Article pubs.acs.org/Organometallics
Structure and Solution Reactivity of (Triethylsilylium)triethylsilane Cations Samantha J. Connelly, Werner Kaminsky, and D. Michael Heinekey* Department of Chemistry, University of Washington, Box 351700, Seattle, Washington, 98195, United States S Supporting Information *
ABSTRACT: Reaction of triphenylmethylium tetrakis(pentafluorophenyl)borate, [Ph3C][B(C6F5)4], with excess neat triethylsilane affords triethylsilylium(triethylsilane) tetrakis(pentafluorophenyl)borate, [(Et3Si)2(μ-H)][B(C6F5)4] (1), identified by X-ray crystallography. In chlorobenzene and fluorobenzene, 1 is observed in solution. When 1 is dissolved in benzene or toluene, the evolved gas was shown to be hydrogen. Isotope labeling experiments demonstrate that the hydrogen arises from the reaction of Et3SiH with the silylium complex of the arene solvent.
E
arly investigations of silylium cations, [R3Si]+, were motivated by the desire to characterize the structure and reactivity of the heavier group 14 analogues of carbocations.1 Silylium species were anticipated to have stability higher than that of their carbocation counterparts due to the larger size, increased polarizability, and decreased electronegativity of silicon relative to carbon.2 [R3Si]+ (R = H, Me, F) was shown to exist in the gas phase by mass spectrometry methods,3 but characterization and isolation of a “free” threecoordinate cation in condensed phases proved difficult.4 The magnitude of the interaction between the cation and its environment is strongly dependent on the solvent and counteranion employed.5 Synthesis of the triethylsilylium toluene complex [Et3Si(C7H8)][B(C6F5)4] (2a) by reaction of triethylsilane with [Ph3C][B(C6F5)4] (3) was first described by Lambert and coworkers.6 The product of this reaction was recrystallized from toluene/hexanes and characterized by NMR spectroscopy and X-ray crystallography. It was determined that the cation was interacting with a toluene solvent molecule, confirming the great difficulty of obtaining a truly three-coordinate silylium cation in the condensed phase. If true π-coordination were observed, the Si−Cring−ring centroid angle would be 90°. In contrast, a σ-bound Wheland intermediate species would remove the positive charge from the silicon and displace it to the para-position of the ring. Complex 2a exhibits a solid-state structure intermediate between the two extremes (Figure 1).1c Silylium−arene complexes continue to generate interest, including recent experimental and computational studies by Schulz, Villinger, and co-workers.7 Solution structures have been studied using 29Si NMR spectroscopy. Computations suggested that three-coordinate silylium cations would have 29Si NMR chemical shifts greater © 2013 American Chemical Society
Figure 1. Comparison of π-coordinated and σ-bound species in the coordination of silicon cations to arene rings.
than 200 ppm.5,8 Such downfield chemical shifts have been observed for sterically congested cations such as trimesitylsilylium.9 For 2a and its benzene analogue [Et3Si(C6H6)][B(C6F5)4] (2b), Lambert and co-workers reported 29Si NMR chemical shifts of ca. 100 ppm.10 In a recent paper, Reed and co-workers determined that uncoordinated [Et3Si]+ does not exist, despite preparation in neat silane according to the procedure of Lambert, in the absence of arene solvent.11 In fact, 1 equiv of the Et3SiH solvent is bound to the [Et3Si]+ species, and the product of the reaction of Et3SiH with 3 in neat Et3SiH is best formulated as the dimeric [(Et3Si)2(μ-H)][B(C6F5)4] (1). This was established by chemical analysis and by the infrared spectrum of 1, which features a broad band at 1800 cm−1 identified as the Si− H−Si stretch, present only when 1 equiv of triethylsilane is bound to the triethylsilylium cation (as Reed reported previously for a trimethylsilylium compound).12 The paper reports that gas evolution was observed upon dissolution of the silylium cation in arene solvents. Reed and Nava suggest that the gas evolved may be triethylsilane. Received: October 3, 2013 Published: November 11, 2013 7478
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Organometallics
Article
In this work, we report the structure of complex 1 as determined by X-ray crystallography. The reaction of 1 with toluene or benzene was examined, and the evolved gas was determined to be hydrogen. Consumption of Et3SiH was found to be catalyzed by the silyl arenium ion.
