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 CO2 release to produce an iron(II) intermediate of a radical dihydroimidazopyridine ligand (L-center dot). The structure of this unprecedented transient iron(II)(L-center dot) complex is supported by UV-vis and Mossbauer spectroscopies, DFT calculations, as well as the X-ray structural characterization of an mu-oxo iron(III) complex of the oxidized derivative of L-center dot, namely, [(Fe2O)-O-III-(Cl) (2)(L+)(2)](ClO4)(4)(MeCN)(2) (L+ = 2-(2-(bis(pyridin-2-ylmethyl) amino)ethyl)-2,3-dihydro-1H-imidazo[1,5-a] pyridin-4-ium). [(Fe2O)-O-III(Cl)(2)(L+)(2)](4+) is obtained only in the absence of O-2. Under aerobic conditions, O-2 will intercept the iron(II)(L-center dot) 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-center dot)](2+) with O-2 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 L-center dot to a ring-opened tautomer to allow for O-2 insertion between the C and Fe atoms of the iron(II) complex.