25 + 65: Un any d'efemèrides per a la química teòrica de Catalunya

La química teòrica de Catalunya va commemorar durant el 2009 dues efemèrides importants: la XXV Reunió Anual i el seixanta-cinquè aniversari del professor Santiago Olivella.The theoretical chemistry community in Catalonia celebrated two important events in 2009: the XXVth Anual Meeting and the 65th birthday of Professor Santiago Olivella

Origins of observed reactivity and specificity in the addition of B2Cl4 and analogues to unsaturated compounds

In 1954 Schlesinger and co-workers observed the direct reaction of diboron tetrachloride with simple organic compounds under mild conditions, the 1,2 addition product being formed with either ethylene or acetylene. In the following 25 years a series of addition reactions to simple alkenes, alkynes and dienes was demonstrated. B2F4 was shown to react in similar manner, albeit under more forcing conditions. Crucially, it was demonstrated that the addition to (E)- or (Z)-but-2-ene occurred with cis-stereospecificity. Only sporadic interest was shown in this field thereafter until catalysed addition reactions of diboron reagents were realized. Encouraged by this revival of interest through the discovery of transition-metal and nucleophilic catalysis of diboryl additions, DFT analysis of uncatalysed additions of B2X4 has been carried out and interpreted. This includes the relative reactivity of several B–B reagents with ethene, and that of B2Cl4vs. B2F4 additions, including benzene, naphthalene and C60 as reactants. This allows the analysis of relative reactivity vis-à-vis substitution on boron, and also direct comparison with hydroboration by HBCl2. [4 + 2] Addition of diboron reagents to dienes with B–B cleavage competes with direct [2 + 2] addition, favourably so for B2F4. The computational results demonstrate that the stereospecific addition to isomeric but-2-enes is a rare concerted [2σs + 2πs] proces

The mechanism of CO2 hydration: a porous metal oxide nanocapsule catalyst can mimic the biological carbonic anhydrase role

Bandeira NAG, Garai S, Müller A, Bo C. The mechanism of CO2 hydration: a porous metal oxide nanocapsule catalyst can mimic the biological carbonic anhydrase role. Chemical Communications. 2015;51(85):15596-15599.The mechanism for the hydration of CO2 within a Keplerate nanocapsule is presented. A network of hydrogen bonds across the water layers in the first metal coordination sphere facilitates the proton abstraction and nucleophilic addition of water. The highly acidic properties of the polyoxometalate cluster are crucial for explaining the catalysed hydration

New graph-based computational methods for dealing with chemical reactivity and catalysis

El creixement exponencial del poder de la computació ha comportat un impuls decisiu en la capacitat de predicció de la química teòrica i computacional. Malgrat aquests avenços, l’estudi de la reactivitat en sistemes químics complexos que implica el tractament de xarxes de reaccions molt denses és dificultós. En aquest article mostrem tres mètodes nous que permeten crear, tractar i processar d’una manera eficient i automàtica cicles catalítics i xarxes de reaccions complexes. En primer lloc presentem una eina per a predir la freqüència de recanvi en cicles catalítics, tant en la catàlisi homogènia com en l’heterogènia. En segon lloc, tractem el que hem anomenat POMSimulator, un mètode que permet determinar mapes de reacció, predir l’especiació en funció del pH i de les concentracions, i també els mecanismes de formació dels òxids moleculars. Finalment, presentem OntoRXN, una nova ontologia orientada a definir formalment els mecanismes de reacció, que facilita el tractament i el processament d’aquesta informació química complexa.The exponential growth of computing power has crystallized in a breakthrough in the predictive capability of theoretical and computational chemistry. In spite of this leap forward, the study of the reactivity of complex chemical systems, implying the treatment of very dense reaction networks, is still a challenge. In this article, we showcase three new methods that allow the generation, treatment and processing of catalytic cycles and complex reaction networks in an efficient automated manner. First, we present a tool for predicting the turnover frequency in catalytic cycles, both for homogeneous and heterogeneous catalysis. Then, we present what we have named the POMSimulator, a method for generating reaction maps and predicting speciation on the basis of pH and concentrations, as well as the formation mechanisms of complex molecular oxides. Lastly, we present OntoRXN, a new ontology for the formal definition of reaction mechanisms whose goal is to simplify the treatment and processing of complex chemical information of this type

