Inorganic Syntheses

PREFACE

This volume of Inorganic Syntheses presents detailed descriptions of the synthesis of more than one hundred compounds drawn from the main group and transition metal elements. More than half of the compounds have been chosen mainly for their synthetic utility, that is to say, they can serve as synthons by simple procedures for a wide range of other compounds. The bis(trimethylsilyl)amido derivatives of manganese, iron, cobalt, or the group 2 or 14 elements are prominent examples of such synthons. In addition, these amides are inexpensive and relatively easily prepared. Furthermore, they are highly useful hydrocarbon‐soluble sources of their masked divalent metal ions.

A further prominent theme in this volume is the synthesis of sterically crowding ligands that have enabled the isolation of species with unusual coordination numbers and multiple bonding. These are exemplified by the terphenyl ligands, which feature a central aryl ring bound to two flanking aryl rings at the ortho positions. The latter rings are further substituted by alkyl groups, thus creating a sterically protected area around the element to which the terphenyl is attached. These terphenyl ligands also bear a steric resemblance to β‐diketiminate or Nacnac ligands that carry aryl groups at their nitrogen atoms and were the subject of a large chapter (Chapter 1) of Volume 35 of Inorganic Syntheses.

A noteworthy inclusion in this volume is the syntheses of species having the first stable well‐characterized examples of magnesium–magnesium and zinc─zinc bonds. These provide a striking illustration of how compounds of a completely new class with unprecedented bonding can be synthesized by relatively straightforward routes using readily accessible ligands, i.e. the abovementioned β‐diketiminate and the well‐known pentamethylcyclopentadienyl ligands.

This volume is organized into eight chapters. Some background and historical perspective are provided in the introduction to each chapter. The opening chapter describes the synthesis and characterization data for the above‐mentioned divalent transition metal silylamides [M{N(SiMe3)2}2]2 (M = Mn, Fe, and Co) and their tetrahydrofuran complexes. The silylamide theme is continued in Chapter 2, where the synthesis of the group 2 compounds [M{N(SiMe3)2}2]2 (M = Ca and Sr) and the monomeric group 14 derivatives M{N(SiMe3)2}2 (M = Ge, Sn, or Pb) are detailed. In addition, the synthesis of some 2,2,6,6‐tetramethylpiperidido and N(Pri) anilido salts of potassium or calcium are described.

The preparations of the abovementioned groundbreaking metal–metal bonded (η⁵‐C5Me5)ZnZn(η⁵‐C5Me5) and NacnacMgMgNacnac complexes are the subject of Chapter 3. Chapter 4 features the synthesis of several sterically crowded main group and transition metal organometallic complexes. These include the simple dimeric, divalent diaryl (FeMes2)2 (Mes = C6H2‐2,4,6‐Me3), and the monomeric bisterphenyl derivatives M(C6H3‐2,6‐Mes2)2 (M = Mn, Fe, and Co). Included also are the syntheses of the precursor iodo and lithium derivatives. Similarly, the bent, highly colored group 14 element congeners M(C6H3‐2,6‐Mes2)2 (M = Ge, Sn, or Pb) are delineated. In addition, the synthesis of the related terphenyl gallium species as well as η⁴‐bonded bisanthracene anionic complexes of iron and cobalt is given.

In Chapter 5, the syntheses of 20 sterically crowded terphenyl compounds are detailed. These include preparations featuring the terphenyl groups ─C6H3‐2,6‐Mes2, ─C6H3‐2,6‐Dipp2 (Dipp = 2,6‐di‐iso‐propylphenyl), and ─C6H3‐2,6‐Trip2 (Trip = 2,4,6‐tri‐iso‐propylphenyl), which include the preparations of their iodo precursors, their lithium salts, azide, aniline, phenol, thiol, and isocyanide derivatives. These derivatives have proven extremely useful in supporting an extensive chemistry of compounds from the s, p, d, and f blocks of the periodic table.

