Comparison of C-H Activation, Hydride Transfer, and Protonation Pathways for SbF5-Mediated Methane 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 Chemistry 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] ion 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 hydrogen-to-deuterium exchange in methane. In contrast to methane, due to the much lower carbocation 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 alkane 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 alkanes, 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 superacids, 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.