Silylamid derivatives

The first examples of compounds with an A-heterocyclic carbene ligand coordinated to a low-valent uranium center were reported in 2004. The synthetic procedures are illustrated in Schemes 7 and 8. Notably, the silylamide derivative (Scheme 8) was isolated in the form of dark blue crystals in excellent yield (>90%). DFT studies indicated a significant degree of 7t-bonding in the U(m) carbene entity.32... 

Consideration of metal-nitrogen bond lengths in light of the ionic-bonding model advanced by Raymond (10) leaves little doubt that the bonding in the binary silylamide derivatives of the lanthanide elements is predominantly ionic (11). Indeed, all of the tris-silylamide derivatives of the p-, d-, and f-block elements can be viewed as being mainly ionic. 

Bis(trimethylsilyl)acetamide. Bis(trimethylsilyl)acetamide (BSA) has been formulated as iV,iV-bis(trimethylsilyl)acetamide as well as the imidate form. The NMR spectra of a number of silylamide derivatives ( C, Si, and 0) have been investigated. Whereas BSA is seen to exist in the imidate form, some analogs, for example bis(trimethylsilyl)formamide, exist in the IVW-bis(trimethylsilyl) form, and the product obtained by the interaction of l,2-bis(chlorodimethylsilyl)ethane with BSA is also clearly in the alternative foim Trimethylsilyl derivatives of hydroxamic acids are formed in reactions using hexamethyl-disilazane and are similar in structure to BSA. This entry will concentrate on examples from the more recent literature. 

The calcium silylamide Ca N(SiMe3)2 2, or its ether adducts, has been used as a synthon to generate numerous other related derivatives. These include phosphanides, arsa-nides," thiolates, " selenolates and tellurolates, as well as a variety of diaza-substimted complexes, some of which were discussed above (see also Ref. 11). 

The first well characterized thallium(I) derivative of a monodentate amide ligand was the silylamide [ TlN(SiMe3)2 2] obtained from LiN(SiMe3)2 and TlCl in toluene. It is a centro-symmetric dimer with an almost perfectly square planar TI2N2 core as shown in Figure 8.12. ... 

The metathesis reaction of metal chloride with alkali alkoxides (method 5) was historically the first approach applied, and it remains the main route to both mono- and bimetallic derivatives of Mn(H). The most efficient approach to Mn(II) derivatives is, however, provided by the alcoholysis of the corresponding silylamide (method 4). The derivatives of Mn(IH) and mixed-valence Mn(H,IH) were obtained by alcoholysis ofMn(III) acetate by polyatomic phenols in the presence of 2,2 -dipyridil accompanied by reduction in the latter case (methods 4 and 4+7). 

V,7V-Diethylcarbamoyl trimethylsilane has been prepared by the reaction of bis(trimethylsilyl) sulphide with bis(A,A-diethylcarbamoyl) mercury (Scheme 30)16. Silylation of the carbamoyl cuprate reagent derived from a lithium amide, by addition of copper(I) cyanide and subsequent exposure to carbon monoxide (1 atm), is also effective75,110. Poor to moderate yields of carbamoyl silanes may be isolated by treatment of lithium silylamides with carbon monoxide and methyl iodide, in a reaction sequence involving a nitrogen to carbon silyl shift in an intramolecular silylation (Scheme 31)111. 

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. 

The present article will focus in particular on structurally characterized complexes and will refer to current trends and potential applications, simultaneously aiming at a coherent picture of the entire family of organometallic lanthanide amides including the inorganic derivatives. The elements Sc, Y, La will be treated as lanthanide elements Ln. Previous reviews cover this subject mostly as an aspect wrapped up under a comprehensive depiction of both metal amides [19] and lanthanide chemistry [20]. Other articles focus on special topics in this field, e.g., inorganic amides [21], silylamides [22], phthalocyanines [23] or porphyrins [24],... 

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. 

Silylamido groups are used throughout the periodic table to stabilize unusual types of bonding, coordination numbers and oxidation states. Lithium silylamides, generally prepared by reaction of the silylamine with alkyl- or aryllithium derivatives, play an important role in the synthesis of such molecules. 

---