Communication pubs.acs.org/Organometallics
A Silylyne Tungsten Complex Having an Eind Group on Silicon: Its Dimer−Monomer Equilibrium and Cycloaddition Reactions with Carbodiimide and Diaryl Ketones Takashi Yoshimoto,† Hisako Hashimoto,*,† Naoki Hayakawa,‡ Tsukasa Matsuo,‡ and Hiromi Tobita*,† †
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 985-8578, Japan Department of Applied Chemistry, Faculty of Science and Engineering, Kindai University, 3-4-1 Kowakae, Higashi-Osaka 577-8502, Japan
‡
S Supporting Information *
ABSTRACT: A tungsten silylyne complex having a fused-ring group (Eind) on Si was synthesized from a hydrido hydrosilylene complex via stepwise abstraction of a proton and a hydride and was isolated as an eight-membered cyclic dimer bound through two isocarbonyl linkages. The IR and variable-temperature NMR analyses revealed that the dimer is in equilibrium with a monomeric silylyne complex in solution. The silylyne complex underwent [2 + 2] cycoaddition reactions with carbodiimide and diaryl ketones, and in the latter case the cycloaddition product further reacted with another molecule of ketone to give a complex formed through C−C and Si−C coupling.
H
synthetic route to heavier analogues of carbyne complexes, although the yield of A was very low (15%). Herein, we report the synthesis of a new silylyne tungsten complex having an Eind group on Si, Cp*(CO)2WSi(Eind) (1; Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl),7 in a manner similar to that of Astepwise abstraction of a proton and a hydride from Cp*(CO)2(H)WSi(H)(Eind) (3) (Scheme 1). Complex 1 is isolated in high yield as a solid in which 1 takes the dimeric form 2. Importantly, the dimer 2 is in dissociation equilibrium with the monomer 1 in solution, and it reacts smoothly with carbodiimide and diaryl ketones via [2 + 2] cycloaddition.
eavier analogues of carbyne complexes are attracting enormous interest due to their unique reactivity originating from their highly unsaturated ME triple bonds (E = Si, Ge, Sn, Pb).1−3 Since the first report on the synthesis of this type of complex, i.e. a germylyne complex, by Simons and Power in 1996,1 stannylyne2a and plumbylyne2b complexes have successively been synthesized. A common process for the synthesis of all these complexes is the coupling of divalent group 14 halides with transition-metal complexes. Finally, Filippou’s group succeeded in synthesizing the first neutral silylyne complex by the coupling of NHC-stabilized halosilylene with an anionic molybdenum complex followed by dissociation of the NHC.3a This silylyne complex is reported to react with some nucleophiles (LiMe, NMe4Cl, etc.) at the silylyne silicon to give anionic silylene complexes.3b Tilley’s group synthesized a cationic silylyne complex having an OsSi bond by a different method: hydride abstraction from a hydrosilylene complex.3d They reported its [2 + 2] cycloaddition reactions with PhCCPh and PCtBu.3d Reactions of silylyne complexes with organic substrates having polarized unsaturated bonds such as Cδ+Oδ− and Cδ+Nδ−, however, have not yet been reported. Our group has recently reported a new synthetic approach for the neutral silylyne complex Cp*(CO)2WSiTsi (A; Cp* = η5-C5Me5, Tsi = C(SiMe3)3).4 It consists of stepwise abstraction of a proton and a hydride from Cp*(CO)2(H)WSi(H)Tsi (B).5 Treatment of B with MeIiPr (1,3diisopropyl-4,5-dimethylimidazol-2-ylidene) gives the anionic silylene complex [Cp*(CO)2WSi(H)C(SiMe3)3][HMeIiPr] (C),6 and subsequent addition of B(C6F5)3 to C leads to silylyne complex A. This approach has opened up a new © XXXX American Chemical Society
Scheme 1
Received: August 22, 2016
A
DOI: 10.1021/acs.organomet.6b00670 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
