Metal bis(trimethylsilyl)amides (often abbreviated as metal silylamides) are coordination complexes composed of a cationic metal M with anionic bis(trimethylsilyl)amide ligands (the −N(Si(CH3)3)2 monovalent anion, or −N(Si(CH3)3)2 monovalent group, 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. [1] The class of ligands and pioneering studies on their coordination compounds were described by Bürger and Wannagat. [2] [3]
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.
Apart from group 1 and 2 complexes, a general method for preparing metal bis(trimethylsilyl)amides entails reactions of anhydrous metal chloride [4] 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 [5] and sodium [6] complexes are trimeric, and the potassium complex is dimeric in solid state. [7] The lithium reagent may be prepared from n-butyllithium and bis(trimethylsilyl)amine: [8]
The direct reaction of these molten metals with bis(trimethylsilyl)amine at high temperature has also been described: [9]
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).
The calcium and barium complexes may be prepared via the general method, by treating calcium iodide or barium chloride with potassium or sodium bis(trimethylsilyl)amide. [10] [11] 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: [12]
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. [13]
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:
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. [14]
Tin(II) bis(trimethylsilyl)amide is prepared from anhydrous tin(II) chloride [15] and is commercially available. It is used to prepare other metal bis(trimethylsilylamide)s via transmetallation. The group 13 [16] and bismuth(III) bis(trimethylsilyl)amides [17] are prepared in the same manner; the aluminium complex may also be prepared by treating strongly basic lithium aluminium hydride with the parent amine: [16]
An alternative synthesis of tetrasulfur tetranitride entails the use of a metal bis(trimethylsilyl)amide [(Me3Si)2N]2S as a precursor with pre-formed S–N bonds. [(Me3Si)2N]2S is prepared by the reaction of lithium bis(trimethylsilyl)amide and sulfur dichloride (SCl2).
The metal bis(trimethylsilyl)amide [((CH
3)
3Si)
2N]
2S reacts with the combination of SCl2 and sulfuryl chloride (SO2Cl2) to form S4N4, trimethylsilyl chloride, and sulfur dioxide: [18]
Tetraselenium tetranitride, Se4N4, is a compound analogous to tetrasulfur tetranitride and can be synthesized by the reaction of selenium tetrachloride with [((CH
3)
3Si)
2N]
2Se. The latter compound is a metal bis(trimethylsilyl)amide and can be synthesized by the reaction of selenium tetrachloride (SeCl4), selenium monochloride (Se
2Cl
2) and lithium bis(trimethylsilyl)amide. [19]
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. [3] There is some variation however, for instance the synthesis Ti{N(SiMe3)2}3 and V{N(SiMe3)2}3 are prepared using the soluble precursors TiCl3(NMe3)2 or VCl3(NMe3)2, respectively. [20] The melting and boiling points of the complexes decrease across the series, with Group 12 metals being sufficiently volatile to allow purification by distillation. [21]
Iron complexes are notable for having been isolated in both the ferrous (II) and ferric (III) oxidation states. Fe[N(SiMe3)2]3 can be prepared by treating iron trichloride with lithium bis(trimethylsilyl)amide [22] and is paramagnetic as the high spin iron(III) contains 5 unpaired electrons.
Similarly, the two coordinate Fe[N(SiMe3)2]2 complex is prepared by treating iron dichloride with lithium bis(trimethylsilyl)amide: [23]
The dark green Fe[N(SiMe3)2]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. [24] In the solid state it forms a dimer with trigonal planar iron centers and bridging amido groups. [25] 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, {(THF)Fe[N(SiMe3)2]2}. [26] Similar behavior can be seen in Mn(hmds)2 and Co(hmds)2, which are monomeric in the gas phase [24] and dimeric in the crystalline phase. [27] [28] Group 11 complexes are especially prone to oligomerization, forming tetramers in the solid phase. [29] [30] [31] The Lewis acid properties of the group 12 complexes have been reported [32] and the improved E and C numbers for the Zn and Cd complexes are listed in the ECW model.
