N,N'-ethylenebis(pyridoxylideneiminato) and N,N'-ethylenebis(pyridoxylaminato): synthesis, characterization, potentiometric, spectroscopic, and DFT studies of their vanadium(IV) and vanadium(V) complexes

The Schiff base N,N'-ethylenebis(pyridoxylideneiminato) (H(2)pyr(2)en, 1) was synthesized by reaction of pyridoxal with ethylenediamine; reduction of H(2)pyr(2)en with NaBH(4) yielded the reduced Schiff base N,N'-ethylenebis(pyridoxylaminato) (H(2)Rpyr(2)en, 2); their crystal structures were determined by X-ray diffraction. The totally protonated forms of 1 and 2 correspond to H(6)L(4+), and all protonation constants were determined by pH-potentiometric and (1)H NMR titrations. Several vanadium(IV) and vanadium(V) complexes of these and other related ligands were prepared and characterized in solution and in the solid state. The X-ray crystal structure of [V(V)O(2)(HRpyr(2)en)] shows the metal in a distorted octahedral geometry, with the ligand coordinated through the N-amine and O-phenolato moieties, with one of the pyridine-N atoms protonated. Crystals of [(V(V)O(2))(2)(pyren)(2)].2 H(2)O were obtained from solutions containing H(2)pyr(2)en and oxovanadium(IV), where Hpyren is the "half" Schiff base of pyridoxal and ethylenediamine. The complexation of V(IV)O(2+) and V(V)O(2) (+) with H(2)pyr(2)en, H(2)Rpyr(2)en and pyridoxamine in aqueous solution were studied by pH-potentiometry, UV/Vis absorption spectrophotometry, as well as by EPR spectroscopy for the V(IV)O systems and (1)H and (51)V NMR spectroscopy for the V(V)O(2) systems. Very significant differences in the metal-binding abilities of the ligands were found. Both 1 and 2 act as tetradentate ligands. H(2)Rpyr(2)en is stable to hydrolysis and several isomers form in solution, namely cis-trans type complexes with V(IV)O, and alpha-cis- and beta-cis-type complexes with V(V)O(2). The pyridinium-N atoms of the pyridoxal rings do not take part in the coordination but are involved in acid-base reactions that affect the number, type, and relative amount of the isomers of the V(IV)O-H(2)Rpyr(2)en and V(V)O(2)-H(2)Rpyr(2)en complexes present in solution. DFT calculations were carried out and support the formation and identification of the isomers detected by EPR or NMR spectroscopy, and the strong equatorial and axial binding of the O-phenolato in V(IV)O and V(V)O(2) complexes. Moreover, the DFT calculations done for the [V(IV)O(H(2)Rpyr(2)en)] system indicate that for almost all complexes the presence of a sixth equatorial or axial H(2)O ligand leads to much more stable compounds.     

Modification of the luminescence properties of an Europium(III) Tris(β-diketonate) Complex by Inclusion in γ-cyclodextrin and 2,3,6-trimethyl-γ-cyclodextrin

A 1:1 inclusion compound between octakis(2,3,6-tri-O-methyl)-γ-cyclodextrin (TRIMEG) and the chelate complex Eu(NTA)₃·2H₂O (NTA=1-(2-naphthoyl)-3,3,3-trifluoroacetonate) was prepared and characterized by powder X-ray diffraction, thermogravimetric analysis and photoluminescence spectroscopy. The results were compared with those obtained for the corresponding native γ-CD adduct. Excitation and emission spectra were measured, and the lifetimes were determined for the Eu³⁺ first excited state (⁵D₀). The results indicate the presence of only one low-symmetry environment for the Eu³⁺ cations in the inclusion compounds. Encapsulation of the Europium complex in the two CDs increases the quantum efficiency of the ligand-to-metal energy transfer pathway, but the efficiency of the Eu³⁺ sensitization was significantly higher with TRIMEG as the host molecule. This may be related with the observation that the two hosts appear to have different influences on the Eu³⁺ coordination environments for the guest molecule.     

Direct evidence of a radical mechanism in the addition reaction of iododifluoroesters to olefins by spin trapping

We present EPR analysis of the reaction of ethyl iododifluoroacetate with 1-tetradecene in the presence of Zn + NiCl2 x 6H2O, confirming the mechanistic studies that provide evidence of a single electron transfer process. We have trapped for the first time the ethoxycarbonyldifluoromethyl radical with a variety of spin traps, such as phenyl tert-butyl nitrone (PBN), 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), 2-methyl-2-nitrosopropane (MNP), and 2-nitro-2-nitrosopropane (NNP), and the EPR spectra of the corresponding adducts have been recorded. In a second step the ethoxycarbonyldifluoromethyl radical adds to the olefin to furnish a second radical intermediate, which can be trapped with NNP. Evidence of this second radical was obtained by EPR only with electron-rich olefins, such as alpha-methylstyrene and 2,4,6-trimethylstyrene, and the new adducts were recorded and interpreted. In addition, we also report the EPR spectra of the corresponding adducts when other alkylating reagents are used, such as ethyl iodoacetate, n-perfluorohexyl iodide, methyl omega-iodohexadecanoate, and n-butyl iodide.     