Scheme 1. Dissociation Reaction Described by Keq
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RESULTS AND DISCUSSION In neat triethylsilane, insoluble yellow [Ph3C][B(C6F5)4] (3) reacted over a period of days to form a white solid product, which was readily dissolved in chlorobenzene or fluorobenzene. The addition of pentane or triethylsilane precipitated X-rayquality crystals of [(Et3Si)2(μ-H)][B(C6F5)4] (1) (Figure 2).
benchmarked, but the exchanging silylium species are more difficult to describe. Complex 2c was independently synthesized by combining equimolar Et3SiH and 3 in chlorobenzene. Reed has previously reported a similar species,12 [iPr3Si(o-dichlorobenzene)][CHB11Cl11], showing that even unreactive solvents can be coordinated due to the extreme electrophilicity of these complexes. The 1H NMR spectrum of this species was highly second order (confirmed by simulation; see the Supporting Information), thereby making quantification of the extent of dissociation of 1 with the 1H NMR signals from the ethyl groups difficult. Instead, the signal for the bridging hydrogen can be monitored. To better understand the exchange reaction, varying amounts of Et3SiH were added to 3 in chlorobenzene. These experiments indicated that some dissociation of triethylsilane from 1 is occurring in solution to form 2c (Scheme 1). If no dissociation were occurring, only the intensity (and not the chemical shift) of the 1H NMR signal for [(Et3Si)2H]+ would change as the amount of triethylsilane added to 3 varied between 1 and 2 equiv. Instead, the signal was not observable until approximately 1.5 equiv of Et3SiH was present. The signal increased in intensity and shifted downfield as more Et3SiH was added. Monitoring the chemical shift indicated that the true chemical shift of [(Et3Si)2H]+ is 1.35 ± 0.12 ppm (see the Supporting Information for data and fit parameters). As noted above, the solution 1H NMR spectrum of 1 exhibits a broad signal at 2.1 ppm. With addition of more triethylsilane, the intensity of this signal increased and the chemical shift moved toward that of free triethylsilane. The signal has been assigned as the coalesced, weighted average of Et3SiH and [(Et3Si)2H]+ exchanging rapidly in solution. Cooling the solution to the freezing point of the solvent (228 K) did not freeze out the exchange process. Using 2.1 ppm as the observed chemical shift and 1.35 and 3.76 ppm as the true chemical shifts of [(Et3Si)2H]+ and Et3SiH, respectively, approximately 31% dissociation is apparent. Further, the exchange process is rapid enough to result in the loss of the 1JSiH coupling. In free triethylsilane, 1JSiH = 177 Hz. It is anticipated that the coupling constant in 1 is on the order of 40 Hz, on the basis of similar species characterized by Müller14 and Nikonov,15 but any coupling is predicted be unobservable when the exchange rate is ≥200 s−1. If the exchange process is sufficiently rapid, complete coalescence of the bridging hydrogen atom in 1 and Et3SiH will result.16 The rapid exchange process bringing 1, 2c, and Et3SiH into equilibrium is confirmed in the 13C{1H} NMR spectra. Because of the complicated ethyl region of the 1H NMR spectra for these species, 13C{1H} NMR spectra are more informative. The equilibrium established between all three triethylsilyl species in Scheme 1 (1, 2c, and Et3SiH) is confirmed by the observation of a single set of resonances in the 13C{1H} NMR spectrum (see the Supporting Information). In contrast to the outcome observed in neat Et3SiH, the in situ reaction of 3 with Et3SiH in arene solvents readily affords
Figure 2. X-ray structure of the cation 1. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms (except bridging hydrogen) and [B(C6F5)4] anion are omitted for clarity. Relevant distances and angles: Si−Si = 3.217(9) Å; ∑C−Si1−C = 347.4(4)°; ∑C−Si2−C = 349.2(4)°. Rint = 0.0573.
