Successive reactions of iron carbonyl cations with dimethyl ether: direct cleavage versus rearrangement

Abstract

The reaction kinetics of electron impact ionization generated Fe(CO)_n⁺ cations with dimethyl ether (DME) is investigated using a triple-cell Fourier Transform Ion Cyclotron Resonance spectrometer. The primary reaction is rapid substitution of CO by DME. One-step substitution of two CO ligands by one DME molecule also occurs for n = 3–4, and for unrelaxed Fe(CO)₂⁺ ions. Successive substitutions in the Fe(CO)₄⁺/DME system lead mainly to Fe(CO)(DME)₂⁺, in which the last CO remains unsubstituted. This ion is slowly converted to Fe(CO)(DME)₃⁺ and Fe(DME)₂⁺. The latter reaction implies formation of neutral CH₃COOCH₃ resulting from iron-promoted CO insertion reaction. Further reaction of Fe(DME)⁺ with DME involves the cleavage of a C–O bond according to two channels, either direct CH₃˙ loss or more exothermic CH₄ loss, implying a molecular rearrangement. The branching ratio is strongly energy-dependent: the latter channel is observed only for Fe(DME)⁺ ions having a very low energy content.