Capillary electrophoretic determination of triamterene, methotrexate, and creatinine in human urine

A capillary zone electrophoresis (CZE) method using a fused-silica capillary (60.2 cm x 75 microm ID) was investigated for the determination of triamterene (TRI), methotrexate (MTX), and creatinine (CREA) in human urine. The separation was performed using a hydrodynamic injection time of 7 s (0.5 psi), a voltage of 25 kV, a capillary temperature of 30 degrees C, and 40 mM phosphoric acid adjusted to pH 2.25 by addition of triethanolamine as separation electrolyte. Under these conditions, analysis takes about 15 min. A linear response over the 0.5-15.0 mg L(-1) concentration range was found for TRI and MTX, and 0.5-80.0 mg L(-1) for CREA. Dilution of the sample (water:urine, 1:1 for TRI and MTX, and 1:25 for CREA determination) was the only step necessary prior to analysis by electrophoresis. The developed method is easy, rapid, and sensitive and has been applied to determine triamterene,methotrexate, and creatinine in urine samples with satisfactory results.     

Oxorhenium(V) Complexes with Thiosemicarbazones

Neutral oxorhenium(V) complexes with thiosemicarbazones derived from 2‐pyridine formamide, HL¹, are formed when [ReOCl₃(PPh₃)₂] reacts with equimolar amounts of the ligands. Reduction of the metal and the formation of rhenium(III) complexes of the composition [Re(L¹)₂]⁺ occurs when an excess of thiosemicarbazones is used and the reaction is performed in boiling toluene for a prolonged period of time. The thiosemicarbazones deprotonate and act as tridentate ligands as has been confirmed by an X‐ray structure of [ReOCl₂(L^1b)], where HL^1b is 2‐pyridineformamide‐N(4)‐ethylthiosemicarbazone and the ligand occupies the equatorial coordination sphere of the complex together with one of the chloro ligands.     

Semiempirical study of electronic and bonding properties of cobalt silicide clusters

The electronic structure of cobalt silicide clusters Co₇Si₇ and Si₇Co₇ was studied in comparison to that of Co₁₉ and Si₁₇ clusters under the scope of the MINDO/SR method. Clusters Co₇Si₇ and Si₇Co₇ represent the environment of a cobalt atom and that of a silicon atom in the cobalt monosilicide bulk, respectively. It is found that the Co? Si bond is essentially sp in character with an indirect participation (by electrostatic interaction) of the cobalt d orbitals. Our calculations show a charge transfer from silicon to the d orbitals of cobalt via sp–sp interaction with an internal sp–d hybridization. The theoretical density of states for cobalt silicide clusters are reported and compared with experimental results of surface spectroscopies. © 1992 by John Wiley & Sons, Inc.     

Oxidative Transformations of Organic Compounds Mediated by Redox Molecular Sieves

Zeolites are viewed by some as the “philosopher's stone” of modern chemistry.^[1] They are more or less indispensable in oil refining and petrochemicals manufacture where they are widely applied as solid acid catalysts. More recently attention has been focused on their use in the manufacture of fine chemicals. The synthetic utility of zeolites and related molecular sieves (zeotypes) has been considerably extended by the incorporation of redox metals into their frameworks. The resulting redox molecular sieves catalyze a variety of selective oxidations under mild conditions in the liquid phase. Their structural diversity–including variation of the redox metal, incorporation of metal complexes, and the size and polarity of the micropores–provides the possibility of designing tailor-made solid catalysts (“mineral enzymes”) for liquid-phase oxidations with clean oxidants such as O₂, H₂O₂, and RO₂H. Hence, they have enormous potential in industrial organic synthesis as environmentally friendly alternatives to traditional oxidations employing inorganic oxidants in stoichiometric amounts. A primary aim of this review is to familiarize organic chemists with the synthetic potential of redox molecular sieves. An outline of their synthesis, structures, and chemical properties, highlighting their unique advantages, is followed by a discussion of general (mechanistic) features that influence the choice of a suitable catalyst for a particular type of oxidation. The main part of the review deals with the oxidation of various substrates of synthetic interest–such as alkanes, alkenes, (alkyl)arenes, alcohols, and amines–and emphasizes the advantages of redox molecular sieves (including selectivity and stability) over their homogeneous counterparts. New directions towards truly biomimetic solid catalysts, for example zeolite-encapsulated chiral metal complexes as heterogeneous catalysts for asymmetric oxidations, are high-lighted.     

The free radical distribution in emulsion polymerization using oil-soluble initiators

A new approach is presented to calculate both the distribution of particles with iradicals and the average number of radicals per particle in emulsion polymerizations carried out using oil-soluble initiators. The convergence and accuracy of the approach were examined. It was found that, in agreement with previously published experimental results, the present approach predicts a kinetic behavior similar to that found for water-soluble initiators. This effect is primarily due to the desorption of initiator radicals from the polymer particles rather than the contribution of the fraction of oil-soluble initiator dissolved in the aqueous phase.