Synthesis of a Tungsten–Silylyne Complex via Stepwise Proton and

Communication pubs.acs.org/Organometallics

Synthesis of a Tungsten−Silylyne Complex via Stepwise Proton and Hydride Abstraction from a Hydrido Hydrosilylene Complex Tetsuya Fukuda, Takashi Yoshimoto, Hisako Hashimoto,* and Hiromi Tobita* Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan S Supporting Information *

ABSTRACT: Treatment of the hydrido hydrosilylene complex Cp*(CO)2(H)WSi(H)Tsi (1; Tsi = C(SiMe3)3) with 1 equiv of MeIiPr (MeIiPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) caused deprotonation to afford the anionic silylene complex [Cp*(CO)2W Si(H)Tsi][HMeIiPr] (2). The hydrogen on the silylene ligand of 2 was then abstracted as a hydride with 2 equiv of B(C6F5)3 to give a mixture of 1 and tungsten−silylyne complex Cp*(CO)2WSiTsi (4), from which 4 was isolated in pure form. The molecular structure of 4 was determined by X-ray crystal structure analysis.

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Scheme 1. Our Synthetic Strategy to a Silylyne Complex

omplexes having a multiple bond between M (transition metal) and Si are attracting considerable attention in the fields of organometallic and coordination chemistry, and great efforts have been made to clarify the nature and reactivity of their multiple bonds. While reports on the synthesis and reactivity of silylene complexes are increasing,1,2 those of silylyne complexes,3,4 i.e. the M−Si triple-bonded species, are still scarce. Tilley and co-workers utilized X− (X = Cl, H) abstraction from halo- or hydrosilylene complexes to obtain the cationic silylyne complexes [Cp*(dmpe)(H)MoSiMes][B(C6F5)4] (A; Cp* = η5-C5Me5, dmpe = PMe2CH2CH2PMe2)3a and [Cp*(PiPr3)(H)OsSiTrip]+ (B),3b the latter of which was unstable at ambient temperature (t1/2 = 30 min). Filippou’s group employed NHC (N-heterocyclic carbene) abstraction from zwitterionic silylene complexes with B(p-tol)3 and isolated the stable silylyne complexes (η5-C5H5)(CO)2MoSi(C6H32,6-Trip2) (C; Trip = C6H2-2,4,6-iPr3)3c and [Cp*(CO)2Cr Si−SIdipp]+ (D; SIdipp = 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene).3d These accomplishments in synthesizing thermally stable silylyne complexes appears to be indebted to a major progress in the methodology for preparation of lowvalent silicon species utilizing NHCs.5 During our research on the reactivity of the neutral silylene complex Cp*(CO)2(H)WSi(H)Tsi (1; Tsi = C(SiMe3)3),6 we recently found that treatment of a pyridine adduct of 1, Cp*(CO)2(H)WSi(H)Tsi(py) (1-py), with MeIiPr (MeIiPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) gave the anionic silylene complex [Cp*(CO)2WSi(H)Tsi][HMeIiPr] (2).7 Interestingly, the νSiH value in the IR spectrum of 2 (1981 cm−1) was greatly shifted to a lower wavenumber in comparison with that of 1 (2052 cm−1), indicating the existence of a very weak Si−H bond in 2. We therefore expected that H− abstraction from an anionic silylene complex would proceed readily to give a neutral silylyne complex (Scheme 1). Here, we report the first synthesis of the tungsten−silylyne complex Cp*(CO)2WSiTsi (4), having an alkyl substituent © XXXX American Chemical Society

on the silylyne silicon, on the basis of this hypothesis. The synthesis of 4 was accomplished by a stepwise proton and hydride abstraction from 1 with MeIiPr and B(C6F5)3. We also isolated [Cp*(CO){(C6F5)3B···OC}WSi(H)Tsi][HMeIiPr] (3), a borane adduct of 2, as an intermediate of the hydride abstraction (Scheme 2). Treatment of 1 with 1 equiv of MeIiPr smoothly gave the anionic hydrosilylene complex 2 in 84% yield via H+ abstraction with MeIiPr. Spectral data of the product agreed well with those of previously reported 2.7 We then tried to abstract the H− Scheme 2. Synthesis of Tungsten−Silylyne Complex 4

