Methoxycyclization of 1,5-Enynes by Coinage Metal Catalysts: Is Gold Always Superior?

Au(I) catalysts promote the intramolecular cycloisomerization of 1,n-enynes, as well as facilitate reactions with alcohols, characteristics that have led to their wide-spread in the synthesis of complex natural products. Literature reports of Cu and Ag species catalyzing the same or similar reactions are much less common. Here, we utilize molecular volcano plots to assess the activity of 45 coinage metal phosphine/phosphite catalysts possessing a variety of stereoelectronic properties for the methoxycyclization reaction of a prototypical 1,5-enyne. Our analysis reveals that the intrinsic properties of gold allow it to be coupled with a wide range of ligands, all of which exhibit high catalytic activity. On the other hand, the coupling of silver and copper with more traditional phosphines, such as triphenylphosphine, leads to catalysts with diminished catalytic ability. The activity of Ag and Cu species, however, can be enhanced by appending ligands with strong π-accepting character, which shifts these species closer to the volcano peak.     

Exploring Coordination Modes: Late Transition Metal Complexes with a Methylene‐bridged Macrocyclic Tetra‐NHC Ligand

A tetranuclear silver(I) N‐heterocyclic carbene (NHC) complex bearing a macrocyclic, exclusively methylene‐bridged, tetracarbene ligand was synthesized and employed as transmetalation agent for the synthesis of nickel(II), palladium(II), platinum(II), and gold(I) derivatives. The transition metal complexes exhibit different coordination geometries, the coinage metals being bound in a linear fashion forming molecular box‐type complexes, whereas the group 10 metals adapt an almost ideal square planar coordination geometry within the ligand's cavity, resulting in saddle‐shaped complexes. Both the Ag^I and the Au^I complexes show ligand‐induced metal–metal contacts, causing photoluminescence in the blue region for the gold complex. Distinct metal‐dependent differences of the coordination behavior between the group 10 transition metals were elucidated by low‐temperature NMR spectroscopy and DFT calculations.     

Surface-enhanced Raman scattering of pyridine using different metals: differences and explanation based on the selective formation of alpha-pyridyl on metal surfaces

A simple method has been developed to produce SERS-active metal surfaces. Six metal surfaces (cadmium, nickel, gold, iron, copper, and silver) have been prepared on an aluminum foil underlayment by chemical reduction and strong surface-enhanced Raman signals have been observed for pyridine species on these surfaces. This permits the direct comparison of pyridine spectra on different metal surfaces prepared by the same chemically clean method. The differences among the SER spectra of the aqueous pyridine species using different metals generally follow the trend of silver, cadmium, nickel, iron, gold, and copper, which can be explained by the selective formation of alpha-pyridyl species and the equilibria between end-on adsorbed pyridines and edge-on adsorbed alpha-pyridyl species on the different metal surfaces.     

Electrochemical oxidation of tertiary phosphines in the presence of camphene

Anodic oxidation of tertiary phosphines (tripropyl-, tributyl-, and triphenylphosphine) in the presence of a bicyclic alkene (camphene) on a platinum and a glassy carbon electrodes was studied. For the first time the voltammetric characteristics of the process of camphene anodic oxidation were obtained. The electrochemical reactions with alkyl and aromatic phosphine were found to be dissimilar. The results of preparative electrooxidation of trialkylphosphines showed that in the course of electrolysis the tertiary phosphine cation-radicals generated on the anode enter into two concurrent reactions: (1) with the parent phosphine to form eventually trialkylphosphonium salts and trialkylphosphine oxides presumably as complex compounds and (2) with camphene to form trialkylcamphenylphosphonium salts and probably phosphonium salts with a monocyclic substituent. Preparative electrochemical oxidation of triphenylphosphine in the presence of camphene affords almost exclusively either triphenylphosphine oxide (in the experiment with platinum anode) or the triphenylphosphine oxide complex with perchloric acid (at the electrolysis on a glassy carbon anode).     

