The reaction of Pt(2)(dba)(3) (dba = bis-dibenzylidene acetone) dispersed in room temperature 1-n-butyl-3-methylimidazolium (BMI) hexafluorophosphate ionic liquid with molecular hydrogen (4 atm) at 75 degrees C leads to stable and isolable nanometric Pt(0) particles. The X-ray diffraction analysis (XRD) of the material indicated that it is constituted of Pt(0). Transmission electron microscopy (TEM) analysis of the particles dispersed in the ionic liquid shows the formation of [Pt(0)](n) nanoparticles of 2.0-2.5 nm in diameter. A detailed examination of the nanoparticles imbibed in the ionic liquid and their environment shows an interaction of the BMI.PF(6) ionic liquid with the Pt(0) nanoparticles. The isolated [Pt(0)](n) nanoparticles can be redispersed in the ionic liquid or in acetone or used in solventless conditions for liquid-liquid biphasic, homogeneous, or heterogeneous hydrogenation of alkenes and arenes under mild reaction conditions (75 degrees C and 4 atm). The recovered platinum nanoparticles can be reused as a solid or redispersed in the ionic liquid several times without any significant loss in catalytic activity.
The controlled decomposition of Pt2(dba)3 (dba = dibenzylideneacetone) dispersed in 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI.BF4) and hexafluorophosphate (BMI.PF6) ionic liquids in the presence of cyclohexene by molecular hydrogen produces Pt0 nanoparticles. The formation of these nanoparticles follows the two-step [A --> B, A + B --> 2B (k1, k2)] autocatalytic mechanism. The catalytic activity in the hydrogenation of cyclohexene is influenced by the nature of the anion rather than the mean-diameter of the nanoparticles. Thus, higher catalytic activity was obtained with Pt0 dispersed in BMI.BF4 containing the less coordinating anion although these nanoparticles possess a larger mean diameter (3.4 nm) than those obtained in BMI.PF6 (2.3 nm). Similar mean diameter values were estimated from in situ XRD and SAXS. XPS analyses clearly show the interactions of the ionic liquid with the metal surface demonstrating the formation of an ionic liquid protective layer surrounding the platinum nanoparticles. SAXS analysis indicated the formation of a semi-organized ionic liquid layer surrounding the metal particles with an extended molecular length of around 2.8 nm in BMI.BF4 and 3.3 nm in BMI.PF6.
The reactivity of a set of prototypical (η6-arene)tricarbonylchromium complexes bearing amino, oxazolyl,
and pyridyl ancillary ligands versus [Cp*RhCl2]2 and [Cp*IrCl2]2 has been investigated. Successful
cyclometalation reactions were achieved essentially in the presence of hydrated sodium acetate with
planar-prochiral 2-phenylpyridine and 3-methyl-2-phenylpyridine complexes with yields ranging from
60% to 92%. The most salient feature of the reported reactions is their stereoselectivity, as the only
diastereomers to be produced are those with the Rh- and Ir-bound chloro ligand located trans with respect
to the Cr(CO)3. According to X-ray diffraction analyses, a relative rac-(pR,T-4-S) configuration may be
assigned to the complexes: the Cp* ligand sits unexpectedly syn with respect to the tricarbonylmetal
moiety. Introduction of the Cr(CO)3 moiety by treatment of cycloiridated 2-phenylpyridine with tricarbonyl(η6-naphthalene)chromium resulted in a unique diastereomer of the same relative configuration. Quantum
calculations using the density functional theory were carried out on models of syn and trans-chloro isomers.
Owing to strong electrostatic repulsion between the chloro ligand and the Cr(CO)3 moiety, the syn-chloro isomers were found less stable by 7−8 kcal/mol than the trans counterparts, suggesting that
cyclorhodation and cycloiridation reactions are thermodynamically controlled.
Pt x Pd 1-x (x ) 1, 0.7, or 0.5) nanoparticles submitted to hydrogen reduction and posterior H 2 S sulfidation at 150 or 300 °C were characterized by in situ X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The in situ XAS measurements allowed monitoring of short-range order changes around the Pt atoms induced by the thermal processes. The surface sensitivity and atom specific characteristics of XPS provided additional information about the chemical state of the atoms present in the outermost layers of the nanoparticles. Our experiments also indicate a Pd migration toward the surface of the nanoparticles driven by the thermal processes. We observed that the reduction process is necessary prior to the occurrence of any sulfur reaction and that the number of chemisorbed sulfur atoms is directly proportional to the quantity of Pd atoms.
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