Facile 1,2-Migration of a Methyl Group on a {Dimethoxy(methyl)silyl

Jul 19, 2010 - Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, Hirosaki, Aomori 036-8560, ...
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Organometallics 2010, 29, 5296–5300 DOI: 10.1021/om100399s

Facile 1,2-Migration of a Methyl Group on a {Dimethoxy(methyl)silyl}tungsten Complex: Formation of a Base-Stabilized (Dimethoxysilylene)(methyl) Complex† Eiji Suzuki,‡ Takashi Komuro,‡ Yuto Kanno,‡ Masaaki Okazaki,§ and Hiromi Tobita*,‡ ‡

Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan, and §Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, Hirosaki, Aomori 036-8560, Japan Received May 1, 2010

Reactions of Cp*(CO)2W(DMAP)Me [1, Cp* = η5-C5Me5, DMAP = 4-(dimethylamino)pyridine] with trialkylsilanes HSiR2Et (R = Me, Et) afforded (DMAP)(trialkylsilyl)tungsten complexes Cp*(CO)2W(DMAP)(SiR2Et) (2a, R = Me; 2b, R = Et). On the other hand, a reaction of 1 with dimethoxy(methyl)silane, HSi(OMe)2Me, gave a DMAP-stabilized (dimethoxysilylene)tungsten complex, cis-Cp*(CO)2W(Me){dSi(OMe)2 3 DMAP} (cis-3), through 1,2-migration of a methyl group from silicon to tungsten. Silylene complex cis-3 isomerized to its trans-isomer trans-3.

Introduction Interconversion between transition-metal silyl and silylene complexes plays important roles in the metal-mediated transformation of organosilicon compounds such as scrambling of the substituents of silanes, dehydrogenative coupling of hydrosilanes, etc.1 One important reaction that enables such interconversion is 1,2-group-migration between a silyl ligand and a metal center.1 Although the 1,2-migration on silyl complexes is known for several kinds of substituent such † Part of the Dietmar Seyferth Festschrift. This paper is dedicated to Professor Dietmar Seyferth for his outstanding contribution as Editor-inChief of Organometallics. *To whom correspondence should be addressed. Tel: þ81-22-7956539. Fax: þ81-22-795-6543. E-mail: [email protected]. (1) (a) Okazaki, M.; Tobita, H.; Ogino, H. Dalton Trans. 2003, 493, and references therein. (b) Ogino, H. Chem. Rec. 2002, 2, 291. (c) Tilley, T. D. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Chapter 24. (d) Eisen, M. S. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998; Vol. 2; Chapter 35. (2) (a) Sharma, H. K.; Pannell, K. H. Chem. Rev. 1995, 95, 1351, and references therein. (b) Ueno, K.; Masuko, A.; Ogino, H. Organometallics 1997, 16, 5023. (c) Ueno, K.; Sakai, M.; Ogino, H. Organometallics 1998, 17, 2138. (d) Tobita, H.; Kurita, H.; Ogino, H. Organometallics 1998, 17, 2844. (e) Ueno, K.; Asami, S.; Watanabe, N.; Ogino, H. Organometallics 2002, 21, 1326. (f) Tobita, H.; Matsuda, A.; Hashimoto, H.; Ueno, K.; Ogino, H. Angew. Chem., Int. Ed. 2004, 43, 221. (g) Hirotsu, M.; Nunokawa, T.; Ueno, K. Organometallics 2006, 25, 1554. (h) Hashimoto, H.; Sato, J.; Tobita, H. Organometallics 2009, 28, 3963. (3) (a) Waterman, R.; Hayes, P. G.; Tilley, T. D. Acc. Chem. Res. 2007, 40, 712, and references therein. (b) Sakaba, H.; Tsukamoto, M.; Hirata, T.; Kabuto, C.; Horino, H. J. Am. Chem. Soc. 2000, 122, 11511. (c) Watanabe, T.; Hashimoto, H.; Tobita, H. Angew. Chem., Int. Ed. 2004, 43, 218. (d) Ochiai, M.; Hashimoto, H.; Tobita, H. Angew. Chem., Int. Ed. 2007, 46, 8192. (4) Yoo, H.; Carroll, P. J.; Berry, D. H. J. Am. Chem. Soc. 2006, 128, 6038. (5) (a) Burger, P.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 10462. (b) Klei, S. R.; Tilley, T. D.; Bergman, R. G. J. Am. Chem. Soc. 2000, 122, 1816. (c) Klei, S. R.; Tilley, T. D.; Bergman, R. G. Organometallics 2001, 20, 3220. (d) Klei, S. R.; Tilley, T. D.; Bergman, R. G. Organometallics 2002, 21, 4648.

