Redox Reactivity of a Mononuclear Manganese-Oxo Complex Binding Calcium Ion and Other Redox-Inactive Metal Ions

Sankaralingam, Muniyandi,Lee, Yong-Min,Pineda-Galvan, Yuliana,Karmalkar, Deepika G.,Seo, Mi Sook,Jeon, So Hyun,Pushkar, Yulia,Fukuzumi, Shunichi,Nam, Wonwoo American Chemical Society 2019 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.141 No.3

<P>Mononuclear nonheme manganese(IV)-oxo complexes binding calcium ion and other redox-inactive metal ions, [(dpaq)Mn<SUP>IV</SUP>(O)]<SUP>+</SUP>-M<SUP><I>n</I>+</SUP> (<B>1</B>-M<SUP>n+</SUP>, M<SUP><I>n</I>+</SUP> = Ca<SUP>2+</SUP>, Mg<SUP>2+</SUP>, Zn<SUP>2+</SUP>, Lu<SUP>3+</SUP>, Y<SUP>3+</SUP>, Al<SUP>3+</SUP>, and Sc<SUP>3+</SUP>) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-<I>N</I>-quinolin-8-yl-acetamidate), were synthesized by reacting a hydroxomanganese(III) complex, [(dpaq)Mn<SUP>III</SUP>(OH)]<SUP>+</SUP>, with iodosylbenzene (PhIO) in the presence of redox-inactive metal ions (M<SUP><I>n</I>+</SUP>). The Mn(IV)-oxo complexes were characterized using various spectroscopic techniques. In reactivity studies, we observed contrasting effects of M<SUP><I>n</I>+</SUP> on the reactivity of <B>1</B>-M<SUP><I>n</I>+</SUP> in redox reactions such as electron-transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. In the OAT and ET reactions, the reactivity order of <B>1</B>-M<SUP><I>n</I>+</SUP>, such as <B>1</B>-Sc<SUP>3+</SUP> ≈ <B>1</B>-Al<SUP>3+</SUP> > <B>1</B>-Y<SUP>3+</SUP> > <B>1</B>-Lu<SUP>3+</SUP> > <B>1</B>-Zn<SUP>2+</SUP> > <B>1</B>-Mg<SUP>2+</SUP> > <B>1</B>-Ca<SUP>2+</SUP>, follows the Lewis acidity of M<SUP><I>n</I>+</SUP> bound to the Mn-O moiety; that is, the stronger the Lewis acidity of M<SUP><I>n</I>+</SUP>, the higher the reactivity of <B>1</B>-M<SUP><I>n</I>+</SUP> becomes. In sharp contrast, the reactivity of <B>1</B>-M<SUP><I>n</I>+</SUP> in the HAT reaction was reversed, giving the reactivity order <B>1</B>-Ca<SUP>2+</SUP> > <B>1</B>-Mg<SUP>2+</SUP> > <B>1</B>-Zn<SUP>2+</SUP> > <B>1</B>-Lu<SUP>3+</SUP>> <B>1</B>-Y<SUP>3+</SUP>> <B>1</B>-Al<SUP>3+</SUP> ≈ <B>1</B>-Sc<SUP>3+</SUP>; that is, the higher is Lewis acidity of M<SUP><I>n</I>+</SUP>, the lower the reactivity of <B>1</B>-M<SUP><I>n</I>+</SUP> in the HAT reaction. The latter result implies that the Lewis acidity of M<SUP><I>n</I>+</SUP> bound to the Mn-O moiety can modulate the basicity of the metal-oxo moiety, thus influencing the HAT reactivity of <B>1</B>-M<SUP><I>n</I>+</SUP>; cytochrome P450 utilizes the axial thiolate ligand to increase the basicity of the iron-oxo moiety, which enhances the reactivity of compound I in C-H bond activation reactions.</P> [FIG OMISSION]</BR>

An amphoteric reactivity of a mixed-valent bis(μ-oxo)dimanganese(<small>III</small>,<small>IV</small>) complex acting as an electrophile and a nucleophile

Sankaralingam, Muniyandi,Jeon, So Hyun,Lee, Yong-Min,Seo, Mi Sook,Ohkubo, Kei,Fukuzumi, Shunichi,Nam, Wonwoo The Royal Society of Chemistry 2016 Dalton Transactions Vol.45 No.1

