GAS-PHASE ION CHEMISTRY OF (ETA(6)-C-C7H8)CR(CO)(3) BY FT-ICR SPECTROSCOPY - BETA-HYDRIDE TRANSFER IN ORGANOMETALLIC IONS

The gas-phase ion chemistry of eta(6)-cycloheptatriene chromium tricarbonyl was studied by Fourier transform ion cyclotron resonance spectroscopy. The well-known trend that fragment-ion reactivity toward the parent neutral increases with increasing electron deficiency on the metal atom was, in general, observed. However, the reactivity of the fragment ions from (eta(6)-c-C7H8)Cr(CO)(3) shows the order Cr+ > C5H6Cr+ > C7H8Cr+ approximate to C7H8Cr(CO)(+) > C7H8Cr(CO)(2)(+) > C7H8Cr(CO)(3)(+), in which C7H8Cr+ has an anomalously low reactivity. Similarly, the secondary ion-molecule product ion, (C7H8Cr)(2)(+), has an anomalously low reactivity when compared with that of (C7H8Cr)(2)(CO)(3)(+). A more quantitative analysis indicates that the ions C7H8Cr+, C7H8Cr2(CO)(+) and C7H8Cr2(CO)(2)(+) are less reactive than expected. All of these low reactivities can be explained by a beta-hydride shift from the cycloheptatriene ring to the chromium atom that converts the cycloheptatriene ligand into an aromatic tropylium ligand. The coordination hapticity to chromium changes from eta(6) to eta(7) + eta(1) and the electron deficiency of chromium drops by 2. Similarly, the reactivity of the fragment ion C5H6Cr+ implies it has a hydrido-pi-cyclopentadienyl structure. An additional driving force for these rearrangements appears to be the aromatization of the seven-membered cycloheptatriene ring to a tropylium ring and the aromatization of the five-membered cyclopentadiene ring to a cyclopentadienyl ring, as the fragment and secondary ion reactivity orders are normal in the otherwise analogous benzene chromium tricarbonyl and cyclopentadienyl vanadium tetracarbonyl systems. These latter systems have no possible corresponding aromatization reaction. Ions that have low reactivity and have metal-hydrogen bonds react with deuterium-labeled methanol to eliminate a molecule of molecular hydrogen in which one hydrogen atom comes from the hydroxyl group of methanol and the other hydrogen atom comes from the organometallic ion. In contrast, ions that have normal reactivity and do not have metal-hydrogen bonds give methanol adduct products when reacted with methanol.