Compounds Probed by Muon Spin Spectroscopy - ACS Publications

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Free Radicals of N‑Donor-Stabilized Silicon(II) Compounds Probed by Muon Spin Spectroscopy Kerim Samedov,† Robert West,*,† Paul W. Percival,‡ Jean-Claude Brodovitch,‡ Lalangi Chandrasena,‡ Mina Mozafari,‡ Reinhold Tacke,§ Konstantin Junold,§ Claudia Kobelt,§ Prinson P. Samuel,∥ Ramachandran Azhakar,∥ Kartik Chandra Mondal,∥ Herbert W. Roesky,∥ Matthias Driess,⊥ and Wenyuan Wang⊥ †

Organosilicon Research Center, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States ‡ Department of Chemistry and TRIUMF, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada § Universität Würzburg, Institut für Anorganische Chemie, Am Hubland, 97074 Würzburg, Germany ∥ Institut für Anorganische Chemie der Universität Göttingen, Tammannstrasse 4, 37077 Göttingen, Germany ⊥ Institute of Chemistry: Metalorganics and Inorganic Materials, Technische Universität Berlin, Strasse des 17. Juni 135, Sekr. C2, D-10623 Berlin, Germany S Supporting Information *

ABSTRACT: Transverse-field muon spin rotation (TF-μSR) spectra have been obtained for muoniated free radicals formed by positive muon irradiation of a series of four-membered cyclic silylenes. Muon-electron hyperfine coupling constants (Aμ) were determined from the spectra; values range from 28.5 to 59.3 MHz. The radicals are formed by muonium (Mu) addition to the silylenes, in a fashion similar to the expected reaction of the H atom. However, it is unclear a priori which is the site of addition. Accordingly, density functional theory (DFT) calculations were carried out for two classes of radicals: silyl radicals formed by H addition to the backbone C atom of the four-membered ring and alkyl radicals formed by H addition to the unsaturated Si(II) center. By comparison of the predicted proton hyperfine constants with the experimentally determined Aμ values it is possible to assign structures to each detected radical. Examples of both addition sites were found.

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INTRODUCTION It is now over three decades since transverse-field muon spin rotation (TF-μSR) was first used to detect muoniated free radicals.1 This rather exotic spectroscopy relies on the use of spin-polarized muons and particle physics techniques, in addition to the principles of magnetic resonance more familiar to chemists. With a lifetime of 2.2 μs, muons are relatively short lived, and it takes specialized facilities to generate the necessary intense beams of muons, direct them to the sample of interest, and monitor the subsequent evolution of the muon spin polarization. Thus, muon spin spectroscopy is available at only a few places in the world. Its application to chemistry is primarily due to the existence of muonium (Mu = [μ+e−]), a single-electron atom with the positive muon (μ+) as its nucleus. Because Mu is chemically equivalent to an H atom (identical electron configuration and similar reduced mass), it can be considered a light isotope of hydrogen.2 Thus, Mu and H undergo similar chemical reactions, such as addition to unsaturated molecules to form free radicals. With its monatomic, nonpolar nature, muonium has been successfully established as an unbiased probe of radical reactivity, and muon © XXXX American Chemical Society

spin spectroscopy has been applied to study a broad range of issues in free radical chemistry.3 The TF-μSR technique relies on the precession of spinpolarized muons in a transverse magnetic field. In common with other types of magnetic resonance, the local field experienced by the spin probe depends on molecular effects as well as the applied field. Thus, different precession frequencies arise from the specific chemical environment of the stopped muon, either as muonium, muoniated radicals, or muons incorporated in diamagnetic compounds. In the studies described here, about 65% of the muons end up in diamagnetic molecules and consequently the associated spin population precesses at the muon Larmor frequency. The remaining muons capture an electron to form muonium atoms, which quickly react to form muoniated radicals. Provided that the rate of reaction is fast enough, the initial muon spin polarization is transferred from the Mu atoms to the muoniated radicals. The spin precession frequencies are extracted by Fourier transReceived: April 17, 2015

