Unexpected Mechanistic Variants in the Thermal Gas-Phase

Review pubs.acs.org/Organometallics

Unexpected Mechanistic Variants in the Thermal Gas-Phase Activation of Methane Helmut Schwarz,* Patricio González-Navarrete, Jilai Li, Maria Schlangen, Xiaoyan Sun, Thomas Weiske, and Shaodong Zhou Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany ABSTRACT: In this review gas-phase studies conducted (mostly) at the Berlin laboratory of the authors are presented. The focus will be on describing mechanistic variants we (and others) came across recently in investigating the thermal activation of methane in the gas phase under idealized conditions. Typical examples include the discussion of those hydrogen-atom-transfer processes that do not follow the wellestablished conventional pathways in which oxyl radicals play a decisive role. This is the case when the spin is located at a metal center, as in [Al2O2]•+, and the C−H bond cleavage follows a proton-coupled electron-transfer mechanism. Also, examples will be presented in which a high spin density at a bridging oxygen atom can be generated by judicious “doping” of the cluster oxides. Further, the particular role Lewis-acidic sites play in the methane activation by closed-shell metal-oxide ions will be highlighted. Then, aspects of the dissociative adsorption of CH4 on rather small cluster ions will be analyzed; here, among other factors, e.g., the role of relativistic bond stabilization, intriguing ligand effects will be reported. Finally, in the context of Fischer− Tropsch-related chemistry, we will describe novel C−C coupling reactions occurring at room temperature with CH4. Common to most systems studied is the synergy between experiment and computational chemistry, and for a few examples remarkable mechanistic commonalities with reactions at a surface were encountered.

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INTRODUCTION For more than a century, the activation and selective functionalization of the most simple alkane, CH4, continue to pose enormous challenges. In fact, these transformations have been regarded in the chemical community as a “Holey Grail”, so to speak.1 For example, current large-scale processes that are both environmentally benign and economically feasible do not exist to bring about at ambient conditions the direct conversion of methane to methanol, eq 1, to couple the two C1 units CH4 and CO2 to form acetic acid, eq 2, or to afford the oxidative dimerization of CH4, eq 3.2 The processes, depicted in eqs 1−3 are rightly viewed as “dream reactions” for supplying the chemical industry with value-added products. CH4 + O → CH3OH

(1)

CH4 + CO2 → CH3CO2 H

(2)

2CH4 → C2H6 − n + Hn (n = 0, 2)

(3)

Rather, often energy-intensive, extreme conditions have to be employed.2 Over the last 30 years, we have been engaged in elucidating the elementary steps and mechanistic features of the reactions depicted in Scheme 1 by performing gas-phase experiments with mass-selected reagents at their electronic ground state Scheme 1. Network for Selected Gas-Phase Transformations of Methanea

No doubt, part of these difficulties is due to the molecular properties of CH4, as indicated by the absence of a dipole moment, the rather small polarizability, the modest proton affinity, the extremely high pKa value, an anomalously high ionization energy, and a negative electron affinity or the significant energies required for both the homo- and heterolytic cleavages of the C−H bond. Thus, ordinary redox reactions or acid−base chemistry is not suitable to convert this overly abundant hydrocarbon into more valuable commodities. © XXXX American Chemical Society

a

[M] stands for a suitable metal and [MO] for a metal oxide (to be specified in the text).

Special Issue: Hydrocarbon Chemistry: Activation and Beyond Received: May 9, 2016

A

DOI: 10.1021/acs.organomet.6b00372 Organometallics XXXX, XXX, XXX−XXX

Review

Organometallics under single-collision conditions.3 While this reductionistic approach never accounts for all the details that prevail at a surface or in solution, when complemented by spectroscopic and adequate computational studies, these gas-phase investigations provide a conceptual framework and permit addressing various mechanistic aspects, for example, which atoms constitute the active part of a single-site catalyst, the socalled “aristocratic atoms”?4 Or, how do factors such as cluster size and stoichiometry, oxidation number, charge and spin states, degree of coordinative saturation, etc., affect the outcome of a chemical process without being obscured by difficult-to-control or poorly defined “environmental” effects?5 As has been demonstrated repeatedly, these gas-phase experiments provide an ideal arena for addressing these problems and for probing the energetics and kinetics of a chemical reaction in an unperturbed fashion at a strictly molecular level.3,5,6 In previous work it has been shown that in the thermal metalmediated dehydrogenation of methane to form metal-carbene complexes, process ①, relativistic effects matter.3b,c,6 Further, in the room-temperature, metal-oxide-induced conversion of methane to methanol, process ②, under thermal conditions excited spin states were found to be involved (“two-state reactivity” (TSR) scenario).7 Finally, for the hydrogen atom transfer (HAT) from methane to a metal oxide, process ③, a high spin density at a terminal oxygen atom is key to permit an efficient C−H bond cleavage under ambient conditions.8 In fact, HAT has been suggested to play a major role in the industrially relevant oxidative coupling of methane.2d,8b,9 In this invited mini-review on gas-phase methane activation, the focus will be on discussing recently discovered, unexpected mechanistic variants that supplement those reported earlier. As requested by the Guest Editors, this survey covers mostly work that has been performed in the Berlin laboratory of the authors. Further, we will refrain from describing any experimental techniques and computational methodologies; these and other details are available from the original work mentioned in the references.

