Metal bis(trimethylsilyl)amides

The bis(trimethylsilyl)amide ligand

Metal bis(trimethylsilyl)amides are coordination complexes of a metal cation with bis(trimethylsilyl)amide ligands. These ligands are often denoted hmds (from hexamethyldisilazide). These compounds are part of a broader category of metal amides.

Metal bis(trimethylsilyl)amide complexes are lipophilic due to the ligand, thus soluble in nonpolar organic solvents. Because of the bulk of the ligands, their complexes are usually discrete and often monomeric. As such they are more reactive than polymeric metal halides. Having a built-in base, these compounds conveniently react with protic ligand precursors to give other metal complexes.^[1] The class of ligands and pioneering studies on their coordination compounds were described by Bürger and Wannagat.^[2][3]

General methods of preparation

Apart from group 1 and 2 complexes, a general method for preparing many metal bis(trimethylsilyl)amides is to react the anhydrous metal chloride^[4] with one of the alkali metal bis(trimethylsilyl)amides:

MCl_x + x Na(hmds) → M(hmds)_x + x NaCl

The alkali metal chloride precipitates, which may be removed by filtration. The resultant metal bis(trimethylsilyl)amide is often purified by distillation or sublimation.

Group 1 complexes

Main articles: lithium bis(trimethylsilyl)amide, sodium bis(trimethylsilyl)amide, and potassium bis(trimethylsilyl)amide

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]

n-C₄H₉Li + HN(SiMe₃)₂ → Li(hmds) + C₄H₁₀

The direct reaction of these molten reactive metals with bis(trimethylsilyl)amine at high temperature has been described as well:^[9]

M + HN(SiMe₃)₂ → MN(SiMe₃)₂ + 1/2 H₂

These reagents are soluble in organic solvent, where they exist as aggregates. They are often used as strong bases. Like lithium diisopropylamide, these reagents are sterically hindered bases. In inorganic chemistry, these reagents are precursors for other bis(trimethylsilyl)amide complexes via metathesis reactions (see below).

Group 2 complexes

The amine N-H is not acidic enough to react with the group 2 metals, but they may be prepared by 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. In the presence of dimethoxyethane, the dimethoxyethane adducts are formed. To obtain the free complexes, non-coordinating solvents like benzene or toluene are used. After filtration, the products crystallize on cooling.^[10]

The calcium complex may also be prepared by reaction of calcium iodide with potassium bis(trimethylsilyl)amide, but 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:^[11]

2 BzK + CaI₂ + THF → Bz₂Ca(thf) + KI

Bz₂Ca(thf) + 2 HN(SiMe₃)₂ → Ca(hmds)₂ + 2 C₆H₅CH₃ + THF

p-Block complexes

Tin(II) bis(trimethylsilyl)amide, an orange-red solid. This compound is commercially available. It is also prepared by reaction of lithium bis(trimethylsilyl)amide with anhydrous tin(II) choride, vacuum distillation, and crystallization.^[12]

The group 13 bis(trimethylsilyl)amides are prepared in the same fashion. These compounds are isolated by filtration, and purified by sublimation. The aluminium complex may also be prepared by treating strongly basic lithium aluminium hydride with the parent amine:^[13]

LiAlH₄ + 4 HN(SiMe₃)₂ → Li(hmds) + Al(hmds)₃ + 4 H₂

d-Block complexes

Frozen zinc bis(trimethylsilyl)amide. This compound melts at 12.5 °C.

The group 12 bis(trimethylsilyl)amides may be prepared by treating zinc chloride, cadmium iodide, and mercury bromide with sodium bis(trimethylsilyl)amide to give the corresponding products as liquids after filtration and evaporation, which are purified by distillation.^[14] The zinc complex is a colorless liquid; it is commercially available.

The manganese(II), nickel(II), copper(II) complexes may be prepared via reaction of the corresponding iodide salt with sodium bis(trimethylsilyl)amide. The cobalt(II) complex is similarly prepared from the chloride. The liquid, blue ferrous derivative is similarly prepared from ferrous chloride. It exists as an equilibrium mixture of monomer and dimer, forming a monomeric three-coordinate adduct with THF.^[15] The dark green iron(III), apple-green chromium(III), and brown vanadium(III) complexes is prepared by treating two equivalents of the metal trichlorides with three equivalents of sodium bis(trimethylsilyl)amide:^[3][16]

FeCl₃ + 3 Na(hmds) → Fe(hmds)₃ + Na₃FeCl₆

The same metathesis method is used for the blue titanium(III) derivative, using the soluble precursor TiCl₃(trimethylamine)₃.

