Metal bis(trimethylsilyl)amides
Metal bis(trimethylsilyl)amides (often abbreviated as metal silylamides) are coordination complexes composed of a cationic metal with anionic bis(trimethylsilyl)amide ligands and are part of a broader category of metal amides.
Due to the bulky hydrocarbon backbone metal bis(trimethylsilyl)amide complexes have low lattice energies and are lipophilic . For this reason, they are soluble in a range of nonpolar organic solvents, in contrast to simple metal halides, which only dissolve in reactive solvents. These steric bulky complexes are molecular, consisting of mono-, di-, and tetramers. Having a built-in base, these compounds conveniently react with even weakly protic reagents. The class of ligands and pioneering studies on their coordination compounds were described by Bürger and Wannagat.
The ligands are often denoted hmds (e.g. M(N(SiMe3)2)3 = M(hmds)3) in reference to the hexamethyldisilazide from which they are prepared.
Apart from group 1 and 2 complexes, a general method for preparing metal bis(trimethylsilyl)amides entails reactions of anhydrous metal chloride with an alkali metal bis(trimethylsilyl)amides via a salt metathesis reaction:
Alkali metal chloride formed as a by-product typically precipitates as a solid, allowing for its removal by filtration. The remaining metal bis(trimethylsilyl)amide is then often purified by distillation or sublimation.
Lithium, sodium, and potassium bis(trimethylsilyl)amides are commercially available. When free of solvent, the lithium and sodium complexes are trimeric, and the potassium complex is dimeric in solid state. The lithium reagent may be prepared from n-butyllithium and bis(trimethylsilyl)amine:
The direct reaction of these molten metals with bis(trimethylsilyl)amine at high temperature has also been described:
Alkali metal silylamides are soluble in a range of organic solvents, where they exist as aggregates, and are commonly used in organic chemistry as strong sterically hindered bases. They are also extensively used as precursors for the synthesis other bis(trimethylsilyl)amide complexes (see below).
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