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International Edition: DOI: 10.1002/anie.201607608 German Edition: DOI: 10.1002/ange.201607608
Alkaline Earth Metals
Synthesis and Reactivity of Discrete Calcium Imides Benjamin M. Wolf, Ccilia Maichle-Mçssmer, and Reiner Anwander* Abstract: Protonolysis of dibenzylcalcium with triphenylsilylamine affords a thf-coordinated tetrametallic calcium imide with a heterocuboid core structure. The use of calcium bis(tetramethylaluminate) as a precursor for tandem salt metathesis/protonolysis reactions with alkali metal amides of 2,6-diisopropylaniline and triphenylsilylamine provides access to Lewis acid stabilized monocalcium imides of the type [(thf)4Ca(m2-NR)(m2-Me)AlMe2]. Treatment of [(thf)4Ca(m2NSiPh3)(m2-Me)AlMe2] with phenylsilane results in HSi addition across the CaN imido bond, producing a homoleptic calcium amidoaluminate complex and putative CaH2 whereas reaction with phenylacetylene leads to protonation of an AlMe moiety to yield the dimeric complex [(thf)Ca{NSiPh3}{AlMe2(CCPh)}]2.
Alkaline-earth metal (Ae) imide chemistry was mentioned as early as 1903, when Meunier examined the reactions of Grignard reagents with aniline derivatives.[1] The first structurally characterized Ae imides, however, were reported only in the 1970s by Cucinella et al., who described heterobimetallic clusters with the composition [(thf)nAe(AlH)3(NtBu)4] (Ae = Mg, n = 1; Ae = Ca, n = 3).[2] It took another 15 years until Power and co-workers described the first homometallic Group 2 imide species, [(thf)Mg(NPh)]6, which was obtained by protonolysis of [MgEt2]n with aniline.[3] Later on, readily available organomagnesium precursors enabled the synthesis, isolation, and full characterization of several other Mg imides, all featuring Mg4 and Mg6 frameworks in the solid state.[4] In contrast, Cucinellas heterobimetallic CaAl3 imide clusters have remained the only examples of a calcium organoimido species characterized by X-ray crystallography to date.[2, 5, 6] Given the enormous progress in organocalcium chemistry and its rapidly growing significance,[7, 8] the virtual non-existence of Ca imide chemistry is surprising and at first glance might be due to the lack of suitable alkyl complex precursors. Even more crucially, the stabilization of highly polarized Ca2+···2NR bonds is anticipated to pose a major challenge for the synthesis of discrete monocalcium imides. To broaden the scope of organoaluminum-assisted imide formation, which was originally applied to rare-earth-metal tetramethylaluminates [Ln(AlMe4)3],[9, 10] we were interested in probing the reactivity of calcium bis(tetramethylaluminate), [Ca(AlMe4)2]n,[11] towards primary amines and their alkali metal salts. To better assess the feasibility/outcome of [*] B. M. Wolf, Dr. C. Maichle-Mçssmer, Prof. Dr. R. Anwander Institut fr Anorganische Chemie Eberhard Karls Universitt Tbingen Auf der Morgenstelle 18, 72076 Tbingen (Germany) E-mail: [email protected] Supporting information for this article can be found under: http://dx.doi.org/10.1002/anie.201607608.
Scheme 1. Synthesis of the tetrameric calcium imide [(thf)Ca(NSiPh3)]4 (1) by protonolysis of dibenzylcalcium with triphenylsilylamine.
Compound 1 crystallized in the monoclinic space group , and is readily soluble in thf and toluene, but only P21 n sparingly soluble in Et2O, and insoluble in n-hexane. Imide complex 1 features a slightly distorted cubic arrangement of calcium and nitrogen atoms similar to that of its lighter Mg congener [(thf)Mg(NSiPh3)]4, which was obtained by Himmel et al. by reacting [Mg(nBu)2] with H2NSiPh3.[4c] Each Ca atom in 1 is coordinated by three m3-imido nitrogen atoms and one thf molecule while the shortest Ca–arene contact is 3.154(2) . The CaN bond lengths in 1 range from 2.298(2) to 2.390(2) and are considerably shorter than those found in [(thf)3Ca(AlH)3(NtBu)4] with a six-coordinate calcium ion (av. 2.490(2) ).[2] For further comparison, dimeric [Ca{N(SiMe3)2}2]2 exhibits two bridging (m2 mode) and two terminal amido ligands with average CaN bond lengths of 2.47 (bridging) and 2.27 (terminal).[19] The heterocubane core structure of 1 exhibits almost perpendicular Ca-N-Ca and N-Ca-N angles ranging from
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these reactions, dibenzylcalcium, [CaBn2]n, was employed as an alternative organocalcium precursor.[12] Moreover, carefully choosing the amino/amido component according to reactivity (pKa), solubility, and steric demand criteria has proven crucial for imide formation.[13] Therefore, we chose 2,6-diisopropylaniline (H2NDipp, Dipp = 2,6-iPr2C6H3) and triphenylsilylamine (H2NSiPh3) as promising imido precursors. As previously shown for protonolysis (Nb),[14] transimination (Ti),[15] and aminolysis reactions (Ti, Zr, Hf)[16, 17] as well as the preparation of a tetrameric magnesium imide,[4c] bulky triphenylsilylamine seems particularly suitable for double deprotonation. Accordingly, when H2NDipp was treated with one equivalent of [CaBn2]n in thf, single deprotonation occurred exclusively (even at elevated temperatures), yielding the bis(amide) complex [(thf)xCa(NHDipp)2] (see the Supporting Information, Figure S5).[18] In contrast, conducting the corresponding [CaBn2]n/H2NSiPh3 reaction under similar conditions gave compound 1, the 1 H NMR spectrum of which did not show any NH resonances. X-ray structure analysis of the slightly yellow crystals obtained from a thf solution of 1 indeed confirmed the formation of the homometallic calcium imide [(thf)Ca(NSiPh3)]4 (Scheme 1 and Figure 1).
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Scheme 2. Syntheses of the heterobimetallic calcium imides [(thf)4Ca(m2-NR)(m2-Me)AlMe2] 2 a (R = Dipp) and 2 b (R = SiPh3) by tandem salt metathesis/protonolysis.
Figure 1. Solid-state structure of [(thf)Ca(NSiPh3)]4 (1). Hydrogen atoms are omitted for clarity. The carbon atoms of the aromatic moieties are shown with reduced radii. All other atoms are represented by atomic displacement ellipsoids set at 50 % probability. The unit cell contains two independent molecules, of which one molecule is shown (see the Supporting Information).
