Oxo Catalysts Research Articles

The spin-forbidden transition in iron(IV)-oxo catalysts relevant to two-state reactivity

Quintet oxoiron(IV) intermediates are often invoked in nonheme iron enzymes capable of performing selective oxidation, while most well-characterized synthetic model oxoiron(IV) complexes have a triplet ground state. These differing spin states lead to the proposal of a two-state reactivity model, where the complexes cross from the triplet to an excited quintet state. However, the energy of this quintet state has never been measured experimentally. Here, magnetic circular dichroism is used to assign the singlet and triplet excited states in a series of triplet oxoiron(IV) complexes. These transition energies are used to determine the energies of the quintet state via constrained fitting of 2p3d resonant inelastic x-ray scattering. This allowed for a direct correlation between the quintet energies and substrate C─H oxidation rates.

Computational and Experimental Characterization of the Ligand Environment of a Ni-Oxo Catalyst Supported in the Metal–Organic Framework NU-1000

Heterogeneous catalysts exhibit significant changes in composition due to the influence of operating conditions, and these compositional changes can have dramatic effects on catalytic performance. For traditional bulk metal heterogeneous catalysts, relationships between composition and catalytic operating conditions are well documented. However, the influence of operating conditions on the compositions of single-site heterogeneous catalysts remains largely unresolved. To address this, we report a combined computational and experimental characterization of a Ni oxo catalyst under catalytic hydrogenation conditions. Specifically, pair distribution function (PDF) analysis is combined with ab initio thermodynamic modeling to investigate ligand environments present on a Ni oxo cluster supported in the metal-organic framework NU-1000. Comparisons of the experimentally observed and simulated Ni-O coordination numbers and Ni-O, Ni···Ni, and Ni···Zr distances provide insight into the Ni ligand environment under H2 (g). These comparisons suggest significant OH and H2O content and, further, that different Ni ions within the cluster and/or NU-1000 structure may comprise subtly different numbers of these ligands. Further, the observation of significant H2O content under H2 (g) suggests that the NU-1000 support supplies H2O to the cluster. Examples of ligand environments that could lead to the observed PDFs are provided. The combination of simulations and experiments provides new insights into the ligand environment for Ni-NU-1000 catalysts that will be useful for understanding the ligand environments of other single-site Ni catalysts as well.

Olefin-Surface Interactions: A Key Activity Parameter in Silica-Supported Olefin Metathesis Catalysts.

Molecularly defined and classical heterogeneous Mo-based metathesis catalysts are shown to display distinct and unexpected reactivity patterns for the metathesis of long-chain α-olefins at low temperatures (<100 °C). Catalysts based on supported Mo oxo species, whether prepared via wet impregnation or surface organometallic chemistry (SOMC), exhibit strong activity dependencies on the α-olefin chain length, with slower reaction rates for longer substrate chain lengths. In contrast, molecular and supported Mo alkylidenes are highly active and do not display such dramatic dependence on the chain length. State-of-the-art two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy analyses of postmetathesis catalysts, complemented by Fourier transform infrared (FT-IR) spectroscopy and molecular dynamics calculations, evidence that the activity decrease observed for supported Mo oxo catalysts relates to the strong adsorption of internal olefin metathesis products because of interactions with surface Si–OH groups. Overall, this study shows that in addition to the nature and the number of active sites, the metathesis rates and the overall catalytic performance depend on product desorption, even in the liquid phase with nonpolar substrates. This study further highlights the role of the support and active site composition and dynamics on activity as well as the need for considering adsorption in catalyst design.

Ethenolysis of Renewable Methyl Oleate Catalyzed by Readily Accessible Supported Group VI Oxo Catalysts

The conversion of methyl oleate by cross-metathesis with ethylene using readily accessible supported group VI oxo species was addressed. TON up to 500 and 5000 were achieved using bisgrafted tungsten- and molybdenum based systems, respectively (≡SiO)2M(═O)R2 (M = W, R = CH2SiMe3, M = Mo, R = CH2CMe3). In the case of molybdenum, these results are comparable to those obtained by Mo-based systems in homogeneous catalysis. Optimization studies showed that ethylene pressure plays a major role in these systems, with inhibition at high pressure resulting from degenerated metathesis involving ethylene and α-olefins.

