Rate Of Disproportionation Research Articles

Optimizing nitroxide-mediated miniemulsion polymerization processes

Living/controlled radical polymerizations provide significant advantages in the control of polymer resin microstructure compared to conventional radical polymerization. Advances in our ability to tailor polymer microstructure will enable improvements in coatings properties and possible new applications of coating technologies. Adapting living radical polymerizations to heterogeneous media such as aqueous-based miniemulsion polymerization presents several challenges related to maintaining the livingness (the fraction of chains that are still “living” at the end of polymerization) of the polymer chains and also developing a commercially viable process. We have studied the nitroxide-mediated polymerization of styrene in miniemulsion, with the intent of maintaining a high degree of livingness by balancing the rates of biradical termination and disproportionation. We can now achieve >95% monomer conversion in less than three hours, while maintaining polydispersities ∼1.3. Monomer conversion can be dramatically increased from about 60–95% by changing the concentration of sodium dodecylbenzenesulfonate (SDBS) surfactant. Conversions in Dowfax 8390 stabilized miniemulsions showed no comparable dependency. Reasons for this potentially commercially important effect are under investigation.

Kinetics of Disproportionation of Inorganic Polysulfides in Undersaturated Aqueous Solutions at Environmentally Relevant Conditions

The rate of the reversible homogeneous disproportionation of polysulfides was studied by following the optical absorbance of polysulfide solutions in a continuous plug flow reactor equipped with an on-line photometric detector. In order to avoid heterogeneous slow reactions involving sulfur colloids or precipitate, the reaction was initiated by an abrupt pH change from an undersaturated solution containing predominantly tetrasulfide species to a pH where pentasulfide is the dominant species. The disproportionation was found to follow first order reversible reaction dynamics. At environmentally relevant conditions the characteristic time of the disproportionation reaction is of the order of 10 s. This characteristic time implies that necessary conditions for speciation of the different polysulfide species by chromatography or another separation and subsequent quantification scheme should be of the order of 1 s.

Controlled Radical Grafting: Nitroxyl-Mediated Maleation of Model Hydrocarbons

An exploration of nitroxyl-mediated polymerization principles for the synthesis of maleated polyethylene (PE) and polybutadiene (PBD) is presented. Thermolysis and disproportionation rates for eigh...

Studies on the disproportionation of trichloromethyltin

AbstractThe rate of disproportionation of trichloromethyltin in various solvents was studied by two methods, derivatization with sodium tetraethylborate and 13C NMR measurements, which gave similar results. The reaction followed second‐order kinetics and was more rapid in coordinating solvents than in uncoordinating ones. Moreover, it was catalyzed by nucleophiles, such as amines or alcohols, which is in favor of a nucleophile‐assisted electrophilic reaction. Copyright © 2003 John Wiley & Sons, Ltd.

Osmium phosphiniminato complexes: synthesis, protonation, structure, and redox-coupled hydrolytic scission of N-p bonds.

The osmium(VI) nitrido complex TpOs(N)Cl(2) [1, Tp = hydrotris(1-pyrazolyl)borate] reacts with triarylphosphines to afford the Os(IV) phosphiniminato complexes TpOs(NPAr(3))Cl(2) [Ar = p-tolyl (tol) (2a), phenyl (2b), p-CF(3)C(6)H(4) (2c)] in nearly quantitative yield. Protonation of 2a-c with 1 equiv of HOTf in MeCN occurs at the phosphiniminato nitrogen to give [TpOs(IV)(NHPAr(3))Cl(2)]OTf (3a-c) in 68-80% yield. Solutions of 2a-c in CH(2)Cl(2) react with excess H(2)O over 1 week to form the disproportionation products 1 (28%), TpOs(III)(NHPAr(3))Cl(2) (4a-c) (60%), and OPAr(3) (35%). Treatment of solutions of 3a-c with H(2)O also affords 1, 4a-c, and OPAr(3). X-ray structures of 2b, 3b, and 4b are presented. Cyclic voltammograms of compounds 2a-c exhibit Os(V)/Os(IV) and Os(IV)/Os(III) couples at approximately 0.3 and -1 V versus Cp(2)Fe(+/0). Protonation to give 3 makes reduction easier by approximately 1.2 V, so that these compounds show Os(IV)/Os(III) and Os(III)/Os(II) couples. In the hydrolytic disproportionation of 2a-c, labeling studies using (18)O-enriched O(2) and H(2)O establish water as the source of the oxygen atom in the OPAr(3) product. The conversions are accelerated by HOTf and inhibited by NaOD. The relative rates of hydrolytic disproportionation of 2a-c vary in the order tol > Ph > p-CF(3)C(6)H(4). The data indicate that protonation of the phosphiniminato nitrogen is required for hydrolysis. The mechanism of the hydrolytic disproportionation is compared to that of the related reaction of the osmium(IV) acetonitrile complex [TpOs(NCMe)Cl(2)](+).

