13657-09-5

When irradiated in a molecular beam at 193 nm, both cyclooctatetraene and styrene dissociate into benzene and acetylene.The average kinetic energy release is 12percent of the available energy in both cases.So that the mechanism of the dissociation of styrene could be determined the ratio of mass 26 to mass 27 (C2H2/C2HD) was measured for C6H5CDCH2 and C6H5CHCH2 and found to be 1.46 +/- 0.10 and 2.29 +/- 0.10, respectively.These numbers rule out cyclooctatetraene as an intermediate in the dissociation of styrene.A bicycloocta-2,4,7-triene intermediate which can tautomerize by 1,3 hydrogen atom jumps in the smaller ring explains all the experimental results.

Pincer complexes of the type (RPCP)IrH2, where (RPCP)Ir is [η3-2,6-(R2PCH2)2 C6H3] Ir, are the most effective catalysts reported to date for the "acceptorle

Molecules capable of both harvesting light and forming new chemical bonds hold promise for applications in the generation of solar fuels, but such first-row transition metal photoelectrocatalysts are lacking. Here we report nickel photoelectrocatalysts for H2 evolution, leveraging visible-light-driven photochemical H2 evolution from bis(diphosphine)nickel hydride complexes. A suite of experimental and theoretical analyses, including time-resolved spectroscopy and continuous irradiation quantum yield measurements, led to a proposed mechanism of H2 evolution involving a short-lived singlet excited state that undergoes homolysis of the Ni–H bond. Thermodynamic analyses provide a basis for understanding and predicting the observed photoelectrocatalytic H2 evolution by a 3d transition metal based catalyst. Of particular note is the dramatic change in the electrochemical overpotential: in the dark, the nickel complexes require strong acids and therefore high overpotentials for electrocatalysis; but under illumination, the use of weaker acids at the same applied potential results in a more than 500 mV improvement in electrochemical overpotential. New insight into first-row transition metal hydride photochemistry thus enables photoelectrocatalytic H2 evolution without electrochemical overpotential (at the thermodynamic potential or 0 mV overpotential). This catalyst system does not require sacrificial chemical reductants or light-harvesting semiconductor materials and produces H2 at rates similar to molecular catalysts attached to silicon.

We report a new system for the silylation of aryl C-H bonds. The combination of [Ir(cod)(OMe)]2 and 2,9-Me2-phenanthroline (2,9-Me2-phen) catalyzes the silylation of arenes at lower temperatures and with faster rates than

A tridentate, NNC-tb (where NNC-tb = 2-(pyridin-2-yl)benzo[h]quinoline) ligated IrIII complex (NNC-tb)Ir(Ph)(4-MePy)(TFA), 11 along with analogues are very active for CH activation as evidenced by rapid catalytic H/D exchange between benzene and trifluoroacetic acid-d1 (DTFA). The complexes were examined with a variety of oxidants for the catalytic conversion of benzene to phenyltrifluoroacetate. Herein, the synthesis and characterization of (NNC-tb)Ir complexes is described along with the reactivity of these complexes towards arenes and alkanes.

In the present study, the nickel-catalyzed dehalogenation of aryl and alkyl halides with iso-propyl zinc bromide or tert-butylmagnesium chloride has been examined in detail. With a straightforward nickel complex as pre-catalyst good to excellent yields and chemoselectivities were feasible for a variety of aryl and alkyl halides.

The synthesis of discrete, air, protic, and thermally stable transition metal NNC complexes that catalyze the CH activation and functionalization of alkanes and arenes is disclosed. Methods for the selective conversion of methane to methanol or methyl esters in acidic and neutral media are disclosed.

Alkyl phenylglyoxylates are shown to be efficient photoinitiators for acrylate polymerization. Benzoyl and phenyl radicals derived from intermolecular hydrogen abstraction are the initiating species. A negative photoimage system was developed based on thi

Hydrogenolyses of hexadeuterobenzene C6D6 and pentadeuterochlorobenzene C6D5Cl have been studied in flow reactors packed with activated carbon (AC) between 300° and 600°C, with void residence times of 4-5 s. At 579°C overall dedeuteration - proceeding in a near statistical manner - was close to 40% in both cases, whereas C6D5Cl is almost completely dechlorinated to deuterated benzenes at 505°C. On a per-site basis, replacement of Cl by H is ca. 20 times faster than that of D. These results clearly demonstrate a catalytic activity of AC in both cases of desubstitution. In the absence of AC, temperatures of 700-900°C are necessary to obtain the same result. Then, replacement of Cl and D have almost equal rates, both reactions occurring via addition of free H atoms. In the AC-mediated reactions, the yields of (D) benzenes are quantitative from C6D6, but around 50% with C6D5Cl. Apparently the latter compound reacts with bonding of its phenyl group to the AC surface, whereas D → H substitutions involve H transfer to a physisorbed (D) benzene.