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DOI: 10.1021/acs.organomet.5b00324 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics formation of the μSR histogram, in a manner similar to NMR signal processing of the free induction decay following an RF pulse. At high magnetic field (>1 kG, a condition well satisfied here) the Fourier spectrum consists simply of a strong diamagnetic peak flanked by two lines corresponding to different electron spin orientations in the muoniated free radical. The frequency difference between the two radical precession signals directly gives Aμ, the muon-electron hyperfine coupling constant (hfc). Further details are given in the Supporting Information and in our earlier papers.4 The underlying physics principles are described more fully in comprehensive review articles.2a,3a,5 An advantage of particular significance to chemical investigations is that the muon is an isotopic traceronly molecules containing Mu are detected. Furthermore, since TFμSR is a single-particle counting technique, it avoids the complications of cross-reactions between muoniated species. Since the muon beam is highly spin polarized, a good spectrum can be accumulated by injecting on the order of 108 muons, with negligible damage to the sample. Given the short lifetime of the muon and the need to conserve spin polarization during reactions, TF-μSR is ideally suited to the study of fast reactions (e.g., H atom addition to low-valent compounds) and the subsequent muoniated free radicals. In the past, the μSR study of free radicals was largely confined to muonium adducts of multiply bonded carbon compounds. Only recently has the field expanded to include muoniated radicals formed from low-coordinate compounds of silicon,3b,6 germanium,3c,6a phosphorus,7 and a few transitionmetal compounds.8 In 2009 we reported the addition of muonium to Nheterocyclic silylenes (NHSis)6a,9 1 and 2 (see Chart 1). These

Scheme 1. Reactions of 2 and 3 with Mu

Chart 2. Further Examples of Silylenes and Their Carbene Complexes Investigated by Muon Spin Spectroscopy

muoniated radicals, and if so, whether the addition would take place at the Si atom, yielding alkyl radicals, or instead at the C atom of the backbone, yielding silyl radicals as was found previously for 7.6c In this paper we present TF-μSR spectra for a series of four-membered-ring silylenes (8−12) stabilized intramolecularly by nitrogen atoms.

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Chart 1. First N-Heterocyclic Silylenes, 1−3, To Be Investigated by Muon Spin Spectroscopy

RESULTS AND DISCUSSION The structures of silylenes 8−12 are shown in Chart 3. The anticipated radicals formed upon muonium addition to the Si Chart 3. Amidinato-Silylenes 8−12 Investigated in the Current Study

silylenes are stabilized by electron donation from the two nitrogen atoms in the five-membered ring. It was found that the primary (i.e., initially formed) muoniated radicals react further with the starting silylene molecules to yield disilanyl radicals (Scheme 1). These were the first examples of secondary radical products detected by μSR. Later, after extending our studies to a series of NHSi molecules with different substituents on the N atoms, we were able to demonstrate that the formation of a secondary muoniated radical is contingent on the steric bulk of the substituents. Thus, the silyl radical 3a formed by Mu addition to silylene 3 could be observed for the first time (see Scheme 1).6b More recently, we investigated the reactivity of the stable zwitterionic NHSi (4) and its carbene complex (5),6d as well as two rare examples of di- (6) and monochlorosilylenes (7)6c (see Chart 2). Very recently, syntheses of stable cyclic amidinato-10−12 and guanidinato-silylenes13 have been reported. We were interested to find out whether these silicon-containing four-membered rings would similarly capture muonium to produce observable

atom (denoted by b) or the C atom of the four-membered ring (denoted by a) are shown in Scheme 2. Note that in the case of Mu addition to the C atom two stereoisomers are possible (denoted by cis and trans). Fourier transform μSR spectra obtained from 8−11 are shown in Figure 1. For each of these cases a single muoniated radical is formed, as evidenced by only one pair of precession frequencies, symmetrically placed around the diamagnetic peak at 196 MHz corresponding to the muon Larmor frequency at 14.5 kG. B


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Organometallics Scheme 2. Possible Paths for the Reaction of Silylenes 8−12 with Mu

Figure 2. Fourier power transverse-field μSR spectrum at 14.5 kG obtained from a 1 M solution of 12′ in THF at −7.8 °C.