In Chapter 6, the isolation of white phosphorus from red phosphorus is given by two methods involving the thermolysis of commercially available red phosphorus. In addition, the synthesis of the unusual species AsP3 from a niobium triphosphide and arsenic trihalide is described. Chapter 7 focuses on the synthesis of various unusual group 13 element derivatives. The synthesis of the pioneering aluminum(I) compound {Al(η⁵‐C5Me5)}4 by two approaches is described, as is that of the unusual Al(C6F5)3∙toluene complex. In addition, the synthesis of various organometallic group 13–15 compounds of relevance to materials chemistry is given.

The final chapter describes the synthesis of a variety of compounds – mainly derivatives of transition metals – that do not fit conveniently into the themes of the earlier chapters. Examples include the (1R,2R‐diaminocyclohexane)oxalatoplatinum(II) or oxaliplatin, which is marketed as the colorectal anticancer drug Eloxatin, the palladium complex tris(dibenzylideneacetone)dipalladium(II), and a series of gold(I) and (II) amidinate complexes. In addition, there are syntheses of chromium(III) acetonitrile complexes, as well as a series of ruthenium dimethylsulfoxide derivatives. From the early transition metal groups, there are the titanium(III) amide tris{(N‐tert‐butyl)(3,5‐dimethylanilido)titanium(III) and the useful tantalum(IV) complex TaCl4(tmeda). The chapter is completed by the synthesis of 1,3,5‐tri‐tert‐butylcyclopentadiene and its sodium and magnesium salts and of a series of tetraalkylammonium salts of tetrafluoroborate and fluoroarylborate salts.

The editor thanks the many (>140!) authors and checkers who contributed to this volume for their hard work and patience. In addition, many other people have helped to bring this volume to completion. Not the least among these are the editor’s undergraduate and graduate coworkers, who contributed greatly to expediting the submission and checking of various syntheses. In addition, the editor gratefully acknowledges the huge contribution of his assistant William Angel for maintaining the organization of the volume as well as the performance of numerous tasks associated with bringing the preparations to a state where they could be submitted to the printer. The editor also thanks Tom Rauchfuss, Greg Girolami, and Al Sattelberger for frequent advice and encouragement.

PHILIP P. POWER

University of California at Davis

Chapter One

DIVALENT MANGANESE, IRON, AND COBALT BIS(TRIMETHYLSILYL)AMIDO DERIVATIVES AND THEIR TETRAHYDROFURAN COMPLEXES

1. INTRODUCTION

The intention of this chapter is to describe in detail reliable synthetic procedures for the uncomplexed metal bissilylamides M{N(SiMe3)2}2 (M = Mn, Fe, or Co) as well as those of their mono‐tetrahydrofuran complexes M{N(SiMe3)2}2(THF). In addition, a synthesis for the bis(THF) complex Mn{N(SiMe3)2}2(THF)2 is given.

The bis(trimethylsilyl)amido group {N(SiMe3)2}−1, 2 is one of the simplest, most versatile, and inexpensive bulky monodentate ligands. Its steric properties were first demonstrated by Bürger and Wannagat via the synthesis of several low‐ (i.e. two‐ or three‐) coordinate transition metal derivatives in the early 1960s. They were prepared by the simple reaction of an alkali metal salt of the amide {N(SiMe3)2}− with chromium, manganese, iron, cobalt, nickel, or copper halides.1–4 The list of new compounds3, 4 included the trivalent complexes M{N(SiMe3)2}3 (M = Cr and Fe), the divalent species M{N(SiMe3)2}2 (M = Mn,⁴ Co,³ and Ni⁴ (unstable)), and the monovalent Cu{N(SiMe3)2}.⁴ The volatility of M(II) and M(III) derivatives supported the notion that the compounds had unassociated molecular structures and were therefore the first stable examples of open‐shell (i.e. d¹–d⁹) transition metal complexes with coordination numbers less than four. The closed‐shell (d¹⁰) Cu(I) derivative, although volatile, proved to be tetrameric {CuN(SiMe3)2}4 with four coppers arranged in a planar array and bridged by silylamido ligands⁵. The trigonal planar coordination of the M(III) species was proven for Fe{N(SiMe3)2}3 via a determination of its crystal structure by Bradley, Hursthouse, and Rodesiler in 1969.⁶ It was shown subsequently that the –N(SiMe3)2 ligand could stabilize three coordination in most of the first‐row transition metals,7–9 lanthanide,9, 10 and some actinide metals.11, 12

