Magnetism and EPR Studies of Binuclear Ruthenium Hydride Binuclear Species Bearing Redox-Active Ligands.

The binuclear complex {[N3]Ru(H)}2(µ-η1:η1-N2) ([N3] = 2,6-(ArylN═CMe)2C5H3N and Aryl = mesityl or xylyl) contains two formally Ru(I), d7 centers linked by a bridging dinitrogen ligand, although the odd electrons are substantially delocalized onto the redox non-innocent pincer ligands. The complex exhibits paramagnetic behavior in solution, but is diamagnetic in the solid state. This difference is attributed to intermolecular "π-stacking" observed in the solid state, which essentially couples unpaired electrons on each half of the complex to form delocalized 22-center-2-electron covalent bonds. Introduction of a bulky t-butyl group on the ligand pyridine ring prevents this intermolecular association and allows further investigation of the magnetic behavior and electronic structure of the binuclear species. The interaction of the unpaired electrons in the two halves of the complex has been probed with magnetic susceptibility and perpendicular and parallel mode EPR measurements, revealing a weakly antiferromagnetically coupled system with a thermally accessible triplet excited state. In addition, the mixed valent, S = 1/2, {[N3]Ru(H)}(µ-η1:η1-N2){[N3]Ru} system has also been observed via perpendicular mode EPR and was used to quantify the growth of the thermally accessible triplet state of the dihydride complex using parallel mode EPR.

Heterogeneous catalysts need not be so "heterogeneous": monodisperse Pt nanocrystals by combining shape-controlled synthesis and purification by colloidal recrystallization.

Well-defined surfaces of Pt have been extensively studied for various catalytic processes. However, industrial catalysts are mostly composed of fine particles (e.g., nanocrystals), due to the desire for a high surface to volume ratio. Therefore, it is very important to explore and understand the catalytic processes both at nanoscale and on extended surfaces. In this report, a general synthetic method is described to prepare Pt nanocrystals with various morphologies. The synthesized Pt nanocrystals are further purified by exploiting the "self-cleaning" effect which results from the "colloidal recrystallization" of Pt supercrystals. The resulting high-purity nanocrystals enable the direct comparison of the reactivity of the {111} and {100} facets for important catalytic reactions. With these high-purity Pt nanocrystals, we have made several observations: Pt octahedra show higher poisoning tolerance in the electrooxidation of formic acid than Pt cubes; the oxidation of CO on Pt nanocrystals is structure insensitive when the partial pressure ratio p(O2)/p(CO) is close to or less than 0.5, while it is structure sensitive in the O(2)-rich environment; Pt octahedra have a lower activation energy than Pt cubes when catalyzing the electron transfer reaction between hexacyanoferrate (III) and thiosulfate ions. Through electrocatalysis, gas-phase-catalysis of CO oxidation, and a liquid-phase-catalysis of electron transfer reaction, we demonstrate that high quality Pt nanocrystals which have {111} and {100} facets selectively expose are ideal model materials to study catalysis at nanoscale.

A versatile route to core-shell catalysts: synthesis of dispersible M@oxide (M=Pd, Pt; oxide=TiO2, ZrO2) nanostructures by self-assembly.

A method, based on self assembly, for preparing core-shell nanostructures that are dispersible in organic solvents is demonstrated for Pd and Pt cores with CeO(2), TiO(2), and ZrO(2) shells. Transmission electron microscopy (TEM) of these nanostructures confirmed the formation of distinct metal cores, approximately 2 nm in diameter, surrounded by amorphous oxide shells. Functional catalysts were prepared by dispersing the nanostructures onto an Al(2)O(3) support; and vibrational spectra of adsorbed CO, together with adsorption uptakes, were used to demonstrate the accessibility of the metal core to CO and the porous nature of the oxide shell. Measurements of water-gas-shift (WGS) rates demonstrated that these catalysts exhibit activities similar to that of conventional supported catalysts despite having lower metal dispersions. Pd-based CeO(2) and TiO(2) core-shell catalysts exhibit significant transient deactivation, which is probably caused by a decrease in the exposed metal surface area due to the ease of reduction of the shells. Alternatively, Pt-based analogous core-shell catalysts do not exhibit such a transient decrease. Both Pd- and Pt-based ZrO(2) core-shell catalysts deactivate at a significantly lower rate due to the less reducible nature of the ZrO(2) shell.

