Communication Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis and Functionalization of a 1,4-Bis(trimethylsilyl)tetrasila1,3-diene through the Selective Cleavage of Si(sp2)−Si(sp3) Bonds under Mild Reaction Conditions Naohiko Akasaka,† Kentaro Fujieda,† Eleonora Garoni,‡,§ Kenji Kamada,‡ Hiroshi Matsui,∥ Masayoshi Nakano,∥ and Takeaki Iwamoto*,† †
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan IFMRI, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan § Dipartimento di Chimica, Università degli Studi di Milano, via Golgi 19, Milan 20133, Italy ∥ Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan ‡
S Supporting Information *
ABSTRACT: Although the oxidative coupling of disilenides, i.e., the disilicon analogues of vinyl anions, represents a promising route to extend the conjugation between SiSi double bonds, previously reported synthetic routes to disilenides involve strongly reducing conditions. Herein, we report a novel synthetic route to disilenides from stable disilenes via the selective cleavage of Si(sp2)−Si(sp3) bonds under milder reaction conditions. Using this method, a 1,4bis(trimethylsilyl)tetrasila-1,3-diene (5) was synthesized from the corresponding silyl-substituted disilene. Moreover, Et3Si-substituted tetrasila-1,3-diene 7 was synthesized via tetrasila-1,3-dien-1-ide 6, which is the first example of a functionalized tetrasila-1,3-diene. he extension of π-electron systems represents a promising strategy for the development of novel π-electron compounds with new electronic and optical properties. Compounds that contain Si−Si double bonds, i.e., disilenes, have attracted substantial interest as π-electron units that exhibit an intrinsically narrow HOMO−LUMO (π−π*) gap, and various derivatives of such disilenes have already been reported.1 Among these, silicon analogues of polyenes, i.e., persilapolyenes, have been predicted theoretically as promising compounds with efficient nonlinear optical properties.2 Nevertheless, so far, conjugated disilenes, in which SiSi double bonds are directly connected to each other by a Si−Si single bond, have remained limited to tetrasila-1,3-dienes 1−4 (Chart 1).3−6 Notably, all synthetic routes to 1−4 involve strongly reducing conditions, e.g. the reduction of halosilanes or stable disilenes with alkali metals. For instance, 1 was obtained from the reduction of Tip2SiSiTip2 (Tip = 2,4,6-triisopropylphenyl) with lithium, which provided the corresponding lithium disilenide Tip2SiSiTipLi, i.e., a disilicon analogue of a vinyl anion, followed by an oxidation with mesityl bromide.3 Compounds 2−4 were obtained from the reduction of the corresponding halosilanes using a one-electron reducing agent. However, such strongly reducing conditions would not be suitable for the synthesis of conjugated disilenes with three or more SiSi double bonds, as these persilapolyenes are anticipated to exhibit low-lying LUMOs that would probably undergo undesired subsequent reductions under these
T
© XXXX American Chemical Society
Chart 1. Tetrasila-1,3-dienes
conditions. For example, the central Si−Si single bond connecting two SiSi bonds in 2 is cleaved upon treatment with t-BuLi or KC8 to afford the corresponding disilenide.4 For the extension of SiSi double bonds under mild conditions, the oxidative coupling of disilenides to form a Si−Si single bond should be promising, as proposed for the conversion of disilenide Tip2SiSiTipLi to 1.1e,f,7 However, the formation of hitherto known disilenides requires strongly reducing agents such as alkali metals.7−9 To generate a disilenide in the absence of a one-electron reducing agent, we Received: December 3, 2017
A
DOI: 10.1021/acs.organomet.7b00864 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics focused our attention on a conversion of silyl-substituted silanes to silyl anions via a desilylation, using a base such as MeLi or t-BuOK.10 In this context, it has been reported that reactions of tetrasilyldisilenes with MeLi provide the corresponding 1,2-adducts across the SiSi double bond,11 while Marschner et al. have reported that the reaction of t-BuOK and 1,2-bis(triisopropylsilyl)-1,2-bis(trimethylsilyl)disilene affords the 1,2-adduct rather than the disilenide.12 On the basis of these results, we anticipated that disilenes which contain sterically more demanding groups on the SiSi double bond should suppress the undesired addition of a base to the SiSi double bond in favor of the selective cleavage of the Si(sp2)−R bond, which would provide the desired disilenide. Herein, we report the selective cleavage of Si(sp2)−Si(sp3) bonds on the SiSi double bond of disilenes by using t-BuOK to afford the corresponding disilenide. Using this reaction, we synthesized 1,4-bis(trimethylsilyl)tetrasila-1,3-diene 5 from the corresponding silyl-substituted disilene, as well as SiEt3-substituted tetrasiladiene 7 through the generation of tetrasila-1,3-dien-1ide 6, which is the first example of a functionalized tetrasila-1,3diene. As shown in Scheme 1, 5 was obtained from (Me3Si)TipSi SiTip(SiMe3) (8).13,14 The reaction of 8 with a stoichiometric
Figure 1. ORTEP drawings of (a) 5ap and (b) 5sc (thermal ellipsoids set at 50% probability; hydrogen atoms omitted for clarity). For 5ap, only one of the three crystallographically independent, but structurally comparable, molecules in the asymmetric unit is shown.
