Bond Activation by MetalâCarbene Complexes in the Gas Phase

Article pubs.acs.org/accounts

Bond Activation by Metal−Carbene Complexes in the Gas Phase Shaodong Zhou, Jilai Li, Maria Schlangen, and Helmut Schwarz* Institut für Chemie, Technische Universität Berlin, 10623 Berlin, Germany CONSPECTUS: “Bare” metal−carbene complexes, when generated in the gas phase and exposed to thermal reactions under (near) single-collision conditions, exhibit rather unique reactivities in addition to the well-known metathesis and cyclopropanation processes. For example, at room temperature the unligated [AuCH2]+ complex brings about efficient C−C coupling with methane to produce C2Hx (x = 4, 6), and the couple [TaCH2]+/CO2 gives rise to the generation of the acetic acid equivalent CH2CO. Entirely unprecedented is the thermal extrusion of a carbon atom from halobenzenes (X = F, Cl, Br, I) by [MCH2]+ (M = La, Hf, Ta, W, Re, Os) and its coupling with the methylene ligand to deliver C2H2 and [M(X)(C5H5)]+. Among the many noteworthy C−N bond-forming processes, the formation of CH3NH2 from [RhCH2]+/NH3, the generation of CH2NH2+ from [MCH2]+/NH3 (M = Pt, Au), and the production of [PtCHNH2]+ from [PtCH2]+/NH3 are of particular interest. The latter species are likely to be involved as intermediates in the platinum-mediated large-scale production of HCN from CH4/NH3 (the DEGUSSA process). In this context, a few examples are presented that point to the operation of co-operative effects even at a molecular level. For instance, in the coupling of CH4 with NH3 by the heteronuclear clusters [MPt]+ (M = coinage metal), platinum is crucial for the activation of methane, while the coinage metal M controls the branching ratio between the C−N bond-forming step and unwanted soot formation. For most of the gas-phase reactions described in this Account, detailed mechanistic insight has been derived from extensive computational work in conjunction with time-honored labeling and advanced mass-spectrometry-based experiments, and often a coherent description of the experimental findings has been achieved. As for some transition metals, in particular those from the third row, the metal−carbene complexes can be formed directly from methane, coupling of the so-generated [MCH2] species with an inert molecule such as CH4, CO2, or NH3 constitutes a route to activate and functionalize methane under ambient conditions. Clearly, while these gas-phase studies cannot be translated directly to formally related processes in solution or those that occur at a surface, they nevertheless provide a conceptual mechanistic understanding and permit researchers to probe directly the remarkable intrinsic features of these elusive molecules and, in a broader context, help to identify the active site of a catalyst, the so-called “aristocratic atoms”.

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INTRODUCTION In recent decades, metal−carbene complexes have been attracting quite some interest in synthetic chemistry,1 not only because of their catalytic performances but also as a result of their role in rearrangement reactions, in particular metathesis processes.2−4 In addition to the plethora of condensed-phase studies on metal carbenes, there exist also a considerable number of reports dealing with various aspects of the gas-phase chemistry and physics of these compounds.5 In the latter, not only have their generation and structural characterization been addressed, but also their often intriguing reactivities have been explored toward a large variety of substrates, including small inert molecules such as CH4, CO2, and NH3. In this Account, we will discuss mostly some recent findings related to the following topics: (1) metal-mediated dehydrogenation of methane, (2) genuine gas-phase metathesis processes and mechanistic variants in cyclopropanation reactions, and (3) unusual couplings of metal carbenes with small molecules giving rise to C−C and C−N bonds. While we will refrain from describing any experimental or computational details, which can be found in the original articles, it should be © XXXX American Chemical Society

stressed that there now exists ample evidence that gas-phase studies on “isolated” reactants provide an ideal arena for probing experimentally the energetics and kinetics as well as the mechanism(s) of a chemical reaction in an unperturbed environment at a strictly molecular level without being obscured by difficult-to-control or poorly defined solvation, aggregation, counterion, and other effects.6−9 Thus, the intrinsic features of reactive species can be probed directly.

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FROM METHANE TO [M]+−CH2

Dehydrogenation of methane to generate [M]CH2 (where [M] stands for a metal atom or a cluster) is of conceptual interest, as the methylene complexes may serve as intermediates in the conversion CH4 to CH2O.10 As the dehydrogenation of methane according to eq 1, [M] + CH4 → [M]CH 2 + H 2

(1)

Received: December 4, 2015

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DOI: 10.1021/acs.accounts.5b00023 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Periodic variations in the reaction efficiencies (k/kc), represented by solid circles, for the room-temperature processes of ground-state atomic cations. The numbers in parentheses indicate the numbers of sequential color-coded reactions observed. Adapted from ref 19. Copyright 2009 American Chemical Society.

