Neutral Penta- and Hexacoordinate N-Heterocyclic ... - ACS Publications

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Organometallics 2009, 28, 6374–6377 DOI: 10.1021/om9007696

Neutral Penta- and Hexacoordinate N-Heterocyclic Carbene Complexes Derived from SiX4 (X = F, Br) Rajendra S. Ghadwal, Sakya S. Sen, Herbert W. Roesky,* Gasper Tavcar, Sebastian Merkel, and Dietmar Stalke Institut fuer Anorganische Chemie der Universitaet Goettingen, Tammannstrasse 4, D 37077 Goettingen, Germany Received September 3, 2009 Summary: Reactions of silicon halides with N-heterocyclic carbene produce neutral penta- or hexacoordinate complexes IPr 3 SiF4 (1), IPr 3 SiF4 3 IPr (2), and IPr 3 SiBr4 (3) (IPr=:C[N(2,6-iPr2-C6H3)CH]2). Molecular structures of 1-3 were determined by X-ray diffraction studies and exhibit interesting structural features. Silicon(IV) halides are well known as Lewis acids, and silicon compounds with a coordination number greater than *Corresponding author. E-mail: [email protected]. (1) Davy, J. Phil. Trans. R. Soc. London 1812, 102, 352–369. (2) (a) Frye, C. L.; Vogel, G. E.; Hall, J. A. J. Am. Chem. Soc. 1961, 83, 996–997. (b) Onan, K. D.; McPhail, A. T.; Yoder, C. H.; Hillyard, R. W. J. Chem. Soc., Chem. Commun. 1978, 209–210. (c) Voronkov, M. G.; Frolov, Yu. L.; D'yakov, V. M.; Chipanina, N. N.; Gubanova, L. I.; Gavrilova, G. A.; Klyba, L. V.; Aksamentova, T. N. J. Organomet. Chem. 1980, 201, 165–177. (d) Corriu, R. J. P.; Royo, G.; De Saxc
e, A. J. Chem. Soc., Chem. Commun. 1980, 892–894. (e) Klebe, G.; Hensen, K.; Fuess, H. Chem. Ber. 1983, 116, 3125–3132. (f) Farnham, W. B.; Whitney, J. F. J. Am. Chem. Soc. 1984, 106, 3992–3994. (g) Corriu, R. J. P.; Mazhar, M.; Poirier, M.; Royo, G. J. Organomet. Chem. 1986, 306, C5–C9. (h) Macharashvili, A. A.; Shklover, V. E.; Struchkov, Yu. T.; Oleneva, G. I.; Kramarova, E. P.; Shipov, A. G.; Baukov, Yu. I. J. Chem. Soc., Chem. Commun. 1988, 683–685. (i) Boyer, J.; Breliere, C.; Carr
