Catalytic Dehydrogenation of Liquid Organic Hydrogen Carrier Model Compounds by CpM⁺(M = Fe, Co, Ni) in the Gas Phase

Although there is much interest in developing liquid organic hydrogen carriers (LOHCs) and new metal catalysts to release hydrogen on demand for application in energy generation, few studies have provided mechanistic insights into the crucial metal-catalyzed dehydrogenation reactions. Here we use multistage mass spectrometry experiments and DFT calculations to examine the dehydrogenation of the LOHC model compounds cyclohexane, pyrrolidine, N-methylpyrrolidine, and piperidine by the half-sandwich cyclopentadienyl cations, [CpM]⁺(M = Fe, Co, and Ni). The [CpM]⁺ions were generated via collision-induced dissociation (CID) of precursor ions generated via electrospray ionization (ESI) of solutions of the compounds CpFe(CO)₂I, Cp₂Ni, and Cp₂Co. The [CpFe]⁺ion was found to catalyze triple dehydrogenation of cyclohexane to benzene via a combination of ion-molecule reactions (IMR) and CID experiments. Reactions of [CpM]⁺(M = Co, Ni) were found to be more complex as dehydrogenation was accompanied by significant cyclohexane ring cleavage. Reactions of [CpFe]⁺with the N-heterocyclic model compounds showed catalytic double dehydrogenation of pyrrolidine/N-methylpyrrolidine and triple dehydrogenation of piperidine. However, multiple side reactions were found, especially in the case of N-methylpyrrolidine and piperidine. They included proton transfer/hydride abstraction to/from the LOHC model methyl loss from the N-methylpyrrolidine complex, and NH₂/NH₃extrusion from the piperidine complex. DFT calculations shed light on the structure of the key intermediates in all these systems.