Aryloxides lanthanide

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... 

Corresponding 5-coordinate samarium complexes can also be accomplished (Table 5) [74,75] and heteroatom stabilization of the strongly reducing Sm(II) was observed in the ate complex [KSm(OC6H3tBu2-2,6-Me-4)3(THF)]n [75]. Indefinite chains or higher aggregations by K arene interactions seems to be a common coordination mode in lanthanide aryloxide complexes. 

Simple CH-alkoxides continue to reveal surprising new structure chemistry and the composition of Ln(OMe)3 and Ln(OEt)3 is not yet determined. Mononuclear lanthanide aryloxide complexes are well-examined and are the only fully characterized 3-coordinate, homoleptic Ln(OR)3 species. The variation of the steric environment in pure CH- and CHF-alkoxides with respect to volatility (molecular mass optimization) seems to be rather exhausted. It was shown that functionalization can ensure volatility and although the entire MOCVD-busi-ness gets somewhat stuck, CHF- and CHdo-alkoxides are promising alternatives to the /J-diketonates. Sol gel technology can fall back on highly soluble CH- and CHdo-alkoxides. 

Scheme 33 Mechanistic scenario for the formation of differently alkylated lanthanide aryloxide complexes (adopted from [188])...

TablelO Polymerization of isoprene with lanthanide aryloxides Ln(OArf Bu Me)3(AlMe3) and Ln(OAr -Pr H)3(AlMe3)2...

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

Organolanthanide catalysts, such as the divalent samarium complex, trivalent organolanthanide alkyls, lanthanide aryloxides, alkoxides, guanidinates, and thiolates,... 

For alkoxides of the Group I and II elements and the lanthanides, typical adducts involve coordinating O-donors such as Et20 or THF. For example, the sparingly soluble, anhydrous aryloxide LiOAr (OAr = 2,6-di-r-butylphenoxide) will dissolve in ethers to give dimeric adducts of the type [(OAr )Li(L)]2 (L = Et20, THF, py), which have been structurally characterized.22 ... 

The same group also showed that mono(cyclopentadienyl) mixed hydride/ aryloxide dimer complexes of several lanthanide elements (Y, Dy, Lu) could be synthesized easily by the acid-base reaction between the mixed hydride/alkyl complexes and an aryl alcohol [144]. These complexes reacted with C02 to generate mixed formate/carboxylate derivatives, which were moderately active initiators for the copolymerization of C02 and cyclohexene oxide, without requiring a co-catalyst. The lutetium derivative 21 was the most active (at 110°C, TOF = 9.4 h ), yet despite a good selectivity (99% carbonate linkages), the molecular weight distribution remained broad (6.15) (Table 6). 

As can be seen from Scheme III, lanthanide halides are suitable precursors for the synthesis of homoleptic derivatives such as silylamides [114], cyclopen-tadienyls [115] and aryloxides [116]. Such organometallies can be readily obtained in a pure form by sublimating them from the reaction mixture. They themselves are important precursors in organometallic transformations (vide infra). Heteroleptic complexes of the type CpxLn(halide)y (x + y = 2,3) are important synthetic precursors with respect to formation of various Ln-X bonds via simple metathesis reactions [2-29]. Fig. 4 indicates the lanthanide element bonds which are involved in these ubiquitous heteroleptic cyclopentadienyl systems. 

Unlike their O-counterparts, namely the aryloxides [92], the anilides have gained entry into lanthanide chemistry only very recently [93]. Their suitability as ligands in /-element chemistry was demonstrated in [K(THF)2][U(NHC6H3iPr2-2,6)5] [94],... 

The synthesis of pure precursor molecules is crucial, e.g., Ln(OR)3 or Ln(SeR)2. Well-defined compounds which contain specific element compositions are attractive for ceramic and electronic materials or catalysts in organic transformations. Hitchcock et al. reported the synthesis of monomeric homo-leptic lanthanide(III) aryloxides according to this route for the first time in 1983... 

A recent development in this field involves the combination of benzamidi-nate ligands with cyclopentadienyl or cyclooctatetraenyl ligands in the coordination sphere of lanthanide ions. The first lanthanide complexes containing both benzamidinate and pentamethylcyclopentadienyl bonded to yttrium was reported by Teuben et al. Yttrium aryloxides as well as anhydrous yttrium trichloride can be used as starting materials in these preparations (Eq. 19, Scheme 5) [27] ... 

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... 

The use of unsubstituted or 4-methyl phenols resulted in the formation of cluster compounds [58]. However, 2,6-di(fcrt-butyl) substituted aryloxide ligands allowed the isolation of mononuclear 3-coordinate homoleptie complexes of the lanthanide elements, the coordination mode of which was first demonstrated with the N(SiMe3)2 ligand [59], The 2,6-substitution pattern is very effective because the alkyl groups are directed towards the metal center and impose a steric coordination number onto the metal which is comparable to the Cp ligand (Cp 2.49 OC6H3rBu2-2,6 2.41) [60],... 

Chart 3 Alkoxide and aryloxide ligands used in lanthanide diene polymerization catalysts and model compounds... 

Aryloxide (phenolate) ligands provide rigid and versatile ancillary ligand sets in order to study the multifunctional reactivity of alkylaluminum and alkylmagnesium reagents toward Ln-OR moieties. Several types of symmetrically substituted phenolate ligands — summarized in Chart 4 — were employed for the synthesis of a variety of heterobimetallic lanthanide(III) and lanthanide(II) metal complexes. Alkylation reactions revealed the preferred... 

Heterogeneous diene polymerization catalysts based on modified and unmodified silica-supported lanthanide complexes are known as efficient gas-phase polymerization catalysts for a variety of support materials and activation procedures (see Sect. 9). Metal siloxide complexes M(()SiR3 )x are routinely employed as molecular model systems of such silica-immobilized/ grafted metal centers [196-199]. Structurally authenticated alkylated rare-earth metal siloxide derivatives are scarce, which is surprising given that structural data on a considerable number of alkylated lanthanide alkoxide and aryloxide complexes with a variety of substitution patterns is meanwhile available. 

A few of these have been made in which lanthanides are refluxed with arenes under Friedel-Crafts conditions, such as [(C6Me6)Sm(AlCl4)3] (Figure 6.22). These appear to have the lanthanide in normal oxidation states the arene group is bound Interactions between a lanthanide and an arene ring are also well-authenticated in what would otherwise be coordinatively unsaturated aryloxides, like [Yb(OC6H3Ph2)3]. 

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