The original divalent transition metal silylamides M{N(SiMe3)2}2 (M = Mn, Co, and Ni) were later expanded to include the iron analogue Fe{N(SiMe3)2}2 by Andersen, Lappert, Haaland, and coworkers in 1988.¹³ With the exception of the Ni species, which is unstable, the M{N(SiMe3)2}2 (M = Mn, Fe, or Co) complexes were shown to have a linear N–M–N structure in the vapor phase by gas electron diffraction in 1988,¹³ in agreement with the original formulation of Bürger and Wannagat.3, 4 Nonetheless, in 1978 it had been shown by Bradley, Hursthouse, and coworkers that the originally reported synthesis of Mn{N(SiMe3)2}2,⁴ which was carried out in tetrahydrofuran, probably described its tetrahydrofuran complex Mn{N(SiMe3)2}2(THF), which could be distilled several times without losing the tetrahydrofuran ligand.¹⁴ This view was supported by an exhaustive study of the manganese(II) silylamides by Horvath in 1979.¹⁵ Despite a 1971 paper¹⁶ that seemed to confirm the monomeric, THF‐free character of Co{N(SiMe3)2}2 (synthesized by the original Bürger and Wannagat route),³ recent work has shown that the synthesis of the silylamides of both cobalt17–19 and nickel²⁰ in tetrahydrofuran also yielded the mono‐tetrahydrofuran complexes M{N(SiMe3)2}2(THF) (M = Co or Ni), which could be distilled directly from the reaction mixture. In addition, it had also been shown in 1991 that the synthesis of Fe{N(SiMe3)2}2 could yield Fe{N(SiMe3)2}2(THF) when carried out in tetrahydrofuran.²¹ The mono‐tetrahydrofuran complexes of the iron and cobalt silylamides are also obtainable by recrystallization of the uncomplexed M{N(SiMe3)2}2 (M = Fe or Co) from tetrahydrofuran. However, for Mn{N(SiMe3)2}2, the recrystallization from tetrahydrofuran produces the bis tetrahydrofuran complex Mn{N(SiMe)2}2(THF)2.²¹

In essence, the improved synthetic characterization methods and readily available crystal handling techniques for X‐ray crystallographic studies available to modern workers have permitted all of Bürger and Wannagat’s originally reported metal bis silylamido compounds to be unambiguously characterized. This work7, 14, 15, 17–20 has shown conclusively that these original divalent compounds are not two‐coordinate species, but are in fact the tetrahydrofuran complexes M{N(SiMe3)2}2(THF) (M = Mn, Co, and Ni). For these mono‐tetrahydrofuran species and their later synthesized iron analogue Fe{N(SiMe3)2}2(THF), the complexed tetrahydrofuran can only be removed with difficulty. For example, the desolvation of Mn{N(SiMe3)2}2(THF) requires heating at 120 °C under argon for 1 h.

The pure tetrahydrofuran‐free amides M{N(SiMe3)2}2 (M = Mn, Fe, Co) can be synthesized by performing the synthesis in diethyl ether, which does not bind to the metal as strongly as tetrahydrofuran and is easily removed. They are monomers in the vapor phase,¹³ but they are crystalline solids at room temperature with amido‐bridged dimeric structures and three‐coordinate metals as shown by X‐ray crystallography.17, 21–24 Variable temperature ¹H NMR studies of their solutions17, 21 have shown that the monomeric and dimeric structures exist in equilibrium with relatively low association energies, so that the major portion of the species present in their solutions consists of the monomers.

