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)]⁺ (<strong>1-Pd</strong>) and [(phen)Pd(CH₃)]⁺ (<strong>5-Pd</strong>) (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)]⁺ (<strong>3-Pd</strong>) 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 <strong>3-Pd</strong> 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 <strong>1-Pd</strong> and <strong>5-Pd</strong> produced by CID of <strong>3-Pd</strong> are allowed to react with hexane again. Ni complexes largely mirrored the chemistry observed for Pd. Both <strong>1-Ni</strong> and <strong>5-Ni</strong> reacted with hexane, forming <strong>3-Ni</strong>, which fragmented under CID conditions in a fashion similar to <strong>3-Pd</strong>. In contrast, only <strong>5-Pt</strong> reacted with hexane to form <strong>3-Pt</strong>, which fragmented predominantly via sequential losses of H₂.