Spin-Controlled Binding of Carbon Dioxide by an Iron Center: Insights from Ultrafast Mid-Infrared Spectroscopy

The influence of the spin on the mode of binding between carbon dioxide (CO₂ ) and a transition-metal (TM) center is an entirely open question. Herein, we use an iron(III) oxalato complex with nearly vanishing doublet-sextet gap, and its ultrafast photolysis, to generate TM-CO₂ bonding patterns and determine their structure in situ by femtosecond mid-infrared spectroscopy. The formation of the nascent TM-CO₂ species according to [L₄ Fe^III (C₂ O₄ )]⁺ + hν → [L₄ Fe(CO₂ )]⁺ + CO₂ , with L₄ =cyclam, is evidenced by the coincident appearance of the characteristic asymmetric stretching absorption of the CO₂ -ligand between 1600 cm^-1 and 1800 cm^-1 and that of the free CO₂ -co-fragment near 2337 cm^-1 . On the high-spin surface (S=5/2), the product complex features a bent carbon dioxide radical anion ligand that is O-"end-on"-bound to the metal. In contrast, on the intermediate-spin and low-spin surfaces, the product exhibits a "side-on"-bound, bent carbon dioxide ligand that has either a partial open-shell (for S=3/2) or fully closed-shell character (for S=1/2).