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

The gas-phase reactions of Fe(CH2O) and Fe(CH2S) 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(CH2O), while C-H insertion is preferred for Fe(CH2S). About 90% of the Fe(CH2O) reaction products are formed by C-C insertion with small alkane loss. For Fe(CH2S), 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 H2S-Fe-olefin complex, which dissociates by H2S elimination. The structures of the reaction products were probed by collisioninduced 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(Fe-C3H6) ) 37 ( 2 kcal/mol < D(Fe-CH2S) < D(Fe-C6H6) ) 49.6 ( 2.3 kcal/mol and D(Fe-CH2O) < D(Fe-C2H4) ) 34 ( 2 kcal/mol. The optimized geometry of Fe(CH2O), obtained by density functional calculations, has C2V symmetry with a nearly undisturbed formaldehyde unit. The Fe-CH2O bonding is found to be predominantly electrostatic with a calculated bond energy of 32.2 kcal/mol. However, the optimized Fe(CH2S) structure has Cs symmetry with dative bonding between Fe+ and CH2S. D(Fe-CH2S) is calculated at 41.5 kcal/mol. The differences in geometry and chemical bonding between Fe(CH2O) and Fe(CH2S) are correlated with the different reaction pathways observed.