Comparison of C-H Activation, Hydride Transfer, and Protonation Pathways for SbF5-Mediated and Isobutane Oxidation Clinton R. King,‡ Ashley Holdaway,‡ George Durrant,‡ Josh Wheeler, Lorna Suaava,‡ Michael M. Kon- nick,† Roy A. Periana,#* and Daniel H. Ess‡*

‡Department of and Biochemistry, Brigham Young University, Provo, UT 84602, †Hyconix, Inc., 4575 Weaver Pkwy, Warrenville, IL 60555, #Department of Chemistry, The Scripps Research Institute, Jupiter, Florida 33458 KEYWORDS. Methane, isobutane, main group, C-H activation, hydride transfer

V ABSTRACT: Sb F5 is generally assumed to oxidize methane through a methanium-to-methyl cation mechanism. However, experi- mentally no H2 is observed, and the mechanism of methane oxidation remains unsolved. Density functional theory calculations were V + used to examine methane oxidation by Sb F5 in the presence of CO leading to the methyl acylium cation ([CH3CO] ). While there is V - + V V - + a low barrier for methane protonation by [Sb F6] [H] (the combination of Sb F5 and HF) to give the [Sb F5] [CH5] pair, H2 dissociation is a relatively higher energy process, even with CO assistance, and so this protonation pathway is reversible. While Sb- mediated hydride transfer has a reasonable barrier, the C-H activation/-bond metathesis mechanism with formation of an SbV-Me intermediate is lower in energy. This pathway leads to the acylium cation by functionalization of the SbV-Me intermediate with CO, and is consistent with no observation of H2. Because this C-H activation/metal-alkyl functionalization pathway is higher in energy than methane protonation it is also consistent with the experimentally observed -to-deuterium exchange in methane. In contrast to methane, due to the much lower hydride affinity, isobutane significantly favors hydride transfer to give tert- butyl carbocation with concomitant SbV to SbIII reduction. In this mechanism, the resulting highly acidic SbV-H intermediate provides a route to H2 through protonation of isobutane.

Introduction V Reaction of methane with Sb F5, especially in the presence of a Brønsted acid (e.g HF or HSO3F), is assumed to induce reversi- + ble formation of methanium ([CH5] ) that fragments to H2 and methyl cation (Scheme 1a), which can further react leading to oligomerization.1 For methane, the first step of this su- peracid-induced reaction pathway is supported by the experi- mental observation of methane hydrogen-to-deuterium (H/D) exchange.2-4 However, density functional theory (DFT) studies reported competitive low energy transition-state structures for alkane protonation and concerted hydrogen exchange, and both pathways can account for H/D exchange in methane.5-8 The sec- ond step in the methanium-to-methyl cation pathway is tenuous 1,9 because H2 is generally not observed. However, H2 is ob- served for larger , such as isobutane, although typically in less than stoichiometric amount.10-16 Scheme 1. a) Outline of methyl cation pathway for reaction of methane with There are several reasons to explore possible alternative mech- V - + V [Sb F6] [H] . b) Overview of methane and isobutane oxidation reactions V anisms for reaction of methane with Sb F5, in particular for con- with Sb F5. c) Possible C-H activation transition state and organoantimony ditions without excess Brønsted acid. Hogeveen17 and de intermediate as the first mechanistic step for the oxidation of alkanes by 18 V V Rege reported that the reaction of methane with neat Sb F5 in Sb F5. the presence of CO at 60 and 80 C led to formation of methyl + III In contrast to the methane reaction, Sommer reported that the acylium cation ([CH3CO] ) and reduced Sb with no observa- V III reaction of isobutane with neat Sb F5 results in Sb F3 with ob- tion of H2 (Scheme 1b). Experiments also showed that while H2 13-15 V III servation of both H2 and tert-butyl cation. While this result is capable of reducing Sb to Sb , under these conditions the would seemingly fit with a protonation-induced carbocation rate of reduction is inconsistent with its formation and rapid re- 18 pathway, Sommer used acetone as a Brønsted base to demon- action without observation. This suggests that the methanium V strate that the small quantity of protons in Sb F5 are unlikely to to methyl cation pathway outlined in Scheme 1a is an unlikely V III + be involved with the Sb to Sb reduction. Alternative to a car- route to [CH3CO] . Additionally, the relatively mild conditions bocation pathway is a pathway that involves direct hydride of this reaction are inconsistent with the ~40 kcal/mol energy 19,20 required to dissociate H2 from methanium. transfer from isobutane to Sb. However, Olah and others dis- Methane oxidation. We began by examining the methanium V - + missed this pathway with citation of the formation of a very pathway outlined in Scheme 1a. With [Sb F6] [H] (the ion pair 4 V weak Sb-H bond, and