Experimental and theoretical studies of FeC 7H 7+ isomers: Fe +-C 7H 7 bond energies and isomeric differentiation

Abstract

Three isomeric structures of Fe(C 7H 7) +, presumably, Fe(tropylium) +, Fe(benzyl) +, and Fe(tolyl) +, are studied with Fourier transform ion cyclotron resonance (FTICR) mass spectrometry and by theoretical calculations. Of particular interest is the role of the metal in stabilizing the tolyl ligand, given that the unmetallated tolyl ion, C 6H 4CH 3 +, rearranges to benzyl ion on the time scale of the FTICR experiment. Ions with the empirical formula Fe(C 7H 7) + were generated by three synthetic routes: (1) reaction of bare metal Fe + with cycloheptatriene, (2) reaction of Fe(phenyl) + with toluene, and (3) reaction of Fe(benzyne) + with methyl iodide. The hypothesis is that these three reactions result in the formation of Fe(tropylium) +, Fe(benzyl) + and Fe(tolyl) +, respectively. Energy resolved collision-induced dissociation, photodissociation, and selected ion/molecule reactions readily distinguish Fe(tropylium) + from the other two structures. Fe(benzyl) + and Fe(tolyl) +, however, are indistinguishable by collision-induced dissociation and photodissociation, but exhibit differences in reactivity with selected reagents. These results indicate that while Fe stabilizes the tolyl ion structure, upon activation the ion can rearrange to the benzyl structure. From photodissociation experiments we infer D∘(Fe +-benzyl) = 63 ± 5 kcal mol −1, D∘(Fe +-tolyl) = 83 ± 10 kcal mol −1, and an upper limit of D∘(Fe +-cycloheptatrienyl) ≤ 129 ± 5 kcal mol −1, while ion-molecule reactions yield the lower limits D∘(Fe +-benzyl) > 55 ± 5 kcal mol −1, D∘(Fe +-tolyl) > 60 ± 5 kcal mol −1, and D∘(Fe +-cycloheptatrienyl) > 73 kcal mol −1. These results are self-consistent and in general agreement with ab initio calculations which yield D∘(Fe +-cycloheptatrienyl) = 86.8 kcal mol −1, D∘(Fe +-tolyl) = 84.4 kcal mol −1, and D∘(Fe +-benzyl) = 69.4 kcal mol −1.