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.

Biphasic Perchlorate Activation Enabled by a Dinuclear Fe-NHC Complex

“…This Fe(IV)=O intermediate is expected to be kinetically active, releasing its oxidative potential through a hydrogen abstraction (HAT) event with C-H substates, resulting in the formation of a Fe(III)-OH species and a carbon centered radical. Subsequently, complex 2 can be regenerated either through the dimerization of Fe(III)-OH species 25 or the re-oxidation of complex 1 by perchlorate anion (Figure 4c). To validate this hypothesis, complex 2(ClO4)4 was treated with an excess amount of 9,10-dihydroanthracene ( 4) and expose to a purple LED (400 nm, 20 W) for 14 h. Remarkably, the major product observed was the homocoupling product 5, rather than the commonly observed O-rebounded or aromatized products when using Fe=O as oxidant 6b,25,26 , as indicated by the 1 H NMR spectrum, suggesting the presence of a hydroanthracene radical intermediate (Figure 4d).…”