Gas-phase activation of methane by ligated transition-metal cations

Ligated Metal Cations

At the outset, let us make a general distinction of the ligands in terms of their electron shell. Mere coordinative, closed-shell ligands L, such as L = H₂O, CO, alkenes, or arenes, do not change the formal oxidation state of the metal. In contrast, open-shell ligands X form (polarized) covalent bonds to the metal and thereby increase the formal oxidation state, e.g., X = F, OH, Cl, Br, I, CH₂, NH, O, and N. Note that some open-shell ligands bear an intermediate behavior, e.g., NO and O₂.

Closed-Shell Ligands L.

In general, coordination of metal ions to closed-shell ligands L lowers the reactivity compared with bare metal cations, which may, however, result in increased selectivities, provided the reactivity is not quenched completely (23, 24). In this respect, Tjelta and Armentrout (25) reported impressive examples for the ability of tuning selectivities in C–H- versus C–C-bond activation of small alkanes by Fe(L)⁺ cations with L = H₂O and CO; the former ligand induces a preferential attack of C–H bonds, and the latter also promotes the cleavage of C–C-bonds in the substrate. Both ligated cations, Fe(H₂O)⁺ and Fe(CO)⁺, are less reactive than bare Fe⁺, however, and none of these three cationic species allows a room-temperature activation of methane.

Albert et al. (26) found a spectacular example for the effect of ligation with a closed-shell ligand, in that Rh₂Ar⁺ can activate methane, whereas the bare dirhodium cation Rh₂⁺ does not. The authors explained the pronounced effect, exerted by no more than a single rare-gas atom, by the increased lifetime of the intermediate encounter complex formed upon exchange of argon by methane in the first step to yield excited [Rh₂CH₄]⁺* that subsequently undergoes dehydrogenation according to Reaction 2; in terms of the classification in Scheme 1, Reaction 2 is an example for route e. Similarly, PtAr⁺ is able to dehydrogenate methane, but here, the ligand damps the reactivity in that the rate constant for bare Pt⁺ is ≈7 times larger; higher PtAr_n⁺ clusters (n = 2–6) do not activate methane any more (27). These simple examples thereby illustrates the general consideration that ligated M(L)⁺ species are much less reactive than the corresponding bare metal cations and often even act as sinks of catalytic cycles.

The seemingly straightforward classification of ligands L encounters some ambiguity with transition metals that can easily change their oxidation state. A metal–olefin complex, for example, can also be viewed as a metallacyclopropane in which the metal formally undergoes a two-electron oxidation. Thus, in the triad of the coinage metals, Ag(C₂H₄)⁺ is a T-shaped π-complex with genuine Ag(I), whereas Au(C₂H₄)⁺ exhibits an auracyclopropane structure with a triangular bonding scheme and a significantly elongated C–C bond; for Cu(C₂H₄)⁺, the situation is in between these extremes (28). Recently, we also could generate the insertion intermediate HNi(OH)⁺, which is experimentally distinguishable from the water complex Ni(H₂O)⁺ (29). Formally, HNi(OH)⁺ corresponds to the unusual oxidation state Ni(III), and it is thus not too surprising that the HNi(OH)⁺ cation is able to activate methane according to Reaction 3. Mechanistically, this process may be viewed as an example for either route c or route d in Scheme 1, depending on whether the formation of dihydrogen is considered as a formal abstraction (via σ-bond metathesis) or whether an insertion intermediate is involved. Formally, Reaction 3 may also be regarded as an example of route a in Scheme 1, i.e., the reaction of a metal hydride with the methane to yield the corresponding methanideconcomitant with dihydrogen. Interestingly, the metal-hydrido species HNi(OH)⁺ was generated by electrospray ionization of a NiI₂ solution in the protic mixture of methanol and water. The details of ion generation are still not resolved, but use of nickel(II) iodide is essential, and the hydridic H-atom stems from the methoxy group of the methanol used as a cosolvent, whereas the hydroxy group stems from water (29, 30).

An even more extreme situation with regard to the ambivalent behavior of ligands evolves for dioxygen complexes of metal cations in which the (open-shell) dioxygen ligand can behave either as a mere π-type ligand or give rise to peroxide- or even dioxide structures (31–34).

Open-Shell Ligands X.

The prototype of this class of covalent ligands is X = F. Because of its large electronegativity and poor donor properties, fluorine forms strongly polarized covalent bonds to metals, and thus, each fluorine increases the metal's formal oxidation by one unit. Even in the very extreme situation of the diatomic trication UF³⁺ (35), fluorine still acts as an electronegative ligand, as demonstrated by population analysis that reveals a formal charge of 3.4 for uranium in UF³⁺. Other electronegative ligands such as X = Cl, Br, I, and OH behave similarly to fluorine in this respect, although the metal fluorides are most reactive.

