Reactivity of an Acyclic Silylsilylene toward ... - ACS Publications

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Reactivity of an Acyclic Silylsilylene toward Ethylene: Migratory Insertion into the Si−Si Bond Daniel Wendel,† Wolfgang Eisenreich,‡ Christian Jandl,§ Alexander Pöthig,§ and Bernhard Rieger*,† †

WACKER-Lehrstuhl für Makromolekulare Chemie and ‡Lehrstuhl für Biochemie, Technische Universität München, Lichtenbergstraße 4, 85747 Garching bei München, Germany § Zentralinstitut für Katalyseforschung, Technische Universität München, Ernst-Otto-Fischer Straße 1, 85747 Garching bei München, Germany S Supporting Information *

ABSTRACT: The reaction of the only known room-temperaturestable, acyclic silylsilylene, Si{NDipp(SiMe3)}{Si(SiMe3)3}, toward ethylene was investigated. A full conversion to the silirane product Si{CH2−CH2}{NDipp(SiMe3)}{Si(SiMe3)3} (1) was observed at ambient temperature. However, heating of the benzene solution under an ethylene atmosphere led to an exceptional Si−Si bond insertion to give the modified silirane Si{CH2−CH2}{NDipp(SiMe3)}{CH2−CH2−Si(SiMe3)3} (2a). With respect to the mechanism, an NMR experiment using C2D4 revealed this reaction to proceed via a migratory insertion of the coordinated ethylene of 1 into the Si−Si bond of the ligand framework and subsequent addition of a second ethylene molecule. Moreover, [4+1] cycloaddition with 2,3-dimethyl-1,3-butadiene to give the corresponding silacyclopent-3-ene ring Si{CH2−CH3CCCH3−CH2}{NDipp(SiMe3)}{Si(SiMe3)3} (3) is reported. All structures were fully characterized by single-crystal X-ray analysis and 1H, 13C, and 29Si NMR spectroscopy.

Chart 1. Room-Temperature-Stable, Monomeric, Acyclic Silylenes I−III

About 50 years ago Goldstein et al. proposed a transient, lowvalent SiMe2 species in the reduction of dimethylchlorosilane with sodium−potassium vapor at elevated temperatures.1 From the time of this pioneering work up to now, exceptional efforts have been made in exploring and elucidating the nature of these elusive silicon compounds.2 The foundation of this development was laid in 1986, when Jutzi and co-workers isolated decamethylsilicocene as the first room-temperature-stable, monomeric Si(II) compound, in which the central silicon atom is protected between two bulky Cp* ligands.3 In 1994 the group of West followed with the groundbreaking discovery of the first room-temperature-stable N-heterocyclic silylene (NHSi),4 whose structural motif has been transferred to a wide variety of stable structures which represent the most investigated silylene class.2 To date, several interesting reactions with small molecules have been reported with NHSis, but more ambitious targets which demand a reactivity more similar to that of transition metals, such as the activation of H2, NH3, CO, and strong C−H bonds, still remain a challenge.2c A step toward this goal was achieved only recently, due to the remarkable work by the groups of Power5 and Aldridge,6 who simultaneously illustrated the highly difficult task of stabilizing silylenes in an acylic framework (Chart 1).7 In contrast to conventional NHSis, acyclic silylenes are ideal candidates for selective small-molecule activation and lead structures for potential metal-free catalysts. This is attributed to their coordinative and oxidative flexibility in connection with the implementation of less π-donating but extremely bulky ligands.2b,8 © XXXX American Chemical Society

A key factor for the evaluation of silylene reactivity is the HOMO−LUMO gap, which in the case of silylenes I and II (2.04 and 1.99 eV, respectively) is assumed to be lowered due to the strong σ-donation from the coordinating boryl/silyl groups.8,9 Both structures have been reported to cleave dihydrogen at ambient temperatures, the prime example demonstrating their enhanced reactivity.6 The HOMO− LUMO gap of bis(arylthio)-substituted silylene III is clearly larger (4.26 eV for IIIa), which presumably prohibits the singlesite activation of hydrogen.5 This compound, however, shows remarkable behavior toward ethylene by reversibly binding the olefin in a dissociation equilibrium with its corresponding silirane ring (silacyclopropane) under ambient conditions.10 This unprecedented behavior by an acyclic, two-coordinate silylene raises the question if such a system is capable of Received: September 17, 2015