Tuning and mechanistic insights of metal chalcogenide molecular catalysts for the hydrogen-evolution reaction

The production of hydrogen through water splitting using earth-abundant metal catalysts is a promising pathway for converting solar energy into chemical fuels. However, existing approaches for fine stoichiometric control, structural and catalytic modification of materials by appropriate choice of earth abundant elements are either limited or challenging. Here we explore the tuning of redox active immobilised molecular metal-chalcoxide electrocatalysts by controlling the chalcogen or metal stoichiometry and explore critical aspects of the hydrogen evolution reaction (HER). Linear sweep voltammetry (LSV) shows that stoichiometric and structural control leads to the evolution of hydrogen at low overpotential with no catalyst degradation over 1000 cycles. Density functional calculations reveal the effect of the electronic and structural features and confer plausibility to the existence of a unimolecular mechanism in the HER process based on the tested hypotheses. We anticipate these findings to be a starting point for further exploration of molecular catalytic systems

Selective Lanthanide Distribution within a Comprehensive Series of Heterometallic [LnPr] Complexes

The preparation of heterometallic, lanthanide-only complexes is an extremely difficult synthetic challenge. By a ligandbased strategy, a complete isostructural series of dinuclear heterometallic [LnPr] complexes has been synthesized and structurally characterized. The two different coordination sites featured in this molecular entity allow study of the preferences of the praseodymium ion for a specific position depending on the ionic radii of the accompanying lanthanide partner. The purity of each heterometallic moiety has been evaluated in the solid state and in solution by means of crystallographic and spectrometric methods, respectively, revealing the limits of this strategy for ions with similar sizes. DFT calculations have been carried out to support the experimental results, confirming the nature of the siteselective lanthanide distribution. The predictable selectivity of this system has been exploited to assess the magnetic properties of the [DyPr] and [LuPr] derivatives, showing that the origin of the slow dynamics observed in the former arises from the dysprosium ion

Thermodynamic stability of heterodimetallic [LnLn] complexes: synthesis and DFT studies

The solid-state and solution configurations of the heterodimetallic complexes (Hpy)[LaEr(HL)(3)(NO3)(py)(H2O)] (1), (Hpy)[CeEr(HL)(3)(NO3)(py)(H2O)] (2), (Hpy)[CeGd(HL)(3)(NO3)(py)(H2O)] (3), (Hpy)[PrSm(HL)(3)(NO3)(py)(H2O)] (4), and (Hpy)(2)[LaYb(HL)(3)(NO3)(H2O)](NO3) (5), in which H3L is 6-(3-oxo-3-(2-hydroxyphenyl)propionyl)pyridine-2-carboxylic acid and py is pyridine, were analyzed experimentally and by using DFT calculations. Complexes 3, 4, and 5 are described here for the first time, and were analyzed by using single-crystal X-ray diffraction and mass spectrometry. The theoretical study was also extended to the [LaCe] and [LaLu] analogues. The results are consistent with a remarkable selectivity of the metal distribution within the molecule in the solid state, enhanced by the size difference between the different ions. This selectivity was reduced in solution, particularly for ions with the most similar radii. This unique entry into 4f-4f heterometallic chemistry establishes for the first time the difference between the selectivity in solution and that in the solid state, as a result of changes to the coordination that follow the dissociation of terminal ligands upon dissolution of the complexes

Structural studies of metal ligand complexes by ion mobility-mass spectrometry

Collision cross sections (CCS) have been measured for three salen ligands, and their complexes with copper and zinc using travelling-wave ion mobility-mass spectrometry (TWIMS) and drift tube ion mobility-mass spectrometry (DTIMS), allowing a comparative size evaluation of the ligands and complexes. CCS measurements using TWIMS were determined using peptide and TAAH calibration standards. TWIMS measurements gave significantly larger CCS than DTIMS in helium, by 9 % for TAAH standards and 3 % for peptide standards, indicating that the choice of calibration standards is important in ensuring the accuracy of TWIMS-derived CCS measurements. Repeatability data for TWIMS was obtained for inter- and intra-day studies with mean RSDs of 1. 1 % and 0. 7 %, respectively. The CCS data obtained from IM-MS measurements are compared to CCS values obtained via the projection approximation, the exact hard spheres method and the trajectory method from X-ray coordinates and modelled structures using density functional theory (DFT) based methods. © 2013 Springer-Verlag Berlin Heidelberg