Simultaneous Formation of cis-and trans-CH3OCu (OH) Intermediates

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Simultaneous Formation of cis- and trans-CH3OCu(OH) Intermediates in Methane Activation by Cu in Solid Ar Yanying Zhao,* Fan Yu, Caixia Wang, and Zhaoman Zhou Department of Chemistry and State Key Laboratory of Advanced Textiles Materials and Manufacture Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China

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ABSTRACT: Understanding the mechanisms of methane activation is an important and active research area of the contemporary catalyzed conversion of hydrocarbons to shippable, valuable feedstock and has invoked close collaborations between experimentalists and theorists. This article describes the trapping of reaction intermediates in a coppercatalyzed direct methane-to-methanol conversion. Specifically, two hydroxy(methoxy)copper(I) [CH3OCu(OH)] isomeric intermediates were distinguished and characterized by matrix isolation infrared spectroscopy and 18O2, CD4, and 13CH4 isotopic substitution experiments combined with quantum chemical calculations. Initially, laserevaporated copper reacted with oxygen to form CuO2. Upon successive broadband UV irradiation, methane activation via the insertion of CuO2 into one of the C−H bonds produced both cis- and trans-CH3OCu(OH). All possible structures of the reactants, intermediates, transition states, minimum-energy crossing points, and products were optimized. The results indicated that the two CH3OCu(OH) isomers are not thermodynamically distinguishable, although both have different frequencies, which agrees with experimental observation. The proposed reaction mechanism involved (i) O−O bond cleavage, (ii) C−H bond activation, and (iii) H and CH3 competitive transfer. The small energy barrier for cis to trans conversion accounts for the simultaneous formation of cisand trans-CH3OCu(OH). These findings indicate that the CH3OCu(OH) species would be a potent precursor in other families of copper-cored oxidants that can trigger methanol formation.

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INTRODUCTION Methane, the main component of natural gas, has been widely used in hydrogen production and energy applications.1,2 It not only has the highest H/C ratio of all hydrocarbons but also is more environmentally friendly than oil- or coal-derived fuels in terms of CO2 emission. However, the activation of methane remains a scientific challenge in contemporary catalysis because of the inertness of the C−H bond, which has a bond dissociation energy of ∼440 kJ/mol.3 Finding solutions to natural gas utilization is exacerbated by problems caused by gases from crude oil production in remote locations. The subsequent conversion of methane to shippable liquid fuels or other safer and more valuable feedstock4−8 is of fundamental significance. The scale-up of the efficient and selective “direct methane-to-methanol” (DMTM) process under mild reaction conditions is of significant scientific and economic interest to both organic chemistry and catalysis research and the petrochemical industry.9−15 Moreover, environmental and economic concerns favor the use of oxidants that are both harmless to the environment and atom-efficient. Thus, significant efforts have been made to understand the mechanisms involved in the process. The C−H bond dissociation energy of methanol is about 47 kJ/mol lower than that of methane, rendering methanol extremely © XXXX American Chemical Society

susceptible to CO2 oxidation under representative reaction conditions. Thus, an effective catalyst would have to promote methane activation while curbing further methanol oxidation. Although the catalytic process consists of a complicated sequence of interrelated reactions, studies of the forward and reverse reactions, CH4 + 1/2O2 + Cat ↔ Cat + CH3OH, can provide quantitative information on the thermodynamics and mechanisms of methane conversion. Several transition-metal-derived catalysts, both heterogeneous and homogeneous, have been developed for the controlled oxidation of methane to methanol.16−18 In particularly, copper-catalyzed catalysis has been regarded as one of the most efficient methods of achieving both high activity and control of selectivity for DMTM conversion.19−23 Research in recent years has greatly enhanced the mechanistic understanding of C−H activation; intensive experimental and theoretical studies of the DMTM mechanism on Cu active sites supported on shape-selective zeolites have profoundly impacted the modeling of copper reactivity in the presence of O2 or other oxidizing reagents.24−30 Using zeolite mordenite samples with different Si/Al ratios, Sushkevich et al. Received: November 29, 2018