Compound 1 crystallizes with no solvent molecules. The closest contact of the silicon atom to the counteranion is well over 3.5 Å. The Si−Si distance is over 3.2 Å, in contrast to Si− Si distances in disilanes such as Si2H6 (ca. 2.3 Å) but corresponding closely to the distances reported for similar compounds (3.17 Å in [Me3Si−H−SiMe3][CHB11Cl11]12 and 2.98 Å in a complex with a naphthyl backbone, [Si2C14H19][B(C6F5)4]·C6F6).13 In nominally unreactive solvents such as chlorobenzene and fluorobenzene, the 1H NMR spectrum of 1 did not exhibit the expected simple spectrum containing an ethyl moiety and a signal for the bridging hydrogen atom with silicon satellites. Instead, the ethyl groups appeared as a complex multiplet and a broad signal at 2.1 ppm was observed without silicon satellites, suggesting an exchange reaction occurring on the NMR time scale. In the 1H−29Si HMQC spectrum with JSiH set to 6 Hz, the ethyl 1H signals were correlated to a 29Si resonance at 58 ppm. In chlorobenzene, we postulate that 1 dissociates to some degree to form [Et3Si(C6H5Cl)][B(C6F5)4] (2c) and free triethylsilane (Scheme 1). In order to define the composition of the system, we attempted to assign the limiting chemical shifts associated with each species. Free triethylsilane is easily 7479
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Organometallics
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the solvent-coordinated triethylsilylium cations. In benzene, toluene, and chlorobenzene, addition of 1 equiv of Et3SiH to 3 resulted in hydride transfer to cleanly form 1 equiv of Ph3CH and 1 equiv of [Et3Si(solvent)][B(C6F5)4] (solvent = toluene (2a), benzene (2b), and chlorobenzene (2c)) (Scheme 2). In the 1H−29Si HMQC spectra, 29Si resonances were observed at lower field (see Table 1).
Scheme 3. Dissolution of 1 in Benzene or Toluene Resulting in Evolution of Hydrogen Gas and Formation of Tetraethylsilane
Scheme 2. Preparation of Complexes 1 and 2a−ca Scheme 4. Presumed Initial Step in the Dissolution of 1 in Benzene
a
The counteranion is [B(C6F5)4], and the solvent is Et3SiH (1), toluene (2a), benzene (2b), or chlorobenzene (2c).
Table 1. Solution NMR Data for Cationic Species cationic speciesa [(Et3Si)2(μ-H)] (1) [(Et3Si)2(μ-H)] (1) [Et3Si(C7H8)] (2a) [Et3Si(C6H6)] (2b) [Et3Si(C6H6)] (2b) [Et3Si(C6H5Cl)] (2c)
Si shiftb (ppm)
29
58 57 85 94 99 115
solvent
PhSiEt3 (the neutral product anticipated to result from deprotonation of the silylarenium ion) was not observed. The principal neutral product of this reaction is tetraethylsilane, as shown by 1H and 13C NMR spectroscopy (in situ) and GC/MS (pentane extraction). This suggests that the highly reactive, Lewis acidic silylium species are promoting redistribution of the ethyl groups. Further, the reaction appears to be catalytic: with addition of more triethylsilane, additional gas evolution and formation of tetraethylsilane was noted. As noted by Reed, the reaction mixtures are biphasic; thus, some products are trapped in a small volume of dense oil at the bottom of the NMR tube. We anticipate that a silylium ion species remains present to facilitate this redistribution reaction, but we cannot conclusively identify the catalytic species or additional silicon-containing products to balance the stoichiometry of this reaction. Further studies are underway to determine the mechanism of this catalysis. One hindrance to full characterization of the catalytic reaction described is the air and water sensitivity of these species. In solution, these ions readily react with trace H2O to form siloxanes. The highly Lewis acidic silylium cations are not stable to long-term storage in the solid state. Even the nominally unreactive [B(C6F5)4]− is susceptible to activation and decomposition over time in the solid state. A solid sample of 1 stored for 3 weeks under Ar showed evidence of darkening, with colorless crystals appearing to sublime to the top of the vial. The sublimate was identified by X-ray crystallography as (C6F5)BF2 (details in the Supporting Information).
