Showing papers in "Angewandte Chemie in 2003"
TL;DR: This review focuses mainly on examples with biological relevance since one of its aims it to enhance the knowledge of molecular recognition forces that is essential for drug development.
Abstract: Intermolecular interactions involving aromatic rings are key processes in both chemical and biological recognition. Their understanding is essential for rational drug design and lead optimization in medicinal chemistry. Different approaches—biological studies, molecular recognition studies with artificial receptors, crystallographic database mining, gas-phase studies, and theoretical calculations—are pursued to generate a profound understanding of the structural and energetic parameters of individual recognition modes involving aromatic rings. This review attempts to combine and summarize the knowledge gained from these investigations. The review focuses mainly on examples with biological relevance since one of its aims it to enhance the knowledge of molecular recognition forces that is essential for drug development.
3,053 citations
TL;DR: The fundamental concepts needed to understand quantum size effects in molecular magnets are reviewed and critically report what has been done in the field to date are critically reported.
Abstract: Molecules comprising a large number of coupled paramagnetic centers are attracting much interest because they may show properties which are intermediate between those of simple paramagnets and classical bulk magnets and provide unambiguous evidence of quantum size effects in magnets. To date, two cluster families, usually referred to as Mn12 and Fe8, have been used to test theories. However, it is reasonable to predict that other classes of molecules will be discovered which have similar or superior properties. To do this it is necessary that synthetic chemists have a good understanding of the correlation between the structure and properties of the molecules, for this it is necessary that concepts such as quantum tunneling, quantum coherence, quantum oscillations are understood. The goal of this article is to review the fundamental concepts needed to understand quantum size effects in molecular magnets and to critically report what has been done in the field to date.
2,452 citations
TL;DR: In this article, the authors highlight the recent developments in the copper-mediated (both stoichiometric and catalytic) reactions of aryl boronic acids as reaction partners in both O- and N-arylation.
Abstract: The copper-mediated C(aryl)N, C(aryl)O, and C(aryl)S bond formation is an important transformation and has been developed to include a wide range of substrates. This Review highlights the recent developments in the copper-mediated (both stoichiometric and catalytic) reactions of aryl boronic acids, aryl halides, iodonium salts, siloxanes, stannanes, plumbanes, bismuthates, and trifluoroborate salts as aryl donors. In particular, the recent introduction of boronic acids as reaction partners in both O- and N-arylation has been a significant discovery and will occupy centre-stage in this review. Clear improvements can be obtained by the correct choice of copper source, base, ligands, and other additives. Mechanistic investigations should provide insight into the catalytically active species, which would aid in the development of milder, more-efficient methods.
2,280 citations
1,966 citations
TL;DR: This work highlights how the interplay between aggregation and crystallization can give rise to mesoscale self-assembly and cooperative transformation and reorganization of hybrid inorganic-organic building blocks to produce single-crystal mosaics, nanoparticle arrays, and emergent nanostructures with complex form and hierarchy.
Abstract: The organization of nanostructures across extended length scales is a key challenge in the design of integrated materials with advanced functions. Current approaches tend to be based on physical methods, such as patterning, rather than the spontaneous chemical assembly and transformation of building blocks across multiple length scales. It should be possible to develop a chemistry of organized matter based on emergent processes in which time- and scale-dependent coupling of interactive components generate higher-order architectures with embedded structure. Herein we highlight how the interplay between aggregation and crystallization can give rise to mesoscale self-assembly and cooperative transformation and reorganization of hybrid inorganic-organic building blocks to produce single-crystal mosaics, nanoparticle arrays, and emergent nanostructures with complex form and hierarchy. We propose that similar mesoscale processes are also relevant to models of matrix-mediated nucleation in biomineralization.
1,697 citations
TL;DR: A low-temperature, large-scale, and versatile synthetic process is needed before ZnO nanowire arrays find realistic applications in solar energy conversion, light emission, and other promising areas, and the ease of commercial scale-up is presented.
