(iPr3P)6Rh6H12]2+: a high-hydride content octahedron that bridges the gap between late and early transition metal clusters

Treatment of [(iPr3P)2Rh(nbd)][Y] {nbd = norbornadiene, Y = B{3,5-(CF3)2C6H3}4- [B(ArF)4] or 1-H-closo-CB11Me11-} with H2 (ca. 4 atm) results in the isolation, in moderate yield, of the octahedral cluster complex [(iPr3P)6Rh6H12][Y]2 1. The cluster (for both anions) has been characterized by NMR, mass spectroscopy, and X-ray crystallography. These show 1 to have 12 edge-bridging hydrogen atoms, and the structure bears more resemblance to clusters of the early transition metals with pi-donor ligands than those of the late transition metals with pi-acceptor ligands. Intermediate complexes on the route to 1, namely, the nonclassical dihydrogen complexes [(iPr3P)2Rh(H)2(eta2-H2)x][B(ArF)4] (x = 1 or 2), have been observed spectroscopically. The high hydride content of 1 makes it a possible model for nanocluster colloidal Rh(0) catalysts that are used in olefin and arene hydrogenation.     

Perfluorinated membranes as catalyst supports

The perfluorinated polymer Nafion and porous PTFE/Nafion composite membranes have been employed as supports for nickel complexes or for platinum and palladium metal particles. The resultant materials have been employed as catalysts in various olefin conversion processes. Supported platinum and palladium metal systems were evaluated as catalysts for the hydrogenation of cyclohexene. Rates of reaction are better than those of commercially available catalysts; turnover numbers in excess of 6000 have been obtained with no poisoning apparent. Catalysts may be regenerated many times. The reduction rate approaches a limit at high pressures of hydrogen and has an activation energy of 13 kJ mol^?1 in neat cyclohexene. Nafion was employed as a strong acid cocatalyst to activate and then support a nickel complex catalyst. The resultant catalyst was active for double-bond-shift isomerization.     

Kinetics and mechanisms of homogeneous catalytic reactions: Part 6. Hydroformylation of 1-hexene by use of Rh(acac)(CO)2/dppe [dppe = 1,2-bis(diphenylphosphino)ethane] as the precatalyst

A kinetic study of the homogeneous hydroformylation of 1-hexene to the corresponding aldehydes (heptanal and 2-methyl-hexanal) was carried out using a rhodium catalyst formed by addition of 1 equiv. of 1,2-bis(diphenylphosphino)ethane (dppe) to Rh(acac)(CO)₂ under mild reaction conditions (80 °C, 1–7 atm H₂ and 1–7 atm CO) in toluene; in all cases linear to branched ratios were close to 2. The reaction rate is first-order in dissolved hydrogen concentration at pressures below 3 atm, but independent of this parameter at higher pressures. In both regimes (low and high H₂ pressure), the initial rate was first-order with respect to the concentration of Rh and fractional order with respect to 1-hexene concentration. Increasing CO pressure had a positive effect on the rate up to a threshold value above which inhibition of the reaction was observed; the range of positive order on CO concentration is smaller when the total pressure is increased. The kinetic data and related coordination chemistry are consistent with a mechanism involving RhH(CO)(dppe) as the active species initiating the cycle, hydrogenolysis of the acyl intermediate as the rate-determining step of the catalytic cycle at low hydrogen pressure, and migratory insertion of the olefin into the metal-hydride bond as rate limiting at high hydrogen pressure. This catalytic cycle is similar to the one commonly accepted for RhH(CO)(PPh₃)₃ but different from previous proposals for Rh-diphosphine catalysts.     

Transition state characterization for the reversible binding of dihydrogen to bis(2,2'-bipyridine)rhodium(I) from temperature- and pressure-dependent experimental and theoretical studies

