Selective Photo-Induced Oxidation with O-2 of a Non-Heme Iron(III) Complex to a Bis(imine-pyridyl)iron(II) Complex

Non-heme iron(II) complexes of pentadentate N4Py (N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) type ligands undergo visible light-driven oxidation to their iron(III) state in the presence of O2 without ligand degradation. Under mildly basic conditions, however, highly selective base catalyzed ligand degradation with O2, to form a well-defined pyridyl-imine iron(II) complex and an iron(III) picolinate complex, is accelerated photochemically. Specifically, a pyridyl-CH2 moiety is lost from the ligand, yielding a potentially N4 coordinating ligand containing an imine motif. The involvement of reactive oxygen species other than O2 is excluded; instead, deprotonation at the benzylic positions to generate an amine radical is proposed as the rate determining step. The selective nature of the transformation holds implications for efforts to increase catalyst robustness through ligand design.
Photoaccelerated oxidation of an aminopyridyl ligand bound to an Fe(III) ion to a well-defined imine-based Fe(II) complex involves initial alkyl C−H deprotonation followed by reaction with O2 to form an alkyl peroxyl radical. Intramolecular C−H abstraction followed by C−N bond cleavage yields picoline aldehyde and a pyridyl-imine complex. The selectivity of the reaction prevents further oxidation and holds implications for ligand degradation under conditions used in oxidation catalysis with peroxides.