Received: February 4, 2016

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DOI: 10.1021/acs.organomet.6b00095 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics from the Si−H moiety of 2 by using 1 equiv of B(C6F5)3. However, instead of the desired H− abstraction, coordination of the borane to one of the CO ligands proceeded at room temperature to give the anionic silylene complex [Cp*(CO){(C6F5)3B···OC}WSi(H)Tsi][HMeIiPr] (3) quantitatively. Complex 3 was isolated in 95% yield as red crystals after recrystallization and was characterized by NMR and IR spectroscopy as well as elemental analysis. The 1H NMR spectrum of 3 shows an SiH signal at 9.99 ppm with two sets of satellite signals caused by coupling with 183W (2JWH = 20.1 Hz) and 29Si (1JSiH = 119.6 Hz). A signal for the aromatic CH proton (8.75 ppm) is observed in the region for the NCHN of imidazolium salts (8.1−11.1 ppm),8 indicating the preservation of the countercation [HMeIiPr]+. In the 29Si NMR spectrum, two resonances are observed at −4.8 and 333.7 ppm. The latter is assigned to the silylene silicon on the basis of its characteristic downfield shift as well as its satellite signals caused by coupling with 183W (1JWSi = 261.6 Hz). The 11B signal of the coordinating B(C6F5)3 moiety (−2.3 ppm) is shifted upfield in comparison with that of free B(C6F5)3 (59.4 ppm).9 Typically, the chemical shift values of four-coordinated borates are lower (shifted more upfield) than those of threecoordinated boranes (four-coordinated, −130 to +10 ppm; three-coordinated, −10 to +90 ppm).10 The observed upfield shift therefore supports the four-coordinated geometry of the boron center in 3. The IR spectrum of 3 shows νCO bands at 1907 cm−1 and around 1535 cm−1. The latter is assignable to the CO ligand coordinated by B(C6F5)3, which is greatly shifted to a lower wavenumber.11 Indeed, this considerable weakening of the C−O bond is attributable to increased dπ(W) → π*(CO) back-donation. In the 13C NMR spectrum of 3, the signal assignable to the CO ligand coordinated by B(C6F5)3 (257.0 ppm) is shifted to a much lower field, which is the region for the carbyne carbons of tungsten−carbyne complexes (255−357 ppm),12 in comparison with that for the other CO ligand (229.8 ppm). These observations indicate the significant contribution of the canonical form 3-II in the resonance hybrid (Chart 1).

Figure 1. ORTEP drawing of 3. The thermal ellipsoids are plotted with 50% probability. Hydrogen atoms, except H48c, and the countercation are omitted for clarity. Selected bond distances (Å) and angles (deg): W1−Si1 = 2.362(3), W1−C1 = 1.932(10), C1−O1 = 1.193(12), W1−C2 = 1.879(9), C2−O2 = 1.249(11), O2−B1 = 1.557(12), Si1−H1 = 1.54(10), Si1−C13 = 1.900(10); C1−W1−C2 = 89.1(4), C1−W1−Si1 = 91.7(3), C2−W1−Si1 = 95.2(3), W1−Si1− C13 = 144.9(3), C13−Si1−H1 = 102(4), H1−Si1−W1 = 113(4), C2−O2−B1 = 130.7(8).