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 are nonreactive because high intrinsic barriers prevent them from engaging in HAT from CH4;8c 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 near the transition state.10 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]+,11 or oligomeric [MgO]n•+ (n ≥ 2).12 As shown computationally, intracomplex spin transfer in these systems is energetically rather disfavored,8a,10c,13 and one may wonder how to alleviate the problem. One way to modify crucial properties, such as the spin density around a reaction center, is by judicious “doping”,14 and the thermal reactions of CH4 with the [MgO]n•+ (n = 1−7) cluster ions under ambient conditions serve as a good example. While diatomic [MgO]•+ initiates HAT even from methane,12a the larger cluster ions are completely inert, even though the hydrogen atom transfer is exothermic. Why is this so? In distinct contrast to [MgO]•+, in dimeric [MgO]2•+ the spin is equally distributed over the two bridging oxygen atoms of the cluster, resulting in a barrier for HAT too high in energy to be accessible at ambient conditions. Thus, HAT is observed only with substrates that have a weaker C−H bond, e.g., propane or butane.12 However, doping the [Mg 2 O 2 ] •+ cluster with Ga 2 O 3 changes the reactivity completely, in that C2H6 and even CH4 undergo HAT at room temperature.15 This dramatic increase in reactivity is due to the fact that in the [Ga2Mg2O5]•+ cluster the spin density of a bridging oxygen atom at the active site of the cluster is increased to 0.896 compared to 0.517 on each oxygen atom in [Mg2O2]•+ and only 0.092 at the O atom in the center of [Ga2Mg2O5]•+. As a consequence, the potential-energy surfaces (PESs) for the HAT reactions are changed such that activation of RH (R = CH3, C2H5) by naked [Ga2Mg2O5]•+ becomes possible at ambient conditions, Figure 1.15 Other examples of how the judicious choices of dopants affect the HAT reactivities of cluster ions can be found in refs 13, 14, and 16. Yet another possibility to bypass high intrinsic barriers in HAT arises by inducing a change in mechanism where the conventional homolytic cleavage of a C−H bond is replaced by proton-coupled electron transfer (PCET).8b,10c,17 That is, while the proton and the electrons originate from the same C−H bond, they are transferred to quite different sites of the acceptor with the proton ending up at an oxo group and the electron at the metal site, Scheme 2. As a consequence, a heterolytic rather than a homolytic cleavage of the C−H bond is operative. As an illustration let us discuss the recently studied [Al2O2]•+/CH4 couple.18 With an efficiency (ϕ) of 10.3%, relative to the collision rate, at room temperature the C−H bond of CH4 is cleaved, eq 4. 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 (see below), in the reaction with methane it is slightly more efficient than the [Al2O3]•+/CH4 couple (ϕ = 7%), which bears a terminal oxyl group.19

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THE ROLE OF SPIN DENSITY IN HYDROGEN-ATOM-TRANSFER: REVISITED Among the general features of an efficient, low-temperature metal-oxide-mediated gas-phase HAT from hydrocarbons, in the present context the most important ones are as follows. (1) Cationic systems are generally more reactive than structurally related neutral or anionic analogues. (2) For the kinetics of HAT, the presence of unpaired, high spin density at the abstracting, preferentially terminal oxygen atom of M−Ot• is crucial. (3) 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.3c,8

[Al 2O2 ]•+ + CH4 → [Al 2O(OH)]+ + CH3• B

(4)


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Organometallics

Figure 1. Potential energy surfaces (kJ mol−1) and key ground-state structures involved in the thermal reactions of [Ga2Mg2O5]•+ with methane and ethane calculated at the G4MP2-6X level of theory. The inset shows the ground-state structure of [Ga 2 Mg 2 O 5 ] •+ (C s symmetry). The blue isosurface indicates the AIM-calculated spin density distribution. Adapted with permission from Angew. Chem., Int. Ed. 2015, 54, 5074−5078. Copyright 2015 Wiley-VCH.

Figure 2. Potential energy profiles for the thermal reaction of [Al 2 O 2 ] • + with CH 4 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/cyan isosurfaces indicate the NBO-calculated spin density distributions. Charges are omitted for the sake of clarity. Adapted with permission from J. Am. Chem. Soc. 2016 DOI: 10.1021/ jacs.6b03798. Copyright 2016 American Chemical Society.

Scheme 2. Schematic Description of HAT versus PCET

thermal 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 EC, a C−H bond is cleaved and the hydrogen atom is transferred to the bridging oxygen atom via transition state TSPCET; the latter is located 28 kJ mol−1 below the separated reactants. This process results in the formation of a rather stable intermediate IPCET, lying below the reaction entrance by 300 kJ mol−1. To generate the formal HAT products, eq 4, 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 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 TSPCET. Here, the basic oxygen moiety abstracts the hydrogen atom as a proton, while the CH3 group moves with the electron pair, as anionic CH3−, and forms a bond with the positively charged Al+; overall, two new bonds, i.e., O−H and Al−CH3, are generated in IPCET. Further, a detailed frontier molecular orbital analysis of the change of the electron density along pathway B also reveals the features of a heterolytic cleavage of the C−H bond via a PCET mechanism.18 Briefly, the electron pair from the σ(C−H) bond of methane is accepted by an empty sp hybrid orbital of the Al atom of the cluster; the latter is generated in the course of coordinating CH4 to one of the aluminum atoms, while the unpaired electron that was delocalized over the two Al atoms in R is shifted to the Al atom opposite to the Al coordination site for methane along the reaction sequence R → EC → TSPCET → IPCET. As to the fate of the hydrogen atom, it is captured as a proton by a lone pair of one of the bridging oxygen atoms. Thus, in the IPCET complex strong σ(O−H) and σ(Al−C)

Mechanistic insight into this puzzling result has been provided by high-level quantum mechanical calculations; the corresponding PESs are shown in Figure 2 and will be discussed next. The most stable structure of [Al2O2]•+ has a rhombus-like geometry with the spin density evenly distributed over the two aluminum atoms (Figure 2, R). For the reaction of [Al2O2]•+ with CH4, two reaction paths, A and B, were located on the doublet ground-state surface. Both sequences start with the formation of an encounter complex, EC; this barrier-free step is exothermic by 121 kJ mol−1. Notably, EC is heavily stabilized by a significant electrostatic interaction between the Lewis-basic carbon atom (δ−) of methane and the Lewis-acidic aluminum coordination site (δ+) 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 a H atom. This is, however, not the case since Al• is a poor H-abstractor due to rather unfavorable thermodynamics. Indeed, the calculations indicate that the energy of the system invariably goes up during a hydrogen atom transfer from methane to the Al atom; in fact, this process is rather endothermic by 120 kJ mol−1, higher in energy relative to the separated reactants. Thus, it is omitted from Figure 2 for the sake of simplicity, and HAT is considered only to be facilitated by the bridging oxygen. However, since the spin is not located on the oxygen atom, 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.10c,17b,18 Indeed, as shown in Figure 2, the HAT starting from EC to IHAT via TSHAT is not accessible under C


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Organometallics bonds are created. An analysis of NBO-calculated spin and charge distributions further reveals that along the reaction coordinate of path B significant negative charge accumulates at the oxygen and carbon atoms in TSPCET; the former facilitates the proton abstraction from the C−H, and the latter is beneficial to the creation of an Al−C bond. These features are energetically much more attractive than forming an energetically demanding three-electron/three-center bond in TSHAT. These findings as well as those obtained computationally for various heteronuclear cluster oxide ions [XYO2]•+ (X, Y = Al, Si, Mg)18 provide an unexpected mechanistic analogy to the recently discussed oxidative coupling of methane on magnesium oxide surfaces. According to elaborate quantummechanical work, it has been suggested that Grignard-type intermediates with a Mg−CH3 structural motif, eq 5, are formed; the latter can be generated upon a heterolytic cleavage of the C−H bond of methane.20a

Figure 3. Potential-energy surfaces for the reactions of [HTiO]+ (singlet state, blue) and [HVO]+ (doublet state, red) with methane, calculated at the B3LYP//def2-QZVP level of theory. All energies are given in kJ mol−1. Charges are omitted for the sake of clarity. C = dark gray, H = light gray, O = red, M = turquoise. Adapted with permission from Angew. Chem., Int. Ed. 2013, 52, 6097−6101. Copyright 2013 Wiley-VCH.

