有序介孔有机金属Au(I)催化剂催化水相炔烃水合制备甲基酮

采用表面活性剂自组装、有机金属硅烷与苯基硅烷共聚法制备了有机金属Au(I)嵌于介孔材料孔壁的非均相催化剂Au(I)-PPh2-PMO(Ph).结果表明,Au(I)-PPh2-PMO(Ph)催化剂的活性位分散均匀,孔道不易堵塞,对反应物的扩散与传质影响较小,其催化活性不仅明显高于后嫁接法制得的Au(I)-PPh2-PMO(Ph)-G和均相Au(PPh3)Cl催化剂,且可以重复使用,显示出较好的应用前景.     

Mechanism of Gold(I)‐Catalyzed Conia‐ene Reaction of β‐Ketoesters with Alkynes: A DFT Study

Density functional calculations have been performed to comparatively investigate two possible pathways of Au(I)‐catalyzed Conia‐ene reaction of β‐ketoesters with alkynes. Our studies find that, under the assistance of trifluoromethanesulfonate (TfO), the β‐ketoester is the most likely to undergo Model II to isomerize into its enol form, in which TfO plays a proton transfer role through a 6‐membered ring transition state. The coordination of the Au(I) catalyst to the alkynes triple bond can enhance the eletrophilic capability and reaction activity of the alkynes moiety, which triggers the nucleophilic addition of the enol moiety on the alkynes moiety to give a vinyl‐Au intermediate. This cycloisomerizaion step is exothermal by 21.3 kJ/mol with an energy barrier of 56.0 kJ/mol. In the whole catalytic process, the protonation of vinyl‐Au is almost spontaneous, and the formation of enol is a rate‐limiting step. The generation of enol and the activation of Au(I) catalyst on the alkynes are the key reasons why the Conia‐ene reaction can occur in mild condition. These calculations support that Au(I)‐catalyzed Conia‐ene reactions of β‐ketoesters with alkynes go through the pathway 2 proposed by Toste.     

Catalysis by supported gold: correlation between catalytic activity for CO oxidation and oxidation states of gold

X-ray absorption near-edge spectra and temperature-programmed oxidation and reduction data demonstrate that Au(I) and Au(0) are both present in working MgO-supported gold catalysts for CO oxidation. EXAFS data indicate gold clusters with essentially the same average diameter (about 30 A) in each catalyst sample. Thus, the results provide no evidence of an effect of gold cluster size on the catalytic activity, but both the catalytic activity and the surface concentration of Au(I) were found to decrease with increasing CO partial pressure (as Au(0) was increasingly formed), demonstrating that the catalytic sites incorporate Au(I).     

Encapsulation of N-heterocyclic carbene–gold (I) catalysts within magnetic core/shell silica gels: A reusable alkyne hydration catalyst

In this study, N-heterocyclic carbene–Au(I) complex, chloro[1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]gold (I), was successfully encapsulated within mesopores of a magnetic core/shell (γ-Fe₂O₃@SiO₂) silica gel through post-pore-size reduction by silylation reactions The post-reduction of the pore size not only minimizes the catalyst leaching during the alkyne hydration reactions but also eliminates any need for covalent modification of the catalyst or support surface. The resulting catalyst exhibits high activity in hydration reactions of various alkynes even under low catalytic loadings. The catalyst can be easily recycled from the reaction mixture using a magnet and can be reused in alkyne hydration reactions up to six times with only 52. wt% Au leaching.     

