Lanthanides alkoxide complexes

ROP of lactones and lactides using lanthanide alkoxide-based initiators is a relatively recent discovery. The first example of lactone polymerization by lanthanide alkoxide complexes was reported in a DuPont patent written by McLain and Drysdale in 1991 [89]. In general, the activity of these catalysts is much higher than that determined for aluminum alkoxides, especially in lactide polymerization [90-92]. Polymers of relatively high molecular weight and narrow MWD are formed. The negative side-reactions such as macrocycle formation, transesterification, and racemization are absent. 

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. 

Lanthanide alkoxide complexes can be prepared using a number of methods. The key difference lies in the nature of lanthanide starting materials. These include elemental metals, halides, alkoxides, amides, carboxylates, hydrides, and organometallic species [1, 11], The organic ligands come from aliphatic alcohols, phenols, or their metal salts. 

Figure 6.1 Coordination modes of alkoxido ligands observed in lanthanide alkoxide complexes.

Using a highly hindered phosphino-alkoxide ligand, lanthanoid metal complexes [Ln(OCBu2CH2PMe2)3l (Ln = Y or Nd) have been obtained [30], These are the first monomeric homoleptic examples of lanthanide alkoxide complexes. The bulky substituents... 

Lanthanide alkoxide complexes are of fundamental interest because of the associated synthetic challenges and the great variety of their structures. They are also an important class of materials, either as catalysts to promote useful but otherwise difficult chemical transformations or as molecular precursors for the realization of high-quality metal oxide-based advanced materials. Selected examples illustrating these applications are discussed below. 

Lanthanide alkoxide complexes have been shown to promote a number of useful chemical reactions, whereby the complex is used catalytically or applied in stoichiometric amount. One such reactions is the Meerwein-Ponndorf-Verley reduction (MPV) or the Oppenauer oxidation, depending on which component is the desired product (Equation 6.5). If the alcohol is the desired product, the reaction is viewed as Meerwein-Ponndorf-Verley Reduction [43]. 

Figure 6.20 Schematic illustration of a generic Meerwein-Ponndorf-Verley reduction catalyzed by a lanthanide alkoxide complex.

Creating metal oxide based advanced materials using lanthanide alkoxide complexes as molecular precursors is another area where the lanthanide alkoxide chemistry has found significant applications [5, 7-10]. A technique heavily used in the semiconductor industry for the growth of metal oxides materials is the process of metal-organic vapor chemical vapor deposition... 

The large number of lanthanide alkoxide complexes featuring the inclusion of oxo or hydroxo groups unmistakably suggests that the coordination chemistry of lanthanide hydroxide is intimately associated with that of the alkoxides. Arguably the simplest and most ubiquitous O-containing ligand, the water molecule, occupies a special position in the development of lanthanide coordination chemistry. Upon coordination to the Lewis acidic lanthanide ion, an... 

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