Electronic Effects on Room-Temperature, Gas-Phase CâH Bond

Perspective pubs.acs.org/JACS

Electronic Effects on Room-Temperature, Gas-Phase C−H Bond Activations by Cluster Oxides and Metal Carbides: The Methane Challenge Helmut Schwarz,*,† Sason Shaik,*,§ and Jilai Li*,†,‡ †

Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel ‡ Institute of Theoretical Chemistry, Jilin University, Changchun 130023, P.R. China §

cally stable and kinetically inert molecules continue to pose enormous challenges. With regard to the valorization of simple hydrocarbons, selective C−H bond activation has been regarded in the chemical community as a “Holy Grail”.2 For example, no large-scale, environmentally benign and economically viable processes are yet available for activating these molecules directly and at ambient conditions. Some examples for the paucity of our knowledge are, the conversion of methane to methanol, eq 1, the coupling of the C(1) units CH4 and CO2 to form acetic acid, eq 2, the oxidative dimerization of CH4, eq 3, or the use of methane as a hydrogen source that produces directly hydrogen peroxide or ammonia, eqs 4 and 5.

ABSTRACT: This Perspective discusses a story of one molecule (methane), a few metal-oxide cationic clusters (MOCCs), dopants, metal-carbide cations, orientedelectric fields (OEFs), and a dizzying mechanistic landscape of methane activation! One mechanism is hydrogen atom transfer (HAT), which occurs whenever the MOCC possesses a localized oxyl radical (M−O•). Whenever the radical is delocalized, e.g., in [MgO]n•+ the HAT barrier increases due to the penalty of radical localization. Adding a dopant (Ga2O3) to [MgO]2•+ localizes the radical and HAT transpires. Whenever the radical is located on the metal centers as in [Al2O2]•+ the mechanism crosses over to proton-coupled electron transfer (PCET), wherein the positive Al center acts as a Lewis acid that coordinates the methane molecule, while one of the bridging oxygen atoms abstracts a proton, and the negatively charged CH3 moiety relocates to the metal fragment. We provide a diagnostic plot of barriers vs reactants’ distortion energies, which allows the chemist to distinguish HAT from PCET. Thus, doping of [MgO]2•+ by Al2O3 enables HAT and PCET to compete. Similarly, [ZnO]•+ activates methane by PCET generating many products. Adding a CH3CN ligand to form [(CH3CN)ZnO]•+ leads to a single HAT product. The CH3CN dipole acts as an OEF that switches off PCET. [MC]+ cations (M = Au, Cu) act by different mechanisms, dictated by the M+−C bond covalence. For example, Cu+, which bonds the carbon atom mostly electrostatically, performs coupling of C to methane to yield ethylene, in a single almost barrier-free step, with an unprecedented atomic choreography catalyzed by the OEF of Cu+.

(1)

CH4 + CO2 → CH3CO2 H

(2)

2CH4 → C2H6 − n + Hn

(n = 0, 2)

(3)

CH4 + 3O2 → 2H 2O2 + CO2

(4)

1.5CH4 + 1.5O2 + N2 → 2NH3 + 1.5CO2

(5)

All these and a few other processes are rightly viewed as “dream reactions” for supplying the chemical industry with value-added commodities. In addition, and not only from an academic point of view, some of the challenges are concerned with unraveling the mechanistic details and determining the elementary steps that are associated with these (and related) transformations. Over the last three decades, the Berlin laboratory of one of the authors of this Perspective has been engaged in this endeavor by performing gas-phase experiments with massselected reagents at their electronic ground states and under single-collision conditions. While this approach never accounts for the many features which may prevail at a surface or in solution, nevertheless when complemented by appropriate and adequate computational studies, these seemingly esoteric gasphase experiments have offered insight that has stood the test of time. They proved meaningful for the very reason that they provide a conceptual framework for addressing questions as for example: the identity of the atoms which constitute the active part of a single-site catalyst, the so-called “aristocratic atoms”,3 the identification of which constitutes one of the intellectual cornerstones in contemporary catalysis. Or, how do factors

1. INTRODUCTION Simple C(1) molecules like CH4 or CO2 continue to make headlines in the media as potent green-house gases. However, these gases have also the potential to serve as the most precious C(1) feedstock for chemical industry, not to speak of their role in extraterrestrial chemistry. No wonder that the number of scientific and popular publications of this trivial molecular stuff is sheer endless. For example, a Web of Science database search for “methane activation” results in more than 15 000(!) published articles covering the most diverse aspects one can possibly imagine.1 Further, for more than a century, the activation and selective functionalization of this thermochemi© 2017 American Chemical Society

CH4 + ⟨O⟩ → CH3OH

Received: September 22, 2017 Published: November 7, 2017 17201

DOI: 10.1021/jacs.7b10139 J. Am. Chem. Soc. 2017, 139, 17201−17212

Perspective

Journal of the American Chemical Society

efficient reagents to induce thermal HAT, eq 7, under singlecollision conditions.

