title: Activation parameters as mechanistic probes in the TAML iron(V)-oxo oxidations of hydrocarbons.
abstract: The results of low-temperature investigations of the oxidations of 9,10-dihydroanthracene, cumene, ethylbenzene, [D10]ethylbenzene, cyclooctane, and cyclohexane by an iron(V)-oxo TAML complex (2; see Figure 1) are presented, including product identification and determination of the second-order rate constants k2 in the range 233-243 K and the activation parameters (ΔH(≠) and ΔS(≠)). Statistically normalized k2 values (log k2') correlate linearly with the C-H bond dissociation energies DC-H, but ΔH(≠) does not. The point for 9,10-dihydroanthracene for the ΔH(≠) vs. DC-H correlation lies markedly off a common straight line of best fit for all other hydrocarbons, suggesting it proceeds via an alternate mechanism than the rate-limiting C-H bond homolysis promoted by 2. Contribution from an electron-transfer pathway may be substantial for 9,10-dihydroanthracene. Low-temperature kinetic measurements with ethylbenzene and [D10]ethylbenzene reveal a kinetic isotope effect of 26, indicating tunneling. The tunnel effect is drastically reduced at 0 °C and above, although it is an important feature of the reactivity of TAML activators at lower temperatures. The diiron(IV) μ-oxo dimer that is often a common component of the reaction medium involving 2 also oxidizes 9,10-dihydroanthracene, although its reactivity is three orders of magnitude lower than that of 2.
authors.affiliation: Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA, 15213 (USA).
authors.forename: Soumen
authors.initials: S
authors.lastname: Kundu
authors.forename: Jasper Van Kirk
authors.initials: JV
authors.lastname: Thompson
authors.forename: Longzhu Q
authors.initials: LQ
authors.lastname: Shen
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Mills
authors.forename: Emile L
authors.initials: EL
authors.lastname: Bominaar
authors.forename: Alexander D
authors.initials: AD
authors.lastname: Ryabov
authors.forename: Terrence J
authors.initials: TJ
authors.lastname: Collins
mesh_terms: D000873:Anthracenes / Q000737:chemistry; D006838:Hydrocarbons / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D007700:Kinetics; D010084:Oxidation-Reduction; D013816:Thermodynamics
chemical_list: D000873:Anthracenes; D006838:Hydrocarbons; C066318:perferryl iron; D007501:Iron
keywords: CH activation; iron; oxidation; oxo species; reaction mechanisms