Metal bis(trimethylsilyl)amides


Metal bisamides are coordination complexes composed of a cationic metal with anionic bisamide ligands and are part of a broader category of metal amides.
Due to the bulky hydrocarbon backbone metal bisamide 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 in reference to the hexamethyldisilazide from which they are prepared.

General methods of preparation

Apart from group 1 and 2 complexes, a [|general method] for preparing metal bisamides entails reactions of anhydrous metal chloride with an alkali metal bisamides 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 bisamide is then often purified by distillation or sublimation.
File:2 rotation.gif|thumb|Space-filling model of Fe2. Color scheme: H is white, Fe is gray, N is blue, Si is blue-green.

Group 1 complexes

Lithium, sodium, and potassium bisamides 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 bisamine:
The direct reaction of these molten metals with bisamine 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 bisamide complexes.

Group 2 complexes

The calcium and barium complexes may be prepared via the general method, by treating calcium iodide or barium chloride with potassium or sodium bisamide. However, this method can result in potassium contamination. An improved synthesis involving the reaction of benzylpotassium with calcium iodide, followed by reaction with bisamine results in potassium-free material:
Magnesium silylamides can be prepared from dibutylmagnesium; which is commercially available as a mixture of n-Bu and s-Bu isomers. It deprotonates the free amine to yield the magnesium bisamide, itself commercially available.
In contrast to group 1 metals, the amine N-H in bisamine is not acidic enough to react with the group 2 metals, however complexes may be prepared via a reaction of tin bisamide with the appropriate metal:
Long reaction times are required for this synthesis and when performed in the presence of coordinating solvents, such as dimethoxyethane, adducts are formed. Hence non-coordinating solvents such as benzene or toluene must be used to obtain the free complexes.

p-Block complexes

Tin bisamide is prepared from anhydrous tin chloride and is commercially available. It is used to prepare other metal biss via transmetallation. The group 13 and bismuth bisamides are prepared in the same manner; the aluminium complex may also be prepared by treating strongly basic lithium aluminium hydride with the parent amine:
An alternative synthesis of tetrasulfur tetranitride entails the use of a metal bisamide 2S as a precursor with pre-formed S-N bonds. 2S is prepared by the reaction of lithium bisamide and sulfur dichloride.
The metal bisamide ' reacts with the combination of SCl2 and sulfuryl chloride to form S4N4, trimethylsilyl chloride, and sulfur dioxide:
Tetraselenium tetranitride, Se4N4, is a compound analogous to tetrasulfur tetranitride and can be synthesized by the reaction of selenium tetrachloride with '. The latter compound is a metal bisamide and can be synthesized by the reaction of selenium tetrachloride, selenium monochloride and lithium bisamide.

d-Block complexes

In line with the general method, bisamides of transition metals are prepared by a reaction between the metal halides and sodium bisamide, some variation does exist however, for instance the synthesis of blue Ti3 using the soluble precursor TiCl33. The melting and boiling points of the complexes decrease across the series, with Group 12 metals being sufficiently volatile to allow purification by distillation.
Iron complexes are notable for having been isolated in both the ferrous and ferric oxidation states. Fe3 can be prepared by treating iron trichloride with lithium bisamide and is paramagnetic as the high spin iron contains 5 unpaired electrons.
Similarly, the two coordinate Fe2 complex is prepared by treating iron dichloride with lithium bisamide:
The dark green Fe2 complex exists in two forms depending on its physical state. At room temperature the compound is a monomeric liquid with two-coordinate Fe centers possessing S4 symmetry, in the solid state it forms a dimer with trigonal planar iron centers and bridging amido groups. The low coordination number of the iron complex is largely due to the steric effects of the bulky bisamide, however the complex will bind THF to give the adduct,. Similar behavior can be seen in Mn2 and Co2, which are monomeric in the gas phase and dimeric in the crystalline phase. Group 11 complexes are especially prone to oligomerization, forming tetramers in the solid phase.

f-Block complexes

can be convenient anhydrous precursors to many bisamides:
However it is more common to see the preparation of lanthanide bisamides from anhydrous lanthanide chlorides, as these are cheaper. The reaction is performed in THF and requires a period at reflux. Once formed, the product is separated from LiCl by exchanging the solvent for toluene, in which Ln3 is soluble but LiCl is not.
Silylamides are important as starting materials in lanthanide chemistry, as lanthanide chlorides have either poor solubility or poor stability in common solvents. As a result of this nearly all lanthanide silylamides are commercially available.
CompoundAppearancem.p. Comment
La3White145-149
Ce3Yellow-brown132-140
Pr3Pale green155-158
Nd3Pale blue161-164
Sm3Pale yellow155-158
Eu3Orange159-162
Gd3White160-163
Dy3Pale green157–160
Ho3Cream161-164
Yb3Yellow162-165
Lu3White167-170

There has also been some success in the synthesis and characterization of actinide bisamides. A convenient synthetic route uses the THF-adducts of the iodide salts AnI34 as starting materials.
CompoundAppearancem.p. Comment
U3Red-purple137–140Sublimates at 80–100 °C
Np3Blue-blackSublimates at 60 °C
Pu3Yellow-orangeSublimates at 60 °C

Safety

Metal bisamides are strong bases. They are corrosive, and are incompatible with many chlorinated solvents. These compounds react vigorously with water, and should be manipulated with air-free technique.