Functionalization of Alkynes by a (Salen)ruthenium(VI) Nitrido Complex

Exploring new reactivity of metal nitrides is of great interest because it can give insights to N₂ fixation chemistry and provide new methods for nitrogenation of organic substrates. In this work, reaction of a (salen)ruthenium(VI) nitrido complex with various alkynes results in the formation of novel (salen)ruthenium(III) imine complexes. Kinetic and computational studies suggest that the reactions go through an initial ruthenium(IV) aziro intermediate, followed by addition of nucleophiles to give the (salen)ruthenium(III) imine complexes. These unprecedented reactions provide a new pathway for nitrogenation of alkynes based on a metal nitride.     

Evidence of an equilibrium between selenides and osmium(VIII) reagents and selenoxides and osmium(VI) reagents

Driving the equilibrium between selenides and osmium(VIII) reagents with selenoxides and osmium(VI) by a subsequent reaction (rearrangement of allyl selenoxides to allyl alcohols or addition of osmium(VIII) species on C=C double bonds) to one side, allows the transformation of methyl geranyl selenides to linalool and of methyl citronellyl selenoxide to 6,7-dihydroxy citronellyl selenide.     

Oxidations catalyzed by osmium compounds. Part 1: Efficient alkane oxidation with peroxides catalyzed by an olefin carbonyl osmium(0) complex

A carbonyl osmium(0) complex with π-coordinated olefin, (2,3-η-1,4-diphenylbut-2-en-1,4-dione)undecacarbonyl triangulotriosmium (1), efficiently catalyzes oxygenation of alkanes (cyclohexane, cyclooctane, n-heptane, isooctane, etc.) with hydrogen peroxide, as well as with tert-butyl hydroperoxide and meta-chloroperoxybenzoic acid in acetonitrile solution. Alkanes are oxidized to corresponding alcohols, ketones (aldehydes) and alkyl hydroperoxides. Thus, heating cyclooctane with the 1-H₂O₂ combination at 70 °C gave products with turnover number as high as 2400 after 6 h. The maximum obtained yield of all products was equal to 20% based on cyclohexane and 30% based on H₂O₂. The oxidation of linear and branched alkanes exhibits very low regio- and bond-selectivity parameters and this testifies that the reaction proceeds via attack of hydroxyl radicals on C-H bonds of the alkane. The oxygenation products were not formed when the reaction was carried out under argon atmosphere and it can be thus concluded that the oxygenation occurs via the reaction between alkyl radicals and atmospheric oxygen. In summary, the Os(0) complex is much more powerful generator of hydroxyl radicals than any soluble derivative of iron (which is an analogue of osmium in the Periodic System).     

Colorimetric determination of osmium (VI) with 1-naphthylamine-3,5,7-trisulfonic acid

An investigation has been made of the organic compound, 1-naphthylamine-3,5,7-trisulfonic acid, for use as a colorimetric reagent for the determination of trace quantities of osniiun(Vl). When this compound is added to an osmium (OsO₄^-2) solution, a violet colored organo-osmium complex is formed which has highest sensitivity at wavelength 560 mμ. The complex reaches maximum color intensity after four hours. The solution should be brought to pH 1.5 and maintained at tins pH with a buffer, after maximum color development. The colored complex conforms to Beer's law over an osmium concentration range of 0 to 6 p.p.m. Its absorbance is not affected by either excess reagent (up to a 7 : 1 ratio) or by temperature variation over the range 15° to 35° C. Many metallic ions interfere with the color reaction and hence require a separation, of osmium from these ions.The nature of the complex in solution was studied by the following three methods: (1) mole ratio, (2) continuous variations and (3) slope ratio. All three methods indicate a ratio of reagent to osmium of 2 to 1.A procedure for the colorimetric determination of osmium(VI) has been developed; it has a sensitivity of 1 part osmium in 15,000,000 parts of solution, with good precision.     

(Salen)Ti(IV) complex catalyzed asymmetric ring-opening of epoxides using dithiophosphorus acid as the nucleophile

A series of chiral bis-Schiff bases were synthesized starting from (1R,2R)-(+)-diaminocyclohexane, (+)-cis-1,2,2-trimethyl-1,3-diaminocyclopentane, (R)-2,2^′-diamino-1,1^′-binaphthalene, and (1S,2S)-diphenyl-1,2-ethanediamine. The enantioselective ring-opening of meso epoxides with dithiophosphorus acids catalyzed by a (salen)Ti(IV) complex formed in situ upon the treatment of Ti(OPr-i)₄ and the aforementioned chiral Schiff base was realized. The resulting products were obtained with low to good enantioselectivities (up to 73% ee). The (salen)Ti(IV) complex containing the backbone of 1,2-diaminocyclohexane exhibited the best enantioselectivity. The substituents in dithiophosphorus acids and those on the salen aromatic ring have a significant influence on the reaction. Moderate enantioselectivity were obtained for the (salen)Ti(IV) complex catalyzed ring-opening of racemic monosubstituted epoxides. High regioselectivity was observed for the alkyl substituted epoxides, whereas poor regioselectivity was obtained for the aryl substituted ones.     

