Electronic and Steric Effects in Ligand Substitution at a Transient Organometallic Species: The Reaction of W(CO)₅(Cyclohexane) with (CH₃)<i>_n</i>THF and (CH₃)<i>_n</i>Furan (<i>n</i> = 1, 2)

Abstract

Time-resolved infrared absorption spectroscopy is used to study the reactions of the solvated transient intermediate W(CO)5(cyclohexane) with the molecules (L) 2-methyltetrahydrofuran (MeTHF), 2,5-dimethyltetrahydrofuran (Me2THF), 2-methylfuran (MeFur), and 2,5-dimethylfuran (Me2Fur). In all four cases, the only reaction observed on the microsecond to millisecond time scale is substitution of the cyclohexane molecule to form W(CO)5(L). From the temperature dependence of the reaction rate constant, measured over the range 15−60 °C, we determine the following activation parameters: for L = MeTHF, ΔH⧧ = 4.0 ± 0.3 kcal mol-1, ΔS⧧ = −14.1 ± 1.0 eu; for L = Me2THF, ΔH⧧ = 4.3 ± 0.3 kcal mol-1, ΔS⧧ = −14.8 ± 1.2 eu; for L = MeFur, ΔH⧧ = 6.0 ± 0.2 kcal mol-1, ΔS⧧ = −10.2 ± 1.2 eu; and for L = Me2Fur, ΔH⧧ = 6.3 ± 0.4 kcal mol-1, ΔS⧧ = −9.8 ± 1.0 eu. We find that the relative rates of reaction of MeTHF and Me2THF can be explained solely in terms of a steric effect, while for MeFur and Me2Fur, the inductive effect of the electron-donating substituents leads to an electronic effect that competes with the steric effect.