Alkane Activation Research Articles

Effect of Mn and Fe on the Reactivity of Sulfated Zirconia toward H2 and n-Butane: A Diffuse Reflectance IR Spectroscopic Investigation

Sulfated zirconia (SZ) and sulfated zirconia promoted with 2 wt % manganese (MnSZ) or iron (FeSZ), all active in n-butane isomerization, were investigated using diffuse reflectance Fourier transform IR spectroscopy (DRIFTS). By adsorption of H(2) at 77 K or of n-butane at room temperature, it was found that the promoters neither enhance the Lewis nor the Brønsted acid strength. SZ and promoted SZ do not exhibit higher acid strength than zeolites. In a batch experiment using 70 hPa of H(2), SZ did not react at 473 K. Reaction of H(2) with MnSZ produced water (band at 5242 cm(-1)) and a decrease in the sulfate groups (multiple bands). Heating of SZ in 10 hPa n-butane to 573 K caused total reduction of sulfate to H(2)S (2583, 2570 cm(-1)) and partial and total oxidation of butane to olefinic species (3062 cm(-1)), CO(2), and water. MnSZ and FeSZ reacted with n-butane already at 373 K; products of skeletal isomerization (methyne CH vibration at 2910 cm(-1)) were detected and sulfate groups were consumed. Rather than increasing the acidity, the promoters enhance the oxidation potential of sulfate and facilitate alkane activation via oxidative dehydrogenation.

Titanium Complexes Bearing a Hemilabile Heteroscorpionate Ligand: Synthesis, Reactivity, and Olefin Polymerization Activity

A series of titanium complexes (bpzmp)TiR1R2R3 (bpzmp = (3,5-tBu2-2-phenoxo)bis(3,5-Me2-pyrazol-1-yl)methane; R1 = R2 = R3 = Cl (2); R1 = R2 = R3 = NMe2 (3); R1 = Cl; R2 = R3 = NMe2 (4); R1 = R2 = Cl; R3= NMe2 (5); R1 = R2 = R3 = Me (6)) has been synthesized and characterized by VT NMR spectroscopy and X-ray diffraction analysis (2, 3, 5). The complexes 2−6 adopt in the solid state an octahedral structure in which the bpzmp ligand is κ3-coordinated to the metal via the phenoxy group and the imino nitrogens of the two pyrazolyl rings. The investigation of the solution structure of 3 by means of VT 1H NMR spectroscopy revealed a fluxional behavior of the bpzmp ligand that produces an equilibrium between the octahedral and tetrahedral form of the titanium complex, the latter resulting from the η1-coordination of the ligand through exclusively the phenolate group. The thermodynamic and kinetic parameters of this process were evaluated by VT NMR spectroscopy. The selective replacement of chloride for dimethylamide in 4 and 5 shifts the equilibrium toward the octahedral complex, which is the favorite configuration at room temperature. Site exchange of the nonequivalent methyl groups in 6 was observed at room temperature in the slow-regime 1H NMR time scale: ΔH⧧ and ΔS⧧ values of 22.3 ± 1.1 kcal·mol-1 and −19.6 ± 3.7 cal mol-1 K-1 were respectively determined for the isomerization process occurring with a rate constant of 3 s-1 and ΔG⧧ of 28.1 ± 0.1 kcal·mol-1 at 293 K. Complexes 2 and 6 are active olefin polymerization catalysts after activation with MAO or [Ph3C][B(C6F5)4]. Linear polyethylene and atactic polypropylene were obtained in the polymerization experiments catalyzed by 6-[Ph3C][B(C6F5)4]. Highest polymerization activities were found with the 2-MAO catalyst, where leaching of the ligand due to MAO excess was suspected. Reaction of 6 with [Ph3C][B(C6F5)4] in the presence of THF readily produces the ionic complex [(bpzmp)TiMe2(THF)][B(C6F5)4], proposed as a model of the active species in this class of olefin polymerization catalysts.

