Cationic Half-Sandwich Iron(II) and Iron(III) Complexes with N‑Heterocyclic Carbene Ligands

The cationic piano-stool iron complexes
containing the N-heterocyclic carbene ligand tethered to a tetramethylcyclopentadienyl
ring [(Cp*-NHC)­Fe­(CO)­(L)]­[X] (L = NCMe, DMSO; X = BF4,
OTf) have been prepared and analyzed by spectroscopic, electrochemical,
and crystallographic methods. Oxidation reactions of the cationic
complex [(Cp*-NHC)­Fe­(CO)­(NCMe)]­[BF4] (2) with
silver tetrafluoroborate and tert-butyl hydroperoxide
in acetonitrile yield the dicationic iron­(III) complexes [(Cp*-NHC)­Fe­(NCMe)2]­[BF4]2 and [(Cp*-NHC)­Fe­(H2O)]­[BF4]2, respectively. When the reaction
of 2 with AgBF4 is performed in dichloromethane,
the cationic complex [(Cp*-NHC)­FeCl]­[BF4] is obtained.
These new iron­(III) complexes have been characterized by Mössbauer
spectroscopy and in one case by X-ray diffraction studies. DFT calculations
were used to rationalize experimental results. The cationic complex
[(Cp*-NHC)­Fe­(CO)­(NCMe)]­[OTf] is catalytically active toward the reduction
of benzaldehyde and acetophenone using phenylsilane under neat conditions
and in the absence of visible light irradiation.