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DOI: 10.1021/acs.organomet.5b00797 Organometallics XXXX, XXX, XXX−XXX

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chemical surroundings in a heteroleptic ligand framework (see the Supporting Information). This statement is in line with analoguous silirane products using Power’s homoleptic silylene III in which only one singlet (IIIa 0.64 ppm and IIIb 0.4 ppm in toluene-d8) for the four chemical equivalent protons is observed.10 The structural details of 1 were determined by single-crystal X-ray analysis and are depicted in Figure 1.

eventually playing a role in catalytic transformations. Still, the chemistry of acyclic silylenes is largely unexplored. The proposed extended reactivity with the chance to use acyclic silylenes as molecular catalysts in macromolecular chemistry inspired our group to investigate the reactivity of the most reactive acyclic silylene, II, toward the simplest olefin monomer, ethylene (Scheme 1). Scheme 1. Reaction Pathway of Silylsilylene II with Ethylene To Afford Siliranes 1 and 2a,b

Herein we report the irreversible reaction of II to the silirane 1 with no dissociation equilibrium at room temperature. Increasing the temperature commonly favors the regeneration of the silylene with olefin elimination.10,11 In this case the unexpected formation of the modified silirane 2a was observed at elevated temperatures. An NMR experiment with C2D4 indicates the mechanism of this process to follow a migratory insertion pathway of the coordinated ethylene into the Si−Si bond with subsequent silirane formation with a deuterated ethylene molecule (Scheme 1; 2b). Moreover, the behavior of II toward the butadiene derivative 2,3-dimethyl-1,3-butadiene was analyzed. Monitoring the reaction via NMR spectroscopy proved the exclusive formation of the [4+1] cycloaddition product 3 with no observable vinylsilirane byproduct (Scheme 2).

Figure 1. Molecular structure of 1. Hydrogen atoms are omitted for clarity, and displacement ellipsoids are set at the 50% probability level. Selected bond lengths (Å) and angles (deg): C1−C2 1.554(3), C1− Si1 1.850(2), C2−Si1 1.847(2), Si2−Si1 2.3439(6), N1−Si1 1.747(1); C2−Si1−C1 49.71(8), C2−C1−Si1 65.0(1), C1−C2−Si1 65.2(1), N1−Si1−Si2 126.88(5).

The central silicon atom is tetracoordinated, with a widened N1−Si1−Si2 angle (126.88(45)°) in comparison to the previous silylene II (116.49°)6b and a much smaller C2− Si1−C1 angle (49.71(8)°), which is in accordance with those of similar siliranes.10,12 Regarding the bond lengths, the newly formed carbon−silicon bonds (C1−Si1 1.850(2) Å and C2− Si1 1.847(2) Å) are very close to standard Si−C single-bond lengths (1.87 Å).13 Similarly, the measured C1−C2 bond (1.554(3) Å) clearly show its single-bond character (standard C−C 1.54 Å).13 A solution of 1 in C6D6 was heated to 60 °C under an ethylene atmosphere and monitored via NMR spectroscopy. The primary idea behind this experiment was the insertion of a second ethylene molecule into the Si−C bond to enlarge the ring system to a silacyclopentane. Ideally, further insertions would then increase the ring chain to build up polyethylene in a fashion similar to that in the polymerization of ethylene with Phillips catalyst.14 Instead, the NMR data after 3 days revealed that a complete conversion to silirane 2a was afforded in which apparently a second ethylene molecule inserted into the Si−Si bond of the ligand. A mechanistic study using C2D4 in an identical experimental setup elucidated that actually the coordinated ethylene unit in 1 migrates into the Si−Si bond and a deuterated ethylene molecule forms the novel silirane ring to give 2b (see the Supporting Information for NMR details). To the best of our knowledge, this transformation is the first reported migratory insertion of an olefin into a Si−Si bond. Related work by Baceiredo et al. illustrated the possibility of a reversible insertion of ethylene into a Si−Sn bond of a stable silylene−phosphine complex.15 Furthermore, a compa-