An interesting aspect of more than a half‐century of work on the compounds is the length of time that was required for the distinction between the bright green Co{N(SiMe3)2}2(THF) and [Co{N(SiMe3)2}2]2 (red/olive) to be delineated. A clear distinction between the two compounds was, in fact, specifically described only in 2013.17, 18 This lengthy period is particularly noteworthy in view of the very sharp contrast between the colors of the two compounds that makes them easily distinguishable. In contrast, the colors of the THF‐free and THF‐complexed amide for each of the metals Mn, Fe, and Ni are similar, which does not allow for easy visual distinction. This author and his group had used [Co{N(SiMe3)2}2]2 (synthesized in diethyl ether solvent) numerous times as a synthon9, 23 over a 30‐year period, and had in fact characterized it structurally using X‐ray crystallography in 1984,²⁴ but he saw the bright green crystals of Co{N(SiMe3)2}2(THF) for the first time (synthesized by graduate student A. M. Bryan) in the fall of 2012.²⁵ One reason for this strange circumstance is that the original Bürger and Wannagat synthetic procedures were all carried out in tetrahydrofuran. This solvent is less commonly used in this author’s lab, owing to the general avoidance of the use of tetrahydrofuran as a solvent if diethyl ether suffices. The Co{N(SiMe3)2}2(THF) complex was rediscovered recently because the magnetic properties17, 19, 26–28 of two‐ and three‐coordinate cobalt complexes, which generally have high orbital magnetism, were being investigated and because of a need for well‐defined cobalt precursor complexes in materials chemistry.¹⁸

References

1. The ligand is obtained by deprotonation of HN(SiMe3)2, itself synthesized via the reaction of ammonia with Me3SiCl: R. O. Sauer, J. Am. Chem. Soc.66, 1707–1710 (1944).

2. For an account of the s‐metal salts of the –N(SiMe3)2 ligand, see M. P. Coles, Coord. Chem. Rev.297–298, 2–23 (2015).

3. H. Bürger and U. Wannagat, Monatsh. Chem.94, 1007–1012 (1963).

4. H. Bürger and U. Wannagat, Monatsh. Chem.95, 1099–1102 (1964).

5. A. M. James, R. K. Laxman, F. R. Fronczek, and A. W. Maverick, Inorg. Chem.37, 3785–3791 (1998).

6. (a) D. C. Bradley, M. B. Hursthouse, and P. F. Rodesiler, J. Chem. Soc. D., Chem. Commun. 14–15 (1969); (b) M. B. Hursthouse and P. F. Rodesiler, J. Chem. Soc., Dalton Trans. 2100–2102 (1972).

7. P. G. Eller, D. C. Bradley, M. B. Hursthouse, and D. W. Meek, Coord. Chem. Rev.24, 1–95 (1977).

8. C. C. Cummins, Prog. Inorg. Chem.47, 685–836 (1998).

9. M. F. Lappert, P. P. Power, A. Protchenko, and A. Seeber, Metal Amide Chemistry, Wiley, Chichester, 2009.

10. J. S. Ghotra, M. B. Hursthouse, and A. J. Welch, Chem. Commun. 669–670 (1973).

11. R. A. Andersen, Inorg. Chem.18, 1507–1509 (1979).

12. L. R. Avens, S. G. Bott, D. L. Clark, A. P. Sattelberger, J. G. Watson, and B. D. Zwick, Inorg. Chem.33, 2248–2256 (1994).

13. R. A. Andersen, K. Faegri, J. C. Green, A. Haaland, M. F. Lappert, W.‐P. Leung, and K. Rypdal, Inorg. Chem.27, 1782–1786 (1988).

14. D. C. Bradley, M. B. Hursthouse, K. M. Abdul Malik, and R. Möseler, Transition Met. Chem.3, 253–254 (1978).

15. B. Horvath, R. Möseler, and E. G. Horvath, Z. Anorg. Allg. Chem.450, 165–177 (1979).

16. D. C. Bradley and K. J. Fisher, J. Am. Chem. Soc.93, 2058–2059 (1971).

17. A. M. Bryan, G. J. Long, F. Grandjean, and P. P. Power, Inorg. Chem.52, 12152–12160 (2013).

18. B. Cormary, F. Dumestre, N. Liakakos, K. Soulantica, and B. Chaudret, Dalton Trans.42, 12546–12553 (2013).

19. A. Eichhöfer, Y. Lan, V. Mereacre, T. Bodenstein, and F. Weigend, Inorg. Chem.53, 1962–1974 (2014).

20. M. Faust, A. M. Bryan, A. Mansikkamäki, P. Vasko, M. M. Olmstead, H. M. Tuononen, and P. P. Power, Angew. Chem. Int. Ed.54, 12914–12917 (2015).

