Metal Nuclei Research Articles

One-step formation of nanostructured gold layers via a galvanic exchange reaction forsurface enhancement Raman scattering

Gold films with various nanostructures on porous silicon substrates have been createdin one step by using a galvanic exchange reaction approach. In this approach,the formation of gold films is initiated by the formation of metal nuclei via anexchange reaction in which both the oxidation of silicon and reduction of goldcomplex occur simultaneously. Results show that the deposition of gold film isdiffusion controlled, and its apparent activation energy was determined to be28.57 kJ mol−1. Therefore, gold nanostructures with text structure, clusters, and dendrites with leaf-likeor branch-like structure can be prepared by adjusting the diffusion conditions such assystem temperature, concentration of gold salt, ultrasonic vibration, and addition ofadditives. The SERS activity of the various porous silicon supported gold nanostructureswas evaluated using pyridine and adenine as molecular probes. Amongst the various poroussilicon supported gold nanostructures, the gold films with leaf-like structures show thehighest SERS activity due to the presence of surface active sites in the nanostructures. Thepresent method is promising for the fabrication of stable gold nanostructures forultrasensitive analytical measurements.

Cross-Coupling of Aryl Halides and Triflates with Group 13-Metal Alkylating Reagents by Palladium-Molybdenum and Palladium-Tungsten Mixed-Metal Catalysts

The palladium-containing clusters {MoPdcp[µ-(CO)2][µ3-(CO)](PPh3)}2 (1) and {PdWcp[µ-(CO)2][µ3-(CO)](PPh3)}2 (2) catalyze the cross-methylation and ethylation of aryl halides and triflates with the respective intramolecularly stabilized dialkylaluminum, gallium and indium reagents 3-7 at temperatures between 78 °C and 120 °C. Unlike the conventional mononuclear palladium catalysts, complexes 1 and 2 promote the coupling of aryl chlorides and show no signs of homocoupling or hydrogenolysis even when the alkylating moieties contain β-hydrogen atoms. The catalytic activity and selectivity of the mixed-metal clusters is attributed to synergism between the different metal nuclei of the catalysts.

Modifying the liquid/liquid interface: pores, particles and deposition

The modification of the liquid/liquid interface with solid phases is discussed in this article. Modified interfaces can be formed with molecular assemblies, but here attention is focussed on solid materials such as mesoscopic particles, or microporous and mesoporous membranes. Charge transfer across the modified liquid/liquid interface is considered in particular. The most obvious consequence of the introduction of such modifying components is their effect on the transport to, and the transfer of material across, the liquid/liquid interface, as measured voltammetrically for example. One particularly interesting reaction is interfacial metal deposition, which can also be studied under electrochemical control: the initial formation of metal nuclei at the interface transforms it from the bare, pristine state to a modified state with very different reactivity. Deposition at interfaces between liquids is compared and contrasted with the cases of metal deposition in bulk solution and conventional heterogeneous deposition on conducting solid surfaces. Comparison is also made with work on the assembly of pre-formed micron and nanometre scale solids at the liquid/liquid interface.

Reduction of Copper in Porous Matrixes. Stepwise and Autocatalytic Reduction Routes

The reduction of Cu(II) oxide species in siliceous matrixes of different porosity (MFI, FAU, MCM-48) and in alumosilicate MFI was studied by temperature-programmed reduction in hydrogen (TPR), by X-ray absorption fine structure (after stationary hydrogen treatments), and by transmission electron microscopy. It was found that the reduction may proceed in one or in two reduction steps. The two-step scheme known for zeolites was observed also for Cu(II) in siliceous microporous matrixes, with similar temperature of Cu(II) reduction onset as for the alumosilicate MFI. Therefore, the two-step scheme cannot be explained by the stabilization of Cu ions by intra-zeolite electrical fields. CuOx clusters in MCM-48 were reduced in a one-step scheme (similar to bulk CuO) at high Cu content (6 wt %) but in a two-step scheme at low Cu content (1 wt %). The two reduction steps observed with most samples cannot be identified with the transitions of all Cu(II) to Cu(I) and of Cu(I) to Cu(0). Instead, Cu(0) nuclei were observed already at low reduction temperatures and were found to coexist with Cu ions over temperature ranges of different extension. This coexistence range was narrow in materials that favor aggregation of the Cu nuclei into particles: Cu-MCM-48 of low Cu content and Cu-ZSM-5. In the latter, metal segregation from the pore system was found to be accompanied by an autocatalytic initiation of the second reduction step. In the siliceous microporous matrixes, the Cu(0) nuclei were observed to coexist with Cu ions over wide temperature ranges (100 K for MFI) at temperatures far above that of Cu reduction in the bulk oxide. These observations suggest that oligomeric Cu metal nuclei which may have been formed, e.g., at the intersections of the MFI channel system, may be unable to activate hydrogen, which would be required for rapid reduction of the coexisting Cu ions.