such as stoichiometry, cluster size, charge and spin states, oxidation number, degree of coordinative saturation, etc. affect the outcome of a chemical transformation without being obscured by ill-defined “environmental effects”? As has been repeatedly demonstrated, these gas-phase experiments provide an ideal arena for addressing many of these (and other) challenging topics and to probe mechanistic aspects of a chemical processincluding industrially related reactionsin an unperturbed fashion at a strictly molecular level.4 This has been shown, for example, for the DEGUSSA process, that is, the large-scale, platinum-mediated coupling of methane and ammonia to generate HCN.5 Mechanistic insight and suggestions on how to improve the catalytic performance were derived initially from mass-spectrometry-based studies;6 later, these predictions were confirmed by in situ experiments.7 Similarly, metal oxides, capable of bringing about activation of methane under single-collision conditions in the gas phase,4b have served as prototypical systems to probe the active sites in heterogeneous catalysis. In particular, metal-oxide-mediated hydrogen-atom transfer from CH4 to generate CH3• is viewed as a decisive step in the oxidative coupling of methane.4e,8 In fact, oxo-metal complexes of high-valent iron and other transition metals act as metalloenzymes and catalysts which perform C−H activation in biological systems and in solution.9 In addition, metal carbide species are proposed to be present in the condensed phase and acts as catalysts for methane activation.10 As such, this Perspective will focus on the discussion of recently discovered, unexpected mechanistic variants related to the problem of HAT initiated by a suitable metal oxide [MO]. This bond activation process is regarded as the key step in numerous chemical transformations, covering homogeneous, heterogeneous, and enzymatic processes,4k,9g−i,11 and, as mentioned above, HAT plays also a role in the oxidative coupling of methane, eq 3.1d,h,i,8a,b,e,12 In addition to addressing the various mechanistic variants of breaking the C−H bond of methane, we will also discuss a few recent examples of C−C coupling processes which are initiated by activation of the C−H bond of CH4. In this combined experimental/computational approach particular attention will also be paid to unexpected doping and ligand effects as well as the effects of OEFs; the latter topic has met recently enormous interest in quite a few areas of chemistry.13 Finally, we refrain here from describing any experimental techniques and computational methodologies as these (and other details) can be found in the original work given in the References.

[MO•] + H−CH3 → [MO−H] + CH3•

In view of the many review articles that describe in detail quite a few examples for the homolytic cleavage of methane’s C−H bond,4g,j,k,16 in the present context it may suffice, to summarize briefly the most pertinent features of these thermal gas-phase processes: (a) Cationic systems are generally more reactive than structurally related neutral or anionic analogues. (b) For the kinetics of HAT, the presence of an unpaired electron (high spin density) on the abstractor site, preferentially a terminal oxygen atom of [M−Ot•] is crucial. (c) Depending on the structure of the metal-oxide clusters, at least two mechanistically distinct scenarios exist. A direct HAT process prevails predominantly for openshell oxide clusters with metal centers in relatively high oxidation states and with coordination numbers that prevent interaction of a hydrocarbon RH with the metal. The indirect, metal-mediated pathway is generally limited to small, often diatomic metal oxides having vacant sites to permit prior coordination of RH to the metal; here, the metal keeps control of the fate of RH from its initial coordination at the metal site through C−H bond scission at the oxyl radical to the eventual liberation of R•. For both scenarios there exist numerous examples.16 A common electronic feature of the many reactive systems, which others and we studied, is their doublet state, which possesses high spin density at the accepting oxyl site of the metal oxides. What happens, one may ask, if there are no oxyl radicals present, as for example in the even-electron oxo species [MO2] (M = Ti, Zr), [MO2]+ (M = V, Nb), or CaO? These systems, in contrast to the reactive oxides [CaO]•+, [TiO2]•+, and [ZrO2]•+, are unreactive because high intrinsic barriers prevent them from engaging in HAT from CH4.16e The reason for this impediment is the penalty of having to generate a “prepared” state by decoupling the MO bond and developing high spin density at the accepting oxygen atom en route to the transition state.9i,16c,17 Similarly, high barriers also result if the spin in open-shell systems is distributed over various bridging oxygen atoms, as for example in aluminum-oxide cluster ions with an odd number of aluminum atoms, e.g., [Al7O12]+,18 or oligomeric magnesium oxides [MgO]n (n ≥ 2).19 According to computational studies, intracomplex spin transfer in these open-shell systems is energetically disfavored.16a−c One way to alleviate the problem and modify crucial properties is by judicious “doping”,20 such as to localize the spin density around a single reaction center. As a typical example, let us discuss the reactivity of [MgO]n•+ (n = 1−7) cluster oxides toward methane. Thus, while diatomic [MgO]•+ initiates HAT even from CH4,19a the larger cluster oxides are completely inert toward this substrate,19b even though HAT is exothermic. Why is this so? In contrast to [MgO]•+, in the dimer [MgO]2•+ the spin is equally distributed over the two bridging oxygen atoms of the clusters (see structure R1 in the inset of Figure 1), resulting in a transition state TS1a/2a too high in energy to be accessible at room temperature.21 However, “doping” the cluster with the Lewis acid Ga2O3 changes the reactivity completely in that even from CH4 a hydrogen atom is abstracted at room temperature. This

2. HAT: THE ROLE OF SPIN AND RADICAL DENSITY IN CLEAVING THE C−H BOND While oxygen-centered radicals, [O]•−, are not likely to act as active sites in “doped” MgO-based catalysts for the oxidative coupling of methane, eq 3,1h,i,g,8d,14 there exists compelling evidence that, if these species are generated, their reactions with hydrocarbons including the rather inert methane will be facile even at ambient conditions. This has already been demonstrated decades ago for the efficient room-temperature reactions of atomic [O]•− with alkanes in the gas phase, eq 6.15 [O]•− + CnH 2n + 2 → [OH]− + CnH 2n + 1•

(7)

(6)

Also, as shown repeatedly, “naked” metal-oxo clusters bearing oxygen-centered radicals are superb, extremely reactive and 17202