energy of the projectile [M], the role of lanthanide contractions,25 and the operation of relativistic effects,26 differences as well as commonalities between first-, secondand third-row transition-metal species in their reactions with methane (and other hydrocarbons) could be described in a coherent fashion (for a detailed discussion, see ref 14). A schematic description of the metal-mediated dehydrogenation of methane is given in Scheme 1. After initial formation of

has been described in quite some detail in previous review articles,11−14 here only a few general aspects will be recalled and complemented by more recent spectroscopic findings. Also, we will not engage in the ongoing terminology disputes as to whether [M]CH2 species are better described as metal carbenes or as carbenoid intermediatesfor a discussion of the particularly intriguing gold(I) carbene complexes, see refs 15−18. The Bohme group19 has conducted a comprehensive study of the thermal reactions of atomic cations with methane, and as shown in Figure 1, only As+, Nb+, Ta+, W+, Os+, Pt+, and Ir+ bring about elimination of molecular hydrogen under ambient conditions. As discussed at great length in a different context,14 the nonreactivity of most of the cations can be traced back to three basic features: (1) Loss of H2 from methane requires the metal−methylidene bond strength D0(M−CH2) to exceed the heat of dehydrogenation of CH4; the latter amounts to 464 kJ mol−1. As shown in a detailed theoretical study,20 this thermochemical requirement is not met by most of the atomic cations studied. (2) The electronic configuration matters as well, as the metal [M] must possess an empty orbital to accept electrons from the σ bond to be broken. Concomitantly, occupied orbitals of [M] having π symmetry may engage in back-donation of electrons into the empty σ* orbital of the C− H bond to be activated. Clearly, if the acceptor orbital of [M] is occupied, a repulsive interaction will be operative at an early stage of the bond activation process. (3) Finally, oxidative insertion of [M] into the C−H bond of a substrate R−H generates an intermediate H−[M]−R. For some metals [M], this is accompanied by a change in spin multiplicity. Thus, features like the promotion energy necessary to achieve a “prepared” state or the efficiency of spin−orbit coupling cannot be ignoredrather, they matter a lot.21−24 Indeed, when additional effects were taken into account, e.g., cluster size, charge, and electronic states, ligand effects, kinetic

Scheme 1. Schematic Description of the Metal-Mediated Dehydrogenation of Methane

an activated encounter complex, the process involves a sequence of oxidative addition, α-hydrogen migration, reductive elimination, and evaporation of H2 to eventually produce a “naked” metal carbene. While it had been assumed for quite a while that for atomic systems the product [M,C,2H]+ of the reaction shown in eq 1 invariably corresponds to a metal−carbene complex, recent infrared multiphoton dissociation spectroscopy experiments revealed a more complex situation.27,28 For the systems [M,C,2H]+ (M = Ta, W, Ir, Pt), it is only the platinum complex [Pt]+−CH2 that exhibits a classic C2v carbene structure;20,27 for M = Ta and W, the presence of empty d orbitals permits both metal ions to form an agostic interaction with a C−H bond that distorts the carbene structure, in line with previous suggestions.29 For the iridium system, only the hydrido−methylidene structure [H−Ir−CH]+ is formed.27 On B


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role of the electrophilicity of the metal center in these processes. Gas-phase metathesis is by no means confined to structurally simple carbene complexes as mentioned above. An interesting example has been reported by the Chen group.37 The cationic ruthenium benzylidene complex at m/z 546 undergoes an extremely efficient thermal metathesis reaction with 1-butene to form neutral styrene and the ion at m/z 498 (Figure 2), as well as ring-opening metathesis with cycloalkenes. Inverse secondary kinetic isotope effects for H/D-labeled substrates support the intermediacy of metallacyclobutane species. Not only the close correspondence of these gas-phase processes with their analogues in solution but also the 104-fold increase in their absolute rates relative to the reactions in solution are remarkable.37,38 Clearly, this enormous acceleration is a result of the energetic penalty caused by the requirement to partially desolvate charged species along the reaction coordinate in solution. More recently, electrospray ionization mass spectrometry studies were conducted to shed further light on intrinsic features of second-generation Hoveyda−Grubbs catalysts and to probe the role of doping these catalysts with alkali cations.39 The Chen group40−43 and Swift and Gronert44 have also reported intriguing findings on the reactions of gold−carbene complexes [LAuCHR]+ with olefins both in the gas phase and in solution. For example, an ion at m/z 679 arises when the carbene complex [IMesAuCHPh]+ (IMes =1,3-bis(2,4,6trimethylphenyl)imidazole-2-ylidene) is reacted with cis-dimethoxyethylene, and three additional signals show up upon collisional activation of the mass-selected species at m/z 679 (Figure 3).40 The signal at m/z 545 has been assigned to the gold carbene complex [IMesAuCHOCH3]+, which demonstrates a metathesis-type reactivity unprecedented for gold. A cyclopropanation reaction, which is typical for the solution chemistry of gold carbenoids,18 also takes place in the gas phase: the signal at m/z 501 corresponds to the transfer of a benzylidene group to the CC double bond. This pathway dominates when the olefin substituent is switched from methoxy to alkyl. Unexpectedly, the metathesis channel is completely switched off for the couple [IMesAuCHPh]+/ C2H5CHCHC2H5, resulting in [IMesAuCHC2H5]+ (Figure 3). Detailed quantum-chemical calculations41 indicate that the metathesis reaction observed for the electron-rich

the basis of detailed theoretical work, a similar structure has been predicted for [H−Os−CH]+.20,28 In the following, we will address the question of what “naked” [M]+−CH2 complexes, not necessarily generated only from methane, can achieve when employed as reagents in thermal reactions in the gas phase.

