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.     

Characterization of a tricationic trigonal bipyramidal iron(IV) cyanide complex, with a very high reduction potential, and its iron(II) and iron(III) congeners

Currently, there are only a handful of synthetic S = 2 oxoiron(IV) complexes. These serve as models for the high-spin (S = 2) oxoiron(IV) species that have been postulated, and confirmed in several cases, as key intermediates in the catalytic cycles of a variety of nonheme oxygen activating enzymes. The trigonal bipyramidal complex [Fe(IV)(O)(TMG(3)tren)](2+) (1) was both the first S = 2 oxoiron(IV) model complex to be generated in high yield and the first to be crystallographically characterized. In this study, we demonstrate that the TMG(3)tren ligand is also capable of supporting a tricationic cyanoiron(IV) unit, [Fe(IV)(CN)(TMG(3)tren)](3+) (4). This complex was generated by electrolytic oxidation of the high-spin (S = 2) iron(II) complex [Fe(II)(CN)(TMG(3)tren)](+) (2), via the S = 5/2 complex [Fe(III)(CN)(TMG(3)tren)](2+) (3), the progress of which was conveniently monitored by using UV-vis spectroscopy to follow the growth of bathochromically shifting ligand-to-metal charge transfer (LMCT) bands. A combination of X-ray absorption spectroscopy (XAS), Mo?ssbauer and NMR spectroscopies was used to establish that 4 has a S = 0 iron(IV) center. Consistent with its diamagnetic iron(IV) ground state, extended X-ray absorption fine structure (EXAFS) analysis of 4 indicated a significant contraction of the iron-donor atom bond lengths, relative to those of the crystallographically characterized complexes 2 and 3. Notably, 4 has an Fe(IV/III) reduction potential of ～1.4 V vs Fc(+/o), the highest value yet observed for a monoiron complex. The relatively high stability of 4 (t(1/2) in CD(3)CN solution containing 0.1 M KPF(6) at 25 °C ≈ 15 min), as reflected by its high-yield accumulation via slow bulk electrolysis and amenability to (13)C NMR at -40 °C, highlights the ability of the sterically protecting, highly basic peralkylguanidyl donors of the TMG(3)tren ligand to support highly charged high-valent complexes.     

Control of nanoparticle aggregation in aerogel hosts

Plasmonic aerogel containing 50 nm gold nanoparticles is made using a modified 2-step method that maintains control over the gel time while preventing nanoparticle aggregation. Strong narrow surface plasmon resonances verify that the nanoparticles are well dispersed within the silica matrix, and enable applications in sensing, SERS, nonlinear optics or plasmonic gain. Discrepancies between measured and simulated resonance wavelengths are attributed to the breakdown of the effective index approximation, due to the short-scale penetration of the resonance electric field into the host medium.     

The effect of density ratio on the near field of a naturally occurring oscillating jet

The triangular oscillating jet nozzle generates a triangular jet partially confined within an axi-symmetric chamber to produce a large scale flow oscillation that has application in thermal processes. Particle image velocimetry and oscillation frequency measurements were conducted to investigate the influence of the jet fluid to ambient fluid density ratio on the resulting oscillating flow. The investigation was conducted with a jet momentum flux of 0.06 kg m s⁻² (Re = 7.3−47.2 × 10³) and density ratios ranging from 0.2 to 5.0. The initial spread and decay of the emerging jet was found to depend upon the density ratio but in a more complex way than does an unconfined jet. Both the spread and decay are strongly influenced by the instantaneous angle of jet deflection, with greater deflection leading to increased spreading and decay of the jet. Decreasing the density ratio below unity results in a rapid decrease in the deflection angle, while increasing the density ratio above unity results in an increase in the deflection angle, albeit with less sensitivity. The frequency of oscillation was also shown to depend on the density ratio with an increase in the density ratio causing a decrease in the dominant oscillation frequency.     

Energy bands,fermi surface and lattice instability in VS

The energy bands of vanadium monosulfide have been calculated by the KKR method in the muffin-tin approximation. The Fermi surface is very complicated, but there are two nearly cylindrical sheets of nearly equal cross-sections centered around Φ and M respectively. We suggest that this Fermi surface feature can support a charge-density-wave (CDW) state in the conduction electron system at low enough temperatures, and the formation of the CDW explains the lattice phase transition in this compound.     

Optical isotope shifts of146Sm and151Sm

We have measured the optical isotope shifts of¹⁴⁶Sm and¹⁵¹Sm by laser resonance fluorescence. From these measurements the changes in the mean square nuclear radii are: Δ〈r²〉 (A=144 to 146)=0.266(10) fm², and Δ〈r²〉 (A=151 to 152)=0.262(10) fm². These results, together with those of the stable isotopes, show that the average nuclear expansion of samarium can be accounted for by the liquid drop model with deformations.     

Trifluoromethylmaleic anhydride

α-Trifluoromethylmaleic anhydride has been prepared by an unusual reaction of hexafluoropropene epoxide (HFPO) with acetic anhydride. It reacted as a dienophile with butadiene, cyclopentadiene, furan, and anthracene.     

Synthesis of (+/-)-3H-epivincamine via a Rh(II)-triggered cyclization/cycloaddition cascade

A synthesis of (+/-)-3H-epivincamine is reported. Important steps include (1) a Rh(II)-catalyzed intramolecular [3+2]-cycloaddition of an alpha-diazo indolo amide, (2) a reductive ring opening of the cycloadduct, (3) a decarboethoxylation reaction, and (4) a base-induced keto-amide ring contraction.