Abstract: Since the first report of ultraviolet lasing from ZnO nanowires, substantial effort has been devoted to the development of synthetic methodologies for one-dimensional ZnO nanostructures. Among the various techniques described in the literature, evaporation and condensation processes are favored for their simplicity and high-quality products, but these gas-phase approaches generally require economically prohibitive temperatures of 800–900 8C. Despite recent MOCVD schemes that reduced the deposition temperature to 450 8C by using organometallic zinc precursors, the commercial potential of gas-phase-grown ZnO nanowires remains constrained by the expensive and/or insulating (for example, Al2O3) substrates required for oriented growth, as well as the size and cost of the vapor deposition systems. A low-temperature, large-scale, and versatile synthetic process is needed before ZnO nanowire arrays find realistic applications in solar energy conversion, light emission, and other promising areas. Solution approaches to ZnO nanowires are appealing because of their low growth temperatures and good potential for scale-up. In this regard, Vayssieres et al. developed a hydrothermal process for producing arrays of ZnO microrods and nanorods on conducting glass substrates at 95 8C. Recently, a seeded growth process was used to make helical ZnO rods and columns at a similar temperature. Here we expand on these synthetic methods to produce homogeneous and dense arrays of ZnO nanowires that can be grown on arbitrary substrates under mild aqueous conditions. We present data for arrays on four-inch (ca. 10 cm) silicon wafers and two-inch plastic substrates, which demonstrate the ease of commercial scale-up. The simple two-step procedure yields oriented nanowire films with the largest surface area yet reported for nanowire arrays. The growth process ensures that a majority of the nanowires in the array are in direct contact with the substrate and provide a continuous pathway for carrier transport, an important feature for future electronic devices based on these materials. Well-aligned ZnO nanowire arrays were grown using a simple two-step process. In the first step, ZnO nanocrystals (5–10 nm in diameter) were spin-cast several times onto a four-inch Si(100) wafer to form a 50–200-nm thick film of crystal seeds. Between coatings, the wafer was annealed at 150 8C to ensure particle adhesion to the wafer surface. The ZnO nanocrystals were prepared according to the method of Pacholski. A NaOH solution in methanol (0.03m) was added slowly to a solution of zinc acetate dihydrate (0.01m) in methanol at 60 8C and stirred for two hours. The resulting nanoparticles are spherical and stable for at least two weeks in solution. After uniformly coating the silicon wafer with ZnO nanocrystals, hydrothermal ZnO growth was carried out by suspending the wafer upside-down in an open crystallizing dish filled with an aqueous solution of zinc nitrate hydrate (0.025m) and methenamine or diethylenetriamine (0.025m) at 90 8C. Reaction times spanned from 0.5 to 6 h. The wafer was then removed from solution, rinsed with deionized water, and dried. A field-emission scanning electron microscope (FESEM) was used to examine the morphology of the nanowire array across the entire wafer, while single nanowires were characterized by transmission electron microscopy (TEM). Nanowire crystallinity and growth direction were analyzed by X-ray diffraction and electron diffraction techniques. SEM images taken of several four-inch samples showed that the entire wafer was coated with a highly uniform and densely packed array of ZnO nanowires (Figure 1). X-ray diffraction (not shown) gave a wurtzite ZnO pattern with an enhanced (002) peak resulting from the vertical orientation of the nanowires. A typical synthesis (1.5 h) yielded wires with diameters ranging between 40–80 nm and lengths of 1.5–2 mm.
1,676 citations
1,258 citations
TL;DR: The authors report on a bulk heterojunction photovoltaic cell in which an isomeric mixt.