Cyclic voltammetric study of the disproportionation reaction of the nitro radical anion from 4-nitroimidazole in protic media

We have chosen 4-nitroimidazole (4-NIm) as a prototype of nitroimidazolic compounds in order to carry out an exhaustive cyclic voltammetric study in protic media, 0.1 M Britton–Robinson buffer+0.3 M KCl with ethanol as a co-solvent. The one electron reduction of 4-NIm in protic media at alkaline pH produces a stable nitro radical anion on the time scale of the cyclic voltammetric technique. The nitro radical anion decays according to a coupled chemical reaction and we have focused the study to follow this reaction using cyclic voltammetric methodology. The one electron reduction of the 4-NIm to form the nitro radical anion and the subsequent reaction of the radical obey an EC 2 mechanism that follows very well the cyclic voltammetry theory for the disproportionation reactions previously described by Olmstead and Nicholson (Anal. Chem. 41 (1969) 862). We have obtained the disproportionation rate constant k 2 which is strongly dependent on both pH and ethanol content according to the following regression equations: log k 2=−0.932pH+12.771 and log(10 -3 k 2)=−1.998log[% EtOH]+3.873. The results obtained from this study in protic media differ substantially from previous studies in aprotic media wherein the nitro radical anion was not stabilized.

Ethylbenzene Isomerization on Bifunctional Platinum Alumina−Mordenite Catalysts. 2. Influence of the Pt Content and of the Relative Amounts of Platinum Alumina and Mordenite Components

Ethylbenzene transformation was carried out over intimate mixtures of Pt/Al2O3 (PtA) and HMOR zeolites under the following conditions: fixed-bed reactor, temperature 683 K, pressures of hydrogen and of ethylbenzene equal to 8 and 2 bar, respectively, weight hourly space velocity between 5 and 200. A significant number of these bifunctional PtA/HMOR catalysts were used, differing by the platinum content of PtA (0.5, 1.1 and 2.3 wt %), the framework Si/Al ratio of the mordenite (from 6.6 to 180), and especially by the relative proportions of the PtA and HMOR components. With all of the bifunctional catalysts, ethylbenzene undergoes isomerization into xylenes, disproportionation, dealkylation, secondary ethylbenzene−xylene transalkylation, hydrogenation followed by ethylcyclohexane isomerization, and secondary cracking of C8 naphthenes. The rate of isomerization which occurs through bifunctional catalysis was shown to depend mainly on the balance between hydrogenating and acid functions, taken here as the ratio between the concentration of accessible platinum atoms and of protonic acid sites (nPt/nH+). The rate of disproportionation per acid site does not depend on the relative proportion of zeolite and of Pt/Al2O3 in the bifunctional catalysts but increases with the density of protonic sites in the mordenite component. This suggests that this bimolecular reaction requires more than one acid site for its catalysis. Ethylbenzene dealkylation does not depend on the density of protonic sites but seems very sensitive to their strength. For these three reactions, the apparent activity of the protonic sites is significantly greater when the HMOR components of the bifunctional catalysts are dealuminated, hence present mesopores; most of the protonic sites of these dealuminated mordenites would be active, whereas for mordenites without mesopores, it would be the case only for the protonic sites of the shell of the crystallites. High selectivities to xylenes (≈75% at 35% conversion) were shown to be obtained on bifunctional catalysts with values of nPt/nH+ sufficiently high to have the acid isomerization of olefinic intermediates as the limiting step of ethylbenzene isomerization.

