The influence of anionic and neutral ligands on the reactivity of cationic iron complexes towards methanol, ethanol, and iso- and n-propanol has been investigated by means of electrospray ionization (ESI) mass spectrometry, and in key questions the experimental results are supported by DFT calculations. The chemical processes taking place when FeCl2+ is coordinated to a variable number of alcohol molecules were investigated by collision experiments, as well as reactivity and labeling studies. The most abundant cations formed upon ESI of FeCl3/ROH mixtures can be described as consisting of a covalently bound FeCl2+ core, which is stabilized by neutral ROH ligands, for example, FeCl2(CH3OH)n+. Indications for a hydrogen-bonding mechanism of ligands in the second coordination sphere are given by the observation of the formally hypercoordinated ion FeCl2(CH3OH)5+ and interpretation of kinetic data for n=4. Further, deuterium labeling experiments disclose a number of hidden hydrogen transfers and imply that complexes with n=1 and 2 can exist in two tautomeric forms. Upon change of the alcohol ligand from methanol to ethanol and propanol, additional reaction pathways become accessible, among which metal-assisted dehydration of the respective alcohols by means of an ion/dipole mechanism is the most important.

Dedicated to Graham Cooks on the occasion of his 60th birthday Reversible b-hydrogen transfer, a key reaction in organometallic chemistry, often relies on an alignment of the alkyl residue in terms of an agostic interaction (Scheme 1). Here, Scheme 1. Reversible b-hydrogen transfer with an agostic interaction.

The n-propyl/isopropyl isomerization operative in Fe+-mediated dehydrations of n-propyl and isopropyl alcohols has been rigorously analyzed by tandem mass spectrometry and density functional calculations. Metastable ion studies of complexes of atomic Fe+ with a set of selectively deuterated propyl alcohols address structural details of the reversibility of β-hydrogen transfer steps. The labeling distributions in the ionic products reveal that the transiently focused, constitutionally equivalent methyl groups remain distinguishable in the course of n-C3H7 → i-C3H7 isomerizations. This asymmetry causes a memory effect operative in the Fe+-mediated dehydration of n-propyl alcohol. While density functional calculations provide detailed insight into the nature of the reaction intermediates and, inter alia, suggest that the experimentally deduced asymmetry can be attributed to agostic interactions of the iron center with a β-hydrogen atom of the emerging methyl group, a comprehensively consistent, quantitative explanation of the experimentally observed effects cannot be provided by the calculations.

Dedicated to Jack D. Dunitz, a distinguished scientist, inspiring colleague, and dear friend, on the occasion of his 80th birthdayThe binding energies, geometries, and electronic structures of cationic ironÀbenzene and ironÀpyridine complexes have been studied by the two hybrid DFT-HF approaches mPW1PW91 and B3LYP, as well as the AQCC and MR-AQCC extension. The AQCC results confirm the experimental binding energies derived from threshold-CID experiments reported by Meyer et al., and Rodgers et al. as well as the previously reported C 2v -symmetric quartet ground state of ironÀbenzene. The ironÀpyridine complex is coordinated via the N-atom lone-pair and has a sextet ground state. Bond energies determined by the kinetic method apparently yield a dissociation energy corresponding to the first excited quartet ironÀpyridine complex. Both DFT methods fail to predict the correct ground state for cationic iron pyridine.

The effects of the oxidation state, the ligand, and the solvent on structures and energetics of cationic iron complexes are investigated by means of electrospray‐ionization mass spectrometry. Insights into the potential‐energy surfaces of FeX+, FeX2+, and XFe(OCH3)+ ions (X = F, Cl, Br, I) with a variable number of coordinated methanol molecules are obtained by means of collision experiments and complementary thermochemical considerations. It is shown that upon change of the halide ligand, the weakly solvated ions respond differentially to the increasing need for stabilization of the partial charge on the metal center. For example, whereas F2Fe(CH3OH)n+ ions tend to lose mainly HF for n = 1–3, the loss of HBr is not observed at all for Br2Fe(CH3OH)n+. Instead, the iron‐bromide cations undergo reductive loss of atomic bromine. Cl2Fe(CH3OH)n+ bears an intermediate position in that both reduction as well as elimination of HCl can occur. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)