Catalytic oxy-functionalization of methane and other hydrocarbons: fundamental advancements and new strategies

TEMPO and a co-reductant mediated aerobic epoxidation of olefins using a new magnetically recoverable iron(III) bis(phenol)diamine complex: experimental and computational studies

A magnetically recoverable catalyst of an iron(III) bis(phenol) diamine complex immobilized onto amine functionalized silica-coated magnetic nanoparticles has been synthesized. The catalyst was characterized using FESEM, TEM and XRD which confirmed the nano structure of the catalyst. The physicochemical techniques of ICP, FT-IR, XPS, EDS and TGA proved the loading of the ligand and metal complex on silica-coated magnetic nanoparticles. Using the prepared heterogeneous catalyst, aerobic epoxidation reactions of different alkenes have been investigated in the presence of SO32- as a reducing agent. Moreover, using TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) to discover the mechanism of the aerobic epoxidation of olefins, a new TEMPO-assisted route has been explored. Both of the reaction pathways led to a moderate to high percentage yield of epoxides in water at room temperature. For further understanding mechanistic aspects, density functional theory (DFT) computational studies have been performed. The DFT calculations confirm the suggested mechanism for the title reaction and show the electron density in the vicinity of Fe(II) in the presence of TEMPO as a co-catalyst was more than that in the presence of SO32-.

Aerobic Partial Oxidation of Alkanes Using Photodriven Iron Catalysis

Photodriven oxidations of alkanes in trifluoroacetic acid using commercial and synthesized Fe(III) sources as catalyst precursors and dioxygen (O₂) as the terminal oxidant are reported. The reactions produce alkyl esters and occur at ambient temperature in the presence of air, and catalytic turnover is observed for the oxidation of methane in a pure O₂ atmosphere. Under optimized conditions, approximately 17% conversion of methane to methyl trifluoroacetate at more than 50% selectivity is observed. It is demonstrated that methyl trifluoroacetate is stable under catalytic conditions, and thus overoxidized products are not formed through secondary oxidation of methyl trifluoroacetate.

Insertion of Unsaturated C-C Bonds into the O-H Bond of an Iridium(III)-Hydroxo Complex: Formation of Phosphorescent Emitters with an Asymmetrical β-Diketonate Ligand

A synthetic methodology to prepare iridium(III) emitters of the class [3b+3b+3b'] with two ortho-metalated 1-phenylisoquinolines and an asymmetrical β-diketonate has been discovered. The abstraction of the chloride ligands of the dimer [Ir(μ-Cl){κ²-C,N-(C₆H₄-isoqui)}₂]₂ (1, C₆H₅-isoqui = 1-phenylisoquinoline) with AgBF₄ in acetone and the subsequent addition of water to the resulting solution affords the water solvate mononuclear complex [Ir{κ²-C,N-(C₆H₄-isoqui)}₂(H₂O)₂]BF₄ (2), which reacts with KOH to give the dihydroxo-bridged dimer [Ir(μ-OH){κ²-C,N-(C₆H₄-isoqui)}₂]₂ (3). Treatment of the latter with dimethyl acetylenedicarboxylate leads to Ir{κ²-C,N-(C₆H₄-isoqui)}₂{κ²-O,O-[OC(CO₂CH₃)CHC(OCH₃)O]} (4), as a result of the anti-addition of the O-H bond of a mononuclear [Ir(OH){κ²-C,N-(C₆H₄-isoqui)}₂] fragment to the C-C triple bond of the alkyne and the coordination of one of the carboxylate substituents to the metal center. Complex 3 also reacts with α,β-unsaturated ketones. The reaction with 3-(4-methylphenyl)-1-phenylprop-2-en-1-one affords Ir{κ²-C,N-(C₆H₄-isoqui)}₂{κ²-O,O-[OC(C₆H₅)CHC(p-C₆H₄Me)O]} (5), whereas methyl vinyl ketone gives a mixture of Ir{κ²-C,N-(C₆H₄-isoqui)}₂{κ²-O,O-[OC(CH₃)CHCHO]} (6) and Ir{κ²-C,N-(C₆H₄-isoqui)}₂{κ²-O,O-[OC(CH₃)CHC(CH═CH₂)O]} (7). Complexes 5 and 6 are the result of the addition of the O-H bond of the mononuclear [Ir(OH){κ²-C,N-(C₆H₄-isoqui)}₂] fragment to the C-C double bond of the α,β-unsaturated ketones and the coordination of the carbonyl group to the iridium center, to generate O,O-chelates which lose molecular hydrogen to aromatize into the asymmetrical β-diketonate ligands. Complexes 4-7 are phosphorescent emitters in the red spectral region (599-672 nm) in doped poly(methyl methacrylate) (PMMA) film at 5 wt % at room temperature and 2-methyltetrahydrofuran at room temperature and 77 K. They display short lifetimes (0.8-2.5 μs) and quantum yields in both doped PMMA films and in 2-methyltetrahydrofuran at room temperature depending on the substituents of the β-diketonate: about 0.6-0.5 for 4 and 6 and ca. 0.35 for 5 and 7.