Thermodynamic and kinetic parameters for the oxidative addition of H2 to [Rh(I)(bpy)2]+ (bpy = 2,2'-bipyridine) to form [Rh(III)(H)2(bpy)2]+ were determined from either the UV-vis spectrum of equilibrium mixtures of [Rh(I)(bpy)2]+ and [Rh(III)(H)2(bpy)2]+ or from the observed rates of dihydride formation following visible-light irradiation of solutions containing [Rh(III)(H)2(bpy)2]+ as a function of H2 concentration, temperature, and pressure in acetone and methanol. The activation enthalpy and entropy in methanol are 10.0 kcal mol(-1) and -18 cal mol(-1) K(-1), respectively. The reaction enthalpy and entropy are -10.3 kcal mol(-1) and -19 cal mol(-1) K(-1), respectively. Similar values were obtained in acetone. Surprisingly, the volumes of activation for dihydride formation (-15 and -16 cm(3) mol(-1) in methanol and acetone, respectively) are very close to the overall reaction volumes (-15 cm(3) mol(-1) in both solvents). Thus, the volumes of activation for the reverse reaction, elimination of dihydrogen from the dihydrido complex, are approximately zero. B3LYP hybrid DFT calculations of the transition-state complex in methanol and similar MP2 calculations in the gas phase suggest that the dihydrogen has a short H-H bond (0.823 and 0.810 Angstroms, respectively) and forms only a weak Rh-H bond (1.866 and 1.915 Angstroms, respectively). Equal partial molar volumes of the dihydrogenrhodium(I) transition state and dihydridorhodium(III) can account for the experimental volume profile found for the overall process.     

Dual functional catalysis for ethylene polymerization to branched polyethylene. I. Evaluation of catalytic systems

Synthesis of low-density polyethylene, that is, a density of less than 0.925 g/cm³, has traditionally been accomplished by the use of free-radical initiators at high ethylene pressures or of an alpha olefin comonomer such as 1-butene at lower pressures. We investigated an alternative route to branched, low-density polyethylene with a single monomer, ethylene, as the feed in conjunction with multicomponent catalyst systems capable of in situ dimerization of ethylene and subsequent copolymerization to produce low-density polyethylene. This article discusses the details of the evaluation of a number of dual-functional systems based on Ziegler-Natta catalysts. Specific, well defined, dual-functional catalyst systems which could easily produce branched, low-density polyethylene with levels of 20–30 branches per 1000 carbon atoms were developed. Variations in the relative number of component catalysts resulted in systematic, predictable changes in the properties of the polyethylene produced, which demonstrated the utility of the dual-functional catalyst concept.     

Iridium-catalyzed hydrogenation of N-heterocyclic compounds under mild conditions by an outer-sphere pathway

A new homogeneous iridium catalyst gives hydrogenation of quinolines under unprecedentedly mild conditions-as low as 1 atm of H(2) and 25 °C. We report air- and moisture-stable iridium(I) NHC catalyst precursors that are active for reduction of a wide variety of quinolines having functionalities at the 2-, 6-, and 8- positions. A combined experimental and theoretical study has elucidated the mechanism of this reaction. DFT studies on a model Ir complex show that a conventional inner-sphere mechanism is disfavored relative to an unusual stepwise outer-sphere mechanism involving sequential proton and hydride transfer. All intermediates in this proposed mechanism have been isolated or spectroscopically characterized, including two new iridium(III) hydrides and a notable cationic iridium(III) dihydrogen dihydride complex. DFT calculations on full systems establish the coordination geometry of these iridium hydrides, while stoichiometric and catalytic experiments with the isolated complexes provide evidence for the mechanistic proposal. The proposed mechanism explains why the catalytic reaction is slower for unhindered substrates and why small changes in the ligand set drastically alter catalyst activity.     

Synthesis of bis(indenyl)zirconium dihydrides and subsequent rearrangement to eta5,eta3-4,5-dihydroindenediyl ligands: evidence for intermediates during the hydrogenation to tetrahydroindenyl derivatives

Exposure of eta9,eta5-bis(indenyl)zirconium sandwich complexes to 4 atm of H2 resulted in facile oxidative addition to furnish the corresponding zirconocene dihydrides, (eta5-C9H5-1,3-R2)2ZrH2 (R = SiMe3, SiMe2Ph, CHMe2). Continued hydrogenation completed conversion to the tetrahydroindenyl derivatives, (eta5-C9H9-1,3-R2)2ZrH2. Deuterium labeling studies established that dihydrogen (dideuterium) addition to the benzo rings is intramolecular and stereospecific, occurring solely from the endo face of the ligand, proximal to the zirconium. In the absence of dihydrogen, the bis(indenyl)zirconium dihydrides rearranged to new zirconium monohydride complexes containing an unusual eta5,eta3-4,5-dihydroindenediyl ligand, arising from metal-to-benzo ring hydrogen transfer. Mechanistic studies, including a normal, primary kinetic isotope effect measured at 23 degrees C, are consistent with a pathway involving regio- and stereoselective insertion of a benzo C=C bond into a zirconium hydride. The stereochemistry of the insertion reaction, and hence the eta5,eta3-4,5-dihydroindenediyl product, is influenced by the presence of donor ligands and controlled by the preferred conformation of the indenyl rings. Exposure of the zirconium hydrides containing the eta5,eta3-4,5-dihydroindenediyl rings to 1 atm of dihydrogen afforded the tetrahydroindenyl zirconium dihydride complexes, establishing the intermediacy of this unusual coordination environment during benzo ring hydrogenation.     

