& Reaction Mechanisms Activation Parameters as Mechanistic Probes in the TAML Iron(V)– Oxo Oxidations of Hydrocarbons Soumen Kundu, Jasper Van Kirk Thompson, Longzhu Q. Shen, Matthew R. Mills, Emile L. Bominaar, Alexander D. Ryabov,* and Terrence J. Collins* [a] Abstract: The results of low-temperature investigations of the oxidations of 9,10-dihydroanthracene, cumene, ethylben- zene, [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 (DH ° and DS ° ). Statistically normalized k 2 values (log k 2 ’) correlate linearly with the C H bond dissociation energies D CH , but DH ° does not. The point for 9,10-dihydroanthracene for the DH ° vs. D CH corre- lation lies markedly off a common straight line of best fit for all other hydrocarbons, suggesting it proceeds via an alter- nate 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 [D 10 ]ethylbenzene reveal a kinetic isotope effect of 26, indicating tunneling. The tunnel effect is drastically reduced at 0 8C and above, although it is an important feature of the reactivity of TAML activators at lower temperatures. The diiron(IV) m-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. Introduction TAML activators such as 1 (Figure 1) were designed at Carnegie Mellon University (CMU) to be functional replicas of peroxidase and cytochrome P450 enzymes. [1–4] In iron TAML systems, mul- tiple Fe IV derivatives in aqueous solutions [5, 6] and iron(V)–oxo species in organic nitriles [7] are readily accessible. The TAML iron(V)–oxo complexes resemble the active sites of peroxidase and cytochrome P450 oxidase enzymes. [8, 9] The reactivity stud- ies that have since followed have all been founded on detailed analyses of the spectroscopic properties of TAML iron(III), -(IV), and -(V) species, especially when generated from 1 by meta- chloroperoxybenzoic acid (mCPBA) in acetonitrile at 40 8C. [10] The elementary reactions, Fe III !Fe IV , Fe IV !Fe V , Fe V !Fe IV , and the Fe III + Fe V comproportionation, were first mapped quantita- tively as prerequisites to substrate reactivity studies. Then the first substrate oxidations were focused on the conversion of organic sulfides to the corresponding sulfoxides. The expected high reactivity of TAML iron(V)–oxo complexes was confirmed and substrate-controlled electron and oxygen-atom transfer mechanisms were revealed. [10] Following this work three years ago, we began a thorough investigation of the reactivity of the iron(V)–oxo species toward hydrocarbons. [11] While this project was being conducted, as fully reported here, related studies were carried out by two other research groups. Firstly, Sen Gupta and co-workers conducted a kinetic investigation of the oxidation of hydrocarbon C H bonds by a room tempera- ture stable, Generation V TAML iron(V)–oxo complex 3. [12] Then, Nam et al. reported on the kinetics and mechanism of oxida- tion of hydrocarbons by 2, [13] duplicating the exact study that we have been engaged in for several years. The approaches employed, the reaction conditions selected, the features of the experimental work emphasized, and the foci of the two investi- gations have turned out to be different such that in this report we will attempt to integrate the combined findings into an op- timal mechanistic assessment. The Nam group’s study and this work were performed at different temperatures—the Nam group selected 0 8C whereas we collected our data in the tem- perature range from 40 to 30 8C. This discrepancy impacts the observed chemistry. The stability of 2, which spontaneous- ly undergoes reduction, Fe V !Fe IV , [10] is considerably higher at the lower temperatures, such that kinetic data collected by monitoring the Fe V !Fe IV transformation at 0 8C for slow C H oxidations could be affected by the spontaneous reduction; we examine this difference in this work. Rate measurements at different temperatures as reported herein allow for the deter- mination of the activation parameters, DH ° and DS ° . These parameters are particularly useful for revealing intimate mechanistic details and for integrating the results from the different research groups. Herein, we establish the importance of tunneling in the C H bond oxidation by 2, which significantly controls the processes at lower temperatures. [a] Dr. S. Kundu, J. V. K. Thompson, Dr. L. Q. Shen, M. R. Mills, Prof. E. L. Bominaar, Prof. A. D. Ryabov, Prof. T. J. Collins Department of Chemistry, Carnegie Mellon University 4400 Fifth Avenue, Pittsburgh, PA, 15213 (USA) E-mail: ryabov@andrew.cmu.edu tc1u@andrew.cmu.edu Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/chem.201405024. Chem. Eur. J. 2015, 21, 1803 – 1810 # 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1803 Full Paper DOI: 10.1002/chem.201405024