Synthesis of lanthanide complexes

The first step in the study of metal complexes is the synthesis or preparation of the complex of interest followed by physicochemical characterization of the complex. Lanthanides rarely behave like transition metal cations and hence the need for different precautions in handling them. The fundamental characteristics and behavior of lanthanides in solution is useful for embarking on the synthesis of lanthanide complexes. Some properties of lanthanides, which are useful, and of direct interest for practical purposes will be recalled here. 

The lanthanide triflates may also be obtained by the addition of triflic acid to lanthanide carbonates in ethanol or acetonitrile [18]. Filtration and concentration of the filtrate yield a solid salt which is washed with solvent. Lanthanide salts of poorly coordinating anions such as PFg, BF4 or BiCftHs) have been also used in the synthesis of lanthanide complexes as starting materials. Adducts of acetonitrile with europium tetrafluoroborate (BF4) and hexafluorophosphate (PF ) have been prepared [19]... 

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. 

The synthesis of lanthanide complexes [244] with multidentate diethylenetriamine (dien) gave rise to two types of complexes, Ln(dien)3(N03>3 for Ln = La-Gd, and Ln(dien)2(N03)3 for Ln = La-Yb. The tris complexes contain ionic nitrate while the bis complexes contain both ionic and coordinated nitrate ions. The coordination number is nine in the tris complexes while it is not known with certainty in the bis complexes. With triethylene triamine (tren) two types of complexes [Ln(tren)(N03)3] and Ln(tren)2(N03)3 have been isolated. In the bis complexes both ionic and coordinated nitrate groups are present for larger lanthanides (La-Nd) but only ionic nitrate for smaller lanthanides (Sm-Yb). When perchlorate is the anion [245] Ln(tren)(C104)3 (Ln = Pr, Gd, Er) and Ln(tren)2(C104)3 for Ln = La, Pr, Nd, Gd, Er complexes were obtained. The monocomplexes contain coordinated perchlorate ions while the bis complexes contain ionic perchlorate ions. 

Scheme 8.1 Three key methods of the synthesis of lanthanide(lll) bis(phthalocyanine) complexes. Homoleptic complexes symmetrical (A, B) and unsymmetrical (C1 and C2, D1). Thereby, the thermal fusion group A can be subdivided into (A1) solvent free fusion (A2) refluxing in the mixture of protic solvents in the presence of organic base - DBU and (A3) MW assisted fusion.

The template methods have also been used for the synthesis of a number of substituted Ln di(naphthalocyanine) complexes, LnNc2 [82-88]. Apart from thermal fusion by conventional heating processes, complexation has been initiated by microwave radiation, although only a few publications are devoted to the template synthesis of lanthanide bis(phthalocyanine) complexes by this method [89, 90]. The use of microwave radiation (MW) reduces the reaction time from several hours to several minutes. Unsubstituted complexes LnPc2 (Ln = Tb, Dy, Lu) were prepared [90] by irradiation (650-700 W) of a mixture of phthalonitrile with an appropriate lanthanide salt for 6-10 min (yields >70%). 

Although potassium complexes of phosphinomethanide ligands have been used in the synthesis of lanthanide phosphinomethanides (143), it is only very recently that a heavier alkali metal phosphinomethanide complex has been isolated and structurally characterized. 

The synthesis of lanthanide chemical shift reagents has been the objective of many groups owing to their effect on NMR spectra simplification. A drawback of the commonly used reagents is their sensitivity to water or acids. Tris(tetraphenylimido diphosphinatojpraseodymium [Pr(tpip)3] has been developed as a CSR for the analysis of carboxylic acids.17 Furthermore, it has been found that dinuclear dicarboxylate complexes can be obtained through reactions with ammonium or potassium salts of carboxylic acids, and these compounds can be used to determine the enantiomer composition of carboxylic acids.18... 

Casnati, A. Baldini, L. Sansone, F. Ungaro, R. Armaroli, N. Pompei, D. Barigelletti, F. Synthesis, complexation and photophysics in protic solvents of lanthanide complexes of novel calix[4]arene polycarboxylic-2,2,-bipyridine mixed ligands. Supramol. Chem. 2002,14(2-3), 281-289. 

In a difference from the above, the majority of metal halides of secondary subgroups of Group I, II, and VII and lanthanides are low-soluble in the examined solvents, so synthesis of their complexes is carried out in comparatively high-polar solvents (water, alcohols, and aqueous-alcohol mixtures). To carry out syntheses in water, the corresponding conditions, necessary to obtaining soluble derivatives of organic compounds (for example, halide hydrates of amines or N-containing hetero-... 

