Activation of methane by Ru+: Experimental and theoretical studies of the thermochemistry and mechanism

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Highlights

  • Activation of CH4 by Ru+ is examined as a function of kinetic energy using guided ion beam tandem MS.

  • Energies for the formation of RuCH2+, RuH+, RuCH3+, RuCH+, and RuC+ are measured.

  • Mechanisms for all pathways are elucidated using ab initio theory.

  • Formation of both RuCH2+ and HRuCH+ products is found to be likely.

  • Results are compared to the reactions of the group 8 congeners, Fe+ and Os+.

Abstract

The reaction of Ru+ with CH4 is studied using a guided-ion beam tandem mass spectrometer. Consistent with previous work, no reactivity is observed at thermal energies. As the collision energy is increased, dehydrogenation to form RuCH2+ + H2 is observed with a relatively low energy threshold. At higher energies, other products, RuH+, RuC+, RuCH+, and RuCH3+, are observed with RuH+ dominating the product spectrum. Modeling of the endothermic cross sections provides thresholds that agree well with thermochemistry determined previously, except in the case of RuC+ where a revised 0 K bond dissociation energy (BDE) of 5.43 ± 0.08 eV is measured here. The experimental BDEs of all products observed are in reasonable agreement with theoretical calculations at the B3LYP, QCISD(T), and CCSD(T,full) levels of theory using several different basis sets. Theory is also used to explore the potential energy surfaces associated with the reaction of Ru+ with methane, with some distinct differences found compared with previous computational results. These calculations reveal that this reaction proceeds through a H‐Ru+‐CH3 intermediate and requires crossing from the quartet spin of the reactants to a doublet spin intermediate, and possibly back to quartet in order to dehydrogenate. Further, both RuCH2+ and HRuCH+ geometries are nearly isoenergetic. Details of the various intermediates, transition states, and crossing points between surfaces are provided. Finally, reactions of Ru+ with methane are compared with those of the first and third-row congeners, Fe+ and Os+, and the differences in behavior and mechanism discussed.

Introduction

Considerable research has been done to study the reactions of first-row [1], [2], [3], [4], [5], [6], second-row [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], and third-row [6], [7], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35] transition-metal cations (M+) with small hydrocarbons. Such studies provide insight into the electronic requirements for the M+ activation of Csingle bondH and Csingle bondC bonds [36], [37], [38], [39], [40], [41], [42], periodic trends in the reactivity [1], [2], [6], and metal-hydrogen and metal-carbon bond dissociation energies (BDEs) [43], [44], [45], [46]. The thermochemistry obtained from these studies is of obvious fundamental interest and also has implications in understanding a variety of catalytic reactions involving transition-metal systems [47]. For the second-row transition metal cations, guided ion beam mass spectrometry (GIBMS) was previously used to study the activation of methane by Y+ [7], Zr+ [19], Nb+ [21], Mo+ [23], Rh+ [48], [49], and Pd+ [50]. This method allows the energetics of the reactions to be determined and mechanisms to be probed. Although others have examined the reaction of methane with Ru+ at room temperature (finding no reactivity and only adduct formation at elevated pressures) [6], [14], [15], the activation of methane by Ru+ has not previously been examined at elevated energies. The present study of the reaction of Ru+ with methane is partially motivated by our recent analogous work on its third-row congener, Os+, reacting with methane [30]. Comparison of these studies along with that of the first-row congener, Fe+ [51], [52], should allow elucidation of the periodic trends in the reactivity of the group 8 transition metal cations.

Although results for the reaction of Ru+ with methane at elevated energies have not previously been reported, we have previously reported such a study of Ru+ reacting with dihydrogen, ethane, propane, isobutane, and cyclopropane [53], [54], primarily to determine appropriate thermochemistry for RuH, RuCH3, RuH+, and RuCHx+ species. Information relevant to the present system is collected in Table 1, and supplants previous studies that reported BDEs for RuH+, RuH, and RuCH3+ [14], [17]. In addition, theoretical calculations have been performed for the BDEs of cationic and neutral ruthenium‐hydrides [55], [56], [57], [58], ruthenium‐methyls [59], [60], and ruthenium‐methylenes [58], [61], [62]. Most relevant to the present work is the density functional theoretical (DFT) study of the activation of methane by Ru+ [63], although several aspects of this work are extended theoretically in the present study at an advanced level of theory, B3LYP/def2-TZVPPD along with CCSD(T) calculations of the reactants and products. Furthermore, the comparison of the experimental results and the theory permits a rather complete elucidation of the reaction mechanism.

Section snippets

General procedures

These experiments were performed using a guided ion beam tandem mass spectrometer described in detail elsewhere [64], [65]. Ions were created in a direct current discharge flow tube source described below, extracted from the source, then accelerated and focused into a magnetic sector momentum analyzer for mass analysis. Ru+ ions containing the 102Ru isotope (31.55% natural abundance) were selected, decelerated to a desired kinetic energy, and focused into an octopole ion guide that traps the

Experimental cross sections for the reaction of Ru+ with methane

Reaction of Ru+ with CH4 yields the product ions shown in Reactions (2)–(6)Ru+ + CH4  RuH+ + CH3            2.86 ± 0.05 eV→RuCH3+ + H             2.82 ± 0.06 eV→RuCH2+ + H2             1.14 ± 0.05 eV→RuCH+ + H2 + H           3.87 ± 0.12 eV→RuC+ + 2H2           3.36 ± 0.11 eV

Endothermicities for each process are shown on the basis of the experimental data in Table 1 (and the literature value for RuC+). Results for CH4 and CD4 are analogous although the larger mass separation between product ions in the CD4 system allows a lower mass resolution to resolve them. In turn, this permits

Mechanism for dehydrogenation of methane by Ru+

On the quartet surface, the energy of 4TS2/4 is the rate limiting step for dehydrogenation. This TS lies at 1.03 eV above ground state reactants, 0.09 eV above ground state products, RuCH2+ (4B2) + H2, at the B3LYP/def2 level. This TS, along with 4TS2/3, 4TS3/4, and the dihydride HRuHCH2+, 43, lie at high energies because the quartet high spin does not allow formation of the four covalent bonds needed to stabilize these species. Therefore, this is unlikely to be the path observed experimentally

Conclusion

Ground-state Ru+ ions are found to react with methane at hyperthermal energies to dehydrogenate and form RuCH2+ + H2. At higher collision energies, this product undergoes further dehydrogenation to form RuC+ and also competes with pathways yielding RuH+ + CH3 (the dominant product channel above 3 eV) and RuCH3+ + H. The latter product rapidly dehydrogenates to form RuCH+. At still higher energies, the RuCH3+ product undergoes loss of a H atom to form RuCH2+.

Analyses of the kinetic energy dependences

Acknowledgement

PBA thanks Prof. Kirk A. Peterson for help with the aug-cc-pwcVTZ-(PP,DK) calculations. This work is supported by the National Science Foundation, Grant No. CHE-1359769. A grant of computer time from the Center for High Performance Computing in University of Utah is gratefully acknowledged.

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