Formation of Nanosized Catalysts Based on Palladium Phosphine Complexes and the Nature of Their Activity

Findings on the formation and features of nanosized particles based on palladium complexes, which are active in hydrogenation catalysis, are summarized. Depending on the nature of a reducing agent, nanosized particles formed by the reduction of palladium(II) phosphine complexes are either metallic nuclei stabilized by organophosphorus ligands or associates of polynuclear phosphido or phosphinideno palladium complexes whose surface contains immobilized Pd(0) clusters. The ensembles of the Pd(0) atoms are active in hydrogenation.     

Reactions of Pd β-Diketonate Complexes with Triethylaluminium

Reactions of Pd(Acac)₂ and Pd(Acac)₂PPh₃ complexes with triethylaluminium in an inert atmosphere are studied by the NMR and IR, electronic microscopy, and X-ray powder diffraction methods. The final products of conversion of the initial Pd(II) complexes are the Pd(0) nanoparticles with the predominant diameter 2–4 nm. The main factors determining the size of Pd(0) particles and the nature of the ligand shell are considered.     

Formation and the nature of activity of Ziegler systems based on palladium β-diketonate complexes in hydrogenation catalysis

The catalytic properties and nature of Ziegler-type Pd(Acac)₂ and Pd(Acac)₂PPh₃ based catalysts are studied in the hydrogenation of unsaturated compounds. The causes of an extremum appearing in the dependence of the specific activity of the catalyst in styrene and phenylacetylene hydrogenation on the proportions of the starting components are considered. The increase in the specific activity of the Pd(Acac)₂ + AlEt₃ catalytic system in hydrogenation as a function of the Al/Pd ratio arises from an increase in the degree of dispersion of the microheterogeneous system, an increase in the fraction of reduced palladium, and changes in the nature of the ligand shell. The inhibiting effect is caused by triethylaluminum adsorption on palladium nanoparticles. Palladium nanoparticle models are suggested.     

Reaction of Palladium Bis(Acetylacetonate) with Phenylphosphine

The reaction between palladium bis(acetylacetonate) and phenylphosphine was performed at different ratios of reagents and studied by the spectral and X-ray powder diffraction methods. It was shown that the reaction of PH₂Ph with Pd(Acac)₂ does not terminate in complexation but is accompanied by the exchange of acido ligands for organophosphorus ligands and the formation of associates of palladium complexes containing the bridging ³-PPh, ²-PHPh, O,O-chelate Acac ligands and the coordinated phenylphosphine molecules. When the reagent ratio is increased to PH₂Ph : Pd(Acac)₂ > 2, Acac^- is fully replaced by organophosphorus ligands.     

Origin of the Activity of Hydrogenation Catalysts Based on Palladium Complexes and Primary Phosphines

The activity of the catalytic system based on palladium bisacetylacetonate and phenylphosphine in hydrogenation catalysis was studied.     

Reactions of Trivalent Phosphorus Compounds with Palladium Bis(acetylacetonate). Complexes Catalytically Active in Hydrogenation

Mechanisms of reactions between palladium bis(acetylacetonate) and tertiary and secondary phosphines and alkali-metal diphenylphosphides are discussed. Depending on the conditions, either complex formation or redox processes occur. The mechanism of formation and the nature of microheterogeneous catalysts for hydrogenation are considered using palladium acetylacetonate complexes with tertiary phosphines as an example. Data of NMR and IR spectroscopies, electron microscopy, and X-ray powder diffraction analysis showed that the catalysts are a palladium organophosphorus matrix with nanosize and with immobilized palladium clusters in the zero oxidation state.     

Formation and Properties of Hydrogenation Catalysts Based on Palladium Complexes with Primary Phosphines

The formation and catalytic properties of hydrogenation catalysts based on palladium(II) complexes with primary phosphines were studied. With the use of IR and UV spectroscopy, XRD analysis and GLC, it was found that the interaction of bis(acetylacetonato)palladium(II) or palladium(II) acetate with primary phosphines in an inert atmosphere resulted in the formation of polynuclear palladium complex associates mainly containing μ³-PR and a coordinated phosphine. Polynuclear palladium complexes and the palladium phosphide Pd₆P, which is formed from these complexes in an atmosphere of hydrogen, serve as supports for Pd(0) clusters. The effects of the ratio between initial components and the nature of the acido ligand at the palladium atom on the optimum conditions of catalyst formation were considered.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 609–614.Original Russian Text Copyright © 2005 by Belykh, Goremyka, Gusarova, Sukhov, Shmidt.     

Hydrogenation Catalysts Based on Polynuclear Palladium Complexes with Organophosphorus Ligands

A new procedure is proposed for the preparation of hydrogenation catalysts. This procedure includes the synthesis of cyclic tetranuclear palladium complexes with bridging diphenylphosphide ligands followed by a reaction with Pd(CH₃COO)₂ in the presence of hydrogen to form nanosized particles. In the test catalysts, the ensembles of palladium atoms (or palladium hydrides) immobilized on supramolecular structures formed by the association of phosphinidene and phosphide complexes of palladium are responsible for the catalytic activity.     

Reaction of Palladium Bis(acetylacetonate) with Diphenylphosphine: Spectral Studies

Interaction of palladium bis(acetylacetonate) with diphenylphosphine is studied by NMR, IR, and UV methods. Reaction between reagents taken in equimolar amounts gives binuclear and trinuclear palladium complexes with bridging diphenylphosphide and the chelate acetylacetonate [Pd(Acac)PPh₂]₂ and [Pd₃(Acac)₂(PPh₂)₄] ligands. With excess PPh₂H, the trinuclear palladium complex, whose composition is supposed to be [Pd₃(PPh₂)₄(PPh₂–PPh₂) · C₆H₆], is isolated and characterized on the basis of the spectral data.