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

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.