Epoxidation of Olefins by Iodosylbenzene Catalyzed by Non-Porphyrin Metal Complexes

Nam Wonwoo,Baek Seung Joong,Kazuko I. Liao,Joan Selverstone Valentine Korean Chemical Society 1994 Bulletin of the Korean Chemical Society Vol.15 No.12

Epoxidation of olefins has been studied using iodosylbenzene (PhIO) as the terminal oxidant and binuclear and mononuclear complexes of $Mn^{2+}$, $Co^{2+}$, and $Cu^{2+}$ as catalysts. Epoxides were the predominant products with trace amounts of allylic oxidation products, and the metal complexes were found to be effective catalysts in the epoxidation reactions. The reactivity of binuclear copper complexes was greater than that of the mononuclear copper complexes, whereas the binuclear and mononuclear complexes of $Mn^{2+}$ and $Co^{2+}$ showed similar reactivities. The nature of the ligands bound to copper did not influence the reactivity of the binuclear copper complexes so long as copper ions were held in close proximity. A metal-iodosylbenzene complex, such as suggested previously for Lewis acidic metal complex-catalyzed epoxidation by iodosylbenzene, is proposed as the active epoxidizing species. Some mechanistic aspects are discussed as well.

Tuning Reactivity and Mechanism in Oxidation Reactions by Mononuclear Nonheme Iron(IV)-Oxo Complexes

Nam, Wonwoo,Lee, Yong-Min,Fukuzumi, Shunichi American Chemical Society 2014 Accounts of chemical research Vol.47 No.4

<title>Conspectus</title><P>Mononuclear nonheme iron enzymes generate high-valent iron(IV)-oxo intermediates that effect metabolically important oxidative transformations in the catalytic cycle of dioxygen activation. In 2003, researchers first spectroscopically characterized a mononuclear nonheme iron(IV)-oxo intermediate in the reaction of taurine: α-ketogultarate dioxygenase (TauD). This nonheme iron enzyme with an iron active center was coordinated to a 2-His-1- carboxylate facial triad motif. In the same year, researchers obtained the first crystal structure of a mononuclear nonheme iron(IV)-oxo complex bearing a macrocyclic supporting ligand, [(TMC)Fe<SUP>IV</SUP>(O)]<SUP>2+</SUP> (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecene), in studies that mimicked the biological enzymes. With these breakthrough results, many other studies have examined mononuclear nonheme iron(IV)-oxo intermediates trapped in enzymatic reactions or synthesized in biomimetic reactions. Over the past decade, researchers in the fields of biological, bioinorganic, and oxidation chemistry have extensively investigated the structure, spectroscopy, and reactivity of nonheme iron(IV)-oxo species, leading to a wealth of information from these enzymatic and biomimetic studies.</P><P>This Account summarizes the reactivity and mechanisms of synthetic mononuclear nonheme iron(IV)-oxo complexes in oxidation reactions and examines factors that modulate their reactivities and change their reaction mechanisms. We focus on several reactions including the oxidation of organic and inorganic compounds, electron transfer, and oxygen atom exchange with water by synthetic mononuclear nonheme iron(IV)-oxo complexes. In addition, we recently observed that the C–H bond activation by nonheme iron(IV)-oxo and other nonheme metal(IV)-oxo complexes does not follow the H-atom abstraction/oxygen-rebound mechanism, which has been well-established in heme systems.</P><P>The structural and electronic effects of supporting ligands on the oxidizing power of iron(IV)-oxo complexes are significant in these reactions. However, the difference in spin states between nonheme iron(IV)-oxo complexes with an octahedral geometry (with an <I>S</I> = 1 intermediate-spin state) or a trigonal bipyramidal (TBP) geometry (with an <I>S</I> = 2 high-spin state) does not lead to a significant change in reactivity in biomimetic systems. Thus, the importance of the high-spin state of iron(IV)-oxo species in nonheme iron enzymes remains unexplained. We also discuss how the axial and equatorial ligands and binding of redox-inactive metal ions and protons to the iron-oxo moiety influence the reactivities of the nonheme iron(IV)-oxo complexes. We emphasize how these changes can enhance the oxidizing power of nonheme metal(IV)-oxo complexes in oxygen atom transfer and electron-transfer reactions remarkably. This Account demonstrates great advancements in the understanding of the chemistry of mononuclear nonheme iron(IV)-oxo intermediates within the last 10 years.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/achre4/2014/achre4.2014.47.issue-4/ar400258p/production/images/medium/ar-2013-00258p_0012.gif'></P>