e, F.; Corriu, R. J. P.; Kpoton, A.; Poirier, M.; Royo, G.; Young, J. C. J. Chem. Soc., Dalton Trans. 1989, 43–51. (j) Gudat, D.; Verkade, J. G. Organometallics 1989, 8, 2772–2779. (k) Kobayashi, J.; Kawaguchi, K.; Kawashima, T. J. Am. Chem. Soc. 2004, 126, 16318– 16319. (l) Pongor, G.; Kolos, Z.; Szalay, R.; Knausz, D. J. Mol. Struct. (THEOCHEM) 2005, 714, 87–97. (m) Szalay, R.; Pongor, G.; Harmat, V.; B€ ocskei, Z.; Knausz, D. J. Organomet. Chem. 2005, 690, 1498–1506. (n) Kalikhman, I.; Gostevskii, B.; Kertsnus, E.; Deuerlein, S.; Stalke, D.; Botoshansky, M.; Kost, D. J. Phys. Org. Chem. 2008, 21, 1029–1034. (o) Metz, S.; Burschka, C.; Tacke, R. Organometallics 2008, 27, 6032–6034. (p) Metz, S.; Burschka, C.; Tacke, R. Organometallics 2009, 28, 2311–2317. (q) Fester, G. W.; Wagler, J.; Brendler, E.; B€ohme, U.; Gerlach, D.; Kroke, E. J. Am. Chem. Soc. 2009, 131, 6855–6864. (r) Kalikhman, I.; KertsnusBanchik, E.; Gostevskii, B.; Kocher, N.; Stalke, D.; Kost, D. Organometallics 2009, 28, 512–516. (3) (a) Bassindale, A. R.; Taylor, P. G. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; John Wiley & Sons Ltd.: New York, 1989; Part 1, pp 839-892. (b) Corriu, R. J. P.; Young, J. C. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; John Wiley & Sons: Chichester, U.K., 1989; Part 2, pp 1241-1288. (c) Johnson, S. E.; Day, R. O.; Holmes, R. R. Inorg. Chem. 1989, 28, 3182–3189. (d) Holmes, R. R. Chem. Rev. 1990, 90, 17–31. (e) Kost, D.; Gostevskii, B.; Kalikhman, I. Pure Appl. Chem. 2007, 79, 1125–1134. (f) Prince, P. D.; Bearpark, M. J.; McGrady, G. S.; Steed, J. W. J. Chem. Soc., Dalton Trans. 2008, 271–282. (g) Kost, D.; Kalikhman, I. Acc. Chem. Res. 2009, 42, 303–314. (4) (a) Marsden, C. J. Inorg. Chem. 1983, 22, 3177–3178. (b) Hu, J.; Schaad, L. J.; Hess, B. A., Jr. J. Am. Chem. Soc. 1991, 113, 1463–1464. (c) Alkorta, I.; Rozas, I.; Elguero, J. J. Phys. Chem. A 2001, 105, 743–749. (d) Ignatyev, I. S.; Schaefer, H. F.III. J. Phys. Chem. A 2001, 105, 7665–7671. (e) Fleischer, H. Eur. J. Inorg. Chem. 2001, 393–404. (f) Davydova, E. I.; Timoshkin, A. Y.; Sevastianova, T. N.; Suvorov, A. V.; Frenking, G. J. Mol. Struct. (THEOCHEM) 2006, 767, 103–111. (g) Pierrefixe, S. C. A. H.; Guerra, C. F.; Bickelhaupt, F. M. Chem.;Eur. J. 2008, 14, 819–828. (h) Couzijn, E. P. A.; van den Engel, D. W. F.; Slootweg, J. C.; de Kanter, F. J. J.; Ehlers, A. W.; Schakel, M.; Lammertsma, K. J. Am. Chem. Soc. 2009, 131, 3741–3751. (i) Hall
oczki, O.; Nyul
aszi, L. Organometallics 2009, 28, 4159–4164. pubs.acs.org/Organometallics