Abstract: The authors report on a bulk heterojunction photovoltaic cell in which an isomeric mixt. of C70 derivs. is used as an electron acceptor in combination with poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylenevinylene] (MDMO-PPV). [70]PCBM, in this case a mixt. of isomeric [6,6]-phenyl- C71- butyric acid Me esters, is the higher fullerene analog of [60]PCBM, and displays improved light absorption in the visible region. Consequently, when this material is used in a photovoltaic cell instead of [60]PCBM, 50 % higher current densities are obtained. The synthesis of [70]PCBM was performed and the monoadduct fraction was isolated from the higher adducts and unreacted C70 by column chromatog. 1H and 13C NMR were performed on the [70]PCBM, as was UV/VIS absorption in toluene soln. Spin-coated composite films were made with MDMO-PPV, and photoinduced absorption measurements give direct spectral evidence for photoinduced charge sepn., not only upon excitation of the polymer, but also after selective excitation of [70]PCBM at 630 nm. Time-resolved photoluminescence measurements ater excitation at 488 mn, obsd. at 570 nm indicate near quant. charge generation upon polymer excitation, but at 720 nm this quenching depends on the processing solvent, chlorobenzene, o-dichlorobenzene, vs. o-xylene. At. force microscopy reveals a difference in film roughness dependent upon spinning solvent. Photovoltaic cells were made by sandwiching the photoactive mixt., consisting of [70]PCBM and MDMO-PPV in a 4.6:1 (wt./wt.) ratio, between charge-selective electrodes of ITO/PEDOT:PSS (ITO: indium tin oxide; PEDOT: poly(3,4-ethylenedioxythiophene); PSS: poly(styrenesulfonate)) and LiF/Al. The external quantum efficiency (EQE) from a 12V, 50 W halogen lamp varied from a max. of 0.2 for chlorobenzene- processed films, to 0.68 for the o-dichlorobenzene- processed films, compared to 0.5 for equiv. films made with [60]PCBM. Photovoltage-photocurrent behavior were also measured. Overall power conversion efficiency was about 3.0%. [on SciFinder (R)]
1,168 citations
TL;DR: Millisecond mixing and transport with no dispersion are achieved by unsteady flows induced in droplets of about 60 pL that travel through winding microfluidic channels, suggesting that arbitrarily complex reaction networks can be created by combining and splitting streams of such droplets.
Abstract: Millisecond mixing and transport with no dispersion are achieved by unsteady flows induced in droplets of about 60 pL that travel through winding microfluidic channels (top). Fluorescence can be used to monitor mixing (bottom) or measure reaction rates. In principle, arbitrarily complex reaction networks can be created by combining and splitting streams of such droplets.
1,153 citations
TL;DR: In this review key observations that resulted in the discovery and development of molybdenum- and tungsten-based metathesis catalysts are outlined and the use of chiral molyBdenum complexes for enantioselective synthesis is focused on.
Abstract: Catalytic olefin metathesis has quickly emerged as one of the most often-used transformations in modern chemical synthesis. One class of catalysts that has led the way to this significant development are the high-oxidation-state alkylidene complexes of molybdenum. In this review key observations that resulted in the discovery and development of molybdenum- and tungsten-based metathesis catalysts are outlined. An account of the utility of molybdenum catalysts in the synthesis of biologically significant molecules is provided as well. Another focus of the review is the use of chiral molybdenum complexes for enantioselective synthesis. These highly efficient catalysts provide unique access to materials of exceptional enantiomeric purity and often without generating solvent waste.
1,047 citations
TL;DR: With the evolution of new catalysts, the selectivity, efficiency, and functional-group compatibility of this reaction have improved to a level that was unimaginable just a few years ago, and this stage where more and more researchers are employing cross-metathesis reactions in multistep procedures and in the synthesis of natural products is reached.
Abstract: Among the many types of transition-metal-catalyzed C-C bond-forming reactions, olefin metathesis has come to the fore in recent years owing to the wide range of transformations that are possible with commercially available and easily handled catalysts. Consequently, olefin metathesis is now widely considered as one of the most powerful synthetic tools in organic chemistry. Until recently the intermolecular variant of this reaction, cross-metathesis, had been neglected despite its potential. With the evolution of new catalysts, the selectivity, efficiency, and functional-group compatibility of this reaction have improved to a level that was unimaginable just a few years ago. These advances, together with a better understanding of the mechanism and catalyst-substrate interactions, have brought us to a stage where more and more researchers are employing cross-metathesis reactions in multistep procedures and in the synthesis of natural products. The recent inclusion of alkynes and hindered bicyclic olefins as viable substrates for bimolecular metathesis coupling, the discovery of enantioselective cross-metathesis and cross-metathesis in water, and the successful marriage of metathesis and solid-phase organic synthesis has further widened the scope of this versatile reaction.
