Search

Asymmetric catalysis is seen as one of the most economical strategies to satisfy the growing demand for enantiomerically pure small molecules in the fine chemical and pharmaceutical industries (1). And visible light has been recognized as an environmentally friendly and sustainable form of energy for triggering chemical transformations and catalytic chemical processes (2-5). For these reasons, visible-light-driven catalytic asymmetric chemistry is a subject of enormous current interest (2-5). Photoredox catalysis provides the opportunity to generate highly reactive radical ion intermediates with often unusual or unconventional reactivities under surprisingly mild reaction conditions (6). In such systems, photoactivated sensitizers initiate a single electron transfer from (or to) a closed-shell organic molecule to produce radical cations or radical anions whose reactivities are then exploited for interesting or unusual chemical transformations. However, the high reactivity of photoexcited substrates, intermediate radical ions or radicals, and the low activation barriers for follow-up reactions provide significant hurdles for the development of efficient catalytic photochemical processes that work under stereochemical control and provide chiral molecules in an asymmetric fashion (7). Here we report a highly efficient asymmetric catalyst that uses visible light for the necessary molecular activation, thereby combining asymmetric catalysis and photocatalysis. We show that a chiral iridium complex can serve as a sensitizer for photoredox catalysis and at the same time provide very effective asymmetric induction for the enantioselective alkylation of 2-acyl imidazoles. This new asymmetric photoredox catalyst, in which the metal centre simultaneously serves as the exclusive source of chirality, the catalytically active Lewis acid centre, and the photoredox centre, offers new opportunities for the 'green' synthesis of non-racemic chiral molecules., Recently, strategies have been developed in which efficient catalytic photochemical processes that work under stereochemical control and provide chiral molecules in an asymmetric fashion can be carried out by two [...]

A sequential chemical separation technique for Cr, Fe, Ni, Zn, and Cu in terrestrial and extraterrestrial silicate rocks was developed for precise and accurate determination of elemental concentration by the isotope dilution method (ID). The technique uses a combination of cation-anion exchange chromatography and Eichrom nickel specific resin. The method was tested using a variety of matrixes including bulk meteorite (Allende), terrestrial peridotite (JP-1), and basalt (JB-lb). Concentrations of each element was determined by thermal ionization mass spectrometry (TIMS) using W filaments and a Si-B-AI type activator for Cr, Fe, Ni, and Zn and a Re filament and silicic acid-H3PO4 activator for Cu. The method can be used to precisely determine the concentrations of these elements in very small silicate samples, including meteorites, geochemical reference samples, and mineral standards for microprobe analysis. Furthermore, the Cr mass spectrometry procedure developed in this study can be extended to determine the isotopic ratios of [sup.53]Cr/[sup.52]Cr and [sup.54]Cr/[sup.52]Cr with precision of ~0.05[epsilon] and ~0.10[epsilon] (1[epsilon] = 0.01%), respectively, enabling cosmochemical applications such as high precision Mn-Cr chronology and investigation of nucleosynthetic isotopic anomalies in meteorites. 10.1021/ac901762a

The discovery two decades ago of metal-free stable carbenes, especially imidazol-2-ylidenes [N-heterocyclic carbenes (NHCs)], has led to numerous breakthroughs in organic and organometallic catalysis. More recently, a small range of complexes has been prepared in which alternative NHC isomers, namely imidazol-5-ylidenes (also termed abnormal NHCs or aNHCs, because the carbene center is no longer located between the two nitrogens), coordinate to a transition metal Here we report the synthesis of a metal-free aNHC that is stable at room temperature, both in the solid state and in solution. Calculations show that the aNHC is more basic than its normal NHC isomer. Because the substituent at the carbon next to the carbene center is a nonbulky phenyl group, a variety of substitution patterns should be tolerated without precluding the isolation of the corresponding aNHC. 10.1126/science.1178206

