The Influence of the Metal Spin State in the Iron-Catalyzed Alkene Isomerization Reaction Studied with Ultrafast Infrared Spectroscopy

The process of iron- and ruthenium-assisted alkene isomerization is investigated with ultrafast time-resolved infrared spectroscopy under ambient conditions. M(CO) 4 (η 2 -1,4-PD) (M = Ru, Fe; PD = pentadiene) was irradiated with a 100 femtosecond pulse of 266 nm light in a solution of cyclohexane, and a single carbonyl was photodissociated. When M = Ru, experimental results indicate that both Ru(CO) 3 (η 4 -1,4-PD) and HRu(CO) 3 (η 3 -allyl) form within a few picoseconds, and their populations do not change significantly out to 800 ps. When M = Fe, the coordinatively unsaturated triplet metal complex, 3 Fe(CO) 3 (η 2 -1,4PD), was observed to form rapidly and rearranged to HFe(CO) 3 (η 3 -allyl) at a rate of (4.8 ± 0.2) × 10 8 s -1 . Surprisingly, the triplet iron complex did not coordinate the free alkene on the pentadiene ligand on the ultrafast time scale. The difference in reactivity between the iron- and the ruthenium-assisted alkene isomerization is due to differences in spin state of the coordinatively unsaturated metal center.