(C(21)···C(78), 3.82(1) Å; C(41)···C(58), 4.01(1) Å; sum of van der Waals radii of two Me groups 4.0 Å11). The W−Si bond lengths are 2.3389(14) and 2.3433(14) Å, which are longer than that of a monomeric silylyne complex A (2.213(3) Å)4 and rather close to the shortest WSi double-bond lengths (∼2.34 Å).5,6,12 The Si−O bond lengths are 1.722(4) and 1.730(4) Å, which are much longer than those of normal Si−O single-bond lengths (1.61−1.67 Å),13 indicating that these are coordinate bonds (CO→Si). In the 29Si{1H} NMR spectrum, a signal of 2 appears at 237.8 ppm with a 183W satellite (1JWSi = 303.9 Hz) at 298 K. Notably, the 1JWSi value is comparable to that of monomeric silylyne complex A (316.1 Hz),4 indicative of high s character of the Si orbital used for formation of the W−Si bonds in 2. In the 13 C{1H} NMR spectrum, the signals for terminal CO ligands and isocarbonyl ligands appear at 227.7 and 245.0 ppm, respectively. The IR spectrum of 2 measured in a KBr pellet showed two νCO bands for terminal CO ligands at 1928 and 1863 cm−1, but those for isocarbonyl ligands could not be assigned because of overlap with other bands. However, the IR spectrum in hexane solution clearly exhibited four terminal νCO bands (1944, 1934, 1873, and 1863 cm−1). This observation suggests that 2 is in dissociation equilibrium with the monomeric silylyne complex 1 in solution, and each species provides two terminal νCO bands. A more direct evidence for the dissociation equilibrium was obtained by NMR at various temperatures. The 1H NMR spectrum of a toluene-d8 solution of 2 (9.2 × 10−3 M) showed not only the signals of 2 but also those of 1 (2:1 ≅ 4:1) at 298 K. When the temperature was raised to 360 K, the signals of 2 almost disappeared and those of 1 became predominant. At this temperature, the 29Si{1H} NMR spectrum showed only a signal for 1 at 302 ppm, which is shifted greatly to low field from that of the dimer 2 (237.8 ppm). The chemical shift is close to those of A (339.1 ppm)4 and other silylyne complexes (289−320 ppm),3a,c,d which confirms that 1 is a monomeric silylyne complex. The 13C{1H} NMR spectrum at 360 K shows a single CO signal, which is also compatible with the monomeric structure of 1 with Cs symmetry. We next examined the reactions of 2 with some organic molecules to elucidate the reactivity of the tungsten−silicon triple bond of 1. Addition of a small excess of N,N′diisopropylcarbodiimide to 2 in toluene at room temperature gave the silylene complex Cp*(CO)2WSi(Eind)N(iPr)C N( iPr) (5) having a W−Si−N−C four-membered-ring structure through [2 + 2] cycloaddition of 1 and the carbodiimide (Scheme 2). Complex 5 was isolated in 80% yield, and its structure was confirmed by X-ray crystallography.10 The 29Si{1H} NMR spectrum showed the signal of the silylene ligand at 192.0 ppm, a typical value for silylene complexes. A signal of the amidine carbon bound to tungsten appears at 159.6 ppm in the 13C{1H} NMR spectrum. Reactions of 2 with excess diaryl ketones also afforded the four-membered cyclic silylene complexes Cp*(CO)2WSi(Eind)OC(4-RC6H4)2 (6-H, R = H; 6-Me, R = Me) almost quantitatively. Isolation of 6 has not been achieved on a large scale because of the existence of the reverse reaction from 6 to 2 and benzophenone or di-p-tolyl ketone14 and also a further reaction with the second molecule of the substrate (vide infra). A single crystal of 6-H, however, was obtained by evaporation of a hexane solution of 2 and benzophenone, and its fourmembered-ring structure was unambiguously determined by Xray crystallography (Figure 2). The W(1)−Si(1) distance is
