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Origin of the Different Reactivity of the Triatomic Anions HMoN− and ZrNH− toward Alkane: Compositions of the Active Orbitals Ji-Chuang Hu,† Lin-Lin Xu,† Xiao-Yu Hou,† Hai-Fang Li,‡ Jia-Bi Ma,*,† and Sheng-Gui He‡ †

The Institute for Chemical Physics, Key Laboratory of Cluster Science, School of Chemistry Beijing Institute of Technology, 100081 Beijing, People’s Republic of China ‡ State Key Laboratory for Structural Chemistry of Unstable and Stable Species Institute of Chemistry, Chinese Academy of Sciences, 100190 Beijing, People’s Republic of China S Supporting Information *

ABSTRACT: The reactivity of the triatomic anions HMoN− and ZrNH− toward alkanes was investigated by means of mass spectrometry in conjunction with density functional theory calculations. HMoN− can activate C−H bond of ethane with the liberation of ethene and hydrogen molecules, and the generation of hydrogen molecules is the major reaction channel; however, no C−H bond activation of ethane was observed over ZrNH− ion, and the density functional theory calculations suggest this pathway is hampered by intrinsic energy barrier. In sharp contrast, another triatomic anion HNbN− can bring about methane activation under thermal conditions, as reported previously. A strong dependence of the chemical reactivity of alkane activation on compositions of active orbitals in the above-mentioned systems is discussed. This combined experimental/ computational study may provide new insights into the importance of compositions of active orbitals and their essential role in the reactions of related systems with alkanes. addition (OA),24−31 and ligand exchange through σ-bond metathesis.22 Anionic species were generally considered to be less reactive than the corresponding cationic counterparts in the reactions with alkanes.19,32,33 The detailed experimental and theoretical studies demonstrated that very few noble-free anions, such as (La2O3)3,4O−,33 FeC6−,34 and Fe(CO)2−,35 are capable of bond activation of methane at room temperature. Some of noble-free anions can activate C−H bonds of ethane under thermal-collision conditions, for instance, MoC3−.36 In contrast with the numerous investigations on the structures and reactivity of transition metal oxide ions, knowledge about structure−property relationships of TMN ions is quite limited. Schwarz et al. reported the thermal reaction of the amidonickel cation Ni(NH2)+ with C2H4,37 and C−N bond was formed during the reaction. Bernstein and coworkers investigated the reactivity of CoxNy toward H2, suggesting the formation of ammonia.38 Quite recently, we reported the first TMN anion, HNbN−, which is capable of activating methane through OA at room temperature, resulting in the formation of H2NbNCH3−.39 More interestingly, the reactivity of HNbN− is quite similar to that of free Pt atom, due to the similar electronic structures of HNbN− and Pt, especially the active orbitals. Therefore, synthesizing TMNs with similar electronic structures to NMs may be one valid way to tailor the design of new and cheap catalysts through partial or full

1. INTRODUCTION In contemporary chemistry, although tremendous efforts have been devoted to elucidate the complex nature of active sites and mechanistic aspects to develop highly active catalysts, many chemical reactions remain to be mastered. For instance, the transformation of nonactivated hydrocarbons into value-added products under ambient conditions is still a challenging task by now,1−3 in which C−H bond activation is the crucial step.2,4−7 In addition to the vast amount of literature on condensed-phase studies of transition metal oxide catalysts, reports on transition metal nitrides (TMNs) show that they are another interesting class of heterogeneous catalysts.8−11 Notably, molybdenum nitride11,12 and other nitrides have been demonstrated to have superior catalytic properties in a number of chemical reactions, including dehydrogenation,8 ammonia decomposition,11 and so on. This kind of catalyst is also known as one of the promising replacements of costly noble metals (NMs), because their activities resemble those of NMs. Well-defined gas-phase studies on isolated molecules can identify the active sites, uncover mechanistic pathways, and establish the structure−function relationships in an unperturbed environment at a strictly molecular level,13−23 which are crucially important for tailoring the design of new and effective catalysts. A literature survey indicates that there are several mechanisms for alkane activation in gas-phase reactions under thermal-collision conditions: activation by oxygen-centered radicals in transition metal oxide ions through hydrogen atom transfer (HAT),17,19 activation by metal centers in atomic ions or in some transition metal carbides/nitrides through oxidative © XXXX American Chemical Society

Received: August 16, 2016 Revised: September 12, 2016

A

DOI: 10.1021/acs.jpca.6b08303 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A substitution of noble metals. This intriguing resemblance in electronic structures of TMN ions and NM atoms and potential applications of TMN materials as well as the relatively limited knowledge about gas-phase TMN ions inspire us to explore more about the properties of these ions, such as how to change their reactivity effectively.