Formula omitted] NMR analysis of nuclear relaxation mechanisms in Pd–H and Pd–Ag–H alloys

Crystalline Pd 1− x Ag x –H y ( x = 0 – 0.35 ) alloys were studied as model systems representing a chemically disordered system for hydrogen-storage materials. Extensive H 1 relaxation rate ( R 1 , R 1 ρ and R 2 ) measurements were carried out in a wide temperature range of 2.4–400 K. Palladium–silver alloys of silver concentration as high as 35 at.% were studied by NMR spectroscopy for the first time. The relaxation mechanisms magnetic dipolar relaxation (hydrogen diffusion), Korringa relaxation, relaxation by paramagnetic impurity ions and cross-relaxation to quadrupolar metal nuclei were identified and characterized. The presence of cross-relaxation between H 1 and quadrupolar Pd 105 nuclear spins was assumed and demonstrated for the first time in Pd–H and Pd 1− x Ag x –H systems. More than one jump processes of diffusing hydrogen were found to be present in the studied palladium–silver alloys. Only lower estimated limit values of Korringa constants could be obtained since no frequency independent R 1 versus T region was found. These values are significantly higher than the ones reported by others.

Electrodeposition of monodisperse copper nanoparticles on highly oriented pyrolytic graphite electrode with modulation potential method

Copper nanoparticles with size of 51 nm and R.S.D. dia of 10% can be obtained by modulation potential electrodeposition: first, a −0.8 V × 120 ms potential step was used to nucleate copper particles on highly oriented pyrolytic graphite (HOPG) electrode surface, where nuclei with a supercritical size were formed, secondly, a 0.082 V × 80 ms strip peak potential ( E s) was used to strip smaller metal nuclei partly, at last, a growth potential, −0.28 V was applied to grow the nearly identical size slowly. Mechanism of copper electrodeposition on HOPG electrode from a solution of 1 mM CuSO 4 and 1.0 M H 2SO 4 has been studied using cyclic voltammogram and chronoamperometry. In copper electrodeposition the charge-transfer step is fast and the rate of growth is controlled by the rate of mass transfer of copper ions to the growing centers. Reduction of Cu(II) ions did not undergo underpotential deposition. The initial deposition kinetics of Cu electrocrystallization corresponds to a model including progressive nucleation and diffusion controlled growth.

Synthesis and structure of the 2-(chlorophenylazo)pyridine chelated tricarbonylrhenium(I) complex and its anion radical derivatives via electrochemical reduction