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as silyl groups,2 hydrogen,3 halogen,4 and hydrocarbyl groups5-7 on a silyl ligand, examples of 1,2-migration of alkyl groups involving the cleavage of a relatively inert Si-C(sp3) bond are quite limited. Bergman, Tilley, and coworkers reported that the reaction of Cp*(PMe3)Ir(OTf)Me with trimethylsilane, HSiMe3, produced a methyl(silyl)iridium complex, Cp*(PMe3)Ir(Me)SiMe2OTf, through 1,2-migration of a methyl group on a silyl ligand of [Cp*(PMe3)IrSiMe3]þ to generate a cationic silyleneiridium intermediate, [Cp*(PMe3)Ir(Me)(dSiMe2)]þ.5 Our group has recently found two types of reactions that are presumed to proceed via 1,2-methyl-migration on neutral silyl complexes: (1) reaction of a silyl(silylene)ruthenium complex, Cp*(CO)Ru(dSiMes2)SiMe3, with ROH (R = Me, Et) in the presence of isocyanide resulted in facile alkoxylation on the SiMe3 ligand through multiple 1,2methyl-migrations to form (trialkoxysilyl)ruthenium complexes,2h and (2) reaction of a tungsten complex having a tridentate Si,Si, O-chelate ligand (xantsil), Cp*(CO)W(κ3Si,Si,O-xantsil)(H), with DMAP led to 1,2-migration of a methyl group on a silicon of the xantsil ligand to give a W-Si-N-C four-membered metallacycle.8 However, neutral silylene complexes formed by 1,2-methyl-migration were neither isolated nor observed in these systems. In our recent research, we found that the reaction of the methyltungsten complex Cp*(CO)2W(DMAP)Me (1) [DMAP = 4-(dimethylamino)pyridine] with aryl-substituted hydrosilanes HSiR2Ar [R2Ar = Me2Ph, Me2(p-Tol), Me(p-Tol)2, and (p-Tol)3] resulted in formation of DMAP-stabilized (aryl)(silylene) complexes Cp*(CO)2W(Ar)(dSiR2 3 DMAP).7b We proposed that this reaction proceeded through 1,2-aryl-migration on a 16-electron silyltungsten intermediate, Cp*(CO)2WSiR2Ar, (6) Okazaki, M.; Suzuki, E.; Miyajima, N.; Tobita, H.; Ogino, H. Organometallics 2003, 22, 4633. (7) (a) Suzuki, E.; Okazaki, M.; Tobita, H. Chem. Lett. 2005, 34, 1026. (b) Suzuki, E.; Komuro, T.; Okazaki, M.; Tobita, H. Organometallics 2009, 28, 1791. (8) Begum, R.; Komuro, T.; Tobita, H. Chem. Lett. 2007, 36, 650. r 2010 American Chemical Society

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Figure 1. ORTEP drawing of 2b. Thermal ellipsoids are drawn at the 50% probability level, and all hydrogen atoms are omitted for clarity. Table 1. Selected Bond Distances (A˚) and Angles (deg) in Cp*(CO)2W(DMAP)SiEt3 (2b) W-Si W-C1 Si-W-N1 Si-W-C2 N1-W-C2

2.6350(15) 1.939(6) 132.39(13) 66.45(18) 82.6(2)

W-N1 W-C2 Si-W-C1 N1-W-C1 C1-W-C2

2.229(5) 1.950(6) 71.00(17) 84.8(2) 105.8(2)

generated by the reaction of 1 with HSiR2Ar.7b This result strongly suggests that 1,2-migration of an aryl group from a silyl ligand to a metal center easily occurs on a coordinatively unsaturated silyl complex, and the resultant aryl(silylene) complex can be trapped by DMAP. This concept prompted us to further explore the possibility of 1,2-migration of an alkyl group by using the same Cp*(CO)2W-silyl system. We report here reactions of complex 1 with trialkylsilanes HSiR2Et (R = Me, Et) and dimethoxy(methyl)silane. As a result, the reaction with the latter silane afforded a (dimethoxysilylene)tungsten complex through 1,2-migration of a methyl group.