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OLEFIN METATHESIS AND CYCLOPROPANATION REACTIONS WITH “NAKED” [M]+−CH2 In his remarkable Nobel Lecture entitled “Olefin Metathesis: The Early Days”,2 while emphasizing the crucial mechanistic role of metal carbenes, Yves Chauvin also hinted at the difficulties of properly characterizing these elusive intermediates under working conditions. This challenge is much less a problem when experiments are conducted under (near) singlecollision conditions in the gas phase and complemented by theoretical studies. In fact, there exist numerous ways to generate “naked” metal−carbene complexes,14,30 to determine their [M]+−CH2 bond strengths,20,27,28,30−33 and to probe their potential in, e.g., metathesis, cyclopropanation, or quite unusual coupling reactions. These aspects will be discussed next. In regard to gas-phase metathesis, as early as 1979 it was reported that [MnCH2]+ reacts with C2D4 to produce [MnCD2]+ efficiently30 and also that the thermal reactions of [MCD2]+ (M = Fe, Co) with C2H4 give rise to the formation of [MCH2]+.34 These processes as well as the one depicted in Scheme 2 clearly point to genuine examples of textbook gasScheme 2. Gas-Phase Metathesis Reactions of [FeCH2]+a

a

Adapted with permission from ref 35. Copyright 1992 Wiley-VCH.

phase metathesis.35 Interestingly, olefin metathesis reactions are not observed under thermal conditions with carbene ligands such as CF233 and CHCN;36 these findings point to the subtle

Figure 2. Thermal metathesis reaction of the mass-selected ruthenium complex at m/z 546 with 1-butene. Adapted with permission from ref 37. Copyright 1998 Wiley-VCH. C


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Figure 3. Daughter-ion mass spectra of mass-selected adducts of [IMesAuCHPh]+ with (top) cis-dimethoxyethylene and (bottom) cis-3-hexene. Adapted from ref 40. Copyright 2008 American Chemical Society.

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C−C AND C−N COUPLING PROCESSES OF [M]+−CH2 Rather unexpected examples of the thermal coupling of a methylene ligand with methane will be presented first. While reactions with the open-shell metal carbenes [PtnCH2]+ (n = 1, 5) have been observed (eq 2), these processes are either not very efficient (n = 1) or do not result in carbon−carbon bond formation upon dehydrogenation of CH4 (n = 5).46,47 Similarly, the efficiency of Fischer−Tropsch-type coupling of methylene units (eq 3) has been reported to depend crucially on the nature of the transition metal M.48,49

dimethoxy-substituted olefin cannot be explained in terms of the classical Chauvin mechanism.2 Rather, a “cyclopropane metathesis” variant has been suggested.41 Cyclopropanation has been conjectured for the gas-phase reactions of a phenanthroline-ligated copper(I) carbene complex with electron-rich olefins (Scheme 3). On the basis Scheme 3. Gas-Phase Cyclopropanation with a Ligated Cu(I) Carbenea

[Pt nCH 2]+ + CH4 → [Pt nC2 , H4]+ + H 2 n = 1,5

[M]+ M = Ta,W,Ir,Os

a

+ nCH4 → [M(CH 2)n ]+ + nH 2 n≤8

(2)

(3)

An entirely different situation has been encountered in the gas-phase reactions of the closed-shell “naked” [AuCH2]+ complex. 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 4).50 Doubleresonance experiments and energetic considerations demonstrated that [AuC2H4]+ does not serve as a precursor in the thermal reaction to form Au+ and C2H6, and labeling studies combined with extensive computational investigations revealed further mechanistic details of this unusual C−C coupling process (Figure 5). In the initial phase, the weakly bound encounter complex 1 undergoes insertion of the methylene unit into the C−H bond of the incoming CH4 substrate (1 → TS1/ 2 → 2). This process profits from the relatively weak Au+−CH2

Adapted with permission from ref 45. Copyright 2015 Wiley-VCH.

of energetic considerations, a dissociation−association pathway has been ruled out in favor of a methylene-transfer process for the CH2 → RCHCH2 ligand exchange at the copper site.45 This study also addressed the problem of how to avoid the wellknown tendency of copper(I) carbenes to undergo intramolecular rearrangements, e.g., Wolff rearrangement or insertion into copper−ligand bonds, by forming copper(I) carbene ylides through complexation with solvents or weak nucleophiles.45 D


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particularly in an environmentally benign and economically feasible way. An early example of this kind of a “Holy Grail” reaction in the gas phase has been realized by the mechanism shown in eqs 5−7,52 Ta + + CH4 → [TaCH 2]+ + H 2

(5)

[TaCH 2]+ + CO2 → [Ta(O)CH 2]+ + CO

(6)

[Ta(O)CH 2]+ + CO2 → [TaO2 ] + + CH 2CO

(7)

corresponding to the overall reaction shown in eq 8: Ta + + CH4 + 2CO2 → [TaO2 ]+ + H 2 + CO + CH 2CO