Published on Web 10/16/2009

four have been known since the early 19th century.1 Highercoordinate silicon compounds have been of great interest both for experimental2,3 and theoretical4 scientists due to their unusual bonding properties,5 distinct reactivity, and potential as synthons6 in preparing interesting silicon compounds. Moreover, higher-coordinate silicon compounds are involved as reactive intermediates7 and as reagents in preparing organosilicon compounds.8 Ionic higher-coordinate silicon compounds3 are well documented, but similar compounds with neutral ligands are limited.2 N-Heterocyclic carbene (NHC) complexes of silicon halides were first synthesized by Kuhn9 and co-workers in 1995, and very recently SiCl4 adducts with sterically bulky NHC ligands have been used,6c,d but there is no report on similar complexes of SiF4. Furthermore, there is no report on hexacoordinate silicon halide complexes with NHC ligands. Very recently, Nyul
aszi and co-workers4i predicted that the higher-coordinate silicon complexes derived from carbene ligands should be stable. Herein we report on the preparation, characterization, and solid state structures of the first penta- and hexacoordinate SiF4 complexes with N-heterocyclic carbenes as ligands.

Results and Discussion Reaction of SiF4 with IPr in toluene led to the formation of neutral penta- and hexacoordinate complexes IPr 3 SiF4 (1) (5) Kocher, N.; Henn, J.; Gostevskii, B.; Kost, D.; Kalikhman, I.; Engels, B.; Stalke, D. J. Am. Chem. Soc. 2004, 126, 5563–5568. (6) (a) So, C.-W.; Roesky, H. W.; Magull, J.; Oswald, R. B. Angew. Chem., Int. Ed. 2006, 45, 3948–3950. So, C.-W.; Roesky, H. W.; Magull, J.; Oswald, R. B. Angew. Chem. 2006, 118, 4052–4054. (b) So, C.-W.; Roesky, H. W.; Gurubasavaraj, P. M.; Oswald, R. B.; Garmer, M. T.; Jones, P. G.; Blaurock, S. J. Am. Chem. Soc. 2007, 129, 12049–12054. (c) Wang, Y.; Xie, Y.; Wei, P.; King, R. B.; Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. Science 2008, 321, 1069–1071. (d) Ghadwal, R. S.; Roesky, H. W.; Merkel, S.; Henn, J.; Stalke, D. Angew. Chem., Int. Ed. 2009, 48, 5683–5686. Ghadwal, R. S.; Roesky, H. W.; Merkel, S.; Henn, J.; Stalke, D. Angew. Chem. 2009, 121, 5793–5796. (7) (a) Tandura, S. N.; Voronkov, M. G.; Alekseev, N. V. Top. Curr. Chem. 1986, 131, 99–189. (b) Chuit, C.; Corriu, R. J. P.; Reye, C.; Young, J. C. Chem. Rev. 1993, 93, 1371–1448. (c) Chuit, C.; Corriu, R. J. P.; Reye, C. In Chemistry of Hypervalent Compounds; Akiba, K., Ed.; Wiley-VCH: New York, 1999; pp 81-146. (d) Kira, M.; Zhang, L. C. In Chemistry of Hypervalent Compounds; Akiba, K., Ed.; Wiley-VCH: New York, 1999; pp 147-170. (e) Katsukiyo, M.; Hosomi, A. In Main Group Metals in Organic Synthesis; Yamamoto, H.; Oshima, K., Eds.; Wiley-VCH: Weinheim, 2004; Vol. 2, pp 409-592. (f) Rendler, S.; Oestreich, M. Synthesis 2005, 1727–1747. (g) Denmark, S. E. J. Org. Chem. 2009, 74, 2915–2927. (8) (a) Denmark, S. E.; Wynn, T.; Beutner, G. L. J. Am. Chem. Soc. 2002, 124, 13405–13407. (b) Denmark, S. E.; Beutner, G. L.; Wynn, T.; Eastgate, M. D. J. Am. Chem. Soc. 2005, 127, 3774–3789. (c) Denmark, S. E.; Fujimori, S. In Modern Aldol Reactions; Mahrwald, R., Ed.; WileyVCH: Weinheim, 2004; Vol. 2, Chapter 7, pp 229-326. (9) Kuhn, N.; Kratz, T.; Bl€aser, D.; Boese, R. Chem. Ber. 1995, 128, 245–250. r 2009 American Chemical Society

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Scheme 1

Scheme 2

Figure 1. Molecular structure of 1. Anisotropic displacement parameters are depicted at the 50% probability level. H atoms are omitted for clarity. Selected bond distances (A˚) and angles (deg): Si(1)-F(1) 1.6462(13), Si(1)-F(2) 1.5736(15), Si(1)-F(3) 1.5730(17), Si(1)-F(4) 1.5836(17), Si(1)-C(1) 2.004(2); F(3)Si(1)-F(2) 122.64(13), F(3)-Si(1)-F(4) 119.63(12), F(2)-Si(1)-F(4) 117.52(12), F(3)-Si(1)-F(1) 91.49(8), F(2)-Si(1)F(1) 91.44(7), F(4)-Si(1)-F(1) 91.61(8), F(3)-Si(1)-C(1) 86.39(8), F(2)-Si(1)-C(1) 90.04(8), F(4)-Si(1)-C(1) 89.09(8), F(1)-Si(1)-C(1) 177.85(9).

