Eduardo L. de Sá

Synthesis, characterization and chemoprotective activity of polyoxovanadates against DNA alkylation

The alkylation of pUC19 plasmid DNA has been employed as a model reaction for the first studies on chemoprotective action by a mixed-valence (+IV/+V) polyoxovanadate. A new, non-hydrothermal route for the high yield preparation of the test compound is described. The deep green, microcrystalline solid A was isolated after a three-day reaction in water at 80°C and 1 atm, while the reaction at 100°C gave green crystals of B. Both solids were structurally characterized by X-ray diffractometry and FTIR, EPR, NMR and Raman spectroscopies. Product A was identified as (NH(4))(2)V(3)O(8), while B corresponds to the spherical polyoxoanion [V(15)O(36)(Cl)](6-), isolated as the NMe(4)(+) salt. The lack of solubility of A in water and buffers prevented its use in DNA interaction studies, which were then carried out with B. Complex B was also tested for its ability to react with DNA alkylating agents by incubation with diethylsulphate (DES) and dimethylsulphate (DMS) in both the absence and presence of pUC19. For DMS, the best results were obtained with 10 mM of B (48% protection); with DES, this percentage increased to 70%. The direct reaction of B with increasing amounts of DMS in both buffered (PIPES 50 mM) and non-buffered aqueous solutions revealed the sequential formation of several vanadium(IV), vanadium(V) and mixed-valence aggregates of different nuclearities, whose relevance to the DNA-protecting activity is discussed.

Novel double alkoxides of titanium(IV) and iron(II)/(III): synthetic, structural and spectroscopic studies

The first structurally characterised titanium and iron polynuclear isopropoxides [FeCl{Ti2(OPri)9}], 1, and [Ti3(OMe)2(OPri)9][Fe4TiCl4(O)(OPri)9], 2, have been prepared from 2 : 1 : 1 mixtures of [Ti(OPri)4], K(OPri) and FeCl2. Their identity was confirmed by a number of chemical and physical methods including X-ray diffractometry. Semiempirical quantum mechanical calculations were consistent with an S = 2 ground state for 1. Structural and spectroscopic data support a mixed-valence formulation for the unique {Fe4Ti} anion in 2.

Investigating the nature of noble gas-copper bonds by the quantum theory of atoms in molecules.

We investigated noble gas-copper bonds in linear complexes represented by the NgCuX general formula in which Ng and X stand for a noble gas (neon, argon, krypton, or xenon) and a halogen (fluorine, chlorine or bromine), respectively, by coupled cluster methods and modified cc-pVQZ basis sets. The quantum theory of atoms in molecules (QTAIM) shows a linear relation between the dissociation energy of noble gas-copper bonds and the amount of electronic charge transferred mainly from the noble gas to copper during complexation. Large changes in the QTAIM quadrupole moments of copper and noble gases resulting from this bonding and a comparison between NgCuX and NgNaCl systems indicate that these noble gas-copper bonds should be better interpreted as predominantly covalent. Finally, QTAIM atomic dipoles of noble gases in NgNaCl systems agree satisfactorily with atomic dipoles given by a simple model for these NgNa van der Waals bonds.

Lactones and quinones from the tubers of Sinningia aggregata.

Three new aromatic epsilon-lactones, aggregatins A (1), B (2), and C (3), a new naphthoquinone derivative, aggregatin D (4), and three known anthraquinones, 2-methylanthraquinone, 7-methoxy-2-methylanthraquinone, and 7-hydroxy-2-methylanthraquinone, were isolated from the tubers of Sinningia aggregata (Gesneriaceae). Compounds 1 and 4 and the anthraquinones showed marginal antimicrobial activity.

The first hetero-binuclear alkoxide of iron and vanadium: structural and spectroscopic features

Abstract The immediate reaction between [V2(μ-OPri)2(OPri)6] (A) and [Fe2(μ-I)2I2(HOPri)4] (B) in toluene/propan-2-ol gives the novel mixed-metal [FeI2(μ-OPri)2V(OPri)2(HOPri)] (C). Structural, spectroscopic and magnetochemical characterisation are all consistent with a non-oxo vanadium(IV)/high-spin iron(II) formulation; variable-temperature X-band EPR spectra reveal an S=1/2↔S=1 equilibrium involving the vanadium centres in propan-2-ol solution.