Results and Discussion Formation of (DMAP)(trialkylsilyl)tungsten Complexes. Reaction of Cp*(CO)2W(DMAP)Me (1)7b with trialkylsilanes HSiR2Et [R = Me (1.5 equiv), Et (1.3 equiv)] in toluene at room temperature gave (DMAP)(silyl) complexes Cp*(CO)2W(DMAP)(SiR2Et) (2a, R = Me; 2b, R = Et) in 56% (for 2a) and 70% (for 2b) isolated yields (eq 1). The 29Si{1H} NMR spectra of 2a and 2b exhibit a signal at 22.9 and 27.0 ppm, respectively. These chemical shifts are comparable with those of the 29Si signals of similar silyltungsten complexes Cp*(CO)2W(DMAP)(SiR2Ar) [15.8-24.3 ppm; R2Ar = Me2Ph, Me2(p-Tol), Me(p-Tol)2, and (p-Tol)3].7 The singlecrystal X-ray analysis revealed that 2b adopts a four-legged piano-stool geometry, where the silyl and the DMAP ligands are located at mutually trans-positions (Figure 1, Table 1). The W-Si bond distance [2.6350(15) A˚] in 2b is comparable with those in the structurally related silyltungsten complexes Cp*(CO)2W(DMAP)(SiR2Ar) [2.6110(13)-2.617(3) A˚; R2Ar = Me2Ph, Me2(p-Tol), and (p-Tol)3].7 The formation of silyl complexes 2a,b by the reaction of 1 with trialkylsilanes is in contrast with that of silylene complexes Cp*(CO)2W(Ar)(dSiR2 3 DMAP) [R = Me, Ar = Ph; R = Me, Ar = p-Tol; R2 = Me(p-Tol), Ar = p-Tol; R = Ar = p-Tol] via 1,2-aryl-migration by the reaction of 1 with arylsilanes HSiR2Ar under similar conditions.7 This clearly shows that

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Figure 2. ORTEP drawing of cis-3. Thermal ellipsoids are drawn at the 50% probability level, and all hydrogen atoms are omitted for clarity. The sites of the methyl ligand and the carbonyl ligand are mutually disordered with the occupancy factors 72:28. The disordered atoms with the higher occupancy factors are labeled with A, namely, O1A, C1A, and C2A, whereas those with the lower occupancy factors are labeled with B, namely, O1B, C1B, and C2B. The former atoms (O1A, C1A, and C2A) are depicted.

1,2-migration of an alkyl group is more difficult than that of an aryl group.

Formation of a DMAP-Stabilized (Dimethoxysilylene)tungsten Complex through 1,2-Methyl-Migration. Silylene complexes are known to be stabilized by introduction of heteroatom substituents on silicon because of the π-donation to the electrophilic silylene silicon atom from a lone pair of the heteroatom.9 We therefore next examined the reaction of 1 with an alkoxy-substituted silane. As a result, complex 1 reacted with dimethoxy(methyl)silane HSi(OMe)2Me (4 equiv) in toluene at room temperature to afford a DMAP-stabilized methyl(silylene) complex, cis-Cp*(CO)2W(Me){dSi(OMe)2 3 DMAP} (cis-3), as yellow crystals in 62% isolated yield (eq 2). This is the first example of the isolation and characterization of a silylene complex formed by 1,2-alkyl-migration.

Characterization of DMAP-Stabilized (Dimethoxysilylene)tungsten Complex cis-3. The structure of silylene complex cis-3 was determined by X-ray crystallography (Figure 2, Table 2). Complex cis-3 adopts a four-legged piano-stool geometry: the (9) (a) Schmedake, T. A.; Haaf, M.; Paradise, B. J.; Millevolte, A. J.; Powell, D. R.; West, R. J. Organomet. Chem. 2001, 636, 17. (b) Straus, D. A.; Grumbine, S. D.; Tilley, T. D. J. Am. Chem. Soc. 1990, 112, 7801. (c) Grumbine, S. D.; Tilley, T. D.; Arnold, F. P.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 7884.

