Roberto Salcedo

Azatrioxa[8]circulenes: Planar Anti-Aromatic Cyclooctatetraenes

We describe herein the first synthesis of a new class of anti-aromatic planar cyclooctatetraenes: the azatrioxa[8]circulenes. This was achieved by treating a suitably functionalised 3,6-dihydroxycarbazole with 1,4-benzoquinones or a 1,4-naphthoquinone. We fully characterised the azatrioxa[8]circulenes by using optical, electrochemical and computational techniques as well as by single-crystal X-ray crystallography. The results of a computational study (NICS) suggest that the central planar cyclooctatetraene is anti-aromatic when the molecules are in neutral or oxidised states (2+), and that the corresponding dianions are aromatic. We discuss the aromatic/anti-aromatic nature of the planar cyclooctatetraenes and compare them with the isoelectronic tetraoxa[8]circulenes.

Nucleus-independent chemical shift criterion for aromaticity in π-extended tetraoxa[8]circulenes

AbstractRecently synthesized π-extended symmetrical tetraoxa[8]circulenes that exhibit electroluminescent properties were calculated at the density functional theory (DFT) level using the quantum theory of atoms in molecules (QTAIM) approach to electron density distribution analysis. Nucleus-independent chemical shift (NICS) indices were used to characterize the aromaticity of the studied molecules. The tetraoxa[8]circulene molecules were found to consist of two antiaromatic perimeters (according to the Hückel “4n” antiaromaticity rule) that include 8 and 24 π-electrons. Conversely, NICS calculations demonstrated the existence of a common π-extended system (distributed like a flat ribbon) in the studied tetraoxa[8]circulene molecules. Thus, these symmetrical tetraoxa[8]circulene molecules provide examples of diatropic systems characterized by the presence of induced diatropic ring currents. FigureSpecial aromaticity of the tetraoxa[8]circulenes

Coordination modes and different hapticities for fullerene organometallic complexes.

The different coordination modes in fullerene organometallic complexes are reviewed. The main modes are η2 and η5, but there are some interesting studies about the other four, all of them are revised in order to show which is the state of art of this kind of compounds with the respect of the hapticity.

Benzene fused five-membered heterocycles. A theoretical approach

The Nuclear Independent Chemical Shift of each ring, as a criterion of aromaticity, is used to explain the stability order of benzopyrrole, benzofuran and benzothiophene, and their isomers. The results indicate that the benzene ring is aromatic in all the systems. The five-membered rings of benzopyrrole, benzofuran and benzothiophene are also aromatic, whereas those of isobenzopyrrole, isobenzofuran and isobenzothiophene are non-aromatic. This could be an explanation of the stability of the former molecules. The molecular orbitals and the condensed Fukui functions derived from the electronic structure calculations are also reported. These reactivity indices explain the expected electrophilic substitution of these compounds. The theoretical structure, ionization energies, order of aromaticity, stability and reactivity are in good agreement with the experimental results. The usefulness of this approach to determine the reactivity is discussed since their stability and reactivity may be understood. The reactivity indices are useful to explain and confirm the experimental information, and for molecules with unknown reactive behavior, this approach could help to predict some of the reactions.

Effects of substituent groups and substituted benzenes on the polymerization of phenylacetylenes initiated by di-μ-pentafluorothiophenolate bis(1,5-cyclooctadiene) rhodium(I)

In order to study the mechanism of polymerization of phenylacetylene initiated by rhodium catalysts, the polymerization of phenylacetylene was carried out in the presence of various substituted benzenes. It was found that the benzenes with electron donor substituents had little effect on the polymerization, while those with carbonyl and nitrile groups had negative effects. However, p-ethynylmethylbenzoate polymerized more rapidly than p-methoxyphenylacetylene and phenylacetylene, suggesting that the acidity of acetylenic hydrogen is most important for the polymerization. A proton transfer mechanism is proposed as a mechanism of the polymerization.