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There are two more aspects, which deserve brief mentioning. (a) H e t e r o n u c l e a r c l u s t e r o x i d e s , fo r e x a m p l e , [V4‑nPnO10]•+,23 [(V2O5)n(SiO2)m]•− (n = 1, 2; m = 1− 4),24 or [VnAlmOo]•± (n + m = 2, 3, 4; o = 3−10),25 all possess a high-spin density located at a terminal oxygen atom that is not bound to the transition-metal vanadium but to the main-group atom, that is [Al−Ot•], [Si−Ot•], or [P−Ot•], respectively. The VOt unit is completely inert. (b) “Doping” of metal oxide clusters can bring about an increase in both reactivity and selectivity! For example, homonuclear [Y2O3]•+ is not capable of activating CH4 at room temperature. The root cause can be ascribed to the fact that the spin density in [Y2O3]•+ is delocalized over two bridging oxygen atoms; thus it lacks a terminal Ot•− group bearing significant spin density.16a,26 In contrast, [Al2O3]•+ reacts and gives rise to HAT and CH2O in a branching ratio of 1:2,27 due to its different electronic structure. The spin density is solely localized on the terminal Ot•−, which acts as a powerful hydrogenatom abstractor from methane. In comparison, the delocalized spin density in [Y2O3]•+ becomes localized on the terminal oxyl group by replacing one Y atom with Al. Therefore, the heteronuclear cluster [YAlO3]•+ in its thermal reaction with CH4 is much more reactive than [Y2O3]•+ and more selective than [Al2O3]•+, giving rise to a single product pair [YAlO2(OH)]+ and CH3•.28

Figure 1. Potential energy surfaces (energies in kJ mol−1 relative to the entrance channels) and key ground-state structures involved in the reactions of [Mg2O2]•+ and [Ga2Mg2O5]•+ with CH4, calculated at the G4MP2-6X level of theory. The insets show the ground state structures of the two cluster oxides; blue isosurfaces indicate the spindensity distributions. Adapted with permission from ref 21. Copyright 2015 Wiley-VCH.

change in reactivity is due to the fact that in [Ga2Mg2O5]•+ (R2) the spin density of a bridging oxygen atom at the active site of the cluster is increased to 0.9 compared to 0.5 on each oxygen atom in R1. As a consequence, the relevant TS1b/2b is lowered to the extent that HAT from CH4 becomes possible at ambient conditions, Figure 1. Effects of “doping”-induced local charge distribution around the active oxyl center, have been investigated by the He group.16a,b,22 As shown in Figure 2, the relative rate constants, krel, for HAT from CH4 correlate quite well with the local positive charges [QL] around the [M−O•] centers. Thus, the enormous differences in HAT reactivity suggest that changing the charge distribution within a small cluster by judicious “doping” can affect significantly the barriers for HAT. We will return to this important finding later in a different context.

3. C−H BOND ACTIVATION BY PROTON-COUPLED ELECTRON TRANSFER (PCET) Yet another possibility to bypass high intrinsic barriers in HAT manifests through a change in mechanism; here, the conventional homolytic cleavage of a C−H bond is replaced by a heterolytic variant using PCET.16c,d,29 That is, while the proton and the electron pair originate from the same C−H bond, they are transferred to different sites of the acceptor with the proton ending up at an oxo group and the electron pair, disguised as R−, at the metal site. As an illustration, let us discuss the recently studied [Al2O2]•+/CH4 couple.30 With an efficiency (ϕ) of 10.3%, relative to the collision rate, at room temperature the C−H bond of CH4 is cleaved. The intramolecular kinetic isotope effect (KIE) derived from the [Al2O2]•+/CH2D2 system amounts to KIE = 2.9, clearly indicating that breaking of the C−H(D) bonds contributes to the rate-limiting step. Although the oxygen-deficient oxide cluster [Al2O2]•+ lacks a reactive oxyl unit, in the reaction with methane it is slightly more efficient than the [Al2O3]•+/CH4 couple (ϕ = 7%); the latter bears a terminal oxyl group.27 The most stable structure of [Al2O2]•+ has a rhombus like geometry with the spin evenly distributed over the two aluminum atoms (R3 in Figure 3). For the reaction of R3 with CH4, two reactions routes, A and B, were located on the doublet ground-state surface. Both start with the formation of an encounter complex 3. Notably, 3 is highly stabilized by a significant electrostatic interaction between the Lewis-basic carbon atom of methane and the Lewis-acidic aluminum coordination site of the cluster oxide. Let us consider first the conventional HAT, pathway A. Since the spin is localized on the Al atoms, one might have thought that they would abstract an H-atom. This is, however, not the case since Al• is a poor H-abstractor due to rather unfavorable

Figure 2. Variation of the experimentally determined relative rate constants (krel) with respect to the calculated local charges (QL) with k rel = k 1 (X + CH 4 )/k 1 (V 4 O 10 + + CH 4 ); X = [Al 8 O 12 ] + ; [V2O5(SiO2)1−4]•+, and [V4−nYnO10−n]•+, n = 0−2. Adapted with permission from ref 22. Copyright 2011 Wiley-VCH. 17203