Scheme 2. Reaction of Silylsilylene II with 2,3-Dimethyl-1,3butadiene To Afford Silacylopentene 3

Exposure of a solution of II in C6D6 to ethylene at room temperature led to the full decolorization of the characteristic purple color within seconds. Subsequent NMR analysis confirmed the sole formation of the silirane 1 with no remaining silylsilylene II. The 29Si NMR spectrum of 1 displays an upfield signal at −80.76 ppm for the central silicon atom, which represents a shift of more than 500 ppm in comparison to the original silylsilylene II signal (two conformers due to the restricted rotation around the Si−N bond at 438.20 and 468.46 ppm).6 Hereby the change in oxidation state and the presence of a single conformer with free rotation around the Si−N bond are demonstrated. Interestingly the 1H NMR spectrum reveals two signal groups for the four ring-bound protons (0.91 and 0.37 ppm) with higher-order multiplets, indicating the different B


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Organometallics rable reaction has been discovered by Marschner et al., who proposed a migratory insertion of phenylacetylene into a Si−Ge bond of a donor-stabilized silylgermylene to build vinylgermylenes.16 In a recent investigation the group of Aldridge conducted the same experiment with silylsilylene II and phenylacetylene but only identified the appropriate silirene (silacyclopropene) as the sole product.17 The newly formed Si−C bonds of 2a give rise to a high-fieldshifted signal for the silirane silicon atom at −51.88 ppm in the 29 Si NMR spectrum, whereas the signal of the quaternary silicon of the hypersilyl group shifts from −118.94 ppm in 1 to −76.47 ppm. The proposed structure of 2a was verified by single-crystal X-ray analysis and is illustrated in Figure 2. The

Figure 3. Molecular structure of 3. Hydrogen atoms are omitted for clarity, and displacement ellipsoids are set at the 50% probability level. Selected bond lengths (Å) and angles (deg): C2−C3 1.336(2), C1− C2 1.518(2), C3−C4 1.515(2), C1−Si1 1.896(2), C4−Si1 1.893(2); C4−Si1−C1 94.50(8), N1−Si1−Si2 121.71(5).

original silylene II (116.49°). The C4−Si1−C1 angle (94.50(8)°) and the measured bond lengths are similar to those obtained in the formation of the silacyclopent-3-ene using IIIb19 and are comparable with related published structures.12,20 In summary, our reactivity study of the promising acyclic silylsilylene II toward ethylene illustrated that under ambient conditions the compound behaves like a classical, transient silylene with full conversion to silirane 1. The original intended ring enlargement to trigger a polymerization reaction of ethylene was not accomplished. However, at elevated temperatures the system performed a unique migratory insertion of the coordinated ethylene of 1 into the Si−Si bond of the ligand framework and subsequently added a second ethylene molecule to give the modified silirane 2a. The proposed mechanism of this process was verified by NMR experiments with C2D4. The separate reactivity study of II toward 2,3-dimethyl-1,3butadiene yielded exclusively the [4+1] cycloaddition product 3. Our work demonstrates the first migratory insertion of an olefin into a Si−Si bond with a low-valent silicon compound. Thereby it re-emphasizes the untapped potential of acyclic silylenes to mimic typical reactions of transition-metal complexes and brings researchers in this field closer to the ultimate goal of metal-free catalysis.