Reacting Re(CO)5Cl with the azopyridine ligand (1) (L) in boiling benzene afford the complex Re(CO)3Cl(L), (2) in excellent yield [L=2-(p-Cl-C6H4NN)C5H4N]. The chelation of the azopyridine ligand accompanied by displacement of the two carbon monoxide ligands furnish a five-membered chelate ring. Structure determination of complex (2) has revealed a distorted octahedral ReC3N2Cl coordination sphere. The Re–N(pyridine) and, Re–N(azo) distances are 2.158(3) and 2.153(6) A respectively, and the N–N length [1.273(4) A], implicate relatively weak Re-azo(π*) back–bonding. The Re(CO)3Cl(L) lattice consists of C–H...Cl hydrogen bonding and Cl...O non-bonded interactions constituting a supramolecular network. Extended Huckel calculations reveal that the LUMO of Re(CO)3Cl(L) is Ca. 57% azo in character. One-electron quasireversible electrochemical reduction of the complex occurs near −0.3 V versus Saturated Calomel electrode(s.c.e.) The redox orbital is believed to belong to the above noted LUMO. Electrogenerated Re(CO)3Cl(L•–) underwent spontaneous solvolytic chloride displacement in MeCN, resulting in the isolation of Re(CO)3(MeCN)(L•–). The latter complex in turn reacted with imidazole and triphenylphosphine, furnishing Re(CO)3(C3H4N2)(L•–) and Re(CO)3(PPh3)(L•–), respectively. The pattern of carbonyl stretching frequencies of these radical anion complexes is similar to that of Re(CO)3Cl(L) but with shifts to lower frequencies by 10–20 cm−1. All three radical anion systems are one-electron paramagnetic (1.7–1.8 μ B ). The unpaired electron is primarily localized on the azoheterocycle ligand in a predominantly azo-π* orbital, but a small metal contribution (185, 187Re, I=5/2) is also present. Thus Re(CO)3(MeCN)(L•–) and Re(CO)3(C3H4N2)(L•–) display six-line e.p.r. spectra (A ˜ 28 G). The line shapes and intensities are characteristic of the presence of g-strain. In the case of Re(CO)3(PPh3)(L•–) seven nearly equispaced lines are observed due to virtually equal coupling between the metal and 31P (I=½) nuclei. The g-values of the radical species are slightly higher than the free-electron value of 2.0023.

Synthesis of Gold Nano- and Microplates in Hexagonal Liquid Crystals

Single-crystalline gold nano- and microplates with triangular or hexagonal shapes are synthesized by reduction of HAuCl(4) in lyotropic liquid crystal (LLC) mainly made of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers and water after adding a small amount of capping agents, cetyltrimethylammonium bromide (CTAB) or tetrabutylammonium bromide (TBAB). During the growth of such plates, capping agents play the crucial role. It is found that there is an optimal value of CTAB or TBAB concentration for producing microplates. The selective adsorption of CTAB or TBAB on certain crystallographic facets may be the key point of the supposed mechanism. Although LLC does not really act as a template, it provides an ordered structure confining CTAB as well as the nascent metal nuclei, which enhances the oriented attachment of nuclei and thus the consequent growth of single-crystal plates.

Selectively aerobic oxidation of C [dbnd]C and allylic C [sbnd]H bonds in α-pinene over simple metalloporphyrins

Simple metalloporphyrins (TPPMnCl, TPPCoCl, TPPFeCl, TPPNi, TPPCu or TPPZn) were employed as the catalysts for the aerobic oxidation of α-pinene in the absence of co-catalyst and solvents, and the catalytic selectivity of metalloporphyrins for aerobic oxidation of double bond and allylic C H of α-pinene was investigated. The research results indicate that the reaction temperature, the metal nuclei, and peripheral substituents of the metalloporphyrins influence the catalytic selectivity of metalloporphyrins in the aerobic oxidation of α-pinene. Over TPPMnCl catalyst, the selectivity regarding the oxidation of C C bond increased with decreasing temperature. Among various metalloporphyrins, TPPMnCl presented the largest selectivity regarding the oxidation of C C bond and TPPFeCl the largest selectivity regarding the oxidation of allylic C H bonds. With increasing the electron-donating abilities of the peripheral substituents in T(p-X)PPMnCl (X Cl, CH 3, OCH 3, OH), the selectivities regarding the oxidation of C C bond decreased. The results may provide an additional evidence for the stepwise mechanism through a radical intermediate.

