Heterobimetallic Complexes of Rhenium and Zinc: Potential Catalysts for Homogeneous Syngas Conversion

6-(Diphenylphosphino)-2,2′-bipyridine (PNN) coordinates to rhenium carbonyls in both κ^1(P) and κ^2(N,N) modes; in the former, the free bpy moiety readily binds to zinc alkyls and halides. [Re(κ^1(P)-PNN)(CO)_5][OTf] reacts with dialkylzinc reagents to form [Re(κ^1(P)-PNN·ZnR)(CO)_4(μ_(2-)C(O)R)][OTf] (R = Me, Et, Bn), in which an alkyl group has been transferred to a carbonyl carbon and the resulting monoalkyl Zn is bound both to the bpy nitrogens and the acyl oxygen. ZnCl_2 binds readily to the bpy group in Re(κ^1(P)-PNN)(CO)_4Me, and the resulting adduct undergoes facile migratory insertion, assisted by the Lewis acidic pendent Zn, to yield Re(κ^1(P)-PNN·ZnCl)(μ_(2-)Cl)(CO)_3(μ_(2-)C(O)Me), in which one of the chlorides occupies the sixth coordination site on Re. Migratory insertion is inhibited by THF or other ethers that can coordinate to ZnCl_2. Migratory insertion is also observed for Re(κ1(P)-PNN)(CO)_4(CH_2Ph) but not for Re(κ^1(P)-PNN)(CO)_4(CH_2OCH_3); coordination of the methoxy oxygen to Zn appears to block its ability to coordinate to the carbonyl oxygen and facilitate migratory insertion. Intramolecular Lewis acid promoted hydride transfer from [(dmpe)_2PtH][PF_6] to a carbonyl in [Re(κ^1(P)-PNN)(CO)_5][OTf] results in formation of a Re–formyl species; additional hydride transfer leads to a novel Re–Zn-bonded product along with some formal dehyde

Transformations of Group 7 Carbonyl Complexes: Possible Intermediates in a Homogeneous Syngas Conversion Scheme

A variety of C−H and C−C bond forming reactions of group 7 carbonyl complexes have been studied as potential steps in a homogeneously catalyzed conversion of syngas to C_(2+) compounds. The metal formyl complexes M(CO)_3(PPh_3)_2(CHO) (M = Mn, Re) are substantially stabilized by coordination of boranes BX_3 (X = F, C_6F_5) in the form of novel boroxycarbene complexes M(CO)_3(PPh_3)_2(CHOBX_3), but these boron-stabilized carbenes do not react with hydride sources to undergo further reduction to metal alkyls. The related manganese methoxycarbene cations [Mn(CO)_(5−x)(PPh_3)_x(CHOMe)]+ (x = 1 or 2), obtained by methylation of the formyls, do react with hydrides to form methoxymethyl complexes, which undergo further migratory insertion under an atmosphere of CO. The resulting acyls, cis- and trans-Mn(PPh_3)(CO)_4(C(O)CH_2OMe), can be alkylated to form the cationic carbene complex [Mn(PPh_3)(CO)_4(C(OR)CH_2OMe)]^+, which undergoes a 1,2 hydride shift to form 1,2-dialkoxyethylene, which is displaced from the metal, releasing triflate or diethyl ether adducts of [Mn(PPh_3)(CO)_4]^+. The acyl can also be protonated with HOTf to form a hydroxycarbene complex, which rearranges to Mn(PPh_3)(CO)_4(CH_2COOMe) and is protonolyzed to yield methyl acetate and [Mn(PPh_3)(CO)_4]^+; addition of L (L = PPh_3, CO) to the manganese cation regenerates [Mn(PPh_3)(CO)_4(L)]^+. Since the original formyl complex can be obtained by the reaction of [Mn(PPh_3)(CO)_5]^+ with [PtH(dmpe)_2]^+, which in turn can be generated from H_2, this set of transformations amounts to a stoichiometric cycle for selectively converting H_2 and CO into a C_2 compound under mild conditions

