Photosynthesis of a Dihydroimidazopyridine Chelate Shines Light on the Reactions of a Photoactivated Iron(III) Complex with O₂

The high-spin (S = ⁵/ ₂) meridional diastereoisomer of [Fe ^III(tpena)] ²⁺(tpena = N,N,N′-tris(2-pyridylmethyl)ethylendiamine-N′-acetate), mer-[Fe(tpena)] ²⁺, undergoes photolytic CO ₂release to produce an iron(II) intermediate of a radical dihydroimidazopyridine ligand (L ^â). The structure of this unprecedented transient iron(II)(L ^â) 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 L ^â, namely, [Fe ^III ₂O(Cl) ₂(L ⁺) ₂](ClO ₄) ₄(MeCN) ₂(L ⁺= 2-(2-(bis(pyridin-2-ylmethyl)amino)ethyl)-2,3-dihydro-1H-imidazo[1,5-a]pyridin-4-ium). [Fe ^III ₂O(Cl) ₂(L ⁺) ₂] ⁴⁺is obtained only in the absence of O ₂. Under aerobic conditions, O ₂will intercept the iron(II)(L ^â) complex to form a putative Fe(III)-alkylperoxide complex which cascades to an iron(II) complex of SBPy3 (SBPy3 = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine). Thus, through different oxidative pathways, the unknown ligand L ⁺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)] ²⁺. This has allowed for kinetic observation of the reaction of [Fe ^II(L ^â)] ²⁺with O ₂which is, unexpectedly, promoted also by light. We propose that this corresponds to the energy needed for the conversion of the ring-closed radical ligand L ^âto a ring-opened tautomer to allow for O ₂insertion between the C and Fe atoms of the iron(II) complex.