![MN(tms)2](https://upload.wikimedia.org/wikipedia/commons/a/a4/MN%28tms%292.png)
Metal bis(trimethylsilyl)amides (often abbreviated as metal silylamides) are
coordination complex
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
es composed of a cationic metal with anionic bis(trimethylsilyl)amide ligands and are part of a broader category of
metal amides Metal amides (systematic name metal azanides) are a class of coordination compounds composed of a metal center with amide ligands of the form NR2−. Amide ligands have two electron pairs available for bonding. In principle, they can be terminal or ...
.
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
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...
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(SiMe
3)
2)
3 = M(hmds)
3) in reference to the
hexamethyldisilazane from which they are prepared.
General methods of preparation
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
A salt metathesis reaction, sometimes called a double displacement reaction, is a chemical process involving the exchange of bonds between two reacting chemical species which results in the creation of products with similar or identical bonding a ...
:
: MCl
+ Na(hmds) → M(hmds)
+ NaCl
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.
Group 1 complexes
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
''n''-Butyllithium C4H9Li (abbreviated ''n''-BuLi) is an organolithium reagent. It is widely used as a polymerization initiator in the production of elastomers such as polybutadiene or styrene-butadiene-styrene (SBS). Also, it is broadly emp ...
and
bis(trimethylsilyl)amine
Bis(trimethylsilyl)amine (also known as hexamethyldisilazane and HMDS) is an organosilicon compound with the molecular formula CH3)3Sisub>2NH. The molecule is a derivative of ammonia with trimethylsilyl groups in place of two hydrogen atoms. An e ...
:
:
nBuLi + HN(SiMe
3)
2 → Li(hmds) + butane
The direct reaction of these molten metals with bis(trimethylsilyl)amine at high temperature has also been described:
: M + HN(SiMe
3)
2 → MN(SiMe
3)
2 + 1/2 H
2
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
The calcium and barium complexes may be prepared via the
general method
A general officer is an officer of high rank in the armies, and in some nations' air forces, space forces, and marines or naval infantry.
In some usages the term "general officer" refers to a rank above colonel."general, adj. and n.". OED On ...
, by treating calcium iodide or barium chloride with potassium or sodium bis(trimethylsilyl)amide. However, this method can result in potassium contamination. An improved synthesis involving the reaction of benzylpotassium with calcium iodide, followed by reaction with bis(trimethylsilyl)amine results in potassium-free material:
:2 BnK + CaI
2 + THF → Bn
2Ca(thf) + KI
:Bn
2Ca(thf) + 2 HN(SiMe
3)
2 → Ca(hmds)
2 + 2 C
6H
5CH
3 + THF
Magnesium silylamides can be prepared from
dibutylmagnesium
Dibutylmagnesium is an organometallic chemical compound of magnesium. Its chemical formula is . Dibutylmagnesium is a chemical compound from the group of organomagnesium compounds. The pure substance is a waxy solid. Commercially, it is marketed ...
; 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.
:Bu
2Mg + 2 HN(SiMe
3)
2 → Mg(hmds)
2 + 2 butane
In contrast to group 1 metals, the amine N-H in
bis(trimethylsilyl)amine
Bis(trimethylsilyl)amine (also known as hexamethyldisilazane and HMDS) is an organosilicon compound with the molecular formula CH3)3Sisub>2NH. The molecule is a derivative of ammonia with trimethylsilyl groups in place of two hydrogen atoms. An e ...
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(SiMe
3)
2 M(hmds)
2 + H
2 (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
Dimethoxyethane, also known as glyme, monoglyme, dimethyl glycol, ethylene glycol dimethyl ether, dimethyl cellosolve, and DME, is a colorless, aprotic, and liquid ether that is used as a solvent, especially in batteries. Dimethoxyethane is misci ...
, adducts are formed. Hence non-coordinating solvents such as benzene or toluene must be used to obtain the free complexes.
p-Block complexes
Tin(II) bis(trimethylsilyl)amide is prepared from anhydrous tin(II) chloride and is commercially available. It is used to prepare other metal bis(trimethylsilylamide)s via
transmetallation Transmetalation (alt. spelling: transmetallation) is a type of organometallic reaction that involves the transfer of ligands from one metal to another. It has the general form:
:M1–R + M2–R′ → M1–R′ + M2–R
where R and R′ can be, but ...
