Jieun Jung

Water-soluble mononuclear cobalt complexes with organic ligands acting as precatalysts for efficient photocatalytic water oxidation

The photocatalytic water oxidation to evolve O2 was performed by photoirradiation (λ > 420 nm) of an aqueous solution containing [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine), Na2S2O8 and water-soluble cobalt complexes with various organic ligands as precatalysts in the pH range of 6.0–10. The turnover numbers (TONs) based on the amount of Co for the photocatalytic O2 evolution with [CoII(Me6tren)(OH2)]2+ (1) and [CoIII(Cp*)(bpy)(OH2)]2+ (2) [Me6tren = tris(N,N′-dimethylaminoethyl)amine, Cp* = η5-pentamethylcyclopentadienyl] at pH 9.0 reached 420 and 320, respectively. The evolved O2 yield increased in proportion to concentrations of precatalysts 1 and 2 up to 0.10 mM. However, the O2 yield dramatically decreased when the concentration of precatalysts 1 and 2 exceeded 0.10 mM. When the concentration of Na2S2O8 was increased from 10 mM to 50 mM, CO2 evolution was observed during the photocatalytic water oxidation. These results indicate that a part of the organic ligands of 1 and 2 were oxidized to evolve CO2 during the photocatalytic reaction. The degradation of complex 2 under photocatalytic conditions and the oxidation of Me6tren ligand of 1 by [Ru(bpy)3]3+ were confirmed by 1H NMR measurements. Dynamic light scattering (DLS) experiments indicate the formation of particles with diameters of around 20 ± 10 nm and 200 ± 100 nm during the photocatalytic water oxidation with 1 and 2, respectively. The particle sizes determined by DLS agreed with those of the secondary particles observed by TEM. The XPS measurements of the formed particles suggest that the surface of the particles is covered with cobalt hydroxides, which could be converted to active species containing high-valent cobalt ions during the photocatalytic water oxidation. The recovered nanoparticles produced from 1 act as a robust catalyst for the photocatalytic water oxidation.

Production of hydrogen peroxide as a sustainable solar fuel from water and dioxygen

Hydrogen peroxide was produced as a solar fuel from water and dioxygen using solar energy by combination of a water oxidation catalyst and a photocatalyst for two-electron reduction of O2 in acidic aqueous solutions. Photocatalytic production of H2O2 occurred under photoirradiation of [RuII(Me2phen)3]2+ (Me2phen = 4,7-dimethyl-1,10-phenanthroline) used as a photocatalyst with visible light in the presence of Ir(OH)3 acting as a water oxidation catalyst in an O2-saturated H2SO4 aqueous solution. Photoinduced electron transfer from the excited state of [RuII(Me2phen)3]2+ to O2 results in the formation of [RuIII(Me2phen)3]3+ and a superoxide radical anion (O2˙−) which is protonated to produce H2O2via disproportionation of HO2˙ in competition with back electron transfer (BET) from O2˙− to [RuIII(Me2phen)3]3+. [RuIII(Me2phen)3]3+ oxidises water with the aid of catalysis of Ir(OH)3 to produce O2. The photocatalytic reactivity of H2O2 production was improved by replacing Ir(OH)3 nanoparticles by [CoIII(Cp*)(bpy)(H2O)]2+ in the presence of Sc(NO3)3 in water. The optimised quantum yield of the photocatalytic H2O2 production at λ = 450 nm was determined using a ferrioxalate actinometer to be 37%. The value of conversion efficiency from solar energy to chemical energy was also determined to be 0.25%.

Light-driven, proton-controlled, catalytic aerobic C-H oxidation mediated by a Mn(III) porphyrinoid complex.

The visible light-driven, catalytic aerobic oxidation of benzylic C-H bonds was mediated by a Mn(III) corrolazine complex. To achieve catalytic turnovers, a strict selective requirement for the addition of protons was established. The resting state of the catalyst was unambiguously characterized by X-ray diffraction as [Mn(III)(H2O)(TBP8Cz(H))](+), in which a single, remote site on the ligand is protonated. If two remote sites are protonated, however, reactivity with O2 is shut down. Spectroscopic methods revealed that the related Mn(V)(O) complex is also protonated at the same remote site at -60 °C, but undergoes valence tautomerization upon warming.

