Hydride Mechanism Research Articles

Rhodium and Iridium β-Diiminate Complexes – Olefin Hydrogenation Step by Step

The bulky β-diiminate ligands [(2,6-C6H3X2)NC(Me)CHC(Me)N(2,6-C6H3X2)]– (X = Me, LMe; X = Cl, LCl) have been found to be effective in stabilizing low coordination numbers (CN) in Rh and Ir complexes. The 14- complex LMeRh(COE) (COE = cyclooctene) has a three-coordinate T-shaped Rh environment and is nonagostic. Coordinative unsaturation is avoided by incorporation of a small ligand (e.g. N2, MeCN, olefins), by the intramolecular coordination of a chlorine atom in LClRh(COE), or by an agostic interaction in LMeRh(norbornene). In solution at room temperature, LMeRh(COE) undergoes rapid isomerization according to the allyl hydride mechanism; the corresponding 2,3-dimethylbutene complex actually prefers the allyl hydride structure. Rhodium(I) complexes of LMe and LCl catalyze olefin hydrogenation; hydrogenation of 2,3-dimethylbutene has been shown to be preceded by isomerization. The shielding properties of the bulky β-diiminate ligands allow direct observation of a number of reactive intermediates or their iridium analogues, including an olefin–dihydrogen complex (with Rh) and an olefin dihydride (with Ir). These observations, together with calculations on simple model systems, provide us with snapshots of a plausible hydrogenation cycle. Remarkably, hydrogenation according to this cycle appears to follow a 14-e/16-e path, in contrast to the more usual 16-e/18-e paths.

Evidence for the hydride mechanism in the methoxycarbonylation of ethene catalysed by palladium–triphenylphosphine complexes

Analysis of the phosphorus containing by-products from the catalytic methoxycarbonylation of ethene using a palladium triphenylphosphine complex with a combination of High Performance Liquid Chromatography (HPLC) and Mass Spectrometry (MS) indicated increasing levels of phosphonium salts through the course of the reaction, major ones being methyltriphenylphosphonium, ethyltriphenylphosphonium and 3-oxopentyltriphenylphosphonium (CH3CH2C(O)CH2CH2PPh3) cations, isolated as the sulfonate salts. The latter are shown to be produced by metal mediated pathways and believed to be indicative of the operation of the hydride mechanism.

Dimerization of 1-butene via zirconium-based Ziegler-Natta catalyst

A zirconium‐based Ziegler–Natta catalytic system has been tested in the dimerization of 1‐butene. It was found that the concentration of Et2AlCl, Ph3P and PhONa as well as the reaction temperature had great influences on the activity and selectivity of the catalyst. Under the optimum reaction conditions, the conversion of 1‐butene is 91.9%, and the selectivity of dimers is 76.7%. Basic ligands such as Ph3P and PhONa can inhibit isomerization of 1‐butene to 2‐butene effectively. In addition, the metal hydride mechanism was also suggested and some indirect evidence was obtained in favor of this mechanism.

Titanium(IV) isopropoxide-catalysed reaction of alkylmagnesium halides with ethyl acetate in the presence of styrene. Non-hydride mechanism of ligand exchange in the titanacyclopropanes

The dependence of the yields of ( E)-1-methyl-2-phenyl-1-cyclopropanol ( 3) on the structure of the organomagnesium compounds and reagents ratio in the reaction of ethyl acetate with Grignard reagents, in the presence of styrene and catalytic amounts of Ti(OPr i ) 4, has been investigated. Butylmagnesium bromide has been found to be the most suitable organomagnesium for the preparation of 3 by this method. The use of (CD 3) 2CHMgBr for the generation of the titanacyclopropane intermediates led to the formation of 3. This result disagreed with the hydride mechanism of the ligand exchange for 2-phenyltitanacyclopropane ( 4) formation.

A unified model of Zircaloy BWR corrosion and hydriding mechanisms

A model explaining the uniform and nodular corrosion and hydriding behaviour in BWRs is presented. As illustration, three materials with different second phase particle (SPP) distributions have been characterised in the un-irradiated and irradiated conditions. One of the materials shows accelerated uniform corrosion at higher burn-ups, despite a similar alloy composition for all three materials. It is concluded that the SPP distribution and chemical composition is controlling the corrosion and hydriding behaviour at all stages, except in the pre-transition stage. It is further concluded that, of several simultaneous corrosion processes, one is normally rate limiting at each stage of corrosion development, but different corrosion processes are rate limiting at each stage.

