1. INTRODUCTION

In this chapter, the synthesis of ─N(SiMe₃)₂ derivatives of some main group 2 and 14 elements, as well as 2,2,6,6‐tetramethylpiperidido and ─N(Ph)Prⁱ salts of potassium and calcium, is described.

The role that the ─N(SiMe₃)₂ ligand has played in main group structural chemistry has been the subject of two recent reviews dealing with metals of the s and p blocks.^{1, 2} A prior report in an earlier volume³ has described the synthesis of the group 1 lithium and sodium bis(trimethylsilyl)amide based on preparations originally reported by Wannagat and coworkers in the early 1960s.^3–5 The lighter group 2 silylamide Be{N(SiMe₃)₂}₂⁶ and various N(SiMe₃)₂ derivatives of magnesium including Mg{N(SiMe₃)₂}₂⁷ were described in 1965 and 1972, respectively. The heavier Ca, Sr, and Ba analogues were not reported⁸ until the 1990s, however. Since that time, their chemistry has been developed in interesting directions that include their use in catalysis,⁹ as metalating agents, and in inverse crown structures.^10–12

The originally described synthetic route to the heaviest group 2 element amides involved a displacement reaction between the group 2 metal (Mg–Ba) and Sn{N(SiMe₃)₂}₂ (see below), which affords the calcium and strontium derivatives in good (>80%) yields.⁸ In the crystalline state, the structures of the compounds proved to be dimeric, with three‐coordinate metals, bridging, and terminal ─N(SiMe₃)₂ groups.^{13, 14} Salt metathesis reactions between the metal diiodide and KN(SiMe₃)₂ in tetrahydrofuran (THF) solvent afforded the bis{bis(trimethylsilylamido)}bis(tetrahydrofuran) metal complexes M{N(SiMe₃)₂}₂(THF)_2.^15–17 Although the possibility of product contamination with KN(SiMe₃)₂ in the case of the reactions is a concern,¹⁸ the key to the avoidance of such problems is the physical properties and condition of the metal diiodide, which affords the desired product when it is in 10 mesh bead form. This salt elimination method of synthesis that affords pure products for the calcium and strontium derivatives is described in this chapter.

The work on the use of the mixed group 1/group 2 metal amides as deprotonating agents and inverse crowns saw extensive use of the 2,2,6,6‐tetramethylpiperidido ligand which has increased basicity in comparison with ─N(SiMe₃)₂. The use of the 2,2,6,6‐tetramethylpiperidido substituent as a bulky ligand in inorganic chemistry dates from its introduction in group 14 chemistry in 1979.¹⁹ The synthesis of the potassium salt of this ligand, as well as the mixed alkyl/aryl amide KN(Ph)Prⁱ and their calcium complexes, is also described.

The syntheses of the divalent group 14 element bis{bis(trimethylsilyl)amido} germanium, tin, and lead complexes constitute the remaining procedures in this chapter. These compounds have played a key role in the development of the molecular chemistry of the divalent state of the group 14 elements.^{20, 21} The three compounds were reported together with derivatives of the related ─N(Bu^t)SiMe₃ ligand in a paper by Lappert and Harris²² in 1974, while Zuckerman and Schaefer²³ reported the preparation of Sn{N(SiMe₃)₂}₂ together with Sn(Cl){N(SiMe₃)₂}, Sn(η⁵‐C₅H₅){N(SiMe₃)₂}, (n = 2, 3, and 4), and related species, essentially simultaneously. Sn{N(SiMe₃)₂}₂ was reported to be monomeric²² or dimeric²³ in benzene solution, on the basis of osmometry^{22, 23} and mass spectrometry.²³ Subsequently, single‐crystal X‐ray crystallographic data showed that the Ge, Sn, and Pb amides are monomers in the solid state.^{24, 25} The monomeric structures of E{N(SiMe₃)₂}₂ (E = Ge, Sn, or Pb) differ from those of the corresponding isoelectronic dialkyls E{CH(SiMe₃)₂}₂,^{26, 27} which are E═E bonded dimers²⁷ with trans‐pyramidalized geometries in the crystalline state. The difference in structure between the two compound classes is a result of the larger energy separation²⁸ of the lone pair and unoccupied p‐orbital of the group 14 atom in the ─N(SiMe₃)₂ series, which weakens the E–E interaction in comparison to their alkyl counterparts.

The compounds have been shown to have an extensive chemistry undergoing a variety of reactions.^{29, 30}

