Rip It off: Nitro to Nitroso Reduction by Iron Half-Sandwich Complexes

Activation of [FeCl(dppe)Cp] (1) by chloride abstraction with Na[BAr^X₄] (X = F, [B(3,5-(CF₃)₂-C₆H₃)₄]; X = Cl, [B(3,5-Cl₂-C₆H₃)₄]) permits reactions with a range of nitro aromatics, RC₆H₄NO₂ (R = halogen, Me, OMe, NO₂ or NMe₂), to give the cationic iron nitroso complexes [Fe{N(O)-C₆H₄R}(dppe)Cp][BAr^X₄]) ([3][BAr^X₄]). Similar reactions of 1 and Na[BAr^X₄] with [Fe(NCC₆H₄NO₂)(dppe)Cp][BAr^X₄] gave bimetallic [{Fe(dppe)Cp}₂{μ-N≡CC₆H₄N(O)}][BAr^F₄]₂. However, reactions of 1 and Na[BAr^X₄] with 4-nitrophenol gave the first example of the bench-stable iron half-sandwich phenolate complex [Fe(OC₆H₄NO₂)(dppe)Cp]⁺ rather than NO₂ activation. The formation of complexes [3]⁺ likely proceeds via the unusual blue bimetallic species [{Fe(dppe)Cp}₂{μ,κ²O,O'-O₂NAr}]²⁺. This compound undergoes N-O bond cleavage, resulting in [3]⁺ and a Fe^IV═O species, which reacts via an internal C-H activation of the dppe ligand to give [Fe^III(κ³O,P,P'-P(2-O-C₆H₄)(Ph)-C₂H₄-PPh₂)Cp]⁺. Complexes [3]⁺ are stable under ambient conditions, are readily purified by column chromatography and can be isolated in up to 50% yield, considering that 0.5 equiv of 1 is required as the oxygen acceptor.