SYNTHESIS0039-78811437-210X Georg Thieme Verlag Stuttgart · New York 2019, 51, 1115–1122 feature 1115 de Syn thesis S. Krieck et al. Feature Straightforward One-Pot Syntheses of Silylamides of Magnesium and Calcium via an In Situ Grignard Metalation Method Sven Krieck Philipp Schüler Jan M. Peschel Matthias Westerhausen* 0000-0002-1520-2401 Friedrich Schiller University Jena, Institute of Inorganic and Analytical Chemistry, Humboldtstraße 8, D-07743 Jena, Germany [email protected] Published as part of the 50 Years SYNTHESIS – Golden Anniversary Issue Received: 11.10.2018 metalation of (H)HMDS with calcium is very challenging Accepted after revision: 13.11.2018 and requires activation of the alkaline earth metal via metal Published online: 13.12.2018 DOI: 10.1055/s-0037-1610407; Art ID: ss-2018-z0683-fa vapor synthesis and co-condensation with the organic sol- 3 License terms: vent. Alternatively, ammonia-saturated solvents can be used for the direct metalation of (H)HMDS with calcium 9 Abstract Calcium bis[bis(trimethylsilyl)amide] (Ca(HMDS)2) is a widely turnings. These procedures are depicted in Scheme 1. used reagent in diverse stoichiometric and catalytic applications. These processes necessitate a straightforward and large-scale access of this Grignard reaction, complex. Calcium does not react with primary and secondary amines, Salt metathesis, Salt metathesis, Metalation Transmetalation but the addition of excess bromoethane to a mixture of calcium turn- M = Mg, R = aryl, alkyl M = Ca, M' = Sn, Hg ings and amines in THF at room temperature yields the corresponding M = Ca, R = aryl calcium bis(amides), calcium bromide and ethane. This in situ Grignard 2 M + 2 RX MX2 + 2 KR M'X2 + 2 KN(SiMe3) metalation method (iGMM) allows the preparation of calcium bis(am- – MX2 – 2 KX 1) – 2 KX ides) from secondary and primary trialkylsilyl-substituted amines and 2) distillation anilines on a multigram scale. R M M'{N(SiMe ) } 1 Background 2 3 2 2 2 The In Situ Grignard Metalation Method (iGMM) – 2 RH+ 2 HN(SiMe3)2 + M – M' 3 Properties of [(thf)2M(HMDS)2] 4 Applications and Perspective [(D)xM{N(SiMe3)2}2] Key words calcium amides, hexamethyldisilazides, silylamides, alka- 1) – 2 KX line earth metal amides, amidocalcium complexes 2) – KyCa{N(SiMe3)2}2+y extraction MX2 + 2 KN(SiMe3) 1 Background M = Mg, Ca; X = Br, I Salt metathesis The development of diverse procedures for the synthe- Scheme 1 Common established procedures for the synthesis of sis of calcium bis[bis(trimethylsilyl)amides] is based on the Mg(HMDS)2 and Ca(HMDS)2 involving several preparation and purifica- tion steps desperate need of soluble organocalcium complexes for var- ious applications. Therefore, different strategies were devel- oped independently at the beginning of the 1990s. The first All these procedures are related to severe drawbacks. syntheses of calcium bis[bis(trimethylsilyl)amide] Transmetalation protocols require the preparation, isola- (Ca(HMDS)2) were performed via transmetalation of tion and distillation of the precursor organometallics 1 2 Hg(HMDS)2 or Sn(HMDS)2, via metathetical approaches of (Hg(HMDS)2 or Sn(HMDS)2). Metathetical approaches ne- AHMDS (A = alkali metal) with calcium alkoxides3 or cessitate an exact stoichiometry to avoid formation of calci- – pseudohalides such as trifluoromethanesulfonates (tri- ates of the type [Ca(HMDS)3] (excess of AHMDS) and ha- flates)4 and arenesulfonates.5 The metathesis reaction of lide-containing product mixtures (substoichiometric 6,7 8 KHMDS with CaI2 in tetrahydrofuran and toluene yields AHMDS). The Ca(HMDS)2 product commonly contains at thf adducts and unsolvated Ca(HMDS)2, respectively. Direct least traces of potassium (potassium calciate formation Georg Thieme Verlag Stuttgart · New York — Synthesis 2019, 51, 1115–1122 1116 Syn thesis S. Krieck et al. Feature and/or traces of potassium iodide) that have to be removed Drawbacks of these procedures are obvious because anoth- by repeated recrystallization efforts. Prior to the direct er metalorganic precursor is required to prepare the meta- metalation of (H)HMDS the calcium metal has to be activat- lation reagent. ed to apply pyrophoric metal powders. In our hands, calci- The bis(trimethylsilyl)amido ligands guarantee not only um pieces are sluggish in reactions in ammonia-saturated solubility of the respective metal complexes in common or- solutions and dull, grayish reaction mixtures are obtained ganic solvents, but variations of this anion allow modifica- which prohibit the isolation of a pure product. These initial tion of the electronic and (especially) steric properties. investigations are summarized and evaluated in more detail Transmetalation of Sn[N(SiMe2CH2)2]2 with calcium in THF elsewhere.10 yields the corresponding tris(thf) adduct of calcium Due to these reasons, diverse strategies have been de- bis(2,2,5,5-tetramethyl-1-aza-2,5-disilylcyclopentanide).15 veloped to accelerate the synthesis of highly pure In contrast, enlargement of the amido