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DOI: 10.1021/acs.inorgchem.8b03322 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Infrared spectra in the 3650−3620, 1480−1000, and 800−700 cm−1 regions from the codeposition of the laser-ablated Cu target with 0.02% O2 and 0.1% CH4 in excess argon. (a) Sample deposition (1 h) at 6 K, (b) after 25 K annealing, (c) after full-arc photolysis for 10 min, and (d) after 30 K annealing. method for preparing simple metal oxides or higher oxides in solid argon for matrix isolation spectroscopic studies.41,42 The experimental setup for pulsed laser evaporation and matrix isolation infrared spectroscopic investigation has been described in detail previously.43,44 Briefly, the 1064 nm fundamental of a Nd:YAG laser (Continuum, Minilite II, 10 Hz repetition rate) was focused onto a rotating copper metal target (Johnson Matthey, 99.9%) through a hole in a CsI window cooled normally to 4 K by means of a closedcycle helium refrigerator. The laser-evaporated metal atoms were codeposited with dioxygen/methane mixtures in excess argon onto the CsI window. In general, matrix samples were deposited for 1 h at a rate of approximately 5 mmol/h. The O2/CH4/Ar mixtures were prepared in a stainless steel vacuum line using a standard manometric technique. Isotopically labeled 18O2 (Cambridge Isotope Laboratories, 99.8%) and CH4 (Isotec, 99%) samples were used without further purification. The infrared absorption spectra of the resulting sample were recorded on a Bruker Vertex 80 V spectrometer at 0.5 cm−1 resolution between 4000 and 400 cm−1 using a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector. After the infrared spectrum of the initial deposition had been recorded, the samples were warmed to a certain temperature and quickly recooled before more spectra were taken. Selected samples were also subjected to broad-band irradiation using a high-pressure mercury arc lamp with glass filters. Quantum chemical calculations were performed using density functional theoretical methods implemented in the Gaussian 09 suite of programs to probe the stationary points and energetics for methane activation by metal copper oxides.45 The three-parameter hybrid functional according to Becke with additional correlation corrections due to Lee, Yang, and Parr (B3LYP) and 6-311++G(3df, 3pd) basis sets were employed to optimize the geometries and obtain vibrational frequencies of complexes along the reaction coordinates.46−49 Hybrid density functional B3LYP has proven to be an accurate method for reproducing metal−ligand bond lengths. The species investigated are all neutral. The DFT geometries were checked to ensure the stability of the wave function by means of the stable = opt keyword. The geometries were fully optimized. The harmonic vibrational frequencies were calculated at the B3LYP level, and zero-point vibrational energies (ZPVE) were derived. The stationary points were confirmed as the minima or transition states (TSs) via the calculation

synthesized and tested catalysts with monomeric and oligomeric copper sites for the DMTM process under both aerobic and anaerobic conditions and found that monomeric copper sites are active only under aerobic conditions.24 Extensive experimental and theoretical studies of the reactions of gas-phase transition-metal monoxide cations with methane revealed that the catalytic efficiency depends strongly on the metal.31−35 For instance, Yoshizawa and Shiota calculated that the bare [CuO]+ cation catalyzed methane-to-methanol conversion at the B3LYP level and proposed that CuO+ is likely to be an excellent mediator for methane hyroxylation.35 After 1 year, Schwarz’s group succeeded experimentally in methane hydroxylation by gaseous CuO+ on the basis of mass spectrometry and density functional theoretical (DFT) calculations, and it was concluded that the low bond energy and positive charge on copper are important factors in effective DMTM conversion.36 Matrix isolation infrared spectroscopy has been conducted to determine the mechanism of catalytic DMTM conversion using the MO + CH4 and M + CH3OH model reactions (M = transition metals).37−40 Several important reactive intermediates, including OM(CH4), M(CH3OH), CH3MOH, CH3M(O)H, and CH3OMH, have been identified via isotopic substitution experiments and DFT calculations, and on the basis of these results, a few unprecedented reaction pathways have been proposed.37−40 In this study, we concentrated on methane oxidation by O2 on laser-ablated Cu in solid argon to produce the cis and trans isomers of hydroxyl(methoxy)copper(I) [CH3OCuOH]. The reaction mechanism was modeled using DFT optimization and accurate CCSD(T) energy calculations.

