Synthesis, structural characterization, and theoretical investigation of compounds containing an Al-O-M-O-Al (M=Ti, Zr) core

We report a facile route to the first molecular compounds with the Al-O-M-O-Al (M=Ti, Zr) structural motif. Synthesis of L(Me)Al(mu-O)M(NMe2)2(mu-O)Al(Me)L [L=CH{N(Ar)(CMe)}2, Ar=2,6-iPr2C6H3; M=Ti (7), Zr (8)] was accomplished by reacting the monometallic hydroxide precursor L(Me)Al(OH) (1) with Ti(NMe2)4 or Zr(NMe2)4 under elimination of Me2NH in good yield. The crystal structural data confirm the trimetallic Al-O-M-O-Al core in both 7 and 8. Preliminary investigation on catalytic activity of these complexes reveals low activity of these complexes in ethylene polymerization as compared to the related oxygen-bridged metallocene-based heterobimetallic complexes L(Me)Al(mu-O)M(Me)Cp2 (M=Ti, Zr) which could be attributed to the relatively lower stability of the supposed cationic intermediate as revealed by DFT calculations.     

Iron-catalyzed C2-C3 bond cleavage of phenylpyruvate with O2: insight into aliphatic C-C bond-cleaving dioxygenases

Iron(II)-phenylpyruvate complexes of tetradentate tris(6-methyl-2-pyridylmethyl)amine (6-Me3-TPA) and tridentate benzyl bis(2-quinolinylmethyl)amine (Bn-BQA) were prepared to gain insight into C-C bond cleavage catalyzed by dioxygenase enzymes. The complexes we have prepared and characterized are [Fe(6-Me3-tpa)(prv)][BPh4] (1), [Fe2(6-Me3-tpa)2(pp)][(BPh4)2] (2), and [Fe2(6-Me3-tpa)2(2'-NO2-pp)][(BPh4)2] (3), [Fe(6-Me3-tpa)(pp-Me)][BPh4] (4), [Fe(6-Me3-tpa)(CN-pp-Et)][BPh4] (5), and [Fe(Bn-bqa)(pp)] (8), in which PRV is pyruvate, PP is the enolate form of phenylpyruvate, 2'-NO2-PP is the enolate form of 2'-nitrophenylpyruvate, PP-Me is the enolate form of methyl phenylpyruvate, and CN-PP-Et is the enolate form of ethyl-3-cyanophenylpyruvate. The structures of mononuclear complexes 1 and 5 were determined by single-crystal X-ray diffraction. Both the PRV ligand in 1 and the CN-PP-Et ligand in 5 bind to the iron(II) center in a bidentate manner and form 5-membered chelate rings, but the alpha-keto moiety is in the enolate form in 5 with concomitant loss of a C-H(beta) proton. The PP ligands of 2, 3, 4, and 8 react with dioxygen to form benzaldehyde and oxalate products, which indicates that the C2-C3 PP bond is cleaved, in contrast to cleavage of the C1-C2 bond previously observed for complexes that do not contain alpha-ketocarboxylate ligands in the enolate form. These reactions serve as models for metal-containing dioxygenase enzymes that catalyze the cleavage of aliphatic C-C bonds.     

<Emphasis Type="Italic">cis</Emphasis>-Dioxomolybdenum(VI) complexes of sterically encumbered phenol-based tetradentate N<Subscript>2</Subscript>O<Subscript>2</Subscript> ligands: Structural,spectroscopic, and electrochemical studies

Two cis-dioxomolybdenum(VI) complexes [MoO₂L] (L: L ¹, 2 and L: L ², 3) in a phenol-based sterically encumbered N₂O₂ ligand environment have been synthesized, and their crystallographic characterizations are reported. The orange crystals of 2 are monoclinic, space group P2₁/a with unit cell dimensions as a=16.2407(17) Å, b=7.2857(8) Å, c=18.400(2) Å, β=98.002(9)°, Z=4, and d _cal=1.486 g cm^?3. The light orange crystals of 3, however, are orthorhombic, space group, Pbcn, with unit cell dimensions a=8.3110(12) Å, b=12.637(3) Å, c=34.673(5) Å, Z=4, and d _cal=1.187 g cm^?3. The structures were refined by a full-matrix least-squares procedure on F ² to a final R=0.046 (0.055 for 3) using 4944 (3677) all independent data. In both the cases, the Mo atom exists in a distorted octahedral geometry defined by a N₂O₄ donor set, which features a cis-Mo(–O)₂ and a trans-Mo(OPh)₂ arrangement. Compound 2 undergoes a quasireversible one-electron reduction at ?1.3 V vs Ag/AgCl reference due to Mo^VIO₂/Mo^VO₂ electron transfer and thus providing a rare example of steric solution to the comproportionation–dimerization problem encountered frequently in the development of valid biomimetic models for the active sites of oxomolybdenum enzymes.     