Two metals are better than one in the gold catalyzed oxidative heteroarylation of alkenes

We present a detailed study of the mechanism for oxidative heteroarylation, based on DFT calculations and experimental observations. We propose binuclear Au(II)-Au(II) complexes to be key intermediates in the mechanism for gold catalyzed oxidative heteroarylation. The reaction is thought to proceed via a gold redox cycle involving initial oxidation of Au(I) to binuclear Au(II)-Au(II) complexes by Selectfluor, followed by heteroauration and reductive elimination. While it is tempting to invoke a transmetalation/reductive elimination mechanism similar to that proposed for other transition metal complexes, experimental and DFT studies suggest that the key C-C bond forming reaction occurs via a bimolecular reductive elimination process (devoid of transmetalation). In addition, the stereochemistry of the elimination step was determined experimentally to proceed with complete retention. Ligand and halide effects played an important role in the development and optimization of the catalyst; our data provides an explanation for the ligand effects observed experimentally, useful for future catalyst development. Cyclic voltammetry data is presented that supports redox synergy of the Au···Au aurophilic interaction. The monometallic reductive elimination from mononuclear Au(III) complexes is also studied from which we can predict a ~15 kcal/mol advantage for bimetallic reductive elimination.     

An efficient method for the synthesis of alkylidenecyclobutanones by gold-catalyzed oxidative ring enlargement of vinylidenecyclopropanes

Rings of gold: Vinylidenecyclopropanes can undergo efficient oxidative ring enlargements under mild conditions to give the corresponding alkylidenecyclobutanone derivatives in good yields. A plausible mechanism for this transformation has been proposed, and a new Au(I) -N complex, which can be used as an efficient catalyst in this oxidation reaction, has been isolated.     

乙炔氢氯化反应中的负载金-离子液体催化剂:离子态金物种的稳定机制

聚氯乙烯(PVC)作为世界通用工程塑料之一,具有优异的物理、化学和机械性能,在工业、农业、建筑、包装、电力等行业中应用广泛.氯乙烯是生产聚氯乙烯的重要单体.氯乙烯的生产主要有电石法和乙烯法两种工艺路线,由于我国“贫油、富煤、少气”的资源现状,电石法产能占全部产能的83%以上.电石法生产氯乙烯的原理是在氯化汞催化剂存在下,将电石水解精制后的乙炔气与氯化氢加成直接合成氯乙烯.随着节能减排及环保要求的逐渐提高和国际涉汞公约的实施,开发新一代绿色无汞催化剂具有重要的战略意义.近年来,金基催化剂是无汞催化剂基础研究和技术开发中最重要的方向.在之前的工作中,我们课题组首先报道了负载离子液体-金催化剂体系(Au-SILP)在电石法生产氯乙烯工艺中的应用,并发现离子液体的存在可以显著提高金活性物种在载体表面的分散度和稳定其化学价态.在后续研究中,我们在负载离子液体-金催化体系中引入金属铜离子(Cu^2+),利用反应过程中Au-Cu之间的氧化还原循环,设计并制备了金属铜基配位离子液体,构建了负载离子液体-金-铜催化剂体系.铜离子的引入形成了一个催化剂自身维持氧化态的微环境,实现了被还原金物种的原位氧化再生.本文在上述研究基础上,利用配位离子液体[Bmim][N(CN)2]中[N(CN)2^–]阴离子和阳离子金之间的强配位作用,构建出比Au-Cl键更稳定的Au–N键,并通过X射线光电子能谱(XPS)、球差校正-扫描透射电镜(AC-STEM)和扩展X射线吸收精细结构(EXAFS)表征证明了Au以单原子状态存在于载体表面.制备的Au-N(CN)2/AC催化剂在乙炔氢氯化反应中表现出比Au-Cl/AC和Au/AC催化剂更高的稳定性和催化活性以及更短的诱导期.进一步表征分析发现,[N(CN)2^–]配体促进了阳离子金和配体之间的电子转移,提高了阳离子金的电子云密度,削弱了乙炔在阳离子金上的吸附强度,抑制了其还原,提高了催化剂的稳定性.更重要的是,与阳离子金配位的[N(CN)2^–]配体使得反应过程中的氯化氢在氮位点发生化学解离,促进了氯化氢活化,同时降低了反应能垒.对负载配位离子液体-金催化体系反应诱导期的分析结果表明,反应诱导期与反应物(乙炔、氯化氢)分子在离子液体层中的溶解度无关,而主要取决于催化剂中Au(Ⅲ)物种的含量和反应物分子在离子液体中的扩散速率.上述研究结果进一步深化了离子液体和活性金物种之间电子的作用机理,建立了负载离子液体-金催化剂体系对反应物的活化机制和反应机理,为进一步开发具有工业应用价值的乙炔氢氯化反应无汞催化剂提供了科学基础和参考.     