Unlike coordinative ligands L, attachment of a covalent ligand X often increases reactivity. Bare Cr⁺ for example, is one of the least reactive transition-metal cations, whereas already CrCl⁺ is significantly more reactive (36), and CrF⁺ is even capable of dehydrogenating propane (37). With respect to the activation of methane, a single fluorine is not sufficient, however, and CrF₂⁺ does not react with CH₄, whereas CrF₃⁺ and CrF₄⁺ are able to activate the C–H bonds of methane (37). In the case of CrF₃⁺, the major product of the reaction with methane corresponds to a methyl cation concomitant with neutral HCrF₃ (or CrF₂ + HF). This reaction hence is an example for hydride transfer according to route b in Scheme 1. Hydride transfer also occurs for bare and ligated transition-metal dications (18), but this is due less to the presence of a transition metal than a general feature in the bimolecular reactions of isolated dications (38). Similar to CrF⁺, FeF⁺ is also more reactive than other FeX⁺ species (X = OH, Cl, Br, I, etc.), but none of these cations can activate methane (39). In the case of NiX⁺, diatomic NiF⁺ promotes C–H bond activation of methane according to Reaction 4, whereas the nickel–halide cations NiCl⁺, NiBr⁺, and NiI⁺ react only with larger alkanes (40). The trends in reactivity can be explained not only in terms of the electronegative character of X, but also by the corresponding reaction enthalpies that are mostly related to the formation of HX, whose bond-dissociation energies are much lower for the heavier halogens. Formation of HX as the thermochemical driving force also accounts for the strong preference of C–H over C–C bond activation in the case of larger alkanes (36, 37, 40).

Also the platinum-chloride and -bromide cations PtX_n⁺ (n = 1, 2) are capable of methane activation under thermal conditions (41). For the chlorides PtCl⁺ and PtCl₂⁺, exclusive losses of HX in analogy to Reaction 4 are observed, whereas in the case of PtBr⁺ formation of the corresponding carbene also takes place to some extent, i.e., Reaction 5, which is analogous to Reaction 1, with M = PtBr. Notably, the formal platinum(IV) species PtCl₃⁺ does not react with methane, most probably because it is coordinatively saturated to an extent that there exists no sufficiently attractive vacant coordination site for the initial binding of methane before its activation within the collision complex. Although the electron affinity of hydrogen is much lower, the corresponding metal-hydride cations MH⁺ behave similar to the metal halides in several respects (42–44). Thus, NiH⁺ and PtH⁺ promote C–H bond activation of methane according to Reaction 6 (29, 41). The obvious explanation for this behavior is the favorable thermochemical driving force associated with the formation of dihydrogen in conjunction with the energetic preference of M–C over M–H bonds. The investigation of the group 10 metal-hydride cations has recently been completed by a study of PdH⁺ (45). Despite the similarities of Reaction 6 for M = Ni, Pd, and Pt, however, the details of the potential-energy surfaces differ decisively, in that the reactions of NiH⁺ and PtH⁺ involve two spin surfaces, whereas σ-bond metathesis in the case of PdH⁺ exclusively proceeds on the singlet surface. Metal-oxide cations comprise an important class of compounds with ligands of the type X that are able to activate methane. Methane activation by gaseous metal-oxide cations MO⁺ generally leads to three different kinds of products, bare metal cations concomitant with methanol (Reaction 7), metal hydroxides via loss of a methyl radical (Reaction 8), and formation of metal-carbene cations, concomitant with water as a neutral product (Reaction 9). Reactions 7 and 8 are of direct relevance for the conversion of methane to more valuable products. The former leads to methanol, and the latter is the first step in the oxidative coupling of methane to ethane. Therefore, the reactions of metal-oxide cations have received great attention and have been reviewed repeatedly (46–48). Here, we concentrate on a few key features that are of particular relevance for the activation of methane. Reactions 7 and 9 proceed via the insertion species (H₃C)M(OH)⁺ as the key intermediates. Although the metal has the formal oxidation state M(III) in MO⁺ and in (H₃C)M(OH)⁺, there exist a fundamental difference in the bonding situations, which has important consequences. Thus, at least 3d and 4d metals cannot effectively support π-type double bonds in MO⁺, which consequently leads to high-spin ground states of the MO⁺ monocations. In contrast, the insertion intermediates primarily involve σ-type bonding, for which perfect pairing and thus low-spin ground states are preferred. This leads to a situation in which the spin surfaces cross each other such that the reaction pathway of lowest energy demand involves a cross-over from the high- to the low-spin surface and eventually back to high spin in the final products. In fact, the reactions of bare metal-oxide cations in the gas phase inter alia led to the development to the concept of two-state reactivity (49), which is now widely accepted in various areas of science far beyond gas-phase ion chemistry, as, for example, in organometallic chemistry and also in the context of metalloenzymes (50). In contrast, Reaction 8 formally corresponds to a simple abstraction hydrogen-atom and therefore does not need to involve a spin change. As outlined further below, these differences in the types of reaction mechanism are also reflected in the kinetic isotope effects associated with the C–H bond activation of methane.

