The rate constant &R for the highly exothermic, spin-allowed reaction F e O + ( W ) + H2 -Fe+(6D) + H 2 0 is 2 orders of magnitude smaller than the collision rate constant &L (&R = 1.6 X 10-11 cm3 molecule -l s-I; kL = 1.5 X cm3 molecule-' s-l). Labeling experiments using HD and D2 reveal that no strong intermolecular kinetic isotope effects are operative on the rate constant of this reaction. Extremely low reactivities are also observed for the reactions of Coo+ and NiO+ with H2 ( k~ = 1.2 and 2.1 X 10-12 cm3 molecule-l s-l), and there are no kinetic isotope effects within experimental error in the reactions of these metal oxides with D2 as well. The experimental data and theoretical considerations point to a reaction mechanism involving a multicentered transition structure in the rate-determining step.
IntroductionThe activation of C-H and C-C bonds of organic molecules by ionic metal oxides, in particular iron oxenoids, is of fundamental interest in organic chemistry as well as biochemistry.] Recently, the C-H bond activation of a series of hydrocarbons? including methane and benzene, by bare FeO+ in the gas phase3 was reported, and mechanistic insight in the elementary steps of these reactions was provided. Here, we describe experiments on the gas-phase reactions of FeO+ with molecular hydrogen. This process, which is analogous to the thoroughly studied reactions of the early transition-metal oxides MO+ (M = Sc, Ti, and V)4 with D2, is, for at least two reasons, of particular importance: (i) In thedetailed study from the Armentrout group4 it is convincingly demonstrated that-in addition to reaction energetics-spin conservation is the prime factor in oxygenation processes. As the ground electronic state of FeO+ corresponds to the W state,5 one should expect that the exothermic, spin-allowed oxidation of molecular hydrogen (eq 1) is a facile process.(ii) If this supposition holds true, one can expect that the simple redox pair described in eq 1 forms the basis for a catalytic cycle (Scheme 1),8 in which molecular hydrogen is oxidized to water by means of FeO+, which itself is formed by reacting Fe+ with convenient oxidants, e.g., N Z O .~~ Although the combustion process H2 + ' / 2 0 2 -H20 is highly exothermic, this reaction is, as is amply known, kinetically hampered.For the purpose of comparison we have also conducted some preliminary experiments with, in terms of electronic structure, the less properly characterized metal oxides MO+ (M = Co, Ni).
Experimental SectionThe experiments were performed with a Spectrospin CMS 47X Fourier transform ion-cyclotron-resonance (FTICR) mass spectrometer; the experimental setup has been described in detail elsewhereO9 In brief, the metal ions M+ (M = Fe, Co. Ni) were generated by laser desorption/laser ionization by focusing the beam of a Nd:YAG laser onto a metal target. The cations were extracted from the source and transferred to the analyzer cell by a system of electric potentials and lenses. The isolation of the metal ions most abundant isotope and all subsequent iso...