unfavorable thermodynamics. from HF coordination to Sb F5) as the ground state, we located While previous DFT studies examined reaction pathways for the methane protonation transition state, TS1 (Scheme 2a,d). V H/D exchange by Sb F5 , there has been no compu- This transition state was previously reported by Dinér, and our tational consideration of mechanisms involving C-H activa- intrinsic reaction coordinate (IRC) calculation shows it con- V - + ‡ tion/metalation (Scheme 1c) or hydride transfer that provide a nects to the [Sb F6] [CH5] ion pair. Our M06 G value is 15.3 route for SbV to SbIII reduction. The possibility of C-H activa- kcal/mol (H‡ = 5.7 kcal/mol). This relatively low barrier for tion with an alkylantimony (Sb-R) intermediate is intriguing be- protonation is consistent with Raugei and Klein’s molecular dy- V cause the most prominent examples of C-H activation involve namics study of reaction of light alkanes with Sb F5 in HF that transition-metal complexes, and only recently has the possibil- identified highly reactive protons and low barriers for alkane V 24,25 ity of p-block, main-group metals facilitating C-H activation protonation resulting from coordination of HF to Sb F5. been suggested.21 While there is a low barrier for methane protonation (see the SI for comparison to the one-step hydrogen exchange transition Here we report DFT calculations that compare C-H activation, 3 state), even with the solvent stabilization, H2 dissociation is un- hydride transfer, and protonation pathways for oxidation of me- V - + V V likely. The G to achieve separated [Sb F6] , [CH3] , and H2 is thane and isobutane with Sb F5. We found that the Sb F5-me- V - + diated C-H activation pathway is viable and results in an SbV- 53.3 kcal/mol, and our estimate for achieving [Sb F6] [CH3] and H2 is ~42 kcal/mol (see SI for discussion about this esti- Me intermediate, which can be functionalized by CO resulting + + III mate). This relatively large value for H2 separation from [CH5] in [CH3CO] and Sb . Because methyl cation is very unstable, is consistent with Dinér’s8 previously calculated value and the this C-H activation pathway is lower in energy than hydride 33 transfer and protonation pathways. In contrast, for isobutane, experimentally measured gas phase value of ~40 kcal/mol. the hydride transfer transition pathway is a significantly lower energy route than C-H activation, which leads to tert-butyl cat- V ion and an Sb -H intermediate that provides a pathway to H2 formation. Results and Discussion V Models and methods. Sb F5 contains small quantities of HF, and protons are produced in the reaction with methane. There- fore, reaction pathways were referenced to the ground state of V - + V - + [Sb F6] [H] (and [Sb 2F11] [H] for a dinuclear model). This refers to the ion pair that forms with coordination of HF to V Sb F5. See the Supporting Information (SI) for discussion of al- V ternative possible ground states, such as F5Sb (CO) and V 8 F5Sb (CH4). Dinér used a similar ground state, and this is con- sistent with Estevez et al’s5-7 previous DFT reports examining V plausible structures for the combination of Sb F5 and HF. Be- V cause of the limited quantity of HF in Sb F5 we did not use the V - + [Sb F6] [H2F] model, which was identified by Kim and Klein’s Car-Parrinello molecular dynamics simulations.22-25 Im- portantly, this ground state allowed direct comparison of proto- nation and Sb-mediated reaction pathways. We used M06/Def2-TZVPDD26-28//M06/6- 31+G**[LANL2DZdp29,30 for Sb]31 in Gaussian 0932 to opti- mize all minima and transition-state structures, and energies correspond to 298 K. We also compared M06 energies with B97X-D and MN15 energies, and the aug-cc-pVTZ basis set. These functionals gave very similar results to M06. Free ener- gies are the sum of E(SCF,Def2-TZVPDD) + EZPE(6-31G**[LANL2DZdp]) + U(6-31G**[LANL2DZdp]) + nRT  TS(6-31G**[LANL2DZdp]) + Gsolv(Def2- TZVPDD). Because the isobutane reaction was carried out in SO2FCl, as an approximation for bulk solvent effects, particu- larly dielectric stabilization, we used the SMD continuum sol- vent model for dimethylsulfoxide. While this approach is crude, inclusion of solvent-induced electrostatic stabilization is im- portant because hydride transfer and protonation pathways tran- Scheme 2. a) Calculated methyl cation pathway that begins with the me- sition states develop significant charge. We also used this con- thane protonation transition state TS1. b) Calculated hydride transfer with tinuum solvent model for the methane oxidation reaction, how- simultaneous CO capture of methyl cation. c) C-H activation and CO func- V tionalization transition states. d) 3D depictions of TS1-TS5.34 ever, because this reaction was reported in neat Sb F5, we also report relevant gas phase values, which is similar to nearly all V previous DFT studies that examined Sb F5.