Hydrogen Atom Transfer Reactions of Mononuclear Nonheme Metal-Oxygen Intermediates

Nam, Wonwoo,Lee, Yong-Min,Fukuzumi, Shunichi American Chemical Society 2018 Accounts of chemical research Vol.51 No.9

<P><B>Conspectus</B></P><P>Molecular oxygen (O<SUB>2</SUB>), the greenest oxidant, is kinetically stable in the oxidation of organic substrates due to its triplet ground state. In nature, O<SUB>2</SUB> is reduced by two electrons with two protons to produce hydrogen peroxide (H<SUB>2</SUB>O<SUB>2</SUB>) and by four electrons with four protons to produce water (H<SUB>2</SUB>O) by oxidase and oxygenase metalloenzymes. In the process of the two-electron/two-proton and four-electron/four-proton reduction of O<SUB>2</SUB> by metalloenzymes and their model compounds, metal-oxygen intermediates, such as metal-superoxido, −peroxido, −hydroperoxido, and −oxido species, are generated depending on the numbers of electrons and protons involved in the O<SUB>2</SUB> activation reactions. The one-electron reduction of metal-oxygen intermediates is coupled with the binding of one proton. Such a hydrogen atom transfer (HAT) is defined as proton-coupled electron transfer (PCET), and there is a mechanistic dichotomy whether HAT occurs via a concerted PCET pathway or stepwise pathways [i.e., electron transfer followed by proton transfer (ET/PT) or proton transfer followed by electron transfer (PT/ET)]. The metal-oxygen intermediates formed are oxidants that can abstract a hydrogen atom (H-atom) from substrate C-H bonds. The H-atom abstraction from substrate C-H bonds by the metal-oxygen intermediates can also occur via a concerted PCET or stepwise PCET pathways. In the PCET reactions, a proton can be provided not only by the substrate itself but also by an acid that is added to a reaction solution.</P><P>This Account describes the reactivities of metal-oxygen intermediates, such as metal-superoxido, −peroxido, −hydroperoxido, and −oxido complexes, in HAT reactions, focusing on the mechanisms of PCET reactions of metal-oxygen intermediates and on the mechanistic dichotomy of concerted versus stepwise pathways. Recent developments in the reactivity studies of Cr-, Fe-, and Cu-superoxido complexes in H-atom and hydride transfer reactions are discussed. Reactivities of an iron(III)-hydroperoxido complex and an iron(III)-peroxido complex binding redox-inactive metal ions are also summarized briefly. Mononuclear nonheme iron(IV)- and manganese(IV)-oxido complexes have shown high reactivities in HAT reactions, and their chemistry in PCET reactions is discussed intensively. Acid-catalyzed HAT reactions of metal-oxygen intermediates are also discussed to demonstrate a unified driving force dependence of logarithm of the rate constants of acid-catalyzed oxidation of various substrates by an iron(IV)-oxido complex and that of PCET from one-electron donors to the iron(IV)-oxido complex. PCET reactions of metal-oxygen intermediates are shown to proceed via a concerted pathway (one-step HAT) or a stepwise ET/PT pathway depending on the ET and PCET driving forces (−Δ<I>G</I>). The boundary conditions between concerted versus stepwise PCET pathways are clarified to demonstrate a switchover of the mechanisms only by changing the reaction temperature in the boundary conditions. This Account summarizes recent developments in the HAT reactions by synthetic mononuclear nonheme metal-oxygen intermediates over the past 10 years.</P> [FIG OMISSION]</BR>

CCB-OCC 시스템을 위한 YOLOv3 기반 패킷 동기화 기술 연구

남원우(Wonwoo Nam),김병욱(Byung Wook Kim) 대한전자공학회 2020 대한전자공학회 학술대회 Vol.2020 No.8

Secure Communications With Asymptotically Gaussian Compressed Encryption

Cho, Wonwoo,Yu, Nam Yul IEEE Signal Processing Society 2018 IEEE signal processing letters Vol.25 No.1