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EXPERIMENTAL METHODS AND COMPUTATIONAL DETAILS

The metal oxide reactants were prepared by the pulsed laser evaporation of bulk metal targets, which has proven to be an effective B


Article

Inorganic Chemistry Table 1. Infrared Absorptions and 12C/13C, 16O/18O, and H/D Isotopic Ratios Observed Experimentally CH4 + O2 3641.4 3634.5 1454.0 1452.8 1073.7 1070.9 773.6 769.5 733.1 719.4

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CH4 + O2 3641.4 3634.5 1450.0 1057.3 1054.6 773.6 769.5 733.1 719.4

CH4 + 18O2

CD4 + O2

3630.6 3623.3 1453.1

2686.9 2681.4

1.003 1.003

1118.0 1032.7 1036.7 767.8 763.9 550.0 554.7

1.002 1.016 1.015 1.000 1.000 1.000 1.000

1060.9 1058.0

728.0 711.4

12

C/13C

16

O/18O

H/D

assignment

1.003 1.003 1.001

1.355 1.355

cis-O−H stretch trans-O−H stretch cis-CH3 (umbrella) trans-CH3 (umbrella) cis-ν(C−O) stretch trans-ν(C−O) stretch cis-(O−Cu−O) sym trans-(O−Cu−O) sym cis-(Cu−O−H) swig trans-(Cu−O−H) swig

1.012 1.012

1.004 1.011

1.299 1.040 1.033 1.007 1.007 1.333 1.297

Figure 2. Infrared spectra in the 3650−3620, 1480−1410, 1340−1275, 1150−1030, 845−760, and 745−700 cm−1 regions from the codeposition of the laser-ablated Cu target with (a) 0.02% O2 with 0.1% CH4, (b) 0.02% O2 with 0.1% 13CH4, and (c) 0.02% 18O2 with 0.1% CH4 in excess argon at 30 K annealing after full-arc photolysis for 10 min.

weak absorption band at 953.7 cm−1 due to the O4− anion (not shown in Figure 1) and strong methane absorption band (trace a). When the as-deposited sample is annealed to 25 K, several sets of product absorption bands are produced (trace b). The 627.7 and 823.0 cm−1 bands previously assigned to CuO and OCuO, respectively,52 are observed (not shown in Figure 1). When the sample is annealed to 30 K after broadband photolysis for 10 min (traces c and d), additional absorptions at 3641.4, 3634.5, 2834.0, 2828.8, 1453.1, 1073.7, 1070.9, 773.6, 769.5, 733.1, and 719.4 cm−1 appear at the expense of the absorption bands at 627.7 and 823.0 cm−1 as a result of copper oxides in experiments with an O2/CH4 mixture in solid argon. No absorptions are observed after the deposition and visible photolysis of samples of laser-ablated copper with O2/ Ar or CH4/Ar. The experiments were repeated under the same conditions using the 16O2 + CH4, 18O2 + CH4, 16O2 + 13CH4, and 16O2 + CD4 samples to aid in product identification on the basis of isotopic shifts and absorption splitting. All infrared absorptions with O-18, C-13, and deuterium substitutions are also listed in Tables 1 and S1. Selected regions of the spectra of the different isotopic samples are shown in Figures 2 and S1. Assignment. Tables 1 and S1 list the observed bands and experimental 12C/13C, 16O/18O, and H/D isotopic ratios. As

of the energy Hessian and the observation of the correct number of imaginary frequencies, zero (0) or one (1), respectively. The TSs were authenticated using intrinsic reaction coordinate (IRC) methods. Optimization and single-point calculations were performed without symmetry restraint and utilized the (un)restricted Kohn− Sham formalism as appropriate. One minimum-energy crossing point (MECP) is searched with the GRRM (global reaction route mapping) program.50 The bonding characteristics, including the electron location function (ELF) distribution, were used to described, analyze, and visualize chemical bonds using Multiwfn package.51 All quoted energetics are Gibbs free energies calculated at 4 K and utilize the B3LYP/6-311++G(3df, 3p) level. Single points were corrected precisely at CCSD(T) using B3LYP optimization geometries.

