Development of versatile and silver-free protocols for gold(I) catalysis

The use of a versatile N‐heterocyclic carbene (NHC) gold(I) hydroxide precatalyst, [Au(OH)(IPr)], (IPr=N,N′‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) permits the in situ generation of the [Au(IPr)]⁺ ion by simple addition of a Brønsted acid. This cationic entity is believed to be the active species in numerous catalytic reactions. ¹H NMR studies in several solvent media of the in situ generation of this [Au(IPr)]⁺ ion also reveal the formation of a dinuclear gold hydroxide intermediate [{Au(IPr)}₂(μ‐OH)], which is fully characterized and was tested in gold(I) catalysis.     

Theoretical investigation of the Ziegler–Natta catalysis in heterogeneous conditions

We present ab initio calculations on the Ziegler–Natta catalysis in heterogeneous conditions. Both cluster and periodic models with different basis sets have been used to explain the chemical reactivity of the various species that are involved. In heterogeneous conditions, several parameters contribute to the catalysis: the oxidation state of the titanium atom, the structure of the titanium site, the interaction with the support, MgCl₂, and the cocatalyst, AlR₃. This article yields to a synthetic view of the catalytic activity. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Similarities and differences between the "relativistic" triad gold, platinum, and mercury in catalysis

Relativistic effects in the valence shell of the elements reach a maximum in the triad Pt-Au-Hg and determine their catalytic activity in organic reactions. In this Review we examine the catalytic activity of Pt, Au, and Hg compounds for some representative reactions, and discuss the respective benefits and disadvantages along with other relevant properties, such as toxicity, price, and availability. For the reactions considered, gold catalysts are generally more active than mercury or platinum catalysts.     

Strong metallophilic interactions in the palladium arylation by gold aryls

It's the second step that counts: arylation of Pd by Au takes place through transition states and intermediates featuring strong Au???Pd metallophilic interactions. However, the aryl transfer from [AuArL] to [PdArClL(2)] is thermodynamically disfavored and will not occur unless an irreversible Ar-Ar coupling from [PdAr(2)L(2)] follows.     

A modular route to nonracemic cyclo-NOBINs. Preparation of the parent ligand for homo- and heterogeneous catalysis

A sequence which combines a nonracemic tether, a naphthyl diol, and an aminonaphthol has been developed leading to the new ligand cyclo-NOBIN, which can easily be mounted onto polystyrene. Opportunities for extending the route to substituted cyclo-NOBINs are also discussed.     

Hydrotalcite-supported gold catalyst for the oxidant-free dehydrogenation of benzyl alcohol: studies on support and gold size effects

Gold nanoparticles with uniform mean sizes (≈3 nm) loaded onto various supports have been prepared and studied for the oxidant-free dehydrogenation of benzyl alcohol to benzaldehyde and hydrogen. The use of hydrotalcite (HT), which possesses both strong acidity and strong basicity, provides the best catalytic performance. Au/HT catalysts with various mean Au particle sizes (2.1-21 nm) have been successfully prepared by a deposition-precipitation method under controlled conditions. Detailed catalytic reaction studies with these catalysts demonstrate that the Au-catalyzed dehydrogenation of benzyl alcohol is a structure-sensitive reaction. The turnover frequency (TOF) increases with decreasing Au mean particle size (from 12 to 2.1 nm). A steep rise in TOF occurs when the mean Au particle size becomes smaller than 4 nm. Our present work suggests that the acid-base properties of the support and the size of Au nanoparticles are two key factors controlling the alcohol dehydrogenation catalysis. A reaction mechanism is proposed to rationalize these results. It is assumed that the activation of the β-C-H bond of alcohol, which requires the coordinatively unsaturated Au atoms, is the rate-determining step.     

Challenges in the use of density functional theory to examine catalysis by M‐doped ceria surfaces

For CeO₂ or M‐doped CeO₂ catalysts, reliable energetics associated with surface reactivity requires accurate representation of oxidized and reduced metal states. Density functional theory (DFT) is used extensively for metals and metal oxides; however, for strongly correlated electron materials, conventional DFT fails to predict both qualitative and quantitative properties. This is the result of a localized electron self‐interaction error that is inherit to DFT. DFT+U has shown promise in correcting energetic errors due to the self‐interaction error, however, its transferability across processes relevant to surface catalysis remains unclear. Hybrid functionals, such as HSE06, can also be used to correct this self‐interaction error. These hybrid functionals are computationally intensive, and especially demanding for periodic surface slab models. This perspective details the challenges in representing the energetics of M‐doped ceria catalyzed processes and examines using DFT extensions to model the localized electronic properties. © 2013 Wiley Periodicals, Inc.     

A novel luminescent ionic trinuclear Cu3I2 cluster cuprous complex supported by a P^N ligand: synthesis,structure characterization,properties and TD–DFT calculations

Luminescent cuprous complexes are an important class of coordination compounds due to their relative abundance, low cost and ability to display excellent luminescence. The title ionic trinuclear Cu₃I₂ complex, tris[μ₂‐diphenyl(pyridin‐2‐yl)phosphane‐κ²P:N]di‐μ₃‐iodido‐tricopper(I)(3 Cu—Cu) hexafluoridophosphate, [Cu₃I₂(C₃₉H₃₂NP)₃]PF₆, conventionally abbreviated as [Cu₃I₂(Ph₂PPy)₃]PF₆, is described. Each Cu^I atom is coordinated by two μ₃‐iodide ligands and by a P and an N atom from two Ph₂PPy ligands, giving rise to a CuI₂PN tetrahedral coordination geometry about each Cu^I centre. The electronic absorption and photoluminescence properties of this trinuclear cluster have been studied on as‐synthesized samples, which had been examined previously by powder X‐ray diffraction. A detailed time‐dependent density functional theory (TD–DFT) study was carried out and showed a green emission derived from a halide‐to‐ligand charge transfer and metal‐to‐ligand charge transfer ³(X+M)LCT excited state.     