Perspective


simplicity, and the only considered HAT is the one mediated by the bridging oxygens. However, since the spin is not located on the oxygen atoms, the [Al2O2]•+ cluster has to undergo electronic reorganization to create an O-centered spin, for which it will have to pay a significant penalty that results in a high barrier. Indeed, as shown in Figure 3, the HAT starting from 3 to 4 via TS3/4 is not accessible at ambient conditions as the reaction is associated with a barrier that lies 88 kJ mol−1 higher in energy than the entrance channel. In contrast, in pathway B, starting from 3, a C−H bond is cleaved and the hydrogen-atom is transferred to the bridging oxygen atom via transition state TS3/5; the latter is located 28 kJ mol−1 below the separated reactants. This process results in the formation of a rather stable intermediate 5. To generate the formal HAT products, the methyl group evaporates from the cluster; this process is accompanied by the opening of the [Al2O2] ring thus forming the linear-shaped hydroxide product ion P2. Pathway B has all features of a heterolytic cleavage of the C−H bond of methane by the [Al+−O−] unit of the cluster. In line with the observed KIE, the rate-determining step corresponds to the activation of a C−H bond via transition state TS3/5. Here, the basic oxygen moiety abstracts the hydrogen as a proton, while the CH3 group moves with the electron pair, as anionic CH3−, and forms a bond with the positively charged aluminum center. The experimental/computational findings on [Al2O2]•+/CH4 as well as those obtained computationally for other heteronuclear cluster oxides [XYO2]•+ (X, Y = Al, Si, Mg)30 provide an unexpected mechanistic analogy to the recently discussed oxidative coupling of methane, catalyzed by magnesium oxide surfaces. Based on elaborate quantummechanical work, it has been suggested that Grignard-like

Figure 3. Potential energy profiles for the reaction of [Al2O2]•+ with CH4 calculated at the CCSD(T)/CBS[AVTZ:AVQZ]//B2GP-PLYP/ def2-TZVP level of theory. Key bond lengths (Å) are also given. The inset shows the ground-state structure of [Al2O2]•+. The yellow and cyan lobes indicate the NBO-calculated spin density distributions. Charges are omitted for the sake of clarity. Adapted with permission from ref 30. Copyright 2016 American Chemical Society.

thermodynamics. Indeed, the calculations indicate that the energy of the system invariably goes up during a hydrogenatom transfer from methane to the Al atom; in fact, this process is rather endothermic by 120 kJ mol−1, relative to the separated reactants. Thus, it is omitted from Figure 3 for the sake of

Figure 4. Potential energy profiles (ΔH, kJ mol−1) and schematized key ground-state structure involved in the reaction of R4 with CH4. All relative energies, except TS-Ot and TS-Al/Oc are calculated at the B2GP-PLYP/def2-QZVPP//B2GP-PLYP/def2-TZVP level; TS-Ot and TS-Al/Oc are based on CCSD(T)/CBS[VTZ:VQZ]//B2GP-PLYP/def2-TZVP calculations. The energies are corrected by ZPVE contributions. Adapted with permission from ref 31. Copyright 2016 American Chemical Society. 17204


Perspective

Journal of the American Chemical Society intermediates with a Mg−CH3 structural motif, eq 8, are formed; the latter can be generated upon a heterolytic cleavage of the C−H bond of CH4.14a [Mg 2 +O2 −]MgO + H−CH3 → [HO−(Mg−CH3)+ ]MgO (8)

A competition between the conventional homolytic HAT process and a PCET variant has been encountered in the experimental/computational investigation of the heteronuclear [Al2Mg2O5]•+/CH4 couple.31 Experimentally, hydrogen-atom abstraction takes place at ambient conditions. As to the mechanism, based on the electronic structure investigation, both PCET and the conventional HAT are feasible and compete with each other. An electronic structure analysis reveals the origin of this competition. For the PCET mechanism, it is the Lewis acid−base pair [Al+−O−] of the cluster that acts as the active site. On the other hand, for the energetically only slightly disfavored but entropically favored HAT (Figure 4, pathway A) the terminal [Al−O•] unit serves as acceptor site for the hydrogen atom. For the PCET mechanism (pathway B), a clear correlation has been established between the electronic nature of the transition state TS8/9, the corresponding barrier height, the Lewis acidity-basicity of the [M+−O−] unit as well as the bond order of the [M+−O−] bond. Quite likely, the knowledge of the factors that control the electronic features of these two competing pathways not only deepen our mechanistic understanding of metal-oxide mediated C−H bond activation, but may also provide conceptual guidance for the rational design of catalysts.31 An example of this helpful insight is the theoretical diagnostic9h of HAT vs PCET that emerges from a plot of the barriers vs the distortion energies of the reactants, which is shown in Figure 5 for heteronuclear clusters reacting with methane.31 It is seen that all the HAT cases (in red color) lie close the line defined by ΔE⧧ = ΔEdef; in contrast, all the PCET cases have distortion energies much higher than the corresponding barriers, showing that the distorted reactants in the TS are highly stabilized by favorable interactions (electronic and electrostatic). Thus, an experimental chemist using this diagnostic plot alongside the experimental results can instantly classify the mechanisms, which transpire during the reaction. Another diagnostics comes from inspection of the singly occupied orbitals of the respective TSs. Taking just two examples, as shown in Scheme 1 HAT is characterized by a singly occupied orbital, σ*(O−H−C), with a node on the H atom in transit. However, in the competing PCET mechanism the unpaired electron occupies a spectator orbital uninvolved in the reactive moiety. The particular mechanisms, i.e., HAT vs PCET, as well as the product distributions are not only affected by the composition of cluster oxides and their “doping”, but also by the ligand to the metal, as observed in a recent study, which exhibited an entirely unexpected ligand effect.32 The room-temperature gas-phase reaction of pristine diatomic [ZnO]•+ with methane, proceeds with an efficiency of 40% (relative to the collision rate), and generate the products shown in eqs 9−11. Remarkably, upon ligation of [ZnO]•+ with CH3CN, the couple [(CH3CN)ZnO]•+/CH4 produces a single product ion, eq 12, with an efficiency of ϕ = 12%. Quite clearly, ligation of bare [ZnO]•+ affects dramatically the selectivity toward CH4 and, as will be shown next, brings

Figure 5. Plot of the sum of the deformation energies of the reactants in the TS (ΔEdef, kJ mol−1) versus the corresponding barriers ΔE⧧ (kJ mol−1) relative to the encounter complexes, for the TSs corresponding to the first C−H bond activation of methane. The line is drawn with a slope of unity such that ΔEdef = ΔE⧧ while the vertical distance from the line gauges the interaction energy between the deformed reactants at the TSs. The black squares correspond to a PCET mechanism, while the red squares correspond to a HAT mechanism. Adapted with permission from ref 31. Copyright 2016 American Chemical Society.