Figure 2. Molecular structure of 2. Hydrogen atoms are omitted for clarity, and displacement ellipsoids are set at the 50% probability. level. Selected bond lengths (Å) and angles (deg): C1−C2 1.584(2), C1− Si1 1.828(1), C2−Si1 1.836(1), C3−Si1 1.863(1), C4−Si2 1.908(1), C3−C4 1.539(2); C2−Si1−C1 51.24(6), C2−C1−Si1 64.64(7), C1− C2−Si1 64.12(7), N1−Si1−C3 112.69(5).

introduced ethyl spacer of 2a causes a considerable contraction of the N1−Si1−C3 angle from 126.88(5)° to 112.69(5)°, thereby releasing tension of the strained silacycle, whose C2− Si1−C1 angle (51.24(6)°) slightly increases in comparison to that of 1 (49.71(8)°). The cleavage of a labile Si−Si bond in combination with the formation of two more stable Si−C single bonds presumably promotes this rearrangement. A diverse reactivity of silylenes with 1,3-dienes has been reported in the literature.12 Here, the bulkiness of the substituents of the silylene turned out to be crucial for product formation to either give silacyclopent-3-enes via [4+1] cycloaddition or vinylsiliranes via [2+1] cycloaddition.18 Therefore, the reactivity of II toward 2,3-dimethyl-1,3butadiene was investigated. A solution of silylsilylene II in C6D6 was added to a Young NMR tube filled with 2,3-dimethyl1,3-butadiene, and the reaction progress was monitored via NMR spectroscopy. The 1H NMR data after 3 days reveal a concerted process to the silacyclopent-3-ene 3 without any sign for a vinylsilirane intermediate (see the Supporting Information). The 29Si NMR spectrum of 3 displays a signal for the central silicon atom at 7.71 ppm, which is shifted to high field in comparison to the signal of the appropriate silacylopent-3ene with Power’s silylene IIIb at 31.49 ppm.19 The molecular structure obtained by single-crystal X-ray analysis is depicted in Figure 3. The N1−Si1−Si2 angle (121.71(5)°) of 3 is considerably widened in comparison to the angle of the

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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.5b00797. Crystallographic data for 1 (CCDC 1424204), 2a (CCDC 1424205), and 3 (CCDC 1424206) (CIF) Detailed synthetic procedures, full spectroscopic analysis, and information for single-crystal X-ray structure determinations (PDF) C


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Organometallics

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Saffon-Merceron, N.; De Cozar, A.; Cossio, F. P.; Baceiredo, A. Angew. Chem., Int. Ed. 2011, 50, 10414−10416. (16) Walewska, M.; Baumgartner, J.; Marschner, C. Chem. Commun. (Cambridge, U. K.) 2015, 51, 276−278. (17) Protchenko, A. V.; Blake, M. P.; Schwarz, A. D.; Jones, C.; Mountford, P.; Aldridge, S. Organometallics 2015, 34, 2126−2129. (18) (a) Zhang, S.; Conlin, R. T. J. Am. Chem. Soc. 1991, 113, 4272− 8. (b) Nag, M.; Gaspar, P. P. Organometallics 2009, 28, 5612−5622. (19) Lips, F.; Mansikkamaki, A.; Fettinger, J. C.; Tuononen, H. M.; Power, P. P. Organometallics 2014, 33, 6253−6258. (20) Xiong, Y.; Yao, S.; Driess, M. Organometallics 2010, 29, 987− 990.

AUTHOR INFORMATION

Corresponding Author

*E-mail for B.R.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are exceptionally grateful to the WACKER Chemie AG for continued financial support. We thank Philipp Pahl, Martin Machat, Benedikt S. Soller, and Matthias Grübel for valuable discussions.

