Thermal stability of organopalladium compounds: Non-radical methyl elimination from [PdXMe(PEt3)2]

The thermolysis of the palladium complexes [PdX(Me){P(C₂H₅)₃}₂] (X = Br, I, CN; Me = CH₃, CD₃) in decalin or toluene under argon, in the temperature range 120–160°C, produces methane, ethane and ethylene, in ratios which vary with the temperature. Deuterium labelling shows that the methane is mainly formed through intramolecular abstraction of hydrogen from the phosphine ligands by the coordinated methyl group and not through homolytic fission of the PdMe bond. The thermal stability and the decomposition mechanisms of the organopalladium complexes are compared with those of the platinum analogues, which are remarkably more stable. At the higher temperatures, the thermal decomposition involves cleavage of the PEt bonds in the phosphine ligands, and this leads to the formation of ethane and ethylene. The rate of generation of methane from the PdMe moieties is increased by a factor of 10 by the presence of an excess of dioxygen. Deuterium isotopic labelling shows that the rate increase is accompanied by a change from an intramolecular to a radical mechanism involving the abstraction of hydrogen by the methyl groups.     

Experimental study of fuel decomposition and hydrocarbon growth processes for practical fuel components in nonpremixed flames: MTBE and related alkyl ethers

Fuel decomposition and hydrocarbon growth processes of methyl tert‐butyl ether (MTBE) and related alkyl ethers have been studied experimentally in soot‐producing nonpremixed flames. Temperature, C1–C12 hydrocarbons, and major species were measured in coflowing methane/air flames whose fuel was separately doped with 5000 ppm of MTBE, n‐butyl methyl ether (NBME), sec‐butyl methyl ether (SBME), ethyl tert‐butyl ether (ETBE), and tert‐amyl methyl ether (TAME; =1,1‐dimethylpropyl methyl ether). The consumption rates of the dopants, several simple kinetic calculations, and the dependence of the observed products on fuel composition indicate that the dominant decomposition process was unimolecular dissociation, not H‐atom abstraction. The dominant dissociations were four‐center elimination of alcohols for the doubly branched ethers (MTBE, ETBE, and TAME) and C? O fission for the linear ether (NBME), while four‐center elimination and C? O fission were comparably important for the singly branched ether (SBME). These dissociations produced alkenes which further reacted to produce alkadienes/alkynes, alkenynes, acetylenic compounds, and aromatics. The dependence of the maximum benzene mole fractions on fuel composition was consistent with benzene formation through reactions of highly‐unsaturated C3 and/or C4 hydrocarbons (C₃H₃, n‐C₄H₃, C₄H₄, n‐C₄H₅, etc.). © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 345–358, 2004     

Tetrabenzo-24-crown-8 tris(1,2-dichloroethane) solvate

The title compound, C₃₂H₃₂O₈·3C₂H₄Cl₂, illustrates how tetrabenzo-24-crown-8 may be solvated by a common solvent-extraction diluent, 1,2-di­chloro­ethane (DCE). Two mol­ecules of DCE occupy the crown cavity, forming weak hydrogen bonds to the ether O atoms and the crown arene rings, while the crown adopts its most commonly observed binding conformation. The asymmetric unit is composed of two mol­ecules of tetrabenzo-24-crown-8 and six mol­ecules of DCE.     

Photochemical studies on η5-cyclopentadienyl(triphenylphosphine)nickelalkyl and -aryl compounds

Photo-induced degradation studies of a series of organonickel complexes of the type (η⁵-C₅H₅)(PPh₃)Ni(R) (R = CH₃, C₂H₅, C₆H₅ and C₆H₄CH₃-p) as well as certain deuterated analogs have been undertaken. Photolysis of the methyl compounds in benzene as well as benzene-d₆ gives methane as the major gaseous product, the photogenerated methyl group abstracting hydrogen from either the cyclopentadienyl ring, from the solvent, or from another methyl group. The photo-induced dealkylation of the ethyl compound gives both ethylene and ethane, and is explained by β-hydride elimination followed by subsequent reaction of the hydrido intermediate with additional ethyl compound. The photolysis of the phenyl and p-tolyl complexes in benzene solution leads to biaryl formation, both from the coupling of two coordinated aryl groups as well as interactions with the solvent. Triphenylphosphine is a product in all of these photo-decomposition studies.     