Scheme 3. Energy landscape comparing methane protonation, hydride transfer, and C-H activation reaction pathways for the mononuclear Sb model. Gibbs free energies (enthalpies) in kcal/mol.

transfer barrier to G‡ = 41.0 kcal/mol and H‡ = 33.1 kcal/mol. ‡ With our best estimates indicating the formation of methyl cat- However, the >40 kcal/mol G value is unlikely to account for V - ion to be unlikely, we considered pathways that circumvent this reactivity at 60 or 80 C. Also, if [(HF)F5Sb H] were to be high-energy intermediate. Therefore, we examined CO induced formed it would likely generate H2 or transfer the hydride to V - + + H2 displacement from [Sb F6] [CH5] through transition state [CH3CO] to give acetaldehyde. Consistent with this argument, - TS2 (Scheme 2a). This transition state provides H2 and the the calculated barrier for protonation of [(HF)F5SbH] with HF + V - V [CH3CO] [Sb F6] ion pair. This pathway is only slightly lower is 50.7 kcal/mol, indicating that an Sb -H bond is highly acidic. V - + ‡ in energy than H2 dissociation from [Sb F6] [CH5] with a G With protonation and hydride transfer pathways being incon- of 49.3 kcal/mol, which suggests that H2 dissociation is not fea- sistent with the relatively mild conditions for conversion of me- + sible. We also located a transition state similar to TS2, but with thane to [CH3CO] , we then considered the possibility of C-H V - V [Sb F6] displacing H2, which was higher in energy than TS2. activation involving an Sb -Me intermediate, which could un- This suggests that methane protonation occurs to achieve dergo SbV to SbIII reduction. The C-H activation transition state V - + [Sb F6] [CH5] , but that this ion pair is impeded from losing H2 involves interaction of the methane C-H bond to the Sb metal and this is unlikely to be a productive avenue for formation of center followed by a -bond metathesis type transition state + V - + [CH3CO] (Scheme 3 pathway), which is consistent with both TS4 (Scheme 2c,d) that leads to [(HF)F5Sb -Me] [H] . Relative V - + ‡ ‡ the experimentally observed H/D exchange in methane and lack to [Sb F6] [H] , the G for TS4 is 35.1 kcal/mol and H is V - + of observed H2. 26.3 kcal/mol. The H for forming [(HF)F5Sb -Me] [H] is With the protonation pathway unlikely to account for formation slightly exergonic with G = -7.8 kcal/mol. Perhaps surpris- + of [CH3CO] , we then explored Sb-mediated hydride transfer, ingly, TS4 is 5.9 kcal/mol lower in Gibbs energy and 6.8 III which could then undergo reductive elimination to give Sb F3 kcal/mol lower in enthalpy compared with TS3. In the gas phase and HF. See the SI for details of reaction pathways for hydro- the G‡ value between TS4 and TS3 increases to 18.3 gen atom transfer (HAT), electron transfer (ET), and proton- kcal/mol. Also, comparison of TS4 and TS3 at 80 C and at 34 coupled electron transfer (PCET) pathways, which are less via- atm of methane and 19 atm of CO (de Rege conditions) did not ble than hydride transfer. While Hogeveen suggested the possi- significantly change this energy difference. All other function- bility of a hydride transfer mechanism between alkanes and als and basis sets examined also show that TS4 is lower in en- V 11,17 4 Sb F5, this mechanism was dismissed by Olah. However, ergy than TS3 (see SI). For example, B97X-D/Def2- ‡ this type of mechanism is generally proposed for carbocation TZVPDD gave a G (TS3-TS4) value of 15.0 kcal/mol. With the V 35 formation for reaction between Sb F5 and alkyl fluorides. For viability of TS4, we examined several variations. For example, V ‡ methane, hydride abstraction of methane by Sb F5 requires G we explored the possibility of a C-H activation transition state ‡ V + = 56.9 kcal/mol (H = 45.3 kcal/mol) and