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REFERENCES

(1) Skell, P. S.; Goldstein, E. J. J. Am. Chem. Soc. 1964, 86, 1442− 1443. (2) For reviews on silylenes see (a) Rivard, E. Chem. Soc. Rev. 2015, DOI: 10.1039/C5CS00365B. (b) Blom, B.; Driess, M. Struct. Bonding (Berlin, Ger.) 2013, 156, 85−123. (c) Yao, S.-L.; Xiong, Y.; Driess, M. Organometallics 2011, 30, 1748−1767. (d) Asay, M.; Jones, C.; Driess, M. Chem. Rev. (Washington, DC, U. S.) 2011, 111, 354−396. (e) Mizuhata, Y.; Sasamori, T.; Tokitoh, N. Chem. Rev. (Washington, DC, U. S.) 2009, 109, 3479−3511. (f) Kira, M.; Iwamoto, T.; Ishida, S. Bull. Chem. Soc. Jpn. 2007, 80, 258−275. (g) Kira, M. J. Organomet. Chem. 2004, 689, 4475−4488. (h) Hill, N. J.; West, R. J. Organomet. Chem. 2004, 689, 4165−4183. (i) Gehrhus, B.; Lappert, M. F. J. Organomet. Chem. 2001, 617−618, 209−223. (j) Tokitoh, N.; Okazaki, R. Coord. Chem. Rev. 2000, 210, 251−277. (k) Haaf, M.; Schmedake, T. A.; West, R. Acc. Chem. Res. 2000, 33, 704−714. (3) Jutzi, P.; Kanne, D.; Kruger, C. Angew. Chem., Int. Ed. Engl. 1986, 25, 164−165. (4) Denk, M.; Lennon, R.; Hayashi, R.; West, R.; Belyakov, A. V.; Verne, H. P.; Haaland, A.; Wagner, M.; Metzler, N. J. Am. Chem. Soc. 1994, 116, 2691−2. (5) Rekken, B. D.; Brown, T. M.; Fettinger, J. C.; Tuononen, H. M.; Power, P. P. J. Am. Chem. Soc. 2012, 134, 6504−6507. (6) (a) Protchenko, A. V.; Birjkumar, K. H.; Dange, D.; Schwarz, A. D.; Vidovic, D.; Jones, C.; Kaltsoyannis, N.; Mountford, P.; Aldridge, S. J. Am. Chem. Soc. 2012, 134, 6500−6503. (b) Protchenko, A. V.; Schwarz, A. D.; Blake, M. P.; Jones, C.; Kaltsoyannis, N.; Mountford, P.; Aldridge, S. Angew. Chem., Int. Ed. 2013, 52, 568−71. (7) A few other stable, acyclic silylenes have been reported that are worth mentioning here (a) Inoue, S.; Leszczynska, K. Angew. Chem., Int. Ed. 2012, 51, 8589−8593. (b) Jutzi, P.; Leszczynska, K.; Mix, A.; Neumann, B.; Rummel, B.; Schoeller, W.; Stammler, H.-G. Organometallics 2010, 29, 4759−4761. (c) Jutzi, P.; Leszczynska, K.; Neumann, B.; Schoeller, W. W.; Stammler, H.-G. Angew. Chem., Int. Ed. 2009, 48, 2596−9. (d) Jutzi, P.; Mix, A.; Rummel, B.; Schoeller, W. W.; Neumann, B.; Stammler, H.-G. Science (Washington, DC, U. S.) 2004, 305, 849−851. (8) Driess, M. Nat. Chem. 2012, 4, 525−526. (9) Kuriakose, N.; Vanka, K. Dalton Trans. 2014, 43, 2194−2201. (10) Lips, F.; Fettinger, J. C.; Mansikkamaki, A.; Tuononen, H. M.; Power, P. P. J. Am. Chem. Soc. 2014, 136, 634−7. (11) (a) Seyferth, D.; Annarelli, D. C. J. Am. Chem. Soc. 1975, 97, 7162−3. (b) Atwell, W. H.; Weyenberg, D. R. J. Am. Chem. Soc. 1968, 90, 3438−43. (12) Ishida, S.; Iwamoto, T.; Kira, M. Heteroat. Chem. 2011, 22, 432− 437. (13) (a) Stark, B. Electron Diffraction Catalogue and Supplement; Van Nostrand Reinhold: London, 1979. (b) Sutton, L. E. Tables of Interatomic Distances and Configuration in Molecules and Ions; Chemical Society: London, 1958. (14) McDaniel, M. P. Adv. Catal. 2010, 53, 123−606. (15) (a) Rodriguez, R.; Contie, Y.; Gau, D.; Saffon-Merceron, N.; Miqueu, K.; Sotiropoulos, J.-M.; Baceiredo, A.; Kato, T. Angew. Chem., Int. Ed. 2013, 52, 8437−8440. (b) Rodriguez, R.; Gau, D.; Kato, T.; D


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