<P>A mixed-valent bis(mu-oxo)dimanganese(III,IV) complex, [(dpaq)Mn-III(O)(2)Mn-IV(dpaq)](+) (1), was prepared by reacting a hydroxomanganese(III) complex, [(dpaq)Mn-III(OH)](+), with hydrogen peroxide in the presence of triethylamine. The mixed-valent bis(mu-oxo)dimanganese(III,IV) complex (1) was well characterised by UV-vis, EPR and CSI-MS techniques. The electrophilic reactivity of 1 was investigated in the oxidation of 2,6-di- tert-butylphenol derivatives by 1, in which the relative rate afforded a good Hammett correlation with a rho value of -1.0. The nucleophilic character of 1 was then investigated in aldehyde deformylation reactions, using 2-phenylpropionaldehyde (2-PPA) and benzaldehyde derivatives as substrates. In contrast to the case of the reaction of 1 with 2,6-di-tert-butylphenol derivatives, a positive rho value of 0.89 was obtained in the Hammett plot, demonstrating that the bis(mu-oxo)-dimanganese(III,IV) complex is an active nucleophilic oxidant. Thus, 1 exhibited an amphoteric reactivity in both electrophilic and nucleophilic oxidative reactions.</P>

A Mononuclear Non-heme Manganese(III)-Aqua Complex as a New Active Oxidant in Hydrogen Atom Transfer Reactions

Sankaralingam, Muniyandi,Lee, Yong-Min,Karmalkar, Deepika G.,Nam, Wonwoo,Fukuzumi, Shunichi American Chemical Society 2018 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.140 No.40

<P>A mononuclear non-heme Mn(III)-aqua complex, [(dpaq)Mn<SUP>III</SUP>(OH<SUB>2</SUB>)]<SUP>2+</SUP> (<B>1</B>, dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-<I>N</I>-quinolin-8-yl-acetamidate), is capable of conducting hydrogen atom transfer (HAT) reactions much more efficiently than the corresponding Mn(III)-hydroxo complex, [(dpaq)Mn<SUP>III</SUP>(OH)]<SUP>+</SUP> (<B>2</B>); the high reactivity of <B>1</B> results from the positive one-electron reduction potential of <B>1</B> (<I>E</I><SUB>red</SUB> vs SCE = 1.03 V), compared to that of <B>2</B> (<I>E</I><SUB>red</SUB> vs SCE = −0.1 V). The HAT mechanism of <B>1</B> varies between electron transfer followed by proton transfer and one-step concerted proton-coupled electron transfer, depending on the one-electron oxidation potentials of substrates. To the best of our knowledge, this is the first example showing that metal(III)-aqua complex can be an effective H-atom abstraction reagent.</P> [FIG OMISSION]</BR>

Amphoteric reactivity of metal–oxygen complexes in oxidation reactions

Sankaralingam, Muniyandi,Lee, Yong-Min,Nam, Wonwoo,Fukuzumi, Shunichi Elsevier Publishing Company 2018 Coordination chemistry reviews Vol.365 No.-

<P><B>Abstract</B></P> <P>Mononuclear metal–oxygen species, such as metal-superoxo, -peroxo, -hydroperoxo and -oxo complexes, are key intermediates involved in dioxygen activation and oxidation reactions catalyzed by a variety of metalloenzymes and their biomimetic compounds. Dioxygen is an electrophile or electron acceptor, but not a nucleophile or electron donor. However, when dioxygen is bound to metal complexes and metal–oxygen species are formed, such as metal-superoxo, -peroxo, -hydroperoxo, alkylperoxo, and -oxo complexes, the metal–oxygen intermediates react as nucleophiles and electron donors as well as electrophiles and electron acceptors. This review is intended to focus on such an amphoteric reactivity of the metal–oxygen complexes in electrophilic and nucleophilic reactions. Both the electronic and steric effects of the ligands have finely tuned the reactivity of metal–oxygen complexes in both the electrophilic and nucleophilic reactions. The amphoteric reactivity of metal–oxygen complexes can also be tuned by binding of redox-inactive metal ions acting as Lewis acids and also by binding of Brønsted acids.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Electrophilic reactivity of metal–oxygen complexes. </LI> <LI> Nucleophilic reactivity of metal–oxygen complexes. </LI> <LI> Tuning the amphoteric reactivity of metal–oxygen complexes. </LI> <LI> Amphoteric reactivity of homo- and heterodinuclear bis(<I>μ</I>-oxo) complexes. </LI> </UL> </P>

A mononuclear manganese(iii)-hydroperoxo complex: synthesis by activating dioxygen and reactivity in electrophilic and nucleophilic reactions

Sankaralingam, Muniyandi,Lee, Yong-Min,Jeon, So Hyun,Seo, Mi Sook,Cho, Kyung-Bin,Nam, Wonwoo The Royal Society of Chemistry 2018 Chemical communications Vol.54 No.10

<P>We report the synthesis of manganese(iii)-peroxo (Mn<SUP>III</SUP>(O2)) and manganese(iii)-hydroperoxo (Mn<SUP>III</SUP>(O2H)) complexes by activating dioxygen (O2) and the amphoteric reactivity of the Mn(iii)-hydroperoxo complex in electrophilic and nucleophilic reactions.</P>

