ENERGETICS AND MECHANISM OF THE THERMAL DECARBOXYLATION OF (CO)4FECOOH- IN THE GAS-PHASE

The energetics and mechanism of decarboxylation of (CO)4FeCOOH- to form CO2 and (CO)4FeH-, a key step in the Fe(CO)5-catalyzed water-gas shift reaction, is investigated using the flowing after glow-triple quadrupole technique. Previous studies of collisional activation of (CO)4FeCOOH- in the gas phase showed only loss of CO ligands, suggesting that base catalysis is necessary for decarboxylation. We have now observed gas-phase decarboxylation of this hydroxycarbonyl ion using energy-resolved collision-induced dissociation. Decarboxylation competes with decarbonylation at translational energies near the reaction threshold, indicating that unimolecular beta-elimination of CO2 can occur. Loss of two carbonyl ligands to form (CO)3FeOH- is the dominant process at somewhat higher energies. The thresholds for loss of CO, 2CO, and CO2 are 21.4 +/- 3.9, 30.2 +/- 2.8, and 18.9 +/- 3.2 kcal/mol, respectively. The latter number corresponds to a barrier for an exothermic reaction. DH[Fe(CO)5-OH-] = 60.8 +/- 3.4 kcal/mol is determined by measurement of the equilibrium constant for hydroxide exchange between Fe(CO)5 and SO2. (CO)4FeSOOH- + CO2 is formed as a side product of this reaction, and the structure of this species is investigated. These data are combined with other thermochemistry to derive a model reaction-energy profile for the Fe(CO)5-catalyzed water-gas shift reaction.