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Table 2. Selected Bond Distances (A˚) and Angles (deg) in cis-Cp*(CO)2W(Me){dSi(OMe)2 3 DMAP} (cis-3) W-Si W-C1B W-C2B Si-O3 Si-N1 Si-W-C1A Si-W-C2A Si-W-C3 C1B-W-C2B W-Si-O4 O3-Si-O4 O4-Si-N1

2.4551(16) 2.26(4) 2.08(3) 1.659(4) 1.913(5) 69.6(3) 75.2(3) 109.8(2) 126.9(14) 115.10(17) 105.1(3) 96.9(2)

W-C1A W-C2A W-C3 Si-O4

2.276(13) 1.949(11) 1.930(6) 1.652(5)

Si-W-C1B Si-W-C2B C1A-W-C2A W-Si-O3 W-Si-N1 O3-Si-N1

78.0(10) 69.6(9) 126.9(5) 122.48(17) 117.98(17) 94.8(2)

tungsten center possesses an η5-C5Me5, a silylene, a methyl, and two carbonyl ligands, in which the methyl ligand and the silylene ligand are located at mutually cis-positions. The W-Si bond distance [2.4551(16) A˚] is within the range of those of base-stabilized silylenetungsten complexes (2.45-2.51 A˚).6 The Si-N1 bond [1.913(5) A˚] is significantly longer than those of normal Si-N covalent bonds (1.70-1.76 A˚).10 This distance is also comparable with those of nitrogen-donor-coordinated silylene complexes [1.908(2)-2.007(9) A˚].2g The sum of the bond angles between the three bonds except the Si-N1 bond around the silicon atom is 342.7(4)°, which is in the middle of the tetrahedral (329°) and trigonal (360°) valence angles. This is characteristic of base-stabilized silylene complexes. The 1H NMR spectrum of cis-3 in C6D6 at room temperature shows one signal assignable to the methyl ligand at 0.46 ppm, which is comparable with that of complex 1 (0.52 ppm).7b The 1 H signal assignable to two mutually diastereotopic methoxy groups on the silylene silicon atom is shown as only one sharp singlet at 3.78 ppm probably due to some dynamic behavior. Since complex cis-3 was thermally unstable in C6D6 even at room temperature, the 29Si{1H} NMR spectrum of cis-3 was measured at 203 K using toluene-d8 as a solvent, in which a signal appeared at 43.9 ppm. The 29Si NMR chemical shift is upfield-shifted compared to those (83.7-86.7 ppm) of the previously reported DMAP-stabilized silylenetungsten complexes Cp*(CO)2W(Ar)(dSiR2 3 DMAP) [R = Me, Ar = Ph, p-Tol; R2 = Me(p-Tol), Ar = p-Tol; and R = Ar = p-Tol].7 This is probably due to a substituent effect of the methoxy groups on silicon in cis-3. A similar upfield shift has been reported for the 29Si signal of the base-stabilized (dialkoxysilylene)chromium complex (CO)5Cr{dSi(O-t-Bu)2 3 HMPA} (12.7 ppm) in comparison with that of a dialkylsilylene analogue (CO)5Cr(dSiMe2 3 HMPA) (101.4 ppm).11 Thermal Isomerization of (Dimethoxysilylene)tungsten Complex cis-3. The cis-silylene complex cis-3 was slowly converted to its trans-isomer trans-Cp*(CO)2W(Me){dSi(OMe)2 3 DMAP} (trans-3) at room temperature in C6D6 (eq 3). The reaction was monitored by 1H NMR spectroscopy. After 3 days, most of cis-3 was consumed and trans-3 was formed as a major product in 53% NMR yield based on the 1H NMR peak intensity. Several small signals of unidentified byproducts were also observed in the 1H NMR spectrum of the reaction mixture. The formation of trans-3 implies that the reaction of 1 with HSi(OMe)2Me initially produced cis-3 as a kinetic-controlled product, and then most of cis-3 isomerized to the thermodynamically more stable trans-3. In a large-scale experiment of the thermal (10) Sheldrick, W. S. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Chapter 3. (11) Leis, C.; Wilkinson, D. L.; Handwerker, H.; Zybill, C.; M€ uller, G. Organometallics 1992, 11, 514.