Selective Oxygenation of Cyclohexene by Dioxygen via an Iron(V)-Oxo Complex-Autocatalyzed Reaction

Sankaralingam, Muniyandi,Lee, Yong-Min,Nam, Wonwoo,Fukuzumi, Shunichi ACS AMERICAN CHEMICAL SOCIETY 2017 Inorganic Chemistry Vol.56 No.9

<P>An iron complex with a tetraamido macrocyclic ligand, [(TAML)Fe-III](-), was found to be an efficient and selective catalyst for allylic oxidation of cyclohexene by dioxygen (O-2); cyclohex-2-enone was obtained as the major product along with cyclohexene oxide as the minor product. An iron(V)-oxo complex, [(TAML)Fe-V(O)](-), which was formed by activating O-2 in the presence of cyclohexene, initiated the autoxidation of cyclohexene with O-2 to produce cyclohexenyl hydroperoxide, which reacted with [(TAML)-Fe-III](-) to produce [(TAML)Fe-V(O)](-) by autocatalysis. Then, [(TAML)Fe-V(O)](-) reacted rapidly with [(TAML)-Fe-III](-) to produce a mu-oxo dimer, [(TAML)Fe-IV(O)Fe-IV(TAML)](2-), which was ultimately converted to [(TAML)Fe-V(O)](-) when [(TAML)Fe-III](-) was not present in the reaction solution. An induction period was observed in the autocatalytic production of [(TAML)Fe-V(O)](-). The induction period was shortened with increasing catalytic amounts of [(TAML)Fe-V(O)](-) and cyclohexenyl hydroperoxide, whereas the induction period was prolonged by adding catalytic amounts of a spin trapping reagent such as 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The allylic oxidation of cycloalkenes was also found to depend on the allylic CH bond dissociation energies, suggesting that the hydrogen atom abstraction from the allylic CH bonds of cycloalkenes is the rate-determining radical chain initiation step. In this study, we have shown that an iron(III) complex with a tetraamido macrocyclic ligand is an efficient catalyst for the allylic oxidation of cyclohexene via an autocatalytic radical chain mechanism and that [(TAML)Fe-V(O)](-) acts as a reactive intermediate for the selective oxygenation of cyclohexene with O-2 to produce cyclohex-2-enone predominantly.</P>

Autocatalytic dioxygen activation to produce an iron(V)-oxo complex without any reductants

Sankaralingam, M.,Lee, Y. M.,Lu, X.,Vardhaman, A.,Nam, W.,Fukuzumi, S. unknown 2017 Chemical communications Vol. No.

<P>An iron(V)-oxo complex with a tetraamido macrocyclic ligand, [(TAML)Fe-V(O)](-), was produced by reacting [(TAML)Fe-III](-) with dioxygen without any electron source in acetone at 298 K. The autocatalytic mechanism of dioxygen activation for the formation of an iron(V)-oxo complex has been clarified based on the autocatalysis by radical chain initiators.</P>

A Mn(iv)-peroxo complex in the reactions with proton donors

Lee, Chien-Ming,Sankaralingam, Muniyandi,Chuo, Chi-He,Tseng, Tzu-Hsien,Chen, Peter P.-Y.,Chiang, Ming-Hsi,Li, Xiao-Xi,Lee, Yong-Min,Nam, Wonwoo The Royal Society of Chemistry 2019 Dalton Transactions Vol.48 No.16

<P>Protons play an important role in promoting O-O or M-O bond cleavage of metal-peroxo complexes. Treatment of side-on O2-bound [PPN][Mn<SUP>IV</SUP>(<SUP>TMS</SUP>PS3)(O2)] (1, PPN = bis(triphenylphosphine)iminium and <SUP>TMS</SUP>PS3H3 = 2,2′,2′′-trimercapto-3,3′,3′′-tris(trimethylsilyl)triphenylphosphine) with perchloric acid (HClO4) in the presence of PR3 (R = phenyl or <I>p</I>-tolyl) results in the formation of neutral five-coordinate Mn<SUP>III</SUP>(OPR3)(<SUP>TMS</SUP>PS3) complexes (R = phenyl, 2a; <I>p</I>-tolyl, 2b), which are confirmed by X-ray crystallography. Isotope labelling experiments demonstrate that the oxygen atom in the phosphine oxide product derives from the peroxo ligand of 1. Reactions of 1 with weak proton donors, such as phenylthiol, phenol, substituted phenol and methanol, are also investigated to explore the reactivity of the Mn<SUP>IV</SUP>-peroxo complex, leading to the isolation of a series of five-coordinate [Mn<SUP>III</SUP>(L)(<SUP>TMS</SUP>PS3)]<SUP>−</SUP> complexes (L = phenylthiolate, phenolate or methoxide). Mechanistic aspects of the reactions of the Mn<SUP>IV</SUP>-peroxo complex with proton donors are discussed as well.</P>

