Catalytic properties of transition metal salts immobilized on nanoporous silica polyamine composites II: hydrogenation

Regulating Pd/Al2O3 catalyst by g-C3N4 toward the enhanced selectivity of isoprene hydrogenation

The catalytic performance of Pd NPs in the selective hydrogenation of isoprene is modulated by g-C₃N₄ deposits on commercial alumina supports.

Lithium-Templated Formation of Polyhedral Oligomeric Silsesquioxanes (POSS)

A coordination complex, lithium hepta(i-butyl)silsesquioxane trisilanolate (1; Li-T₇), a stable intermediate in silsesquioxane (SQ) syntheses, was successfully isolated in 65% yield and found to be highly soluble in nonpolar solvents such as hexane. The structure of Li-T₇ was confirmed by NMR, IR spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry, electrospray ionization mass spectrometry, and computational simulation, providing detailed elucidation of the intermolecular self-association of the SQ cage with a box-shaped Li₆O₆ polyhedron through strong coordination bonds. After acid treatment, Li-T₇ undergoes lithium-proton cationic exchange, yielding hepta(i-butyl)silsesquioxane trisilanol (2; H-T₇) quantitatively. The high yield of H-T₇ seems to be influenced by Li-O bonding in the Li-T₇ complex that affects the selective formation of hepta(i-butyl)silsesquioxane trisilanolate and the bulky i-butyl groups which may prevent decomposition or SQ cage-rearrangement even at reflux under alkaline conditions. Single-crystal X-ray crystallography confirms the presence of the dumbbell-shaped SQ partial cages through strong intermolecular hydrogen bonds. Interestingly, lowering the polarity of the reaction solution by adding dichloromethane results in formation of the cubic octa(i-butyl)silsesquioxane (3; T₈) cage in a good yield (47%), which is isolated by crystallization from the reaction solution.

New Heterogeneous Rh-Containing Catalysts Immobilized on a Hybrid Organic-Inorganic Surface for Hydroformylation of Unsaturated Compounds

Anchoring Rh complexes to the surface of a silica polyamine composite, which has a poly(allylamine) covalently grafted to the surface of amorphous silica gel, yielded a material that proved to be an effective and novel heterogeneous catalyst for hydroformylation of unsaturated compounds. Surface amino groups of the material were modified with phosphines by covalent and ionic coupling. The modified materials were then treated with Rh(acac)(CO)₂, giving the catalysts K-1 and K-2. Catalysts were characterized by solid-state NMR spectroscopy, IR spectroscopy, XPS, TEM, and elemental analysis. The activity and stability of K-1 and K-2 were then studied for the hydroformylation of selected unsaturated compounds. Hydroformylation of terminal double bonds occurred selectively in the presence of internal double bonds. Characterization of the catalysts and the problems encountered with the supported catalysts are discussed. Catalyst K-1 is reusable and can be applied to the hydroformylation of linear olefins, styrene, 4-vinylcyclohexene, and dienes, as well as representative terpenes and other unsaturated hydrocarbons in a batch reactor.

Pd nanoparticles in dendrimers immobilized on silica-polyamine composites as catalysts for selective hydrogenation

New heterogeneous hydrogenation catalysts, based on Pd nanoparticles and polypropyleneimine (PPI) dendrimers of the third generation that have been covalently grafted to a silica surface modified with polyallylamine (PAA) have been synthesized. The final products were characterized by TEM, XPS, and solid-state NMR spectroscopy. The synthesized materials are effective catalysts for selective hydrogenation of dienes to monoenes and phenyl acetylene to styrene at very high substrate/Pd ratios with turnover rates higher than related Pd nanoparticle catalysts. The synthesized catalysts can be reused without any loss of activity in the case of styrene and isoprene.

Surface-Bound Ruthenium Diimine Organometallic Complexes: Excited-State Properties

Ruthenium complexes of the general formula [Ru(CO)(H)(L₂)(L′₂)][PF₆] (L₂ = trans-2PPh₃, L′ = η²-4,4′-dicarboxybipyridine (1); L₂ =trans-2Ph₂PCH₂CH₂COOH, L′₂ = bipyridine (2); L₂ = Ph₂PCHCHPPh₂, L′ = η²-5-amino-1,10-phenanthroline (3); L₂ = trans-2PPh₃, L′₂ = η²-4-carboxaldehyde-4′-methylbipyridine (4)) have been shown to have longer emission lifetimes and higher quantum yields in solution compared with more symmetrical molecules such as [Ru(bpy)₃][Cl]₂. Compound 4 is obtained as a mixture with the corresponding acetal, 4′. These less symmetrical complexes have been covalently immobilized on the surface of silica polyamine composites, and their photophysical properties have been studied. The surface-bound complexes have been characterized by solid-state CPMAS ¹³C, ³¹P, and ²⁹Si NMR, UV–vis, and FT-IR spectroscopies. Excited-state lifetime studies revealed that, in general, the lifetimes of the immobilized complexes are 1.4 to 8 times longer than in solution and are dependent on particle size (300–500 μm versus 10–20 nm average diameter silica gels), polymer structure (linear poly(allylamine) versus branched poly(ethylenimine)), and the type of surface tether. One exception to this trend is the previously reported complex [Ru(bpy)₂(5-amino-1,10-phenanthroline)][PF₆]₂ (5), where only a slight increase in lifetime is observed. Only minor changes in emission wavelength are observed for all the complexes. This opens up the possibility for enhanced heterogeneous electron transfer in photocatalytic reactions.