Vanadium-Catalyzed Coupling of Allenols with Electrophilic Halide Sources for the Formation of α-Halo-α′,β′-unsaturated Ketones

A vanadium-catalyzed coupling of allenylic alcohols with electrophilic halide sources to form α-halo-α',β'-unsaturated ketones is described. The process proceeds through a metal enolate formed from the 1,3-transposition of an allenol that is initiated by a cheap and earth-abundant vanadium oxo catalyst. Fluorine, chlorine, and bromine electrophiles can be utilized, and the resulting products can give rise to the introduction of nitrogen, oxygen, sulfur, and iodine nucleophiles α to the ketone through substitution chemistry.

Silica-Supported Tungsten Neosilyl Oxo Precatalysts: Impact of the Podality on Activity and Stability in Olefin Metathesis

In order to establish structure–reactivity relationships in propylene metathesis as a function of the podality of tungsten oxo species bearing neosilyl ligands, we targeted the parent tris alkyl [(≡SiO)WOR3] and bis alkyl oxo [(≡SiO)2WOR2] derivatives prone to carbene formation. Thus, [WO(CH2SiMe3)3Cl] (1) was grafted onto silica dehydroxylated at 700 °C (SiO2–700), proceeding via W–Cl cleavage to yield well-defined monopodal species [(≡SiO)WO(CH2SiMe3)3] (2a) along with HCl release. On the other hand, the corresponding bipodal species [(≡SiO)2WO(CH2SiMe3)2] (2b) was obtained on SiO2–200 by release of both HCl and TMS. The formation of these species were demonstrated by mass balance analysis, elemental analysis, IR, advanced solid-state NMR (1D and 2D 1H, 13C, 29Si, and 17O), and EXAFS. Furthermore, DFT calculations allowed understanding and rationalizing the experimental results regarding grafting selectivity. Materials 2a and 2b proved to lead to stable and efficient supported tungsten oxo catalysts for...

Accessing Realistic Models for the WO3–SiO2 Industrial Catalyst through the Design of Organometallic Precursors

In order to access realistic models to the industrial olefin metathesis catalyst WO3/SiO2, which is the bigrafted tungsten oxo alkylidene species [(≡SiO)2WO(═CHR)], we targeted the parent bis-alkyl oxo derivative [(≡SiO)2WOR2] prone to carbene formation. Thus, grafting of [WO(CH2EMe3)3Cl] (E = C, 1-Np; E = Si, 1-Ns) onto silica dehydroxylated at 200 °C was performed. While 1-Np affords the monopodal species [(≡SiO)WO(CH2CMe3)3] (2-Np), the neosilyl derivative 1-Ns reacts to yield the well-defined bipodal species [(≡SiO)2WO(CH2SiMe3)2] (2-Ns), via consecutive HCl and SiMe4 release. This was demonstrated by mass balance analysis, elemental analysis, IR, advanced solid-state NMR (1D and 2D 1H, 13C, 29Si, and 17O), and EXAFS. Furthermore, DFT calculations allowed understanding and rationalizing the experimental results regarding grafting selectivity. The material 2-Ns proved to lead to the most stable and efficient supported tungsten oxo catalyst for propene metathesis under dynamic conditions at 80 °C.

Strongly σ Donating Thiophenoxide in Silica-Supported Tungsten Oxo Catalysts for Improved 1-Alkene Metathesis Efficiency

We report the synthesis of tungsten oxo alkylidene complexes bearing bulky thiophenoxide ligands [W(═O)(═CHtBu)(SHMT)2(PMe2Ph)] and [W(═O)(═CHtBu)(SHMT)2] (SHMT = 2,6-dimesitylthiophenoxide) and their grafting on partially dehydroxylated silica, affording the supported complexes [(≡SiO)W(═O)(═CHtBu)(SHMT)(PMe2Ph)] and [(≡SiO)W(═O)(═CHtBu)(SHMT)]. While the molecular precursors are not significantly active in the metathesis of alkenes, the grafted analogue without bound phosphine ligands displays activity comparable to that of its aryloxide analogue [(≡SiO)W(═O)(═CHtBu)(OHMT)] (OHMT = 2,6-dimesitylphenoxide). It is worth noting that [(≡SiO)W(═O)(═CHtBu)(SHMT)] showed unprecedented activity in the metathesis of 1-alkenes, probably because of the lower stability of metallacyclobutane intermediates.