References

1. 1. M. P. Coles, Coord. Chem. Rev. 297–298, 2–23 (2015).
2. 2. M. P. Coles, Coord. Chem. Rev. 297–298, 24–39 (2015).
3. 3. E. H. Amonoo‐Neizer, R. A. Shaw, D. O. Skovlin, and B. C. Smith, Inorg. Synth. 8, 19–21 (1966).
4. 4. C. R. Krüger and H. Neiderprüm, Inorg. Synth. 8, 15–17 (1966).
5. 5. U. Wannagat and H. Neiderprüm, Chem. Ber. 94, 1540–1547 (1961).
6. 6. H. Bürger, C. Forker, and J. Goubeau, Monatsh. Chem. 96, 597–601 (1965).
7. 7. U. Wannagat, H. Autzen, H. Kuckertz, and H.‐J. Wismar, Z. Anorg. Allg. Chem. 394, 254–262 (1972).
8. 8. M. Westerhausen, Inorg. Chem. 30, 96–101 (1991).
9. 9. M. S. Hill, D. J. Liptrot, and C. Weetman, Chem. Soc. Rev. 45, 972–988 (2016).
10. 10. R. E. Mulvey, Organometallics 25, 1060–1075 (2006).
11. 11. R. E. Mulvey, F. Mongin, M. Uchiyama, and Y. Kondo, Angew. Chem. Int. Ed. 46, 3802–3824 (2007).
12. 12. R. E. Mulvey, Acc. Chem. Res. 42, 743–755 (2009).
13. 13. M. Westerhausen and W. Schwarz, Z. Anorg. Allg. Chem. 604, 127–140 (1991).
14. 14. M. Westerhausen and W. Schwarz, Z. Anorg. Allg. Chem. 606, 127–140 (1991).
15. 15. P. S. Tanner, D. J. Bunkey, and T. P. Hanusa, Polyhedron 14, 331–333 (1995).
16. 16. J. M. Boncella, C. J. Coston, and J. K. Cammack, Polyhedron 10, 769–770 (1991).
17. 17. X. He, B. C. Noll, A. Beatty, R. E. Mulvey, and K. W. Henderson, J. Am. Chem. Soc. 126, 7444–7445 (2004).
18. 18. A. M. Johns, S. C. Chmely, and T. P. Hanusa, Inorg. Chem. 48, 1380–1384 (2009).
19. 19. M. F. Lappert, P. P. Power, M. J. Slade, L. Hedberg, K. Hedberg, and V. Schomaker, Chem. Commun. 369–370 (1979).
20. 20. M. F. Lappert, P. P. Power, A. Protchenko, and A. Seeber, Metal Amide Chemistry, Wiley, Chichester, 2009, Chapter 9.
21. 21. Y. Mizuhata, T. Sasamori, and N. Tokitoh, Chem. Rev. 109, 3479–3511 (2009).
22. 22. D. H. Harris and M. F. Lappert, Chem. Commun. 895–896 (1974).
23. 23. C. D. Schaeffer and J. J. Zuckerman, J. Am. Chem. Soc. 96, 7160–7162 (1974).
24. 24. T. Fjeldberg, H. Hope, M. F. Lappert, P. P. Power, and A. J. Thorne, Chem. Commun. 639–641 (1983).
25. 25. R. N. Chorley, P. B. Hitchcock, M. F. Lappert, W. P. Leung, P. P. Power, and M. M. Olmstead, Inorg. Chim. Acta 198–200, 203–209 (1992).
26. 26. P. J. Davidson and M. F. Lappert, Chem. Commun. 317 (1973).
27. 27. D. E. Goldberg, D. H. Harris, M. F. Lappert, and K. M. Thomas, Chem. Commun. 261–262 (1976).
28. 28. D. H. Harris, M. F. Lappert, J. B. Pedley, and G. J. Sharp, Dalton Trans. 945–950 (1976).
29. 29. (a) M. J. S. Gynane, M. F. Lappert, S. J. Miles, and P. P. Power, J. Chem. Soc. Chem. Commun. 256–257 (1976); (b) M. F. Lappert and P. P. Power, Adv. Chem. Ser. 157, 70–81 (1976);(c) M. J. S. Gynane, M. F. Lappert, S. J. Miles, and P. P. Power, J. Chem. Soc., Chem. Commun. 192–193 (1977); (d) M. F. Lappert and P. P. Power, J. Chem. Soc., Dalton Trans. 51–57 (1985); (e) M. F. Lappert, M. C. Misra, M. Onyszchuk, R. S. Rowe, P. P. Power, and M. J. Slade, J. Organomet. Chem. 330, 31–46 (1987).
30. 30. (a) K. A. Miller, T. W. Waton, J. E. Bender, M. M. Banaszak Holl, and J. W. Kampf, J. Am. Chem. Soc. 123, 982–983 (2001); (b) K. A. Miller, J. M. Bartolin, R. M. O’Neill, R. D. Sweeder, T. M. Owens, J. W. Kampf, M. M. Banaszak Holl, and N. J. Wells, J. Am. Chem. Soc. 125, 8986–8987 (2003); (c) J. M. Bartolin, A. Kavara, J. W. Kampf, and M. M. Banaszak Holl, Organometallics 25, 4738–4740 (2006).

2. POTASSIUM (2,2,6,6‐TETRAMETHYLPIPERIDIDE), BIS(2,2,6,6‐TETRAMETHYLPIPERIDIDO) (N,N,N′,N′‐TETRAMETHYLETHYLENEDIAMINE)CALCIUM(II), POTASSIUM (N‐ISOPROPYLANILIDO), AND BIS(N‐ISOPROPYLANILIDO) TRIS(TETRAHYDROFURAN)CALCIUM(II)

Submitted by CARSTEN GLOCK,* SVEN KRIECK,* and MATTHIAS WESTERHAUSEN*

Checked by CATHERINE M. LAVIN,^† MIRIAM M. GILLETT-KUNNATH,^† and KARIN RUHLANDT^†

*Institute of Inorganic and Analytical Chemistry, Friedrich-Schiller-Universität Jena, 00743, Germany

^†Department of Chemistry, Syracuse University, Syracuse, NY13244

Calcium bis{bis(trimethylsilyl)amide} represented the first calcium bis(amide) that is soluble in common organic solvents.¹ The bulky trimethylsilyl groups significantly reduce basicity, nucleophilicity, and reactivity of this amide. A similar observation is true for calcium bis(diphenylamide) where the negative charge is delocalized into a phenyl group.² However, alkyl‐substituted amides of calcium exhibit enhanced reactivity. These alkaline earth metal amides represent valuable reagents for diverse applications for, e.g. amide transfer and deprotonation reactions.^{3, 4} In general, due to the insolubility of KI in common organic solvents, the metathesis reaction of potassium amide with calcium iodide allows the isolation of calcium bis(amides), whereas calcium metal is not reactive enough for a direct calciation of primary and secondary amines. The reactivity of these amides can further be enhanced via formation of heterobimetallic dipotassium tetrakis(amino)calciates that are even effective catalysts for the hydroamination of alkynes.⁵

Herein we report the synthesis of alkyl‐substituted amides of potassium and calcium as well as that of anhydrous {CaI₂(THF)₄}.