21. M. M. Olmstead, P. P. Power, and S. C. Shoner, Inorg. Chem.30, 2547–2551 (1991).

22. D. C. Bradley, M. B. Hursthouse, A. A. Ibrahim, K. M. Abdul Malik, M. Motevalli, R. Möseler, H. Powell, J. D. Runnacles, and A. C. Sullivan, Polyhedron19, 2959–2964 (1990).

23. P. P. Power, Chemtracts ‐ Inorg. Chem.6, 181–195 (1994).

24. B. D. Murray and P. P. Power, Inorg. Chem.23, 4594–4588 (1984).

25. Photographs of crystals of [Co{N(SiMe3)2}2]2, Co{N(SiMe3)2}2(THF), and some other three coordinate Co(II) species are illustrated in the Supplementary Information of Reference 17.

26. R. A. Layfield, Organometallics33, 1084–1099 (2014).

27. B. M. Day, K. Pal, T. Pugh, J. Tuck, and R. A. Layfield, Inorg. Chem.53, 10578–10584 (2014).

28. A. Massart, P. Braunstein, A. D. Danopoulos, S. Choua, and P. Rabu, Organometallics34, 2429–2438 (2015).

2. BIS{BIS(TRIMETHYLSILYL)AMIDO}IRON(II) DIMER: [Fe{N(SiMe3)2}2]2

Submitted by RICHARD A. ANDERSEN*

Checked by AIMEE M. BRYAN,† MICHELLE FAUST,† and PHILIP P. POWER†

*Department of Chemistry, University of California, Berkeley, CA 94720

†Department of Chemistry, University of California, Davis, CA 95616

The hydrocarbon‐soluble ironsilylamide, Fe{N(SiMe3)2}2,¹ is a useful starting material for the synthesis of inorganic, coordination, and organometallic compounds utilizing proton‐transfer reactions, since the pKa of HN(SiMe3)2 is approximately 26 in THF.² A comprehensive description of the compounds that may be prepared using this methodology is available in an exhaustive review of two‐coordinate compounds.³ The silylamide is a useful precursor for solid‐state materials⁴ and catalysts.⁵ The synthesis of Fe{N(SiMe3)2}2 has been described in an earlier volume⁶ of this series. Here we provide related syntheses by two routes and provide details of its electronic, mass, IR, and electronic spectra.

General Procedures

All reactions are performed under an atmosphere of nitrogen. Ether solvents are distilled from sodium–benzophenone and hydrocarbons are distilled from sodium under an atmosphere of nitrogen. It is important that the distillation of Fe{N(SiMe3)2}2 is carried out in an all‐glass distillation apparatus directly connected to a diffusion‐pump vacuum system with greased, ground‐glass joints. The FeBr2(THF)2 is obtained by Soxhlet extraction of anhydrous FeBr2 with THF as described in the literature.⁷ The ratio of THF to FeBr2 is determined by combustion analysis or hydrolysis of a known mass suspended in C6D6 containing a reference, such as ferrocene, with D2O in a NMR tube and integrating the resulting ¹H NMR spectrum. Crystalline LiN(SiMe3)2(0.80 Et2O) is prepared by dropwise addition of HN(SiMe3)2 in diethyl ether to n‐butyllithium in hexane in a 1 : 1 molar ratio at 0 °C followed by crystallization by cooling a concentrated solution to −20 °C. The ratio of HN(SiMe3)2 to Et2O is obtained by hydrolysis of a crystal dissolved in C6D6 with D2O in a NMR tube and integrating the resulting ¹H NMR spectrum.

box Caution. n‐Butyllithium is pyrophoric in air and reacts rapidly and exothermically with water.

