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

Abstract

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 [L4FeIII(C2O4)]+ + hν → [L4Fe(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).