A. POTASSIUM 2,2,6,6‐TETRAMETHYLPIPERIDIDE

Caution. Butyllithium is pyrophoric in air, as well as highly reactive toward moisture, and should be handled exclusively under dry nitrogen or argon. It is important to vent the reaction. Using standard Schlenk techniques an oil bubbler is required to avoid an overpressure of butane.

Procedure

The synthesis of K(Tmp) is a slightly modified version of the literature procedure.⁶ In a 50 mL Schlenk flask, a solution of 2,2,6,6‐tetramethylpiperidine (HTmp; dried over CaH₂ and distilled prior to use; 5.2 mL, 30 mmol) in 15 mL of hexane is cooled to ca. −78 °C in an isopropanol/dry ice bath. Then 18.75 mL of a 1.6 M BuⁿLi solution in hexane (30 mmol) is added within 5 min followed by stirring for 0.5 h at this temperature. Thereafter, the solution is warmed to room temperature and stirred for 1 h. KOBu^t (5.15 g, 45 mmol) is added to the thus prepared Li(Tmp) solution. Vigorous stirring for 15 h yields a beige‐colored precipitate of very reactive K(Tmp) that is collected on a frit and washed three times with 10 mL portions of pentane. Yield: 4.3 g (24 mmol, 80%).

Properties

K(Tmp) is a highly pyrophoric solid that should be handled only in an inert gas atmosphere. Upon contact with O₂ or H₂O, solid potassium piperidide will immediately deflagrate. One has to be careful even with small amounts of dust that could be carried out of the flask with an inert gas flow and ignite immediately. K(Tmp) is readily soluble in polar aprotic solvents (THF, TMEDA) and insoluble in aromatic and aliphatic hydrocarbons. ¹H NMR (d₈‐THF, 200 MHz, 300 K): 1.05 (s, 12H, CH₃), 1.25–1.31 (m, 4H, β‐CH₂), 1.56–1.68 (m, 2H, γ‐CH₂). ¹³C NMR (d₈‐THF, 50 MHz, 300 K): 19.2 (γ‐CH₂), 32.1 (CH₃), 39.1 (β‐CH₂), 49.9 (α‐C).

B. DIIODOTETRAKIS(TETRAHYDROFURAN)CALCIUM(II)

Caution. Activated calcium metal is pyrophoric in the presence of air and moisture and has to be handled under strictly anaerobic conditions in an atmosphere of nitrogen or argon.

Procedure

In a 500 mL Schlenk flask, calcium metal (3.40 g, 84.83 mmol) is activated according to a literature procedure⁷ and suspended in 200 mL of THF. This mixture is cooled to ca. 4 °C, and freshly sublimed iodine (20.45 g, 80.59 mmol, 0.95 equiv) is added in small portions. After shaking for 0.5 h at ambient temperature, the mixture is filtered, and the off‐white solid is extracted with ca. 200 mL of THF in a Soxhlet extraction apparatus yielding colorless crystals that are collected on a Schlenk frit and dried under reduced pressure. Yield: 38.5 g (66 mmol, 82%, the calcium content is determined via hydrolysis of an aliquot and complexometric titration using Erio‐T as indicator).

Properties {CaI₂(THF)₄}

{CaI₂(THF)₄} is hygroscopic and sparingly soluble in ethers (~2 mmol in 50 mL of THF) and almost insoluble in hydrocarbons. In this complex the central calcium atom is bound to two iodine atoms in a trans arrangement and four thf molecules, leading to a distorted octahedral coordination sphere. The title compound undergoes a phase transition upon cooling from ambient temperature (monoclinic, P2₁/c, a 8.201(7), b 14.262(9), c 10.008(6) Å, β 93.93(6)°) to 100 K (triclinic, P–1, a 8.442(2), b 9.858(2), c 13.610(3) Å, α 80.16, β 98.41, γ 87.88°).⁸ During phase transition the crystals become dull.

C. BIS(2,2,6,6‐TETRAMETHYLPIPERIDIDO)(N,N,N′,N′‐TETRAMETHYLETHYLENEDIAMINE)CALCIUM(II)

Procedure

A 35 mL Schlenk flask is charged with KTmp (1.170 g, 6.52 mmol) and 20 mL of benzene. This suspension is treated with 4 mL of TMEDA to give a brown solution. Solvent‐free and anhydrous CaI₂ (1.115 g, 3.79 mmol) or the equivalent amount of {CaI₂(THF)₄} is then added, followed by subsequent stirring for 12 h at ambient temperature. To separate from fine white KI precipitate, the solution is filtered over Celite. All volatile materials are removed under reduced pressure and the residue is redissolved in 10 mL of toluene. Cooling to −20 °C yields a first crop of crystalline {(TMEDA)Ca(Tmp)₂}; another crop of crystals is obtained by reducing the volume of the mother liquor to one‐third of the original volume and cooling to −20 °C. Yield: 0.791 g (1.81 mmol, 48%).

