Michael N. Vargaftik

Giant palladium clusters as catalysts of oxidative reactions of olefins and alcohols

Abstract Giant cationic palladium clusters approximated as Pd561L60(OAc)180 (L = phen, bipy) and Pd561phen60O60(PF6)60 were synthesized and characterized with high resolution TEM, SAXS, EXAFS, IR and magnetic susceptibility data. The results of these studies suggest that L and O ligands are bound to Pd atoms located on the surfaces of close-packed metal skeletons of the clusters, while OAc−1 and PF6− anions are outer-sphere ligands. Under mild conditions (293–363 K, l atm) the giant palladium clusters catalyze oxidative acetoxylation of ethylene into vinyl acetate, propylene into allyl acetate, and toluene into benzyl acetate; oxidation of primary aliphatic alcohols into esters; and conversion of aldehydes into acetals. Kinetics of oxidative acetoxylation of ethylene and propylene in solutions of the giant clusters were studied. A mechanism for these reactions is proposed, which includes the following steps: coordination of reagents by the cluster, oxidative addition of alkene to a Pd—Pd fragment and transfer of an electron pair from a Pd—H, Pd—vinyl, or Pd—allyl fragment to a coordinated molecule of oxidant (O2, peroxide, Pd(II)).

Catalysis with a palladium giant cluster: phenol oxidative carbonylation to diphenyl carbonate conjugated with reductive nitrobenzene conversion

Giant palladium-561 clusters were found to be effective in catalysis of the oxidative carbonylation of phenol to diphenyl carbonate, conjugated with the reductive carbonylation and reduction of nitrobenzene under CO pressure.

Giant palladium clusters: synthesis and characterization

A series of palladium clusters containing from four to several hundred Pd atoms in the metal skeleton has been prepared and characterized with respect to structure and chemical properties, including catalytic activity. For smaller clusters good agreement was observed. between single-crystal X-ray and EXAFS structural data. Giant clusters approximating to Pd561L60(OCOCH3)180(L = phen, bipy) and Pd561phen60O60X60(X = PF6, ClO4, BF4, CF3CO2), were characterized with TEM, SAXS, EXAFS, NMR and magnetic succeptibility data. These clusters contain a closepacked metal core and ligands L and X that are located at the periphery of a cluster. They are very soluble in water and polar organic solvents and can be considered as a bridge between low molecular clusters and particles of colloidal metals. Giant Pd clusters were found to be active homogeneous catalysts for various organic reactions, e.g. oxidative acetoxylation of alkenes and alkylarenes; oxidation of alkenes, formic acid and alcohols; dehydration of alcohols and formation of acetals. The kinetics and mechanism of the homogeneous oxidation of alkenes and HCO2H in solutions of giant clusters were elucidated.

Palladium clusters: Stoichiometric and catalytic reactions

The mechanistic aspects of organic reactions catalyzed with palladium clusters and stoichiometric reactions of carbonyl and carbene clusters are discussed. Palladium carbonyl carboxylates Pd4(CO)4 (OCOR)4(RMe, CMe3, Ph, CF3, CCl3) undergo thermolysis above 110–130°C, giving rise to CO2, CO and diacyls. In solutions of aromatic compounds the insertion of carbon dioxide into the aromatic CH bond or activated CH bond of alkylaromatic compounds was observed in the course of the thermolysis. The decomposition of palladium carbene carboxylate cluster Pd4(Ph2C)4(OAc)4 at 80°C has been found to involve inner sphere carbene oxidation during which an oxygen atom is transferred from the carboxylate group to the carbene ligand. Analogously, the reaction of [Pd(OAc)2PPh3]2 with formic acid, a reaction involving intermediate cluster formation, includes the transfer of an oxygen atom from the formate droup to the P atom of a phosphorus containing ligand, supposedly a diphenylphosphido bridging group. Positional and geometric α-alkene isomerization in aqueous PdCl2−2 solution has been found to be catalyzed by palladium (I) complexes of type Pd2Cl2−4. Colloidal clusters containing more than 500 palladium atoms in the metal core, which are soluble in polar organic solvents, have been found to catalyze the oxidative reactions of alkenes, toluene, alcohols and formic acid. Alcohols bearing at least one hydrogen atom in α-position undergo dehydration under mild conditions in non-acidic solution containing a Pd, Mo octanuclear anionic cluster [Pd4Mo4(CO)12Cp]2−. The reaction of benzyl alcohol gives rise to trans-stilbene. All these catalytic reactions can be rationalized within a scheme including the oxidative addition of the substrates across cluster metal-metal bond as a key step.

Long- and short-distance ordering of the metal cores of giant Pd clusters

Abstract Three types of metal cores for giant Pd clusters — octahedral or cuboctahedral, icosahedral (or multiply twinned), and random ones — have been identified by high resolution electron microscopy. Electron diffraction data confirm the fcc-like structure for the Pd particles regardless of their type; the lattice parameters are very close to those of metallic palladium. Structural transformations in the Pd clusters are found to be accompanied by changes in their catalytic activity and magnetic properties towards the size-induced paramagnetic-diamagnetic transition.