C6H5Cl C6H5F C7D8 C6D6 C6H5F C6H5Cl
a The counteranion is [B(C6F5)4]. bAs determined by a HMQC NMR experiment; coupling constant 6 Hz.
29
Si−1H
Reaction of equimolar Et3SiH with 3 in fluorobenzene did not proceed to completion. All of the triethylsilane reacted with half of the available compound 3, and a signal consistent with 1 was observed in the 29Si−1H HMQC spectrum. This result suggests that Et3SiH is a stronger ligand for the electrophilic cation than is fluorobenzene. Complex 1 released triethylsilane from the exchange reaction upon addition of a stronger nucleophile. When Ph3P was added to a solution of 1 in chlorobenzene, 1 equiv of free triethylsilane was observed in the 1H NMR spectrum. The resulting free Et3SiH resonance was not broadened by exchange, and 1JSiH = 177 Hz was readily observable. In a report by Reed and Nava, release of triethylsilane gas upon the dissolution of 1 in benzene or toluene is described. We postulate instead that the boiling point of Et3SiH (107 °C) prevents its evolution as a gas and that it remains in solution (as has been shown for chlorobenzene above). Monitoring of the reactions by 1H NMR spectroscopy identifies the gas evolved as hydrogen. This was confirmed by isotope-labeling experiments with benzene-d6 and toluene-d8, where evolution of HD gas was observed in the 1 H NMR spectrum. Similarly, when [(Et3Si)2(μ-D)][B(C6F5)4] (1-d1) was dissolved in protiobenzene or toluene, HD gas was evolved (Scheme 3). Dissolution of 1 in benzene results in displacement of Et3SiH by benzene to form 2b (Scheme 4). This is supported by the observed crystallization of 2a upon dissolution of 1 in toluene. We postulate that 2b is sufficiently acidic to protonate Et3SiH, evolving hydrogen (the superacidity of silylarenium cations has been previously discussed by Reed).17 The isotope labeling experiments described above, where HD was evolved, support this proposed mechanism. Attempts to identify the siliconcontaining products of this reaction were undertaken. When the neutral products were isolated by extraction into pentane,
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CONCLUSION While the [Et3Si]+ cation is readily generated in neat Et3SiH solvent, we have confirmed the observation of Reed and Nava that this cation cannot be isolated under these conditions. The silane complex of the cation, the dimeric complex 1, has been characterized by X-ray crystallography. A nonreactive solvent could not be identified. In chlorobenzene, partial dissociation of 1 to form 2c and Et3SiH could be observed. Dissolution of 1 in benzene or toluene is accompanied by hydrogen evolution. The solvent is involved in the formation of hydrogen, as demonstrated by the observation of HD in the reaction of 1 with C6D6. These results suggest that halide abstraction reactions with 1 should not be performed in benzene or toluene, since the chemistry is much more complex. 7480
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Article
EXPERIMENTAL SECTION
material is available free of charge via the Internet at http:// pubs.acs.org.
