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Dimethylcalcium Benjamin M. Wolf, Christoph Stuhl, Cäcilia Maichle-Mössmer, and Reiner Anwander J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12984 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018
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Journal of the American Chemical Society
Dimethylcalcium Benjamin M. Wolf, Christoph Stuhl, Cäcilia Maichle-Mössmer, and Reiner Anwander* Institut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany Calcium, Alkyl, Methyl, Organocalcium, Heavy Grignard
ABSTRACT: The salt metathesis reaction between homoleptic calcium bis(trimethylsilyl)amide [Ca{N(SiMe3)2}2]2 and "halide-free" methyllithium MeLi allowed for the isolation of X-ray amorphous dimethylcalcium [CaMe2]n in good yields and purities. The formation of [CaMe2]n was proven by microanalysis and NMR/FTIR spectroscopic methods as well as a series of derivatization reactions. Although slowly decomposing thf, [CaMe2]n could be crystallized from chilled thf solutions as the heptametallic adduct [(thf)10Ca7Me14]. Reaction of [CaMe2]n with CaI2 in thf led to the dimeric complex [(thf)3Ca(Me)(I)]2 whereas in tetrahydropyran (thp) the trinuclear complex [(thp)5Ca3(Me)5(I)] was obtained, both representing the first crystallographically characterized heavyGrignard compounds with methyl groups as the hydrocarbyl ligand. While protonolysis of [CaMe2]n with the superbulky proligand HTptBu,Me in non-polar solvents gave homoleptic (TptBu,Me)2Ca, reaction in donor solvents (thf, thp) afforded the monomeric complexes [(TptBu,Me)CaMe(thf)] and [(TptBu,Me)CaMe(thp)], which are the first examples bearing terminal Ca−CH3 functionalities. Grignard-type nucleophilic methyl-group transfer to hexamethylacetone gave access to the dimeric alkoxide complexes [(thf)Ca(OCtBu2Me)2]2 and [(tBu2CO)Ca(µ2-OCtBu2Me)3Ca(OCtBu2Me)]. Finally, addition of the Lewis acid GaMe3 to [CaMe2]n led to the corresponding tetramethylgallate compound [Ca(GaMe4)2]n.
Introduction Ever since Frankland’S epoch-making discovery of the spontaneously inflammable ZnEt2,1 scientists have been intrigued by the feasibility of such pure organometallics, and in particular the quest for thermally stable methyl derivatives.2,3 Archetypal compounds comprise [Me3PtX]4 (X = Me, I),4 alkylmagnesium halides RMgX ("Grignard reagents", Nobel prize in 1912),5 MeLi,6 AlMe37 or WMe6,8 of which the highly nucleophilic Grignard and organolithium reagents are widely used in organic synthesis. In the early days of metal alkyl chemistry, another milestone was set by Wilhelm Schlenk, who discovered that homoleptic dialkylmagnesium species MgR2 could be obtained from RMgX by simple variation of the donor solvent (known as Schlenk equilibrium).9 Alkyl derivatives of the higher group 2 homologues have gained increasing attention over the past decade.10 This is particularly true for the alkaline-earth metal (Ae) calcium and organometallics derived from. Whereas simple alkyl compounds like [AeMe2] are well established for the lighter alkaline-earth metals Be11 and Mg,12 these species remained elusive for the heavier metals Ca, Sr and Ba. As for magnesium, the beginnings of organocalcium chemistry strongly relates to heteroleptic complexes RCaX, often termed heavyGrignard or post-Grignard reagents.13 Initial reports can be traced back till 1905, when Beckmann stated that mixtures of elemental calcium and PhI/EtI perform in classical Grignardtype reactions with CO2 and PhCHO.14 In the ca. 80 years to come, various research groups focused on the synthesis of heavy-Grignard compounds but the outcomes of these studies remained controversial.15–26 The relatively low reactivity of
calcium metal and, at the same time, underestimated high reactivity of the corresponding organocalcium compounds turned out to be one of the main problems. Especially, Wurtztype coupling reactions and the degradation of ethereal solvents obviously led to misinterpretations of analytical data, consisting mainly of the hydrocarbon content formed via hydrolysis as well as indirect evidence by derivatization reactions. In 1958 Payne and Sanderson reported on the syntheses of homoleptic dimethylcalcium, dimethylstrontium, and dimethylbarium via reaction of CH3I with elemental Ae in anhydrous pyridine.27 The compounds were described as pyrophoric solids, pale in color and stable even at elevated temperatures as high as 400 °C. Again, analytics were limited to measurements of the evolved gas upon hydrolysis and the gravimetric determination of metal and iodide contents. This work could not be reproduced by others.28–30 It was not until 1991 when Lappert et al. reported on the successful isolation and characterization of [(diox)2Ca{CH(SiMe3)2}2] as the first dialkylcalcium derivative. The monomeric complex was obtained by co-condensation of calcium vapor with the respective bromoalkane followed by addition of 1,4-dioxane, exploiting the Schlenk-type equilibrium.31 This seminal piece of work revealed that organocalcium compounds can be accessed by using sterically demanding and electronically stabilized silylalkyl ligands, which not only enhance solubility in common solvents but also prevent the complexes from selfdecomposition or rapid ether cleavage. The strategy allowed for the isolation of congeneric compounds like [Ca{C(SiMe3)3}2],32 [K[Ca{CH(SiMe3)2}2],33 34 [(thf)2Ca{CH(SiMe3)2}2], [(thf)2Ca{C(SiMe2H)3}2],35 and only very recently [(do)xCa(CH2SiMe3)2]36 (do = thp, x = 4; do =
