An unusual chemoselective hydrogenation of quinoline compounds using supported gold catalysts.

The pursuit of modern sustainable chemistry has stimulated the development of innovative catalytic processes that enable chemical transformations to be performed under mild and clean conditions with high efficiency. Herein, we report that gold nanoparticles supported on TiO(2) catalyze the chemoselective hydrogenation of functionalized quinolines with H(2) under mild reaction conditions. Our results point toward an unexpected role for quinolines in gold-mediated hydrogenation reactions, namely that of promoter; this is in stark contrast to what prevails in the traditional noble metal Pd-, Pt-, and Ru-based catalyst systems, in which quinolines and their derivatives typically act as poisons. As a result of the remarkable promotional effect of quinoline molecules to H(2) activation over supported gold, the transformation can proceed smoothly under very mild conditions (even at temperatures as low as 25 °C). Of practical significance is that various synthetically useful functional groups including halogens, ketone, and olefin remain intact during the hydrogenation of quinolines. Moreover, the protocol also shows promise for the regiospecific hydrogenation of the heterocyclic ring of a variety of other biologically important heteroaromatic nitrogen compounds, such as isoquinoline, acridine, and 7,8-benzoquinoline, in a facile manner. Apart from its importance in catalytic hydrogenation, we believe that this intriguing self-promoted effect by reactant molecules may have fundamental implications for the broad field of gold catalysis and form the basis for development of new catalytic procedures for other key transformations.

Efficient subnanometric gold-catalyzed hydrogen generation via formic acid decomposition under ambient conditions.

Formic acid (FA) has tremendous potential as a safe and convenient source of hydrogen for sustainable chemical synthesis and renewable energy storage, but controlled and efficient dehydrogenation of FA by a robust solid catalyst under ambient conditions constitutes a major challenge. Here, we report that a previously unappreciated combination of subnanometric gold and an acid-tolerant oxide support facilitates the liberation of CO-free H(2) from FA. Applying an ultradispersed gold catalyst comprising TEM-invisible gold subnanoclusters deposited on zirconia to a FA-amine mixture affords turnover frequencies (TOFs) up to 1590 per hour and a turnover number of more than 118,400 at 50 °C. The reaction was accelerated at higher temperatures, but even at room temperature, a significant H(2) evolution (TOFs up to 252 h(-1) after 20 min) can still be obtained. Preliminary mechanistic studies suggest that the reaction is unimolecular in nature and proceeds via a unique amine-assisted formate decomposition mechanism on Au-ZrO(2) interface.

Matrix isolation spectroscopic and theoretical study of carbon dioxide activation by titanium oxide molecules.

The reactions of titanium monoxide and dioxide molecules with carbon dioxide were investigated by matrix isolation infrared spectroscopy. It was found that the titanium monoxide molecule is able to activate carbon dioxide to form the titanium dioxide-carbon monoxide complex upon visible light excitation via a weakly bound TiO(η(1)-OCO) intermediate in solid neon. In contrast, the titanium dioxide molecule reacted with carbon dioxide to form the titanium monoxide-carbonate complex spontaneously on annealing. Theoretical calculations predicted that both activation processes are thermodynamically exothermic and kinetically facile.

The conformational transition pathway of ATP binding cassette transporter MsbA revealed by atomistic simulations.

ATP binding cassette transporters are integral membrane proteins that use the energy released from ATP hydrolysis at the two nucleotide binding domains (NBDs) to translocate a wide variety of substrates through a channel at the two transmembrane domains (TMDs) across the cell membranes. MsbA from Gram-negative bacteria is a lipid and multidrug resistance ATP binding cassette exporter that can undergo large scale conformational changes between the outward-facing and the inward-facing conformations revealed by crystal structures in different states. Here, we use targeted molecular dynamics simulation methods to explore the atomic details of the conformational transition from the outward-facing to the inward-facing states of MsbA. The molecular dynamics trajectories revealed a clear spatiotemporal order of the conformational movements. The disruption of the nucleotide binding sites at the NBD dimer interface is the very first event that initiates the following conformational changes, verifying the assumption that the conformational conversion is triggered by ATP hydrolysis. The conserved x-loops of the NBDs were identified to participate in the interaction network that stabilizes the cytoplasmic tetrahelix bundle of the TMDs and play an important role in mediating the cross-talk between the NBD and TMD. The movement of the NBD dimer is transmitted through x-loops to break the tetrahelix bundle, inducing the packing rearrangements of the transmembrane helices at the cytoplasmic side and the periplasmic side sequentially. The packing rearrangement within each periplasmic wing of TMD that results in exposure of the substrate binding sites occurred at the end stage of the trajectory, preventing the wrong timing of the binding site accessibility.

Elastic network model-based normal mode analysis reveals the conformational couplings in the tripartite AcrAB-TolC multidrug efflux complex.

