Photoinduced formation of a platina-α-lactone – a carbon dioxide complex of platinum. Insights from femtosecond mid-infrared spectroscopy

The binding of carbon dioxide to a transition metal is a complex phenomenon that involves a major redistribution of electron density between the metal center and the triatomic ligand. The chemical reduction of the ligand reveals itself unambiguously by an angular distortion of the CO2-molecule as a result of the occupation of an anti-bonding π-orbital and a shift of its antisymmetric stretching vibration, ν3, to lower wavenumbers. Here, we generate a carbon dioxide complex of the heavier group-10 metal, platinum, by ultrafast electronic excitation and cleavage of CO2 from the photolabile oxalate precursor, oxaliplatin, and monitored the ensuing primary dynamics with ultrafast mid-infrared spectroscopy. A neutral and thermally relaxed CO2-molecule is detected in the ν3-region within 5 ps after impulsive excitation with 266 nm light. Concurrently, an induced absorption peaking at 1717 cm─1 is observed, which is distinctly up-shifted relative to the oxalate stretching bands of the precursor and which resembles the C=O stretching absorption of organic ketones. Accompanying density functional theory suggests that the 1717 cm─1-absorption arises from a Pt-CO2 product complex featuring a side-on binding mode, which can indeed be regarded as a ketone ; specifically, as the metalla-α-lactone, 1-oxa-3-platinacyclopropan-2-one.