Nuclear Factor-κB Activated by Capacitative Ca2+ Entry Enhances Muscarinic Receptor-mediated Soluble Amyloid Precursor Protein (sAPPα) Release in SH-SY5Y Cells

Gq/11 protein-coupled muscarinic receptors are known to regulate the release of soluble amyloid precursor protein (sAPPα) produced by α-secretase processing; however, their signaling mechanisms remain to be elucidated. It has been reported that a muscarinic agonist activates nuclear factor (NF)-κB, a transcription factor that has been shown to play an important role in the Alzheimer disease brain, and that NF-κB activation is regulated by intracellular Ca2+ level. In the present study, we investigated whether NF-κB activation plays a role in muscarinic receptor-mediated sAPPα release enhancement and contributes to a changed capacitative Ca2+ entry (CCE), which was suggested to be involved in the muscarinic receptor-mediated stimulation of sAPPα release. Muscarinic receptor-mediated NF-κB activation was confirmed by observing the translocation of the active subunit (p65) of NF-κB to the nucleus by the muscarinic agonist, oxotremorine M (oxoM), in SH-SY5Y neuroblastoma cells expressing muscarinic receptors that are predominantly of the M3 subtype. NF-κB activation and sAPPα release enhancement induced by oxoM were inhibited by NF-κB inhibitors, such as an NF-κB peptide inhibitor (SN50), an IκBα kinase inhibitor (BAY11-7085), a proteasome inhibitor (MG132), the inhibitor of proteasome activity and IκB phosphorylation, pyrrolidine dithiocarbamate, the novel NF-κB activation inhibitor (6-amino-4-(4-phenoxyphenylethylamino) quinazoline), and by an intracellular Ca2+ chelator (TMB-8). Furthermore, both oxoM-induced NF-κB activation and sAPPα release were antagonized by CCE inhibitors (gadolinium or SKF96365) but not by voltage-gated Ca2+-channel blockers. On the other hand, treatment of cells with NF-κB inhibitors (SN50, BAY11-7085, MG132, or pyrrolidine dithiocarbamate) did not inhibit muscarinic receptor-mediated CCE. These findings provide evidence for the involvement of NF-κB regulated by CCE in muscarinic receptor-mediated sAPPα release enhancement.

Photocatalytic Oxygenation of 10-Methyl-9,10-dihydroacridine by O2 with Manganese Porphyrins

Photocatalytic oxygenation of 10-methyl-9,10-dihydroacridine (AcrH2) by dioxygen (O2) with a manganese porphyrin [(P)MnIII: 5,10,15,20-tetrakis-(2,4,6-trimethylphenyl)porphinatomanganese(III) hydroxide [(TMP)MnIII(OH)] (1) or 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinatomanganese(III) acetate [(TPFPP)MnIII(CH3COO)] (2)] occurred to yield 10-methyl-(9,10H)-acridone (Acr=O) in an oxygen-saturated benzonitrile (PhCN) solution under visible light irradiation. The photocatalytic reactivity of (P)MnIII in the presence of O2 is in proportion to concentrations of AcrH2 or O2 with the maximum turnover numbers of 17 and 6 for 1 and 2, respectively. The quantum yield with 1 was determined to be 0.14%. Deuterium kinetic isotope effects (KIEs) were observed with KIE = 22 for 1 and KIE = 6 for 2, indicating that hydrogen-atom transfer from AcrH2 is involved in the rate-determining step of the photocatalytic reaction. Femtosecond transient absorption measurements are consistent with photoexcitation of (P)MnIII, resulting in intersystem crossing from a tripquintet excited state to a tripseptet excited state. A mechanism is proposed where the tripseptet excited state reacts with O2 to produce a putative (P)MnIV superoxo complex. Hydrogen-atom transfer from AcrH2 to (P)MnIV(O2•–) generating a hydroperoxo complex (P)MnIV(OOH) and AcrH• is likely the rate-determining step, in competition with back electron transfer to regenerate the ground state (P)MnIII and O2. The subsequent reductive O–O bond cleavage by AcrH• may occur rapidly inside of the reaction cage to produce (P)MnV(O) and AcrH(OH), followed by the oxidation of AcrH(OH) by (P)MnV(O) to yield Acr=O with regeneration of (P)MnIII.

