Effect of ligand backbone on the reactivity and mechanistic paradigm of non-heme iron(IV)-oxo during olefin epoxidation.

Oxygen atom transfer (OAT) reactivity of a non-heme [Fe IV (2PyN2Q)(O)] 2+ ( 2 ) containing sterically bulky quinoline-pyridine pentadentate ligand (2PyN2Q) has been thoroughly studied with different olefins. The ferryl-oxo complex 2 shows excellent OAT reactivity during epoxidations. The steric encumbrance and electronic effect of the ligand influence the mechanistic shuttle between oxygen atom transfer pathway I (OAT) and isomerization pathway II (during the reaction stereo pure olefins), resulting in a mixture of cis - trans epoxide products. On the contrary, the sterically less hindered and electronically different [Fe IV (N4Py)(O)] 2+ ( 1 ) provides only cis -stilbene epoxide. The Hammett study (with different para -substituted styrene derivatives), i.e. log( k H / k X ) against σ P + (considering polarity and resonance effect) shows a liner plot with reaction constant, ρ + = -1 suggesting the role of dominant inductive electronic as well as resonance effect during electron transfer from olefin to 2 in the rate-limiting step . Additionally, the computational study supports the involvement of stepwise pathways during olefin epoxidation. The ferryl bend due to the bulkier ligand incorporation leads to destabilization of both d z 2 and d x 2 -y 2 orbital, leading to a very small quintet-triplet gap and enhanced reactivity for [Fe IV (2PyN2Q)(O)] 2+ compared to [Fe IV (N4Py)(O)] 2+ . Thus, the present study unveils the role of steric and electronic effects of the ligand towards mechanistic modification during olefin epoxidation.