central Si−Si single bonds that link the two double bonds of 5ap (2.2801(12)−2.2862(11) Å) are significantly shorter than those of 5sc (2.2902(2) Å) and other acyclic tetrasiladienes (2.321(2)−2.3470(12) Å).3−5 These structural features suggest that, in single crystals of 5ap, the π conjugation between the two SiSi double bonds is more effective in comparison to the conjugation between those in other tetrasiladienes, although contributions from steric effects cannot be ruled out completely at this point. The electronic properties of 5 in solution were examined by UV−vis absorption spectroscopy. As two conformers of 5 (5ap and 5sc) exist in solution, the observed absorption spectrum should be the sum of the individual absorption spectra, and accurate molar coefficients could thus not be estimated (for details, see the Supporting Information). The longest-wavelength absorption band (537 nm) is substantially red shifted in comparison to those of disilene 8 (394 nm) and other acyclic tetrasiladienes (438−531 nm),3−5 indicative of effective π conjugation between the SiSi double bonds of 5 in solution. It has been known that the intermediate open-shell nature of a molecule enhances its third-order nonlinear optical processes such as the two-photon absorption.16 Thus, we examined the multiphoton absorption property of 5 experimentally by the femtosecond open-aperture Z-scan method.17 We found that 5 only exhibits a weak multiphoton absorption band for 5 at ∼1000 nm (∼4 GM in the two-photon absorption cross section) in hexane or THF. To the best of our knowledge, this is the first observation of a multiphoton absorption band of disilenes. This result is consistent with the results of a previously reported theoretical study which suggests that the parent tetrasila-1,3-diene exhibits a weak longitudinal second hyperpolarizability on account of its weak open-shell nature.2b In order to examine the details of the structural features of 5, we used DFT calculations at the B3PW91-D3 level of theory on model compound 5′, in which the Tip groups were replaced by 2,6-diisopropylphenyl (Dip) groups. The results thus obtained were in good agreement with those experimentally obtained for 5. Two conformers of 5′ that adopt antiperiplanar (5′ap, δ = 163.6°) and synclinal conformations (5′sc, δ = 65.5°) were located as local minima, and 5′sc was only 5.2 kJ/mol more stable in free energy (298.15 K) than 5′ap, which is consistent with the notion of assigning two conformers on the basis of the NMR spectra of 5. The longer SiSi and shorter Si−Si distances of 5′ap (2.1645 and 2.276 Å) in comparison to those of 5′sc (2.157 and 2.284 Å) are also in good agreement with the structural features obtained from the XRD analysis of 5. The
Scheme 1. Synthesis of 5 from 8 via Disilenide 9
amount of t-BuOK afforded the corresponding desilylated disilene (9), which was characterized by a combination of multinuclear NMR spectroscopy and its reactivity. In this reaction, the 1,2-addition of t-BuOK across the SiSi double bond was not observed. To the best of our knowledge, this is the first example of the conversion of a stable disilene to a disilenide in the absence of a one-electron reducing agent.7−9 After THF was replaced with toluene, treatment of 9 with 1,2dibromoethane afforded purple crystals of 5 in 77% yield. The NMR spectra of 5 exhibit two sets of signals due to two species, whose ratio depends on the temperature (Figure S12 in the Supporting Information). This result indicates an equilibrium between two conformers of 5, which differ with respect to the spatial arrangement of the SiSi double bond relative to the Si−Si single bonds. Unfortunately, resolving the