Abstract
The present study explores the feasibility of utilizing acetylene clusters as model reactors for screening the catalytic activity of transition metal ions towards the polymerization of acetylene. Laser vaporization/ionization is used to generate atomic transition metal cations (M+center dot) which interact with neutral acetylene clusters to generate M+(C2H2)(n) cluster ions. Evidence is presented for C-H bond activation by low-lying excited states of the laser-generated V+, Fe+, Co+ and Ni+ ions. A sequential addition of acetylene molecules to the activated acetylene monomer within the clusters is proposed as a possible mechanism for polymerization of acetylene, initiated by C-H bond activation. The proposed mechanism suggests the intermediacy of C4H3+ in the generation of higher hydrocarbon species such as C6H4+, C6H5+, C8H6+ and C8H7+. Based on thermodynamic considerations it is expected that the observed hydrocarbon ions would have cyclic structures equivalent to benzene and styrene fragments. Enhanced ion intensities have been observed for V+(C2H2)(2), Cr+(C2H2)(2), Fe+(C2H2)(4), Fe+(C2H2)(4), Co+(C2H2)(3), Ni+(C2H2)(3), and Cu+(C2H2)(3) consistent with the formation of stable covalent products formed by metal ion catalyzed polymerization of acetylene clusters. DFT calculations identify the structures of the initially formed trimer ion clusters Fe+(C2H2)(3), Co+(C2H2)(3) and Ni+(C2H2)(3) where the metal cation is embedded in a "cage" created by the three acetylene molecules. Isomerization of these cluster ions into the more stable metal ion-benzene adducts is suggested and the energy needed to surpass the isomerization barriers is likely to come from the laser-generated excited state metal ions. A remarkable even-odd alternation has been observed for Co+(C2H2)(n) clusters with enhanced ion intensities for n=3, 6, 9 and 12, which could be explained by multiple isomerization events resulting in the formation of Co+(benzene)(n) clusters with n=1-4. The combination of the excited state energy of the TM ions and the unique cluster environment which promotes concerted multi-monomer interactions with the metal ions could lead, under favorable conditions, to TM ion-mediated cyclotrimerization of acetylene molecules resulting in the formation of benzene and other polycyclic aromatic hydrocarbons. (C) 2010 Elsevier B.V. All rights reserved.