The dramatic effect of NH 3 co‐ligation on the FE + ‐assisted activation of carbon dioxide in the gas phase: From bare metal ions to complexes

The catalytic efficiency of Fe + ion over the CO 2 decomposition in the gas phase has been extensively investigated with the help of electronic structure calculation methods. Potential‐energy profiles for the activation process Fe + + CO 2 → CO + FeO + along two rival potential reaction paths, namely the insertion and addition pathways , originating from the end‐on κ 1 ‐O and κ 2 ‐O,O coordination modes of CO 2 with the metal ion, respectively, have been explored by DFT calculations. For each pathway the potential energy surfaces of the high‐spin sextet ( S = 5/2) and the intermediate‐spin quartet ( S = 3/2) spin‐states have been explored. The complete energy reaction profile calculated by a combination of ab initio and density functional theory (DFT) computational techniques reveals a two‐state reactivity, involving two spin inversions, for the decomposition process and accounts well for the experimentally observed inertness of bare Fe + ions towards CO 2 activation. Furthermore, the coordination of up to three extra ancillary NH 3 ligands with the Fe + metal ion has been explored and the geometric and energetic reaction profiles of the CO 2 activation processes Fe + + n ·NH 3 + CO 2 → [Fe(NH 3 ) n (CO 2 )] + → [Fe(NH 3 ) n (O)(CO)] + → CO + [Fe(O)(NH 3 ) n ] + ( n = 1, 2 or 3) have thoroughly been scrutinized for both the insertion and the addition mechanisms. Inter alia , the geometries and energies of the various states of the [Fe(NH 3 ) n (CO 2 )] + and [Fe(NH 3 ) n (O)(CO)] + complexes are explored and compared. Finally, a detailed analysis of the coordination modes of CO 2 in the cationic [Fe(NH 3 ) n (CO 2 )] + ( n = 0, 1, 2 and 3) complexes is presented. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008