Synthesis lanthanide complexes

Danishefsky et al. were probably the first to observe that lanthanide complexes can catalyze the cycloaddition reaction of aldehydes with activated dienes [24]. The reaction of benzaldehyde la with activated conjugated dienes such as 2d was found to be catalyzed by Eu(hfc)3 16 giving up to 58% ee (Scheme 4.16). The ee of the cycloaddition products for other substrates was in the range 20-40% with 1 mol% loading of 16. Catalyst 16 has also been used for diastereoselective cycloaddition reactions using chiral 0-menthoxy-activated dienes derived from (-)-menthol, giving up to 84% de [24b,c] it has also been used for the synthesis of optically pure saccharides. 

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). 

Her present research interests are focused onto the design, synthesis of new lanthanide complexes as contrast agents for magnetic resonance imaging (MRI), as well as the development of a novel series of pH and p02 indicators for 111 NMR spectroscopic imaging ( II-MRSI). More recently, she has been involved in synthesis and evaluation of nanostructurated contrast agents. 

Whilst there have been a number of excellent reviews on the design, synthesis, structural determination, and photophysics of luminescent lanthanide complexes (1-6), detailed information for coordination chemists on how these compounds may be used as analytes in the biochemical and biological sciences has been somewhat less readily... 

Costes, J.-P. Dupuis, A. Commenges, C. Lagrave, S Laurent, J.-P. Mononuclear lanthanide complexes of tripodal ligands synthesis and spectroscopic studies. Inorg. Chim. Acta 1999, 285, 49-54. 

Prodi, L. Pivari, S. Bolletta, F. Hissler, M. Ziessel, R. Synthesis of functionalized calix[4]arene ligands incorporating bipyridine N.N -dioxide chromo-phores and luminescence of their lanthanide complexes. Eur. J. Inorg. Chem. 1998, 1959-1965. 

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. 

Hydrocarbonyl compounds, lanthanide complexes, 4, 4 ( -Hydrocarbyl)bis(zirconocene), preparation, 4, 906 Hydrocarbyl-bridged cyclopentadienyl-amido complexes, with Zr(IV), 4, 864 Hydrocarbyl complexes bis-Cp Ti hydrocarbyls reactions, 4, 551 structure and properties, 4, 551 synthesis, 4, 542 cobalt with rf-ligands, 7, 51 cobalt with rf-ligands, 7, 56 cobalt with ]4-ligands, 7, 59 cobalt with rf-ligands, 7, 71 heteroleptic types, 4, 192 homoleptic types, 4, 192 into magnetic metal nanoparticles via ligand stabilization, 12, 87 via polymer stabilization, 12, 87 into noble metal nanoparticles... 

Lactones, via indium compounds, 9, 686 Lactonizations, via ruthenium catalysts, 10, 160 Ladder polysilanes, preparation and properties, 3, 639 Lanthanacarboranes, synthesis, 3, 249 Lanthanide complexes with alkenyls, 4, 17 with alkyls, 4, 7 with alkynyls, 4, 17 with allyls, 4, 19 with arenes, 4, 119, 4, 118 and aromatic C-F bond activation, 1, 738 bis(Cp ), 4, 73... 

Tei, L., Baum, G., Blake, A.J., Fenske, D., and Schrooder, M. Lanthanide complexes of a new nonadentate ligand derived from 1,4,7-triazacyclononane synthesis, structural characterisation and NMR spectroscopic studies,/. Chem. Soc., Dalton Trans. (2000), 2793-2799. 

Insofar as asymmetric synthesis is concerned, some pioneering work was conducted by Marks and co-workers through their demonstration of enantioselectivity in the samarium or lanthanide complex-catalyzed cyclization of aminoalkenes, an analogy of the simple reaction exemplified in Scheme 36. As before, the reaction works best for Se-unsaturated amines where the product of an exocyclic ring closure pathway is a cyclopentylamine. In the most favorable cases, high turnover to the desired product is observed [110] (Scheme 37). 

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. 

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. 

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 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 leading references for the synthesis and properties of various lanthanide complexes are given below. 

The ligand p-t-butylcalix[4]arene fitted with phosphinoyl arms is synthesized by refluxing the tetrasodium derivative of p-t-butylcalix[4]arene and chloro(dimethylphosphinoyl) methane in toluene or xylene [88]. The resulting ligand L is reacted with lanthanum perchlorate in acetonitrile to obtain 1 1 and 1 2 complexes, LaL and LaL2- The majority of the supramolecular complexes of rare earths involves the use of organic solvents like toluene, hexane or acetonitrile for the synthesis of both the ligands and lanthanide complexes. 

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. 

The potential use of non-solvated lanthanide cyclopentadienyl hydride complexes as catalysts in alkene C-H bond activation, hydrogenation of alkynes led to synthesis of aluminum hydride organo lanthanide complexes. Examples of such complexes with polymeric structure and chain structure have been characterized [251]. 

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. 

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