Investigations of the catalytic properties of manganese oxides for the oxygen reduction reaction in alkaline media

The oxygen reduction reaction (ORR) was studied in KOH electrolyte on different manganese oxides, dispersed on a carbon powder (MnO_x/C). The oxides were prepared by different methods, for producing MnO, Mn₃O₄ and MnO₂ as major phases dispersed on the Vulcan XC-72 carbon. The oxides were characterized by XRD (X-ray diffraction) and in situ XANES (X-ray absorption near edge structure). The electrochemical measurements were made using cyclic voltammetry and steady state polarization curves carried out in an ultra-thin layer rotating ring/disk electrode. The results have shown lower activity for the ORR on the MnO_x/C species compared to that on Pt/C, but higher activity compared to that of pure Vulcan carbon. Formation of involving 2e⁻ per O₂ molecule is the main path of the ORR in the studied MnO_x/C catalysts but, at low overpotentials and rotation rates the number of electrons is raised to 4 due to the occurrence of a disproportionation reaction. Large differences of electrocatalytic activity were seen for the different oxide species, and these were related to the presence of a Mn(IV) phase and the occurrence of a mediation processes involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen.     

Rydberg states with quantum Monte Carlo

Calculations on Rydberg states are performed using quantum Monte Carlo methods. Excitation energies and singlet-triplet splittings are calculated for two model systems, the carbon atom (3P and 1P) and carbon monoxide ((1Sigma and 3Sigma). Kohn-Sham wave functions constructed from open-shell localized Hartree-Fock orbitals are used as trial and guide functions. The fixed-node diffusion quantum Monte Carlo (FN-DMC) method depends strongly on the wave function's nodal hypersurface. Nodal artefacts are investigated for the ground state of the carbon atom. Their effect on the FN-DMC results can be analyzed quantitatively. FN-DMC leads to accurate excitation energies but to less accurate singlet-triplet splittings. Variational Monte Carlo calculations are able to reproduce the experimental results for both the excitation energies and the singlet-triplet splittings.     

Chiral Lewis base promoted trichlorosilane reduction of ketimines. An enantioselective organocatalytic synthesis of chiral amines

The reduction of the carbon-nitrogen double bond is an important transformation. Here we report our studies on a family of chiral organic catalysts able to promote the stereoselective reduction of ketimines with trichlorosilane. The very cheap, metal-free catalysts were easily prepared in one step from commercially available products, namely a chiral aminoalcohol and picolinic acid derivatives. The catalyst structure was extensively modified in a study that allowed to identify an extremely active species, able to promote the reduction on a large variety of substrates with high efficiency (up to 95% ee). A three component methodology has been also developed, where the enantiomerically enriched amine was isolated after performing the reaction by mixing a ketone and an amine in the presence of trichlorosilane and the catalyst. Its synthetic potentiality was demonstrated by employing the present metal-free catalytic procedure in the preparation of (S)-metolachlor, a potent and widely used herbicide.     

Site selectivity in the reactions of the hexanuclear platinum cluster [Pt6(mu-PtBu2)4(CO)6][CF3SO3]2

The previously reported hexanuclear cluster [Pt(6)(mu-PtBu(2))(4)(CO)(6)](2+)[Y](2) (1-Y(2): Y=CF(3)SO(3) (-)) contains a central Pt(4) tetrahedron bridged at each of the opposite edges by another platinum atom; in turn, four phosphido ligands bridge the four Pt-Pt bonds not involved in the tetrahedron, and, finally, one carbonyl ligand is terminally bonded to each metal centre. Interestingly, the two outer carbonyls are more easily substituted or attacked by nucleophiles than the inner four, which are bonded to the tetrahedron vertices. In fact, the reaction of 1-Y(2) with 1 equiv of [nBu(4)N]Cl or with an excess of halide salts gives the monochloride [Pt(6)(mu-PtBu(2))(4)(CO)(5)Cl](+)[Y], 2-Y, or the neutral dihalide derivatives [Pt(6)(mu-PtBu(2))(4)(CO)(4)X(2)] (3: X=Cl; 4: X=Br; 5: X=I). Moreover, the useful unsymmetrically substituted [Pt(6)(mu-PtBu(2))(4)(CO)(4)ICl] (6) was obtained by reacting equimolar amounts of 2 and [nBu(4)N]I, and the dicationic derivatives [Pt(6)(mu-PtBu(2))(4)(CO)(4)L(2)](2+)[Y](2) (7-Y(2): L=(13)CO; 8-Y(2): L=CNtBu; 9-Y(2): L=PMe(3)) were obtained by reaction of an excess of the ligand L with 1-Y(2). Weaker nitrogen ligands were introduced by dissolving the dichloride 3 in acetonitrile or pyridyne in the presence of TlPF(6) to afford [Pt(6)(mu-PtBu(2))(4) (CO)(4)L(2)](2+)[Z](2) (Z=PF(6) (-), 10-Z(2): L=MeCN; 11-Z(2): L=Py). The "apical" carbonyls in 1-Y(2) are also prone to nucleophilic addition (Nu(-): H(-), MeO(-)) affording the acyl derivatives [Pt(6)(mu-PtBu(2))(4)(CO)(4)(CONu)(2)] (12: Nu=H; 13: Nu=OMe). Complex 12 is slowly converted into the dihydride [Pt(6)(mu-PtBu(2))(4)(CO)(4)H(2)] (14), which was more cleanly prepared by reacting 3 with NaBH(4). In a unique case we observed a reaction involving also the inner carbonyls of complex 1, that is, in the reaction with a large excess of the isocyanides R-NC, which form the corresponding persubstituted derivatives [Pt(6)(mu-tPBu(2))(4)(CN-R)(6)](2+)[Y](2), (15-Y(2): R=tBu; 16-Y(2) (2-): R=-C(6)H(4)-4-C triple bond CH). All complexes were characterized by microanalysis, IR and multinuclear NMR spectroscopy. The crystal and molecular structures of complexes 3, 5, 6 and 9-Y(2) are also reported. From the redox viewpoint, all complexes display two reversible one-electron reduction steps, the location of which depends both upon the electronic effects of the substituents, and the overall charge of the original complex.     

