salts. For example, dimethoxypropane has been used to prepare both (CH2)4SO (65) and rePr2SO (56) complexes of the lanthanide nitrates. An alternative dehydrating agent is ethyl orthoformate [Eq. (14)]. 

Many crystal structures of hydrated lanthanide salts show simultaneous bonding of water molecules and one or more anions to M(III). It is not possible to infer the constituents of the first coordination sphere from the composition of salt hydrates. 

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]. 

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. 

Since Schiff bases are relatively acidic, the complexes can be prepared with a hydrated lanthanide salt in pure ethanol. A mild base such as sodium acetate or triethylamine can be added slowly to achieve deprotonation [59]. 

Lanthanide complexes of poorly coordinating ligands such as aldehydes and ketones [24], esters [25], and ligands containing sulphur and phosphorus [26] have been prepared. The procedure consists of mixing a solution of hydrated lanthanide salt with a solution of the... 

Lanthanide complexes with macrocyclic polyethers [64] have been obtained by mixing solutions of the ligand and solution of a hydrated lanthanide salt [65]. 

Rare earth cryptates (table 5) are more stable than coronates and are highly kinetically inert towards dissociation in aqueous solutions. They can therefore be studied in water, but hydrolysis hinders their synthesis in this solvent. On the contrary, the preparation of unsolvated 1 1 complexes requires anhydrous conditions (Gansow and Triplett, 1981). The hydrated lanthanide salt solutions in... 

The chlorides, bromides, nitrates, bromates, and perchlorate salts ate soluble in water and, when the aqueous solutions evaporate, precipitate as hydrated crystalline salts. The acetates, iodates, and iodides ate somewhat less soluble. The sulfates ate sparingly soluble and ate unique in that they have a negative solubitity trend with increasing temperature. The oxides, sulfides, fluorides, carbonates, oxalates, and phosphates ate insoluble in water. The oxalate, which is important in the recovery of lanthanides from solutions, can be calcined directly to the oxide. This procedure is used both in analytical and industrial apptications. 

Amongst the known examples of this arrangement are a number of [M(H20)9] + hydrates of lanthanide salts and [ReH9] . The latter is... 

A number of complexes of urea and substituted ureas with various lanthanide salts have been isolated. The lanthanide acetates give both anhydrous and hydrated complexes with urea (67, 68). The hydrated complexes could be dehydrated by drying the complexes over CaCl2 or P4Oi0 (68). It is interesting to note that in the complexes of substituted ureas like EU (70) and CPU (71), the L M is independent of the anion. The anions in these complexes with a L M of 8 1 are apparently nonco-ordinated. Seminara et al. (72) have reported complexes of lanthanide chlorides with DMU and DEU which contain five and three molecules of the ligand respectively per... 

In this section are discussed hydrates of lanthanide salts where the water molecules form a majority of the coordination sphere and where there is evidence concerning the number or... 

The lanthanide salt hydrates and D were taken in the ratio Ln D 1 (2 3). Their mixture and 12 ceramic balls (d= 8-10 mm) were put into a corundum thick-wall reactor (volume 150mL) and exposed to the vibration (50 Hz, 1.5 kW) for 3-15 min. Then small amounts of solvents (ethanol, water, hexane, 3-10 mL) were added. The formed solid was recrystallized from acetone. Yields 85-92%. 

One of the methods involves the addition of an excess of triethylorthoformate to a lanthanide salt in acetonitrile and refluxing for 5 hours [2]. In another procedure the hydrated salt in acetonitrile or n-butanol is refluxed through a Soxhlet extractor packed with 3 A molecular sieves [3]. After 12 hours the molecular sieves are replaced and the water content analysed by Karl Fischer titration. The process is repeated until the solution is practically free from water. 

The dehydration of lanthanide perchlorates to obtain the anhydrous salt has been studied [13-15]. Lighter lanthanide perchlorate lose the water of hydration readily at 200°C under vacuum while the heavier lanthanide salts produced insoluble basic salts. Anhydrous heavier lanthanide perchlorates have been obtained by extraction with anhydrous acetonitrile. Utmost precaution should be exercised in the purification of lanthanide perchlorate, since the mixture of lanthanide perchlorate and acetonitrile can lead to an explosion. An alternate approach involves the addition of triethylorthoformate to the mixture or refluxing the solvent through a Soxhlet extractor packed with molecular sieves [3], In view of the hazardous nature of perchlorates, alternate materials such as lanthanide trifluoromethane sulfonates have received some attention. Lanthanide triflates are thermally stable, soluble in organic solvents, unreactive to moisture and are weak coordinating agents. Triflic acid is stronger than perchloric acid [17]. Lanthanide perchlorates and triflate have the same reduction potentials in aprotic solvents and the dissociation of the triflates is less than the perchlorates in acetonitrile [17],... 

Lewis bases like aliphatic or aromatic amines, sulphoxides, phosphorous derivatives form adducts with / -diketonates, by recrystallization of the hydrated lanthanide fi-diketonates in a solution of the substrate [51,52]. The adducts have also been prepared by extraction of lanthanide salt with a stoichiometric mixture of the /5-diketone and a Lewis base. Alternatively it can be prepared by increasing the pH of an ethanolic-water solution of lanthanide salt, /5 -diketone and the organic substrate. 

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. 

In contrast to the alkali metals, the lanthanides do not form crown ether complexes readily in aqueous solution, due to the considerable hydration energy of the Ln + ion. These complexes are, however, readily synthesized by operating in non-aqueous solvents. Because many studies have been made with lanthanide nitrate complexes, coordination numbers are often high. Thus 12-coordination is found in La(N03)3(18-crown-6) (Eigure 4.4), 11 coordination in La(N03)3(15-crown-5), and 10 coordination is found in La(N03)3(12-crown-4). Other complexes isolated include Nd(18-crown-6)o.75(N03)3, which is in fact [ Nd(18-crown-6)(N03)2 +]3 psid(N03)6]. Other lanthanide salts complex with crown ethers small crowns like 12-crown-4 give 2 1 complexes with lanthanide perchlorates, though the 2 1 complexes are not obtained with lanthanide nitrates where the anion can... 

It was only in 1995 that the first structure of a hydrated salt of scandium containing only water molecules in its coordination sphere was reported. Refluxing scandium oxide with triflic acid leads to the isolation of hydrated scandium triflate Sc(03SCF3)3 9H20. It is isomorphous with the hydrated lanthanide triflates, containing tricapped trigonally prismatic coordinate scandium in the [Sc(H20)g] ions, with Sc—O (vertices) = 2.171(9) A and Sc—O (face capped) 2.47(2) A. 

Hafnium and zirconium mononitrate tris(triflate), [M(H20)x(N03)](0Tf)35 were prepared from their tetrachlorides in analogous fashion to the lanthanide salts. Much to our delight these (deliquescent) salts displayed A, nitrate stretching frequencies in their IR spectra at 1651 and 1650 cm1 respectively. Armed with this pleasing information and with a specific programme aim of nitrating o-nitrotoluene (ONT) to dinitrotoluenes (DNTs) with these catalyst types, (catalytic quantities of ytterbium(III) triflate were essentially ineffective for this transformation) hafnium(IV) and zirconium(IV) triflate were prepared as their hydrated salts via metathesis of their tetrachlorides with silver triflate in water. 

Thermal studies of other lanthanide salts have been reviewed by Niinisto and Leskela (1987). For most salts the total dehydration occurs in a single-step reaction, although for a few systems (e.g. the selenate and iodate hydrates) dehydration occurs via a multi-step process. 

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