The spectra derived from irradiation of silylenes 8−11 show sharp and clear signals at 25 °C, with hfcs ranging from 28.5 MHz for 8 to 59.3 MHz for 11 (see Figure 1). The signals become less intense as the temperature decreases, consistent with slower radical formation. Interestingly, the spectrum derived from muon irradiation of silylene 12 displays much weaker and broader signals (see Figure 2). Only at lower temperatures is it possible to discern a pair of radical precession signals and determine the hfc (48.4 MHz at −7.8 °C). This can be explained in terms of rapid radical decay that is naturally faster at higher temperatures and contributes to line broadening and loss in signal intensity. The lower stability of this radical indicates some inherent differences in geometry and electronic structure from the other muoniated radicals (vide infra). The muon hfcs extracted from the TF-μSR spectra are summarized in Table 1. (Details of precession frequencies are given in Table S1 of the Supporting Information.) Also shown

are hfcs predicted from DFT calculations on the isotopomeric radicals containing H instead of Mu. Generally, using DFT for prediction of muon hfcs is legitimate, because within the Born− Oppenheimer approximation the electronic structures and equilibrium geometries of the protiated and muoniated radicals should be the same. Molecular geometries of possible radicals were optimized at either the UB3LYP/6-311+G(2d,p) or UPBE0/6-311+G(2d,p) level, as indicated in Table 1, and subsequently the corresponding hfcs and single-point energies were calculated at the UB3LYP/cc-pVDZ or UB3LYP/ccpVTZ level. The computationally predicted hfcs were then scaled by the ratio of muon/proton magnetic moments (3.183). hfc values for each of the three possible isomers (denoted by 8a-cis, 8a-trans, 8b..., etc.) that could be formed upon muonium addition to the two different sites are given for each silylene

Figure 1. Fourier power transverse-field μSR spectra at 14.5 kG and 25 °C obtained from solutions of various silylenes in THF: (a) 8 (saturated solution); (b) 9 (1.2 M); (c) 10 (0.7 M); (d) 11 (0.5 M). The truncated peak at 196 MHz is due to muons incorporated in diamagnetic molecules. C


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Organometallics

allows a more or less unambiguous identification of the observed radical species in the μSR spectrum of silylene 9 as a mixture of 9a-cis and 9a-trans isomers. In contrast, it is not immediately obvious what radical species might be responsible for the signals in the spectra arising from silylenes 810b and 10.10a The observed hfc for the radical species derived from silylene 8 (28.5 MHz) is ca. 40% higher than that predicted for the alkyl radical 8b formed by Mu addition to the Si atom in 8 (17 MHz) and ca. 30% higher than the vibrationally averaged hfc of the isomer mixture consisting of 8a-cis and 8a-trans (20 MHz). Although the hfc for 8b is some 10% further off the observed hfc value than the averaged value, 8b turns out to be 25.3 kJ/ mol more stable than 8a-trans and 30 kJ/mol lower in energy in comparison to 8a-cis. Furthermore, given the rather “heavy” substituent on the Si atom in silylene 8, it appears unlikely that any rapid interconversion between 8a-cis and 8a-trans would actually take place, if muonium were to add to the backbone C atom. A direct consequence of muonium addition to the backbone would thus be formation of two distinct radicals with hfcs of 6 MHz for 8a-trans and 130 MHz for 8a-cis. While the former value would result in unobservable signals, completely hidden by the intense and broad diamagnetic signal D, the latter is too far off the observed hfc value of ca. 28.5 MHz. These considerations leave alkyl radical 8b formed by muonium addition to the Si atom as the only viable alternative. The fact that the calculated hfc of 17 MHz is within the potential 40% margin caused by isotope effects supports this assignment. Similarly, the hfc observed for the radical species derived from silylene 10 (42.05 MHz) is ca. 30% higher than that predicted for the alkyl radical 10b formed by Mu addition to the Si atom in 10 (29 MHz) and ca. 22% higher than the vibrationally averaged hfc of the isomer mixtures consisting of 10a-cis and 10a-trans (33 MHz). However, 10b is only 11.5 kJ/ mol more stable than 10a-trans and 17.3 kJ/mol lower in energy than 10a-cis. Furthermore, vibrational analysis of 10atrans and 10a-cis, which are structurally related to each other by a simple flip of the exocyclic Si−N bond, indicates the presence of three low-frequency wagging modes of the Si−N bond, hence making rapid interconversion between 10a-trans and 10a-cis a possibility. Given that and the rather marginal difference in the thermodynamic stabilities of 10b and 10a-cis/ 10a-trans, one would expect to observe two distinct radical species if 10b were formed. However, only one type of radical is observed in the spectrum obtained from 10. Although the above facts alone are not enough to conclusively exclude formation of 10b, it appears less likely to be the observed radical species. Silylene 1112 is different from the other four silylenes in this study in that it possesses significantly bulkier Dipp substituents on the N atoms stabilizing the Si(II) center. This might influence its reactivity toward muonium and the relative stability of the muoniated radicals formed. The hfc of the radical derived from 11 is the largest (59.3 MHz) of the set. Remarkably, in contrast to the results of DFT calculations for 8−10 (see Table 1), the predicted averaged hfc for 11a-cis and 11a-trans is somewhat higher (68 MHz) than the observed hfc. Since the calculated hfc is usually an underestimate, due to neglect of zero-point vibrational effects, this finding seems to indicate that the observed spectrum should not be assigned to 11a-cis and 11a-trans. In contrast, Mu addition to the Si atom of 11 would result in a muoniated alkyl radical 11b with a predicted hfc of 51 MHz, only 4% smaller than the observed

