Revealing the Influence of Secondary Metals on Redox-Active Palladium Complexes

Incorporation of redox-inactive metals into redox-active complexes and catalysts attracts attention for engendering new reactivity modes, but this strategy has not been extensively investigated beyond the first-row of the transition metals. Here, the isolation and characterization of the first series of heterobimetallic complexes of palladium with mono-, di-, and tri-valent redox-inactive metal ions are reported. A Reinhoudt-type heteroditopic ligand with a salen-derived [N2,O2] binding site for Pd and a crown-ether-derived [O6] site has been used to prepare isolable adducts of the Lewis acidic redox-inactive metal ions (Mn+). Comprehensive data from single-crystal X-ray diffraction analysis reveal distinctive trends in the structural properties of the heterobimetallic species, including an uncommon dependence of the Pd•••M distance on Lewis acidity. The reorganization energy associated with reduction of the heterobimetallic species is strongly modulated by Lewis acidity, with the slowest heterogeneous electron transfer kinetics associated with the strongest incorporated Lewis acids. This hitherto unexplored reorganization energy penalty for electron transfer contrasts with prior thermodynamic studies, revealing that kinetic parameters should be considered in studies of reactivity involving heterobimetallic species.

Utilization of BODIPY-based redox events to manipulate the Lewis acidity of fluorescent boranes

This report describes the incorporation of BODIPY dyes into the ligand framework of a borane to generate a family of molecular boranes that are capable of exhibiting tunable Lewis acidities through BODIPY-based redox events.

Review: chemical approaches toward catalytic lignin degradation

Lignin is a potentially high natural source of biological aromatic substances. However, decomposition of the polymer has proven to be quite challenging, as the complex bonds are fairly difficult to break down chemically. This article is intended to provide an overview of various recent methods for the catalytic chemical depolymerization of the biopolymer lignin into chemical products. For this purpose, nickel-, zeolite- and palladium-supported catalysts were examined in detail. In order to achieve this, various experiments of the last years were collected, and the efficiency of the individual catalysts was examined. This included evaluating the reaction conditions under which the catalysts work most efficiently. The influence of co-catalysts and Lewis acidity was also investigated. The results show that it is possible to control the obtained product selectivity very well by the choice of the respective catalysts combined with the proper reaction conditions.

Use of Novel Non-Toxic Bismuth Catalyst for the Preparation of Flexible Polyurethane Foam

Foam products are one of the largest markets for polyurethane (PU) and are heavily used in many sectors. However, current PU formulations use highly toxic and environmentally unfriendly production processes. Meanwhile, the increasing environmental concerns and regulations are intensifying the research into green and non-toxic products. In this study, we synthesized flexible polyurethane foam (PUF) using different weight percentages (0.025%, 0.05% and 0.1%) of a non-toxic bismuth catalyst. The bismuth-catalyzed foams presented a well evolved cellular structure with an open cell morphology. The properties of the bismuth-catalyzed flexible PUF, such as the mechanical, morphological, kinetic and thermal behaviors, were optimized and compared with a conventional tin-catalyzed PUF. The bismuth-catalyst revealed a higher isocyanate conversion efficiency than the stannous octoate catalyst. When comparing samples with similar densities, the bismuth-catalyzed foams present better mechanical behavior than the tin-catalyzed sample with similar thermal stability. The high solubility of bismuth triflate in water, together with its high Lewis acidity, have been shown to benefit the production of PU foams.

Copper Phosphotungstate-Catalyzed Microwave-Assisted Synthesis of 5-Hydroxymethylfurfural In A Biphasic System

In this paper, we have described a novel route to produce 5-hydroxymethylfurfural (HMF), a valuable platform-molecule obtained from biomass, using transition metal-exchanged Keggin heteropolyacid salts as catalysts, in microwave-assisted reactions carried out in a water-ethyl acetate biphasic system. To avoid the use of homogenous Brønsted acid catalysts, which are corrosive and difficult to be reused, we have exchanged the protons of the Keggin heteropolyacids by transition metal cations. These salts were evaluated in the fructose dehydration, being the Cu3/2PW12O40 the most active and selective catalyst, achieving 81 % of HMF yield, after 15 min reaction at 413 K under microwave irradiation (MWI). The effects of metal cation, anion, and heteropolyanion present in the catalyst were evaluated. The greatest efficiency of Cu3/2PW12O40 was attributed to its high Lewis acidity strength, which allows that it coordinate with water molecules, consequently generating H3O+ ions in the reaction medium. Even though the catalyst has been water-soluble, it was easily reused removing the extracting phase, and adding a new load of the substrate to the remaining aqueous phase. This way, it was successfully reused without loss activity.

Electrochemical Modeling of Iodide Oxidation in Metal-Halide Molten Salts

The oxidation of iodide in NaI-AlBr3, NaI-AlCl3, and NaI-GaCl3 molten salts was analyzed using simulation software to extract relevant kinetic parameters. The experimental oxidation potentials were ordered AlCl3 < AlBr3 < GaCl3, with higher oxidation potentials correlating with softer Lewis acidity of the metal halide. An iodide oxidation and metal halide speciation model was developed and simulated to fit the electrochemical response, enabling determination of electrochemical charge transfer parameters and chemical equilibrium constants. NaI-AlBr3 displayed the fastest electron transfer rates yet showed the lowest current densities. All salts revealed smaller than expected current densities, explained by equilibrium between various species, where some are not electrochemically active at the studied potentials. These equilibrium reactions are due to the various metal halide species, controlling the reactant concentration of iodide and the resultant current. We hypothesize the electrochemically active iodide species, present as a metal halide monomer (MX3I−), is decreased dramatically from the expected concentration, sequestered as a more stable metal halide dimer species (M2X6I−) with a higher oxidation potential. Traditional Tafel analysis of the experimental data supports the validity of the simulations. These results increase understanding of iodide oxidation in low-temperature Lewis acidic molten salts and inform task-specific molten salt design.