and IPr 3 SiF4 3 IPr (2) (Scheme 1). The formation of these complexes depends on the amount of SiF4 that is added. However, complex 1 is less soluble in toluene and can be isolated as the first recrystallized product of a mixture of 1 and 2. Alternatively, complex 1 can be prepared exclusively by passing an excess of SiF4 gas through the toluene solution of IPr, whereas the reaction of 1 with an equimolar amount of IPr afforded complex 2 (Scheme 2). The five-coordinate SiBr4 complex IPr 3 SiBr4 (3) was prepared by the reaction of SiBr4 with 1 equiv of IPr (Scheme 1). Complexes 1-3 are stable under an inert atmosphere. They are soluble in common organic solvents and can be crystallized as colorless blocks from each of their toluene solutions. 1-3 were characterized by elemental analyses and 1 H, 13C, 19F, and 29Si NMR spectroscopic studies. 1H and 13 C NMR spectra of 1-3 show resonances for the NHC ligand coordinated to the silicon atom. All fluorine atoms of complex 1 appear as a sharp singlet in the 19F NMR spectrum at δ -119.62 ppm with silicon satellite (JSi-F = 202.03 Hz). Complex 1 shows fluxional behavior in solution due to a rapid exchange process on the NMR time scale.10 Even at -78 °C the 19F NMR spectrum of 1 exhibits a sharp singlet. This indicates that even at this temperature a rapid exchange on the NMR time scale is observed. Complex 2 exhibits a 19F NMR resonance at δ -100.32 ppm. 29Si NMR resonances for 1 and 2 appear at δ -141.01 and -184.01 ppm, respectively, consistent with five- and six-coordinate11 silicon complexes. Single-Crystal Structures. The molecular structures of complexes 1, 2, and 3 were determined by single-crystal X-ray

Figure 2. Molecular structure of 2. Anisotropic displacement parameters are depicted at the 50% probability level. H atoms are omitted for clarity. Selected bond distances (A˚) and angles (deg): Si(1)-F(1) 1.6465(17), Si(1)-F(2) 1.6648(17), Si(1)-F(3) 1.6539(17), Si(1)-F(4) 1.6628(16), Si(1)-C(1) 2.0088 (14), Si(1)-C(26) 2.0109 (15); F(1)-Si(1)-F(2) 90.15(13), F(1)-Si(1) -F(4) 89.85(13), F(3)-Si(1)-F(2) 89.97(12), F(3)-Si(1)-F(4) 90.02(12), F(1)-Si(1)-F(3) 179.54(9), F(2)-Si(1)-F(4) 178.66(9), F(1)-Si(1)-C(1) 90.37(7), F(2)-Si(1)-C(1) 90.94(7), F(3)-Si(1)C(1) 90.07(7), F(4)-Si(1)-C(1) 90.41(7), F(1)-Si(1)-C(26) 88.94(7), F(2)-Si(1)-C(26) 90.33(7), F(3)-Si(1)-C(26) 90.61(7), F(4)-Si(1)-C(26) 88.33(7), C(1)-Si(1)-C(26) 178.56(6).

(10) (a) Klanberg, F.; Muetterties, E. L. Inorg. Chem. 1968, 7, 155– 160. (b) Marat, R. K.; Janzen, A. F. Can. J. Chem. 1977, 55, 3845–3849. (c) Brownstein, S. Can. J. Chem. 1980, 58, 1407–1411. (d) Johnson, S. E.; Payne, J. S.; Day, R. O.; Holmes, J. M.; Holmes, R. R. Inorg. Chem. 1989, 28, 3190–3198. (11) (a) Takeuchi, Y.; Takayama, T. In The Chemistry of Organosilicon Compounds; Rappoport, Z.; Apeloig, Y. Eds.; Wiley: New York, 1998; Vol. 2, Chapter 6, pp 267-354. (b) Grimmer, A. -R.; Von-Lampe, V.; M€agi, M. Chem. Phys. Lett. 1986, 132, 549–553.