The mechanism of the catalytic behaviour of platinum triphenylphosphine complexes in the ethylene hydrocarbonylation

High resolution 1 H, 13C and 31P NMR was used for step-by-step investigation of ethylene hydrocarbonylation into diethyl ketone (DEK) in the ‘ P 2PtX 2 (or P 4Pt)CF 3COOH H 2O ’ (P = PPh 3, X = CF 3COO −; [H 2O] ≤ 30 vol.%) catalytic systems. The so-called ‘hydride mechanism’ of the reaction was unambiguously determined. Six key Pt(II) intermediates, namely, [HPtP 3] +, 1, trans-[HPt(X)P 2], 2, trans-[HPt(CO)P 2] +, 5, trans-[HPt(C 2H 4)P 2] +, 6, trans-[C 2H 5Pt(C 2H 4)P 2] +, 7, and trans-[C 2H 5Pt(CO)P 2] +, 9, were identified and characterized, and in addition their reactivity was studied. The relative thermodynamic stability of the Pt(II) hydride complexes turned out to determine the catalytic behaviour of the platinum systems investigated and was found to increase in CF 3COOH H 2O solutions in the order: 2 < 6 1 < 5. In accordance with this, the ‘ P 4PtCF 3COOH H 2O ’ system, in which rapid and quantitative conversion of initially formed complex 1 into the extremely inert to C 2H 4 complex 5 occurred under the reaction conditions, exhibited no detectable activity in DEK formation. The initial activity of the ‘ P 2PtX 2CF 3COOH H 2O ’ system in DEK formation was suggested to result from the generation of complex 9, in addition to 5, at the initial unsteady-state period of the reaction as a result of the kinetic competition of two rapid reaction sequences: 2 → 6 → 7 → 9 against 2 → 5. Isomerization of 9 into the corresponding platinum propionyl complex appeared to be the rate-determining stage of DEK formation. The kinetic regularities of the model catalytic reactions of ethylene hydrogenation and hydrogen isotopic heteroexchange with the ‘ P 2PtX 2CF 3COOH H 2O ’ and ‘ P 4PtCF 3COOH H 2O ’ systems were also studied. The former system was found to exhibit a much higher initial activity due to formation of a more active platinum hydride complex 2, while in the latter system a less active complex 1 was formed initially. The activities of both systems, however, leveled off during the reactions due to 2 1 equilibration.

Migration of hydrogen through thin films of ZrO2 on Zr–Nb alloy

The barrier properties of ZrO2 to inward migration of deuterium have been investigated with a view to understanding the hydriding mechanisms of a Zr-2.5% Nb alloy used in CANDU nuclear reactors fuel channels. Thin film oxide specimens, grown in steam to ∼1 μm thickness, have been heated to 350 °C and exposed to deuterium gas at pressures ranging from 6×10−3 Pa to 101 kPa (1 atm) and times from 10 to 810 min. Some irreversible uptake can be measured for all exposures using secondary ion mass spectrometry. At low exposures, the shape of the deuterium concentration profile is can be fitted to a Fickian relationship. During longer exposures, the rate of deuterium ingress is sharply curtailed, presumably due to passivated outer oxide surface. Reactions between D2O vapor and the thin film oxide in the 10−3 Pa pressure region and above show a sharply higher uptake of deuterium than in the equivalent pressure of D2 gas. This is ascribed to a more efficient decomposition of D2O on the ZrO2 surface compared to D2.

The effect of thermal cycling on the hydriding rate of MmNi4.5Al0.5

Abstract The hydriding kinetic mechanism and the change in the hydriding reaction rate of MnNi 4.5 Al 0.5 during thermally induced hydrogen absorption-desorption cycling are investigated. Comparison of the reaction rate data obtained by the pressure sweep method with the theoretical rate equations suggests that the hydriding rate-controlling step has changed from dissociative chemisorption of hydrogen molecules at the surface to hydrogen diffusion through the hydride phase with increasing hydriding fraction. This hydriding kinetic mechanism does not change during cycling. However, the intrinsic hydriding reaction rate of MmNi 4.5 Al 0.5 after 5500 cycles increases significantly compared with that of the activated alloy. It is suggested that the change in the hydriding kinetic behaviour due to the intrinsic degradation of MmNi 4.5 Al 0.5 can be explained by the formation of nickel clusters at the surface of the sample and the exchange of host metal atoms in the bulk by thermal cycling.