thermodynamic parameters for this equilibrium was unsuccessful due to the overlapping signals. Recrystallization of 5 from Et2O at −35 °C furnished purple and red single crystals. The molecular structure of these single crystals15 was determined by single-crystal X-ray diffraction analysis (Figure 1). In the purple and red crystals, 5 adopts antiperiplanar (5ap) and synclinal (5sc) conformations, respectively. The asymmetric unit of 5ap contains three crystallographically independent molecules. The dihedral angles of the SiSi double bonds in the three molecules of 5ap (δ = 165.65(6), 161.41(6), and 156.57(6)°) demonstrate higher levels of coplanarity for the two double bonds of 5ap in comparison to those of 5sc (δ = 60.81(6)°) and other reported tetrasiladienes. The SiSi double bonds in 5ap (2.1599(12)− 2.1755(12) Å) are slightly longer than those of 5sc (2.1592(7), 2.1617(7) Å) and 8 (2.152(3) Å) and fall within the range of typical SiSi double bonds (2.118−2.289 Å).1 Conversely, the B
DOI: 10.1021/acs.organomet.7b00864 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics π(SiSi) and π*(SiSi) orbitals of 5′ap and 5′sc are split due to substantial π conjugation between the two SiSi double bonds, whereby the HOMO and LUMO levels are higher and lower in energy, respectively, in comparison to those of the model compound for monodisilene 8 (8′) calculated at the same level of theory (Figure S29 in the Supporting Information). This, in turn, is consistent with the bathochromically shifted absorption bands observed for 5. The substantial π conjugation in 5′ap and 5′sc is furthermore consistent with the results of the approximately spin-projected UHF/cc-pVTZ// B3LYP/cc-pVTZ calculations on the diradical character, y,18 of the parent tetrasiladiene (5H), which exhibits a non-negligible amplitude, and is not significantly affected by the variation of δ (y ≈ 0.15−0.19 for δ = 0−180°). These results stand in contrast to the behavior of all-carbon butadienes (cf. the Supporting Information). The y values of 5H at δ = 160° (0.187) and 60° (0.192) are slightly smaller than that at δ = 180° (0.193), which indicates a slight decrease and increase of the π-bond order of the SiSi and central Si−Si bonds, respectively, in these antiperiplanar and synclinal conformations. In order to further extend the SiSi double bonds, we investigated the desilylation of 5. When we treated 5 with tBuOK in the presence of 18-crown-6 ether at −50 °C in THFd8, the solution turned from the typical red-purple of 5 to deep blue. The resulting solution exhibited two 29Si signals assignable to SiMe3 groups at −12.3 and −11.9 ppm, as well as eight signals assignable to unsaturated silicon nuclei at 37.1, 37.5, 45.7, 52.8, 90.2, 93.1, 207.1, and 232.5 ppm, which suggests the formation of two conformers of tetrasiladienyl anion 6 (Scheme 2). Similar to the case for 5′, antiperiplanar and synclinal
In conclusion, we have developed a selective Si(sp2)−Si(sp3) cleavage reaction for silyl-substituted disilenes to furnish a disilenide, which represents a potentially useful reaction for the extension of SiSi double bonds under mild reaction conditions. Using this method, we synthesized tetrasiladiene 5 from disilene 8, as well as the first tetrasila-1,3-dien-1-ide (6) from 5 without cleaving the central Si−Si bond that connects the SiSi double bonds or reducing the SiSi double bonds. Using a combination of carefully designed silyl-substituted disilenes and the present synthetic route may provide access to novel persilapolyenes such as octasilatetraenes and hexasilabenzenes,20 as well as to extended silicon-based π-electron systems that contain two or more SiSi double bonds.