While this sequence of reactions demonstrates that at the atomic level activation and coupling of the two C1 fragments to form an acetic acid equivalent (i.e., ketene) can be achieved, the reaction, whose intriguing mechanism involving several changes of multiplicity along the reaction coordinate has been explored computationally,53,54 suffers from the drawback that Ta+ is used in stoichiometric amounts and is irreversibly consumed as [TaO2]+. In fact, in a rather detailed computational survey it was recently shown55 that heteronuclear clusters, e.g., doped magnesium oxides, are more promising to achieve this demanding goal. Here, a late transition metal is exploited in the C−H bond activation of methane and an early one helps to bring about breaking of the strong CO bond of CO2. The final example concerns an unprecedented extrusion of a carbon atom from the ipso position of a substituted benzene ring and its coupling with a methylene ligand to form acetylene (eq 9):5,56On the basis of labeling experiments, consideration

Figure 4. Au+-catalyzed coupling of a carbene ligand with methane to form C−C bonds. Adapted with permission from ref 50. Copyright 2016 Wiley-VCH.

bond (D0 = 357 kJ mol−1)51 and the reduced electron density of the carbene ligand (+0.33e);50 the latter reflects the rather high electronegativity of Au+.26 Complex 2 can either dissociate directly to Au+ and C2H6 (P1) or rearrange further along the sequence 2 → 3 → 4 (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 3 ⇄ 5 ⇄ 5′. Furthermore, according to Figure 5 atomic Au+ is predicted to bring about thermal C−H bond activation of C2H6 because all of the intermediates and transition structures along the sequence P1 →→ P2 are located below the entrance channel P1. This prediction has been verified experimentally.50 Another topical reaction of enormous economic relevance is the coupling of methane and carbon dioxide to produce acetic acid (eq 4), CH4 + CO2 → CH3CO2 H

(8)

of thermochemical features (i.e., the bond-dissociation energies of the C−X and La+−X bonds), collision experiments, and extensive density functional theory (DFT) calculations, the branching ratios of eqs 9 and 10 could be explained; more

(4)

Figure 5. Potential energy surface for the reactions of [Au[CH2)]+ (1A1) with CH4 calculated at the CCSD(T)/BSI//BMK/BSI level of theory. Zero-point-corrected energies are given in kJ mol−1, and charges have been omitted for the sake of clarity. Adapted with permission from ref 50. Copyright 2016 Wiley-VCH. E


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Accounts of Chemical Research importantly, a detailed mechanistic scenario was presented that described all of the experimental findings in a coherent fashion. As one of the central intermediates, species 6 was identified (Scheme 4); 6 corresponds to a complex of atomic La+ Scheme 4. Carbon-Atom Extrusion and C−C Bond Couplinga

a

Adapted with permission from ref 56. Copyright 2016 Wiley-VCH. Figure 6. Simplified potential energy surface for the reaction [Pt(CH2)]+ + NH3 → [Pt(CHNH2)]+ + H2 calculated for the doublet-spin surface at the B3LYP/TZP//B3LYP/DZP level of theory; relative energies are given in kJ mol−1 and have been corrected for zero-point vibrational energies. For the sake of clarity, charges have been omitted. Color code: red, platinum; gray, carbon; blue, nitrogen; white, hydrogen. Adapted with permission from ref 57. Copyright 2014 Wiley-VCH.

interacting with a benzyl halide. In fact, those lanthanides Ln+ having a promotion energy small enough to attain a 4fn5d16s1 electronic configuration are also reactive and form both [LnCl]+ and [Ln(C5H5)Cl]+ (Ln = La, Ce, Gd, Tb, Lu) when exposed to benzyl chloride. While the carbene complexes of the third-row transition-metal ions Hf, Ta, W, Re, and Os also bring about this unusual transformation in their reactions with C6H5Cl, variation of the substituent X demonstrates that the carbon-atom extrusion is limited to X = halogen. For substrates C6H5X with X = H, CN, NO2, OCH3, or CF3, no reaction takes place or products that reflect the high oxophilicity or fluorine affinity of the lanthanides are formed.56 Finally, the gold−carbene complex [AuCH2]+ once more displays rather unique chemistry upon exposure to C6H5Cl (eqs 11 and 12): The formation of these two organic cations rather

mediated N−H and C−H bond activations to generate 10 via intermediate 8 is energetically favored over the alternative path commencing with C−H bond activation (7 → 9 → 10); the concerted, metal-free hydrogen migration/elimination path (7 → TS 7/11 → 11) is significantly higher in energy because it is symmetry-forbidden.58 In fact, the combined experimental/computational exploration of the Pt+/CH4/NH3 system provided a rather detailed view of the elementary steps involved in the DEGUSSA process (Scheme 5).58 Central is the generation of a [Pt]−CH2 carbene complex directly from methane. The alternative N−H bond activation of ammonia to produce initially [Pt]−NH is much less likely to occur. Even if it does occur, this intermediate is prone to undergo facile metathesis with methane to regenerate

than [AuCl]+ or [AuH]+ most likely reflects the high stabilities of AuCl and AuH as well as the large ionization energies of AuX (X = H, halogen); the latter (>9 eV) are much higher than those of C6H5CH2• (7.24 eV) and C6H5CHCl• (8.0 eV).56 In any case, the occurrence of the reactions shown in eqs 11 and 12 demonstrates the coupling of the methylene ligand with a phenyl ring. Metal carbenes are also capable of engaging in C−N coupling when reacted with NH3 under thermal conditions (for a recent review, see ref 57). We got interested in this topic in the context of the DEGUSSA process, which is the largescale platinum-mediated coupling of CH4 and NH3 to generate HCN (eq 13):

