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 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 + -C3H6) ) 37 ( 2 kcal/mol < D 0 (Fe + -CH2S) < D 0 (Fe + -C6H6) ) 49.6 ( 2.3 kcal/mol and D 0 (Fe + -CH2O) < D 0 (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 0 (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