Scheme 1. Singly Occupied Orbitals (Quasi-restricted Orbitals) for Transition States in the HAT and PCET Pathways of the Heteronuclear [Al2Mg2O5]•+/CH4 Couplea

a

Adapted with permission from ref 31. Copyright 2016 American Chemical Society.

about a mechanistic switch. For both couples, the intramolecular kinetic isotope effect (KIE) amounts to 2.5, thus indicating that the C−H(D) bond cleavage contributes to the rate-limiting step. [(CH3CN)ZnO]•+ + CH4 100%

⎯⎯⎯⎯→ [(CH3CN)Zn(OH)]+ + CH3•

(12)

Mechanistic insight has been provided by high-level quantum chemical calculations (Figure 6) which were confined to the doublet states of [ZnO]•+ and [(CH3CN)ZnO]•+. As the corresponding quartet states are higher in energy by 115 and 17205


Perspective


Most interestingly, the effect of the ligand on both the branching ratio as well as the actual mechanism, i.e., HAT versus PCET could be mimicked by replacing the CH3CN ligand with an oriented external electric field (negative point charge). This finding suggests that both the mechanistic switch and the selectivity of a catalyst may be tuned at will by an oriented external electric field (OEEF).13a−c,e−i,32 Such a technique had been utilized recently to catalyze a Diels− Alder reaction in an elegant experiment.33

4. METHANE ACTIVATION BY CLOSED-SHELL REAGENTS While the majority of HAT reactions involve radical abstractors, there has been a growing repertoire of reactions in solution wherein the cleavage of a C−H bond (however, mostly from relatively weak C−H bonds) involves closed-shell molecules, e.g., CrO2Cl2, MnO4−, etc. The high barriers for a conventional HAT process involving these and other closed-shell abstractors are due to the additional promotion energy that is required in order to create a radical center along the reaction coordinate and prepare the abstractor for pulling away a hydrogen atom from the hydrocarbon RH. “Mixing” of the PCET states into the HAT states mitigates, however, these high barriers; in the extreme, the conventional homolytic HAT process is replaced by a heterolytic cleavage of the C−H bond via a PCET mechanism.34 How do things look like in the gas phase? Until a few years ago, no single even-electron metal-oxo species was known to bring about thermal hydrogen abstraction from methane. However, even-electron [CuO]+ having 16 valence electrons, reacts with CH4 with high efficiency to produce CH3• and CH3OH.35 This is due to the fact that [CuO]+ corresponds to [CuII−O•]+ with a triplet ground state, and the reaction is initiated by the oxyl center. In a metal-mediated HAT process, the CH3• radical is generated. For the formation of CH3OH, however, along the reaction coordinate via spin−orbit coupling a switch from the triplet ground state to the excited singlet state occurs. Thus, the overall reaction, eq 13, follows a two-state reactivity (TSR) scenario.36

Figure 6. Simplified potential-energy surfaces (energies in kJ mol−1 relative to the entrance channels and corrected for zero-point energy and thermal contributions) and some relevant structural information on the couples [ZnO]•+/CH4 and [(CH3CN)/ZnO]•+/CH4 as calculated at the CCSD(T)/CBS//BMK level of theory. Bond lengths are given in Å and bond angles in degrees; for the sake of clarity, charges are omitted. Adapted with permission from ref 32. Copyright 2017 Wiley-VCH.

281 kJ mol−1, most likely, they are not accessed in the experiment. For the [ZnO]•+/CH4 couple, the central intermediate corresponds to [(CH3)Zn[OH)]•+ (11). From 11, in entropydriven reactions, both OH• and CH3• can be liberated. While the generation of CH3OH is thermochemically favored, the requisite rebound step via the tight transition state TS11/12 is, however, kinetically hampered. The rate-limiting step for all three decomposition channels, eqs 9−11, is associated with activation of the C−H bond, i.e., 10 → 11, in line with the observed KIE > 1. In contrast, for the ligated system [(CH3CN)ZnO]•+/CH4, the most favored process is the one that generates the HAT intermediate 14 from which CH3• can be eliminated with ease. All other conceivable decomposition paths are significantly more energy demanding.32 This is in line with the experimental finding that the only product generated corresponds to the expulsion of CH3•, eq 12. In a detailed computational analysis, the electronic origin of this remarkable ligand effect and the mechanistic aspects were addressed. In short, based on an analysis of the NBO charge distributions, the Wiberg bond indices, and a ΔE‡/ΔEdef analysis it was concluded that for the [ZnO]•+/CH4 couple the energetically preferred path via TS*10/11 corresponds to a PCET step. In contrast, for the ligated system the relevant TS13/14 has the features of a classical HAT process. The absence or presence of a ligand at the axial position of Zn is decisive for the bond strengths of Zn−C and the ensuing reactions. In the bare [ZnO]•+ system, the incoming ligand occupies the vacant axial position of the Zn atom resulting in a rather strong dative bond in 10. Upon ligation, the energy of the acceptor orbital of the Zn moiety is pushed up and the charge of the metal center is reduced. As a consequence, complexation of the incoming CH4 molecule is much less exothermic (−31 versus −139 kJ mol−1), and also the Lewis acidity of the metal center, so crucial for PCET, is reduced.

