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Activation Parameters as Mechanistic Probes in the TAML Iron(V)–Oxo Oxidations of Hydrocarbons
Author(s) -
Kundu Soumen,
Thompson Jasper Van Kirk,
Shen Longzhu Q.,
Mills Matthew R.,
Bominaar Emile L.,
Ryabov Alexander D.,
Collins Terrence J.
Publication year - 2015
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201405024
Subject(s) - ethylbenzene , chemistry , cyclooctane , homolysis , cumene , cyclohexane , reaction rate constant , reactivity (psychology) , kinetic isotope effect , hydrogen atom abstraction , dissociation (chemistry) , photochemistry , dimer , bond cleavage , bond dissociation energy , kinetics , organic chemistry , radical , benzene , deuterium , catalysis , medicine , physics , alternative medicine , pathology , quantum mechanics
The results of low‐temperature investigations of the oxidations of 9,10‐dihydroanthracene, cumene, ethylbenzene, [D 10 ]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 k 2 in the range 233–243 K and the activation parameters (Δ H ≠ and Δ S ≠ ). Statistically normalized k 2 values (log k 2 ′) correlate linearly with the CH bond dissociation energies D CH , but Δ H ≠ does not. The point for 9,10‐dihydroanthracene for the Δ H ≠ vs. D CH 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 CH 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 [D 10 ]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 .