Thermal Activation of CH4 and H2 as Mediated by the Ruthenium Oxide Cluster Ions [RuOx ]+ (x=1-3): On the Influence of Oxidation States

Thermal gas-phase reactions of the ruthenium-oxide clusters [RuO_x ]⁺ (x=1-3) with methane and dihydrogen have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. For methane activation, as compared to the previously studied [RuO]⁺ /CH₄ couple, the higher oxidized Ru systems give rise to completely different product distributions. [RuO₂ ]⁺ brings about the generations of [Ru,O,C,H₂ ]⁺ /H₂ O, [Ru,O,C]⁺ /H₂ /H₂ O, and [Ru,O,H₂ ]⁺ /CH₂ O, whereas [RuO₃ ]⁺ exhibits a higher selectivity and efficiency in producing formaldehyde and syngas (CO+H₂ ). Regarding the reactions with H₂ , as compared to CH₄ , both [RuO]⁺ and [RuO₂ ]⁺ react similarly inefficiently with oxygen-atom transfer being the main reaction channel; in contrast, [RuO₃ ]⁺ is inert toward dihydrogen. Theoretical analysis reveals that the reduction of the metal center drives the overall oxidation of methane, whereas the back-bonding orbital interactions between the cluster ions and dihydrogen control the H-H bond activation. Furthermore, the reactivity patterns of [RuO_x ]⁺ (x=1-3) with CH₄ and H₂ have been compared with the previously reported results of Group 8 analogues [OsO_x ]⁺ /CH₄ /H₂ (x=1-3) and the [FeO]⁺ /H₂ system. The electronic origins for their distinctly different reaction behaviors have been addressed.

Understanding trends in methane oxidation to formaldehyde: statistical analysis of literature data and based hereon experiments

A regression tree analysis on selective oxidation of methane to methanol/formaldehyde was applied to identify fundamentals affecting catalyst performance. The electronegativity correlates with methane activation energy and formaldehyde selectivity.

A review of plasma-assisted catalytic conversion of gaseous carbon dioxide and methane into value-added platform chemicals and fuels

CO2 and CH4 contribute to greenhouse gas emissions, while the production of industrial base chemicals from natural gas resources is emerging as well. Such conversion processes, however, are energy-intensive and introducing a renewable and sustainable electric activation seems optimal, at least for intermediate-scale modular operation. The review thus analyses such valorisation by plasma reactor technologies and heterogeneous catalysis application, largely into higher hydrocarbon molecules, that is ethane, ethylene, acetylene, propane, etc., and organic oxygenated compounds, i.e. methanol, formaldehyde, formic acid and dimethyl ether. Focus is given to reaction pathway mechanisms, related to the partial oxidation steps of CH4 with O2, H2O and CO2, CO2 reduction with H2, CH4 or other paraffin species, and to a lesser extent, to mixtures' dry reforming to syngas. Dielectric barrier discharge, corona, spark and gliding arc sources are considered, combined with (noble) metal materials. Carbon (C), silica (SiO2) and alumina (Al2O3) as well as various catalytic supports are examined as precious critical raw materials (e.g. platinum, palladium and rhodium) or transition metal (e.g. manganese, iron, cobalt, nickel and copper) substrates. These are applied for turnover, such as that pertinent to reformer, (reverse) water-gas shift (WGS or RWGS) and CH3OH synthesis. Time-on-stream catalyst deactivation or reactivation is also overviewed from the viewpoint of individual transient moieties and their adsorption or desorption characteristics, as well as reactivity.

Multiple C-H Bond Activations and Ring-Opening C-S Bond Cleavage of Thiophene by Dirhenium Carbonyl Complexes

The reaction of Re₂(CO)₈(μ-C₆H₅)(μ-H) (1) with thiophene in CH₂Cl₂ at 40 °C yielded the new compound Re₂(CO)₈(μ-η²-SC₄H₃)(μ-H) (2), which contains a bridging σ-π-coordinated thienyl ligand formed by the activation of the C-H bond at the 2 position of the thiophene. Compound 2 exhibits dynamical activity on the NMR time scale involving rearrangements of the bridging thienyl ligand. The reaction of compound 2 with a second 1 equiv of 1 at 45 °C yielded the doubly metalated product [Re₂(CO)₈(μ-H)]₂(μ-η²-2,3-μ-η²-4,5-C₄H₂S) (3), formed by the activation of the C-H bond at the 5 position of the thienyl ligand in 2. Heating 3 in a hexane solvent to reflux transformed it into the ring-opened compound Re(CO)₄[μ-η⁵-η²-SCC(H)C(H)C(H)][Re(CO)₃][Re₂(CO)₈(μ-H)] (4) by the loss of one CO ligand. Compound 4 contains a doubly metalated 1-thiapentadienyl ligand formed by the cleavage of one of the C-S bonds. When heated to reflux (125 °C) in an octane solvent in the presence of H₂O, the new compound Re(CO)₄[η⁵-μ-η²-SC(H)C(H)C(H)C(H)]Re(CO)₃ (5) was obtained by cleavage of the Re₂(CO)₈(μ-H) group from 4 with formation of the known coproduct [Re(CO)₃(μ₃-OH)]₄. All new products were characterized by single-crystal X-ray diffraction analyses.

