A structural and corrosion study of triethoxysilyl functionalized POSS coatings on AA 2024 alloy

A novel bifunctional polyhedral oligomeric silsesquioxane (POSS) based silane precursor R(x)R'(y)(SiO(3/2))(8), (x + y = 8), bearing 3-(N-(3-triethoxysilylpropyl)ureido)propyl (ureasil - U) and isooctyl (IO) groups (i.e., U(2)IO(6) POSS) was synthesized, and the corresponding coatings, prepared under the acid hydrolysis conditions, were studied in order to assess their corrosion inhibition of the AA 2024-T3 alloy. The U(2)IO(6) POSS precursor was made in two steps: in the first, an appropriate stoichiometric (2:6) mixture of 3-aminopropyltriethoxysilane (AP(2)) and isooctyltrimethoxysilane (IO(6)) was autoclaved under basic hydrolysis conditions giving AP(2)IO(6)(SiO(3/2))(8) cubes, which were reacted in the second step with 3-isocyanatopropyltriethoxysilane (ICPTES), leading to the bis end-capped sol-gel precursor U(2)IO(6) POSS having a cube-like structure. Coatings were made from sols catalyzed with acidified water. IR and (29)Si NMR spectroscopic studies combined with mass spectrometric measurements were employed to confirm the cube-like structure of AP(2)IO(6) and U(2)IO(6) POSS. The structure and morphology of the U(2)IO(6) POSS coatings were studied with the help of infrared reflection-absorption (IR RA) spectroscopic measurements combined with XPS and AFM measurements, providing information about the formation of partially self-assembled coatings. The degree of corrosion inhibition was assessed from the potentiodynamic measurements showing around 10 times smaller current densities for the coatings only 30-40 nm thick. Ex situ IR RA spectroelectrochemical measurements were performed by consecutive measurements of the IR RA spectra of U(2)IO(6) POSS coatings which were chronocoulometrically charged at different potentials. At potentials more positive than the corrosion potential (E(corr) approximately -0.5 V), the amide I bands shifted, indicating the formation of new urea-urea aggregations and associations, with the newly formed strong band at 1680-1690 cm(-1) suggesting the formation of amidonium ions. These results showed that the urea groups represented the weakest part of the coatings due to their tendency to protonation.     

Structural determination of salsolins A and B, new antioxidant polyoxygenated triterpenes from Salsola baryosma, by 1D and 2D NMR spectroscopy

Salsolins A (1) and B (2), the new triterpenes, have been isolated from the chloroform soluble fraction of Salsola baryosma along with 2alpha,3beta,23,24-tetrahydroxyurs-12-en-28-oic acid (3) reported for the first time from this species. Their structures have been assigned from 1H and 13C NMR spectra, DEPT and by 2D COSY, NOESY, HMQC and HMBC experiments. The compounds 1-3 showed significant antioxidant activity.     

Development and implementation of <Emphasis Type="Italic">k</Emphasis><Subscript>0</Subscript>-INAA standardization at PINSTECH

The k ₀ method has been introduced at the 30 kW miniaturized neutron source reactor (MNSR) at the Pakistan Institute of Nuclear Science & Technology (PINSTECH). It involved the full energy peak efficiency calibration of the HPGe detector for different counting geometries and the characterization of the neutron flux at four inner irradiation channels. The latter involved the determination of the thermal to the epithermal flux ratio, epithermal flux shape factor, the modified spectral index, Westcott’s g-factor, the Maxwellian neutron temperature and the fast flux. The method was validated by analyzing IAEA-SL1 (Lake Sediment) and NIST-SRM-1572 (Citrus Leaves) reference materials. All calculations were performed in Excel, including the optimization step. The results revealed that most of the elements were estimated with less than 10% relative deviation from the certified value.     

Potentiostatic voltammetry at spherical electrodes and microelectrodes in the presence of product

An explicit analytical solution for the complete anodic–cathodic I–E–t response of a reversible electrode process in presence/absence of amalgamation under potentiostatic conditions is presented. To obtain this solution we have taken into account the electrode curvature, using in the case of amalgamation, Koutecký’s approximation (i.e. the finite electrode volume has not been considered). Explicit expressions for the concentration profiles and the surface concentrations have been also deduced. All the results are applicable to electrodes of any radius including ultramicroelectrodes when both species are soluble in the electrolytic solution for any value of the ratio D_O/D_R. When amalgamation takes place, analytical results were compared with the numerical ones deduced by using the rigorous condition of null net flux in the electrode centre, pointing out that Koutecký’s approximation remains valid even for electrode radius and time values at which the diffusion layer reaches the electrode centre. For high electrode sphericities the I–t curves present a cross-linking for applied potential values higher than the equilibrium potential which is the cause of the appearance of a “peak” near to the anodic limit of the I–E curve.     

An efficient and expeditious microwave‐assisted synthesis of quinoxaline derivatives via nucleophilic heteroaromatic substitution

A series of 2‐substituted quinoxaline derivatives including five novel compounds have been successfully synthesized from 2‐chloroquinoxaline using microwave methodology. The yields of the quinoxalines synthesized through this method, were an improvement over the thermal methods usually employed.     

Palladium-catalyzed intramolecular C-arylation of benzylic carbon: synthesis of 3-benzoxazolylisoindolinones by a sequence of Ugi-4CR/postfunctionalization

Submitting Ugi-adduct 4 to two consecutive metal-catalyzed intramolecular reactions, namely copper-catalyzed O-arylation and palladium-catalyzed C-arylation of benzylic carbon developed in the course of this study, affords the benzoxazolylisoindolinones in good to excellent yields.     

