Innocent and less-innocent solvent ligands: a systematic investigation of cationic iron chloride/alcohol complexes by electrospray ionization mass spectrometry complemented by DFT calculations.

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.