The preparation and characterization of novel cocylic(arylene disulfide) oligomers

A series of homo‐ and cocyclic(arylene disulfide) oligomers were synthesized under high dilution conditions by the catalytic oxidation of arylenedithiols with oxygen in the presence of a copper‐amine catalyst in DMAc. The aryl groups contained moieties such as sulfone, ether, and ketone. The free radical ring‐opening polymerization of these cyclic(arylene disulfide) oligomers led to the formation of linear poly(thio arylene)s. The homo‐ and cocyclic(arylene disulfide) oligomers were characterized by gradient high pressure liquid chromatography (HPLC), get permeation chromatography (GPC), ¹H‐NMR, and differential scanning calorimetry (DSC) methods. These cocyclic(arylene disulfide) oligomers except those containing sulfone moiety had lower melt flow temperature as low as 140 °C and therefore could readily undergo free radical ring‐opening polymerization under mild conditions. The glass transition temperatures of these cocyclics ranged from 72.3 to 190.0 °C, while the glass transition temperatures of the polydisulfides derived from these cocyclics ranged from 78.4 to 194.5 °C. In this article, a new method of preparing arylene dithiols 4,4′‐oxybis(benzenethiol) and diphenylmethane‐4,4′‐dithiol is reported. Copyright © 2003 John Wiley & Sons, Ltd.     

Theoretical studies on the redox potentials of Fe dinuclear complexes as models for hydrogenase

Density Functional calculations have been performed at the uB3LYP and uBP86 levels to calculate the one-electron redox potentials for a series of small models based on the diiron hydrogenase enzymes in the presence of acetonitrile (MeCN). The solvation effects in MeCN are incorporated via a self-consistent reaction field (SCRF) using the polarized continuum model (PCM). The calculated redox potentials reproduce the trends in experimental data with an average error of only 0.12 V using the BP86 functional, whereas comparing results with the B3LYP functional require a systematic shift of -0.82 and -0.53 V for oxidation and reduction, respectively. The bonding orbitals and d-electron populations were examined using Mulliken population analysis, and the results were used to rationalize the calculated and observed redox potentials. These studies demonstrate that the redox potential correlates with the empirical spectrochemical series for the ligands, as well as with the amount of electron density donated by the ligand onto the Fe centers.     

Complete 1H, 13C and 15N NMR assignment of tirapazamine and related 1,2,4-benzotriazine N-oxides

1H, 13C and 15N NMR measurements (1D and 2D including 1H--15N gs-HMBC) have been carried out on 3-amino-1, 2,4-benzotriazine and a series of N-oxides and complete assignments established. N-Oxidation at any position resulted in large upfield shifts of the corresponding N-1 and N-2 resonances and downfield shifts for N-4 with the exception of the 3-amino-1,2,4-benzotriazine 1-oxide in which a small upfield shift of N-4 was observed. Density functional GIAO calculations of the 15N and 13C chemical shifts [B3LYP/6-31G(d)//B3LYP/6-311+G(2d,p)] gave good agreement with experimental values confirming the assignments. The combination of 13C and 15N NMR provides an unambiguous method for assigning the 1H and 13C resonances of N-oxides of 1,2,4-benzotriazines.     

Organic-inorganic hybrid nanoparticles: surface characteristics and interactions with a polyester resin

Organic-inorganic hybrid nanoparticles, derived from silica precursors with different organic functionalities (methyl, ethyl, vinyl, and phenyl) synthesized via a modified St?ber method have been investigated. These particles are intended as modifiers for polymers and polymer matrix composites. Therefore, the characteristics of a polyester matrix have also been determined, and the likely interactions with the particles have been proposed. Particles have been characterized using inverse gas chromatography (IGC), X-ray photoelectron spectroscopy (XPS), and infrared spectroscopy (FT-IR). The particles show two different sets of characteristics, with methyl, ethyl, and vinyl modified silicas showing one type of behavior and the phenyl modified silica behaving rather differently. The methyl, ethyl, and vinyl groups exhibit the appearance of uniform coverage, as they are comparatively small and tightly packed, which will prevent interaction of matrix resin with retained silanol groups. The phenyl group, which is comparatively large, is not able to pack as closely, which results in a reduction of the presence and availability of silanol groups, compared to an unmodified fumed silica, but not complete inaccessibility as far as the matrix resin is concerned.     

Alkali-metal mediated zincation of N-heterocyclic substrates using the lithium zincate complex, (THF)Li(TMP)Zn(tBu)2 and applications in in situ cross coupling reactions

This study investigates the ability of the mixed-metal reagent [Li(TMP)Zn(tBu)₂] 1 to promote direct Zn-H exchange reactions (zincations) of a wide range of N-heterocyclic molecules. The generated metallated intermediates from these reactions are intercepted with I₂ and some of them are also employed as precursors in Pd-catalysed Negishi cross-coupling applications. A comparison with recent precedents in metallation chemistry reveals that for some of these heterocycles, 1 allows improved conversions, under milder conditions and in certain cases, even gives unique regioselectivities.     

A flexible synthesis of C-6 and N-1 analogues of a 4-amino-1,3-dihydroimidazo[4,5-c]pyridin-2-one core

A flexible route which enables access to derivatives of 4-amino-1,3-dihydroimidazo[4,5-c]pyridin-2-ones is described. Issues of selectivity, reaction safety, and low yields in original routes are overcome with the key improvements to the route, including a Negishi cross-coupling and use of a carbamate as a protecting group and intrinsic carbonyl source. The new route enables variation of C-6 and N-1 substituents.     