crystallography and are shown in Figures 1-3. Complexes 1-3 were crystallized as colorless crystals, containing free toluene molecules as lattice solvent. Crystallographic data for 1-3 are summarized in Table 1. Complex 1 crystallizes in the monoclinic space group C2/c. The silicon atom in 1 is fivecoordinate, and the sum of the bond angles around silicon exhibits a distorted trigonal-bipyramidal geometry. Interestingly, in contrast to the reported SiCl4 complexes6c,9 where the

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Ghadwal et al. Table 1. Crystallographic Data and Structure Refinement for 1-3 1 3 1.5C6H5CH3 formula CCDC no. fw cryst size/mm

Figure 3. Molecular structure of 3. Anisotropic displacement parameters are depicted at the 50% probability level. H atoms are omitted for clarity. Selected bond distances (A˚) and angles (deg): Si(1)-Br(1) 2.4106(7), Si(1)-Br(2) 2.2360(4), Si(1)-Br(3) 2.3765(7), Si(1)-C(1) 1.935(2); Br(1)-Si(1)-Br(2) 90.447(19), Br(2)-Si(1)-Br(3) 91.374(19), Br(1)-Si(1)-Br(3) 176.53(3), Br(2)-Si(1)-Br(20 ) 116.67(3), C(1)-Si(1)-Br(1) 86.07(7), C(1)-Si(1)-Br(3) 90.46(7), C(1)-Si(1)-Br(2) 121.611(15).

NHC ligand was at an equatorial position, the NHC ligand in the fluoro analogue IPr 3 SiF4 (1) resides at an axial position. Computational studies4i for NHC 3 SiF4 systems showed low energy difference between the axial and equatorial positioned carbene. Furthermore, the preference of the equatorial position of SiCl4 complexes4i by a NHC ligand is larger than that for the SiF4 analogues. As expected, the Si(1)-F(1) (axial) bond distance (1.6462(13) A˚) is larger than those of the equatorial Si-F bond distances (av 1.5767(17) A˚). The Si-C bond distance (2.004(2) A˚) in 1 is close to the calculated value4i but larger than that observed for the SiCl4 complexes.6c,9 The bond angles of C(1)-Si(1)-F(1), 177.85(9)°, and F-Si-F (trigonal plane, av 119.93(13)°) are slightly deviated from the regular trigonal-bipyramidal geometry. Compound 2 crystallizes in the triclinic space group P1. The NHC ligand and the F atoms in 2 are arranged in a distorted octahedral geometry. The NHC ligands occupy trans positions and the imidazole rings of NHC are tilted by 41.7° with respect to each other, because the sterical demand of the isopropyl substituents is effective even across the planar SiF4 moiety. The Si(1)-C(1) and Si(1)-C(26) bond distances (av 2.0098(15) A˚) are close to that observed for 1. All fluorine atoms around the silicon are present in a distorted tetragonal plane. The mean value (1.657(17) A˚) of the Si-F bond distances in 2 is close to the Si(1)-F(1) (axial) bond length (1.6462(13) A˚) of 1. The C(1)-Si(1)-C(26) bond angle (178.56(6)°) is nearly linear. Complex 3 crystallizes in the orthorhombic space group Pnma. It features a trigonal-bipyramidal geometry at the 5-fold coordinate silicon atom with the NHC ligand occupying an equatorial position like in the SiCl4 complex.6c The Si(1)-C(1) bond distance of 1.935(5) A˚ is smaller than that of 1 but comparable with that of IPr 3 SiCl4. Like in the IPr 3 SiCl4 compound, the Si-Br bond distances for axial Br atoms (av 2.3936(7) A˚) are slightly larger than those of equatorial Si-Br bond lengths (2.2360(4) A˚). Therefore, compound 3 is structurally close to the SiCl4 analogue.

cryst syst space group a/A˚ b/A˚ c/A˚ R/deg β/deg γ/deg V/A˚3 Dcalcd/g cm-3 Z abs coeff/mm-1 θ range/deg reflns collected/ indep reflns max. and min. transmn final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole/e A˚3

