A Remarkable End-On Activation of Diazoalkane and Cleavage of

Article pubs.acs.org/Organometallics

A Remarkable End-On Activation of Diazoalkane and Cleavage of Both C−Cl Bonds of Dichloromethane with a Silylene to a Single Product with Five-Coordinate Silicon Atoms† Sakya S. Sen, Jakob Hey, Daniel Kratzert, Herbert W. Roesky,* and Dietmar Stalke* Institut für Anorganische Chemie, Georg-August-Universität, Tammannstrasse 4, D-37077, Göttingen, Germany S Supporting Information *

ABSTRACT: The 1:1 reaction of benzamidinato-stabilized chlorosilylene PhC(NtBu)2SiCl (1) with CH(SiMe3)N2 resulted in the formation of colorless [PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2 (2), which consists of a four-membered Si2N2 ring. Surprisingly, N2 elimination from the diazoalkane did not occur, but rather an end-on activation of the nitrogen was observed. For the mechanism, we propose the formation of a silaimine complex A as an intermediate, which is formed during the reaction and dimerized under [2 + 2] cycloaddition to 2. In contrast, treatment of 1 with dichloromethane afforded a 2:1 product, [{PhC(NtBu)2Si(Cl2)}2CH2] (3), which is obviously formed by oxidative addition under cleavage of both C−Cl bonds and formation of two Si−Cl and two Si−C bonds. Both silicon atoms in 3 are five-coordinate. Compounds 2 and 3 were characterized by single-crystal X-ray studies, multinuclear NMR spectroscopy, and EI-mass spectrometry.

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INTRODUCTION One of the research fields that has blossomed since the advent of silylenes is their novel reaction chemistry. This is a highly interdisciplinary enterprise that has gathered efforts from synthetic chemists and theoreticians and leads to an exponentially increasing number of publications.1 This emanates from the understanding of chemical properties and bonding characteristics of silylenes that contain an electrophilic Si(II) center. Silylenes have a propensity toward oxidation reactions due to the conversion from the formally divalent to the thermodynamically stable tetravalent state. The past few years have witnessed that silylenes are potent Lewis bases,2 can be oxidized by chalcogens,3 can form adducts with boranes4 and carbenes,5 and can take part in cyclization reactions with alkynes.6 Our formal entry in silylene chemistry started in 2006, following the synthesis of the first monochlorosilylene (PhC(NtBu)2SiCl) (1) by the reduction of the corresponding trichlorosilane (PhC(NtBu)2SiCl3) with finely divided potassium.7 However, due to yield constraints, we were not able to investigate its chemistry. In 2010, an improved synthesis of 1 resulted in 90% yield by using LiN(SiMe3)2 as a dehydrochlorinating agent,8 and its systematic reactions with homo-8 and heteroalkynes,9 ketone,10 diketone,11 isocyanate,12 carbodiimide,12 imine,13 diimine,12 COT,12 diazobenzene,14 borane,15 and metal carbonyls16 were carried out. © 2011 American Chemical Society

Another very interesting class of compounds with which the chemistry of silylenes so far is not well-studied are diazoalkanes. The interaction of diazoalkanes and transition-metal compounds leads to a variety of products, depending on the electronic properties of the diazoalkanes and the formal charge on the metal center. These reactions are extremely interesting given the utility for the generation of alkylidene complexes that are active for catalytic cyclopropanation of alkene metathesis.17 In contrast, the reaction of diazoalkane with main group metals is scarcely known in the literature.18 In many cases, N2 elimination is not observed and stable diazoalkane complexes are isolated.19 In 2004, we reported the synthesis of a diiminylaluminum(III) compound L1Al(NCPh2)2 (L1 = HC{(CMe)(NAr)}2, Ar = 2,6-iPr2C6H3) by the treatment of L1Al(I) with Ph2CN2.20 The analogous diiminylsilane was obtained by Driess et al. from the reaction of L2Si(II) (L2 = Ar−N−C(Me)CH−C(CH2)−N−Ar) with Ph2CN2.21 Very recently, we described a unprecedented end-on nitrogen insertion of a diazo compound into a L 1 Ge(II)−H bond.22 Moreover, we also showed that the reaction of a N-heterocyclic germylene (L 2Ge(II)) with CH(SiMe3)N 2 afforded a diazogermylene L 1GeC(N 2)SiMe 3, which slowly rearranged to isonitrile-trimethylsilyl germanium(II) amide Received: October 25, 2011 Published: December 27, 2011 435

