Tapan Kanti PAINE
Indian Assocation for the Cultivation of Science, India
Biomimetic Approaches to Selective Catalytic Oxidations by Nonheme Iron Complexes
Dioxygen activating nonheme iron enzymes catalyze a myriad of biological oxidations such as hydroxylation of aliphatic C-H bonds, cis-dihydroxylation/epoxidation of alkenes, halogenations etc. High-valent metal-oxo intermediates, capable of transferring oxygen atoms into the C–H and C=C bonds of substrates, have been implicated as the active oxidants in enzymatic reactions. Inspired by the diverse biological oxidations by nonheme enzymes, several synthetic iron catalysts have been developed.[2,3] However, the selective electron transfer for the reductive activation of dioxygen without quenching the active species remains a major issue in performing bio-inspired oxidation catalysis with dioxygen by iron-based catalyst. Hence, most of the bio-inspired catalysts developed so far had to shunt this reduction process with peroxides or other ready oxidants. But many of the catalytic systems often exhibit non-selective oxidation of substrates due to the involvement of free radicals generated in the catalytic conditions.
To address these challenges, we have been exploring several biomimetic approaches towards catalyst design. The desired properties for iron catalysts are possible to achieve (i) by using sacrificial reductants coordinated to the metal center for controlled reduction of dioxygen, and (ii) by taking the advantage of secondary sphere coordination environment through non-covalent interactions similar to the protein environment surrounding metalloenzyme active sites. In that direction, we have developed several iron(II)-α-hydroxy acid complexes of nitrogen rich ligands that activate dioxygen leading to the generation of iron-oxygen oxidants.[5a-c] The O2-derived oxidants exhibit versatile reactivity toward hydrocarbon substrates including cis-dihydroxylation of alkenes and aliphatic C-H bond hydroxylation. Additionally, the role of secondary interaction on the stability and reactivity of iron(III)-alkylperoxo intermediates with a supporting ligand bearing urea group has been evaluated.[5d] The reactivity of the dioxygen/peroxide-derived iron-oxygen oxidants and the catalytic activity of the complexes will be presented in the talk.
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 (a) Chatterjee S. and Paine T. K., Angew. Chem. Int. Ed., 54, (2015), 9338; (b) Chatterjee S. and Paine T. K., Angew. Chem. Int. Ed., 55, (2016), 7717; (c) Chatterjee S., Bhattacharya S. and Paine T. K., Inorg. Chem., 57, (2018), 10160; (d) Ghosh I. and Paine T. K., 2018, submitted manuscript.