[Mg 2 +O2 −]MgO + H−CH3 → [HO−(Mg−CH3)+ ]MgO (5)

Interestingly, heterolytic cleavage of the C−H bond of methane has also been reported to be brought about by Zn2+-doped MFI-type zeolites.20b

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VARIATION OF A THEME: REACTIONS OF METHANE WITH CLOSED-SHELL IONS Yet another, perhaps surprising, reactivity scenario has been encountered when [HMO]+ (M = Ti, V) is reacted with CH4; in contrast to their inert isomers [M(OH)]+ the former bring about efficient C−H bond activation at ambient conditions, eq 6.21 + [HMO]+ + CH4 → [(H3C)MO] + H 2 M= Ti,V

bonds in [HMO]+ and [(CH3)MO]+ are weakened by the presence of an oxo ligand compared to [HM]+ and [(CH3)M]+, respectively. However, this effect is more pronounced for [HMO]+ than for [(CH3)MO]+, thus rendering the ligand exchange, eq 6, exothermic.21 Also the metal-free, closed-shell cluster oxide [OSi(OH)]+ in its thermal reaction with CH4 exhibits unexpected mechanistic features.23 With an efficiency of ϕ = 22%, relative to the collision rate, the couple [OSi(OH)]+/CH4 delivers the products shown in eq 8. The reaction is not affected by a

(6)

According to DFT calculations, for both oxide cations the ligand exchange involves their electronic ground states, i.e., the singlet for 1[HTiO]+ and the doublet for 2[HVO]+; higher spin states need not be considered on energetic grounds, and also a two-state reactivity scenario can be ruled out. As 1[HTiO]+ lacks a radical site and the penalty to decouple the TiO double bond and to access the triplet state 3 [HTiO]+ is much too demanding, the system [HTiO]+/CH4 bypasses the classical HAT route; rather, it engages in a σmetathesis-like ligand exchange, as shown in Figure 3.21 Also quite telling is the behavior of the open-shell oxide 2[HVO]•+. This oxide does possess a high spin density; however, as has been shown for the oxides of [Al2O2]•+ and [Mo(O)2]•+,18,22 for 2[HVO]•+, again, the spin is not located at an oxygen atom. Rather, in 2[HVO]+ the spin resides at the vanadium atom (1.25 μB) and amounts to only −0.19 μB for oxygen. Further, as the formation of yet another, relatively weak V−H bond cannot provide a thermochemical driving force to help breaking the strong C−H bond of methane, the classical HAT channel is not accessible. Instead, for 2[HVO]•+ the ligand exchange, eq 6, proceeds via a σ-metathesis route. Interestingly, although the oxygen atom of [HMO]+ is not directly involved in the bond making and bond breaking and thus may be viewed as an inert spectator ligand, its very presence enables the activation of methane according to eq 6. This conclusion is supported by the finding that for the [MH]+/CH4 couples (M = Ti, V) the ligand switch, eq 7, is endothermic and not observed. Why is this so? DFT calculations reveal that both the M−H and M−CH3

kinetic isotope effect. Mechanistic insight into the details of the methane activation step by [OSi(OH)]+ has been provided by high-level quantum chemical calculations. The most favorable pathways were located on the singlet potential-energy surface as shown in Figure 4. The encounter complex 3 is initially formed from the reactants; this barrier-free step is exothermic by 104 kJ mol−1, thus indicating a rather strong interaction between the positively charged silicon atom and methane (the charge on the silicon atom of [OSi(OH)]+ amounts to 2.5 |e| based on an NBO analysis). Subsequently, one C−H bond of the incoming hydrocarbon substrate is activated and a hydrogen atom is transferred to the oxo group of [OSi(OH)]+ via transition state TS3/4 to form the rather stable silyl cation 4. Next, rather than homolytically splitting the strong Si−C bond of 4, the methyl group migrates via TS4/5 to one of the hydroxide ligands, thus forming complex 5. This intermediate then serves as a branching point either to liberate CH3OH under the formation of [Si(OH)]+ or to bring about H2O loss accompanied by the generation of [Si(OCH3)]+. For the latter route, complex 5 first isomerizes to intermediate 6 via TS5/6. As this step and the evaporation of H2O to produce [Si(OCH3)]+ are much less energy demanding than the formation of [Si(OH)]+ and D


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Organometallics

Figure 4. PES and selected structural information on the associated species for the reaction of singlet [OSi(OH)]+ with CH4 at the CCSD(T)/C// BMK/B level of theory. Zero-point-corrected relative energies are given in kJ mol−1, and bond lengths in Å; charges are omitted for the sake of clarity. Adapted with permission from Chem.−Eur. J. 2016, DOI: 10.1002/chem.201601981. Copyright 2016 Wiley-VCH.

CH3OH, a higher branching ratio in favor of [Si(OCH3)]+ is expected; this is in line with the experimental findings. In the overall activation of CH4 by [OSi(OH)]+, the energetically most demanding steps correspond to the transition states TS2/ 3 and TS4/5; however, as both are located ca. 60 kJ mol−1 below the entrance channel, in line with the experimental results, facile activation of CH4 can occur under ambient conditions. Interestingly, the structurally related oxide [OC(OH)]+ exhibits a completely different reaction scenario when exposed to CH4. The only product amounts to a slightly exothermic proton transfer, eq 9. [OC(OH)]+ + H−CH3 → CO2 + [CH5]+

bond contributes to the rate-limiting step of the oxidative insertion. As a driving force, the large dipole moment of 19.2 D of the linear [FeC6]− cluster has been identified. This dipole moment helps to polarize the C−H bond and facilitates its eventual cleavage. In a way, this notion resembles the interesting suggestion that properly oriented C−H bonds can be activated by external electric fields.26 While the energy gained in forming an encounter complex is sufficient to overcome the barriers associated with the steps 7 → 8 → 9, Scheme 3, expulsion of CH3• to produce any chemically Scheme 3. Schematic Presentation of the Oxidative Insertion of [FeC6]− in the C−H Bond of Methanea

