Ultrafast X-ray spectroscopy (XAS) allows visualizing transient electronic and geometric changes accompanying a sequence of redox reactions occurring during molecular catalysis. We have used it to study a high-valent Fe complex, containing the metal in an unusually high oxidation state (+V), which belongs to a class of compounds considered as model systems of vital reaction intermediates found in several important biocatalytic reactions. However, octahedral Fe+V complexes have not yet been detected for biologically relevant conditions (room-temperature solutions). Only recently picosecond IR studies of Fe+V formation, following UV photodetachment of N2 from the azido-iron(III) complex in CH3CN solution, extracted a subsequent formation of the high-valent complex on a 10 ps time scale. The photochemical reaction cycle of the parent complex 1 involves competing reaction pathways throughout the UV-VIS range with the two most relevant reaction products 2 and 3 (Fig. 1a). Here we employed ultrafast XAS to study the photoinduced dynamics of both long-lived photoproducts and ultrafast reaction intermediates, which may involve changes in the electronic and geometric structures of the molecule. For this purpose we carried out a combined study following photoexcitation across UV and VIS ranges and using both 100 ps hard X-ray pulses from a synchrotron (Figs 1b and 1c) and furthermore fs X-ray from an X-ray Free Electron Laser. Time-resolved XAS with both ps and fs temporal resolution helped to identify key steps during this light-triggered reaction. Oxidation state changes together with the local environment around the Fe atom were extracted via the XANES, since it is sensitive to chemical shifts via the so-called pre-edge resonance transitions, whereas X-ray emission spectroscopy allowed for further identifying the spin states of the involved intermediate excited states.

Ultrafast X-ray spectroscopy of transition metal compounds relevant for catalysis: a study case of a high-valent Fe complex

256th National Meeting (2018)

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