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, [FeIII(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 ηO,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 ηCO2-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 ηO,bent1-to-ηCO2 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.