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 ligands are often denoted hmds (e.g. M(N(SiMe3)2)3 = M(hmds)3) in reference to the hexamethyldisilazane from which they are prepared.
General methods of preparation
- MCl + Na(hmds) → M(hmds) + NaCl
Alkali halide 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.
Group 1 complexes
- nBuLi + HN(SiMe3)2 → Li(hmds) + butane
The direct reaction of these molten metals with bis(trimethylsilyl)amine at high temperature has also been described:
- M + HN(SiMe3)2 → MN(SiMe3)2 + 1/2 H2
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).
Group 2 complexes
- 2 BnK + CaI2 + THF → Bn2Ca(thf) + KI
- Bn2Ca(thf) + 2 HN(SiMe3)2 → Ca(hmds)2 + 2 C6H5CH3 + THF
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 bis(trimethylsilyl)amide, itself commercially available.
- Bu2Mg + 2 HN(SiMe3)2 → Mg(hmds)2 + 2 butane
In contrast to group 1 metals, the amine N-H in bis(trimethylsilyl)amine is not acidic enough to react with the group 2 metals, however complexes may be prepared via a reaction of tin(II) bis(trimethylsilyl)amide with the appropriate metal:
- M + 2 HN(SiMe3)2 M(hmds)2 + H2 (M = Mg, Ca, Sr, Ba)
- M + Sn(hmds)2 → M(hmds)2 + Sn
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
- LiAlH4 + 4 HN(SiMe3)2 → Li(hmds) + Al(hmds)3 + 4 H2
An alternative synthesis of tetrasulfur tetranitride entails the use of a metal bis(trimethylsilyl)amide (Me3Si)2N2S as a precursor with pre-formed S–N bonds. (Me3Si)2N2S is prepared by the reaction of lithium bis(trimethylsilyl)amide and sulfur dichloride (SCl2).
- 2 (CH3)3Si2NLi + SCl2 → ((CH3)3Si)2N2S + 2 LiCl
The metal bis(trimethylsilyl)amide reacts with the combination of SCl2 and sulfuryl chloride (SO2Cl2) to form S4N4, trimethylsilyl chloride, and sulfur dioxide:
- 2((CH3)3Si)2N2S + 2SCl2 + 2SO2Cl2 → S4N4 + 8 (CH3)3SiCl + 2SO2
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 bis(trimethylsilyl)amide and can be synthesized by the reaction of selenium tetrachloride (SeCl4), selenium monochloride () and lithium bis(trimethylsilyl)amide.
d-Block complexes
Iron complexes are notable for having been isolated in both the ferrous (II) and ferric (III) oxidation states. FeN(SiMe3)23 can be prepared by treating iron trichloride with lithium bis(trimethylsilyl)amide and is paramagnetic as the high spin iron(III) contains 5 unpaired electrons.
- FeCl3 + 3LiN(SiMe3)2 → FeN(SiMe3)23 + 3LiCl
Similarly, the two coordinate FeN(SiMe3)22 complex is prepared by treating iron dichloride with lithium bis(trimethylsilyl)amide:
- FeCl2 + 2LiN(SiMe3)2 → FeN(SiMe3)22 + 2LiCl
f-Block complexes
- Ln(OTf)3 + 3 M(hmds) → Ln(hmds)3 + 3 MOTf (M = Li, Na, K; Ln = La, Nd, Sm, Er)
However it is more common to see the preparation of lanthanide bis(trimethylsilyl)amides 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 Ln(hmds)3 is soluble but LiCl is not.
- Ln(Cl)3 + 3 HMDS + 3 nBuLi → Ln(hmds)3 + 3 LiCl + 3 butane (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Ho, Yb, and Lu)
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.
There has also been some success in the synthesis and characterization of actinide bis(trimethylsilyl)amides. A convenient synthetic route uses the THF-adducts of the iodide salts AnI3(THF)4 as starting materials.
| Sublimates at 80–100 °C ( ca. 10−3 torr) |
| Sublimates at 60 °C ( ca. 10−4 torr) |
| Sublimates at 60 °C ( ca. 10−4 torr) |
Safety
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