Properties

The TMEDA complex of Ca(Tmp)₂ is readily soluble in ethers and aromatic hydrocarbons and is slightly soluble in aliphatic hydrocarbons. Whereas ethers are degraded within a few hours, aromatic hydrocarbon solutions are stable for weeks. ¹H NMR ([d₆]benzene, 400 MHz, 300 K): δ 1.45 (s, 24H, CH₃ (Tmp)), 1.52–1.55 (m, 12H, β‐CH₂ (Tmp) + CH₂ (TMEDA)), 1.87 (s, 12H, CH₃ (TMEDA)), 2.04–2.10 (m, 4H, γ‐CH₂ (Tmp)). ¹³C NMR (d₆-benzene, 100 MHz, 300 K): δ 21.1 (γ‐CH₂ (Tmp)), 35.3 (CH₃ (Tmp)), 40.9 (β‐CH₂ (Tmp)), 47.3 (CH₃ (TMEDA)), 52.7 (α‐C (Tmp)), 56.7 (CH₂ (TMEDA)).

D. POTASSIUM N‐{ISOPROPYL(PHENYL)AMIDE} (POTASSIUM N‐ISOPROPYLANILIDE)

Procedure

Commercially available potassium bis(trimethylsilyl)amide (95% purity, Aldrich) has to be purified prior to use. Therefore a 50 mL Schlenk flask with a Schlenk frit is used. Under an inert gas atmosphere, potassium bis(trimethylsilyl)amide (3 g, 15 mmol) is added to the frit, and subsequent dissolution with two 10 mL amounts of toluene with filtration through the frit leaves a shallow yellow filter cake on the frit, which is discarded (alternatively, commercially available solutions of KN(SiMe₃)₂ in toluene can be used). Into this colorless clear filtrate, N‐isopropylaniline (2.2 mL, 15 mmol, ρ = 0.937 g/cm³) is added via syringe to give a fine, pale yellow, amorphous precipitate of the potassium anilide. The suspension is stirred for additional 2 h to complete the reaction before the product is collected on a frit and washed twice with 10 mL portions of toluene and finally with 10 mL of pentane. After final drying under reduced pressure, 2.5 g of KN(Ph)Prⁱ (14.2 mmol, 95%) is isolated.

Properties

Solvent‐free potassium anilides are extremely sensitive toward moisture and air and even pyrophoric upon exposition to air.⁹ They are readily soluble in polar aprotic solvents such as ethers and insoluble in hydrocarbon solvents. This procedure could also be applied to aniline, N‐methylaniline, and diphenylamine. ¹H NMR (d₈‐THF, 200 MHz, 300 K): δ 6.84 (2H, m‐H, ³J_H,H = 7.8 Hz), 6.27 (2H, o‐H, ³J_H,H = 8.0 Hz), 6.04 (1H, p‐H, ³J_H,H = 7.0 Hz), 3.47 (1H, hept, CH, ³J_H,H = 6.2 Hz), 1.18 (6H, d, CH₃, ³J_H,H = 6.2 Hz). ¹³C NMR (d₈‐THF, 50 MHz, 300 K): δ 154.5 (i‐C), 129.8 (m‐C), 112.6 (o‐C), 110.6 (b, p‐C), 45.9 (CH), 23.9 (CH₃).

E. BIS{N‐ISOPROPYL(PHENYL)AMIDO} TRIS(TETRAHYDROFURAN)CALCIUM(II)

Procedure

Potassium N‐isopropylanilide (0.409 g, 2.36 mmol) is dissolved in 15 mL of THF before solvent‐free CaI₂ (0.338 g, 1.15 mmol; alternatively also the THF adduct {CaI₂(THF)₄} can be used) is added. During the reaction CaI₂ dissolves, while the by‐product KI precipitates as a very fine white solid. After 2 h of stirring at room temperature, the suspension is filtered over Celite. Crystalline material of this nearly quantitative reaction can be obtained after reduction of the volume to ca. 20% of the original volume, subsequent addition of 2 mL of toluene and cooling to −20 °C. Yield: 0.310 g (0.59 mmol, 51.4%).

Properties

¹H NMR (d₈‐THF, 200 MHz, 300 K): δ 6.73 (4H, m‐H, ³J_H,H = 7.6 Hz), 6.11 (4H, o‐H, ³J_H,H = 7.8 Hz), 5.85 (2H, p‐H, ³J_H,H = 6.8 Hz), 3.42 (2H, hept, CH, ³J_H,H = 6.2 Hz), 1.13 (12H, d, CH₃, ³J_H,H = 6.4 Hz). ¹³C NMR (d₈‐THF, 50 MHz, 300 K): δ 157.1 (i‐C), 126.6 (m‐C), 109.6 (o‐C), 104.9 (b, p‐C), 44.8 (CH), 21.4 (CH₃).

F. BIS[{BIS(TETRAHYDROFURAN)POTASSIUM}BIS{μ‐N(ISOPROPYL)(PHENYL)AMIDO}]CALCIUM(II)

Procedure

In a 35 mL Schlenk flask, potassium N‐isopropylanilide (527 mg, 3.04 mmol) is dissolved in 15 mL of THF. Solvent‐free calcium diiodide (223 mg, 0.76 mmol; alternatively the THF complex {CaI₂(THF)₄} can be employed) is added, and the reaction mixture stirred for 2 h. The fine white precipitate of KI is removed over Celite before the volume of the solution is reduced until crystallization begins. After redissolving at 50 °C, the solution is stored at −20 °C for 12 h to yield the product as colorless crystalline material. After decanting via syringe the product is dried in vacuo. The volume of the mother liquor can be reduced to one‐third of the original volume. Storage at −20 °C yields another crop of crystals. Yield: 0.512 g (0.51 mmol, 67%) of isolated crystalline product.