Naphthochromenes and Related Constituents from the Tubers of Sinningia allagophylla

Chemical investigation of the tubers of Sinningia allagophylla led to the isolation of two new chromenes, (2S)-12-hydroxylapachenole (1) and (3R)-3,4-dihydro-3-hydroxy-4-oxo-8-methoxylapachenole (2), and three new dimeric chromenes, allagophylldimers A-C (3-5). Thirteen known compounds, 6-methoxy-7,8-benzocoumarin (6), lapachenole, 8-methoxylapachenole, tectoquinone, 7-hydroxytectoquinone, dunniol, α-dunnione, dunnione, 8-hydroxydunnione, aggregatin E, cedrol, oleanolic acid, and halleridone, were also identified. 6-Methoxy-7,8-benzocoumarin (6) has been isolated for the first time from a natural source.

2-Mercaptobenzoxazole pentacyanoferrate(II/III) complexes: UV-Visible, Mössbauer, electron paramagnetic resonance, electrochemistry and molecular modeling

2-Mercaptobenzoxazole pentacyanoferrate(II/III) complexes, [FeII/III(CN)5(bzoxs)]3-/2- , were prepared in MeOH/H2O 75:25% solutions and characterized by spectroscopic UV-Vis, Mossbauer, electron paramagnetic resonance (epr) and electrochemical-cyclic voltammetry- techniques. UV-Vis and epr spectra along with the electrochemical behavior suggested the coordination of the multi-functional N,S,O- donor ligand, bzoxs, to iron(III) through the sulfur atom. The crystal field parameters, DqL and Dt, calculated for the iron(II) complex, in addition to the reversible redox process FeIII-bzoxs + e- ® FeII-bzoxs also pointed to coordination via the sulfur atom. The results were compared with the chemical properties of pentacyanoferrate complexes containing other monodentate N-, S- and O-donor ligands. Ab initio calculations revealed the composition of the frontier orbitals of bzoxs and are in agreement with the mode of coordination proposed from the experimental data.

New titanium(IV) and vanadium(IV) haloalkoxides: synthetic route and structural characterisation

The novel binuclear titanium(IV) complex [{TiCl(OPri)2(HOPri )}2(m-OPri)2], 1, and the first mononuclear vanadyl(IV) chloroalcoholate [VCl(O)(HOPri)4]Cl, 2, have been prepared from 2:1:1 mixtures of [Ti(OPri)4], KOPri and VCl3. Products were characterised by elemental analysis, FTIR, EPR, NMR and electronic spectroscopies, magnetic susceptibility measurements and single crystal X-ray diffractometry. A third product was found to be polynuclear and to contain both titanium and vanadium. Semi-empirical quantum-mechanical calculations were carried out for the evaluation of the electronic structure of 1.

Catalytic Activity of a Titanium(IV)/Iron(II) Heterometallic Alkoxide in the Ring‑Opening Polymerization of ε‑Caprolactone and rac‑Lactide

The activity of the heterometallic alkoxide [FeCl{Ti2(OiPr)9}] (1) towards polymerization of rac-lactide (rac-LA) and e-caprolactone (e-CL) was investigated in toluene solution and in bulk at various temperatures, monomer/heterometallic alkoxide molar ratio and reaction times. The alkoxide 1 was active in solution for e-CL and in bulk for both monomers. Polymers were obtained in good yields with molecular weights ranging from 3890 to 15000 g moL-1 and polydispersity indexes (PDI) values varying from 1.3 to 2.5. Based on the 1H nuclear magnetic resonance (NMR) end-group analysis of polymers, a coordination-insertion mechanism was suggested for both monomers. The average number of growing chains per molecule of initiator (4 to 5 for rac-LA and 7 to 8 for e-CL) indicates that both bridging and terminal isopropoxides are active initiating groups. Kinetic studies with e-CL indicated that the polymerization rate is first order with respect to monomer concentration. The catalytic properties of 1 were compared to those found for other titanium(IV) and iron(II) complexes using e-CL as a model monomer.

Termocromismo em soluções de alcóxidos de Vanádio(IV): uma abordagem pela modelagem molecular

The thermochromic behavior exhibited by vanadium(IV) alkoxides, [V2(μ-OPri)2(OPri) 6] and [V2(μ-ONep)2(ONep)6 ], OPri = isopropoxide and ONep = neopentoxide, was studied by molecular modeling using DFT, TDDFT and INDO/S methods. The vibrational and electronic spectra calculated for [V2(μ-OPri)2(OPri) 6] were very similar to the experimental data registered for crystalline samples of the complex and for its solutions at low temperature ( 315 K) were compatible with those calculated for the monomeric form, [V(OPri)4]. These results consistently point to a monomer/dimer equilibrium as an explanation for the solution thermochromism of {V(OPri)4}n. In spite of the structural similarity between [V2(μ-ONep)2(ONep)6 ] and [V2(μ-OPri)2(OPri) 6] in the solid state, the thermochromic behavior of the former could not be explained by the same model, and the possibility of tetranuclear aggregation at low temperatures was also investigated.