Facile Access to the Functionalized N-Donor Stabilized Silylenes PhC

Article pubs.acs.org/Organometallics

Facile Access to the Functionalized N-Donor Stabilized Silylenes PhC(NtBu)2SiX (X = PPh2, NPh2, NCy2, NiPr2, NMe2, N(SiMe3)2, OtBu) Ramachandran Azhakar, Rajendra S. Ghadwal,* Herbert W. Roesky,* Hilke Wolf, and Dietmar Stalke* Institut für Anorganische Chemie der Universität Göttingen, Tammannstrasse 4, 37077 Göttingen, Germany S Supporting Information *

ABSTRACT: Reactions of silylenes with organic substrates generally lead to silicon(IV) compounds. Ligand substitution at the silicon(II) atom of silylene, without changing the formal +2 oxidation state, is very rare. We report herein a straightforward route to functionalized silylenes LSiX (L = PhC(NtBu)2 and X = PPh2 (1), NPh2 (2), NCy2(3), NiPr2 (4), NMe2 (5), N(SiMe3)2 (6), OtBu (7)). Silylenes 1−7 have been prepared in quantitative yield by a modified ligand exchange reaction of PhC(NtBu)2SiCl (LSiCl) with the corresponding lithium or potassium salts. Compounds 1−7 were characterized by spectroscopic and spectrometric techniques. Single-crystal X-ray structures of 1, 3, and 4 were determined.

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KC(SiMe3)3 with LSiCl.7i By that time Driess et al. reported a bis-silylene and studied its pincer ligand properties.18 Generally the reaction of silylenes with alkali-metal salts yielded silyl alkali-metal compounds and also in some cases they underwent a rearrangement of the SiMe3 substituent to the silicon center.6b Representative examples are given in Chart 1. Herein, we report on the treatment of monochlorosilylene LSiCl with alkali-metal amides, phosphide, and alkoxide in toluene as a solvent, which resulted in facile formation of the functionalized silylenes PhC(NtBu)2SiX (LSi(X); X = PPh2 (1), NPh2 (2), NCy2 (3), NiPr2 (4), NMe2 (5), N(SiMe3)2 (6), OtBu (7)) in high yields.

INTRODUCTION N-heterocyclic carbenes (NHC's) have been studied extensively, and they find applications not only as excellent σ-donor ligands to stabilize some compounds with unusual oxidation states but also function as organocatalysts.1−5 NHC's have found increasing use as reagents for asymmetric organocatalysis and in organic transformations. However, their stereoelectronic properties are important, especially for applications in catalytic reactions. Considering the great importance of stable NHCs, the analogous N-heterocyclic silylenes (NHSi's)6,7 are still scarce, due to difficult and low-yield synthetic methods. Interestingly, silylenes possess an ambiphilic character and behave as Lewis acids as well as Lewis bases because of two nonbonding electrons in the HOMO and an empty p orbital as the LUMO, which feature both nucleophilic and electrophilic reactive sites at the silicon atom.8 Many noteworthy reactivity studies of stable NHSi's toward a variety of substrates have been reported, such as insertion,9−14 addition,10c,15 metal complexes,16 and Lewis acids.17 A challenge in the case of NHSi's is the tedious synthetic procedure available for the preparation of NHSi's.6 The conventional synthetic route for the preparation of NHSi's involves the use of strong reducing agents, such as potassium metal and potassium graphite. These reducing agents and harsh conditions often lower the yields and also require careful handling. In 2006, we reported the synthesis of the amidinato ligand stabilized monochlorosilylene LSiCl (L = PhC(NtBu)2) in 10% yield.7b Then, we established a convenient and safe method to prepare LSiCl in high yield by reductive dehydrochlorination of the corresponding chlorosilane, using the mild reducing agent LiN(SiMe3)2.7c The highyield availability of LSiCl allowed us and others to investigate its chemistry to a great extent.6c Recently we reported on the monoalkylsilylenes LSitBu and LSi[C(SiMe3)3], which were prepared by the facile metathesis reaction of LitBu and © 2012 American Chemical Society