<P>In this letter, we study the security of a cryptosystem over wireless channels that employs the asymptotically Gaussian compressed encryption. We investigate the indistinguishability and the energy sensitivity of the cryptosystem, where the total variation (TV) distance is examined as a statistical measure for the indistinguishability. To characterize the TV distance, we compute the Hellinger distance between probability distributions of ciphertexts, each of which can be modeled as a circularly symmetric complex Gaussian random vector with a constraint on plaintexts. Using the distance metrics, we show that the cryptosystem can be a promising option for secure wireless communications by guaranteeing the indistinguishability against an eavesdropper, as long as each plain text has constant energy.</P>

Photofunctional triplet excited states of cyclometalated Ir(<small>III</small>) complexes: beyond electroluminescence

You, Youngmin,Nam, Wonwoo Royal Society of Chemistry, etc 2012 Chemical Society reviews Vol.41 No.21

<P>The development of cyclometalated Ir(<SMALL>III</SMALL>) complexes has enabled important breakthroughs in electroluminescence because such complexes permit the efficient population of triplet excited states that give rise to luminescent transitions. The triplet states of Ir(<SMALL>III</SMALL>) complexes are advantageous over those of other transition metal complexes in that their electronic transitions and charge-transfer characteristics are tunable over wide ranges. These favorable properties suggest that Ir(<SMALL>III</SMALL>) complexes have significant potential in a variety of photofunctions other than electroluminescence. In this <I>critical review</I>, we describe recent photonic applications of novel Ir(<SMALL>III</SMALL>) complexes. Ir(<SMALL>III</SMALL>) complexes have been shown to affect the exciton statistics in the active layers of organic photovoltaic cells, thereby improving the photon-to-current conversion efficiencies. Nonlinear optical applications that take advantage of the strong charge-transfer properties of triplet transitions are also discussed. The tunability of the electrochemical potentials facilitates the development of efficient photocatalysis in the context of water photolysis or organic syntheses. The photoredox reactivities of Ir(<SMALL>III</SMALL>) complexes have been employed in studies of charge migration along DNA chains. The photoinduced cytotoxicity of Ir(<SMALL>III</SMALL>) complexes on live cells suggests that the complexes may be useful in photodynamic therapy. Potential biological applications of the complexes include phosphorescence labeling and sensing. Intriguing platforms based on cyclometalated Ir(<SMALL>III</SMALL>) complexes potentially provide novel protein tagging and ratiometric detection. We envision that future research into the photofunctionality of Ir(<SMALL>III</SMALL>) complexes will provide important breakthroughs in a variety of photonic applications.</P> <P>Graphic Abstract</P><P>Principles of triplet state photofunctionalities of cyclometalated Ir(<SMALL>III</SMALL>) complexes are explained with recent examples of a variety of photoelectronic applications. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cs35171d'> </P>

Synthesis and Structure of the New Fe complex, [FeIICl3(PEt3)]-

Jaehong Han*,Wonwoo Nam 대한화학회 2004 Bulletin of the Korean Chemical Society Vol.25 No.6

A theoretical study into a <i>trans</i>-dioxo Mn^V porphyrin complex that does not follow the oxygen rebound mechanism in C–H bond activation reactions

Cho, Kyung-Bin,Nam, Wonwoo The Royal Society of Chemistry 2016 Chemical communications Vol.52 No.5

<P>Previous experimental results revealed that the C-H bond activation reaction by a synthetic trans-dioxo Mn-V porphyrin complex, [(TF(4)TMAP)(OMnO)-O-V](3+), does not occur via the well-known oxygen rebound mechanism, which has been well demonstrated in (FeO)-O-IV porphyrin pi-cation radical reactions. In the present study, theoretical calculations offer an explanation through the energetics involved in the C-H bond activation reaction, where a multi-spin state scenario cannot be excluded.</P>

Cyclometalated Iridium(III) Complexes for Phosphorescence Sensing of Biological Metal Ions

You, Youngmin,Cho, Somin,Nam, Wonwoo American Chemical Society 2014 Inorganic Chemistry Vol.53 No.4