Scheme 5. Schematic Representation of Elementary Steps Involved in the Platinum-Mediated DEGUSSA Process CH4 + NH3 → HCN + 3H2a

[Pt]

CH4 + NH3 ⎯→ ⎯ HCN + 3H 2

(13)

Mass-spectrometry-based experiments suggested the key role of a platinum carbene. Its thermal reactions with NH3 are given in eqs 14−16, and for the dehydrogenation path (eqs 15), DFT calculations demonstrated how the platinum center is exploited as a catalyst (Figure 6). For example, the sequence of metal-

a

F

Adapted from ref 58. Copyright 1999 American Chemical Society. DOI: 10.1021/acs.accounts.5b00023 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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NH3 and form [Pt]−CH2. Oligomerization of CH4 at the [Pt]−CH2 site, which may give rise to unwanted soot formation, is a side process that can be controlled by doping the catalyst with coinage metals (see below). Interestingly, many of the predictions derived from these mass-spectrometric studies were later confirmed by in situ experiments.59 C−N coupling in the reactions of [MCH2]+ with NH3 is not confined to M = Pt. While the carbenes of the 3d metals Fe+ and Co+ are completely unreactive toward NH3, those of the 4d and 5d metals Rh, W, Os, and Ir exhibit moderate to high efficiencies (ϕ = 10−40%).57 For the [RhCH2]+/NH3 couple, the major process corresponds to the formation of Rh+ and CH3NH2. This reaction requires metal carbenes with D0(M+− CH2) ≤ 364 kJ mol−1, and this requirement is met only for [RhCH2]+ with D0 = 355 kJ mol−1. The behavior of the [AuCH2]+/NH3 couple is again quite unique. With an efficiency of ϕ = 60%, the only product pair generated corresponds to [CH2NH2]+ and AuH. Breaking the relatively weak Au+−CH2 bond and making a strong Au−H bond can be viewed as the driving forces.57 Finally, the coinage metals M = Cu, Ag, and Au were identified to play a particular role in the reaction of NH3 with the heteronuclear cluster carbenes [MPtCH2]+.60 While the platinum atom is involved in the activation of CH4 to bring about its dehydrogenation, it is the coinage metal M that controls the branching ratio between C−N coupling and unwanted soot formation, as convincingly shown by labeling experiments and relativistic DFT calculations.61,62 Thus, even at a strictly molecular level cooperative effects in heteronuclear cluster catalysis seem to exist, and as discussed in a different context, each atom counts!9

Shaodong Zhou received his Ph.D. under the supervision of Professor Xinzhi Chen in 2013 at Zheijang University. In 2013 he joined the Schwarz research group at TU Berlin as a postdoctoral fellow. Jilai Li obtained his Ph.D. in 2007 under the supervision of Professor Xuri Huang at Jilin University. From 2009 to 2011 he served as an Associate Professor of Chemistry at Jilin University, which was followed by postdoctoral studies with Ulf Ryde at Lund University and Helmut Schwarz at TU Berlin. Maria Schlangen studied chemistry in Köln and Berlin and conducted her Ph.D. research at TU Berlin under the supervision of Helmut Schwarz. Since 2007 she has been in charge of the mass spectrometry facility of the Institute of Chemistry at TU Berlin. Helmut Schwarz majored in chemistry at the Technische Universität Berlin in 1971. After completing his habilitation at TU Berlin in 1974, he spent some time at ETH Zürich, MIT, and Cambridge University before being appointed Professor of Chemistry at TU Berlin in 1978. Since January 2008 he has served as President of the Alexander von Humboldt Foundation (Bonn).

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ACKNOWLEDGMENTS We sincerely thank the Deutsche Forschungsgemeinschaft (UniCat) and the Fonds der Chemischen Industrie for financial support. Conceptual and practical contributions of past and current co-workers are appreciated, as is technical assistance by Andrea Beck. Dr. Thomas Weiske is thanked for many helpful comments.

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DEDICATION This Account is dedicated to the memory of Ernst Otto Fischer and Yves Chauvin.

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CONCLUDING REMARKS While it is obvious that the study of “naked” gas-phase species will never account for the precise kinetic and mechanistic details that prevail at a surface or in solution, it nevertheless holds true that when complemented by appropriate computational studies, gas-phase experiments are extremely meaningful. They permit a systematic approach to conduct experiments at a strictly molecular level, help to characterize the active site of a catalyst (the so-called “aristocratic atoms”),63 and more importantly, offer a conceptual framework. Furthermore, comparison with related work in the condensed phase provides insight into the role of the “environment”. In addition, because of the higher activity of the “naked” species, entirely unexpected reactions can be encountered, as for example the unprecedented extrusion of a carbon atom from a benzene ring by an [M]+−CH2 complex5,56 or the abstraction of methylene from CH2Cl2 and its coupling with a cyclopentadienyl ligand by [FeC5H5]+.64 Clearly, since the seminal work on these elusive carbene species by E. O. Fischer and his school65 and the visionary explanation of Y. Chauvin2 to recognize their role in metathesis processes,66 metal−carbene complexes now occupy a central, well-accepted position in chemistry, be that in solution or in the gas phase.

