Organometallics 2009, 28, 1985–1987
1985
Reaction of the (Pentamethylcyclopentadienyl)silicon(II) Cation with a Sterically Encumbered β-Diketiminato Anion: Unexpected Formation of a Tricyclic Silicon(IV) Compound Peter Jutzi,* Kinga Leszczyn´ska, Andreas Mix, Beate Neumann, Wolfgang W. Schoeller,† and Hans-Georg Stammler Fakulta¨t fu¨r Chemie, UniVersita¨t Bielefeld, UniVersita¨tstrasse 25, 33615 Bielefeld, Germany ReceiVed February 23, 2009 Summary: Reaction of the Cp*Si+ cation with the β-diketiminato anion [HC(CMeNAryl)2]- (Aryl ) 2,6-iPr2C6H3) did not lead to the expected silicon(II) compound Cp*[HC(CMeNAryl)2]Si (3) but to the constitutional isomer 7 containing a silicon(IV) center. Calculations were performed for 3, for the conformational isomer 3* containing a η3-Cp* ligand, and for less substituted species. During the last few decades, several room-temperature-stable derivatives of the highly reactive silylene SiH2 have been prepared using the concept of kinetic and/or thermodynamic stabilization.1,2 Only recently has it even been possible to synthesize stable derivatives of the silyliumylidene cation SiH+, by using the π-bonded pentamethylcyclopentadienyl ligand in the compound Me5C5Si+B(C6F5)4- (1)3 or a chelating β-diketiminato substituent in the compound HC(CMeNAryl)2Si+B(C6F5)4(2; Aryl ) 2.6-iPr2C6H3).4 Thermodynamic stabilization is realized in the cation 1a by three-dimensional and in the cation 2a by two-dimensional aromaticity (see Figure 1); thus, in the true sense of the word, both cations do not represent silyliumylidene cations, because they contain higher coordinated silicon atoms. In principle, both cations should lead to novel neutral silicon(II) compounds simply by the addition of appropriate anionic nucleophiles. For 1a, this has already been shown by some examples.5,6 We were interested in the result of combining the Cp* and the β-diketiminato group in a silicon(II) compound to answer the question of which bonding situation at silicon * To whom correspondence should be addressed. E-mail: peter.jutzi@ uni-bielefeld.de. Tel: +49-5211066163. † Current address: University of California at Riverside, Department of Chemistry, Riverside, CA 92521-0403. E-mail:
[email protected]. (1) Silylenes: (a) Denk, M.; Lennon, R.; Hayashi, R.; West, R.; Haaland, A.; Belyakov, H.; Verne, P.; Wagner, M. N. J. Am. Chem. Soc. 1994, 116, 2691–2692. (b) Gehrhus, B.; Lappert, M. F.; Heinicke, J.; Boese, R.; Blaser, D. J. Chem. Soc., Chem. Commun. 1995, 1931–1932. (c) Heinicke, J.; Oprea, A.; Kindermann, M. K.; Karpati, T.; Nyulaszi, L.; Veszpremi, T. Chem. Eur. J. 1998, 4, 541–545. (d) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc. 1999, 121, 9722. (e) Driess, M.; Yao, S.; Brym, M.; van Wu¨llen, C.; Lentz, D. J. Am. Chem. Soc. 2006, 128, 9628. Reviews: (f) Haaf, M.; Schwedake, A.; West, R. Acc. Chem. Res. 2000, 33, 704. (g) Gerhus, B.; Lappert, M. F. J. Organomet. Chem. 2001, 617-618, 209. (2) Higher coordinated silicon(II) compounds: (a) Jutzi, P.; Kanne, D.; Kru¨ger, C. Angew. Chem. 1986, 98, 163; Angew. Chem., Int. Ed. Engl. 1986, 25, 164. (b) Karsch, H. H.; Keller, U.; Gamper, S.; Mu¨ller, G. Angew. Chem. 1990, 102, 297; Angew. Chem., Int. Ed. Engl. 1990, 29, 295. (c) So, C. W.; Roesky, H. W.; Magull, J.; Oswald, E. B. Angew. Chem. 2006, 118, 4052; Angew. Chem., Int. Ed. 2006, 45, 3948. (d) So, C. W.; Roesky, H. W.; Gurubasavaraj, P. M.; Oswald, R. B.; Gamer, M. T.; Jones, P. G.; Blaurock, S. J. Am. Chem. Soc. 2007, 129, 12049–12054. (3) Jutzi, P.; Mix, A.; Rummel, B.; Schoeller, W. W.; Neumann, B.; Stammler, H.-G. Science 2004, 305, 849. (4) Driess, M.; Yao, S.; Brym, M.; van Wu¨llen, C. Angew. Chem. 2006, 118, 6882; Angew. Chem., Int. Ed. 2006, 45, 6730.
