From the Iron Pentacarbonyl Cation to Heteroleptic η⁶‐arene Carbonyls and bis‐η⁶‐arene Cations

Abstract

Partial ligand substitution at the iron pentacarbonyl radical cation generates novel half‐sandwich complexes of the type [Fe(η6‐arene)(CO)2]•+ (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)2]•+ was fully characterized by XRD analysis, IR, NMR, cw‐EPR, Mössbauer spectroscopy and cyclic voltammetry as the [Al(ORF)4]− (RF = C(CF3)3) salt. Chemical electronation, i.e., the single electron reduction, with decamethylferrocene generates neutral [Fe(mes*)(CO)2], whereas further deelectronation under CO‐pressure leads to a dicationic three‐legged [Fe(mes*)(CO)3]2+ salt with [Al(ORF)4]– counterion. The full substitution of the carbonyl ligands in [Fe(CO)5]•+[Al(ORF)4]− mainly resulted in disproportionation reactions, giving solid Fe(0) and the dicationic bis‐arene salts [Fe(η6‐arene)2]2+([Al(ORF)4]–)2 (arene = 1,3,5‐trimethylbenzene, benzene and fluorobenzene). Only by employing the very large fluoride bridged anion [F‐{Al(ORF)3}2]−, it was possible to isolate an open shell bis‐arene cation salt [Fe(C6H6)2]•+[F‐{Al(ORF)3}2]−. The highly reactive cation was characterized by XRD analysis, cw‐EPR, Mössbauer spectroscopy and cyclic voltammetry. The disproportionation of [Fe(C6H6)2]•+ salts to give solid Fe(0) and [Fe(C6H6)2]2+ 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.