Infrared spectroscopy of physisorbed and chemisorbed O2 on Pt(111)

The adsorption of O2 on the Pt(111) surface, with particular emphasis on the influence of substrate temperature, has been studied by infrared reflection absorption spectroscopy (IRAS). In the temperature range 30-90 K the IRAS spectra reveal three different molecular adsorption states. A physisorbed state appears below 40 K while chemisorbed peroxo- and superoxo-like states are observed in the whole temperature range, the characteristic vibrational frequencies are at full coverages of 16O2, 1543 cm(-1) and around 700 cm(-1) and 870 cm(-1), respectively. Flash heating from 30 K to 45 K reveal that the physisorbed state acts as a precursor to the superoxo chemisorption. Theoretical calculations suggest that peroxo molecules may occupy both fcc and hcp threefold sites on the Pt(111) surface. However, within the high resolution of the IRAS measurements we only observe one peroxo state in the temperature range 45-90 K, assigned to occupy the fcc site. The peroxo adsorption probability is significantly lower at 45 K than at 90 K, presumably due to reduced thermal activation from the physisorbed precursor state. A longer lifetime in this precursor state at the low temperature results in formation of larger superoxo islands already at low oxygen coverage.     

Effect of intrinsic properties of metals on the adsorption behavior of molecules: benzene adsorption on Pt group metals

Investigation of benzene adsorption on different metal surfaces closer to a practical system appears to be a very important intermediate stage to utilize the conclusion obtained on single-crystal surfaces. In this paper, we studied the electrochemical adsorption behaviors of benzene on roughened Pt group electrodes using surface enhanced Raman spectroscopy (SERS). The effects of potential, surface roughness, and benzene concentration were investigated. Significant difference in surface Raman spectra of benzene on Ru, Rh, Pd, and Pt surfaces were found. On Pt surfaces, the parallel-chemisorbed benzene, the vertical dissociated chemisorbed benzene, and the physisorbed benzene were observed, evidenced by the ring vibration mode appearing at 872, 1012, and 991 cm(-1), respectively. On Pd, only parallel-chemisorbed benzene and physisorbed benzene were found. On Rh and Ru, the SERS signals were mainly from the parallel-chemisorbed benzene, with an extremely weak signal from the physisorbed benzene. The potential dependent study reveals that the parallel-chemisorbed species interacts strongest with the substrate, while the physisorbed species can be easily removed at positive potentials. The models for the adsorbed benzene were given to account for the different types of benzene on these Pt group metals. The difference in the atomization heat of the four metals was used to interpret the different interaction strength of benzene with Pt group metals.     

Synthesis and molecular structure of gold triarylcorroles

A number of third-row transition-metal corroles have remained elusive as synthetic targets until now, notably osmium, platinum, and gold corroles. Against this backdrop, we present a simple and general synthesis of β-unsubstituted gold(III) triarylcorroles and the first X-ray crystal structure of such a complex. Comparison with analogous copper and silver corrole structures, supplemented by extensive scalar-relativistic, dispersion-corrected density functional theory calculations, suggests that "inherent saddling" may occur for of all coinage metal corroles. The degree of saddling, however, varies considerably among the three metals, decreasing conspicuously along the series Cu > Ag > Au. The structural differences reflect significant differences in metal-corrole bonding, which are also reflected in the electrochemistry and electronic absorption spectra of the complexes. From Cu to Au, the electronic structure changes from noninnocent metal(II)-corrole(?2-) to relatively innocent metal(III)-corrole(3-).     

XPS study of the effect of the conditions of peptide chemisorption to gold and silver coated polymer surfaces

Chemisorption of peptides and active moieties onto gold and silver coated surfaces is an attractive method for studying the effect of fundamental surface properties on biological interactions. In theory, the monolayers formed have a high density of the active group of interest, and the relatively mild conditions associated with chemisorption should allow biological activity to be maintained. While the conditions for chemisorption are widely reported in the literature, significant differences exist between research groups and the effects of changing these conditions on the resultant surface have not been fully examined. Furthermore, comparisons have not been made between gold and silver as potential substrates in these applications. In the current work, glutathione and cysteine were chemisorbed onto gold- and silver-coated polymers. The effect of varying the chemisorption conditions was evaluated by XPS analysis of the resultant surfaces. Factors identified as having potentially significant effects on chemisorption procedure included chemisorption time, peptide concentration, peptide, pH of the chemisorption solution, mixing and material of the incubation container. Factor significance was evaluated using a two level fractional factorial design of experiments (DOE), with sulfur content determined by XPS used as a measure of chemisorption effectiveness. Significant differences were noted between the silver and gold-coated surfaces, with a higher amount of sulfur and hence, by inference, peptide found in general on silver surfaces reacted under identical conditions. On the gold surfaces, peptide concentration, peptide type, and chemisorption time were found to have a significant effect on the composition of the resulting surface. On the silver surfaces, factor specific differences were not as significant but there were a number of two factor interactions. The results provide further evidence of the differences in interactions with thiol between silver and gold and suggest that changes in the chemisorption conditions can dramatically affect the resultant surface composition.     