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REFERENCES

(1) Riener, K.; Haslinger, S.; Raba, A.; Högerl, M. P.; Cokoja, M.; Herrmann, W. A.; Kühn, F. E. Chemistry of Iron N-Heterocyclic Carbene Complexes: Syntheses, Structures, Reactivities, and Catalytic Applications. Chem. Rev. 2014, 114, 5215−5272 and references therein. (2) Chauvin, Y. Olefin Metathesis: The Early Days. Angew. Chem., Int. Ed. 2006, 45, 3740−3747. (3) Schrock, R. R. Multiple Metal−Carbon Bonds for Catalytic Metathesis Reactions. Angew. Chem., Int. Ed. 2006, 45, 3748−3759. (4) Grubbs, R. H. Olefin-Metathesis Catalysts for the Preparation of Molecules and Materials. Angew. Chem., Int. Ed. 2006, 45, 3760−3765. (5) For many references, see: Zhou, S.; Schlangen, M.; Li, J.; Wu, X.N.; Schwarz, H. Carbon-Atom Extrusion from Halobenzenes and Its Coupling with a Methylene Ligand to Form Acetylene. Chem. - Eur. J. 2015, 21, 9629−9631. (6) Böhme, D. K.; Schwarz, H. Gas-Phase Catalysis by Atomic and Cluster Metal Ions: The Ultimate Single-Site Catalysts. Angew. Chem., Int. Ed. 2005, 44, 2336−2354. (7) Lang, S. M.; Bernhardt, T. M. Gas phase metal cluster model systems for heterogeneous catalysis. Phys. Chem. Chem. Phys. 2012, 14, 9255−9269. (8) Schlangen, M.; Schwarz, H. Effects of Ligands, Cluster Size, and Charge State in Gas-Phase Catalysis: A Happy Marriage of Experimental and Computational Studies. Catal. Lett. 2012, 142, 1265−1278. (9) Schwarz, H. Doping Effects in Cluster-Mediated Bond Activation. Angew. Chem., Int. Ed. 2015, 54, 10090−10100. (10) Sun, M.; Zhang, J.; Putaj, P.; Caps, V.; Lefebvre, F.; Pelletier, J.; Basset, J.-M. Catalytic Oxidation of Light Alkanes (C1−C4) by Heteropoly Compounds. Chem. Rev. 2014, 114, 981−1019.

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The authors declare no competing financial interest. G


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and density functional study. Int. J. Mass Spectrom. Ion Processes 1998, 175, 1−14. (34) Jacobson, D. B.; Freiser, B. S. Reactions of FeCH2+ and CoCH2+ with olefins in the gas phase. Studies involving olefin metathesis. J. Am. Chem. Soc. 1985, 107, 2605−2612. (35) Stöckigt, D.; Schwarz, H. On the Reactions of Fe(CH2)+ with 1,7-Octadiene in the Gas Phase. Chem. Ber. 1992, 125, 2817−2819. (36) Wilson, K. L.; Cooper, B. T.; Buckner, S. W. Bond activation by d0 metal carbene complexes in the gas phase. Rapid Commun. Mass Spectrom. 1993, 7, 844−847. (37) Hinderling, C.; Adlhart, C.; Chen, P. Olefin Metathesis of a Ruthenium Carbene Complex by Electrospray Ionization in the Gas Phase. Angew. Chem., Int. Ed. 1998, 37, 2685−2689. (38) Schwab, P.; Grubbs, R. H.; Ziller, J. W. Synthesis and Applications of RuCl2(=CHR‘) (PR3)2. J. Am. Chem. Soc. 1996, 118, 100−110. (39) Wang, H.-Y.; Yim, W.-L.; Guo, Y.-L.; Metzger, J. O. ESI-MS Studies and Calculations on Second-Generation Grubbs and Hoveyda−Grubbs Ruthenium Olefin Metathesis Catalysts. Organometallics 2012, 31, 1627−1634. (40) Fedorov, A.; Moret, M.-E.; Chen, P. Gas-Phase Synthesis and Reactivity of a Gold Carbene Complex. J. Am. Chem. Soc. 2008, 130, 8880−8881. (41) Fedorov, A.; Batiste, L.; Bach, A.; Birney, D. M.; Chen, P. Potential Energy Surface for (Retro-)Cyclopropanation: Metathesis with a Cationic Gold Complex. J. Am. Chem. Soc. 2011, 133, 12162− 12171. Also see: Fedorov, A.; Chen, P. Mechanistic Insights from the Gas-Phase Reactivity of Phosphorus-Ylid-Supported Benzylidene Gold Complexes. Organometallics 2010, 29, 2994−3000. (42) Ringger, D. H.; Kobylianskii, I. J.; Serra, D.; Chen, P. Quantitative Description of Structural Effects on the Stability of Gold(I) Carbenes. Chem. - Eur. J. 2014, 20, 14270−14281. (43) Fedorov, A.; Chen, P. Electronic Effects in the Reactions of Olefin-Coordinated Carbene Complexes. Organometallics 2009, 28, 1278−1281. (44) Swift, C. A.; Gronert, S. Formation and Reactivity of Gold Carbene Complexes in the Gas Phase. Organometallics 2014, 33, 7135−7140. (45) Aldajaei, J. T.; Keeffe, J. R.; Swift, C. A.; Gronert, S. Are Copper(I) Carbenes Capable Intermediates for Cyclopropanations? The Case for Ylide Intermediates. Chem. - Eur. J. 2015, 21, 12702− 12708. (46) Koszinowski, K.; Schröder, D.; Schwarz, H. Reactions of Platinum−Carbene Clusters PtnCH2+ (n = 1 - 5) with O2, CH4, NH3, and H2O: Coupling Processes versus Carbide Formation. Organometallics 2003, 22, 3809−3819. (47) Koszinowski, K.; Schröder, D.; Schwarz, H. Reactivity of Small Cationic Platinum Clusters. J. Phys. Chem. A 2003, 107, 4999−5006. (48) Irikura, K. K.; Beauchamp, J. L. Osmium tetroxide and its fragment ions in the gas phase: reactivity with hydrocarbons and small molecules. J. Am. Chem. Soc. 1989, 111, 75−85. (49) Irikura, K. K.; Beauchamp, J. L. Electronic structure considerations for methane activation by third-row transition-metal ions. J. Phys. Chem. 1991, 95, 8344−8351. (50) Zhou, S.; Li, J.; Wu, X.-N.; Schlangen, M.; Schwarz, H. Efficient Room-Temperature, Au+-Mediated Coupling of a Carbene Ligand with Methane to Generate C2Hx (x = 4, 6). Angew. Chem., Int. Ed. 2016, 55, 441−444. (51) Li, F.-X.; Armentrout, P. B. Activation of methane by gold cations: Guided ion beam and theoretical studies. J. Chem. Phys. 2006, 125, 133114. (52) Wesendrup, R.; Schwarz, H. Tantalum-Mediated Coupling of Methane and Carbon Dioxide in the Gas Phase. Angew. Chem., Int. Ed. Engl. 1995, 34, 2033−2035. (53) Sändig, N.; Koch, W. Mechanism of the Ta+-Mediated Activation of the C−H Bond in Methane. Organometallics 1997, 16, 5244−5251.