Table 1. Comparison of Calculated and Experimentally Determined Muon Hyperfine Constants Aμ for Various Muoniated Free Radicals Derived from Silylenes 7−12 Aμ/MHz compound

exptl

g

30.2 28.5 46.2 42.0 59.3

7 8 9 10 11 12 12′

a

48.4f

calcd for C-Mub

calcd for Si-Muc

34 20 43 33d 68 146d 50d

−12 17 22 29e 51 32e 28e

Determined from the transverse-field μSR spectrum; at 25−26 °C except where otherwise stated. bBoltzmann-weighted average of the corresponding hfcs for the cis and trans isomers scaled from the proton hyperfine constant computed for the analogous protiated radicals at the equilibrium geometry using UB3LYP/cc-pVDZ//UB3LYP/6311+G(2d,p) except where otherwise stated. cScaled from the proton hyperfine constant calculated for the analogous protiated radical at the equilibrium geometry using UB3LYP/cc-pVDZ//UB3LYP/6-311+G(2d,p). dBoltzmann-weighted average of the corresponding hfcs for the cis and trans isomers scaled from the proton hyperfine constant computed for the analogous protiated radicals at the equilibrium geometry using UB3LYP/cc-pVTZ//UPBE0/6-311+G(2d,p). eScaled from the proton hyperfine constant calculated for the analogous protiated radical at the equilibrium geometry using UB3LYP/ccpVTZ// UPBE0/6-311+G(2d,p). fDetermined at −7.8 °C. gData from ref 6c. a

separately in the Supporting Information in Tables S2−S19. As mentioned earlier, in the case of Mu addition to the C atom of the backbone in 8−12, two stereoisomers can be formed. On the basis of our previous experience,6c the current findings that the energy difference between cis and trans isomers ranges only between 1.4 and 5.8 kJ/mol (see the Supporting Information), and the fact that only one radical species is observed in each spectrum, we propose that a rapid equilibrium exists between the two stereoisomers, resulting in an averaged hfc for radicals derived from the silylenes bearing “lighter” substituents (NMe2, NPh2) on the Si atom (9−11). The average hfc depends on the values for the two stereoisomers weighted according to their individual energies. Obviously, no such considerations apply to radicals resulting from Mu addition to the Si atom. With the exception of the average over the stereoisomers discussed above, the computational predictions do not take account of vibrational averaging over normal modes involving H(Mu). Thus, the predicted values correspond to properties of the equilibrium structures at 0 K. Hyperfine constants are sensitive to zero-point energy, particularly for anharmonic potentials, and such effects are further enhanced by the light mass of Mu. Observed hfcs can thus be up to 40% higher than those predicted for the equilibrium structure.14 No attempt was made to quantify the isotope effects in this work. A look at Table 1 reveals that, with the exception of 11 and 12 (vide infra), the observed hfcs are in better agreement with the hfcs predicted for the addition products to the backbone C atom than to the Si atom. This is also in accord with our previous findings.6c Silylene 910a displays the smallest difference between predicted vibrationally averaged and observed hfcs (7%). Muonium addition to the Si atom in 9 would result in the muoniated alkyl radical 9b with a predicted hfc of 22 MHz, which would be ca. 50% smaller than the observed value. This D


