Carbon Dioxide Insertion into Bridging Iron Hydrides: Kinetic and Mechanistic Studies

The reduction of CO₂ to formic acid by transition metal hydrides is a potential pathway to access reactive C1 compounds. To date, no kinetic study has been reported for insertion of a bridging hydride in a weak-field ligated complex into CO₂; such centers have relevance to metalloenzymes that catalyze this reaction. Herein, we report the kinetic study of the reaction of a tri(μ-hydride)triiron(II/II/II) cluster supported by a tris(β-diketimine) cyclophane (1) with CO₂ monitored by ¹H-NMR and temperature-controlled UV-vis spectroscopy. We found that 1 reacts with CO₂ to traverse the reported monoformate (1-CO ) and a diformate complex (1-2CO ) at 298 K in toluene, and ultimately yields the triformate species (1-3CO ) at elevated temperature. The second order rate constant, H/D kinetic isotope effect, ∆H ^‡,and ∆S ^‡for formation of 1-CO were determined as 8.4(3)×10^-4 M^-1·s^-1, 1.08(9), 11(1) kcal·mol^-1, and -3(1)×10 cal·mol^-1·K^-1, respectively at 298 K. These parameters suggest that CO₂ coordination to the iron centers does not coordinate prior to the rate controlling step whereas Fe-H bond cleavage does.