■
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00864. Experimental details (PDF) Cartesian coordinates for calculated structures (XYZ) Accession Codes
CCDC 1588783−1588785 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
■
Scheme 2. Generation of 6a
AUTHOR INFORMATION
Corresponding Author
*E-mail for T.I.:
[email protected]. ORCID
Hiroshi Matsui: 0000-0001-8730-520X Masayoshi Nakano: 0000-0002-3544-1290 Takeaki Iwamoto: 0000-0002-8556-5785 a
Notes
18-c-6 = 18-crown-6 ether.
The authors declare no competing financial interest.
■
conformers of model compounds 6′ap and 6′sc were identified as local minima, and both compounds exhibited very small energetic differences (6′ap is 0.4 kJ/mol higher in free energy (298.15 K) than 6′sc at the B3PW91-D3/6-31+G(d) level of theory), while the theoretical 29Si chemical shifts were comparable to the experimentally observed 29Si NMR signals for 6.19 Compound 6 is stable at −50 °C for at least several hours, although it decomposes above −50 °C to afford a complicated product mixture. The formation of 6 was confirmed by the generation of tetrasiladiene 7 after addition of Et3SiCl at −50 °C. Compound 7 was isolated in 65% yield, and its structure was confirmed by a combination of NMR spectroscopy, elemental analysis, and a single-crystal XRD analysis (Figure S25 in the Supporting Information). To the best of our knowledge, this is the first example of the generation, characterization, and documentation of the chemical reactivity of a functionalized tetrasila-1,3-diene, i.e., a tetrasila-1,3-dien-1-ide. Unfortunately, the oxidation of 6 with dibromoethane at −50 °C in THF furnished a red complex mixture that did not show any 29Si NMR signals assignable to unsaturated silicon nuclei, which indicates that the targeted octasilatetraene was not formed.
ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI grants JP15J02977 (N.A.), JP15H00966 (K.K.), JP26107004 (K.K.), JP25248007 (M.N.), JP17H05157 (M.N.), JP15J05489 (H.M.), JP24655024 (T.I.), and JP15K13634 (T.I.). The authors thank Prof. Shintaro Ishida for helpful discussions.
■
REFERENCES
(1) For recent comprehensive reviews on disilenes, see: (a) Lee, V. Ya.; Sekiguchi, A. Organometallic Compounds of Low-Coordinate Si, Ge, Sn, and Pb: From Phantom Species to Stable Compounds; Wiley: Chichester, U.K., 2010. (b) Fischer, R. C.; Power, P. P. Chem. Rev. 2010, 110, 3877−3923. (c) Iwamoto, T.; Ishida, S. Struct. Bonding (Berlin, Ger.) 2013, 156, 125−202. For reviews on functionalized disilenes, see: (d) Abersfelder, K.; Scheschkewitz, D. Pure Appl. Chem. 2010, 82, 595−602. (e) Scheschkewitz, D. Chem. Lett. 2011, 40, 2−11. (f) Präsang, C.; Scheschkewitz, D. Chem. Soc. Rev. 2016, 45, 900−921. (2) (a) Delhalle, J.; Champagne, B.; Dory, M.; Fripiat, J. G.; André, M. J. Bull. Soc. Chim. Belg. 1989, 98, 811−815. (b) Matsui, H.; Fukuda, K.; Ito, S.; Nagami, T.; Nakano, M. J. Phys. Chem. A 2016, 120, 948− 955. (c) Matsui, H.; Nagami, T.; Takamuku, S.; Ito, S.; Kitagawa, Y.; Nakano, M. Molecules 2016, 21, 1540. C