2. METHODS Experimental Methods. A reflectron time-of-flight mass spectrometer (TOF-MS)40 equipped with a laser ablation ion source, a quadrupole mass filter (QMF),41 and a linear ion-trap (LIT) reactor42 were used to study the reactions of HMoN− and ZrNH− with alkanes. The HMoN− and ZrNH− ions were generated by laser ablation of a molybdenum disk (compressed isotope-enriched 98Mo powder, 99.45%, Trace Science International) and a zirconium disk, respectively, in the presence of ∼0.1% NH3 seeded in a He carrier gas with a backing pressure of 5 atm. The ions of interest were mass-selected by the QMF and entered into the LIT reactor, where they were confined and thermalized by collisions with a pulse of He gas and then interacted with a pulse of alkanes (CH4, C2H6 or C3H8) for a period of time, on the order of a few to 10 ms. It has been proved that the ions are thermalized to (or close to) room temperature before reactions in our previous works.42,43 The ions ejected from the LIT were detected by a reflection TOFMS. The method to derive the rate constants are described in detail in ref 44. Deuterated ethane (C2D6; 98% D, Cambridge Isotope Laboratories, Inc) was also used to verify the assignments of the reaction channels and to study the isotope effect on the reaction. Theoretical Methods. All DFT calculations were performed with the Gaussian 0945 program package employing the BMK46 density functional method (42% Hartree−Fock exchange). For all the reaction pathways, the 6-311+G(d) basis sets47 were selected for N, C, H atoms, and the def2TZVP basis sets48 were selected for Mo and Zr atoms. Among the 20 tested methods, BMK gives good prediction of Mo−H, Mo−N, Mo−C, H−C2H5, and N−H bonds, as listed in Table S1. In addition, the BMK functional was used in the previously reported HNbN−/alkane systems, and to make the reactions of HMoN− and ZrNH− comparable with that of HNbN−, BMK was also adopted in the DFT calculations herein. The coupledcluster method with single, double, and perturbative triple excitations method [CCSD(T)]49,50 was also used. The reaction mechanism calculations involve geometry optimization of reaction intermediates (IMs) and transition states (TSs). Vibrational frequency calculations were performed to check that the IMs or TSs have zero and only one imaginary frequency, respectively. The intrinsic reaction coordinate (IRC) calculations51,52 were performed to make sure that a TS connects two appropriate minima. The reported energies (ΔH0K in eV) are corrected with zero-point vibrations. The natural population analysis was performed with NBO 5.9,53 and the program Multiwfn54 was employed to analyze orbital compositions by natural atomic orbital method.

Figure 1. TOF-MS for the reactions of mass-selected H98MoN− (a) with CH4 (b) for 8.5 ms, with C2H6 (c) and C2D6 (d) for 6 and 14.5 ms, respectively, as well as mass-selected 90ZrNH− (e) with C2H6 (f) for 6 ms. The effective reactant gas pressures are shown. The label +X denotes HMoNX− or ZrNHX− (X = O, C2H4, etc). (d) The peak (m/ z 146) marked with an asterisk may come from reactions with impurities.