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L1GeN(SiMe3)NC.23 These striking results prompted us to probe the reaction of 1 with diazoalkane because, with respect to the other reported N-heterocyclic silylenes, 1 possesses a remarkably different pattern of reactivity toward unsaturated organic substrates.8−16 Herein, we report the 1:1 reaction of CH(SiMe3)N2 with 1 and the isolation of the four-membered Si2N2 ring compound 2.

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RESULT AND DISCUSSION Addition of 1 equiv of CH(SiMe3)N2 to a toluene solution of 1 at −40 °C, followed by recrystallization and storing at −32 °C in a freezer, furnished [PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2 (2) as colorless crystals (see Scheme 1). In the 1H NMR Scheme 1. Preparation of [PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2 (2) Figure 1. Crystal structure of 2·2C7H8. Hydrogen atoms and two toluene molecules are not shown for clarity. Anisotropic displacement parameters are depicted at the 50% probability level. Selected bond lengths (Å) and bond angles (deg): Si(1)−N(3) 1.7350(16), Si(1)− N(3)#1 1.8090(15), Si(1)−N(1) 1.8227(16), Si(1)−N(2) 1.9445(16), Si(1)−Cl(1) 2.1431(7); N(3)−Si(1)−N(3)#1 80.33(7), N(3)−Si(1)−N(1) 123.62(8), N(3)#1−Si(1)−N(1) 106.90(7), N(3)−Si(1)−N(2) 100.30(7), N(3)#1−Si(1)−N(2) 176.09(7), N(1)−Si(1)−N(2) 69.50(7). Symmetry generator #: −x, y, z + 1/2.

monoclinic space group P21/n.24 A two-fold symmetry axis passes through the centroid of the four-membered Si2N2 ring. The core structure consists of a rhombic-shaped Si2N2 unit featuring alternating Si and N atoms. The coordination geometry around the Si atoms can be described by trigonalbipyramidal geometry. To explain the axial and equatorial arrangement, we selected Si(1). N(1), Cl(1), and N(3) which reside in the equatorial positions, whereas N(2) and N(3)#1 occupy the axial positions of the TBP geometry. The Si−Si interatomic separation in the four-membered Si2N2 ring is 2.7089(10) Å, which indicates that there is no bond between the two Si atoms. The Si−N bond lengths in 2 are 1.7350(16) and 1.8227(16) Å for equatorial nitrogen atoms, while the axial Si−N bond lengths are 1.8090(15) and 1.9445(16) Å.25 The Si(1)−Cl(1) bond length in 2 is 2.1431(7) Å, which is very close to that of 1 (2.156(1) Å).7 The average N−N bond length in 2 is 1.38 Å, which is consistent with the N−N single bond length.26 Reactions of silylenes with haloalkanes have been studied in detail by different research groups owing to the possibility of a direct synthesis of alkylhalosilanes. It has been observed that CH2Cl2 reacts in one or the other way with different silylenes,27 and therefore, these provocative reactions need further attention. Specially, it remains to be examined how CH2Cl2 reacts with 1, which already contains a Si(II)−Cl bond. As shown in Scheme 3, the reaction of 1 with excess CH2Cl2 at room