Compound	Appearance	m.p. (°C)	b.p. (°C)	Comment
Group 3 complexes
Sc(hmds)3[17]	Colorless solid	172-174
Y(hmds)3	White solid	180-184	105°C/10 mmHg (subl.)	Commercially available
Group 4 complexes
Ti(hmds)3[17]	Bright blue solid			Paramagnetic. Prepared from TiCl3(N(CH3)3)2
Group 5 complexes
V(hmds)3[17]	Brown solid			Prepared from VCl3(N(CH3)3)2
Group 6 complexes
Cr(hmds)3[3][17]	Apple-green solid	120	110 / 0.5 mmHg (subl.)	Paramagnetic
Group 7 complexes
Mn(hmds)2[3]	Beige solid		100 / 0.2 mmHg
Group 8 complexes
Fe(hmds)2[18]	Light green solid		90-100 / 0.01 mmHg
Fe(hmds)3[17]	Dark green solid		120 / 0.5 mmHg (subl.)	Paramagnetic
Group 9 complexes
Co(hmds)2[2]	Green solid	73	101 / 0.6 mmHg
Group 10 complexes
Ni(hmds)2[3]	Red liquid		80 / 0.2 mmHg
Group 11 complexes
Cu(hmds)[3]	Colorless solid		180 / 0.2 mmHg (subl.)
Group 12 complexes
Zn(hmds)2[14]	Colorless liquid	12.5	82 / 0.5 mmHg	Commercially available
Cd(hmds)2[14]	Colorless liquid	8	93 / 0.5 mmHg
Hg(hmds)2[14]	Colorless liquid	11	78 / 0.15 mmHg

Lanthanides

Lanthanide triflates can be convenient anhydrous precursors to many bis(trimethylsilyl)amides:^[19]

Ln(OTf)₃ + 3 M(hmds) → Ln(hmds)₃ + 3 MOTf (M = Li, Na, K; Ln = La, Nd, Sm, Er)

Compound	Appearance	m.p. (°C)	b.p. (°C)	Comment
La(hmds)3		149-153		Commercially available
Ce(hmds)3		132-140		Commercially available
Nd(hmds)3		156-159		Commercially available
Sm(hmds)3		93-106		Commercially available
Eu(hmds)3		147-154		Commercially available
Gd(hmds)3		67-90		Commercially available
Ho(hmds)3		165-169		Commercially available
Tm(hmds)3		159-164		Commercially available
Yb(hmds)3		162-164		Commercially available

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.