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General Considerations. All experiments were carried out under an argon atmosphere using standard Schlenk techniques and a glovebox. Solvents were dried over CaH2 and then stored over 4.0 Å molecular sieves (halogenated solvents) or sodium metal (benzene/ toluene). All solvents were added to reaction mixtures by vacuum transfer immediately prior to use. The silylium cations are remarkably water sensitive, and it is imperative to thoroughly dry all glassware and reagents. Glassware was silylated18 to remove surface hydroxides. Detailed descriptions of all synthetic procedures can be found in the Supporting Information. 1 H, 13C{1H}, and 29Si{1H} NMR spectra were recorded using Bruker AV700, AV500, DRX500, and AV300 spectrometers equipped with 5 mm solution probes. Spectra were recorded at 298 K unless otherwise noted in C6D6, C6H6, C7D8, C6H5F, and C6H5Cl. 1H and 13 C{1H} spectra were referenced to solvent signals (see the Supporting Information); 29Si{1H} (recorded using an inverse-gated decoupling pulse program) and 29Si−1H HMQC experiments were referenced externally to a sample of neat tetramethylsilane. [Ph3C][B(C6F5)4] (3). This compound is commercially available but is readily synthesized. In a procedure modified from literature reports,11,19 Ph3CCl (0.28 g, 1.0 mmol) and K[B(C6F5)4] (0.72 g, 1.0 mmol) were combined and stirred for 45 min in dichloromethane. Filtration through Celite removes the KCl byproduct, and 3 can be precipitated by layering with pentane. Yield: 76%. [(Et3Si)2(μ-H)][B(C6F5)4] (1). This compound was synthesized by stirring 3 (0.085 g, 0.092 mmol) in neat triethylsilane (∼2−3 mL, 12− 18 mmol) for 24−72 h. During this time, yellow 3 was replaced by a white solid. The Et3SiH/Ph3CH solution was removed via cannula, and the white solid was washed with pentane or heptane (3 × 4 mL). Yield: 95−98%. Dissolving 1 in chlorobenzene and layering the solution with alkane afforded X-ray-quality crystals over the course of several hours. The yield of isolated crystalline material ranged from 35 to 45%. See the NMR spectra in the Supporting Information. [Et3Si(solvent)][B(C6F5)4] (2a−c). Stoichiometric addition of Et3SiH to [Ph3C][B(C6F5)4] to generate 2a−c in situ was achieved in one of two ways outlined below. Upon addition of triethylsilane (2.56 mg, 0.022 mmol), a solution of 3 (20 mg, 0.022 mmol) turned colorless. Triphenylmethane and the silylium product could be observed by NMR spectroscopy. (a) Stock solutions of 0.75−1.0 M Et3SiH in each solvent were made and placed in a Teflon screw-top NMR tube containing the trityl cation solution via microliter syringe with strong argon counterflow. (b) A small amount of triethylsilane was placed in a screw top NMR tube of known mass via vacuum transfer. The calculated corresponding amount of [Ph3C][B(C6F5)4] required was then placed in a second screw-top NMR tube, dissolved in the appropriate solvent, and degassed. The known amount of triethylsilane was then added to the frozen reaction mixture by vacuum transfer. Isolation of compounds 2a−c often led to oily solids and decomposition upon any contact with moisture. Isolation was attempted by layering the reaction mixture with alkane (added to degassed and frozen reaction mixture by vacuum transfer). Lambert’s reported structure of 2b was replicated by this method.10 Dissolution of 1 (20 mg, 0.022 mmol) in benzene or toluene (∼0.5 mL, added by vacuum transfer) vigorously evolved hydrogen gas (as identified by an NMR resonance at 4.47 and 4.50 ppm, respectively). In deuterated solvents, HD was observed. The signal appeared as the characteristic 1:1:1 triplet with 1JHD = 43 Hz. Formation of a neutral species (identified by 1H, 13C{1H}, and 1H−29Si HMQC NMR and GC/MS experiments to be tetraethylsilane) was observed. The reaction mixture catalytically consumed Et3SiH with evolution of hydrogen. Tetraethylsilane could be isolated by extraction of the reaction mixture into pentane (Supporting Information).
