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.