Cyclopentadienyls- lanthanides

Marks and coworkers developed a series of cyclopentadienyl-lanthanide complexes. In the initial investigations on achiral catalysts 36a and 36b (Fig. 29.21), TOFs greater than 100000 IT1 were observed in the hydrogenation of 1,2-disub-stituted unfunctionalized alkenes [48]. 

An approach other than steric hindrance has been used to overcome the previously mentioned instability of the actinide homoalkyls. It was found that the inclusion of jT-bonding ligands in the coordination sphere considerably enhanced the stability of the alkyl complex. Recently, the same line of reasoning has also yielded a new series of 7r-cyclopentadienyl lanthanide alkyls (C5H5)2LnR where Ln =Gd, Er, Yb and R = C=C, and CH3 120,121). The infrared data for these complexes are consistent with u-bonded structures and the room temperature magnetic susceptibilities are very close to the free ion values. The actinide complexes (75,... 

Tris(i/5-cyclopentadienyl)lanthanides were the first authentic organolanth-anides to be prepared1 and bis(t/5-cyclopentadienyl)lanthanide(II) compounds have played a germinal part in the development of lower oxidation state organolanthanide chemistry.2 These cyclopentadienyls are sources of coordination compounds of structural interest and are reagents for the synthesis of other organolanthanides, for example, bis- and mono(t)5-cyclopentadienyl)lanthanide(III) derivatives.2... 

In the following sections we detail transmetalation syntheses from ( /5-C5H5)T1 (see Ref. 12) of three (i/5-cyclopentadienyl)lanthanide complexes, which can be isolated analytically pure in nearly quantitative yields simply by evaporation of the filtered reaction solvent. These simplify and optimize our reported syntheses.6,7... 

Fig. 1. Apparatus for the preparation of ( 5-cyclopentadienyl)lanthanide complexes from the metallic elements. 

The homoleptic lanthanide(III) benzamidinates 20-23 can be regarded as analogues of the well known tris(cyclopentadienyl)lanthanide complexes (C5H5)3Ln [7, 8]. One of the most characteristic reactions of the homoleptic cyclopentadienyls is the formation of 1 1 adducts with Lewis bases such as ethers, nitriles, esters etc. [7, 8], Recently it was discovered that the homoleptic lanthanide benzamidinates [PhC(NSiMe3)2]3Ln (20) form similar adducts with THF and nitrile ligands such as acetonitrile or benzonitrile [59]. The molecular and crystal structures of two benzonitrile adducts (26g, h) have been determined by X-ray diffraction. Figure 8 depicts the molecular structure of the europium... 

The properties of cyclopentadienyl lanthanide compounds are influenced markedly by the relationship between the size of the lanthanide atoms and the steric demand of the Cp group. The former varies from La to Lu according to lanthanide contraction, while the latter varies from the least bulky Cp to highly substituted Cp, which is appreciably larger. The Sc and Y complexes are very similar to those of lanthanides with proper allowance for the relative atomic sizes. 

Tris(cyclopentadienyl)lanthanide complexes with steri-cally more crowded Cp ligands such as C5Me4R (R = Me, Et, Tr, and SiMe3) are not assessable by simple metathesis between lanthanide trihalides and the respective alkali metal salt of the bulky Cp ligand. For instance, Cp 3Sm, obtainable from Cp 2Sm and cyclooctatetraene, reacts with THF with ring-opening forming Cp 2Sm[0(CH2)4Cp ](THF) (equation 14). 

The organometalhc chemistry of the rare earths deals mainly with bis(cyclopentadienyl) derivatives due to the easy available bis(cyclopentadienyl) rare earth chlorides and other halides via reaction of the rare earth trichlorides with two equivalents of a cyclopentadienyl alkali salt. Bis(cyclopentadienyl)lanthanide chlorides are formed as chloride-bridged dimers (Figure 28a), as monomers stabihzed by a donor molecule like THF (Figure 28b) or as ate complexes with alkali hahdes (Figure 28c). 

Another approach to bis(cyclopentadienyl)lanthanide alkoxides is the cleavage of ethers by bis(cyclopentadienyl) lanthanide hydrides. DiaUcyl ethers, ROR, are cleaved by (Cp 2LnH)2 (Ln = Y, La, Ce) to form Cp 2LnOR, Cp 2LnOR, R H, and RH. The extent to which either of the C-0 bonds of asymmetrically substituted dialkyl ethers ROR ... 

A recent development in the low-valent lanthanide area is the synthesis and X-ray structural determination of the unsolvated complex (C5Me5)2Sm 121). This species is the first structurally characterized bis(cyclopentadienyl)lanthanide species which has no other ligands in the metal coordination sphere. As such, it is the closest lanthanide analogue of the bis(ring)metallocene sandwich compounds like ferrocene. Samarocene was obtained by desolvation of (C5Me5)2Sm(thf)2 under high vacuum and sublimation of the product [Eq. (58)]. The sublimed crystals have a bent... 

The compound can be isolated in pentane, recrystallized, and characterized by IR, H NMR and x-ray diffraction. The bis(cyclopentadienyl)lanthanide(f -allyl) complexes are also stable decomposition occurs at greater than 200°C... 

Following the synthesis of tris(cyclopentadienyl) lanthanide complexes, the first examples of organolanthanide derivatives, a vast number of organolanthanide it-complexes with cyclopen-tadienyl (Cp), indenyl (Ind), cyclooctatetraenyl (COT), and related substituted derivatives have been prepared and extensively reviewed [3]. Here we will briefly discuss cyclopentadienyl and allyl derivatives as representatives of organolanthanide K-complexes. 

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]. 

The cyclopentadienyl ring in the even less sterically bulky tris(cyclopentadienyl) lanthanide complexes, (C5H5)3Ln and (CH3CsH4)3Ln, has been found to be able to functionalize by a direct nucleophilic addition reaction under suitable reaction conditions. Reaction of (C5H5)3Ln... 

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],... 

Cleavage of Ln-C a-bonds of lanthanide alkyls and aryls by a hydrogen molecule at ambient pressure and room temperature is a popular method for the synthesis of neutral lanthanide hydride complexes (Equation 8.25). The first structurally characterized neutral lanthanide hydrides were prepared by hydrogenolysis of bi(cyclopentadienyl) lanthanide alkyl... 

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