Preeyanuch Sangtrirutnugul

Tunable Porosity of Cross-Linked-Polyhedral Oligomeric Silsesquioxane Supports for Palladium-Catalyzed Aerobic Alcohol Oxidation in Water

Polyhedral oligomeric silsesquioxane (POSS)-based materials, poly-POSS-Tn [n = 8 (1), 10 (2), 12 (3), and mix (4)], were prepared in high yields via free radical polymerization of corresponding pure forms of methacrylate-functionalized POSS monomers, MMA-POSS-Tn (n = 8, 10, 12), and the mixture form, MMA-POSS-Tmix. Powder X-ray diffraction (XRD) spectra and BET analysis indicate that 1-4 are amorphous materials with high surface areas (683-839 m2 g-1). The surface areas and total pore volumes follow the trend: poly-POSS-T12 > poly-POSS-T10 > poly-POSS-Tmix > poly-POSS-T8. In addition, on the basis of Barrett-Joyner-Halenda (BJH) analysis, poly-POSS-T12 contains the highest amount of mesopores. The Pd nanoparticles immobilized on poly-POSS-Tn [n = 8 (5), 10 (6), 12 (7), and mix (8)] are well dispersed with 4-6 wt % Pd content and similar average particle sizes of 6.2-6.5 nm, according to transmission electron microscopy-energy dispersive X-ray analysis (TEM-EDX) and microwave plasma-atomic emission spectroscopy (MP-AES). At 90 °C, the stabilized Pd nanoparticles in 5-8 catalyzed aerobic oxidation of benzyl alcohol to benzaldehyde in 72-100% yields at 6 h using a mixture of a H2O/Pluronic (P123) solution. The PdNp@poly-POSS-T8 catalyst (5) exhibited the lowest catalytic activity, as a result of its lowest surface areas, total pore volumes, and amounts of mesopores. With the catalyst 8, various benzyl alcohol derivatives were converted to the corresponding aldehydes in good to excellent yields. However, with alcoholic substrates featuring electron-withdrawing substituents, high conversions were achieved with 1 equiv of K2CO3 additive and longer reaction times.

Pyridine–triazole ligands for copper-catalyzed aerobic alcohol oxidation

A series of Cu(NN′)2(OTf)2 complexes containing pyridine–triazole ligands [OTf = OSO2CF3; NN′ = NN′Ph (1), NN′hex (2), NN′py (3)] with different substituents at the triazole N4 position or 2,2′-bipyridine (bpy; 4) have been synthesized. Crystal structures of 1 and 3 reveal a trans-isomer with strong preference for regular-type triazole coordination (for 3) whereas the Cu–bipyridine complex 4 is more stable in a cis-form. Cyclic voltammetry of 1–4 suggest that the electron-donating strength follows the trend: bpy > NN′py > NN′hex ∼ NN′Ph. The catalyst systems consisting of 5 mol% Cu(OTf)2/NN′/TEMPO (TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxy) in the presence of 2 × 2.0 cm2 Cu0 sheets as a reducing agent and 10 mol% N-methylimidazole (NMI) exhibit good activities for aerobic oxidation of benzyl alcohol to benzaldehyde. Catalytic studies have shown that the activities were higher with more electron-rich N-based ligands. Furthermore, oxidation of aliphatic alcohols such as 1-hexanol and 2-methyl-1-pentanol using the Cu catalyst system with the NN′py ligand at room temperature afforded the corresponding aldehydes in >99% and 46% yields, respectively after 24 h.

Effects of appended hydroxyl groups and ligand chain length on copper coordination and oxidation activity

Treatment of a series of (imino)pyridine ligands bearing appended hydroxyl groups 2-((pyridin-2-ylmethylene)amino)phenol (Hpyph), 2-((pyridin-2-ylmethylene)amino)ethanol (Hpyet), and 3-((pyridin-2-ylmethylene)amino)propanol (Hpypr) with one equiv. of CuCl2·2H2O afforded the corresponding Cu(II) complexes in low to moderate yields. The crystal structure of (μ-Cl)2[CuCl(κ2-N,N-Hpyet)]2 reveals a symmetric dinuclear structure with the bidentate N,N-coordination mode of (imino)pyridine with no Cu–OH interaction. On the other hand, the dinuclear Cu(II) complex of the related propyl ligand Hpypr possesses a significantly different crystal structure involving nucleophilic addition of the hydroxyl group to the aldehyde group of 2-pyridinecarboxaldehyde. The Cu complex/Cu0/TEMPO/Na2CO3 (TEMPO = 2,2,6,6-tetramethylpiperidinyl-1-oxyl) catalyst system generally exhibited good activity for aerobic oxidation of benzyl alcohol to benzaldehyde in H2O at room temperature. The dinuclear Cu(II) complex (μ-Cl)2[CuCl(κ2-N,N-Hpyet)]2 was demonstrated as an effective catalyst toward aerobic oxidation of various benzyl alcohol derivatives, cinnamyl alcohol, and 2-thiophenemethanol.

Bis[2,6-bis­(2-meth­oxy­phen­yl)pyridinium] di-μ-bromido-bis­[dibromidocuprate(II)]

The title salt, (C19H18NO2)2[Cu2Br6], was obtained from an attempt to synthesize the copper(II) complex of 2,6-bis(2-methoxyphenyl)pyridine (L) from a reaction between CuBr2 and one equivalent of L in CH2Cl2 at room temperature. The resulting compound is the salt of the 2,6-bis(2-methoxyphenyl)pyridinium cation and 0.5 equivalents of a hexabromidodicuprate(II) dianion. Both methoxy groups of the cationic pyridinium moiety are directed towards the N atom of the pyridine ring as a result of intramolecular N—H⋯O hydrogen bonds. The centrosymmetric hexabromidodicuprate dianion possesses a distorted tetrahedral geometry at the copper ion. The Cu—Br bond lengths are 2.3385 (7) and 2.3304 (7) Å for the terminal bromides, whereas the bond length between the Cu atom and two bridging bromides is slightly longer [2.4451 (6) Å].