generation of HMoNO− (HMoN− + H2O → HMoNO− + H2). HMoN− is inert toward the most stable alkane, CH4 (Figure 1b), but can activate the C−H bonds of C2H6 (Figure 1c) with the formation of H2 molecules (reaction 1). A weak signal corresponding to H3MoN− (reaction 2) can be observed at longer reaction time (Figure S1, Supporting Information). Labeling experiment with C2D6 (Figure 1d) results in ion mass distributions exhibiting the expected shifts, confirming reaction 1; the peak of HD2MoN− is absent due to the small branching ratio for this channel and the large kinetic isotope effect (KIE). Upon the interaction of HMoN− with C3H8 in the reactor, the channel leading to H3MoN− is much more obvious (Figure S2 in Supporting Information). It is hard to prove experimentally that the appearance of the H3MoN− peak is due to the reaction with C2H6 by now (Figure S1). However, a comparison of reactions with C2H6 (Figure 1c) and C3H8 (Figure S2b) at similar reaction times (6 vs 5.5 ms) and reactant pressures (0.19 vs 0.20 Pa) indicates that H3MoN− comes from reaction with alkanes rather than impurities; otherwise, H3MoN− would also be present in Figure 1c. Density functional theory (DFT) calculations as discussed later further support this conclusion. In addition, zirconium has five stable isotopes: 51.5% 90Zr, 11.2% 91Zr, 17.1% 92Zr, 17.4% 94Zr, and 2.8% 96Zr. The peak with mass of 105 amu contains both 90ZrNH− and 91ZrN−, which are with the same mass-to-charge ratios; then, this peak was mass-selected and reacted with C2H6. As shown in Figure 1e,f, ZrNH− is inert or reacts very slowly with C2H6 at thermal conditions. According to the simulated isotopic pattern of ZrNH− and the experimentally observed peak intensity of 90 ZrNH−, the proportions of 90ZrNH− and 91ZrN− in the 105 amu are 43% and 57%, respectively, (Figure S3).

3. RESULTS AND DISCUSSION Experimental Results. The H98MoN− and 90ZrNH− anions were generated, mass-selected, confined, and cooled, and then they interacted with CH4 and C2H6 in an LIT reactor.42 As shown in Figure 1a, HMoN− ions can react with a trace amount of water impurity in the LIT, leading to the

HMoN− + C2H6 → HMoNC2H4 − + H 2

(1)

HMoN− + C2H6 → H3MoN− + C2H4

(2)

−

ZrNH + C2H6 → no reaction B

(3) DOI: 10.1021/acs.jpca.6b08303 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Figure 2. DFT-calculated potential-energy profiles for the reactions of C2H6 with HMoN− (a) and ZrNH− (b). Selected bond lengths are given (in pm). The zero-point vibration corrected energies (ΔH0K in eV) of the reaction intermediates, transition states, and products with respect to the separated reactants are given. All structures given refer to the doublet surface. The superscripts indicate the spin states of species, and those without superscripts are doublet spin states. (b) The details of the reaction steps connecting 6 and P3 as well as 6 and P4 are not calculated.

of the metal atom into the C−H bond through TS1/2 (−0.01 eV, with respect to the separated reactants), with the formation of intermediate 2. A spin conversion from quartet state to doublet state may take place along IS1 + C2H6 → 1. The details of spin flip are not considered herein. This high reaction barrier TS1/2 is consistent with the experimentally observed low rate constant (k1 = (7.0 ± 0.6) × 10−13 cm3 molecule−1 s−1). Starting from 2, two different reaction channels A (liberation of H2) and B (liberation of C2H4) are accessible. In pathway A, one more hydrogen atom transfers from CH2CH3 unit to the molybdenum-bound hydrogen atom, bringing about the formation of HMoNC2H4−···H2 (3) via TS2/3; intermediate 3 then dissociates to HMoNC2H4− and H2 (P1). Pathway B results in the formation of H3MoN− and C2H4 (TS2/4 → 4 → P2). The energies of TS2/3 (−0.85 eV) and TS2/4 (−0.89 eV) are comparable, but P1 is with 0.5 eV more exothermic than P2; thus, the path A is theoretically predicted to be favored over the path B, which is in line with the small branching ratio for reaction 2 as observed experimentally. Other possibilities for reactions 1 and 2 have also been considered, which are not favorable kinetically or thermodynamically (Figures S6 and S7). Different from those of HNbN− and HMoN−, the lowestenergy structure of ZrNH− (2IS2) is determined to have a doublet state with linear geometry, and the isomer with a bent structure (2IS4) is 0.54 eV higher in energy than the ground state (Figure S5b). In 2IS2, the H atom is bonded with the N atom rather than the Zr atom. Figure 2b shows the PES of 2IS2 + C2H6. In the first step, the ZrNH− ion and C2H6 form the encounter complex ZrNH−···C2H6 (5); in the following C−H bond activation step, one barrier (TS5/6, 1.04 eV with respect to the separated reactants) is located above the entrance channel, which prevents the product formation. The quartet TS corresponding to the first HAT step (4TS5/6) is also very high

