Showing papers on "Osmium published in 1986"

Carbene and Carbyne Complexes of Ruthenium, Osmium, and Iridium

Abstract: Publisher Summary This chapter discusses the carbene and carbyne complexes of ruthenium, osmium, and iridium. The importance of transition-metal carbene complexes and of transition-metal carbyne complexes is now well appreciated. The wealth of empirical information collected for transition-metal carbene and carbyne complexes may be best interpreted within the framework of sound theoretical models for these compounds. Theoretical studies of metal–carbene complexes have been undertaken by several groups. The chemistry of transition metal–carbyne complexes is rather less developed than the chemistry of carbene complexes. The development of the chemistry of carbene complexes of the Group 8A metals, Ru, Os, and Ir, parallels chemistry realized initially with transition metals from Groups 6 and 7. Although transition-metal alkylidene complexes––that is, carbene complexes––containing only hydrogen or carbon-based substituents were first recognized over 15 years ago, it is only relatively recently that Ru, Os, and Ir alkylidene complexes have been characterized. In 1980, a stable dichlorocarbene complex of osmium (II) was described, and since then a large number of dihalocarbene complexes of ruthenium, osmium, and iridium has been prepared. Transition-metal carbyne complexes are still relatively uncommon as only a few synthetic approaches to these compounds have proved generally applicable. M=C and M=C bonds are now well-established features of the chemistry of Ru, Os, and Ir. Many exciting possibilities exist for using these functions in further reactions.

Chemistry and catalytic properties of ruthenium and osmium complexes. 3. Development of highly active systems for the homogeneous hydrogenation of aldehydes and ketones

Abstract: The catalytic hydrogenation of aldehydes and ketones to yield the corresponding alcohols exclusively is efficiently achieved by use of a series of 19 ruthenium and 4 osmium complexes containing hydride, phosphine, and carboxylate ligands, under moderate reaction conditions. For complexes MHX(CO)(PR/sub 3/)/sub 3/ the catalytic activity is dependent on X (halide) and independent of R. Evidence is presented for a mechanism involving MHX(CO)(PR/sub 3/)/sub 2/ as the active species and alkoxy-metal intermediates in the cycle. Carboxylate species MX(OCOR)(CO)(PPh/sub 3/)/sub 2/ show a catalytic behavior dependent on the stereochemistry of the complex, on X, and on the electronic nature of the R group. Correlations between k/sub obsd/ for the catalytic reaction and pK/sub a/ of the acid from which carboxylate ligands are derived have been found. This is explained in terms of a mechanism involving a bidentate-monodentate equilibrium for the carboxylate as a key step in the catalysis.

NMR Studies of Simple Molecules on Metal Surfaces

Abstract: In recent years, improvements in the sensitivity of nuclear magnetic resonance have made it possible to detect progressively smaller numbers of nuclei. Experiments and studies previously thought to be impractical can now be undertaken, for example, the study of phenomena at surfaces. Nuclear magnetic resonance has been applied to study simple molecules (carbon monoxide, acetylene, and ethylene) adsorbed on metal surfaces (ruthenium, rhodium, palladium, osmium, iridium, and platinum). The metals, in the form of clusters 10 to 50 angstroms in diameter, supported on alumina, are typical of real catalysts. The experiments provide information about the bonding of the molecules to the metal, the structures the molecules assume after adsorption, the motion of molecules on the surface, the breakup of molecules induced by heating, and the products of such breakup.

Conductive polymers derived from iron, ruthenium, and osmium metalloporphyrins: The shish-kebab approach.

Abstract: The synthesis and characterization of pyrazine-bridged polymers of iron(II/III), ruthenium(II/III), and osmium(II/III) octaethylporphyrin (dubbed “shish-kebab” polymers) are presented. Optical and dc conductivity measurements reveal that the ruthenium and osmium polymers, when partially oxidized, are highly conductive. Electrochemical and ESR results are presented that indicate the existence of an interesting metal-centered conduction pathway. Unlike most of the previously reported porphyrinic molecular metals in which the conduction electrons are macrocyclic-based, electron transport in these materials proceeds exclusively along the metal-pyrazine backbone.

Molecular organoosmium chemistry and catalysis on the basic magnesium oxide surface

Abstract: The complex (H/sub 2/Os(CO)/sub 4/) is deprotonated upon adsorption on the basic surface of magnesia. The resulting (HOs(CO)/sub 4/)/sup -/ is held strongly to the surface by a localized carbonyl-Mg/sup 2 +/ interaction analogous to the contact anion ion pairing observed for (Na)(HOs(CO)/sub 4/) in THF. The surface-bound complex is the precursor of surface species that are catalytically active for CO hydrogenation to give methane and higher hydrocarbons at 275 /sup 0/C and 10 atm. During catalysis, the mononuclear anion is transformed into the stable (H/sub 3/Os/sub 4/(CO)/sub 12/)/sup -/ and (Os/sub 10/C(CO)/sub 24/)/sup 2 -/, which become the only surface species detected by infrared spectroscopy and by extraction of surface-bound anions. The chemistry of the molecular condensation reactions of osmium carbonyls on the basic magnesia surface closely parallels that of osmium carbonyls in basic solution. The tetranuclear osmium cluster is suggested to be involved in the CO hydrogenation catalysis. 51 references, 12 figures, 1 table.

New method for the measurement of osmium isotopes applied to a New Zealand Cretaceous/Tertiary boundary shale

Abstract: The determination of osmium content and isotopic abundances in geological materials has received increasing attention in recent years following the proposal of Alvarez et al.1 that mass extinctions at the end of the Cretaceous period were caused by the impact of a large (∼10km) meteorite which left anomalously high iridium levels as a geochemical signature in the boundary shales. Here we report a new and simple method for measuring osmium in geological materials, involving fusion of the sample with sodium peroxide, distillation of the osmium as the tetroxide using perchloric acid, extraction into chloroform, and absorption of the chloroform extract onto graphite powder before instrumental neutron activation analysis. In a variant of this technique, the chloroform extract is back-extracted into an aqueous phase and the osmium isotopes are determined by plasma-source mass spectrometry (ICPMS). We have used this method on the Woodside Creek (New Zealand) Cretaceous/Tertiary boundary clay and have obtained the first osmium content (6g ng g−1) for this material. The 187Os/186Os ratio is 1.12±0.16, showing a typical non-crustal signature. This combined distillation–extraction–ICPMS method will prove to be useful for measuring osmium isotopes in other geological materials.

Reactions of osmium(IV) complexes of PAC ligands with azide species

Abstract: Etude des reactions de trans-Os(η 4 −HBA−B)(PPh 3 ) 2 avec des azides organiques. Les mecanismes sont differents suivant l'azide

Intra- and intermolecular activation of carbon−hydrogen bonds in a tetrakis(trimethylphosphine)osmium(II) system

Abstract: Activation des liaisons C-H, de facon intramoleculaire ou intermoleculaire par un intermediaire genere par thermolyse du cis-L 4 Os(H)R avec L=(CH 3 ) 3 P et R=CH 3 , CH 2 C(CH 3 ) 3 et CH 2 Si(CH 3 ) 3