Site-selective catalysis of phenyl thionoformate transfer as a tool for regioselective deoxygenation of polyols

We report the application of peptide-embedded imidazoles as catalysts for the site-selective delivery of the phenyl thionoformate unit as a prelude to deoxygenation reactions of polyols. Methodology was developed that allows for the synthesis of thiocarbonyl derivatives based on a combination of additives that include N-alkylimidazoles and FeCl3 as co-catalysts. The use of this reagent combination leads to increased reaction rates and efficient yields relative to those of simple base-mediated reactions. In terms of controlling regioselectivity during the course of polyol modification, we found that histidine-containing peptides, in combination with FeCl3, could lead to modulation of the product distribution. Through screening of peptides and control of reaction conditions, products could be observed that reflected both the inherent preference of substrates and also reversal of inherent selectivity.     

Robust and efficient, yet uncatalyzed, synthesis of trialkylsilyl-protected cyanohydrins from ketones

High-yielding cyanosilylation of ketones with NaCN and various chlorotrialkylsilanes in DMSO proceeds smoothly without catalysis to give silyl-protected ketone cyanohydrins. The unique role of DMSO consists in rendering naked cyanide anions that reversibly add to the C double bond O bond at the rate-determining step followed by fast trapping of the transient tertiary sodium cyanoalcoholates with chlorotrialkylsilanes or in situ generated cyanotrialkylsilanes. Preparatively, the reaction matches the best known catalytic cyanosilylation systems applying expensive Me(3)SiCN and demonstrates unprecedented efficiency in the synthesis of sterically congested trialkylsilyl-protected cyanohydrins.     

Exploiting soft and hard X-ray absorption spectroscopy to characterize metallodrug/protein interactions: the binding of [trans-RuCl4(Im)(dimethylsulfoxide)][ImH] (Im = imidazole) to bovine serum albumin

The reaction of bovine serum albumin (BSA) with [ trans-RuCl 4(Im)(dimethylsulfoxide)][ImH] (Im = imidazole) (NAMI-A), an experimental ruthenium(III) anticancer drug, and the formation of the respective NAMI-A/BSA adduct were investigated by X-ray absorption spectroscopy (XAS) at the sulfur and chlorine K-edges and at the ruthenium K- and L 3-edges. Ruthenium K and L 3-edge spectra proved unambiguously that the ruthenium center remains in the oxidation state +3 after protein binding. Comparative analysis of the chlorine K-edge XAS spectra of NAMI-A and NAMI-A/BSA, revealed that the chlorine environment is greatly perturbed upon protein binding. Only modest changes were observed in the sulfur K-edge spectra that are dominated by several protein sulfur groups. Overall, valuable information on the nature of this metallodrug/protein adduct and on the mechanism of its formation was gained; XAS spectroscopy turns out to be a very suitable method for the characterization of this kind of systems.     

CuI complexes with N,N',S,S' scorpionate ligands: evidence for dimer-monomer equilibria

The heteroscorpionate N, N', S, S' donor ligands 4-methoxy-3,5-dimethyl-2-(3-(methylthio)-1-(3-(2-(methylthio)phenyl)-1H-pyrazol-1-yl)propyl)pyridine (L(a)) and 4-methoxy-3,5-dimethyl-2-(2-(methylthio)-1-(3-(2-(methylthio)phenyl)-1H-pyrazol-1-yl)ethyl)pyridine (L(b)) were prepared. The Cu(I) complexes [Cu(L(a))]2(BF4)2 (a2(BF4)2) and [Cu(L(b))]2(BF4)2 (b2(BF4)2) were synthesized and characterized by X-ray crystallography. Both compounds exhibit a dinuclear structure, presenting each Cu(I) center in a distorted N, N', S, S' tetrahedral environment. On the basis of nuclear magnetic resonance (NMR) and ESI-mass data, the presence of a mononuclear complex in equilibrium with the dimer was hypothesized for both complexes. The dimerization constants of the processes, 2a(+) = a2(2+) and 2b(+) = b2(2+) , were obtained by (1)H NMR dilution experiments (fast-exchange regime) in CD 3CN: log K(a2(2+)) = 3.55(6) and log K(b2(2+)) = 3.23(5) at 300 K. Thermodynamic parameters were determined by a van't Hoff analysis (280-310 K temperature range): DeltaH(0)(a2(2+)) = -12(1) kJ mol (-1), DeltaH(0)(b2(2+)) = -10(1) kJ mol(-1), DeltaS(0)(a2(2+)) = +27(4) kJ mol (-1), and DeltaS(0)(b2(2+)) = +28(4) kJ mol (-1). Pulsed gradient spin-echo (PGSE) NMR experiments provided the weighted-average hydrodynamic volume (VH) of the species present in CD 3CN solution at different copper concentrations (CCu). Nonlinear interpolation of VH as a function of C Cu for a dimer-monomer equilibrium led to the hydrodynamic volumes of both monomers (VH(0)(M)) and dimers (VH(0)(D)): VH(0)(a(+)) = 620(40) A(3), VH(0)(b(+)) = 550(10) A(3), VH(0)(a2(2+)) = 950(20) A(3), and VH(0)(b2(2+)) = 900(10) A(3). Cyclic voltammetry experiments performed in CH3CN and CH2Cl2 showed a quasi-reversible to irreversible behavior of the Cu(I)/Cu(II) redox couple for both complexes.