[Me₂Si] Ansa bridged complexes of permethyltitanocene: synthesis and structural characterization of fulvene derivatives with trialkylidenemethane character †

Abstract

A series of permethylated [Me₂Si] ansa bridged titanocene complexes has been synthesized and structurally characterized by X-ray doffraction; the dialkyl complexes [Me₂Si(C₅Me₄)]TiR₂ are thermally unstable towards elimination of alkane (RH), thereby yielding fulvene derivatives [Me₂Si(C₅Me₄)(C₅Me₃CH₂)]TiR.

References

1. H. Lee, P. J. Desrosiers, I. Guzei, A. L. Rheingold and G. Parkin, J. Am. Chem. Soc., 1998, 120, 3255.
2. P. Jutzi and R. Dickbreder, Chem. Ber., 1986, 119, 1750.
3. Other [Me₂Si(C₅Me₄)₂]TiX₂ derivatives include [Me₂Si(C₅Me₄)₂]TiCl and [Me₂Si(C₅Me₄)₂]Ti[η₂-C₂(SiMe₃)₂]. See: V. Varga, J. Hiller, R. Gyepes, M. Polasek, P. Sedmera, U. Thewalt and K. Mach, J. Organomet. Chem., 1997, 538, 63.
4. For unsubstituted analogs, [Me₂Si(C₅H₄)₂]TiR₂, see: (a) R. Gómez, T. Cuenca, P. Royo, W. A. Herrmann and E. Herdtweck, J. Organomet. Chem., 1990, 382, 103; (b) R. Gómez, T. Cuenca, P. Royo and E. Hovestreydt, Organometallics, 1991, 10, 2516.
5. Alternatively, [Me₂Si(C₅Me₄)₂]Ti[(C₆H₄)₂] may also be obtained by reaction of [Me₂Si(C₅Me₄)₂]TiPh₂ with PhLi.
6. H. Lee, J. Cordaro and J. B. Bonanno, unpublished work.
7. Non-bridged fulvene analogs Cp*(C₅Me₄CH₂)TiR (R = H, Me, CH₂CMe₃, CH₂SiMe₃, Ph, CH=CH₂) have also been reported. See, for example: (a) C. McDade, J. C. Green and J. E. Bercaw, Organometallics, 1982, 1, 1629; (b) G. A. Luinstra, P. H. P. Brinkmann and J. H. Teuben, J. Organomet. Chem., 1997, 532, 125; (c) G. A. Luinstra and J. H. Teuben, Organometallics, 1992, 11, 1793; (d) J. L. Polse, R. A. Andersen and R. G. Bergman, J. Am. Chem. Soc., 1996, 118, 8737; (e) J. L. Polse, A. W. Kaplan, R. A. Andersen and R. G. Bergman, J. Am. Chem. Soc., 1998, 120, 6316; (f) R. Beckhaus, J. Oster and T. Wagner, Chem. Ber., 1994, 127, 1003; (g) R. Beckhaus, J. Oster, I. Sang, I. Strauβ and M. Wagner, Synlett, 1997, 241.
8. The CH₂ groups in [Me₂Si(C₅Me₄)(C₅Me₃CH₂)]TiR are characterized by ¹³C NMR signals at ca. δ 80, with ¹J_C–H coupling constants of ca. 150 Hz: Me (δ 78.3, 149 and 154 Hz), Ph (δ 82.5, 150 and 153 Hz) and CH₂SiMe₃(δ 80.2 ppm, 149 and 151 Hz). These values are comparable to those for Cp*(C₅Me₄CH₂)TiR derivatives (see, for example, ref. 7).
9. The Ti–C bond lengths for the unmetallated cyclopentadienyl group range from 2.29 Å to 2.46 Å.
10. For a review of bonding in fulvene complexes, see: J. A. Bandy, V. S. B. Mtetwa, K. Prout, J. C. Green, C. E. Davies, M. L. H. Green, N. J. Hazel, A. Izquierdo and J. J. Martin-Polo, J. Chem. Soc., Dalton Trans., 1985, 2037.
11. For further comparison, the longest Ti–C bond length in the Ti(III) complex Cp*(C₅Me₄CH₂)Ti is 2.47 Å. See: J. M. Fischer, W. E. Piers and V. G. Young, Jr., Organometallics, 1996, 15, 2410.
12. For trialkylidene methane derivatives of zirconium, see: G. Rodriguez and G. C. Bazan, J. Am. Chem. Soc., 1997, 119, 343.