TL;DR: The self-limiting growth mechanism characteristic to atomic layer deposition (ALD) facilitates the control of film thickness at the atomic level and allows deposition on large and complex surfaces, which makes ALD a very promising technique for future integrated circuits.
Abstract: New materials, namely high-k (high-permittivity) dielectrics to replace SiO2, Cu to replace Al, and barrier materials for Cu, are revolutionizing modern integrated circuits. These materials must be deposited as very thin films on structured surfaces. The self-limiting growth mechanism characteristic to atomic layer deposition (ALD) facilitates the control of film thickness at the atomic level and allows deposition on large and complex surfaces. These features make ALD a very promising technique for future integrated circuits. Recent ALD research has mainly focused on materials required in microelectronics. Chemistry, in particular the selection of suitable precursor combinations, is the key issue in ALD; many interesting results have been obtained by smart chemistry. ALD is also likely to find applications in other areas, such as magnetic recording heads, optics, demanding protective coatings, and micro-electromechanical systems, provided that cost-effective processes can be found for the materials required.
TL;DR: The ocean is an overlooked habitat from which to isolate important microorganisms, and the rate of discovery of new biologically active compounds from common soil actino-mycetes has been falling.
Abstract: thus the discovery of a major new group of thesebacteria in marine sediments suggests that the ocean repre-sents an overlooked habitat from which to isolate theseimportant microorganisms. Given thatthe rate of discovery ofnew biologically active compounds from common soil actino-mycetes has been falling,
TL;DR: The approaches discussed in this review allow screening of compound libraries as well as a detailed identification of the groups involved in the binding events.
Abstract: Binding events of ligands to receptors are the key for an understanding of biological processes. Gaining insight into protein-protein and protein-ligand interactions in solution has recently become possible on an atomic level by new NMR spectroscopic techniques. These experiments identify binding events either by looking at the resonance signals of the ligand or the protein. Ideally, both techniques together deliver a complete picture of ligand binding to a receptor. The approaches discussed in this review allow screening of compound libraries as well as a detailed identification of the groups involved in the binding events. Also, characterization of the binding strength and kinetics is possible, competitive binding as well as allosteric effects can be identified, and it has even been possible to identify ligand binding to intact viruses and membrane-bound proteins.
TL;DR: The Haber-Bosch synthesis was the first heterogeneous catalytic system employed in the chemical industry and is still in use today and a fundamental criterion for scientific development in catalysis research.
Abstract: Nitrogen atoms are essential for the function of biological molecules and thus are and important component of fertilizers and medicaments. Bonds to nitrogen also find nonbiological uses in dyes, explosives, and resins. The synthesis of all these materials requires ammonia as an activated nitrogen building block. This situation is true for natural processes and the chemical industry. Knowledge of the various techniques for the preparation of ammonia is thus of fundamental importance for chemistry. The Haber-Bosch synthesis was the first heterogeneous catalytic system employed in the chemical industry and is still in use today. Understanding the mechanism and the translation of the knowledge into technical perfection has become a fundamental criterion for scientific development in catalysis research.
TL;DR: This new approach to heterogeneous catalysis can bring molecular insight to the design of new catalysts and even allow the discovery of new reactions (Ziegler-Natta depolymerization and alkane metathesis).
Abstract: Surface organometallic chemistry is an area of heterogeneous catalysis which has recently emerged as a result of a comparative analysis of homogeneous and heterogeneous catalysis. The chemical industry has often favored heterogeneous catalysis, but the development of better catalysts has been hindered by the presence of numerous kinds of active sites and also by the low concentration of active sites. These factors have precluded a rational improvement of these systems, hence the empirical nature of heterogeneous catalysis. Catalysis is primarily a molecular phenomenon, and it must involve well-defined surface organometallic intermediates and/or transition states. Thus, one must be able to construct a well-defined active site, test its catalytic performance, and assess a structure-activity relationship, which will be used, in turn-as in homogeneous catalysis-to design better catalysts. By the transfer of the concepts and tools of molecular organometallic chemistry to surfaces, surface organometallic chemistry can generate well-defined surface species by understanding the reaction of organometallic complexes with the support, which can be considered as a rigid ligand. This new approach to heterogeneous catalysis can bring molecular insight to the design of new catalysts and even allow the discovery of new reactions (Ziegler-Natta depolymerization and alkane metathesis). After more than a century of existence, heterogeneous catalysis can still be improved and will play a crucial role in solving current problems. It offers an answer to economical and environmental problems faced by industry in the production of molecules (agrochemicals, petrochemicals, pharmaceuticals, polymers, basic chemicals).