Microkinetic modeling of propane aromatization over HZSM-5

Reaction kinetic studies of propane conversion to aromatics were conducted on an HZSM-5 zeolite at a pressure of 1 atm, temperatures in the range 793–823 K, and different space times (0–12 g cat h/mol). The rates of production of methane, ethane, ethene, propene, propane, butane, butene, benzene, toluene, and xylene are reported. A kinetic model has been postulated that considers surface species as neutral alkoxides, reactions of these alkoxide species by carbenium ion-like transition states, and alkane activation by carbonium ion-like transition states. The associated elementary steps, categorized within the reaction types adsorption, desorption, unimolecular protolytic cracking and dehydrogenation, β-scission, oligomerization, hydride transfer, alkylation, dealkylation, and cyclization, were parsed into reaction families based on an equal reactivity assumption. A total of 311 reaction steps were grouped into 37 reaction families, and the number of unknown parameters was reduced to 25 using adsorption parameters for n-alkanes and relative rates for β-scission and hydride transfer from the literature. It is proposed that this kinetic model describes the reaction behavior over an HZSM-5 catalyst in terms of relevant rate and equilibrium constants and activation energies.

The Road to Early‐Transition‐Metal Phosphinimide Olefin Polymerization Catalysts

AbstractFor Abstract see ChemInform Abstract in Full Text.

Constrained geometry complexes of titanium (IV) and zirconium (IV) involving cyclopentadienyl fused to thiophene ring

Constrained geometry complexes (CGCs) of titanium (IV) and zirconium (IV) containing isomeric cyclopentadienyls fused to thiophene fragment, i.e., 4,5-dimethylcyclopenta[ b]thienyl and 5,6-dimethylcyclopenta[ b]thienyl, have been prepared and unambiguously characterized. The molecular structure of the titanium complex [η 5-(5,6-dimethylcyclopenta[ b]thienyl)SiMe 2(N t Bu)-η 1]TiCl 2 was established by X-ray crystal structure analysis. Preliminary studies showed that the studied CGCs/MAO are active olefin polymerization catalysts.

Well-Defined Surface Tungstenocarbyne Complexes through the Reaction of [W(⋮CtBu)(CH2tBu)3] with Silica

The molecular complex [W(&3bond; CtBu)(CH(2)tBu)(3)], 1, reacts with SiO2-(700) to give as major species 2a, [(&3bond; SiO)W(CtBu)(CH(2)tBu)(2)], while a bisgrafted surface species 3a, [(&3bond; SiO)(2)W-(&3bond; CtBu)(CH(2)tBu)], is obtained on SiO2-(200). As in molecular organometallic chemistry, the alkylalkylidyne tautomeric form is favored. Despite these structural features, these surface organometallic complexes are very active olefin metathesis catalysts, as reported earlier, and it is very likely that the necessary metallocarbene intermediates are generated under the reaction conditions.

The Competition between Allene and Butadiene in the Carbon−Hydrogen Bond Activation Initiated by a Tungsten Allyl Complex: A DFT Study

Mechanisms for H-transfer from 1,1-dimethylallyl (DMA) to neopentyl (CH2t-Bu) in the complex Cp*W(NO)(CH2t-Bu)(η3-DMA) are examined by B3LYP-DFT calculations. Either the mid-H or the methyl H on the DMA can transfer to the W-bound neopentyl to generate neopentane (2,2-dimethylpropane) and Cp*W(NO)(η2-1,1-dimethylallene) or Cp*W(NO)(η2-2-methylbuta-1,3-diene) intermediates, respectively. The open coordination site created by the neopentane loss is the site of the subsequent C−H bond activation of alkanes and arenes. The similarities in the overall enthalpic barriers for these mechanisms, 23−24 kcal/mol, explain the observed deuterium distribution (65% on the mid-carbon and 35% on the methyls of the DMA) when benzene-d6 reacts as substrate.