Properties

Ca[{μ‐N(Ph)Prⁱ}₂K(THF)₂]₂ is readily soluble in polar aprotic solvents like THF and nearly insoluble in aromatic and aliphatic hydrocarbons. It loses coordinated THF readily when the solid is heated in vacuo. Also two molecules of THF can be substituted by one tmeda ligand.¹⁰¹H NMR (d₈‐THF, 200 MHz, 300 K): δ 6.81 (8H, m‐H, b), 6.32 (8H, o‐H, b), 5.89 (4H, p‐H, b), 3.46 (4H, hept, CH, ³J_H,H = 6.2 Hz), 1.18 (12H, d, CH₃, ³J_H,H = 6.0 Hz). ¹³C NMR (d₈‐THF, 50 MHz, 300 K): δ 159.9 (i‐C), 129.9 (m‐C), 112.7 (o‐C), 107.7 (b, p‐C), 47.9 (CH), 26.2 (CH₃).

Acknowledgment

We thank Steffen Ziemann for checking the procedures for the synthesis of K(Tmp) and Ca(Tmp)₂.

References

1. 1. (a) M. Westerhausen, Inorg. Chem. 30, 96–101 (1991);(b) M. Westerhausen, Coord. Chem. Rev. 176, 157–210 (1998);(c) M. Westerhausen, Trends Organomet. Chem. 2, 89–105 (1997).
2. 2. M. Gärtner, R. Fischer, J. Langer, H. Görls, D. Walther, and M. Westerhausen, Inorg. Chem. 46, 5118–5124 (2007).
3. 3. A. Torvisco, A. Y. O’Brien, and K. Ruhlandt‐Senge, Coord. Chem. Rev. 255, 1268–1292 (2011).
4. 4. M. Westerhausen, J. Langer, S. Krieck, and C. Glock, Rev. Inorg. Chem. 31, 143–184 (2011).
5. 5. C. Glock, H. Görls, and M. Westerhausen, Chem. Commun. 48, 7094–7096 (2012).
6. 6. D. R. Armstrong, D. V. Graham, A. R. Kennedy, R. E. Mulvey, and C. T. O’Hara, Chem. Eur. J. 14, 8025–8034 (2008).
7. 7. R. Fischer, M. Gärtner, H. Görls, and M. Westerhausen, Organometallics 25, 3496–3500 (2006).
8. 8. (a)M. J. McCormick, S. C. Sockwell, C. E. H. Davies, T. P. Hanusa, and J. C. Huffman, Organometallics 8, 2044–2049 (1989); (b) K. F. Tesh, D. J. Burkey, and T. P. Hanusa, J. Am. Chem. Soc. 116, 2409–2417 (1994);(c) K. W. Henderson, J. A. Rood, and B. C. Noll, Acta Cryst. E61, m2006–m2007 (2005);(d) J. Langer, S. Krieck, R. Fischer, H. Görls, and M. Westerhausen, Z. Anorg. Allg. Chem. 636, 1190–1198 (2010).
9. 9. C. Glock, H. Görls, and M. Westerhausen, Eur. J. Inorg. Chem. 5288–5298 (2011).
10. 10. C. Glock, H. Görls, and M. Westerhausen, Dalton Trans. 40, 8108–8113 (2011).

3. BIS{BIS(TRIMETHYLSILYL)AMIDO}CALCIUM(II) DIMER, [Ca{N(SiMe₃)₂}₂]₂, AND BIS{BIS(TRIMETHYLSILYL)AMIDO}STRONTIUM(II) DIMER, [Sr{N(SiMe₃)₂}₂]₂

Submitted by MICHAEL S. HILL,* MATHEW D. ANKER, and ANDREW S. S. WILSON

Checked by CATHERINE WEETMAN^† and POLLY L. ARNOLD^†

*Department of Chemistry, University of Bath, Bath, BA2 7AY, UK

^†School of Chemistry, The University of Edinburgh, Edinburgh, EH9 3FJ, UK

Amide derivatives of the heavier alkaline earth metals (Ae = Ca, Sr, and Ba) are finding increasing use as reagents in both materials synthesis and homogeneous molecular catalysis.¹ Due to their stability, ease of handling, and solubility in a wide range of organic media, the dimeric, homoleptic bis(trimethylsilyl)amides [Ae{N(SiMe₃)₂}₂]₂ provide a particularly prominent entry point into heavier alkaline earth chemistry.² The initial report of these compounds was provided by Westerhausen who prepared all three derivatives in high yield by redox transmetalation of the pre‐distilled and activated group 2 elements with [Sn{N(SiMe₃)₂}₂].³ Although use of this route has, in our hands, provided viable quantities of the target compounds, the necessary distillation step is inconvenient, and the isolated yields are inconsistent. More attractive, therefore, are salt metathesis reactions between [KN(SiMe₃)₂] and commercially available AeI₂ reagents. Performance of these reactions in THF invariably provides the persistent four‐coordinate and mononuclear THF adducts [Ae{N(SiMe₃)₂(THF)₂]. As initially described by Boncella (Ae = Ba),⁴ Hanusa (Ae = Ca),⁵ and Mulvey and Henderson (Ae = Ca),⁶ however, we have found that similar reactions carried out in diethyl ether yield labile adducts that readily desolvate under dynamic vacuum. A subsequent and more recent report by Hanusa has highlighted that contamination by significant quantities of the calciate K[Ca{N(SiMe₃)₂}₃] presents a potential issue during the metathetical synthesis of [Ca{N(SiMe₃)₂}₂]₂.⁷ While we would guard against similar production of K[Ba{N(SiMe₃)₂}₃] during the synthesis of [Ba{N(SiMe₃)₂}₃]₂, we have encountered no such difficulties during the synthesis of either the calcium or strontium bis(trimethylsilyl)amides by the procedure described below. In this case the success of the reaction is vitally dependent on the use of the group 2 iodide reagents in −10 mesh bead form, which provides analytically pure quantities of both the calcium and strontium derivatives in high isolated yield.

