RESUMO
The controlled synthesis of gold nanoparticles (AuNPs), incarcerated in a semicrystalline nanoporous polymer matrix that consisted of a syndiotactic polystyrene-co-cis-1,4-polybutadiene multi-block copolymer is described. This catalyst was successfully tested in the oxidation of primary and secondary alcohols, in which we used dioxygen as the oxidant under mild conditions. Accordingly, (±)-1-phenylethanol was oxidised to acetophenone in high yields (96%) in 1 h, at 35 °C, whereas benzyl alcohol was quantitatively oxidised to benzaldehyde with a selectivity of 96% in 6 h. The specific rate constants calculated from the corresponding kinetic plots were among the highest found for polymer-incarcerated AuNPs. Similar values in terms of reactivity and selectivity were found in the oxidation of primary alcohols, such as cinnamyl alcohol and 2-thiophenemethanol, and secondary alcohols, such as indanol and α-tetralol. The remarkable catalytic properties of this system were attributed to the formation, under these reaction conditions, of the nanoporous ε crystalline form of syndiotactic polystyrene, which ensures facile and selective accessibility for the substrates to the gold catalyst incarcerated in the polymer matrix. Moreover, the polymeric crystalline domains produced reversible physical cross-links that resulted in reduced gold leaching and also allowed the recovery and reuse of the catalyst. A comparison of catalytic performance between AuNPs and annealed AuNPs suggested that multiple twinned defective nanoparticles of about 9 nm in diameter constituted the active catalyst in these oxidation reactions.
RESUMO
New zinc complexes (LOMe)ZnCl(2) (1) and (LOH)ZnCl(2)(2) of the heteroscorpionate ligands 1-[(3,5-di-tert-butyl-2-methoxyphenyl)(3,5-dimethyl-pyrazol-1-yl)methyl)]-3,5-dimethyl-pyrazole (LOMe) and 2,4-di-tert-butyl-6-[bis(3,5-dimethyl-pyrazol-1-yl)methyl]phenol (LOH) have been synthesized. The X-ray molecular structure of 2 was reported and compared with the one of the iron(II) complex (LOH)FeI(2) (3). The complexes 2-3 adopt a tetrahedral structure in the solid state in which the LOH ligand is kappa(2)-coordinated to the metal via the imino nitrogens of the two pyrazolyl rings. The hydroxyl phenyl group is not coordinated to the metal but found to be involved in an intermolecular hydrogen bond. The solution structures of 1 and 2 are consistent with this tetrahedral C(S) symmetric geometry. Dilution and (1)H-(1)H Nuclear Overhauser Effect Spectroscopy (NOESY) experiments revealed that the free ligands LOMe and LOH are involved in intra- and intermolecular hydrogen bonding interactions. Coordination of LOMe and LOH to ZnCl(2) was investigated by NMR titration methods. Association constants (K(a)) of (8.6 +/- 0.4) x 10(2) M(-1) and (7.8 +/- 0.3) x 10(2) M(-1) were obtained in methanol/water solutions (95:5) for LOMe and LOH, respectively. Coordination of bis(3,5-dimethyl-pyrazol-1-yl)methane (bpm) ligand to ZnCl(2) is weaker, as evidenced by the lower value of the association constant (5.3 +/- 0.3) x 10(2) M(-1). When bpm was added to solutions of 1 or 2, an equilibrium shifted toward the (bmp)ZnCl(2) species was observed. The thermodynamic parameters for this reaction were determined by VT NMR analysis. The optical properties of the ligands (LOMe, LOH) and of the corresponding zinc complexes 1 and 2 were also investigated by means of UV-vis and fluorescence spectroscopy to assess the potential use of these ligands as fluorescent sensors for Zn(2+) detection.