Oxo vs Imido Alkylidene d0-Metal Species: How and Why Do They Differ in Structure, Activity, and Efficiency in Alkene Metathesis?

Density functional calculations have been carried out to analyze the origin of the differences in reactivity, selectivity, and stability toward deactivation in metathesis of d0 oxo alkylidene complexes vs their isoelectronic imido counterparts. DFT calculations show that the elementary steps and geometries of the extrema are similar for the oxo and imido complexes, but that the energy profiles are different, the greatest difference occurring for the deactivation pathway. For the alkene metathesis pathway, replacing the imido by an oxo ligand slightly lowers the energy barrier for alkene coordination but raises that for the [2+2]-cycloaddition and cycloreversion; it also destabilizes the trigonal bipyramidal (TBP) metallacyclobutane intermediate with respect to the separated reactants. The isomeric square-based pyramid (SP) metallacyclobutane is in general more stable, and its stability relative to the separated reactants is similar for oxo and imido systems. Consequently, the oxo complex is associated with a slightly larger energy difference between the lowest energy intermediate (SP or separated reactants) and the highest energy transition state (cycloreversion) than the imido complex, which accounts for a slightly lower activity. Changing the imido into an oxo ligand disfavors strongly the deactivation pathway by raising considerably the energy barrier of the β-H transfer at the SP metallacycle that begins the entry into the channel for deactivation and byproduct formation as well as that of the subsequent ethene insertion. This makes the oxo catalysts more selective and stable toward deactivation than the corresponding imido catalysts, when dimerization can be avoided.

Electrochemical generation of hydrogen from acetic acid using a molecular molybdenum–oxo catalyst

We recently reported the catalytic generation of hydrogen from water mediated through the in situ reduction of the molybdenum(IV)–oxo complex [(PY5Me2)MoO]2+ (1; PY5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine) at a mercury electrode. To gain further insight into this unique molecular motif for hydrogen production, we have now examined the competence of this complex for the catalytic reduction of protons on an alternative electrode material. Herein, we demonstrate the ability of the molybdenum–oxo complex 1 to reduce protons at a glassy carbon electrode in acidic organic media, where the active catalyst is shown to be diffusing freely in solution. Cyclic and rotating disk voltammetry experiments reveal that three reductive electrochemical processes precede the catalytic generation of hydrogen, which occurs at potentials more negative than −1.25 V vs. SHE. Gas chromatographic analysis of the bulk electrolysis cell headspace confirms that hydrogen is generated at a Faradaic efficiency of 99%. Under pseudo-first order conditions with an acid-to-catalyst ratio of >290, a rate constant of 385 s−1 is calculated for the reduction of acetic acid in acetonitrile. Taken together, these data show that metal–oxo complex 1 is a competent molecular motif for catalytic generation of hydrogen from protons under soluble and diffusion-limited conditions.

A radical non-metal solution

Converting methane into more useful and readily transportable compounds has previously required the use of metal-based oxo catalysts, but now sulfur and phosphorus are showing their mettle.