Synthesis and Structure of N-Heterocyclic Carbene Complexes with Tethered Olefinic Groups: Application of the Ruthenium Catalyst in Olefin Metathesis

Silver(I) and rhodium(I) complexes bearing the bisallyl-substituted N-heterocyclic carbene ligand (4R,5S)-4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (allyl2SIMes) have been prepared in a straightforward synthesis. The reaction of (4R,5S)-4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro-3H-imidazol-1-ium tetrafluoroborate (1a) with Ag2O affords the ionic biscarbene complex [(allyl2SIMes)2Ag]+BF4- (2), while the reaction of (4R,5S)-4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro-3H-imidazol-1-ium chloride (1b) with Ag2O leads to the monocarbene complex (allyl2SIMes)AgCl (3). Sequential treatment of 1a with KOtBu and dimeric [RhCl(cod)]2 (cod = cyclooctadiene) yields the rhodium carbene complex (allyl2SIMes)RhCl(cod) (4). However, the reaction of 1a with the first-generation Grubbs catalyst (PCy3)2Cl2RuC(H)Ph (Cy = cyclohexyl) leads to ring-closing metathesis of the two allylic groups, yielding 1,3-bis(2,4,6-trimethylphenyl)-3a,4,7,7a-tetrahydro-3H-benzimidazol-1-ium tetrafluoroborate (5). Subsequent reaction of this new imidazolium salt with KOtBu and 1 equiv of (PCy3)Cl2RuC(H)(C6H4OiPr-2) forms [1,3-bis(2,4,6-trimethylphenyl)-3a,4,7,7a-tetrahydro-3H-benzimidazolin-2-ylidene]dichloro(2-isopropanolatobenzylidene)ruthenium(II) (8). All new complexes have been thoroughly characterized, including X-ray crystallographic analyses of 2, 3, and 8. The most intriguing feature of 8 is the presence of an innocent CC bond that is part of a highly active olefin metathesis catalyst, which offers many options for further functionalization of the ligand backbone. The catalytic activity of complex 8 has been evaluated for the ring-closing metathesis of N,N-diallyl-4-toluenesulfonamide.

NaIO 4-mediated C–H activation of alkylbenzenes and alkanes with LiBr

NaIO 4 oxidizes lithium bromide efficiently under acidic conditions to functionalize alkylbenzenes and alkanes and produce the corresponding bromo and acetoxy derivatives in excellent yields. The protocol also demonstrates the direct conversion of cyclohexane into trans-1,2-dibromocyclohexane in moderate yield.

In situ diffuse-reflectance infrared spectroscopic investigation of promoted sulfated zirconia catalysts during n-butane isomerization

Sulfated zirconia, unpromoted or promoted with either 2 wt% iron or manganese, was investigated by in situ diffuse reflectance IR spectroscopy during activation, n-butane isomerization (323–378 K, 1–5 kPa n-butane), and regeneration. During activation at 773 K in N 2 or O 2, more than 95% of the adsorbed water was removed, and comparison of the resulting pattern of sulfur–oxygen vibrations with DFT calculations (A. Hofmann and J. Sauer, J. Phys. Chem. B 108 (2004) 14652) suggests S 2 O 7 2 − as a major surface species. All catalysts underwent an induction period before maximum rates were measured, which at 1 kPa n-butane were 350 and 180 μmol g −1 h −1 for the Fe- or Mn-promoted catalyst (323 K) and 5 μmol g −1 h −1 for sulfated zirconia (358 K). During isomerization several bands increased in the range of 1700–1600 cm −1; the growth of a band at 1600 cm −1 was restricted to the period of increasing catalytic activity. The rate of isomerization was linearly correlated with the total area increase of the bands in this region, indicating that the absorbing species are either active surface intermediates or side products (possibly water) formed in an amount proportional to the active surface species. The rate per such species was much higher in the presence of Mn; that is, promoters facilitate reaction steps after the initial alkane activation. The promoted materials deactivated within hours under the applied conditions, but complete recovery of the spectral and catalytic properties of Mn-promoted sulfated zirconia was possible through regeneration in O 2 at 773 K. Attempts to regenerate in N 2 produced unsaturated surface species absorbing at 1532 and 1465 cm −1.