(9)

While the formation of [CH3CO]+ under expulsion of H2O from [OC(OH)]+/CH4 is exothermic, a substantial intrinsic barrier prohibits the generation of the central intermediate [(CH3)C(OH)2]+. As this barrier is ca. 70 kJ mol−1 above the entrance channel, under thermal conditions it is not accessible. A comparison of the two systems [OM(OH)]+/CH4 (M = C, Si) reveals that the much higher Lewis acidity of the siliconcontaining couple permits a significantly stronger interaction of this oxide cluster ion with the incoming hydrocarbon substrate as compared to the carbon analogue.23,24 As a consequence, for [OSi(OH)]+/CH4 the crucial transition states to generate 4 and 5 are pulled below the entrance channel.

a

Adapted with permission from J. Phys. Chem. Lett. 2015, 6, 2287− 2291. Copyright 2015 American Chemical Society.

conceivable [FeC6H]− isomers is endothermic, relative to the entrance channel.25 Finally, in this context it should be mentioned that neither the carbon-free atom Fe or Fe+ nor the iron-free carbon cluster [C6]− is capable of inserting in the C−H bond of CH4.25 For the iron systems only a physical adsorption of the hydrocarbon had been reported earlier.27 (2) In a combined experimental/computational study,28 the naked [HNbN]− cluster anion was demonstrated to exhibit reactivities with CH4 similar to the one observed for the electronically related atomic platinum.29 Commonalities in the electronic structures, especially the active orbitals, are the root cause for this remarkable resemblance of [HNbN]− to atomic Pt. Photoelectron imaging spectroscopy reveals that the laser-

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DISSOCIATIVE ADSORPTION OF METHANE ON SMALL CLUSTER IONS Dissociative adsorption of small, inert molecules on surfaces is a well-known phenomena in heterogeneous catalysis. Can this bond activation also occur when, for example, methane is “adsorbed” on a small cluster in the gas phase? Four recently studied examples will be discussed next to illustrate the point. (1) The iron-carbide anion [FeC6]− inserts at room temperature in the C−H bond of methane.25 A kinetic isotope effect of KIE = 11 ± 3 demonstrates that breaking the C−H E


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Organometallics generated [HNbN]− anion is produced in both its triplet and singlet states. While the former is slightly more stable than the latter, it is only the excited singlet electromer 1[HNbN]− that, in a two-state-reactivity scenario, enables oxidative insertion in the C−H bond of CH4, Scheme 4.28 The kinetic isotope effect Scheme 4. Insertion of 1[HNbH]− in the C−H bond of CH4a

a

Adapted with permission from Angew. Chem., Int. Ed. 2016, 55, 4947−4951. Copyright 2016 Wiley-VCH.

amounts to KIE = 2.0. Liberation of H2 from the insertion intermediate, however, requires external energy supply in the form of, for example, increasing the center-of-mass energy of the [HNbN]−/CH4 couple up to ca. 20 kJ mol−1. Interestingly, although the hydrogen atom of [HNbN]− is not directly involved in the activation of CH4 and, thus, may be viewed as an innocent spectator ligand, its presence is crucial to achieve the C−H bond activation. It does so by lowering the energy of the singlet-spin state of [HNbN]−; further, it renders the Nb site of the cluster nitride less negatively charged, thus aiding in the approach of the hydrocarbon, Scheme 4: 111 → TS11/12. (3) A remarkable ligand effect was identified to be key in the thermal activation of CH4 by [XHfO]+ (X = F, Cl, Br).30 While the open-shell oxide [HfO]•+ is inert toward methane, the halogen-ligated closed-shell oxide ions convert CH4 to form the insertion product [Hf(X)(OH)(CH3)]+. These insertion processes are accompanied by inverse kinetic isotope effects, kH/kD < 1. While the inertness of [HfO]•+ is due to the lack of an oxyl site (the spin density at the oxygen atom in [HfO]•+ amounts to only 0.01 |e| and an unfavorable thermochemistry for HAT, see Figure 5), the role of the halogen atom becomes obvious when inspecting the PES of the [XHfO]+/CH4 system as shown in Figure 5 for X = F.30 The most favorable pathways for the [FHfO]+/CH4 system were located on the singlet potential-energy surface, which is much lower in energy than the triplet-PES. The transformation starts from forming the encounter complex 13a, in which CH4 is strongly bound to the Hf atom. By passing the metathesislike transition state TS13a/14a, which is 38 kJ mol−1 located below the entrance channel, the insertion intermediate 14a is generated. The latter species corresponds to the global minimum on the PES; its dissociation to liberate a methyl radical by breaking the Hf−CH3 bond requires an external energy supply. Further, a rebound step to convert 14a to the complex [Hf(F)(CH3OH)]+ is not possible, as the related transition structure is energetically too demanding to be accessed. In addition, also the elimination of HF is thermally unavailable for [FHfO]+/CH4, as this process is heavily endothermic. So, what is the fate of the rovibrationally hot 14a? (i) Intermediate 14a may be trapped in a deep potential well provided radiative stabilization via IR photon emission is efficient, or (ii) 14a dissociates back to [FHfO]+ and CH4. The observed, rather low efficiency (0.03%) points to an inefficient radiative stabilization of 14a. However, when introducing argon via a pulse valve during the reaction delay, considerably more adduct is generated due to collisional cooling, thus preventing the back reaction to R.30

Figure 5. PESs and selected structural information for the reactions of [HfO]•+ and [FHfO]+ with CH4 as calculated at the CCSD(T)/ BSI//B3LYP/BSI level of theory. Zero-point-corrected energies are given in kJ mol−1, and bond lengths in Å; charges are omitted for the sake of clarity. Adapted with permission from Angew. Chem., Int. Ed. 2016, DOI: 10.1002/anie.201602312. Copyright 2016 Wiley-VCH.