TL;DR: The power of self-organization may be harnessed most usefully in a number of nanotechnology applications, which include the preparation of nanoparticles, the templating of nanostructures, nanomotor design, the exploitation of biomineralization, and the development of functionalized delivery vectors.
Abstract: Nature exploits self-organization of soft materials in many ways, to produce cell membranes, biopolymer fibers and viruses, to name just three. Mankind is now able to design materials at the nanoscale, whether through atom-by-atom or molecule-by-molecule methods (top-down) or through self-organization (bottom-up). The latter method encompasses soft nanotechnology. Self-organization of soft materials can be exploited to create a panoply of nanostructures for diverse applications. The richness of structures results from the weak ordering because of noncovalent interactions. Thus, thermal energy is important as it enables transitions between phases with differing degrees of order. The power of self-organization may be harnessed most usefully in a number of nanotechnology applications, which include the preparation of nanoparticles, the templating of nanostructures, nanomotor design, the exploitation of biomineralization, and the development of functionalized delivery vectors.
TL;DR: A method that has proven very successful is the formation of in organic structures under the influence of (bio)organic templates, which has resulted in the generation of a large variety of structured inorganic structures that are currently unattainable through any other method.
Abstract: Mankind's fascination with shapes and patterns, many examples of which come from nature, has greatly influenced areas such as art and architecture. Science too has long since been interested in the origin of shapes and structures found in nature. Whereas organic chemistry in general, and supramolecular chemistry especially, has been very successful in creating large superstructures of often stunning morphology, inorganic chemistry has lagged behind. Over the last decade, however, researchers in various fields of chemistry have been studying novel methods through which the shape of inorganic materials can be controlled at the micro- or even nanoscopic level. A method that has proven very successful is the formation of inorganic structures under the influence of (bio)organic templates, which has resulted in the generation of a large variety of structured inorganic structures that are currently unattainable through any other method.
TL;DR: The chemistry of the closely related and structurally rather unique alkaloid roseophilin is summarized, a cytotoxic agent that recently became the focal point of many innovative total syntheses.
Abstract: The pyrrole alkaloids of the prodigiosin family make up an unusual chapter in the chemistry of natural products. Owing to the characteristic red color of these secondary metabolites, colonies of the Gram-negative-producing bacteria may strikingly resemble droplets of blood. This phenomenon caused considerable confusion in the past and was likely responsible for many seemingly miraculous (prodigious) events. After the eventual transition from superstition to science, the prodigiosins started to attract considerable attention because of their promising physiological properties. Most interesting are the immunosuppressive activities at doses that are not cytotoxic, in particular since in vivo studies suggest that the prodigiosins act synergistically with cyclosporine A or FK 506, which are presently the dominant drugs in clinical immunosuppressive regimens. Furthermore, the chemistry of the closely related and structurally rather unique alkaloid roseophilin is summarized, a cytotoxic agent that recently became the focal point of many innovative total syntheses.
TL;DR: In this Review, the various synthetic approaches now being employed to produce expanded porphyrins as well as their various applications-related aspects are discussed.
Abstract: Expanded porphyrins are synthetic analogues of the porphyrins, and differ from these and other naturally occurring tetrapyrrolic macrocycles by containing a larger central core with a minimum of 17 atoms, while retaining the extended conjugation features that are a hallmark of these quintessential biological pigments. The result of core expansion is to produce systems with novel spectral and electronic features, interesting and, often unprecedented, cation-coordination properties, and, in many cases, an ability to bind anions in certain protonation states. Also adding to the appeal of expanded porphyrins is their central role in addressing issues of aromaticity. In many cases, they also display structural features, such as decidedly nonplanar "figure-eight" motifs, that have no antecedents in the chemistry of porphyrins or related macrocyclic compounds. In this Review, the various synthetic approaches now being employed to produce expanded porphyrins as well as their various applications-related aspects are discussed.