spectrum of 2, a broad resonance appears at δ = 1.35 ppm for the 36 tBu protons, which is shifted downfield compared with that of 1 (δ = 1.08 ppm).8 A sharp resonance at δ = 0.43 ppm corresponds to 18 SiMe3 protons. The two alkene CH protons appear at δ = 8.29 ppm. Two resonances are observed at δ = −10 and −70.2 ppm in the 29Si NMR spectrum. The former resonance corresponds to the SiMe3 moiety, whereas the signal at δ = −70.2 ppm is attributed to the two five-coordinate Si atoms. In the EI-mass spectrum, the molecular ion is observed at m/z 816.4, although with low intensity. However, the most abundant peak exhibited at m/z 801.4, which corresponds to [2-Me]. The mechanism of the reaction seems to be obvious. The reaction of 1 with CH(SiMe3)N2 initially afforded a silaimine complex A through end-on oxidative addition of CH(SiMe3)N2 at the silicon atom. However, this complex is kinetically unstable due to the absence of any bulky substituents at the α-nitrogen atom and consequently dimerizes in situ under a [2 + 2] cycloaddition reaction to form [PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2 (2) with five-coordinate silicon atoms (see Scheme 2). Scheme 2. Mechanism for the Formation of [PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2 (2)

Scheme 3. Preparation of [{PhC(NtBu)2Si(Cl2)}2CH2] (3)

Single crystals suitable for X-ray diffraction studies were obtained from a toluene solution of [PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2 (2) at −32 °C in a freezer (see Figure 1). 2 crystallizes in the 436

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temperature occurred smoothly to give the 2:1 oxidative addition product 3 in 68% yield. No 1:1 oxidative addition product was observed in the reaction mixture even though the concentration of 1 was much lower than that of CH2Cl2. Formation of similar 2:1 products has been observed in the reactions of dialkyl silylene27b and bis[di(trimethylsilyl)methyl]stannylene with dichloromethane.28 However, the resulting product contains four-coordinate silicon atoms,27b whereas 3 exhibits two five-coordinate silicon atoms. Species of this type have not been reported so far. In the EI-mass spectrum of 3, no molecular ion peak is observed, but it exhibits the most abundant peak with the highest relative intensity at m/z 601, which corresponds to [M+ − 2Cl]. In the 29Si NMR, a sharp resonance exhibits at δ = −79.5 ppm, which can be attributed to the two five-coordinate silicon atoms. The mechanism of the reaction is still not clear, but it is obvious that it cannot be explained by a radical mechanism because a single dichloromethane molecule reacts with two silylene molecules even in the presence of a large excess of dichloromethane. As shown in Scheme 4, we can propose that

Figure 2. Molecular structure of [{PhC(NtBu)2Si(Cl2)}2CH2] (3). Hydrogen atoms except at C(31) are not shown for clarity. Anisotropic displacement parameters are depicted at 50% probability level. Selected bond lengths [Å] and angles [deg]: N(1)−Si(1) 1.8155(8), N(2)−Si(1) 1.9414(8), Cl(1)−Si(1) 2.2172(4), Cl(2)− Si(1) 2.1079(4), C(31)−Si(1) 1.8674(10); N(1)−Si(1)−C(31) 128.51(4), N(1)−Si(1)−N(2) 69.94(3), C(31)−Si(1)−N(2) 98.56(4), N(1)−Si(1)−Cl(2) 118.83(3), C(31)−Si(1)−Cl(2) 111.19(3), N(2)−Si(1)−Cl(2) 91.21(3), N(1)−Si(1)−Cl(1) 98.01(3), C(31)−Si(1)−Cl(1) 93.54(3), N(2)−Si(1)−Cl(1) 166.63(3), Cl(2)−Si(1)−Cl(1) 89.717(14), Si(1)−C(31)−Si(2) 128.29(5).

Scheme 4. Tentative Mechanism for the Formation of 3

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CONCLUSION We describe the reactions of base-stabilized chlorosilylene PhC(NtBu)2SiCl with CH(SiMe3)N2 and CH2Cl2. The unique feature of the former reaction is the end-on activation of CH(SiMe3)N2 under preservation of the N2 moiety in the product [PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2 (2) and formation of a four-membered Si2N2 ring. In the latter reaction, 2 equiv of silylene 1 react with CH2Cl2 to yield exclusively [{PhC(NtBu)2Si(Cl2)}2CH2] (3), which contains five-coordinate silicon atoms.

