Photoinduced O-2-Dependent Stepwise Oxidative Deglycination of a Nonheme Iron(III) Complex

The iron(III) complex [Fe(tpena)](2+) (tpena = N, N, N'-tris(2-pyridylmethyl)ethylendiamine-N'-acetate) undergoes irreversible O-2-dependent N-demethylcarboxylation to afford [Fe-II(SBPy3)(MeCN)](2+) (SBPy3 = N, N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine), when irradiated with near-UV light. The loss of a mass equivalent to the glycyl group in a process involving consecutive C-C and C-N cleavages is documented by the measurement of the sequential production of CO2 and formaldehyde, respectively. Time-resolved UV-vis absorption, Mossbauer, EPR, and Raman spectroscopy have allowed the spectroscopic characterization of two iron-based intermediates along the pathway. The first of these, proposed to be a low-spin iron(II)-radical ligand complex, reacts with O-2 in the rate-determining step to produce a putative alkylperoxide complex. DFT calculations suggest that this evolves into an Fe(IV)-oxo species, which can abstract a hydrogen atom from a cis methylene group of the ligand to give the second spectroscopically identified intermediate, a high-spin iron(III)-hydroxide of the product oxidized ligand, [Fe-III(OH)(SBPy3)](2+). Reduction and exchange of the cohydroxo/water ligand produces the crystallographically characterized products [Fe-II(SBPy3)(X)](2+/3+), X = MeCN, [Zn(tpena)](+).