Disproportionation Equilibrium of a μ-Oxodiiron(III) Complex Giving Rise to C-H Activation Reactivity: Structural Snapshot of a Unique Oxoiron(IV) Adduct.

Abstract

μ-Oxodiiron(III) species are air-stable and unreactive products of autoxidation processes of monomeric heme and non-heme iron(II) complexes. Now, the organometallic [(LNHC )FeIII -(μ-O)-FeIII (LNHC )]4+ complex 1 (LNHC is a macrocyclic tetracarbene) is shown to be reactive in C-H activation without addition of further oxidants. Studying the oxidation of dihydroanthracene, it was found that 1 thermally disproportionates in MeCN solution into its oxoiron(IV) (2) and iron(II) components; the former is the active species in the observed oxidation processes. Possible cleavage scenarios for 1 are shown by scrambling experiments and structural characterization of an unprecedented adduct of 1 and oxoiron(IV) complex 2. Kinetic analysis gave an equilibrium constant for the disproportionation of 1, which is very small (Keq =7.5±2.5×10-8 m). Increasing Keq might by a useful strategy for circumventing the formation of dead-end μ-oxodiiron(III) products during Fe-based homogeneous oxidation catalysis.