Kinetic evaluation of highly active supported gold catalysts prepared from monolayer-protected clusters: an experimental Michaelis-Menten approach for determining the oxygen binding constant during CO oxidation catalysis

Thiol monolayer-protected Au clusters (MPCs) were prepared using dendrimer templates, deposited onto a high-surface-area titania, and then the thiol stabilizers were removed under H2/N2. The resulting Au catalysts were characterized with transmission electron microscopy, X-ray photoelectron spectroscopy, and infrared spectroscopy of adsorbed CO. The Au catalysts prepared via this route displayed minimal particle agglomeration during the deposition and activation steps. Structural data obtained from the physical characterization of the Au catalysts were comparable to features exhibited from a traditionally prepared standard Au catalyst obtained from the World Gold Council (WGC). A differential kinetic study of CO oxidation catalysis by the MPC-prepared Au and the standard WGC catalyst showed that these two catalyst systems have essentially the same reaction order and Arrhenius apparent activation energies (28 kJ/mol). However, the MPC-prepared Au catalyst shows 50% greater activity for CO oxidation. Using a Michaelis-Menten approach, the oxygen binding constants for the two catalyst systems were determined and found to be essentially the same within experimental error. To our knowledge, this kinetic evaluation is the first experimental determination of oxygen binding by supported Au nanoparticle catalysts under working conditions. The values for the oxygen binding equilibrium constant obtained from the Michaelis-Menten treatment (ca. 29-39) are consistent with ultra-high-vacuum measurements on model catalyst systems and support density functional theory calculations for oxygen binding at corner or edge atoms on Au nanoparticles and clusters.     

Intra- and intermolecular reactions of indoles with alkynes catalyzed by gold

Indoles react intramolecularly with alkynes in the presence of gold catalysts to give from six- to eight-membered-ring annulated compounds. The cationic Au(I) complex [Au(P{C(6)H(4)(o-Ph)}(tBu)(2))(NCMe)]SbF(6) is the best catalyst for the formation of six- and seven-membered rings by 6-endo-dig, 6-exo-dig, and 7-exo-dig cyclizations. Indoloazocines are selectively obtained with AuCl(3) as catalyst in a rare 8-endo-dig process. In this process allenes or tetracyclic annulated derivatives are also formed as a result of an initial fragmentation reaction. The intermolecular reaction of indoles with alkynes proceeds to form 3-alkenylated intermediates that react with a second equivalent of indole to give bisindolyl derivatives. Indoles that are substituted at the 3-position react intermolecularly with alkynes to give 2-alkenylated intermediates that can be trapped intramolecularly with the appropriate nucleophiles.     

Structural, photophysical, and catalytic properties of Au(I) complexes with 4-substituted pyridines

Ionic gold(I) complexes with general formula of [Au(Py)2][AuCl2] and [Au(Py)2][PF6] (Py = 4-substituted pyridines) have been synthesized. Structures of five Au(I) complexes and a Ag(I) complex were determined by single crystal X-ray diffraction. Evidence for cationic aggregation of [Au(py)2][PF6] complexes in solution was obtained by conductivity measurements and by the isosbestic point observed from variable temperature UV-visible absorption spectra. All compounds were luminous in the solid state. Calculations employing density functional theory were performed to shed light on the nature of the electronic transitions. While the [Au(4-dmapy)2][AuCl2] (4-dmapy = 4-dimethylaminopyridine) and [Au(4-pic)2][AuCl2] (4-pic = 4-picoline) emissions were found to be mainly ligand in nature, their [PF6](-) counterparts involved a Au...Au-interaction imbedded in the highest occupied molecular orbital. [Au(4-dmapy)2][AuCl2] was found to be an efficient catalyst for Suzuki cross-coupling of aryl bromide and phenylboronic acid.     

