James M. Boncella

Synthesis of Imido Analogs of the Uranyl Ion

Here we describe the synthesis of two imido analogs of the uranyl ion, UO2+2, in which the oxygens are replaced by divalent alkyl or aryl nitrogen groups: U(NtBu)2I2(THF)2 (1) and U(NPh)2I2(THF)3 (2) (where tBu is tert-butyl and THF is tetrahydrofuran). Both compounds have been fully characterized by standard analytical techniques, including x-ray crystallography, and the chemical bonding between the metal center and the nitrogen ligands was quantified by using hybrid density functional theory calculations. As expected for a uranyl analog, these complexes exhibit linear N-U-N linkages and very short U-N bonds. In addition, the theoretical calculations show strong involvement of the 5f and 6d electrons in the U-N bonding.

Near-infrared electroluminescence from conjugated polymer/lanthanide porphyrin blends

Near-infrared-emitting polymer light-emitting diodes (PLEDs) have been fabricated using blends of conjugated polymers and lanthanide tetraphenylporphyrin complexes. Host polymers include MEH–PPV and a bis-alkoxy-substituted poly(p-phenylene) (PPP–OR11), and the lanthanide complexes include Yb(TPP)acac and Er(TPP)acac (where TPP=5,10,15,20-tetraphenylporphyrin and acac=acetylacetonate). Electroluminescence (EL) is observed at 977 nm from devices fabricated using MEH–PPV or PPP–OR11 blended with Yb(TPP)acac, and EL is observed at 1560 nm from a device fabricated using a blend of MEH–PPV and Er(TPP)acac. Visible EL from the host polymers is strongly suppressed in all of the devices, however, in the device fabricated using the PPP–OR11 polymer blue emission from the host is completely quenched. Very efficient quenching of the EL from the host in the PPP–OR11 device is believed to occur due to efficient Forster energy transfer, which is facilitated by the excellent spectral overlap between the PPP–OR11 fluores...

Cation–Cation Interactions, Magnetic Communication, and Reactivity of the Pentavalent Uranium Ion [U(NtBu)2]+

Communication is important: The dimeric bis(imido) uranium complex [{U(NtBu)(2)(I)(tBu(2)bpy)}(2)] (see picture; U green, N blue, I red) has cation-cation interactions between [U(NR)(2)](+) ions. This f(1)-f(1) system also displays f orbital communication between uranium(V) centers at low temperatures, and can be oxidized to generate uranium(VI) bis(imido) complexes.

Chemical Evolution of Amphiphiles: Glycerol Monoacyl Derivatives Stabilize Plausible Prebiotic Membranes

The self-assembly of simple amphiphiles like fatty acids into cell-like membranous structures suggests that such structures were available on prebiotic Earth to support the origin of cellular life. However, the composition of primitive membranes remains unclear because the physical properties of the aqueous environment in which they assembled are relatively unconstrained in terms of temperature, pH, and ionic concentrations. It seems likely that early membranes were composed of mixtures of various amphiphiles in an aqueous medium warmed by geothermal activity prevalent in the Archean era. To better understand the properties of mixed bilayers formed by binary mixtures of single-chain amphiphiles under these conditions, we conducted stability experiments, using membranes composed of various fatty acids having hydrocarbon chain length between 8 and 18 carbons, in mixtures with their glycerol monoacyl amphiphile derivatives (GMAs). The parameters investigated were critical vesicle concentration (CVC), encapsulation, and temperature-dependent stability. We found that hydrocarbon chain length and the presence of GMAs were major factors related to membrane stability. As chain length increased, GMA additions decreased the CVC of the mixtures 4- to 9-fold. Encapsulation ability also increased significantly as a function of chain length, which reduced permeation of small marker molecules. However, long exposures to temperatures in excess of 60 degrees C resulted in a total release of encapsulated solutes and extensive mixing of the membrane components between vesicles. We conclude that GMAs can significantly increase the stability of mixed amphiphile membranes, but further studies are required to establish model membranes that are stable at elevated temperatures.

Tetrahalide complexes of the [U(NR)2]2+ ion: synthesis, theory, and chlorine K-edge X-ray absorption spectroscopy.

