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

The reduction of CO 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 CO; such centers have relevance to metalloenzymes that catalyze this reaction. Herein, we report the kinetic study of the reaction of a tri(-hydride)triiron(II/II/II) cluster supported by a tris(β-diketimine) cyclophane () with CO monitored by H-NMR and temperature-controlled UV-vis spectroscopy. We found that reacts with CO to traverse the reported monoformate ( ) and a diformate complex ( ) at 298 K in toluene, and ultimately yields the triformate species ( ) at elevated temperature. The second order rate constant, H/D kinetic isotope effect, ∆ ,and ∆ for formation of were determined as 8.4(3)×10 M·s, 1.08(9), 11(1) kcal·mol, and -3(1)×10 cal·mol·K, respectively at 298 K. These parameters suggest that CO coordination to the iron centers does not coordinate prior to the rate controlling step whereas Fe-H bond cleavage does.