Complexation and Disproportionation of Np(V) in K10P2W17O61 Solutions

Complexation of NpO2 + and NpO2 2 + with unsaturated K n P2W17O61 n - 1 0 (L x-) heteropolyanion and disproportionation of Np(V) in the presence of L x- were studied spectrophotometrically. The logarithms (logK) of the formation constants of NpO2 VL and NpO2 V IL are ∼3 and 7, respectively. The K+ and Na+ cations bind the L x- anions, thus decreasing the yield of the complexes. Neptunium(V) disproportionation in K10P2W17O61 solutions containing 1 M (HClO4 + NaClO4) (pH from 0 to 4) and free from NaClO4 (pH 2-6.5) was studied. The disproportionation rate is described by the equation -d[Np(V)]/dt = k[Np(V)][L x-]. The pH dependence of the rate constant passes through a maximum at pH 1. The rate constant decreases with increasing [Na+]. The reaction is inhibited by its product, Np(IV). The Np(V) complex is not involved in disproportionation; the reactive species is NpO2 + aqua ion, which is probably converted into NpO3 +L x-. Then NpO3 +L x- rapidly reacts with NpO2 +, which occurs simultaneously with, or is preceded by release of the second oxygen atom.

Ethylbenzene Isomerization on Bifunctional Platinum Alumina–Mordenite Catalysts: 1. Influence of the Mordenite Si/Al Ratio

The transformation of ethylbenzene was carried out on intimate mixtures of 0.5 wt% Pt/Al2O3 (75 wt%) with a platinum dispersion of 100% and of HMOR zeolites (25 wt%) with Si/Al ratios between 6.6 and 180. Products result from six main transformations of ethylbenzene or/and of the reaction products: the desired isomerization of ethylbenzene into xylenes (reaction 1), disproportionation of ethylbenzene (2), dealkylation followed by hydrogenation of ethylene (3), secondary ethylbenzene–xylene transalkylation (4), hydrogenation of ethylbenzene followed by ethylcyclohexane isomerization (5), and secondary cracking of C8 naphthenes (6). From the comparison of the product distributions over Pt/Al2O3, over HMOR and over the bifunctional catalysts, reactions 2–4 were confirmed to occur through acid catalysis and hydrogenation through metal catalysis, and reactions 1 and 6 through bifunctional catalysis. Whatever the catalyst, the selectivity to isomers increases with ethylbenzene conversion, which is due to thermodynamic limitations in reactions 2 and 5. A significant increase in selectivity to isomers is also observed when the Si/Al ratio of the mordenite component increases from 20 to 180. With these bifunctional catalysts, the initial rate of disproportionation is proportional to the square of the concentration of protonic sites, which suggests that this bimolecular reaction requires two protonic sites for its catalysis; furthermore, the rate of dealkylation (per protonic site) depends only on the strength of the protonic sites and the change of the isomerization rate with the balance between the metallic and acidic functions is the one expected from a classical bifunctional mechanism. For the bifunctional catalysts with HMOR6.6 and −10 as acidic components, the rates of disproportionation, dealkylation, and isomerization are lower than expected and the selectivity to isomers is higher. This can be explained by strong diffusion limitations in these zeolites which, in contrast to the other samples, do not present mesopores in addition to micropores.