The precursor complex 3 was prepared by the reaction of [Cp*(CO)2W(NC5H5)Me]8 with (Eind)SiH37c in toluene and isolated as red crystals in 63% yield. Characteristic features of the NMR spectra of 3 are essentially similar to those of B.5 Namely, in the 29Si{1H} NMR spectrum, the signal of 3 appears at relatively low field (262.2 ppm; for B, δSi 275.3 ppm). The 1 H NMR spectrum of 3 shows the signals of SiH and WH at 10.62 and −8.97 ppm, respectively, which are also similar to those of B (δH 10.39 ppm (SiH), −10.67 ppm (WH)). The JSiH value (2JSiH = 24.4 Hz) obtained from the 29Si satellite signals of the WH resonance is larger than 20 Hz,9 which implies the existence of a weak interligand interaction between the hydrido and silylene ligands, as has been observed in B.5 In addition, some signals of ethyl groups of 3 are slightly broadened in the 1 H and 13C{1H} NMR spectra at 298 K, probably due to the hindered Si−C(Eind) bond rotation. Treatment of 3 with MeIiPr in THF led to proton abstraction from the tungsten center to give the anionic silylene complex [Cp*(CO)2WSi(H)(Eind)][HMeIiPr] (4) in 78% yield (Scheme 1). The 29Si{1H} NMR signal of the WSi group of 4 was observed in THF-d8 at 173 K as a broad signal at 248 ppm, at low field characteristic of silylene complexes. The signal broadening at this low temperature may be caused by WSi bond rotation and/or hindered Si−C bond rotation. In the 1H NMR spectrum, the signal of SiH is observed at 10.75 ppm and that of the imidazolium proton in [HMeIiPr+] appears at 9.24 ppm. These chemical shifts are comparable with those of complex C (δH 9.99 (SiH), 8.75 ppm (imidazolium-CH)).6 Complex 4 was then treated with B(C6F5)3 in toluene to abstract a hydride on Si, which led to a yellow-orange solution. From this mixture, dimeric silylyne complex 2 was isolated as yellow crystals in 66% yield together with the colorless byproduct [HB(C6F5)3−][HMeIiPr+] in 59% yield. Both 2 and the byproduct were fully characterized spectroscopically and crystallographically.10 The crystal structure of 2 in Figure 1
Figure 1. Molecular structure of 2·0.5C6H14 with thermal ellipsoids at the 50% probability level. H atoms and a hexane molecule (crystal solvent) are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): W(1)−Si(1) 2.3389(14), W(1)−C(2) 1.841(5), W(1)− C(1) 1.942(4), Si(1)−O(4) 1.722(4), W(2)−Si(2) 2.3433(14), W(2)−C(3) 1.968(6), W(2)−C(4) 1.840(5), Si(2)−O(2) 1.730(4); W(1)−Si(1)−C(5) 142.00(16), W(1)−Si(1)−O(4) 122.48(13), W(2)−Si(2)−C(6) 144.16(17), W(2)−Si(2)−O(2) 121.79(13).
reveals that 2 is a dimeric silylyne complex having an eightmembered-ring structure constructed by two isocarbonyl linkages (C(4)−O(4)→Si(1) and C(2)−O(2)→Si(2)). The planar Eind groups enable the dimerization by securing a suitable space around the Si atoms, though there is some steric repulsion between the Cp* groups and the Eind groups B
DOI: 10.1021/acs.organomet.6b00670 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics Scheme 2
Figure 3. Molecular structure of 7-Me·Et2O with thermal ellipsoids at the 50% probability level. H atoms (except for H(1)) and the Et2O molecule are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): W(1)−O(1) 1.946(5), W(1)−C(6) 1.951(10), W(1)− C(7) 1.948(9), O(1)−Si(1) 1.632(5), Si(1)−C(5) 1.893(8), C(5)− C(4) 1.528(10), C(4)−C(3) 1.413(11), C(3)−C(2) 1.538(11), C(2)−O(2) 1.450(9), Si(1)−O(2) 1.649(5); W(1)−O(1)−Si(1) 149.3(4).
that the coupling of two diaryl ketones occurred through o-C− H activation, C−C bond formation, and CO bond cleavage. In the 29Si{1H} NMR spectra, signals of 7-H (−16.7 ppm) and 7-Me (−17.0 ppm) were observed in the normal region for siloxy groups. In the 1H NMR spectra, the signals of the methine bound to Si were observed at 4.55 ppm (7-H) and 4.54 ppm (7-Me). A possible formation mechanism of 7 is depicted in Scheme 3. The first step is a [2 + 2] cycloaddition between 1 and the ketone that gives 6. Then, coordination of another ketone to
Figure 2. Molecular structure of 6-H with thermal ellipsoids at the 50% probability level. H atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): W(1)−Si(1) 2.386(2), W(1)− C(3) 1.944(9), W(1)−C(4) 1.956(9), Si(1)−O(1) 1.617(6), O(1)− C(1) 1.518(10), W(1)−C(1) 2.435(9); W(1)−Si(1)−C(2) 144.1(3), W(1)−Si(1)−O(1) 102.8(2), Si(1)−O(1)−C(1) 95.5(5), W(1)− C(1)−O(1) 104.0(5), Si(1)−W(1)−C(1) 57.6(2), C(3)−W(1)− C(4) 78.0(4).
