MECHANISTIC STUDY OF A RUTHENIUM HYDRIDE COMPLEX OF TYPE [RuH(CO)(N-N)(PR3)2]+ AS CATALYST PRECURSOR FOR THE HYDROFORMYLATION REACTION OF 1-HEXENE

Indexación: Web of Science; Scopus; Scielo.The catalytic activity of systems of type [RuH(CO)(N-N)(PR3)(2)](+) was evaluated in the hydroformylation reaction of 1-hexene. The observed activity is explained through a reaction mechanism on the basis of the quantum theory. The mechanism included total energy calculations for each of the intermediaries of the elemental steps considered in the catalytic cycle. The deactivation of the catalyst precursors takes place via dissociation of the polypyridine ligand and the subsequent formation of thermodynamically stable species, such as RuH(CO)(3)(PPh3)(2) and RuH3(CO)(PPh3)(2), which interrupt the catalytic cycle. In addition, the theoretical study allows to explain the observed regioselectivity which is defined in two steps: (a) the hydride migration reaction with an anti-Markovnikov orientation to produce the alkyl-linear-complex (3.1a), which is more stable by 19.4 kJ/mol than the Markovnikov orientation (alkyl-branched-complex) (3.1b); (b) the carbon monoxide insertion step generates the carbonyl alkyl-linear specie (4.1a) which is more stable by 9.5 kJ/mol than the alternative species (4.1b), determining the preferred formation of heptanal in the hydroformylation of 1-hexene. Palabras clavehttp://ref.scielo.org/db4yc

Preparation, spectral characterization, structural study, and evaluation of antibacterial activity of Schiff base complexes for VOII, CrIII, MnII, ZnII,CdII and CeIII

A new series of metal ions complexes of VO(II), Cr(III), Mn(II), Zn(II), Cd(II) and Ce(III) have been synthesized from the Schiff bases (4-chlorobenzylidene)-urea amine (L1) and (4-bromobenzylidene)-urea amine (L2). Structural features were obtained from their elemental microanalyses, magnetic susceptibility, molar conductance, FT-IR, UV–Vis, LC-Mass and 1HNMR spectral studies. The UV–Vis, magnetic susceptibility and molar conductance data of the complexes suggest a tetrahedral geometry around the central metal ion except, VOII complexes that has square pyramidal geometry, but CrIII and CeIII octahedral geometry. The biological activity for the ligand (L1) and its Vanadium and Cadmium complexes were studied. Structural geometries of compounds also were suggested in gas phase by using theoretical treatments, using Hyper Chem-6 program for the molecular mechanics and semi-empirical calculations. The heat of formation (?Hf ?) and binding energy (?Eb) in the temperature of 298K for the free ligand (L1) and their metal complexes were calculated by PM3 and ZINDO/I methods. The electrostatic potential of the free ligands were calculated to investigate the reactive sites of the molecules.Bacteriological evaluation of considerable number of these compounds were maintained using organisms Escherichia coli and Staphylococcus aureus,and they were found to exhibit the high effect of activity. This may be attributed to the impact of both the Schiff bases and the metal present in these complexes

Synthesis, Characterization, Thermal Analysis and Structural Studies of New Complexes with Tetradentate Ligand

Some new complexes of 4-(5-(1,5-dimethyl-3-oxo-2-phenyl pyrazolidin-4- ylimino)-3,3-dimethyl cyclohexylideneamino) -1,5- dimethyl-2- phenyl -1H- pyrazol -3(2H) –one (L) with Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Pd(II), Re(V) and Pt(IV) were prepared. The ligand and its metal complexes were characterized by phisco- chemical spectroscopic techniques. The spectral data were suggested that the (L) as a neutral tetradentate ligand is coordinated with the metal ions through two nitrogen and two oxygen atoms. These studies revealed Octahedral geometries for all metal complexes, except square planar for Pd(II) complex. Moreover, the thermodynamic activation parameters, such as ?E*, ?H, ?S, ?G and K are calculated from the TGA curves using Coats –Redfern method. Hyper Chem -8 program has been used to predict structural geometries for compounds in gas phase. The heat of formation (?Hf) and bin ding energy (?Eb) at 298 °K for the free ligand and its metal complexes were calculated by PM3 method. The synthesized ligands and their metal complexes were screened for their biological activity against bacterial species, two Gram positive bacteria (Bacillus subtilis and Staphylococcus aureus) and two Gram negative bacteria (Escherichia coli and Pseudomonas aeruginosa)

A Priori Assessment of the Stereoelectronic Profile of Phosphines and Phosphites

This article discusses research that has demonstrated the utility of a rigorously calibrated, molecular mechanics/semiempirical quantum mechanical protocol for developing stereoelectronic (Tolman) maps for phosphine ligands

The development of hybrid quantum classical computational methods for carbohydrate and hypervalent phosphoric systems