Oxidation, Redox Disproportionation and Chain Termination Reactions Catalysed by the Pd-561 Giant Cluster

Redox disproportionation of benzyl alcohol to benzaldehyde and toluene catalysed by the Pd561phen60(OAc)180 (phen=1,10-phenanthroline) giant cluster 1 under anaerobic conditions was found, whereas in an O2 atmosphere cluster 1 catalyses the oxidation of benzyl alcohol to benzaldehyde and inhibits further oxidation of the latter. A study of the AIBN-initiated and non-initiated oxidation of benzyl alcohol, sec-butyl alcohol and styrene in the presence of cluster 1 revealed that cluster 1 performs three functions in the oxidation reactions: 1) catalysis of polar oxidation of the substrates with O2, 2) termination of the chains of radical oxidation, and 3) catalysis of redox disproportionation.

Mechanism of reaction of palladium(II) carboxylates with carbon monoxide in nonaqueous media

Abstract The reduction of Pd II carboxylates by carbon monoxide in benzene and AcOH solutions has been studied. In the case of Pd II acetate the reaction is found to proceed through an intermediate complex with a Pd-CO-CH 3 group and the Pd I carbonylacetate [Pd(CO)(OAc)] 4 (PCA). A reductive condensation type of mechanism is suggested for the PCA decomposition on the base of kinetic data. The metal cluster skeleton is proposed to increase stepwise as the oxidation state of Pd decreases. A number of Pd I complexes, PCA analogs having composition [Pd(CO)(OCOR)] 4 , where R = C 2 H 5 , C 6 H 5 , CF 3 , CCl 3 , CH 2 Cl, have been prepared.

Giant cluster catalysis: possible intermediacy of nitrene and carbene species

Abstract Coordinated carbene and nitrene species were suggested as the key intermediates in hydrogen-transfer reduction of multiple bonds and reduction of nitroaromatics by CO catalysed with giant Pd-561 clusters as well as in the reaction of Pd (I) carbonylacetate cluster with nitrosobenzene. In the last reaction, a palladium complex with phenyl- o -nitrosophenylamide was obtained whose structure gives evidence for the intermediacy of nitrene ligands in Pd-catalysed reactions.

IMIDO/NITRENE LIGANDS IN THE COMPLEXES OF PLATINUM METALS

A synthetic search for palladium and platinum complexes with nitrene ligands via reaction of tetranuclear cluster Pd 4 ( μ 2 -CO) 4 ( μ 2 -OAc) 4 with nitrosobenzene and reactions of mononuclear complexes [MX 4 ] 2− (M=Pt, Pd; X=Cl, NO 2 ) with o -phenylenediamine and its N -phenyl derivative have been carried out. The complexes obtained, [Pd(OAc) o -(PhN)(NO)C 6 H 4 ] 2 , Pd[ o -(NPh)(N)C 6 H 4 ] 2 , Pt[ o -(NPh)(N)C 6 H 4 ] 2 , Pt 2 [ o -(NPh)(N)C 6 H 4 ] 4 [O 3 SCF 3 ] 2 , and Pt 2 [ o -(NPh)(N)C 6 H 4 ] 4 Br 2 , have been structurally characterized by single crystal X-ray diffraction studies. The oxidation of complex Pt[ o -(NPh)(N)C 6 H 4 ] 2 by excess AgO 3 SCF 3 afforded a heterotricyclic quinoxaline derivative, whose structure has been determined by X-ray analysis. The data obtained suggest that labile nitrene species coordinated to Pd and Pt complexes are possible intermediates in the reactions under study.

High-resolution electron microscopy and electron energy-loss spectroscopy of giant palladium clusters

Combined structural and chemical characterization of cationic polynuclear palladium coordination compounds Pd561L60(OAc)180, where L=1,10-phenantroline or 2,2′-bipyridine has been carried out by high-resolution electron microscopy (HREM) and analytical electron microscopy methods including electron energy-loss spectroscopy (EELS), zero-loss electron spectroscopic imaging, and energy-dispersive X-ray spectroscopy (EDX). The cell structure of the cluster matter with almost completely uniform metal core size distributions centered around 2.3 ±0.5 nm was observed. Zero-loss energy filtering allowed to improve the image contrast and resolution. HREM images showed that most of the palladium clusters had a cubo-octahedral shape. Some of them had a distorted icosahedron structure exhibiting multiple twinning. The selected-area electron diffraction patterns confirmed the face centered cubic structure with lattice parameter close to that of metallic palladium. The energy-loss spectra of the populations of clusters contained several bands, which could be assigned to the delayed Pd M4, 5-edge at 362 eV, the Pd M3-edge at 533 eV and the Pd M2-edge at 561 eV, the NK-edge at about 400 eV, the O K-edge at 532 eV overlapping with the Pd M3-edge and the carbon C K-edge at 284 eV. Background subtraction was applied to reveal the exact positions and fine structure of low intensity elemental peaks. EELS evaluations have been confirmed by EDX. The recorded series of the Pd M-edges and the N K-edge in the spectra of the giant palladium clusters obviously were related to Pd-Pd- and Pd-ligand bonding.