Dual Reactivity of a Stable Zwitterionic N ... - ACS Publications

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Dual Reactivity of a Stable Zwitterionic N-Heterocyclic Silylene and Its Carbene Complex Probed with Muonium Paul W. Percival,*,† Brett M. McCollum,‡ Jean-Claude Brodovitch,† Matthias Driess,§ Amitabha Mitra,∥,⊥ Mina Mozafari,† Robert West,∥ Yun Xiong,§ and Shenglai Yao§ †

Department of Chemistry and TRIUMF, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada Department of Chemical and Biological Sciences, Mount Royal University, Calgary, Alberta T3E 6K6, Canada § Institute of Chemistry: Metalorganics and Inorganic Materials, Sekretariat C2, Technische Universität Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany ∥ Organosilicon Research Center, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States ‡

S Supporting Information *

ABSTRACT: The reactivity of the multifunctional cyclic silylene 4 and its carbene complex 5 have been investigated by a combination of muon spin spectroscopy and computation. The free radicals formed by muonium (Mu) addition to 4 were identified, showing that there are two dominant sites of free radical attack: on the Si atom and on the exocyclic methylene carbon. Reaction of muonium with 5 also produced two radicals, but with markedly different hyperfine constants. For both compounds avoided level-crossing resonance spectra and calculation of hyperfine constants show that one of the radicals results from Mu addition to the methylene group, yielding radicals 4a and 5a. Each contains a muoniated methyl group, −CH2Mu, which undergoes restricted rotation with respect to the plane of the ring. For 4 the second product is readily assigned as the muoniated silyl radical 4b, on the grounds of its high muon hyperfine constant (716 MHz). The second product from 5 shows instead a very small coupling constant, 19 MHz, assignable to the muoniated complex 5b, in which the spin density has been transferred from the silicon to the carbenic carbon.

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INTRODUCTION Silylenes are divalent dicoordinate silicon species and are generally viewed as heavier analogues of carbenes. Because of their high reactivity, they have mostly been studied as transient species in the gas phase1 or liquid solution2 or by matrix isolation.3 However, the advent of isolable silylenes4 has opened up a rich field of synthetic organosilicon chemistry5 and enabled their use as ligands in transition-metal complexes with valuable catalytic properties.6 A key factor in the stability of isolable silylenes is the presence of a singlet electronic ground state, with the lone pair in an orbital of largely s character. The vacant silicon p orbital has high reactivity and must be protected in some fashion to achieve an isolable silylene. The stability of N-heterocyclic silylenes 1 (NHSis) and carbenes (NHCs) is widely understood in terms of the electronwithdrawing ability of the α-nitrogens in the σ-system and donation through the π-system. Incorporation of the latter feature in an extended π-system is often described as aromaticity (1′ in Chart 1), but it is clearly absent for the saturated silylenes 2, which are indeed less stable. A third factor affecting silylene stability (kinetic rather than thermodynamic) is the protection afforded by bulky substituents on the atoms adjacent to the reaction center. This is clearly a crucial factor in the case of Kira’s silylene 3.7 © 2012 American Chemical Society

Chart 1. Isolable Silylenes with Five-Membered Rings

Another isolable silylene, 4, has the dual advantages of αnitrogens and a six-membered ring.8 Surprisingly, despite the sp2-hybridized ring carbons, 4 is nonaromatic:8,9 i.e., the zwitterionic form 4′ (Chart 2) is less important to its reactivity than the dual features of the silylene center and the butadiene moiety with its exocyclic methylene group. The reactions of 4 have been explored with a wide variety of reagents.10 In addition, the NHC complex 5 has a highly nucleophilic Si center, which opens up new synthetic pathways.11 In recent years we have employed muonium (Mu) to explore the reactivity of silylenes and silenes.12 Muonium is a singleelectron atom whose nucleus is the positive muon; it is Special Issue: F. Gordon A. Stone Commemorative Issue Received: October 14, 2011 Published: January 11, 2012 2709

dx.doi.org/10.1021/om200966p | Organometallics 2012, 31, 2709−2714

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(The low amplitude of the latter peak is a consequence of the limited time resolution of the apparatus.) The presence of two pairs clearly signals the existence of two distinct muoniated radicals, and the separation of precession frequencies gives their muon hyperfine constants (150.7 MHz for the inner pair and 717.1 MHz for the outer pair of peaks). The results are summarized in Table 1 (a detailed list of

