Gas-Phase Reactions of Fe(CH₂O)⁺ and Fe(CH₂S)⁺ with Small Alkanes:  An Experimental and Theoretical Study

Abstract

The gas-phase reactions of Fe(CH 2 O) + and Fe(CH 2 S) + with a series of aliphatic alkanes were studied by Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. Like bare Fe +, C−C insertion, particularly terminal C−C insertion, is predominant for the reactions of Fe(CH 2 O) +, while C−H insertion is preferred for Fe(CH 2 S) + . About 90% of the Fe(CH 2 O) + reaction products are formed by C−C insertion with small alkane loss. For Fe(CH 2 S) +, after initial C−H insertion, the proposed mechanism includes hydrogen transfer to sulfur, followed by migratory insertion of methylene into the metal−alkyl bond and formation of an activated H 2 S−Fe + −olefin complex, which dissociates by H 2 S elimination. The structures of the reaction products were probed by collision-induced dissociation, ion−molecule reactions, and use of labeled compounds, yielding information about the reaction mechanism. Collision-induced dissociation and ligand displacement reactions yield the brackets D 0 (Fe + −C 3 H 6 ) = 37 ± 2 kcal/mol < D 0 (Fe + −CH 2 S) < D 0 (Fe + −C 6 H 6 ) = 49.6 ± 2.3 kcal/mol and D 0 (Fe + −CH 2 O) < D 0 (Fe + −C 2 H 4 ) = 34 ± 2 kcal/mol. The optimized geometry of Fe(CH 2 O) +, obtained by density functional calculations, has C 2 v symmetry with a nearly undisturbed formaldehyde unit. The Fe + −CH 2 O bonding is found to be predominantly electrostatic with a calculated bond energy of 32.2 kcal/mol. However, the optimized Fe(CH 2 S) + structure has C s symmetry with dative bonding between Fe + and CH 2 S. D 0 (Fe + −CH 2 S) is calculated at 41.5 kcal/mol. The differences in geometry and chemical bonding between Fe(CH 2 O) + and Fe(CH 2 S) + are correlated with the different reaction pathways observed.