Role of the Support in Catalysis: Activation of a Mononuclear Ruthenium Complex for Ethene Dimerization by Chemisorption on Dealuminated Zeolite Y

A set of supported ruthenium complexes with systematically varied ratios of chemisorbed to physisorbed species was formed by contacting cis‐[Ru(acac)₂(C₂H₄)₂] ( I ; acac=C₅H₇O₂^?) with dealuminated zeolite Y. Extended X‐ray absorption fine structure (EXAFS) spectra used to characterize the samples confirmed the systematic variation in the loadings of the two supported species and demonstrated that removal of bidentate acac ligands from I accompanied chemisorption to form [Ru(acac)(C₂H₄)₂]⁺ attached through two Ru? O bonds to the Al sites of the zeolite. A high degree of uniformity in the chemisorbed species was demonstrated by sharp bands in the infrared (IR) spectrum characteristic of ruthenium dicarbonyls that formed when CO reacted with the anchored complex. When the ruthenium loading exceeded 1.0 wt % (Ru/Al≈1:6), the additional adsorbed species were simply physisorbed. Ethene ligands on the chemisorbed species reacted to form butenes when the temperature was raised to approximately 393 K; acac ligands remained bonded to Ru. In contrast, ethene ligands on the physisorbed complex simply desorbed under the same conditions. The chemisorption activated the ruthenium complex and facilitated dimerization of the ethene, which occurred catalytically. IR and EXAFS spectra of the supported samples indicate that 1) Ru centers in the chemisorbed species are more electron deficient than those in the physisorbed species and 2) Ru–ethene bonds in the chemisorbed species are less symmetric than those in the physisorbed species, which implies the presence of a preferred configuration for the catalytic dimerization.     

Adsorption mechanisms of sulfathiazole on gold,silver and copper surfaces studied by SERS

Surface-enhanced Raman scattering (SERS) of sulfathiazole was studied in gold, silver and copper colloids as well as on a gold plate. SERS spectra of sulfathiazole in gold and silver colloids indicated chemisorption of molecules on the metal nanoparticles through the amide nitrogen, with the phenyl moiety orthogonally placed and the thiazole ring almost parallel positioned towards the metal surface. Although selectively enhanced phenyl bands pointed to a very similar position of the sulfathiazole molecules on the copper colloid, a chemical bonding was not implied. Unlike adsorption mechanisms and position of the molecules on the colloid metal surfaces, a sideway adsorption of sulfathiazole on the gold plate was proposed. Hereby, both, the amide nitrogen and the thiazole nitrogen were considered responsible for approaching of sulfathiazole to the gold enhancing surface.     

Embedded cluster model studies of impurities at metal surfaces

A knowledge of the electronic properties of impurities at metal surfaces is of great value in the understanding of such important phenomena as chemisorption and surface segregation in alloys. We have adopted here a unified approach based on an Embedded Cluster model to study the properties of surface impurities. We have mainly concentrated on hydrogen impurities either adsorbed above the surface or incorporated into the bulk of metals. We have also considered the case of substitutional metal impurities at the surface of host metals.For hydrogen chemisorption we have considered such substrates as free-electron, transition and noble metals as well as bimetallic substrates composed of a single metal impurity in a host matrix or a metallic overlayer on a metal support. The electronic structure of the chemisorbed system is compared to photoemission data when available, from which interpretation of the details of the experimental spectra may be made. It is found that hydrogen adsorption on transition and noble metals results in the formation of a pair of bonding/antibonding resonances on either side of the metal d-band, while for hydrogen on free-electron metals a single hydrogen induced resonance is observed. One-electron energy differences between the H on jellium and H on metal systems are estimated and trends in such energies across the 3d and 4d transition series are compared to the trends in experimental chemisorption energies for H on these metals. The change in hydrogen chemisorption capacity of an inert substrate due to the introduction of chemically active impurities is investigated. The different properties of Pd overlayers with respect to Pd surfaces are also investigated. Interaction energies between adatoms on surfaces are estimated in order to predict the geometry of ordered structures on surfaces.One-electron heats of segregation for binary alloys are calculated. These show a strong solute surface segregation for noble metal impurities in group VIII metals, which is due to the higher d-band occupancy of the noble metal.     