Etude de l'hydruration d'un alliage uranium-vanadium a 0.2% en poids

Knowledge of the hydriding mechanism of the U-0.2wt.%V alloy led us to study the influence of temperature (from room temperature to about 200 °C) and hydrogen pressure up to 10 5 Pa) on the reaction kinetics, using ultra-high vacuum technology to minimive the effect of gaseous impurities. After a short nucleation stage, the reaction rate remains constant and takes values higher than those already reported in the literature. The hydriding process cannot be described by a single activation mechanism. A complev hydrided vone is observed under the spall front of the reaction, displaying a double-layer structure; thin hydride precipitates are detected in the underlying metal up to a depth of a few microns.

Hydriding mechanism of Mg2Ni in the presence of oxygen impurity in hydrogen

For the case where oxygen is present as an impurity in hydrogen, a hydriding mechanism is proposed along with hydriding cycling of the Mg2Ni alloy. The large chemical affinity of magnesium for oxygen leads to the selective oxidation of magnesium and to the segregation of the more noble nickel component. The consequence is a progressive decrease in hydrogen storage capacity of Mg2Ni along with hydriding cycling. The segregated nickel provides active sites for chemisorption of oxygen and hydrogen. The chemisorbed oxygen accelerates the surface segregation of nickel or can form water vapour with hydrogen, or (least favourably) directly oxidizes nickel. The chemisorbed hydrogen can form water vapour with oxygen, can hydride Mg2Ni or can reduce the nickel oxide eventually formed. All these reactions are exothermic, causing an increase in temperature during the hydriding process.

Rhodium(I) catalyzed E/Z isomerization of dimethyl butenedioate

Hydride complexes of Rh(I) represent highly effective homogeneous catalysts of the isomerization of (Z)-dimethyl butenedioate (I) yielding (E)-dimethyl butenedioate (II) in benzene at 25 °C. The reaction catalyzed by RhH(P(C6H5)3)4 is first order both in I and in the catalyst, k = 0.51 l mol-1 s-1, Ea = 48 kJ mol-1, ΔS≠ = -46 J mol-1 K-1. At high substrate-to-catalyst ratios the catalyst is inactivated, which consists mainly in deoxygenation and decarbonylation of the E- and Z-esters with formation of methyl 2-butenoate, triphenylphosphine oxide, and carbonylocomplexes of Rh(I). Statistical redistribution of deuterium during the isomerization of equimolar mixture of I and [2,3-2H2]-I and other experimental evidence are consistent with the addition-elimination hydride mechanism of the isomerization involving σ-alkyl rhodium complexes as the intermediates and RhH(P(C6H5)3)2 as the catalytically active species.

A New Technique for Investigating the Electrochemical Behavior of Electroless Plating Baths and the Mechanism of Electroless Nickel Plating

The electrochemical potential of electroless nickel and electroless cobalt plating baths can be altered by the addition of a variety of chemical compounds to the baths. At the same time, the metal‐to‐phosphorus ratio in the deposit is altered. This behavior has been studied for a series of baths using typical accelerators including thiourea, glycine, and formate; each of these represents a different type of accelerator characteristic. A Tafel‐like behavior was observed to apply for the deposition of nickel (or cobalt) and also for the deposition of phosphorus. Therefore, given a suitable reference point, the plating rate and metal‐to‐phosphorus ratio can be predicted from the measured potential. A model based on a modified hydride mechanism is proposed for the electroless deposition of nickel‐phosphorus. With the help of this model, the hydrogen evolved during deposition can be quantitatively accounted for.

Disintegration of Lead Cathodes, II

In continuation of previous work, the disintegration of lead cathodes at high current densities at 25°C was studied. The solutions were acid, basic, and neutral combinations of sulfuric and phosphoric acids and ammonium and alkali sulfates, chlorides, and phosphates. The results support the hydride mechanism and contradict the alkali alloy hypothesis. The data are interpreted to indicate that water molecules are reduced at current densities below those needed for visible disintegration and that the surface is saturated with adsorbed hydrogen atoms at the start of disintegration.