Ligand Design for Isomer-Selective Oxorhenium(V) Complex Synthesis.

Recently, N,N-trans Re(O)(L)X (L = monoanionic N-O chelates; X = Cl or Br prior to being replaced by solvents or alkoxides) complexes have been found to be superior to the corresponding N,N-cis isomers in the catalytic reduction of perchlorate via oxygen atom transfer. However, reported methods for Re(O)(L)X synthesis often yield only the N,N-cis complex or a mixture of trans and cis isomers. This study reports a geometry-inspired ligand design rationale that selectively yields N,N-trans Re(O)(L)Cl complexes.

Configuration Control in the Synthesis of Homo- and Heteroleptic Bis(oxazolinylphenolato/thiazolinylphenolato) Chelate Ligand Complexes of Oxorhenium(V): Isomer Effect on Ancillary Ligand Exchange Dynamics and Implications for Perchlorate Reduction Catalysis.

This study develops synthetic strategies for N,N-trans and N,N-cis Re(O)(LO-N)2Cl complexes and investigates the effects of the coordination spheres and ligand structures on ancillary ligand exchange dynamics and catalytic perchlorate reduction activities of the corresponding [Re(O)(LO-N)2](+) cations. The 2-(2'-hydroxyphenyl)-2-oxazoline (Hhoz) and 2-(2'-hydroxyphenyl)-2-thiazoline (Hhtz) ligands are used to prepare homoleptic N,N-trans and N,N-cis isomers of both Re(O)(hoz)2Cl and Re(O)(htz)2Cl and one heteroleptic N,N-trans Re(O)(hoz)(htz)Cl. Selection of hoz/htz ligands determines the preferred isomeric coordination sphere, and the use of substituted pyridine bases with varying degrees of steric hindrance during complex synthesis controls the rate of isomer interconversion.

Critical review of Pd-based catalytic treatment of priority contaminants in water.

Catalytic reduction of water contaminants using palladium (Pd)-based catalysts and hydrogen gas as a reductant has been extensively studied at the bench-scale, but due to technical challenges it has only been limitedly applied at the field-scale. To motivate research that can overcome these technical challenges, this review critically analyzes the published research in the area of Pd-based catalytic reduction of priority drinking water contaminants (i.e.

Enhanced activity and selectivity of carbon nanofiber supported Pd catalysts for nitrite reduction.

Pd-based catalyst treatment represents an emerging technology that shows promise to remove nitrate and nitrite from drinking water. In this work we use vapor-grown carbon nanofiber (CNF) supports in order to explore the effects of Pd nanoparticle size and interior versus exterior loading on nitrite reduction activity and selectivity (i.e.

Heterogeneous catalytic reduction of perchlorate in water with Re-Pd/C catalysts derived from an oxorhenium(V) molecular precursor.

The molecular Re(V) complex, chlorobis(2-(2'-hydroxyphenyl)-2-oxazoline)-oxorhenium(V), Re(O)(hoz)(2)Cl, has been investigated as a suitable precursor, when combined with activated carbon powder containing 5 wt % Pd, to provide a heterogeneous catalyst for the reduction of aqueous perchlorate by hydrogen. Two general methods for catalyst preparation have been adopted: first, by a standard "incipient wetness" impregnation of the carbon powder with handling under largely aerobic conditions for convenience and, second, by a completely anaerobic procedure maintaining a hydrogen atmosphere during adsorption of the complex in water onto the powder. Both types of catalyst were efficient for the complete reduction of perchlorate to chloride within a few hours at room temperature over a range of initial concentrations (2-200 ppm) under 1 atm of H(2) and acidic conditions (pH 2.

Influence of rhenium speciation on the stability and activity of Re/Pd bimetal catalysts used for perchlorate reduction.

Recent work demonstrates reduction of aqueous perchlorate by hydrogen at ambient temperatures and pressures using a novel rhenium-palladium bimetal catalyst immobilized on activated carbon (Re/Pd-AC). This study examines the influence of Re speciation on catalyst activity and stability. Rates of perchlorate reduction are linearly dependent on Re content from 0-6 wt %, but no further increases are observed at higher Re contents.

Enhancement of oxyanion and diatrizoate reduction kinetics using selected azo dyes on Pd-based catalysts.

Azo dyes are widespread pollutants and potential cocontaminants for nitrate; we evaluated their effect on catalytic reduction of a suite of oxyanions, diatrizoate, and N-nitrosodimethylamine (NDMA). The azo dye methyl orange significantly enhanced (less than or equal to a factor of 5.24) the catalytic reduction kinetics of nitrate, nitrite, bromate, perchlorate, chlorate, and diatrizoate with several different Pd-based catalysts; NDMA reduction was not enhanced.

Ligand-enhanced reduction of perchlorate in water with heterogeneous Re-Pd/C catalysts.