On the Nature of Carbon−Hydrogen Bond Activation at Rhodium and Related Reactions

Over the past 20 years, substantial progress has been made in the understanding of the activation of C-H and other strong bonds by reactive metal complexes in low oxidation states. This paper will present an overview of the use of pentamethylcyclopentadienyl and trispyrazolylborate rhodium complexes for the activation of arene and alkane C-H bonds. Insights into bond strengths, kinetic and thermodynamic selectivities, and the nature of the intermediates involved will be reviewed. The role of eta-2 arene complexes will be shown to be critical to the C-H activation reactions. Some information about the fleeting alkane sigma-complexes will also be presented. In addition, use of these complexes with thiophenes has shown the ability to cleave C-S bonds. Mechanistic information has been obtained indicating coordination through sulfur prior to cleavage. Relevant examples of nickel-based C-S cleavage will also be given.

The Road to Early-Transition-Metal Phosphinimide Olefin Polymerization Catalysts

This paper chronicles our work examining early-transition-metal systems incorporating phosphinimide ligands for use as olefin polymerization catalysts. Incorporation of these ancillary ligands affords unique deactivation pathways which are readily overcome by specific activation procedures and contrived ligand modification. In these ways, a new family of highly active olefin polymerization catalysts was successfully developed.

Direct Observation of a 14-Electron Ruthenacyclobutane Relevant to Olefin Metathesis

The 14-electron ruthenium phosphonium alkylidene complex [(IH2Mes)Cl2Ru=CH(PCy3)][B(C6F5)4], 1b, a highly active olefin metathesis catalyst, reacts with stoichiometric quantities of ethylene at -50 degrees C in CD2Cl2 to generate the ruthenacyclobutane complex [(IH2Mes)Cl2RuCH2CH2CH2], 2, and [CH2=CH(PCy3)][B(C6F5)4] in quantitative yield by NMR spectroscopy. 1H and 13C NMR spectroscopies on 2 and 2-13C3 are consistent with a symmetrical C2v structure, providing the first experimental information concerning this crucial intermediate in ruthenium-mediated olefin metathesis. At -50 degrees C, exchange with free ethylene takes place on the chemical time scale. Complex 2 decomposes in solution upon warming to room temperature, generating propene and unknown ruthenium product(s).

Metallocenes with Cp Ring-Fused to Chalcogen Heterocycles: Synthesis and Polymerization Results

Substituted metallocene ligands containing cyclopentadiene ring fused to either selenophene or benzo[b]tellurophene were prepared following methods previously developed for analogs containing sulfur. 5-Methyl-4,5-dihydro-cyclopenta[b]-selenophen-6-one ( 2 ) and 3-Methyl-3,4-dihydro-benzo[b]cyclopenta[d]telluraphen-2-one ( 7 ) (major isomers) were prepared by polyphosphoric acid catalyzed Friedel-Crafts acylation/Nazarov cyclization of methacrylic acid onto selenophene and respectively benzo[b]tellurophene. Following reduction of the ketone to alcohol, then dehydration, the chalcogen-containing cyclopentadiene olefins were prepared. The olefins were deprotonated with n-butyllithium followed by either bridging with dichlorodimethylsilane, deprotonation and metallation, or deprotonation and direct metallation. In this manner, isomeric mixtures of –rac/-meso dimethylsilanediylbis(η5-5-Methyl-cyclopenta[b]selenophen-yl)zirconium dichloride ( 5 ) Bis(η5−2-methyl-cyclopenta[b][1]benzotelluraphen-yl)zirconium dichloride ( 9 ) were prepared and characterized. Complexes formed active olefin polymerization catalyst when activated with methylalumoxane. Polymerization results with ethylene and propylene are included.

CH Activation of Alkanes and Arenes Catalyzed by an O‐Donor Bis(tropolonato)iridium(III) Complex

Stable and swift: The title complex is stable to air and water under thermal conditions. It efficiently activates the CH bonds of alkanes and arenes at rates 50 times higher than does the only previously reported O-donor IrIII complex (see picture).