gives the from the more reactive [F4Sb ] . However, fluoride dissociation − + [(HF)F5SbH] [CH3] ion pair. While this relatively large barrier is very unfavorable (G = 86.3 kcal/mol) and while intramolec- V for hydride abstraction is perhaps not unexpected based on the ular fluoride transfer can occur for Sb 2F10, the fluoride bridges hydride affinity of methyl cation (312 kcal/mol),36 we examined and does provide a vacancy around Sb for methane coordination if hydride transfer would be more feasible if facilitated by CO and activation. leading to overall hydride substitution. TS3 (Scheme 2b,d) with a relatively bent Sb-H-C angle and highly stretched C-H partial V - bond length provides a one-step route to [(HF)F5Sb H] + [CH3CO] . Indeed, the presence of CO does lower the hydride

Scheme 4. Energy landscape comparing methane protonation, hydride transfer, and C-H activation reaction pathways for the dinuclear Sb model. Gibbs free energies (enthalpies) in kcal/mol. Italicized values are gas phase.

ion pair is low in energy, but H2 dissociation via TS2 is rela- + tively high in energy at 41.3 kcal/mol. While this relatively Upon C-H activation, a plausible route to [CH3CO] involves large free energy fits with the experimental lack of H2 for- SbV-Me bond functionalization with CO. We located TS5 mation, and is close to the experimental barrier for H2 dissocia- (Scheme 2c,d) where there is displacement of reduced Sb with + tion from [CH5] , the entropy penalty is likely overestimated. simultaneous C-C bond formation. The overall reaction thermo- + III Importantly, even though TS2 is an ion pair, in the gas phase dynamics leading to [CH3CO] and Sb is G = -37.7 kcal/mol. this transition state is significantly higher in energy, with G‡ = While the SbV-Me intermediate is slightly exergonic relative to  ‡ the starting ground state, which indicates it is potentially ob- 68.7 kcal/mol and H = 50.0 kcal/mol. servable, the barrier for functionalization is relatively low. The Similar to the mononuclear energy surface, the dinuclear hy- G‡ for TS5 is 25.9 kcal/mol (H‡ = 19.2) relative to the SbV- dride and C-H activation transition states are lower in energy Me intermediate. This suggest that at the temperature and pres- than TS2. The C-H activation transition state is lower in Gibbs sure conditions required to undergo C-H activation that the SbV- free energy and enthalpy, but only by 2.8 and 2.3 kcal/mol, re- Me intermediate should rapidly undergo CO functionalization. spectively. While there remains a clear preference for the C-H We also explored alternative SbV-Me bond functionalization activation pathway, the smaller energy difference with the di- pathways that were found to be higher in energy. For example, nuclear model could mean that the hydride pathway may occur SbV-Me bond heterolysis without CO is 24.3 kcal/mol higher in as a competitive pathway. In the gas phase, however, as ex- energy than TS5 and SbV-Me bond homolysis requires >60 pected due to the lack of stabilization for the charge build up in kcal/mol. TS3, the C-H activation transition state is lower in Gibbs free With C-H activation and CO functionalization identified as a energy and enthalpy by more than 10 kcal/mol. We also exam- + ined the relative energies of TS3 and TS4 using M06-2X and plausible pathway to explain the observation of [CH3CO] with- B97X-D functionals. For M06-2X/Def2-TZVPPD and M06- out forming H2, we continued to examine several other less plausible pathways. For example, we examined the possibility 2X/aug-cc-PVTZ the C-H activation transition state TS4 is V III lower in Gibbs free energy by 5.3 and 5.7 kcal/mol, respec- of Sb F5 undergoing reductive elimination to give Sb F3 and tively. For B97X-D/Def2-TZVPPD and