Chemistry of tricarbonylrhenium(I) chelates of azopyrimidines, azoimidazoles and anion radicals thereof

The azopyrimidine and azoimidazole ligands (general abbreviations, RL) used in the present work are 2-( p-R-C 6H 4NN)C 4H 3N 2, RL pm (R=H, Cl) and 2-( p-R-C 6H 4NN)-1-(Me)C 3H 2N 2, RL im (R=Me, Cl), respectively. The reaction of Re(CO) 5Cl with a slight excess of RL in boiling benzene has furnished blue–violet complexes of type Re(CO) 3Cl(RL) which have been spectrally characterized. In Re(CO) 3Cl(HL pm) and Re(CO) 3Cl(ClL im) the Re–N h, Re–N a distances are 2.173(6), 2.136(6) Å and 2.150(5), 2.166(5) Å, respectively (N h and N a are heterocyclic and azo N atoms, respectively). Their N–N lengths (1.271(8), 1.281(7) Å) implicate relatively weak Re-azo(π *) back-bonding. In the lattice of Re(CO) 3Cl(HL pm), pair-wise C–H⋯O hydrogen bonding between symmetry related molecules is present (C⋯O; 3.264(9) Å, H⋯O; 2.460(10) Å; C–H⋯O; 130.6(5)°). The lattice of Re(CO) 3Cl(ClL im) also consists of centrosymmetric dimers held by aromatic π–π stacking between parallely placed pendant aryl rings (centroid⋯centroid distance, 3.781(9) Å). Extended Hückel calculations reveal that the LUMO of Re(CO) 3Cl(RL) is ∼60% azo in character. One-electron quasireversible electrochemical reduction occurs near −0.1 and −0.4 V vs. SCE in the cases of Re(CO) 3Cl(RL pm) and Re(CO) 3Cl(RL im), respectively. The redox orbital is believed to be to the above noted LUMO. Electrogenerated Re(CO) 3Cl(RL − ) underwent spontaneous solvolytic chloride displacement in MeCN furnishing Re(CO) 3(MeCN)(RL − ) which has been isolated. The latter in turn reacted with imidazole and triphenyl phosphine furnishing Re(CO) 3(C 3H 4N 2)(RL − ) and Re(CO) 3(PPh 3)(RL − ), respectively. The pattern of carbonyl stretching frequencies of these radical anion complexes is similar to that of Re(CO) 3Cl(RL) but for shifts to lower frequencies by 10–40 cm −1. All the three radical anion systems are one-electron paramagnets (1.7–1.8 μ B). The unpaired electron is primarily localized in a predominantly azo-π * orbital. A small metal contribution ( 185, 187Re, I=5/2) is present and both Re(CO) 3(MeCN)(RL − ) and Re(CO) 3(C 3H 4N 2)(RL − ) display six-line EPR spectra ( A∼28 G). The line shapes and intensities are characteristic of the presence of g-strain. In the case of Re(CO) 3(PPh 3)(RL − ) seven nearly equispaced lines are observed due to virtually equal coupling with metal and 31P ( I=1/2) nuclei. The g values of the radical species span the range 2.0033–2.0066.

Rovibrational corrections to transition metal NMR shielding constants.

Beyond static molecules: The effect of zero-point motion on NMR parameters of transition-metal complexes (for example, see graphic), as assessed by a perturbational approach, can be sizeable; for example, for the metal nucleus in iron pentacarbonyl, a deshielding of about 100 ppm relative to the equilibrium geometry is predicted.

Structures and ligand exchange of N-confused porphyrin dimer complexes with group 12 metals.

N-confused 5,20-diphenylporphyrin (NCDPP, 1) formed 2:2 dimer complexes with group 12 metals both in the solid state and in solution. X-ray single-crystal analyses of the Zn(II) and Cd(II) complexes (7, 8) revealed that each metal ion is coordinated with three inner core nitrogens and a peripheral nitrogen of the other NCDPP in the pair. In the (1)H NMR spectra of 7, 8, and the Hg(II) complex (9), the outer alpha-H signals of the confused pyrrole ring appeared in the upfield region at 2.57, 3.44, and 3.60 ppm, respectively, due to the ring current effect by the coordinated porphyrins. In the case of the Cd(II) and Hg(II) complexes (8, 9), additional magnetic couplings with the metal nuclei of the partner rings were observed. The equilibrium constants (K) of the monomer exchange reaction at 25 degrees C were determined to be 2.5, 1.3, and 0.6 for the (Zn-Cd), (Cd-Hg), and (Zn-Hg) heterodimer complexes, respectively, from the (1)H NMR spectra of a solution containing two different dimers. Furthermore, a metal-transfer reaction from a Zn(II) NCP dimer complex to the free base porphyrin was demonstrated.