While there is so far no spectroscopic support to assign 14a as the structure of [Hf,C,O,F,H4]+, circumstantial evidence is indeed in favor of 14a: (i) If only the encounter complex 13a were generated, upon collisional activation, on both energetic and entropic grounds, over a wide range of collisional energies [FHfO(CH4)]+ will dissociate exclusively back to [FHfO]+ and CH4; (ii) if [Hf,C,O,F,H4]+ corresponds to a mixture of 13a and 14a, the branching ratio of generating [FHfO]+ versus [FHf(OH)]+ should not depend heavily on the collision energy; (iii) if, however, 14a constitutes the major (if not only) product, a reasonable explanation of the experimental findings is at hand. When Ecoll is low, only the path 14a → TS13a/14a → 13a → R is accessible; upon increasing the collision energy, the energetically favored but entropically disfavored path back to R will face competition with the energetically more demanding but entropically strongly favored process 14a → [FHf(OH)]+ + CH3. This is, indeed, observed experimentally. In addition, the measured inverse kinetic isotope effect most likely results from an increased lifetime of the rovibrationally excited complex [FHf(OD)(CD3)]+ as compared with the unlabeled system. The significant strengthening of the Hf−C interaction by adding a fluoro ligand, and thus pulling down TS13a/14a, has its origin in the different electronic structures of [Hf(OH)]+ and [Hf(F)(OH]•+, respectively. Thus, the electron pair at the Hf atom in closed-shell [Hf(OH)]+ has to be uncoupled to bind to a CH3• radical; in contrast, this “promotion energy” is not required for the open-shell doublet [Hf(F)(OH)]•+. As a consequence, the bond dissociation energies (BDEs) of the Hf−C bonds differ with BDE((HO)Hf+−CH3) = 278 kJ mol−1 versus BDE((HO)(F)Hf+−CH3) = 321 kJ mol−1.30 In addition, F


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Organometallics

19, at 93.7 kJ mol−1, relative to the entrance channel, is much too high to be accessed. Also, the homolytic cleavage of the strong Ta−C bond to generate CH3• and [TaO(OH)]+ (P) requires external energy input. So, the ion is trapped in a deep potential well as previously discussed for the [XHf(OH)(CH3)]+ insertion products. The uniqueness of the [TaO2]+/CH4 couple is further evidenced when being compared with the related dioxides [MO2]+, which are not capable of inserting into the C−H bond of methane.32 The binding energy of CH4 to these metal oxides for M = V, Nb is significantly smaller than for M = Ta; further, the transition states to convert the encounter complexes to the insertion products are, for M = V, Nb, located above the entrance channel. In general, it is the much stronger metal− carbon interaction for Ta, as compared to that for V and Nb, that is responsible for the increased reactivity of [TaO2]+ toward CH4. As shown earlier in a broader context,6 this strengthening results from the lanthanide contraction that leads to a tightening of the valence s and p orbitals, resulting in a stabilization of the 6s orbital energy relative to 5d, thus causing higher BDEs(M−C) for 5d metals. These features can be traced back to the operation of a relativistic effect, which matters much more for 5d elements, especially the late 5d transition metals, as compared with their lighter congeners.6

by adding a halogen atom to the [HfO]+ system the electron transfer from the σ(H3C−H) bond to the HfO unit is eased. (4) As shown in a combined experimental/computational study, the distinct behavior of [TaO2]+,31 as compared to its lighter congeners [MO2]+ (M = V, Nb),32 in their thermal reactions with CH4, can be traced back to a sizable relativistic effect operative in the [TaO2]+/CH4 couple. Oxidative insertion of the cluster oxide in the C−H bond, according to eq 10, occurs with an efficiency of ϕ = 0.25%, and the inverse kinetic isotope effect amounts to KIE = 0.53.31 [TaO2 ]+ + CH4 → [Ta(O)(OH)(CH3)]+

(10)

Mechanistic insight is, once more, provided by a detailed quantum chemical analysis. As shown in Figure 6, in the

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FISCHER-TROPSCH-RELATED C−C COUPLING Fischer−Tropsch-type33 coupling of methylene units has been reported for the thermal gas-phase reactions of atomic Ta+, W+, Ir+, and Os+ with CH4, eq 11.34 Similarly, dehydrogenation of methane was also observed for the open-shell cluster carbenes [PtnCH2)]+, eq 12, for n = 1 and 5.35 However, for n = 1 the generation of a C2H4 ligand occurred with only a rather low efficiency (ϕ = 2%), while for n = 5 the efficiency was higher (ϕ = 26%), and dehydrogenation of CH4 did not result in C−C bond coupling.35 [M]+ M= Ta,W,Ir,Os

+ nCH4 → [M(CH 2)n ]+ + nH 2 (11)

n≤8

[Pt n(CH 2)]+ + CH4 → [Pt n, C2 , H4]+ + H 2 n = 1,5

(12)

An entirely different situation has been encountered in the gasphase reactions of the closed-shell “naked” [Au(CH2)]+ carbene.36 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 7.37a Doubleresonance 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 8). 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)38 and the reduced electron density of the carbene ligand (+0.33 IeI);37 the latter reflects the rather high electronegativity of Au+.6,39 Complex 22 can either dissociate directly to Au+ and C2H6 (P1) or rearrange 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 8, atomic Au+ is predicted to

Figure 6. PESs and selected structural information for the thermal reaction of [TaO2]+ with CH4 as calculated at the CCSD(T)/ BSII//PBE0/BSI level of theory. Zero-point-corrected energies are given in kJ mol−1, and charges are omitted for the sake of clarity. Adapted with permission from Chem.−Eur. J. 2016, 22, 7225−7228. Copyright 2016 Wiley-VCH.

energetically most favorable pathway only the singlet state of the [TaO2]+/CH4 system is involved. The related triplet surface is so much higher in energy that this PES will not be accessible under ambient conditions. On the singlet PES the interaction of 1 [TaO2]+ with CH4 commences with the generation of a rather stable encounter complex, 117; its formation profits from a strong interaction of the tantalum−carbon atoms. The energy gained in generating 117 is more than sufficient to cross 1TS17/ 18, thus producing the insertion intermediate 118; this species constitutes the global minimum on the potential energy surface of [Ta,C,O2H4]+. Again, what is the fate of rovibrationally hot 1 18? A rebound step to form the [OTa(CH3OH)]+ complex 1 19 is energetically out of the question, as the related 1TS18/ G


Review

Organometallics

atoms of the encounter complex have an equal probability of being incorporated in the acetylene molecule eliminated. Mechanistic details about the nature or the intermediates of the reaction sequence are not available yet.41 Finally, in the thermal reaction of CH4 with diatomic [MoC]+ the major product corresponds to the loss of C2H4 under the regeneration of atomic [Mo]+.42 DFT-calculations suggest a two-state-reactivity scenario with a doublet → quartet transition. The overall thermochemically and kinetically favored process is initiated by a metal-mediated C−H bond activation to form a strong Mo−C bond. Along the reaction coordinate various features were identified bearing some resemblance to the gas-phase process with the activation of methane by Modoped zeolites in the condensed phase.42,43 Essential steps involve, inter alia, transfer and migration of a hydrogen atom as well as C−C coupling, and the presence of a metal seems to be key in these transformations, Scheme 5.

