Synthesis and Characterization of Chiral Group 4 Metallocene Alkyne

Organometallics 2009, 28, 915–918

915

Notes Synthesis and Characterization of Chiral Group 4 Metallocene Alkyne Complexes: (η5-menthyl-C5H4)2M(η2-Me3SiC2SiMe3), M ) Ti, Zr⊥ Marcus Klahn, Wolfgang Baumann, Perdita Arndt, Vladimir V. Burlakov,§ Thomas Schareina, Anke Spannenberg, and Uwe Rosenthal* Leibniz-Institut fu¨r Katalyse e. V. an der UniVersita¨t Rostock, Albert-Einstein-Strasse 29a, D-18059 Rostock, Germany ReceiVed October 7, 2008 Summary: Reaction of the menthyl-substituted metallocene dichlorides (η5-menthyl-C5H4)2MCl2 (M ) Ti (1-Ti), Zr (1-Zr)) with magnesium in the presence of Me3SiC2SiMe3 in THF giVes the chiral group 4 metallocene alkyne complexes of titanium and zirconium (η5-menthyl-C5H4)2M(Me3SiC2SiMe3) (M ) Ti (2-Ti), Zr (2-Zr)). Both complexes show a similar complexation behaVior and reactiVity to their achiral metallocene congeners. For example, the reaction with ethylene was studied; whereas the zirconium complex 2-Zr yields the zirconacyclopentane 3 in a well-defined reaction, the titanium analogue 2-Ti produces only an as yet unseparable mixture of different compounds. 2-Ti, 2-Zr, and 3 were characterized by X-ray crystal structure analysis. For stoichiometric and catalytic reactions of organometallic compounds it is necessary to have suitable complex fragments that are coordinatively and electronically unsaturated. In group 4 chemistry such complex fragments are titanocene “Cp′2Ti”, zirconocene “Cp′2Zr”, and hafnocene “Cp′2Hf” (Cp′ ) substituted or unsubstituted η5-cyclopentadienyl), which are considered as generally unstable 14-electron compounds having a d2 configuration.1 The aim is to find an adequate precursor that liberates the very reactive core complex under mild conditions. There are several systems known that are able to generate metallocene fragments.2 We used acetylene ligands to stabilize the metallocene fragments. The complexation of bis(trimethylsilyl)acetylene by metallocenes leads to three-membered metallacyclopropenes Cp′2M(L)(η2-Me3SiC2SiMe3), for example, Cp2Zr(py)(η2-Me3SiC2SiMe3).1 The acetylene is easily exchange⊥ This work is dedicated to John M. Birmingham on the occasion of his 80th birthday. * Corresponding author. Tel: ++49-381-1281-176. Fax: + +49-3811281-51176. E-mail: [email protected]. § On leave from the A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov St. 28, 117813, Moscow, Russia. (1) (a) Rosenthal, U.; Burlakov, V. V. Titanium and Zirconium in Organic Synthesis; Marek, I., Ed.; Wiley-VCH, 2002; p 355. (b) Rosenthal, U.; Burlakov, V. V.; Arndt, P.; Baumann, W.; Spannenberg, A. Organometallics 2003, 22, 884. (c) Rosenthal, U.; Burlakov, V. V.; Arndt, P.; Baumann, W.; Spannenberg, A.; Shur, V. B. Eur. J. Inorg. Chem. 2004, 24, 4739. (2) Examples:(a) Negishi, E.; Cederbaum, F. E.; Takahashi, T. Tetrahedron Lett. 1986, 27, 2829. (b) Denhez, C.; Me´de´gan, S.; He´lion, F.; Namy, J.-L.; Vasse, J.-L.; Szymoniak, J. Org. Lett. 2006, 8, 2945. (c) Chirik, P. J. Dalton Trans. 2007, 16, and references therein. (d) Pun, D.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2008, 130, 6047.

