Kei Unoura

Reversible half-wave potentials of reduction processes on nitroxide radicals

Abstract The reversible half-wave potential ( E 1 2 rev ) of the reduction process was determined for six nitroxide radicals by cyclic voltammetry as the midpoint potential between reduction and reoxidation peaks. The variations of E 1 2 rev with pH were interpreted as the protonation induced by the one-electron reduction of nitroxide radicals.

Substituent effects of cis-dioxobis(dithiocarbamato) molybdenum(VI) on redox properties: redox potentials for one-electron reduction and second-order rate constants for oxygen atom transfer

Abstract The electrochemical properties of a series of cis-dioxobis(dithiocarbamato)molybdenum(VI) complexes [MoO2(RR′dte)2] (RR′dte = N,N-disubstituted dithiocarbamate) and the oxygen atom transfer reactions between [MoO2(RR′dte)2] and PPh3 in 1,2-C2H4Cl2 at 25°C are systematically investigated. A good correlation between logarithmic values of second-order rate constants k1 of the oxygen atom transfer reactions and redox potentials E°′ for [MoO2OP2RR′dte)2]01couples is observed over the wide range of k1 (from 0.012 to 0.61 M s+2); the values of log k1 linearly increase with the increase of E0. The departure from the correlation line is exactly observed for [MoO2(Bz2dte)2/PPh2 [MoO2(Ph2dte)2]/PPh1 and [MoO2(BzPhdte)2]PPh1 systems. A negative activation entropy (ΔS′ = −114 J mol+1K+1) of the oxygen atom transfer reaction obtained for the [MoO2(Et2dte)2]/PPh4 system indicates that the reaction proceed through an associative process. These results suggest that an attack of a lone pair of PPh4 on an oxo ligand and a negative charge flow to the molybdenum center occurs to form an activated complex. The anomaly observed for [MoO2(Bz2dte)2]/PPh4, [MoO2(Ph2dte)2]/Pph4 and [MoO2(RR′dte)2]. Other physicochemical properties of the complexes such as 11C NMR and Mo O stretching frequencies in solutions and the X-ray crystal structure of cis-dioxibis(thiomorpholydithiocarbamato)molybdenum(VI) are also reported.

Aminopeptidase function of dinuclear zinc(II) complexes of phenol-based dinucleating ligands: effect of p-substituents

Abstract The aminopeptidase functions of newly obtained dinuclear zinc(II) complexes [Zn 2 (bonp)(MeCO 2 ) 2 ]BPh 4 ( 1 ) and [Zn 2 (bocp)(MeCO 2 ) 2 ]BPh 4 ( 2 ) were estimated using l -leucine– p -nitroanilide as a substrate [H(bonp): 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-nitrophenol; H(bocp): 4-chloro-2,6-bis[bis(2-methoxyethyl)aminomethyl]phenol]. Both complexes showed aminopeptidase activities, and a second-order rate equation was obtained as v = k [complex][substrate]. The second-order rate constants k were 5.9(5)×10 −1 for complex 1 , and 2.7(1)×10 −2 dm 3 mol −1 s −1 for complex 2 . Compared to the previously reported complex, [Zn 2 (bomp)(MeCO 2 ) 2 ]BPh 4 ( 3 ) [H(bomp): 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenol], the aminopeptidase activity was improved 250 times by the substitution of the p -methyl group into p -nitro group. The rate constants were found to be ordered as 1 > 2 > 3 (250:10:1), and this order is a decreasing order of the electron-withdrawing effect of the p -substituents of the dinucleating ligands 1 (–NO 2 )> 2 (–Cl)> 3 (–CH 3 ).

Synthesis and redox property of the binuclear Pt(II) complexes bridged by thieno[3,2-b]thiophenes

The binuclear Pt(II) complexes bridged by thieno[3,2-b]thiophenes were prepared and their redox behavior was measured by cyclic voltammetry. The complexes showed two one-electron quasi-reversible waves and their first redox potentials are comparable with that of ferrocene. The oxidation potentials of the complexes and the related complexes are explained by the result of the MO calculation. The one-electron oxidation of some complexes gave stable oxidized complexes which showed no IT band in the near infrared region. The electronic and EPR spectra suggest that an unpaired electron is localized on the Pt atom.

Voltammetric studies of cis-dioxobis(dithiocarbamato) molybdenum(VI) complexes

Abstract The electrochemical behaviour of cis -[MoO 2 (R 2 dtc) 2 ] [R = CH 3 , C 2 H 5 , i-C 3 H 7 (i-Pr), i-C 4 H 9 , C 6 H 5 CH 2 , C 6 H 5 ] was investigated in dichloromethane by conventional semi-infinite cyclic voltammetry, thin-layer cyclic voltammetry and coulometry at low temperature. Each complex undergoes a one-electron quasi-reversible reduction followed by a homogeneous chemical reaction to give the free ligand and a mono-oxo-bridged Mo(V) dinuclear complex [Mo 2 O 3 (R 2 dtc) 4 ], except for [MoO 2 (i-Pr 2 dtc) 2 ]. The first reduction of [MoO 2 (i-Pr 2 dtc) 2 ] gives the free ligand, but does not give the mono-oxo-bridged Mo(V) dinuclear complex. The formal potentials for the reduction of these complexes correlate with the σ * values of the substituents of the dithiocarbamate ligands except for diphenyldithiocarbamate (σ * denotes Tafts polar substituent constant).