The AcrAB-TolC drug efflux system, energized by proton movement down the transmembrane electrochemical gradient, is responsible for the resistance of the organism to a wide range of drugs. Experimental data suggest functional roles of each part of the assembly, but the detailed working mechanism of this machinery remains elusive. We used elastic network-based normal mode analysis (NMA) to explore the conformational dynamics of the AcrAB-TolC complex. The intrinsic flexibilities of the pore domain in AcrB monomer conform to the previously proposed three-step functionally rotating mechanism for asymmetric AcrB trimer. Conformational couplings across monomers in the AcrB trimer were observed, and the coupling between the transmembrane domain and the other parts of AcrB are strengthened through trimeric assembly. In the tripartite AcrAB-TolC assembly obtained through molecular docking, concerted motions were observed not only at the direct contact interfaces between various components but also between distant parts of the whole complex. The presence of AcrA was shown to significantly strengthen the motional couplings between AcrB and TolC. Overall, NMA revealed an allosteric network in the AcAB-TolC efflux system, which provides hints to our understanding of its detailed working mechanism.

Liquefaction of solid-state BH3NH3 by gaseous NH3.

This paper reports for the first time that under ammonia atmosphere, ammonia borane (AB) reversibly absorbs up to at least 6 equiv of NH(3), forming liquid AB(NH(3))(n) (n = 1-6) complexes at 0 °C. Reasonable structures for AB(NH(3))(n) were identified via density functional theory calculations, which indicate that the strong classical hydrogen bond formed between the lone pair of NH(3) and the -NH(3) of AB is the driving force for the absorption of ammonia by AB. By use of the van't Hoff equation, the enthalpy change (ΔH) for AB to absorb one NH(3) was determined to be -2.24 kcal/mol, which is in good agreement with the theoretical calculations. Other organic amines were screened to further confirm the role of the N lone pair; only 1,4-diazabicyclo[2.2.2]octane (DABCO) formed a stable adduct, which X-ray structural analysis showed was the DABCO-BH(3) species. Finally, Raman spectra of AB(NH(3))(n) were collected, and its unique spectral features are also discussed.

CO oxidation catalyzed by a single gold atom: benchmark calculations and the performance of DFT methods.

Quantum chemical calculations were carried out on CO oxidation catalyzed by a single gold atom. To investigate the performance of density functional theory (DFT) methods, 42 DFT functionals have been evaluated and compared with high-level wavefunction based methods. It was found that in order to obtain accurate results the functionals used must treat long range interaction well. The double-hybrid mPW2PLYP and B2PLYP functionals are the two functionals with best overall performance. CAM-B3LYP, a long range corrected hybrid GGA functional, also performs well. On the other hand, the popular B3LYP, PW91, and PBE functionals do not show good performance and the performance of the latter two are even at the bottom of the 42 functionals. Our accurate results calculated at the CCSD(T)/aug-cc-pVTZ//mPW2PLYP/aug-cc-pVTZ level of theory indicate that Au atom is a good catalysis for CO oxidation. The reaction follows the following mechanism where CO and O(2) adsorb on Au atom forming an Au(OCOO) intermediate and subsequently O(2) transfer one oxygen atom to CO to form CO(2) and AuO. Then AuO reacts with CO to form another CO(2) to complete the catalytic cycle. The overall energy barrier at 0 K for the first CO oxidation step (Au + CO + O(2)→ AuO + CO(2)) is just 4.8 kcal mol(-1), and that for the second CO oxidation step (AuO + CO → Au + CO(2)) is just 1.6 kcal mol(-1).

A hybrid sol­gel synthesis of mesostructured SiC with tunable porosity and its application as a support for propane oxidative dehydrogenation.

Porous silicon carbide (SiC) is of great potential as catalyst support in several industrially important reactions because of its unique thermophysical characteristics. Previously porous SiC was mostly obtained by a simple sol­gel or reactive replica technique which can only produce a material with low or medium surface area (< 50 m2 g(−1)). Here we report a new hybrid sol­gel approach to synthesize mesostructured SiC with high surface area (151­345 m2 g(−1)) and tunable porosity. The synthesis route involves a facile co-condensation of TEOS and alkyloxysilane with different alkyl-chain lengths followed by carbothermal reduction of the as-prepared alkyloxysilane precursors at 1350 °C. The resulting materials were investigated by X-ray diffraction, N2 adsorption-desorption, transmission electron microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. A mechanism for the tailored synthesis of mesostructured SiC was tentatively proposed. To demonstrate the catalytic application of these materials, vanadia were loaded on the mesostructured SiC supports, and their catalytic performance in oxidative dehydrogenation of propane was evaluated. Vanadia supported on the mesostructured silicon carbide exhibits higher selectivity to propylene than those on conventional supports such as Al2O3 and SiO2 at the same propane conversion levels, mainly owing to its outstanding thermal conductivity which makes contributions to dissipate the heat generated from reaction thus alleviating the hot spots effect and over-oxidation of propylene.