Scheme 3
2.386(2) Å, which is in the range of reported WSi doublebond lengths (2.34−2.47 Å).5,6,12 The Si(1)−O(1) and O(1)− C(1) distances are 1.617(6) and 1.518(10) Å, respectively. The sum of bond angles around Si(1) (358.3(5)°) is close to 360°, showing that the Si(1) is sp2 hybridized. The four atoms forming the four-membered ring are almost coplanar, as indicated by the dihedral angle W(1)−Si(1)−C(1)−O(1) (−178.0(5)°). It is noteworthy that crystal structure determinations of the [2 + 2] cycloaddition products between metal− element triple-bonded complexes and CO compounds are rare even for carbyne complexes.15 The 29Si{1H} NMR spectra of 6 show the signals of the silylene ligands at low field (6-H, 175.1 ppm; 6-Me, 174.4 ppm) comparable to that of 5 (192.0 ppm). Complexes 6 further reacted with another molecule of ketone slowly to give the siloxy complexes Cp*(CO)2WOSi(Eind){OC(4-RC6H4)2(C6H3R)CH(4-RC6H4)} (7-H, R = H; 7-Me, R = Me) in 63−66% yields after 1 week (Scheme 2). These complexes were isolated by recrystallization from diethyl ether, and the X-ray crystal structure analysis of 7-Me (Figure 3) revealed the existence of a silicon-containing six-membered ring in a siloxy ligand. The formation of this ring system means C
DOI: 10.1021/acs.organomet.6b00670 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
(6) Fukuda, T.; Hashimoto, H.; Sakaki, S.; Tobita, H. Angew. Chem., Int. Ed. 2016, 55, 188−192. (7) (a) Fukazawa, A.; Li, Y.; Yamaguchi, S.; Tsuji, H.; Tamao, K. J. Am. Chem. Soc. 2007, 129, 14164−14165. (b) Matsuo, T.; Suzuki, K.; Fukawa, T.; Li, B.; Ito, M.; Shoji, Y.; Otani, T.; Li, L.; Kobayashi, M.; Hachiya, M.; Tahara, Y.; Hashizume, D.; Fukunaga, T.; Fukazawa, A.; Li, Y.; Tsuji, H.; Tamao, K. Bull. Chem. Soc. Jpn. 2011, 84, 1178−1191. (c) Kobayashi, M.; Hayakawa, N.; Nakabayashi, K.; Matsuo, T.; Hashizume, D.; Fueno, H.; Tanaka, K.; Tamao, K. Chem. Lett. 2014, 43, 432−434. (8) Sakaba, H.; Tsukamoto, M.; Hirata, T.; Kabuto, C.; Horino, H. J. Am. Chem. Soc. 2000, 122, 11511−11512. (9) Schubert, U. Adv. Organomet. Chem. 1990, 30, 151−187. (10) For details, see the Supporting Information. (11) Pauling, L. The Nature of The Chemical Bond, 3rd ed.; Cornell University Press: New York, 1960; pp 260−261. (12) (a) Mork, B. V.; Tilley, T. D. J. Am. Chem. Soc. 2001, 123, 9702−9703. (b) Schmedake, T. A.; Haaf, M.; Paradise, B. J.; Millevolte, A. J.; Powell, D. R.; West, R. J. Organomet. Chem. 2001, 636, 17−25. (c) Ueno, K.; Asami, S.; Watanabe, N.; Ogino, H. Organometallics 2002, 21, 1326−1328. (d) Takanashi, K.; Lee, V. Y.; Yokoyama, T.; Sekiguchi, A. J. Am. Chem. Soc. 2009, 131, 916−917. (13) Kaftory, M.; Kapon, M.; Botoshansky, M. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2, Part 1, Chapter 5, pp 181−265. (14) When iPrNCNiPr was added to a C6D6 solution containing 6-H, all 6-H reacted with the carbodiimide quickly at room temperature to give 5 and benzophenone. Therefore, it is suggested that 6-H is in dissociation equilibrium with 1 and benzophenone, and the formed 1 reacts with carbodiimide to give 5. See section 2.6.4 in the Supporting Information. (15) (a) Weiss, K.; Schubert, U.; Schrock, R. R. Organometallics 1986, 5, 397−398. (b) Freudenberger, J. H.; Schrock, R. R. Organometallics 1986, 5, 398−400. (c) Fischer, E. O.; Filippou, A. C.; Alt, H. G.; Thewalt, U. Angew. Chem., Int. Ed. Engl. 1985, 24, 203− 205. (16) We previously proposed the formation of a metallogermylene intermediate similar to b in the reactions of a germylyne complex (an analogue of A) with aldehydes,16a which was supported by theoretical calculations.16b (a) Fukuda, T.; Hashimoto, H.; Tobita, H. Chem. Commun. 2013, 49, 4232−4234. (b) Ye, X.; Yang, L.; Li, Y.; Huang, J.; Zhou, L.; Lei, Q.; Fang, W.; Xie, H. Eur. J. Inorg. Chem. 2014, 2014, 1502−1511.
