Proximity Effects of Methyl Group on Ligand Steric Interactions and Colloidal Stability of Palladium Nanoparticles

Water-Soluble Pd Nanoparticles for the Anti-Markovnikov Oxidation of Allyl Benzene in Water

The catalytic activity and selectivity of two different water-soluble palladium nanoparticles capped with 5-(trimethylammonio)pentanethiolate and 6-(carboxylate)hexanethiolate ligands are investigated using the catalytic reaction of allyl benzene. The results show that the regioselective transformation of allyl benzene to 3-phenylpropanal occurs at room temperature and under atmospheric pressure in neat water via a Tsuji-Wacker type oxidation. Conventionally, the Tsuji-Wacker oxidation promotes the Markovnikov oxidation of terminal alkenes to their respective ketones in the presence of dioxygen. Water-soluble Pd nanoparticles, however, catalyze the anti-Markovnikov oxidation of allyl benzene to 3-phenylpropanal in up to 83% yields. Catalytic results of other aromatic alkenes suggest that the presence of benzylic hydrogen is a key to the formation of a p-allyl Pd intermediate and the anti-Markovnikov addition of H₂O. The subsequent b-H elimination and tautomerization contribute to the formation of aldehyde products. Water-soluble Pd nanoparticles are characterized using nuclear magnetic resonance (NMR), UV-vis spectroscopy, thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). Catalysis results are examined using ¹H NMR and/or GC-MS analyses of isolated reaction mixtures.

Effects of lipid bilayer encapsulation and lipid composition on the catalytic activity and colloidal stability of hydrophobic palladium nanoparticles in water

This article shows the preparation of a lipid-nanoparticle assembly (LNA) which contains hydrophobic palladium nanoparticles (PdNPs) within the hydrophobic regions of the liposomal micelles. To understand the colloidal stability and catalytic activity of LNAs, the structure–property relationships of LNAs are investigated by manipulating the lipid composition and reaction temperature. The studies of LNAs using dynamic light scattering (DLS), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM) show decreased colloidal stability with the incorporation of PdNPs compared to their counterpart 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) liposomes without PdNPs. LNAs with PdNPs catalyze the hydrogenation of 1-octene and its isomers to octane under one atm hydrogen gas and at room temperature within 24 h. The kinetic studies show that the isomerization of 1-octene to 2-octene occurs more favorably in the early stage of the reactions, which is followed by the subsequent hydrogenation of all octene isomers. The studies on temperature effects indicate that there is a significant increase in conversion yield of substrates when the reaction temperature increases from 22 to 37 °C, which correspond to room temperature and biological temperature, respectively. Phase transition of DSPC-PdNP LNAs from gel to liquid crystalline phase changing the fluidity of the bilayer is proposed to be the main reason for dramatic increases in the catalytic activity of the LNAs. It is also found that the rate of hydrogenation is dependent on the lipid composition of LNAs with the presence of cholesterol having a negative influence on the catalytic activity of LNAs while increasing their colloidal stability.

Hydrophobic micellization effect and dynamic lipid bilayer–substrate interactions enhance the catalytic activity of hydrophobic Pd nanoparticles embedded in liposomal assemblies.