Addition–substitution competition in liquid-phase chlorinations

The rate constants of liquid-phase hydrogen abstraction by chlorine atoms from 1,2-C₂H₄Cl₂ relative to those of addition to C₂HCl₃ and to C₂Cl₄ and to that of hydrogen abstraction from cyclohexane have been measured between 250 and 345°K. Assuming a zero activation energy for the addition reactions permits one to calculate the corresponding values for the liquid-phase hydrogen abstraction from the chlorinated ethanes. These values are discussed and compared with the gas-phase data.     

Time-resolved mass spectrometric study of the reaction H + Trans-2-Butene

The title reaction was studied in a discharge flow system using mass flow and modulated molecular beam sampling with phase-sensitive detection in order to obtain time-resolved mass spectrometric analysis. At total conversion exceeding 30%, the major products are methane and ethane when initially hydrogen atoms are in excess; when butene is in excess, the major products are ethane and propylene. No hydrocarbons with more than 4 carbon atoms were detected in the products. The reaction is a complicated one since the simplest reaction scheme that successfully simulates the experimental results comprises 20 elementary reactions. The simulation, coupled with sensitivity analysis, shows that with hydrogen atoms in excess, significant amounts of propylene formed in the initial decomposition of the butyl radical react further with hydrogen atoms to form methane and ethane. When butene is in excess, approximately [C₃H₆] ≈ [CH₄] + ½[C₂H₆] which means that this propylene does not react further and almost all methyl radicals end up as CH₄ or C₂H₆. At small conversion, simulation shows that the major product by far is propylene regardless of the [H]/[butene] ratio. The absence of higher hydrocarbons in the products is at variance with earlier results of Rabinovitch and coworkers; however the present work leads to a comparable value for the average rate constant ??_a = ωD/S where D and S is the amount of products arising from the decomposition and stabilization, respectively, of the butyl radical and ω is the collision frequency.     

The UV photolysis (λ = 185 nm) of liquid t-butyl methyl ether

t-Butyl methyl ether has been UV photolysed (λ = 185 nm) to a maximal conversion of less than 0·1%. A study of the products (quantum yields) has been made: methanol (0·40₅), t-butanol (0·20), isobutene (0·17₈), t-butyl neopentyl ether (0·14₂), t-butyl ethyl ether (0·13₄), 1,2-di-t-butoxyethane (0·097), methane (0·056), isobutane (0·046), isopropenyl methyl ether (0·030), hydrogen (0·020), neopentane (0·016), ethane (0·015), formaldehyde (0·012), 2-methoxy-2-methyl-4-t-butoxybutane (0·005), hexamethylethane (0·0048), 2-methoxy-2-methylbutane (0·0027), 2-methoxy-2-methyl-3-t-butoxypropane (0·002), isopropyl methyl ether (0·0015), formaldehyde t-butyl methyl acetal (0·001), formaldehyde di-t-butyl acetal (0·001), 2-methoxy-2-methyl-4,4-dimethylpentane (0-001), 2-methoxy-2-methyl-3,3-dimethylbutane (0·0003), 2,5-dimethoxy-2,5-dimethylhexane (0·0002), di-t-butyl ether (5 · 10^?5), 2,2-dimethyloxirane (?, <- 0·001). There is no decomposition of the t-BuO radical into acetone (< 5 · 10^?4) and CH₃. Cyclisation reactions leading to α,α-dimethyloxetane (< 10^?4) and 1-methoxy-1-methylcyclopropane (< 10^?4) do not occur. The material balance yields C₅H_11·97O_1·018.The main modes of fragmentation (ca 82%) are represented by the homolytic CO bond split, either into t-butyl and methoxy (ca 52%) or into t-butoxy and methyl (ca 30%), Fragmentation into methanol and isobutene (8·5%) as well as into formaldehyde and isobutane (2%) are further modes of decomposition. The break of a CC linkage (4·5%) mainly occurs by elimination of molecular methane. A CH bond split has a probability of ca 3% with the methoxy CH bond the more likely one to break.     