(11) Schröder, D.; Schwarz, H. Gas-phase activation of methane by ligated transition-metal cations. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 18114−18119. (12) Roithová, J.; Schröder, D. Selective Activation of Alkanes by Gas-Phase Metal Ions. Chem. Rev. 2010, 110, 1170−1211. (13) Schwarz, H. Chemistry with Methane: Concepts Rather than Recipes. Angew. Chem., Int. Ed. 2011, 50, 10096−10115. (14) Schwarz, H. How and Why Do Cluster Size, Charge State, and Ligands Affect the Course of Metal-Mediated Gas-Phase Activation of Methane? Isr. J. Chem. 2014, 54, 1413−1431. (15) Benitez, D.; Shapiro, N. D.; Tkatchouk, E.; Wang, Y.; Goddard, W. A., III; Toste, F. D. A bonding model for gold(I) carbene complexes. Nat. Chem. 2009, 1, 482−486. (16) Seidel, G.; Fürstner, A. Structure of a Reactive Gold Carbenoid. Angew. Chem., Int. Ed. 2014, 53, 4807−4811. (17) Wang, Y.; Muratore, M. E.; Echavarren, A. M. Gold Carbene or Carbenoid: Is There a Difference? Chem. - Eur. J. 2015, 21, 7332− 7339. (18) Qian, D.; Zhang, J. Gold-catalyzed cyclopropanation reactions using a carbenoid precursor toolbox. Chem. Soc. Rev. 2015, 44, 677− 698. (19) Shayesteh, A.; Lavrov, V. V.; Koyanagi, G. K.; Bohme, D. K. Reactions of Atomic Cations with Methane: Gas Phase RoomTemperature Kinetics and Periodicities in Reactivity. J. Phys. Chem. A 2009, 113, 5602−5611. (20) Zhang, X.; Schwarz, H. Bonding in Cationic MCH2+ (M = K La, Hf - Rn): A Theoretical Study on Periodic Trends. Chem. - Eur. J. 2010, 16, 5882−5888. (21) Armentrout, P. B. Chemistry of Excited Electronic States. Science 1991, 251, 175−179. (22) Weisshaar, J. C. Bare transition metal atoms in the gas phase: reactions of M, M+, and M2+ with hydrocarbons. Acc. Chem. Res. 1993, 26, 213−219. (23) Schröder, D.; Shaik, S. S.; Schwarz, H. Two-State Reactivity as a New Concept in Organometallic Chemistrys. Acc. Chem. Res. 2000, 33, 139−145. (24) Schwarz, H. On the spin-forbiddeness of gas-phase ion− molecule reactions: a fruitful intersection of experimental and computational studies. Int. J. Mass Spectrom. 2004, 237, 75−105. (25) Cornehl, H. H.; Heinemann, C.; Schröder, D.; Schwarz, H. GasPhase Reactivity of Lanthanide Cations with Hydrocarbons. Organometallics 1995, 14, 992−999. (26) Schwarz, H. Relativistic Effects in Gas-Phase Ion Chemistry: An Experimentalist’s View. Angew. Chem., Int. Ed. 2003, 42, 4442−4454. (27) Lapoutre, V. J. F.; Redlich, B.; van der Meer, A. F. G.; Oomens, J.; Bakker, J. M.; Sweeney, A.; Mookherjee, A.; Armentrout, P. B. Structures of the Dehydrogenation Products of Methane Activation by 5d Transition Metal Cations. J. Phys. Chem. A 2013, 117, 4115−4126. (28) Armentrout, P. B.; Parke, L.; Hinton, C.; Citir, M. Activation of Methane by Os+: Guided-Ion-Beam and Theoretical Studies. ChemPlusChem 2013, 78, 1157−1173. (29) Simon, A.; Lemaire, J.; Boissel, P.; Maître, P. Competition between agostic WCH2+ and HWCH+: A joint experimental and theoretical study. J. Chem. Phys. 2001, 115, 2510−2518. (30) Stevens, A. E.; Beauchamp, J. L. Properties and reactions of manganese methylene complexes in the gas phase. The importance of strong metal−carbene bonds for effective olefin metathesis catalysts. J. Am. Chem. Soc. 1979, 101, 6449−6450. (31) Metz, R. B. Photofragment spectroscopy of covalently bound transition metal complexes. A window into C-H and C-C bond activation by transition metal ions. Int. Rev. Phys. Chem. 2004, 23, 79− 108. (32) Perera, M.; Metz, R. B.; Kostko, O.; Ahmed, M. Vacuum Ultraviolet Photoionization Studies of PtCH2 and H-Pt-CH3: A Potential Energy Surface for the Pt+CH4 Reaction. Angew. Chem., Int. Ed. 2013, 52, 888−891. (33) Chen, Q.; Auberry, K. J.; Freiser, B. S. Reactions of FeCF2+ and CoCF2+ with simple alkanes and olefins in the gas phase: An FTICR H


