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1 Supporting Information Electronic Origins of the Variable Efficiency of Room-Temperature Methane Activation by Homo- and Heteronuclear Cluster Oxide Cations [XYO 2 ] + (X, Y = Al, Si, Mg): Competition between Proton-Coupled Electron Transfer and Hydrogen-Atom Transfer Jilai Li,, Shaodong Zhou, Jun Zhang, Maria Schlangen, Thomas Weiske, Dandamudi Usharani, ± Sason Shaik $, * Helmut Schwarz,, * Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135, Berlin, Germany Institute of Theoretical Chemistry, Jilin University, Changchun , People s Republic of China Institute of Theoretical Chemistry, University of Cologne, Greinstraße 4, Cologne, Germany ± Department of Lipid Science, CSIR-Central Food Technological Research Institute, Mysore, , India $ Institute of Chemistry and the Lise-Meitner-Minerva Center for Computational Quantum Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel *Corresponding Authors Helmut.Schwarz@tu-berlin.de (HS); sason@yfaat.ch.huji.ac.il (SS) Table of Content 1. Additional experimental results S2 2. Additional computational results S4 3. References S6 4. Schemes S7 5. Figures S8 6. Tables S12 7. Coordinates S15 S1

2 1. EXPERIMENTAL RESULTS 1.1 [Mg 2 O 2 ] + /C 2 H 6 SYSTEM The Fourier transform ion-cyclotron resonance mass spectra, Figure S1, show the reactions of mass-selected, thermalized [Mg 2 O 2 ] + ions (m/z = 80) with C 2 H 6 and its isotopomers; the reaction with noble gas argon has also been recorded to provide a reference spectrum. As shown in Figure S1a, the hydrogen-atom transfer (HAT) product ion [Mg 2 O 2 H] + is formed even if the ICR cell is filled only with argon, i.e. [Mg 2 O 2 ] + reacts with background water. 1-3 However, the intensity of the product ion [Mg 2 O 2 H] + is much higher when C 2 H 6 is leaked into the ICR cell, Figure S1b; in addition, a signal with Δm = +2 relative to the precursor [Mg 2 O 2 ] + appears, which can be assigned to product ion [Mg 2 O 2 H 2 ] +. Notably, as shown in Figure S1b, the intensities of product ions [Mg 2 O 2 H] + and [Mg 2 O 2 H 2 ] + are comparable; by removing the background contribution, the [Mg 2 O 2 H 2 ] + is expected to be predominant. The C H bond scission was confirmed by isotopic labeling experiments with CH 3 CD 3 and C 2 D 6, in which product ions with Δm = +3 and +4 are observed, respectively, Figure S1c and S1d. The product ion with Δm = +3 in Figure S1c and S1d is expected to be mainly [Mg 2 O 2 HD] +, and Δm = +4 can be assigned to [Mg 2 O 2 D 2 ] +. All these product ions are formed in primary reactions as revealed by doubleresonance experiments. For example, constant removal of the product ion [Mg 2 O 2 H] + (Δm = +2) from the reaction cell does not affect the formation of product ion with Δm = +3 and/or +4. In addition, [Mg 2 O 2 H] + has been mass selected to react with ethane to make sure that double hydrogen atom transfer product origins from primary reactions. In summary, experimental data show that the cluster oxide [Mg 2 O 2 ] + can only be capable of activating ethane, not methane, Reactions (S1). The rate constant (k([mg 2 O 2 ] + /C 2 H 6 ) is estimated to cm 3 molecule 1 s 1, corresponding to a rather low collision efficiency of ϕ = 0.2%, relative to the collision rate. 4-6 [Mg 2 O 2 ] + + C 2 H 6! [Mg 2 O 2 H] + + C 2 H 5 (S1) S2