TL;DR: It is apparent that the chalcogen amino acids cysteine, methionine, selenocysteines, and selenomethionine exhibit a unique biological chemistry that is the source of exciting research opportunities.
Abstract: Sulfur and selenium occur in proteins as constituents of the amino acids cysteine, methionine, selenocysteine, and selenomethionine. Recent research underscores that these amino acids are truly exceptional. Their redox activity under physiological conditions allows an amazing variety of posttranslational protein modifications, metal free redox pathways, and unusual chalcogen redox states that increasingly attract the attention of biological chemists. Unlike any other amino acid, the "redox chameleon" cysteine can participate in several distinct redox pathways, including exchange and radical reactions, as well as atom-, electron-, and hydride-transfer reactions. It occurs in various oxidation states in the human body, each of which exhibits distinctive chemical properties (e.g. redox activity, metal binding) and biological activity. The position of selenium in the periodic table between the metals and the nonmetals makes selenoproteins ideal catalysts for many biological redox transformations. It is therefore apparent that the chalcogen amino acids cysteine, methionine, selenocysteine, and selenomethionine exhibit a unique biological chemistry that is the source of exciting research opportunities.
TL;DR: Recently, the halogen-magnesium exchange has considerably extended the range of functionalized Grignard reagents available for synthetic purposes and new applications of organomagnesium reagents in cross-coupling reactions and amination reactions will be covered in this Review.
Abstract: Organomagnesium reagents occupy a central position in synthetic organic and organometallic chemistry. Recently, the halogen-magnesium exchange has considerably extended the range of functionalized Grignard reagents available for synthetic purposes. Functional groups such as esters, nitriles, iodides, imines, or even nitro groups can be present in a wide range of aromatic and heterocyclic organomagnesium reagents. Also various highly functionalized alkenyl magnesium species can be prepared. These recent developments as well as new applications of organomagnesium reagents in cross-coupling reactions and amination reactions will be covered in this Review.
TL;DR: A detailed review of metal sigma complexes can be found in this article, with a focus on the development of metal alkynyl sigma sigma-complexes, the progress of which is summarized in this review.
Abstract: Metal alkynyl complexes hold a fascination for synthetic chemists, structural chemists, and materials scientists alike. Harnessing the unique overlap of metal and carbon orbitals is a challenge that can be overcome in many ways and hence, there are many synthetic routes toward M-C=C-bond-forming reactions that utilize a wide variety of transition-metal and alkynyl reagents. Some methods can be widely applied, while others are specific to a particular metal or compound. The linear geometry of the alkynyl unit and its pi-unsaturated character have led to metal alkynyls becoming attractive building blocks for molecular wires and polymeric organometallic materials, which can possess interesting properties, such as optical nonlinearity, luminescence, liquid crystallinity, and electrical conductivity. A unique, multifaceted approach, often combining talents from all three of the above chemical disciplines, has served as a driving force behind the intense research into the development of metal alkynyl sigma complexes, the progress of which, particularly in the last ten years, is summarized in this review.
TL;DR: By judicious choice of the templating procedure, unprecedented control of the structure and texture on length scales between nanometers and micrometers has been achieved over the last few years.
Abstract: Porous and high-surface-area materials are of interest to many scientific communities Templating pathways can be used to synthesize such materials with a high degree of control over their structural and textural properties As templates molecular or supramolecular units are added to the synthesis mixture They are occluded in the growing solid and leave a pore system after their removal For such templates the term "endotemplate" is introduced Alternatively, the templates can be materials with structural pores in which another solid is created, thus providing a scaffold for the synthesis After removal of the scaffold, a porous or finely divided solid remains, depending on the connectivity of the template Such a template is termed an "exotemplate" By judicious choice of the templating procedure, unprecedented control of the structure and texture on length scales between nanometers and micrometers has been achieved over the last few years
TL;DR: This Review focuses on the comparison of the different strategies to assemble the bicyclo[3.2.1]octane core, to introduce the bridgehead quaternary C20 to form the pyrrolidine moiety, to construct the oxindole residue, and to close the tetrahydropyran ring en route to gelsemine.