Theoretical study of O--O single bond formation in the oxidation of water by the ruthenium blue dimer

The first key step in the oxidation of water to O(2) by the oxidized species [(bpy)(2)(O)Ru(V)ORu(V)(O)(bpy)(2)](4+) of the Ru blue dimer is studied using density functional theory (DFT) and an explicit solvent treatment. In the model reaction system [L(2)(O)Ru(V)ORu(V)(O)L(2)](4+)·(H(2)O)(4)·W(76), the surrounding water solvent molecules W are described classically while the inner core reaction system is described quantum mechanically using smaller model ligands (L). The reaction path found for the O--O single bond formation involves a proton relay chain: direct participation of two water molecules in two proton transfers to yield the product [L(2)(HOO)Ru(IV)ORu(IV)(OH)L(2)](4+)·(H(2)O)(3)·W(76). The calculated ～3 kcal/mol reaction free energy and ～15 kcal/mol activation free energy barrier at 298 K are consistent with experiment. Structural changes and charge flow along the intrinsic reaction coordinate, the solvent's role in the reaction barrier, and their significance for water oxidation catalysis are examined in detail.     

Role of the uranyl oxo group as a hydrogen bond acceptor

Density functional theory calculations have been used to evaluate the geometries and energetics of interactions between a number of uranyl complexes and hydrogen bond donor groups. The results reveal that although traditional hydrogen bond donors are repelled by the oxo group in the [UO(2)(OH(2))(5)](2+) species, they are attracted to the oxo groups in [UO(2)(OH(2))(2)(NO(3))(2)](0), [UO(2)(NO(3))(3)](-), and [UO(2)Cl(4)](2-) species. Hydrogen bond strength depends on the equatorial ligation and can exceed 15 kcal mol(-1). The results also reveal the existence of directionality at the uranyl oxo acceptor, with a weak preference for linear U═O---H angles.     

Application of Phosphorus-31 Nuclear Magnetic Resonance Spectrosocopy for the Study of Human Diabetic Cataractogenesis

Intact human Senses incubated at 5.5 mM (normal) and 35.5 mM glucose were examined by phosphorus-31 nuclear magnetic resonance (31P NMR) spectroscopy. Lense in 35.5 mM glucose showed an altered metabolic steady-state characterized by a lowered adenosine triphosphate/inorganic orthophosphate ratio. ³¹P NMR spectroscopy can be used to measure metabolic changes in the lens. This model offers an important means to study dynamic metabolism in the human lens in the setting of diabetic cataractogenesis.     

Systematic studies of early actinide complexes: thorium(IV) fluoroketimides

Reaction of (C5Me5)2Th(CH3)2 with 2 equiv of NC-ArF gives the corresponding fluorinated thorium(IV) bis(ketimide) complexes (C5Me5)2Th[-N=C(CH3)(ArF)]2 (where ArF = 3-F-C6H4 (4), 4-F-C6H4 (5), 2-F-C6H4 (6), 3,5-F2-C6H3 (7), 3,4,5-F3-C6H2 (8), 2,6-F2-C6H3 (9), 2,4,6-F3-C6H2 (10), and C6F5 (11)). The complexes have been characterized by a combination of single-crystal X-ray diffraction, cyclic voltammetry and NMR, and UV-visible absorption and low-temperature luminescence spectroscopies. Density functional theory (DFT) and time-dependent DFT (TD-DFT) results are reported for complexes 5, 11, and (C5Me5)2Th[-N=C(Ph)2]2 (1) for comparison with experimental data and to guide in the interpretation of the spectroscopic results. The most significant structural perturbation imparted by the fluorine substitution in these complexes is a rotation of the fluorophenyl group (ArF) out of the plane defined by the N=C(CMe)(Cipso) fragment in complexes 9-11 when the ArF group possesses two ortho fluorine atoms. Excellent agreement is obtained between the optimized ground state DFT calculated structures and crystal structures for 11, which displays the distortion, as well as 5, which does not. In complexes 9-11, the out-of-plane rotation results in large interplanar angles (phi) between the planes formed by ketimide atoms N=C(CMe)(Cipso) and the ketimide aryl groups in the range phi = 49.1-88.8 degrees , while in complexes 5, 7, and 8, phi = 5.7-34.9 degrees . The large distortions in 9-11 are a consequence of an unfavorable steric interaction between one of the two ortho fluorine atoms and the methyl group [-N=C(CH3)] on the ketimide ligand. Excellent agreement is also observed between the experimental electronic spectroscopic data and the TD-DFT predictions that the two lowest lying singlet states are principally of nonbonding nitrogen p orbital to antibonding C=N pi* orbital (pN-->pi*C=N or npi*) character, giving rise to moderately intense transitions in the mid-visible spectral region that are separated in energy by less than 0.1 eV. Low-temperature (77 K) luminescence from both singlet and triplet excited states are also observed for these complexes. Emission lifetime data at 77 K for the triplet states are in the range 50-400 mus. These emission spectral data also exhibit vibronic structure indicative of a small Franck-Condon distortion in the ketimide M-N=C(R1)(R2) linkage. Consistent with this vibronic structure, resonance enhanced Raman vibrational scattering is also observed for (C5Me5)2Th[-N=C(Ph)(CH2Ph)]2 (2) when exciting into the visible excited states. These systems represent rare examples of Th(IV) complexes that engender luminescence and resonance Raman spectral signatures.