3 [Mg 2 O 2 ] + + C 2 H 6! [Mg 2 O 2 H 2 ] + + C 2 H 4 (S2) [Mg 2 O 2 ] + + CH 3 CD 3! [Mg 2 O 2 HD] + + C 2 H 2 D 2 (S3) [Mg 2 O 2 ] + + C 2 D 6! [Mg 2 O 2 D 2 ] + + C 2 D 4 (S4) 1.2 [Si 2 O 2 ] + /C 2 H 6 /C 3 H 8 SYSTEM The Fourier transform ion-cyclotron resonance mass spectra, Figure S2, show the reactions of mass-selected, thermalized [Si 2 O 2 ] + ions (m/z = 88) with C 3 H 8 and C 3 D 8 ; the reaction with noble gas argon has also been recorded to provide a reference spectrum. [Si 2 O 2 ] + is not capable of activating C 2 H 6 under thermal conditions; the mass spectra was not recorded therefore. As shown in Figure S2a, [Si 2 O 2 ] + reacts with background water; a signal with Δm = +17 relative to the precursor [Si 2 O 2 ] + appears, which can be assigned to product ion [Si 2 O 2 (OH)] +. When C 3 H 8 is leaked into the ICR cell, a signal with Δm = +3 relative to the precursor [Si 2 O 2 ] + appears, which can be assigned to hydrogen atom transfer product ion [Si 2 O 2 H 3 ] +, Figure S2b. The C H bond scission was confirmed by isotopic labeling experiments with C 3 D 8, in which product ions with Δm = +6 is observed, as shown in Figure S2c, which can be assigned to [Si 2 O 2 D 3 ] +. Notably, as shown in Figure S2b and S2c, there are other ions generated. As it is not a goal of this study we ignored to identify them. In summary, experimental data show that the cluster oxide [Si 2 O 2 ] + can only be capable of activating propane or higher alkanes. S3

4 2. COMPUTATIONAL RESULTS 2.1 [Mg 2 O 2 ] + /CH 4 SYSTEM As shown in Figure S3, in the reaction of [Mg 2 O 2 ] + with CH 4, an encounter complex 1 is formed initially from the reactants (R and CH 4 ); this step is exothermic by -65 kj/mol. Subsequently, one C H bond is activated and the corresponding hydrogen atom is transfered to the O b unit of R via transition state TS HAT which is 15 kj/mol higher in energy compared to the separated reactants. This HAT process results in the formation of the association product 3 (-79 kj/mol), in which the methyl radical is loosely coordinated to the cluster via the H atom of the newly formed OH group. Finally, the hydroxide product ion P1 is generated under evaporation of a methyl radical. Alternatively, the reaction can proceed via TS PCET by overcoming a 55 kj/mol barrier, and delivers 2, in which the methyl group is coordinated to the magnesium atom of the cluster. The elimination of a methyl group yields hydroxide product ion P1. As shown in Figure S3, both the rate-determining steps corresponding to the transition state TS HAT and TS PCET are not accessible under thermal conditions, in line with the experimental observation. 2.2 [Si 2 O 2 ] + /CH 4 SYSTEM The structure of [Si 2 O 2 ] + also corresponds to a rhombus-like structure with the spin density evenly distributed over the two silicon atoms (Figure S4, R). As shown in Figure S4, the reaction stems from an association of the isolated reactants, resulting in encounter complex 1. In 1, the carbon atom lies out of the plane of the cluster [Si 2 O 2 ] + moiety, different from the [Mg 2 O 2 ] + and [Al 2 O 2 ] + systems. Subsequently, the C-H cleavage of methane can proceed by two pathways, via TS HAT or TS PCET by surmounting 100 kj/mol or 67 kj/mol barriers, respectively. After the transition states, the system reaches 2 or 3. The methyl coordination to the silicon atom of the hydroxide moiety [Si 2 O 2 H] + in 2 lies, 165 kj/mol in energy, lower than that of loosely bound complex 3. Finally, both complexes 2 and 3 can yield separated product P1 by eliminating a methyl group. Since the rate-limiting barriers and S4