New bimetallic heterogeneous catalysts for the reduction of perchlorate in water with dihydrogen have been prepared in two ways: (1) by impregnating 5 wt % Pd on activated carbon powder with one of the complexes trans-[ReO(2)(py-X)(4)](+) (py-X = 4-substituted pyridine; X = H, Me, OMe, NMe(2)) or (2) by adsorbing perrhenate onto the Pd/C powder in the presence of the pyridine ligand under a hydrogen atmosphere. Both sets of catalysts are highly active at pH 2.7-3.

Rapid reduction of N-nitrosamine disinfection byproducts in water with hydrogen and porous nickel catalysts.

There is a need for new technologies to rapidly and economically treatwater contaminated with N-nitrosodimethylamine (NDMA) and related compounds because of their high toxicity and recent detection in drinking water sources as a consequence of industrial releases and chlorine disinfection of wastewater effluent Treatment of N-nitrosamines with H2 in conjunction with a high surface area porous nickel material, a model nonprecious metal catalyst, has been evaluated. Experiments show that NDMA is reduced rapidly and catalytically to dimethylamine and N2 (e.g.

Rapid metal-catalyzed hydrodehalogenation of Iodinated x-ray contrast media.

Iodinated X-ray contrast media (ICM) are detected in natural waters at high concentrations relative to other pharmaceuticals due to extensive use in medical diagnostics and high recalcitrance during conventional wastewater treatment. This study examines, for the first time, reductive treatment of ICM in water with hydrogen gas in combination with supported palladium and porous nickel catalysts. Kinetic experiments demonstrate rapid and complete hydrodehalogenation of both ionic (diatrizoate) and neutral (iopromide) ICM.

Regeneration of sulfur-fouled bimetallic Pd-based catalysts.

Pd-based catalysts provide efficient and selective reduction of several drinking water contaminants, but their long-term application requires effective treatments for catalyst regeneration following fouling by constituents in natural waters. This studytested alumina-supported Pd-Cu and Pd-In bimetallic catalysts for nitrate reduction with H2 after sulfide fouling and oxidative regeneration procedures. Both catalysts were severely deactivated after treatment with microM levels of sulfide.

Efficient heterogeneous catalytic reduction of perchlorate in water.

A new heterogeneous catalyst that promotes the reduction by hydrogen of perchlorate ion in water under mild conditions has been developed. The catalyst is prepared by adsorption of a rhenium(VII) precursor (either ammonium perrhenate or methylrhenium trioxide) onto carbon powder containing 5% palladium by weight. Under standard batch conditions of room temperature, 1 bar of hydrogen, and 200 ppm perchlorate (as HClO4), reduction proceeded to less than 1 ppm in as little as 5 h.

Metal-catalyzed reduction of N-nitrosodimethylamine with hydrogen in water.

There is considerable need for the rapid destruction of N-nitrosodimethylamine (NDMA) in water because current alternative treatment methods are relatively inefficient. Powdered metal catalysts in conjunction with hydrogen gas showed notable potential for rapid destruction of N-nitrosodimethylamine (NDMA) in water. Palladium, copper-enhanced palladium, and nickel catalysts showed significant efficacy for NDMA reduction, with observed half-lives on the order of hours using 10 mg L(-1) catalyst metal.

Effects of natural water ions and humic acid on catalytic nitrate reduction kinetics using an alumina supported Pd-Cu catalyst.

Catalytic nitrate reduction was evaluated for the purpose of drinking water treatment. Common anions present in natural waters and humic acid were evaluated for their effects on NO3(-) hydrogenation over a bimetallic supported catalyst (Pd-Cu/gamma-Al2O3). Groundwater samples, with and without powder activated carbon (PAC) pretreatment, were also evaluated.

Synthesis and Properties of IrRe(2)(&mgr;-H)(2)(CO)(9)(eta(5)-C(9)H(7)).

The slow addition of Re(2)(&mgr;-H)(2)(CO)(8) to a solution of Ir(CO)(eta(2)-C(8)H(14))(eta(5)-C(9)H(7)) in hexane at reflux provides IrRe(2)(&mgr;-H)(2)(CO)(9)(eta(5)-C(9)H(7)) (1) in 80% yield. The molecular structure of 1 shows an IrRe(2) triangle incorporating one Ir(CO)(eta(5)-C(9)H(7)) and two Re(CO)(4) fragments. The strongly different Ir-Re distances suggest that one hydride ligand bridges one Ir-Re edge and the other hydride bridges the Re-Re edge.

Synthesis, Characterization, and Comparative Properties of [PPN](2)[Re(6)C(CO)(18)Mo(CO)(4)] and [PPN](2)[Re(6)C(CO)(18)Ru(CO)(3)].

The reaction of [PPN](2)[Re(6)C(CO)(19)] with Mo(CO)(6) and Ru(3)(CO)(12) under sunlamp irradiation provided the new mixed-metal clusters [PPN](2)[Re(6)C(CO)(18)Mo(CO)(4)] and [PPN](2)[Re(6)C(CO)(18)Ru(CO)(3)], which were isolated in yields of 85% and 61%, respectively. The compound [PPN](2)[Re(6)C(CO)(18)Mo(CO)(4)] crystallizes in the monoclinic space group P2(1)/c with a = 20.190 (7) Å, b = 16.