The photoredox reaction of trisoxalato cobaltate (111) has been studied by means of ultrafast extended x-ray absorption fine structure and optical transient spectroscopy after excitation in the charge-transfer band with 267-nm femtosecond pulses. The Co--O transient bond length changes and the optical spectra and kinetics have been measured and compared with those of ferrioxalate. Data presented here strongly suggest that both of these metal oxalato complexes operate under similar photoredox reaction mechanisms where the primary reaction involves the dissociation of a metal--oxygen bond. These results also indicate that excitation in the charge-transfer band is not a sufficient condition for the intramolecular electron transfer to be the dominant photochemistry reaction mechanism. photoreduction | organometallic | ultrafast spectroscopy | time-resolved EXAFS | photodissociation

Results from the examination of cofactor usage of putative ancient redox proteins showed that the redox functions of primitive proteins depended more or organic cofactors than transition metals. The study implies that the primitive redox proteins mainly consisted of neutral or near-neutral amino acids and organic cofactors, which are further confirmed by the relative amino acid abundance in the primordial world.

The description of two novel double-hybrid functionals, B2K-PLYP and mPW2K-PLYP, is presented. The proposed functionals depict excellent results for main-group thermochemistry, main-group thermochemical kinetics, and late transition metal reactions.

The electron density of 4-methyl-[4]helicene is analyzed with an emphasis on the fjord region as metabolic activation is initiation in this region. The analysis has shown that the highly twisted nature of helicenes have entailed the appearance of a weak closed-shell hydrogen-hydrogen (H-H) bonding interaction in the fjord region of the molecule.

Recent work has proposed that enhanced efficiency can be achieved in photovoltaic cells through implementation of the intermediate band (IB) concept in which a narrow band inserted within the band gap of a semiconductor is used to make the combined absorption of two sub-band gap energy photons lead to the generation of current at the higher voltage corresponding to the full band gap. Trying to tell which materials could have the IB properties necessary for this, quantum calculations within density functional theory at the generalized gradient approximation level have been carried out in this work for chalcopyrite-type copper gallium sulphide structures where gallium is partially substituted by transition metal atoms (Ti, V, Cr, Mn), as these materials are presumed to be candidates for developing the IB electronic structure able to realize this concept. The computed electronic structures characterized by density-of-state curves and band dispersion diagrams, show transition metal-induced spin-polarized characteristics and additional bands appearing in the band gap of the parent sulphide compound. In the results reported here for these compounds, the multiplicity, degree of filling, and energy position of the electronic levels depends on the number of electrons and the electronegativity of the transition element, and they are analyzed in terms of the crystal field splitting produced by the crystalline structure in the metal 3d orbital manifold. For the Ti- and Cr-derived structures (the more interesting ones from the point of view of the intermediate band photovoltaic concept), the stability of these materials is also assessed by computing the energetics of their decomposition in appropriate, known stable compounds. Although this decomposition is found to be favorable, the corresponding energy difference is relatively small, and it is envisaged that they can be made effective experimentally. With these results, the suitability of these materials for use as IB compounds in photovoltaic cells is discussed, and the Ti-substituted one is proposed as the best candidate of this type. [DOI: 10.1115/1.2735345]

The synthesis, X-ray structures, and magnetic properties of a family of [[M.sub.14][(L).sub.6][O.sub.6][(OMe).sub.16][X.sub.6]] cluster where (M = [Fe.sup.III], [Cr.sup.III] and [V.sup.III/IV]) is reported.

A theoretical study of the reaction of acetaldehyde with [Ni.sup.+] is reported. The results reveal that both C-C and aldehyde C-H activations could result in the decarbonylation of C[H.sub.3]CHO with [Ni.sup.+] and [Fe.sup.+], where potential minimum [M.sup.+](H)(CO)(C[H.sub.3]) is located to be a common species on the decarbonylation pathways.