C-X（X = Cl，Br，I）键解离能作为烧结Au/AC催化剂再分散的描述因子及相关蕴意

负载型 Au基催化剂在工业过程中具有非常广泛的潜在应用,如催化加氢/脱氢过程、精细化学品合成、能源催化转化及环境保护等过程,表现出很高的催化活性和选择性. Au基催化剂活性物种或活性中心基本由纳米粒子或化合物构成,但在应用过程中因 Ostwald熟化效应或粒子迁移作用,尤其是高温高压等苛刻反应条件下,均随应用时间延长从小尺寸粒子逐渐长为大粒子,造成活性降低或完全失活,这也是负载型催化剂失活的最主要原因之一.其中因成本、稀缺等特性,负载型 Au催化剂的烧结问题是影响和制约其应用的主要因素.除可通过载体改性、助剂和官能团配位稳定等方法来延缓其失活过程外,对已烧结催化剂的高效、快捷和绿色的再分散/再生过程也具有基础和应用研究的重要意义.活性炭载 Au催化剂(Au/AC)广泛应用于乙炔氢氯化反应中,以期替代高毒性的汞基催化剂,但在反应过程中因高活性的 Au3+物种易被还原而形成 Au0物种进而烧结导致失活;如新鲜 Au/AC催化剂表面的 Au粒子尺寸为1-2 nm,经乙炔氢氯化反应后变为33 nm左右;随之在453 K、0.1 MPa、乙炔体积空速(GHSV)为600 h-1、氯化氢与乙炔摩尔比为1.1的反应条件下,乙炔转化率从81.8%降至11.2%.如何有效对大粒子 Au再分散/再生可为其应用提供有力支撑.有研究表明,气相 CH3I在甲醇羰基化反应过程中明显改变 Au/AC表面的 Au粒子尺寸;或采用浓盐酸或王水也可将烧结的 Au/AC催化剂进行再分散/再生.但已有的 Au基催化剂再分散/再生过程均伴随着强酸、强氧化或高毒性在分散剂的应用,对环境的影响及后续处理有明显的局限性,且再分散机理尚不明确.在前期工作基础上,本文采用系列卤代烃(碘代烃、溴代烃和氯代烃)对烧结的 Au/AC进行再分散/再生研究.结果表明,在室温常压条件下 CHI3可以快捷高效地对烧结 Au/AC催化剂进行再分散/再生,具有最优的再分散性能;通过对系列碘代烃 C-I键的解离能分析,发现 C-I解离能越低越有利于大粒子 Au的再分散.同时,溴代烃和氯代烃对烧结的 Au/AC催化剂也具有再分散能力,但比碘代烃的再分散效率低. C-X键的解离能与再分散效率有高相关性,即 C-X键的解离能越低越有利于 Au的再分散.总体上,三类卤代烃再分散效率高低顺序为 C-I＞C-Br＞C-Cl.进而,通过不同分散过程中 Au粒子分散状态推测了卤代烃对 Au粒子的再分散机理,即卤代烃先在 Au粒子表面化学吸附,然后 C-X键解离,形成 Au-X物种,小粒子 Au在 AC表面聚集并稳定,最后形成高分散 Au粒子(粒径<1 nm)催化剂.以乙炔氢氯化反应考察了再生 Au/AC催化剂性能,结果表明,该催化剂上乙炔转化率可达79.4%,基本恢复至初始水平,且该方法可对失活催化剂进行多次高效再生.     