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RESULTS AND DISCUSSION Infrared Spectra. The copper oxides and copper superoxides produced by the reaction of laser-evaporated copper with dioxygen in solid argon have been identified through an analysis of isotopic substitution effects on infrared spectra and theoretical calculations.52,53 The Cu−O stretching region of the infrared spectra obtained for the codeposition of laserevaporated Cu atoms with 0.02% O2 and 0.1% CH4 in argon is shown in Figure 1, and the product absorption bands are listed in Table 1. After 1.5 h of sample deposition, no product absorption bands are observed in the spectrum except for a C


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Inorganic Chemistry

Figure 3. Potential-energy surface profile for methane activation by copper dioxide and the associated structures of the intermediates and transition states optimized at the B3LYP/6-311++G(3df, 3pd) level of theory. Relative energies corrected for zero-point energy contributions are given in kcal/mol; bond lengths, in Å; and angles, in degrees at the B3LYP/6-311++G(3df, 3pd) (blue in brackets) and CCSD(T)//B3LYP/6-311+ +G(3df, 3pd) levels.

observed for the 773.6, 769.5, 733.1, and 719.4 cm−1 bands; however, these bands shift to 767.8, 763.9, 550.0, and 554.7 cm−1, respectively, in the experiment with the 16O2 + CD4 sample. Owing to the experimental limit, the shift of the 733.6 cm−1 band cannot be observed for the 18O2 + CH4 sample. On the other hand, for the 18O2 + CH4 sample, the 733.1 cm−1 band is red-shifted to 728.0 cm−1, and the 12C/13C, 16O/18O, and H/D isotopic frequency ratios are 1.000, 1.004, and 1.333, respectively. These experimental observations indicate that the carbon atom is not involved in the absorptions at 773.6 and 733.1 cm−1; therefore, they should undoubtedly be assigned to the CuOH mode, as listed in Table 1. The other bands in the low-frequency region do not correspond to known group absorption bands. Accordingly, we tentatively assigned the group absorption bands to the [CH3OCuOH] species. To accurately evaluate the assignments, all possible isomers that include the CH3O, OCuO, and CuOH units were explored by DFT calculations using the B3LYP functional. The results indicate that two stable optimized CH3OCuOH molecules have pseudolinear OCuO and planar COCuOH groups. The methyl group and hydrogen atom of the OH group are located on either the same or opposite sides of OCuO, and the isomers are designated as cis-CH3OCuOH

shown in Figure 2, absorption bands at 3641.4 and 3634.5 cm−1 are shifted to 3630.6 and 3623.3 cm−1 and 2686.9 and 2681.4 cm−1 for the 18O2 + CH4 and 16O2 + CD4 samples, respectively. No shift is observed for the O2 + 13CH4 sample. The 16O/18O (1.003) and H/D isotopic frequency ratios (1.355) are characteristic of O−H stretching vibrations. Almost no shifts are displayed by the 2834.0, 2828.8, and 1453.1 cm−1 bands in the 18O2 + CH4 sample. The spectra of the 16O2 + 13CH4 and 16O2 + CD4 samples show three absorption bands related to the −CH3 modes. The 12C/13C (1.002) and H/D (1.327) isotopic frequency ratios indicate that both 2834.0 and 2828.8 cm−1 bands correspond to the asymmetric −CH3 stretching vibration. The 1452.8 cm−1 band is tentatively assigned to the umbrella −CH3 mode because of the small 13C (1.002) and large deuterium isotopic frequency (1.299), which cause red shifts to 1450.0 and 1118 cm−1, respectively. The moderately strong absorptions at 1073.7 and 1070.9 cm−1 indicate 13C, 18O, and deuterium shifts of about 16, 13, and 41 cm−1, respectively. The isotopic frequency ratios (12C/13C = 1.012 and 1.015, 16O/18O = 1.012 and 1.015, and H/D = 1.040) indicate that the 1073.7 and 1070.9 cm−1 bands correspond to H3C−O stretching with a long C−O bond distance. In the Cu−O vibrational region, no 13C shifts are D