lengths for four-coordinated aryl borates (1.43−1.69 Å), while it obviously falls outside the range for three-coordinated aryl boranes (1.29−1.41 Å).13 Using this borane adduct 3 as a precursor, we next tried the H− abstraction from its Si−H moiety. Reaction of 3 with B(C6F5)3 in C6D6 proceeded at room temperature to give the desired silylyne complex Cp*(CO)2WSiTsi (4) together with the starting silylene complex 1 (4:1 ≅ 1:1 by 1H NMR) and some unidentified compounds. The formation of a compound having a B−H moiety, which was tentatively assigned to [HMeIiPr][HB(C6F5)3],14 was also observed, indicating that the hydrogen of the Si−H moiety of 3 was abstracted as a hydride. The reason for the formation of neutral complex 1 remains unclear. In a large-scale experiment, silylyne complex 4 was eventually isolated as air-sensitive orange crystals in 15% yield by recrystallization from n-hexane and was fully characterized on the basis of NMR and IR spectra and elemental analysis. The 1H NMR spectrum of 4 shows only two singlet signals, assigned to the SiMe3 group (0.35 ppm) and the η5-C5Me5 ligand (2.11 ppm). In the 13C NMR spectrum of 4, the C(SiMe3)3 quaternary carbon signal appears at 54.8 ppm, which is shifted significantly downfield from the corresponding signal of 1 (24.0 ppm).6 A similar tendency was also observed in their germanium analogues (C(SiMe3)3: 32.6 ppm for Cp*(CO)2(H)WGe(H){C(SiMe3)3} (E),15 66.2 ppm for Cp*(CO)2WGe{C(SiMe3)3} (F)).16 A sharp singlet assignable to two CO ligands appears at 224.9 ppm, indicating that this molecule is Cs symmetric. The 29Si NMR spectrum of 4 displays two signals at −5.2 and 339.1 ppm. The signal at 339.1 ppm is assignable to the silylyne silicon, whose chemical shift is comparable to those of reported silylyne complexes B and C (B, 321 ppm;3b C, 320 ppm3c). This silylyne silicon signal is accompanied by satellite signals with a large 1JWSi coupling constant (316.2 Hz), which is much larger than those for basefree tungsten−silylene complexes (91−155 Hz),6,17 indicating that the W and Si atoms in 4 use the orbitals having very high s character to form the W−Si bond.18 In the IR spectrum, the two CO stretching absorption bands appear at 1900 and 1832 cm−1.

Chart 1. Possible Canonical Structures for 3

The B(C6F5)3-coordinated structure of 3 was confirmed by an X-ray diffraction study (Figure 1). The W−Si length (2.362(3) Å) is slightly greater than that in the borane-free anionic silylene complex 2 (2.3367(17) Å); these are the shortest W−Si double bonds ever reported.6,13 The W−C length for the borane-coordinated CO ligand (1.879(9) Å) is significantly shortened in comparison with that for the terminal CO ligand (1.932(10) Å) and is nearly comparable to those in tungsten−carbyne complexes (1.81−1.87 Å).12 On the other hand, the C−O length in the borane-coordinated CO ligand (C2−O2 1.249(11) Å) is much longer than that in the terminal CO ligand (C1−O1 1.193(12) Å). These observations clearly indicate that the π-back-donation from W to a CO ligand is strongly enhanced by coordination of a B(C6F5)3 to the oxygen atom of the CO. The B1−O2 length (1.557(12) Å) lies roughly in the middle of the upper and lower ends of B−O bond B

DOI: 10.1021/acs.organomet.6b00095 Organometallics XXXX, XXX, XXX−XXX

Organometallics

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The crystal structure of 4 (Figure 2) was unambiguously determined by X-ray crystallography. The W−Si bond length of

Communication

AUTHOR INFORMATION

Corresponding Authors

*E-mail for H.H.: [email protected]. *E-mail for H.T.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grants-inaid for Scientific Research Nos. 24109011, 15H03782, 15K05444, and 25·1537) and JSPS Research Fellowships for Young Scientists (T.F.).

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Figure 2. ORTEP drawing of 4. The thermal ellipsoids are plotted with 50% probability. The Cp* ligand is disordered at two positions with occupancy factors for both at 50%. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): W1−Si1 = 2.2297(9), W1−C1 = 1.961(4), W1−C2 = 1.950(4), Si1−C23 = 1.833(3), C(23)−Si(2) = 1.928(8), C(23)−Si(3) = 1.926(3), C(23)− Si(4) = 1.910(3); C1−W1−C2 = 92.18(16), C1−W1−Si1 = 87.45(11), C2−W1−Si1 = 85.20(11), W1−Si1−C23 = 173.71(11).

4 (2.2297(9) Å) is 5.9% shorter than that of 1 (2.3703(11) Å)6 and is the shortest known W−Si bond.13 The W−Si−C angle is almost linear (173.71(11)°) and is close to the corresponding angles of reported silylyne complexes (C, 173.49(8)°;3c D, 169.76(9)°3d). The DFT-optimized structure of a model complex of 4, Cp(CO)2WSi{C(SiH3)3} (4′), well reproduces the X-ray crystal structure of 4. NBO analysis and Kohn− Sham molecular orbitals of 4′ clearly indicate the presence of one σ bond and two π bonds between the W and Si atoms (see Tables S7 and S8 and Figure S18 in the Supporting Information). All of these results confirm that 4 is a tungsten−silylyne complex. In conclusion, we succeeded in synthesizing the first example of a tungsten−silylyne complex, 4, via stepwise proton and hydride abstraction from the hydrido hydrosilylene complex 1 with MeIiPr and B(C6F5)3. Complex 4 is also the first example of a silylyne complex having an alkyl substituent on the silylyne silicon. A B(C6F5)3 adduct of an anionic silylene complex, 3, was also isolated and characterized as an intermediate of the hydride abstraction. This stepwise methodology will provide further insight into the synthesis of silylyne complexes and their congeners. Research on the reactivity of 4 toward organic substrates and small molecules is in progress.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00095. Experimental procedures, spectral and crystallographic data, and computational details (PDF) Crystallographic data for 3 (CIF) Crystallographic data for 4 (CIF) Cartesian coordinates for the computed structures (XYZ) C

