Alkali metal aryloxides

Lanthanide aryloxides have proved to be excellent precursors to homoleptic lanthanide alkyls (B, Eq. 13) [140], The reaction can be conducted in non-polar solvents because of the good solubility of the starting compounds. The formation of insoluble alkali metal aryloxides is the driving force (kinetic control). Complexes derived from aliphatic alcohols [141] and acetylacetonates [131] are... 

A much more widely used synthetic method entails the metathetical exchange reaction between alkali metal aryloxides and the metal halide. This procedure has been applied to the synthesis of lanthanide, actinide, and d-block metal aryloxides as well as derivatives of the main group metals (Eqs 6.23, 6.24, ° 6.25, " 6.26, 6.27, 6.28, 6.29, and 6.30 ° ). 

The elimination of H2 by addition of phenolic reagents to metal hydrides is an excellent method for the synthesis of alkali metal aryloxide compounds (Eqs 6.54, ° 6.55, and 6.56 ). 

Overall, the alkali metal alkoxide and aryloxide systems are excellent examples in demonstrating the effects of steric influences on both molecular aggregation and also on the nature of any extended network architecture adopted. The large database of O-M complexes that have now been identified has led to a good deal of predictability regarding the coordination chemistry of these species. 

Alkoxide and aryloxide ligands are excellent ligands for the actinides. As a result, these ligands have been studied extensively in the coordination chemistry and reactivity of tri-, tetra-, penta-, and hexavalent actinides. The alkoxides and aryloxides can be synthesized by a variety of routes the two most popular routes include direct reaction of actinide halides with alkali metal salts of the alkoxide or aryloxide of interest and protonolysis of actinide amides by alcohols. 

The reaction of metal halides with an alkoxide or aryloxide (most commonly an alkali metal alkoxide), can resnlt in metathetic exchange to give the required complex (equation 6). Such reactions can also result in the formation of heterometallic species (such as NaZr2(OR)4) this can be a problem when homometallic aUcoxides are desired, but it is an important route to heterometallic alkoxides and aryloxides. The nature of the product can be influenced by the alkali metal, the alkoxide ligand, and the relative amounts of the reactantsd ... 

Many alkoxides in particular have been known since the 1960s, but interest in them has been stimulated recently by their potential use as precursors for deposition of metal oxides using the sol-gel or MOCVD process. A review covering the literature to 1990 has appeared. " Traditionally, alkoxides are made by salt-elimination reactions of lanthanide chlorides with alkali metal alkoxides (or aryloxides) which sometimes causes chloride retention... 

Although metal alkoxides were successfully used as a synthon for the synthesis (see Scheme 9) of more interesting homo- and heteroleptic metal alkyls, they have not attained the same importance as their sterically hindered aryloxide analogues. This finding might be due to the general solubility of both the products [i.e., desired metal alkyls and alkali metal (generally lithium) alkoxides] in hydrocarbon solvents. This limitation has made a cleaner separation of the products more difficult. 

The unsubstituted para-t-butyl calixarenes themselves complex metals via their aryloxide groups. Since aryloxide complexes are frequently oligomeric, the simple calixarenes do not give monomeric complexes. Aryloxides are hard ligands, therefore they readily form complexes with oxo-philic hard metal ions such as alkali metals, early transition metals, lanthanides, and actinides. Complexation is often inferred because the calixarene acts as a carrier for the metal ion from an aqueous to an organic phase. With the /wa-/-butylcalix[ ]arenes in alkaline solution, a value of n = 6 gives the best carrier for lithium(I), sodium(I), and potassium(I), with a value of n 8 giving the best carrier for rubidium(I) and caesium(I).15,16 Titanium(IV) complexes have been characterized,17-19 as well as those of niobium(V) and tantalum(V).20-22 These complexes are classified as... 

Scheme 8. Reaction behavior of lanthanide aryloxide and alkoxide complexes towards alkali metal alkyl reagents (OAr=C6H3fBu2-2,6)...

Synthesis.- The basic strategy - condensation of the separate halves of the ring by nucleophilic displacement - remains the same, The role of ion pairing of aryloxide ions and alkali-metal cations was studied. It was found that such ion pairs facilitated cyclisation, except when lithium was the cation. Catalytic efficiency decreases in the same order as for free cations. 

The alkali metal is typically bound to the aryloxide oxygen atoms, although in a number of cases it has been shown that interactions between sodium and the phenoxy ring of aryloxide ligand are an important aspect of the bonding in the mixed metal cluster (Eq. 6.34 ). 

It is also common that mixed metal derivatives of the lanthanides and group 1 metals are the products of synthesis. This can occur for small aryloxides leading to clusters, e.g. [La2Na3(/r4-OAr)3(/z2-OAr)6(dioxane)5] (OAr = OQH4Me-4), as well as in the generation of more discrete species with bulkier aryloxides, e.g. [(thf)Li(/i2-OAr)2La(OAr)2(THF)] (OAr = OC6H3Pr -2,6). In some situations the alkali metal interacts with the aryloxide jr-nucleus, e.g. [Cs( 7r-Ar-0)2La(OAr)2] (OAr = OC6H3P1I-... 

Aryloxides can be used instead of chlorides in the reactions with alkali metal amides [49] ... 

Metal exchange reactions have been used in the preparation of heavy alkali organometallics by reaction of alkali metal alkoxide or aryloxide with an organolithium reagent under precipitation of lithium alkoxide/aryloxide. For these reactions, careful ligand choice enables the separation of the two solid reaction products. Two variants of this reaction for alkaline-earth metal organometallics... 

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