Long-range order in the spatial distribution of electrodeposited copper and silver nuclei on glassy carbon

The features of spatial distributions of metal nuclei (Cu, Ag) electrodeposited on glassy carbon (GC) were studied under potentiostatic conditions. A new approach was suggested to estimate inter-nuclei distances, which allows, considering the size of nuclei, to the changes in average distances between the nearest neighbor nuclei during their formation to be analyzed. To evaluate effectively the degree of deviation of experimental spatial distributions from Poisson and simulated homogeneous distributions, the statistical quantitative criterion of Romanovsky's correspondence was applied. The analysis of radial distribution functions of inter-nuclei distances and spatial distributions of average distances between the nearest neighbors during their formation showed that metal nuclei (Cu, Ag) electrodeposited at low and average overpotentials are distributed statistically ordered on the surface of GC. The ordering is obvious to virtue of the fact that specific inter-nuclei distances for which the value of Romanovsky's correspondence criterion is larger or equal to 3 obey a geometrical progression with a ratio q≈1.41. It was determined that the formation of the long-range order in the spatial distribution of copper and silver nuclei occurs under different mechanisms. It was demonstrated that the effect of the nearest neighbors determines the character of the spatial distribution of nuclei.

Formation of fine particles from residual oil combustion: Reducing nuclei through the addition of inorganic sorbent

The potential use of sorbents to manage emissions of ultrafine metal nuclei from residual oil combustion was investigated by using an 82-kW-rated laboratory-scale refractory-lined combustor. Without sorbent addition, baseline measurements of the fly ash particle size distribution (PSD) and chemical composition indicate that most of the metals contained in the residual oil form ultrafine particles (∼0.1 μm diameter). These results are consistent with particle formation via mechanisms of ash vaporization and subsequent particle nucleation and growth. Equilibrium calculations predict metal vaporization at flame temperatures and were used to define regions above the dew point for the major metal constituents (iron [Fe], nickel [Ni], vanadium [V], and zinc [Zn]) where vapor-phase metal and solidphase sorbents could interact. The addition of dispersed kaolinite powder resulted in an approximate 35% reduction in the ultrafine nuclei as determined by changes to the PSDs as well as the size-dependent chemical composition.

Nucleation and Growth of Platinum Clusters in Solution and on Biopolymers

The molecular mechanisms of platinum cluster nucleation and growth in solution and on biopolymers have been investigated by means of first-principles molecular dynamics. In contrast with the classical picture where clusters nucleate by aggregation of metallic Pt(0) atoms, it was found that Pt-Pt bonds can form between dissolved Pt(II) complexes after only a single reduction step. Furthermore, small clusters were observed to grow by addition of unreduced [PtCl2(H2O)2] complexes, in agreement with an autocatalytic growth mechanism. Moreover, Pt(II) ions covalently bound to biopolymers were found to act as preferential nucleation sites for the formation of clusters. This is a consequence of the presence of heterocyclic donor ligands which both enhance the electron affinity of the metal nuclei and induce the formation of metal-metal bonds that are stronger than those obtained in solution. In fact, in metallisation experiments a clean and purely heterogeneous metallisation of single DNA molecules leading to thin and uniform Pt cluster chains extending over several microns was obtained.

Modification of PADC through electron–target collision

The stopping power of an impinging electron beam depends on the atomic number and the atomic weight of the target material. Both the scattering and the slowing down of electrons increase with increasing target atom mass. Moreover high-energy electrons in the vicinity of heavy metal nuclei emits bremsstrahlung. In the present work, the energy loss of electrons when it traverses the layered metallic targets, has been calculated as the total energy loss which is the sum of the energy loss by ionisation and that due to bremsstrahlung emission. The effect of electron energy loss is observed in the modification in track properties of polyallyldiglycol carbonate (PADC). The etching behaviour of PADC was improved which might be due to the effect of bremsstrahlung emission. A comparative study has been carried out to study the effect of metallic targets of different atomic numbers on the energy loss phenomena.