2 3 C6H5CH3

3 3 2C6H5CH3

C37.50H48F4N2Si 746183 630.87 0.55  0.25  0.01 monoclinic C2/c 22.332(5) 14.576(4) 24.406(8) 90 114.188(3) 90 7250(1) 1.156 8 0.112 1.72 to 26.55 76 619/7534

C61H80F4N4Si 746185 937.38 0.32  0.14  0.14 triclinic P1 12.7184(11) 14.6505(11) 20.0103(16) 73.5780(10) 85.8090(10) 74.8090(10) 3451.5(5) 0.937 2 0.078 1.06 to 27.12 79 555/15 220

C41H52Br4N2Si 746184 920.58 0.40  0.30  0.15 orthorhombic Pnma 16.0120(16) 14.9509(15) 17.2127(17) 90 90 90 4120.6(7) 1.484 4 3.966 2.21 to 28.35 91 720/5286

[R(int) = 0.0435] 0.9989 and 0.9460 0.0526

[R(int) = 0.0448] 0.9892 and 0.9755 0.0520

[R(int) = 0.0277] 0.5877 and 0.2999 0.0232

0.0744

0.0713

0.0292

0.836 and -0.518

0.403 and -0.320

0.613 and -0.482

Comparison of NHC Complexes of SiF4, SiCl4, and SiBr4. The order of Si-C bond distances (A˚) of 1.928 (SiCl4) ≈ 1.935 (SiBr4) is in good agreement with the theoretical4i ones. The NHC ligands occupy trans positions in distorted octahedral complex 2, and imidazole rings are present in different planes with a dihedral angle of 41.7°. Recently formation of an ionic product of composition [SiBr3(IPr)]Br was reported12 by the reaction of SiBr4 with IPr. The crystal structure of [SiBr3(IPr)]Br shows the presence of tetrahedral [SiBr3(IPr)]þ cations with well-separated Br- counterions, whereas complex 3 exhibits a trigonal-bipyramidal geometry at the five-coordinate silicon. Obviously, the energy differences between the covalent and the ionic species are small. Therefore the generation of single crystals from different solvents might result in different structures.

Conclusion Higher-coordinate silicon complexes continue to generate a great deal of interest due to their unusual structure and reactivity. N-Heterocyclic carbene complexes of SiF4 (1 and 2) and SiBr4 (3) are reported. Formation and structures of complexes 1-3 provide experimental evidence for the theoretically predicted N-heterocyclic carbene-silicon complexes. Preference by an N-heterocyclic carbene to adopt axial and equatorial positions, respectively, in pentacoordinate SiF4 and SiBr4 (also SiCl4) complexes is demonstrated experimentally. The reason for preference for axial or equatorial position by IPr in these complexes may be steric encumbrance. This effect is apparently larger at the axial positions and therefore destabilizes chloro and bromo ana(12) Filippou, A. C.; Chernov, O.; Schnakenburg, G. Angew. Chem., Int. Ed. 2009, 48, 5687–5690. Filippou, A. C.; Chernov, O.; Schnakenburg, G. Angew. Chem. 2009, 121, 5797–5800.

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logues when compared with the fluoro analogue. A comparison of structure and stability of SiF4, SiCl4, and SiBr4 complexes with NHCs is discussed.

