Mauricio Isaacs

Electrocatalytic reduction of CO2 by aza-macrocyclic complexes of Ni(II), Co(II), and Cu(II). Theoretical contribution to probable mechanisms

Abstract The electrocatalyzed reduction of carbon dioxide was investigated with cobalt(II), nickel(II) and copper(II) complexes of hexaaza-macrocycle ligands. The macrocyclic ligands were obtained via the condensation of phenanthroline or bipyridine. The X-ray structure of a triflate derivative was determined and it showed the less common “CuN4O2” centrosymmetric octahedral configuration with apically elongated CuO bonds. A preference for a neutral, mononuclear, octahedral configuration is exhibited by the planar macrocycle with an extended π-electron system. Its structure is characterized by shorter CuN bonds than those of Cu complexes with saturated macrocycles. The reduction of the complexes has been studied by cyclic voltammetry and UV–Vis spectroscopy. The results show that the metal center in its lower oxidation state, i.e. M(I), is the active site. The metal center is oxidized to M(II) in the presence of CO2 with the concomitant formation of an adduct between the metal center and CO2. A further one electron reduction regenerates the catalyst and produces carbon monoxide and/or formic acid. A theoretical model is proposed for a simple mechanism that involves initial coordination of CO2 to the metal center.

Antifungal Activity of Resveratrol against Botrytis cinerea Is Improved Using 2-Furyl Derivatives

The antifungal effect of three furyl compounds closely related to resveratrol, (E)-3,4,5-trimethoxy-β-(2-furyl)-styrene (1), (E)-4-methoxy-β-(2-furyl)-styrene (2) and (E)-3,5-dimethoxy-β-(2-furyl)-styrene (3) against Botrytis cinerea was analyzed. The inhibitory effect, at 100 µg ml−1 of compounds 1, 2, 3 and resveratrol on conidia germination, was determined to be about 70%, while at the same concentration pterostilbene (a dimethoxyl derivative of resveratrol) produced complete inhibition. The title compounds were more fungitoxic towards in vitro mycelial growth than resveratrol and pterostilbene. Compound 3 was the most active and a potential explanation of this feature is given using density functional theory (DFT) calculations on the demethoxylation/demethylation process. Compound 3 was further evaluated for its effects on laccase production, oxygen consumption and membrane integrity of B. cinerea. An increase of the laccase activity was observed in the presence of compound 3 and, using Sytox Green nucleic acid stain, it was demonstrated that this compound altered B. cinerea membrane. Finally, compound 3 partially affected conidia respiration.

Characterization of a neutral polysaccharide with antioxidant capacity from red wine.

A neutral fraction (PS-SI) (0.3g/L) with MW of 74kDa, which contained galactose, arabinose, mannose, and glucose in the molar ratio of 1.0:0.6:0.4:0.2 was obtained by treatment of the whole polysaccharide extracted from red wine with cetrimide, followed by gel permeation chromatography. Spectroscopic and methylation analyses indicated that PS-SI is a mixture of neutral polysaccharides, consisting mainly of beta (1-->3)-linked galactopyranosyl residues, with side chains of galactopyranosyl residues at positions O-6. Arabinofuranosyl residues linked alpha (1-->5), alpha-mannopyranosyl and glucosyl residues appear to be components of different polysaccharides. The in vitro antioxidant capacity of fractions of wine polysaccharide was studied by hydroxyl radical scavenging and ORAC assays. Fraction PS-SI presented the strongest effect on hydroxyl radicals (IC(50)=0.21).

Toward a pKa Scale of N-base Amines in Ionic Liquids

An electrochemical technique was used to investigate pKa values of some substituted secondary alicyclic (SA) amines, pyridines (py), anilines (AN), and triethylamine (Et3N) in different ionic liquids. The method involves cyclic voltammetry at a platinized Pt electrode. The experimental data were correlated with pKa values reported previously in aqueous solution, and Hammett parameters were correlated with pKa values in ionic liquids to determine ρ values in these media.

Electrocatalytic reduction of carbon dioxide on a cobalt tetrakis(4-aminophenyl)porphyrin modified electrode in BMImBF4

Electro-catalytic reduction of carbon dioxide has been studied with cobalt 5,10,15,20 tetrakis(4-aminophenyl)porphyrin forming a conducting polymer on an indium tin oxide electrode, using an ionic liquid as a novel reaction media. Electrocatalytic reduction of CO2 has been carried out, the potential at which this occurs has been reduced from −2.4 V vs. Ag/AgCl reported before, to −0.8 V vs. Ag/AgCl which means a significant energy gain regarding the electrochemical fixation of CO2. Electrochemical impedance spectroscopy (EIS) experiments have been carried out in order to understand the electrical behavior of the modified electrode in ionic liquid medium. Spectroelectrochemical evidence suggests that the reactive species which promotes the reaction is an electrochemically generated Co(I) porphyrin species which can be detected in an inert atmosphere but not in the carbon dioxide saturated medium.