Synthetic routes to salts containing uranium bis-imido tetrahalide anions [U(NR)(2)X(4)](2-) (X = Cl(-), Br(-)) and non-coordinating NEt(4)(+) and PPh(4)(+) countercations are reported. In general, these compounds can be prepared from U(NR)(2)I(2)(THF)(x) (x = 2 and R = (t)Bu, Ph; x = 3 and R = Me) upon addition of excess halide. In addition to providing stable coordination complexes with Cl(-), the [U(NMe)(2)](2+) cation also reacts with Br(-) to form stable [NEt(4)](2)[U(NMe)(2)Br(4)] complexes. These materials were used as a platform to compare electronic structure and bonding in [U(NR)(2)](2+) with [UO(2)](2+). Specifically, Cl K-edge X-ray absorption spectroscopy (XAS) and both ground-state and time-dependent hybrid density functional theory (DFT and TDDFT) were used to probe U-Cl bonding interactions in [PPh(4)](2)[U(N(t)Bu)(2)Cl(4)] and [PPh(4)](2)[UO(2)Cl(4)]. The DFT and XAS results show the total amount of Cl 3p character mixed with the U 5f orbitals was roughly 7-10% per U-Cl bond for both compounds, which shows that moving from oxo to imido has little effect on orbital mixing between the U 5f and equatorial Cl 3p orbitals. The results are presented in the context of recent Cl K-edge XAS and DFT studies on other hexavalent uranium chloride systems with fewer oxo or imido ligands.

Near-infrared organic light emitting diodes

Near-IR emitting organic light emitting diodes that contain an active material consisting of a blend of a poly(paraphenylene) and Ln-porphyrins are prepared and characterized.

THE SYNTHESIS OF BIS(HEXAMETHYLDISILYLAMIDO)BARIUM(II)

Abstract Bis(hexamethyldisilylamido)barium(II) (1) was synthesized in the reaction between BaCl2 and 2 equivalents of NaN(SiMe3)2. Compound 1 forms adducts with a variety of Lewis bases and can be used as a precursor to YBa2Cu3O7−x.

Imido Exchange in Bis(imido) Uranium(VI) Complexes with Aryl Isocyanates

Addition of 1 and 2 equiv of ArNCO (ArPh, 2,4,6-Me3C6H2) to U(NtBu)2(I)2(OPPh3)2 yields the aryl−imido complexes U(NAr)(NtBu)(I)2(OPPh3)2 and U(NAr)2(I)2(OPPh3)2, respectively. Unlike analogous transition metal reactions this imido exchange reaction does not proceed through a metal oxo intermediate. Density functional theory calculations and 15N-labeling studies suggest this transformation involves the [2 + 2] cycloaddition of the aryl isocyanate CN bond across the UN imido ligand to form an N,N-bound ureato intermediate.

Stability of Cations for Anion Exchange Membrane Fuel Cells

The hydrothermal stability of quaternary ammonium hydroxides was evaluated to better understand the degradation of anion exchange membranes used in alkaline fuel cells. Benzyltrimethylammonium hydroxide and phenyltrimethylammonium hydroxide were examined as representative cations for membrane materials. The benzyltrimethylammonium hydroxides displayed much better stability than the phenyltrimethylammonium hydroxides under similar conditions. Additionally, as the concentration of the ammonium hydroxides in water increased, the stability of the cation decreased.

Prebiotically relevant mixed fatty acid vesicles support anionic solute encapsulation and photochemically catalyzed trans-membrane charge transport

The spontaneous assembly of amphiphile-based compartments in aqueous solution is widely viewed as a key step in models for the abiotic formation of primitive cell-like structures. Proposed organic components for such systems consist of mixed short chain fatty acids (FA) and polycyclic aromatic hydrocarbon (PAH) species, the composition of which have been modeled after organic extracts of carbonaceous meteorites. Self-assembly of amphiphiles from these extracts into aqueous suspensions of bilayer structures was long ago demonstrated, although little has since been reported concerning the stability and potential functionality of these complex mixtures. This work explores the thermodynamic and kinetic stability of vesicles prepared from complex mixtures of short chain FA species (CH3COOH–C9H19COOH) with membrane solubilized PAH species. Critical vesicle concentration measurements and ultrafiltration analyses of decanoic acid in the presence of other shorter chain FA species indicate the formation of mixed component vesicle phases composed mainly of C10–C8 FA components. An electrostatic barrier to trans-membrane diffusion of negative charges allows observation of stably encapsulated poly-anionic solutes inside these vesicles. As a model for primitive energy transduction, trans-membrane electron transfer between EDTA and encapsulated ferricyanide was demonstrated, driven catalytically via PAH photochemistry without substantial decomposition of the chromophores or vesicles. These results indicate a plausible role for compartmentalization and catalysis by short chain fatty acids and PAH species in prebiotic vesicle-encapsulated systems.