The antiestrogen [2‐(4‐benzyl­phenoxy)­ethyl]­diethyl­ammonium chloride

The crystal structure of the title compound, C₁₉H₂₆NO⁺·Cl^? (common name: N,N‐diethyl‐2‐[(4‐phenyl­methyl)phenoxy]‐ethan­amine hydro­chloride), contains one mol­ecule in the asymmetric unit. The planes through the two phenyl rings are roughly perpendicular. Protonation occurs at the N atom, to which the Cl^? ion is linked via an N—H?Cl hydrogen bond. The mol­ecule adopts an eclipsed rather than extended conformation.     

Four related diethyl [(arylamino)(4‐ethynylphenyl)methyl]phosphonates

Crystal structures are reported for four related diethyl [(arylamino)(4‐ethynylphenyl)lmethyl]phosphonate derivatives, namely diethyl [(4‐bromoanilino)(4‐ethynylphenyl)methyl]phosphonate, C₁₉H₂₁BrNO₃P, (I), diethyl ((4‐chloro‐2‐methylanilino){4‐[2‐(trimethylsilyl)ethynyl]phenyl}methyl)phosphonate, C₂₃H₃₁ClNO₃PSi, (II), diethyl ((4‐fluoroanilino){4‐[2‐(trimethylsilyl)ethynyl]phenyl}methyl)phosphonate, C₂₂H₂₉FNO₃PSi, (III), and diethyl [(4‐ethynylphenyl)(naphthalen‐2‐ylamino)methyl]phosphonate, C₂₃H₂₄NO₃P, (IV). The conformation of the anilinobenzyl group is very similar in all four compounds. The P—C bond has an approximately staggered conformation, with the aniline and ethynylphenyl groups in gauche positions with respect to the P=O double bond. The two six‐membered rings are almost perpendicular. The sums of the valence angles about the N atoms vary from 344 (2) to 351 (2)°. In the crystal structures, molecules of (I), (III) and (IV) are arranged as centrosymmetric or pseudocentrosymmetric dimers connected by two N—H...O=P hydrogen bonds. The molecules of (II) are arranged as centrosymmetric dimers connected by C_methyl—H...O=P hydrogen bonds. The N—H bond of (II) is not involved in hydrogen bonding.     

Thermal reactions of ethane-d6–acetylene and D2–acetylene mixtures behind shock waves

A study of the thermal decomposition of an acetylene–ethane-d₆ mixture indicates that the rate constant for hydrogen abstraction from acetylene by methyl is more than 20 times less than for abstraction from ethane. Isotopic exchange is initiated by a rapid reaction between product D atoms and C₂H₂. A series of experiments involving the reactions of a D₂–acetylene mixture indicated that a molecular exchange process was also occurring, and it was shown that d[C₂HD]/dt = k[D₂]^0.7[C₂H₂]^0.3, effective activation energy = 15.8 kcal/mol. This mechanism made an insignificant contribution to isotope exchange in C₂H₂–C₂D₆ mixtures.     

Conversion of Methane over Ag<Superscript>+</Superscript>-exchanged Zeolite in the Presence of Ethene