Figure 7. Au+-mediated coupling of the carbene ligand of [Au(CH2)]+ with methane to form C−C bonds. Adapted with permission from Angew. Chem., Int. Ed. 2016, 55, 441−444. Copyright 2016 WileyVCH.

Scheme 5. Suggested Mechanism for the Methane → Ethylene Conversion Mediated by Mo-Doped Zeolitesa

a Adapted with permission from Chem.−Eur. J. 2014, 20, 4163−4169. Copyright 2014 Wiley-VCH.

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Figure 8. PES for the reactions of [Au(CH2)]+ (1A1) with CH4 calculated at the CCSD(T)/BSI//BMK/BSI level of theory. Zeropoint-corrected energies are given in kJ mol−1, and charges are omitted for the sake of clarity. Adapted with permission from Angew. Chem., Int. Ed. 2016, 55, 441−444. Copyright 2016 Wiley-VCH.

CONCLUSION As shown in this review, there exist many more mechanistic variants in the gas-phase thermal activation of methane than previously anticipated. Striking examples include the switch from a conventional hydrogen atom abstraction process, proceeding via homolytic cleavage of the C−H bond, to a heterolytic mechanism; the latter is controlled by protoncoupled electron transfer. Or, how by the deliberate “doping” of a cluster oxide one induces an intracomplex spin transfer that produces a high spin density at an otherwise unreactive bridging oxygen atom, thus enabling HAT to occur. We also came across quite a few unexpected mechanistic commonalties between gas-phase processes and reactions occurring at a surface, such as in the oxidative coupling of methane by magnesium oxides, for which Grignard-type structural motifs with a metal−carbon bond have been recently suggested.20a Similarly, Fischer−Tropsch-related coupling reactions exhibit features comparable to those observed for small cluster ions. Quite unexpected are also the mechanistic variants when comparing closed- and open-shell systems in their reactivities with methane. Recent examples include σ-metathesis-type processes or the extraordinary roles played in some systems by sizable relativistic effects, intriguing ligand effects, or the presence of strong Lewis-acidic sites in the cluster oxide ions. Common to all these effects is that they favor the generation of a highly stabilized encounter complex between methane and the gas-phase reagents. As a consequence, at an early stage of

bring about thermal C−H bond activation of C2H6 because all of the intermediates and transition structures along the sequence P1 → 22 → P2 are located below the entrance channel P1. This prediction has been verified experimentally.37 An entirely unexpected cluster-size effect in the context of methane activation according to eq 13 has been reported by Lang et al.40 While atomic ground-state Au+ (1S0) as well as dimeric [Au2]+ are unreactive to methane under single-collision conditions, the [Au2]+ cluster ion catalyzes C−C coupling of CH4 at increased substrate pressure and slightly elevated temperature to yield C2H4. [Au 2]+ + 2CH4 → [Au 2]+ + C2H4 + 2H 2

(13)

Not only do atomic 5d elements, eq 11, or metal carbene complexes, eq 12 and Figure 7, bring about C−C coupling by activating methane, but also some metal carbides facilitate this reaction. For example, the tantalum carbide cluster ions [TaCn]+ (n ≤ 14) react with CH4 under liberation of C2H2, eq 14.41 [TaCn]+ + CH4 → [TaCn − 1H 2]+ + C2H 2

(14)

13

Quite surprisingly, when CH4 is employed as a substrate, the isotope distribution in the products reveals that all carbon H


