Entity

Time filter

Source Type

CQB
Havana, Cuba

Veiros L.F.,University of Lisbon | Honzicek J.,New University of Lisbon | Romao C.C.,New University of Lisbon | Calhorda M.J.,CQB
Inorganica Chimica Acta | Year: 2010

Spiro[2,4]hepta-4,6-diene and spiro[4,4]nona-1,3-diene react with [Mo(CO)2Cp'(NCCH3)2][BF4] (Cp' = Cp, Ind; Ind = η5-C9H7) to afford the corresponding diene complex [Mo(diene)(CO)2Cp′]+. When Cp' = Ind, the reaction proceeded forward leading to ring opening in the case of the small spiro ring. Although this and another product resulting from migration of the side arm to the carbonyl were detected when Cp' = Cp, they did not form from the diene complex. A DFT/PBE1PBE study was carried out and showed a kinetically controlled reaction pathway leading from the [Mo(diene)(CO)2Ind]+ to the reaction product, with an activation barrier of 21.3 kcal mol-1. The thermodynamic preferred species was the non-observed complex (insertion), and its formation required higher barriers. In the presence of Cp, all the barriers increased significantly, explaining the inertness of the initial diene complex. The interpretation of this behaviour is associated with the ease of the η5 → η3 haptotropic rearrangement of the indenyl, which helps to lower some relevant barriers. This route is not available for the Cp analogue. © 2009 Elsevier B.V. All rights reserved. Source


Ribeiro-Claro P.J.A.,University of Aveiro | Vaz P.D.,CQB | Nolasco M.,University of Aveiro
Journal of Molecular Structure: THEOCHEM | Year: 2010

In this work, the usefulness of the Pairs in Molecular Materials (PiMM) approach to elucidate unknown structures of molecular crystals is assessed. This methodology aims to assist the prediction of probable crystal structures from the vibrational spectra, through the identification of the most relevant motifs in the potential energy landscape of the crystal. The method is successfully applied to the elucidation of 4-(dimethylamino)benzaldehyde crystal structure, considered to be "unknown" to illustrate the applicability of the PiMM approach. The proposed crystal structure, based on weak CH⋯O hydrogen bonding and π-stacking interactions, is in full agreement with the recently reported crystal structure. © 2010 Elsevier B.V. All rights reserved. Source


Paixao V.,Polytechnic Institute of Lisbon | Carvalho A.P.,CQB | Rocha J.,University of Aveiro | Fernandes A.,University of Lisbon | Martins A.,Polytechnic Institute of Lisbon
Microporous and Mesoporous Materials | Year: 2010

The effect of several desilication experimental parameters (base concentration, temperature and time) on the characteristics of MOR zeolite was studied. The samples were characterized by X-ray diffraction, 27Al and 29Si MAS-NMR, chemical analysis, and FTIR (framework vibration region). The textural characterization was made by N2 adsorption and the acidity was evaluated by pyridine adsorption followed by FTIR and by the catalytic model reaction of n-heptane cracking. The alkaline treatments promoted the Si extraction from the zeolite framework, without considerable loss of crystallinity and, as it was envisaged, an important increase of the mesoporous structure was attained. A linear correlation between the number of framework Si per unit cell, NSi, and the asymmetric stretching wavenumber, ν1, was observed. The acidity characterization shows that the desilicated samples exhibit practically the same acid properties than the parent HMOR zeolite. The optimum desilication conditions were those used to obtain sample M/0.2/85/2, i.e., sample treated with 0.2 M NaOH solution at 85 °C for 2 h. © 2010 Elsevier Inc. All rights reserved. Source


Melato A.I.,CQB | Abrantes L.M.,CQB | Botelho do Rego A.M.,Institute of Nanoscience and Nanotechnology IN
Thin Solid Films | Year: 2010

Poly(3,4-ethylenedioxythiophene) (PEDOTh) films were potentiodynamically deposited on platinum from tetrabutylammonium hexafluorophosphate/acetonitrile solutions. Polymers prepared with different number of cycles showed reversible redox behaviour and the X-ray Photoelectron Spectroscopy (XPS) characterization revealed the existence of zones with different conductivity and confirmed the presence of PF6 -. The incorporation of the metallic complex Fe(CN)6 3- in the PEDOTh films was made in one step, after the electrosynthesis of a film grown with a given number of potential cycles, and by means of polymerization/incorporation sequences. XPS data confirmed the presence of the inorganic species and the highest Fe 2p3/2 peak intensity was observed for thin films (30 cycles) when the one step incorporation has been used. The XPS results suggest a partial dissociation of the Fe-(C-N) bond of the complex in the modified electrodes. © 2009 Elsevier B.V. All rights reserved. Source


Quintal S.,CQB | Concepcion Gimeno M.,University of Zaragoza | Laguna A.,University of Zaragoza | Calhorda M.J.,CQB
Journal of Organometallic Chemistry | Year: 2010

The reactions between five ferrocenyl derivatives containing both a C{double bond, long}O and at least an imidazole or pyridine nitrogen atom and AgPF6, AgOTf, or [Cu(NCCH3)4]PF6 precursors were studied. The ligand {[bis(2-pyridyl)amino]carbonyl}ferrocene (L3), derived from (2-pyridyl)amine, favored tetrahedral coordination of Ag+ (with two ligands) and of Cu+ (with two acetonitrile ligands left from the precursor). In all the other ligands, both metal centers coordinated linearly to two ligands, preferring the imidazole or pyridinic nitrogen to other nitrogen atoms (amine) or oxygen donors. When the counter anions were triflate, the crystal structure showed a dimerization of the complex, with the ferrocenyl moieties occupying cis positions, by means of a weak Ag⋯Ag interaction. This was shown experimentally in the crystal structure of complex [Ag(L1)2]OTf (L1 = ferrocenyl imidazole) and in the presence of peaks corresponding to {Ag2(L2)3(OTf)}+ and {Ag2(L2)4(OTf)}+ in the mass spectra of [Ag(L2)2]OTf (L2 = ferrocenyl benzimidazole). In all complexes containing PF6, there was no evidence for dimerization. Indeed, in the crystal structure of [Ag(L2)2]PF6, the ferrocenyl moieties occupy trans positions and the metal centers are far from each other. DFT calculations showed that the energy of the cis and trans conformers is practically the same and the balance of crystal packing forces leads to dimerization when triflate is present. © 2009 Elsevier B.V. All rights reserved. Source

Discover hidden collaborations