B97X-D /aug-cc- F2, which could oxidize methane. While the G for this reaction is exergonic, one-step and two-step reductive elimination tran- PVTZ the C-H activation transition state TS4 is lower in Gibbs sition states require transition states with very large barriers free energy by 5.2 and 5.7 kcal/mol, respectively. All of these (G‡ >80 kcal/mol). We also ruled out fluorine and Sb centered calculations suggest that while hydride transfer could be com- petitive it would likely contribute in a minor amount to forming radical pathways resulting from Sb-F bond homolysis, which + has a bond strength of >80 kcal/mol. [CH3CO] . Again, similar to the mononuclear energy surface, while the SbV-Me intermediate is slightly exergonic the barrier While the mononuclear Sb model suggests that C-H activation for functionalization is <10 kcal/mol on the Gibbs surface and is a lower energy route to methane oxidation than either proto- <5 kcal/mol on the enthalpy surface. However, these very low nation or hydride transfer pathways, we wondered if a dinuclear barriers are due to solvent stabilization of the polarized transi- Sb model would alter this conclusion. Therefore, we examined tion state TS5. these pathways beginning with the ground state model V - + V Isobutane oxidation. As outlined in Scheme 1b, reaction of [Sb 2F11] [H] (dinuclear bridged Sb 2F10 with HF coordina- V isobutane with Sb F5 was reported without CO, and results in tion). Scheme 4 presents the solvent stabilized and gas phase III Sb F3 with observation of both H2 and tert-butyl cation. While dinuclear pathway energies. The qualitative shape of this dinu- the C-H activation pathway with an SbV-R intermediate pro- clear surface is similar to the mononuclear surface. Again, me- V - + V - + vides an explanation for the methane oxidation, it is unlikely thane protonation [Sb 2F11] [H] leading to the [Sb2 F11] [CH5]

that C-H activation would result in isobutane oxidation and gen- pathways to become favored where they are more dramatically erate H2. Therefore, for isobutane, using mononuclear and di- influenced by the alkane structure, as is demonstrated here for nuclear Sb models, we examined protonation and hydride trans- hydride transfer. fer, and compared these pathways to the C-H activation We examined several possible pathways for formation of H2 af- (Scheme 5). ter generation of the anionic SbV-H, and several plausible path- To us, it was surprising that the barrier for protonation of iso- ways emerged. The most viable is by SbV-H protonation of iso- butane through TS6 (G‡ = 15.2 kcal/mol and H‡ = 2.2 butane through TS10 (Scheme 5d). Importantly, the anionic kcal/mol for the dinuclear case, Scheme 5a) is similar to the SbV-H hydrogen is highly acidic, and we found an extremely barrier for methane protonation. However, this is consistent low barrier for protonation of isobutane. TS10 has a G‡ value with previous computational results.5-8 In contrast to the more of <5 kcal/mol relative the SbV-H intermediate. This protona- V III than 50 kcal/mol endergonic value to forming methyl cation and tion also results in Sb to Sb reduction. Generation of H2 fol- + H2 from [CH5] , it is exergonic to form to form tert-butyl cation lows by dissociation by TS7. This proposed pathway for H2 for- + - and H2 from [C4H11] . With [Sb2F11] as a weak assisting nucle- mation is consistent with experients that show that the ratio of 10-16 ophile, there is a low barrier for H2 dissociation by TS7 H2 to isobutane is <1. (Scheme 5a). While there are relatively low barriers for isobutane protonation and H2 dissociation, there is a lower barrier for hydride transfer. PCET, ET, and HAT have significantly larger barriers or unfa- vorable thermodynamics compared to hydride