Review

Organometallics

Angew. Chem., Int. Ed. 2012, 51, 5544−5555. (c) Schwarz, H. Chem. Phys. Lett. 2015, 629, 91−101. (9) (a) Lunsford, J. H. Catal. Today 2000, 63, 165−174. (b) Zavyalova, U.; Holena, M.; Schlögl, R.; Baerns, M. ChemCatChem 2011, 3, 1935−1947. (c) Arndt, S.; Laugel, G.; Levchenko, S. V.; Horn, R.; Baerns, M.; Scheffler, M.; Schlögl, R.; Schomäcker, R. A. Catal. Rev.: Sci. Eng. 2011, 53, 424−514. (d) Hammond, C.; Conrad, S.; Hermans, I. ChemSusChem 2012, 5, 1668−1686. (e) Schlögl, R. Angew. Chem., Int. Ed. 2015, 54, 3465−3520. (10) (a) Shaik, S. S.; Shurki, A. Angew. Chem., Int. Ed. 1999, 38, 586− 625. (b) Ye, S.; Neese, F. Curr. Opin. Chem. Biol. 2009, 13, 89−98. (c) Lai, W.; Li, C.; Chen, H.; Shaik, S. S. Angew. Chem., Int. Ed. 2012, 51, 5556−5578. (11) Feyel, S.; Döbler, J.; Höckendorf, R. F.; Beyer, M. K.; Sauer, J.; Schwarz, H. Angew. Chem., Int. Ed. 2008, 47, 1946−1950. (12) (a) Schröder, D.; Roithová, J. Angew. Chem., Int. Ed. 2006, 45, 5705−5708. (b) Kwapien, K.; Sierka, M.; Döbler, J.; Sauer, J.; Haertelt, M.; Fielicke, A.; Meijer, G. Angew. Chem., Int. Ed. 2011, 50, 1716− 1719. (13) Zhao, Y.-X.; Ding, X.-L.; Ma, Y.-P.; Wang, Z.-C.; He, S.-G. Theor. Chem. Acc. 2010, 127, 449−465. (14) Schwarz, H. Angew. Chem., Int. Ed. 2015, 54, 10090−10100. (15) Li, J.; Wu, X.-N.; Schlangen, M.; Zhou, S.; González-Navarrete, P. A.; Tang, S.; Schwarz, H. Angew. Chem., Int. Ed. 2015, 54, 5074− 5078. (16) (a) Ma, J.-B.; Wang, Z.-C.; Schlangen, M.; He, S.-G.; Schwarz, H. Angew. Chem., Int. Ed. 2012, 51, 5991−5994. (b) Zhao, Y.-X.; Li, Z.Y.; Yuan, Z.; Li, X.-N.; He, S.-G. Angew. Chem., Int. Ed. 2014, 53, 9482−9486. (c) According to a personal communication of Prof. S. Shaik to H.S. (October 2015), any Lewis acid coordinated to [Mg2O2]•+ will act in a way similar to Ga2O3.. (17) (a) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961−7001. (b) Li, C.; Danovich, D.; Shaik, S. S. Chem. Sci. 2012, 3, 1903−1918. (c) Saouma, C. T.; Mayer, J. M. Chem. Sci. 2014, 5, 21−31. (d) Migliore, A.; Polizzi, N. F.; Therien, M. J.; Beratan, D. N. Chem. Rev. 2014, 114, 3381−3465. (e) Hammes-Schiffer, S. J. Am. Chem. Soc. 2015, 137, 8860−8871. (f) Li, J.; Wu, X.-N.; Zhou, S.; Tang, S.; Schlangen, M.; Schwarz, H. Angew. Chem., Int. Ed. 2015, 54, 12298−12302. (18) Li, J.; Zhou, S.; Zhang, J.; Schlangen, M.; Weiske, T.; Usharani, D.; Shaik, S.; Schwarz, H. J. Am. Chem. Soc. 2016, DOI: 10.1021/ jacs.6b03798. (19) Wang, Z.-C.; Dietl, N. P. R.; Kretschmer, R.; Ma, J.-B.; Weiske, T.; Schlangen, M.; Schwarz, H. Angew. Chem., Int. Ed. 2012, 51, 3703− 3707. (20) (a) Kwapien, K.; Paier, J.; Sauer, J.; Geske, M.; Zavyalova, U.; Horn, R.; Schwach, P.; Trunschke, A.; Schlögl, R. Angew. Chem., Int. Ed. 2014, 53, 8774−8778. (b) Oda, A.; Torigoe, H.; Itadani, A.; Ohkubo, T.; Yumura, T.; Kobayashi, H.; Kuroda, Y. J. Phys. Chem. C 2013, 117, 19525−19534; J. Phys. Chem. C 2014, 118, 15234−15241. (21) Kretschmer, R.; Schlangen, M.; Schwarz, H. Angew. Chem., Int. Ed. 2013, 52, 6097−6101. (22) Kretzschmar, I.; Fiedler, A.; Harvey, J. N.; Schröder, D.; Schwarz, H. J. Phys. Chem. A 1997, 101, 6252−6264. (23) Sun, X.; Zhou, S.; Schlangen, M.; Schwarz, H. Chem. - Eur. J. 2016, 22, 3073. (24) For a comparison of the open-shell systems [XO]+/CH4 (X = C, Si), see: Dietl, N. P. R.; Troiani, A.; Schlangen, M.; Ursini, O.; Angelini, G.; Apeloig, Y.; de Petris, G.; Schwarz, H. Chem. - Eur. J. 2013, 19, 6662−6669. (25) Li, H.-F.; Li, Z.-Y.; Liu, Q.-Y.; Li, X.-N.; Zhao, Y.-X.; He, S.-G. J. Phys. Chem. Lett. 2015, 6, 2287−2291. (26) (a) Yang, H.-Q.; Hu, C.-W.; Qin, S. Chem. Phys. 2006, 330, 343−348. (b) Hirao, H.; Chen, H.; Carvajal, M. A.; Wang, Y.; Shaik, S. S. J. Am. Chem. Soc. 2008, 130, 3319−3327. (27) (a) Chiodo, S.; Rivalta, I.; Michelini, M.; Russo, N.; Sicilia, E.; Ugalde, J. M. J. Phys. Chem. A 2006, 110, 12501−12511. (b) Wang, Z.C.; Wu, X.-N.; Zhao, Y.-X.; Ma, J.-B.; Ding, X.-L.; He, S.-G. Chem. Phys. Lett. 2010, 489, 25−29.

the reaction sequence the rovibrationally quite hot intermediates have an internal energy high enough to drive the reactions toward product formation. While many competing mechanistic scenarios have been identified recently,44 a heterolytic cleavage of the C−H bond of methane via the abstraction of a hydride has not yet been realized;45 however, on the basis of the work described in this review one would not be too surprised to see this being achieved in the near future.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous financial support by the Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft (“UniCat”) is appreciated. Dr. P. González-Navarrete acknowledges support from the Alexander von Humboldt-Stiftung in the form of a Postdoctoral Research Fellowship. We are grateful to Andrea Beck for her efforts to convert the authors’ notes into a manuscript. This work is dedicated, with admiration, to Prof. Pierre Braunstein.

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REFERENCES

(1) (a) Barton, D. H. R. Aldrichim. Acta 1990, 23, 3−10. (b) Crabtree, R. H. Chem. Rev. 1995, 95, 987−1007. (c) Heyduk, A. F.; Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. ACS Symp. Ser. 2004, 885, 250−263. (d) Olah, G. A. Angew. Chem., Int. Ed. 2005, 44, 2636−2649. (e) Bergman, R. G. Nature 2007, 446, 391−393. (f) Hammond, C.; Conrad, S.; Hermans, I. ChemSusChem 2012, 5, 1668−1686. (g) Cavaliere, V. N.; Mindiola, D. J. Chem. Sci. 2012, 3, 3356−3365. (h) Hashiguchi, B. G.; Bischof, S. M.; Konnick, M. M.; Periana, R. A. Acc. Chem. Res. 2012, 45, 885−898. (i) Kothandaraman, J.; Goeppert, A.; Czaun, M.; Olah, G. A.; Prakash, G. K. S. J. Am. Chem. Soc. 2016, 138, 778−781. (2) (a) Fokin, A. A.; Schreiner, P. R. Chem. Rev. 2002, 102, 1551− 1593. (b) Lersch, M.; Tilset, M. Chem. Rev. 2005, 105, 2471−2526. (c) Olah, G. A.; Goeppert, A.; Prakash, G. K. S. Beyond Oil and Gas: The Methanol Economy; Wiley-VCH: Weinheim, 2009. (d) Copéret, C. Chem. Rev. 2010, 110, 656−680. (e) Guo, Z.; Liu, B.; Zhang, Q.; Deng, W.; Wang, Y.; Yanhui, Y. Chem. Soc. Rev. 2014, 43, 3480−3524. (f) Haynes, C. A.; Gonzalez, R. Nat. Chem. Biol. 2014, 10, 331−339. (g) Horn, R.; Schlögl, R. Catal. Lett. 2015, 145, 23−39. (h) Li, J.; González-Navarrete, P. A.; Schlangen, M.; Schwarz, H. Chem. - Eur. J. 2015, 21, 7780−7789. (i) Schwarz, H. Coord. Chem. Rev. 2016, DOI: 10.1016/j.ccr.2016.03.009. (j) Olivos-Suarez, A. I.; Szécsényi, À .; Hensen, E. J. M.; Ruiz-Martinez, J.; Pidko, E. A.; Gascon, J. ACS Catal. 2016, 6, 2965−2981. (3) (a) Böhme, D. K.; Schwarz, H. Angew. Chem., Int. Ed. 2005, 44, 2336−2354. (b) Schwarz, H. Angew. Chem., Int. Ed. 2011, 50, 10096− 10115. (c) Schwarz, H. Isr. J. Chem. 2014, 54, 1413−1431. (4) (a) Taylor, H. S. Proc. R. Soc. London, Ser. A 1925, 108, 105−111. (b) Taylor, H. S. J. Phys. Chem. 1926, 30, 145−171. (5) (a) Lang, S. M.; Bernhardt, T. M. Phys. Chem. Chem. Phys. 2012, 14, 9255−9269. (b) Schlangen, M.; Schwarz, H. Catal. Lett. 2012, 142, 1265−1278. (6) Schwarz, H. Angew. Chem., Int. Ed. 2003, 42, 4442−4454. (7) (a) Schröder, D.; Shaik, S. S.; Schwarz, H. Acc. Chem. Res. 2000, 33, 139−145. (b) Schwarz, H. Int. J. Mass Spectrom. 2004, 237, 75− 105. (c) Shaik, S. S. Int. J. Mass Spectrom. 2013, 354−355, 5−14. (d) Harvey, J. N. WIREs Comput. Mol. Sci. 2014, 4, 1−14. (8) (a) Ding, X.-L.; Wu, X.-N.; Zhao, Y.-X.; He, S.-G. Acc. Chem. Res. 2012, 45, 382−390. (b) Dietl, N. P. R.; Schlangen, M.; Schwarz, H. I