Theoretical study of the novel sandwich compound [Au3Cl3Tr2]2+

AbstractA theoretical study of a sandwich compound with a metal monolayer sheet between two aromatic ligands is presented. A full geometry optimization of the [Au3Cl3Tr2]2+ (1) compound, which is a triangular gold(I) monolayer sheet capped by chlorines and bounded to two cycloheptatrienyl (Tr) ligands was carried out using perturbation theory at the MP2 computational level and DFT. Compound (1) is in agreement with the 18–electron rule, the bonding nature in the complex may be interpreted from the donation interaction coming from the Tr rings to the Au array, and from the back-donation from the latter to the former. NICS calculations show a strong aromatic character in the gold monolayer sheet and Tr ligands; calculations done with HOMA, also report the same aromatic behavior on the cycloheptatrienyl fragments giving us an insight on the stability of (1). The Au –Au bond lengths indicate that an intramolecular aurophilic interaction among the Au(I) cations plays an important role in the bonding of the central metal sheet. Figure(a) Ground state geometry of complex 1; (b) Top view of compound 1 and Wiberg bond orders computed with the MP2/B1 computational method; (c) Lateral view of compound 1 and NICS values calculated with the MP2/B1 method; the values in parenthesis were obtained at the VWN/TZP level

Construction of simplified models to simulate estrogenic disruptions by esters of 4-hydroxy benzoic acid (parabens)

Four parabens (methyl, n-butyl, benzyl and isobutylparaben) were theoretically studied in order to evaluate their estrogenic activity through simplified models. The experimental structure of the human estrogen receptor ligand-binding domain in complex with 17beta-estradiol was used as the starting point to construct the models. The complex between 17beta-estradiol and three fragments of the estrogenic receptor (Arg, Glu and His), resulted in a reasonable simplified model of interaction. The replacement of 17-beta-estradiol by parabens was evaluated by conformational analyses and interaction energy calculations at BHandHLYP/cc-PVTZ(-f)+ level of theory. According with the calculated interaction energies, methylparaben is the paraben with higher estrogenic activity, which is in agreement with experimental studies of extraction and quantification of parabens in tumors. The antibacterial activity of parabens was also explored considering the formation of potassium salts in the phenolic OH groups. From the obtained relative energy values, methylparaben is the most active preservative.

Synthesis and polymerization of β,β-dibromo-4-ethynylstyrene ; preparation of a new polyconjugated, hyperbranched polymer

The new monomer β,β-dibromo-4-ethynylstyrene was synthesized and polymerized by the Heck reaction to give a partially soluble, conjugated hyperbranched polymer. The polymer structure was elucidated using standard spectroscopic techniques and with the aid of model compound synthesis. Theoretical calculations using the AM1 method were carried out and they showed that conjugation in the polymer is partially disrupted by twisting of the benzene rings. Both the model compound and the polymer showed luminescence.

Theoretical study on the series of [Au3Cl3M2] complexes, with M = Li, Na, K, Rb, Cs

AbstractThe prediction of the series of complexes [Au3Cl3M2] with M = Li, Na, K, Rb and Cs, has been achieved at the ab initio level of theory. All geometries were fully optimized at the MP2 level of theory; the central Au3 cluster is capped by chlorine atoms and the alkaline metals lie above and below the plane of the central ring; aurophilic interactions were found on the metal cluster, and also a strong aromatic character coming from the delocalized d-electrons of the Au atoms according to nuclear independent chemical shift calculations. On the other hand, the chemical hardness parameter was used to test the stability of the series of complexes, and the Fukui indexes of electrophilic and nucleophilic attack were employed to explore possible sites where chemical reactivity may play a role. FigureMolecular representations of the series of complexes [Au3Cl3M2] (M = Li, Na, K, Rb, Cs) and their corresponding chemical hardness

Electronic structure in different environments for vanadyl porphyrinate molecules present in crude oil

The role of lateral chains in vanadyl porphyrinate is analyzed from a theoretical point of view. Three different vanadyl porphyrinate molecules are studied in different solutions, one with aliphatic substituents another with aromatic substituents and the molecule without substituents for comparison. The analysis of the results is mainly based on the behavior in different solvents, aromaticity measures, frontier molecular orbital changes and charge behavior. The isolated molecule is compared with the same molecule immersed in three different solvents, with low, medium and high values of dipole moment, tetrachloromethane, pyridine and acetone, respectively. The reactivity of the complexes is improved with the presence of the substituents without relevant aliphatic or aromatic nature, therefore, it is expected that the substituted molecules will be more pollutant to the catalysts than the molecule without substituents.