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

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 (η 2 -1-pentene) with a bimolecular rate constant of k a = (4 ± 1) × 10 - 10 cm 3 molecule - 1 s - 1 . Rapid β-hydrogen transfer, by way of intramolecular C−H bond insertion to form HFe(CO) 3 (η 3 -C 5 H 9 ), follows rate-limiting addition of 1-pentene to Fe(CO) 3 and proceeds with a lower bound of k 1 ≥ 10 9 s - 1 . Under experimental conditions, HFe(CO) 3 (η 3 -C 5 H 9 ) decays on a millisecond time scale with concurrent formation of Fe(CO) 3 (η 2 -pentene) 2 by addition of 1-pentene to an Fe(CO) 3 (η 2 -pentene) intermediate. It is Fe(CO) 3 (η 2 -pentene) that is in equilibrium with HFe(CO) 3 (η 3 -C 5 H 9 ) that adds 1-pentene to form Fe(CO) 3 (η 2 -pentene) 2, which may contain an isomerized olefin. CO may add to Fe(CO) 3 (η 2 -pentene) that is in equilibrium with HFe(CO) 3 (η 3 -C 5 H 9 ) to form Fe(CO) 4 (η 2 -pentene). Fe(CO) 4 (η 2 -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 (η 3 -C 5 H 9 ) and Fe(CO) 3 (η 2 -pentene) than for the corresponding species in the propene system.