Liesegang patterns engineered by a chemical reaction assisted by complex formation.

Liesegang rings based on a chemical reaction, not a conventional precipitation reaction, have been developed by appropriate design of the nucleation dynamics in a system involving complex formation in a matrix. The periodic and concentric rings consisted of well-dispersed Ag nanoparticles with diameters of a few nanometers. The approach modeled here could be applied to form novel micropatterns out of inorganic salts, metal nanoparticles, organic nanocrystals, or polymeric fibers, and it could also offer a scaffold for novel models of a wide variety of reaction-diffusion phenomena in nature.

Correlation between 95Mo NMR chemical shifts and rate constants of oxygen atom transfer reactions of various cis-dioxobis(dithiocarbamato)molybdenum(VI) complexes with PPh3

Abstract The 95 Mo NMR properties of cis -dioxomolybdenum(VI) complexes with various dithiocarbamates [MoO 2 (R 2 dtc) 2 ] (R 2 dtc − : dithiocarbamates, R = CH 3 , C 2 H 5 i-C 3 H 7 , i-C 4 H 9 , C 6 H 5 , C 6 H 5 CH 2 ) have been investigated. The chemical shifts varied from δ = 151 (R = CH 3 ) to δ = 216 (R = i-C 3 H 7 ) depending on the nature of the substituents on the dithiocarbamate ligands, and they were found to be in a good correlation with the rate constants for the oxygen atom transfer reaction between [MoO 2 (R 2 dtc) 2 ] and triphenylphosphine in 1,2-C 2 H 4 ,Cl 2 .

Electrochemical studies of dinuclear molybdenum(V) complexes with EDTA in aprotic solvents

Abstract The electrochemical behaviour of the dinuclear complexes [Mo2O4-nSn(edta)]2− (n = 0–2) was investigated in various aprotic solvents, acetonitrile (CH3CN), dimethyl sulphoxide (DMSO), N,N-di-methylformamide (DMF) and N,N-dimethylacetamide (DMA). These complexes undergo quasi-reversible one-electron reduction and quasi-reversible one-electron oxidation in CH3CN. This behaviour is quite different from that in aqueous media. The formal potentials for the reduction of these complexes, as well as the formal potentials for the oxidation of these complexes, shift in the positive direction with an increase in the number of bridging sulphides. The electrochemical behaviour of these complexes in DMF and DMA is similar to that in CH3CN. The only exception is the oxidation processes of [Mo2O4edta)]2−. in which an irreversible and nearly two-electron transfer occurs in DMF or DMA. The complexes of [Mo2O4−nSnedta)]2− (n = 0–2) also undergo quasi-reversible one-electron reduction in DMSO. However, anomalies were observed for the oxidation process in this solvent.

Activity of Keggin and Dawson polyoxometalates toward model cell membrane

We present direct information showing that polyoxometalates (POMs) show destructive activity toward the model cell membrane, via the rapid adsorption of POMs on the vesicle surface. The crucial step in causing the vesicle destruction is the structural change into stable POM–lipid conjugates, which are known as surfactant-encapsulated clusters.

Electrochemical and 95Mo NMR studies of triply-bridged dinuclear oxomolybdenum(V) complexes with various dithiocarbamates

The electrochemical and 95 Mo NMR behaviour of triply-bridged oxomolybdenum(V) dinuclear complexes with a series of dithiocarbamate ligands [Mo 2 O 3 (OC 2 H 4 S)(RR′dtc) 2 ] (RR′dtc − =dithiocarbamate) has been investigated. [Mo 2 O 3 (OC 2 H 4 S)(RR′dtc) 2 ] undergoes two successive one-electron reductions in dichloromethane, acetonitrile, dimethylsulfoxide, N , N -dimethylformamide, and N , N -dimethylacetamide. The first reduction of [Mo 2 O 3 (OC 2 H 4 S)(RR′dtc) 2 ] is a quasi-reversible one-electron reduction followed by a homogeneous chemical reaction. The formal potentials of the first reduction of these complexes correlate well with the values of σ* of the substituents on the dithiocarbamate ligands except for [Mo 2 O 3 (OC 2 H 4 S)(Ph 2 dtc) 2 ] (σ* denotes Taft’s polar substituent constant) and become more positive with increase in acceptor number of the solvents. The chemical shifts of 95 Mo NMR of the complexes varied from δ =329 (RR′=Me 2 ) to δ =354 (RR′=i-Pr 2 ) depending on the nature of the dithiocarbamate ligands. The linewidth of the NMR signal apparently increases with increase in steric bulkiness of the alkyl groups on the dithiocarbamate ligands. The X-ray crystal structure of [Mo 2 O 3 (OC 2 H 4 S)(i-Bu 2 dtc) 2 ] is also reported.