Article

Accounts of Chemical Research (54) Sändig, N.; Koch, W. A Quantum Chemical View on the Mechanism of the Ta+-Mediated Coupling of Carbon Dioxide with Methane. Organometallics 1998, 17, 2344−2351. (55) Li, J.; González-Navarrete, P.; Schlangen, M.; Schwarz, H. Activation of Methane and Carbon Dioxide Mediated by TransitionMetal Doped Magnesium Oxide Clusters [MMgO]+/0/− (M = Sc- Zn). Chem. - Eur. J. 2015, 21, 7780−7789. (56) Zhou, S.; Li, J.; Schlangen, M.; Schwarz, H. Breaking and Making of Carbon-Carbon Bonds by Lanthanides and Third-row Transition Metals. Chem. - Eur. J. 2016, DOI: 10.1002/ chem.201504571. (57) Kretschmer, R.; Schlangen, M.; Schwarz, H. Mechanisms of Metal-Mediated C−N Coupling Processes: A Synergistic Relationship between Gas-Phase Experiments and Computational Chemistry. In Understanding Organometallic Reaction Mechanisms and Catalysis; Ananikov, V. P., Ed.; Wiley-VCH: Weinheim, Germany, 2014; pp 1−16. (58) Diefenbach, M. A.; Brönstrup, M.; Aschi, M.; Schröder, D.; Schwarz, H. HCN Synthesis from Methane and Ammonia: Mechanisms of Pt+-Mediated C−N Coupling. J. Am. Chem. Soc. 1999, 121, 10614−10625. (59) Horn, R.; Mestl, G.; Thiede, M.; Jentoft, F. C.; Schmidt, P. M.; Bewersdorf, M.; Weber, R.; Schlögl, R. Gas phase contributions to the catalytic formation of HCN from CH4 and NH3 over Pt: An in situ study by molecular beam mass spectrometry with threshold ionization. Phys. Chem. Chem. Phys. 2004, 6, 4514−4521. (60) Koszinowski, K.; Schröder, D.; Schwarz, H. C−N Coupling of Methane and Ammonia by Bimetallic Platinum−Gold Cluster Cations. Organometallics 2004, 23, 1132−1139. (61) Koszinowski, K.; Schröder, D.; Schwarz, H. Coupling of Methane and Ammonia by Dinuclear Bimetallic Platinum-CoinageMetal Cations PtM+. Angew. Chem., Int. Ed. 2004, 43, 121−124. (62) Xia, F.; Chen, J.; Cao, Z. Relativistic density-functional study on the dehydrogenation reactivity of PtMCH2+ (M = Cu; Ag; Au; Pt) toward NH3. Chem. Phys. Lett. 2006, 418, 386−391. (63) Schlögl, R. Heterogeneous Catalysis. Angew. Chem., Int. Ed. 2015, 54, 3465−3520. (64) Troiani, A.; Rosi, M.; Garzoli, S.; Salvitti, C.; de Petris, G. IronPromoted C−C Bond Formation in the Gas Phase. Angew. Chem., Int. Ed. 2015, 54, 14359−14362. (65) Fischer, E. O.; Maasböl, A. On the Existence of a Tungsten Carbonyl Carbene Complex. Angew. Chem., Int. Ed. Engl. 1964, 3, 580−581. (66) For an early description of metathesis processes and a mechanistic explanation, see the following by-and-large not adequately credited publication: Calderon, N.; Chen, H. Y.; Scott, K. W. Olefin metathesis - A novel reaction for skeletal transformations of unsaturated hydrocarbons. Tetrahedron Lett. 1967, 8, 3327−3329.

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