No abstract

No abstract

In comparison with the separate components, the combination on the same support material of dispersed palladium nanoparticles and grafted molecular rhodium complexes has provided evidence of improved activity in the hydrogenation of arenes.[1] It was proposed that the catalytic efficiency is a consequence of a hydrogen-spillover process that would enhance specifically the hydrogenation activity of the molecular catalyst.[1] A recent study of the hydrogenation of arenes with a catalyst obtained by silica sol-gel coentrapment of metallic palladium and [Rh(cod)(m-Cl)] 2 disagrees with the hydrogen-spillover hypothesis and suggests that the action of both metals is caused by a synergistic effect; [2] however, no explanation of the nature of this effect has been forthcoming. Herein, we provide a rationale for this synergistic effect, and show that the grafted rhodium complex speeds up the hydrogenation of arenes to cyclic dienes.Three different heterogeneous catalysts were tested in the hydrogenation of some arenes in n-pentane at relatively low temperature (40-60 8C) and 30 bar H 2 . A highly dispersed palladium metallic phase and the Rh I complex [Rh(cod)(sulphos)] (cod = cyclooctadiene; sulphos = À O 3 S(C 6 H 4 )CH 2 -C(CH 2 PPh 2 ) 3 ) were separately supported on high-surfacearea porous silica . A calcination/reduction procedure of silica-supported PdCl 2 was employed to prepare Pd 0 /SiO 2 , 1, with a metal content of about 10 wt %, while the Rh I complex was grafted by using a known procedure involving hydrogen bonds between silanols of the support and sulfonate groups from the sulphos ligand.[3] The single-site catalyst, [Rh(cod)-(sulphos)]/SiO 2 , 2 (Figure 1 A) with a metal content of about 0.5 wt %, was previously employed to catalyze the hydrogenation and hydroformylation of olefins in either gas-solid or liquid-solid phase.[3a] Upon hydrogenation, the cod ligand

In this work are described the syntheses of several new dppp-like ligands (dppp =1,3-bis(diphenylphosphino)propane) bearing different substituents on the carbon backbone and of their palladium(II) complexes with acetate or trifluoroacetate coligands (L). The complexes exhibit the general formula Pd(P−P)(L)2 and have been employed as catalyst precursors for the copolymerization of ethene and carbon monoxide in MeOH under experimental conditions that are comparable to those reported in the relevant literature and patents for dppp-based Pd(II) copolymerization catalysts. It has been found that the introduction of alkyl substituents in the 2-position of the carbon backbone of dppp does not significantly improve the performance of the corresponding catalyst precursors (highest productivity value 6.2 kg of copolymer (g of Pd h)-1 vs 5.4 kg of copolymer (g of Pd h)-1 for Pd(dppp)(L)2). In contrast, the productivity increases remarkably when methyl groups are introduced in both 1-positions of the diphosphine ligand, particularly with R,S (S,R) stereochemistry as in meso-CH2(CH3CHPPh2)2 (productivity of 8.0 kg of copolymer (g of Pd h)-1). On the basis of NMR and cyclic voltammetric studies of the catalyst precursors, it is suggested that the increased productivity provided by the C1-substituted ligands is both electronic and steric in nature. In situ high-pressure NMR experiments in sapphire tubes equipped with Ti alloy valves showed that the only phosphorus-containing species visible on the NMR time scale in effective copolymerization conditions are Pd(II) complexes with the formula Pd(diphosphine)X2 (X = p-toluenesulfonate, trifluoroacetate, or MeOH). It has been proposed that these Pd(II) complexes act as a reservoir of [Pd(diphosphine)]2+ moieties which may either be delivered into the catalysis cycle by action of various reagents (MeOH, H+, H2O, H2) or be withdrawn after the termination step and watched over deactivation paths.

The water-soluble diphosphine (NaO3S(C6H4)CH2)2C(CH2PPh2)2 (Na2DPPPDS) was employed to prepare the bis-trifluoroacetate Pd(II) complex Pd(Na2DPPPDS)(CO2CF3)2·2THF (1). The catalytic performance of 1 in the co- and terpolymerization of CO and ethene and propene in water has been studied in different experimental conditions. In combination with both a protic acid, commonly p-toluenesulfonic acid, and an organic oxidant such as 1,4-benzoquinone, 1 forms the most efficient catalyst systems ever reported for the copolymerization of CO and ethene in water. Under comparable conditions, the activity of 1 is similar to that of the industrial Pd(II) 1,3-bis(diphenylphosphino)propane catalysts in MeOH. Unlike the latter which yield a mixture of copolymers bearing diketone, keto-ester, or diester end groups, the copolymers and terpolymers produced with the Na2DPPPDS-based catalysts have exclusively ketonic end groups for average molecular weights ranging from 10 to 30 kg mol-1 depending on the reaction conditions. In situ high-pressure NMR (HPNMR) studies have been performed in actual copolymerization conditions using D2O as solvent. The only palladium complex visible on the NMR time scale contains the diphosphine ligand and TsO- or water groups. It is suggested that this palladium complex acts as a reservoir of “(diphosphine)Pd(II)” moieties which are delivered into the catalysis cycle as Pd−H species by reaction with water and/or H+. A catalysis cycle is proposed on the basis of HPNMR experiments, the structure of the copolymers, and the occurrence of the water-gas-shift reaction.