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88.65(6)8 to 93.23(6)8 and from 86.50(6)8 to 91.20(6)8, respectively. All attempts to disrupt the cubane framework of 1 to generate lower aggregated imide species have failed thus far; neither complexation with neutral chelating ligands (e.g., 2,2’bipyridine, 1,10-phenanthroline) nor addition of Lewis acids such as AlMe3 were expedient. Treatment of 1 with various amounts of AlMe3 in thf gave no reaction at all whereas in toluene, a mixture of products was obtained, consisting mainly of imidoalanes and [Ca(AlMe4)2]n. [Ca(AlMe4)2]n, on the other hand, can be used as a precursor for tandem salt metathesis/protonolysis reactions in combination with alkali metal amides [M{NH(R)}]. Compared to the double deprotonation of the amine, the clear benefit of this approach is that a tetramethylaluminate salt, [M(AlMe4)], is formed as a byproduct instead of the more reactive AlMe3. This hypothesis was corroborated by the formation of complex product mixtures in reactions of [Ca(AlMe4)2]n with various primary amines.[20] Owing to the insolubility of [Ca(AlMe4)2]n in aliphatic and aromatic organic solvents and the absence of a stabilizing ancillary ligand, thf was used as a donor solvent in the following syntheses. The equimolar reaction of [Ca(AlMe4)2]n with [Li{NH(Dipp)}] proceeded within 30 hours at ambient temperature, with the evolution of methane being indicative of the formation of an imide species. Under the same conditions, [K{NH(SiPh3)}] reacted considerably faster, and methane evolution was again observed. As confirmed by X-ray structure analysis, monocalcium imide species, [(thf)4Ca(m2-NR)(m2-Me)AlMe2] (2 a: R = Dipp; 2 b: R = SiPh3), had been formed (Scheme 2). Whereas 2 a was crystallized from a mixture of thf and toluene in 57 % yield (monoclinic, Cc), 2 b was collected from the reaction mixture as a white precipitate in 78 % yield and subsequently crystallized from a saturated thf solution ). The molecular connectivity of these (monoclinic, P21 n heterobimetallic complexes (Figure 2) can be described as Lewis acid (AlMe3) stabilized terminal calcium imide species. Similar AlMe3 masked imides are already known from rarewww.angewandte.org
Figure 2. Solid-state structures of the heterobimetallic calcium imides [(thf)4Ca(m2-NR)(m2-Me)AlMe2] 2 a (left, R = Dipp) and 2 b (right, R = SiPh3). Hydrogen atoms are omitted for clarity. Atoms are represented by atomic displacement ellipsoids set at 50 % probability.
earth-metal chemistry. For example, Mindiola et al. obtained the scandium imide complex [(PNP)Sc(m2-NDipp)(m2Me)AlMe2] (PNP = N[2-P-(CHMe2)2-4-methylphenyl]2) by treatment of the anilide precursor [(PNP)Sc(NHDipp)(CH3)] with AlMe3.[21] Recently, we described the dimeric species [Ln2(m2-NDipp)(m3-NDipp){(m2-Me)2AlMe}(AlMe4)2] (Ln = Y, La, Ce, Nd)[10] and the monomeric rare-earth-metal complexes [TptBu,MeLn(m2-NR)(m2-Me)AlMe2] (TptBu,Me = tris(3-Me-5-tBu-pyrazolyl)borate; Ln = Y, R = tBu, Ad, 2,6(CH3)2C6H3 ; Ln = Ho, R = tBu, Ad),[22, 23] which feature similar connectivities. The Ca atoms in 2 a and 2 b are coordinated in a distorted octahedral fashion by bridging imido and methyl groups as well as by four molecules of thf. As expected for m2-bridging imido ligands, the CaN bonds in 2 a (2.236(2) ) and 2 b (2.280(2) ) are even shorter than those in the homometallic imide 1. The CaC(m-CH3) bonds (2.651(2) (2 a) and 2.668(2) (2 b)) are longer than those in the six-coordinate bis(tetramethylaluminate) complex [(phen)Ca(AlMe4)2][11] (av. 2.584(7) ) and similar to the distances in the structurally related amide complex [Ca{N(SiMe3)2(GaEt3)}2][24] (2.656(5) and 2.697(4) ). The AlN bond lengths of 1.849(2) (2 a) and 1.831(2) (2 b) match those of comparable imidoalanes.[25–28] It should be mentioned that such homometallic imidoalanes could indeed be synthesized from alkylaluminum or alane precursors and primary amines. Compared to the preparation from tetramethylaluminate precursors, however, the second deprotonation step usually has to be carried out under harsher conditions. For example, the dimeric imidoa-
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Scheme 3. Reactivity of 2 b towards phenylsilane and phenylacetylene. Angew. Chem. Int. Ed. 2016, 55, 1 – 6
formation of colloidal calciumhydride [CaH2(L)x]y was similarly observed by Okuda and co-workers during the hydrogenolysis of [(thf)4Ca(SiPh3)2] (d = 3.9–5.4 ppm in [D8]thf; L = neutral Lewis base).[31, 32] To assess the robustness of the imido versus the organoaluminum moiety, we next examined the reactivity of 2 b towards moderately CH acidic phenylacetylene. When a thf solution of 2 b was treated with one equivalent of PhCCH, instant methane evolution was observed (Scheme 3). Not surprisingly, the 1H NMR spectrum of the new compound 4, which was obtained in 72 % yield, did not show any NH resonances, indicating that the imido moiety stayed intact while one of the methyl groups was protonated. X-ray crystallographic analysis revealed the formation of the dimeric complex [(thf)Ca{NSiPh3}{AlMe2(CCPh)}]2 (4; see the Supporting Information for its molecular structure), in which the imido moiety moved to a bridging position between two Ca centers and one Al atom. One methyl group (m2-h1:h1) as well as the phenylacetylido moiety (m2-h1:h2) also act as bridges of one Al and distinct Ca centers. Similar observations were made by Schmidt et al. during the generation of [Al(CCPh)(NBn)]6 from imidoalane [Al(H)(NBn)]6 and PhCCH by hydrogen evolution.[33] In conclusion, we have prepared the first homometallic calcium imide, [(thf)Ca(NSiPh3)]4, from dibenzylcalcium and H2NSiPh3. Whereas the reaction of [CaBn2]n with H2NDipp resulted in the formation of the undesired bis(amide) complex [(thf)xCa(NHDipp)2], the use of [Ca(AlMe4)2]n in tandem salt metathesis/protonolysis reactions with either [Li{NH(Dipp)}] or [K{NH(SiPh3)}] led to the heterobimetallic imide species [(thf)4Ca(m2-NR)(m2-Me)AlMe2] (R = Dipp, SiPh3), respectively. The present study underlines both the great potential of alkaline-earth-metal tetraalkylaluminates in imide formation reactions and the versatility of the robust SiPh3 backbone for unveiling Ca=NR reactivity.
