Metal bis(trimethylsilyl)amides

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The bis(trimethylsilyl)amide ligand

Metal bis(trimethylsilyl)amides (often abbreviated as metal silylamides) are coordination complexes composed of a cationic metal with anionic bis(trimethylsilyl)amide ligands and are part of a broader category of metal amides.

Metal bis(trimethylsilyl)amide complexes are lipophilic due to the ligand and hence are soluble in a range of nonpolar organic solvents, this often makes them more reactive than the corresponding metal halides, which can difficult to solubilise. The steric bulk of the ligands causes their complexes to be discrete and monomeric (with the exception of group 1 complexes). 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]

The ligands are often denoted hmds (e.g. M(N(SiMe3)2)3 = M(hmds)3) 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 many metal bis(trimethylsilyl)amides is to react the anhydrous metal chloride[4] with one of the alkali metal bis(trimethylsilyl)amides via a salt metathesis reaction:

MClx + x Na(hmds) → M(hmds)x + x NaCl

Alkali metal chloride is formed as a by-product but typically precipitates, allowing for its removal by filtration. The remaining metal bis(trimethylsilyl)amide is then often purified by distillation or sublimation.

Space-filling model of Fe[N(tms)2]2. Color scheme: H is white, Fe is gray, N is blue (barely visible), Si is blue-green.

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]

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:[9]

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

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]

2 BzK + CaI2 + THF → Bz2Ca(thf) + KI
Bz2Ca(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.[13]

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.[14]

p-Block complexes

Tin(II) bis(trimethylsilyl)amide is prepared from anhydrous tin(II) choride[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]

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.

In line with the general method, bis(trimethylsilyl)amides of transition metals are prepared by a reaction between the metal halides (typically chlorides) and sodium bis(trimethylsilyl)amide,[3] some variation does exist however, for instance the synthesis of blue Ti(N(SiMe3)2)3 using the soluble precursor TiCl3(Et3N)3.[18] The melting and boiling points of the complexes decrease across the series, with Group 12 metals being sufficiently volatile to allow purification by distillation.[19]

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[20] and is paramagnetic as the high spin iron(III) contains 5 unpaired electrons.

FeCl3 + 3LiN(SiMe3)2 → Fe[N(SiMe3)2]3 + 3LiCl

Similarly, the two coordinate Fe[N(SiMe3)2]2 complex is prepared by treating iron dichloride with lithium bis(trimethylsilyl)amide:[21]

FeCl2 + 2LiN(SiMe3)2 → Fe[N(SiMe3)2]2 + 2LiCl

The dark green Fe[N(SiMe3)2]2 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,[22] in the solid state it forms a dimer with trigonal planar iron centers and bridging amido groups.[23] 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}.[24] Similar behavior can be seen in Mn(hmds)2 and Co(hmds)2, which are monomeric in the gas phase[22] and dimeric in the crystalline phase.[25][26] Group 11 complexes are especially prone to oligomerization, forming tetramers in the solid phase.[27][28][29]

Compound Appearance m.p. (°C) b.p. (°C) Comment
Group 3 complexes
Sc(hmds)3[30] Colorless solid 172-174
Y(hmds)3 White solid 180-184 105°C/10 mmHg (subl.) Commercially available
Group 4 complexes
Ti(hmds)3[30] Bright blue solid Paramagnetic. Prepared from TiCl3(N(CH3)3)2
Group 5 complexes
V(hmds)3[30] Brown solid Prepared from VCl3(N(CH3)3)2
Group 6 complexes
Cr(hmds)3[3][30] Apple-green solid 120 110 / 0.5 mmHg (subl.) Paramagnetic
Group 7 complexes
Mn(hmds)2[3] Beige solid 100 / 0.2 mmHg
Mn(hmds)3[31] Violet solid 108-110
Group 8 complexes
Fe(hmds)2[32] Light green solid 90-100 / 0.01 mmHg
Fe(hmds)3[30] Dark green solid 120 / 0.5 mmHg (subl.) Paramagnetic
Group 9 complexes
Co(hmds)2[2] Green solid 73 101 / 0.6 mmHg
Co(hmds)3[31] Dark olive green solid 86-88
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.)
Ag(hmds)[28] Colorless solid Insoluble in hydrocarbons and diethyl ether
Au(hmds)[29] Colorless solid
Group 12 complexes
Zn(hmds)2[19] Colorless liquid 12.5 82 / 0.5 mmHg Commercially available
Cd(hmds)2[19] Colorless liquid 8 93 / 0.5 mmHg
Hg(hmds)2[19] Colorless liquid 11 78 / 0.15 mmHg

f-Block complexes

Lanthanide triflates can be convenient anhydrous precursors to many bis(trimethylsilyl)amides:[33]

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,[34] 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.