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EXPERIMENTAL SECTION

All reactions and handling of reagents were performed under an atmosphere of dry nitrogen or argon using standard Schlenk techniques or a glovebox where the O2 and H2O levels were usually kept below 1 ppm. CH(SiMe3)N2 was purchased from Sigma-Aldrich and used as received. Dichloromethane was dried over CaH2. Compound 1 was prepared according to literature methods.8 NMR spectra were recorded on Bruker Avance 200, Bruker Avance 300, and Bruker Avance 500 MHz NMR spectrometers. C6D6 was dried by stirring for 2 days over Na/K alloy, followed by distillation in vacuum and degassed. EI-MS spectra were obtained with a Finnigan MAT 8230 or a Varian MAT CH5 instrument (70 eV) by EI-MS methods. Elemental analyses were performed by the Analytisches Labor des Instituts für Anorganische Chemie der Universität Göttingen. Synthesis of [PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2 (2). To the toluene solution (20 mL) of 1 (0.29 g, 1.00 mmol) (trimethylsilyl)diazomethane (0.49 mL, 1.01 mmol, 2 M in n-hexane) was added at −40 °C. The reaction mixture was slowly warmed to room temperature and stirred for 4 h. The solvent was removed under reduced pressure and the crude product was extracted with toluene (10 mL). Concentration and storing at −32 °C in a freezer afforded colorless crystals of 2 (0.48 g, 29.3%); mp: 160−165 °C. 1H NMR (200 MHz, C6D6, 25 °C): δ 0.43 (s, 18H, TMS), 1.35 (br, 36 H, tBu), 6.83−7.45 (m, 10H, Ph), 8.29(s, 2H, CH(TMS)) ppm. 29Si{1H} NMR (99.36 MHz, C6D6, 25 °C): δ −10, −70.2 ppm. EI-MS: m/z: 816.4 (4%) (M+), 801.4 (M+ − Me) (100%). For the elemental analysis, the crystals of 2·2C7H8 were kept in a vacuum overnight to remove two molecules of toluene. Elemental analysis for C38H66Cl2N8Si4: calcd. C, 55.78; H, 8.13; N, 13.69; found, C, 55.16; H, 8.12; N, 12.10.

the reaction proceeds through a conventional acid−base complex intermediate, with a chlorine atom donating a lone pair of electrons to silicon. The methylene carbon in the initial LSiCl− dichloromethane complex (L = PhC(NtBu)2) is attacked by another LSiCl to form a dichlorosilyl anion and a (chloromethyl)silyl cation stabilized by intramolecular complexation, followed by a nucleophilic attack of the silyl anion to the chloromethyl carbon to afford [{PhC(NtBu)2Si(Cl2)}2CH2] (3).27b,28,29 The molecular graph of [{PhC(NtBu)2Si(Cl2)}2CH2] (3) is shown in Figure 2. Compound 3 crystallizes in the monoclinic space group P21/c.24 Both Si atoms in 3 adopt a distorted trigonalbipyramidal geometry. To assign the axial and equatorial positions, we selected Si(1). C(31), Cl(1), and N(1) reside at the equatorial position of Si(1), whereas N(2) and Cl(1) occupy the axial positions with a N(2)−Si(1)−Cl(1) bond angle of 166.63(3)°. It is noticeable that the Si−N and Si−Cl bond lengths differ significantly depending on whether equatorial or axial positions are occupied (cf. Si−Nax = 1.9414(8) and 1.9935(8) Å, Si−Neq = 1.8155(8) and 1.8118(8) Å; Si−Clax = 2.2172(4) Å, Si−Cleq = 2.1079(4) Å). The two Si−C bond lengths are almost the same (1.8674(10) and 1.8726(10) Å), which correspond to the Si−C single bonds and are similar to those reported in the literature.25c,30 The geometry of C(31) is best described as heavily distorted tetrahedral where the sum of the bond angles is 420.8°. 437