A molecular orbital investigation of ru-oxo-catalyzed alcohol oxidations

The initial interaction between substrate and catalyst in the oxidation of alcohols by an octahedral Ru(IV)–oxo complex, cis-[Ru(HNCH—HCNH)2(NH3)(O)]2+, was researched using the INDO/1 method. Activation of the C—H bond, of the alcohol, by the oxo oxygen of the Ru(IV)–oxo complex was explored by the construction of two potential energy surfaces. First, the energetics for a triangular C—H—Oxo interaction geometry was subject to calculation and found to be repulsive at all distances. Second, the linear C—H—Oxo interaction geometry was probed and found to be more favorable than the triangular geometry. A loosely bound intermediate, corresponding to an ion(RuN5O2+)–molecule(ROH) complex and resembling a hydrogen-bonded species, was isolated on the linear potential energy surface. Several degrees of freedom for the linear ion-molecule minimum were studied in an attempt to reduce the oxo—H distance to a covalent value, all of which led to large increases in the total energy. Inner-sphere coordination shell expansion of the Ru–oxo catalyst was also considered. Three seven-coordinate structures—the pentagonal bipyramid, the capped octahedron, and the capped trigonal prism—were studied. The energetic and electronic similarity of seven-coordinate complexes, and lower calculated energy of activation (9.6 vs. 15.6 kcal mol–1), led us to only investigate the bonding of methanol to the pentagonal bipyramidal precursor in depth. The bonding of the pentagonal bipyramidal precursor with the oxygen atom of methanol leads to a decrease in the total energy as the metal–oxygen distance decreases, until a covalently bound species is formed. The subsequent conversion of the coordinated methanol to carbonyl products is considered in qualitative terms.

Combined Experimental and Theoretical Study on Aromatic Hydroxylation by Mononuclear Nonheme Iron(IV)−Oxo Complexes

The hydroxylation of aromatic compounds by mononuclear nonheme iron(IV)-oxo complexes, [FeIV(Bn-tpen)(O)]2+ (Bn-tpen=N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine) and [FeIV(N4Py)(O)]2+ (N4Py=N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), has been investigated by a combined experimental and theoretical approach. In the experimental work, we have performed kinetic studies of the oxidation of anthracene with nonheme iron(IV)-oxo complexes generated in situ, thereby determining kinetic and thermodynamic parameters, a Hammett rho value, and a kinetic isotope effect (KIE) value. A large negative Hammett rho value of -3.9 and an inverse KIE value of 0.9 indicate that the iron-oxo group attacks the aromatic ring via an electrophilic pathway. By carrying out isotope labeling experiments, the oxygen in oxygenated products was found to derive from the nonheme iron(IV)-oxo species. In the theoretical work, we have conducted density functional theory (DFT) calculations on the hydroxylation of benzene by [FeIV(N4Py)(O)]2+. The calculations show that the reaction proceeds via two-state reactivity patterns on competing triplet and quintet spin states via an initial rate determining electrophilic substitution step. In analogy to heme iron(IV)-oxo catalysts, the ligand is noninnocent and actively participates in the reaction mechanism by reshuttling a proton from the ipso position to the oxo group. Calculated kinetic isotope effects of C6H6 versus C6D6 confirm an inverse isotope effect for the electrophilic substitution pathway. Based on the experimental and theoretical results, we have concluded that the aromatic ring oxidation by mononuclear nonheme iron(IV)-oxo complexes does not occur via a hydrogen atom abstraction mechanism but involves an initial electrophilic attack on the pi-system of the aromatic ring to produce a tetrahedral radical or cationic sigma-complex.

Selective 1,3-Isomerization of Allylic Alcohols via Rhenium Oxo Catalysis

A protocol featuring mild conditions, short reaction times and low catalyst loading has been developed for the 1,3-isomerization of allylic alcohols using a rhenium oxo catalyst. Enantioenriched starting materials undergo iso­merization with only a small loss of optical purity.

Comparison of silyl chromate and chromium oxide based olefin polymerization catalysts

AbstractA comparison of the ethylene polymerization performance conducted with an oxo and a triphenylsilyl chromate catalyst on silica was performed. The oxo catalyst has higher activity and better comonomer response. The silylchromate catalyst has a much longer induction time and made a much broader molecular weight distribution polymer compared to the oxo analogue. Performance similar to silylchromate on silica was observed when triphenylsilanol (TPS) was added to the oxo chromium catalyst. The oxo catalyst was converted to the silyl chromate catalyst by ligand substitution. Analysis of the catalyst components when TPS was added to the oxo chrome analogue showed that bis triphenylsilyl chromate can form and be removed from the support.

