Sequential Gas-Phase Activation of Carbon Dioxide and Methane by [Re(CO)_2]~+: The Sequence of Events Matters!

摘要

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 CO_2, [Re(CO)_2]~+ abstracts one oxygen atom from this substrate spontaneously at ambient conditions. Based on ~(13)C and ~(18)O 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 CH_4 to produce the dihydridomethylene complex [ORe(CO)(CH_2)(H)_2]~+. However, coupling of the CO and CH_2 ligands to form CH_2=C=O does not take place. Further, the complexes [Re(CO)_x(CH_2)]~+ (x = 1, 2), generated in the thermal reaction of [Re(CO)_x]~+ (x = 1, 2) with CH_4, are inert toward CO_2. Mechanistic insight on the origin of this remarkable reactivity pattern has been derived from detailed quantum chemical calculations.