Elongated dihydrogen versus compressed dihydride complexes: the temperature dependence of the H-D spin-spin coupling constant as a criterion to distinguish between them

To be able to propose experimental tests to distinguish elongated dihydrogen transition-metal complexes from compressed dihydride transition-metal complexes, a thorough density functional study of the electronic structure in combination with quantum nuclear dynamics calculations have been performed for complexes [Cp*Ru(H2PCH2PCH2(H2)]+ and [CpRe(CO)2H2]. The results of this study suggest that elongated dihydrogen complexes and compressed dihydride complexes have different properties and that it should be possible to distinguish between them experimentally. In particular, different behavior is predicted with respect to 1) the sign of the isotope geometric effect on the H-H distance at 0 K, 2) the temperature dependence of the H-H distance, and 3) the temperature dependence of the H-D spin-spin coupling constant in 1H NMR spectroscopy.     

Effect of pressure on the production of hydrogen and nanofilamentous carbon by the catalytic pyrolysis of methane on Ni-containing catalysts

An experimental study of the production of hydrogen and nanofilamentous carbon (NFC) by methane pyrolysis on Ni- and Ni + Cu-containing catalysts at 675°C and pressures of up to 5 atm is reported. As the pressure and the copper content of the catalyst are increased, the stable service life of the catalyst and the hydrogen and NFC yield per unit weight of the catalyst, as well as the specific surface area and micropore volume of the resulting NFC, increase markedly.     

Mechanistic comparison of ruthenium olefin metathesis catalysts: DFT insight into relative reactivity and decomposition behavior

The reaction mechanism and substrate-induced decomposition behavior of three ruthenium olefin metathesis catalysts, viz. first- and second-generation catalysts and the recently developed Phoban catalyst (“Phobcat”) are compared by constructing ΔG surfaces at 298.15 K and 1 atm for the complete ligand systems. From these calculations fundamental insight is gained into the reactivity and stability observed experimentally for the three catalysts. In particular, the higher conversions obtained for the first-generation derived Phobcat catalyst, compared to conventional first-generation catalysts, is attributed to its similarity to the second-generation catalysts instead of first-generation catalyst. Important differences between the calculated ΔG surfaces and previously reported total electronic energy (ΔE) surfaces for the metathesis mechanism with complete ligand complexes are discussed.     

Hydration of acetylenic compounds without using mercury

Copper(II), palladium(II) and silver(I), in conjunction with non-coordinating anion environments, have been found to catalyze the hydration of a variety of functionalized and nonfunctionalized alkynes. Suitable anions include trifluoromethane sulfonate, tetrafluoroborate, and sulfonate groups on perfluorinated ion-exchange resins. These catalyst systems are the first reported catalysts that are active for the hydration of acetylenic alcohols in high selectivity and do not contain mercury. The problems of dehydration and byproduct formation, observed when other catalysts are used with these alcohols, are minimized.     

Direct Vapor Phase Carbonylation of Methanol over NiCl2/C Catalyst

IntroductionThe carbonylation of alcohols via homogenous catalysis is important in manufacturingacetic acid and higher carboxylic acids and their esters[1 ,2 ] . The main route to produce aceticacid is to make methanol carbonylated by means of the Monsanto and BP process in which ahomogeneous rhodium catalystis used.Although the homogeneous carbonylation ofmethanolis a highly selective process,itisaffected by the disadvantagesassociated with a highly corro-sive reaction medium due to the use …     

Acrylate Esters by Ethenolysis of Maleate Esters with Ru Metathesis Catalysts: an HTE and a Technoeconomic Study

