An Iron Complex with a Bent, O-Coordinated CO₂ Ligand Discovered by Femtosecond Mid-Infrared Spectroscopy

The activation of carbon dioxide by transition metals is widely recognized as a key step for utilizing this greenhouse gas as a renewable feedstock for the sustainable production of fine chemicals. However, the dynamics of CO₂ binding and unbinding to and from the ligand sphere of a metal have never been observed in the time domain. The ferrioxalate anion is used in aqueous solution as a unique model system for these dynamics and femtosecond UV-pump mid-infrared-probe spectroscopy is applied to explore its photoinduced primary processes in a time-resolved fashion. Following optical excitation, a neutral CO₂ molecule is expelled from the complex within about 500 fs to generate a highly intriguing pentacoordinate ferrous dioxalate that carries a bent carbon dioxide radical anion ligand, that is, a reductively activated form of CO₂ , which is end-on-coordinated to the metal center by one of its two oxygen atoms.