Reactivity and photochemistry of transition-metal-bound aromatic compounds in the gas phase.

Abstract

The mechanisms and energetics of gas phase reactions between aromatic and heteroaromatic compounds and the groups three and five transition metal ions have been studied by Fourier Transform ion cyclotron resonance mass spectrometry. The results have application to arene exchange, arene activation, hydrodenitrogenation (HDN), hydrodesulphanation (HDS), and cyclotrimerization in these systems. A photoactivation pathway for arene displacement with simple Lewis bases has been demonstrated. It is possible to distinguish both associative and dissociative mechanisms. For example, V(C₆H₆)₂⁺ reacts by a dissociative mechanism, undergoing initial photodissociation to form V(C₆H₆)⁺ which slowly attaches CH₃CN to form V(C₆H₆) (CH₃CN)⁺. CH₃CN does not displace benzene from V(C₆H₆)⁺ at thermal energies. However, upon photoexcitation at 604 nm, [V(C₆H₆)⁺]* undergoes associative ligand displacement to generate V(CH₃CN)⁺. The associative reaction occurs on a single minimum potential surface with an endothermic barrier. Tantalum cation activates benzene ring cleavage at thermal energies. The other group three and group five metals studied (with the exception of vanadium) form metal coordinated benzynes by dehydrogenation. A ring cleavage pathway becomes accessible for these metals if pyridine is introduced as an alternate substrate. The primary cleavage products are, in all cases, metallacyclopentadienes. Evidence suggests the cleavage mechanisms for benzene and the heterocycles differ. A metallanorbornadiene intermediate is favored for benzene cleavage, while a seven-membered ring lies on the reaction coordinate for heterocyclic ring cracking. Toluene is deactivated to ring cleavage by tantalum, however all metal cations studied eliminate acetonitrile from picoline.