The Reactive Sites of Methane Activation: A Comparison of IrC3+ with PtC3+

The activation reactions of methane mediated by metal carbide ions MC3+ (M = Ir and Pt) were comparatively studied at room temperature using the techniques of mass spectrometry in conjunction with theoretical calculations. MC3+ (M = Ir and Pt) ions reacted with CH4 at room temperature forming MC2H2+/C2H2 and MC4H2+/H2 as the major products for both systems. Besides that, PtC3+ could abstract a hydrogen atom from CH4 to generate PtC3H+/CH3, while IrC3+ could not. Quantum chemical calculations showed that the MC3+ (M = Ir and Pt) ions have a linear M-C-C-C structure. The first C–H activation took place on the Ir atom for IrC3+. The terminal carbon atom was the reactive site for the first C–H bond activation of PtC3+, which was beneficial to generate PtC3H+/CH3. The orbitals of the different metal influence the selection of the reactive sites for methane activation, which results in the different reaction channels. This study investigates the molecular-level mechanisms of the reactive sites of methane activation.


Introduction
Methane has attracted attention as the main component of natural gas and the conversion of methane into value-added chemicals is very important [1,2]. However, methane is extremely stable, with high C-H bond strengths (439 kJ/mol), negligible electron affinity and low polarizability [3]. At present, most of the catalytic conversion of methane needs to be carried out under high-temperature or high-pressure conditions [4]. Metal carbides are a kind of molecules with the potential to activate methane and have been studied by several groups [5]. Research on the mechanisms of the activation of methane by metal carbides is of great value and it may be helpful to find new catalysts of metal carbides [6,7]. At the same time, it is very difficult to study the activation mechanisms of methane. In recent years, people have found that the study of the gas-phase reaction of methane is an important means to study related reaction mechanisms [8,9].
Past studies have shown that gas-phase clusters are an ideal model for studying the reaction mechanisms in condensed-phase systems. The study of gas-phase clusters can reveal the specific reaction mechanisms, including the active sites, and provide references for condensed-phase catalytical processes [10]. At present, researchers are concerned about the reactions of methane with metal ions such as Os + [11], Pt + [12], Ta + [13] and Rh(0) [14] and metal oxides such as MgO + [15] [7]. These studies have explored some possible mechanisms for the activation of methane by metal carbide clusters. For example, the study of AuC + reveals a special hydride-transfer mechanism (HT) [22], while the study of FeC 3 shows that methane and atomic clusters generate CC-coupling reaction products at high temperatures and explained the possible mechanism of non-oxidized methane aromatization at the molecular level [6]. The study of FeC 4 + shows that the cluster can activate the C-H bonds of methane via the hydrogen-atom transfer (HAT) mechanism at ambient temperature; the study used the frontier orbital theory to explain the root cause of the HAT reaction [23]. Although there have been some studies on the mechanisms of metal carbides to activate methane in the past, there is still no clear investigation on the reactive sites of methane activation. Therefore, further research is needed to study the reactive sites for the activation of methane. Here, we reported the reactions of MC 3 + (M = Pt and Ir) with methane.

Experimental and Computational Methods
The experiments were performed using an ion trap mass spectrometer equipped with a laser vaporization-supersonic expansion ion source that was reported previously [24,25]. The MC 3 + (M = Pt and Ir) ions were generated by pulsed laser ablation of a rotating and translating metal/carbon (metal:carbon = 1:4) target. The nascent ablated plasma was entrained by a helium carrier gas with a backing pressure of about 0.5 MPa. The ions were mass-selected by a quadrupole and then were sent into a linear ion trap, where the ions were accumulated and cooled by helium gas. The MC 3 + ions reacted with CH 4 , CD 4 and 13 CH 4 , introduced by a pulsed valve. After a 10 ms reaction, the trapped ions were ejected for mass detection.
Theoretical calculations were performed using the Gaussian 09 package [26]. All of the calculations were performed using the BMK functional with the def2-TZVP basis sets [27,28]. Vibrational frequency calculations were employed to identify the nature of reaction intermediates, transition states (TSs) and products. The Molclus program [29] was used to search for the possible stable structures of the MC 3 + , MC 2 H 2 + , MC 4 H 2 + and MC 3 H + (M = Pt and Ir). The low-lying stable isomers were then re-optimized at the BMK/def2TZVP level to confirm the relative energy sequence. Transition-state optimizations were performed with the synchronous transit-guided quasi-Newton (STQN) method and were verified through intrinsic reaction coordinate (IRC) calculations [30,31]. Vibrational frequency calculations were performed to identify the nature of reaction intermediates, transition states (TSs) and products.