Pressure-dependent Raman spectroscopy of metal-oxo multiple bonds in rhenium(V) and osmium(VI) complexes

The pressure dependence of stretching vibrations involving metal-oxo multiple bonds is reported and discussed. Metal–ligand vibrational frequencies involving double and triple bonds increase with pressure according to the nature of the metal center and ancillary ligands. Pressure-dependent luminescence spectra, at room temperature, of a trans-dioxo rhenium(V) complex exhibit resolved vibronic structure with a progression in a high frequency metal-oxo stretching mode that shows a change of the intensity distribution with pressure. Raman and luminescence spectra are used to rationalize the unusual decrease of the metal-oxo stretching frequency observed for two complexes at pressures over 30 kbar.     

Highly enantioselective addition of Me2Zn to aldehydes catalyzed by ClCr(Salen)

High enantiomeric excesses are obtained in the addition of Me2Zn catalyzed by commercially available ClCr(Salen). Broad scope, simple procedure, room temperature, low catalyst loading are the characteristics of this new enantioselective process, which uses the rather unreactive Me2Zn. Enantiomeric excesses in the range of 71-99% are obtained with all the aldehydes tested.     

Reaction of an osmium(VI) nitrido complex with cyanide: formation and reactivity of an osmium(III) hydrogen cyanamide complex

Reaction of [Os(VI)(N)(L(1))(Cl)(OH(2))] (1) with CN(-) under various conditions affords (PPh(4))[Os(VI)(N)(L(1))(CN)(Cl)] (2), (PPh(4))(2)[Os(VI)(N)(L(2))(CN)(2)] (3), and a novel hydrogen cyanamido complex, (PPh(4))(2)[Os(III){N(H)CN}(L(3))(CN)(3)] (4). Compound 4 reacts readily with both electrophiles and nucleophiles. Protonation and methylation of 4 produce (PPh(4))[Os(III)(NCNH(2))(L(3))(CN)(3)] (5) and (PPh(4))[Os(III)(NCNMe(2))(L(3))(CN)(3)] (6), respectively. Nucleophilic addition of NH(3), ethylamine, and diethylamine readily occur at the C atom of the hydrogen cyanamide ligand of 4 to produce osmium guanidine complexes with the general formula [Os(III){N(H)C(NH(2))NR(1)R(2)}(L(3))(CN)(3)](-) , which have been isolated as PPh(4) salts (R(1) = R(2) = H (7); R(1) = H, R(2) = CH(2)CH(3) (8); R(1) = R(2) = CH(2)CH(3) (9)). The molecular structures of 1-5 and 7 and 8 have been determined by X-ray crystallography.     

Kinetics and mechanism of osmium(VIII) catalyzed oxidation of tellurium(IV) by cerium(IV)

The reaction is first order in substrate and catalyst and zero order in cerium(IV). The rate decreases with increasing [H⁺] as well as with increasing ionic strength. H and S have been found to be 44.8 kJ mol^–1 and 161.8 JK^–1 mol^–1 respectively. A mechanism is proposed.

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(IV). [H⁺], . , H S 44,8 ·^–1 161,8 ·^–1–1, . .

     

Hydrolytic Kinetic Resolution of Terminal Epoxides catalyzed by Novel Bimetallic Chiral Co (Salen) Complexes

Novel bimetallic chiral Co (salen) complexes bearing transition‐metal salts have been synthesized. The easily prepared complexes exhibited very high catalytic reactivity and enantioselectivity in hydrolytic kinetic resolution (HKR) of racemic terminal epoxides and consequently provided enantiomerically enriched epoxides (up to 99% ee).     

Cationic nitridoruthenium(VI) catalyzed hydrosilylation of ketones and aldehydes

The first example of a ruthenium nitrido compound as hydrosilylation catalyst is described, using phenylsilane as reductant. A variety of ketones and aldehydes are reduced to alcohols with good to high isolated yields. Some mechanistic insight on this new system is provided on the basis of the available experimental findings.     

Ab initio study of the comparative stability of ruthenium(VI) and osmium(VI) oxo hydroxo complexes with different coordination numbers

The comparative stability of oxohydroxoruthenium(VI) and oxohydroxoosmium(VI) anions with different coordination numbers has been calculated by the relativistic density functional theory method with inclusion of spin-orbit coupling. In line with experimental data, the computations point to the stability of the ruthenium complex RuO₃(OH) ₂ ^2? ? with CN = 5, whereas osmium is stabilized in a six-coordinate environment (OsO₂(OH) ₄ ^2? ). The spin state of the stable Ru complex is close to triplet and that of the Os complex, to singlet.     