Experimental Section Complex 1: A solution of H2NSiPh3 (137 mg, 0.50 mmol) in toluene (1 mL) and 10 drops of thf were added to a suspension of [CaBn2]n (111 mg, 0.50 mmol) in toluene/Et2O (1:1 v/v, 4 mL). The resulting yellowish suspension was stirred at 40 8C for 60 h, which eventually led to a clear yellow solution. The volume of the reaction mixture was reduced to about 1 mL, and n-hexane (3 mL) was added to precipitate a slightly yellow solid, which was separated, washed with n-hexane (2 1 mL), and dried under reduced pressure. Yield: 133 mg, 0.09 mmol, 69 %. Single crystals suitable for X-ray structure analysis were grown from a saturated thf solution at ambient temperature. 1 H NMR (400 MHz, [D8]thf, 26 8C): d = 7.53 (m, 6 H, m-ArH), 7.21 (m, 6 H, o-ArH), 7.14 (m, 3 H, p-ArH), 3.62 (m, 4 H, OCH2 thf), 1.78 ppm (m, 4 H, CH2 thf). 13C{1H} NMR (101 MHz, [D8]thf, 26 8C): d = 150.6 (ipso-ArC), 135.9 (o-ArC), 128.3 (m-ArC), 127.7 (p-ArC), n ¼3054 (m), 3006 (w), 68.4 (OCH2 thf), 26.6 ppm (CH2 thf). DRIFT: ~ 2983 (m), 2881 (vw), 1956 (vw), 1888 (vw), 1822 (vw), 1771 (vw), 1652 (vw), 1560 (vw), 1474 (w), 1423 (m), 1372 (vw), 1295 (vw), 1259 (w), 1180 (w), 1152 (vw), 1091 (s), 1035 (s), 984 (vs), 910 (w), 882 (m), 740 (s), 704 (vs), 686 (s), 621 (vw), 522 (vs), 510 (s), 503 (s), 457 (s), 447 (s), 418 cm1 (m). Elemental analysis calcd (%) for C88H92N4Ca4O4Si4 (1542.38 g mol1): C 68.53, H 6.01, N 3.63; found: C 68.57, H 6.24, N 3.61. Complex 2 a: [Ca(AlMe4)2]n (79 mg, 0.37 mmol) was suspended in thf (2 mL), and a solution of [Li{NH(Dipp)}] (67 mg, 0.37 mmol) in
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lane [Mes*AlNSiPh3]2 (Mes* = 2,4,6-tBu3C6H2) was synthesized from neat [Mes*AlH2]2 and H2NSiPh3 at 135 8C in 15 % yield.[28] Another example is the thermolysis of [Me2AlN(H)SiPh3]2 at 185 8C, which yielded the tetrameric aluminum imide [MeAlNSiPh3]4.[29] The 1H NMR spectra of 2 a and 2 b in [D8]thf show a sharp singlet at around 1.0 ppm with an integral ratio of 9H (Al(CH3)3), indicating fast exchange of the bridging and terminal methyl groups at ambient temperature. Variable-temperature 1H NMR spectra, however, revealed that even at temperatures as low as 90 8C, these resonances did not decoalesce, corroborating the rapid exchange of the bridging and terminal methyl groups. With the discrete Ca imides 1 and 2 in hand, we envisaged initial reactivity studies. It turned out that 2 b benefits from the SiPh3 backbone, which enhances the solubility in ethereal and aromatic solvents and minimizes CH bond activation pathways as observed for the iPr groups in 2 a. Chen et al. have nicely demonstrated that scandium imido moieties engage in silane activation.[30] The terminal imide [LSc(NDipp)(DMAP)] (DMAP = 4-dimethylaminopyridine, L = [MeC{N(Dipp)}CHC{Me}{NCH2CH2NMe}]) readily reacts with PhSiH3 to yield the putative anilide hydride complex [LSc(H){N(Dipp)(SiH2Ph)}], which could be trapped by subsequent reaction with a carbodiimide, RN=C=NR.[30] Similar 1,2-additions were probed with the CaII imides 1 and 2 b. Whereas reactions of tetrametallic 1 with various amounts of PhSiH3 in thf or toluene gave ill-defined products, the clean conversion of 2 b into the new complex 3 in the presence of one equivalent of PhSiH3 was monitored by 1 H NMR spectroscopy in [D8]thf and occurred within six hours at ambient temperature (Scheme 3 and Figure S26 for in situ NMR spectra). Upscaling this reaction allowed for the isolation of crystalline complex 3 in 83 % yield (see the Supporting Information for the molecular structure of 3). Apparently, HSi addition across the CaN bond furnished the homoleptic calcium amidoaluminate [(thf)4Ca{(m2Me)AlMe2(N(SiPh3)(SiH2Ph))}2] (3) and putative calcium hydride [CaH2(L)x]y (L = neutral Lewis base) via a Schlenktype ligand rearrangement. All efforts to isolate these transient calcium hydride species have failed thus far. However, an in situ 1H NMR spectroscopic study showed a very broad resonance between 4.1 and 4.7 ppm ([D8]thf), which we ascribed to a hydride species (Figure S26). The
