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

Abstract

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/NH3 extrusion from the piperidine complex. DFT calculations shed light on the structure of the key intermediates in all these systems.

Thermodynamic evaluations of the acceptorless dehydrogenation and hydrogenation of pre-aromatic and aromatic N-heterocycles in acetonitrile

“…The 28 pre-aromatic N-heterocycles included 6H 2 (−0.4 kcal mol −1 ), 7H 2 (−0.8 kcal mol −1 ), 8H 2 (2.9 kcal mol −1 ), 15H 2 (2.7 kcal mol −1 ), 23H 2 (−1.9 kcal mol −1 ), 24H 2 (0.6 kcal mol −1 ), 29H 2 (1.9 kcal mol −1 ), 30H 2 (−0.2 kcal mol −1 ), 31H 2 (3.5 kcal mol −1 ), 32H 2 (−0.1 kcal mol −1 ), 38H 2 (0.7 kcal mol −1 ), 40H 2 (2.8 kcal mol −1 ), 41H 2 (0.8 kcal mol −1 ), 46H 2 (−2.5 kcal mol −1 ), 48H 2 (−1.1 kcal mol −1 ), 49H 2 (0.2 kcal mol −1 ), 51H 2 (−0.1 kcal mol −1 ), 52H 2 (−1.3 kcal mol −1 ), 53H 2 (3.0 kcal mol −1 ), 61H 2 (0.3 kcal mol −1 ), 64H 2 (−2.3 kcal mol −1 ), 65H 2 (−1.4 kcal mol −1 ), 66H 2 (1.6 kcal mol −1 ), 68H 2 (2.4 kcal mol −1 ), 70H 2 (0.6 kcal mol −1 ), 72H 2 (1.0 kcal mol −1 ), 76H 2 (2.2 kcal mol −1 ), and 78H 2 (0.9 kcal mol −1 ), which need further validation and support in experimental work. Examining the chemical structures of the investigated N-heterocycles in previous literature, 3–19 they were not exactly the same pre-aromatic N-heterocycle structure, and most cases involved two H 2 molecules release and acceptance from N-heterocycles, which could not provide direct experimental data to validate the thermodynamic model.…”

“…From Scheme 5, the following valuable conclusions could be made: (1) the Δ G H 2 R ( Y I H 2 ) scale covered from −15.8 kcal mol −1 to 22.0 kcal mol −1 , which spanned the widest thermodynamic range by 37.8 kcal mol −1 among the 4 groups of amines Y I H 2 –Y IV H 2 ; (2) according to the Δ G H 2 R ( YH 2 ) scales of Y II H 2 (−11.5 to 8.5 kcal mol −1 ), Y III H 2 (−7.4 to 9.5 kcal mol −1 ), and Y IV H 2 (10.3–18.3 kcal mol −1 ), the dehydrogenation abilities decreased in the order of Y II H 2 ≈ Y III H 2 > Y IV H 2 ; (3) in view of the relations between sets, it was found that the Δ G H 2 R ( YH 2 ) values displayed the following regular pattern of {Δ G H 2 R ( Y II H 2 ) ∪ Δ G H 2 R ( Y III H 2 ) ∪ Δ G H 2 R ( Y IV H 2 )} ⊆ Δ G H 2 R ( Y I H 2 ); (4) for the pre-aromatic N-heterocycles, the Δ G H 2 R ( YH 2 ) scales ranged from −15.8 to 22.0 kcal mol −1 for Y I H 2 , from −11.5 to 8.5 kcal mol −1 for Y II H 2 , and from −7.4 to 9.5 kcal mol −1 for Y III H 2 , respectively. Since the Δ G H 2 R ( YH 2 ) scales of Y I H 2 , Y II H 2 , and Y III H 2 crossed negative and positive values (−15.8 to 22.0 kcal mol −1 ), it was indicated that not all the acceptorless dehydrogenation of pre-aromatic N-heterocycles is a thermodynamically uphill or downhill process under ambient conditions, 3–19 and not all pre-aromatic N-heterocycles are thermodynamically feasible to serve as chemical hydrogen-storage materials. Δ G H 2 R ( YH 2 ) is absolutely an important thermodynamic parameter to guide chemists to discover more potentially excellent chemical hydrogen-storage materials.…”

“…Many groups have focused on the reversible dehydrogenation and hydrogenation of pre-aromatic and aromatic N-heterocycles, mainly including discovering better pre-aromatic N-heterocycles carriers and developing novel metal–organic catalysts. 1–19 Since we have obtained so much valuable thermodynamic data of pre-aromatic N-heterocycles dehydrogenation and aromatic N-heterocycles hydrogenation, it seemed meaningful to clarify why the reversible dehydrogenation and hydrogenation of pre-aromatic and aromatic N-heterocycles could happen, as well as why they possess the typical thermodynamic feature of the corresponding pre-aromatic N-heterocycles.…”

“…In fact, N-heterocycles have already been verified as an important type of chemical hydrogen-storage materials. 12–19 The acceptorless dehydrogenation and hydrogenation of N-heterocycles as organic hydrogen carriers have been broadly studied, mainly focusing on the catalyst synthesis and design, investigation of the redox mechanisms, and extension of substrate scope. 3–19…”

“…Undoubtedly, this work focuses on new thermodynamic parameters, which is an important supplement to our previous work to offer precise insights into the chemical hydrogen storage and hydrogenation reactions of pre-aromatic N-heterocycles. 1–19…”