Oxidative properties of a nonheme Ni(<small>II</small>)(O₂) complex: Reactivity patterns for C–H activation, aromatic hydroxylation and heteroatom oxidation

Latifi, Reza,Tahsini, Laleh,Kumar, Devesh,Sastry, G. Narahari,Nam, Wonwoo,de Visser, Sam P. Royal Society of Chemistry 2011 Chemical communications Vol.47 No.38

<P>Density functional theory calculations on the reactivity of a Ni(<SMALL>II</SMALL>)-superoxo complex in C–H bond activation, aromatic hydroxylation and heteroatom oxidation reactions have been explored; the Ni(<SMALL>II</SMALL>)-superoxo complex is able to react with substrates with weak C–H bonds and PPh<SUB>3</SUB>.</P> <P>Graphic Abstract</P><P>Density functional theory calculations on the reactivity of a Ni(<SMALL>II</SMALL>)-superoxo complex in C–H bond activation, aromatic hydroxylation and heteroatom oxidation reactions have been explored. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c1cc13993b'> </P>

Regioselectivity of aliphatic <i>versus</i> aromatic hydroxylation by a nonheme iron(<small>II</small>)-superoxo complex

Latifi, Reza,Tahsini, Laleh,Nam, Wonwoo,de Visser, Sam P. The Royal Society of Chemistry 2012 Physical chemistry chemical physics Vol.14 No.7

<P>Many enzymes in nature utilize molecular oxygen on an iron center for the catalysis of substrate hydroxylation. In recent years, great progress has been made in understanding the function and properties of iron(<SMALL>IV</SMALL>)-oxo complexes; however, little is known about the reactivity of iron(<SMALL>II</SMALL>)-superoxo intermediates in substrate activation. It has been proposed recently that iron(<SMALL>II</SMALL>)-superoxo intermediates take part as hydrogen abstraction species in the catalytic cycles of nonheme iron enzymes. To gain insight into oxygen atom transfer reactions by the nonheme iron(<SMALL>II</SMALL>)-superoxo species, we performed a density functional theory study on the aliphatic and aromatic hydroxylation reactions using a biomimetic model complex. The calculations show that nonheme iron(<SMALL>II</SMALL>)-superoxo complexes can be considered as effective oxidants in hydrogen atom abstraction reactions, for which we find a low barrier of 14.7 kcal mol<SUP>−1</SUP> on the sextet spin state surface. On the other hand, electrophilic reactions, such as aromatic hydroxylation, encounter much higher (>20 kcal mol<SUP>−1</SUP>) barrier heights and therefore are unlikely to proceed. A thermodynamic analysis puts our barrier heights into a larger context of previous studies using nonheme iron(<SMALL>IV</SMALL>)-oxo oxidants and predicts the activity of enzymatic iron(<SMALL>II</SMALL>)-superoxo intermediates.</P> <P>Graphic Abstract</P><P>Calculations show that iron(<SMALL>II</SMALL>)-superoxo is a possible oxidant in hydrogen abstraction reactions but not in aromatic hydroxylation reactions. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cp23352e'> </P>

Effect of Porphyrin Ligands on the Regioselective Dehydrogenation versus Epoxidation of Olefins by Oxoiron(IV) Mimics of Cytochrome P450^†

Kumar, Devesh,Tahsini, Laleh,de Visser, Sam P.,Kang, Hye Yeon,Kim, Soo Jeong,Nam, Wonwoo American Chemical Society 2009 The Journal of physical chemistry A Vol.113 No.43

<P>The cytochromes P450 are versatile enzymes involved in various catalytic oxidation reactions, such as hydroxylation, epoxidation and dehydrogenation. In this work, we present combined experimental and theoretical studies on the change of regioselectivity in cyclohexadiene oxidation (i.e., epoxidation vs dehydrogenation) by oxoiron(IV) porphyrin complexes bearing different porphyrin ligands. Our experimental results show that meso-substitution of the porphyrin ring with electron-withdrawing substituents leads to a regioselectivity switch from dehydrogenation to epoxidation, affording the formation of epoxide as a major product. In contrast, electron-rich iron porphyrins are shown to produce benzene resulting from the dehydrogenation of cyclohexadiene. Density functional theory (DFT) calculations on the regioselectivity switch of epoxidation vs dehydrogenation have been performed using three oxoiron(IV) porphyrin oxidants with hydrogen atoms, phenyl groups, and pentachlorophenyl (ArCl<SUB>5</SUB>) groups on the meso-position. The DFT studies show that the epoxidation reaction by the latter catalyst is stabilized because of favorable interactions of the substrate with halogen atoms of the meso-ligand as well as with pyrrole nitrogen atoms of the porphyrin macrocycle. Hydrogen abstraction transition states, in contrast, have a substrate-binding orientation further away from the porphyrin pyrrole nitrogens, and they are much less stabilized. Finally, the regioselectivity of dehydrogenation versus hydroxylation is rationalized using thermodynamic cycles.</P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp9028694'>ACS Electronic Supporting Info</A></P>

How Does the Axial Ligand of Cytochrome P450 Biomimetics Influence the Regioselectivity of Aliphatic versus Aromatic Hydroxylation?

de Visser, Sam P.,Tahsini, Laleh,Nam, Wonwoo WILEY-VCH Verlag 2009 Chemistry Vol.15 No.22