DOI: 10.1021/acs.organomet.6b00095 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics (11) The assignment of νCO values is supported by theoretical calculations on a model of 3 having Cp instead of Cp*: calcd 1960 (CO) and 1595 (CO···B) cm−1. The difference (53−60 cm−1) between the experimental and calculated values is ascribable to the replacement of Cp* by Cp in the model. (12) For references on tungsten−carbyne complexes containing a (η5-C5R5)(CO)WCR′ fragment, see: (a) Huang, H.; Hughes, R. P.; Rheingold, A. L. Dalton Trans. 2011, 40, 47. (b) Sakaba, H.; Yoshida, M.; Kabuto, C.; Kabuto, K. J. Am. Chem. Soc. 2005, 127, 7276. (c) Hulkes, A. J.; Hill, A. F.; Nasir, B. A.; White, A. J. P.; Williams, D. J. Organometallics 2004, 23, 679. (d) Filippou, A. C.; Portius, P.; Jankowski, C. J. Organomet. Chem. 2001, 617-618, 656. (e) Filippou, A. C.; Portius, P.; Winter, J. G.; Kociok-Köhn, G. J. Organomet. Chem. 2001, 628, 11. (f) Carter, J. D.; Kingsbury, K. B.; Wilde, A.; Schoch, T. K.; Leep, C. J.; Pham, E. K.; McElwee-White, L. J. Am. Chem. Soc. 1991, 113, 2947. (g) Greaves, W. W.; Angelici, R. J.; Helland, B. J.; Klima, R.; Jacobson, R. A. J. Am. Chem. Soc. 1979, 101, 7618. (h) Fischer, E. O.; Hollfelder, H.; Friedrich, P.; Kreissl, F. R.; Huttner, G. Angew. Chem., Int. Ed. Engl. 1977, 16, 401. (13) Bond lengths have been retrieved from the Cambridge Structural Database (CSD version 5.35). (14) Although we were not able to isolate the borane compound, 1H, 11 B, and 19F NMR spectra support the formula [HMeIiPr][HB(C6F5)3] for it. See section 2-3(a) and Figure S9 in the Supporting Information. (15) Hashimoto, H.; Tsubota, T.; Fukuda, T.; Tobita, H. Chem. Lett. 2009, 38, 1196. (16) Hashimoto, H.; Fukuda, T.; Tobita, H.; Ray, M.; Sakaki, S. Angew. Chem., Int. Ed. 2012, 51, 2930. (17) (a) Watanabe, T.; Hashimoto, H.; Tobita, H. Chem. - Asian J. 2012, 7, 1408. (b) Ueno, K.; Asami, S.; Watanabe, N.; Ogino, H. Organometallics 2002, 21, 1326. (18) NBO analysis for the model silylyne complex (η5-C5H5) (CO)2WSi{C(SiH3)3} (4′) indicates high s character of the W and Si orbitals used for forming the W−Si σ bond (W, 31.4% s, 24.1% p, 44.5% d; Si, 59.5% s, 40.5% p). Especially, the s character of the Si orbital of 4′ is higher than those of anionic silylene complexes [(η5C5H5)(CO)2WSi(H){C(SiH3)3}]− (2′) (Si, 50.9% s, 48.9% p)7 and [(η5-C5H5)(CO){(C6F5)3B···OC}WSi(H)C(SiH3)3}]− (3′) (Si, 47.9% s, 51.9% p) undoubtedly because of the difference in hybridization. For details, see Table S6 (for 3′) and Table S8 (for 4′) in the Supporting Information.

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DOI: 10.1021/acs.organomet.6b00095 Organometallics XXXX, XXX, XXX−XXX