Metal acetylene cluster ions M +(C 2H 2) n as model reactors for studying reactivity of laser-generated transition metal cations

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 +*) which interact with neutral acetylene clusters to generate M +(C 2H 2) 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 C 4H 3 + in the generation of higher hydrocarbon species such as C 6H 4 +, C 6H 5 +, C 8H 6 + and C 8H 7 +. 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 +(C 2H 2) 2, Cr +(C 2H 2) 2, Fe +(C 2H 2) 4, Fe +(C 2H 2) 4, Co +(C 2H 2) 3, Ni +(C 2H 2) 3, and Cu +(C 2H 2) 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 +(C 2H 2) 3, Co +(C 2H 2) 3 and Ni +(C 2H 2) 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 +(C 2H 2) 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.