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Inorganic Chemistry (2P-cis) and trans-CH3OCuOH (2P-trans), respectively, as shown in Figure 3. The bond lengths and bond angles of the two isomers are very similar except for the OCuO bond angle, which is 173.8° for cis-CH3OCuOH and 179.7° for transCH3OCuOH. More interestingly, single-point energy calculations indicate very small energy differences of 0.1 and 0.2 kcal/mol between the isomers at the B3LYP and CCSDT// B3LYP levels, respectively; thus, the isomers are virtually indistinguishable from a thermodynamic perspective. However, vibrational analysis including the isotopic ratios clearly reveals that the characteristic vibrational frequencies of cis- and trans-CH3OCuOH are easily distinguishable, as shown in Figure S2 and Tables S2−S6. Table S7 compares the experimental and calculated [B3LYP/6-311++G(3df, 3pd)] differences (|Δcis − trans|) between the wavenumbers of cis- and trans-CH3OCuOH. For the O−H stretching, −CH3 umbrella, and C−O stretching modes, the calculated |Δcis − trans| values are 3.3, 2.2, and 3.4 cm−1, respectively, which correspond to the experimental |Δcis − trans| values of 6.9, 1.2, and 2.8 cm−1. On the other hand, the calculated values for O−Cu−O symmetric stretching and Cu−O−H swigging modes are much smaller than the experimental values. This indicates that more accurate theoretical methods are necessary. Reaction Mechanism. DFT is also a valuable tool for elucidating reaction mechanism, including that of (i) O−O bond cleavage, (ii) C−H bond activation, and (iii) H and CH3 competitive transfer. It provides a means to track the formation of cis- and trans-CH3OCuOH and the isomerization of intermediates and transition states as well as generate reaction potential energy surfaces (PESs). The reaction mechanisms can then be further investigated using high-level CCSD(T) energy calculations. The most energetically favorable PESs and selected structural information for relevant species in the reaction of CuO2 with methane are shown in Figure 3. Initially, the lowest-energy mechanism for O2 activation by copper was determined at the B3LYP level (Figure S3). The first intermediate doublet Cu−O2 complex (21) rearranges to copper peroxide (22) via 2TS1/2. Intermediate 22 then passes through transition state 2TS2/3 with an energy barrier of more than 40 kcal/mol (CCSD(T)//B3LYP) to form linear doublet CuO2 (23). The large energy barrier indicates that O−O bond cleavage is the rate-determining step in O2 activation. In addition, there are two spin states for CuO2, and for the reaction with methane, only key features are shown in Figures 3 and S3. At the CCSD(T)//B3LYP level, the energy of 43 is only 2.8 kcal/mol higher than that of the corresponding doublet, which is consistent with previous reports indicating that the electronic ground state of CuO2 is the latter.54−59 Thus, 23 was taken as the starting point of the reaction. When methane approaches 23, hydrogen bonding complex 44 becomes more stable than the corresponding doublet complex in the PES. Thereafter, to further activate methane, the system has to transit back to the doublet state. During methane activation, there are two MECPs, as shown in Figure 3. Thus, complex 44 goes through transition state 4TS4/5, which has a short H···O bond length of 1.248 Å, and subsequently MECP2, to finally generate intermediate 25. Intermediate 25 is the H3C···HOCuO complex, which has a remote C−H distance of 2.322 Å and an O−H bond length of 0.967 Å. The IRC paths are shown in Figures S4 and S5. Subsequently, isomerization of the C−H bond of 25 produces stable species 26 via a very low barrier. IRC further indicate that the isomerization process from 5 to 6 is barrierless, as shown in Figure S6.

From the latter, methyl migration occurs in two steps: (1) methyl transfer from the O1 atom to the copper center via a three-center (O1, Cu, C) transition state, 2TS6/7, to form a four-center (O1, Cu, O2, C) intermediate, 27 and (2) the formation of 2P-trans via a three-centered (Cu, C, O2) transition state, 2TS7/P-trans. The energy barrier of methyl transfer exceeds 40 kcal/mol, which is close to that of the oxygen activation step. The validity of 2TS6/7 was confirmed by IRC, and the results are shown in Figures S7 and S8. More strikingly, methyl transfer on the doublet surface is not thermodynamically possible, as the most energetically favorable pathway, 26 → 27, is rather endothermic; however, almost no energy barrier exists to form 2P-trans from 27. On the other hand, hydrogen transfer behaves rather differently. One-step hydrogen transfer from O1 to the terminal O2 atom occurs via a four-center transition state, 2TS6/P-cis, to form the more stable 2P-cis. Hydrogen transfer in 26 has an energy barrier of 52 kcal/mol, as shown in Figures 3 and S8. It is worth mentioning that this is 11.5 kcal/mol higher than that of methyl transfer. That is the reason that we give priority to the methyl transfer to the terminal oxygen. IRC calculations60,61 for 2TS6/P-cis indicate that it directly connects certain conformers of 2P-cis through a particularly unusual long-haul proton transfer, as shown in Figure S9. Finally, IRC calculations for 2TS-P, which directly connect cis- to trans-CH3OCuOH, were performed to obtain the reaction path. In Figure S10, the abscissa indicates the IRC distance measured from the TS and the ordinate indicates the potential energy. IRC calculations demonstrate that both the H−O−Cu−O and C−O−Cu−O dihedral angles change significantly during isomerization. Three important structures, including that of 2TS-P on this reaction path, are also shown in Figure 3. The reaction profile of the isomerization is obtained through a relaxed potential energy scan along the H−O−Cu− C dihedral angle as shown in Figure 4. The small energy barriers of 2.9 and 4.2 kcal/mol at the B3LYP and CCSD(T)//B3LYP levels, respectively, for the cis- to transCH3OCuOH conversion indicate the similar stabilities of the two species. Thus, for the whole reaction process, the highest activation energy exists in the O−O bond cleavage stage.