Compound | Appearance | m.p. (°C) | b.p. (°C) | Spin | Comment |
---|---|---|---|---|---|
Group 3 complexes | |||||
Sc(hmds)3 [33] | Colorless solid | 172-174 | S = 0 | ||
Y(hmds)3 | White solid | 180-184 | 105 °C/10 mmHg (subl.) | S = 0 | Commercially available |
Group 4 complexes | |||||
Ti(hmds)3 [33] | Bright blue solid | S = 1/2 | Prepared from TiCl3(N(CH3)3)2 | ||
Group 5 complexes | |||||
V(hmds)3 [34] | Dark violet solid | 174-176 | S = 1 | Prepared from VCl3(N(CH3)3)2 | |
Group 6 complexes | |||||
Cr(hmds)3 [3] [33] | Apple-green solid | 120 | 110 / 0.5 mmHg (subl.) | S = 3/2 | |
Group 7 complexes | |||||
Mn(hmds)2 [3] [24] | Beige solid | 100 / 0.2 mmHg | S = 5/2 | ||
Mn(hmds)3 [35] | Violet solid | 108-110 | S = 2 | ||
Group 8 complexes | |||||
Fe(hmds)2 [36] | Light green solid | 90-100 / 0.01 mmHg | |||
Fe(hmds)3 [33] | Dark green solid | 120 / 0.5 mmHg (subl.) | S = 5/2 | ||
Group 9 complexes | |||||
Co(hmds) [37] | Black Solid | Tetrameric in the solid state | |||
Co(hmds)2 [2] | Green solid | 73 | 101 / 0.6 mmHg | ||
Co(hmds)3 [35] | Dark olive green solid | 86-88 | S = 2 | ||
Group 10 complexes | |||||
Ni(hmds) [38] | Black solid | >250 | Tetrameric in the solid state | ||
Ni(hmds)2 [3] | Red liquid | 80 / 0.2 mmHg | |||
Group 11 complexes | |||||
Cu(hmds) [3] | Colorless solid | 180 / 0.2 mmHg (subl.) | S = 0 | ||
Ag(hmds) [30] | Colorless solid | S = 0 | Insoluble in hydrocarbons and diethyl ether | ||
Au(hmds) [31] | Colorless solid | S = 0 | |||
Group 12 complexes | |||||
Zn(hmds)2 [21] | Colorless liquid | 12.5 | 82 / 0.5 mmHg | S = 0 | Commercially available |
Cd(hmds)2 [21] | Colorless liquid | 8 | 93 / 0.5 mmHg | S = 0 | |
Hg(hmds)2 [21] | Colorless liquid | 11 | 78 / 0.15 mmHg | S = 0 |
Lanthanide triflates can be convenient anhydrous precursors to many bis(trimethylsilyl)amides: [39]
However it is more common to see the preparation of lanthanide bis(trimethylsilyl)amides from anhydrous lanthanide chlorides, [40] 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.
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.
Compound | Appearance | m.p. (°C) | Comment |
---|---|---|---|
La(hmds)3 | White | 145-149 | |
Ce(hmds)3 | Yellow-brown | 132-140 | |
Pr(hmds)3 | Pale green | 155-158 | |
Nd(hmds)3 | Pale blue | 161-164 | |
Sm(hmds)3 | Pale yellow | 155-158 | |
Eu(hmds)3 | Orange | 159-162 | |
Gd(hmds)3 | White | 160-163 | |
Dy(hmds)3 [41] | Pale green | 157–160 | |
Ho(hmds)3 | Cream | 161-164 | |
Yb(hmds)3 | Yellow | 162-165 | |
Lu(hmds)3 | White | 167-170 |
There has also been some success in the synthesis and characterization of actinide bis(trimethylsilyl)amides. [42] [43] A convenient synthetic route uses the THF-adducts of the iodide salts AnI3(THF)4 as starting materials.
