Abstract
High-valent metal-oxo complexes have been extensively studied over the years due to their intriguing properties and their abundant catalytic potential. The majority of the catalytic reactions performed by these metal-oxo complexes involves a C-H activation step and extensive efforts over the years have been undertaken to understand the mechanistic aspects of this step. The C-H activation by metal-oxo complexes proceeds via a hydrogen atom transfer reaction and this could happen by multiple pathways, (i) via a proton-transfer followed by an electron transfer (PT-ET), (ii) via an electron-transfer followed by a proton transfer (ET-PT), (iii) via a concerted proton-coupled electron transfer (PCET) mechanism. Identifying the right mechanism is a surging topic in this area and here using [Mn(III)H3buea(O)](2-) (1) and [Mn(IV)H3buea(O)](-) (2) species (where H3buea = tris[(N'-tert-butylureaylato)-N-ethylene]aminato) and its C-H activation reaction with dihydroanthracene (DHA), we have explored the mechanism of hydrogen atom transfer reactions. The experimental kinetic data reported earlier (T. H. Parsell, M.-Y. Yang and A. S. Borovik, J. Am. Chem. Soc., 2009, 131, 2762) suggests that the mechanism between 1 and 2 is drastically different. By computing the transition states, reaction energies and by analyzing the wavefunction of the reactant and transitions states, we authenticate the proposal that the Mn(III)=O undergoes a step wise PT-ET mechanism where as the Mn(IV)=O species undergo a concerted PCET mechanism. Both the species pass through a [Mn(III)-OH] intermediate and the stability of this species hold the key to the difference in the reactivity. The electronic origin for the difference in reactivity is routed back to the strength and basicity of the Mn-oxo bond and the computed results are in excellent agreement with the experimental results.
Highlights
Selective C–H bond oxidative cleavage of aromatic/aliphatic hydrocarbons is one of the important synthetic transformations in enzymatic and industrial processes
The C–H activation by metal–oxo complexes proceeds via a hydrogen atom transfer reaction and this could happen by multiple pathways, (i) via a proton-transfer followed by an electron transfer (PT-ET), (ii) via an electron-transfer followed by a proton transfer (ET-PT), (iii) via a concerted proton-coupled electron transfer (PCET) mechanism
Identifying the right mechanism is a surging topic in this area and here using [MnIIIH3buea(O)]2− (1) and [MnIVH3buea(O)]− (2) species (where H3buea = tris[(N’-tert-butylureaylato)-N-ethylene]aminato) and its C–H activation reaction with dihydroanthracene (DHA), we have explored the mechanism of hydrogen atom transfer reactions
Summary
Target other metal based oxo complexes for achieving the C–H bond oxidative cleavage.[5,6,7,8,9] Among the reported other metal based biomimetic model complexes, the manganese based oxo complexes have shown to possess prominent catalytic ability towards the C–H bond activation and O-atoms transfer reactions when compared to ferryl–oxo species and these model complexes are capable of mimicking the catalytic properties of lipooxygenase, cytochrome P450 and ironoxido reductase enzyme.[10,11,12,13]. The C–H bond activation by [MnIIIH3buea(O)]2− (1) suggested to proceed through a two step mechanism, proton transfer followed by electron transfer whereas in [MnIVH3buea(O)]− (2) the reaction proceeds in a single step, i.e. via proton-coupled electron transfer (PCET) step This PCET mechanism is suggested as the possible mechanism for iron and manganese lipooxygenase and cytochrome P450 enzymes.[19,20,21,22,23,24] Further the mechanistic studies suggest that 1 follows anionic mechanism attributed to the strong basic character of the oxo group (larger pKa) compared to species 2. The energy decomposition analysis (EDA)[41,42,43] was performed at the same level of theory using AOMIX software.[44,45,46]
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