Photochemical Oxidation of a Manganese(III) Complex with Oxygen and Toluene Derivatives to Form a Manganese(V)-Oxo Complex

Visible light photoirradiation of an oxygen-saturated benzonitrile solution of a manganese(III) corrolazine complex [(TBP8Cz)Mn(III)] (1): [TBP8Cz = octakis(p-tert-butylphenyl)corrolazinato(3-)] in the presence of toluene derivatives resulted in formation of the manganese(V)-oxo complex [(TBP8Cz)Mn(V)(O)]. The photochemical oxidation of (TBP8Cz)Mn(III) with O2 and hexamethylbenzene (HMB) led to the isosbestic conversion of 1 to (TBP8Cz)Mn(V)(O), accompanied by the selective oxidation of HMB to pentamethylbenzyl alcohol (87%). The formation rate of (TBP8Cz)Mn(V)(O) increased with methyl group substitution, from toluene, p-xylene, mesitylene, durene, pentamethylbenzene, up to hexamethylbenzene. Deuterium kinetic isotope effects (KIEs) were observed for toluene (KIE = 5.4) and mesitylene (KIE = 5.3). Femtosecond laser flash photolysis of (TBP8Cz)Mn(III) revealed the formation of a tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state was shown to be the key, activated state that reacts with O2 via a diffusion-limited rate constant. The data allow for a mechanism to be proposed in which the tripseptet excited state reacts with O2 to give the putative (TBP8Cz)Mn(IV)(O2(•-)), which then abstracts a hydrogen atom from the toluene derivatives in the rate-determining step. The mechanism of hydrogen abstraction is discussed by comparison of the reactivity with the hydrogen abstraction from the same toluene derivatives by cumylperoxyl radical. Taken together, the data suggest a new catalytic method is accessible for the selective oxidation of C-H bonds with O2 and light, and the first evidence for catalytic oxidation of C-H bonds was obtained with 10-methyl-9,10-dihydroacridine as a substrate.

Capacitative Ca2+ entry is involved in regulating soluble amyloid precursor protein (sAPPα) release mediated by muscarinic acetylcholine receptor activation in neuroblastoma SH‐SY5Y cells

Previous studies have demonstrated that stimulation of phospholipase C‐linked G‐protein‐coupled receptors, including muscarinic M1 and M3 receptors, increases the release of the soluble form of amyloid precursor protein (sAPPα) by α‐secretase cleavage. In this study, we examined the involvement of capacitative Ca2+ entry (CCE) in the regulation of muscarinic acetylcholine receptor (mAChR)‐dependent sAPPα release in neuroblastoma SH‐SY5Y cells expressing abundant M3 mAChRs. The sAPPα release stimulated by mAChR activation was abolished by EGTA, an extracellular Ca2+ chelator, which abolished mAChR‐mediated Ca2+ influx without affecting Ca2+ mobilization from intracellular stores. However, mAChR‐mediated sAPPα release was not inhibited by thapsigargin, which increases basal [Ca2+]i by depletion of Ca2+ from intracellular stores. While these results indicate that the mAChR‐mediated increase in sAPPα release is regulated largely by Ca2+ influx rather than by Ca2+ mobilization from intracellular stores, we further investigated the Ca2+ entry mechanisms regulating this phenomenon. CCE inhibitors such as Gd3+, SKF96365, and 2‐aminoethoxydiphenyl borane (2‐APB), dose dependently reduced both Ca2+ influx and sAPPα release stimulated by mAChR activation, whereas inhibition of voltage‐dependent Ca2+ channels, Na+/Ca2+ exchangers, or Na+‐pumps was without effect. These results indicate that CCE plays an important role in the mAChR‐mediated release of sAPPα.

Catalytic Two-Electron Reduction of Dioxygen by Ferrocene Derivatives with Manganese(V) Corroles