The density functional theory (DFT) calculations are used to investigate the possibility of stabilizing the unstable acepentalene (acp) molecule through coordination to transition metals. The comparison of the results with pentalene complexes has indicated that their acp homologues are slightly less stable, suggesting that they can be synthesized and isolated

A flowing afterglow instrument is used to measure the reaction products and rates for the gas-phase reactions of selected first row transition metal ions and both CO and C[O.sub.2]. The formation and description of products and reaction mechanisms are presented and an ionization mechanism is proposed for the formation of the dications.

The possible transition between icosahedral (ico) and cuboctahedral (fcc) structures in lead nanoclusters of sizes up to 309 atoms was studied by ab inito methods. Spontaneous fcc-to-ico transition in [Pb.sub.13] was observed in the ab inito molecular dynamics (MD) simulations and the calculated energy surface of the transition in [Pb.sub.13] showed that the transition is barrierless.

The complexes formed by the simplest amino acid, glycine with different bare and hydrated metal ions ([Mn.sup.2}, [Fe.sup.2}, [Co.sup.2}, [Ni.sup.2}, [Cu.sup.2}, [Zn.sup.2}) were studied in the gas phase and in solvent. Results in the gas phase showed that the zwitterion of glycine is the form present in the most stable complexes of all ions and addition of water molecules in the number necessary to realize an octahedral hydration shell changes the coordination type of the metals.

The analogous group 4 metal reactions with OCS have resulted in SMCO, OMCS and Mn([[eta].sub.2]-CO)S products. The relatively higher energy of the CO insertion product with respect to the CS insertion product is manifested in lower product yields for the former and the product identifications are verified by isotopic substitution and by density functional vibrational frequency calculations.

The structure and bonding of halogens on various metal low-index surfaces was studied by means of density functional theory (DFT) calculations using periodic slabs to model the surface. It was seen that periodic trends in bond ionicities and metal -halogen vibrational frequencies are in harmony with data derived in electrochemical environment, indicating that the metal-halogen bonding in vacuum share some features with the electrode metal surface-halogen bonding.

The well-known photoreduction of polyoxometalates by alcohols are exploited to produce the reduced species, which cannot be directly produced electrochemically in nonaqueous solvents. Photochemistry is a reliable means to produce stable reduced transition metal substituted heteropoly tungstates in nonpolar media, where they hold promise as multielectron reduction catalysts.

Adventures involving the synthesis and characterization of substances containing s,p- and d-block metals in negative oxidation states are described. A classification of these species and discussions of formal oxidation state assignments for low-valent transition metals in complexes are included, along with comments on the innocent character of ligands in metalates.

A detailed study of the nature of the bottom t(sub 2g)-block bands of layered perovskites with d(super x) transition metals based on accurate first-principles calculations is reported. An important feature that must be taken into account for some of the systems in which the t(2g)-block bands exhibit low band fillings is disorder.

The redox-active species that facilitate H2O2/transition metals and NO/O2 and O2(super -)-mediated Ras guanine nucleotide dissociation in the presence and absence of CO2 are identified and characterized. The effect of redox agents on Ras GNE (GDP- exchange for GTP) and binding interactions with the Ras-binding domain (RBD) of Raf-1 are examined to assess the action of redox agents in vitro under conditions that might mimic redox-mediated Ras-signaling processes observed in vivo.