5 product lie above the reaction entrance, so the reaction is not favorable under thermal conditions. 2.3 [Mg 2 O 2 ] + /C 2 H 6 SYSTEM We also explored the reaction mechanism for the system [Mg 2 O 2 ] + /C 2 H 6 since there is a little difference with previous experimental report (Figure S5). 7 For the system [Mg 2 O 2 ] + /C 2 H 6 as shown in Figure S5, initially, the reaction of [Mg 2 O 2 ] + with C 2 H 6 first forms an encounter complex (EC) 1 from the separated reactants (R and C 2 H 6 ); this step is exothermic by -76 kj/mol. Subsequently, one C H bond is activated and the corresponding hydrogen atom is transferred to the O b unit of R via transition state TS1/2 which is -8 kj/mol lower in energy compared to the separated reactants. This HAT process results in the formation of the association product 2 (-175 kj/mol), in which the ethyl radical is loosely coordinated to one of the Mg atoms via the carbon atom of the newly formed ethyl group. Finally, the hydroxide product ion P1 is generated under evaporation of an ethyl radical. Alternatively, another hydrogen atom transfer occurs from CH 3 -end of the ethyl group to the bridging oxygen via TS2/3 by surmounting a 20 kj/mol barrier, resulting in 3. Subsequently, ethylene dissociates from 3, resulting in the generation of [Mg 2 O 2 H 2 ] +. As shown in Figure S6, the rate-determining step corresponds to the transition state TS HAT which is accessible under thermal conditions, in line with our experimental results. S5

6 3. References (1) Li, J.; Zhou, S.; Wu, X.-N.; Tang, S.; Schlangen, M.; Schwarz, H. Angew. Chem. Int. Ed. 2015, 54, (2) Li, J.; Wu, X.-N.; Zhou, S.; Tang, S.; Schlangen, M.; Schwarz, H. Angew. Chem. Int. Ed. 2015, 54, (3) Li, J.; Wu, X.-N.; Schlangen, M.; Zhou, S.; González-Navarrete, P.; Tang, S.; Schwarz, H. Angew. Chem. Int. Ed. 2015, 54, (4) Kummerlöwe, G.; Beyer, M. K. Int. J. Mass. Spectrom. 2005, 244, (5) Su, T.; Bowers, M. T. J. Chem. Phys. 1973, 58, (6) Bowers, M. T.; Laudenslager, J. B. J. Chem. Phys. 1972, 56, (7) Schröder, D.; Roithová, J. Angew. Chem. Int. Ed. 2006, 45, S6

7 4. SCHEMES Scheme S1. The most-stable rhombus-like structures of [XYO2]+ (X, Y = Mg, Al, Si) calculated at the B2GP-PLYP/def2-TZVP level of theory. S7

8 5. FIGURES Figure S1. Mass spectra for the thermal reaction of [Mg 2 O 2 ] + with a) Ar, b) C 2 H 6, c) CH 3 CD 6 and d) C 2 D 6, at a pressure of mbar after a reaction time of 15s. * represents [Mg 2 O 2 H(H 2 O)] +. The unit for the x axes is always m/z. Figure S2. Mass spectra for the thermal reaction of [Si 2 O 2 ] + with a) Ar, b) C 3 H 8, and c) C 3 D 8 at a pressure of mbar after a reaction time of 5s, 5s and 10s, respectively. The unit for the x axes is always m/z. S8

9 Figure S3. Potential energy surfaces for the reaction of [Mg2O2] + with CH4 at the CCSD(T)/CBS[AVTZ:AVQZ]//B2GP-PLYP/def2-TZVP level of theory. Key bond lengths (Å) are also given. The inset shows the ground-state structure of [Mg2O2] +. S9

10 Figure S4. Potential energy surfaces for the reaction of [Si2O2] + with CH4 at the CCSD(T)/CBS[AVTZ:AVQZ]//B2GP-PLYP/def2-TZVP level of theory. Key bond lengths (Å) are also given. The inset shows the ground-state structure of [Si2O2] +. The yellow isosurface indicates the NBO-calculated spin density distribution. S10

11 Figure S5. Potential energy surfaces for the reaction of [Mg2O2] + with C2H6 at the CCSD(T)/CBS[AVTZ:AVQZ]//B2GP-PLYP/def2-TZVP level of theory. Key bond lengths (Å) are also given. S11