A. BIS{BIS(TRIMETHYLSILYL)AMIDO}CALCIUM(II) DIMER, [Ca{N(SiMe₃)₂}₂]₂, AND BIS{BIS(TRIMETHYLSILYL)AMIDO}STRONTIUM(II) DIMER, [Sr{N(SiMe₃)₂}₂]₂

Two 250 mL Schlenk flasks equipped with PTFE (Teflon) magnetic stirring bars were oven dried at 150 °C overnight and brought directly, while hot, into a glovebox and allowed to cool under vacuum in the antechamber. The reactants, calcium iodide (2.0 g, 6.8 mmol, anhydrous beads, Sigma‐Aldrich, 99.999% trace metals basis, −10 mesh; use of CaI₂ powder leads to significantly reduced yields and increased levels of impurity, most likely through calciate complex formation) or strontium iodide (2.32 g, 6.8 mmol, anhydrous beads, Sigma‐Aldrich, 99.99% trace metals basis, −10 mesh) and potassium hexamethyldisilazide⁷ (2.7 g, 13.6 mmol), are placed into the separate Schlenk flasks, which are sealed and removed from the glovebox. 20 mL aliquots of diethyl ether are added via cannula to the Schlenk flasks at room temperature. The potassium hexamethyldisilazide dissolves immediately to give clear solutions with the calcium or strontium iodide beads resting at the bottom. The reactants are then stirred for 3 days during which time white precipitates (potassium iodide) are observed to form. All volatile materials are removed under reduced pressure, and the resulting colorless solids are extracted with hexanes (ca. 40 mL), providing colorless solutions with white precipitates. The solutions are filtered via cannula to afford clear, colorless solutions. The hexanes are removed under reduced pressure to yield [Ca{N(SiMe₃)₂}₂(Et₂O)₂] and [Sr{N(SiMe₃)₂}₂(Et₂O)₂] as colorless solids. The solids are heated to 80 °C under dynamic vacuum (ca. 10⁻² mmHg) and maintained under these conditions for 12 h. Redissolution in the minimum amount of hexane (ca. 50 mL) and cooling to −35 °C for 16 h provide [Ca{N(SiMe₃)}₂]₂ and [Sr{N(SiMe₃)}₂]₂ in the form of colorless crystals in yields of 2.04 g (83%) and 2.49 g (90%), respectively.^a

[Ca{N(SiMe₃)}₂]₂: m.p. 148–152 °C. ¹H NMR (toluene‐d₈): δ 0.26 (s, 18H, SiCH₃), 0.30 (s, 18H, SiCH₃); ¹³C{¹H} NMR (toluene‐d₈) δ 6.4 (SiCH₃, ¹J_SiC = 52.7 Hz), 6.9 (SiCH₃, ¹J_SiC = 52.7 Hz).

[Sr{N(SiMe₃)}₂]₂: m.p. 148–152 °C. ¹H NMR (toluene‐d₈): 0.23 (s, 36H, SiCH₃); ¹³C{¹H} NMR (toluene‐d₈) δ 6.5 (SiCH₃, ¹J_SiC = 52.4 Hz).

Properties

The calcium(II) and strontium(II) bis(silylamide)s are both air and moisture sensitive but can be stored inside an argon‐filled glovebox indefinitely without noticeable decomposition.

References

1. 1. (a) M. P. Coles, Coord. Chem. Rev. 297–298, 2–23 (2015); (b) M. S. Hill, D. J. Liptrot and C. Weetman, Chem. Soc. Rev. 45, 972–988 (2016).
2. 2. M. Westerhausen, Coord. Chem. Rev. 176, 157–210 (1998).
3. 3. M. Westerhausen, Inorg. Chem. 30, 96–101 (1991).
4. 4. J. M. Boncella, C. J. Coston, and J. K. Cammack, Polyhedron 10, 769–770 (1991).
5. 5. P. S. Tanner, D. J. Burkey, and T. P. Hanusa, Polyhedron 14, 331–333 (1995).
6. 6. X. He, B. C. Noll, A. Beatty, R. E. Mulvey, and K. W. Henderson, J. Am. Chem. Soc. 126, 7444–7445 (2004).
7. 7. A. M. Johns, S. C. Chmely, and T. P. Hanusa, Inorg. Chem. 48, 1380–1384 (2009).

4. DIVALENT GROUP 14 METAL BIS(TRIMETHYLSILYLAMIDES), M{N(SiMe₃)₂}₂ (M = Ge, Sn, Pb)

Submitted by ALEX J. VEINOT,* DARCIE L. STACK,* JASON A. C. CLYBURNE,* and JASON D. MASUDA*

Checked by DIANE A. DICKIE,^† UJWAL CHADHA,^† and RICHARD A. KEMP*^,†

*Department of Chemistry and The Atlantic Centre for Green Chemistry, Saint Mary’s University, Halifax, NS, B3H 3C3, Canada

^†Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, NM, 87131

^‡Advanced Materials Laboratory, Sandia National Laboratories, Albuquerque, NM, 87106