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RESULTS AND DISCUSSION Compounds 1−7 were obtained in high yields by one-pot reactions of LSiCl with LiPPh2 (for 1), LiNPh2 (for 2), LiNCy2 (for 3), LiNiPr2 (for 4), LiNMe2 (for 5), LiN(SiMe3)2 (for 6), and KOtBu (for 7) in toluene in a 1:1 ratio (Scheme 1) at ambient temperature by a facile metathesis reaction. Previously we reported compounds 5 (yield ∼40%) and 7 (yield ∼50%),7e which were obtained in moderate yield by alkali-metal reduction of the appropriate chlorosilane, and compound 6, obtained by treatment of LSi(H)Cl2 with 2 equiv of KN(SiMe3)2.7g In this study we report on the high-yield access to silylenes 1−7 by treatment of LSiCl with the corresponding alkali-metal phosphide, amide, or alkoxide. Compounds 1−7 are soluble in common organic solvents. They were fully characterized by spectroscopic methods. The molecular structures of compounds 1, 3, and 4 were established unequivocally by single-crystal X-ray structural analysis. Received: May 4, 2012 Published: June 13, 2012 4588

dx.doi.org/10.1021/om3003762 | Organometallics 2012, 31, 4588−4592

Organometallics

Article

Chart 1

Scheme 1. Synthesis of 1−7

The 29Si NMR spectrum of 1 shows a double resonance of equal intensity which is centered at δ 34.3 ppm (JSiP = 152 Hz), and is shifted downfield in comparison with that of LSiCl (δ 14.6 ppm).7b This chemical shift is comparable with the reported value of δ 56.2 ppm for LSiPiPr2.7e The 31P NMR spectrum of compound 1 exhibits a resonance at δ −31.11 ppm, with two silicon satellites with JSiP = 152 Hz. The tBu protons for compound 1 in its 1H NMR spectrum appear as a singlet at δ 0.99 ppm, which is shifted upfield in comparison to that of LSiCl (δ 1.08 ppm).7b Moreover, 1 shows its molecular ion for [M+] in its mass spectrum at m/z 444. The 29Si NMR spectra of 2−4 display a single resonance at δ −20.50 (2), −5.8 (3), and −6.5 ppm (4), respectively, which are shifted upfield and correspond to LSiCl.7b The tBu protons exhibit a single resonance at δ 1.04 (2), 1.24 (3), and 1.21 ppm (4) in their respective 1H NMR spectra. Compounds 2−4 display each their molecular ion [M+] in their mass spectra at m/z 427, 439, and 359, respectively. The formation of compounds 5−7 by facile metathesis reactions was established by comparing the spectral data obtained from NMR spectroscopy and EI-MS spectrometry reported in the literature.7e,g The molecular structures of compounds 1, 3, and 4 are shown in Figures 1−3. In their molecular structures, the silicon atom is three-coordinate and features a trigonal-pyramidal geometry. The CN2 plane and the Si−P bond include an angle of 105.3°, emphasizing the predominant s character of the silicon-centered lone pair.7h Compound 1 crystallizes in the triclinic space group P1̅ (Table 1, Supporting Information). The coordination environment of silicon is derived from two nitrogen atoms of the supporting amidinato ligand and one phosphorus atom. The

Si−P bond distance in 1 is 2.3111(6) Å, which is comparable to the value of 2.307(8) Å in PhC(NtBu)2SiPiPr27e but slightly longer compared to the 2.2838 Å reported for PhC(NtBu)2SiP(SiMe3)2.8f The bond lengths between the chelating nitrogens

Figure 1. Molecular structure of 1. The anisotropic displacement parameters are depicted at the 50% probability level. Selected bond lengths (Å) and bond angles (deg): N2−Si1 = 1.8614(11), N1−Si1 = 1.8626(11), P1−Si1 = 2.3111(6); N1−Si1−N2 = 69.81(4), N2−Si1− P1 = 105.12(4), N1−Si1−P1 = 97.30(4). 4589

