Chivukula V. Sastri

Visible light assisted photocatalytic hydrogen generation and organic dye degradation by CdS–metal oxide hybrids in presence of graphene oxide

Single step hydrothermal synthesis of CdS/Oxide (Oxide = ZnO, Al2O3) was demonstrated and examines their photocatalytic activity in presence of graphene oxide (GO). CdS/Al2O3/GO and CdS/ZnO/GO both exhibits enhanced photocatalytic activity for hydrogen generation with apparent quantum yields (AQY) of 14% and 30% respectively. Moreover, CdS/Oxide/GO displayed efficient photodegradation of an organic dye; ∼90% for CdS/Al2O3/GO and ∼99% for CdS/ZnO/GO within 60 min of time interval. Superior photocatalytic properties are attributed to the enhanced surface area and effective separation of photoinduced charge carriers due to the presence of GO. The present study highlights the potential application of graphene based materials in the field of energy conversion and environment remediation.

Nonheme ferric hydroperoxo intermediates are efficient oxidants of bromide oxidation

This work presents the first combined experimental and computational study that gives evidence of the electrophilic reactivity of a nonheme iron(III)-hydroperoxo species. We show that in contrast to their heme counterparts the nonheme iron(III)-hydroperoxo complexes are catalytically much more active and even more so than nonheme iron(IV)-oxo species.

Comparison of the Reactivity of Nonheme Iron(IV)–Oxo versus Iron(IV)–Imido Complexes: Which is the Better Oxidant?

Which is better? The first detailed comparison of the reactivity of nonheme iron(IV)-imido versus nonheme iron(IV)-oxo intermediates with substrates is presented. The iron(IV)-imido variant reacts with sulfides five times faster than iron(IV)-oxo, whereas the reverse trend is observed for hydrogen atom abstraction. These observed trends are analyzed and explained.

Mechanistic insight into halide oxidation by non-heme iron complexes. Haloperoxidase versus halogenase activity

This work presents the first detailed study on mechanistic aspects of halide oxidation by non-heme iron complexes. We show that while iron(III)-hydroperoxo complexes oxidise halides via oxygen atom transfer, the corresponding iron(IV)-oxo complex reacts via electron transfer.

Differences and Comparisons of the Properties and Reactivities of Iron(III)–hydroperoxo Complexes with Saturated Coordination Sphere

Heme and nonheme monoxygenases and dioxygenases catalyze important oxygen atom transfer reactions to substrates in the body. It is now well established that the cytochrome P450 enzymes react through the formation of a high-valent iron(IV)–oxo heme cation radical. Its precursor in the catalytic cycle, the iron(III)–hydroperoxo complex, was tested for catalytic activity and found to be a sluggish oxidant of hydroxylation, epoxidation and sulfoxidation reactions. In a recent twist of events, evidence has emerged of several nonheme iron(III)–hydroperoxo complexes that appear to react with substrates via oxygen atom transfer processes. Although it was not clear from these studies whether the iron(III)–hydroperoxo reacted directly with substrates or that an initial O–O bond cleavage preceded the reaction. Clearly, the catalytic activity of heme and nonheme iron(III)–hydroperoxo complexes is substantially different, but the origins of this are still poorly understood and warrant a detailed analysis. In this work, an extensive computational analysis of aromatic hydroxylation by biomimetic nonheme and heme iron systems is presented, starting from an iron(III)–hydroperoxo complex with pentadentate ligand system (L52). Direct C–O bond formation by an iron(III)–hydroperoxo complex is investigated, as well as the initial heterolytic and homolytic bond cleavage of the hydroperoxo group. The calculations show that [(L52)FeIII(OOH)]2+ should be able to initiate an aromatic hydroxylation process, although a low-energy homolytic cleavage pathway is only slightly higher in energy. A detailed valence bond and thermochemical analysis rationalizes the differences in chemical reactivity of heme and nonheme iron(III)–hydroperoxo and show that the main reason for this particular nonheme complex to be reactive comes from the fact that they homolytically split the O–O bond, whereas a heterolytic O–O bond breaking in heme iron(III)–hydroperoxo is found.

Influence of ligand architecture on oxidation reactions by high-valent nonheme manganese oxo complexes using water as a source of oxygen.

Mononuclear nonheme Mn(IV)=O complexes with two isomers of a bispidine ligand have been synthesized and characterized by various spectroscopies and density functional theory (DFT). The Mn(IV)=O complexes show reactivity in oxidation reactions (hydrogen-atom abstraction and sulfoxidation). Interestingly, one of the isomers (L(1) ) is significantly more reactive than the other (L(2) ), while in the corresponding Fe(IV)=O based oxidation reactions the L(2) -based system was previously found to be more reactive than the L(1) -based catalyst. This inversion of reactivities is discussed on the basis of DFT and molecular mechanics (MM) model calculations, which indicate that the order of reactivities are primarily due to a switch of reaction channels (σ versus π) and concomitant steric effects.

Influence of Ligand Architecture in Tuning Reaction Bifurcation Pathways for Chlorite Oxidation by Non-Heme Iron Complexes

Reaction bifurcation processes are often encountered in the oxidation of substrates by enzymes and generally lead to a mixture of products. One particular bifurcation process that is common in biology relates to electron transfer versus oxygen atom transfer by high-valent iron(IV)-oxo complexes, which nature uses for the oxidation of metabolites and drugs. In biomimicry and bioremediation, an important reaction relates to the detoxification of ClOx- in water, which can lead to a mixture of products through bifurcated reactions. Herein we report the first three water-soluble non-heme iron(II) complexes that can generate chlorine dioxide from chlorite at ambient temperature and physiological pH. These complexes are highly active oxygenation oxidants and convert ClO2- into either ClO2 or ClO3¯ via high-valent iron(IV)-oxo intermediates. We characterize the short-lived iron(IV)-oxo species and establish rate constants for the bifurcation mechanism leading to ClO2 and ClO3- products. We show that the ligand architecture of the metal center plays a dominant role by lowering the reduction potential of the metal center. Our experiments are supported by computational modeling, and a predictive valence bond model highlights the various factors relating to the substrate and oxidant that determine the bifurcation pathway and explains the origins of the product distributions. Our combined kinetic, spectroscopic, and computational studies reveal the key components necessary for the future development of efficient chlorite oxidation catalysts.