An entirely different mechanistic variant is encountered in the thermal activation of CH4 with [HTiO]+, eq 14.37 This reaction is confined to the electronic singlet ground state, and the ligand switch is brought about by a σ-metathesis process.38 The oxygen ligand in [HTiO]+ is not entirely innocent, in that it weakens the Ti−H bond more than the newly formed Ti− CH3 bond. Thus, the overall exchange of ligands is exothermic. In fact, the oxygen-free metal hydride does not engage in this reaction, eq 15.37 [HTiO]+ + CH4 → [(CH3)TiO]+ + H 2

(14)

[HTi]+ + CH4 →̷[(CH3)Ti]+ + H 2

(15)

In the anionic and closed-shell Au-“doped” titanium oxide cluster [AuTi3O7]−, it is the cooperative interaction of the separated Au+ and O2− ions that initiate a PCET mechanism which brings about the heterolytic C−H bond activation of CH4, eq 16.39 17206


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Journal of the American Chemical Society [AuTi3O7 ]− + CH4 → [Ti3O6 (OH)]− + AuCH3

(16)

A remarkably variable reactivity of the closed-shell [AlCeOn]+ (n = 2−4) cluster ions toward CH4 as a function of oxygen content was recently reported.40 Whereas the cluster ions with n = 2, 4 are unreactive, hydrogen abstraction, eq 17, has been observed for n = 3; the reaction efficiency is good (ϕ = 7%), and the process is subject to KIE = 2.4. [AlCeO3 ]+ + CH4 → [AlCeO2 (OH)]•+ + CH3•

(17)

+

The behavior of [AlCeO3] is in distinct contrast to that of other closed-shell species, such as [HTiO]+,37 [LHfO]+ (L = F, Cl, Br),41 [TaOn]+ (n = 2, 3],42 [Al2TaO5]+,43 [AuTi3On]− (n = 7, 8),39 or [AuV2O6]+.44 Whereas these ions thermally activate CH4, they do not afford single H-atom abstraction with concomitant expulsion of a methyl radical. The origin of the unprecedented C−H bond activation by [AlCeO3]+ has been explored by high-level ab initio electronic structure calculations.40 For the unreactive couples [AlCeOn]+/CH4 (n = 2, 4) various C−H bond scenarios were explored, but common to all paths is that their respective transition structures are located well above the entrance channel; thus, at thermal energies they are not accessible on energetic grounds. For the reactive pair [AlCeO3]+/CH4, the initial C−H bond activation proceeds in a PCET manner taking advantage of the Lewis acidity of aluminum in forming a relatively strong Al−C bond. As shown by the calculations, this Lewis acidity is highest for clusters with n = 3. In addition, along the reaction coordinate, the cage cluster undergoes a rather extensive structural reorganization in the course of which and upon liberation of CH3• the CeIV atom is reduced to its favorable oxidation state CeIII. It is this redox step that contributes to some extent to the exothermicity of the reaction. Yet, entirely different reactivity scenarios have been encountered in the thermal reactions of protonated [XO2]+ (X = Si, C) with CH4.45 While [OC(OH)]+ is not capable of activating CH4, the cluster [OSi(OH)]+ reacts with CH4 (ϕ = 22%) and two unexpected products are generated, i.e., [Si(OH)]+ and [Si(OCH3)]+, eq 18.

Figure 7. Simplified PES (relative energies are given in kJ mol−1 and are corrected for zero-point energy contributions) and selected structural information (bond lengths in Å) for the reactions of singlet [OSi(OH)]+ with CH4 calculated at the CCSD(T)//BMK level of theory. Charges are omitted for the sake of clarity. Adapted with permission from ref 45. Copyright 2016 Wiley-VCH.

analysis, the Lewis acidity of the carbon atom in [OC(OH)]+ iscompared with that of Si in [OSi(OH)]+much too low to provide a driving force for pulling down the transition state of methane activation.

5. FISCHER−TROPSCH-RELATED C−C BOND FORMATION IN THE GAS PHASE Fischer−Tropsch-analogous C−C coupling of methylene motifs has been reported for the thermal reactions of atomic Ta+, W+, Ir+, and Os+ with CH4, eq 19.46 Similarly, dehydrogenation of CH4 was also observed for the open-shell cluster carbenes [Ptn(CH2)]•+ (n = 1, 5).47 However, for n = 1 the generation of a C2H4 ligand occurred with an efficiency of only ϕ = 2%, while for n = 5, ϕ was as high as 26% but dehydrogenation of methane did not yield any C−C bond coupling, eq 20.47 [M]+ + nCH4 → [M(CH 2)n ]+ + nH 2 , M = Ta, W, Ir, Os,

n≤5

(19)

[Pt n(CH 2)]•+ + CH4 → [Pt n(CH 2)(CH 2)]•+ + H 2 n = 1, 5

Extensive labeling studies complemented by CCSD(T)based calculations provided mechanistic insight (Figure 7): In the [OSi(OH)]+/CH4 couple, after formation of an encounter complex 15, in a σ-bond metathesis-like reaction the C−H bond of the incoming methane substrate is activated giving rise to a rather stable silyl cation 16. Next, rather than homolytically cleaving the strong Si−C bond (ca. 430 kJ mol−1) of 16, the methyl group migrates intramolecularly to one of the hydroxide ligands (16 → TS16/17 → 17). Intermediate 17 then serves as a branching point to liberate either CH3OH with the formation of [Si(OH)]+ (20) or to engage in the loss of H2O, via TS17/18 → 18, and accompanied by the formation of [Si(OCH3)]+ (21). For the carbon analogue [OC(OH)]+/CH4 it is the high energy demand of the initial C−H bond activation step, in forming the insertion intermediate [C(OH)2CH3]+, which prevents C−H cleavage to occur. Based on a detailed NBO

