Ultrafast "end-on"-to-"side-on" binding-mode isomerization of an iron-carbon dioxide complex

Carbon dioxide (CO₂) binding by transition metals is a captivating phenomenon with a tremendous impact in environmental science and technology, most notably, for establishing circular economies based on greenhouse gas emissions. The molecular and electronic structures of coordination compounds containing CO₂ can be studied in great detail using photochemical precursors bearing the photolabile oxalato-ligand. Here, we study the photoinduced elementary dynamics of the ferric complex, [Fe^III(cyclam)(C₂O₄)]⁺, in dimethyl sulfoxide solution using femtosecond mid-infrared spectroscopy following oxalate-to-iron charge transfer excitation with 266 nm pulses. The pump-probe response in the ν₃-region of carbon dioxide gives unequivocal evidence that a CO₂-molecule is detached from the metal within only 500 fs and with a primary quantum yield of 38%. Simultaneously, a primary ferrous product is formed that carries a carbon dioxide radical anion ligand absorbing at 1649 cm^-1, which is linked to the metal in a bent-O-"end-on" fashion. This primary η_O,bent¹-product is formed with substantial excess vibrational energy, which relaxes on a time scale of several picoseconds. Prior to full thermalization, however, a fraction of the ferrous primary product can structurally isomerize at a rate of 1/(3.5 ps) to a secondary η_CO²-product absorbing at 1727 cm^-1, which features a bent carbon dioxide ligand that is linked to the metal in a "side-on" fashion. The η_O,bent¹-to-η_CO² isomerization requires an intersystem crossing from the sextet to the quartet state, which rationalizes a partial trapping of the system in the metastable bent-O-"end-on" geometry. Finally, a fraction (62%) of the initially photoexcited complexes can return without structural changes to the parent's electronic ground state, but dressed with excess kinetic energy, which relaxes again on a time scale of several picoseconds.