Self-assembly of a Rh(I) complex on Au(111) surfaces and its electrocatalytic activity toward the hydrogen evolution reaction

The self-assembly of a Wilkinson type of catalyst molecule, trans-RhCl(CO)(PPh3)2, on Au(111) surfaces and its electrocatalytic properties toward the hydrogen evolution reaction (HER) are investigated by employing scanning tunneling microscopy (STM), cyclic voltammetry (CV), and X-ray photoelectron spectroscopy (XPS). The self-assembled monolayers of RhCl(CO)(PPh3)2 are prepared from either dichloromethane or aqueous solutions, but the ordered structures are observed only in atmospheric conditions after solvents evaporate. In the electrolyte solutions, disordered yet uniformly sized spherical clusters of individual molecules are observed as a result of the conformational change of the molecule by the solvation effect of water. The immobilized Rh(I) molecular clusters are electrochemically stable in a wide potential window and exhibit remarkable electrocatalytic activity toward HER in perchloric acid solutions. Several comparative experiments involving similar types of immobilized complexes containing Ru(I) and Ir(I) centers and solution species of RhCl(CO)(PPh3)2 are performed. However, none of them are found to be electroactive to HER. The Tafel slope of HER on the Rh(I) complex modified Au(111) electrode in 0.1 M HClO4 is determined to be -0.061 V, which is almost in the middle of those on bare Au(111) (-0.093 V) and Rh covered (thetaRh approximately 0.3) Au(111) (-0.034 V) electrodes. XPS measurements reveal a valence change of Rh(I) to Rh(0), and an oxidative addition and reductive elimination mechanism is suggested for the enhancement of HER.     

An ultra-sensitive electrochemical sensor for ascorbic acid based on click chemistry

A highly selective and sensitive electrochemical sensor for ascorbic acid (AA) assay has been prepared through Cu(I) catalyzed azide-alkyne cycloaddition reaction (CuAAC). The catalyst, Cu(I) species, is acquired from the reduction of Cu(II) by AA in situ. In the presence of Cu(I) catalyst, the azide modified Au electrode surface is shown to react quantitatively with terminal propargyl-functionalized ferrocene forming 1,2,3-triazoles. The electrochemical response of propargyl-functionalized ferrocene modified Au electrode surface can be monitored using differential pulse voltammetry (DPV) technique. Under optimal conditions, it is found that the current intensity has a linear relationship with the logarithm of AA concentration in the range of 5.0 × 10(-12) to 1.0 × 10(-9) M. Furthermore, the proposed electrochemical sensor shows a good stability (RSD 4.2%), high selectivity and low detection limit for AA detection. In addition, it also demonstrates that the proposed sensor can be applied to detect AA in real urine samples with satisfactory results.     

Platinum-catalysed bisindolylation of allenes: a complementary alternative to gold catalysis

Pt versus Au: Platinum-catalysed addition of nucleophiles to allenes follows a distinctly different pathway to the process catalysed by gold(I) complexes; the platinum catalyst leads to different products with indoles involving a bisindolylation reaction, whereas gold(I) gives allyl indoles from a single addition (see scheme).     

Gold nanoparticles supported on ionic liquid‐modified cellulose as an efficient and recyclable catalyst for the oxidation of alcohols to aldehydes/ketones and reduction of nitroarenes

A novel catalyst of gold nanoparticles supported on cellulose fibres with the ionic liquid framework (Au NPs@CL‐IL) has been shown to be a highly active and recyclable catalyst for the oxidation of primary and secondary alcohols and reduction of nitroarenes in aqueous media. The reusability of this catalyst is high, and it can be reused ten times without a significant decrease in its catalytic activity. Furthermore, transmission electron micrographs of the recovered catalyst show the presence of well‐distributed Au NPs on the CL‐IL fibres without any aggregation.     

氮杂环卡宾双核金络合物催化的胺芳基化反应

联萘胺出发合成了氮杂环卡宾双核和单核金络合物, 通过X射线的单晶衍射确定了它们的结构, 并将其应用于催化胺芳基化反应中, 以高达95%的收率得到吡咯烷类化合物. 综合上述实验结果, 发现氮杂环卡宾双核金络合物4b中存在着Au(I)-Au(I)间相互弱作用力, 而且这种弱相互作用可能对该催化反应起重要的作用, 以高收率得到吡咯烷类化合物.     