Manganese(II), iron(II), cobalt(II), and copper(II) derivatives of two inherently chiral, Tris(bipyridyl) cages (L and L') of type [ML]-[(P[F.sub.6]).sub.2][(solvent).sub.n] and [FeL'][(CI[O.sub.4]).sub.2] are reported, where L is the hexa-tertiary butyl-substituted derivative of L'. These products were obtained by using the free cage and metal template procedures; the latter involved the reductive amination of the respective Tris-dialdehyde precursor complexes of iron(II), cobalt(II), or nickel(II). Electrochemical, EPR, and NMR studies have been used to probe the nature of the individual complexes. X-ray structures of the manganese(II), iron(II), and copper(II) complexes of L and the iron(II) complex of L' are presented; these are compared with the previously reported structures of the corresponding nickel(II) complex and metal-free cage (L). In each complex the metal cation occupies the cage's central cavity and is coordinated to six nitrogens from the three bipyridyl groups. The cations [[MnL].sup.2+] and [[FeL].sup.2+] are isostructural but both exhibit a different arrangement of the bound cage to that observed in the corresponding nickel(II) and copper(II) complexes. The latter have an exo-exo arrangement of the bridgehead nitrogen lone pairs, with the metal inducing a triple helical twist that extends [approximately equal to] 22 [Angstrom] along the axial length of each complex. In contrast, [[MnL].sup.2+] and [[FeL].sup.2+] have their terminal nitrogen lone pairs directed endo, causing a significant change in the configuration of the bound ligand. In [[FeL'].sup.2+], the cage has both bridgehead nitrogen lone pairs orientated exo. Semiempirical calculations indicate that the observed endo-endo and exo-exo arrangements are of comparable energy. encapsulation | template | transition metal | triple helix

The total synthesis of rac-heliophenanthrone was achieved by a convergent approach, making use of a transition-metal-catalyzed domino process with an intramolecular Diels-Alder reaction at an isobenzopyrylium cation as key step. The transition-metal-catalyzed formation of isobenzopyrylium salts with a subsequent Diels-Alder reaction was proven to be very valuable method for the construction of highly functionalized carbocyclic ring systems, with heliophenanthrone as a representative example.

The efforts to prepare new Zintl compounds with transition metals resulted in the discovery of Sr2MnSb2, which features a unique two-dimensional corrugated anionic framework. The synthesis, crystal structure, and magnetic properties of Sr2MnSb2 are reported.

The study shows that the manganacarborane dianion in [N(PPh3)2][NEt4][1,1,1-(CO)3-2-Ph-closo-1,2-MnCB9H9] reacts with cationic transition metal-ligand fragments to give products in which the electrophilic metal groups (M') are exo-polyhedrally attached to the {closo-1,2-MnCB9} cage system. Details of the x-ray diffraction studies carried out to elucidate the structures encountered in compounds are discussed.

Nitrogen containing 15-membered triacetylenic macrocycles known as 1,6,11-tris(arylsulfonyl)-1,6,11-triazacyclopentadeca-3,8,13-triynes and enediynic macrocycles called 1,6,11-tris(arylsulfonyl)-1,6,11-triazacyclopentadeca-3-ene-8,13-diynes were prepared to test [2+2+2] cycloisomerization processes catalyzed by transition metals. The RhCl(CO)(PPh3)2 complex was found to catalyze the cycloisomerization reaction giving the desired cycloadducts in high yields.

The transition-metal-incorporated nickel phosphate molecular sieves (TMI-VSB-5) are hydrothermally synthesized at 453 K in weak basic conditions under microwave irradiation. The study demonstrated the nanoporous VSB-5 has a versatile framework to accommodate various TMIs with several oxidation states and symmetries.

Cyclopentadienes, 2H-phospholes can act both as dienes and dienophiles. 2H-phospholes can readily deprotonated to give highly aromatic anions and can readily react with transition metal derivatives to give n5-complexes representing a diagonal phosphorus-carbon analogy.

Stable phases of nitroprussides of divalent transition metals forming a family of microporous materials with three typical crystal structures and behaviors are investigated. The stable phases of this family of materials belong to following crystal structures, orthorhombic (Pnma) (Mn(super 2+), Fe(super 2+), Zn(super 2+), and Cd(super 2+)), cubic (Fm3m) (Co(super 2+) and Ni (super 2+)) and orthorhombic (Amm2) (Cu(super 2+))

The topological analysis of the electron localization function (ELF) is applied to explain the electronic structure and the geometry of the M-CO complexes and to derive simple rules based on the topological properties of the latter function. It is shown that the interaction between a transition metal and CO should be characterized as a dative bond, in which the monosynaptic basin of the carbon plays the role of the disynaptic basin connecting the metal core to the carbon atom.