A high throughput experimentation (HTE) study identified active Ru metathesis catalysts and reaction conditions for the ethenolysis of maleate esters to the respective acrylate esters. Catalysts were tested at various loadings (75–10’000 ppm) and temperatures (30–60 °C) with maleate esters dissolved in toluene (up to ca. 44 wt-%) or neat and at variable partial pressures of ethylene (0.2–10 bar). Ruthenium catalysts containing a PCy₃ ligand, such as 1st or 2nd generation Grubbs catalysts, as well as the state-of-the-art catalysts containing cyclic alkyl amino carbene (CAAC) ligands, are generally inferior to Hoveyda–Grubbs 2nd generation catalyst in ethenolysis of maleates. Productive turnover numbers could exceed 1900 if the ethenolysis reaction is performed at low ethylene pressure (0.2–3 bar) and reach 5200 when a polymeric phenol additive was used. Such catalytic performance falls well within the window practiced in industry. Moreover, a crude technoeconomic analysis finds similar production cost for the ethenolysis route and conventional technology, that is, propene oxidation followed by esterification, justifying research to further improve the ethenolysis route.     

Carborane as a tunable tag for Ru catalysts: generating an anion-appended recyclable and robust catalyst suitable for the noncovalent binding concept

Ruthenium carbene complexes 9 with a closo-1,2-C(2)B(10)H(11) tag and 10 with an ionic [nido-7,8-C(2)B(9)H(11)](-) tag were synthesized. Both 9 and 10 are highly reactive catalysts for olefin metathesis reactions. Importantly, 10 is a robust and recyclable anion-appended catalyst that was suitable for noncovalent binding with many cationic resins. At least ten recycles were achieved for RCM of the selected substrate using 10 as the catalyst in ionic liquids.     

Preparation of cyclic molecules bearing “strained” olefins using olefin metathesis

Recent advancements in metathesis catalyst design have allowed chemists to re-examine olefin metathesis as a route to systems bearing strained olefins embedded in their skeletons. Such ring systems include various azabicyclo [3.3.1] and [4.2.1] rings systems, the unique tricyclic ring system of the natural product ingenol, and strained macrocyclic systems exhibiting atropisomerism. Several examples of forming strained aromatic systems is also presented. The variety of different catalysts that have been developed allows for the possibility to select a catalyst having the necessary level of reactivity to access a strained system but also to avoid catalysts which may be so reactive as to favour ring-opening of the desired ring system.     

Dihydrogen complexes of rhodium: [RhH2(H2)x (PR3)2]+ (R = Cy, iPr; x = 1, 2)

Addition of H2 (4 atm at 298 K) to [Rh(nbd)(PR3)2][BAr(F)4] [R = Cy, iPr] affords Rh(III) dihydride/dihydrogen complexes. For R = Cy, complex 1a results, which has been shown by low-temperature NMR experiments to be the bis-dihydrogen/bis-hydride complex [Rh(H)2(eta2-H2)2(PCy3)2][BAr(F)4]. An X-ray diffraction study on 1a confirmed the {Rh(PCy3)2} core structure, but due to a poor data set, the hydrogen ligands were not located. DFT calculations at the B3LYP/DZVP level support the formulation as a Rh(III) dihydride/dihydrogen complex with cis hydride ligands. For R = iPr, the equivalent species, [Rh(H)2(eta2-H2)2(P iPr3)2][BAr(F)4] 2a, is formed, along with another complex that was spectroscopically identified as the mono-dihydrogen, bis-hydride solvent complex [Rh(H)2(eta2-H2)(CD2Cl2)(P iPr3)2][BAr(F)4] 2b. The analogous complex with PCy3 ligands, [Rh(H)2(eta2-H2)(CD2Cl2)(PCy3)2][BAr(F)4] 1b, can be observed by reducing the H2 pressure to 2 atm (at 298 K). Under vacuum, the dihydrogen ligands are lost in these complexes to form the spectroscopically characterized species, tentatively identified as the bis hydrides [Rh(H)2(L)2(PR3)2][BAr(F)4] (1c R = Cy; 2c R = iPr; L = CD2Cl2 or agostic interaction). Exposure of 1c or 2c to a H2 atmosphere regenerates the dihydrogen/bis-hydride complexes, while adding acetonitrile affords the bis-hydride MeCN adduct complexes [Rh(H)2(NCMe)2(PR3)2][BAr(F)4]. The dihydrogen complexes lose [HPR3][BAr(F)4] at or just above ambient temperature, suggested to be by heterolytic splitting of coordinated H2, to ultimately afford the dicationic cluster compounds of the type [Rh6(PR3)6(mu-H)12][BAr(F)4]2 in moderate yield.     