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],... 

The initial starting materials for the preparation of lanthanide complexes must be such that the counterions do not compete with the ligand in the formation of complexes. Among the lanthanide salts the complex formation tendency is in the order SO - > NO3 > CIOJ and hence lanthanide perchlorates are the preferred starting materials for the synthesis of complexes. 

The ratio of the size of the metal ion and the radius of the internal cavity of the macrocyclic polyether determines the stoichiometry of these complexes. The stoichiometry of these complexes also depends on the coordinating ability of the anion associated with the lanthanide. For example, 12-crown-4 ether forms a bis complex with lanthanide perchlorate in acetonitrile while a 1 1 complex is formed when lanthanide nitrate is used in the synthesis [66]. Unusual stoichiometries of M L are observed when L = 12 crown-4 ether and M is lanthanide trifluoroacetate [67]. In the case of 18-crown-6 ligand and neodymium nitrate a 4 3 stoichiometry has been observed for M L. The composition of the complex [68] has been found to be two units of [Nd(18-crown-6)(N03)]2+ and [Nd(NCh)<--)]3. A similar situation is encountered [69] when L = 2.2.2 cryptand and one has [Eu(N03)5-H20]2- anions and [Eu(2.2.2)N03]+ cations. It is important to note that traces of moisture can lead to polynuclear macrocyclic complexes containing hydroxy lanthanide ions. Thus it is imperative that the synthesis of macrocyclic complexes be performed under anhydrous conditions. 

With iodide the complexes [222] are Ln(DMSO)8l3 for Ln = La-Er. When PFfi is the anion one has Ln(DMSO)y(PFg )3 for both heavy and lighter lanthanides. It appears that experimental conditions have to be controlled very carefully in the synthesis of these complexes. 

As noted earlier, pentamethyl substitution in the cyclopentadienyl ligand confers some stability allowing ligand redistribution and the synthesis of chloride complexes of lanthanides. Stabilization also occurs by coordination with Lewis bases or alkali halides. 

X-ray analysis of the Yb complex [249] shows the two coordination tetrahedra around Yb and Ce are linked by a CO bridge. A large number of lanthanide complexes with Ln-OC-M bridges have been characterized [259]. Tetrathiometallate anions have been used for the synthesis of mixed metal [261], Mo-Sm, W-Sm complexes such as [Cp Sm]2Mo(/r-S)4]PPh4. 

CVD) of metallic copper films.20 The search for precursors for (potentially superconducting) LnMl1 Cu307-x phases has led to synthesis of heteronuclear complexes containing Cu11 combined with lanthanide and/or alkaline earth (Mn) ions.21... 

The discussion below starts with the general synthesis of lanthanide alkoxides, followed by a summary of the OR (R = aliphatic or aryl) coordination modes. Selected examples of complexes will then be presented in order to illustrate the coordination chemistry unique to each class of these ligands (aliphatic alkoxido, aryloxido, and macrocyclic polyaryloxido). Toward the end, catalytic and materials applications of lanthanide alkoxide complexes will be discussed. 

Tris(cyclopentadienyl) lanthanide complexes can be used as precursors for the synthesis of lanthanide derivatives via a protonolysis reaction (Figure 8.7) [15,16]. The biggest advantage of this method is that it excludes the formation of lanthanide -ate compounds [17],... 

Synthesis and Reactivity of Lanthanide Complexes Containing Ln-C o-Bonds... 

The most convenient method for the synthesis of lanthanide dialkyl complexes is the alkane elimination reaction of lanthanide trialkyl complexes with a monoanionic preligand (L H) (Equation 8.15). The monoanionic preligands, which have tunable steric and electronic features, are favored for modifying the stability and reactivity of the complexes. Various bulky substituted cyclopentadienyl [4] and non-cyclopentadienyl derivatives have been used as the spectator ligands. The most common types of monoanionic non-cyclopentadienyl ancillary ligands are summarized in Figure 8.10 [36 8]. 

Figure 8.12 Synthesis of lanthanide monoaUcyl complex via metathesis reaction.

Burton-Pye, B.P., Heath, S.L., and Faulkner, S. (2005) Synthesis and luminescence properties of lanthanide complexes incorporating a hydralazine-derived chromophore. Dalton Transactions, 146. 

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