Methane is shown to react with ethene over silver-exchanged zeolites at around 673 K to form higher hydrocarbons. Methane conversion of 13.2% is achieved at 673 K over Ag–ZSM−5 catalyst. Under these conditions, H–ZSM−5 does not catalyze the methane conversion, only ethene being converted into higher hydrocarbons. Zeolites with extra-framework metal cations such as In and Ga also activate methane in the presence of ethene. Using ¹³C-labeled methane as a reactant, propene is shown to be a primary product from methane and ethane. ¹³C atoms were not found in benzene molecules produced, indicating that benzene is entirely originated from ethane. On the other hand, in toluene, ¹³C atoms are found in both the methyl group and the aromatic ring. This implies that toluene is formed by the reaction of propene with butenes formed by the dimerization of ethene, and also by the reaction of benzene with methane. The latter path was confirmed by direct reaction of ¹³CH₄ with benzene. In this case, ¹³C atoms are found only in methyl groups of toluene produced. The heterolytic dissociation of methane over Ag⁺-exchanged zeolites is proposed as a reaction mechanism for the catalytic conversion of methane, leading to the formation of silver hydride and CH₃^δ+ species, which reacts with ethene and benzene to form propene and toluene, respectively. The conversion of methane over zeolites loaded with metal cations other than Ag⁺ is also described. The reaction of methane with benzene over indium-loaded ZSM−5 afforded toluene and xylenes in yields of up to 7.6% and 0.9% at 623 K when the reaction was carried out in a flow reactor.     

Synthesis and Properties of Brominated 6,6'-Dimethyl-[2,2'-bi-1H-indene]-3,3'-diethyl-3,3'-dihydroxy-1,1 '-diones

Photochromic 6‐bromomethyl‐6′‐methyl‐[2,2′‐bi‐1H‐indene]‐3,3′‐diethyl‐3,3′‐dihydroxy‐1,1′‐dione ( 2 ), 6,6′‐ bis(bromomethyl)‐[2,2′‐bi‐1H‐indene]‐3,3′‐diethyl‐3,3′‐dihydroxy‐1,1′‐dione ( 3 ) and 6,6′‐bis(dibromomethyl)‐[2,2′‐ bi‐1H‐indene]‐3,3′‐diethyl‐3,3′‐dihydroxy‐1,1′‐dione ( 4 ) have been synthesized from 6,6′‐dimethyl‐[2,2′‐bi‐1H‐ indene]‐3,3′‐diethyl‐3,3′‐dihydroxy‐1,1′‐dione ( 1 ). The single crystal of 4 was obtained and its crystal structure was analyzed. The results indicate that in crystal 4 , molecular arrangement is defective tightness compared with its precursor 1 . Besides, UV‐Vis absorption spectra in CH₂Cl₂ solution, photochromic and photomagnetic properties in solid state of 2 , 3 and 4 were also investigated. The results demonstrate that when the hydrogen atoms in the methyl group on the benzene rings of biindenylidenedione were substituted by bromines, its properties could be affected considerably.     

Symmetrical Bis‐Phosphorus Compounds and Macrocycles with two 1,3,2‐Benzodiazaphosphorinone Units – Oxidation and X‐Ray Structure Analysis of Selected Compounds

The hydrolysis of 1,2‐bis(5,6‐benzo‐1‐methyl‐2‐chloro‐1,3,2‐diazaphosphorin‐4‐on‐3‐yl)ethane ( 1 ) and its 1,3‐propane derivative ( 2 ) with excess water led, without decomposition, to the formation of the bis‐phosphoryl compounds 3 and 4 . Reaction of 1 and 2 with bis(trimethylsiloxy)ethane formed the symmetrical macrocycles 5 and 6 , which could readily be oxidized by (H₂N)₂C(:O) · H₂O₂ or elemental sulfur, leading to the formation of the phosphoryl compounds 7 and 10 , and the thiophosphoryl derivatives 9 and 11 , respectively. The influence of the ring size on the reaction rate of the oxidation was investigated. For the sulfurization of 6 , the stepwise addition of sulfur to phosphorus was proved by NMR spectroscopy. All compounds exist as single conformers in common organic solvents such as toluene, diethyl ether, dichloromethane or chloroform. For compounds 7 (dichloromethane solvate) and 9 , single crystal X‐ray structure analyses were conducted; both diastereomeric molecules were shown to display RR/SS configuration. In both structures one short non‐classical hydrogen bond was observed.     