Nanostructured PtRu/C as anode catalysts prepared in a pseudomicroemulsion with ionic surfactant for direct methanol fuel cell

Nanostructured PtRu/C catalysts have been prepared from a water-in-oil pseudomicroemulsion with the aqueous phase of a mixed concentrated solution of H(2)PtCl(6), RuCl(3), and carbon powder, oil phase of cyclohexane, ionic surfactant of sodium dodecylbenzene sulfonate (C(18)H(29)NaO(3)S), and cosurfactant n-butanol (C(4)H(10)O). Two different composing PtRu/C nanocatalysts (catalyst 1, Pt 20 wt %, Ru 15 wt %; catalyst 2, Pt 20 wt %, Ru 10 wt %) were synthesized. The catalysts were characterized by transmission electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, and thermogravimetric analysis, and the particles were found to be nanosized (2-4 nm) and inherit the Pt face-centered cubic structure with Pt and Ru mainly in the zero valance oxidation state. The ruthenium oxide and hydrous ruthenium oxide (RuO(x)()H(y)()) were also found in these catalysts. The cyclic voltammograms (CVs) and chronoamperometries for methanol oxidation on these catalysts showed that catalyst 1 with a higher Ru content (15 wt %) has a higher and more durable electrocatalytic activity to methanol oxidation than catalyst 2 with low Ru content (10 wt %). The CV results for catalysts 1 and 2 strongly support the bifunctional mechanism of PtRu/C catalysts for methanol oxidation. The data from direct methanol single cells using these two PtRu/C as anode catalysts show the cell with catalyst 1 has higher open circuit voltage (OCV = 0.75 V) and maximal power density (78 mW/cm(2)) than that with catalyst 2 (OCV = 0.70 V, P(max) = 56 mW/cm(2)) at 80 degrees C.     

Stopped-flow kinetic analysis of the interaction of cyclo[8]pyrrole with anions

The on and off rates corresponding to the binding of two test anions (acetate, AcO(-), and dihydrogen phosphate, H(2)PO(4)(-), studied as their tetrabutylammonium salts) to diprotonated cyclo[8]pyrrole have been determined in CH(3)CN using stopped-flow analyses carried out at various temperatures. For dihydrogen phosphate, this afforded the activation enthalpies and entropies associated with both off and on processes. The different dynamic behavior seen for these test anions underscores the utility of kinetic analyses as a possible new tool for the advanced characterization of anion receptors.     

Computational study of ion distributions at the air/liquid methanol interface

Molecular dynamic simulations with polarizable potentials were performed to systematically investigate the distribution of NaCl, NaBr, NaI, and SrCl(2) at the air/liquid methanol interface. The density profiles indicated that there is no substantial enhancement of anions at the interface for the NaX systems, in contrast to what was observed at the air/aqueous interface. The surfactant-like shape of the larger more polarizable halide anions, which is part of the reason they are driven to air/aqueous interfaces, was compensated by the surfactant nature of methanol itself. These halide anions had on average an induced dipole of moderate magnitude in bulk methanol. As a consequence, methanol hydroxy groups donated hydrogen bonds to anions where the negatively charged side of the anion induced dipole pointed, and methyl groups interacted with anions where the positively charged side of the anion-induced dipole pointed. Furthermore, salts were found to disrupt the surface structure of methanol. For the neat air/liquid methanol interface, there is relative enhancement of methyl groups at the outer edge of the air/liquid methanol interface in comparison with hydroxy groups, but with the addition of NaX this enhancement was reduced somewhat. Finally, with the additional of salts to methanol, the computed surface potentials decreased, which is in contrast to what is observed in corresponding aqueous systems, where the surface potential increases with the addition of salts. Both of these trends have been indirectly observed with experiments. The surface potential trends were found to be due to the greater propensity of anions for the air/water interface that is not present at the air/liquid methanol interface.     

Multiple anion binding by a zinc-containing tetratopic cyclen-resorcinarene

The synthesis and binding properties of a new tetratopic anion receptor are reported. The resorcinarene ligand bearing four cyclen moieties is able to bind four Zn²⁺ ions and subsequently bind anions. NMR titrations show proton shifts during the binding of the first one or two anions. Isothermal titration calorimetry (ITC) titrations show that two or more anions bind to one tetramer. The tetratopic receptor in methanol has high affinity for dihydrogen phosphate, acetate, and halide ions and weak affinity for nitrate and perchlorate.