American Chemical Society
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Interplay of Experiment and Theory in Elucidating Mechanisms of Oxidation Reactions by a Nonheme RuIVO Complex

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journal contribution
posted on 2015-07-08, 00:00 authored by Sunder N. Dhuri, Kyung-Bin Cho, Yong-Min Lee, Sun Young Shin, Jin Hwa Kim, Debasish Mandal, Sason Shaik, Wonwoo Nam
A comprehensive experimental and theoretical study of the reactivity patterns and reaction mechanisms in alkane hydroxylation, olefin epoxidation, cyclohexene oxidation, and sulfoxidation reactions by a mononuclear nonheme ruthenium­(IV)–oxo complex, [RuIV(O)­(terpy)­(bpm)]2+ (1), has been conducted. In alkane hydroxylation (i.e., oxygen rebound vs oxygen non-rebound mechanisms), both the experimental and theoretical results show that the substrate radical formed via a rate-determining H atom abstraction of alkanes by 1 prefers dissociation over oxygen rebound and desaturation processes. In the oxidation of olefins by 1, the observations of a kinetic isotope effect (KIE) value of 1 and styrene oxide formation lead us to conclude that an epoxidation reaction via oxygen atom transfer (OAT) from the RuIVO complex to the CC double bond is the dominant pathway. Density functional theory (DFT) calculations show that the epoxidation reaction is a two-step, two-spin-state process. In contrast, the oxidation of cyclohexene by 1 affords products derived from allylic C–H bond oxidation, with a high KIE value of 38(3). The preference for H atom abstraction over CC double bond epoxidation in the oxidation of cyclohexene by 1 is elucidated by DFT calculations, which show that the energy barrier for C–H activation is 4.5 kcal mol–1 lower than the energy barrier for epoxidation. In the oxidation of sulfides, sulfoxidation by the electrophilic Ru–oxo group of 1 occurs via a direct OAT mechanism, and DFT calculations show that this is a two-spin-state reaction in which the transition state is the lowest in the S = 0 state.