Experimental Section General Procedures. All reactions and manipulations were performed under inert atmosphere using standard Schlenk line techniques or a glovebox. The solvents used were purified by MBRAUN solvent purification system MB SPS-800. The Nheterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)imidazol2-ylidene (IPr)13 and SiF4 gas14 were prepared by literature methods. All chemicals received from Aldrich were used without further purification. C6D6 was dried over Na metal and distilled under nitrogen prior to use. 1H, 13C, 19F, and 29Si NMR spectra were recorded using Bruker Avance DPX 200 or Bruker Avance DRX 500 spectrometers. Elemental analyses were obtained from the Analytical Laboratory of the Institute of Inorganic Chemistry at the University of G€ ottingen. Melting points were measured in sealed capillary tubes under nitrogen and were not corrected. Preparation of 1 and 2. To a toluene (100 mL) solution of IPr (5.16 g, 13.28 mmol) was added SiF4 gas at -78 °C for 15 min. The solution turned immediately from light brown to colorless and ultimately resulted in a white slurry. After overnight stirring at room temperature a colorless solution was obtained. Removal of volatiles gave a white solid material, which shows 1H NMR resonances for complexes IPr 3 SiF4 (1) and IPr 3 SiF4 3 IPr (2). Recrystallization from saturated toluene (30 mL) solution at -35 °C afforded complex 1 (3.39 g, 52%) as colorless crystals after 20 h. Finally, the mother liquor was concentrated to 10 mL, and 10 mL of n-hexane was added and the solution was stored at -35 °C for 7 days in a freezer. Colorless crystals of 2 were obtained (1.00 g, 17%). IPr 3 SiF4 (1): colorless crystals. Mp: 235 °C. Anal. Calcd for C27H36F4N2Si (M = 492.67): C, 65.82; H, 7.37; N, 5.69. Found: C, 65.72; H, 7.62; N, 5.67. 1H NMR (200.13 MHz, C6D6, 298 K): δ 1.00 (d, J=6.91 Hz, 12H, CHMe2), 1.32 (d, J=6.77 Hz, 12H, CHMe2), 2.64 (m, 4H, CHMe2), 6.32 (s, 2H, NCH), 7.04-7.25 (m, 6H, C6H3) ppm. 13C{1H} NMR (125.76 MHz, C6D6, 298 K): δ 22.70 (CH(CH3)2), 25.56 (CH(CH3)2), 29.02 (CH(CH3)2), 123.47 (NCH), 124.06 (m-C6H3), 130.62 (p-C6H3), 134.73 (oC6H3), 145.67 (ipso-C6H3) ppm. 19F NMR (188.27 MHz, CFCl3, C6D6, 298 K): δ -119.62 (JSi-F = 202.03 Hz) ppm. 29 Si NMR (99 MHz, C6D6, 298 K): δ -141.01 ppm (quintet, JSi-F = 202.03 Hz). IPr 3 SiF4 3 IPr (2): colorless crystals. Mp: 240 °C. Anal. Calcd for C54H72F4N4Si (M = 881.26): C, 73.60; H, 8.24; N, 6.36. Found: C, 73.49; H, 8.31; N, 6.25. 1H NMR (200 MHz, C6D6, 298 K): δ 1.06 (d, J = 6.86 Hz, 24H, CHMe2), 1.15 (d, J = 6.54 Hz, 24H, CHMe2), 2.77 (m, 8H, CHMe2), 6.18 (s, 4H, NCH), 7.07-7.26 (m, 12H, C6H3) ppm. 13C{1H} NMR (75.47 MHz, C6D6, 298 K): δ 23.21 (CH(CH3)2), 25.65 (CH(CH3)2), 28.68 (CH(CH3)2), 122.47 (NCH), 122.97 (m-C6H3), 128.64 (p-C6H3), 137.90 (o-C6H3), 145.79 (ipso-C6H3) ppm. 19F NMR (188.27 MHz, CFCl3, C6D6, 298 K): δ -100.32 (JSi-F = 182.34 Hz) ppm. 29Si NMR (99 MHz,C6D6, 298 K): δ -184.01 (JSi-F = 182.34 Hz) ppm. Alternative Synthesis of 1. To a toluene (100 mL) solution of IPr (3.58 g, 9.21 mmol) was added SiF4 gas at -78 °C for 30 min. The resulting white slurry was brought to room temperature and (13) Jafarpour, L.; Stevens, E. D.; Nolan, S. P. J. Organomet. Chem. 2000, 606, 49–54. (14) Booth, H. S.; Swinehart, C. F. J. Am. Chem. Soc. 1935, 57, 1333– 1337. (15) (a) Kottke, T.; Stalke, D. J. Appl. Crystallogr. 1993, 26, 615–619. (b) Stalke, D. Chem. Soc. Rev. 1998, 27, 171–178.

