Hydrogenation of arenes over silica-supported catalysts that combine a grafted rhodium complex and palladium nanoparticles: evidence for substrate activation on Rh(single-site)-Pd(metal) moieties

The complex Rh(cod)(sulfos) (Rh(I); sulfos = (-)O(3)S(C(6)H(4))CH(2)C(CH(2)PPh(2))(3); cod = cycloocta-1,5-diene), either free or supported on silica, does not catalyze the hydrogenation of benzene in either homogeneous or heterogeneous phase. However, when silica contains supported Pd metal nanoparticles (Pd(0)/SiO(2)), a hybrid catalyst (Rh(I)-Pd(0)/SiO(2)) is formed that hydrogenates benzene 4 times faster than does Pd(0)/SiO(2) alone. EXAFS and DRIFT measurements of in situ and ex situ prepared samples, batch catalytic reactions under different conditions, deuterium labeling experiments, and model organometallic studies, taken together, have shown that the rhodium single sites and the palladium nanoparticles cooperate with each other in promoting the hydrogenation of benzene through the formation of a unique entity throughout the catalytic cycle. Besides decreasing the extent of cyclohexa-1,3-diene disproportionation at palladium, the combined action of the two metals activates the arene so as to allow the rhodium sites to enter the catalytic cycle and speed up the overall hydrogenation process by rapidly reducing benzene to cyclohexa-1,3-diene.     

Cationic η3-Triphenylcyclopropenylnickel complexes with tridentate ligands containing N,P, As as the donor atoms. Molecular structure of η3-triphenylcyclopropenyl-1,1,1-tris(diphenylphosphinomethyl)ethanenickel perchlorate

The [(C₃Ph₃)Ni(PPh₃)₂]ClO₄ complex reacts with the tridentate ligands, 1,1,1-tris(dimethylphosphinomethyl)ethane, 1,1,1-tris(diphenylphosphinomethyl)ethane, (bis(2-diphenylphosphino)ethyl)phenylphosphine, (bis(2-diphenylphosphino)ethyl)-n-propylamine, and 1,1,1-tris(diphenylarsinomethyl)ethane to give cationic η³-triphenylcyclopropenyl complexes of formula [(C₃Ph₃)NiL]Y (Y = ClO₄, BPh₄). An uncharged derivative with the formula [(C₃Ph₃)Ni(hb(3,5-me₂Pz)₃)] (hb(3,5-me₂Pz)₃ = hydrotris(3,5-dimethyl-1-pyrazolyl)borate) has also been prepared. The molecular structure of [(C₃Ph₃)Ni(triphos)]ClO₄ has been determined from counter diffraction data. The crystals are monoclinic, space group P2₁/n with cell dimensions: a 17.750(5), b 17.629(5), c 16.509(4) Å; β 92-59(9)°, D_c = 1.359 g cm^?3 for Z = 4. Full matrix least-squares refinement led to the conventional R factor of 0.064 for 2556 observed reflections. The molecular structure consists of [(C₃Ph₃)Ni(triphos)]⁺ cations and ClO₄^? anions. The nickel atom is coordinated to the three phosphorus atoms of the triphos ligand, and to the C₃Ph₃ fragment in a symmetric η³ fashion.     

Limits on the production of large transverse momentum direct photons deduced from the measurement of low-mass electron pairs

The hadronic production of electron pairs with masses between 200 and 500 MeV and large transverse momentum has been measured at the CERN Intersecting Storage Rings (ISR). The expected relation between low-mass electron pairs and real photons is used to determine the direct hadronic production of photons. Contrary to indications from some previous experiments, the observed spectrum is consistent with expectations from the decay of known mesons, and leads to a value for the ratio of direct photons to π⁰ of γ/π⁰ = (0.55 ± 0.92)% for 2 < p_T < 3 GeV and 〈√s〉 = 55 GeV.     

Azimuthal correlation of a high pTπ0 and η with a second π0 produced in pp collisions at the ISR

The correlation between a high transverse momentum π⁰and η and a second π⁰ have been measured using an apparatus with large azimuthal acceptance. The angular distribution of α, measuring the deviation from collinearity in azimuth, peaks near zero. The r.m.s. of the distribution is found to have an inverse logarithmic rather than a $\text{1}\text{p}$ fall-off.     

Modulated optical lattice as an atomic fabry-perot interferometer

We propose to engineer the atomic band structure in optical lattices in order to design a Fabry-Perot interferometer with large mode spacing and strong nonlinear coupling to be employed in atom optics. The use of an optical lattice allows for a significant reduction of the atomic effective mass, while the slow modulation of its parameters spatially confines the matter waves on a length scale of a few dozen optical wavelengths. As a consequence, the mode spacing in such a cavity would be as high as one-tenth of the recoil energy, allowing for a very efficient filter action, while the nonlinear coupling due to interatomic interactions could lead to bistability and limiting effects in the transmission of the atomic beam.