Organolanthanide compounds

The central point in this consideration is the Ln-OH moiety, the preferred formation of which is considered to be a dilemma in organolanthanide chemistry. Organolanthanide compounds containing Ln-X a-bonds such as alkyls, amides and alkoxides readily hydrolyze when exposed to moist air, with the formation of the hydroxides. Lanthanide complexes with Ln-C linkages are considered to be oversensitive compounds [89]. Even ligands with lower pKa values than water, as exemplified by substituted phenol ligands, tend to hydrolyze in organic solvents because the insoluble hydroxides formed act as... 

Scheme III. Synthetic strategies towards organolanthanide compounds...

Despite the kinetic lability of the Ln-X-cr-bonds (even the thermodynamically very stable Ln-OR bond is subject to rapid ligand exchange reactions [49]) organolanthanide compounds are thermally very robust over a wide range of temperature (Fig. 5) [114, 116, 139, 144-151]. Thermal stability is not only favorable in catalytic transformations at elevated temperatures [47], for the support of volatile molecular precursors is of fundamental importance in chemical vapor deposition techniques the sublimation behavior is a criterion of thermal stability and suitability for these processes (Fig. 5). 

Organolanthanide compounds display a remarkable chemistry with the simplest carbonyl derivative, namely carbon monoxide [280-287]. Organolanthanide activation of carbon monoxide was first observed by employing the monomeric alkyl complex Cp2 Lu(f Bu)(TH F) (Scheme 37) [281a]. Reaction of one equivalent CO yielded single insertion of CO into the Ln-fBu bond to form a r 2-acyl complex. However, excess CO yielded a multiple insertion and coupling of four molecules of CO led to an enedione diolate moiety which bridges two lutetium atoms (Table 18). 

In contrast to the extensive carbonyl chemistry of the d-transition metals, lanthanide metals do not form complexes with CO under normal conditions. All organolanthanide compounds are highly sensitive to oxygen and moisture, and in some cases they are also thermally unstable. 

Catalyst mixtures with discrete rare-earth metal components featuring preformed Ln - H bonds have not been employed in diene polymerization. Therefore, in this section the interaction of purely inorganic aluminohydrides (alanes) with organolanthanide compounds will be only briefly addressed, as well as the utilization of borohydrides in the fabrication of polydienes. 

It should be noted that all organolanthanide compounds are markedly sensitive to oxygen and moisture. 

This was a clear example of the potential of organolanthanides to display unique reactivity patterns. The tert-butyl results also demonstrated that one need not necessarily expect the principles of organotransition metal chemistry to apply exactly to organolanthanide compounds. Indeed, one of the exciting aspects of this young organolanthanide area is that the principles of reactivity are still being established, i.e., current experiments... 

Lanthanide(lII) chlorides or some organolanthanide compounds catalyzed aldol reactions of ketene silyl acetals with aldehydes were reported, (a) Takai, K. Heathcock, C. H. J. Org. Chem. 1985, 50, 3247. (b) Vougioukas, A.E. Kagan, H.B. Tetrahedron Lett. 1987, 28, 5513. (c) Gong,... 

Scheme 2. Synthetic strategies towards organolanthanide compounds [A amine elimination reactions, e.g. silylamide route B alkylation via alkoxide precursor, e.g. aryloxide route C alkylation via amide precursor D hydrogenolysis of alkyl moieties]...

Piets first attempt to prepare organolanthanide compounds marked the dawn of a new area of chemistry (2). Since then, a wide variety of complexes of f-block elements have been synthesized and their structures have been satisfactorily explained in terms of "steric congestion" of ligands around f-metal ions (3,4,5,6). However, the question of whether, the 4f valence orbitals participate in bonding has not as yet been answered. 

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