47-Electron organometallic clusters derived by chemical and electrochemical oxidation of trihydrido(alkylidyne)triruthenium clusters: II. Disproportionation mechanism for decomposition

The decomposition of 47-electron clusters, generated by chemical or electrochemical oxidations of 48-electron H 3Ru 3(μ 3-CX)(CO) 9− n L n (X=OMe; L=PPh 3; n=0–3: X=OMe, SEt; L=dppm; L=PPh 3; n=3: X=SEt, NMeBz; L=PR 3, SbPh 3; n=2,3) occurs by disproportionation back to the 48-electron precursor and very unstable 46-electron species. Both 47/48- and 46/47-e redox potentials display similar ligand additivity trends, which are correlated with HOMO energies determined by Fenske–Hall MO calculations. Decomposition of [H 3Ru 3(μ 3-COMe)(CO) 6(PPh 3) 3] 1+ in the presence of added PPh 3 forms 48-e H 3Ru 3(μ 3-COMe)(CO) 6(PPh 3) 3, 46-e [H 3Ru 3(CO) 7(PPh 3) 3] 1+, and [MePPh 3] 1+; the rate law is second order with respect to concentration of the 47-e cluster and displays a small dependence on PPh 3 concentration. Decompositions of [H 3Ru 3(μ 3-COMe)(CO) 7(PPh 3) 2] 1+, [H 3Ru 3(μ 3-CSEt)(CO) 7(dppm)] 1+, and [H 3Ru 3(μ 3-CSEt)(CO) 6(PPh 3) 3] 1+ are also second order, although only the 48-e product could be characterized. The mechanism is proposed to involve rate-limiting outer-sphere electron transfer, the slow rate of disproportionation is a consequence of the fact that the reaction involves transfer of an electron from one half-filled bonding orbital to another half-filled bonding orbital, thus requiring considerable reorganization energy. Electrochemical studies of the 2-e oxidations of H 3Ru 3(μ 3-COMe)(CO) 9− n (PPh 3) n , n=0 and 1, indicate that the initial 46-e cluster rearranges very rapidly to a new 46-e species, which decomposes rapidly to electrochemically inactive products.

Effects of denaturation on soy protein–xanthan interactions: comparison of a whipping–rheological and a bubbling method

The effect of xanthan on foam formation and on physical mechanisms of destabilization involved in the breakdown of foams made from native and denatured soy protein at neutral pH was studied by a bubbling and a whipping–rheological method. Parameters describing foam formation and destabilization by liquid drainage and disproportionation obtained by the two methods showed that the addition of xanthan was accompanied by delayed rates of drainage and disproportionation and reduced foam height decay (collapse). Drainage showed the largest reduction, mainly because of the increased bulk viscosity. In the absence of xanthan, protein denaturation enhanced foam formation and stability against drainage and disproportionation, but increased the collapse of foams. In the presence of xanthan, differences in foam formation and drainage/disproportionation stability between native and denatured soy protein were greatly reduced. However, differences in foam collapse were greatly enhanced. The increased stability of foams in the presence of xanthan could not be explained purely in terms of increased aqueous phase viscosity. More specific interactions of xanthan and soy proteins at the air–water interface influencing the surface rheology, and the protein composition and aggregation, are involved.

Oxygen and sulfur isotope fractionation during anaerobic bacterial disproportionation of elemental sulfur

Bacterial disproportionation of elemental sulfur is an important process in the sulfur cycle of natural sediments and leads to the formation of hydrogen sulfide and sulfate. The oxygen atoms in sulfate during this anaerobic process are completely derived from water according to the overall reaction: 4H 2O + 4S 0 → 3H 2S + SO 4 2− + 2H +In the present study, stable oxygen isotope fractionation during formation of sulfate via this reaction was experimentally investigated for a pure culture (Desulfocapsa thiozymogenes) and an enrichment culture (“Kuhgraben”) at 28°C. Synthetic FeCO 3 and FeOOH were used as scavengers for hydrogen sulfide to keep the disproportionation reaction exergonic and to suppress polysulfide formation and isotope exchange between elemental sulfur and hydrogen sulfide. Compared to water, dissolved sulfate was enriched in 18O by +17.4 ± 0.1‰ ( Desulfocapsa thiozymogenes) and +16.6 ± 0.5‰ (Kuhgraben) at cell specific sulfur disproportionation rates of 10 −15.4 ± 0.4 mol S° cell −1 h −1 and 10 −14.4 ± 0.9 mol S° cell −1 h −1, respectively. Oxygen isotope fractionation was not influenced by the type of iron-bearing scavenger used, corroborating earlier findings that H 2S oxidation by FeOOH yields elemental sulfur as the dominant oxidation product. Sulfite is suggested to be formed as a metabolic intermediate to facilitate isotope exchange with water. Due to bacterial disproportionation, dissolved sulfate was also enriched in 34S compared to elemental sulfur by +11.0 to +18.4‰ ( D. thiozymogenes) and +12.7 to +17.9‰ (Kuhgraben). FeS was depleted in 32S compared to elemental sulfur by −3.7 to −5.3‰ ( D. thiozymogenes).