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Communications thf (2 mL) was added slowly. The reaction mixture was stirred for 36 h at ambient temperature, and then toluene (3 mL) was added to the clear solution. At 35 8C, the product crystallized as colorless crystals, which were separated from the solution and evaporated to dryness at ambient temperature under inert atmosphere. Crystalline yield: 121 mg, 0.21 mmol, 57 %. Single crystals suitable for X-ray structure determination were grown from a saturated thf/toluene solution (1:1 v/v) at 35 8C. 1H NMR (400 MHz, [D8]thf, 26 8C): d = 6.56 (d, 2 H, 3 JHH = 7.4 Hz, m-ArH), 5.93 (t, 1 H, 3JHH = 7.4 Hz, p-ArH), 4.20 (sep, 2 H, 3JHH = 6.8 Hz, iPr-CH), 3.62 (m, 16 H, OCH2 thf), 1.77 (m, 16 H, CH2 thf), 0.99 (d, 12 H, 3JHH = 7.0 Hz, iPr-CH3), 1.00 ppm (s, 9 H, AlCH3). 13C{1H} NMR (101 MHz, [D8]thf, 26 8C): d = 162.1 (ipsoArC), 139.2 (o-ArC), 122.0 (m-ArC), 109.3 (p-ArC), 68.4 (OCH2 thf), 26.5 (CH2 thf), 26.4 (iPr-CH), 26.4 (iPr-CH3), 2.0 ppm (AlCH3). DRIFT: ~n ¼2952 (m), 2908 (m), 2803 (w), 1577 (w), 1539 (vw), 1456 (m), 1400 (s), 1371 (w), 1325 (m), 1255 (m), 1218 (w), 1192 (w), 1150 (m), 1100 (vw), 1033 (vs), 982 (vw), 880 (s), 803 (w), 765 (w), 748 (m), 709 (s), 668 (vs), 618 (s), 575 (m), 561 (w), 546 (w), 475 (m), 438 (w), 422 cm1 (m). Elemental analysis calcd (%) for C31H58NAlCaO4 (575.87 g mol1): C 64.66, H 10.15, N 2.43; found: C 64.44, H 10.21, N 2.56. Complex 2 b: [Ca(AlMe4)2]n (172 mg, 0.80 mmol) was suspended in thf (2 mL), and a solution of [K{NH(SiPh3)}] (251 mg, 0.80 mmol) in thf (2 mL) was added slowly. Stirring the clear and colorless reaction mixture for 16 h at ambient temperature led to the formation of a white precipitate, which was filtered off, washed with cold thf (2 mL), and evaporated to dryness under reduced pressure. Yield: 421 mg, 0.62 mmol, 78 %. Single crystals suitable for X-ray structure analysis were grown from a saturated thf solution at 35 8C. 1H NMR (400 MHz, [D8]thf, 26 8C): d = 7.60 (m, 6 H, ArH), 7.04 (m, 9 H, ArH), 3.62 (m, 16 H, OCH2 thf), 1.77 (m, 16 H, CH2 thf), 1.08 ppm (s, 9 H, AlCH3). 13C{1H} NMR (101 MHz, [D8]thf, 26 8C): d = 150.3 (ipsoArC), 137.0 (o-ArC), 126.8 (m-ArC), 126.6 (p-ArC), 68.4 (OCH2 thf), n ¼3042 (vw), 2984 26.6 (CH2 thf), 1.05 ppm (AlCH3). DRIFT: ~ (vw), 2883 (w), 2800 (vw), 1479 (vw), 1457 (vw), 1424 (w), 1255 (vw), 1180 (vw), 1150 (w), 1096 (m), 1032 (s), 934 (w), 877 (m), 739 (m), 705 (vs), 694 (s), 667 (m), 618 (w), 568 (vw), 511 (s), 502 (m), 460 (vw), 424 cm1 (vw). Elemental analysis calcd (%) for C37H56NAlCaO4Si (674.00 g mol1): C 65.94, H 8.38, N 2.08; found: C 66.24, H 8.14, N 2.18. Full experimental and analytical details of compounds 1–4 are available in the Supporting Information. CCDC 1495862 (1), 1495864 (2 a), 1495865 (2 b), 1495866 (3), and 1495863 (4) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.
Acknowledgements We gratefully acknowledge support from the German Science Foundation (Grant AN 238/15-1) and the Carl Zeiss Foundation (PhD scholarship for B.M.W.). Keywords: alkyl groups · aluminum · calcium · imides · protonolysis
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[1] L. Meunier, C. R. Hebd. Seances Acad. Sci. 1903, 136, 758. [2] [(thf)nCa(AlH)3(NtBu)4] was obtained from a mixture of Ca(AlH4)2, [AlH3(NMe3)], and tBuNH2 in thf with release of H2 ; see: a) S. Cucinella, G. Dozzi, G. Perego, A. Mazzei, J. Organomet. Chem. 1977, 137, 257 – 264; b) G. Del Piero, M. Cesari, S. Cucinella, A. Mazzei, J. Organomet. Chem. 1977, 137, 265 – 274.
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[20]
[21]
[22] [23]
[24] [25] [26] [27]
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[28] R. J. Wehmschulte, P. P. Power, Inorg. Chem. 1998, 37, 6906 – 6911. [29] D. M. Choquette, M. J. Timm, J. L. Hobbs, T. M. Nicholson, M. M. Olmstead, R. P. Planalp, Inorg. Chem. 1993, 32, 2600 – 2603. [30] J. Chu, E. Lu, Y. Chen, X. Leng, Organometallics 2013, 32, 1137 – 1140. [31] V. Leich, T. P. Spaniol, L. Maron, J. Okuda, Chem. Commun. 2014, 50, 2311 – 2314. [32] For NMR chemical shifts of discrete CaHCa entities, see: a) V. Leich, T. P. Spaniol, L. Maron, J. Okuda, Angew. Chem. Int. Ed. 2016, 55, 4794 – 4797; Angew. Chem. 2016, 128, 4872 – 4876; b) V. Leich, T. P. Spaniol, J. Okuda, Inorg. Chem. 2015, 54, 4927 – 4933; c) P. Jochmann, J. P. Davin, T. P. Spaniol, L. Maron, J. Okuda, Angew. Chem. Int. Ed. 2012, 51, 4452 – 4455; Angew. Chem. 2012, 124, 4528 – 4531; d) S. Harder, J. Brettar, Angew. Chem. Int. Ed. 2006, 45, 3474 – 3478; Angew. Chem. 2006, 118, 3554 – 3558. [33] N. D. Reddy, H. W. Roesky, M. Noltemeyer, H.-G. Schmidt, Inorg. Chem. 2002, 41, 2374 – 2378.