Study on the interaction of platinum(IV), gold(III) and silver(I) ions with DNA

Fluorescence, absorption spectra have been produced by the interaction of platinum(IV), silver(I) and gold(III) ions with the berberine–DNA system (berberine, Scheme 1). Platinum(IV) and gold(III) ions show different effects from that of silver(I) ion on the spectral characteristics of the berberine–DNA system. Quenching fluorescence is seen with platinum(IV) and gold(III) ions addition, whereas increasing fluorescence is observed for silver(I) ions. The addition of gold(III) and silver(I) ions cause an increase in absorption of the berberine–DNA system. The above results suggest that different metal ions exhibit different affinities when binding to DNA correlates well with the ions’ charge, structure and the coordination ability.     

D-penicillamine adsorption on gold: an in situ ATR-IR spectroscopic and QCM study

The adsorption of penicillamine from ethanol on gold was studied in situ by attenuated total reflection infrared (ATR-IR) and quartz crystal microbalance (QCM) experiments. Both ATR-IR and QCM reveal a fast mass uptake. In ethanol, the molecule adopts a zwitterionic form. Upon adsorption, part of the molecules deprotonate at the amine group, which is a relatively slow process that goes along with a strong shift of the nu(as)(COO(-)) mode. Both ATR-IR and QCM confirm a physisorbed layer. ATR-IR furthermore shows that the latter consists of zwitterionic molecules only, whereas both zwitterionic and anionic species are found in the chemisorbed layer. The infrared spectra of the physisorbed and chemisorbed layers are rather different, and the molecules within both layers seem to be oriented with respect to the surface. The ATR-IR spectra furthermore indicate that all three functional groups of penicillamine (i.e., thiol, carboxylate, and amine) interact with the surface, and density functional theory calculations support this finding. QCM also shows that the molecule uses considerably more space on the surface than molecules of similar size, which supports a three-point interaction. The latter leads to a strong anchoring of the molecule to the metal, which may explain the exceptional capability of penicillamine to bind metals.     

Facile solid-state synthesis of oxidation-resistant metal nanoparticles at ambient conditions

A simple and scalable method for the synthesis of metal nanoparticles in the solid-state was developed, which can produce nanoparticles in the absence of solvents. Nanoparticles of coinage metals were synthesized by grinding solid hydrazine and the metal precursors in their acetates and oxides at 25 °C. The silver and gold acetates converted completely within 6 min into Ag and Au nanoparticles, respectively, while complete conversion of the copper acetate to the Cu sub-micrometer particles took about 2 h. Metal oxide precursors were also converted into metal nanoparticles by grinding alone. The resulting particles exhibit distinctive crystalline lattice fringes, indicating the formation of highly crystalline phases. The Cu sub-micrometer particles are better resistant to oxidation and exhibit higher conductivity compared to conventional Cu nanoparticles. This solid-state method was also applied for the synthesis of platinum group metals and intermetallic Cu₃Au, which can be further extended to synthesize other metal nanoparticles.     

The effect of severe thermal pretreatment on the redox behaviour of gold in aqueous acid solution

Severe thermal pretreatment of gold wire electrodes in an inert gas atmosphere resulted in the appearance of dramatic premonolayer oxidation responses, which in some instances commenced at 0.25 V (RHE), for the resulting active gold electrodes in aqueous acid media. Similar behaviour was reported earlier for platinum and gold activated by cathodic pretreatment in acid solution; these active noble metal surfaces are evidently more susceptible to oxidation than bulk copper. Such behaviour was attributed to the effect of surface disorder; many of the metal atoms at the interface are assumed to be in a very active, metastable state possessing quite low lattice stabilization energy. Premonolayer oxidation responses are again correlated with electrocatalytic behaviour and the existence of unusual chemisorption behaviour for molecules reacting at highly active states of metals is outlined. Electronic Publication     

Stability of Gold and Platinum Nanowires on Graphite Edges

The stability of coinage and noble metal nanowires supported on graphite steps is examined by density functional theory. In particular, we study the stability of supported gold and platinum wires and compare their chemical properties with those of surfaces and bare wires. A substantially stronger bond with graphite was found for platinum wires due to unfilled antibonding states, which are occupied in the case of gold. This difference has direct consequences for the adsorption of hydrogen. This reaction can occur either on the wire or directly on graphite steps. In the case of gold, the reaction is favoured on steps, while on platinum wires, it has no thermodynamical preferences. Our results suggest that, in early stages of wire formation, hydrogen could desorb gold from graphite, but not platinum.     

Surface-enhanced vibrational spectroscopy of B vitamins: what is the effect of SERS-active metals used?