Figure 4. Profile of the relaxed potential energy surface scan of the reaction on the transfer between cis- and trans-CH3OCuOH along the dihedral angles of HOCuC. The energy values were obtained at the B3LTP/6-311+G(3df, 3pd) level. E


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Inorganic Chemistry

Figure 5. Color-filled plots of the valence electron density and electron local function (ELF) as well as contour line plots of the electron energy density and deformation density (ρdef) of cis- and trans-CH3OCuOH. In the color-filled map, the region with a value higher than the upper limit of the color scale is depicted as white. The X and Y axes correspond to coordinates. In the contour line map, a solid black line and a blue dashed line represent positive and negative regions, respectively.

catalyzed methane oxidation.20,21,24−26,29,30,35,36,62−65 To unravel the nature of bonding in CH3OCu(OH), the electronic structure was calculated, and the electron densities, atoms-inmolecules (AIM) critical points, ELFs, and isosurfaces were determined using the AIM66 and ELF67−70 approaches. These methods have been proven to be useful tools for obtaining

Bonding Analysis. Cheap copper-catalyzed DMTM conversion has fascinated scientists for a long time, and recently, there has been much enthusiasm devoted to various experimental and theoretical studies on aspects such as the mechanism. To date, only the copper cation is considered to be key to obtaining insights into the reactivity of copperF


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Inorganic Chemistry

Figure 6. Natural atomic charges (blue) at the B3LYP/6-311++G(3df, 3dp) level and Mayer bond orders (red) at the B3LYP/6-31G(d, p) level for cis- (a) and trans-CH3OCuOH (b) molecules.

to that in trans-CH3OCu(OH) (∠OCuO = 179°). Moreover, the valence electron density plots in Figure 5a,b show that the two lone pairs on oxygen are responsible for ∠OCuO. For cisCH3OCu(OH), the stronger repulsion due to the lone pair electrons being on the same side of the OCuO plane leads to the ∠OCuO torsion, which can largely reduce the repulsive force. However, because the lone pair electrons are located on the opposite sides of the OCuO plane in trans-CH3OCu(OH), the repulsion is weaker, which accounts for the nearly linear ∠OCuO. Natural population analysis indicates that the valence electronic configuration of the central Cu is 3d9.44s0.5 and 3d9.64s0.3 for cis- and trans-CH3OCu(OH), respectively, although the total electron numbers are both close to 10. This sufficiently indicates the loss of one electron from atomic Cu (3d94s2 or 3d104s1) to yield a valence of +1 on the copper core. As shown in Figure 6, the positive charge of 0.99 further confirms that copper has a +1 valence. This three-center π bond can also be verified by examining the Mayer bond order. Thus, considering the +1 valence on the copper core, both cisand trans-CH3OCu(OH) would be potent precursors for triggering subsequent methanol formation on the basis of previous studies.20,21,24−26,29,30,35,36,62−65,75−80