Anthocyanin composition in aged Chilean Cabernet Sauvignon red wines

Anthocyanins in aged Cabernet Sauvignon red wines were studied by HPLC-MS. The major anthocyanin in 6, 7, and 8year old red wine extracts was the dimer vitisin A produced by condensation of malvidin-3-O-glucoside mediated by pyruvic acid. In aged wine, the content of malvidin-3-O-glucoside decreased with age with a concomitant increase of vitisin A. The latter is accompanied by several condensation products with molecular weight up to 1500m/z. Differential pulse voltammetry indicated that aged wines have lower antioxidant capacity than young wine (400mV), but higher than malvidin-3-O-glucoside (483mV). Tafels plots showed that the electrochemical process occurring in aged wines is different from young wines. Six, seven and eight year old wines show similar behaviour with plots of 234, 177 and 188mV/dec, respectively. These values are higher than the expected 120mV/dec corresponding to a first electronic transfer but smaller compared to the 523mV/dec corresponding to young wine.

Photoelectrochemical Reduction of Carbon Dioxide on Quantum-Dot-Modified Electrodes by Electric Field Directed Layer-by-Layer Assembly Methodology

The photoelectrocatalytic reduction of CO2 on ITO-modified electrodes, with electrostatic assemblies (polycations/quantum dots) (polycations=poly diallyldimethylammonium, PDDA, and poly(2-trimethylammonium)ethyl methacrylate, PMAEMA)), is described in the present work. Nanoparticles of CdTe (5.6 nm) were incorporated through an electric field directed layer-by-layer assembly method on ITO electrodes. Modified surfaces were active toward the reduction of CO2 at -400 mV vs Ag/AgCl; the activity of ITO-(PMAEMA/QDs)6 electrodes was enhanced by 300 mV under irradiation conditions. The photoelectrocatalytic effect was associated with the structure of the polycation and its influence in the assembly. Reaction products at -450 mV were H2, CO, CH3OH, and HCOH.

Electrocatalytic reduction of carbon dioxide on conducting glass electrode modified with polymeric porphyrin films containing transition metals in ionic liquid medium

Electrocatalytic reduction of carbon dioxide has been studied using modified electrodes with conducting polymers from tetra-amino-phenyl porphyrins containing transition metals (Zn(II) and Fe(III)), on an indium tin oxide electrode in BMImBF4, as solvent and supporting electrolyte. Electropolymerized Fe porphyrin is active toward the reaction under survey, while Zn derivative shows poor activity. Spectroelectrochemistry experiments on electropolymerized Fe porphyrin films have shown intermediary species like Fe–CO2 and Fe–CO at open-circuit potential. Potential-controlled bulk electrolysis carried out in ionic liquid shows that only carbon monoxide can be detected as reaction product in the gas phase and that Fe polymeric film shows a turnover number of 9.18, while the Zn film shows a value of 2.74, corroborating the poor activity observed in cyclic voltammetry. Graphical Abstract

Influence of the ionic liquid on the rate and the mechanism of reaction of p-nitrophenyl acetate with secondary alicyclic amines†

The reaction of p-nitrophenyl acetate (1) with piperidine was studied in nine organic solvents and in nine ionic liquids. The aminolysis of 1 by four different secondary amines was also studied in [Bmim]BF4 and their basicities were determined in this solvent using two different methods. Medium effects were analysed and interpreted by comparing the rate constants of aminolysis for the two sets of solvents.

CHARACTERIZATION OF PHENOLIC ACIDS PROFILE FROM CHILEAN RED WINES BY HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY

The success of Chilean wine in the international market has led to an important attention on the development of science and technology for wine differentiation, productivity, and quality. In order to report the profile of phenolic acid compounds in Chilean red wines, an SPE technique over ODS column was used followed by a reverse phase HPLC program that was developed to separate, characterize and quantify six phenolic acids (caffeic, gallic, p-coumaric, protocatechuic, syringic and vanillic acids) in four red Chilean wines varieties (Cabernet Sauvignon, Carmenere, Merlot and Syrah) from grapes cultivated in the central zone of Chile. The separation was carried out on an RP C-18 column using a binary gradient elution and diode array detection.