Received: August 5, 2016 Published online: && &&, &&&&
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[19]
see: C. Glock, F. M. Younis, S. Ziemann, H. Gçrls, W. Imhof, S. Krieck, M. Westerhausen, Organometallics 2013, 32, 2649 – 2660. M. Westerhausen, W. Schwarz, Z. Anorg. Allg. Chem. 1991, 604, 127 – 140. In a related study, Mitzel and co-workers found that the reaction of Ca(AlEt4)2 with 2 equiv of HNiPr2 did not result in the desired product Ca[(NiPr2)(AlEt3)]2 but in [Ca(NiPr2)(AlEt4)]2 and [AlEt2(NiPr2)]2, which underpins the non-innocent nature of released AlEt3 ; see: M. Hlsmann, B. Neumann, H.-G. Stammler, N. W. Mitzel, Eur. J. Inorg. Chem. 2012, 4200 – 4209. J. Scott, F. Basuli, A. R. Fout, J. C. Huffman, D. J. Mindiola, Angew. Chem. Int. Ed. 2008, 47, 8502 – 8505; Angew. Chem. 2008, 120, 8630 – 8633. D. Schdle, C. Maichle-Mçssmer, C. Schdle, R. Anwander, Chem. Eur. J. 2015, 21, 662 – 670. D. Schdle, M. Meermann-Zimmermann, C. Schdle, C. Maichle-Mçssmer, R. Anwander, Eur. J. Inorg. Chem. 2015, 1334 – 1339. M. Westerhausen, S. Weinrich, M. Oßberger, N. W. Mitzel, Z. Anorg. Allg. Chem. 2003, 629, 575 – 577. W. Uhl, A. Vogelpohl, Eur. J. Inorg. Chem. 2009, 93 – 97. J. Lçbl, A. Y. Timoshkin, T. Cong, M. Necas, H. W. Roesky, J. Pinkas, Inorg. Chem. 2007, 46, 5678 – 5685. R. J. Wehmschulte, P. P. Power, J. Am. Chem. Soc. 1996, 118, 791 – 797.
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Communications Alkaline Earth Metals B. M. Wolf, C. Maichle-Mçssmer, R. Anwander* &&&&—&&&&
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Synthesis and Reactivity of Discrete Calcium Imides
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Tandem salt metathesis/protonolysis reactions of [Ca(AlMe4)2]n and [K{NH(SiPh3)}] provide access to Lewis acid
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(AlMe3) stabilized calcium imides. Their Ca=NR multiple-bond character was revealed by SiH addition (see scheme).
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International Edition: DOI: 10.1002/anie.201607608 German Edition: DOI: 10.1002/ange.201607608
Alkaline Earth Metals
Synthesis and Reactivity of Discrete Calcium Imides Benjamin M. Wolf, Ccilia Maichle-Mçssmer, and Reiner Anwander* Abstract: Protonolysis of dibenzylcalcium with triphenylsilylamine affords a thf-coordinated tetrametallic calcium imide with a heterocuboid core structure. The use of calcium bis(tetramethylaluminate) as a precursor for tandem salt metathesis/protonolysis reactions with alkali metal amides of 2,6-diisopropylaniline and triphenylsilylamine provides access to Lewis acid stabilized monocalcium imides of the type [(thf)4Ca(m2-NR)(m2-Me)AlMe2]. Treatment of [(thf)4Ca(m2NSiPh3)(m2-Me)AlMe2] with phenylsilane results in HSi addition across the CaN imido bond, producing a homoleptic calcium amidoaluminate complex and putative CaH2 whereas reaction with phenylacetylene leads to protonation of an AlMe moiety to yield the dimeric complex [(thf)Ca{NSiPh3}{AlMe2(CCPh)}]2.
Alkaline-earth metal (Ae) imide chemistry was mentioned as early as 1903, when Meunier examined the reactions of Grignard reagents with aniline derivatives.[1] The first structurally characterized Ae imides, however, were reported only in the 1970s by Cucinella et al., who described heterobimetallic clusters with the composition [(thf)nAe(AlH)3(NtBu)4] (Ae = Mg, n = 1; Ae = Ca, n = 3).[2] It took another 15 years until Power and co-workers described the first homometallic Group 2 imide species, [(thf)Mg(NPh)]6, which was obtained by protonolysis of [MgEt2]n with aniline.[3] Later on, readily available organomagnesium precursors enabled the synthesis, isolation, and full characterization of several other Mg imides, all featuring Mg4 and Mg6 frameworks in the solid state.[4] In contrast, Cucinellas heterobimetallic CaAl3 imide clusters have remained the only examples of a calcium organoimido species characterized by X-ray crystallography to date.[2, 5, 6] Given the enormous progress in organocalcium chemistry and its rapidly growing significance,[7, 8] the virtual non-existence of Ca imide chemistry is surprising and at first glance might be due to the lack of suitable alkyl complex precursors. Even more crucially, the stabilization of highly polarized Ca2+···2NR bonds is anticipated to pose a major challenge for the synthesis of discrete monocalcium imides. To broaden the scope of organoaluminum-assisted imide formation, which was originally applied to rare-earth-metal tetramethylaluminates [Ln(AlMe4)3],[9, 10] we were interested in probing the reactivity of calcium bis(tetramethylaluminate), [Ca(AlMe4)2]n,[11] towards primary amines and their alkali metal salts. To better assess the feasibility/outcome of [*] B. M. Wolf, Dr. C. Maichle-Mçssmer, Prof. Dr. R. Anwander Institut fr Anorganische Chemie Eberhard Karls Universitt Tbingen Auf der Morgenstelle 18, 72076 Tbingen (Germany) E-mail: [email protected] Supporting information for this article can be found under: http://dx.doi.org/10.1002/anie.201607608.
Scheme 1. Synthesis of the tetrameric calcium imide [(thf)Ca(NSiPh3)]4 (1) by protonolysis of dibenzylcalcium with triphenylsilylamine.