Convenient Esterification of Carboxylic Acids by SN2 Reaction Promoted by a Protic Ionic-Liquid System Formed in Situ in Solvent-Free Conditions

The reaction of esterification of benzoic acid with benzyl chloride was chosen as a model reaction to study the esterification by S_N2 promoted by tertiary amine as deprotonating agent. The use of ionic liquid (IL) 1,3-dimethylimidazolium methanesulfonate [MMIm][OMs] as reaction medium has proven to give quantitative yield of the ester, but interestingly the reaction does occur even in solvent-free conditions, where the acid + the amine form a liquid system (a protic IL) in situ. This last methodology was extended to several carboxylic acids in conditions of atom economy (i.e., without excess of any reagent), giving moderately good yields of esters (54–78%) recovered by weight in pure form.     

Simulation of mesogenic diruthenium tetracarboxylates: Development of a force field for coordination polymers of the MMX type

A classical molecular mechanics force field, able to simulate coordination polymers (CP) based on ruthenium carboxylates (Ru₂(O₂CR_eq)₄L_ax) (eq = equatorial group containing aliphatic chains, L_ax= axial ligand), has been developed. New parameters extracted from experimental data and quantum calculations on short aliphatic chains model systems were included in the generalized AMBER force field. The proposed parametrization was evaluated using model systems with known structure, containing either short or long aliphatic chains; experimental results were reproduced satisfactorily. This modified force field, although in a preliminary stage, could then be applied to long chain liquid crystalline compounds. The resulting atomistic simulations allowed assessing the relative influence of the factors determining the CP conformation, determinant for the physical properties of these materials. © 2013 Wiley Periodicals, Inc.     

Synthesis and thermal crosslinking of carbosiloxane and oligo(oxyethylene) polymers

Acyclic diene metathesis (ADMET) polymerization has been used in the synthesis of telechelic materials using alkoxy‐functionalized carbosiloxane or oligo(oxyethylene)‐based polymers, varying from internal to terminal cured materials or the combination of them. Previous investigations demonstrated that introduction of chain‐end crosslinking improves the stress–strain behavior of such materials. A series of saturated and unsaturated carbosiloxane and oligo(oxyethylene)‐based polymers were synthesized by ADMET polymerization using silacyclobutane as chain‐end, thermally induced crosslinker. The carbosiloxane derivatives presented pure amorphous behavior, whereas the oligo(oxyethylene) polymers were semicrystalline. The thermal curing process was monitored by differential scanning calorimetry via the exotherm between 160 and 210 °C. Mechanical properties on thermoset polymers were measured, where cured polymers showed moduli from 0.6 to 9.3 MPa, tensile strengths from 0.3 to 1.0 MPa, and elongations from 12 to 76%. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Atomistic simulation of InGaN/GaN quantum disk LEDs

In this work electronic and optoelectronic properties of InGaN/GaN nanocolumn quantum disk LEDs have been studied with the multiscale simulation tool tiberCAD. Calculations have been performed with an atomistic tight-binding model. Results shows that emission energies have a minor dependence on the nanocolumn dimension while In concentration in the active region is a critical parameter.     

MCRs reshaped into a switchable microwave-assisted protocol toward 5-aminoimidazoles and dihydrotriazines

A tunable microwave-assisted protocol for the synthesis of two biologically relevant families of heterocycles has been designed. Via a simple switch of reaction conditions, the same starting materials can be engaged in either an improved synthesis of the dihydrotriazine scaffold or a novel, first-in-class MCR to render the challenging 5-aminoimidazole nucleus in a single step. An additional first-in-class MCR is also reported utilizing guanidines to afford 2,5-aminoimidazoles.