Surface-enhanced Raman scattering (SERS) spectroscopy and surface-enhanced infrared absorption (SEIRA) spectroscopy are analytical tools suitable for the detection of small amounts of various analytes adsorbed on metal surfaces. During recent years, these two spectroscopic methods have become increasingly important in the investigation of adsorption of biomolecules and pharmaceuticals on nanostructured metal surfaces. In this work, the adsorption of B-group vitamins pyridoxine, nicotinic acid, folic acid and riboflavin at electrochemically prepared gold and silver substrates was investigated using Fourier transform SERS spectroscopy at an excitation wavelength of 1,064 nm. Gold and silver substrates were prepared by cathodic reduction on massive platinum targets. In the case of gold substrates, oxidation–reduction cycles were applied to increase the enhancement factor of the gold surface. The SERS spectra of riboflavin, nicotinic acid, folic acid and pyridoxine adsorbed on silver substrates differ significantly from SERS spectra of these B-group vitamins adsorbed on gold substrates. The analysis of near-infrared-excited SERS spectra reveals that each of B-group vitamin investigated interacts with the gold surface via a different mechanism of adsorption to that with the silver surface. In the case of riboflavin adsorbed on silver substrate, the interpretation of surface-enhanced infrared absorption (SEIRA) spectra was also helpful in investigation of the adsorption mechanism.     

Neutral gold(I) metallosupramolecular compounds: synthesis and characterization, photophysical properties, and density functional theory studies

The reaction of the tris-indole InTREN ligand (L) with different gold phosphine fragments allows the construction of new gold(I) complexes with different geometries depending on the chosen phosphine. A metallodendrimeric structure is obtained when the gold atom is linked to a triphenylphosphine ligand, and neutral gold(I) metallocryptands are constructed when a triphosphine is used. Characterization of the compounds was accomplished by 31P{1H} and 1H NMR, IR, absorption, and fluorescence spectroscopies, electrospray ionization mass spectrometry (ESI-MS(+)), and elemental analysis, and their geometry was optimized using density functional theory (B3LYP). Time-dependent density functional theory (TD-DFT) calculations have been used to assign the lowest energy absorption bands to LMCT N(p, tertiary amine)-->Au transitions. Photophysical characterization of the complexes shows strong luminescence in the solid state. The formation of heterobimetallic species has been detected in solution in the presence of equimolar quantities of metal cations, and their structures have been identified by a combination of spectroscopic methods and mass spectrometry.     

Chiral Phosphines: The Effect of Asymmetric Carbon Centers α or β to Phosphorus on the 31P NMR Spectra

Abstract

Chiral alkylphosphines containing asymmetric carbon centers either alpha or beta to the phosphorus are formed from the free radical addition of phosphine to prochiral olefins. The phosphorus NMR spectra of the primary phosphines have only one signal for the enantiomeric pair while the spectra of the secondary phosphines have three signals in a 1:2:1 ratio. This is the statistical ratio expected for the enantiomeric pair and two meso forms. Four enantiomeric pairs of tertiary phosphines can be formed from one prochiral olefin. One pair has alkyl groups of the same configuration while the remaining pairs have mixed configurations. The phosphorus NMR spectra show two signals in a 1:3 ratio. Dilution of the secondary phosphines with a polar solvent results in only one signal in the phosphorus NMR spectra. The chemical shifts and diluent effect can be explained in terms of the gamma shielding effect (1).     

Nanoscale patterning of two metals on silicon surfaces using an ABC triblock copolymer template

Patterning technologically important semiconductor interfaces with nanoscale metal films is important for applications such as metallic interconnects and sensing applications. Self-assembling block copolymer templates are utilized to pattern an aqueous metal reduction reaction, galvanic displacement, on silicon surfaces. Utilization of a triblock copolymer monolayer film, polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) (PS-b-P2VP-b-PEO), with two blocks capable of selective transport of different metal complexes to the surface (PEO and P2VP), allows for chemical discrimination and nanoscale patterning. Different regions of the self-assembled structure discriminate between metal complexes at the silicon surface, at which time they undergo the spontaneous reaction at the interface. Gold deposition from gold(III) compounds such as HAuCl4(aq) in the presence of hydrofluoric acid mirrors the parent block copolymer core structure, whereas silver deposition from Ag(I) salts such as AgNO3(aq) does the opposite, localizing exclusively under the corona. By carrying out gold deposition first and silver second, sub-100-nm gold features surrounded by silver films can be produced. The chemical selectivity was extended to other metals, including copper, palladium, and platinum. The interfaces were characterized by a variety of methods, including scanning electron microscopy, scanning Auger microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy.