Showing papers in "Nature Chemistry in 2009"
TL;DR: The first steps towards using computational methods to design new catalysts are reviewed and how, in the future, such methods may be used to engineer the electronic structure of the active surface by changing its composition and structure are discussed.
Abstract: Over the past decade the theoretical description of surface reactions has undergone a radical development. Advances in density functional theory mean it is now possible to describe catalytic reactions at surfaces with the detail and accuracy required for computational results to compare favourably with experiments. Theoretical methods can be used to describe surface chemical reactions in detail and to understand variations in catalytic activity from one catalyst to another. Here, we review the first steps towards using computational methods to design new catalysts. Examples include screening for catalysts with increased activity and catalysts with improved selectivity. We discuss how, in the future, such methods may be used to engineer the electronic structure of the active surface by changing its composition and structure.
3,023 citations
TL;DR: A new set of ORR electrocatalysts consisting of Pd or Pt alloyed with early transition metals such as Sc or Y, identified using density functional theory calculations as being the most stable Pt- and Pd-based binary alloys with ORR activity likely to be better than Pt.
Abstract: The widespread use of low-temperature polymer electrolyte membrane fuel cells for mobile applications will require significant reductions in the amount of expensive Pt contained within their cathodes, which drive the oxygen reduction reaction (ORR). Although progress has been made in this respect, further reductions through the development of more active and stable electrocatalysts are still necessary. Here we describe a new set of ORR electrocatalysts consisting of Pd or Pt alloyed with early transition metals such as Sc or Y. They were identified using density functional theory calculations as being the most stable Pt- and Pd-based binary alloys with ORR activity likely to be better than Pt. Electrochemical measurements show that the activity of polycrystalline Pt(3)Sc and Pt(3)Y electrodes is enhanced relative to pure Pt by a factor of 1.5-1.8 and 6-10, respectively, in the range 0.9-0.87 V.
2,588 citations
TL;DR: This work has devised a complete reduction process through chemical conversion by sodium borohydride and sulfuric acid treatment, followed by thermal annealing that is particularly effective in the restoration of the π-conjugated structure, and leads to highly soluble and conductive graphene materials.
Abstract: Graphite oxide is one of the main precursors of graphene-based materials, which are highly promising for various technological applications because of their unusual electronic properties Although epoxy and hydroxyl groups are widely accepted as its main functionalities, the complete structure of graphite oxide has remained elusive By interpreting spectroscopic data in the context of the major functional groups believed to be present in graphite oxide, we now show evidence for the presence of five- and six-membered-ring lactols On the basis of this chemical composition, we devised a complete reduction process through chemical conversion by sodium borohydride and sulfuric acid treatment, followed by thermal annealing Only small amounts of impurities are present in the final product (less than 05 wt% of sulfur and nitrogen, compared with about 3 wt% with other chemical reductions) This method is particularly effective in the restoration of the π-conjugated structure, and leads to highly soluble and conductive graphene materials
2,311 citations
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TL;DR: The concept of the cooperative integration of 'softness' and 'regularity' and the relationship between the structures and properties of these materials in view of their practical applications are discussed.
Abstract: Encapsulating guest molecules inside host structures ranging from soft, flexible enzymes to rigid, porous zeolites has led to developments in many areas, including catalysis, sensing and separation. This Review highlights how metal–organic frameworks — materials formed by linking metal centres with organic ligands — can combine softness with regularity to produce dynamic, yet crystalline, structures that may prove useful for a range of applications.
The field of host–guest complexation is intensely attractive from diverse perspectives, including materials science, chemistry and biology. The uptake and encapsulation of guest species by host frameworks are being investigated for a wide variety of purposes, including separation and storage using zeolites, and recognition and sensing by enzymes in solution. Here we focus on the concept of the cooperative integration of 'softness' and 'regularity'. Recent developments on porous coordination polymers (or metal–organic frameworks) have provided the inherent properties that combine these features. Such soft porous crystals exhibit dynamic frameworks that are able to respond to external stimuli such as light, electric fields or the presence of particular species, but they are also crystalline and can change their channels reversibly while retaining high regularity. We discuss the relationship between the structures and properties of these materials in view of their practical applications.
1,936 citations
TL;DR: With energy swiftly rising to the top of the world's agenda, Harry B. Gray at the California Institute of Technology looks at how chemistry can help to harness the power of the Sun to meet the world't energy needs.
Abstract: With energy swiftly rising to the top of the world's agenda, Harry B. Gray at the California Institute of Technology looks at how chemistry can help to harness the power of the Sun to meet the world's energy needs.
1,464 citations
TL;DR: The fundamentals of the technique are discussed, along with how it can be used to synthesize macromolecules with controlled molecular architecture, and how their self-assembly can create nanostructured functional materials.
Abstract: The simplicity and broad applicabilty of atom transfer radical polymerization make it a rapidly developing area of synthetic polymer chemistry. Here, the fundamentals of the technique are discussed, along with how it can be used to synthesize macromolecules with controlled molecular architecture, and how their self-assembly can create nanostructured functional materials.
1,138 citations
TL;DR: Piezochromic luminescent materials - which change the colour of their luminescence in response to mechanical stimuli - are described and have potential for various applications, including sensors, memory and displays.
Abstract: Altering the shape and properties of a material through external factors such as heat, light, pressure, pH, electric or magnetic fields, or the introduction of a guest molecule, is an attractive prospect. In this Perspective, piezochromic luminescent materials - which change the colour of their luminescence in response to mechanical stimuli - are described. Such piezochromism has been observed for a few molecular materials that contain luminescent cores in liquid-crystalline and crystalline solid states, as well as for polymeric materials doped with dyes. These changes in photoluminescent colour can be activated by various types of mechanical pressure such as shearing, grinding or elongation, which can trigger different mechanisms of producing the colour. Such stimuli-responsive materials have potential for various applications, including sensors, memory and displays.
1,032 citations
TL;DR: This work presents a new analysis method, ion mobility coupled with mass spectrometry, for determination of in vitro oligomer distributions and the qualitative structure of each of the aggregates, which provides a candidate in the Aβ42 dodecamer for the primary toxic species in Alzheimer's disease.
Abstract: In recent years, small protein oligomers have been implicated in the aetiology of a number of important amyloid diseases, such as type 2 diabetes, Parkinson's disease and Alzheimer's disease. As a consequence, research efforts are being directed away from traditional targets, such as amyloid plaques, and towards characterization of early oligomer states. Here we present a new analysis method, ion mobility coupled with mass spectrometry, for this challenging problem, which allows determination of in vitro oligomer distributions and the qualitative structure of each of the aggregates. We applied these methods to a number of the amyloid-β protein isoforms of Aβ40 and Aβ42 and showed that their oligomer-size distributions are very different. Our results are consistent with previous observations that Aβ40 and Aβ42 self-assemble via different pathways and provide a candidate in the Aβ42 dodecamer for the primary toxic species in Alzheimer's disease.
853 citations
TL;DR: Na(3)(2,4,6-trihydroxy-1,3,5-benzenetrisulfonate) (named β-PCMOF2), a MOF that conducts protons in regular one-dimensional pores lined with sulfonate groups is reported.
Abstract: Metal organic frameworks (MOFs) are particularly exciting materials that couple porosity, diversity and crystallinity. But although they have been investigated for a wide range of applications, MOF chemistry focuses almost exclusively on properties intrinsic to the empty frameworks; the use of guest molecules to control functions has been essentially unexamined. Here we report Na(3)(2,4,6-trihydroxy-1,3,5-benzenetrisulfonate) (named β-PCMOF2), a MOF that conducts protons in regular one-dimensional pores lined with sulfonate groups. Proton conduction in β-PCMOF2 was modulated by the controlled loading of 1H-1,2,4-triazole (Tz) guests within the pores and reached 5 × 10(-4) S cm(-1) at 150 °C in anhydrous H(2), as confirmed by electrical measurements in H(2) and D(2), and by solid-state NMR spectroscopy. To confirm its potential as a gas separator membrane, the partially loaded MOF (β-PCMOF2(Tz)(0.45)) was also incorporated into a H(2)/air membrane electrode assembly. The resulting membrane proved to be gas tight, and gave an open circuit voltage of 1.18 V at 100 °C.
680 citations
TL;DR: A host of crystallographic and reactivity data, as well as theoretical results, that indicate a highly non-canonical bonding situation in many members of this series, which may well affect the understanding of organic chemistry in general.
Abstract: A systematic variation of ligand properties allows an in-depth experimental and theoretical study of a highly non-canonical bonding situation in certain organic compounds, and provides insight into the criteria that must be fulfilled for such compounds to be truly considered as carbon(0)-containing entities.
678 citations
TL;DR: The rise of FBDD is reviewed, including its application to discovering clinical candidates against targets for which other chemistry approaches have struggled, and how it requires fewer compounds to be screened and, despite the lower initial potency of the screening hits, offers more efficient and fruitful optimization campaigns.
Abstract: Fragment-based drug discovery is an approach that relies on the ability to identify weakly binding drug fragments using sophisticated screening techniques. Binding can be optimized while maintaining favourable physical properties of the drug, which should have a positive impact on the attrition rates of new drug candidates.
The search for new drugs is plagued by high attrition rates at all stages in research and development. Chemists have an opportunity to tackle this problem because attrition can be traced back, in part, to the quality of the chemical leads. Fragment-based drug discovery (FBDD) is a new approach, increasingly used in the pharmaceutical industry, for reducing attrition and providing leads for previously intractable biological targets. FBDD identifies low-molecular-weight ligands (∼150 Da) that bind to biologically important macromolecules. The three-dimensional experimental binding mode of these fragments is determined using X-ray crystallography or NMR spectroscopy, and is used to facilitate their optimization into potent molecules with drug-like properties. Compared with high-throughput-screening, the fragment approach requires fewer compounds to be screened, and, despite the lower initial potency of the screening hits, offers more efficient and fruitful optimization campaigns. Here, we review the rise of FBDD, including its application to discovering clinical candidates against targets for which other chemistry approaches have struggled.
TL;DR: It is demonstrated that microwave heating in combination with the screening of comonomer reactant ratios can be used to obtain donor-acceptor copolymers with high average molecular weights and properties that make them suitable for solar cell incorporation.
Abstract: The most efficient plastic solar cells comprise a blend of conjugated polymer and a suitable electron acceptor, typically a fullerene derivative. Therefore narrow-bandgap conjugated polymers are currently sought for the fabrication of such devices. A significant challenge is being able to predict device function and performance from consideration of the molecular connectivity and dimensions of the partners within the active layer. Improved chemical syntheses are therefore required to make structurally varied polymers and enable the delineation of structure–function relationships with the aim of improving power conversion efficiencies. Here, we demonstrate that microwave heating in combination with the screening of comonomer reactant ratios can be used to obtain donor–acceptor copolymers with high average molecular weights and properties that make them suitable for solar cell incorporation. Furthermore, we highlight the importance of high molecular weight and the contribution of solubilizing side groups in determining the final device properties.
TL;DR: On the basis of pH-tunable spectral overlap of donors and acceptors, the donor-loaded polymerized vesicles display pH-dependent fluorescence resonance energy transfer from the encapsulated donors to the bilayer dye membrane, providing ultrasensitive pH information on their aqueous environment with fluorescence colour changes covering the whole visible light range.
Abstract: Water-soluble, self-assembled nanocapsules composed of a functional bilayer membrane and enclosed guest molecules can provide smart (that is, condition responsive) sensors for a variety of purposes. Owing to their outstanding optical and redox properties, perylene bisimide chromophores are interesting building blocks for a functional bilayer membrane in a water environment. Here, we report water-soluble perylene bisimide vesicles loaded with bispyrene-based energy donors in their aqueous interior. These loaded vesicles are stabilized by in situ photopolymerization to give nanocapsules that are stable over the entire aqueous pH range. On the basis of pH-tunable spectral overlap of donors and acceptors, the donor-loaded polymerized vesicles display pH-dependent fluorescence resonance energy transfer from the encapsulated donors to the bilayer dye membrane, providing ultrasensitive pH information on their aqueous environment with fluorescence colour changes covering the whole visible light range. At pH 9.0, quite exceptional white fluorescence could be observed for such water-soluble donor-loaded perylene vesicles.
TL;DR: Insight is provided into the dynamics of diffusion in cells, which is pertinent to drug delivery, cell signalling and intracellular mass transport, and the effect of this viscosity increase is observed directly in the diffusion-dependent kinetics of the photosensitized formation and decay of a key cytotoxic agent.
Abstract: Diffusion-mediated cellular processes, such as metabolism, signalling and transport, depend on the hydrodynamic properties of the intracellular matrix. Photodynamic therapy, used in the treatment of cancer, relies on the generation of short-lived cytotoxic agents within a cell on irradiation of a drug. The efficacy of this treatment depends on the viscosity of the medium through which the cytotoxic agent must diffuse. Here, spectrally resolved fluorescence measurements of a porphyrin-dimer-based molecular rotor are used to quantify intracellular viscosity changes in single cells. We show that there is a dramatic increase in the viscosity of the immediate environment of the rotor on photoinduced cell death. The effect of this viscosity increase is observed directly in the diffusion-dependent kinetics of the photosensitized formation and decay of a key cytotoxic agent, singlet molecular oxygen. Using these tools, we provide insight into the dynamics of diffusion in cells, which is pertinent to drug delivery, cell signalling and intracellular mass transport.
TL;DR: The invention of chemoselective methodologies is crucial to the execution of 'protecting-group-free' synthesis, and recent advances in this area are also highlighted.
Abstract: The constant pressure to prepare compounds in a more efficient manner has placed the process by which traditional synthetic chemistry is conducted under scrutiny. Areas that have the potential to be improved must be highlighted and modified, so that we can approach the criterion of the 'ideal synthesis'. One area that offers this prospect is the minimization of the use of protecting groups in synthesis. A protection/deprotection event introduces at least two steps into a sequence, incurring costs from additional reagents and waste disposal, and generally leads to a reduced overall yield. Here we present relevant historical context and highlight recent (post-2004) total syntheses that have developed new chemistry in an effort to exclude protecting groups. The invention of chemoselective methodologies is crucial to the execution of 'protecting-group-free' synthesis, and recent advances in this area are also highlighted.
TL;DR: It is found that non-covalent interactions between hydrated alkali metal cations M(+)(H(2)O)(x) and adsorbed OH (OH(ad)) species increase in the same order as the hydration energies of the corresponding cations, which suggests that the clusters block the platinum active sites for electrocatalytic reactions.
Abstract: The classic models of metal electrode-electrolyte interfaces generally focus on either covalent interactions between adsorbates and solid surfaces or on long-range electrolyte-metal electrostatic interactions. Here we demonstrate that these traditional models are insufficient. To understand electrocatalytic trends in the oxygen reduction reaction (ORR), the hydrogen oxidation reaction (HOR) and the oxidation of methanol on platinum surfaces in alkaline electrolytes, non-covalent interactions must be considered. We find that non-covalent interactions between hydrated alkali metal cations M(+)(H(2)O)(x) and adsorbed OH (OH(ad)) species increase in the same order as the hydration energies of the corresponding cations (Li(+) >> Na(+) > K(+) > Cs(+)) and also correspond to an increase in the concentration of OH(ad)-M(+)(H(2)O)(x) clusters at the interface. These trends are inversely proportional to the activities of the ORR, the HOR and the oxidation of methanol on platinum (Cs(+) > K(+) > Na(+) >> Li(+)), which suggests that the clusters block the platinum active sites for electrocatalytic reactions.
TL;DR: Charge transport through symmetric tetraphenyl and non-symmetric diblock dipyrimidinyldiphenyl molecules covalently bound to two electrodes is studied to study diode behaviour in single molecules.
Abstract: In the molecular electronics field it is highly desirable to engineer the structure of molecules to achieve specific functions. In particular, diode (or rectification) behaviour in single molecules is an attractive device function. Here we study charge transport through symmetric tetraphenyl and non-symmetric diblock dipyrimidinyldiphenyl molecules covalently bound to two electrodes. The orientation of the diblock is controlled through a selective deprotection strategy, and a method in which the electrode-electrode distance is modulated unambiguously determines the current-voltage characteristics of the single-molecule device. The diblock molecule exhibits pronounced rectification behaviour compared with its homologous symmetric block, with current flowing from the dipyrimidinyl to the diphenyl moieties. This behaviour is interpreted in terms of localization of the wave function of the hole ground state at one end of the diblock under the applied field. At large forward current, the molecular diode becomes unstable and quantum point contacts between the electrodes form.
TL;DR: The results challenge the currently accepted mechanism for oxidative palladium catalysis via Pd(II)–Pd(IV) redox cycles and implicate bimetallic palladium complexes in redox catalysis.
Abstract: Palladium is a common transition metal for catalysis, and the fundamental organometallic reactivity of palladium in its 0, I, II and IV oxidation states is well established. The potential role of Pd(III) in catalysis has not been investigated because organometallic reactions that involve Pd(III) have not been reported previously. In this article we present the formation of carbon–heteroatom bonds from discrete bimetallic Pd(III) complexes and show the synergistic involvement of two palladium atoms of the bimetallic core during both oxidation and reductive elimination. Our results challenge the currently accepted mechanism for oxidative palladium catalysis via Pd(II)–Pd(IV) redox cycles and implicate bimetallic palladium complexes in redox catalysis. The new mechanistic insight provides an opportunity to explore rationally the potential of bimetallic palladium catalysis for synthesis.
TL;DR: It is found that silver(I) complexes of polymer-functionalized N-heterocyclic carbenes, which are latent organocatalysts, catalyse a transesterification reaction when exposed to ultrasound in solution and ultrasonic activation of a ruthenium biscarbene complex with appended polymer chains results in catalysis of olefin metathesis reactions.
Abstract: Homogeneously catalysed reactions can be 'switched on' by activating latent catalysts. Usually, activation is brought about by heat or an external chemical agent. However, activation of homogeneous catalysts with a mechanical trigger has not been demonstrated. Here, we introduce a general method to activate latent catalysts by mechanically breaking bonds between a metal and one of its ligands. We have found that silver(I) complexes of polymer-functionalized N-heterocyclic carbenes, which are latent organocatalysts, catalyse a transesterification reaction when exposed to ultrasound in solution. Furthermore, ultrasonic activation of a ruthenium biscarbene complex with appended polymer chains results in catalysis of olefin metathesis reactions. In each case, the catalytic activity results from ligand dissociation, brought about by transfer of mechanical forces from the polymeric substituents to the coordination bond. Mechanochemical catalyst activation has potential applications in transduction and amplification of mechanical signals, and mechanically initiated polymerizations hold promise as a novel repair mechanism in self-healing materials.
TL;DR: Synthetic oligosaccharides and glycoconjugates are increasingly used as probes for biological research and as lead compounds for drug and vaccine discovery, and the power of carbohydrate chemistry is highlighted by an ability to synthesize glycoproteins.
Abstract: Synthetic oligosaccharides and glycoconjugates are increasingly used as probes for biological research and as lead compounds for drug and vaccine discovery. These endeavors are, however, complicated by a lack of general methods for the routine preparation of this important class of compounds. Recent development such as one-pot multi-step protecting group manipulations, the use of unified monosaccharide building blocks, the introduction of stereoselective glycosylation protocols, and convergent strategies for oligosaccharide assembly, are beginning to address these problems. Furthermore, oligosaccharide synthesis can be facilitated by chemo-enzymatic methods, which employ a range of glycosyl transferases to modify a synthetic oligosaccharide precursor. Glycosynthases, which are mutant glycosidases, that can readily form glycosidic linkages are addressing a lack of a wide range glycosyltransferases. The power of carbohydrate chemistry is highlighted by an ability to synthesize glycoproteins.
TL;DR: This work presents a practical synthesis for ultrafine subnanometre platinum clusters using a spherical macromolecular template with no disorder in molecular weight or structure, and finds that the clusters exhibit very high catalytic activity for the four-electron reduction of oxygen molecules.
Abstract: Colloidal platinum nanoparticles with diameters of 2-5 nm on carbon supports are currently regarded as the best catalysts for the oxygen reduction reaction. However, the particle size is limited by the conventional preparation methods that are used to synthesize small platinum particles; the inherent activity of ultrasmall nanoparticles has not yet been revealed. We present a practical synthesis for ultrafine subnanometre platinum clusters using a spherical macromolecular template with no disorder in molecular weight or structure. The template, a phenylazomethine dendrimer, offers control of the number of metal complexes in an assembly through stepwise complexation, allowing the complexes to accumulate in discrete nano-cages. Subsequent reduction of Pt(IV) chloride to Pt(0) results in the formation of platinum clusters composed of a defined number of atoms. As a result of exceptionally small particle size, the clusters exhibit very high catalytic activity for the four-electron reduction of oxygen molecules.
TL;DR: The demonstration of robust, uniform organic functionalization of epitaxial graphene presents opportunities for graphene-based molecular electronics and sensors.
Abstract: Graphene, a two-dimensional sheet of carbon atoms, is a promising material for next-generation technology because of its advantageous electronic properties, such as extremely high carrier mobilities. However, chemical functionalization schemes are needed to integrate graphene with the diverse range of materials required for device applications. In this paper, we report self-assembled monolayers of the molecular semiconductor perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) formed on epitaxial graphene grown on the SiC(0001) surface. The molecules possess long-range order with a herringbone arrangement, as shown by ultra-high vacuum scanning tunnelling microscopy at room temperature. The molecular ordering is unperturbed by defects in the epitaxial graphene or atomic steps in the underlying SiC surface. Scanning tunnelling spectra of the PTCDA monolayer show distinct features that are not observed on pristine graphene. The demonstration of robust, uniform organic functionalization of epitaxial graphene presents opportunities for graphene-based molecular electronics and sensors.
TL;DR: A sensor based on a hybrid synthetic-biomolecule that uses arrays of green fluorescent protein and nanoparticles to detect proteins at biorelevant concentrations in both buffer and human serum is reported.
Abstract: There is a direct correlation between protein levels and disease states in human serum, which makes it an attractive target for sensors and diagnostics. However, this is challenging because serum features more than 20,000 proteins, with an overall protein content greater than 1 mM. Here we report a sensor based on a hybrid synthetic-biomolecule that uses arrays of green fluorescent protein and nanoparticles to detect proteins at biorelevant concentrations in both buffer and human serum. Distinct and reproducible fluorescence-response patterns were obtained from five serum proteins (human serum albumin, immunoglobulin G, transferrin, fibrinogen and a-antitrypsin), both in buffer and when spiked into human serum. Using linear discriminant analysis we identified these proteins with an identification accuracy of 100% in buffer and 97% in human serum. The arrays were also able to discriminate between different concentrations of the same protein, as well as a mixture of different proteins in human serum.
TL;DR: A dinuclear ruthenium(II) polypyridyl system that works as a multifunctional biological imaging agent staining the DNA of eukaryotic and prokaryotic cells for both luminescence and transition electron microscopy is presented.
Abstract: In the search for new biological imaging agents, metal coordination compounds able to emit from triplet metal-to-ligand charge transfer (MLCT) states offer many advantages as luminescent probes of DNA structure. However, poor cellular uptake restricts their use in live cells. Here, we present a dinuclear ruthenium(II) polypyridyl system that works as a multifunctional biological imaging agent staining the DNA of eukaryotic and prokaryotic cells for both luminescence and transition electron microscopy. This MLCT 'light switch' complex directly images nuclear DNA of living cells without requiring prior membrane permeabilization. Furthermore, inhibition and transmission electron microscopy studies show this to be via a non-endocytotic, but temperature-dependent, mechanism of cellular uptake in MCF-7 cells, and confocal microscopy reveals multiple emission peaks that function as markers for cellular DNA structure.
TL;DR: Clear differentiation among 19 different toxic industrial chemicals within two minutes of exposure at IDLH (immediately dangerous to life or health) concentration has been demonstrated and excellent detection limits have been demonstrated, generally below the PELs (permissible exposure limits).
Abstract: We have developed a simple colorimetric sensor array that detects a wide range of volatile analytes and then applied it to the detection of toxic gases. The sensor consists of a disposable array of cross-responsive nanoporous pigments with colours that are changed by diverse chemical interactions with analytes. Although no single chemically responsive pigment is specific for any one analyte, the pattern of colour change for the array is a unique molecular fingerprint. Clear differentiation among 19 different toxic industrial chemicals (TICs) within two minutes of exposure at concentrations immediately dangerous to life or health were demonstrated. Based on the colour change of the array, quantification of each analyte was accomplished easily, and excellent detection limits were achieved, generally below the permissible exposure limits. Different TICs were identified readily using a standard chemometric approach (hierarchical clustering analysis), with no misclassifications over 140 trials.