A. BIS{BIS(TRIMETHYLSILYL)AMIDO}IRON(II) DIMER: [Fe{N(SiMe3)2}2]2

The lithium silylamide, LiN(SiMe3)2 (0.80 Et2O) (10.4 g, 0.046 mol), dissolved in diethyl ether (100 mL) is added by cannula to a suspension of FeBr2(THF)2 (8.3 g, 0.023 mol) in diethyl ether at ca. 0 °C, and the suspension is stirred at 0 °C for 12 h.a The diethyl ether is removed under reduced pressure. The green‐yellow residue is extracted with pentane (2 × 50 mL), and the combined red filtrates are taken to dryness under reduced pressure, resulting in a dark red viscous oil. The oil is dissolved in a small amount of pentane (ca. 7–8 mL) and transferred to a distillation apparatus, and the volatile material (including residual THF) is removed at 20 °C under dynamic diffusion‐pump vacuum. After all of the volatile material is removed, the red oil is distilled at ca. 0.01 mmHg over the temperature range of 80–90 °C (bath temperature 115–125 °C) into a receiver flask cooled in an ice bath. The distillate is a green‐yellow mobile liquid that slowly solidifies to a soft green‐yellow solid. The yield is 6.0 g (70%).b

Properties

The [Fe{N(SiMe3)2}2]2 complex is air and moisture sensitive, but it can be stored in a stoppered flask inside of a dry box for extended periods of time. The solid is soluble in hydrocarbons and gives a monomeric molecular ion in the mass spectrum, M+ m/z (calculated intensity, found relative intensity): 376 (100, 100), 377 (36.8, 33.6), 378 (19.8, 17.3), and 379 (4.93, 3.54). The infrared spectrum recorded as a Nujol mull between CsI windows has absorption at 1250 (sh, s), 1240 (s), 1175 (w), 1020 (sh, m), 990 (s), 970 (s), 845 (s), 825 (s), 783 (s), 745 (m), 700 (w), 657 (m), 628 (w), 605(m), and 355 (s) cm−1. The ¹H NMR spectrum in C7D8 (30 °C) is a broad resonance at δ = 63. The structure of Fe{N(SiMe3)2} in the gas phase is a linear monomer with two‐coordinate iron atoms.¹ The structure in the solid state is dimeric with two silylamide groups bridging the three‐coordinate iron atoms. In toluene solution a monomer–dimer equilibrium exists, for which ΔG = +3 kcal/mol at 300 K.¹¹

References

1. R. A. Andersen, K. Faegri, Jr., J. C. Green, A. Haaland, M. F. Lappert, W.‐P. Leung, and K. Rypdal, Inorg. Chem.27, 1782–1786 (1988).

2. R. R. Fraser, T. S. Mansour, and S. Savard, J. Org. Chem.20, 3232–3234 (1985).

3. P. P. Power, Chem. Rev.112, 3482–3507 (2012).

4. F. Dumestre, B. Chaudret, C. Amien, P. Renard, and P. Fejes, Science303, 821–823 (2004).

5. J. Yang and T. D. Tilley, Angew. Chem. Int. Ed.49, 10186–10188 (2010).

6. Y. Ohki, S. Ohta, and K. Tatsumi, Inorg. Synth.35, 138–140 (2010).

7. S. D. Ittel, A.D. English, C. A. Tolman, and J. P. Jesson, Inorg. Chim. Acta33, 101–106 (1979).

8. B. Horvath, R. Möseler, and E. G. Horvath, Z. Anorg. Allg. Chem.450, 165–177 (1979).

9. D. L. J. Broere, I. Corić, A. Brosnahan, and P. J. Holland, Inorg. Chem.56, 3140–3143 (2017).

10. L. C. H. Maddock, T. Cadenbach, A. R. Kennedy, I. Borilovic, G. Aromi, and E. Hevia, Inorg. Chem.54, 9201–9210 (2015).

11. M. M. Olmstead, P. P. Power, and S. C. Shoner, Inorg. Chem.30, 2547–2551 (1991).

12. C.‐Y. Lin, J. C. Fettinger, and P. P. Power, Inorg. Chem.56, 9892–9902 (2017).

Notes

a The checkers report that the synthesis may also be performed on the same scale, without the use of THF or FeBr2(THF)2, by reacting two equivalents of in situ generated LiN(SiMe3)2 in diethyl ether with anhydrous FeCl2, freshly generated by dehydrating FeCl2⋅4H2O by the method of Horvath⁸ (cf. also the preparation of THF‐free manganese(II) and cobalt(II) silylamides in this volume). There are three other syntheses of [Fe{N(SiMe3)2}2]2 from LiN(SiMe3)2 and FeCl26, 9 or FeBr2¹⁰ in diethyl ether.

