# From C─H Bond Insertion to Hydrogen Atom Transfer: Tuning the Reaction Mechanisms of Methane Activation by the Oxidation of Ta2 +

## TL;DR

This study shows how oxidation changes the way tantalum compounds react with methane, switching from dehydrogenation to hydrogen atom transfer.

## Contribution

The paper reveals a new hydrogen atom transfer mechanism in tantalum oxides and links it to spin density on oxygen atoms.

## Key findings

- Ta2O2+ promotes methane dehydrogenation via Ta–C–Ta bridging motifs.
- Ta2O5+ enables hydrogen atom transfer driven by oxygen spin density.
- Oxidation tunes tantalum compounds' reactivity from metallic to oxidic behavior.

## Abstract

The activation of methane under mild conditions is a challenging but rewarding goal; the underlying key parameters, however, remain elusive. In this study on isolated tantalum Ta2
+ compounds exposed to methane in a ring‐electrode ion trap, strong changes in the reactivity are observed depending on the compound's degree of oxidation. While the general reaction behavior is presented for species ranging from Ta2
+ to Ta2O6
+ based on experimental kinetic studies, we focus in more detail on the dehydrogenation reactions occurring on Ta2O2
+ and the hydrogen atom transfer (HAT) on Ta2O5
+, for which density functional theory calculations were performed. In the first part, we elucidate the role of Ta–C–Ta bridging motifs in product structures as driving forces for the dehydrogenation of methane on Ta2O2
+; in the second part, we investigate the origins of the HAT – a hitherto unknown reaction scheme for binary tantalum oxides. For the latter, we show that the reactivity originates from the spin density on oxygen atoms, which is a typical characteristic of the reaction on other metal oxides. This reflects a change in the reactivity from oxidized metallic systems to metal oxides and demonstrates that chemical modifications of tantalum compounds can achieve different methane activation schemes.

In a combined experimental and theoretical study, it is observed that the continued oxidation from Ta2
+ to Ta2Ox
+ (with x = 1 – 6) changes the reactivity with methane from a typical metallic to a typical oxidic behavior. While the bridging motifs in Ta2O2
+ enable the dehydrogenation of CH4, the spin density on oxygen atoms in Ta2O5
+ facilitates a HAT reaction.