Structure and reactivity of the prototype iron-oxide cluster Fe2O2+

The iron-oxide cluster Fe2O2+ is synthesized in the gas phase from a mixture of Fe(CO)(5) and O-2, and its gas-phase reactivity is subsequently probed using sector-field and Fourier-transform mass spectrometers. The experimental findings are in accord with, but do not establish that Fe2O2+ has a rhombic structure with two equivalently bound oxo-ligands. Although the reactivity studies conducted with Fe2O2+ are by and large consistent with previous literature data and the few thermochemical data available for ligated iron clusters, a severe discrepancy exists for the Fe2O2+/C2H4 couple. Although exclusive dehydrogenation to afford Fe2C2H2O2+ has been reported in a previous work [Gehret and Irion, Chem. fur. J. 2 (1996) 598], this product constitutes a very minor channel in the present study; instead, various C-H and C-C bond activation processes as well as O-atom transfer from Fe2O2+ to ethene are observed. A possible explanation is that two isomers and/or states of Fe2O2+ cation were probed in the different experiments, thereby highlighting an important structural ambiguity in reactivity studies of transition-metal clusters. Assuming that the most stable Fe2O2+ species is monitored in the present experiments, the reactions observed in combination with complementary thermochemical information imply heals of formation of Delta H-f(o)(Fe2O2- -32 +/- 12 kcal/mol, Delta H-f(o)(Fe2O2) = 22 +/- 12 kcal/mol, and Delta H-f(o)(Fe2O2+) = 216 +/- 9 kcal/mol for anionic, neutral, and cationic di-iron dioxide. (Int J Mass Spectrom 204 (2001) 233-245) (C) 2001 Elsevier Science B.V.