valuable information about the electron distribution and bonding in molecules. Chemical bonding analysis that focuses on CH3OCu(OH) was performed using electronic properties in position space such as bonding critical points and the Multiwfn51 program to examine the features of the valence electron density, ELF, electron energy density, deformation density, natural charges, and Mayer bond orders, as shown in Figures 5 and 6. For comparison, the total electron density is also plotted in Figure S11. Figure 5 unveils the qualitatively similar electronic structures of cis- and trans-CH3OCuOH, namely, the strong O−H, C−O, and C−H covalent bonds. These bonds can be vividly recognized in the valence electron density maps, wherein the valence electron densities in the corresponding regions are remarkably greater than those in the surrounding regions. Moreover, each oxygen atom evidently has a lone pair, marked as the red region on the opposite side of the substituent, as shown in Figure 5a,b. It can also be seen from the ELF maps in Figure 5c,d that very high values exist at the terminal hydrogen and oxygen atoms. However, although electrons are also localized on copper, the local electron distribution between Cu and O is evidently very low. This observation suggests that charge-shift bonding71−74 between Cu and O is strong, which is also further confirmed by the positive ρ(r) and ∇2ρ(r) shown in Table S8. Moreover, there is evident covalent interaction between Cu and O based on the negative electron energy density and deformation density. The four plots show that the four Cu−O bonds are highly polar and have mixed ionic and covalent character, as the greatest electron concentration and highest valence electron distribution along the bond are on oxygen. In addition, the calculated values of the |V(r)|/G(r) ratio, ∼1.2, are closer to 1 than to 2, which indicates that the ionic character is more prominent. Thus, the O−Cu−O center is evidently a three-center π bond, and there is quite sufficient evidence that the bond is formed by the 3d orbital of Cu and the 2p orbitals of O, as shown in Figure S12. Subsequently, adaptive natural density partitioning (AdNDP) analysis75 was performed on the threecenter π bond, and the results are shown in Figure S13. The O−Cu−O substructure exhibits the classical 3c-2e d-pπ bonding. The occupation number for both isomers is exactly 1.998|e|, which is composed of 39.00% Cu 3d, 41.90% O(H) 2p, and 16.84% O(CH3) 2p for cis-CH3OCu(OH) and 39.32% Cu 3d, 47.51% O(H) 2p, and 9.86% O(CH3) 2p for transCH3OCu(OH). The deformation of the electron distribution in the AdNDP plot may be mainly due to the larger twist angle of O−Cu−O in cis-CH3OCu(OH) (∠OCuO = 173°) relative

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CONCLUSIONS In the copper-catalyzed oxidation process, we provided a stringent test to characterize the CH3OCu(OH) reaction intermediates, which would be even more ambitious copper core catalysts for methane to methanol conversion. Initially, the two CH3OCu(OH) isomeric intermediates from coppercatalyzed methane activation were characterized and distinguished by matrix isolation infrared spectroscopy and 18O2, CD4, and 13CH4 isotopic substitution experiments combined with DFT calculations. Subsequently, the mechanism of formation of the CH3OCu(OH) isomers involving (i) O−O bond cleavage, (ii) C−H bond activation, and (iii) H and CH3 competitive transfer was determined. Laser-evaporated copper reacted with oxygen to form linear copper dioxide, which further reacted with methane to produce both cis- and transCH3OCu(OH) upon broadband UV irradiation. All possible structures of reactants, intermediates, transition states, MECPs, and products were optimized by DFT. The results indicated that the two isomers can be easily distinguished because of their different calculated and experimental frequencies, although the more stable one cannot be determined. The small energy barrier for cis to trans isomerization can also be interpreted as the simultaneous formation of the two isomers. G


Article

Inorganic Chemistry

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CH3OCuOH is likely an important intermediate that further rearranges to CH3OH. From the combined analyses, it can be concluded that both cis- and trans-CH3OCu(OH) have the effective cationic copper core necessary to carry out subsequent methanol formation. This research significantly reveals the connection between computation and experiment and highlights the contribution of the former to our understanding of catalytic methane oxidation based on copper-assisted C−H activation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the Web site. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.inorgchem.8b03322.

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Additional experimental and quantum chemical data and Cartesian coordinates of optimized geometries (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-571-868-436-27. ORCID

Yanying Zhao: 0000-0002-2783-2443 Author Contributions

All authors contributed to refining ideas, carrying out additional analyses, and finalizing the article. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (grant nos. 21473162 and 21273202) and the National Basic Research Program of China (2013CB834604). Y.Z. is grateful to the Project Grants 521 Talents Cultivation of Zhejiang Sci-Tech University. This work was also supported by the Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology. We thank Prof. Mingfei Zhou of Fudan University for significant beneficial discussions.

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REFERENCES

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Simultaneous Formation of cis-and trans-CH3OCu (OH) Intermediates

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