<P>Phosphorescence signaling provides a valuable alternative to conventional bioimaging based on fluorescence. The benefits of using phosphorescent molecules include improved sensitivity and capabilities for effective elimination of background signals by time-gated acquisition. Cyclometalated Ir(III) complexes are promising candidates for facilitating phosphorescent bioimaging because they provide synthetic versatility and excellent phosphorescence properties. In this Forum Article, we present our recent studies on the development of phosphorescence sensors for the detection of metal ions based on cyclometalated iridium(III) complexes. The constructs contained cyclometalating (C^N) ligands with the electron densities and band-gap energies of the C^N ligand structures systematically varied. Receptors that chelated zinc, cupric, and chromium ions were tethered to the ligands to create phosphorescence sensors. The alterations in the C^N ligand structures had a profound influence on the phosphorescence responses to metal ions. Mechanistic studies suggested that the phosphorescence responses could be explained on the basis of the modulation of photoinduced electron transfer (PeT) from the receptor to the photoexcited iridium species. The PeT behaviors strictly adhered to the Rehm–Weller principle, and the occurrence of PeT was located in the Marcus-normal region. It is thus anticipated that improved responses will be obtainable by increasing the excited-state reduction potential of the iridium(III) complexes. Femtosecond transient absorption experiments provided evidence for the presence of an additional photophysical mechanism that involved metal-ion-induced alteration of the intraligand charge-transfer (ILCT) transition state. Utility of the mechanism by PeT and ILCT has been demonstrated for the phosphorescence sensing of biologically important transition-metal ions. In particular, the phosphorescence zinc sensor could report the presence of intracellular zinc pools by using confocal laser scanning microscopy and photoluminescence lifetime imaging microscopy techniques. We hope that the significant knowledge gained from our studies will be of great help in the design of new molecules as phosphorescence sensors.</P><P>Cyclometalated iridium(III) complexes serve as useful platforms for the creation of phosphorescence sensors for the detection of biological metal ions. Control in the ligand structures of the iridium(III) complexes is a versatile approach for maximizing the biosensing capabilities.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/inocaj/2014/inocaj.2014.53.issue-4/ic4013872/production/images/medium/ic-2013-013872_0014.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ic4013872'>ACS Electronic Supporting Info</A></P>

The Effect and Influence of <i>cis</i>-Ligands on the Electronic and Oxidizing Properties of Nonheme Oxoiron Biomimetics. A Density Functional Study^†

de Visser, Sam P.,Nam, Wonwoo American Chemical Society 2008 The Journal of physical chemistry A Vol.112 No.50

<P>Density functional theory studies on the nature of the cis effect and cis influence of ligands on oxoiron nonheme complexes have been performed. A detailed analysis of the electronic and oxidizing properties of [Fe<SUP>IV</SUP>═O(TPA)L]<SUP>+</SUP> with L = F<SUP>−</SUP>, Cl<SUP>−</SUP>, and Br<SUP>−</SUP> and TPA = tris-(2-pyridylmethyl)amine are presented and compared with [Fe<SUP>IV</SUP>═O(TPA)NCCH<SUB>3</SUB>]<SUP>2+</SUP>. The calculations show that the electronic cis effect is determined by favorable orbital overlap between first-row elements with the metal, which are missing between the metal and second- and third-row elements. As a consequence, the metal 3d block is split into a one-below-two set of orbitals with L = Cl<SUP>−</SUP> and Br<SUP>−</SUP>, and the HOMO/LUMO energy gap is widened with respect to the system with L = F<SUP>−</SUP>. However, this larger HOMO/LUMO gap does not lead to large differences in electron affinities of the complexes. Moreover, a quantum mechanical analysis of the binding of the ligand shows that it is built up from a large electric field effect of the ligand on the oxoiron species and a much smaller quantum mechanical effect due to orbital overlap. These contributions are of similar strength for the three tested halogen cis ligands and result in similar reactivity patterns with substrates. The calculations show that [Fe<SUP>IV</SUP>═O(TPA)L]<SUP>+</SUP> with L = F<SUP>−</SUP>, Cl<SUP>−</SUP>, and Br<SUP>−</SUP> have closely lying triplet and quintet spin states, but only the quintet spin state is reactive with substrates. Therefore, the efficiency of the oxidant will be determined by the triplet−quintet spin state crossing of the reaction. The reaction of styrene with a doubly charged reactant, that is, [Fe<SUP>V</SUP>═O(TPA)L]<SUP>2+</SUP> with L = F<SUP>−</SUP>, Cl<SUP>−</SUP>, and Br<SUP>−</SUP> or [Fe<SUP>V</SUP>═O(TPA)NCCH<SUB>3</SUB>]<SUP>3+</SUP>, leads to an initial electron transfer from the substrate to the metal followed by a highly exothermic epoxidation mechanism. These reactivity differences are mainly determined by the overall charge of the system rather than the nature of the cis ligand.</P>