12 6. TABLES Table S1. NBO charges development along the reaction coordinates in the PCET and HAT pathways of the [XYO 2 ] + /CH 4 couples (X, Y = Mg, Al, Si). SR, isolated oxide cluster and substrate; TS PCET, transition state in the PCET pathways; TS HAT, transition state in the HAT pathways. O b, the hydrogen abstracting atom; O, the spectator oxygen; H t, the transferring hydrogen. CHARGE R1 R2 R3 R4 R5 R6 R7 R8 R9 SR X Y O b O C H t H H H TS PCET X Y O b O C H t H H H TS HAT X Y O b O C H t H H H S12

13 Table S2. NBO spin density development along the reaction coordinates in the PCET and HAT pathways of the [XYO 2 ] + /CH 4 couples (X, Y = Mg, Al, Si). SR, isolated oxide cluster and substrate; TS PCET, transition state in the PCET pathways; TS HAT, transition state in the HAT pathways. O b, the hydrogen abstracting atom; O, the spectator oxygen; H t, the transferring hydrogen. SPIN R1 R2 R3 R4 R5 R6 R7 R8 R9 SR X Y O b O C H t H H H TS PCET X Y O b O C H t H H H TS HAT X Y O b O C H t H H H S13

14 Table S3. Relative energies (kj mol -1 ) of key stationary points on the HAT and PCET pathways of the [XYO 2 ] + /CH 4 (X, Y = Al, Si, Mg) systems calculated at the B2GP- PLYP/def2-TZVP level of theory. The energies are given relative to the separated reactants, i.e. [XYO 2 ] + and CH 4. EC: encounter complex; TS PCET and TS HAT : transition structures of the HAT and PCET reactions, respectively; I PCET and I HAT : intermediates following TS PCET and TS HAT, respectively. Numbers in brackets correspond to the results obtained from calculations at the CCSD(T)/CBS[AVTZ:AVQZ]//B2GP-PLYP/def2-TZVP level of theory. a) X Y EC TS PCET I PCET TS HAT I HAT R1 Mg Mg (55) (15) -81 R2 Mg Al -96 4(26) (124) 9 R3 Mg Si (56) (113) 5 R4 Al Mg (-43) (124) 9 R5 Al Al (-28) (97) 28 R6 Al Si (7) (103) 124 R7 Si Mg (80) (113) 5 R8 Si Al (99) (103) 124 R9 Si Si (67) (100) 34 a) See Scheme 1 for assignments of the [XYO 2 ] + /CH 4 couples. S14

15 7. Cartesian coordinates of species calculated in this report. Note: Cartesian coordinates (x, y, z) are in Å. E B2GP-PLYP : the uncorrected QM energy, obtained from the geometry optimizations at the B2GP- PLYP/def2-TZVP level of theory with dispersion correction. H thermal : the thermal enthalpy correction including the zero-point energy. E CCSD(T)/CBS : the single-point energy, calculated at the CCSD(T)/CBS[AVTZ:ZVQZ] level of theory by using the B2GP-PLYP structure. 7.1 Separated reactants and products CH 4 C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = CH 3 C H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = C 2 H 6 C H H H C H H H E B2GP- PLYP = H thermal = E CCSD(T)/CBS = C 2 H 5 C H H S15

16 C H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = C 2 H 4 C H H C H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = [Mg 2 O 2 ] + Mg Mg O O E B2GP-PLYP = H thermal = E CCSD(T)/CBS = [MgAlO 2 ] + and [AlMgO 2 ] + Mg Al O O E B2GP-PLYP = H thermal = E CCSD(T)/CBS = [MgSiO 2 ] + and [SiMgO 2 ] + Mg Si O O E B2GP-PLYP = H thermal = E CCSD(T)/CBS = [Al 2 O 2 ] + Al S16

17 Al O O E B2GP-PLYP = H thermal = E CCSD(T)/CBS = [AlSiO 2 ] + and [SiAlO 2 ] + Al Si O O E B2GP-PLYP = H thermal = E CCSD(T)/CBS = [Si 2 O 2 ] + Si Si O O E B2GP-PLYP = H thermal = E CCSD(T)/CBS = [Mg 2 O 2 H] + (P1) of [Mg 2 O 2 ] + /CH 4 /C 2 H 6 O Mg O Mg H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = [Al 2 O 2 H] + (P1) of [Al 2 O 2 ] + /CH 4 Al Al O O H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = [Al 2 O 2 H] + (P2) of [Al 2 O 2 ] + /CH 4 Al Al S17