Group 14 metal bis‐trimethylsilylamides were initially reported by Lappert (Ge, Sn, Pb)^1–3 and Zuckerman (Sn)⁴ in the 1970s via metathesis of the metal(II) dihalides with lithium bis‐trimethylsilylamide. The bent, two‐coordinate nature of the metals was confirmed by both single‐crystal X‐ray^{5, 6} and gas‐phase electron diffraction.⁵ These metallylenes have a singlet configuration, leading to behavior that is related to that of singlet carbenes such as N‐heterocyclic carbenes and their acyclic analogues^7–9; however the lone pair effect is more apparent in the heavier congeners. The preparation of these compounds has been widely reported throughout the literature,⁷ often used as convenient starting materials for metathesis reactions as well as precursors for applications such as the preparation of nanowires¹⁰ and other nanomaterials.^{11, 12}

A. BIS{BIS(TRIMETHYLSILYL)AMIDO}GERMANIUM(II), Ge{N(SiMe₃)₂}₂

In a glovebox, lithium bis(trimethylsilyl)amide (7.224 g, 43.17 mmol) is carefully added to 125 mL of diethyl ether stirred with a PTFE (Teflon)‐coated magnetic stir bar in a 250 mL round‐bottom flask. ( Caution. The solvation of the lithium bis(trimethylsilyl)amide is exothermic, watch for boiling diethyl ether (b.p. 34.6 °C); addition of diethyl ether to solid lithium bis(trimethylsilyl)amide can be problematic and may result in excessive diethyl ether vapor formation and boiling over of materials.) The resulting slightly cloudy mixture is transferred to a 250 mL pressure‐equalizing dropping funnel (24/40 joints). The top of the funnel is fitted with a gas adapter containing a stopcock. In a 250 mL round‐bottom flask (fitted with a 24/40 joint and a stopcock sidearm) equipped with a PTFE‐coated magnetic stir bar, GeCl₂·dioxane (5.002 g, 21.65 mmol) is combined with 20 mL of diethyl ether, and the flask is fitted with the dropping funnel. Both the dropping funnel and germanium‐containing flask are then transferred from the glovebox to the Schlenk line. The diethyl ether mixture of lithium bis(trimethylsilyl)amide is then added to the rapidly stirring GeCl₂·dioxane/diethyl ether mixture over 45 min. The mixture slowly transitioned from colorless to faint orange. After 4.5 h the stirring is stopped and the resulting LiCl is allowed to settle over 30 min. The mixture is then cannula filtered into a 250 mL round‐bottom flask fitted with a 24/40 joint and a stopcock sidearm. The solvent is removed under reduced pressure, resulting in an orange‐colored crude oil. In a glovebox, the oil is then transferred to a 25 mL round‐bottom flask equipped with a PTFE‐coated magnetic stir bar for distillation. The oil is vacuum distilled using a one‐piece, short‐path distillation apparatus without water cooling. ( Caution. The distillate may solidify in the condenser if water cooling is used.) Bis(bistrimethylsilylamido)germanium(II) is collected as a bright orange‐colored oil (70–75 °C, 0.15 Torr) and a small amount of thick, brown liquid remains in the still pot. Yield: 6.402 g (75%).^c The oil solidifies upon standing to yellow/orange crystals, m.p. 32–33 °C.^d

¹H NMR (300.3 MHz, C₆D₆): δ 0.31 ppm. ¹³C{¹H} NMR (75.5 MHz, C₆D₆): δ 5.42 ppm.

B. BIS{BIS(TRIMETHYLSILYL)AMIDO}TIN(II), Sn{N(SiMe₃)₂}₂

In a glovebox, lithium bis(trimethylsilyl)amide (17.65 g, 0.1055 mol) is carefully added to 230 mL of diethyl ether stirred with a PTFE (Teflon)‐coated magnetic stir bar in a 250 mL round‐bottom flask. ( Caution. Solvation of the lithium bis(trimethylsilyl)amide is exothermic, watch for boiling diethyl ether (b.p. 34.6 °C); addition of diethyl ether to solid lithium bis(trimethylsilyl)amide can be problematic and may result in excessive diethyl ether vapor formation and boiling over of materials.) The resulting slightly cloudy mixture is transferred to a 250 mL pressure‐equalizing dropping funnel (24/40 joints), and an additional 20 mL of diethyl ether is added to bring the entire volume to 250 mL. The top of the funnel is fitted with a gas adapter containing a stopcock, and the bottom of the funnel is fitted with a 50 mL round‐bottom flask to facilitate removal of the dropping funnel from the glovebox. In a 500 mL round‐bottom flask (fitted with a 24/40 joint and a stopcock sidearm) equipped with a PTFE‐coated magnetic stir bar, SnCl₂ (10.00 g, 0.0528 mol) is combined with 40 mL of diethyl ether and rapidly stirred to suspend the SnCl₂. Then, 20 mL of THF is added, resulting in almost complete dissolution of the SnCl₂. The flask is fitted with a septum. Both the dropping funnel and tin‐containing flask are then transferred from the glovebox to the Schlenk line. Using proper Schlenk techniques, the dropping funnel is attached to the 500 mL flask. The diethyl ether solution of lithium bis(trimethylsilyl)amide is then added dropwise to the stirring SnCl₂ mixture over 45 min. The mixture slowly transitioned from colorless to orange. After 3 h the stirring is stopped and the resulting LiCl is allowed to settle over 30 min. The mixture is then cannula filtered into a 500 mL round‐bottom flask fitted with a 24/40 joint and a stopcock sidearm. The solvent is removed in vacuo resulting in an orange‐colored crude oil. In a glovebox, the oil is then transferred to a 25 mL round‐bottom flask equipped with a PTFE‐coated magnetic stir bar for distillation. The oil is vacuum distilled using a one‐piece, short‐path distillation apparatus without water cooling. ( Caution. The distillate may solidify in the condenser if water cooling is used.) Bis{bis(trimethylsilyl)amido}tin(II) is collected as a red‐orange‐colored oil (76–84 °C/0.2 Torr) and a small amount of thick, dark brown liquid remains in the still pot. Yield: 20.31 g (73%).^e

¹H NMR (300.3 MHz, C₆D₆): δ 0.29 ppm. ¹³C{¹H} NMR (75.5 MHz, C₆D₆): δ 5.81 ppm.

