Photoinduced de(thio)carboxylation dynamics monitored by time-resolved mid-IR-to-near-UV spectroscopies. From transition metals to main-group elements

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The photochemistry of 3d-transition metal (TM) oxalates is of significant importance for a variety of processes in nature and technology. In the past, this project was dedicated to elucidating the photochemical processes induced upon ligand-to-metal charge-transfer excitation of such species using ultrafast non-linear laser spectroscopy. The major findings during the first funding period are: (I) When attached to a TM, the bidentate oxalato ligand becomes susceptible to heterolytic CC-bond cleavage leading to the formation of a neutral CO2 that departs promptly from the ligand sphere and a carbonite anion (CO2^2─) that remains bound to the metal center as a redox-active ligand. (II) This photoreactivity pattern establishes an entry into the chemistry of TM-CO2-complexes. (III) For the high-spin system, ferrioxalate, a primary product complex was discovered, which features an exceptional bent, O-end-on-bound CO2●─ radical anion ligand that is ferromagentically coupled to an Fe(II)-center. (IV) For a low-spin monooxalato-iron(III) model complex, the primary product features the classical side-on binding mode of CO2. (V) The results obtained in previous project strongly suggest that the CO2-binding mode can be controlled by the spin and hence, by a careful design of the TM’s ligand sphere.Driven by its relevance for the topic of CO2-activation and inspired by biological carbon fixation, the continuation project now intends to expand ist research toward (1) the photo-induced decarboxylation of coordination compounds of main-group elements, specifically of Al and Si oxalates. To learn more about the nature of the resultant open-shell ligands and their interaction with the coordinating center, the project shall also expand ist research towards the photochemistry associated with intricate bifunctional carboxylate ligands attached to TMs and main-group elements. An emphasis is therefore laid here on (2) the photo-induced decarboxylation of higher dicarboxylates and amino carboxylates of TMs and on (3) the photo-induced de(thio)carboxylation of the thiooxalates, all of which attached to TMs and main-groups elements. Preliminary quantum-chemical calculations suggest that the CO2-loss of an S-coordinated thiooxalate can generate a terminal sulfido ligand. This project may thus open up an elegant route to late-TM-sulfides – a class of coordination compounds whose isolation is exceedingly rare. All three lines of research shall be tackled again using ultrafast non-linear laser spectroscopy in the mid-infrared, the visible, and the near ultraviolet spectral regions.