able by substrate molecules. The presence of the additional donor “L” depends on either the metal or the nature of the cyclopentadienyl. Such titanium and zirconium compounds show a very broad chemistry: by using different metals, Cp ligands, and additional ligands a fine-tuning of the complexes is possible. A chiral modification was tried by using (S)-(-)-nicotine as additional ligand in the complex rac-(ebthi)Zr(L)(η2-Me3SiC2SiMe3), with ebthi ) 1,2-ethylene-1,1′-bis(η5-tetrahydroindenyl).3 Recently, Gansa¨uer et al. published a convenient method to synthesize chiral metallocene dichlorides Cp′2MCl2 (Cp′ ) η5menthyl-C5H4) (1-M, M ) Ti, Zr), which have menthylsubstituted Cp ligands and were first synthesized by Kagan.4a,b,d Very recently, we used a modified procedure to prepare the metallocene dichlorides and the corresponding difluorides Cp′2MF2 (M ) Ti, Zr, Hf).4c These complexes do not undergo any racemization. This offers the possibility for enantioselective reactions with enantiomeric pure complexes.

Results and Discussion The reduction of (η5-menthyl-C5H4)2TiCl2 (1-Ti) with equimolar amounts of magnesium in the presence of Me3SiC2SiMe3 in THF at 50 °C gave, after workup, yellow prisms of (η5-menthylC5H4)2Ti(η2-Me3SiC2SiMe3) (2-Ti) in 93% yield. Under analogous conditions, the reaction of (η5-menthyl-C5H4)2ZrCl2 (1-Zr) afforded blue-green plates of (η5-menthyl-C5H4)2Zr(η2Me3SiC2SiMe3) (2-Zr) in 70% yield (Scheme 1). For 2-Zr, similar to rac-(ebthi)Zr(η2-Me3SiC2SiMe3), no additional donor was found, whereas zirconocene alkyne complexes with an unsubstituted cyclopentadienyl such as Cp2Zr(L)(η2-Me3SiC2SiMe3) contain an additional stabilizing ligand (e.g., L ) THF, pyridine, acetone).3,5 (3) (a) Lefeber, C.; Baumann, W.; Tillack, A.; Kempe, R.; Go¨rls, H.; Rosenthal, U. Organometallics 1996, 15, 3486. (b) Peulecke, N.; Lefeber, C.; Ohff, A.; Baumann, W.; Tillack, A.; Kempe, R.; Burlakov, V. V.; Rosenthal, U. Chem. Ber. 1996, 129, 959. (4) (a) Gansa¨uer, A.; Narayan, S.; Schiffer-Ndene, N.; Bluhm, H.; Oltra, J. E.; Cuerva, J. M.; Rosales, A.; Nieger, M. J. Organomet. Chem. 2006, 691, 2327. (b) Cesarotti, E.; Kagan, H. B.; Goddard, R.; Kru¨ger, C. J. Organomet. Chem. 1978, 162, 297. (c) Klahn, M.; Arndt, P.; Spannenberg, A.; Gansa¨uer, A.; Rosenthal, U. Organometallics 2008, 27, 5846. (d) Gansa¨uer, A.; Bluhm, H.; Pierobon, M.; Keller, M. Organometallics 2001, 20, 914.

10.1021/om8009624 CCC: $40.75  2009 American Chemical Society Publication on Web 01/15/2009

916 Organometallics, Vol. 28, No. 3, 2009

Notes

Scheme 1. Formation of the Alkyne Complexes 2-M (M ) Ti, Zr)

Table 1. Structural and Spectroscopic Data for the Coordinated Alkyne in 2-Ti and 2-Zr M-C1/Å C1-C1A/Å C1A-C1-Si1/deg 13 C(CtC)/ppm ν(CtC)/cm-1

2-Ti

2-Zr

2.113(2) 1.295(4) 140.67(6) 242.2 1658

2.224(3) 1.332(5) 139.05(9) 245.6 1565

The alkyne ligand in 2-Ti and 2-Zr is coordinated symmetrically to the metal center. As expected, an elongation of the C1-C2 bond length is observed due to the complexation to the metal, but its magnitude depends on the used metal core. Within the group 4 metals the coordination strength rises from titanium toward hafnium. This effect was observed before and can be seen at the different bonding motifs: reduced bond order of the triple bond and increased bending of the trimethylsilyl groups.1b,6 Both compounds 2-Ti and 2-Zr support the former observation; in Table 1 their structural and spectroscopic data are compiled. The compounds 2-Ti and 2-Zr are isomorphous and crystallize from n-hexane in the monoclinic space group C2. The molecular structure of 2-Ti is shown exemplarily for both complexes in Figure 1. The Supporting Information contains CD spectra of the compounds 2-Ti, 2-Zr, and 3 compiled in one figure. There are some similarities in the CD spectra, but one should use them with caution to assign absolute or relative configurations because any change of the complex results in CD curves with different shapes (see CD spectra of 1-M, M ) Ti, Zr, in the literature4b). Another method to get information about chiral properties of the complexes is based on the single-