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stirred overnight. Then the solution was filtered, and the filtrate was concentrated to 40 mL and stored at -35 °C for 12 h in a freezer. Colorless crystals of 1 were obtained (3.19 g, 70%). Alternative Synthesis of 2. To a flask (100 mL) containing 1 (1.02 g, 2.07 mmol) and IPr (0.81 g, 2.08 mmol) was added toluene (30 mL), and the resulting colorless solution was stirred for 12 h and then concentrated to 15 mL. Addition of n-hexane (10 mL) and storing the solution at -35 °C for 5 days in a freezer gave colorless crystals of complex 2 (0.87 g, 48%). Preparation of IPr 3 SiBr4 (3). To a toluene (100 mL) solution of IPr (3.30 g, 8.49 mmol) was added SiBr4 (1.10 mL, 8.85 mmol), and the resulting white suspension was allowed to stir at room temperature overnight. Volatiles were removed under vacuum, and the white residue was washed with n-hexane (40 mL) and dried to obtain the white solid compound 3 (mp 265 °C) (5.65 g, 90%). Suitable crystals for X-ray crystallography were obtained on storing the saturated toluene solution of 3 at -35 °C for 12 h in a freezer. Anal. Calcd for C27H36Br4N2Si (M=736.29): C, 44.04; H, 4.93; N, 3.80. Found: C, 43.81; H, 4.97; N, 3.69. 1H NMR (200 MHz, C6D6, 298 K): δ 0.93 (d, J = 6.96 Hz, 12H, CHMe2), 1.49 (d, J=6.70 Hz, 12H, CHMe2), 3.31 (m, 4H, CHMe2), 6.49 (s, 2H, NCH), 7.03-7.20 (m, 6H, C6H3) ppm. 29Si NMR (99 MHz, C6D6, 298 K): δ -89.62 ppm. X-ray Structure Determination of 1-3. For the data acquisition of compounds 1-3 crystals were selected at low temperatures and fixed in a drop of oil at the tip of a fiber.15 A Bruker SMART-APEX II diffractometer with a D8 goniometer equipped with a Bruker TXS Mo rotating anode and Quazar mirrors was employed in data collection. The data were integrated with SAINT,16 and an empirical absorption correction (SADABS17 or TWINABS18) was applied. The structures were solved by direct methods (SHELXS-9719) and refined by fullmatrix least-squares against F2 (SHELXL-9720). 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 of their pivot atoms for terminal sp3-carbon atoms and 1.2 times for all other carbon atoms. Disordered moieties were refined using bond length restraints, rigid bond restraints, similarity restraints, and ADP restraints. In the case of 2 only one toluene molecule of the lattice solvent could be described anisotropically, and the program Squeeze of the PLATON21 program suite was used to describe residual electron density from additional solvent molecules located in channels of the crystal. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre. The CCDC numbers are 746183, 746184, and 746185. Copies of the data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Acknowledgment. Financial support from the Deutsche Forschungsgemeinschaft is highly acknowledged. G.T. thanks the Alexander von Humboldt Stiftung for a Fellowship. Supporting Information Available: Crystallographic data for complexes 1-3 as a CIF file. This material is available free of charge via the Internet at http://pubs.acs.org. (16) SAINT v7.46A in Bruker APEX v2.2-0; Bruker AXS Inst. Inc.: Madison, WI, 2005. (17) Sheldrick, G. M. SADABS 2008/1; G€ottingen: Germany, 2007. (18) Sheldrick, G. M. TWINABS 2007/5; G€ottingen: Germany, 2007. (19) Sheldrick, G. M. SHELXS in SHELXTL v6.10; Bruker AXS Inst. Inc.: Madison, WI, 2000. (20) Sheldrick, G. M. Acta Crystallogr. Sect. A 2008, 64, 112–122. (21) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.