Theoretical studies on concerted versus two steps hydrogen atom transfer reaction by non-heme Mn-IV/III=O complexes: how important is the oxo ligand basicity in the C-H activation step?

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)H(3)buea(O)](2-) (1) and [Mn(IV)H(3)buea(O)](-) (2) species (where H(3)buea = 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.