The pseudo-first-order rate constant (k1) of the reaction of HMoN− with C2H6 is estimated to be (7.4 ± 0.4) × 10−13 cm3 molecule−1 s−1, corresponding to efficiency (ø)55,56 of ∼0.04%. The KIE defined as k1(HMoN− + C2H6)/k1(HMoN− + C2D6) amounts to 5. The signal dependence of product ions on reactant gas pressures can be derived and well-fitted with the experimental data (Figure S4). When the other fitting model, in which the possibility of containing unreactive HMoN− ions in the cluster source is considered, is applied to the experimental data, the relative intensity of the unreactive component of HMoN− is estimated to be 1%, implying that most of the generated HMoN− anions have a uniform structure (see Page S3 in Supporting Information for details). Theoretical Results. To interpret the experimental observations, DFT calculations with BMK functional were performed to study the structures of MNH− (M = Mo and Zr) and the reaction mechanisms. HMoN− ions have two isomers: one bent and one linear structure (Figure S5, Supporting Information). For HMoN−, the bent isomer with a quartet state (4IS1) is more stable than its doublet electromer (2IS1) by 0.01 eV, and the linear isomers are higher in energy. The energy gap between 4IS1 and 2IS1 is increased to 0.20 eV by using the CCSD(T) level of theory. As the ground state, 4IS1 with quartet state is not capable of bond activation of C2H6 due to the positive reaction barrier of 1.07 eV (Figure 2), but the excited state, 2IS1, can achieve the reaction. This phenomenon is not unexpected, because the OA mechanism, also applicable to the HMoN−/C2H6 system, is more favorable in low-spin than in high-spin electronic states of transition metal carbide36 and nitride ions.39 The potential-energy surfaces (PESs) and relevant structural information on the most favorable pathways for reactions 1 and 2 are shown in Figure 2a. The chemical transformations commence with the formation of the weakly bound encounter complex 1, and the following step is insertion C

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The Journal of Physical Chemistry A in energy. Since the reaction of ZrNH− with C2H6 is hampered by intrinsic energy barrier, the detailed steps for the product generation from 6 are not considered. The theoretical calculations are in good agreement with the experimental results. Molecular Orbital Analysis for HMoN− and ZrNH− and Comparison with HNbN−. We reported the reactions of HNbN− with alkanes previously, and the thorough comparison of the electronic structures of MNH− (M = Zr, Nb, Mo) turned out to be rather informative. In sharp contrast to the reactivity of HNbN− toward CH4 and C2H6,39 the reactivity of HMoN− is decreased, which can activate ethane rather than methane, and ZrNH− exhibits no reactivity in terms of HAT or OA toward C2H6. Mo (4d55s1) and Zr (4d25s2) atoms possess one more and one less electron than Nb atom (4d45s1), respectively, and the orbital analysis indicates compositions of active orbitals play key roles in changing the reactivity of MNH−. The HNbN− anions with 12 valence electrons (VEs) can react with CH4 and C2H6; HMoN−, with 13 VEs, is reactive toward C2H6 rather than methane, and ZrNH− ion, containing 11 VEs, is totally inert toward CH4 and C2H6. The intrinsic reason can be understood from a molecular orbital analysis. As pointed out previously, the highest occupied molecular orbital (HOMO) of HNbN− is mainly composed of the Nb 5s (58%) and 4dz2 (25%), that is, 4d/5s hybrid orbital, which has primary contribution to the alkane activation reactions.39 As shown in Figure 3, the HOMO of HMoN− derives primarily from Mo