Chart 2. An Isolable NHSi with a Six-Membered Ring and Its NHC Adducta

Table 1. Muon Hyperfine Constants Determined for Radicals Formed from 4 (0.7 M in THF) a

Ar = 2,6-diisopropylphenyl.

chemically equivalent to H but has only one-ninth the mass.13 Lacking a dipole moment, an atom such as H or Mu is an unbiased probe of alternative reaction sites. Muon spin spectroscopy can be used to identify the muoniated radical(s) formed by Mu addition to an unsaturated molecule, allowing one to determine the relative reactivity of different sites within a molecule toward radical attack. This paper describes such an investigation of silylene 4 and its NHC complex 5. Likely sites of Mu addition are the Si atom and the exocyclic methylene carbon, but the other carbons in the butadiene moiety are also possibilities. (The possibility of Mu addition to the aryl group is discounted on the basis of previous results.12d) The carbene in 5 offers additional sites for Mu addition, and it is not clear a priori whether the complex will remain bound when Mu adds, particularly if this occurs at the Si.

a

temp/°C

hfc 1/MHza

hfc 2/MHza

4.2 23.2 41.8 57.7

150.67(3) 148.77(2) 147.07(3) 145.66(3)

717.13(7) 715.74(5) 714.54(6) 713.44(6)

The numbers in parentheses represent statistical uncertainties.

precession frequencies used in the analysis is given in the Supporting Information). The smaller hfc is consistent with Mu addition to the terminal carbon of a butadiene group;14 the larger value is in line with expectations for Mu addition to the silicon.12 Equivalent μSR investigation of 5 (0.25 M in tetrahydrofuran) also reveals the generation of two muoniated radicals (Figure 2 and Table 2), but both with smaller muon hyperfine

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MUON SPIN SPECTROSCOPY A solution of 4 (0.7 M in tetrahydrofuran) was irradiated with positive muons, and transverse-field muon spin rotation (μSR) spectra were recorded at several temperatures in the range 4− 58 °C. An example is shown in Figure 1. The strongest peak in

Figure 2. Fourier power transverse-field μSR spectrum at 8.68 kG obtained from 5 (0.25 M in THF at 25.4 °C).

Table 2. Muon Hyperfine Constants Determined for Radicals Formed from 5 (0.25 M in THF)


a

the spectrum (truncated in this display) is due to muons which end up in a diamagnetic environment (roughly 65%) and which therefore precess at the muon Larmor frequency (104.5 MHz for 7.71 kG applied field). The remaining four peaks arise from muons incorporated in free radicals. This is evident from the pattern of precession frequenciespairs symmetrically placed about the diamagnetic peak. Quadrature detection and complex Fourier transformation allows us to distinguish between positive and negative precession frequencies; this is important for the correct assignment of the 248 MHz precession frequency, which is negative and complements the +469 MHz signal.

temp/°C

hfc 1/MHza

hfc 2/MHza

7.2 25.4 40.6

134.16(5) 132.78(2) 131.60(16)

19.43(6) 18.97(3) 18.78(12)

The numbers in parentheses represent statistical uncertainties.

constants (19.0 and 132.8 MHz at 25 °C). Does this mean that NHC complexation protects the Si atom from attack by Mu? If so, what is the identity of the radical with the small hfc? A computational investigation was undertaken to answer these questions. In addition, a second muon spin spectroscopy technique, avoided-level-crossing resonance (μLCR), was applied to test the provisional assignment of the other radical to Mu addition at the exocyclic methylene. In μLCR each resonance is due to the mixing of spin levels which arise from the muon and one other magnetic nucleus. The resonance field position therefore depends on the muon 2710


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hfc and the hfc of the interacting nucleus. Since the muon hfc is already known (from μSR), the other nuclear hfc can be calculated from the resonance field. Figure 3 shows spectra obtained