the Si center of 6 forms a and o-C−H bond activation on the unsaturated W center in a occurs to give the metallosilylene b.16 A subsequent 1,4-H shift from W to the carbonyl carbon to give c followed by aryl migration from W to the carbonyl carbon of the second ketone forms the W−Si−O threemembered-ring intermediate d. Finally, 1,2-alkyl migration from O to Si and W−Si bond cleavage yield 7. Because no deuterium kinetic isotope effect (kH/kD = 1.0) was observed in an experiment using OC(C6D5)2,10 the rate-determining step (rds) is not the C−H bond cleavage (a → b) but is considered to be the coordination of the second ketone (6 → a). This is supported by the observation of 6 as an intermediate. In conclusion, silylyne complex 1 was synthesized by stepwise abstraction of a proton by MeIiPr and a hydride by B(C6F5)3 from the hydrido(hydrosilylene) complex 3. Complex 1 was isolated as its dimer 2, which was proved to be in dissociation equilibrium with 1 in solution. The monomer 1 in solution reacted with a carbodiimide and diaryl ketones to give four-membered cyclic silylene complexes 5 and 6, respectively, through [2 + 2] cycloaddition reactions. The four-memberedring complex 6 further reacted with another ketone molecule to give siloxy complexes 7 through the coupling of two ketone molecules. Reactions of complex 2 with other organic molecules are now under active investigation.
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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.6b00670. Experimental procedures, spectral data, and crystallographic data (PDF) Crystallographic data for 4, 2, [HMeIiPr][HB(C6F5)3], 5, 6-H, and 7-Me (CIF)
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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 JSPS KAKENHI Grant Nos. JP24109003, JP24109011, JP15H03782, and JP15K05444. REFERENCES
(1) Simons, R. S.; Power, P. P. J. Am. Chem. Soc. 1996, 118, 11966− 11967. (2) (a) Filippou, A. C.; Portius, P.; Philippopoulos, A. I.; Rohde, H. Angew. Chem., Int. Ed. 2003, 42, 445−447. (b) Filippou, A. C.; Rohde, H.; Schnakenburg, G. Angew. Chem., Int. Ed. 2004, 43, 2243−2247. (3) (a) Filippou, A. C.; Chernov, O.; Stumpf, K. W.; Schnakenburg, G. Angew. Chem., Int. Ed. 2010, 49, 3296−3300. (b) Filippou, A. C.; Chernov, O.; Schnakenburg, G. Angew. Chem., Int. Ed. 2011, 50, 1122−1126. (c) Mork, B. V.; Tilley, T. D. Angew. Chem., Int. Ed. 2003, 42, 357−360. (d) Hayes, P. G.; Xu, Z.; Beddie, C.; Keith, J. M.; Hall, M. B.; Tilley, T. D. J. Am. Chem. Soc. 2013, 135, 11780−11783. (4) Fukuda, T.; Yoshimoto, T.; Hashimoto, H.; Tobita, H. Organometallics 2016, 35, 921−924. (5) (a) Watanabe, T.; Hashimoto, H.; Tobita, H. Angew. Chem., Int. Ed. 2004, 43, 218−221. (b) Watanabe, T.; Hashimoto, H.; Tobita, H. Chem. - Asian J. 2012, 7, 1408−1416. D
DOI: 10.1021/acs.organomet.6b00670 Organometallics XXXX, XXX, XXX−XXX