(20)

An entirely different situation has been encountered in the gas-phase reactions of the closed-shell [Au(CH2)]+ carbene.48 At room temperature, C−C coupling of the methylene ligand with methane occurs with an efficiency of ϕ = 29%, relative to the collision rate, Figure 8. Double resonance experiments and energetic considerations demonstrate that [Au(C2H4)]+ does not serve as a precursor to form Au+, and labeling studies combined with extensive computational investigations revealed further mechanistic details of this C−C coupling process (Figure 9). In the initial phase, the weakly bound encounter complex 21 undergoes insertion of the methylene unit into the C−H bond of the incoming CH4 substrate (21 → TS21/22 → 22). This process profits from the relatively weak Au+−CH2 bond (D0 = 357 kJ mol−1)49 and the reduced electron density of the carbene ligand (+0.33 |e|);48 the latter reflects the rather high electronegativity of Au+.50 Complex 22 can either 17207


Perspective


as ro-vibrational energy in 28 is more than sufficient to liberate C2H4.

Figure 8. Au+-mediated coupling of the carbene ligand of [Au(CH2)]+ with methane to form C−C bonds. Adapted with permission from ref 48. Copyright 2016 Wiley-VCH.

Figure 10. (a) CCSD(T)-REL//B2GP-PLYP calculated PES for the reaction of singlet [AuC]+ with methane. Zero-point energy corrected relative energies are given in kJ mol−1, and bond lengths in Å. (b) Schematic MO diagrams as represented by quasi-restricted orbitals for the reactant pair, intermediate 26, and TS26/27 are given. Adapted with permission from ref 51a. Copyright 2016 Wiley-VCH.

Further insight in mechanistic aspects of the initial phase of the C−C coupling was derived from an analysis of the quasirestricted orbitals (Figure 10b). Accordingly, in the electrophilic attack of the C−H bond by [AuC]+, the actual C−C coupling is accompanied by a hydride transfer and not a conventional HAT mechanism. This is further evidenced by the two-electron, three-center, bent transition state TS26/27 and the absence of a singly occupied nonbonding orbital with a node on the hydrogen atom in transit.51a Moving on to the reaction mechanism of the [CuC]+/CH4 couple, we encountered an entirely unexpected and a unique mechanism, which proceeds with simultaneous activation of two C−H bonds (Figure 11).51b The stepwise reaction, observed for the [AuC]+/CH4 pair (Figure 10), is less favored than the synchronous process for [CuC]+/CH4 by >30 kJ mol−1. As shown in Figure 11, in a TSR scenario, the two surfaces cross at the minimum-energy crossing point (MECP), and the subsequent reaction proceeds on the singlet surface in an unprecedented way. The electrophilic carbon atom of [Cu− C]+ simultaneously inserts into the two C−H bonds of methane via 1TS-diH. In the same step, the nascent CH2 group bends backward, while the CH2 group remaining of methane rotates by 90° along the C−C axis, thus generating the [Cu(C2H4)]+ complex; all are in a single step and in a barrier free reaction. 1 TS-diH is really unique for it involves not only the simultaneous breaking of two C−H bonds of methane, but also the generation of two new C−H bonds and a CC double bond, while transforming a σ(Cu−C) bond into a π-interaction of the copper ion with the newly formed CC double bond. The root cause for the distinct mechanistic differences of the two coinage metal carbides in their reaction with CH4 has been traced back to the large difference between the bond dissociation energies (BDEs) of the two carbides with BDE[Au−C(1D)]+ = 501 and BDE[Cu−C(1D)] = 329 kJ

Figure 9. PES for the reaction of [Au(CH2)]+ (1A1) with CH4 calculated at the CCSD(T)//BMK level of theory. Zero-point corrected relative energies are given in kJ mol−1, and charges are omitted for the sake of clarity. Adapted with permission from ref 48. Copyright 2016 Wiley-VCH.

dissociate directly to Au+ and C2H6 (P1) or rearranges further along the sequence 22 → 23 → 24 → (P2). The experimentally observed H/D scrambling between the methylene ligand and CH4 as well as the kinetic isotope effects can be explained by the degenerate process 23 ⇄ 25 ⇄ 25′. Furthermore, according to Figure 9 atomic Au+ is predicted to efficiently activate C2H6. This prediction has been verified experimentally.48 Finally, recent studies demonstrate that also diatomic metal carbides [MC]+ can afford thermal coupling with methane to yield C2H4, eq 21, with efficiencies of 22% (M = Au) and 44% (M = Cu), respectively.51 [M−C]+ + CH4 → M+ + C2H4 M = Au, Cu

(21) 51,52

The coupling reaction of the singlet ground state of [AuC]+ with CH4 can be described in a sequence of the electrophilic insertion of the positively charged carbon atom into a C−H bond (26 → 27) to be followed by an intracomplex hydride migration to yield 28 (Figure 10). The internal energy gained along the reaction coordinate and stored 17208


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over the role in activating atomic carbon in its reaction with CH4.17a,51b As shown in Figure 12, the energy difference between the encounter complex 1EC and the transition state 1 TS-diH depends very much on the magnitude of the point charge, varied between +1e to +2e, and the distance between the point charge and the carbon atom. Increasing the magnitude of the charge and getting the charge closer to atomic C(1D) diminishes significantly the barrier.51b The positive point charge approaching the carbon center causes a significant charge transfer from methane toward the carbon atom compared to the nonpolar, unassisted reaction of C(1D) with CH4. As the field strength of the point charge increases, the dipole moment of the transition state 1TS-diH increases as well (Figure 12). As a consequence, the weights of ionic structures in the transition state along the reaction axis are augmented, leading to electrostatic stabilization of the transition state.17a,53 The OEEF-induced dipole moment is obviously affected by its corresponding orientation. Therefore, the enormous acceleration effect caused by this coinage metal can actually be modeled by a positive point charge serving as a reagent,13c,h and the mechanistic switch could be accomplished at will by tuning the direction and intensity of the OEEF.