The gas-phase removal rate constants for the reaction of ground state palladium (a(super 1)S(sub 0) with oxygen in argon buffer was reported as a function of temperature and pressure. It is reported that palladium atoms were produced by the photodissociation of palladium trifluoroacete, which were detected by laser-induced fluorescence.

The reactions of vinyl chloride (VC) with representative late metal, single-site olefin dimerization and polymerization catalysts are investigated. Termination of chain growth by beta-Cl eliminations is the most significant obstacle of metal-catalyzed insertion polymerization of VC.

Study was conducted to examine the adsorption of acetylene and hydrogen on the (111) surface planes of Pd, Pt, Ni, and Rh using plane-wave density functional theory and extended Huckel calculations. Hydrogen is found to interact most favorably with 3-fold hollow sites on the surface, although the potential-energy surface is relatively flat for all metals, especially Pt.

Reaction products of laser-ablated palladium, platinum, silver and gold atoms' reaction with molecular oxygen in excess neon during condensation at 4K are identified from isotopic shifts and splitting in their matrix infrared spectra. The reaction mechanism and comparison with argon matrix spectra are discussed.

The reversible proton dissociation and geminate recombination of photoacids as a function of temperature in monols, diols, and glycerol is studied. The experimental data are analyzed and solved numerically.

The oxides TiO, CrO, and CoO, formed by reaction of the laser-ablated metal toms and O2 in excess argon during condensation at 10 K are laser desorbed/ionized from solid argon with 308 nm radiation for observation by TOF mass spectrometry. The laser desorption/ionoztion process in cryogenic experiments with low ionization energy subject atoms and molecules is assisted by the organic acid and bromobenzene chromopores.

The effects of water vapor on the temperature-dependent surface oxidation of Pt-group metals in ambient-pressure gaseous oxygen environment are analyzed by means of surface-enhanced Raman spectroscopy (SERS). Findings are believed to provide clear-cut evidence demonstrating the important role played by gaseous water in facilitating the O2-induced surface oxidation of Pt-group metals.

Titanium is incorporated into mesoporous silica using novel one-pot procedures based on the true liquid crystal templating (TLCT) synthesis method. It was demonstrated that the TLCT synthesis route could be used to incorporate Ti catalytic sites into the silica mesoporous products in a controllable way.

A novel KO'-Bu-promoted three-component coupling reaction of aldehydes and alkynes without a transition-metal catalyst is developed. This tandem protocol represents an extremely concise, practical, and economical synthesis of (E)-1-en-4-yn-3-ols and (Z)-2-en-4-yn-1-ols, which are of versatile utilities in organic synthesis.

Computational studies are used for verifying the role of the Kubas interaction in transition metal grafted mesoporous silicas and to rationalize unusual rising enthalpy trends with surface coverage by hydrogen in these systems. The high enthalpies predicted for organometallic fragments containing hydride ligands and the low affinity of Cr(III) for hydrogen have shown that the features are not ideal for a practical hydrogen storage system.

Unexpected reactivity on the part of oxide nanoparticles that has enabled their transformation into phosphides or sulfides by solution-phase reaction with trioctylphosphine (TOP) or sulfur is described. The use of oxide nanoparticles in lieu of organometallic reagents, which may or may not be transformed into metal nanoparticles, has simplified the production of nanoscale phosphides and sulfides.

First principles calculations have displayed the high hydrogen storage capacity of metallacarboranes, where the transition metal (TM) atoms can bind up to 5 [H.sub.2]-molecules. The presence of carbon atom in the cages has permitted linking the metallacarboranes to form metal organic frameworks, which are able to adsorb hydrogen via Kubas interaction along with van der Waals physisorption.