Femtosecond solvation dynamics in a neat ionic liquid and ionic liquid microemulsion: excitation wavelength dependence

Solvation dynamics in a neat ionic liquid, 1-pentyl-3-methyl-imidazolium tetra-flouroborate ([pmim][BF4]) and its microemulsion in Triton X-100 (TX-100)/benzene is studied using femtosecond up-conversion. In both the neat ionic liquid and the microemulsion, the solvation dynamics is found to depend on excitation wavelength (lambda(ex)). The lambda(ex) dependence is attributed to structural heterogeneity in neat ionic liquid (IL) and in IL microemulsion. In neat IL, the heterogeneity arises from clustering of the pentyl groups which are surrounded by a network of cation and anions. Such a nanostructural organization is predicted in many recent simulations and observed recently in an X-ray diffraction study. In an IL microemulsion, the surfactant (TX-100) molecules aggregate in form of a nonpolar peripheral shell around the polar pool of IL. The micro-environment in such an assembly varies drastically over a short distance. The dynamic solvent shift (and average solvation time) in neat IL as well as in IL microemulsions decreases markedly as lambda(ex) increases from 375 to 435 nm. In a [pmim][BF4]/water/TX-100/benzene quaternary microemulsion, the solvation dynamics is slower than that in a microemulsion without water. This is ascribed to the smaller size of the water containing microemulsion. The anisotropy decay in an IL microemulsion is found to be faster than that in neat IL.     

Organocatalytic highly enantioselective nitroaldol reaction of alpha-ketophosphonates and nitromethane

[reaction: see text] The first organocatalytic highly enantioselective nitroaldol reaction of alpha-ketophosphonates and nitromethane has been realized by using cupreine (2) or 9-O-benzylcupreine (3) as the catalyst. Both catalysts are highly reactive and highly enantioselective. alpha-Hydroxy-beta-nitrophosphonates have been synthesized in good yields and excellent enantioselectivities (>or=90% ee) at a low catalyst loading (5 mol %). These nitroaldol products may be reduced to the biologically significant beta-amino-alpha-hydroxyphosphonates with complete retention of the stereochemistry.     

Fluorescent Sensors for Hg2+ and Cu2+ Based on Condensation Products of 4H-1,2,4 Triazole-4-Amine and Carboxylated Benzoic Acids

Journal of Fluorescence - Mercury (Hg) causes serious health issues in its all forms. Deficiency as well as excess of copper ion (Cu2+) in human body is hazardous. A series of four compounds have...     

Novel phosphonated polymer without anhydride formation for proton exchange membrane fuel cells

Proton exchange membrane fuel cells(PEMFCs)are regarded as one of the most promising clean energy technology because of their high energy density,silent emission-free operation,and wide applications[1].Recently,anion exchange membrane fuel cells(AEMFCs)has emerged as an alternative to PEMFCs.     

Dispersion polymerization of pyrrole using ethylhydroxy-ethylcellulose as a stabilizer

Oxidative polymerization of pyrrole has been studied using FeCl₃ or (NH₄)₂S₂O₈ (APS) as oxidant, ethylhydroxyethylcellulose (EHEC) as a steric stabilizer and water or aqueous ethanol as the dispersion medium. Transmission electron micrographic images of the particles from the as-prepared dialysed dispersions in aqueous ethanol show small as well as large particles (about a decade larger) when FeCl₃ is used as the oxidant but only large particles when APS is used as the oxidant. Small particles are not found when the dispersions are prepared in water, irrespective of the oxidant used. The particle size decreases with an increase in molecular weight of the stabilizer for the same stabilizer concentration. The minimum amount of stabilizer required to support dispersion polymerization decreases upon increasing the alcohol content of the medium. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3723–3729, 1999     

Polycyclic phosphonate resins: Thermally cross-linkable intermediates for flame-retardant materials

Two series of polycyclic phosphonates were synthesized by reacting p-substituted phenol–formaldehyde resins with excess phosphonic dichlorides under dilute conditions in a polar aprotic solvent. MALDI–TOF was used to detect the absolute masses of different species formed under these conditions. The presence of phosphorus-containing species was also confirmed by ³¹P-NMR. The polycyclics were subjected to ring-opening polymerization simultaneously with a transesterification reaction with commercial polycarbonates in order to cross-link the polymers and to impart flame retardancy to the final thermosetting materials. Thermal properties of the polycarbonates containing phosphonate moieties were determined by thermogravimetric analysis and differential scanning calorimetry techniques. Percent char yields of the final thermosetting materials were considerably higher than that of the linear polycarbonates. Polycyclic phosphonates, therefore, can potentially be used to impart flame retardance to polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1911–1918, 1998