Reactivity of [CpFe(CO)2]+ with Nitrogen-Containing Heterocyclic Compounds in the Gas Phase: Ligand Exchange and Dehydrogenation.

Abstract

A previous gas-phase study has uncovered formal catalytic cycles for the dehydrogenation of model liquid organic hydrogen carriers (LOHCs) pyrrolidine, N-methylpyrrolidine, and piperidine by the coordinatively unsaturated half-sandwich cyclopentadienyl iron cation, [CpFe]⁺. That work is extended here to the well-known condensed-phase [CpFe(CO)₂]⁺ cation, which was generated via electrospray ionization for gas-phase reactions with model LOHCs in a linear ion trap mass spectrometer, in which the helium bath gas was seeded with 0.1% carbon monoxide. The initial ion-molecule reaction (IMR) was exothermic enough to expel one CO molecule from the complex to form [CpFe(CO)L]⁺ (L = pyrrolidine, N-methylpyrrolidine, or piperidine). Collision-induced dissociation (CID) of these cations revealed two fragmentation pathways: (i) removal of another CO molecule leading to the species [CpFeL]⁺ that was studied previously; (ii) dehydrogenation of the ligand L (except for L= N-methylpyrrolidine). Two new formal catalytic cycles (for dehydrogenation of pyrrolidine and piperidine) were found that operate via a combination of IMR and CID experiments and which rely on the presence of CO for re-ligation of iron complexes. Density functional theory calculations were performed to compute the structures of all species observed as well as the reaction energetics.