Figure 1. Stable silicon(II) cations.
might be preferred: (a) a π-Cp* and a monocoordinate ketiminato group. (b) a chelating β-diketiminato and a σ-Cp* group, or (c) a π-Cp* and a chelating β-diketiminato group. In this context, we have performed the reaction of compound 1 with the lithium compound Li[HC(CMeNAryl)2] · DME (Aryl ) 2.6-iPr2C6H3)7-9 (described as LiNacnac · DME in Scheme 1) in DME as solvent at -78 °C (see Scheme 1). The reaction mixture was warmed to room temperature. After evaporation of DME and addition of hexane, the salt LiB(C6F5)4 was separated. Instead of the desired silicon(II) species, the silicon(IV) compound 7 was obtained in high yields after concentration of the remaining solution and crystallization from hexane. A proposed pathway for the formation of compound 7 is given in Scheme 1. The desired silicon(II) compound 3, containing a σ-Cp* substituent and a chelating β-diketiminato group, is formed as a reactive intermediate which rearranges on reductive cleavage of the N-Cimine bond of the Nacnac unit, to form the silicon(IV) compound 4, containing an imino group bound to silicon. This type of redox rearrangement is known from comparable titanium(II) β-diketiminato complexes.10 The zwitterionic structure 5 represents a resonance structure of 4. A further stabilization occurs by C-C bond formation between the carbocation in 5 and an olefinic carbon atom of the Cp* unit, which leads to the five-membered zwitterionic silacycle 6, containing a 1,2,3-trimethylallyl-type cation and an anionic (deprotonated) arylamido group in the exo position; finally, a (5) Jutzi, P.; Mix, A.; Neumann, B.; Rummel, B.; Stammler, H.-G. Chem. Commun. 2006, 3519–21. (6) Jutzi, P.; Leszczyn´ska, K.; Neumann, B.; Schoeller, W. W.; Stammler, H.-G. Angew. Chem. 2009, 121, in press; Angew. Chem., Int. Ed. 2009, DOI 200805749. (7) The DME complex was prepared following the procedure for the synthesis of comparable complexes published in detail by Power et al.8 (8) Stender, M.; Wright, R. J.; Eichler, B. E.; Prust, J.; Olmstead, M. M.; Roesky, H. W.; Power, P. P. Dalton Trans. 2001, 3465–3469. (9) The X-ray crystal structure data of the DME complex are given in the Supporting Information; the parameters are comparable to those obtained for complexes described in ref 8 (10) (a) Basuli, F.; Kilgore, U. J.; Brown, D.; Huffman, J. C.; Mindiola, D. J. Organometallics 2004, 23, 6166–6175. (b) Hamahaki, H.; Takeda, N.; Tokitoh, N. Organometallics 2006, 25, 2457–2464. (c) Bai, G.; Wei, P.; Stephan, D. W. Organometallics 2006, 25, 2649–2655.