Scheme 1. Possible Formation Mechanism of Trialkylsilyl Complexes 2a,b and Dimethoxysilylene Complex cis-3

reaction of cis-3 in toluene (eq 3), trans-3 was isolated from the reaction mixture as a yellow powder in 36% yield.

Characterization of Silylene Complex trans-3. Silylene complex trans-3 was characterized by NMR and IR spectroscopic analysis. The 1H NMR spectrum of trans-3 shows a signal of the methyl ligand on the tungsten center at 0.81 ppm, which is downfield-shifted compared to that of cis-3 (0.46 ppm). A similar downfield shift of a 1H signal of a methyl ligand is known for the cis/trans-isomers of the (isocyanide)(methyl)tungsten complexes (η5-C9H7)(CO)2W(Me){CN(t-Bu)} (cis: -0.09 ppm, trans: 0.33 ppm)12a and Cp*(CO)2W(Me){CNSi(p-Tol)3} (cis: -0.04 ppm, trans: 0.01 ppm).12b The 13C{1H} NMR signal of the methyl ligand is observed at -23.7 ppm with 183W satellites (1JWC = 49 Hz). In the 29Si{1H} NMR spectrum, a signal appears at 56.6 ppm with 183W satellites (1JWSi = 148 Hz), which is downfieldshifted compared to that of cis-3 (43.9 ppm). The much larger coupling constant between 183W and 29Si nuclei (1JWSi = 148 Hz) compared to those of 2a (27 Hz) and 2b (28 Hz) is attributable to the strong s-character of the hybrid orbital of silicon used for the W-Si bonding. The IR spectrum of trans-3 shows two bands for a symmetric and an antisymmetric CO vibration, where the intensity of the latter band (1788 cm-1) is stronger than that of the former one (1876 cm-1). This supports the mutually trans arrangement of the two carbonyl ligands in trans-3. Possible Formation Mechanism of Silyl Complexes 2a,b and Silylene Complex cis-3. A possible formation mechanism of 2a,b and cis-3 in the reactions of 1 with hydrosilanes is depicted in Scheme 1. The mechanism is common until the formation of 16-electron silyl complex A, that is (1) dissociation of DMAP from 1, (2) oxidative addition of hydrosilane, and (3) methane elimination to generate complex A. After that, when all the substituents on the silyl ligand of A are alkyl groups, DMAP is simply coordinated to the tungsten (12) (a) Amador, U.; Daff, P. J.; Poveda, M. L.; Ruiz, C.; Carmona, E. J. Chem. Soc., Dalton Trans. 1997, 3145. (b) Suzuki, E.; Komuro, T.; Kanno, Y.; Okazaki, M.; Tobita, H. Organometallics 2010, 29, 1839.

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center to give 2a,b. On the other hand, when the silyl ligand has two methoxy groups and one methyl group, the methyl group undergoes 1,2-migration to generate methyl(silylene)complex B, and then DMAP is coordinated to the silylene silicon to yield cis-3. The driving force for the conversion of A to B is probably the π-donating effect of methoxy groups, which strongly stabilizes the silylene complex intermediate B. The latter mechanism is closely related to that of the formation of aryl(silylene) complexes from the reactions of 1 with arylsilanes that we have reported previously.7

Conclusion (Dimethoxysilylene)tungsten complex cis-3 was formed by the reaction of (DMAP)(methyl) complex 1 with HSi(OMe)2Me through 1,2-methyl-migration on a (dimethoxymethylsilyl)tungsten intermediate. In contrast, the reaction of 1 with trialkylsilanes HSiR2Et (R = Me, Et) gave (DMAP)(silyl) complexes 2a,b without 1,2-alkyl-migration. Thus, the 1,2-alkyl-migration is facilitated by the presence of methoxy substituents on silicon. Silylene complex cis-3 isomerized to the thermodynamically more stable trans-silylene complex trans-3. Further study on the mechanism of 1,2-group-migration on coordinatively unsaturated silyl complexes is in progress.