In principle, there is ambiguity in the assignments, since there are two radicals and two possible types of spin-active nuclei. In practice, there is little doubt, since alternative assignments would produce unreasonable values for the nuclear hfcs. For example, assignment of the 26.3 kG resonance to protons would require a proton hfc in excess of 220 MHz for a 716 MHz muon hfc or −340 MHz if associated with the 149 MHz muon hfc. Similarly assignment of the 6.1 kG resonances to nitrogen would imply a negative hfc, inconsistent with previous work. Furthermore, our assignment of these resonances to protons is consistent with radical structure assignments, temperature dependence of hfcs, and computational predictions, as explained below. Spectra similar to Figure 3a were obtained from 5 (0.25 M in tetrahydrofuran); the analysis is summarized in Table 4. Table 4. Analysis of μLCR Data Obtained from 5 (0.25 M in THF) T/°C

Aμ/MHz

BLCR/kGa

7.4

134.16

25.4

132.78

40.0

131.60

5.515(3) 5.621(5) 5.441(2) 5.529(3) 5.388(2) 5.472(4)

nucleus 1

H 1 H 1 H 1 H 1 H 1 H

Ak/MHz 31.1(1) 29.1(1) 31.1(1) 29.4(1) 30.9(1) 29.3(1)

a

The numbers in parentheses represent statistical uncertainties derived from fits.

No μLCR resonances were assigned to nitrogen nuclei in the case of 5; subsequent analysis based on computational predictions shows that such resonances would lie close to zero field and would be undetectable. In addition to the two abundant radicals evident in Figure 2, there is some indication of weaker μSR signals from 5, as shown by the inset in Figure 4. These features are discussed

Figure 3. Muon avoided-level-crossing spectra obtained from 4 (0.7 M in THF at 23.3 °C): (a) a pair of close-lying resonances attributed to protons; (b) a pair of resonances attributed to 14N nuclei. The vertical lines denote data points and their statistical errors; the curves through the data represent the best fits of field-modulated Lorentzian line shapes.

from 4; analysis of the resonance field positions is summarized in Table 3. The spectrum shown in Figure 3a is clearly attributed Table 3. Analysis of μLCR Data Obtained from 4 (0.7 M in THF) T/°C 3.3

Aμ/MHz 150.67

23.3

148.77

58.0

145.66

23.3

715.74

BLCR/kGa 6.162(3) 6.282(4) 6.070(3) 6.179(3) 5.917(2) 6.015(3) 26.261(2)b 26.297(2)

nucleus 1

H H 1 H 1 H 1 H 1 H 14 N 14 N 1

Ak/MHz 35.5(1) 33.3(1) 35.3(1) 33.3(1) 35.0(1) 33.2(1) 15.6(1)b 16.5(1)


below, after the prominent signals have been unambiguously assigned.

a

The numbers in parentheses represent statistical uncertainties derived from fits. bA single resonance fit gives BLCR = 26.279(4) kG, from which Ak = 16.1(2) MHz.

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COMPUTATIONAL RESULTS AND DISCUSSION Inspection of the structure of 4 reveals five potential sites for H atom addition. Apart from the silylene site, there are four carbon atoms in the butadiene moiety, although the normal expectation is for preferential radical attack at the ends of a conjugated bond system.14,15 However, since C(3) has a methyl

to a pair of close-lying resonances (each signal has a differentiallike shape due to field modulation). The situation is less clear for Figure 3b. However, as discussed later, there is good reason to expect a pair of signals, and indeed the fit is slightly better with this premise. 2711