10.1021/om9001457 CCC: $40.75 2009 American Chemical Society Publication on Web 03/19/2009
1986 Organometallics, Vol. 28, No. 7, 2009
Communications
Scheme 1. Proposed Reaction Sequence for the Formation of Compound 7
proton from the neighboring methyl group in the allyl-type cation is transferred to the amido group to give the stable silacycle 7. The tricyclic compound 7 possesses stereocenters at the silicon atom Si(1) and at the carbon atoms C(1), C(2), and C(11), which are formed stereospecifically, and a further stereocenter at the exocyclic nitrogen atom, which leads to the presence of two isomers due to the nonrigidity of the amino group. Figure 2 shows a representation of the structure of one isomer of 7. Aryl, methyl, methylene, and amino groups represent exo substituents. Further structure data are given as Supporting Information. The 1H, 13C, and 29Si NMR data of 7 are consistent with the described structure (see the Experimental Section). To learn more about the intermediate silicon(II) compound containing a Cp* and a β-diketiminato group, theoretical
Figure 2. Structure of one isomer of compound 7. The aryl groups at the N atoms are omitted for clarity.
calculations have been performed on the RI-BP86/TZVP level.11 They reveal for the fully substituted compound two energy minimum structures, 3 and 3* (Figure 3): the first structure possesses a chelating β-diketiminato ligand and a σ-bound Cp* group (Si-C ) 2.066 Å; Si-N ) 1.957, 1.987 Å) and the second a chelating β-diketiminato ligand and a η3-bound Cp* group (Si-C ) 2.253 Å; Si-N ) 2.034, 2.007 Å). The latter, 3*, can be viewed as a weakly stable interaction complex of the NacNac- ligand with 1a, which readily rearranges to 3. The energetic preference of 3 over 3* by -13.1 kcal/mol is reflected in the comparatively shorter Si-N and Si-C bonds in the former compound caused by the diminished steric interaction of the Cp* with the ketiminato ligand. Nevertheless, there still remains a pronounced steric pressure even in 3 (see the rather long Si-C and Si-N distances), so that both species represent high-energy minima and thus are reactive intermediates on the way to the insertion product 4. The structural parameters for 3 and 3* and for some less substituted species in this class of compounds are given in the Supporting Information. In summary, the combination of the Cp* group with the bulky [HC(CMeNAryl)2] (Aryl ) 2.6-iPr2C6H3) substituent does not lead to a stable silicon(II) compound. A further conclusion is, that, depending on the steric requirements, a β-diketiminato ligand might easily undergo a redox-rearrangement process comparable to that already observed in the chemistry of divalent titanium compounds. Experimental Section. Synthesis of LiNacnac · DME.7 n-BuLi (1.6 M in hexane, 2.4 mL, 3.8 mmol) was added by syringe to a cooled (-78 °C) solution of NacnacH (1.60 g, 3.8 mmol) in 20 mL of hexane. The reaction mixture was warmed to room temperature and stirred overnight. Then it
Figure 3. Calculated structures of 3 and of 3*. Hydrogen atoms are omitted for clarity.
