Near-UV photolysis of a mixture of Fe(CO)5 and C3H6 in the gas phase produces the (π-allyl) metal hydride complex, HFe(CO)3(η3-C3H5), an intermediate implicated in iron carbonyl-catalyzed olefin isomerization. The formation of HFe(CO)3(C3H5) is rate limited by the addition of C3H6 to Fe(CO)3 which has a rate constant of (2.6 ± 0.3) × 10–10 cm3 molecules–1 s–1 at 296 K. Subsequent to propene addition, the unimolecular rearrangement of Fe(CO)3(η2-C3H6) →k1HFe(CO)3(η3-C3H5) takes place with a lower bound for k1 of 1010 s–1. With the assumption of a preexponential for k1 of 1013 s–1, the activation enthalpy associated with k1 is <3.5 kcal mol–1. The data are consistent with the establishment of an equilibrium between HFe(CO)3(η3-C3H5) and Fe(CO)3(η2-C3H6) with an equilibrium constant of 2.4 × 10–5 and an isotope effect of KeqH/KeqD = 0.45 at 296 K. Fe(CO)3(C3H6)2 forms by addition of propene to Fe(CO)3(η2-C3H6), which is in equilibrium with HFe(CO)3(η3-C3H5), with a phenomenological rate constant Keqk+ = (4.5 ± 0.1) × 10–16 cm3 molecules–1 s–1. A van’t Hoff plot gives ΔH = 7.2 ± 0.6 kcal mol–1 and ΔS = 3 ± 2 cal K–1 mol–1 for the process HFe(CO)3(η3-C3H5) → Fe(CO)3(η2-C3H6), assuming that the rate constant for addition of C3H6 to Fe(CO)3(C3H6), k+, is ∼ 1.6 × 10–11 cm3 molecules–1 s–1. These results can be quantitatively related to a catalytic cycle for olefin isomerization.
ASJC Scopus subject areas
- Colloid and Surface Chemistry