The Silylamide Route

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 

In particular the redox stability of lanthanide cations makes the route superior to, e.g., the usual metathesis reaction involving lanthanide halides. The advantages are evident  

A limiting factor of this specific btsa route is the steric bulk of the btsa ligand. Hence the exchange reaction with other bulky ligands such as Cp H, tritox-H or phosphanes can be totally suppressed or is incomplete as in, e.g., KSmCpJ [N(SiMe3)2] [148]. 

The influence of steric factors was thoroughly studied in the reaction of Ln(btsa)3 with the alcohol tritox-H. While the reaction takes place with larger lanthanides like neodymium to yield the homoleptic alkoxide complexes (Eq. 18) [264], the analogous reaction does not work with smaller metals like yttrium and thulium (Eq. 19). However, variation of the reaction conditions to a stoichiometric solid reaction yielded a fully exchanged product along with an unexpected and unusual silylamine degradation [265] (Eq. 20). This degradation reaction seems to be sterically forced and points out N-Si bond disruptions and C-Si bond formations under mild conditions [114]. 

back to the tritox-reaction. The reactivity-determining influence of the ligand constitution has been examined by introduction of the sterically less crowded bdsa amide ligand (Fig. 27) [114]. This ligand offers a valuable extension of the silylamide route  

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Considering the organometallic side of Scheme III (right), usually all the compounds located on this side are able to produce the neighboring systems on the left by Broensted acid-base-type reactions, e.g., alkyls might readily react with amines, cyclopentadienes and alcohols to yield amide, alkoxide and cyclo-pentadienyl complexes, respectively. The silylamide route has attracted enormous attention for the synthesis of pure alkoxides (A in Scheme III, Eq. 12)... 

Ln(btsa)2(PR2). The diphenylphosphido complex La(btsa)2(PPh2)(OPPh3)2 (La-P, 3.165(6) A) was prepared according to the silylamide route [130b]. 

The silylamide route is also transferable to lanthanide(II) chemistry. However, now steric effects are of minor importance due to the enhanced steric flexibility of Ln(II) amides. Eu(II) and Yb(II) silylamides are accessible to exchange reactions for all reagents listed in Scheme 6. For example... 

Yb(tritox)2]2 is readily formed [150]. The strongly reducing and thermally unstable Sm(II) amides tend to get oxidized by alcohols under the usual conditions [268], However, aryloxides have been isolated, especially when they are stabilized by ate complexation as in the case of [KSm(OC6H3-2,6-tBu2)3 (THF)]n [269]. In addition, the latter complex represents another approach of the silylamide route, namely the generation of mixed metallic complexes by retention of the original metal composition of the amide. 

Synthetic strategies to alkoxide complexes have been covered in full by previous reviews [14]. The silylamide route proved to be an advantageous method of preparation, especially in the case of homoleptic derivatives [15]. The group (IIIA) elements - scandium, yttrium and lanthanum - are considered as lanthanides on the basis of their general chemical similarity to the true lanthanides. 

At this point the author would like to emphasize the importance and flexibility of the silylamide route. Ln(tritox)3 complexes are readily available from Ln[N(SiMe3)2]3 only for the larger lanthanide elements. However, the decreased steric bulk in Ln[N(SiHMe2)2]3(THF)2 amide precursors also allows the isolation of Ln(tritox)3 complexes of the smaller lanthanide elements [16, 40]. 

Use of less basic triphenylsiloxide ligands also allowed the isolation of mononuclear complexes (Table 1) [46,47]. In addition to the silylamide route, anhydrous nitrates and isopropoxides were employed as synthetic precursors (Eqs. 1,2). The siloxide bridges in the solvent-free dimeric systems Ln2(OSiPh3)6 are readily disrupted by donor solvents like THF, OP Bu3 or DME. The m s-THF adducts of lanthanum, cerium (Fig. 5) and yttrium adopt an approximately /ac-octahedral geometry. 

CHF-alkoxides were prepared from anhydrous LnCl3 and alcohol in the presence of ammonia or according to the silylamide route. In both cases coordination of ammonia to the metal center could be observed. In the latter procedure ammonia is generated by excess of alcohol according to Eq. (8). 

The interplay of steric constraints and surplus Do functionalities create a ligand atmosphere which, for the first time, allowed the isolation of defined mixed ligand (amide/alkoxide) systems according the silylamide route. Both... 

A stabilizing option of the functionalized and highly hygroscopic ligand 2,4,6-tris(dimethylaminomethyl)phenol was demonstrated by the isolation of the remarkable water adduct complex Pr[0C6H2(CH2NMe2)3-2,4,6]3(H20)2 according to the silylamide route [129]. 

Solvent free LnL complexes (Type I) resulted from the silylamide route [184]. Crystals of Yb(trac) could be grown by sublimation (180 °C/10 2 Torr). The heptadentate ligand is wrapped around the ytterbium emerging in a disordered geometry. Both enantiomers, the right- (A) and left-handed (A) screw, are present in the unit cell. 

Factors which often make the silylamide route superior to traditional salt metathesis reactions are (i) the reaction in non-coordinating solvents due to the high solubility of the monomeric metal amides, (ii) mild reaction conditions often at ambient temperature, (iii) avoidance of halide contamination, (iv) ease of product purification [removal of the released amine along with the solvent under vacuum (bp HN(SiMe3)2 125 °C)], (v) base-free products (coordination of the sterically demanding, released amine is disfavored), (vi) quantitative yield , and (vii) the facile availability of mono- and heterobimetallic amide precursors. 

Scheme 7. Heterobimetallic Sm(II) complexes according to the silylamide route...

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