Compound | Appearance | m.p. (°C) | Comment |
---|---|---|---|
U(hmds)3 | Red-purple | 137–140 | Sublimates at 80–100 °C (ca. 10−3 torr) |
Np(hmds)3 | Blue-black | Sublimates at 60 °C (ca. 10−4 torr) | |
Pu(hmds)3 | Yellow-orange | Sublimates at 60 °C (ca. 10−4 torr) |
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.
Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula Li[AlH4] or LiAlH4. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters, carboxylic acids, and amides. The solid is dangerously reactive toward water, releasing gaseous hydrogen (H2). Some related derivatives have been discussed for hydrogen storage.
In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. Organolithium reagents are used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers. They have also been applied in asymmetric synthesis in the pharmaceutical industry. Due to the large difference in electronegativity between the carbon atom and the lithium atom, the C−Li bond is highly ionic. Owing to the polar nature of the C−Li bond, organolithium reagents are good nucleophiles and strong bases. For laboratory organic synthesis, many organolithium reagents are commercially available in solution form. These reagents are highly reactive, and are sometimes pyrophoric.
Europium(III) chloride is an inorganic compound with the formula EuCl3. The anhydrous compound is a yellow solid. Being hygroscopic it rapidly absorbs water to form a white crystalline hexahydrate, EuCl3·6H2O, which is colourless. The compound is used in research.
Lithium diisopropylamide is a chemical compound with the molecular formula LiN(CH 2)2. It is used as a strong base and has been widely utilized due to its good solubility in non-polar organic solvents and non-nucleophilic nature. It is a colorless solid, but is usually generated and observed only in solution. It was first prepared by Hamell and Levine in 1950 along with several other hindered lithium diorganylamides to effect the deprotonation of esters at the α position without attack of the carbonyl group.
Sodium bis(trimethylsilyl)amide is the organosilicon compound with the formula NaN(Si 3)2. This species, usually called NaHMDS, is a strong base used for deprotonation reactions or base-catalyzed reactions. Its advantages are that it is commercially available as a solid and it is soluble not only in ethers, such as THF or diethyl ether, but also in aromatic solvents, like benzene and toluene by virtue of the lipophilic TMS groups.
Trimethylsilyl chloride, also known as chlorotrimethylsilane is an organosilicon compound, with the formula (CH3)3SiCl, often abbreviated Me3SiCl or TMSCl. It is a colourless volatile liquid that is stable in the absence of water. It is widely used in organic chemistry.
Lithium amide or lithium azanide is an inorganic compound with the chemical formula LiNH2. It is a white solid with a tetragonal crystal structure. Lithium amide can be made by treating lithium metal with liquid ammonia:
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:
Bis(trimethylsilyl)amine (also known as hexamethyldisilazane and HMDS) is an organosilicon compound with the molecular formula [(CH3)3Si]2NH. The molecule is a derivative of ammonia with trimethylsilyl groups in place of two hydrogen atoms. An electron diffraction study shows that silicon-nitrogen bond length (173.5 pm) and Si-N-Si bond angle (125.5°) to be similar to disilazane (in which methyl groups are replaced by hydrogen atoms) suggesting that steric factors are not a factor in regulating angles in this case. This colorless liquid is a reagent and a precursor to bases that are popular in organic synthesis and organometallic chemistry. Additionally, HMDS is also increasingly used as molecular precursor in chemical vapor deposition techniques to deposit silicon carbonitride thin films or coatings.
In organic chemistry, the Buchwald–Hartwig amination is a chemical reaction for the synthesis of carbon–nitrogen bonds via the palladium-catalyzed coupling reactions of amines with aryl halides. Although Pd-catalyzed C–N couplings were reported as early as 1983, Stephen L. Buchwald and John F. Hartwig have been credited, whose publications between 1994 and the late 2000s established the scope of the transformation. The reaction's synthetic utility stems primarily from the shortcomings of typical methods for the synthesis of aromatic C−N bonds, with most methods suffering from limited substrate scope and functional group tolerance. The development of the Buchwald–Hartwig reaction allowed for the facile synthesis of aryl amines, replacing to an extent harsher methods while significantly expanding the repertoire of possible C−N bond formations.