Adsorption of small molecules on helical gold nanorods: A relativistic density functional study

We study the adsorption of a variety of small molecules on helical gold nanorods using relativistic density functional theory. We focus on Au₄₀ which consists of a central linear strand of five gold atoms with seven helical strands of five gold atoms on a coaxial tube. All molecules preferentially adsorb at a single low‐coordinated gold atom on the coaxial tube at an end of Au₄₀. In most cases, there is significant charge transfer (CT) between Au₄₀ and the adsorbate, for CO and NO₂, there is CT from the Au₄₀ to adsorbate while for all other molecules there is CT from the adsorbate to Au₄₀. Thus, Au₄₀‐adsorbate can be described as a donor–accepter complex and we use charge decomposition analysis to better understand the adsorption process. We determine the adsorption energy order to be C₅H₅N >NO₂ > CO > NH₃ > CH₂?CH₂ > CH₂?CH? CHO > NO > HC?CH > H₂S > SO₂ > HCN > CH₃OH > H₂C?O > O₂ > H₂O > CH₄ > N₂. We find that the Au? C, Au? N, Au? S, and Au? O bonds are surprisingly strong, with clear implications for reactivity enhancement of the adsorbate. The Au? H bond is relatively weak but, for interactions via an H atom that is bonded to a carbon atom (e.g., CH₄), we find that there is large charge polarization of the Au? H? C moiety and partial activation of the inert C? H bond. Although the Au? S and Au? O bonds are generally weaker than the Au? C and Au? N bonds, we find that adsorption of H₂S or H₂O causes greater distortion of Au₄₀ in the binding region. However, the degree of distortion is small and the helical structure is retained, demonstrating the stability of the helical Au₄₀ nanorod under perturbations. © 2014 Wiley Periodicals, Inc.     

Low energy structures of gold nanoclusters in the size range 3–38 atoms

Using a combination of first principles calculations and empirical potentials we have undertaken a systematic study of the low energy structures of gold nanoclusters containing from 3 to 38 atoms. A Lennard-Jones and many-body potential have been used in the empirical calculations, while the first principles calculations employ an atomic orbital, density functional technique. For the smaller clusters (n=3–5) the potential energy surface has been mapped at the ab initio level and for larger clusters an empirical potential was first used to identify low energy candidates which were then optimised with full ab initio calculations. At the DFT-LDA level, planar structures persist up to six atoms and are considerably more stable than the cage structures by more than 0.1 eV/atom. The difference in ab initio energy between the most stable planar and cage structures for seven atoms is only 0.04 eV/atom. For larger clusters there are generally a number of minima in the potential energy surface lying very close in energy. Furthermore our calculations do not predict ordered structures for the magic numbers n=13 and 38. They do predict the ordered tetrahedral structure for n=20. The results of the calculations show that gold nanoclusters in this size range are mainly disordered and will likely exist in a range of structures at room temperature.     

Self‐Relay Gold(I)‐Catalyzed Pictet–Spengler/Cyclization Cascade Reaction for the Rapid Elaboration of Pentacyclic Indole Derivatives

Gold‐catalyzed cascade reactions allow the rapid elaboration of pentacyclic indolo[2,3‐a]quinolizidines from N‐allyl tryptamines and ortho‐alkynylarylaldehydes. The tandem process combines a gold‐catalyzed Pictet‐Spengler reaction and a cyclization occurring concomitantly with an allyl transfer from the nitrogen atom to the stilbene function. Various substituted allyls were successfully transferred, furnishing the products in yields typically ranging from 60–98 % in high diastereoselectivity. Tryptamines bearing a butenol chain undergo an additional cyclization to chiral hemiaminals in high diastereoselectivities.     

A Unique Au–Ag–Au Triangular Motif in a Trimetallic Halonium Dication: Silver Incorporation in a Gold(I) Catalyst

As a result of explorations into the solution chemistry of silver/gold mixtures, a unique diphosphine trimetallic chloronium dication was discovered that incorporates silver–arene chelation and a triangular mixed gold/silver core in the solid state. Notably, it was isolated from a Celite prefiltered solution initially thought to be silver‐free. The crystal structure also incorporates the coordination to silver of one fluorine atom of one SbF₆^? counterion. The structure was compared to two new, but well‐precedented, phosphine digold chloride cations. DFT calculations supported significant silver–halide and silver–arene interactions in the mixed gold/silver complex and metallophilic interactions in all three complexes. Comparison of computed data revealed that the ωB97X‐D functional, which has a long‐range corrected hybrid with atom–atom dispersion corrections, gave a better fit to the experimental data compared with the PBE0 functional, which has previously failed to capture aurophilic interactions. Preliminary studies support the presence of the mixed gold/silver structure in solution.