Figure 1. Molecular structure of compound 2-Ti. The thermal ellipsoids correspond to 30% probability. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ti1-C1 2.113(2), C1-C1A 1.295(4), C1A-C1-Si1 140.67(6), C1-Ti-C1A 35.7(1); C1A and Si1A are generated by symmetry operation -x+1, y, -z+1.

crystal X-ray diffraction analysis, where the Flack parameter is used to estimate the absolute configuration (x ) 0.00(2) for 2-Ti; x ) -0.05(2) for 2-Zr; x ) 0.00(2) for 3).7 The NMR spectra of 2-Ti and 2-Zr are quite similar. All relevant resonances, including those due to the menthyl groups, could be assigned. There is a unique feature observed in the proton spectra, viz., a pronounced shift to low frequencies for the methylene protons in position 6 (2-Ti: 0.03 ppm axial and -1.42 ppm equatorial, 2-Zr: 0.41 ppm axial and -0.89 ppm equatorial). In solution this effect is found neither for the respective dihalides4c,d nor for the zirconacyclopentane 3. Therefore, it must be related to the presence of the metallacyclopropene core, for which an aromatic character was discussed.1b,8 This would create a magnetic anisotropy in its surrounding. Since the cyclopentadienyl and the cyclohexane ring take a roughly perpendicular orientation relative to each other [evident from an NOE between the cyclopentadienyl R-H (6.65 ppm) and 1-Hax and between the other R-H (5.35 ppm) and 6-Hax and 2-Hax on the opposite face of the cyclohexane], the methylene group 6 and in particular its equatorial hydrogen atom enter the sector above or below the metallacyclic plane and thus become shielded. In this way, the relation characteristic for cyclohexane derivatives, δ(Hax) < δ(Heq), is reversed. Cooling a solution of 2-Zr in toluene to -75 °C does not lead to significant spectroscopic changes. The slightly stronger alkyne complexation in 2-Zr compared to 2-Ti results in a varied reactivity. Here, the reaction with ethylene is shown representatively for their different reactivity. It is known that zirconocene bis(trimethylsilyl)acetylene complexes form with ethylene zirconacyclopentanes,9 whereas the analogous titanium compounds do not react to the desired titanacyclopentane. The complex 2-Zr reacts spontaneously with ethylene at room temperature to give the zirconacyclopentane 3 with menthyl-substituted cyclopentadienyl ligands. Compound 3 is not stable in solution; there is a competing equilibrium between 3 and 2-Zr, which depends on either the ethylene or alkyne concentration in the solution (Scheme 2). Due to elimination of ethylene, the isolation of analytically pure 3, especially for elemental analysis, without decomposition products is challenging and has not been accomplished so far for useful amounts of this complex. Nevertheless, suitable single (5) (a) Rosenthal, U.; Ohff, A.; Michalik, M.; Go¨rls, H.; Burlakov, V. V.; Shur, V. B. Angew. Chem. 1993, 105, 1228; Angew. Chem., Int. Ed. Engl. 1993, 32, 1193. (b) Rosenthal, U.; Ohff, A.; Baumann, W.; Tillack, A.; Go¨rls, H.; Burlakov, V. V.; Shur, V. B. Z. Anorg. Allg. Chem. 1995, 621, 77. (c) Peulecke, N.; Baumann, W.; Kempe, R.; Burlakov, V. V.; Rosenthal, U. Eur. J. Inorg. Chem. 1998, 419. (6) Beweries, T.; Burlakov, V. V.; Bach, M. A.; Arndt, P.; Baumann, W.; Spannenberg, A.; Rosenthal, U. Organometallics 2007, 26, 247. (7) Flack, H. D. Acta Crystallogr. 1983, A39, 876. (8) (a) McDonald, J. W.; Corbin, J. L.; Newton, W. E. J. Am. Chem. Soc. 1975, 97, 1970. (b) Shur, V. B.; Burlakov, V. V.; Vol’pin, M. E. J. Organomet. Chem. 1988, 347, 77. (c) Eisch, J. J.; Otieno, P. O. Eur. J. Org. Chem. 2004, 3269. (9) Mansel, S.; Thomas, D.; Lefeber, C.; Heller, D.; Kempe, R.; Baumann, W.; Rosenthal, U. Organometallics 1997, 16, 2886.