Bond Activation by Nitrogen Ligated Pt(II)/Pt(IV) Complexes

Activation and functionalization of hydrocarbons by platinum has been studied using chelating nitrogen donor ligands to stabilize Pt(II) and Pt(IV) complexes. These bidentate and tridentate ligands allowed examination of oxidative addition of R-H (R = C, Si) bonds and reductive elimination of R′-H (R′ = C, H) bonds. In addition we have investigated functionalization of hydrocarbons by dehydrogenation of alkanes and by coupling of alkynes. Stoichiometric dehydrogenation of alkanes or simple ethers can be accomplished by stirring Me4Pt2(μ-SMe2)2 and nacnacH (nacnac = bis-N-aryl-β-diiminate) in the solvent to be dehydrogenated. This reaction yields Pt(II) alkene hydride complexes, (nacnac)Pt(H)(η2- alkene). The dimeric Pt reagent is protonated by nacnacH and ensuing methane elimination forms the (nacnac)PtMe fragment which then binds and cleaves a solvent C-H bond through oxidative addition to Pt. Methane elimination followed by β-H elimination from the activated solvent molecule yield the Pt alkene hydride complex, (nacnac)Pt(H)(η2-alkene). Linear alkanes undergo selective C-H activation of primary C-H bonds to form α-olefin complexes, but ethers add selectively through the secondary C-H α to oxygen; this is due to coordination of the ether oxygen to Pt preceeding C-H activation. The alkene hydride complex readily exchanges free and bound olefins. This facile ligand exchange has allowed us to explore the reactivity of the (nacnac)Pt(H) fragment. iv Triphenylsilane displaces pentene from (nacnac)Pt(H)(1-pentene) and oxidative addition of the Si-H bond forms a stable five-coordinate Pt(IV) silyldihydride complex (nacnac)Pt(H)2(SiPh3). This five-coordinate complex was characterized crystallographically and appears to be a square based pyramid with the vacant coordination site trans to the silyl ligand. If acetylene or phosphaalkyne is mixed with the five-coordinate species the triple bond inserts across the vacant site on the Pt and the central CH on the nacnac ligand. Alkynes easily displace pentene from (nacnac)Pt(H)(1-pentene) and rapidly undergo insertion into the Pt-H bond to initiate a reaction cascade. The identity of the final platinum product depends on the substituents on the alkyne reagent. Alkynes with propargylic protons bond favor allyl formation. Terminal silyl alkynes such as R3SiC≡CH (R3Si = Me3Si, Ph3Si, Ph2MeSi) insert into the Pt-H bond in a 2,1 fashion placing the silyl group and Pt on the same carbon, subsequent C-H activation of the silicon substituents, either methyl or phenyl, forms chelated vinyl silane products. Terminal alkynes with no propargylic hydrogens such as PhC≡CH and t-BuC≡CH insert into the Pt-H bond in a 1,2, rather than a 2,1, fashion, placing the substituent β to the metal and precluding C-H activation of the alkyne substituent. Instead, a second alkyne binds to the metal and inserts into the Pt-vinyl bond in a 1,2 fashion forming chelated η1-η2-butadienyl ligands with R groups at the 2 and 4 positions. Acid assisted reductive elimination of hydrogen from Tp'PtH3 (Tp' = hydridotris(3,5- dimethylpyrazolyl)borate) was examined. Loss of H2 is observed from Tp'Pt(H)3 upon protonation and addition of CO. No formation of hydrogen is observed if the reaction is conducted in the absence of CO. In contrast to most reductive eliminations from Pt(IV) which occur from five-coordinate intermediates, here reductive elimination occurs directly from the 6-coordinate [κ2 -(HTp')Pt(H)3(CO)][BAr'4] species

Solar Fuels Generator

The solar fuels generator includes an ionically conductive separator between a gaseous first phase and a second phase. A photoanode uses one or more components of the first phase to generate cations during operation of the solar fuels generator. A cation conduit is positioned provides a pathway along which the cations travel from the photoanode to the separator. The separator conducts the cations. A second solid cation conduit conducts the cations from the separator to a photocathode

Genomic distribution of H3K9me2 and DNA methylation in a maize genome

DNA methylation and dimethylation of lysine 9 of histone H3 (H3K9me2) are two chromatin modifications that can be associated with gene expression or recombination rate. The maize genome provides a complex landscape of interspersed genes and transposons. The genome-wide distribution of DNA methylation and H3K9me2 were investigated in seedling tissue for the maize inbred B73 and compared to patterns of these modifications observed in Arabidopsis thaliana. Most maize transposons are highly enriched for DNA methylation in CG and CHG contexts and for H3K9me2. In contrast to findings in Arabidopsis, maize CHH levels in transposons are generally low but some sub-families of transposons are enriched for CHH methylation and these families exhibit low levels of H3K9me2. The profile of modifications over genes reveals that DNA methylation and H3K9me2 is quite low near the beginning and end of genes. Although elevated CG and CHG methylation are found within gene bodies, CHH and H3K9me2 remain low. Maize has much higher levels of CHG methylation within gene bodies than observed in Arabidopsis and this is partially attributable to the presence of transposons within introns for some maize genes. These transposons are associated with high levels of CHG methylation and H3K9me2 but do not appear to prevent transcriptional elongation. Although the general trend is for a strong depletion of H3K9me2 and CHG near the transcription start site there are some putative genes that have high levels of these chromatin modifications. This study provides a clear view of the relationship between DNA methylation and H3K9me2 in the maize genome and how the distribution of these modifications is shaped by the interplay of genes and transposons.The research was supported by a grant from the National Science Foundation (IOS-1237931) to MWV and NMS. This work also used resources or cyberinfrastructure provided by iPlant Collaborative. The iPlant Collaborative is funded by a grant from the National Science Foundation (DBI-0735191; www. iplantcollaborative.org). Start-up funds from the University of Georgia and a research grant from the National Science Foundation (IOS-1339194) to RJS supported aspects of this study

