Chromium Dioxide Cation OCrO+ in the Gas Phase: Structure, Electronic States, and the Reactivity with Hydrogen and Hydrocarbons1

The potential-energy surface of the triatomic cation [Cr,O 2 ] + has been examined by means of ion-cyclotron resonance and sector-field mass spectrometry as well as high-level theoretical methods. Chromium(V) dioxide cation OCrO + can be generated by electron ionization of CrO 2 Cl 2 and exhibits a doublet ground state. The experimentally determined IE(CrO 2 ) of 9.7 ± 0.2 eV leads to Δ H f (OCrO + ) = 209 ± 12 kcal/mol which compares well with a theoretical prediction of 217 kcal/mol. The high-valent chromium(V) dioxide OCrO + slowly reacts with H 2 to form CrO + and H 2 O as products. Activation of saturated and unsaturated hydrocarbons, including methane and benzene, is much more efficient and involves C−H as well as C-C bond activation. In the reaction of OCrO + with CH 4 the by-product OCr(OCH 2 ) + points to the operation of a stepwise mechanism in that initially a single oxo unit in OCrO + is activated in terms of a [2 + 2] cycloaddition of methane across the Cr−O double bond. A structurally different [Cr,O 2 ] + cation can be generated by chemical ionization of a mixture of Cr(CO) 6 and O 2 . The experimental findings together with the computations propose that the ions formed consist of a mixture of doublet and quartet states of the dioxide OCrO + and the dioxygen complex Cr(O 2 ) + ( 6 A‘‘); in the latter the dioxygen unit is end-on coordinated to the metal. Due to spin conservation, the direct formation of the doublet ground state OCrO + ( 2 A 1 ) from the ground state reactants Cr + ( 6 S) and O 2 ( 3 Σ g - ) is not possible; rather, two curve crossings from the sextet via the quartet to the doublet surface in the sequence 6 Cr + + 3 O 2 → 6 Cr(O 2 ) + → 4 OCrO + → 2 OCrO + are suggested.