b The checkers report that the melting point of [Fe{N(SiMe3)2}2]2 (recrystallized from pentane) is 35–37 °C (cf. 36–38 °C in Ref. 6). Its UV–Vis spectrum in hexane features absorptions at 380 nm (ε = 810 M−1 cm−1) and 626 nm (ε = 5 M−1 cm−1). Its Mössbauer spectrum at 80 K features a δ = 0.59 mm/s and |ΔEQ| of 1.02 mm/s.⁹ Furthermore, it forms the complex Fe{N(SiMe3)2}2(THF) (m.p. 42–45 °C) when recrystallized from THF¹¹ (the binding constant in hexanes was determined to be 7.8 × 10³ ± 1.4% at 25 °C¹²).

3. BIS{BIS(TRIMETHYLSILYL)AMIDO}COBALT(II) DIMER, [Co{N(SiMe3)2}2]2, AND BIS{BIS(TRIMETHYLSILYL)AMIDO}(TETRAHYDROFURAN)COBALT(II), Co{N(SiMe3)2}2(THF)

Submitted by AIMEE M. BRYAN* and PHILIP P. POWER*

Checked by RICHARD A. ANDERSEN†

*Department of Chemistry, University of California, Davis, CA 95616

†Department of Chemistry, University of California, Berkeley, CA 94720

In the early 1960s, Bürger and Wannagat reported a series of low‐coordinate first‐row transition metal complexes using the silylamido ligand –N(SiMe3)2. The complexes were synthesized via the reaction of NaN(SiMe3)2 with the respective halides in THF solution.1, 2 The resulting hydrocarbon‐soluble amido complexes proved to be useful sources of M²+ and M³+ ions for various inorganic and organometallic reactions.3, 4 However, subsequent work5–7 showed that the original route of Bürger and Wannagat, involving the use of CoI2 and NaN(SiMe3)2 in THF solvent, yielded the complex Co{N(SiMe3)2}2(THF), and not Co{N(SiMe3)2}2 as originally reported. The divalent Co(II) silylamide, Co{N(SiMe3)2}2, which can be most conveniently obtained by carrying out the synthesis in diethyl ether, has been shown to be monomeric in the gas phase,⁸ to be dimeric in the solid state,⁹ and to exist in a monomer–dimer equilibrium in hydrocarbon solution.⁵ Although its crystal structure was reported in 1984,⁹ the physical, magnetic, and spectroscopic properties of THF‐free [Co{N(SiMe3)2}2]2 were not described in detail until 2013.⁵ The spectroscopic,5, 6 magnetic,5–7 and structural5–7 characterization of Co{N(SiMe3)2}2(THF) were described in three different publications in 2013–2014.

General Procedures

All reactions are performed with the use of modified Schlenk techniques or in a Vacuum Atmospheres dry box under nitrogen or argon atmosphere. Solvents are dried and collected using a Grubbs‐type solvent purification system (Glass Contour)¹⁰ and degassed by using the freeze–pump–thaw method.

box Caution. n‐Butyllithium is pyrophoric in air and should be handled under a nitrogen or argon atmosphere. In addition, n‐butyllithium reacts rapidly and exothermically with water. n‐BuH is released during the addition of n‐BuLi to protic reagents. Such reactions should be vented through an oil bubbler.

