Lanthanide complexes 5-diketonates

Assays based on luminescent lanthanide ions were developed initially in the 1970s, when instrumentation became available which could distinguish long-lived luminescence from a shortlived background. Leif and co-workers reported the first attempts to use lanthanide complexes (in this case europium complexes with 1,10-phenanthroline and 7-diketonates, i.e., [Eu(phen)(diketo-nate)3]) as tags for antibodies.107 These proved kinetically unstable in the pH regime required... 

The Eu—0 (diketonate) distances vary from 2.32 to 2.37 A and agree very well with other lanthanide-/S-diketonate complexes and with eight coordinated Eu(DPM)3(Py)2 complex (see later). Eu—0 (sulphoxide) distance is 2.40 A. The sulphoxide ring is puckered by 35° and the S—0 has equitorial conformation. 

The efficient and selective catalysis of some Diels-Alder reactions by lanthanide P-diketonate complexes has been known since 1975 [226, 227]. The fluorinated p-diketonate complexes Ln(fod)3 (cf. Scheme 12.5) selectively catalyze the Danishefsky transformation (Scheme 12.23) as a consequence of their mild Lewis acidity. Importantly, zeolites and Lewis acid modified silica or alumina also catalyze Diels-Alder reactions [228-232]. 

Under this section are considered lanthanide complexes of /3-diketones and their adducts, but work specifically in the area of lanthanide shift reagents is dealt with in Section 39.2.9. Of course, the majority of lanthanide shift reagents are lanthanide /3-diketonates, and when they function as shift reagents they do so by forming adducts in solution. Furthermore, interest in shift reagents has directly stimulated a considerable amount of fundamental research on lanthanide /3-diketonates and their adducts. Much of this fundamental work was, therefore, carried out in the early 1970s. AH this means that the division between this section and Section 39.2.9 is not entirely clear cut. 

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... 

The formation of luminescent lanthanide complexes relies on a number of factors. The choice of coordinating ligand and the method by which the antenna chromophore is attached to it, as well as the physical properties of the antenna, are important. In order to fully coordinate a lanthanide ion, either a high-level polydentate ligand such as a cryptate 1 or a number of smaller ligands (such as 1,3-diketones, 2) working in cooperation are required. Both 1 and 2 are two of the simplest coordination complexes possible for lanthanide ions. In both cases there are no antennae present. However, the number of bound solvent molecules is decreased considerably from nine (for lanthanide ions in solution) to one to two for the cryptate and three for the 1,3-diketone complexes. 

Lipophilic lanthanide complexes of fluorinated 3-diketonate ligands were demonstrated to bind unprotected amino acids under neutral conditions. It is not clear whether amino acids are bound as anions or zwitterions. Chiral ligands 63-66 have been prepared and tested for extraction of amino acids from water into dichloromethane [84] (Table 7). NMR and CD spectroscopic... 

However, the lanthanide p-diketonates shown in scheme 4 cannot be used as labels, since there is no active binding group on the p-diketonate ligands and these complexes are not very stable with stability constants in the order of 103-106 only, therefore the complexes dissociate in highly diluted solutions and the luminescence intensity decreases. Recently, three chloro-sulfonylated tetradentate p-diketones were synthesized by Yuan and Matsumoto (1996,1997) and Yuan et al. (1998a, 1998b) (scheme 5). They differ from other p-diketones, because the emission intensity of their Eu3+ complexes is not weakened by the presence of the sulfonyl... 

Lanthanide complexes with diketones and Schiff bases.266... 

Mixed complexes such as lanthanide porphyrin diketonates have been prepared and these are soluble in organic solvents [83,84]. These mixed complexes are prepared in organic solvents like 4-phenylpyridine, or 1,2,4-trichlorobenzene with refluxing. After the reaction is complete, the solvent is removed by distillation under reduced pressure and the product purified by column chromatography. 

Up to about the 1960 s, elemental analysis coupled with absorption spectra and infrared spectra and X-ray crystallography were the primary methods used in the studies of complexes. Later on with the developments in nuclear magnetic resonance (NMR) spectroscopy, especially multinuclear NMR, this technique has been invariably used in the studies of structural features of lanthanide complexes. To illustrate these points some references to literature are herein pointed out. The studies on the rare earth 1,3-diketonates, where 1,3-diketones are acetyl acetone, benzoyl acetone, dibenzoyl methane and 2-thienoyl tri-fluoroacetone totally relied on elemental analysis, UV-Vis and IR spectra to establish the nature of the complexes [89]. The important role played by X-ray crystallography in the elucidation of the structures of lanthanide complexes has been extensively discussed in Chapter 5 and the use of this technique goes as far back as the 1960 s. Nevertheless it continues to play a major role in the studies of lanthanide complexes. 

Comparative absorption spectrophotometry has been used in the studies of lanthanide /J-diketonatc complexes in solids as well as in solutions. The neutral hexacoordinated lanthanide tris diketonate on dissolution in a polar nonaqueous solvent, increased its coordination to eight by accepting two solvent molecules [204]. The addition of water or other oxygenated solvent to Nd(diket)3 in solution resulted in significant changes in the shape and intensity of the band due to 4l9/2 —> 4Gis/2, transition. These changes have been attributed to an increase in coordination number of Nd(III) from 6 to 8 by the coordination of two solvent molecules [238-241]. 

These observations suggest an interaction between the n electron cloud located at second and third carbon atoms of the diols and the lanthanide metal orbitals. Similar behaviour was observed in the mixed complexes of lanthanide /1-diketonates and the substituted diols [233],... 

Many lanthanide shift reagents are now available commercially which are soluble in common organic solvents. Most of the reagents are lanthanide complexes of -diketones having the basic 2,4-pentanedione structure. Some common reagents are Pr(fod)3, Eu(tfa)3, Yb(hfa)3 and also associated permutations. The shift reagents are tris-yS-diketonates of... 

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