Thermal Decomposition of bis(cyclopentadienyl)hafnium compounds and their deuterated analogues

Thermal decomposition ranges of Cp₂HfR_, (R = Me, Ph) have been found by the DTA method. The thermal stability of hafnium derivatives greatly exceeds the stability of analogous titanium and zirconium compounds. Decomposition of Cp₂HfR₂ occurs by abstraction of σ-bonded groups which convert into RH. Hydrogen donors for the RH formation are both π-cyclopentadienyl and σ-bonded groups. The initial π-Cp₂Hf structure rearranges to form the (η⁵-Cp)-(η⁵,η¹-C₅H₄)Hf fragment. These react with HCl to produce Cp₂HfCl₂. It has been established that hydrogen exchange between cyclopentadienyl rings and methyl groups occurs during the thermal decomposition of Cp₂HfMe₂. As a result of the exchange process on thermal decomposition of Cp₂HfMe₂-d6, deuterium insertion into the cyclopentadienyl ring has been shown. The participation of solvent during the decomposition process of the hafnium derivatives has been studied.     

Photocatalytic conversions of ethanol,initiated by processes of electron phototransfer,in alkoxide-chloride complexes of titanium

A study has been made of the composition of reaction mixtures obtained in the process of photocatalytic formation of molecular hydrogen in ethanol solutions of titanium tetrachloride. It has been established that this process is accompanied by reactions of oxidation and decomposition of the ethanol molecules. Methane, ethane, acetaldehyde, and ethyl chloride have been identified. Kinetic relationships have been found for the formation of these compounds, and the quantum yields have been determined. In the initial stage of irradiation, CH₄, C₂H₆,and CH₃CHO are obtained. In the second stage, when most of the titanium ions are already in the +3 state of oxidation, the formation of methane and acetaldehyde ends, and H₂ and C₂H₆ become the main products. Possible reasons for the change in composition of the photolysis products are examined, and the basic features of the mechanism of this photocatalytic process are discussed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 1, pp. 44–51, January–February, 1986.     

3,5‐Bis(4‐methoxy­benzyl­idene)‐1‐methyl‐4‐piperidone and 3,5‐bis(4‐methoxy­benzyl­idene)‐1‐methyl‐4‐oxopiperidinium chloride: potential biophotonic materials

In the title compound 3,5‐bis(4‐methoxy­benzyl­idene)‐1‐methyl‐4‐piperidone, C₂₂H₂₃NO₃, (I), the central heterocyclic ring adopts a flattened boat conformation, while in the related salt 3,5‐bis(4‐methoxy­benzyl­idene)‐1‐methyl‐4‐oxopiperidin­ium chloride, C₂₂H₂₄NO₃⁺·Cl⁻, (II), the ring exhibits a `sofa' conformation in which the N atom deviates from the planar fragment. The pendant benzene rings are twisted from the heterocyclic ring planes in both mol­ecules in the same direction, the range of dihedral angles between the ring planes being 24.5 (2)–32.7 (2)°. The dominant packing motif in (I) involves centrosymmetric dimers bound by weak intermolecular C—H⋯O hydrogen bonds. In (II), cations and anions are linked by strong N—H⋯Cl hydrogen bonds, while weak C—H⋯O and C—H⋯Cl hydrogen bonds link the cations and anions into a three‐dimensional framework.     

Comparative Study of the Decomposition of CCl4 in Cold and Thermal Plasma

Decomposition of carbon tetrachloride was studied in an inductively coupled thermal plasma reactor and in a low temperature, non-equilibrium plasma reactor, in neutral and oxidative conditions, respectively. In neutral conditions formation of solid soot, aliphatic- and cyclodienes was observed in equilibrium, and products, such as Cl₂ and C₂Cl₆ were detected in non-equilibrium plasma. Feeding of oxygen into the thermal plasma reactor depressed both soot and dienes formation and induced the formation of oxygen containing intermediates and products. GC-MS analyses of the gaseous products and the extract of the soot referred to as complex decomposition and recombination mechanism at given conditions. Presence of oxygen in the low temperature plasma reactor results in the formation of carbonyl compounds as intermediers. CO₂ and Cl₂ revealed as final products of CCl₄ decomposition in cold plasma.     