Organoactinide chemistry is the science exploring the properties, structure, and reactivity of organoactinide compounds, which are organometallic compounds containing a carbon to actinide chemical bond.
Lithium bis(trimethylsilyl)amide is a lithiated organosilicon compound with the formula LiN(Si(CH3)3)2. It is commonly abbreviated as LiHMDS or Li(HMDS) (lithium hexamethyldisilazide - a reference to its conjugate acid HMDS) and is primarily used as a strong non-nucleophilic base and as a ligand. Like many lithium reagents, it has a tendency to aggregate and will form a cyclic trimer in the absence of coordinating species.
Potassium bis(trimethylsilyl)amide (commonly abbreviated as KHMDS, Potassium(K) HexaMethylDiSilazide) or potassium hexamethyldisilazane is the chemical compound with the formula ((CH3)3Si)2NK. It is a strong, non-nucleophilic base with an approximate pKa of 26 (compare to lithium diisopropylamide, at 36).
Silylation is the introduction of one or more (usually) substituted silyl groups (R3Si) to a molecule. Silylations are core methods for production of organosilicon chemistry. Silanization involves similar methods but usually refers to attachment of silyl groups to solids.
Metal amides (systematic name metal azanides) are a class of coordination compounds composed of a metal center with amide ligands of the form NR2−. Amido complexes of the parent amido ligand NH2− are rare compared to complexes with diorganylamido ligand, such as dimethylamido. Amide ligands have two electron pairs available for bonding.
Tris(trimethylsilyl)amine is the simplest tris(trialkylsilyl)amine which are having the general formula (R3Si)3N, in which all three hydrogen atoms of the ammonia are replaced by trimethylsilyl groups (-Si(CH3)3). Tris(trimethylsilyl)amine has been for years in the center of scientific interest as a stable intermediate in chemical nitrogen fixation (i. e. the conversion of atmospheric nitrogen N2 into organic substrates under normal conditions).
Tris(trimethylsilyl)phosphine is the organophosphorus compound with the formula P(SiMe3)3 (Me = methyl). It is a colorless liquid that ignites in air and hydrolyses readily.
Bis(trimethylsilyl)sulfur diimide is the organosulfur compound with the formula S(NSiMe3)2 (Me = CH3). A colorless liquid, it is a diaza analogue of sulfur dioxide, i.e., a sulfur diimide. It is a reagent in the synthesis of sulfur nitrides. For example, it is a precursor to C2(N2S)2.
(Trimethylsilyl)methyllithium is classified both as an organolithium compound and an organosilicon compound. It has the empirical formula LiCH2Si(CH3)3, often abbreviated LiCH2TMS. It crystallizes as the hexagonal prismatic hexamer [LiCH2TMS]6, akin to some polymorphs of methyllithium. Many adducts have been characterized including the diethyl ether complexed cubane [Li4(μ3-CH2TMS)4(Et2O)2] and [Li2(μ-CH2TMS)2(TMEDA)2].
In chemistry, salt-free reduction describes methodology for reduction of metal halides by electron-rich trimethylsilyl reagents. Traditional reductions of metal halides are accomplished with alkali metals, a process that cogenerates alkali metal salts. Using the salt-free reduction, the reduction of metal halides is accompanied by formation of neutral organic compounds that can be easily removed from the inorganic or organometallic product. In addition to the reduction of metal halides, the reagents associated with this methodology are applicable to deoxygenation of organic substrates.
{{cite book}}
: CS1 maint: multiple names: authors list (link){{cite book}}
: |journal=
ignored (help)