Sequential Gas-Phase Activation of Carbon Dioxide and Methane by [Re(CO)₂]⁺: The Sequence of Events Matters!

The potential of carbonyl rhenium complexes in activating and coupling carbon dioxide and methane has been explored by using a combination of gas-phase experiments (FT-ICR mass spectrometry) and high-level quantum chemical calculations. While the complexes [Re(CO)_x]⁺ (x = 0, 1, 3) are thermally unreactive toward CO₂, [Re(CO)₂]⁺ abstracts one oxygen atom from this substrate spontaneously at ambient conditions. Based on ¹³C and ¹⁸O labeling experiments, the newly generated CO ligand is preferentially eliminated, and two mechanistic scenarios are considered to account for this unexpected finding. The oxo complex [ORe(CO)₂]⁺ reacts further with CH₄ to produce the dihydridomethylene complex [ORe(CO)(CH₂)(H)₂]⁺. However, coupling of the CO and CH₂ ligands to form CH₂═C═O does not take place. Further, the complexes [Re(CO)_x(CH₂)]⁺ (x = 1, 2), generated in the thermal reaction of [Re(CO)_x]⁺ (x = 1, 2) with CH₄, are inert toward CO₂. Mechanistic insight on the origin of this remarkable reactivity pattern has been derived from detailed quantum chemical calculations.