CH Activation of Alkanes and Arenes Catalyzed by an O‐Donor Bis(tropolonato)iridium(III) Complex

Stabil und flink: Der an Luft und in Wasser auch bei höheren Temperaturen stabile Titelkomplex aktiviert effektiv die C-H-Bindungen von Alkanen und Arenen, und zwar mit Umsatzzahlen, die 50-mal höher sind als die mit dem einzigen früher beschriebenen O-Donor-IrIII-Komplex erreichten (siehe Bild).

Alkane activation by silica supported Group VB metal hydrides. A quantum-chemical study

A quantum-chemical study of alkane hydrogenolysis and metathesis on silica-supported Group VB metal hydrides (M = V, Nb, Ta) was carried out. Using a model fragment with the β-crystobalite structure permits a correct description of the properties of the grafted organometallic complexes. Alkane hydrogenolysis and metathesis reactions involve (i) π-complexes and carbene complexes of MV and (ii) alkyl derivatives of MIII as possible intermediates. The character of the potential energy surfaces of the key alkane transformations points to an important role of triplet states in the catalytic processes involving niobium and vanadium. An alternative low-energy reaction pathway characterized by a change in the spin state was found.

Mechanisms of Reactions Related to Selective Alkane Oxidation by Pt Complexes

AbstractFor Abstract see ChemInform Abstract in Full Text.

Activation of alkanes on zeolites: A DFT study of hydride abstraction and dehydrogenation on extra‐framework aluminum species

AbstractAn AlOH2+ · T6 cluster was used to represent, at zeroth order, an extra‐framework aluminum species coordinated with the zeolite Y structure. This cluster was used to study the hydride abstraction from alkanes, at B3LYP/6‐311++G(d,p)//B3LYP/6‐31G(d,p) level, to form an alkoxide and an Al‐hydride · T6 species. It was found that all reactions were endothermic with respect to the isolated reactants and presented high activation energy. The endothermicity and activation energy depend on the type of hydride being abstracted. Methane showed the highest values (ΔH = +41.5 kcal/mol and ΔH‡ = 91.7 kcal/mol); isobutane has the lowest (ΔH = +36.8 kcal/mol and ΔH‡ = 46.7 kcal/mol). Dehydrogenation to form an alkene and an adsorbed hydrogen molecule on the AlOH2+ · T6 cluster was also investigated and computed slightly more endothermic than the respective hydride abstraction. However, dehydrogenation showed a significantly lower activation enthalpy, especially for linear alkanes. For isobutane the results indicated a similar energy barrier for hydride abstraction and dehydrogenation. Larger, more realistic clusters and periodic reactivity studies are nevertheless needed to describe, in a more refined way, the energetics of these relevant catalytic reactions in zeolitic systems. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

On two alternative mechanisms of ethane activation over ZSM-5 zeolite modified by Zn2+ and Ga1+ cations

The activation of ethane over zinc- and gallium-modified HZSM-5 dehydrogenation catalysts was studied by diffuse reflectance infrared spectroscopy. Hydrocarbon activation on HZSM-5 modified by bivalent Zn and univalent Ga cations proceeds via two distinctly different mechanisms. The stronger molecular adsorption of ethane by the acid-base pairs formed by distantly separated cationic Zn2+ and basic oxygen sites results already at room temperature in strong polarizability of adsorbed ethane and subsequent heterolytic dissociative adsorption at moderate temperatures. In contrast, molecular adsorption of ethane on Ga+ cations is weak. At high temperatures dissociative hydrocarbon adsorption takes place, resulting in the formation of ethyl and hydride fragments coordinating to the cationic gallium species. Whereas in the zinc case a Brønsted acid proton is formed upon ethane dissociation, decomposition of the ethyl fragment on gallium results in gallium dihydride species and does not lead to Brønsted acid protons. This difference in alkane activation has direct consequences for hydrocarbon conversions involving dehydrogenation.