Anaerobic sulfide oxidation and stable isotope fractionation associated with bacterial sulfur disproportionation in the presence of MnO 2

The sulfur and oxygen isotope effects associated with anaerobic bacterial disproportionation of elemental sulfur by a pure culture ( Desulfocapsa thiozymogenes) and an enrichment culture were investigated experimentally in the presence of synthetic Mn(IV)oxides. During bacterial disproportionation, 34S/ 32S were fractionated in dissolved sulfate compared to elemental sulfur by −0.6 to +2.0‰ ( D. thiozymogenes) and −0.2 to +1.1‰ (enrichment culture) at cellular sulfur disproportionation rates of 10 −16 mol S°/cell/h and 10 −17 mol S°/cell/h, respectively. The measured sulfur isotope effects are much smaller than those observed previously for the same cultures in the presence of Fe(III) and Fe(II) compounds, indicating that microbial isotope fractionation was superimposed by the chemical re-oxidation of hydrogen sulfide by MnO 2 to sulfate. Significant re-oxidation of H 2S to sulfate was additionally confirmed by the oxygen isotopic composition of sulfate, which was enriched in 18O compared to water by +8 to +12‰. These new experimental results imply that the overall influence of bacterial disproportionation on stable isotope partitioning in natural surface sediments depends on the proportion and relative recycling rates of reactive Fe(III) to Mn(IV)(oxyhydr)oxides.

Novel dinuclear manganese(III) complexes with bi- or tridentate and bridging tetradentate Schiff base ligands: preparation, properties and catalase-like function

Novel mono- and dinuclear manganese(III) complexes: [Mn III(acac)(PA) 2] ( 1), [Mn III(acac)( N-OPh-sal)(EtOH)] ( 2), [Mn III 2(PA) 4(sal- m-xylylene)] ( 3), [Mn III 2( N-OPh-sal) 2(X-sal- m-xylylene)] (X=H ( 4a), 5-MeO ( 4b), 3-MeO ( 4c), 5-Br ( 4d), and 5-NO 2 ( 4e)), and [Mn III 2( N-OPh-sal) 2(X-salpentn)] (X=H ( 5a) and 5-MeO ( 5b)) have been prepared by ligand substitution reactions and characterized, where Hacac, HPA, N-HOPh-Hsal, H 2X-sal- m-xylylene, and H 2X-salpentn denote acetylacetone, picolinic acid, N-hydroxyphenyl-salicylideneamine, N, N′-di-substituted-salicylidene- m-xylylenediamine, and N, N′-di-substituted-salicylidene-1,5-pentanediamine, respectively. Single crystals of complexes 1, 2, 4a, and 5a were used for X-ray crystallographic determination. Complexes 1 and 2 have a mononuclear structure, in which the central manganese(III) ions adopt a distorted octahedral geometry having an elongated axial bond compared to the equatorial bonds due to the Jahn–Teller effect. For complexes 4a and 5a, their detailed structures could not be clarified owing to a disorder of their molecules and quick degradation of the crystals in air. However, the results suggested that the two manganese(III) ions in these complexes are bridged by one sal- m-xylylene or salpentn ligand to form a dinuclear complex, and each complex has an arrangement similar to that of the mononuclear complex 2. The reactivities of these manganese(III) complexes toward H 2O 2 have been found that the dinuclear complexes 4a– e, 5a, and 5b can decompose excess amounts of H 2O 2 (H 2O 2/Mn<100) almost without degradation of the complexes, whereas complex 3 decomposes with an equimolar amount of H 2O 2. Moreover, the disproportionation rates of H 2O 2 have been found to depend on the bridging tetradentate ligands, the substituents on the phenyl rings of the bridging ligands, and the solvents used. On the basis of EPR spectroscopic studies, a series of complexes 4a– e, 5a, and 5b cycle their oxidation levels between Mn 2(III, III) and Mn 2(II, II) with dioxygen evolution during the decomposition of H 2O 2.

