From the Iron Pentacarbonyl Cation to Heteroleptic η-arene Carbonyls and bis-η-arene Cations.

Partial ligand substitution at the iron pentacarbonyl radical cation generates novel half-sandwich complexes of the type [Fe(η-arene)(CO)]⋅ (arene=1,3,5-tri-tert-butylbenzene, 1,3,5-trimethylbenzene, benzene and fluorobenzene). Of those, the bulkier 1,3,5-tri-tert-butylbenzene (mes*) derivative [Fe(mes*)(CO)]⋅ was fully characterized by XRD analysis, IR, NMR, cw-EPR, Mössbauer spectroscopy and cyclic voltammetry as the [Al(OR)] (R=C(CF)) salt. Chemical electronation, i. e., the single electron reduction, with decamethylferrocene generates neutral [Fe(mes*)(CO)], whereas further deelectronation under CO-pressure leads to a dicationic three-legged [Fe(mes*)(CO)] salt with [Al(OR)] counterion. The full substitution of the carbonyl ligands in [Fe(CO)]⋅[Al(OR)] mainly resulted in disproportionation reactions, giving solid Fe(0) and the dicationic bis-arene salts [Fe(η-arene)]([Al(OR)]) (arene=1,3,5-trimethylbenzene, benzene and fluorobenzene). Only by employing the very large fluoride bridged anion [F-{Al(OR)}], it was possible to isolate an open shell bis-arene cation salt [Fe(CH)]⋅[F-{Al(OR)}]. The highly reactive cation was characterized by XRD analysis, cw-EPR, Mössbauer spectroscopy and cyclic voltammetry. The disproportionation of [Fe(CH)]⋅ salts to give solid Fe(0) and [Fe(CH)] salts was analyzed by a suitable cycle, revealing that the thermodynamic driving force for the disproportionation is a function of the size of the anion used and the polarity of the solvent.