C. BIS{BIS(TRIMETHYLSILYL)AMIDO}LEAD(II), Pb{N(SiMe₃)₂}₂

In a glovebox, lithium bis(trimethylsilyl)amide (12.03 g, 71.92 mmol) is carefully added to 200 mL of diethyl ether stirred with a PTFE (Teflon)‐coated magnetic stir bar in a 250 mL round‐bottom flask. ( Caution. Solvation of the lithium bis(trimethylsilyl)amide is exothermic, watch for boiling diethyl ether (b.p. 34.6 °C); addition of diethyl ether to solid lithium bis(trimethylsilyl)amide can be problematic and may result in excessive diethyl ether vapor formation and boiling over of materials.) The resulting slightly cloudy mixture is transferred to a 250 mL pressure‐equalizing dropping funnel (24/40 joints). The top of the funnel is fitted with a gas adapter containing a stopcock, and the bottom of the funnel is fitted with a 50 mL round‐bottom flask to facilitate removal of the dropping funnel from the glovebox. In a 500 mL round‐bottom flask (fitted with a 24/40 joint and a stopcock sidearm) equipped with a PTFE‐coated magnetic stir bar, PbCl₂ (10.00 g, 35.96 mmol) is combined with 40 mL of diethyl ether, and the flask is fitted with a septum. Both the dropping funnel and lead‐containing flask are then transferred from the glovebox to the Schlenk line. Using proper Schlenk techniques, the dropping funnel is attached to the 500 mL flask. The diethyl ether mixture of lithium bis(trimethylsilyl)amide is then added to the stirred PbCl₂ mixture over 40 min. The mixture slowly transitioned from colorless to red/orange. After 3 h the stirring is stopped and the resulting LiCl is allowed to settle over 30 min. The mixture is then cannula filtered into a 500 mL round‐bottom flask fitted with a 24/40 joint and a stopcock sidearm. The solvent is removed in vacuo, resulting in a red‐colored crude oil. In a glovebox, the oil is then transferred to a 25 mL round‐bottom flask equipped with a PTFE‐coated magnetic stir bar for distillation. The oil is vacuum distilled using a one‐piece, short‐path distillation apparatus without water cooling. ( Caution. The distillate may solidify in condenser if water cooling is used.) Bis (bistrimethylsilylamido)lead(II) is collected in two fractions as a bright yellow‐colored oil (98–103 °C/0.3 Torr), and approximately 2–3 mL of black‐colored liquid remains in the still pot. Note: During the latter half of the distillation, a small amount of lead metal is deposited in the still pot and distillation column, giving a rainbow metallic hue to the glassware. Yield: 15.31 g (69%).

¹H NMR (300.3 MHz, C₆D₆): δ 0.24 ppm. ¹³C{¹H} NMR (75.5 MHz, C₆D₆): δ 5.64 ppm.

References

1. 1. J. D. Cotton, C. S. Cundy, D. H. Harris, A. Hudson, M. F. Lappert, and P. W. Lednor, J. Chem. Soc. Chem. Commun. 651–652 (1974).
2. 2. D. H. Harris and M. F. Lappert, J. Chem. Soc. Chem. Commun. 895–896 (1974).
3. 3. M. J. S. Gynane, D. H. Harris, M. F. Lappert, P. P. Power, P. Riviere, and M. Riviere‐Baudet, J. Chem. Soc. Dalton Trans. 2004–2009 (1977).
4. 4. C. D. Schaeffer and J. J. Zuckerman, J. Am. Chem. Soc. 96, 7160–7162 (1974).
5. 5. T. Fjeldberg, H. Hope, M. F. Lappert, P. P. Power, and A. J. Thorne, J. Chem. Soc. Chem. Commun. 639–641 (1983).
6. 6. R. W. Chorley, P. B. Hitchcock, M. F. Lappert, W. Leung, P. P. Power, and M. M. Olmstead, Inorg. Chim. Acta. 198–200, 203–209 (1992).
7. 7. Y. Mizuhata, T. Sasamori, and N. Tokitoh, Chem. Rev. 109, 3479–3511 (2009).
8. 8. M. F. Lappert, P. P. Power, A. Protchenko, and A. Seeber, Metal Amide Chemistry, Wiley, Chichester, 2009.
9. 9. M. F. Lappert, P. P. Power, A. R. Sanger, and R. C. Srivastava, Metal and Metalloid Amides, Ellis Horwood Ltd., Chichester, 1980.
10. 10. M. S. Seifner, F. Biegger, A. Lugstein, J. Bernardi, and S. Barth, Chem. Mater. 27, 6125–6130 (2015).
11. 11. B. Hernandez‐Sanchez, T. J. Boyle, H. D. Pratt, M. A. Rodriguez, L. N. Brewer, and D. R. Dunphy, Chem. Mater. 20, 6643–6656 (2008).
12. 12. M. R. Buck, A. J. Biacchi, E. J. Popczun, and R. E. Schaak, Chem. Mater. 25, 2163–2171 (2013).