Thermal Activation of Methane and Ethene by Bare MO^.+ (M=Ge, Sn, and Pb): A Combined Theoretical/Experimental Study

Abstract

The thermal ion/molecule reactions (IMRs) of the Group 14 metal oxide radical cations MO(·+) (M=Ge, Sn, Pb) with methane and ethene were investigated. For the MO(·+)/CH(4) couples abstraction of a hydrogen atom to form MOH(+) and a methyl radical constitutes the sole channel. The nearly barrier-free process, combined with a large exothermicity as revealed by density functional theory (DFT) calculations, suggests a fast and efficient reaction in agreement with the experiment. For the IMR of MO(·+) with ethene, two competitive channels exist: hydrogen-atom abstraction (HAA) from and oxygen-atom transfer (OAT) to the organic substrate. The HAA channel, yielding C(2)H(3·) and MOH(+) predominates for the GeO(·+)/ethene system, while for SnO(·+) and PbO(·+) the major reaction observed corresponds to the OAT producing M(+) and C(2)H(4)O. The DFT-derived potential-energy surfaces are consistent with the experimental findings. The behavior of the metal oxide cations towards ethene can be explained in terms of the bond dissociation energies (BDEs) of MO(+)-H and M(+)-O, which define the hydrogen-atom affinity of MO(+) and the oxophilicity of M(+), respectively. Since the differences among the BDEs(MO(+)-H) are rather small and the hydrogen-atom affinities of the three radical cations MO(·+) exceed the BDE(CH(3)H) and BDE(C(2)H(3)-H), hydrogen-atom abstraction is possible thermochemically. In contrast, the BDEs(M(+)-O) vary quite substantially; consequently, the OAT channel becomes energetically less favorable for GeO(·+) which exhibits the highest oxophilicity among these three group 14 metal ions.

Thermal hydrogen-atom transfer from methane: A mechanistic exercise

“…While the heavier congeners of [MO] •+ , with M = Si, Ge, Sn, and Pb, exhibit features typical for the operation of an indirect, metalmediated HAT [54,55], as shown exemplarily in Figure 4b •+ /CH 4 couple, the reaction proceeds as it has been reported for almost all diatomic metal-oxide cations in an indirect way, the approach of methane at the oxygen site of [CO] •+ triggers a so-called intramolecular spin-density transfer [57] and leads to the eventual translocation of a hydrogen atom to the oxygen atom. Spin-density transfer had also been suggested to account for the efficient thermal HAT in the [SO 2 ] •+ /CH 4 couple [58].…”

Mechanistic Aspects of Gas‐Phase Hydrogen‐Atom Transfer from Methane to [CO].+ and [SiO].+: Why Do They Differ?

“…It prevails predominantly for open-shell oxide clusters mostly with coordinatively saturated metal or non-metal centers in relatively high oxidation states, for example, [SO 2 ]C + , [9] [Ce 2 O 4 ]C + , [10] [V x P 4Àx O 10 ]C + (x = 0, 2-4), [11] [(Al 2 O 3 ) x ]C + (x = 3-5), [12] [VAlO 4 ]C + , [13] or [(V 2 O 5 ) x -A C H T U N G T R E N N U N G (SiO 2 ) y ]C + (x = 1, 2; y = 1-4). [20] In the context of interstellar chemistry, also the reactivity of cationic carbon monoxide [CO]C + towards CH 4 was studied quite some time ago. So far, this mechanism has only been observed for diatomic metal oxides possessing a vacant coor-Keywords: C À H activation · computational chemistry · gas-phase reactions · hydrogen-atom transfer · reaction mechanisms Abstract: The reactivity of the two diatomic congeneric systems [CO]C + and [SiO]C + towards methane has been investigated by means of mass spectrometry and quantum-chemical calculations.…”

“…On the other hand, this pathway has to compete with the thermochemically much more preferred abstraction of the hydrogen atom by the C site of [CO]C + , for which no intramolecular spin-density transfer is required. [20] In the subsequent transition state TS7-8, the C À H bond scission occurs at the reactive oxo site, followed by a recoil of the newly formed methyl group, thus forming the intermediate [CH 3 C···Si-OH] + (8). Furthermore, the cleavage of the C À H bond of methane is with r(C-H) = 1.539 much more advanced compared to the one in intermediate 2.…”

“…In intermediate 4, the spin is almost completely transferred to the carbon atom of the methyl group (83 %) with the remaining spin localized at the carbon atom of the CO fragment. [20] The overall HAT reaction is completed by the liberation of the CH 3 · radical from 8 to form oxygen-protonat-ed silicon-oxide 9; the computed overall exothermicity amounts to À174 kJ mol À1 . Finally, while 4 is about 94 kJ mol À1 lower in energy than 2, the expulsion of the methyl radical to complete the reaction requires only 63 kJ mol À1 , much less energy-demanding than the alternative route.…”

High reactivity of nanosized niobium oxide cluster cations in methane activation: A comparison with vanadium oxides

“…14, 15 The active species include many metal [16][17][18][19][20][21][22][23][24][25] (such as vanadium, V) or heteronuclear (such as X-V, in which X = Al, 26 Si, 27 P, 28,29 Y, 30 or Nb 31 ) oxide cluster cations and a few anionic ones, such as La 6 O 10 −32 and PtAl 2 O 4 − . 33 Most of these oxide clusters are with particular stoichiometry to satisfy ∆ = 1, in which ∆ is defined as ∆ = 2 y − nx + q for an oxide cluster M x O y q to indicate its oxygen-richness or poorness (q is the charge number and n counts the highest oxidation state of element M).…”