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