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typical isotope effect for this type of radical.14,16 The temperature dependence of the muon hfc (hfc 1 in Table 1) is also consistent with the assignment of a β-substituted CH2Mu group in the radical. Because the C−Mu bond eclipses the SOMO in the lowest energy conformation, increasing temperature results in contributions of torsional states with greater dihedral angles and consequently smaller muon hfcs. Similarly, the two proton hfcs of the CH2Mu group are predicted to increase with temperature in a complementary fashion, so that the average hfc for the CH2Mu group is temperature independent.16 Our results agree with this prediction within the precision of the data. Unlike 4a, radical 4b has unpaired spin density mostly localized on the silicon atom. This is evident from the predicted 29 Si hfcs; unfortunately the low natural abundance of that spinactive isotope precludes measurement by μLCR. A secondary indication of the unpaired spin distribution is the increase in predicted 14N hfcs. It is noteworthy that 4b lacks the symmetry of 4a, so that two different hfcs are predicted. The spectrum displayed in Figure 3b was originally analyzed as a single resonance, but given its assignment to 4b, we chose to interpret the spectrum as two close-lying resonances, recognizing that the poor signal to noise ratio does not allow a clear distinction between the two possibilities. The most obvious spectral feature distinguishing 4b from 4a is the large muon hfc of 4b. This is partly due to the localization of unpaired spin density on the Si but also due to the σ-character of the silyl 4b in contrast to the extended π-system of 4a. After adjustment for the larger magnetic moment, the measured muon hfc is 20% higher than the predicted value, consistent with the isotope effect noted for other muoniated silyl radicals.12 In considering the assignment of signals arising from the silylene−carbene complex 5, we start with the assumption that the two radicals detected are 5a,b (Scheme 2), the complexed

substituent, it is likely disfavored relative to the exo methylene carbon. These factors suggest that the two muoniated radicals detected correspond to Mu addition at Si and the exoCH 2 (Scheme 1). Computation of optimized geometries Scheme 1. Muonium Addition to Silylene 4a

a


(UB3LYP/6-31G(d)) for all five H adduct radicals shows that H addition is exoergic in all cases and that (neglecting H/Mu isotope effects) the two lowest energy isomers are 4a,b, with the former 41.6 kJ mol−1 lower in energy (see the Supporting Information for details). Single-point DFT calculations (UB3LYP/cc-pVDZ) were carried out for the optimized geometries (UB3LYP/6-31G(d)) of 4a,b (with H replacing Mu) to predict nuclear hyperfine constants. For muon hfcs the computed proton hfc was scaled by a factor of 3.183 to account for the differing magnetic moments of the muon and proton. The predicted values are compared with experimental results in Table 5. Optimized Table 5. Comparison of Predicted and Measured Hyperfine Constants (in MHz) for Radicals 4a,b hfc radical

site

4a

Si: N1, N3 ⟨CH3⟩ CH2Mu CH2Mu Si−H N1, N3 Si−H Si−Mu

4b

a

nucleus 29

Si 14 N 1 H 1 H μ 29 Si 14 N 1 H μ

computed 3.4 6.4 32.1 a

102.3 −572.4 16.35, 17.94 187.2 596.0a

exptl

35.3 33.3 148.8

Scheme 2. Muonium Addition to Silylene 5a

15.6, 16.1 715.8

Computed proton hfc scaled by a factor of 3.183. a

geometries correspond to frozen conformations; therefore, three different proton hfc values are predicted for the methyl groups in 4a (note also that, without Mu substitution, 4a is symmetrical with two identical methyl groups). However, under the conditions of our experiments it is reasonable to assume that there is relatively free methyl rotation; thus, only the average of the predicted proton hfcs is reported in the table. Muonium in CH2Mu breaks the symmetry of the methyl group, and it has been well established that the C−Mu bond eclipses the SOMO in the lowest energy conformation.14,16 A consequence of this is that the muon hfc typically has a larger value and the proton hfc has a lower value than predicted for completely free rotation. Thus, the measured value of 35.3 MHz is assigned to the unsubstituted CH3 protons, and the 33.3 MHz value is assigned to protons in the CH2Mu group. After correction for the magnetic moments, the muon hfc measured for 4a is 32% higher than the proton hfc determined for the unsubstituted methyl in the same radical, a


versions of 4a,b. This is in accord with the computational results, which show that H addition to 5 is exoergic at multiple sites, including at the carbene C, but the two lowest energy radical products are 5a,b. Table 6 compares predicted hfcs with those determined experimentally. Assignment of the muon and two proton hfcs to the exocyclic CH2Mu and CH3 groups of 5a parallels that for 4a. All three hfcs are reduced by 11% in the complex. This can be interpreted as transfer of this fraction of the unpaired spin density from the silyl radical to the attached carbene. Similarly, the temperature dependence of hfcs assigned to 5a parallels that of 4a. A dramatic difference between the H adduct radicals of 4 and 5 is the tiny proton hfc predicted for 5b. In this radical the principal site of unpaired spin density is on the carbene carbon, consistent with a tetracoordinate silicon. This also results in larger nitrogen hfcs on the imidazole ring than on the ring 2712