References

1. ^ Michael Lappert, Andrey Protchenko, Philip Power, Alexandra Seeber (2009). Metal Amide Chemistry. Weinheim: Wiley-VCH. doi:10.1002/9780470740385. ISBN 0470721847. 
2. ^ ^{a b} H. Bürger and U. Wannagat (1963). "Silylamido-Derivate von Eisen und Kobalt". Monatshefte für Chemie 94: 1007–1012. doi:10.1007/BF00905688. 
3. ^ ^{a b c d e f} H. Bürger and U. Wannagat (1963). "Silylamido-Derivate von Chrom, Mangan, Nickel und Kupfer". Monatshefte für Chemie 95: 1099–1102. doi:10.1007/BF00904702. 
4. ^ Many metal chlorides may be dried by refluxing in thionyl chloride. See Alfred R. Pray, Richard F. Heitmiller, Stanley Strycker (1990). Anhydrous Metal Chlorides. Inorg. Synth.. 28. pp. 321–323. doi:10.1002/9780470132593.ch80. ISBN 9780470132593. 
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6. ^ Driess, Matthias; Pritzkow, Hans; Skipinski, Markus; Winkler, Uwe (1997). "Synthesis and Solid State Structures of Sterically Congested Sodium and Cesium Silyl(fluorosilyl)phosphanide Aggregates and Structural Characterization of the Trimeric Sodium Bis(trimethylsilyl)amide". Organometallics 16 (23): 5108–5112. doi:10.1021/om970444c. 
7. ^ Tesh, Kris F.; Hanusa, Timothy P.; Huffman, John C. (1990). "Ion pairing in [bis(trimethylsilyl)amido]potassium: The x-ray crystal structure of unsolvated [KN(SiMe3)2]2". Inorg. Chem. 29 (8): 1584–1586. doi:10.1021/ic00333a029. 
8. ^ Amonoo-Neizer, E. H.; Shaw, R. A.; Skovlin, D. O.; Smith, B. C.; Rosenthal, Joel W.; Jolly, William L. (1966). "Lithium Bis(Trimethylsilyl)Amide and Tris(Trimethylsilyl)Amine". Inorg. Synth. 8: 19–22. doi:10.1002/9780470132395.ch6. ISBN 9780470132395. 
9. ^ US 5420322 
10. ^ Westerhausen, Matthias. (1991). "Synthesis and spectroscopic properties of bis(trimethylsilyl)amides of the alkaline-earth metals magnesium, calcium, strontium, and barium". Inorg. Chem. 30: 96–101. doi:10.1021/ic00001a018. 
11. ^ Johns, Adam M.; Chmely, Stephen C.; Hanusa, Timothy P. (2009). "Solution Interaction of Potassium and Calcium Bis(trimethylsilyl)amides; Preparation of Ca[N(SiMe3)2]2from Dibenzylcalcium". Inorg. Chem. 48 (4): 1380–1384. doi:10.1021/ic8012766. 
12. ^ Schaeffer, Charles D.; Myers, Lori K.; Coley, Suzanne M.; Otter, Julie C.; Yoder, Claude H. (1990). "Preparation, analysis, and reactivity of bis[N,N-bis(trimethylsilyl)amino]tin(II): An advanced undergraduate laboratory project in organometallic synthesis". J. Chem. Ed. 67 (4): 347. doi:10.1021/ed067p347. 
13. ^ Bürger, H (1971). "Beiträgezur chemie der silicium-stickstoff-verbindungen CVII. Darstellung, schwingungsspektren und normalkoordinatenanalyse von disilylamiden der 3. Gruppe: M[N(SiMe3)2]3 mit M = Al, Ga und In". J. Organomet. Chem. 33: 1–12. doi:10.1016/s0022-328x(00)80797-4. 
14. ^ ^{a b c d} Bürger, H (1965). "Darstellung und schwinkungsspektren von silylamiden der elemente zink, cadmium und quecksilber". J. Organomet. Chem. 3 (2): 113–120. doi:10.1016/s0022-328x(00)84740-3. 
15. ^ Y. Ohki, S. Ohta, and K. Tatsumi "Monomeric Iron(II) Complexes Having Two Sterically Hindered Arylthiolates" Inorg. Syn. 2010, vol. 35, 137. doi:10.1002/9780470651568.ch7
16. ^ D. C. Bradley and Copperthwaite "Transition Metal Complexes of Bis(trimethylsilyl)amine" Inorg. Synth. 1978, vol. 18, 112–120 (1978)). doi:10.1002/9780470132494.ch18
17. ^ ^{a b c d e} D. C. Bradley and R.G. Copperthwaite (1978). Transition Metal Complexes of Bis(trimethylsilyl)amine. Inorg. Synth.. 18. pp. 112–120. doi:10.1002/9780470132494.ch18. ISBN 9780470132494. 
18. ^ Y. Ohki, S. Ohta, and K. Tatsumi (2010). Monomeric Iron(II) Complexes Having Two Sterically Hindered Arylthiolates. Inorg. Synth.. 35. pp. 137. doi:10.1002/9780470651568.ch7. ISBN 9780470651568. 
19. ^ Schuetz, Steven A.; Day, Victor W.; Sommer, Roger D.; Rheingold, Arnold L.; Belot, John A. (2001). "Anhydrous Lanthanide Schiff Base Complexes and Their Preparation Using Lanthanide Triflate Derived Amides". Inorg. Chem. 40 (20): 5292–5295. doi:10.1021/ic010060l.