Insight into ethylene interactions with molybdenum suboxide cluster anions from photoelectron spectra of chemifragments

Abstract

Recent studies on reactions between Mo_xO_y^- cluster anions and H₂O/C₂H₄ mixtures revealed a complex web of addition, hydrogen evolution, and chemifragmentation reactions, with chemifragments unambiguously connected to cluster reactions with C₂H₄. To gain insight into the molecular-scale interactions along the chemifragmentation pathways, the anion photoelectron (PE) spectra of MoC₂H₂^-, MoC₄H₄^-, MoOC₂H₂^-, and MoO₂C₂H₂^- formed directly in Mo_xO_y^- + C₂H₄ (x > 1; y ≥ x) reactions, along with supporting CCSD(T) and density functional theory calculations, are presented and analyzed. The complexes have spectra that are all consistent with η²-acetylene complexes, though for all but MoC₄H₄^-, the possibility that vinylidene complexes are also present cannot be definitively ruled out. Structures that are consistent with the PE spectrum of MoC₂H₂^- differ from the lowest energy structure, suggesting that the fragment formation is under kinetic control. The PE spectrum of MoO₂C₂H₂^- additionally exhibits evidence that photodissociation to MoO₂^- + C₂H₂ may be occurring. The results suggest that oxidative dehydrogenation of ethylene is initiated by Lewis acid/base interactions between the Mo centers in larger clusters and the π orbitals in ethylene.