Notes

Organometallics, Vol. 28, No. 3, 2009 917 Scheme 2. Equilibrium in Solution between 2-Zr and 3

Experimental Section

Figure 2. Molecular structure of compound 3. The thermal ellipsoids correspond to 30% probability. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å]: Zr1-C1 2.307(2), C1-C2 1.543(3), C2-C3 1.520(3), C3-C4 1.533(3), Zr1-C4 2.302(2).

crystals for X-ray analysis were obtained from an n-pentane solution, and the molecular structure is shown in Figure 2. Comparing the structural properties of 3 with known zirconacyclopentanes, no relevant differences can be found.9,10 In fact, complex 3 shows a similarity to rac-(ebthi)Zr(C4H8).

Conclusion The synthesized bis(trimethylsilyl)acetylene complexes of titanium and zirconium 2-Ti and 2-Zr follow in their complexation behavior the trend of an increasing interaction between metal center and alkyne that was already found for this compound class with other cyclopentadienyl ligands. Therefore, they show a similar reactivity to their congeners of titanium and zirconium, respectively. The complexes 2-Ti and 2-Zr have the potential to generate the coordinatively and electronically unsaturated chiral complex fragments “(η5-menthyl-C5H4)2M” as intermediates in reactions with substrates. As an example, the reaction with ethylene was studied, and it was found that only the zirconium complex yielded the zirconacyclopentane 3 in a well-defined reaction. The same reaction with the titanium analogue gave an as yet undefined mixture of different compounds. (10) (a) Takahashi, T.; Fischer, R.; Xi, Z.; Nakajima, K. Chem. Lett. 1996, 357. (b) Podubrin, S. Ph.D. Thesis, University of Kaiserslautern, 1993.