Formation of Excited Uranyl in Oxidation of U(IV) with Oxygen in HClO4 Aqueous Solutions: II. Catalytic Effect of UO22+

Strong effect of uranyl ion on the kinetic and chemiluminescence characteristics of U(IV) oxidation with oxygen in weakly acidic HClO4 solutions is found. For instance, in the presence of 8 × 10-4 M UO22+ the reaction rate constant, maximal intensity of the chemiluminescence, and chemiluminescence efficiency increase by factors of 50, 100, and 8, respectively. The catalytic effect of uranyl ion is probably caused by both formation of the complex UO22+·UO2+ (which decreases the rate of disproportionation of UO2+ participating in the chain propagation) and generation of UO2+ in the reaction of UO22+ with U(IV).

Kinetics and mechanism of hydrogen-induced disproportionation of ZrCo

The kinetics and mechnism of the hydrogen-induced disproportionation of ZrCo was studied experimentally. In general, the time dependence of the disproportionation has a sigmoidal shape. The disproportionation rate increased with increasing initial hydrogen pressure and showed a 3/2-power dependence on the pressure. As for the temperature dependence, the rate increased with temperature only up to 815 K and then decreased at higher temperatures. Namely, a volcano-type pattern was observed in the Arrhenius plot of the rate constant. In addition, a fairly large isotope effect was observed for the rate of disproportionation between hydrogen and deuterium. These results were explained well by a nucleation and nuclei growth model, in which hydrogen solubility in the ZrCo phase was assumed to play an important role.

Thermophilic sulfate and sulfite reduction with methanol in a high rate anaerobic reactor

Thermophilic sulfite and sulfate reduction offers good prospects as part of an alternative technology to conventional off-gas desulfurization technologies. Thermophilic sulfate and sulfite reduction with methanol as the sole carbon and energy source for the sulfate reducing bacteria was studied in lab-scale Expanded Granular Sludge Bed (EGSB) reactors operated at 65 °C and pH 7.5. At a hydraulic retention time (HRT) of 4 hr, sulfite and sulfate elimination rates of up to 0.22 mol-S.l-1.day-1 (100% elimination) and 0.15 mol-S.l-1.day-1 (80% elimination), respectively, were achieved. Sulfite and sulfate reduction accounted for 85–90% of the electrons released during degradation of methanol. In addition, 10–13% and 1–2% of the consumed methanol was converted to acetate and methane, respectively. Acetate was not utilized as electron donor for sulfate reduction. Acetate production seemed to be linearly correlated to the amount of sulfite and sulfate reduced. Sulfite disproportionating activity of the sludge was demonstrated by the simultaneous appearance of sulfide and sulfate in batch tests with sulfite. However, sulfite disproportionation rates were 4 times lower than sulfate reduction rates with methanol. The results clearly demonstrate that methanol can be efficiently used as electron and carbon source to obtain high sulfite and sulfate elimination rates in thermophilic bioreactors.

Hydrogenation-disproportionation processes in Nd 14.01Hf 0.08Fe 78.91B 6.99 alloys

The kinetics of the reactions undergone by the system Nd 14.01Hf 0.08Fe 78.91B 6.99 submitted to thermal treatments under H 2 atmosphere were studied in detail. Constant number of moles, constant pressure ( P≈119 kPa), isothermal or high heating rate experiments were performed at temperatures between 240 and 980°C on samples of ∼200 mg. The state after each treatment was characterized by Mössbauer effect spectroscopy and X-ray diffraction. Three temperature regions were identified. In the first one ( T≤580°C) two processes were observed: the formation of Nd-hydride at grain boundaries (2.6 H/Nd) and a solid solution Nd 2Fe 14BH x in the matrix phase. Activation energy of 36.20±0.02 kJ mol −1 for absorption process was determined. In the second temperature region (680°C≤ T≤800°C) two processes of H 2 absorption were found: (1) φ-Nd 2Fe 14B hydrogenation and formation of grain border Nd-hydride, and (2) disproportionation regime. For T≥800°C disproportionation rates are slower for increasing temperatures, the φ-phase incorporates less than 0.6 H/Nd and a strong dependence with the experimental conditions was found in this temperature range. Between 800 and 875°C the disproportionation reaction is fast but saturates, leaving only a small non-disproportionated fraction. This may constitute the best temperature interval for the hydrogenation-disproportionation processes.