Enhanced Electron-Transfer Reactivity of a Long-Lived Photoexcited State of a Cobalt-Oxygen Complex

Saracini, Claudio,Malik, Deesha D.,Sankaralingam, Muniyandi,Lee, Yong-Min,Nam, Wonwoo,Fukuzumi, Shunichi American Chemical Society 2018 Inorganic Chemistry Vol.57 No.17

<P>Photodynamics and electron-transfer reactivity of an excited state derived from an earth-abundant mononuclear cobalt-oxygen complex ground state, [(TAML)Co<SUP>IV</SUP>(O)]<SUP>2-</SUP> (<B>1</B>; H<SUB>4</SUB>TAML = 3,4,8,9-tetrahydro-3,3,6,6,9,9-hexamethyl-1<I>H</I>-1,4,8,11-benzotetraazo-cyclotridecane-2,5,7,10-(6<I>H</I>, 11<I>H</I>)tetrone), prepared by electron-transfer oxidation of Li[(TAML)Co<SUP>III</SUP>]·3(H<SUB>2</SUB>O) (<B>2</B>) in a 1:1 acetonitrile/acetone solvent mixture at 5 °C, were investigated using a combination of femtosecond and nanosecond laser absorption spectroscopy. Visible light photoexcitation of <B>1</B> (λ<SUB>exc</SUB> = 393 nm) resulted in generation of the excited state <B>S<SUB>2</SUB>*</B> (lifetime: 1.4(4) ps), detected 2 ps after laser irradiation by femtosecond laser spectroscopy. The initially formed excited state <B>S<SUB>2</SUB>*</B> converted to a lower-lying excited state, <B>S<SUB>1</SUB>*</B> (λ<SUB>max</SUB> = 580 nm), with rate constant <I>k</I><SUB>c</SUB> = 7(2) × 10<SUP>11</SUP> s<SUP>-1</SUP> (<B>S</B><SUB><B>2</B></SUB><B>*</B> → <B>S</B><SUB><B>1</B></SUB><B>*</B>). <B>S<SUB>1</SUB>*</B> exhibited a 0.6(1) ns lifetime and converted to the initial ground state <B>1</B> with rate constant <I>k</I><SUB>d</SUB> = 1.7(3) × 10<SUP>9</SUP> s<SUP>-1</SUP> (<B>S</B><SUB><B>1</B></SUB><B>*</B> → <B>1</B>). The same excited state dynamics was observed when <B>1</B> was generated by electron-transfer oxidation of <B>2</B> using different one-electron oxidants such as Cu(OTf)<SUB>2</SUB> (OTf<SUP>-</SUP> = triflate anion), [Fe(bpy)<SUB>3</SUB>]<SUP>3+</SUP> (bpy = 2,2′-bipyridine), and tris(4-bromophenyl)ammoniumyl radical cation (TBPA<SUP>•+</SUP>). The electron-transfer reactivity of <B>S</B><SUB><B>1</B></SUB><B>*</B> was probed by nanosecond laser photoexcitation of <B>1</B> in the presence of a series of electron donors with different one-electron oxidation potentials (<I>E</I><SUB>ox</SUB> vs SCE): benzene (2.35 V), toluene (2.20 V), <I>m</I>-xylene (2.02 V), and anisole (1.67 V). The excited state <B>S<SUB>1</SUB>*</B> engaged in electron-transfer reactions with <I>m</I>-xylene and anisole to generate π-dimer radical cations of <I>m</I>-xylene and anisole, respectively, observed by nanosecond laser transient absorption spectroscopy, whereas no reactivity was observed toward benzene and toluene. Such differential electron-transfer reactivity depending on the <I>E</I><SUB>ox</SUB> values of electron donors allowed the estimation of the one-electron reduction potential of <B>S<SUB>1</SUB>*</B> (<I>E</I><SUB>red</SUB>*) as 2.1(1) V vs SCE, which is much higher than that of the ground state (<I>E</I><SUB>red</SUB> = 0.86 V vs SCE).</P><P>An excited state of a cobalt(IV)-oxo complex (<B>1</B>), <B>S</B><SUB><B>1</B></SUB><B>*</B>, was generated and detected for the first time by femtosecond laser transient absorption spectroscopy from photoexcitation of <B>1</B>. This study has paved a new way to generate reactive excited states (e.g., <B>S</B><SUB><B>1</B></SUB><B>*</B>) from earth-abundant transition metal-oxo complexes (e.g., cobalt(IV)-oxo) that can be exploited to perform intermolecular electron transfer with organic substrates for solar-driven oxidation reactions, which would otherwise be impossible to be achieved by the ground state.</P> [FIG OMISSION]</BR>