Cross‐Coupling of Aryl Halides and Triflates with Intramolecularly Stabilized Group 13‐Metal Alkylating Reagents in the Presence of Mixed‐Metal Catalysts

AbstractFor Abstract see ChemInform Abstract in Full Text.

Metal and ligand hyperfine couplings in transition metal complexes: The effect of spin–orbit coupling as studied by coupled perturbed Kohn–Sham theory

The derivation, implementation, and validation of a method to calculate spin–orbit coupling (SOC) contributions to the hyperfine coupling (HFC) is reported. Due to this development it is now possible to calculate the complete ligand and metal nuclei HFCs at a consistent level of theory, i.e., spin unrestricted Hartree–Fock (HF) or density functional theory (DFT). The SOC contributions to the HFC are calculated as a second-order property and are treated by a coupled-perturbed HF or Kohn–Sham treatment. The method is applied to a representative collection of first row transition metal complexes. The calculated metal HFCs are in reasonable to good agreement with the experimental data, with the main error being probably due to the Fermi contact term that is not well represented by present day DFT functionals. Similarities and differences to the closely related field of electronic g-tensor calculations are discussed.

Cross-Coupling of Aryl Halides and Triflates with Intramolecularly Stabilized Group 13-Metal Alkylating Reagents in the Presence of Mixed-Metal Catalysts

In the presence of tertiary phosphines, the palladium-containing mixed-metal complexes [(CO) 4 Fe(μ-PPh 2 )Pd(μ-Cl)] 2 (1) and [(CO) 3 Co] 2 (μ-CO)Pd[μ-(Ph 2 PCH 2 ) 2 ] (2) catalyze the cross-coupling of aryl triflates and halides (including chlorides) with intramolecularly stabilized dialkylaluminum, -gallium and -indium reagents 3-8. The reactions are high yielding, and homocoupling and hydrodehalogenation processes are minimal even when the alkyl moieties of the alkylating reagents contain β-hydrogen atoms. As the components of the mixed-metal complexes are either poor catalysts, or completely inactive, the high catalytic activity of 1 and 2 is attributed to synergism between the different metal nuclei of the catalysts.

A 95Mo and 13C solid-state NMR and relativistic DFT investigation of mesitylenetricarbonylmolybdenum(0) – a typical transition metal piano-stool complex

The chemical shift (CS) and electric field gradient (EFG) tensors in the piano-stool compound mesitylenetricarbonylmolybdenum(0), 1, have been investigated via95Mo and 13C solid-state magic-angle spinning (MAS) NMR as well as relativistic zeroth-order regular approximation density functional theory (ZORA-DFT) calculations. Molybdenum-95 (I = 5/2) MAS NMR spectra acquired at 18.8 T are dominated by the anisotropic chemical shift interaction (Ω = 775 ± 30 ppm) rather than the 2nd-order quadrupolar interaction (CQ = −0.96 ± 0.15 MHz), an unusual situation for a quadrupolar nucleus. ZORA-DFT calculations of the 95Mo EFG and CS tensors are in agreement with the experimental data. Mixing of appropriate occupied and virtual d-orbital dominated MOs in the region of the HOMO-LUMO gap are shown to be responsible for the large chemical shift anisotropy. The small, but non-negligible, 95Mo quadrupolar interaction is discussed in terms of the geometry about Mo. Carbon-13 CPMAS spectra acquired at 4.7 T demonstrate the crystallographic and magnetic nonequivalence of the twelve 13C nuclei in 1, despite the chemical equivalence of some of these nuclei in isotropic solutions. The principal components of the carbon CS tensors are determined via a Herzfeld–Berger analysis, and indicate that motion of the mesitylene ring is slow compared to a rate which would influence the carbon CS tensors (i.e. tens of μs). ZORA-DFT calculations reproduce the experimental carbon CS tensors accurately. Oxygen-17 EFG and CS tensors for 1 are also calculated and discussed in terms of existing experimental data for related molybdenum carbonyl compounds. This work provides an example of the information available from combined multi-field solid-state multinuclear magnetic resonance and computational investigations of transition metal compounds, in particular the direct study of quadrupolar transition metal nuclei with relatively small magnetic moments.