Experimental Section General Procedures. All manipulations were carried out under dry nitrogen in a glovebox or using a standard high-vacuum line. Benzene-d6, toluene, toluene-d8, and hexane were dried over calcium hydride and distilled before use. All commercially available reagents were used as received. Cp*(CO)2W(DMAP)Me (1) was prepared according to the literature method.7b Physical Measurements. 1H, 13C{1H}, and 29Si{1H} NMR spectra were recorded on a Bruker AVANCE-300 or AVANCE600 spectrometer. 29Si{1H} NMR measurements for products except cis-3 were performed using the DEPT pulse sequence. The 29Si{1H} NMR measurement for cis-3 was carried out using an inverse gate decoupling pulse sequence. The residual proton (C6D5H, 7.15 ppm) and the carbon resonances (C6D6, 128.0 ppm) of deuterated solvents were used as internal references for 1H and 13C resonances, respectively. The pyridyl proton or carbon is abbreviated as py-H or py-C. The 1H signals of pyridyl protons of DMAP were shown as AA0 XX0 multiplets, but coupling constants of the signals could not be determined because peak tops of each signal are poorly resolved. 29Si{1H} NMR chemical shifts were referenced to SiMe4 as an external standard. The NMR data were collected at room temperature unless indicated otherwise. Infrared spectra were recorded with a C6D6 solution placed between KBr plates in a liquid cell or with a KBr pellet using a HORIBA FT-730 spectrometer. Highresolution mass spectra (HRMS) were recorded on a Bruker Daltonics APEX-(III) spectrometer operating in the electrospray ionization (ESI) mode with the addition of NaI to the sample. Measurement of a 29Si{1H} NMR spectrum, mass spectrometric analysis, and elemental analysis were performed at the Research and Analytical Center for Giant Molecules, Tohoku University. Synthesis of Cp*(CO)2W(DMAP)SiMe2Et (2a). A mixture of complex 1 (109 mg, 0.213 mmol) and HSiMe2Et (29 mg, 0.33 mmol) in toluene (2 mL) was allowed to stand at room temperature for 2 days. On cooling the resulting solution at -35 °C, yellow crystals precipitated out of the solution. After removal of the mother liquor, the crystals were washed with hexane (1 mL  2) and dried under vacuum to give 2a 3 0.5toluene as yellow crystals (77 mg, 0.12 mmol) in 56% yield. 1H NMR (300 MHz, C6D6): δ 1.03 (s, 6H, SiMe2),

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Table 3. Crystallographic Data for 2b and cis-3

formula fw cryst size/mm cryst color cryst syst space group a/A˚ b/A˚ c/A˚ β/deg V/A˚3 Z Dcalcd/g cm-3 F(000) μ(Mo KR)/mm-1 reflns collected unique reflns (Rint) refined params R1, wR2 (all data) R1, wR2 [I > 2σ(I)] GOF largest residual peak, hole/e A˚-3

2b

cis-3

C25H40N2O2SiW 612.53 0.11  0.11  0.09 yellow orthorhombic Pbca (No. 61) 13.4319(2) 28.3133(3) 13.60540(10)

C22H34N2O4SiW 602.45 0.39  0.31  0.25 yellow monoclinic P21/n (No. 14) 8.9831(4) 16.8823(9) 15.7119(10) 94.7457(11) 2374.6(2) 4 1.685 1200 4.95 18 997 5405 (0.097) 278 0.045, 0.106 0.039, 0.099 1.15 1.21, -1.29

5174.15(10) 8 1.573 2464 4.53 39 956 5908 (0.055) 290 0.060, 0.096 0.046, 0.085 1.46 1.43, -1.67