Compound 1 crystallized in the monoclinic space group , and is readily soluble in thf and toluene, but only P21 n sparingly soluble in Et2O, and insoluble in n-hexane. Imide complex 1 features a slightly distorted cubic arrangement of calcium and nitrogen atoms similar to that of its lighter Mg congener [(thf)Mg(NSiPh3)]4, which was obtained by Himmel et al. by reacting [Mg(nBu)2] with H2NSiPh3.[4c] Each Ca atom in 1 is coordinated by three m3-imido nitrogen atoms and one thf molecule while the shortest Ca–arene contact is 3.154(2) . The CaN bond lengths in 1 range from 2.298(2) to 2.390(2) and are considerably shorter than those found in [(thf)3Ca(AlH)3(NtBu)4] with a six-coordinate calcium ion (av. 2.490(2) ).[2] For further comparison, dimeric [Ca{N(SiMe3)2}2]2 exhibits two bridging (m2 mode) and two terminal amido ligands with average CaN bond lengths of 2.47 (bridging) and 2.27 (terminal).[19] The heterocubane core structure of 1 exhibits almost perpendicular Ca-N-Ca and N-Ca-N angles ranging from
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these reactions, dibenzylcalcium, [CaBn2]n, was employed as an alternative organocalcium precursor.[12] Moreover, carefully choosing the amino/amido component according to reactivity (pKa), solubility, and steric demand criteria has proven crucial for imide formation.[13] Therefore, we chose 2,6-diisopropylaniline (H2NDipp, Dipp = 2,6-iPr2C6H3) and triphenylsilylamine (H2NSiPh3) as promising imido precursors. As previously shown for protonolysis (Nb),[14] transimination (Ti),[15] and aminolysis reactions (Ti, Zr, Hf)[16, 17] as well as the preparation of a tetrameric magnesium imide,[4c] bulky triphenylsilylamine seems particularly suitable for double deprotonation. Accordingly, when H2NDipp was treated with one equivalent of [CaBn2]n in thf, single deprotonation occurred exclusively (even at elevated temperatures), yielding the bis(amide) complex [(thf)xCa(NHDipp)2] (see the Supporting Information, Figure S5).[18] In contrast, conducting the corresponding [CaBn2]n/H2NSiPh3 reaction under similar conditions gave compound 1, the 1 H NMR spectrum of which did not show any NH resonances. X-ray structure analysis of the slightly yellow crystals obtained from a thf solution of 1 indeed confirmed the formation of the homometallic calcium imide [(thf)Ca(NSiPh3)]4 (Scheme 1 and Figure 1).
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Scheme 2. Syntheses of the heterobimetallic calcium imides [(thf)4Ca(m2-NR)(m2-Me)AlMe2] 2 a (R = Dipp) and 2 b (R = SiPh3) by tandem salt metathesis/protonolysis.
Figure 1. Solid-state structure of [(thf)Ca(NSiPh3)]4 (1). Hydrogen atoms are omitted for clarity. The carbon atoms of the aromatic moieties are shown with reduced radii. All other atoms are represented by atomic displacement ellipsoids set at 50 % probability. The unit cell contains two independent molecules, of which one molecule is shown (see the Supporting Information).
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88.65(6)8 to 93.23(6)8 and from 86.50(6)8 to 91.20(6)8, respectively. All attempts to disrupt the cubane framework of 1 to generate lower aggregated imide species have failed thus far; neither complexation with neutral chelating ligands (e.g., 2,2’bipyridine, 1,10-phenanthroline) nor addition of Lewis acids such as AlMe3 were expedient. Treatment of 1 with various amounts of AlMe3 in thf gave no reaction at all whereas in toluene, a mixture of products was obtained, consisting mainly of imidoalanes and [Ca(AlMe4)2]n. [Ca(AlMe4)2]n, on the other hand, can be used as a precursor for tandem salt metathesis/protonolysis reactions in combination with alkali metal amides [M{NH(R)}]. Compared to the double deprotonation of the amine, the clear benefit of this approach is that a tetramethylaluminate salt, [M(AlMe4)], is formed as a byproduct instead of the more reactive AlMe3. This hypothesis was corroborated by the formation of complex product mixtures in reactions of [Ca(AlMe4)2]n with various primary amines.[20] Owing to the insolubility of [Ca(AlMe4)2]n in aliphatic and aromatic organic solvents and the absence of a stabilizing ancillary ligand, thf was used as a donor solvent in the following syntheses. The equimolar reaction of [Ca(AlMe4)2]n with [Li{NH(Dipp)}] proceeded within 30 hours at ambient temperature, with the evolution of methane being indicative of the formation of an imide species. Under the same conditions, [K{NH(SiPh3)}] reacted considerably faster, and methane evolution was again observed. As confirmed by X-ray structure analysis, monocalcium imide species, [(thf)4Ca(m2-NR)(m2-Me)AlMe2] (2 a: R = Dipp; 2 b: R = SiPh3), had been formed (Scheme 2). Whereas 2 a was crystallized from a mixture of thf and toluene in 57 % yield (monoclinic, Cc), 2 b was collected from the reaction mixture as a white precipitate in 78 % yield and subsequently crystallized from a saturated thf solution ). The molecular connectivity of these (monoclinic, P21 n heterobimetallic complexes (Figure 2) can be described as Lewis acid (AlMe3) stabilized terminal calcium imide species. Similar AlMe3 masked imides are already known from rarewww.angewandte.org
Figure 2. Solid-state structures of the heterobimetallic calcium imides [(thf)4Ca(m2-NR)(m2-Me)AlMe2] 2 a (left, R = Dipp) and 2 b (right, R = SiPh3). Hydrogen atoms are omitted for clarity. Atoms are represented by atomic displacement ellipsoids set at 50 % probability.