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Synthesis of [{PhC(NtBu)2Si(Cl2)}2CH2](3). The chlorosilylene 1 (0.05 g, 0.17 mmol) was added to 5 mL of dichloromethane at room temperature. The reaction mixture was allowed to stay at room temperature overnight. Attempts to grow the crystals from the mother liquor failed due to the high solubility of 3 in CH2Cl2. The solvent was then removed under vacuum, and toluene (10 mL) was added to obtain colorless crystals of 3 (0.15 g, 68.22%); mp: 150−155 °C. 1H NMR (200 MHz, C6D6, 25 °C): δ 1.27 (br, 36H, tBu), 1.4 (s, 2H, CH2), 6.68−7.49 (m, 10 H, Ph) ppm. 29Si{1H} NMR (99.36 MHz, C6D6, 25 °C): δ −79.5 ppm. EI-MS: m/z: 601 (M+ − 2Cl) (100%). Elemental analysis for C31H48Cl4N4Si2: calcd. C, 55.18; H, 7.17; N, 8.30; found, C, 55.16; H, 7.12; N, 9.10. Crystal Structure Determination of [PhC(NtBu)2Si(Cl){N2CH(SiMe3)}]2 (2) and [{PhC(NtBu)2Si(Cl2)}2CH2] (3). Shock-cooled crystals were selected and mounted under a nitrogen atmosphere using the X-TEMP2. The data of 2 and 3 were measured on an Bruker APEX II quazar with a INCOATEC Mo Microsource with mirror optics.31 The diffractometer was equipped with a low-temperature device and used Mo Kα radiation, λ = 0.71073 Å. The data sets were integrated with SAINT32 and an empirical absorption (SADABS)33 was applied. The structures were solved by direct methods (SHELXS97)34 and refined by full-matrix least-squares methods against F2 (SHELXL-97). 34 All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were refined isotropically on calculated positions using a riding model with their Uiso values constrained to equal to 1.5 times the Ueq of their pivot atoms for terminal sp3 carbon atoms and 1.2 times for all other carbon atoms. The positions of the hydrogen atoms at C(31) of 3 where refined freely with distance restraints. Crystallographic data (excluding structure factors) for the structures35 reported in this paper have been deposited with the Cambridge Crystallographic Data Centre. The crystal data are available from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information contains the CIF files for 2 and 3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Authors *Tel: +49551393001, +49551393000. Fax: +49551393373. E-mail: [email protected] (H.W.R.), [email protected] (D.S.).

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ACKNOWLEDGMENTS H.W.R. thanks the Deutsche Forschungsgemeinschaft (DFGRO 224/55-3) for financial support. D.S. is grateful for funding from the DFG Priority Programme 1178, the DNRF funded Center for Materials Crystallography (CMC) for support, and the Land Niedersachsen for providing J.H. and D.K. with a fellowship in the Catalysis for Sustainable Synthesis (CaSuS) Ph.D. program.

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DEDICATION This paper is dedicated to Professor Alan H. Cowley.