1.40-1.58 (m, 5H, SiEt), 1.76 (s, 15H, Cp*), 1.99 (s, 6H, NMe2), 5.32-5.41 (m, 2H, py-H), 8.20-8.29 (m, 2H, py-H). 13C{1H} NMR (75.5 MHz, C6D6): δ 3.1 (SiMe2), 10.9 (SiEt), 11.1 (C5Me5), 14.7 (SiEt), 38.0 (NMe2), 100.9 (C5Me5), 108.0, 153.1, 159.7 (pyC), 243.1 (CO). 29Si{1H} NMR (59.6 MHz, C6D6): δ 22.9 [1JWSi(satellite) = 27 Hz]. IR (C6D6, cm-1): 1871 (w, νCOsym), 1786 (s, νCOasym). HRMS (ESI): m/z calcd for [C23H36N2O2SiW þ Na]þ 607.1948, found 607.1949 [M þ Na]þ. Anal. Calcd for C26.5H40N2O2SiW (2a 3 0.5toluene): C, 50.48; H, 6.39; N, 4.44. Found: C, 50.18; H, 6.19; N, 4.55. Synthesis of Cp*(CO)2W(DMAP)SiEt3 (2b). The title compound was synthesized by a procedure similar to that for 2a using a mixture of 1 (109 mg, 0.213 mmol) and HSiEt3 (31 mg, 0.27 mmol) in toluene (2 mL). 2b was obtained as yellow crystals (93 mg, 0.15 mmol) in 70% yield. 1H NMR (300 MHz, C6D6): δ 1.58 (s, 15H, SiEt3), 1.77 (s, 15H, Cp*), 1.97 (s, 6H, NMe2), 5.34-5.43 (m, 2H, py-H), 8.23-8.33 (m, 2H, py-H). 13C{1H} NMR (75.5 MHz, C6D6): δ 11.1 (C5Me5), 12.0, 12.2 (SiEt3), 38.0 (NMe2), 100.9 (C5Me5), 108.1, 153.1, 159.5 (py-C), 244.4 (CO). 29 Si{1H} NMR (59.6 MHz, C6D6): δ 27.0 [1JWSi(satellite) = 28 Hz]. IR (C6D6, cm-1): 1871 (w, νCOsym), 1788 (s, νCOasym). HRMS (ESI): m/z calcd for [C25H40N2O2SiW þ Na]þ 635.2261, found 635.2263 [M þ Na]þ. Anal. Calcd for C25H40N2O2SiW: C, 49.02; H, 6.58; N, 4.57. Found: C, 48.92; H, 6.86; N, 4.53. Synthesis of cis-Cp*(CO)2W(Me){dSi(OMe)2 3 DMAP} (cis-3). To a solid of 1 (0.15 g, 0.29 mmol) was added a toluene (4 mL) solution of HSi(OMe)2Me (0.13 g, 1.2 mmol). The suspension was exposed to ultrasonic waves for 3 min, and the mixture was stirred at room temperature for 1 h. The resulting dark yellowish-brown solution was concentrated under vacuum, and then the solution was cooled at -30 °C. Yellow crystals precipitated out of the solution. After removal of the mother liquor, the crystals were washed with hexane (1 mL  2) and dried under vacuum to give cis-3 as yellow crystals (0.11 g, 0.18 mmol) in 62% yield. 1H NMR (300 MHz, C6D6): δ 0.46 (s, 3H, WMe), 1.78 (s, 6H, NMe2), 2.11 (s, 15H, Cp*), 3.78 [s, 6H, Si(OMe)2], 5.49-5.56 (m, 2H, py-H), 8.22-8.28 (m, 2H, py-H). 29Si{1H} NMR (119 MHz, toluene-d8, 203 K): δ 43.9. IR (KBr pellet, cm-1): 1867 (s, νCOsym), 1774 (s, νCOasym). Anal. Calcd for C22H34N2O4SiW: C, 43.86; H, 5.69; N, 4.65. Found: C, 43.92; H, 5.65; N, 4.57. The 1H NMR spectrum of a C6D6 solution of the isolated yellow crystals of cis-3 showed some weak signals of byproducts, i.e., unidentified products and trans-3 (see Figure S1 in Supporting Information). Because of thermal instability and insufficient solubility of cis-3 in toluene-d8, we could