earth-metal chemistry. For example, Mindiola et al. obtained the scandium imide complex [(PNP)Sc(m2-NDipp)(m2Me)AlMe2] (PNP = N[2-P-(CHMe2)2-4-methylphenyl]2) by treatment of the anilide precursor [(PNP)Sc(NHDipp)(CH3)] with AlMe3.[21] Recently, we described the dimeric species [Ln2(m2-NDipp)(m3-NDipp){(m2-Me)2AlMe}(AlMe4)2] (Ln = Y, La, Ce, Nd)[10] and the monomeric rare-earth-metal complexes [TptBu,MeLn(m2-NR)(m2-Me)AlMe2] (TptBu,Me = tris(3-Me-5-tBu-pyrazolyl)borate; Ln = Y, R = tBu, Ad, 2,6(CH3)2C6H3 ; Ln = Ho, R = tBu, Ad),[22, 23] which feature similar connectivities. The Ca atoms in 2 a and 2 b are coordinated in a distorted octahedral fashion by bridging imido and methyl groups as well as by four molecules of thf. As expected for m2-bridging imido ligands, the CaN bonds in 2 a (2.236(2) ) and 2 b (2.280(2) ) are even shorter than those in the homometallic imide 1. The CaC(m-CH3) bonds (2.651(2) (2 a) and 2.668(2) (2 b)) are longer than those in the six-coordinate bis(tetramethylaluminate) complex [(phen)Ca(AlMe4)2][11] (av. 2.584(7) ) and similar to the distances in the structurally related amide complex [Ca{N(SiMe3)2(GaEt3)}2][24] (2.656(5) and 2.697(4) ). The AlN bond lengths of 1.849(2) (2 a) and 1.831(2) (2 b) match those of comparable imidoalanes.[25–28] It should be mentioned that such homometallic imidoalanes could indeed be synthesized from alkylaluminum or alane precursors and primary amines. Compared to the preparation from tetramethylaluminate precursors, however, the second deprotonation step usually has to be carried out under harsher conditions. For example, the dimeric imidoa-
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Scheme 3. Reactivity of 2 b towards phenylsilane and phenylacetylene. Angew. Chem. Int. Ed. 2016, 55, 1 – 6
formation of colloidal calciumhydride [CaH2(L)x]y was similarly observed by Okuda and co-workers during the hydrogenolysis of [(thf)4Ca(SiPh3)2] (d = 3.9–5.4 ppm in [D8]thf; L = neutral Lewis base).[31, 32] To assess the robustness of the imido versus the organoaluminum moiety, we next examined the reactivity of 2 b towards moderately CH acidic phenylacetylene. When a thf solution of 2 b was treated with one equivalent of PhCCH, instant methane evolution was observed (Scheme 3). Not surprisingly, the 1H NMR spectrum of the new compound 4, which was obtained in 72 % yield, did not show any NH resonances, indicating that the imido moiety stayed intact while one of the methyl groups was protonated. X-ray crystallographic analysis revealed the formation of the dimeric complex [(thf)Ca{NSiPh3}{AlMe2(CCPh)}]2 (4; see the Supporting Information for its molecular structure), in which the imido moiety moved to a bridging position between two Ca centers and one Al atom. One methyl group (m2-h1:h1) as well as the phenylacetylido moiety (m2-h1:h2) also act as bridges of one Al and distinct Ca centers. Similar observations were made by Schmidt et al. during the generation of [Al(CCPh)(NBn)]6 from imidoalane [Al(H)(NBn)]6 and PhCCH by hydrogen evolution.[33] In conclusion, we have prepared the first homometallic calcium imide, [(thf)Ca(NSiPh3)]4, from dibenzylcalcium and H2NSiPh3. Whereas the reaction of [CaBn2]n with H2NDipp resulted in the formation of the undesired bis(amide) complex [(thf)xCa(NHDipp)2], the use of [Ca(AlMe4)2]n in tandem salt metathesis/protonolysis reactions with either [Li{NH(Dipp)}] or [K{NH(SiPh3)}] led to the heterobimetallic imide species [(thf)4Ca(m2-NR)(m2-Me)AlMe2] (R = Dipp, SiPh3), respectively. The present study underlines both the great potential of alkaline-earth-metal tetraalkylaluminates in imide formation reactions and the versatility of the robust SiPh3 backbone for unveiling Ca=NR reactivity.
Experimental Section Complex 1: A solution of H2NSiPh3 (137 mg, 0.50 mmol) in toluene (1 mL) and 10 drops of thf were added to a suspension of [CaBn2]n (111 mg, 0.50 mmol) in toluene/Et2O (1:1 v/v, 4 mL). The resulting yellowish suspension was stirred at 40 8C for 60 h, which eventually led to a clear yellow solution. The volume of the reaction mixture was reduced to about 1 mL, and n-hexane (3 mL) was added to precipitate a slightly yellow solid, which was separated, washed with n-hexane (2 1 mL), and dried under reduced pressure. Yield: 133 mg, 0.09 mmol, 69 %. Single crystals suitable for X-ray structure analysis were grown from a saturated thf solution at ambient temperature. 1 H NMR (400 MHz, [D8]thf, 26 8C): d = 7.53 (m, 6 H, m-ArH), 7.21 (m, 6 H, o-ArH), 7.14 (m, 3 H, p-ArH), 3.62 (m, 4 H, OCH2 thf), 1.78 ppm (m, 4 H, CH2 thf). 13C{1H} NMR (101 MHz, [D8]thf, 26 8C): d = 150.6 (ipso-ArC), 135.9 (o-ArC), 128.3 (m-ArC), 127.7 (p-ArC), n ¼3054 (m), 3006 (w), 68.4 (OCH2 thf), 26.6 ppm (CH2 thf). DRIFT: ~ 2983 (m), 2881 (vw), 1956 (vw), 1888 (vw), 1822 (vw), 1771 (vw), 1652 (vw), 1560 (vw), 1474 (w), 1423 (m), 1372 (vw), 1295 (vw), 1259 (w), 1180 (w), 1152 (vw), 1091 (s), 1035 (s), 984 (vs), 910 (w), 882 (m), 740 (s), 704 (vs), 686 (s), 621 (vw), 522 (vs), 510 (s), 503 (s), 457 (s), 447 (s), 418 cm1 (m). Elemental analysis calcd (%) for C88H92N4Ca4O4Si4 (1542.38 g mol1): C 68.53, H 6.01, N 3.63; found: C 68.57, H 6.24, N 3.61. Complex 2 a: [Ca(AlMe4)2]n (79 mg, 0.37 mmol) was suspended in thf (2 mL), and a solution of [Li{NH(Dipp)}] (67 mg, 0.37 mmol) in
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lane [Mes*AlNSiPh3]2 (Mes* = 2,4,6-tBu3C6H2) was synthesized from neat [Mes*AlH2]2 and H2NSiPh3 at 135 8C in 15 % yield.[28] Another example is the thermolysis of [Me2AlN(H)SiPh3]2 at 185 8C, which yielded the tetrameric aluminum imide [MeAlNSiPh3]4.[29] The 1H NMR spectra of 2 a and 2 b in [D8]thf show a sharp singlet at around 1.0 ppm with an integral ratio of 9H (Al(CH3)3), indicating fast exchange of the bridging and terminal methyl groups at ambient temperature. Variable-temperature 1H NMR spectra, however, revealed that even at temperatures as low as 90 8C, these resonances did not decoalesce, corroborating the rapid exchange of the bridging and terminal methyl groups. With the discrete Ca imides 1 and 2 in hand, we envisaged initial reactivity studies. It turned out that 2 b benefits from the SiPh3 backbone, which enhances the solubility in ethereal and aromatic solvents and minimizes CH bond activation pathways as observed for the iPr groups in 2 a. Chen et al. have nicely demonstrated that scandium imido moieties engage in silane activation.