transfer (see SI). V - + Relative to [Sb 2F11] [H] and isobutane, TS8 (Scheme 5b) has a G‡ value of 7.8 kcal/mol, which is ~26 kcal/mol lower in energy than TS3 for methane, and is consistent with the signif- icantly lower hydride affinity of tert-butyl cation compared to methyl cation. The G of the anionic SbV-H and tert-butyl cat- ion ion pair is -20.2 kcal/mol, which is consistent with its ex- perimental observation. To confirm that C-H activation is a higher energy pathway than hydride transfer and protonation for isobutane, we calculated TS9 (Scheme 5c). Although the tertiary C-H bond of isobutane is homolytically weaker than the methane C-H bonds, the C-H activation G‡ of 40.3 kcal/mol (Scheme 5, TS9) for isobutane is somewhat larger than the barrier for methane, likely due to isobutane repulsion. The slightly larger isobutane C-H activa- Scheme 5. a) Calculated tert-butyl cation pathway that begins with proto- tion barrier compared to methane is consistent with Cundari’s nation of isobutane. b) Isobutane hydride transfer transition state. c) Isobu- tane C-H activation transition state. d) Protonation of isobutane by SbV-H previous computational study on Co-mediated alkane C-H acti- formed from hydride transfer. vation.37 Because the barrier for C-H activation is relatively in- sensitive to the alkane structure this allows non-C-H activation Corresponding Author Conclusions *[email protected] *[email protected] DFT calculations were presented that support a new mechanism V for oxidation of methane with Sb F5. A low barrier for methane V - + Funding Sources protonation by [Sb F6] [H] was found, but this pathway is a This work was supported by the U.S. Department of Energy, Of- dead end and reversible because barrier for H2 dissociation is very large because of the instability of methyl cation. Reversi- fice of Science, Basic Energy Sciences, Catalysis Science Pro- ble methane protonation is consistent with the experimentally gram, under Award # DE-SC0018329. observed hydrogen-to-deuterium exchange. As a very viable al- ACKNOWLEDGMENT ternative to the methyl cation pathway, the C-H activation/- bond metathesis mechanism results in an SbV-Me intermediate, We thank Hyconix Inc. + which can be functionalized by CO resulting in [CH3CO] . In contrast to methane, due to the much lower carbocation hydride ABBREVIATIONS affinity, hydride transfer is a much lower pathway for isobutane DFT, Density Functional Theory. and results in tert-butyl carbocation and H2 is formed by the resulting SbV-H protonation of isobutane. NOTES The authors declare no competing financial interests. ASSOCIATED CONTENT Supporting Information. Xyz coordinates, thermochemistry, gas REFERENCES phase optimized energies, H2 reduction of SbF5, and discussion of 1. Olah, G. A., “Electrophilic methane conversion.” Acc. Chem. alternative reaction pathways. Res. 1987, 20, 422-428. 2. Olah, G. A.; Klopman, G.; Schlosberg, R. H., “Super acids. III. AUTHOR INFORMATION Protonation of alkanes and intermediacy of alkanonium ,

+ penta-coordinated cations of CH5 type. Hydrogen ex- “Main-Group Compounds Selectively Oxidize Mixtures of change, protolytic cleavage, hydrogen abstraction; polycon- Methane, Ethane, and Propane to Alcohol Esters.” Science densation of methane, ethane, 2,2-dimethylpropane and 2014, 343, 1232. 