Electron transfer from octamethylferrocene (Me8Fc) to the manganese(V) imidocorrole complex (tpfc)Mn(V)(NAr) [tpfc = 5,10,15-tris(pentafluorophenyl)corrole; Ar = 2,6-Cl2C6H3] proceeds efficiently to give an octamethylferrocenium ion (Me8Fc(+)) and [(tpfc)Mn(IV)(NAr)](-) in acetonitrile (MeCN) at 298 K. Upon the addition of trifluoroacetic acid (TFA), further reduction of [(tpfc)Mn(IV)(NAr)](-) by Me8Fc gives (tpfc)Mn(III) and ArNH2 in deaerated MeCN. TFA also results in hydrolysis of (tpfc)Mn(V)(NAr) with residual water to produce a protonated manganese(V) oxocorrole complex ([(tpfc)Mn(V)(OH)](+)) in deaerated MeCN. [(tpfc)Mn(V)(OH)](+) is rapidly reduced by 2 equiv of Me8Fc in the presence of TFA to give (tpfc)Mn(III) in deaerated MeCN. In the presence of dioxygen (O2), (tpfc)Mn(III) catalyzes the two-electron reduction of O2 by Me8Fc with TFA in MeCN to produce H2O2 and Me8Fc(+). The rate of formation of Me8Fc(+) in the catalytic reduction of O2 follows zeroth-order kinetics with respect to the concentrations of Me8Fc and TFA, whereas the rate increases linearly with increasing concentrations of (tpfc)Mn(V)(NAr) and O2. These kinetic dependencies are consistent with the rate-determining step being electron transfer from (tpfc)Mn(III) to O2, followed by further proton-coupled electron transfer from Me8Fc to produce H2O2 and [(tpfc)Mn(IV)](+). Rapid electron transfer from Me8Fc to [(tpfc)Mn(IV)](+) regenerates (tpfc)Mn(III), completing the catalytic cycle. Thus, catalytic two-electron reduction of O2 by Me8Fc with (tpfc)Mn(V)(NAr) as a catalyst precursor proceeds via a Mn(III)/Mn(IV) redox cycle.

Photocatalytic Oxygenation of Substrates by Dioxygen with Protonated Manganese(III) Corrolazine

UV-vis spectral titrations of a manganese(III) corrolazine complex [Mn(III)(TBP8Cz)] with HOTf in benzonitrile (PhCN) indicate mono- and diprotonation of Mn(III)(TBP8Cz) to give Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] with protonation constants of 9.0 × 10(6) and 4.7 × 10(3) M(-1), respectively. The protonated sites of Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] were identified by X-ray crystal structures of the mono- and diprotonated complexes. In the presence of HOTf, the monoprotonated manganese(III) corrolazine complex [Mn(III)(OTf)(TBP8Cz(H))] acts as an efficient photocatalytic catalyst for the oxidation of hexamethylbenzene and thioanisole by O2 to the corresponding alcohol and sulfoxide with 563 and 902 TON, respectively. Femtosecond laser flash photolysis measurements of Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] in the presence of O2 revealed the formation of a tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state of Mn(III)(OTf)(TBP8Cz(H)) reacted with O2 with a diffusion-limited rate constant to produce the putative Mn(IV)(O2(•-))(OTf)(TBP8Cz(H)), whereas the tripseptet excited state of [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] exhibited no reactivity toward O2. In the presence of HOTf, Mn(V)(O)(TBP8Cz) can oxidize not only HMB but also mesitylene to the corresponding alcohols, accompanied by regeneration of Mn(III)(OTf)(TBP8Cz(H)). This thermal reaction was examined for a kinetic isotope effect, and essentially no KIE (1.1) was observed for the oxidation of mesitylene-d12, suggesting a proton-coupled electron transfer (PCET) mechanism is operative in this case. Thus, the monoprotonated manganese(III) corrolazine complex, Mn(III)(OTf)(TBP8Cz(H)), acts as an efficient photocatalyst for the oxidation of HMB by O2 to the alcohol.

Switchover of the Mechanism between Electron Transfer and Hydrogen-Atom Transfer for a Protonated Manganese(IV)-Oxo Complex by Changing Only the Reaction Temperature.

Hydroxylation of mesitylene by a nonheme manganese(IV)-oxo complex, [(N4Py)Mn(IV) (O)](2+) (1), proceeds via one-step hydrogen-atom transfer (HAT) with a large deuterium kinetic isotope effect (KIE) of 3.2(3) at 293 K. In contrast, the same reaction with a triflic acid-bound manganese(IV)-oxo complex, [(N4Py)Mn(IV) (O)](2+) -(HOTf)2 (2), proceeds via electron transfer (ET) with no KIE at 293 K. Interestingly, when the reaction temperature is lowered to less than 263 K in the reaction of 2, however, the mechanism changes again from ET to HAT with a large KIE of 2.9(3). Such a switchover of the reaction mechanism from ET to HAT is shown to occur by changing only temperature in the boundary region between ET and HAT pathways when the driving force of ET from toluene derivatives to 2 is around -0.5 eV. The present results provide a valuable and general guide to predict a switchover of the reaction mechanism from ET to the others, including HAT.