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[35] 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.[36][37] 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)

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 0-470-72184-7. 
  2. ↑ 2.0 2.1 H. Bürger and U. Wannagat (1963). "Silylamido-Derivate von Eisen und Kobalt". Monatshefte für Chemie 94 (6): 1007–1012. doi:10.1007/BF00905688. 
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 H. Bürger and U. Wannagat (1963). "Silylamido-Derivate von Chrom, Mangan, Nickel und Kupfer". Monatshefte für Chemie 95 (4–5): 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". Inorganic Syntheses. Inorg. Synth. 28. pp. 321–323. doi:10.1002/9780470132593.ch80. ISBN 978-0-470-13259-3. 
  5. ↑ Mootz, D.; Zinnius, A.; Böttcher, B. (1969). "Assoziation im festen Zustand von Bis(trimethylsilyl)amidolithium und Methyltrimethylsilanolatoberyllium". Angew. Chem. 81 (10): 398–399. doi:10.1002/ange.19690811015. 
  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. Inorganic Syntheses 8: 19–22. doi:10.1002/9780470132395.ch6. ISBN 978-0-470-13239-5. 
  9. ↑ US 5420322 
  10. ↑ Boncella, J. M.; Coston, C. J.; Cammack, J. K. (1991). "The synthesis of bis(hexamethyldisilylamido)barium(II)". Polyhedron 10 (7): 769. doi:10.1016/s0277-5387(00)83767-5. 
  11. ↑ Tanner, P. S.; Burkey, D. J.; Hanusa, T. P. (1995). "Cyclopentadienyl ring metathesis with bis(pentamethylcyclopentadienyl)calcium as a route to mixed ring organolanthanide complexes; the crystal structure of (C5Me5)2Nd(C5H5)". Polyhedron 14 (2): 331. doi:10.1016/0277-5387(94)00316-7. 
  12. ↑ 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. 
  13. ↑ LM Engelhardt, BS Jolly, PC Junk, CL Raston, BW Skelton, AH White (1986). "Highly Hindered Amido-Lithium and Amido-Magnesium Complexes. Crystal-Structures of [Li(μ-N(SiMe3)2)(Tetrahydrofuran)]2 and [MgBus(μ-N(SiMe3)2)]2". Aust. J. Chem. 39 (9): 1337. doi:10.1071/CH9861337. 
  14. ↑ 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. 
  15. ↑ 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. Bibcode:1990JChEd..67..347S. doi:10.1021/ed067p347. 
  16. ↑ 16.0 16.1 Bürger, H; Cichon, J; Goetze, U; Wannagat, U; Wismar, H.J (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. 
  17. ↑ Vehkam�Ki, Marko; Hatanp��, Timo; Ritala, Mikko; Leskel�, Markku (2004). "Bismuth precursors for atomic layer deposition of bismuth-containing oxide films". J. Mater. Chem. 14 (21): 3191. doi:10.1039/b405891g. 
  18. ↑ 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
  19. ↑ 19.0 19.1 19.2 19.3 Bürger, H; Sawodny, Wolfgang; Wannagat, Ulrich (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. 
  20. ↑ Douglas, Bodie E. (1978). Inorganic Syntheses, 18. Hoboken: John Wiley & Sons. ISBN 978-0-470-13284-5. 
  21. ↑ Rauchfuss, Thomas B. (2010). Inorganic syntheses. [Hoboken, N.J.]: John Wiley & Sons. ISBN 9780470651568. 
  22. ↑ 22.0 22.1 Andersen, R. A.; Faegri, Knut; Green, Jennifer C.; Haaland, Arne; Lappert, M. F.; Leung, Wing Por; Rypdal, Kristin (1 May 1988). "Synthesis of bis[bis(trimethylsilyl)amido]iron(II). Structure and bonding in M[N(SiMe3)2]2 (M = manganese, iron, cobalt): two-coordinate transition-metal amides". Inorganic Chemistry 27 (10): 1782–1786. doi:10.1021/ic00283a022. 
  23. ↑ Rauchfuss, Thomas B, ed. (2010). Inorganic Syntheses. Inorganic Syntheses. doi:10.1002/9780470651568. ISBN 9780470651568. 
  24. ↑ Sulway, Scott A.; Collison, David; McDouall, Joseph J. W.; Tuna, Floriana; Layfield, Richard A. (21 March 2011). "Iron(II) Cage Complexes of N-Heterocyclic Amide and Bis(trimethylsilyl)amide Ligands: Synthesis, Structure, and Magnetic Properties". Inorganic Chemistry 50 (6): 2521–2526. doi:10.1021/ic102341a. PMID 21314147. 