Benzene and (1,4)-dimethyl-cyclohexane monolayers were physisorbed on graphite covered Pt(111) surfaces. Exposure of benzene monolayers at 125 K to D atoms (1700 K) initially hydrogenates sp2 hybridized C atoms with a cross section of ca. 8 A2 producing C6H6D intermediates. Additional D atom reactions either transform this intermediate via a second hydrogenation reaction to cyclohexadiene-d2, C6H6D2, or restore benzene, C6H5D, via H abstraction. Once the aromaticity is broken, successive hydrogenation of the diene occurs rapidly generating the saturated cyclohexane-d6, C6H6D6. The C6H5D reaction product can undergo further H/D exchange reactions and, at any level of deuteration, the benzene species might get hydrogenated. Monolayers of the saturated hydrocarbon (1,4)-dimethyl-cyclohexane (DMCH) that are exposed to D atoms produce deuterated DMCH via successive abstraction/hydrogenation reactions. Thermal desorption mass spectra revealed that H atoms at the ring were exchanged with an apparent cross section of 1.7 A2. Methyl groups H atoms were exchanged much more slowly than ring H atoms. It was also observed that D exposed molecules/radicals exhibit a tendency to desorb from the surface, which is ascribed to the exothermicity of the reactions which lead to these species.

Mixtures of C6D6 and C6H5X, X = CH3, CF3, OH, Cl, and F, have been thermolyzed in H2 in a tubular flow system at atmospheric pressure between 898 and 1039 K.Removal of D or X occurs via hydrogen atom attack and lower deuterated benzenes and C6H6 are formed.Mass spectral analyses for (deuterio)benzenes have been used to determine the rates of desubstitution C6H5X + H --> C6H6 relative to H + C6D6 --> C6D5H (9).For X = D, CH3, CF3, and OH, desubstitution occurs by addition of a H atom to the ring followed by loss of X. from the substituted cyclohexadienyl intermediate.For X = Cl direct abstraction also takes place and for X = F abstraction is the only operative mechanism.No evidence for hydrogen migration around the ring in cyclohexadienyl intermediates was found.On a per-site basis, the dedeuteration rates of C6D6, C6D5H, C6D4H2, ..., C6DH5 were found to be equal.On the basis of log k9/L mol-1 s-1 = 10.69 - 2350/2.3T, we have found the following log k(benzene)/L mol-1 s-1 for C6H5X + H --> C6H6: X = CH3, 10.19 - 2914/2.3T; CF3, 9.79 - 2832/2.3T; OH, 9.93 - 2945/2.3T; Cl, 10.48 - 4148/2,3T; F, 10.31 - 5066/2.3T.Results are compared with relevant literature data.

Multiplexed temperature programmed reaction measurements have been used to examine mechanistic details of the acetylene yields benzene reaction on Pd(111). Results obtained from (C//2H//2 plus C//2D//2) experiments are in quantitative accord with a nondissociative reaction pathway; H/D scrambling in the reactants is not of significance, but at higher temperatures some H/D scrambling is observed between product molecules. The effects of surface crowding by unreacted acetylene on the binding and conformation of the benzene product are investigated, and it is shown that all benzene desorption is the result of the same initial reactive events. (C//2D//2 plus C//6H//6) data demonstrate that benzene desorption is not reaction-rate limited, consistent with the above conclusion.

Dilute mixtures (3-20 ppm) of C6D6 (benzene-d6) were pyrolyzed behind reflected shock waves at temperatures of 1630-1940 K and total pressures of 2-3 atm.Progress of the reaction was followed by analysis for D atoms using resonance absorption spectroscopy.Appearance of D atoms was a first-order process with respect to benzene concentration, and with respect to time during the first part of each experiment.An Arrhenius equation for the formation of D atoms, based on 34 experiments, is kD = 9.7E12 exp(-87100 cal/RT) s-1 with an estimated uncertainty of a factor of 1.5.From measurements of H atoms during pyrolysis of chlorobenzene under similar conditions at 1570-1790 K, the first-order rate constant for the dissociation of chlorobenzene to chlorine atoms and phenyl radicals was found to be k6 = 1.2E14 exp(-90000 cal/RT) s-1, and that for the dissotiation of phenyl radicals to H atoms and other products k3a = 1.2E15 exp(-82000 cal/RT) s-1.With this information, the rate constant for dissociation of benzene-d6 to phenyl-d5 and D atoms was found to be k1D = 4.6E13 exp(-95000 cal/RT) s-1.The rate constant for the exchange reaction H + C6D6 -> C6D5H + D was found to be k4a = 3.2E13 exp(-4200 cal/RT) mol-1 cm3 s-1 over the range 300-1400 K by combining our results with others at lower temperatures.A very simple kinetic model based on a reaction chain with H as carrier can relate our data to other shock-tube work at higher benzene concentrations.

The surface chemistry of benzene, toluene, cyclohexane, cyclohexene, and cyclohexadienes on Pt(100) is described.Benzene chemisorption was largely molecular although H-D exchange between chemisorbed C6H6 and C6D6 was observed at temperatures of 100 deg C and above.Toluene chemisorbed with bond breaking to give Pt-(100)-benzyl.This benzyl species (C6D5CD2) underwent H-D exchange with chemisorbed hydrogen.Exchange was more facile at the CH2 site than at aromatic C-H sites.Cyclohexane, cyclohexene, and cyclohexadienes chemisorbed on Pt(100) to form benzene with expected relative ease of dehydrogenation of cyclohexadienes >/= cyclohexene > cyclohexane.

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