A Study of the Mechanism of Iron Carbonyl-Catalyzed Isomerization of 1-Pentene in the Gas Phase Using Time-Resolved Infrared Spectroscopy

Abstract

After generation of Fe(CO)3 by 308-nm gas-phase photolysis of Fe(CO)5, 1-pentene adds to Fe(CO)3 to form Fe(CO)3(η2-1-pentene) with a bimolecular rate constant of ka = (4 ± 1) × 10-10 cm3 molecule-1 s-1. Rapid β-hydrogen transfer, by way of intramolecular C−H bond insertion to form HFe(CO)3(η3-C5H9), follows rate-limiting addition of 1-pentene to Fe(CO)3 and proceeds with a lower bound of k1 ≥ 109 s-1. Under experimental conditions, HFe(CO)3(η3-C5H9) decays on a millisecond time scale with concurrent formation of Fe(CO)3(η2-pentene)2 by addition of 1-pentene to an Fe(CO)3(η2-pentene) intermediate. It is Fe(CO)3(η2-pentene) that is in equilibrium with HFe(CO)3(η3-C5H9) that adds 1-pentene to form Fe(CO)3(η2-pentene)2, which may contain an isomerized olefin. CO may add to Fe(CO)3(η2-pentene) that is in equilibrium with HFe(CO)3(η3-C5H9) to form Fe(CO)4(η2-pentene). Fe(CO)4(η2-pentene) remains stable on the time scale of catalytic turnover and its formation serves as a termination pathway for thermal catalysis. This system is compared to the analogous propene system (Long, G. T.; Wang, W.; Weitz, E. J. Am. Chem. Soc. 1995, 117, 12810). The major difference in behavior between these systems is attributed to an ∼3 orders of magnitude shift in the equilibrium constant toward HFe(CO)3(π-allyl) relative to Fe(CO)3(olefin) when the starting olefin is 1-pentene instead of propene. The magnitude of the equilibrium constants indicates that there is an ∼4 kcal mol-1 greater enthalpy difference between HFe(CO)3(η3-C5H9) and Fe(CO)3(η2-pentene) than for the corresponding species in the propene system.