. The group 13
and bismuth(III) bis(trimethylsilyl)amides are prepared in the same manner; the aluminium complex may also be prepared by treating strongly basic
lithium aluminium hydride
Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula Li Al H4. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic ...
with the parent amine:
: LiAlH
4 + 4 HN(SiMe
3)
2 → Li(hmds) + Al(hmds)
3 + 4 H
2
An alternative synthesis of
tetrasulfur tetranitride
Tetrasulfur tetranitride is an inorganic compound with the formula . This gold-poppy coloured solid is the most important binary sulfur nitride, which are compounds that contain only the elements sulfur and nitrogen. It is a precursor to many S-N ...
entails the use of a metal bis(trimethylsilyl)amide
3Si)2N">Me3Si)2Nsub>2S as a precursor with pre-formed S–N bonds.
3Si)2N">Me3Si)2Nsub>2S is prepared by the reaction of lithium bis(trimethylsilyl)amide and
sulfur dichloride
Sulfur dichloride is the chemical compound with the formula . This cherry-red liquid is the simplest sulfur chloride and one of the most common, and it is used as a precursor to organosulfur compounds. It is a highly corrosive and toxic substance, ...
(SCl
2).
:2
3)3Si">CH3)3Sisub>2NLi + SCl
2 →
3)3Si)2N">(CH3)3Si)2Nsub>2S + 2 LiCl
The metal bis(trimethylsilyl)amide reacts with the combination of SCl
2 and sulfuryl chloride (SO
2Cl
2) to form S
4N
4,
trimethylsilyl chloride
Trimethylsilyl chloride, also known as chlorotrimethylsilane is an organosilicon compound ( silyl halide), with the formula (CH3)3SiCl, often abbreviated Me3SiCl or TMSCl. It is a colourless volatile liquid that is stable in the absence of water. ...
, and sulfur dioxide:
:2
3)3Si)2N">(CH3)3Si)2Nsub>2S + 2SCl
2 + 2SO
2Cl
2 → S
4N
4 + 8 (CH
3)
3SiCl + 2SO
2
Tetraselenium tetranitride
Tetraselenium tetranitride is the inorganic compound with the formula . Like the analogous tetrasulfur tetranitride , is an orange solid. It is however less soluble and more shock-sensitive than .
As determined by X-ray crystallography, adopts a ...
, Se
4N
4, 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
Selenium tetrachloride is the inorganic compound composed with the formula SeCl4. This compound exists as yellow to white volatile solid. It is one of two commonly available selenium chlorides, the other example being selenium monochloride, Se2 ...
(SeCl
4),
selenium monochloride () and lithium bis(trimethylsilyl)amide.
d-Block complexes
![Ti(HMDS)3_(left)_and_V(HMDS)3_(right)](https://upload.wikimedia.org/wikipedia/commons/5/5f/Ti%28HMDS%293_%28left%29_and_V%28HMDS%293_%28right%29.jpg)
In line with the general method, bis(trimethylsilyl)amides of transition metals are prepared by a reaction between the metal halides (typically chlorides) and an alkali metal bis(trimethylsilyl)amide.
There is some variation however, for instance the synthesis Ti
3 and V
3 are prepared using the soluble precursors TiCl
3(
NMe3)
2 or VCl
3(
NMe3)
2, respectively. 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 (II) and ferric (III) oxidation states. Fe
3)2">(SiMe3)2sub>3 can be prepared by treating iron trichloride with lithium bis(trimethylsilyl)amide and is
paramagnetic
Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, d ...
as the high spin iron(III) contains 5 unpaired electrons.
: FeCl
3 + 3LiN(SiMe
3)
2 → Fe
3)2">(SiMe3)2sub>3 + 3LiCl
Similarly, the two coordinate Fe
3)2">(SiMe3)2sub>2 complex is prepared by treating iron dichloride with lithium bis(trimethylsilyl)amide:
: FeCl
2 + 2LiN(SiMe
3)
2 → Fe
3)2">(SiMe3)2sub>2 + 2LiCl
![FeHMDS3](https://upload.wikimedia.org/wikipedia/commons/5/54/FeHMDS3.jpg)
The dark green Fe
3)2">(SiMe3)2sub>2 complex exists in two forms depending on its physical state. In the gas phase, the compound is a monomeric with two-coordinate Fe 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 bis(trimethylsilyl)amide, however the complex will bind THF to give the adduct, . Similar behavior can be seen in Mn(hmds)
2 and Co(hmds)
2, 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.
The Lewis acid properties of the group 12 complexes have been reported and the improved E and C numbers for the Zn and Cd complexes are listed in the
ECW model.
f-Block complexes
Lanthanide triflates can be convenient anhydrous precursors to many bis(trimethylsilyl)amides:
: 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 AnI
3(THF)
4 as starting materials.
Safety
Metal bis(trimethylsilyl)amides 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 Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of compounds that are air-sensitive. These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less comm ...
.
References
{{Coordination complexes
*
Coordination complexes