Communications
was cooled to 0 °C and DME (0.5 mL) was added. The reaction mixture was stirred for 0.5 h at room temperature. The product precipitated as a white solid, which was washed with hexane (∼5 mL) and dried at reduced pressure. Concentration of the hexane filtrate to ∼10 mL and storage at 0 °C resulted in the isolation of an additional portion of the product as colorless crystals. Total yield: 1.65 g (84%). Crystals suitable for an X-ray analysis were grown from hexane at 0 °C. 1 H NMR (C6D6): δ 1.19, 1.30 (2d, 2 × 12H, CH(CH3)2, 3JHH ) 6.9 Hz), 1.87 (s, 6H, CH3), 2.54 (s, 4H, OCH2), 2.65 (s, 6H, OCH3), 3.42 (sept, 4H, CH(CH3)2, 3JHH ) 6.9 Hz), 4.88 (s, 1H, γ-CH), 7.0-7.2 (m, 6H, Ar H). 13C NMR (C6D6): δ 23.5 (CH3), 24.2, 24.4 (CH(CH3)2), 27.7 (CH(CH3)2), 57.8 (OCH2), 69.4 (OCH3), 92.5 (γ-CH), 122.6, 123.0, 141.0, 150.1 (Ar C), 163.3 (CN). 7Li NMR (C6D6): δ 1.76 ppm. Synthesis of 7. A cooled suspension of LiNacnac · DME (0.170 g, 0.33 mmol) in DME (∼4 mL) was added by syringe to a cooled solution of Cp*Si+B(C6F5)4- (0.278 g, 0.33 mmol) in DME (∼3 mL) (bath temperature -70 °C). The color of the solution changed to deep green. With stirring, the reaction mixture was slowly warmed to room temperature (∼2 h) and maintained at room temperature for ∼2 h. At ∼-10 °C a color change to yellow was observed. After evaporation of the solvent at reduced pressure, the resulting solid was extracted with hexane (∼7 mL). The hexane filtrate was concentrated to about 0.3 mL and left to crystallize at room temperature. Compound 7 was isolated as pale yellow crystals (0.120 g, 63%). Crystals suitable for an X-ray analysis were grown from hexane at room temperature. 1 H NMR (C6D6): δ 0.47, 0.71, 1.03, 1.05 (4d, 4 × 3H, CH(CH3)2, 3JHH ) 6.9 Hz), 1.16 (s, 3H, CH3), 1.18, 1.25,
Organometallics, Vol. 28, No. 7, 2009 1987
1.27, 1.40 (4d, 4 × 3H, CH(CH3)2, 3JHH ) 6.9 Hz), 1.43, 1.47, 1.48, 1.63, 1.81 (5s, 5 × 3H, CH3), 2.75, 2.99 (2 sept, 2 × 1H, CH(CH3)2, 3JHH ) 6.9 Hz), 3.11 (s, 1H, NH), 3.73, 3.93 (2 sept, 2 × 1H, CH(CH3)2, 3JHH ) 6.9 Hz), 4.40 (s, 1H, CdCH2), 4.43 (s, 1H, CdCH2), 4.81 (s, 1H, CHdCN), 6.83-7.22 (m, 6H, Ar H). 13C NMR (C6D6): δ 10.6, 14.2, 15.4, 17.1, 20.6, 22.4, 23.3, 23.4, 23.6, 24.4, 25.0, 27.6, 28.4, 28.5, 31.7 (CH3, CH(CH3)2, CH(CH3)2), 50.0, 53.8, 63.7 (CIV), 94.5 (CdCH2), 111.4 (CHdC(CH3)N), 122.4, 122.7, 123.4, 123.6, 124.7, 127.2, 135.5, 137.4, 137.7, 144.3, 144.9, 145.0, 148.1, 148.7, 149 (CdC, Ar C), 161.6 (CN). 29Si NMR (C6D6): δ -9.14 ppm. IR (KBr, cm-1): 3305 (sharp) NH. Anal. Calcd for C39H56N2Si: C, 80.6; H, 9.7; N, 4.8. Found: C, 80.6; H, 9.5; N, 4.5.
Acknowledgment. The financial support of the University of Bielefeld, the Deutsche Forschungsgemeinschaft, and the Fonds der Chemischen Industrie is gratefully acknowledged. Supporting Information Available: Text, tables, and figures giving details of the computational results for 3 and its derivatives and structural data for 7 and for Li[HC(CMeNAryl)2] · DME (Aryl ) 2.6-iPr2C6H3) and CIF files giving crystal structure data for 7 and for Li[HC(CMeNAryl)2] · DME. This material is available free of charge via the Internet at http://pubs.acs.org. OM9001457
(11) All computational results were obtained by full structural optimization at the RI-BP86 level utilizing a triple-ζ (TZVP) AO basis set; structures were identified as energy minima by vibrational analysis. For details and references see the Supporting Information.