Abstract: Gelsemine and 21-oxogelsemine have been synthesized through several routes. This Review focuses on the comparison of the different strategies to assemble the bicyclo[3.2.1]octane core, to introduce the bridgehead quaternary C20 to form the pyrrolidine moiety, to construct the oxindole residue, and to close the tetrahydropyran ring en route to gelsemine.
TL;DR: TPR measurements of CO oxidation have shown that the smallest gold cluster that catalyzes the reaction is Au8, and while Au4 is catalytically inert, the doped cluster Au3Sr is active, and findings are revealed that underlying the aforementioned remarkable chemical size-sensitivity is the nature of bonding and the activation of molecular oxygen by these nanocluster catalysts.
Abstract: Metal clusters exhibit unique size-dependent physical[1] and chemical properties[2] that differ from those of bulk materials. While inert as a bulk material, gold nanoparticles and clusters have attracted considerable interest lately as active catalysts for a number of industrially relevant reactions.[3–6] Unlike supported particles of larger size or extended solid surfaces,[7–10] size-selected small metal clusters adsorbed at specific sites of a support material (e.g. oxygen vacancies in the case of a MgO(100) surface) exhibit unique properties that originate from the highly reduced dimensions of the individual metal aggregates. These properties underlie the remarkable, newly found catalytic activity of small gold clusters, and they include: 1) dynamic structural fluxionality that exhibits itself through the propensity of small clusters to transform, in the course of chemical reactions, between various energetically accessible structural isomers, thus enhancing the rates for overcoming reaction barriers, 2) quantum size effects that are reflected in size-dependent characteristics of the electronic spectra of small gold clusters, and in charge transfer from the support to the clusters, 3) impurity-doping effects that allow modification and control of the electronic structure, and consequently the chemical reactivity, of small supported clusters, through incorporation of judiciously chosen impurity atoms in otherwise inert clusters. Herein, we focus on gaining fundamental insights into the above size-dependent “nanocatalytic factors”, and illustrate through experimental and theoretical investigations the manner in which such fundamental understanding may guide the design and atomic-scale modifications of nanocatalysts. Recently, a set of model catalysts have been prepared by soft-landing[11] of mass-selected Aun and AunSr cluster ions onto well-characterized MgO(100) thin films. These substrate films contained a low concentration (typically 5 6 1013 cm 1) of oxygen vacancies (surface F-centers, FC), that act as strong trapping sites for the clusters at low temperatures.[12–14] Temperature-programmed reaction (TPR) measurements of CO oxidation (CO+ =2O2!CO2) have shown that the smallest gold cluster that catalyzes the reaction is Au8. Furthermore, it has been found that while Au4 is catalytically inert, the doped cluster Au3Sr is active. These findings, in conjunction with ab initio calculations, have revealed that underlying the aforementioned remarkable chemical size-sensitivity is the nature of bonding and the activation of molecular oxygen by these nanocluster catalysts. The measured chemical activity is summarized in Figure 1, which shows typical TPR spectra for selected samples (a–e). The total CO2 yield per cluster obtained in a one-cycle heating experiment for Aun and AunSr with 1 n 9 is shown in the inset. The active model systems (n 8 for the pure Aun and
TL;DR: Some potential applications of organophosphorus chemistry in the areas of molecular materials and homogeneous catalysis are presented.
Abstract: Since the beginning of the seventies, organophosphorus chemistry has been completely rejuvenated by the discovery of stable derivatives in which phosphorus has the coordination numbers one or two. The chemistry of these compounds mimics the chemistry of their all-carbon analogues. In this Review article this analogy is discussed for the phosphorus counterparts of alkenes, alkynes, and carbenes. In each case, the synthesis, reactivity, and coordination modes are briefly examined. Some special electronic configurations are also discussed, which include one-electron Pbond;P bonds, strained bonds, and aromatic systems. To conclude, some potential applications of this chemistry in the areas of molecular materials and homogeneous catalysis are presented.