Photosynthesis of a Dihydroimidazopyridine Chelate Shines Light on the Reactions of a Photoactivated Iron(III) Complex with O.

The high-spin ( = /) meridional diastereoisomer of [Fe(tpena)] (tpena = -tris(2-pyridylmethyl)ethylendiamine--acetate), [Fe(tpena)], undergoes photolytic CO release to produce an iron(II) intermediate of a radical dihydroimidazopyridine ligand (). The structure of this unprecedented transient iron(II)() complex is supported by UV-vis and Mössbauer spectroscopies, DFT calculations, as well as the X-ray structural characterization of an μ-oxo iron(III) complex of the oxidized derivative of , namely, [FeO(Cl)()](ClO)(MeCN) ( = 2-(2-(bis(pyridin-2-ylmethyl)amino)ethyl)-2,3-dihydro-1-imidazo[1,5-]pyridin-4-ium). [FeO(Cl)()] is obtained only in the absence of O. Under aerobic conditions, O will intercept the iron(II)() complex to form a putative Fe(III)-alkylperoxide complex which cascades to an iron(II) complex of SBPy3 (SBPy3 = -bis(2-pyridylmethyl)amine--ethyl-2-pyridine-2-aldimine). Thus, through different oxidative pathways, the unknown ligand or SBPy3 forms by loss of a one-carbon-atom or a two-carbon-atom unit, respectively, from the glycyl arm of tpena. Acceleration of the photodecarboxylation step is achieved by addition of thiocyanate because of transient formation of a more photoreactive NCS adduct of [Fe(tpena)]. This has allowed for kinetic observation of the reaction of [Fe()] with O which is, unexpectedly, promoted also by light. We propose that this corresponds to the energy needed for the conversion of the ring-closed radical ligand to a ring-opened tautomer to allow for O insertion between the C and Fe atoms of the iron(II) complex.