Microwve induced catalytic reactions of carbon dioxide and water: mimicry of photosynthesis

In an attempt to emulate the highly successful photosynthetic recycling of CO₂ to form energetically useful fuels in nature this study investigates the microwave induced reaction of carbon dioxide and water in a continuous flow system using a supported nickel catalyst and 2.45 GHz microwave radiation with an average incident power of 2.2 kW. The major reaction products were methane, ethane, methanol, acetone, C₃ and C₄ alcohols. The yields of methane, C₃ and C₄ alcohols reached maximum values after 30 s of irradiation, while the yields of ethane, methanol and acetone were proportional to the irradiation time within the investigated range.     

Pyrolysis of methyl chloride,a pathway in the chlorine-catalyzed polymerization of methane

The reaction of CH₄ + Cl₂ produces predominantly CH₃Cl + HCl, which above 1200 K goes to olefins, aromatics, and HCl. Results obtained in laboratory experiments and detailed modeling of the chlorine-catalyzed polymerization of methane at 1260 and 1310 K are presented. The reaction can be separated into two stages, the chlorination of methane and pyrolysis of methylchloride. The pyrolysis of CH₃Cl formed C₂H₄ and C₂H₂ in increasing yields as the degree of conversion decreased and the excess of methane increased. Changes of temperature, pressure, or additions of HCl had little effect. In the absence of CH₄ C₂H₄ and C₂H₂ are formed by the recombination of ?H₃ and ?H₂Cl radicals. With added CH₄ recombination of ?H₃ forms C₂H₆, which dehydrogenates to C₂H₄ + H₂. C₂H₄ in turn dehydrogenates to C₂H₂ + H₂. While HCl, C, CH₄, and H₂ are the ultimate stable products, C₂H₄, C₂H₂, and C₆H₆ are produced as intermediates and appear to approach stationary concentrations in the system. Their secondary reactions can be described by radical reactions, which can lead to soot formation. ?H₃ - initiated polymerization of ethylene is negligible relative to the ?₂H₃ formation through H abstraction by Cl. The fastest reaction of ?₂H₃ is its decomposition to C₂H₂. About 20% of the consumption of C₂H₂ can be accounted for by the addition of ?₂H₃ to it with formation of the butadienyl radical. The addition of the latter to C₂H₂ is slow relative to its decomposition to vinylacetylene. Successive H abstraction by Cl from C₄H₄ leading to diacetylene has rates compatible with the experimental values. About 10% of ?₄H₅ abstracts H from HCl and forms butadiene. Successive additions of ?₂H₃ to butadiene and the products of addition can account for the formation of benzene, styrene, naphthalene, and higher polyaromatics. The following rate parameters have been derived on the basis of the experimentally measured reaction rates, the estimated frequency factors, and the currently available heat of formation of the ?₂H₃ radical (69 kcal/mol):      

Methane conversion by various metal, metal oxide and metal carbide catalysts

The progress in the field of methane conversion into higher hydrocarbons including aromatics and oxygenated compounds in the recent five years will be reviewed shortly, together with a new type of the methane conversion reaction with carbon monoxide at lower temperatures (600–700 K) by supported group VIII metal catalysts. Benzene was formed selectively among hydrocarbons in the CH₄–CO reaction over silica-supported Rh, Ru, Pd and Os catalysts under atmospheric pressure. Both CH₄ and CO were required for benzene formation, and only ethane and ethylene were formed besides benzene. The amount of C₃–C₅ hydrocarbons was negligible, which suggests that a completely different mechanism from the CO–H₂ reaction may be operating over these catalysts despite of the similarity in the reaction conditions with the CO–H₂ reaction. The mechanism of benzene formation was studied deeply by means of kinetical investigation as well as infrared spectroscopy and isotopic tracer method in connection with that of CO hydrogenation.