DOI: 10.1021/acs.organomet.7b00864 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics (3) Weidenbruch, M.; Willms, S.; Saak, W.; Henkel, G. Angew. Chem., Int. Ed. Engl. 1997, 36, 2503−2504. (4) (a) Ichinohe, M.; Sanuki, K.; Inoue, S.; Sekiguchi, A. Organometallics 2004, 23, 3088−3090. (b) Ichinohe, M.; Sanuki, K.; Inoue, S.; Sekiguchi, A. Silicon Chem. 2007, 3, 111−116. (5) Uchiyama, K.; Nagendran, S.; Ishida, S.; Iwamoto, T.; Kira, M. J. Am. Chem. Soc. 2007, 129, 10638−10639. (6) Suzuki, K.; Matsuo, T.; Hashizume, D.; Fueno, H.; Tanaka, K.; Tamao, K. Science 2011, 331, 1306−1309. (7) Scheschkewitz, D. Angew. Chem., Int. Ed. 2004, 43, 2965−2967. (8) Inoue, S.; Ichinohe, M.; Sekiguchi, A. Chem. Lett. 2005, 34, 1564−1565. (9) (a) Meltzer, A.; Majumber, M.; White, A. J. P.; Huch, V.; Scheschkewitz, D. Organometallics 2013, 32, 6844−6850. (b) Abersfelder, K.; Zhao, H.; White, A. J. P.; Praesang, C.; Scheschkewitz, D. Z. Anorg. Allg. Chem. 2015, 641, 2051−2055. (10) (a) Gilman, H.; Smith, C. L. Chem. Ind. (London) 1965, 848− 849. (b) Sakurai, H.; Okada, A.; Kira, M.; Yonezawa, K. Tetrahedron Lett. 1971, 12, 1511−1514. (c) Marschner, C. Eur. J. Inorg. Chem. 1998, 1998, 221−226. For recent reviews on silyl anions, see: (d) Marschner, C. Organometallics 2006, 25, 2110−2125. (e) Präsang, C.; Scheschkewitz, D. Struct. Bonding (Berlin, Ger.) 2013, 156, 1−47. (f) Marschner, C. In Organosilicon Compounds Theory and Experiment (Synthesis); Lee, V. Ya., Ed.; Academic Press: Oxford, U.K., 2017; pp 295−360. (11) (a) Iwamoto, T.; Sakurai, H.; Kira, M. Bull. Chem. Soc. Jpn. 1998, 71, 2741−2747. (b) Ichinohe, M.; Kinjo, R.; Sekiguchi, A. Organometallics 2003, 22, 4621−4623. (12) Zirngast, M.; Flock, M.; Baumgartner, J.; Marschner, C. J. Am. Chem. Soc. 2008, 130, 17460−17470. (13) While West et al. have photochemically synthesized 8 from TipSi(SiMe3)3 as a mixture of E and Z isomers,14 the reductive debromination of (Me3Si)(Tip)SiBr2 delivers exclusively the E isomer of 8. (14) Archibald, R. S.; van den Winkel, Y.; Millevolte, A. J.; Desper, J. M.; West, R. Organometallics 1992, 11, 3276−3281. (15) Single crystals of 5ap and 5sc, suitable for XRD analysis, were obtained by recrystallization (−35 °C) from dimethoxyethane and diethyl ether, respectively. (16) (a) Nakano, M.; Kishi, R.; Nitta, T.; Kubo, T.; Nakasuji, K.; Kamada, K.; Ohta, K.; Champagne, B.; Botek, E.; Yamaguchi, K. J. Phys. Chem. A 2005, 109, 885−891. (b) Kamada, K.; Ohta, K.; Kubo, T.; Shimizu, A.; Morita, Y.; Nakasuji, K.; Kishi, R.; Ohta, S.; Furukawa, S.; Takahashi, H.; Nakano, M. Angew. Chem., Int. Ed. 2007, 46, 3544− 3546. (c) Nakano, M.; Champagne, B. J. Phys. Chem. Lett. 2015, 6, 3236−3256. (17) For experimental details and methods, see the Supporting Information. (18) (a) Yamaguchi, K. Chem. Phys. Lett. 1975, 33, 330−335. (b) Nakano, M. Top. Curr. Chem. 2017, 375, 47−113. (19) The 29Si chemical shifts for unsaturated silicon nuclei in THF calculated at the HCTH407/B//B3PW91-D3/6-31+G(d) level of theory (B: 6-311G(3d) for Si and 6-31G(d) for C and H) are 28.4, 56.1, 97.9, and 218.7 ppm for 6′ap as well as 42.6, 49.2, 76.8, and 197.9 ppm for 6′sc. (20) For a recent prediction, see: Benedek, Z.; Szilvási, T.; Veszprémi, T. Dalton Trans. 2014, 43, 1184−1190.
D
DOI: 10.1021/acs.organomet.7b00864 Organometallics XXXX, XXX, XXX−XXX