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(52 ± 2%, 48 ± 2%) but slightly less for the silyl when complexed: i.e., 5a and 5b yields of 60 ± 5% and 40 ± 5%, respectively. Given that Mu (H) addition to alkenes is known to be very fast, particularly for dienes, the finding of equal yields for 4a,b may simply be a demonstration that both reaction channels are diffusion-limited. The small reduction in yield for 5b compared with that for 5a might reflect a change in reactivity of the complexed silylene, but it could also be simply due to steric interference with the diffusion-limited addition reaction. The weak signal displayed at −93.4 MHz in Figure 4 was only detected at elevated temperature. The precession frequency is consistent with radical 4b, and so we assign it to the partial dissociation of the complex 5. The signal is too weak for accurate analysis, but the Fourier power amplitude is consistent with about 20% dissociation of the complex at 40 °C.

Table 6. Comparison of Predicted and Measured Hyperfine Constants (in MHz) for Radicals 5a,b hfc radical

site

5a

Si: N1, N3 ⟨CH3⟩ CH2Mu CH2Mu Si−H N1, N3 carbene Ns Si−H Si−Mu

5b

a

nucleus 29

Si 14 N 1 H 1 H μ 29 Si 14 N 14 N 1 H μ

computed 3.4 2.6, 10.3 28.2 89.8a −40.7 5.6, 12.1 15.8, 18.0 1.6 5.0a

exptl

31.1 29.5 132.8

19.0

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Computed proton hfc scaled by a factor of 3.183.

CONCLUSIONS Muon spin spectroscopy accompanied by theoretical calculations establishes that silylene 4 and its carbene complex 5 both react with muonium to form radicals in which Mu is attached to the exocyclic methylene group, 4a and 5a. Silylene 4 also yields a product with the muon attached to silicon, 4b, as shown by its very large muon hyperfine coupling constant. The silylene− carbene complex 5 also yields a second muoniated product, 5b, but in this case the unpaired electron density is largely transferred to the carbenic carbon atom, greatly reducing the hyperfine splitting constant. Computational results and avoided-level-crossing muon spin resonance experiments confirm these structural conclusions. The reactivity of 4 and 5 toward radicals is currently under investigation. The present results form a basis for the interpretation of results obtained by using other radical sources.

containing silicon. Inspection of the optimized molecular geometry (Figure 5b) shows that the Si−Mu bond is essentially

Figure 5. Optimized molecular geometries of the two radicals formed by Mu addition to the silylene−carbene complex: (a) 5a; (b) 5b. Color scheme: Si, violet; N, yellow; C, gray; H, blue; Mu, red.

parallel to the imidazole ring and is thus in the nodal plane of the MO containing the unpaired electron. Vibrational averaging about this minimum energy configuration should result in a larger proton hfc for Si−H than predicted, and this effect should be enhanced for the increased zero-point vibration of Si−Mu. The measured muon hfc of 19 MHz is therefore consistent with assignment to 5b, although the structure is better drawn as in 5b′ (Chart 3)

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EXPERIMENTAL METHODS

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ASSOCIATED CONTENT

Silylene 4 and its NHC complex 5 were synthesized following previously reported experimental procedures.8,11c The samples were dissolved in distilled and degassed THF and sealed under vacuum in stainless steel cells. Muon spin spectroscopy experiments were carried out at the M20 beamline at the TRIUMF cyclotron facility. The muon momentum (28 MeV/c) was low enough to ensure that the muons all stopped in the sample, which was mounted in the HELIOS spectrometer with its magnetic field aligned along the beam direction. The muon beam was tuned for transverse spin polarization in the case of μSR spectra and longitudinal polarization for μLCR experiments. Molecular geometries and hyperfine coupling constants were calculated in Gaussian 09 using the WestGrid high performance computing facilities of Compute/Calcul Canada. Additional calculations were performed at Mount Royal University.