Titanium oxide complexes with dinitrogen. Formation and characterization of the side-on and end-on bonded titanium oxide-dinitrogen complexes in solid neon.

The reactions of titanium oxide molecules with dinitrogen have been studied by matrix isolation infrared spectroscopy. The titanium monoxide molecule reacts with dinitrogen to form the TiO(N(2))(x) (x = 1-4) complexes spontaneously on annealing in solid neon. The TiO(η(1)-NN) complex is end-on bonded and was predicted to have a (3)A'' ground state arising from the (3)Δ ground state of TiO. Argon doping experiments indicate that TiO(η(1)-NN) is able to form complexes with one or more argon atoms. Argon atom coordination induces a large red-shift of the N-N stretching frequency. The TiO(η(2)-N(2))(2) complex was characterized to have C(2v) symmetry, in which both the N(2) ligands are side-on bonded to the titanium metal center. The tridinitrogen complex TiO(η(1)-NN)(3) most likely has C(3v) symmetry with three end-on bonded N(2) ligands. The TiO(η(1)-NN)(4) complex was determined to have a C(4v) structure with four equivalent end-on bonded N(2) ligands. In addition, evidence is also presented for the formation of the TiO(2)(η(1)-NN)(x) (x = 1-4) complexes, which were predicted to be end-on bonded.

Carbon dioxide coordination and activation by niobium oxide molecules.

Carbon dioxide coordination and activation by niobium oxide molecules were studied by matrix isolation infrared spectroscopy. It was found that the niobium monoxide molecule reacted with carbon dioxide to form the niobium dioxide carbonyl complex NbO(2)(η(1)-CO) spontaneously on annealing in solid neon. The observation of the spontaneous reaction is consistent with theoretical predictions that this carbon dioxide activation process is both thermodynamically exothermic and kinetically facile. In contrast, four niobium dioxide-carbon dioxide complexes exhibiting three different coordination modes of CO(2) were formed from the reactions between niobium dioxide and carbon dioxide, which proceeded with the initial formation of the η(1)-O bound NbO(2)(η(1)-OCO) and NbO(2)(η(1)-OCO)(2) complexes on annealing. The NbO(2)(η(1)-OCO) complex rearranged to the η(2)-O,O bound NbO(2)(η(2)-O(2)C) isomer under visible light irradiation, while the NbO(2)(η(1)-OCO)(2) complex isomerized to the NbO(2)(η(1)-OCO)(η(2)-OC)O structure involving an η(2)-C,O ligand under IR excitation. In these niobium dioxide carbon dioxide complexes, the η(1)-O coordinated CO(2) ligand serves as an electron donor, whereas both the η(2)-C,O and η(2)-O,O coordinated CO(2) ligands act as electron acceptors.

Fe(x)O(y)@C spheres as an excellent catalyst for Fischer-Tropsch synthesis.

We demonstrate a one-pot hydrothermal cohydrolysis-carbonization process using glucose and iron nitrate as starting materials for the fabrication of carbonaceous spheres embedded with iron oxide nanoparticles. It is verified by TEM, (57)Fe Mossbauer, and Fe K-edge XAS that iron oxide nanoparticles are highly dispersed in the carbonaceous spheres, leading to a unique microstructure. A formation mechanism is also proposed. This route is also applicable to a range of other naturally occurring saccharides and metal nitrates. A catalytic study revealed the remarkable stability and selectivity of the reduced Fe(x)O(y)@C spheres in the Fischer-Tropsch synthesis, which clearly exemplifies the promising application of such materials.

Validation of density functional methods for the calculation of small gold clusters.

The performance of various density functional theory methods on the geometries and energetics of Au(2), Au(3), Au(4), and Au(5) has been systematically evaluated. The results were compared with those from experiments or high-level wave function theory methods. In the present study, spin-orbit (SO) coupling was considered. It was found that SO coupling plays a very important role in the calculation of both the atomization energies and relative stability of the isomers of gold clusters. Functionals including SO coupling effect will overestimate the atomization energies of gold clusters compared with those just including the scalar relativistic (SC) effect. On the other hand, hybrid functionals will underestimate the atomization energies compared with those of the corresponding pure functionals. For the calculation of the relative stability of the different isomers, many functionals not including SO coupling will predict the wrong stability order. In addition, SO correction to the atomization energy of the cluster (ΔE(SO)) has a weak dependence on the choice of functional. A linear relationship was established between ΔE(SO) and the number of Au atoms and Au-Au bonds in the cluster. The relationship indicates that inclusion of SO coupling will favor the isomer with more Au-Au bonds. Among all of the functionals evaluated, the SO TPSSh method has the best overall performance, and SC M06-L also performs well, although it predicts that the two isomers of Au(3) are almost degenerate in energy.