Selective Oxidation in Supercritical Carbon Dioxide Using Clean Oxidants

We have systematically investigated heterogeneous catalytic oxidation of different substrates in supercritical carbon dioxide (SC-CO2). Three types of catalysts, a metal complex {cis-[Fe(dmp)2(H2O)2] (CF3SO3)2} (dmp = 2,9-dimethyl-1,10-phenanthroline), 0.5% platinum γ-alumina, and 0.5% palladium γ-alumina, were used at a pressure of 200 bar, at temperatures from 60 to 150 °C, and for 3−18 h for partial oxidation of organic substrates. The metal−oxo catalyst used gave as high as 3.9% conversion of cycloxene to its oxides and epoxides, with the major products being the ketone and alcohol. The oxidation of cyclohexene strongly depended on the concentration of oxygen, whereas length of reaction (3−18 h) and temperature (60−150 °C) were shown to be less significant on the conversion and selectivity of the reaction. Oxidation of cyclohexene over Pd and Pt/Al2O3 catalysts resulted in a mixture of dehydrogenation and oxygenated products.

Toroid NMR probes for the in situ examination of homogeneous cobalt hydroformylation catalysts at high pressures and temperatures

Toroid NMR probes can be routinely operated at 300 atm and up to 250°C. Such operating conditions make these probes well suited for the in situ examination of catalytic industrial processes. These probes have been employed to examine phosphine substituted (Shell-type) and unsubstituted cobalt carbonyl oxo catalysts under catalytic and non-catalytic conditions in a variety of solvents. The solvents include cyclohexane, benzene, dioxane, 2-methylpropanol, supercritical carbon dioxide (scCO 2), and scCO 2/toluene mixtures. The high-pressure/high-temperature probes allowed the observation of precatalytic equilibria under high carbon monoxide pressures in the presence and absence of hydrogen and/or free phosphine.

Reconsidered mechanism in Ru III(hedta)-catalyzed epoxidation of stilbenes

The cis-stilbene oxide/ trans-stilbene oxide isomer assignment given formerly in Inorg. Chim. Acta, 193 (1992) 217 should be reversed. This assignment was based on the 1H NMR spectra of the stilbene oxides given in The Varian High Resolution NMR Spectral Catalog, Vol. 2, which is in error. The large amount of trans-oxide versus cis-oxide (5.5:1) from the cis-stilbene epoxidation by [Ru III(hedta)]/t-BuOOH, and 100% trans-oxide from trans- stilbene, is reinterpreted as indicative of a radicaloid intermediate in ⩾85% of the reaction channels. Comparisons are made with 15 ruthenium oxo catalysts which epoxidize stilbenes. Two categories are observed which are (A) stereoretentive Ru IVO catalysts or sterically hindered Ru VIO porphyrin oxidants, and (B) those which give isomer mixtures for Z-olefins and which are less hindered Ru VO and Ru IVO catalysts. The mechanistics aspects of these epoxidations are discussed.

The use of solid polymeric ligands in a new oxo catalyst recovery and recycle system

Solid, poly-2-vinylpyridine polymers are used to complex cobalt carbonyls in a unique and reversible way, which provides the basis for a new oxo catalyst recovery and recycle system. In a hydrogen-carbon monoxide atmosphere, both the HCo(CO) 4 and Co 2(CO) 8 cobalt species compete for sites within the polymer matrix. However, Co 2(CO) 8 is absorbed slowly and to a limited extent while HCo(CO) 4 is absorbed rapidly and to a much greater extent. In a carbon monoxide atmosphere (without hydrogen), the absorbed HCo(CO) 4 is rapidly released and converted to Co 2(CO) 8 which can be carried back into the oxo reaction zone by an olefin feed. Thus, cobalt carbonyls are either absorbed by or released from the solid polymer phase depending only on the presence of absence of hydrogen. This recycle system is one which carries the active catalyst through the cycle without consuming chemical reagents or producing unwanted by-products.