Molecular mechanism(s) for UV-B irradiation-induced glutathione depletion in cultured human keratinocytes

Glutathione (GSH) plays a central role in maintenance of cellular redox homeostasis and protection against oxidative injury. Ultraviolet B (UV-B) irradiation-induced GSH depletion is believed to be involved in the pathogenesis of several cutaneous disorders. In this study, the molecular mechanism(s) of UV-B-induced GSH depletion was investigated in cultured human keratinocytes, HaCaT cells. We found that UV-B irradiation caused GSH depletion in a dose- and time-dependent manner in HaCaT cells. The mechanistic studies showed that UV-B-induced GSH depletion did not result from the GSH efflux. UV-B irradiation appeared to cause a slight decrease in enzymatic activity of gamma-glutamate cysteine ligase (GCL), a rate-limiting enzyme in GSH biosynthesis. UV-B irradiation resulted in the GCL cleavage through the activation of a caspase cascade. Inhibition of total caspase activity by the general caspase inhibitor, zVAD-fmk, partially reversed the UV-B-induced GSH depletion. More importantly, we found that UV-B irradiation could dramatically decrease the cystine uptake through the functional inhibition of the system Xc(-), a cystine transporter on the cell membrane. The results suggest that the inactivation of cystine transporter system Xc(-) was a major contributor to the UV-B-mediated decrease of GSH levels in human keratinocytes.     

General synthesis of (salen)ruthenium(III) complexes via N...N coupling of (salen)ruthenium(VI) nitrides

Reaction of [Ru (VI)(N)(L (1))(MeOH)] (+) (L (1) = N, N'-bis(salicylidene)- o-cyclohexylenediamine dianion) with excess pyridine in CH 3CN produces [Ru (III)(L (1))(py) 2] (+) and N 2. The proposed mechanism involves initial equilibrium formation of [Ru (VI)(N)(L (1))(py)] (+), which undergoes rapid N...N coupling to produce [(py)(L (1))Ru (III) N N-Ru (III)(L (1))(py)] (2+); this is followed by pyridine substituion to give the final product. This ligand-induced N...N coupling of Ru (VI)N is utilized in the preparation of a series of new ruthenium(III) salen complexes, [Ru (III)(L)(X) 2] (+/-) (L = salen ligand; X = H 2O, 1-MeIm, py, Me 2SO, PhNH 2, ( t )BuNH 2, Cl (-) or CN (-)). The structures of [Ru (III)(L (1))(NH 2Ph) 2](PF 6) ( 6), K[Ru (III)(L (1))(CN) 2] ( 9), [Ru (III)(L (2))(NCCH 3) 2][Au (I)(CN) 2] ( 11) (L (2) = N, N'-bis(salicylidene)- o-phenylenediamine dianion) and [N ( n )Bu 4][Ru (III)(L (3))Cl 2] ( 12) (L (3) = N, N'-bis(salicylidene)ethylenediamine dianion) have been determined by X-ray crystallography.     

On the hydrolysis step in osmium catalyzed asymmetric dihydroxylations

In order to obtain information about the most important features that affect the efficiency of osmium catalyzed asymmetric dihydroxylation, a series of substituted styrenes have been studied by using a Hammett type approach as well as solvent kinetic isotope effects. A concave shaped Hammett plot with a minimum at X=H revealed a change in the mechanism going from electron-donating to electron-withdrawing substituents for both NaClO2 and K3[Fe(CN)6] asymmetric dihydroxylations. The Hammett plot together with solvent isotope effect results indicates that osmium (mono)glycolates of styrenes with electron-withdrawing substituents are hydrolyzed by a stepwise attack of the nucleophile to the electrophilic osmium-center and subsequent protonation of the alkaline intermediate. Osmium (mono)glycolates in dihydroxylation, using NaClO2 as the stoichiometric oxidant of styrenes with electron-donating substituents, are hydrolyzed by specific acid catalysis. The rate-limiting step is an A1 type process. Differences in the rho values in the Hammett plots for NaClO2 and K3[Fe(CN)6] asymmetric dihydroxylations indicate that in dihydroxylations with NaClO2 as the secondary oxidant, the reactive osmium(VI) mono(glycolate) is oxidized to osmium(VIII) mono(glycolate) prior to hydrolysis. The reaction rate was found to have an effect on the enantioselectivity in asymmetric dihydroxylation. If the hydrolysis step is slow enough, a competitive bis(glycolation) deteriorates the enantioselectivity in K3[Fe(CN)6] asymmetric dihydroxylations and even more so in NaClO2 asymmetric dihydroxylations.     

Osmium (VI) catalyzed chemoselective oxidation of allylic and benzylic alcohols

A mild and highly chemoselective approach to oxidation of allylic, electron rich/deficient benzylic, and heterocyclic alcohols employing catalytic quantities of K₂[OsO₂(OH)₄] (3 mol %) and chloramine-T (50 mol %) is described. The protocol offers short reaction times (25 min–2 h), controlled oxidation, and tolerance to a variety of substrates. A systematic mechanistic study based on the LC-ESI-MS/MS reveals the presence of imidotriooxoosmium species which further reacts with alcohol to give the oxidized product.     

Platinum catalyzed reduction of uranium(VI) with hydrazine in sulfuric acid media

The process of platinum-catalyzed uranium(VI) reduction with hydrazine in sulfuric acid media has been studied. The influence of sulfuric acid and hydrazine concentrations, and temperature on the reaction rate were investigated. A new reaction mechanism is proposed. The process allows to obtain pure U(IV) sulfate solutions with concentrations of up to 0.3 mol·l^–1. Further increase of initial U(VI) concentrations is limited by the reductant solubility in sulfuric acid media.