General Procedures. All operations were carried out under argon with standard Schlenk techniques. Prior to use nonhalogenated solvents were freshly distilled from sodium tetraethylaluminate and stored under argon. Deuterated solvents were treated with sodium tetraethylaluminate, distilled, and stored under argon. The following spectrometers were used. Mass spectra: Finnigan MAT 95-XP. NMR spectra: Bruker AV 300/AV 400. Chemical shifts (1H, 13C, 29 Si) are given relative to SiMe4 and are referenced to signals of the used solvent: C6D6 (δH 7.16, δC 128.0), toluene-d8 (δH 2.03, δC 20.4). The spectra of 2-Zr were assigned with the help of DEPT, INEPT, and shift correlation experiments; data for the other compounds were assigned analogously. The numbering system refers to Scheme 1. Melting points: sealed capillaries, Bu¨chi 535 apparatus. Elemental analyses: Leco TruSpec Micro elemental analyzer. The CD spectra were recorded on a JASCO J-710 spectropolarimeter. Preparation of 2-Ti. (η5-Menthyl-C5H4)2TiCl24c (0.617 g, 1.174 mmol) and magnesium turnings (0.031 g, 1.275 mmol) were dissolved in 15 mL of THF, followed by the addition of bis(trimethylsilyl)acetylene (0.17 mL, 1.174 mmol). The solution was stirred 2 h at 50 °C and additionally 1 h at room temperature. The color of the solution changed within this time from red to light brown. The solvent was removed under vacuum, and the obtained residue was extracted twice with 15 mL of n-hexane. Leaving the n-hexane solution at -78 °C gave 0.685 g (1.096 mmol, 93%) of the product as fine, yellow crystals, which were dried under vacuum. Mp: 154-155 °C (dec) under Ar. Anal. Calcd for C38H64Si2Ti: C, 73.03; H, 10.32. Found: C,73.09; H, 10.13. IR (Nujol mull, cm-1): 1658 (CtC). NMR (298 K, C6D6) 1H (300 MHz): δ -1.42 (dq, 2H, 3J ) 12.5 Hz, 6-Heq), -0.09 (s, 18H, SiMe3), 0.03 (q, 2H, 2J ≈ 3J ≈ 12 Hz, 6-Hax), 0.62 (d, 6H, 3J ) 6.5 Hz, CH3-menthyl), 1.06 (d, 6H, 3J ) 7.0 Hz, CH3-menthyl), 1.19 (d, 6H, 3J ) 6.9 Hz, CH3menthyl), 0.75 (dq, 2H, 2J ≈ 3J ≈ 12 Hz, 3/4-Hax), 0.95 (hidden, 2H, 5-Hax), 1.01 (dq, 2H, 2J ≈ 3J ≈ 13 Hz, 3/4-Hax), 1.31 (tm, 2H, 3 J ) 11/12 Hz, 2-Hax), 1.58 (dm, 2H, 2J ) 12.2 Hz, 3/4-Heq), 1.68 (dm, 2H, 2J ) 13 Hz, 3/4-Heq), 2.41 (ddd, 2H, 3J ) 12/11/3 Hz, 1-Hax), 2.80 (dsept, 2H, 3J ) 6.8/2.7 Hz, 8-H), 5.15, 7.87 (2 dt, 2H each, R-C5H4), 5.47, 7.42 (2 dt, 2H each, β-C5H4). 13C (300 MHz): δ 1.1 (SiMe3), 16.2, 22.0, 22.3 (3 CH3), 25.1 (C3), 28.2 (C8), 32.5 (C5), 35.2 (C4), 41.1 (C6), 42.1 (C1), 51.1 (C2), 108.9, 112.5, 115.2, 126.0 (4 CH, Cp), 137.1 (Cq, Cp), 242.2 (CtC). MS (70 eV, m/z): 624 [M]+; 454 [M - alkyne]+, 170 [alkyne]+. Preparation of 2-Zr. (η5-Menthyl-C5H4)2ZrCl24c (1.000 g, 1.758 mmol) and magnesium turnings (0.047 g, 1.934 mmol) were dissolved in 15 mL of THF, followed by the addition of bis(trimethylsilyl)acetylene (0.40 mL, 1.758 mmol). The solution was stirred at 50 °C until the color of the solution changed from white to green. The solvent was removed under vacuum, and the obtained residue was extracted with 20 mL of n-hexane. Leaving the n-hexane solution at -78 °C gave 0.818 g (1.224 mmol, 70%) of the product as fine, blue-green crystals, which were dried under vacuum. Mp: 158-159 °C (dec) under Ar. Anal. Calcd for C38H64Si2Zr: C, 68.29; H, 9.65. Found: C, 68.21; H, 9.55. IR (Nujol mull, cm-1): 1565 (CtC). NMR (297 K, toluene-d8) 1H (400 MHz): δ -0.89 (dq, 2H, 2J ) 12 Hz, 6-Heq), 0.07 (s, 18H, SiMe3), 0.41 (q, 2H, 2J ≈ 3J