Figure 11. CCSD(T)//B2PLYP-calculated potential energy profile for the synchronous two C−H bond insertion step of [Cu−C]+/CH4. Key ground-state structures with selected geometric parameters are also provided. Enthalpies (in kJ mol−1), corrected for contributions of zeropoint vibrational and thermal energies, of the reaction intermediates and transition states are given relative to the separated reactants 3 [Cu−C]+ and CH4. Relative electronic structure energies (ΔE in kJ mol−1), without zero-point vibrational and thermal energies, are also provided in parentheses. Charges are omitted for the sake of clarity. The inset shows the schematic diagram of the bond-breaking/making in a single step along IRC calculations. Adapted with permission from ref 51b. Copyright 2017 American Chemical Society.

6. CONCLUDING REMARKS AND OUTLOOK Activation of methane; what could be simpler? This might be the first thought that comes to mind. But as this Perspective demonstrates such a thought would be entirely misleading! The gas-phase chemistry of methane activation by small positively charged clusters poses the explorer with a dizzying mechanistic landscape, which in retrospect can be understood and rationalized. As this Perspective shows, the mechanistic manifold embraces anything from the simple HAT, when oxyl radical centers are available, all the way to the PCET, when such radical centers are absent or delocalized. And more: When a reagent is prone to PCET, and leads to a plethora of products, a mere ligation of the metal center by CH3CN quenches all the

mol−1,51b,52 as well as to the qualitative difference of the M−C bonding: being largely covalent for M = Au but almost devoid of covalence for M = Cu. In line with this bonding difference, the computational results strongly suggest that a positive point charge can take

Figure 12. (a) Plot of the relative energies (kJ mol−1) between the transition state and the encounter complex under the influence of a positive point charge. Charge magnitudes (1.0e−2.0e) are colored from black to gray; distances between the positive point charge and C(1D) are in the range of 1.6−5.0 Å. Note that here the positive point charge is placed at the metal position of the [Cu−C]+/CH4 system, but the corresponding structures in the C(1D) + CH4 reaction are used. (b) The x-axis refers to the direction of the dipole moment of the transition state under the influence of a positive point charge. Color code and distance are the same as shown in panel a. Adapted with permission from ref 51b. Copyright 2017 American Chemical Society. 17209


Perspective


and special thanks goes to Andrea Beck for superb technical assistance in the final preparation of the manuscript. This article is dedicated to the memory of the late Professor George A. Olah.

channels except for HAT. Other clusters lead to metatheses reactions, which produce H2, or to C−C coupling via Fischer− Tropsch type, while metal carbide cations proceed by hydride transfer or via a concerted methane conversion to ethylene in a single step, with hardly any barrier, and with an astounding atomic choreography. This latter reaction can be mimicked using OEEFs, which may act as invisible reagents and catalyze such unusual transformations.13h,33,54 During this mechanistic journey we could not avoid the feeling that scientists often have, that reality is always much more complex and wide ranging than one’s experience which is reminiscent of the famous elephant’s parable in which a score of blind men hopelessly attempt to define an elephant by touching and probing its parts.55 Being aware of this admonition, we tried to provide insight for these mechanistic switchovers throughout the Perspective. Thus, by simply tracing the oxyl centers, one can predict when HAT will occur, and when will it be replaced by PCET. Furthermore, we present a simple diagnostic (Figure 5) plot,9g,h which enables the experimentalist to distinguish between HAT and PCET, without much ado. We believe this insight may be helpful to navigate in this rich mechanistic land of methane activation. Further inspiration from our combined experimental/ computational experience leads us to ask: Have we really explored the entire mechanistic land? We therefore intend to exploit the acquired insight and investigate the [MC] + complexes of the entire 3d-5d transition metal series, and try to understand when does one expect to get coupling with methane to produce ethylene, and when other reactions will transpire and lead to new insight. In addition to metal carbide cations, we plan to probe the heavier analogues, [ME]+, E = Si, Ge, etc. Will we get H2C = EH2? Will the use of richer carbon clusters, [MCn]+, result in terminal olefins which are practically important? How can one use the invisible reagent, OEEF, to stimulate these and other reactions? These are some of the questions in our future outlook.

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] ORCID

Helmut Schwarz: 0000-0002-3369-7997 Sason Shaik: 0000-0001-7643-9421 Jilai Li: 0000-0002-3363-9164 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was sponsored by the Deutsche Forschungsgemeinschaft (DFG), in particular the Cluster of Excellence “Unifying Concepts in Catalysis”, and the Fonds der Chemischen Industrie. The work at Jilin University has been supported by the National Natural Science Foundation of China (Nos. 21473070 and 21773085) and incubation program of Jilin University for China National Funds for Distinguished Young Scientists. The research at the Hebrew University of Jerusalem has been sponsored by the Israel Science Foundation (ISF grant 1183/13). We are extremely grateful for practical and conceptual contributions by past and present co-workers, 17210


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