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69.81(4)°. The Si atom is shifted out of the plane defined by N1−C1−N2 of 0.3515(24) Å. Compounds 3 and 4 crystallize in the monoclinic space groups P21/c and C2/c, respectively. The coordination environment of silicon in 3 and 4 is derived from three nitrogen atoms: two from the supporting amidinato ligand and one from the amide group. The bond lengths of the chelating nitrogen atoms of the amidinato ligands to the silicon atom are Si1−N1 = 1.8939(13) Å and Si1−N2 = 1.8940(13) Å in 3 and Si1−N1 = 1.9003(12) Å and Si1−N2 = 1.9002(13) Å in 4, which are slightly longer in comparison with those of 1. The included angle of the CN2 plane and the Si−N(amide) bond in 3 is 105.8° and in 4 is 106.5°; hence, the angle hardly responds to the various steric bulks and this underlines the s orbital character of the lone pair, governing the geometry. The bite angle N1−Si1−N2 is 68.34(5)° in 3 and 68.10(5)° in 4. The distance between the silicon and nitrogen atom of the amide group is 1.7432(12) Å in 3 and 1.7434(12) Å in 4. This is comparable to the reported value of 1.724(2) Å in LSiNMe2 (5)7e and is shorter in comparison with that of 1.8776(10) Å in LSiN(SiMe3)2 (6).7g The Si atom is shifted out of the plane defined by N1−C1−N2 by 0.3554(33) Å in 3 and 0.3957(34) Å in 4. Figure 2. Molecular structure of 3. The anisotropic displacement parameters are depicted at the 50% probability level. Selected bond lengths (Å) and bond angles (deg): N1−Si1 = 1.8939(13), N2−Si1 = 1.8940(13), N3−Si1 = 1.7432(12); N1−Si1−N2 = 68.34(5), N1− Si1−N3 = 103.04(5), N2−Si1−N3 = 103.03(5).

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CONCLUSION

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EXPERIMENTAL SECTION

The chlorosilylene LSiCl is a versatile building block due to the easily substituted chlorine ligand. Therefore, fine tuning of the coordination environment around silicon(II) is possible, which leads to an electronic effect, thereby greatly influencing the reactivity of the molecule. Currently we are investigating the properties of the reported heteroleptic silylenes which might lead to compounds with fascinating new silicon bonds.

atom and silicon are nearly identical (Si1−N2 = 1.8614(11) Å and Si1−N1 = 1.8626(11) Å) and are longer than Si−N single bonds in silicon(IV) amines19 and present lone-pair-driven dative N→Si bonds.20 The bite angle N1−Si1−N2 is

Syntheses were carried out under an inert gas atmosphere of dinitrogen in oven-dried glassware using standard Schlenk techniques, and other manipulations were accomplished in a dinitrogen-filled glovebox. Solvents were purified with the MBRAUN solvent purification system MB SPS-800. All chemicals were purchased from Aldrich and used without further purification. LSiCl,7c LiPPh2,21 and LiNPh222 were prepared as reported in the literature. 1H, 13C, and 29Si NMR spectra were recorded with a Bruker Avance DPX 200, Bruker Avance DRX 300, or Bruker Avance DRX 500 spectrometer, using C6D6 as solvent. Chemical shifts δ are given relative to SiMe4. EI-MS spectra were obtained using a Finnigan MAT 8230 instrument. Elemental analyses were performed by the Institut für Anorganische Chemie, Universität Göttingen. General Procedure for the Synthesis of LSi(X) (X = PPh2 (1), NPh2 (2), NCy2 (3), NiPr2 (4), NMe2 (5), N(SiMe3)2 (6), OtBu (7)). The respective LSi(X) compounds were prepared by treating an equivalent amount of LSiCl with LiPPh2 (for 1), LiNPh2 (for 2), LiNCy2 (for 3), LiNiPr2 (for 4), LiNMe2 (for 5), LiN(SiMe3)2 (for 6), and KOtBu (for 7) in toluene (60 mL) at room temperature. The mixture was stirred for 14 h at room temperature. The solution was then filtered and the solvent was removed in vacuo to afford the corresponding LSi(X). Compound 1. Quantity used: LSiCl, 1.01 g (3.42 mmol); LiPPh2, 0.66 g (3.43 mmol). Yield: 1.30 g (86%). Anal. Calcd for C27H33N2PSi (444.62): C, 72.94; H, 7.48; N, 6.30. Found: C, 72.72; H, 7.43; N, 6.28. 1H NMR (500 MHz, C6D6, 25 °C): δ 0.99 (s, 18H, C(CH3)3), 6.82−7.21 (m, 15H, C6H5) ppm. 13C NMR (125.75 MHz, C6D6, 25 °C): δ 31.31, 53.39, 126.21, 127.52, 127.86, 128.33, 128.62, 129.69, 130.95, 133.77, 134.03, 139.84, 159.53 ppm. 31P{1H} NMR (121.49 MHz, C6D6, 25 °C): δ −31.11 (JSiP = 152 Hz) ppm. 29Si{1H} NMR (99.36 MHz, C6D6, 25 °C): δ 34.3 (d, JSiP = 152 Hz) ppm. EI-MS: m/ z 444 [M+].