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AUTHOR INFORMATION
Corresponding Author
*D.M.H.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the University of Washington. Partial support was provided by the U.S. National Science Foundation (CHE-0750503).
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REFERENCES
(1) Reviews: (a) Lambert, J. B.; Zhao, Y.; Zhang, S. M. J. Phys. Org. Chem. 2001, 14, 370−379. (b) Lambert, J. B.; Kania, L.; Zhang, S. Chem. Rev. 1995, 95, 1191−1201. (c) Reed, C. A. Acc. Chem. Res. 1998, 31, 325−332. (2) Lickiss, P. D. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998; Vol. 2, Part 1, pp 557−594. (3) For an overview see: Schwarz, H. in The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Part 1, Vol. 1, pp 445−510. (4) See reviews in ref 1, as well as: (a) Pauling, L. Science 1994, 263, 983. (b) Olah, G. A.; Rasul, G.; Li, X-y.; Buchholz, H. A.; Sandford, G.; Surya Prakash, G. K. Science 1994, 263, 983−984. (c) Lambert, J. B.; Zhang, S. Science 1994, 263, 984−985. (d) Reed, C. A.; Xie, Z. Science 1994, 263, 985−986. (5) Arshadi, M.; Johnels, D.; Edlund, U.; Ottosson, C.-H.; Cremer, D. J. Am. Chem. Soc. 1996, 118, 5120−5131. (6) Lambert, J. B.; Zhang, S. M.; Stern, C. L.; Huffman, J. C. Science 1993, 260, 1917−1918. (7) Ibad, M. F.; Langer, P.; Schulz, A.; Villinger, A. J. Am. Chem. Soc. 2011, 133, 21016−21027. (8) (a) Olah, G. A.; Field, L. D. Organometallics 1982, 1, 1485−1487. (9) (a) Lambert, J. B.; Zhao, Y. Angew. Chem., Int. Ed. 1997, 36, 400− 401. (b) Kim, K.-C.; Reed, C. A.; Elliott, D. W.; Mueller, L. J.; Tham, F.; Lin, L.; Lambert, J. B. Science 2002, 297, 825−827. (10) Lambert, J. B.; Zhang, S.; Ciro, S. M. Organometallics 1994, 13, 2430−2443. (11) Nava, M.; Reed, C. A. Organometallics 2011, 30, 4798−4800. (12) Hoffmann, S. P.; Kato, T.; Tham, F. S.; Reed, C. A. Chem. Commun. 2006, 767−769. (13) Panisch, R.; Bolte, M.; Müller, T. J. Am. Chem. Soc. 2006, 128, 9676−9682. (14) Müller, T. Angew. Chem. 2001, 113, 3123−3126. (15) Khalimon, A. Y.; Lin, Z. H.; Simionescu, R.; Vyboishchikov, S. F.; Nikonov, G. I. Angew. Chem. 2007, 119, 4614−4617 (reports several 1JSiH coupling constants, with values of ca. 45 Hz for cationic Si). For further studies of this system, see: Tussupbayev, S.; Nikonov, G. I.; Vyboishchikov, S. F. J. Phys. Chem. A 2009, 113, 1199−1209. (16) Simulations were done using gNMR (version 5.1) software developed by P. H. M. Budzelaar. Details are given in the Supporting Information. (17) Reed, C. A. Chem. Commun. 2005, 1669−1677. (18) Hughes, R. P.; Rose, P. R.; Rheingold, A. L. Organometallics 1993, 12, 3109−3117. (19) (a) Scott, V. J.; Celenligil-Cetin, R.; Ozerov, O. V. J. Am. Chem. Soc. 2005, 127, 2852−2853 (Supporting Information, method 2).. (b) Romanato, R.; Duttwyler, S.; Linden, A.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 2010, 132, 7828−7829.
ASSOCIATED CONTENT
S Supporting Information *
Text, figures, tables, and a CIF file giving NMR spectra, spectral simulations, crystallographic data, and preparative details. This 7481
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