Results and Discussion
The mass spectra for the reactions of mass-selected MC 3 + (M = Pt and Ir) ions with He (a1 and a2), CH 4 (b1 and b2), CD 4 (c1 and c2) and 13 CH 4 (d1 and d2) in the ion trap at room temperature are shown in Figure 1. No product ion was observed in the mass spectra when using pure He as reactant gas, while two main peaks, at m/z = 219 and 243 for the Ir-system, and m/z = 222 and 246 for the Pt-system for the reactions with CH 4 , which can be attributed to the product ions with chemical formulas MC 2 H 2 + and MC 4 H 2 + (M = 196 Pt and 193 Ir), were observed to be the major reaction products. A weak mass peak at m/z = 233 assigned to the ion with chemical formula PtC 3 H + for PtC 3 + /CH 4 was also observed, while no IrC 3 H + was found. The mass spectra suggest that two main reaction channels for both systems and one hydrogen-atom abstraction channel for PtC 3 + were observed. The first channel is the formation of the MC 2 H 2 + cation with the release of a neutral C 2 H 2 (reaction 1). The second channel is the generation of the MC 4 H 2 + ion with concomitant elimination of a dihydrogen molecule (reaction 2). The third channel is the generation of the PtC 3 H + ion with the release of CH 3 as shown in reaction 3. Impurity PtC 3 •H 2 O + and IrCO + ions were formed due to the small amount of contaminant in the chamber of the instrument. Isotopic-labeling experiments conducted using the CD 4 sample showed that there were peaks of IrC 2 D 2 + , IrC 4 D 2 + , PtC 2 D 2 + , PtC 4 D 2 + and PtC 3 D + , which demonstrates that the H atoms in the products were all from methane. Both the PtC 2 H 2 + and Pt 13 CCH 2 + product ions were observed to have approximately the same intensity, when using 13 CH 4 , indicating that one or both carbon atoms of the eliminated C 2 H 2 neutral molecule came from the PtC 3 + ion. The intensity of the peak of Ir 13 CCH 2 + is similar with that of IrC 2 H 2 + , which eliminates the background peak. The peaks of Ir 13 CC 3 H 2 + and Pt 13 CC 3 H 2 + were observed, which confirms the reaction channels. In order to gain insight into the reaction mechanisms, the various possible structures of the products MC 3 + (M = Pt and Ir) were obtained by calculations at the BMK/def2-TZVP level and are shown in Figure S1.  Figure S2.
In order to gain insight into the reaction mechanism, the potential energy profiles (PESs) were calculated. All of the three Reactions (1)-(3) leading to the stable structures of the products were exothermic. The pathways for the IrC 3 + + CH 4 reaction leading to the IrC 2 H 2 + /C 2 H 2 and IrC 4 H 2 + /H 2 products are shown in Figure 2 and details are given in Figures S3-S4. An encounter complex ( 3 I1), that is −1.51 eV lower in energy than the ground state reactants, is formed initially. The Ir atom serves as the active site of the reaction and intermediate 1 I2 is formed via the first C-H bond activation. Then, the CH 3 moiety is transferred to C 3 to form a C-C bond; meanwhile, one H atom is transferred to the Ir atom ( 1 I2 → 1 TS2 → 1 I3). Subsequently, two hydrogen atoms transfer from the metal Ir atom to the C atom to form two new C-H bonds ( 1 I3 → 1 TS3 → 1 I4 → 1 TS4 → 1 I5  Figure S4. The pathways for the PtC 3 + + CH 4 reaction are shown in Figure 3. Details and other possible pathways are given in Figures S5-S7. The reaction pathway starts from the encounter complex 2 I1, followed by the formation of a stable intermediate 2 I2 via the approaching of CH 4 to the terminal carbon of PtC 3 + ( 2 I1 → 2 TS1 → 2 I2), where the first C-H activation takes place. The CH 3 moiety of 2 I2 is transferred to form 2 I3, where CH 3 is loosely coordinated to PtC 3 H + . The final product P1 (−0.46 eV) is generated with the liberation of CH 3 . The energy of P1 is higher than the transition states from 2 I2 to P2/P3, which can explain the weak peak of PtCH  For the reactions of the IrC 3 + and PtC 3 + with CH 4 , we could find the hydrogen abstraction reaction products PtC 3 H + and no IrC 3 H + was observed. Based on our calculations, the reactive sites play a key role. There are two possible sites, the metal and terminal carbon atom (the other two carbon atoms are fully bonded). For IrC 3 + /CH 4 , the barrier to the first C-H activation on the terminal C atom (+0.04 eV; see Figure S4) is higher in energy than 1 TS1 (−0.68 eV). Based on the lack of IrC 3 H + observed, we confirm that the reactive site of the first C-H activation is the Ir atom. For PtC 3 + /CH 4 , the energy required for the first C-H bond activation on Pt and C atoms is +0.01 ( 2 TSX3) and +0.07 eV ( 2 TS1), given in Figure 3 and Figure S6, respectively. Though the Pt atom, as a reactive site, is favorable, the transition state of H atom transfer as the next step (+0.19 eV, 2 TSX4; Figure S4) is higher in energy than 2 TS1. Based on the observation of PtC 3 H + in the experiment, we confirm that the terminal carbon atom can be a reactive site for PtC 3 + /CH 4 . As we all know, metals have a stronger adsorption capacity for methane than nonmetals. The different reactive sites can be explained by the analysis of the orbital. In the PtC 3 + species, platinum uses two of the six valence orbitals (s and d) to form a σand π-bond with the adjacent carbon atom. This leaves seven electrons occupying the four remaining non-bonding orbitals on platinum, such that there are no empty orbitals on the metal. Therefore, it is hard for the C−H bond to donate electron density to the metal and the energy of the transition state 2 TSX3 is a little higher than the 2 TS1 in Figure 3. For the IrC 3 + species, Ir uses two of the six valence orbitals (s and d) to form a bond with the adjacent carbon atom, which leaves six electrons occupying the four remaining non-bonding orbitals on the Ir atom, such that there are enough empty orbitals on the metal, which is beneficial for the C-H bond activation of CH 4 . This can explain why the barrier to the first C-H activation on the terminal C atom (+0.04 eV) is higher than that of 1 TS1 (−0.68 eV). Our work demonstrates that the orbitals of the metal influence the selection of the reactive sites for the activation of methane. The different reactive sites result in different reaction channels.
In conclusion, the activation of methane mediated by MC 3 + (M = Ir and Pt) were comparatively studied at room temperature by gas-phase experiments with theoretical calculations. Mass spectrometric studies on the reactions of the MC 3 + (M = Ir and Pt) ions with CH 4 show that two main reaction channels were observed. The first channel is the formation of the MC 2 H 2 + cation with the release of neutral C 2 H 2 . The second channel is the generation of the MC 4 H 2 + ion with concomitant elimination of a dihydrogen molecule. Besides that, PtC 3 H + could be found in the experiments, while IrC 3 H + could not. Quantum chemical calculations suggest that the MC 3 + (M = Ir and Pt) ions have a linear M-C-C-C + structure. The Ir atom is the reactive site for the reaction of IrC 3 + /CH 4 . The generation of the PtC 3 H + and PESs can confirm that the terminal carbon atom is the reactive site for PtC 3 + /CH 4 . The orbitals of the metal influence the selection of the reactive sites for methane activation, which results in the different reaction channels. Our work is helpful for understanding reactive sites of methane activation.

Supplementary Materials:
The following are available online, Figure S1: The optimized geometries of the [IrC 3 ] + and [PtC 3 ] + isomers at the BMK/def2-TZVP level. The relative energies relative to the global minimum structure (in eV), symmetry and electronic states are shown; Figure S2: H] + (bottom) isomers at the BMK/def2-TZVP level. The relative energies relative to the global minimum structure (in eV), symmetry and electronic states are shown; Figure S3: The detailed potential energy profiles of the reaction of IrC 3 + and CH 4 in Figure 2. The energies are given in eV; Figure S4: The other possible potential energy profiles of the reaction of IrC 3 + and CH 4 . The energies are given in eV; Figure S5. The detailed potential energy profiles of the reaction of PtC 3 + and CH 4 in Figure 3. The energies are given in eV; Calculated geometries and potential energy profiles. This material is available free of charge via the Internet.