Includes bibliographical references.Ab initio, density functional theory, and semi-empirical methods serve as major computational tools for quantum mechanical calculations of medium to large molecular systems. Semi-empirical methods are most effectively used in a hybrid quantum mechanics/molecular mechanics (QM/MM) dynamics framework. However, semi-empirical methods have been designed to provide accurate results for organic molecules, but often fail to treat hypervalent species accurately due to their use of an sp basis. Recently, significant breakthroughs have been made with the incorporation of d-orbitals into the semi-empirical framework, thereby allowing for accurate modeling of both hypervalent and transition metal systems. Here I consider two methods that adopt this new methodology, namely AM1/d-PhoT and AM1*. Our major focus is the simulation of chemical biological and more specifically chemical glycobiological problems of biochemical interest. When I tested the ability of both AM1/d-PhoT and AM1* to reproduce key metrics in chemical glycobiology (i.e., sugar ring pucker, phosphate participation in transferase reactions) these methods, in combination with the published parameters, performed very poorly. Using the AM1/d-PhoT and AM1* Hamiltonians I set out to re-parameterize these methods aiming to produce holistic biochemical QM/MM toolsets able to simulate fundamental problems of binding and enzyme reactivity in chemical glycobiology. We called these methods AM1/d-CB1 and AM1*-CB1. In the development of these parameter sets I focused specifically on proton transfer, carbohydrate ring puckering, bond polarization, amino acid interactions, and phosphate interactions (facets important to chemical glycobiology). Both AM1/d-CB1 and AM1*-CB1 make use of a variable property optimization parameter approach for the glycan molecular class and its chemical environment. The accuracy of these methods is evaluated for carbohydrates, amino acids and phosphates present in catalytic domains of glycoenzymes, and the are shown to be more accurate for key performance indices (puckering, etc.) and on average across all simulation derived properties (QM/MM polarization, protein performance, etc.) than all other NDDO semiempirical methods currently being used. A major objective of the newly developed AM1/d-CB1 and AM1*-CB1 is to provide a platform to accurately model reactions central to chemical glycobiology using hybrid QM/MM molecular dynamics (MD) simulations. AM1/d-CB1 is applied to a well-known reaction involving purine nucleoside phosphorylase (PNP) and results lead me to conclude that the method shows promise for modelling glycobiological QM/MM systems

Applications of density functional theory (DFT) to investigate the structural, spectroscopic and magnetic properties of lanthanide(III) complexes

[Abstract] Density functional theory (DFT) has become a general tool to investigate the structure and properties of complicated inorganic molecules, such as lanthanide(III) coordination compounds, due to the high accuracy that can be achieved at relatively low computational cost. Herein, we present an overview of different successful applications of DFT to investigate the structure, dynamics, vibrational spectra, NMR chemical shifts, hyperfine interactions, excited states, and magnetic properties of lanthanide(III) complexes. We devote particular attention to our own work on the conformational analysis of LnIII-polyaminocarboxylate complexes. Besides, a short discussion on the different approaches used to investigate lanthanide(III) complexes, i. e. all-electron relativistic calculations and the use of relativistic effective core potentials (RECPs), is also presented. The issue of whether the 4f electrons of the lanthanides are involved in chemical bonding or not is also shortly discussed.Ministerio de Educación y Ciencia; CTQ2009-10721Xunta de Galicia; IN845B-2010/06

Relative energetics of acetyl-histidine protomers with and without Zn²⁺ and a benchmark of energy methods

We studied acetylhistidine (AcH), bare or microsolvated with a zinc cation by simulations in isolation. First, a global search for minima of the potential energy surface combining both, empirical and first-principles methods, is performed individually for either one of five possible protonation states. Comparing the most stable structures between tautomeric forms of negatively charged AcH shows a clear preference for conformers with the neutral imidazole ring protonated at the N-epsilon-2 atom. When adding a zinc cation to the system, the situation is reversed and N-delta-1-protonated structures are energetically more favorable. Obtained minima structures then served as basis for a benchmark study to examine the goodness of commonly applied levels of theory, i.e. force fields, semi-empirical methods, density-functional approximations (DFA), and wavefunction-based methods with respect to high-level coupled-cluster calculations, i.e. the DLPNO-CCSD(T) method. All tested force fields and semi-empirical methods show a poor performance in reproducing the energy hierarchies of conformers, in particular of systems involving the zinc cation. Meta-GGA, hybrid, double hybrid DFAs, and the MP2 method are able to describe the energetics of the reference method within chemical accuracy, i.e. with a mean absolute error of less than 1kcal/mol. Best performance is found for the double hybrid DFA B3LYP+XYG3 with a mean absolute error of 0.7 kcal/mol and a maximum error of 1.8 kcal/mol. While MP2 performs similarly as B3LYP+XYG3, computational costs, i.e. timings, are increased by a factor of 4 in comparison due to the large basis sets required for accurate results

Preparation, spectral characterization, structural study, and evaluation of antibacterial activity of Schiff base complexes for VOII, CrIII, MnII, ZnII,CdII and CeIII

A new series of metal ions complexes of VO(II), Cr(III), Mn(II), Zn(II), Cd(II) and Ce(III) have been synthesized from the Schiff bases (4-chlorobenzylidene)-urea amine (L1) and (4-bromobenzylidene)-urea amine (L2). Structural features were obtained from their elemental microanalyses, magnetic susceptibility, molar conductance, FT-IR, UV–Vis, LC-Mass and 1HNMR spectral studies. The UV–Vis, magnetic susceptibility and molar conductance data of the complexes suggest a tetrahedral geometry around the central metal ion except, VOII complexes that has square pyramidal geometry, but CrIII and CeIII octahedral geometry. The biological activity for the ligand (L1) and its Vanadium and Cadmium complexes were studied. Structural geometries of compounds also were suggested in gas phase by using theoretical treatments, using Hyper Chem-6 program for the molecular mechanics and semi-empirical calculations. The heat of formation (?Hf ?) and binding energy (?Eb) in the temperature of 298K for the free ligand (L1) and their metal complexes were calculated by PM3 and ZINDO/I methods. The electrostatic potential of the free ligands were calculated to investigate the reactive sites of the molecules.Bacteriological evaluation of considerable number of these compounds were maintained using organisms Escherichia coli and Staphylococcus aureus,and they were found to exhibit the high effect of activity. This may be attributed to the impact of both the Schiff bases and the metal present in these complexes