18 O O H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = [Si 2 O 2 H] + (P1) of [Si 2 O 2 ] + /CH 4 O Si O Si H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = [Mg 2 O 2 H 2 ] + (P2) of [Mg 2 O 2 ] + /C 2 H 6 Mg Mg O O H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = Encounter complexes, intermediates, and transition states [Mg 2 O 2 ] + /CH 4 EC Mg Mg O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = TS HAT Mg Mg S18

19 O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = TS PCET Mg Mg O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = I HAT Mg Mg O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = I PCET Mg Mg O O C H H H S19

20 H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = [MgAlO 2 ] + /CH 4 EC Mg Al O O C H H H H E B2GP-PLYP = H thermal = TS HAT Mg Al O O H C H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = TS PCET Mg Al O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = S20

21 I HAT Mg Al O O H C H H H E B2GP-PLYP = H thermal = I PCET Mg Al O O C H H H H E B2GP-PLYP = H thermal = [MgSiO 2 ] + /CH 4 EC Mg Si O O C H H H H E B2GP-PLYP = H thermal = TS HAT Mg Si O O C H S21

22 H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = TS PCET Mg Si O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = I HAT Mg Si O O C H H H H E B2GP-PLYP = H thermal = I PCET Mg Si O O C H H H H E B2GP-PLYP = H thermal = S22

23 7.2.4 [AlMgO 2 ] + /CH 4 EC Al Mg O O C H H H H E B2GP-PLYP = H thermal = TS HAT Al Mg O O H C H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = TS PCET Al Mg O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = I HAT Mg Al O O H S23

24 C H H H E B2GP-PLYP = H thermal = I PCET Al Mg O O C H H H H E B2GP-PLYP = H thermal = [Al 2 O 2 ] + /CH 4 EC Al Al O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = TS HAT Al Al O O H C H H H E B2GP-PLYP = H thermal = S24

25 E CCSD(T)/CBS = TS PCET Al Al O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = I HAT Al Al O O H H H H C E B2GP-PLYP = H thermal = E CCSD(T)/CBS = I PCET Al Al O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = [AlSiO 2 ] + /CH 4 EC Al S25

26 Si O O C H H H H E B2GP-PLYP = H thermal = TS HAT Al Si O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = TS PCET Al Si O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = I HAT Si Al O O C H H H S26

27 H E B2GP-PLYP = H thermal = I PCET Al Si O O C H H H H E B2GP-PLYP = H thermal = [SiMgO 2 ] + /CH 4 EC Si Mg O O C H H H H E B2GP-PLYP = H thermal = TS HAT Si Mg O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = TS PCET Si S27

28 Mg O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = I HAT Mg Si O O C H H H H E B2GP-PLYP = H thermal = I PCET Si Mg O O C H H H H E B2GP-PLYP = H thermal = [SiAlO 2 ] + /CH 4 EC Si Al O O C H H S28

29 H H E B2GP-PLYP = H thermal = TS HAT Si Al O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = TS PCET Si Al O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = I HAT Si Al O O C H H H H E B2GP-PLYP = H thermal = I PCET Si S29

30 Al O O C H H H H E B2GP-PLYP = H thermal = [Si 2 O 2 ] + /CH 4 EC Si Si O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = TS HAT Si Si O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = TS PCET Si Si O O C H S30

31 H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = I HAT Si Si O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = I PCET Si Si O O C H H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = [Mg 2 O 2 ] + /C 2 H 6 1 Mg Mg O O C H H H C H H S31

32 H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = TS HAT Mg Mg O O C H H H C H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = Mg Mg O O C H H H C H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = TS 2/3 Mg Mg O O C H H H S32

33 C H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = Mg Mg O O C H H H C H H H E B2GP-PLYP = H thermal = E CCSD(T)/CBS = S33