Selective modification of the acid-base properties of ceria by supported Au

Au supported on CeO(2) prepared by deposition-precipitation with urea leads to a basic catalyst. Au acts in two ways as surface modifier. First, Au selectively interacts with Ce(4+) cations by either blocking access to or reducing Ce(4+) to Ce(3+). Second, the resulting Au atoms (presumably as Au(+) ions) act as soft, weak Lewis acid sites stabilizing carbanion intermediates and enhancing hydride abstraction in the dehydrogenation of alcohols. In consequence, the thus-synthesized basic catalyst catalyzes the dehydrogenation of propan-2-ol to acetone with high efficiency and without notable deactivation. Additionally, the dehydration pathway of propan-2-ol is eliminated, as Au also quantitatively blocks access to strongly acidic Ce(4+) ions or reduces them to Ce(3+).     

Active and Recyclable Gold Metal Nanoparticles Catalyst Supported on Nitrogen-Doped Mesoporous Carbon for Chemoselective Hydrogenation of Cinnamaldehyde to Cinnamyl Alcohol

Several supported gold metal catalysts with different Au nanoparticles sizes were prepared and evaluated for the chemoselective hydrogenation of cinnamaldehyde (CA) to cinnamyl alcohol (CAL). To investigate the structure-activity relationship, stability of catalyst, heterogeneity and recyclability, the structural characteristics of materials and Au catalysts (fresh and spent catalysts) were studied by employing variety of physico-chemical techniques. The interrelationship among Au nanoparticles size (nm) with turnover frequency (h⁻¹) of Au catalysts has also been explored. Among the various Au catalysts tested, nitrogen-doped mesoporous carbon (NMC) supported Au catalyst having homogeneously dispersed (78.8%) Au nanoparticles (1.6 nm) synthesized by sol-immobilization method (Au-NMC-SI) demonstrated improved catalytic activity affording 78% CAL selectivity and 94.2% CA conversion without using any promoter. Moreover, Au-NMC-SI catalyst exhibited good recyclability and stability. The catalyst synthesis approach described in this investigation opens up a novel strategy for the design of highly efficient metal nano-catalysts supported on NMC materials.     

Identical-location Transmission Electron Microscopy of Pt Nanoparticle Electrocatalysts Using Iridium-coated Au Grids for Fuel Cell Durability Tests Simulating Start-up and Shutdown Conditions

In an identical-location TEM (IL-TEM) analysis of an electrocatalyst, an Au grid is usually selected owing to its chemical stability under potential cycling conditions. A potential cycle between 1.0 and 1.5 V is applied to the catalyst-cast Au grid for cathode durability testing that simulates the start-up and shutdown conditions of fuel cells. Because of the redox potential of Au (1.36 V vs. RHE), the grid dissolves and redeposits on the catalyst under the applied potential, making it complicated to evaluate the catalyst nanoparticle shape and size after degradation. We fabricated an Au grid coated with an iridium oxide layer (Ir-coated Au grid) to suppress the dissolution of Au. The Ir-coated Au grid with carbon support was compared to the Au grid to confirm the effect of the iridium oxide layer. No Au deposition was observed, even after 3000 cycles at 60 °C for the Ir-coated Au grid, but Au was deposited on carbon on the Au grid after 1000 cycles. Consequently, the alkylamine-modified Pt nanoparticle catalyst (unwashed catalyst) was observed along with the Ir-coated Au grid using IL-TEM under durability tests simulating start-up and shutdown conditions. A catalyst with less alkylamine content was prepared by butylamine washing (washed catalyst) and observed using IL-TEM for comparison. The Pt nanoparticles of the washed catalyst aggregated and changed their morphology after 1000 cycles, while the alkylamine-modified nanoparticles of the unwashed catalyst almost maintained their original size and shape up to 1000 cycles. The Ir-coated gold grids allow proper IL-TEM analysis of catalysts in durability tests without the interference of Au dissolution.