A. BIS{BIS(TRIMETHYLSILYL)AMIDO}COBALT(II) DIMER: [Co{N(SiMe3)2}2]2

A diethyl ether suspension of LiN(SiMe3)211, 12 is synthesized in situ by adding n‐BuLi (16 mL, 2.5 M solution in hexanes, 0.040 mol) dropwise to HN(SiMe3)2 (8.4 mL, 6.47 g, 0.040 mol) in diethyl ether (40 mL), cooled in an ice bath.a The solution is allowed to come to room temperature and stirring is continued for 12 h. The resulting colorless suspension is added dropwise via cannula over 30 min to a diethyl ether (40 mL) slurry of CoCl2 (2.86 g, 0.022 mol) chilled in an ice bath. An immediate color change of the slurry from blue to dark green is observed. When the addition is complete, the suspension is warmed to ca. 35 °C and stirred for 12 h. The ether is removed under reduced pressure, and the resulting dark green solids are extracted with hexanes (ca. 40 mL), which results in a dark green solution with a gray precipitate. The solution is then filtered through a Celite‐padded filter stick to afford a clear dark green solution. The hexanes are removed under reduced pressure to give a dark green oil. The oil is distilled as a dark green vapor at ca. 100 °C (0.05 Torr) using a short‐path distillation apparatus. Upon cooling, the vapor solidifies to a red/olive mass. The solid is redissolved in hexanes (ca. 30 mL) at ca. 65 °C which affords an olive‐green solution. Cooling slowly to 0 °C gives a precipitate of [Co{N(SiMe3)2}2]2 in the form of red/olive dichroic crystals with a yield of 5.7 g (7.5 mmol, ca. 68%).

Properties

The cobalt(II) bis(silylamide) is both air and moisture sensitive but can be stored inside a nitrogen or argon dry box for several months without noticeable decomposition. If decomposition does occur, redistillation and then recrystallization in hexanes can be used to purify the compound. The compound is soluble in hydrocarbon solvents but exists in a monomer–dimer equilibrium with an association energy (∆Greacn) of −0.30(20) kcal mol−1 at 300 K in benzene solution.⁵ As a solid, the red/olive dichroic compound is dimeric with bridging silylamide ligands between two three‐coordinate cobalt(II) ions.5, 9 In the gas phase, Co{N(SiMe3)2}2 is monomeric with strictly linear coordination at the cobalt(II) ion.⁸ M.p. 89–90 °C. UV–Vis/NIR (hexane, 298 K, nm [ε, M−1 cm−1]): 209 [3000], 223 [11 000], 281 [3400], 324 [6500], 604 [140], 668 [200]. IR (Nujol, CsI, cm−1): 3140, 2890, 2710, 2650, 1450, 1368, 1357, 1340, 1290, 1250, 1239, 1150, 1070, 1010, 955, 918, 880, 840, 828, 810, 794, 726, 710, 657, 600, 348, 265. μeff = 4.7(2) μB (C6D6, 298 K, Evans’ method). ¹H NMR (295 K, 400 MHz, C6D6): 8.97 (br s, −SiMe3, [Co{N(SiMe3)2}2]2), 0.51 (s, −SiMe3 (terminal), [Co{N(SiMe3)2}2]2), −4.22 (br s, −SiMe3 (bridging), [Co{N(SiMe3)2}2]2) ppm.

B. BIS{BIS(TRIMETHYLSILYL)AMIDO}(TETRAHYDROFURAN)COBALT(II): Co{N(SiMe3)2}2(THF)

Using a preparation similar to that described originally by Bürger and Wannagat, solid LiN(SiMe3)2¹¹ (6.68 g, 40 mmol) is added to a rapidly stirred suspension of CoCl2 (2.6 g, 20 mmol) in THF (ca. 40 mL) with cooling in an ice bath. After 1 h, the ice bath is removed and stirring is continued for 12 h to afford a green solution. The THF is pumped off under reduced pressure, and the residue is extracted with pentane (50 mL) and filtered using a Celite‐padded medium frit. The pentane is removed under reduced pressure, and the residual oily green solid is sublimed at ca. 70 °C under reduced pressure (ca. 0.02 Torr) to afford the product Co{N(SiMe3)2}2(THF) as bright green crystals with a yield of 6.23 g, 69%.b Co{N(SiMe3)2}2(THF) can also be obtained by dissolving [Co{N(SiMe3)2}2]2 (2.5 g, 3.29 mmol, see Section 3.A) in THF (ca. 30 mL), which affords a bright green solution. The solution is concentrated under reduced pressure until a green precipitate is formed. Gentle warming to redissolve the solids and cooling for 12 h in a ca. 4 °C refrigerator affords the product [Co{N(SiMe3)2}2(THF)] (2.72 g, ca. 91%).c