Chart 3. Improved Description of Radical 5b

S Supporting Information *

It is interesting to compare the relative signal amplitudes of 5a,b with those of 4a,b. After correction for signal loss due to limited frequency resolution (only significant above 300 MHz) and spin dephasing during reaction (very unlikely here), relative signal amplitudes are good measures of relative radical yields, which in turn are indicative of relative rates of formation. The spectra shown in Figures 1 and 2 display Fourier power: i.e., they represent squares of signal amplitudes. However, other factors affect such Fourier transforms; therefore, it is best to extract initial signal amplitudes from fits to μSR spectra in the time domain. By this means we find equal yields for 4a and 4b

Text, tables, and formulas detailing the calculation of hyperfine constants from transverse-field muon spin resonance spectra and muon avoided-crossing spectra, and energies and Cartesian coordinates of optimized geometries. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 2713


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Present Address

Driess, M. Organometallics 2009, 28, 1927−1933. (g) Jana, A.; Schulzke, C.; Roesky, H. W. J. Am. Chem. Soc. 2009, 131, 4600−4601. (11) (a) Xiong, Y.; Yao, S. L.; Driess, M. J. Am. Chem. Soc. 2009, 131, 7562−7563. (b) Yao, S.; Xiong, Y.; Driess, M. Chem. Eur. J. 2010, 16, 1281−1288. (c) Xiong, Y.; Yao, S. L.; Driess, M. Chem. Asian J. 2010, 5, 322−327. (12) (a) McCollum, B. M.; Abe, T.; Brodovitch, J.-C.; Clyburne, J. A. C.; Iwamoto, T.; Kira, M.; Percival, P. W.; West, R. Angew. Chem., Int. Ed. 2008, 47, 9772−9774. (b) McCollum, B. M.; Brodovitch, J.-C.; Clyburne, J. A. C.; Mitra, A.; Percival, P. W.; Tomasik, A.; West, R. Chem. Eur. J. 2009, 15, 8409−8412. (c) McCollum, B. M.; Brodovitch, J.-C.; Clyburne, J. A. C.; Percival, P. W.; West, R. Physica B 2009, 404, 940−942. (d) Mitra, A.; Brodovitch, J.-C.; Krempner, C.; Percival, P. W.; Vyas, P.; West, R. Angew. Chem., Int. Ed. 2010, 49, 2893−2895. (e) Percival, P. W.; Brodovitch, J.-C.; Mozafari, M.; Mitra, A.; West, R; Ghadwal, R. S.; Azhakar, R.; Roesky, H. W. Chem. Eur. J. 2011, 17, 11970−11973. (13) (a) Roduner, E. The Positive Muon as a Probe in Free Radical Chemistry; Springer-Verlag: Berlin, 1988; Lecture Notes in Chemistry 49. (b) Walker, D. C. J. Chem. Soc., Faraday Trans. 1 1998, 94. (c) Rhodes, C. J. J. Chem. Soc., Perkin Trans. 2 2002, 1379. (d) McKenzie, I.; Roduner, E. Naturwissenschaften 2009, 96, 873− 887. (e) West, R.; Percival, P. W. Dalton Trans. 2010, 39, 9209−9216. (14) Roduner, E.; Strub, W.; Burkhard, P.; Hochmann, J.; Percival, P. W.; Fischer, H.; Ramos, M.; Webster, B. C. Chem. Phys. 1982, 67, 275−285. (15) Bennett, J. E.; Mile, B. J. Chem. Soc., Faraday Trans. 1 1973, 69, 1398−1414. (16) Percival, P. W.; Brodovitch, J. C.; Leung, S. K.; Yu, D.; Kiefl, R. F.; Luke, G. M.; Venkateswaran, K.; Cox, S. F. J. Chem. Phys. 1988, 127, 137−147.

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Wacker Chemical Corporation, 3301 Sutton Road, Adrian, MI 49221, United States.

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ACKNOWLEDGMENTS We thank G. Langille for assistance with the muon experiments and the staff of the Centre for Molecular and Materials Science at TRIUMF for technical support. This research was supported by the Natural Sciences and Engineering Research Council of Canada, by the Organosilicon Research Center at the University of WisconsinMadison, and by the Deutsche Forschungsgemeinschaft. R.W. is grateful for support from the WCU program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R33-10082). B.M.M. expresses appreciation for support from the MRU Research Reserve Fund.

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