918 Organometallics, Vol. 28, No. 3, 2009 ≈ 12 Hz, 6-Hax), 0.72 (d, 6H, 3J ) 6.4 Hz, 7-H), 0.79 (hidden q, 2H, 4-Hax), 0.87 (d, 6H, 3J ) 7.2 Hz, 10-H), 0.89 (d, 6H, 3J ) 6.8 Hz, 9-H), 0.93 (hidden q, 2H, 3-Hax), 1.13 (hidden t, 2H, 2-Hax), 1.14 (hidden t, 2H, 5-Hax), 1.56 (m, 4H, 3-Heq and 4-Heq), 2.08 (dsept, 2H, 3J ) 7/2.7 Hz, 8-H), 2.39 (ddd, 2H, 3J ) 12/11/3 Hz, 1-Hax), 5.35, 5.59, 6.61, 6.65 (4 dt, 2H each, C5H4; assignment according to X-ray numbering from the Supporting Information, SI: 6-H, 5-H, 4-H, 3-H, respectively). 13C (100 MHz): δ 1.6 (SiMe3), 16.0 (C9), 21.7 (C10), 22.3 (C7), 25.1 (C3), 28.0 (C8), 32.6 (C5), 35.4 (C4), 40.2 (C6), 41.9 (C1), 50.6 (C2), 104.0, 106.3, 110.1, 118.3, 134.9 (C5H4; assignment according to X-ray numbering from the SI: C6, C4, C5, C3, C2, respectively), 245.6 (CtC). 29Si (79.5 MHz): δ -11.4. MS (70 eV, m/z): 666 [M]+; 170 [alkyne]+. Preparation of 3. (η5-Menthyl-C5H4)2Zr(Me3SiC2SiMe3) (0.155 g, 0.232 mmol) was dissolved in 15 mL of toluene. The solution was warmed to 40 °C, and afterward the argon atmosphere was replaced carefully by ethylene. An immediate color change from dark green to yellow was oberserved. The solution was stirred over 1 h. Then all volatiles were removed under vacuum to obtain the crude product. Recrystallization in n-pentane at -30 °C yielded 0.118 g (0.213 mmol, 92% determined by NMR) of the product as yellow crystals. Mp: 115 °C (dec) under Ar. Anal. Calcd for C34H54Zr: C, 73.71; H, 9.82. Found: C, 69.37; H, 9.50 (because of the instability and the behavior in solution of this compound, no better data could be collected). NMR (294 K, toluene-d8, excess ethylene) 1H (300 MHz): δ 0.56 (m, 4H, 6-H), 0.76 (d, 6H, 3J ) 6.5 Hz, 7-H), 0.78 (hidden, 2H, 4-Hax), 0.82 (d, 6H, 3J ) 7.0 Hz, 10-H), 0.85 (hidden, 2H, ZrCH), 0.86 (d, 6H, 3J ) 7.1 Hz, 9-H), 0.95 (m, 2H, 3-Hax), 0.98 (m, 2H, 2-Hax), 1.12 (m, 2H, 5-Hax), 1.29 (m, 2H, ZrCH), 1.58 (m, 2H, 3-Heq), 1.61 (m, 2H, 4-Heq), 1.83 (dsept, 2H, 3J ) 7/2 Hz, 8-H), 1.92 (m, 2H, ZrCCH), 2.06 (m, 2H, ZrCCH), 2.41 (m, 2H, 1-Hax), 5.12, 5.40, 6.41, 6.71 (4 dt, 2H each,

Notes C5H4; assignment according to X-ray numbering from the SI: 5-H, 6-H, 7-H, 8-H, respectively). 13C (75 MHz): δ 15.9 (C9), 21.9 (C10), 22.8 (C7), 25.1 (C3), 27.8 (C8), 30.1 (ZrCC), 33.0 (C5), 35.5 (C4), 41.8 (C1), 41.9 (ZrC), 42.8 (C6), 51.5 (C2), 102.4, 106.1, 114.4, 115.8, 133.7 (C5H4; assignment according to X-ray numbering from the SI: C5, C6, C7,C8, C9, respectively). MS (70 eV, m/z): 552 [M]+. Diffraction data were collected on a STOE IPDS diffractometer using graphite-monochromated Mo KR radiation. The structures were solved by direct methods (SHELXS-9711) and refined by fullmatrix least-squares techniques on F2 (SHELXL-9712). XP (Bruker AXS) was used for graphical representations. The absolute configuration of 2-Ti, 2-Zr, and 3 in the solid state has been determined by refinement of the Flack parameter (x ) 0.00(2) for 2-Ti; x ) -0.05(2) for 2-Zr; x ) 0.00(2) for 3).7

Acknowledgment. We thank our technical staff and in particular Regina Jesse and Petra Bartels for assistance. Support by the Deutsche Forschungsgemeinschaft (SPP 1118 and GRK 1213) is acknowledged. Supporting Information Available: Figures, tables, CD spectra, and CIF file giving detailed information of the compounds 2-Ti, 2-Zr, and 3. This material is available free of charge via the Internet at http://pubs.acs.org. OM8009624 (11) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure Solution; University Go¨ttingen: Germany, 1997. (12) Sheldrick, G. M., SHELXL-97, Program for Crystal Structure Refinement; University Go¨ttingen: Germany, 1997.