13. Thus, the Ti–C12 bond deviates by only 2.5° from the Ti–centroid vector.
14. (a) L. E. Schock, C. P. Brock and T. J. Marks, Organometallics, 1987, 6, 232; (b) A. R. Bulls, W. P. Schaefer, M. Serfas and J. E. Bercaw, Organometallics, 1987, 6, 1219.
15. For example, the wide range of M–C bond lengths in the zirconium and hafnium complexes, Cp*(C₅Me₄CH₂)ZrPh (2.28–2.62 Å)^14a and Cp*(C₅Me₄CH₂)HfCH₂Ph (2.25–2.60 Å),^14b suggests that they may also be considered to possess trialkylidenemethane character.
16. Specifically, [Me₂Si(C₅Me₄)₂]Ti(CD₃)₂ yields principally the isotopomers [Me₂Si(C₅Me₄)(C₅Me₃CH₂)]Ti(CD₂H) and CD₄, rather than [Me₂Si(C₅Me₄)(C₅Me₃CH₂)]Ti(CD₃) and CD₃H.
17. C. McDade, J. C. Green and J. E. Bercaw, Organometallics, 1982, 1, 1629.
18. For [Me₂Si(C₅Me₄)₂]TiMe₂: ΔH‡= 27.8(8) kcal mol^–1, ΔS‡=–5(2) e.u. (1 e.u. = 4.184 J K^–1 mol^–1). For Cp*₂TiMe₂: ΔH‡= 27.6(3) kcal mol^–1, ΔS‡=–2.9(7) e.u. (ref. 17).
19. Interestingly, values of k_H/k_D≈ 5 have also been observed for elimination of methane from Cp*₂Ti(CH₃)(C₆D₅)(5.7 at 33 °C) and Cp*₂Ti(CH₃)(CD=CD₂)(5.1 at 80 °C). See ref. 7(c).
20. A benzyne intermediate has also been proposed in the formation of Cp*(C₅Me₄CH₂)ZrPh by thermal elimination of PhH and H₂ from Cp*₂ZrPh₂^14a and Cp*₂Zr(Ph)H (F. D. Miller and R. D. Sanner, Organometallics, 1998, 7, 818), respectively.
21. Cp₂TiPh₂ has been proposed to decompose via ortho-hydrogen abstraction by the other phenyl group generating a benzyne intermediate, [Cp₂Ti(η²-C₆H₄)]. See: (a) J. Dvorak, R. J. O'Brien and W. Santo, Chem. Commun., 1970, 411; (b) C. P. Boekel, J. H. Teuben and H. J. de Liefde Meijer, J. Organomet. Chem., 1975, 102, 161.
22. At 40 °C, the rate constants for elimination of RH from [Me₂-Si(C₅Me₄)₂]TiR₂ are: Ph [1.30(1)× 10^–4s^–1], CH₂SiMe₃[9.85(9)× 10^–5s^–1] and Me [2.0 × 10^–8s^–1]. The value for the methyl derivative is that determined by the activation parameters listed in ref. 18.
23. The numbering system is such that C₁ is the ring carbon attached to silicon.
24. D. J. Sikora, M. D. Rausch, R. D. Rogers and J. L. Atwood, J. Am. Chem. Soc., 1981, 103, 1265.
25. ESR spectroscopic studies suggest that 2-methyltetrahydrofuran binds more strongly to [Me₂Si(C₅Me₄)₂]TiCl than to Cp*₂TiCl, providing further evidence for enhanced electrophilicity of the ansa titanocene system. See ref. 3.
26. It should, however, be noted that computational studies suggest that ring slippage and tilt introduced by incorporation of bulky substituents are not the principal factors responsible for modification of the electron density at a metal center in a series of non-bridged titanocene complexes; rather the changes in electron density merely reflect the inductive effects of the various substituents. See: B. E. Bursten, M. R. Callstrom, C. A. Jolly, L. A. Paquette, M. R. Sivik, R. S. Tucker and C. A. Wartchow, Organometallics, 1994, 13, 127.
27. T. C. McKenzie, R. D. Sanner and J. E. Bercaw, J. Organomet. Chem., 1975, 102, 457.