Modeling Metal-Catalyzed Polyethylene Depolymerization: [(Phen)Pd(X)]⁺ (X = H and CH₃) Catalyze the Decomposition of Hexane into a Mixture of Alkenes via a Complex Reaction Network

Abstract

The ternary Pd complexes [(phen)­Pd­(H)]+ (1-Pd) and [(phen)­Pd­(CH3)]+ (5-Pd) (where phen = 1,10-phenanthroline) both react with hexane in a linear ion trap mass spectrometer, forming the C–H activation product [(phen)­Pd­(C6H11)]+ (3-Pd) and releasing H2 and CH4, respectively. Density functional theory (DFT) calculations agree well with the experiments in predicting low barriers for these reactions proceeding via a metathesis mechanism. Species 3-Pd undergoes extensive fragmentation, or “cracking”, of the hydrocarbon chain when sufficient energy is supplied via collision-induced dissociation (CID), resulting in the extrusion of a mixture of alkenes, methane, and hydrogen. DFT calculations show that Pd “chain-walking” from α (terminal carbon) to β and from β to γ positions can proceed with barriers sufficiently below those required for chain “cracking”. The fragmentation reactions can be made catalytic if 1-Pd and 5-Pd produced by CID of 3-Pd are allowed to react with hexane again. Ni complexes largely mirrored the chemistry observed for Pd. Both 1-Ni and 5-Ni reacted with hexane, forming 3-Ni, which fragmented under CID conditions in a fashion similar to 3-Pd. In contrast, only 5-Pt reacted with hexane to form 3-Pt, which fragmented predominantly via sequential losses of H2.

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

“…Similar to the work of the Schwarz group on alkane dehydrogenation using related Pt complexes, we found that these complexes can catalyze dehydrogenation of ethane (Scheme , left cycle) . We also found that for hexane, cracking of the chain via C–C bond activation reactions can compete with dehydrogenation; Ni and Pd were found to favor cracking (Scheme , right cycle), while Pt favored dehydrogenation . Here, we explore the activation of cyclohexane by [(phen)­M­(X)] + complexes.…”

“…23 We also found that for hexane, cracking of the chain via C−C bond activation reactions can compete with dehydrogenation; Ni and Pd were found to favor cracking (Scheme 1, right cycle), while Pt favored dehydrogenation. 24 Here, we explore the activation of cyclohexane by [(phen)M(X)] + complexes. Since the C−H bond dissociation energy in cyclohexane is similar to those of the 2-and 3positions in hexane, it is highly likely that [(phen)M(X)] + will activate C−H bonds in cyclohexane.…”

“…The cyclic nature of some products found in naphtha causes mechanistic differences in the C−H and C−C bond activation when compared to linear hydrocarbons like hexane. 24 To access the C−C bonds within cyclohexane, after the C−H bond activation shown in Figure 1 has proceeded, a subsequent ring opening must occur. A series of neutral losses from the collision-induced dissociation (CID) of 2-Pd can be seen in Figure 2.…”

Palladium-Mediated CO2 Extrusion Followed by Insertion of Allenes: Translating Mechanistic Studies to Develop a One-Pot Method for the Synthesis of Alkenes

“…The unimolecular reaction of the product ion was subjected to CID (Figure S2). Multiple fragmentation pathways were observed involving deinsertion of n -pentylallene; loss of phenanthroline; loss of the allyl group; and cleavage of the aliphatic chain . Unlike the phenyl complex 2a , the ortho -dimethoxy-substituted aryl-palladium complex 2b has much lower reactivity with phenylallene, and no reaction with n -pentylallene was observed.…”

Green Chemistry and Process Intensification: Milestones on a Sustainable Development

“…Chamas, et al [104] described benchmarks for decay rates of usual plastics that will assist in prioritizing future study attempts on chemical decay to close the loop. Parker, et al [105] investigated the depolymerization of ethylene with palladium catalysts to comprehend techniques to remediation of polyolefins. Liu, et al [106] focused on the recuperation of lithium from spent batteries employing an acid-free mechanochemical method, which uses salt and sodium carbonate as the sole reagents.…”