Figure 3. Molecular structure of 4. The anisotropic displacement parameters are depicted at the 50% probability level. Selected bond lengths (Å) and bond angles (deg): N1−Si1 = 1.9003(12), N2−Si1 = 1.9002(13), N3−Si1 = 1.7434(12); N1−Si1−N2 = 68.10(5), N1− Si1−N3 = 103.52(6), N2−Si1−N3 = 103.85(5). 4590

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Compound 2. Quantity used: LSiCl, 1.04 g (3.53 mmol); LiNPh2, 0.62 g (3.54 mmol). Yield: 1.35 g (89%). Anal. Calcd for C27H33N3Si (427.66): C, 75.83; H, 7.78; N, 9.83. Found: C, 75.76; H, 7.62; N, 9.82. 1H NMR (300 MHz, C6D6, 25 °C): δ 1.04 (s, 18H, C(CH3)3), 6.86−6.97 (m, 5H, C6H5), 7.14−7.27 (m, 10H, 2 × C6H5) ppm. 13C NMR (75.47 MHz, C6D6, 25 °C): δ 31.21, 53.06, 125.37, 126.10, 127.81, 127.87, 128.89, 129.51, 129.97, 134.12, 161.95 ppm. 29Si{1H} NMR (59.63 MHz, C6D6, 25 °C): δ −20.5 ppm. EI-MS: m/z 427 [M+]. Compound 3. Quantity used: LSiCl, 1.00 g (3.39 mmol); LiNCy2, 0.64 g (3.42 mmol). Yield: 1.22 g (82%). Anal. Calcd for C27H45N3Si (439.75): C, 73.74; H, 10.31; N, 9.56. Found: C, 73.72; H, 10.22; N, 9.60. 1H NMR (200 MHz, C6D6, 25 °C): δ 0.92−2.11 (m, cy), 1.24 (s, C(CH3)3), 2.32−2.57 (m, cy), 2.77−2.91 (m, cy), 6.86−7.09 (m, C6H5), 7.28−7.35 (m, C6H5) ppm. 13C NMR (125.75 MHz, C6D6, 25 °C): δ 25.46, 26.39, 26.56, 26.73, 27.34, 27.96, 31.86, 32.55, 34.12, 34.86, 40.05, 52.92, 54.43, 54.70, 127.36, 127.62, 128.29, 129.13, 130.80, 135.28, 161.21 ppm. 29Si{1H} NMR (59.63 MHz, C6D6, 25 °C): δ −5.8 ppm. EI-MS: m/z 439 [M+]. Compound 4. Quantity used: LSiCl, 1.02 g (3.46 mmol); LiNiPr2, 0.37 g (3.46 mmol). Yield: 1.15 g, 93%. Anal. Calcd for C21H37N3Si (359.62): C, 70.14; H, 10.37; N, 11.68. Found: C, 70.12; H, 10.29; N, 11.62. 1H NMR (500 MHz, C6D6, 75 °C): δ 1.21 (s, 18H, C(CH3)3), 1.67 (d, 2H, J = 7 Hz, NCH(CH3)2), 3.31 (m, 1H, NCH(CH3)2), 3.44 (m, 1H, NCH(CH3)2), 6.82−7.08 (m, 5H, C6H5) ppm. 13C NMR (75.47 MHz, C6D6, 25 °C): δ 23.27, 28.77, 31.87, 43.94, 45.11, 52.98, 127.29, 127.60, 128.99, 129.03, 130.77, 135.23, 160.80 ppm. 29Si{1H} NMR (59.63 MHz, C6D6, 25 °C): δ −6.5 ppm. EI-MS: m/z 359 [M+]. Compound 5.7e Quantity used: LSiCl, 1.00 g (3.39 mmol); LiNMe2, 0.17 g (3.33 mmol). Yield: 0.90 g (88%). 1H NMR (300 MHz, C6D6, 75 °C): δ 1.13 (s, 18H, C(CH3)3), 2.80 (s, 18H, N(CH3)2), 6.86−7.09 (m, 5H, C6H5) ppm. 29Si{1H} NMR (59.63 MHz, C6D6, 25 °C): δ −2.7 ppm. EI-MS: m/z 304 [M+]. Compound 6.7g Quantity used: LSiCl, 1.00 g (3.59 mmol); LiN(SiMe3)2, 0.59 g (3.53 mmol). Yield: 1.30 g (92%). 