ligand and the use of Ca(HMDS)2. The reaction of calcium with (H)HMDS in THF Sn[N(SiMe3)Si(SiMe3)3]2 in the transmetalation procedure can be ensured via addition of BiPh3 under ultrasonica- only gives the aminyl radicals and finally the amine 11 tion. The organometallic metalation of (H)HMDS with HN(SiMe3)Si(SiMe3)3, but a calcium bis(amide) cannot be dibenzylcalcium12 circumvents the necessity of metal acti- isolated. Suitable procedures for the preparation of vation prior to use. Arylcalcium reagents (heavy Grignard Ca[N(SiHMe2)2]2 are the metathetical approach with 13 reagents) are easily available and represent suitable meta- KN(SiHMe2)2 and CaI2 as well as the transamination of 14 16 lation reagents for the syntheses of calcium amides. Ca(HMDS)2 with HN(SiHMe2)2. The metathesis reaction of the potassium amides with calcium iodide offers access to solvated bulky calcium bis(amides) with amido ligands like Biographical Sketches Sven Krieck (left) studied chemistry at Frie- Philipp Schüler (second from left) studied ny, and studied for his Ph.D. thesis at the drich Schiller University Jena, Germany, and the synthesis and coordination chemistry of University of Stuttgart, Germany, under the graduated in 2007 with his diploma thesis in alkaline earth metal bis[bis(trimethylsi- supervision of Professor Gerd Becker. In the field of electrochemical studies on het- lyl)amides] at Friedrich Schiller University Je- 1987/88, he worked as a postdoctoral fellow erosupramolecular aggregates in the group na, Germany, and obtained his M.Sc. degree with Professor Robert T. Paine at the Univer- of Professor Günter Kreisel. He then joined in 2018. He is currently a Ph.D. student in sity of New Mexico in Albuquerque, USA, in the group of Professor Matthias Westerhau- the Westerhausen group at the same institu- the field of phosphanylboranes. Back at the sen and completed his Ph.D. in April 2010 tion. University of Stuttgart, he finished his habili- working on the stabilization of organocalci- tation in the Institute of Inorganic Chemistry um compounds. His thesis was awarded with Jan M. Peschel (second from right) studied in December 1994 and received the venia the university prize as best thesis in 2010. chemistry at Friedrich Schiller University Je- legendi for Inorganic Chemistry in February After a postdoctoral fellowship with Profes- na, Germany, and obtained his B.Sc. degree 1995. From 1996 to 2004 he was a professor sor Nadia Mösch-Zanetti at Karl Franzens in 2017. During his laboratory courses he in- at Ludwig Maximilians University Munich University in Graz, Austria, he returned to vestigated the synthesis and coordination where he was also vice rector from 2001 to the Westerhausen group in 2011. Today he chemistry of alkaline earth metal 2004. He was awarded the Teaching Excel- is lecturer at the Institute of Inorganic and bis[bis(trimethylsilyl)amides]. lence Award of the State of Bavaria in 1998. Analytical Chemistry at Friedrich Schiller Uni- Since 2004, he has been teaching and re- Matthias Westerhausen (right) obtained versity Jena, Germany. searching at Friedrich Schiller University Je- his diploma degree in chemistry in 1983 na, Germany. from Philipps University in Marburg, Germa- Georg Thieme Verlag Stuttgart · New York — Synthesis 2019, 51, 1115–1122 1117 Syn thesis S. Krieck et al. Feature N(SiMe3)(SiMe2tBu), N(SiMe3)(SiPh2tBu), and havior of aromatic and aliphatic hydrocarbyl iodides and 17 N(SiMe3)(SiPh3). The salt metathesis reaction even allows bromides R–X (X = Br, I) lies in the nature of the hydrocarbyl the synthesis of unsolvated calcium bis[bis(triisopropylsi- halide radical anions after the first electron transfer from lyl)amide] with a nearly linear N–Ca–N fragment of the metal onto R–X. Without stabilization of the negative 172.62(11)°.18 This metathetical approach also provides ac- charge (via delocalization into the aryl group or via negative cess to substituted N-trimethylsilylanilides with bulky aryl hyperconjugation to the trimethylsilyl groups) the C–X groups like 2,4,6-trimethylphenyl (mesityl, Mes)19 and 2,6- bond is elongated much more, easing dissociation into the diisopropylphenyl (Dipp).20 halide anion and the alkyl radical. Furthermore, the co-ligands of Ca(HMDS)2 can be varied Calcium is unable to directly metalate (H)HMDS in com- to also influence the shielding of the reactive Ca–N bonds. mon organic solvents (aliphatic and aromatic hydrocar- Structural studies have been reported for dimeric bons, ethers) regardless of a prior activation procedure. 21 [Ca(HMDS)2]2 as well as for the ether solvates Therefore, commercially available calcium granules and this 21 22 [(dme)Ca(HMDS)2], [(thf)2Ca(HMDS)2], and amine were placed together in a Schlenk flask in THF. Next, 23 [(thp)2Ca(HMDS)2]. Electroneutral co-ligands with nitro- ethyl bromide, dissolved in THF, was added dropwise at gen donor atoms like tmeda24 and imidazole25 form stable room temperature and the reaction mixture was stirred for complexes too.