“Bare” Iron Methoxide Cation: A Simple Model To Probe the Mechanism of β-Hydrogen Transfer in Organometallic Compounds

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Abstract

Iron methoxide cation, Fe(OCH3)+ (1), and its tautomer, the formaldehyde complex of the iron hydride cation, HFe(OCH2)+ (2), have been examined in combined mass spectrometric and computational studies. Although the experimental methods used for ion generation yield two isomers, largely because intermolecular isomerization is facile, differentiation of them is straightforward. Fe(OCH3)+ corresponds to the global minimum of the [Fe,C,H3,O]+ potential-energy hypersurface with an experimentally determined bond-dissociation energy of 69 ± 2 kcal/mol for the Fe+−OCH3 bond. In the gas phase, Fe(OCH3)+ can isomerize via a β-hydrogen transfer to HFe(OCH2)+, which is experimentally found to be 15 ± 4 kcal/mol less stable than Fe(OCH3)+. The experiments suggest and the calculations predict that the two isomers are separated by a significant activation barrier. According to the calculations both species exhibit quintet ground states and the transition structure associated with their interconversion on the quintet potential-energy hypersurface is 37 kcal/mol above Fe(OCH3)+. Consideration of the excited triplet surface indicates that the barrier for the β-hydrogen transfer connecting both isomers may be lowered substantially by additional ligands. Moreover, in the complexes Fe(L)(OCH3)+ (L = C2H4, CH2O) direct H-transfer from the OCH3 ligand to L may occur without involving an iron hydride as an intermediate.

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