2,2,3,3-tetramethylbutane in FSO3H-SbF5.” J. Amer. Chem. 22. Kim, D.; Klein, M. L., “Liquid with an Ex- Soc. 1969, 91, 3261-3268. cess Proton: Ab Initio Molecular Dynamics Study of a Super- 3. Olah, G. A.; Halpern, Y.; Shen, J.; Mo, Y. K., “Electrophilic acid.” J. Amer. Chem. Soc. 1999, 121, 11251-11252. reactions at single bonds. III. H-D exchange and protolysis 23. Kim, D.; Klein, M. L., “Ab Initio Molecular Dynamics Study (deuterolysis) of alkanes with superacids. The mechanism of of the System SbF5/HF Solution.” J. Phys. Chem. B acid-catalyzed hydrocarbon transformation reactions involving 2000, 104, 10074-10079. the σ electron pair donor ability of single bonds via three-center 24. Raugei, S.; Klein, M. L., “Structure of the strongly associated bond formation.” J. Amer. Chem. Soc. 1971, 93, 1251-1256. liquid : An ab initio molecular dynam- 4. Olah, G. A.; Halpern, Y.; Shen, J.; Mo, Y. K., “Electrophilic ics study.” J. Chem. Phys. 2002, 116, 7087-7093. reactions at single bonds. XII. Hydrogen-deuterium exchange, 25. Raugei, S.; Klein, M. L., “Hydrocarbon Reactivity in the Su- protolysis (deuterolysis), and oligocondensation of alkanes peracid SbF5/HF: an ab Initio Molecular Dynamics Study.” J. with superacids.” J. Amer. Chem. Soc. 1973, 95, 4960-4970. Phys. Chem. B 2002, 106, 11596-11605. 5. Esteves, P. M.; Alberto, G. G. P.; Ramirez-Solis, A.; Mota, C. 26. Metz, B.; Stoll, H.; Dolg, M., “Small-core multiconfiguration- J. A., “The alkane σ-bond basicity scale revisited.” J. Am. Dirac–Hartree–Fock-adjusted pseudopotentials for post-d Chem. Soc. 1999, 121, 7345-7348. main group elements: Application to PbH and PbO.” J. Chem. 6. Esteves, P. M.; Ramírez-Solís, A.; Mota, C. J. A.; “A Theoret- Phys. 2000, 113, 2563-2569. ical Study of Alkane Protonation in HF/SbF5 Superacid Sys- 27. Rappoport, D.; Furche, F., “Property-optimized Gaussian basis tem.” J. Braz. Chem. Soc. 2000,11, 345-348. sets for molecular response calculations.” J. Chem. Phys. 2010, 7. Esteves, P. M.; Ramirez-Solis, A.; Mota, C. J. A., “DFT Cal- 133, 134105. culations on the Protonation of Alkanes on HF/SbF5 Superac- 28. Weigend, F.; Ahlrichs, R., “Balanced basis sets of split va- ids Using Cluster Models.” J. Phys. Chem. B 2001, 105, 4331- lence, triple zeta valence and quadruple zeta valence quality for 4336. H to Rn: Design and assessment of accuracy.” Phys. Chem. 8. Dinér, P., “Superacid-Promoted Ionization of Alkanes Without Chem. Phys. 2005, 7, 3297-3305. Formation: A Density Functional Theory 29. Check, C. E.; Faust, T. O.; Bailey, J. M.; Wright, B. J.; Gilbert, Study.” J. Phys. Chem. A 2012, 116, 9979-9984. T. M.; Sunderlin, L. S., “Addition of Polarization and Diffuse 9. Hogeveen, H.; Gaasbeek, C. J., “Chemistry and spectroscopy Functions to the LANL2DZ Basis Set for P-Block Elements.” in strongly acidic solutions. XV. Electrophilic substitution at J. Phys. Chem. A 2001, 105, 8111-8116. methane.” Recl. Trav. Chim. Pays-Bas 1968, 87, 319-320. 30. Wadt, W. R.; Hay, P. J., “Ab initio effective core potentials for 10. Hogeveen, H.; Bickel, A. F., “Formation of trimethylcarbon- molecular calculations. Potentials for main group elements Na ium ions from isobutane and protons. Basicity of isobutene.” to Bi.” J. Chem. Phys. 1985, 82, 284-298. Recl. Trav. Chim. Pays-Bas 1967, 86, 1313-1315. 31. Schuchardt, K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; 11. Hogeveen, H.; Bickel, A. F., “Chemistry and spectroscopy in Gurumoorthi, V.; Chase, J.; Li, J.; Windus, T. L., “Basis Set strongly acidic solutions. Electrophilic substitution at alkane- Exchange: A Community Database for Computational Sci- carbon by protons.” Chem. Comm. 1967, 635-636. ences.” J. Chem. Inf. Mod. 2007, 47, 1045-1052. Basis sets 12. Olah, G. A.; Schlosberg, R. H., “Chemistry in super acids. I. were obtained from the basis set exchange: https://www.ba- Hydrogen exchange and polycondensation of methane and al- sissetexchange.org/ kanes in FSO3H-SbF5 ("magic acid") solution. Protonation of 32. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; + alkanes and the intermediacy of CH5 and related hydrocarbon Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; ions. The high chemical reactivity of "paraffins" in ionic solu- Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; tion reactions.” J. Amer. Chem. Soc. 1968, 90, 2726-2727. Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; 13. Culmann, J. C.; Sommer, J., “Hydrocarbon oxidation by anti- Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, mony pentafluoride.” J. Am. Chem. Soc. 1990, 112, 4057-4058. R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, 14. Sommer, J.; Bukala, J., “Selective electrophilic activation of O.; Nakai, H.; Vreven, T.; Montgomery Jr., J. A.; Peralta, J. E.; alkanes.” Acc. Chem. Res. 1993, 26, 370-376. Ogliaro, F.; Bearpark, M. J.; Heyd, J.; Brothers, E. N.; Kudin, 15. Sommer, J.; Bukala, J.; Hachoumy, M.; Jost, R., “Reversible K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Protonation of Isobutane in Liquid Superacids in Competition Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; with Protolytic Ionization.” J. Am. Chem. Soc. 1997, 119, Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; 3274-3279. Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; 16. Olah, G. A.; Prakash, G. K. S.; Molnár, Á.; Sommer, J. Super- Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; acid Chemistry, Second Edition, John Wiley & Sons, Inc., Ho- Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; boken, New Jersey, 2009, 511-517. Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; 17. Hogeveen, H.; Lukas, J.; Roobeek, C. F., “Trapping of the me- Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; thyl cation by carbon monoxide; formation of acetic acid from Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gauss- methane.” J. Chem. Soc. D 1969, 920-921. ian 09, Gaussian, Inc.: Wallingford, CT, USA, 2009. 18. De Rege, P. J. F.; Gladysz, J. A.; Horvath, I. T., “Facile and 33. Hiraoka, K.; Kebarle, P., “An application of ion equilibria selective carbonylation of methane in superacids.” Adv. Synth. measurements: Protonated alkanes, protolysis of alkanes and Catal. 2002, 344, 1059-1062. hydride transfer reactions between alkyl cations and alkanes.” 19. Hiraoka, K.; Kebarle, P., “Stabilities and energetics of penta- Rad. Phys. Chem. (1977) 1982, 20, 41-49. coordinated carbonium ions. The isomeric protonated ethane 34. CYLview, 1.0b; Legault, C. Y., Université de Sherbrooke, ions and some higher analogs: protonated propane and proto- 2009 (http://www.cylview.org) nated butane.” J. Amer. Chem. Soc. 1976, 98, 6119-6125. 35. Olah, G. A.; DeMember, J. R.; Schlosberg, R. H., “Friedel- 20. Raghavachari, K.; Whiteside, R. A.; Pople, J. A.; Schleyer, P. Crafts chemistry. III. Methyl fluoride-antimony pentafluoride, V. R., “Molecular orbital theory of the electronic structure of a powerful new methylating agent. Methylation reactions and organic molecules. 40. Structures and energies of C1-C3 car- the polycondensation of methyl fluoride.” J. Am. Chem. Soc. bocations including effects of electron correlation.” J. Amer. 1969, 91, 2112-2113. Chem. Soc. 1981, 103, 5649-5657. 36. Anslyn, E. V. D., Dennis A., page 89 in Modern Physical Or- 21. Hashiguchi, B. G.; Konnick, M. M.; Bischof, S. M.; Gustafson, ganic Chemistry. University Science Books: Sausalito, Califor- S. J.; Devarajan, D.; Gunsalus, N.; Ess, D. H.; Periana, R. A., nia, 2006; pp 1099.

37. a) Bakhoda, A.; Jiang, Q.; Badiei, Y. M.; Bertke, J. A.; Cund- Bellows, S. M.; Cundari, T. R.; Jones, W. D., “Methane Is the ari, T. R.; Warren, T. H., “Copper-Catalyzed C(sp3)−H Ami- Best Substrate for C(sp3)–H Activation with dation: Sterically Driven Primary and Secondary C−H Site-Se- Cp*(PMe3)Co(Me)(OTf): A Density Functional Theory lectivity.” Angew. Chem. Inter. Ed. 2019, 58, 3421-3425. b) Study.” Organometallics 2015, 34, 4032-4038.