TL;DR: It is proposed that the carbon-gold bond in these intermediates is comprised of varying degrees of both σ and π-bonding; however, the overall bond order is generally less than or equal to unity.
Abstract: An analysis of key intermediates relevant to gold(I) catalysis has been performed using density functional theory. A bonding model is proposed whereby the reactivity of gold(I)-coordinated carbenes is dependent on carbene substituents and ancillary ligands that dictate where these gold structures lie on a continuum ranging from a metal-stabilized singlet carbene to a metal-coordinated carbocation.
TL;DR: The optimum geometry of an X-bond can be predicted from the pattern of H-bonds in a folded protein, enabling X- bonds to be introduced to improve ligand affinities without disrupting these structurally important interactions.
Abstract: Halogen bonds (X-bonds) are shown to be geometrically perpendicular to and energetically independent of hydrogen bonds (H-bonds) that share a common carbonyl oxygen acceptor. This orthogonal relationship is accommodated by the in-plane and out-of-plane electronegative potentials of the oxygen, which are differentially populated by H- and X-bonds. Furthermore, the local conformation of a peptide helps to define the geometry of the H-bond and thus the oxygen surface that is accessible for X-bonding. These electrostatic and steric forces conspire to impose a strong preference for the orthogonal geometry of X- and H-bonds. Thus, the optimum geometry of an X-bond can be predicted from the pattern of H-bonds in a folded protein, enabling X-bonds to be introduced to improve ligand affinities without disrupting these structurally important interactions. This concept of orthogonal molecular interactions can be exploited for the rational design of halogenated ligands as inhibitors and drugs, and in biomolecular engineering.
TL;DR: An anionic MOF material built from In(iii) centres and tetracarboxylic acid ligands (H4L) in which kinetic trapping behaviour—where hydrogen is adsorbed at high pressures but not released immediately on lowering the pressure—is modulated by guest cations.
Abstract: Metal-organic frameworks (MOFs)--microporous materials constructed by bridging metal centres with organic ligands--show promise for applications in hydrogen storage, which is a key challenge in the development of the 'hydrogen economy'. Their adsorption capacities, however, have remained insufficient for practical applications, and thus strategies to enhance hydrogen-MOF interactions are required. Here we describe an anionic MOF material built from In(III) centres and tetracarboxylic acid ligands (H(4)L) in which kinetic trapping behaviour--where hydrogen is adsorbed at high pressures but not released immediately on lowering the pressure--is modulated by guest cations. With piperazinium dications in its pores, the framework exhibits hysteretic hydrogen adsorption. On exchange of these dications with lithium cations, no hysteresis is seen, but instead there is an enhanced adsorption capacity coupled to an increase in the isosteric heat of adsorption. This is rationalized by the different locations of the cations within the pores, determined with precision by X-ray crystallography.
TL;DR: Small anions can be used to modulate the physical properties of supramolecular gels by interacting with the low-molecular-weight gelators from which such materials are composed, and a better understanding will aid in the rational design of responsive gels that may prove useful for a number of practical applications.
Abstract: Small anions can be used to modulate the physical properties of supramolecular gels by interacting with the low-molecular-weight gelators from which such materials are composed. A better understanding of this anion-tuning effect will aid in the rational design of responsive gels that may prove useful for a number of practical applications.
TL;DR: This work examines recent advances in metal-catalysed diamination reactions and their asymmetric variants and suggests that they will find extensive application in the construction of natural products and drug molecules in the near future.
Abstract: The 1,2-diamine motif is present in a number of natural products with interesting biological activity and in many important pharmaceutical agents. Chiral 1,2-diamines are also widely used as the control elements in asymmetric synthesis and catalysis. Such compounds are thus an attractive target for the synthetic chemist. Although the diamination of an alkene seems an obvious route to these structures, far less research has been devoted to it than to the analogous dihydroxylation or aminohydroxylation reactions that are well-established processes in asymmetric synthesis. Here, we examine recent advances in metal-catalysed diamination reactions and their asymmetric variants. Given the prevalence of these structures, it seems likely that they will find extensive application in the construction of natural products and drug molecules in the near future.