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.