Evidence for ligand non-innocence in a formally ruthenium(I) hydride complex.

Formally zerovalent, dinitrogen-bridged ruthenium complex, {[N(3)(Xyl)]Ru}(2)(mu-eta(1):eta(1)-N(2)) (1), where [N(3)(Xyl)] = 2,6-(XylN horizontal lineCMe)(2)C(5)H(3)N, reacts with excess H(2) to give the binuclear hydride species, {[N(3)]Ru(H)}(2)(mu-eta(1):eta(1)-N(2)) (2) bearing a single hydrogen per ruthenium. Complex 2 is an unusual example of a structurally characterized paramagnetic transition metal hydride, and the first such example for ruthenium. Structural data and DFT calculations suggest unpaired electron density is strongly delocalized onto the non-innocent [N(3)] ligand, with a relatively small degree of the metalloradical character implied by the Ru(I) formal oxidation state, and that the [N(3)](-)/Ru(II) formalism may be more informative. Consistent with an effective oxidation state greater than Ru(I), further reaction of 2 with excess H(2) to give metal dihydride species ([N(3)]RuH(2)(L)) is not observed. The magnetic moment of 2 (3.50 mu(B)) in solution is consistent with one unpaired electron per [N(3)]Ru moiety; however, 2 is diamagnetic in the solid due to close (3.26 A) head-to-tail contact between Ru pyridine planes of neighboring molecules. Although the geometry is reminiscent of the weak "pi-stacking" observed for closed-shell aromatic ring systems, DFT calculations indicate the structure and associated spin pairing result from in-phase overlap of the delocalized SOMOs on neighboring molecules-that is the interaction is best viewed as a weak covalent bond delocalized over 22 atoms.

Novel embedded Pd@CeO(2) catalysts: a way to active and stable catalysts.

1-wt% Pd-CeO(2) catalysts were prepared by co-precipitation of Pd nanoparticles with ceria (Pd@CeO(2)-CP), by a microemulsion procedure (Pd@CeO(2)-ME), and by normal impregnation of Pd salts (Pd/CeO(2)-IMP) in order to test the concept that Pd-CeO(2) catalysts could be more stable for the water-gas-shift (WGS) reaction when the Pd is embedded in CeO(2). Initial WGS rates measured at 250 degrees C were similar for the Pd@CeO(2)-CP and Pd/CeO(2)-IMP, indicating that Pd was accessible for gas-phase reactions on both catalysts. Pd@CeO(2)-CP exhibited better stability for WGS than did Pd/CeO(2)-IMP but exposure to the WGS environment at 400 degrees C still caused a decrease in activity. Physical characterization of the Pd@CeO(2)-ME implied that the core-shell nanoparticles underwent condensation that resulted in a low surface area and poor Pd accessibility. However, the Pd@CeO(2)-ME sample exhibited good stability for WGS, suggesting that more effective encapsulation of Pd can limit the sintering of the metal phase, thus resulting in stable catalysts under high temperature reaction conditions.

Synthesis of dispersible Pd@CeO(2) core-shell nanostructures by self-assembly.

A methodology is described for the preparation of Pd@CeO(2) core-shell nanostructures that are easily dispersible in common organic solvents. The method involves the synthesis of Pd nanoparticles protected by a monolayer of 11-mercaptoundecanoic acid (MUA). The carboxylic groups on the nanoparticle surfaces are used to direct the self-assembly of a cerium(IV) alkoxide around the metal particles, followed by the controlled hydrolysis to form CeO(2). The characterization of the nanostructures by means of different techniques, in particular by electron microscopy, allowed us to demonstrate the nature of core-shell systems, with CeO(2) nanocrystals forming a shell around the MUA-protected Pd core. Finally, an example of the use of these nanostructures as flexible precursors for the preparation of heterogeneous catalysts is reported by investigating the reactivity of Pd@CeO(2)/Al(2)O(3) nanocomposites toward CO oxidation, water-gas shift (WGS), and methanol steam reforming reactions. Together with CO adsorption data, these observations suggest the accessibility of the Pd phase in the nanocomposites.