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REFERENCES

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Organometallics

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(22) Jana, A.; Sen, S. S.; Roesky, H. W.; Schulzke, C.; Dutta, S.; Pati, S. K. Angew. Chem. 2009, 121, 4310−4312; Angew. Chem., Int. Ed. 2009, 48, 4246−4248. (23) Jana, A.; Objartel, I.; Roesky, H. W.; Stalke, D. Inorg. Chem. 2009, 48, 7645−7649. (24) (a) Kottke, T.; Stalke, D. J. Appl. Crystallogr. 1993, 26, 615−619. (b) Stalke, D. Chem. Soc. Rev. 1998, 27, 171−176. (25) (a) Mitzel, N. W. Z. Naturforsch. 2003, 58b, 369−375. (b) Blake, A. J.; Ebsworth, E. A. V.; Rankin, D. W. H.; Robertson, H. E.; Smitj, D. E.; Welch, A. J. J. Chem. Soc., Dalton Trans. 1986, 91− 95. (c) Sen, S. S.; Gašper, T.; Roesky, H. W.; Kratzert, D.; Hey, J.; Stalke, D. Organometallics 2010, 29, 2343−2347. (26) (a) The N−N bond length in (H3Si)2NN(SiH3)2 is 1.46 Å: Glidewell, C.; Rankin, D. W. H.; Robiette, A. G.; Sheldrick, G. M. J. Chem. Soc. A 1970, 318−320. (b) Veith, M.; Rammo, A. Z. Anorg. Allg. Chem. 2001, 627, 662−668. (c) Sen, S. S.; Kratzert, D.; Stern, D.; Roesky, H. W.; Stalke, D. Inorg. Chem. 2010, 49, 5786−5788. (27) (a) Gehrhus, B.; Lappert, M. F. J. Organomet. Chem. 2001, 617− 618, 209−223. (b) Ishida, S.; Iwamoto, T.; Kabuto, C.; Kira, M. Chem. Lett. 2001, 1102−1103. (c) Moser, D. F.; Naka, A.; Guzei, I. A.; Müller, T.; West, R. J. Am. Chem. Soc. 2005, 127, 14730−14738. (d) Xiong, Y.; Yao, S.; Driess, M. Organometallics 2009, 28, 1927− 1933. (e) Szilvási, T.; Nyíri, K.; Veszprémi, T. Organometallics 2011, 30, 5344−5351. (28) Gynane, M. J. S.; Lappert, M. F.; Miles, S. J.; Carty, A. J.; Taylor, N. J. J. Chem. Soc., Dalton Trans. 1977, 2009−2015. (29) Köcher, J.; Lehnig, M.; Neumann, W. P. Organometallics 1988, 7, 1201−1207. (30) Kaftory, M.; Kapon, M.; Botoshansky, M. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2, Chapter 5. (31) Schulz, T.; Meindl, K.; Leusser, D.; Stern, D.; Ruf, M.; Sheldrick, G. M.; Stalke, D. J. Appl. Crystallogr. 2009, 42, 885−891. (32) SAINT V7.68A; Bruker AXS Inc.: Madison, WI, 2005. (33) Sheldrick, G. M. SADABS 2008/2; Universität Göttingen: Göttingen, Germay, 2008. (34) Sheldrick, G. M. Acta Crystallogr., Sect. A. 2008, 64, 112−122. (35) Crystal data for 2·2C7H8:C52H82Cl2N8Si4; fw, 1002.51; T, 200(2) K; monoclinic; space group, P21/n; a = 12.6130(5) Å, b = 11.3070(5) Å, c = 21.1483(9) Å, β = 104.573(2)°; V = 2919.0(2) Å3; Z = 2; ρcalcd = 1.141 mg/m3; μ = 0.233 mm−1; Mo Kα; F(000), 1080; crystal size, 0.30 × 0.20 × 0.04 mm; θ range for data collection, 1.71− 27.12°; reflections collected/unique observed reflections [I > 2σ(I)], 35865/6388; [R(int), 0.0335]; data/restraints/parameters, 6388/0/ 308; GOF on F2, 1.060; R1 [I > 2σ(I)] = 0.0423; wR2 (all data) = 0.1167; largest diff peak/hole, 0.319 and −0.308 e/Å3; CCDC No 843607. Crystal data for 3: C31H48Cl4N4Si2; fw, 674.71; T, 100(2) K; monoclinic; space group, P21/c; a = 14.8427(7) Å, b = 13.6256(7) Å, c = 17.5591(8) Å, β = 100.029(2)°; V = 3496.9(3) Å3; Z = 4; ρcalcd = 1.382 mg/m3; μ = 0.434 mm−1; Mo Kα; F(000), 1432; crystal size, 0.20 × 0.20 × 0.18 mm; θ = 1.39−29.82°; reflections collected/unique observed reflections [I > 2σ(I)], 98691/10034; [R(int), 0.0291]; data/ restraints/parameters, 10034/2/436; GOF on F2, 1.029; R1 [I > 2σ(I)] = 0.0251; wR2 (all data) = 0.0678; largest diff peak/hole = 0.417 and −0.192 e/Å3; CCDC No 843608.

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dx.doi.org/10.1021/om201031n | Organometallics 2012, 31, 435−439