Asymmetric conformational flexibility in the ATP-binding cassette transporter HI1470/1.

Putative metal-chelate-type ABC transporter HI1470/1 is homologous with vitamin B(12) importer BtuCD but exhibits a distinct inward-facing conformation in contrast to the outward-facing conformation of BtuCD. Normal-mode analysis of HI1470/1 reveals the intrinsic asymmetric conformational flexibility in this transporter and demonstrates that the transition from the inward-facing to the outward-facing conformation is realized through the asymmetric motion of individual subunits of the transporter. This analysis suggests that the asymmetric arrangement of the BtuC dimer in the crystal structure of the BtuCD-F complex represents an intermediate state relating HI1470/1 and BtuCD. Furthermore, a twisting motion between transmembrane domains and nucleotide-binding domains encoded in the lowest-frequency normal mode of this type of importer is found to contribute to the conformational transitions during the whole cycle of substrate transportation. A more complete translocation mechanism of the BtuCD type importer is proposed.

Gold supported on hydroxyapatite as a versatile multifunctional catalyst for the direct tandem synthesis of imines and oximes.

Two birds with one auric stone: The title system acts as a highly efficient heterogeneous catalyst for the one-pot tandem synthesis of imines or oximes from alcohols and the corresponding amines under mild conditions (see scheme; HAP = hydroxyapatite).

The conformational coupling and translocation mechanism of vitamin B12 ATP-binding cassette transporter BtuCD.

ATP-binding cassette transporter BtuCD mediating vitamin B(12) uptake in Escherichia coli couples the energy of ATP hydrolysis to the translocation of vitamin B(12) across the membrane into the cell. Elastic normal mode analysis of BtuCD demonstrates that the simultaneous substrate trapping at periplasmic cavity and ATP binding at the ATP-binding cassette (BtuD) dimer proceeds readily along the lowest energy pathway. The transport power stroke is attributed to ATP-hydrolysis-induced opening of the nucleotide-binding domain dimer, which is coupled to conformational rearrangement of transmembrane domain (BtuC) helices leading to the closing at the periplasmic side and opening at the cytoplasmic gate. Simultaneous hydrolysis of two ATP is supported by the fact that antisymmetric movement of BtuD dimer implying alternating hydrolysis cannot induce effective conformational change of the translocation pathway. A plausible mechanism of translocation cycle is proposed in which the possible effect of the association of periplasmic binding protein BtuF to the transporter is also considered.

Synergy between crystal strain and surface energy in morphological evolution of five-fold-twinned silver crystals.

In polyol synthesis at relatively low concentrations of polyvinylpyrrolidone (PVP, surface-capping agent), some micrometer-size Ag truncated decahedra with five-fold-twinned structure were clearly observed. This result indicates that the internal strain in the five-fold-twinned crystals is not sufficient to restrict their lateral growth. On the basis of first-principles calculation of their surface energies before and after the adsorption of surface-capping agent, it is proposed that the surface energy difference between the (100) and (111) facets (Delta phi(100)-(111)) plays an essential role in the aggressive growth of lateral (100) facets, overcoming the strain restriction. When enough PVP is adsorbed on the surfaces of Ag crystals, the Delta phi(100)-(111) will evidently decrease, and then the strain restriction on the lateral growth becomes predominant, resulting in pentagonal anisotropic structures. Following this mechanism, diameter-tunable Ag wires have been synthesized by controlling the axial growth and the lateral growth separately at higher and lower PVP concentrations, respectively.

Efficient and chemoselective reduction of carbonyl compounds with supported gold catalysts under transfer hydrogenation conditions.

A new heterogeneous catalytic transfer hydrogenation (CTH) system, consisting of a non-flammable supported Au catalyst along with 2-propanol as the hydrogen donor, was proven to be effective for chemoselective reduction of a wide range of aromatic ketones and aldehydes to the corresponding alcohols.

Is the FeO2(-) anion bent or linear?

FeO(2)(-) anions were produced by co-condensation of laser-ablated iron atoms and electrons with dioxygen in excess argon at 6 K. A photosensitive absorption at 870.6 cm(-1) is assigned to the antisymmetric OFeO stretching vibration (nu(3)) of the inserted FeO(2)(-) anion trapped in solid argon. On the basis of the observed nu(3) vibrational frequencies for Fe(16)O(2) and Fe(18)O(2), the anion is estimated to be linear. Due to the severe symmetry-breaking problems of the reference wave function, calculations with single-reference methods, including various DFT and post-HF methods, are unreliable for this molecule. However, the state-averaged multireference MRCI method, which incorporates both dynamical and nondynamical correlations, predicted that the anion has a linear doublet ground state, consistent with the experimental observations.