[30] The terminal imide [LSc(NDipp)(DMAP)] (DMAP = 4-dimethylaminopyridine, L = [MeC{N(Dipp)}CHC{Me}{NCH2CH2NMe}]) readily reacts with PhSiH3 to yield the putative anilide hydride complex [LSc(H){N(Dipp)(SiH2Ph)}], which could be trapped by subsequent reaction with a carbodiimide, RN=C=NR.[30] Similar 1,2-additions were probed with the CaII imides 1 and 2 b. Whereas reactions of tetrametallic 1 with various amounts of PhSiH3 in thf or toluene gave ill-defined products, the clean conversion of 2 b into the new complex 3 in the presence of one equivalent of PhSiH3 was monitored by 1 H NMR spectroscopy in [D8]thf and occurred within six hours at ambient temperature (Scheme 3 and Figure S26 for in situ NMR spectra). Upscaling this reaction allowed for the isolation of crystalline complex 3 in 83 % yield (see the Supporting Information for the molecular structure of 3). Apparently, HSi addition across the CaN bond furnished the homoleptic calcium amidoaluminate [(thf)4Ca{(m2Me)AlMe2(N(SiPh3)(SiH2Ph))}2] (3) and putative calcium hydride [CaH2(L)x]y (L = neutral Lewis base) via a Schlenktype ligand rearrangement. All efforts to isolate these transient calcium hydride species have failed thus far. However, an in situ 1H NMR spectroscopic study showed a very broad resonance between 4.1 and 4.7 ppm ([D8]thf), which we ascribed to a hydride species (Figure S26). The
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Communications thf (2 mL) was added slowly. The reaction mixture was stirred for 36 h at ambient temperature, and then toluene (3 mL) was added to the clear solution. At 35 8C, the product crystallized as colorless crystals, which were separated from the solution and evaporated to dryness at ambient temperature under inert atmosphere. Crystalline yield: 121 mg, 0.21 mmol, 57 %. Single crystals suitable for X-ray structure determination were grown from a saturated thf/toluene solution (1:1 v/v) at 35 8C. 1H NMR (400 MHz, [D8]thf, 26 8C): d = 6.56 (d, 2 H, 3 JHH = 7.4 Hz, m-ArH), 5.93 (t, 1 H, 3JHH = 7.4 Hz, p-ArH), 4.20 (sep, 2 H, 3JHH = 6.8 Hz, iPr-CH), 3.62 (m, 16 H, OCH2 thf), 1.77 (m, 16 H, CH2 thf), 0.99 (d, 12 H, 3JHH = 7.0 Hz, iPr-CH3), 1.00 ppm (s, 9 H, AlCH3). 13C{1H} NMR (101 MHz, [D8]thf, 26 8C): d = 162.1 (ipsoArC), 139.2 (o-ArC), 122.0 (m-ArC), 109.3 (p-ArC), 68.4 (OCH2 thf), 26.5 (CH2 thf), 26.4 (iPr-CH), 26.4 (iPr-CH3), 2.0 ppm (AlCH3). DRIFT: ~n ¼2952 (m), 2908 (m), 2803 (w), 1577 (w), 1539 (vw), 1456 (m), 1400 (s), 1371 (w), 1325 (m), 1255 (m), 1218 (w), 1192 (w), 1150 (m), 1100 (vw), 1033 (vs), 982 (vw), 880 (s), 803 (w), 765 (w), 748 (m), 709 (s), 668 (vs), 618 (s), 575 (m), 561 (w), 546 (w), 475 (m), 438 (w), 422 cm1 (m). Elemental analysis calcd (%) for C31H58NAlCaO4 (575.87 g mol1): C 64.66, H 10.15, N 2.43; found: C 64.44, H 10.21, N 2.56. Complex 2 b: [Ca(AlMe4)2]n (172 mg, 0.80 mmol) was suspended in thf (2 mL), and a solution of [K{NH(SiPh3)}] (251 mg, 0.80 mmol) in thf (2 mL) was added slowly. Stirring the clear and colorless reaction mixture for 16 h at ambient temperature led to the formation of a white precipitate, which was filtered off, washed with cold thf (2 mL), and evaporated to dryness under reduced pressure. Yield: 421 mg, 0.62 mmol, 78 %. Single crystals suitable for X-ray structure analysis were grown from a saturated thf solution at 35 8C. 1H NMR (400 MHz, [D8]thf, 26 8C): d = 7.60 (m, 6 H, ArH), 7.04 (m, 9 H, ArH), 3.62 (m, 16 H, OCH2 thf), 1.77 (m, 16 H, CH2 thf), 1.08 ppm (s, 9 H, AlCH3). 13C{1H} NMR (101 MHz, [D8]thf, 26 8C): d = 150.3 (ipsoArC), 137.0 (o-ArC), 126.8 (m-ArC), 126.6 (p-ArC), 68.4 (OCH2 thf), n ¼3042 (vw), 2984 26.6 (CH2 thf), 1.05 ppm (AlCH3). DRIFT: ~ (vw), 2883 (w), 2800 (vw), 1479 (vw), 1457 (vw), 1424 (w), 1255 (vw), 1180 (vw), 1150 (w), 1096 (m), 1032 (s), 934 (w), 877 (m), 739 (m), 705 (vs), 694 (s), 667 (m), 618 (w), 568 (vw), 511 (s), 502 (m), 460 (vw), 424 cm1 (vw). Elemental analysis calcd (%) for C37H56NAlCaO4Si (674.00 g mol1): C 65.94, H 8.38, N 2.08; found: C 66.24, H 8.14, N 2.18. Full experimental and analytical details of compounds 1–4 are available in the Supporting Information. CCDC 1495862 (1), 1495864 (2 a), 1495865 (2 b), 1495866 (3), and 1495863 (4) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.
Acknowledgements We gratefully acknowledge support from the German Science Foundation (Grant AN 238/15-1) and the Carl Zeiss Foundation (PhD scholarship for B.M.W.). Keywords: alkyl groups · aluminum · calcium · imides · protonolysis
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[1] L. Meunier, C. R. Hebd. Seances Acad. Sci. 1903, 136, 758. [2] [(thf)nCa(AlH)3(NtBu)4] was obtained from a mixture of Ca(AlH4)2, [AlH3(NMe3)], and tBuNH2 in thf with release of H2 ; see: a) S. Cucinella, G. Dozzi, G. Perego, A. Mazzei, J. Organomet. Chem. 1977, 137, 257 – 264; b) G. Del Piero, M. Cesari, S. Cucinella, A. Mazzei, J. Organomet. Chem. 1977, 137, 265 – 274.
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Chemie
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Communications Alkaline Earth Metals B. M. Wolf, C. Maichle-Mçssmer, R. Anwander* &&&&—&&&&
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Synthesis and Reactivity of Discrete Calcium Imides
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Tandem salt metathesis/protonolysis reactions of [Ca(AlMe4)2]n and [K{NH(SiPh3)}] provide access to Lewis acid
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(AlMe3) stabilized calcium imides. Their Ca=NR multiple-bond character was revealed by SiH addition (see scheme).
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