1H NMR (300 MHz, C6D6, 25 °C): δ 0.43 (s, 9H, Si(CH3)3), 0.63 (s, 9H, Si(CH3)3), 1.22 (s, 18H, C(CH3)3), 6.86−6.97 (m, 5H, C6H5) ppm. 29Si{1H} NMR (59.63 MHz, C6D6, 25 °C): δ −8.1 (SiNSi(CH3)3), 2.8 (Si(CH3)3), 3.7 (Si(CH3)3) ppm. EI-MS: m/z 419 [M+]. Compound 7.7e Quantity used: LSiCl, 1.10 g (3.73 mmol); KOtBu, 0.42 g (3.74 mmol). Yield: 1.10 g (89%). 1H NMR (200 MHz, C6D6, 25 °C): δ 1.20 (s, 18H, C(CH3)3), 1.58 (s, 18H, OC(CH3)3), 6.80− 7.10 (m, 5H, C6H5) ppm. 29Si{1H} NMR (99.36 MHz, C6D6, 25 °C): δ −5.0 ppm. EI-MS: m/z 333 [M+]. Crystal Structure Determination. Suitable single crystals for Xray structural analysis of 1, 3, and 4 were obtained from toluene solutions stored at 0 °C in a freezer. The crystals were mounted at low temperature in inert oil under an argon atmosphere by applying the XTemp 2 device.23 The diffraction data were collected at 100 K on a Bruker D8 three-circle diffractometer equipped with a SMART APEX II CCD detector and a rotating anode with INCOATEC Quazar mirror optics (λ = 0.710 73 Å).24 The data were integrated with SAINT,25 and an empirical absorption correction with SADABS26 was applied. The structures were solved by direct methods (SHELXS-97) and refined against all data by full-matrix least-squares methods on F2 (SHELXL-97). 27 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 1.5 times the Ueq value of their pivot atoms for terminal sp3 carbon atoms and 1.2 times for all other carbon atoms.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.W.R.); [email protected] (R.S.G.); [email protected] (D.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to the Deutsche Forschungsgemeinschaft and Prohama, Ludwigshafen, Germany, for supporting this work. R.A. is thankful to the Alexander von Humboldt Stiftung for a research fellowship. D.S. and H.W. are grateful to the DNRF funded Center for Materials Crystallography (CMC) for support and the Land Niedersachsen for providing a fellowship in the Catalysis of Sustainable Synthesis (CaSuS) Ph.D. program.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

CIF files and a table giving crystal data and details of the structure solution and refinement for 1, 3, and 4. This material is available free of charge via the Internet at http://pubs.acs.org. 4591

dx.doi.org/10.1021/om3003762 | Organometallics 2012, 31, 4588−4592

Organometallics

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