<P>How deep is your orbital? Density functional theory studies on the axial ligand effect of aliphatic versus aromatic hydroxylation of ethylbenzene by iron–oxo complexes with a variable axial ligand show that strong (anionic) ligands pull the metal inside the plane of the haeme and destabilise cationic intermediates through orbital interactions (see picture). <img src='wiley_img/09476539-2009-15-22-CHEM200802234-content.gif' alt='wiley_img/09476539-2009-15-22-CHEM200802234-content'> </P><P>The catalytic activity of high-valent iron–oxo active species of heme enzymes is known to be dependent on the nature of the axial ligand trans to the iron–oxo group. In a similar fashion, experimental studies on iron–oxo porphyrin biomimetic systems have shown a significant axial ligand effect on ethylbenzene hydroxylation, with an axial acetonitrile ligand leading to phenyl hydroxylation products and an axial chloride anion giving predominantly benzyl hydroxylation products. To elucidate the fundamental factors that distinguish this regioselectivity reversal in iron–oxo porphyrin catalysis, we have performed a series of density functional theory calculations on the hydroxylation of ethylbenzene by [Fe<SUP>IV</SUP>&n.dbond;O(Por<SUP>+.</SUP>)L] (Por=porphyrin; L=NCCH<SUB>3</SUB> or Cl<SUP>−</SUP>), which affords 1-phenylethanol and p-ethylphenol products. The calculations confirm the experimentally determined product distributions. Furthermore, a detailed analysis of the electronic differences between the two oxidants shows that their reversed regioselectivity is a result of differences in orbital interactions between the axial ligand and iron–oxo porphyrin system. In particular, three high-lying orbitals (π*<SUB>xz</SUB>, π*<SUB>yz</SUB> and a<SUB>2u</SUB>), which are singly occupied in the reactant complex, are stabilised with an anionic ligand such as Cl<SUP>−</SUP>, which leads to enhanced HOMO–LUMO energy gaps. As a consequence, reactions leading to cationic intermediates through the two-electron reduction of the metal centre are disfavoured. The aliphatic hydroxylation mechanism, in contrast, is a radical process in which only one electron is transferred in the rate-determining transition state, which means that the effect of the axial ligand on this mechanism is much smaller.</P> <B>Graphic Abstract</B> <P>How deep is your orbital? Density functional theory studies on the axial ligand effect of aliphatic versus aromatic hydroxylation of ethylbenzene by iron–oxo complexes with a variable axial ligand show that strong (anionic) ligands pull the metal inside the plane of the haeme and destabilise cationic intermediates through orbital interactions (see picture). <img src='wiley_img/09476539-2009-15-22-CHEM200802234-content.gif' alt='wiley_img/09476539-2009-15-22-CHEM200802234-content'> </P>

An Investigation on Constant Convection Coefficient Assumption in Decoupled Conjugate Heat Transfer

Kimia Sadafi,Amir Mahdi Tahsini,Fatemeh Ghavidel Mangodeh 한국항공우주학회 2024 International Journal of Aeronautical and Space Sc Vol.25 No.2

The accuracy of using a constant convective heat transfer coefficient of the flow field in conjugate heat transfer problems over various surface temperatures and geometries is numerically investigated. The complexity of analytical solutions as well as the acute computational cost necessitates decoupling the solid–gas interactions. The utilization of decoupled equations demands the convective heat transfer coefficient of the flow field to be independent of the surface temperature. The investigation is carried out over a number of conventional flow geometries as the laminar and turbulent flows over the flat plate, the turbulent flow behind a backward-facing step, and the laminar flow over the blunt nose in different Mach numbers. The results demonstrate that the convective heat transfer coefficient is substantially influenced by the surface temperature of the flat plate case at supersonic flows yet is barely affected by the surface temperature of the blunt nose case at subsonic flows. It is illustrated that the validation of the decoupling approach should be questioned in different flow geometries to guarantee the accuracy of simulations in practical applications.

The Axial Ligand Effect on Aliphatic and Aromatic Hydroxylation by Non-heme Iron(IV)-oxo Biomimetic Complexes.

de Visser, Sam P,Latifi, Reza,Tahsini, Laleh,Nam, Wonwoo Wiley-VCH 2011 Chemistry - An Asian Journal Vol.6 No.2

<P>Iron(IV)-oxo heme cation radicals are active species in enzymes and biomimetic model complexes. They are potent oxidants in oxygen atom transfer reactions, but the reactivity is strongly dependent on the ligand system of the iron(IV)-oxo group and in particular the nature of the ligand trans to the oxo group (the axial ligand). To find out what effect the axial ligand has on the reactivity of non-heme iron(IV)-oxo species, we have performed a series of density functional theory (DFT) calculations on aliphatic and aromatic hydroxylation reactions by using [Fe(IV) 전(TMC)(L)](n+) (TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, and L=acetonitrile or chloride). The studies show that the regioselectivity of aliphatic over aromatic hydroxylation is preferred. The studies are in good agreement with experimental product distributions. Moreover, the system with the acetonitrile axial ligand is orders of magnitude more reactive than that with a chloride axial ligand. We have analyzed our results and we have shown that the metal-ligand interactions influence the orbital energies and as a consequence also the electron affinities and hydrogen atom abstraction abilities. Thermodynamic cycles explain the regioselectivity preferences.</P>