New N=2 Superconformal Field Theories in Four Dimensions

New examples of N=2 supersymmetric conformal field theories are found as fixed points of SU(2) N=2 supersymmetric QCD. Relations among the scaling dimensions of their relevant chiral operators, global symmetries, and Higgs branches are understood in terms of the general structure of relevant deformations of non-trivial N=2 conformal field theories. The spectrum of scaling dimensions found are all those compatible with relevant deformations of a y^2 = x^3 singular curve.Comment: 17 pages, harvma

The UV, Optical, and IR Properties of SDSS Sources Detected by GALEX

We discuss the UV, optical, and IR properties of the SDSS sources detected by GALEX as part of its All-sky Imaging Survey Early Release Observations. Virtually all of the GALEX sources in the overlap region are detected by SDSS. GALEX sources represent ~2.5% of all SDSS sources within these fields and about half are optically unresolved. Most unresolved GALEX/SDSS sources are bright blue turn-off thick disk stars and are typically detected only in the GALEX near-UV band. The remaining unresolved sources include low-redshift quasars, white dwarfs, and white dwarf/M dwarf pairs, and these dominate the optically unresolved sources detected in both GALEX bands. Almost all the resolved SDSS sources detected by GALEX are fainter than the SDSS 'main' spectroscopic limit. These sources have colors consistent with those of blue (spiral) galaxies (u-r<2.2), and most are detected in both GALEX bands. Measurements of their UV colors allow much more accurate and robust estimates of star-formation history than are possible using only SDSS data. Indeed, galaxies with the most recent (<20 Myr) star formation can be robustly selected from the GALEX data by requiring that they be brighter in the far-UV than in the near-UV band. However, older starburst galaxies have UV colors similar to AGN, and thus cannot be selected unambiguously on the basis of GALEX fluxes alone. With the aid of 2MASS data, we construct and discuss median 10 band UV-optical-IR spectral energy distributions for turn-off stars, hot white dwarfs, low-redshift quasars, and spiral and elliptical galaxies. We point out the high degree of correlation between the UV color and the contribution of the UV flux to the UV-optical-IR flux of galaxies detected by GALEX.Comment: 35 pages, 11 figures, 3 tables; to appear in the AJ. PS with better figures available from http://www.astro.washington.edu/agueros/pub

The antecedents of biliary cancer: a primary care case–control study in the United Kingdom

In a case–control study using a large UK primary care database, we found that non-steroidal anti-inflammatory drugs had no protective effect against biliary carcinomas (cholangiocarcinoma and gall bladder cancer). Increased risks were observed for cigarette smoking, diabetes, gallstone disease and obesity

Epigenetic and Genetic Influences on DNA Methylation Variation in Maize Populations

DNA methylation is a chromatin modification that is frequently associated with epigenetic regulation in plants and mammals. However, genetic changes such as transposon insertions can also lead to changes in DNA methylation. Genome-wide profiles of DNA methylation for 20 maize (Zea mays) inbred lines were used to discover differentially methylated regions (DMRs). The methylation level for each of these DMRs was also assayed in 31 additional maize or teosinte genotypes, resulting in the discovery of 1966 common DMRs and 1754 rare DMRs. Analysis of recombinant inbred lines provides evidence that the majority of DMRs are heritable. A local association scan found that nearly half of the DMRs with common variation are significantly associated with single nucleotide polymorphisms found within or near the DMR. Many of the DMRs that are significantly associated with local genetic variation are found near transposable elements that may contribute to the variation in DNA methylation. Analysis of gene expression in the same samples used for DNA methylation profiling identified over 300 genes with expression patterns that are significantly associated with DNA methylation variation. Collectively, our results suggest that DNA methylation variation is influenced by genetic and epigenetic changes that are often stably inherited and can influence the expression of nearby genes