Cracking and Dehydrogenation of Cyclohexane by [(phen)M(X)]⁺ (M = Ni, Pd, Pt; X = H, CH₃) in the Gas Phase

Abstract

Gas-phase cationic ternary complexes of group 10 metals of the formula [(phen)M(X)] + , where phen = 1,10phenanthroline (M = Ni, Pd, or Pt; and X = H or CH 3 ), react with cyclohexane via C−H activation, forming the respective cyclohexyl species [(phen)M(c-C 6 H 11 )] + . Upon collisional activation, these species undergo two key competing processes: (i) ring opening followed by "cracking" of the hydrocarbon chain leading to extrusion of propylene and ethylene as major products among other hydrocarbons; and (ii) dehydrogenation of the cyclohexyl ring leading to the loss of one, two, or three hydrogen molecules, with subsequent loss of cyclohexenes or benzene. The relative prevalence of these two pathways strongly depends on the metal ion, with Pt preferring dehydrogenation over ring opening. The multiple catalytic cycles operating within both pathways are described. Density functional theory (DFT) calculations are used to shed light on mechanistic aspects associated with the experimental results.

Sustainable production of hydrogen with high purity from methanol and water at low temperatures

“…Afterward, various liquid organic carriers have been proposed to store hydrogen in the dense liquid phase, which offers a promising methodology with high capacity and safe re-distributions on demands. Among various liquid hydrogen carriers (formic acid 3 – 5 , N -heteroarenes 6 – 8 , cyclohexane 9 , etc. ), methanol, as a sustainable, inexpensive, and readily available hydrogen source derived from biomass and/or CO 2 hydrogenation, can give 18.8 wt.% H 2 gravimetric density via reforming with H 2 O to release three equivalent amounts of H 2 .…”

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

“…Those include [CpNi­(C 3 H 6 )] + ( m / z 165) and [CpNi­(C 4 H 6 )] + ( m / z 177) corresponding to the neutral losses of undetermined fragment(s) . This observation is consistent with the work of Goll and Müller as well as our recent work showing that Ni­(II) complexes can extrude smaller hydrocarbons from a cyclohexyl ring. Additional peaks at m / z 127 possibly representing [C 5 H 9 Ni] + and its cyclohexane adduct [C 5 H 9 Ni­(C 6 H 12 )] + ( m / z 209) (due to IMR with the neutral present in the ion trap) would have to come from more complex transformations.…”

“…Multiple groups have investigated molecular catalysts involving the dehydrogenation of not only homocyclic compounds, but also N-heterocycles. For example, our recent work showed that the [(phen)­Pt­(H)] + cationic complex (where phen = 1,10-phenathroline) can activate and catalytically dehydrogenate cyclohexane via eq in the gas phase, as shown in Scheme . Søgaard et al have also shown the dehydrogenation of indole derivatives in biphasic (molten salt/organic) conditions with Crabtree’s iridium-based catalyst .…”

Ion‐molecule reactions of mass‐selected ions

“…A similar catalytic process was used by Parker et al (2021) to model the depolymerization of polyethylene in the gas‐phase. Polymerization and depolymerization reactions constitute a large economic field where metal catalysts play a vital role.…”

“…Work recently published by Parker and coworkers (2021) has described the ability of ternary group 10 hydride and methide cationic complexes to activate C–H bonds in cyclohexane and, through subsequent CID experiments, elucidate the ring opening and fragmentation reaction pathways (Equations and ). Differences in products formed was dependent on the identity of the metal used.…”