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not measure the 13C{1H} NMR spectrum and 183W satellites of the 29 Si{1H} NMR signal. Monitoring a Thermal Isomerization of cis-Silylene Complex cis-3. An NMR tube with a Teflon valve (5 mm o.d.) was charged with cis-3 (5.6 mg, 9.3 μmol), (C5H5)2Fe (less than 1 mg) as an internal standard, and C6D6 (1 mL). The isomerization of cis-3 at room temperature was monitored by 1H NMR spectroscopy. After 3 days at room temperature, the reaction was almost completed. The trans-silylene complex trans-Cp*(CO)2W(Me){dSi(OMe)2 3 DMAP} (trans-3) was formed as a major product in 53% NMR yield. Several small 1H signals of unidentified byproducts were also observed. The NMR yield of trans-3 was determined by comparing the intensity of the Si(OMe)2 signal of trans-3 in the 1H NMR spectrum to that of (C5H5)2Fe (internal standard). Synthesis of trans-Cp*(CO)2W(Me){dSi(OMe)2 3 DMAP} (trans-3). A suspension of cis-3 (94 mg, 0.16 mmol) in toluene (2 mL) was stirred at room temperature for 3 days. Hexane (2 mL) was added to the reaction mixture, and the mixture was allowed to stand at room temperature for 1 day. After removal of the mother liquor, the residue was washed with hexane (1 mL  2) and dried under vacuum to give trans-3 3 0.5toluene as a yellow powder (36 mg, 0.056 mmol) in 36% yield. 1H NMR (300 MHz, C6D6): δ 0.81 [s, 3H, 2JWH(satellite) = 5.0 Hz, WMe], 1.76 (s, 6H, NMe2), 2.09 (s, 15H, Cp*), 3.84 [s, 6H, Si(OMe)2], 5.49-5.54 (m, 2H, py-H), 8.32-8.38 (m, 2H, py-H). 13 C{1H} NMR (75.5 MHz, C6D6): δ -23.7 [1JWC(satellite) = 49 Hz, WMe], 11.0 (C5Me5) 38.1 (NMe2), 51.0 [Si(OMe)2], 99.4 (C5Me5), 105.7, 144.6, 155.3 (py-C), 237.1 (CO). 29Si{1H} NMR (59.6 MHz, C6D6): δ 56.6 [1JWSi(satellite) = 148 Hz]. IR (KBr pellet, cm-1): 1876 (m, νCOsym), 1788 (s, νCOasym). HRMS (ESI): m/z calcd for [C22H34N2O4SiW þ Na]þ 625.1689, found

625.1691 [M þ Na]þ. Anal. Calcd for C25.5H38N2O4SiW (trans-3 3 0.5toluene): C, 47.23; H, 5.91; N, 4.32. Found: C, 46.83; H, 5.74; N, 4.30. The 1H NMR spectrum of a C6D6 solution of the isolated yellow powder of trans-3 3 0.5toluene showed some weak signals of byproducts (see Figure S2 in Supporting Information). X-ray Crystal Structure Determination. Selected crystallographic data for 2b and cis-3 are listed in Table 3. X-ray quality single crystals of 2b and cis-3 were obtained from toluene as yellow block crystals at room temperature. Intensity data for the analysis were collected on a Rigaku RAXIS-RAPID imaging plate diffractometer with graphite-monochromated Mo KR radiation (λ = 0.71069 A˚) under a cold nitrogen stream (T = 150 K). Numerical absorption corrections were applied to the data. The structures were solved by the Patterson method using the DIRDIF-99 program13 and refined by full matrix leastsquares techniques on all F2 data with SHELXL-97.14 For cis-3, the sites of the methyl ligand and a carbonyl ligand were mutually disordered with the occupancy factors 72:28. Except for the disordered atoms, anisotropic refinements were applied to all non-hydrogen atoms, and all the hydrogen atoms were put at calculated positions. The disordered atoms were isotropically refined, and no hydrogen atoms on these atoms were included. CCDC reference numbers: 778344 (2b) and 778345 (cis-3). Crystallographic data are available as a CIF file.

(13) Beurskens, P. T.; Beurskens, G.; de Gelder, R.; Garcia-Granda, S.; Gould, R. O.; Israel, R.; Smits, J. M. M. The DIRDIF-99 Program System; Crystallography Laboratory, University of Nijmegen: The Netherlands, 1999. (14) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.

Supporting Information Available: 1H NMR spectra of complexes cis-3 and trans-3 as a PDF file; X-ray crystallographic data as a CIF file. These materials are available free of charge via the Internet at http://pubs.acs.org.

Acknowledgment. This work was supported by Grants-in-Aid for Scientific Research (Nos. 18350027, 18064003, and 20750040) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. One of the authors (T.K.) is grateful to JGC-S Scholarship Foundation for financial support.