Synthesis lanthanide alkoxides

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... 

Routes to monomeric , mononuclear , monolanthanide alkoxides, enolates, siloxides and aryloxides - an expanded title which will put the scope of the article in a more concrete form. The synthesis of mononuclear alkoxides, in particularly homoleptic derivatives [1], was decisively stimulated by the discovery of high temperature superconducting ceramics based on YBa2Cu307<, where yttrium represents the lanthanide elements [2]. The support of volatile and highly soluble molecular precursors is a prerequisite for synthesizing thin films of these materials by means of MOCVD [3] and sol gel processes [4], respectively. More recently, lanthanide alkoxide reagents became established in... 

Asymmetric ate complexes of type Na3Ln((S)-BINOL)3 6THF H20 (Ln = Pr, Nd, Eu) could be crystallized from THF and exist only in the /1-form [158], Their stability against moisture had an enormous impact on application of lanthanide alkoxides in organic synthesis (Sect. 6.2.1). 

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. 

Anwander, R. (1996) Routes to monomeric lanthanide alkoxides. Topics in Current Chemistry, 179 (Organolanthoid Chemistry Synthesis Structure Catalysis) 149-245. 

Other small-scale laboratory procedures have been developed for the direct synthesis of the more reactive THF adducts, avoiding inconvenient high temperature treatment [59-62]. For example, the preparation of LnCl3(thf)x from metal powder and hexachloroethane is facilitated by sonication [Eq. (1)] [59]. Additional metal-based synthetic routes include the redox transmetallation with mercury(II) halides [Eq. (2)] [60] and the reaction with trimethylsilyl chloride and anhydrous methanol [Eq. (3)] [61]. Ammonia has been employed as an alternative donating solvent in the synthesis of lanthanide alkoxides starting from lanthanide chlorides [63]. 

The strategy of metal alkoxides synthesis is entirely related to the electronegativity of the element concerned. Some electropositive metals, such as alkali metals, alkaline earth metals, and lanthanides, react directly with alcohols. But some less electropositive metals such as magnesium and aluminum require a catalyst (I or HgCy for successful reaction with alcohols. Use of electrochemical synthesis by anodic dissolution of some metals or metalloids (Sc, Y, Ti, Zr, Nb, Ta, Fe, Co, Ni, Cu, Pb, Si, Ge, etc.) in dry alcohol performs a promising procedure because it does not produce any by-products except hydrogen gas. Another applicable method for the synthesis of some alkoxides (B, Si, Ti, Zr, Hf, Nb, Ta, Fe, etc.) is the reaction of their chlorides with alcohols which require a base such as... 

Evans W.E., Sollberger M.S., Hanusa T. Synthesis and stmctnre ofthe polymetallic yttrium alkoxide complex Y3(/x-OCMe3)( U3-Cl)( Lt-OCMe3)3(OCMe3)4(THF)2 and related complexes Ln3(/u,3-OR)(jU3-X)( i-OR)3 building blocks in yttrium and lanthanide alkoxide chemistry. J. Am. Chem. Soc. 1988,110 1841... 

Two principle strategies have been employed for the synthesis of siloxide-containing molecular precursors. The first involves a silanolysis, or condensation, reaction of the Si - OH groups with a metal amido, alkyl, hahde, or alkoxide complex. The second method involves salt metathesis reactions of an alkali metal siloxide with a metal hahde. Much of our work has been focused on formation of tris(tert-butoxy)siloxide derivatives of the early transition metals and main group elements. The largely imexplored regions of the periodic table include the lanthanides and later transition metals. 

The addition of a base, typically ammonia, to mixtures of transition metal halides and alcohols allows the synthesis of homoleptic alkoxides and phenoxides for a wide range of metals. Anhydrous ammonia was first used in the preparation of titanium alkoxides where the reaction is forced to completion by the precipitation of ammonium chloride.41 Although useful for the synthesis of simple alkoxides and phenoxides of Si, Ge, Ti, Zr, Hf, V, Nb, Ta and Fe, as well as a number of lanthanides,42-47 the method fails to produce pure /-butoxides of a number of metals.58 Presumably, secondary reactions between HC1 and Bu OH take place. However, mixing MC14(M = Ti, Zr) with the Bu OH in the presence of pyridine followed by addition of ammonia proves successful, giving excellent yields of the M(OBul)4 complexes.59... 

Attempted synthesis of LnCp according to Scheme 19 resulted in THF cleavage and generation of a n-butoxide ligand terminally functionalized with pentamethylcyclopentadiene (Table 14) [202,203]. The in situ generated alkoxide ligand is bonded in a monodentate fashion to the lanthanide elements. 

Lanthanides in homogeneous systems As organometallics As cerium(IV) salts As coordination complexes As nitrates, chlorides, alkoxides etc. For olefin polymerization For olefin hydrogenation For free radical polymerization For Diels-Alder reactions For olefin polymerization In organic synthesis... 

It is a commonplace to say that there has been explosive growth in the use of lanthanides in organic chemistry. For many years, the use of cerium(iv) compounds as oxidants was widespread, but more recently a whole range of other compounds have made their appearance. Thus samarium(ii) compounds are now routinely used as one-electron reducing agents and the use of trifluoromethanesulfonate ( triflate ) salts of scandium and the lanthanides as water-soluble Lewis acid catalysts is widespread. Beta-diketonate complexes and alkoxides have also come into use there are even applications of mischmetal in organic synthesis. 

Sheng, H., Xu, R, Yao, Y, Zhang, Y, and Shen, Q. (2007) Novel mixed-metal alkoxide clusters of lanthanide and sodium synthesis and extremely active catalysts for the polymerization of e-caprolactone and trimethylene carbonate. Inorganic Chemistry, 46, 7722-7724. 

Tetrahydrofuran complexes of the lanthanide chlorides have attracted considerable attention. The anhydrous trichlorides are themselves very useful starting materials in the synthesis of compounds such as alkoxides and aryloxides, alkylamides, and organometallic compounds in... 

This route has been recently utilized for the synthesis of soluble volatile alkoxides of trivalent bismuth (101) and lanthanides (28, 102, 103) in low-coordination states as well as three-coordinate aryloxides of lanthanides (104) and actinides (105), for example ... 

This method has been utilized for the synthesis of alkoxides of zinc and cadmium as well as the lanthanides with some specially hindered alcohols (cf. Section IV). 

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