bonding interaction between the HOMO of C2H6 and the dxz of the Mo atom increases the energy of singly occupied molecular orbital of 2TS1/2, which is not favorable for the system energy; thus, the reactivity of HMoN− toward alkanes is lowered in comparison to that of HNbN−. Orbital analysis for ZrNH− and the 2TS5/6 of the reaction toward C2H6 further supports sd hybrid orbital plays a pivotal role in alkane activation mediated by MNH−. The HOMO of doublet ZrNH− ion is mainly composed of the Zr 5s orbital (86%), and the percentage of the Zr 4dz2 orbital is only 5% (Figure 3c). In TS5/6 as shown in Figure 3d, the Zr 5s orbital dominates the α-HOMO, and the reaction of ZrNH− + C2H6 is hampered by this intrinsic energy barrier. Interestingly, ethane activation also cannot be observed in the reaction with NbN−,39 which has the same VE value as ZrNH−, and the HOMOs of these two anions are with high percentages of Zr/Nb 5s orbitals. In the related transition states, the orbital interaction of the HOMOs with the LUMOs of ethane molecules is an important factor. In 2TS1/2 shown in Figure 3b, the bonding interactions between Mo and H1 as well as Mo and C1 atom is strong through the more favorable orbital overlap between sd hybrid orbital and σ*C−H. This also holds true for HNbN−/ C2H6 system. In comparison, the orbital overlap between Zr and C2 as well as Zr and H2 atom is weaker in 2TS5/6 (Figure 3d). Analogously, the feature of frontier orbitals in the transition state of NbN−/C2H6 (Figure S9) is similar to that in 2TS5/6. Therefore, compared to s orbital, sd hybrid orbital overlaps better with ethane orbital, producing an energetically favorable transition state.

4. CONCLUSIONS The reactions of mass-selected triatomic anions HMoN− and ZrNH− with alkanes in an ion-trap reactor under thermal collision conditions have been investigated experimentally and theoretically. Compared to the previously reported HNbN−, which is reactive toward CH4, HMoN− and ZrNH− show quite different reactivities in alkane activation reactions, although these two ions carry one more and one less electron than HNbN−, respectively. HMoN− can activate C−H bond of ethane, producing ethene and hydrogen molecules, and ZrNH− is inert or reacts very slowly with C2H6. The molecular orbital analysis indicates the sd hybrid orbital of the metal atom in MNH− is a pivotal factor for alkane activation by transition metal nitride ions, and the reactivity of ions can be changed efficiently by adjusting compositions of active orbitals. This may point the way to find approaches in tailoring the design of catalysts with controllable reactivity.

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Figure 3. Parts of electronic structures of the doublet HMoN− (2IS1, panel a), 2TS1/2 (b), ZrNH− (2IS2, panel c), and 2TS5/6 (d). The superscripts indicate the spin states of species.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b08303. DFT-calculated and experimental bond dissociation energies, TOF MS for reactions, variation of ion intensities with respect to reactant gas pressures, distribution of HxZrN− anions within mass range of m/ z 102-113, DFT-calculated structures and relative energies, other possible relaxed potential-energy curves for reaction pathways with ethane, parts of electronic structures, transition state for the first hydrogen atom transfer (PDF)

4d/5s hybrid orbital, which is quite similar to the reactive orbital of HNbN−; in addition, the excess VE in HMoN− relative to HNbN− is located on the Mo dxz orbital (αHOMO−1). Along the reaction pathway of HMoN− with C2H6, the valence molecular orbital picture of 2TS1/2 is different from that of the transition state for C−H activation by HNbN−. Unlike the frontier orbital picture of HNbN−, in which the 4d/5s orbital (HOMO) is the only active one,39 both of the HOMO and α-HOMO−1 orbitals in HMoN − simultaneously participate in bonding with C2H6 (Figure S8). The schematic diagram in Figure S8c indicates that this D

DOI: 10.1021/acs.jpca.6b08303 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-01-81381387. Fax: +86-01-68914780-804. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21503015) and the Institute of Chemistry, Chinese Academy of Sciences.

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DOI: 10.1021/acs.jpca.6b08303 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpca.6b08303 J. Phys. Chem. A XXXX, XXX, XXX−XXX