So far, thermal methane activation has been achieved by the mononuclear transition-metal oxide cations MnO⁺ (51), FeO⁺ (52, 53), NiO⁺ (54), OsO⁺ (55), and PtO⁺ (5). Interestingly, the branching ratios of Reactions 7–9 very much depend on the position of the metal in the periodic table. Thus, MnO⁺ leads to exclusive hydrogen-atom abstraction (Reaction 8), for FeO⁺ Reactions 7 and 8 compete, whereas NiO⁺ only undergoes Reaction 7. In OsO⁺, oxygen serves as a mere spectator ligand, and the osmium center activates methane in analogy to Reaction 1, with M = OOs (55, 56). Finally, PtO⁺ reacts with a collision rate with a competition of Reactions 7 and 9 in favor of the latter because of the operation of relativistic stabilization of the PtCH₂⁺ product (57). Furthermore, several high-valent, higher metal-oxide cations were found to activate methane in the gas phase under thermal conditions, i.e., CrO₂⁺ (33, 58), HOFeO⁺ (59), MoO₃⁺ (60), OsO_n⁺ with n = 2, 4 (55), and PtO₂⁺ (61). Also notable is the main-group metal-oxide cation MgO⁺, which efficiently activates methane with a strong preference for hydrogen-atom abstraction to afford MgOH⁺ according to Reaction 8 (62).

Metal-imido cations MNH⁺ and metal carbenes MCH₂⁺ are formally isolobal to MO⁺ species; in general, however, the latter are found to be much more reactive. FeNH⁺, for example, can activate methane to afford Fe⁺ and methylamine (63), but the reaction is much less efficient than for FeO⁺. Bare metal-nitrido cations MN⁺, which could potentially activate methane, could so far not be generated experimentally in yields sufficient for subsequent reactivity studies.

Combinations of L and X.

With regard to bridging the gap to condensed-phase catalysts, more promising systems are perhaps those in which the metal species bear additional coordinative ligands and yet have preserved their intrinsic reactivity. Unfortunately, precisely the opposite behavior is often observed. For example, the reactive FeO⁺ cation, which oxidizes almost any substrate, including such inert compounds as acetonitrile or C₆F₆ (64), does not activate alkanes any more if attached to an arene ligand, e.g., (C₆H₆)FeO⁺ (65–67). These arene complexes of FeO⁺ react only with activated compounds such as olefins. A related, but conceptually different, example is represented by the cation (phen)CuO⁺ (phen = 1,10-phenanthroline), which is a formal Cu(III) compound (68). In this case, the bare CuO⁺ cation itself is too reactive to be generated in yields sufficient for gas-phase reactivity studies. The phen ligand can stabilize the Cu(III) center and (phen)CuO⁺ is one of the few ligated metal-oxide cations that is capable of activating small alkanes, although methane itself does not react with (phen)CuO⁺. The key step in the gas-phase synthesis of (phen)CuO⁺ is the homolytic decomposition of the nitrato ligand in the copper(II) precursor species (phen)Cu(NO₃)⁺ according to Reaction 10. A gas-phase activation of methane itself at room temperature has recently been achieved by related complexes of platinum(II) with aromatic nitrogen ligands (phen and bipy, where the latter stands for 2,2′-bipyridyl). These complexes of the type (H₃C)Pt(L)⁺ (69) react with methane via degenerate σ-bond metathesis that is detected only by use of isotopic labeling, Reaction 11. Within the lifetime of the encounter complex [(H₃C)Pt(L)(CD₄)]⁺*, C–H(D) bond activation of methane can occur repeatedly, leading to a substantial amount of H/D scrambling in the overall reaction. The intermediate steps in Reaction 11 are partially reversible such that the evaluation of the respective kinetic parameters requires explicit kinetic modeling (70, 71) of the sequential steps that reveals moderate kinetic isotope effects for the C–H(D) bond activation of methane by this cationic platinum complex. Similar to the generation of the reactive Cu(III) species (phen) CuO⁺, an attempt was made recently to generate a free metal–nitrido complex in the gas phase, where a polydendate cyclamacetato ligand was used for the stabilization of the FeN⁺ center containing formal iron(V). Analysis of the experimental findings suggests that the nitren [(cyclamacetato)FeN]⁺ was indeed generated initially upon decomposition of the azidocomplex [(cyclamacetato)Fe(N₃)]⁺ used as the precursor, but before any subsequent bimolecular reactions, the intermediate nitrido complex promotes sacrificial self-oxidation of the ligand (72). Notably, the elusive [(cyclamacetato)FeN]⁺ cation could be trapped in the condensed phase via matrix-isolation techniques (73). Very recently, a FeN²⁺ unit with formal iron(V), stabilized by a polydendate aminoligand, could be generated in the gas phase in a similar manner (Scheme 2). The resulting species is able to transfer the nitrido-nitrogen to organic substrates such as dienes, yet it does not react with alkanes (74). Future work in this direction will show whether the design of other multiply ligated systems for the room-temperature activation of methane can be realized successfully.