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equilibrium geometries of analogous radicals containing H instead of Mu. While the muoniated radical formed from compound 9 has an hfc value that is close to that calculated for the radical resulting from Mu addition to the backbone C atom of the four-membered ring, the experimentally observed hfc for muoniated radicals formed from 8 and 10 are 20−40% higher than those predicted for either of the possible addition products. This is most likely due to vibrational isotope effects, which are particularly important for anharmonic modes involving the much lighter Mu. On the basis of the energetics of the possible muoniated radicals derived from 8 and 10 as determined by DFT calculations, we tentatively assign the spectrum derived from silylene 8 to the radical formed by Mu addition to the Si atom and the spectrum derived from silylene 10 to the radical formed by Mu addition to the backbone C atom of the four-membered ring. In contrast, comparison of the experimentally obtained hfc and DFT predictions for 11 suggests that the observed radical is formed by Mu addition to the Si atom rather than the backbone C atom. The hfc observed for the radical formed from silylene 12 was obviously inconsistent with the values predicted by DFT calculations. The contradictions can be resolved by taking into account the existence of the closed form 12′ (four-coordinate silicon(II) center) in solution. Assuming that Mu adds preferably to the backbone C atom of the four-membered ring in 12′ leads to predictions that are in excellent agreement with the experimentally observed hfc.

value. Although it is not entirely evident how and why larger substituents on the N atoms in 11 would favor formation of one radical species (here 11b) over the other (11a-cis and 11atrans), the above conclusion is further supported by the computed radical energies: 11b is almost 70 kJ/mol more stable than 11a-cis or 11a-trans. Taking these facts into account, we assign the radical species observed in the μSR spectrum from silylene 11 as the alkyl radical 11b. Silylene 1211a is significantly different from the other four silylenes in that it can be described as a donor-stabilized silylene with both a bidentate and a monodentate amidinato ligand. With the imino group of its monodentate ligand as a potential donor, this Si(II) species offers an interesting reactivity profile, and distinct structural and conformational flexibility. Surprisingly, the experimentally observed hfc for the radical derived from it (48.4 MHz) was well in line with those observed for 8− 11. However, as shown in Table 1, the results of the DFT calculations could not be reconciled satisfactorily with either the weighted average hfc amounting to 146 MHz from 12a-cis (147 MHz) and 12a-trans (17.5 MHz) or the alkyl radical 12b (32 MHz). A look at the relative energies of silyl radicals 12a-cis and 12a-trans explains the high prediction as follows: while for all other silylenes a-trans isomers are somewhat lower in energy than a-cis isomers (from 1.2 to 5.8 kJ/mol), it is the other way around for 12, resulting in a higher contribution of the hfc for the 12a-cis isomer to the weighted average. The discrepancy between calculations and observations can be resolved by taking into account that 12 can exist both in an open form, in which the imino group of its monodentate ligand points away from the Si atom (this is how 12 is found to exist in the solid state),11a and in its cyclic form 12′, in which the imino group of its monodentate ligand coordinates to the Si atom, yielding a bicyclic butterfly-like structure with a four-coordinate Si atom (see Scheme 3). As a matter of fact, according to recent

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Compounds 8−12 were synthesized according to the published procedures.10−12 All samples were dissolved in dry and oxygen-free tetrahydrofuran (THF) and sealed under an inert atmosphere in stainless-steel cells equipped with a thin stainless-steel window for muon spin spectroscopy experiments. The solution concentrations were typically 0.5−1.2 M. Muon spin spectroscopy experiments were carried out at TRIUMF in Vancouver, Canada, at the M20 beamline using standard equipment and techniques described elsewhere.4 All geometry optimizations of protiated radicals were carried out without symmetry restraints using Gaussian 09, Revision D.01,15 at the density functional level of theory (DFT) using either the hybrid threeparameter functional of Becke16 and the correlation functional of Lee, Yang, and Parr17 (UB3LYP) or the hybridized18 density functional of Perdew, Burke, and Ernzerhof19 (UPBE0) for unrestricted systems as implemented in Gaussian 09. The 6-311+G(2d,f) basis set was used for all atoms. The nature of stationary points on the PES (potential energy surface) was determined by calculations of a full Hessian matrix followed by frequency calculations. All structures reported were found to be true minima on the PES with no imaginary frequencies. Stationary-point calculations of isotropic hyperfine coupling constants and energies were performed using UB3LYP/cc-pVDZ20 or UB3LYP/ cc-pVTZ.21 The polarizable continuum model (PCM) was employed to account for possible solvent effects in the geometry optimization routine.22 Muon hyperfine coupling constants were scaled from the proton hyperfine constants by the ratio of the muon/proton magnetic moment (3.1833).