  25. ↑ Bradley, Donald C.; Hursthouse, Michael B.; Malik, K. M. Abdul; Möseler, Reinhold (1978). "The crystal molecular structure of "bis(hexamethyldisilylamido) manganese"". Transition Metal Chemistry (3): 253–254. doi:10.1007/BF01393560. 
  26. ↑ Murray, Brendan D.; Power, Philip P. (1984). "Three-Coordinate Metal Amides of Manganese(II) and Cobalt(II): Synthesis and X-ray Structure of the First Tris(silylamide) of Manganese and the X-ray Crystal Structures of [M2(N(SiMe3)2)4] (M = Mn, Co)". Inorganic Chemistry (23): 4584–4588. doi:10.1021/ic00194a036. 
  27. ↑ James, Alicia M.; Laxman, Ravi K.; Fronczek, Frank R.; Maverick, Andrew W. (1998). "Phosphorescence and Structure of a Tetrameric Copper(I)-Amide Cluster". Inorganic Chemistry (37): 3785–3791. doi:10.1021/ic971341p. 
  28. ↑ 28.0 28.1 Hitchcock, Peter B.; Lappert, Michael F.; Pierssens, Luc J.-M. (1996). "Synthesis and X-ray molecular structures of the silver(I) amides [{Ag[μ-N(SiMe3)2]}4] and [{Ag[μ-NCMe2(CH2)3CMe2]}4]". Chemical Communications (10): 1189–1190. doi:10.1039/CC9960001189. 
  29. ↑ 29.0 29.1 Bunge, Scott D.; Just, Oliver; Rees, William S., Jr. (2000). "[{Au[μ-N(SiMe3)2]}4]: The First Base-Free Gold Amide". Angewandte Chemie International Edition (39): 3082–3084. doi:10.1002/1521-3773(20000901)39:17<3082::AID-ANIE3082>3.0.CO;2-2. 
  30. ↑ 30.0 30.1 30.2 30.3 30.4 D. C. Bradley and R.G. Copperthwaite (1978). "Transition Metal Complexes of Bis(Trimethyl-silyl)Amine (1,1,1,3,3,3-Hexamethyldisilazane)". Transition Metal Complexes of Bis(trimethylsilyl)amine. Inorg. Synth. 18. pp. 112–120. doi:10.1002/9780470132494.ch18. ISBN 978-0-470-13249-4. 
  31. ↑ 31.0 31.1 Ellison, Jeffrey J.; Power, Philip P.; Shoner, Steven C. (1989). "First Examples of Three-Coordinate Manganese(III) and Cobalt(III): Synthesis and Characterization of the Complexes M[N(SiMe3)2]3 (M = Mn or Co)". Inorganic Chemistry (111): 8044–8046. doi:10.1021/ja00202a071. 
  32. ↑ Y. Ohki, S. Ohta, and K. Tatsumi (2010). "Bio-Inspired Iron and Nickel Complexes". Monomeric Iron(II) Complexes Having Two Sterically Hindered Arylthiolates. Inorg. Synth. 35. p. 137. doi:10.1002/9780470651568.ch7. ISBN 978-0-470-65156-8. 
  33. ↑ 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. 
  34. ↑ Bradley, Donald C.; Ghotra, Joginder S.; Hart, F. Alan (1973). "Low co-ordination numbers in lanthanide and actinide compounds. Part I. The preparation and characterization of tris{bis(trimethylsilyl)-amido}lanthanides". Journal of the Chemical Society, Dalton Transactions (10): 1021. doi:10.1039/DT9730001021. 
  35. ↑ Herrmann, Wolfgang A.; Anwander, Reiner; Kleine, Matthias; Scherer, Wolfgang (1992). "Lanthanoiden-Komplexe, I Solvensfreie Alkoxid-Komplexe des Neodyms und Dysprosiums. Kristall- und Molekülstruktur von trans-Bis(acetonitril)tris(tri-tert-butylmethoxy)neodym". Chem. Ber. 125 (9): 1971–1979. doi:10.1002/cber.19921250902. 
  36. ↑ Andersen, Richard A. (1979). "Tris((hexamethyldisilyl)amido)uranium(III): Preparation and Coordination Chemistry". Inorganic Chemistry (6): 1507–1509. doi:10.1021/ic50196a021. 
  37. ↑ Avens, Larry R.; Bott, Simon G.; Clark, David L.; Sattelberger, Alfred P.; Watkin, John G.; Zwick, Bill D. (1993). "A Convenient Entry into Trivalent Actinide Chemistry: Synthesis and Characterization of AnI3(THF)4 and An[N(SiMe3)2]3 (An = U, Np, Pu)". Inorganic Chemistry (10): 2248–2256. doi:10.1021/ic00088a030. 
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