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tmeda, x = 2). Moreover, benzyl ligands give access to stable hydrocarbyl complexes [(thf)2Ca{C(SiMe3)2Ph}2],37 38 [(thf)2Ca{CH(SiMe3)Ar}2], [(thf)2Ca(CHMeAr)2]39 (Ar = C6H4(NMe2)-2), and [(thf)4Ca(CH2Ar)2]40 (Ar = Ph, C6H4tBu-4) through delocalization of the negative charge at the carbanion as reflected in decreased basicity/reactivity. Using similar concepts and activated metallic calcium (e.g. by dissolution of Ca in liquid NH3 followed by complete removal of NH3) Westerhausen and co-workers finally were able to advance the field of heavy Grignard reagents. Accordingly, compounds of the type [(do)nCa(R)(X)] (R = CH2SiMe3, aryl, alkenyl; X = halide) were synthesized in good yields and were characterized comprehensively.10d By shifting Schlenk-type equilibria through addition of KOtBu it was also possible to obtain halide-free diarylcalcium complexes.41 It must be stated, however, that simple, unfunctionalized alkyl-calcium compounds of the type CaR2 and RCaX (R = CnH2n+1) remained elusive. Aside from high reactivity, solubility issues for the smallest alkyls, ßH elimination reactions as well as the lack of suitable precursors and synthesis protocols hampered their preparation so far. For example, while the reaction of CaI2 with benzylpotassium in thf smoothly gives [(thf)4Ca(CH2Ph)2] and KI,40 similar salt-metathesis protocols are not effective for alkali-metal methyls like MeLi or methylpotassium because of a unfavorable combination of solubility issues, degradation reactions and inseparability of product mixtures. The redox transmetallation of organomercury compounds HgR2 and elemental metal as well as transmetallation by ligand exchange introduced by Schlenk represent convenient methods for the preparation of metal alkyl species like organolithium compounds (Scheme 1, I and II).6 Even though route I was claimed feasible for calcium,42 there are reports indicating that calcium and HgR2 do not react well.17 Application of zinc or tin alkyls also did not yield homoleptic CaR2 but led to the formation of complex zincates and stannides, respectively.43,44 Scheme 1. Historical Valuable Strategies for the Synthesis of Lithium/Metal Alkyls via Redox Transmetallation (I), Transmetallation by Ligand Exchange (II), and Salt Metathesis (III).
Salt metathesis of alkyllithium species with alkaline metal alkoxides, and in particular tert-butoxides as mentioned by Lochmann, early revealed a high potential for the preparation of alkylsodium compounds (Scheme 1, III).45 Like method II, this route especially benefits from the insolubility of the generated metal alkyl MRn, driving the reaction to completion. By using this method Weiss and co-workers actually were able to synthesize and structurally characterize the methyl derivatives of Na,46 K,47 Rb,48 and Cs.48 On the other hand, welldefined calcium alkoxides are rather rare and putative calcium tert-butoxide [Ca(OtBu)2] is seemingly unknown, which further complicated research in this field. We now succeeded in adopting the Lochmann protocol49 for calcium bis(trimethylsilyl)amide [Ca{N(SiMe3)2}2]250 instead of using an alkoxide as the soluble calcium precursor which allowed for
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the synthesis and characterization of dimethylcalcium [CaMe2]n.51 The existence of [CaMe2]n is further evidenced by derivatization reactions.
Results and Discussion Preparation and Properties of [CaMe2]n. When treating a diluted solution of [Ca{N(SiMe3)2}2]2 in Et2O with two equivalents of a diluted solution of commercially available methyllithium (MeLi, ~1.6 M in Et2O) a white amorphous precipitate was formed immediately. These white solids were reproducibly obtained and mainly consisted of [CaMe2]n, but unsurprisingly significant contaminations by variable amounts of lithium, chloride, and Et2O were detected. Subsequent assessment of the reaction parameters and precursors revealed that it is possible to drastically reduce the amount of impurities which finally led to high-purity [CaMe2]n (1). Since the calcium silylamide precursor was employed in very pure crystalline form, it could be excluded as a source of impurities (cf., experimental section).52 Methyllithium, however, turned out to be problematic concerning purity and base content. Regardless of whether MeLi is prepared from chloromethane (MeCl) and lithium metal in a direct synthesis as described by Ziegler and Colonius or commercially available "low-halide" solutions in Et2O were used, these reagents incorporate up to 5-10 mol% of halide, mainly chloride.53,54 If treated with a soluble calcium precursor in Et2O the chloride impurities will mostly co-precipitate as inseparable calcium- or lithiumchloride species, producing contaminated [CaMe2]n (1con, see ESI). This problem may be solved by employing "halide-free" methyllithium, but unfortunately all methods known for the preparation of halide-free MeLi require highly toxic dimethylmercury (Scheme 1, I and II).6,55 The mercury-based routes became inevitable when the absence of halide was a crucial factor, however.46,47b,47c,56 In order to avoid HgMe2 we developed a method whereby the chloride content of commercially available MeLi solutions in Et2O could be drastically reduced. After determination of the base content, the chloride content was checked by potentiometric titration with silver nitrate and was found to lie within the range of 6-9 mol%, depending on the MeLi batch. Crucially, addition of K[N(SiMe3)2] to an ethereal MeLi solution triggers the fractional precipitation of chloride as KCl, thus effectively minimizing the chloride content to about 1 mol%. Concomitantly formed Li[N(SiMe3)2] does not interfere with the salt metathesis between MeLi and [Ca{N(SiMe3)2}2]2 later on as it is formed at this point anyway (Scheme 2).57 [CaMe2]n (1) obtained by this procedure was found to contain about 3 mol% Et2O,