Scheme 3. Equilibrium between the Three-Coordinate Silylene Open Structure 12 and the Isomer 12′ in THF Solution

studies11b 12 exists in solution almost exclusively as 12′. Assuming that Mu adds preferably to the backbone C atom of the two four-membered rings leads to a predicted averaged hfc of 50 MHz, which is in excellent agreement with the experimentally observed value. Muonium addition to the Si atom would yield alkyl radicals with an hfc value (28 MHz) that appears to be rather inconsistent with the experimental observation.

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S Supporting Information *

CONCLUSION In summary, transverse-field μSR spectra have been obtained for muoniated radicals derived from the five silylenes 8−12 containing an unsaturated three-coordinate (8−11) or fourcoordinate Si(II) center incorporated in a four-membered ring. The muon hyperfine coupling constants range from 28.5 to 51.3 MHz. These experimentally observed hfcs were compared with values predicted using DFT calculations on the

Text, a table, and figures giving formulas detailing the calculation of muon hfcs from transverse-field muon spin resonance spectra, muon spin precession frequencies and hfcs at different temperatures for radicals derived from silylenes 8− 12, and optimized geometries (Cartesian coordinates), energies, and hfcs calculated for possible muoniated radicals derived from 8−12. The Supporting Information is available E


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(8) (a) McKenzie, I. Phys. Chem. Chem. Phys. 2014, 16, 10600− 10606. (b) Macrae, R. M. Phys. B 2006, 374−375, 307−309. (c) McKenzie, I., Unpublished results. (d) Jayasooriya, U. A.; Grinter, R.; Hubbard, P. L.; Aston, G. M.; Stride, J. A.; Hopkins, G. A.; Camus, L.; Reid, I. D.; Cottrell, S. P.; Cox, S. F. J. Chem. - Eur. J. 2007, 13, 2266−2276. (9) McCollum, B. M.; Brodovitch, J.-C.; Clyburne, J. A. C.; Percival, P. W.; West, R. Phys. B 2009, 404, 940−942. (10) (a) Azhakar, R.; Ghadwal, R. S.; Roesky, H. W.; Wolf, H.; Stalke, D. Organometallics 2012, 31, 4588−4592. (b) Wang, W.; Inoue, S.; Yao, S.; Driess, M. J. Am. Chem. Soc. 2010, 132, 15890−15892. (11) (a) Junold, K.; Baus, J. A.; Burschka, C.; Tacke, R. Angew. Chem., Int. Ed. 2012, 51, 7020−7023. (b) Junold, K.; Nutz, M.; Baus, J. A.; Burschka, C.; Fonseca Guerra, C.; Bickelhaupt, F. M.; Tacke, R. Chem. - Eur. J. 2014, 20, 9319−9329. (12) Tacke, R.; Kobelt, C.; Baus, J. A.; Bertermann, R.; Burschka, C. Dalton Trans. 2015, accepted. (13) Mück, F. M.; Junold, K.; Baus, J. A.; Burschka, C.; Tacke, R. Eur. J. Inorg. Chem. 2013, 2013, 5821−5825. (14) (a) Roduner, E.; Reid, I. D. Isr. J. Chem. 1989, 29, 3−11. (b) Boxwell, M. A.; Claxton, T. A.; Cox, S. F. J. J. Chem. Soc., Faraday Trans. 1993, 89, 2957−2960. (c) Webster, B. J. J. Chem. Soc., Faraday Trans. 1997, 93, 205−210. (15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F. O.; Bearpark, M. J.; Heyd, J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, C. d. n.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09; Gaussian, Inc., Wallingford, CT, USA, 2009. (16) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (17) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (18) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158−6170. (19) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (20) Woon, D. E.; Dunning, T. H. J. J. Chem. Phys. 1993, 98, 1358− 1371. (21) Kendall, R. A.; Dunning, T. H.; Harrison, R. J. J. Chem. Phys. 1992, 96, 6796−6806. (22) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999−3094.

free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00324.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the staff of the TRIUMF Centre for Molecular and Materials Science (CMMS) for technical support. TRIUMF is operated by a consortium of Canadian universities, under a contribution from the National Research Council of Canada. CMMS is also supported by the Natural Sciences and Engineering Research Council of Canada. R.T. thanks the Deutsche Forschungsgemeinschaft for financial support (TA 75/16-1). Quantum chemical calculations were supported in part by National Science Foundation Grant CHE-0840494.

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REFERENCES

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