Stable Paramagnetic Half-Sandwich Mo(V) and W(V) Polyhydride Complexes. Structural, Spectroscopic, Electrochemical, Theoretical, and Decomposition Mechanism Studies of [Cp*MH3(dppe)]+ (M = Mo, W)

Compounds Cp*MH3(dppe) (M = Mo, 1; W, 2) are oxidized chemically and electrochemically to the corresponding 17-electron cations 1+ and 2+. Analogous oxidations of 1-d3 and 2-d3 provide 1+-d3 and 2+-d3, respectively. Complex 2+ is stable in CH2Cl2, THF, and MeCN at room temperature. A single-crystal X-ray analysis of the PF6- salt of 2+ shows a geometry for the cation which is intermediate between octahedral and trigonal prismatic, which is reproduced by geometry optimization of the [CpWH3(PH2CH2CH2PH2)]+ model at the B3LYP/LANL2DZ level. Identical calculations on the neutral analogue also reproduce the previously reported trigonal prismatic structure for 1. A blue shift in the M−H stretching vibrations upon oxidation for both Mo and W compounds indicates that a M−H bond strengthening accompanies the oxidation process. The DFT calculations (M−H bond lengths, BDE, and stretching frequencies) are in good agreement with the experimental results. Complex 1+ decomposes in solution at room temperature by one or more of three different mechanisms depending on conditions: H2 reductive elimination, solvent-assisted disproportionation, or deprotonation. In THF or CH2Cl2, a reductive elimination of H2 affords the stable paramagnetic monohydride Cp*MoH(dppe)PF6 (3), which adds a molecule of solvent in CH2Cl2, THF, and MeCN. EPR studies show that the CH2Cl2 molecule coordinates in a bidentate mode to afford a 19-electron configuration. A solvent dependence of the decomposition rate [k(CH2Cl2) ≈ 7.8k(THF) at 0 °C] and an inverse isotope effect [kH/kD = 0.50(3) in CH2Cl2 at 0 °C] indicate the nature of 1+ as a classical trihydride and suggest a decomposition mechanism which involves equilibrium conversion to a nonclassical intermediate followed by a rate-determining associative exchange of H2 with a solvent molecule. In MeCN at 20 °C, a solvent-assisted disproportionation (rate = kdisp[1+]2, kdisp = 3.98(9) × 103 s-1 M-1) and a deprotonation by residual unoxidized 1 (rate = kdeprot[1+][1], kdeprot = 2.8(2) × 102 s-1 M-1) take place competitively, as shown by detailed cyclic voltammetric and thin-layer cyclic voltammetric studies. The stoichiometric chemical oxidation of 1 in MeCN leads to a mixture of [Cp*MoH2(dppe)(MeCN)]+ and [Cp*MoH(dppe)(MeCN)2]2+ by the disproportionation mechanism.

Fast reactions of*CH2(CH3)NCHO radicals from the photolysis of Feaq 3+ in the presence ofN,N-dimethylformamide

Laser flash photolysis was used to study the photochemistry of Feaq3+ complex in aqueous solutions with additions ofN,N-dimethylformamide. An electronic absorption band with a maximum at 380 nm belonging to the*CH2(CH3)NCHO radical was recorded. The recombination (or disproportionation) rate constants of*CH2(CH3)NCHO radicals and their reaction with the Feaq3+ complex were estimated to be (3.4±0.8)×109 M−1s−1 and (4.3±0.6)×107 M−1s−1, respectively. The mechanism of radical formation is discussed.