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Scheme 2.

In the context of multiply ligated metal species, we further note that with V₄O₁₀⁺ the first transition-metal oxide cluster was also found to activate methane via hydrogen-atom abstraction (75). Most recently, H-atom abstraction from methane has also been achieved with the cationic aluminum-oxide clusters Al_2nO_3n⁺ (n = 3–5) (76).

Intramolecular Kinetic Isotope Effects (KIEs) as a Mechanistic Probe for Methane Activation.

Finally, let us address the intramolecular KIEs associated with the C–H(D) bond activation of methane by cationic metal fragments in the gas phase; hence, KIE = k_C–H/k_C–D; investigation of CH₂D₂ is most suited for this purpose. Before a more detailed discussion of the results, it is important to point out that intramolecular KIEs provide direct mechanistic insight, whereas intermolecular KIEs (e.g., the rate constants for oxidation of CH₄ and CD₄, respectively) might be affected by a difference in collision rates, diffusion effects, etc. (53, 77, 78). This is particularly true for cases of two-state reactivity in which the cross-over between two spin surface can be the rate-determining step, such that the resulting intermolecular KIEs may be rather low, whereas intramolecular KIEs can still be significant. With the increased knowledge gathered in recent years, both experimentally and theoretically, some mechanistic guidelines for the interpretation of the primary, intramolecular KIEs associated with the C–H(D) bond activation of methane can be provided, however.

For the platinum species listed in the left side of Table 1, the KIEs are generally quite low, which is consistent with the view that for platinum not the activation step itself, but the release of the products is rate-determining (5, 27, 79). In contrast, a sizable KIE occurs in the case of the nickel(II) fluoride cation NiF⁺, which is consistent with the view that this reaction proceeds as a σ-bond metathesis in which C–H(D) bond cleavage and H(D)-F bond formation contribute to the rate-determining step (40). For the metal-oxide cations (right side of Table 1), the reaction of the cluster cation V₄O₁₀⁺ proceeds as a genuine H-atom abstraction (75) that is close to the KIE of ≈1.3 reported for H-atom abstraction by free OH radicals (80). H-atom abstraction from methane by MnO⁺ (51) and MgO⁺ (62) is also promoted by the radical character of the oxygen atom, although theory implies a significant participation of the metal and the formation of an insertion intermediate (62, 81), as is reflected by the significant KIEs. MoO₃⁺ exhibits a KIE of the same magnitude, thus pointing to the operation of a similar mechanism (60). In the case of OFeOH⁺ (59) and FeO⁺ (52, 53), even larger KIEs are observed that, at least in the latter case, can be ascribed to the formation of an intermediate insertion species in conjunction with spin cross-over (81–85).

In a more general perspective, the knowledge collected so far allows the consideration of the experimentally measured intramolecular KIEs associated with the activation of CH₂D₂ as a mechanistic probe for the activation of methane. Thus, KIEs <1.5 can be attributed either to reactions in which the initial C–H(D) bond cleavage does not contribute to the rate-determining step (i.e., the Pt species in Table 1) or to very exothermic reactions that proceed as simple H-atom abstraction (i.e., V₄O₁₀⁺). Medium KIEs (1.5–2.5) indicate the occurrence of a metal-assisted mechanism (i.e., MnO⁺, MgO⁺, and MoO₃⁺), which is, however, still governed by the radical character of the attacking oxygen atom. In contrast, large KIEs (>2.5) are indicative of concerted reactions with the formation of a covalently bound insertion intermediate (i.e., FeO⁺) and/or occurrence of σ-bond metathesis (i.e., NiF⁺).