Review

Organometallics (28) Ma, J.-B.; Xu, L.-L.; Liu, Q.-Y.; He, S.-G. Angew. Chem., Int. Ed. 2016, 55, 4947−4951. (29) (a) Carroll, J. J.; Weisshaar, J. C.; Siegbahn, P. E. M.; Wittborn, C. A. M.; Blomberg, M. R. A. J. Phys. Chem. 1995, 99, 14388−14396. (b) Adlhart, C.; Uggerud, E. Chem. Commun. 2006, 2581−2582. (c) Cho, H.-G.; Andrews, L. Organometallics 2009, 28, 1358−1368. (d) Perera, M.; Metz, R. B.; Kostko, O.; Ahmed, M. Angew. Chem., Int. Ed. 2013, 52, 888−891. (30) Zhou, S.; Li, J.; Schlangen, M.; Schwarz, H. Angew. Chem., Int. Ed. 2016, DOI: 10.1002/anie.201602312. (31) Zhou, S.; Li, J.; Schlangen, M.; Schwarz, H. Chem. - Eur. J. 2016, 22, 7225−7228. (32) Harvey, J. N.; Diefenbach, M. A.; Schröder, D.; Schwarz, H. Int. J. Mass Spectrom. 1999, 182−183, 85−97. (33) (a) Fischer, F.; Tropsch, H. Brennst. Chem. 1928, 3, 39−46. (b) Rofer-DePoorter, C. K. Chem. Rev. 1981, 81, 447−474. (34) (a) Irikura, K. K.; Beauchamp, J. L. J. Am. Chem. Soc. 1989, 111, 75−85. (b) Irikura, K. K.; Beauchamp, J. L. J. Phys. Chem. 1991, 95, 8344−8351. (35) (a) Koszinowski, K.; Schröder, D.; Schwarz, H. Organometallics 2003, 22, 3809−3819. (b) Koszinowski, K.; Schröder, D.; Schwarz, H. J. Phys. Chem. A 2003, 107, 4999−5006. (36) For a recent review on bond activation by metal-carbene complexes in the gas phase, see: Zhou, S.; Li, J.; Schlangen, M.; Schwarz, H. Acc. Chem. Res. 2016, 49, 494−502. (37) (a) Zhou, S.; Li, J.; Wu, X.-N.; Schlangen, M.; Schwarz, H. Angew. Chem., Int. Ed. 2016, 55, 441−444. (b) Li, J.; Zhou, S.; Schlangen, M.; Weiske, T.; Schwarz, H. ChemistrySelect 2016, 1, 444− 447. (38) Li, F.-X.; Armentrout, P. B. J. Chem. Phys. 2006, 125, 133114. (39) Pyykkö, P. Angew. Chem., Int. Ed. 2004, 43, 4412−4456. (40) Lang, S. M.; Bernhardt, T. M.; Barnett, R. N.; Landman, U. Angew. Chem., Int. Ed. 2010, 49, 980−983. (41) Cassady, C. J.; McElvany, S. W. J. Am. Chem. Soc. 1990, 112, 4788−4797. (42) Li, Z.-Y.; Yuan, Z.; Zhao, Y.-X.; He, S.-G. Chem. - Eur. J. 2014, 20, 4163−4169. (43) (a) Li, W.; Meitzner, G. D.; Borry, R. W.; Iglesia, E. J. Catal. 2000, 191, 373−383. (b) Liu, H.; Bao, X.; Xu, Y. J. Catal. 2006, 239, 441−450. (44) For example, recently He and co-workers described the unexpected heterolytic cleavage of CH4 by the closed-shell, clusteroxide anion [AuTi3O7]− to give rise to [Ti3O7H]− and AuCH3. Again, it is a Lewis acid−base chemistry that in combination with a strong relativistic effect of the electronegative Au center constitutes the root cause of this rather efficient bond activation. For details, see: Zhao, Y.X.; Li, X.-N.; Yuan, Z.; Liu, Q.-L.; Shi, Q.; He, S.-G. Chem. Sci. 2016, DOI: 10.1039/C6SC00539J. (45) For examples of gas-phase hydride abstraction from larger hydrocarbons (≥C3H8), see: (a) Schlangen, M.; Schröder, D.; Schwarz, H. Chem. - Eur. J. 2007, 13, 6810−6816. (b) Dietl, N.; Engeser, M.; Schwarz, H. Chem. - Eur. J. 2009, 15, 11100−11104. (c) Wu, X.-N.; Zhao, H.-T.; Li, J.-L.; Schlangen, M.; Schwarz, H. Phys. Chem. Chem. Phys. 2014, 16, 26617−26623.

J


Unexpected Mechanistic Variants in the Thermal Gas-Phase

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