Multiple aromatic C-H bond activations by a dirhenium carbonyl complex

Multiple aromatic CH activations by a dirhenium complex have provided meta-related dimetallated arene rings in the complexes Re2(CO)8(μ-H)(μ-η2-1,2-μ-η2-3,4-C14H8)Re2(CO)8(μ-H), 3 and Re2(CO)8(μ-H)(μ-η1-1,μ-η1-3-C6H4)Re2(CO)8(μ-H), 4. Both products were characterized structurally by single crystal X-ray diffraction and DFT molecular orbital analyses.

Thermal Methane Activation by the Metal-Free Cluster Cation [Si2 O4 ]

The thermal reaction of methane with the metal-free cluster cation [Si₂ O₄ ]^.+ has been examined by using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. In addition to generating a methyl radical via hydrogen-atom abstraction, [Si₂ O₄ ]^.+ can selectively oxidize methane to formaldehyde. The mechanisms of these rather efficient reactions have been elucidated by high-level quantum-chemical calculations.

Methane conversion into different hydrocarbons or oxygenates: current status and future perspectives in catalyst development and reactor operation

This Perspective highlights recent developments in methane conversion into different hydrocarbons and C₁-oxygenates. Our analysis identified possible directions for further research to bring the above approaches to a commercial level.

Thermal Methane Activation by [Si2 O5 ].+ and [Si2 O5 H2 ].+ : Reactivity Enhancement by Hydrogenation

The thermal reactions of methane with the oxygen-rich cluster cations [Si₂ O₅ ]^⋅+ and [Si₂ O₅ H₂ ]^⋅+ have been examined using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry in conjunction with state-of-the-art quantum chemical calculations. In contrast to the inertness of [Si₂ O₅ ]^.+ towards methane, the hydrogenated cluster [Si₂ O₅ H₂ ]^.+ brings about hydrogen-atom transfer (HAT) from methane with an efficiency of 28 % relative to the collision rate. The mechanisms of this process have been investigated in detail and the reasons for the striking reactivity difference of the two cluster ions have been revealed.

On the Activation of Methane and Carbon Dioxide by [HTaO](+) and [TaOH](+) in the Gas Phase: A Mechanistic Study

The thermal reactions of [Ta,O,H](+) with methane and carbon dioxide have been investigated experimentally and theoretically by using electrospray ionization mass spectrometry (ESI MS) and density functional theory calculations. Although the activation of methane proceeds by liberation of H2 , the activation of CO2 gives rise to the formation of [OTa(OH)](+) under the elimination of CO. Computational studies of the reactions of methane and carbon dioxide with the two isomers of [Ta,O,H](+) , namely, [HTaO](+) and [Ta(OH)](+) , have been performed to elucidate mechanistic aspects and to explain characteristic reaction patterns.

Alkane C-H functionalization and oxidation with molecular oxygen

The application of environmentally benign, cheap, and economically viable oxidation procedures is a key challenge of homogeneous, oxidative alkane functionalization. The typically harsh reaction conditions and the propensity of dioxygen for radical reactivity call for extraordinary robust catalysts. Mainly three strategies have been applied. These are (1) the combination of a catalyst responsible for C-H activation with a cocatalyst responsible for dioxygen activation, (2) transition-metal catalysts, which react with both hydrocarbons and molecular oxygen, and (3) the introduction of very robust main-group element catalysts for C-H functionalization chemistry. Herein, these three approaches will be assessed and exemplified by the reactivity of chelated palladium (N-heterocyclic carbene) catalysts in combination with a vanadium cocatalyst, the methane functionalization by cobalt catalysts, and the reaction of group XVII compounds with alkanes.

Partial oxidation of light alkanes by periodate and chloride salts

The efficient and selective partial oxidation of light alkanes using potassium periodate and potassium chloride is reported.

Arene C–H activation using Rh(i) catalysts supported by bidentate nitrogen chelates

New Rh(i) complexes as precursors to efficient catalytic arene C–H activation in trifluoroacetic acid.