Study on some metal–metal quadruple bonds

In the present paper, the role of (n ? 1)? orbitals in metal–metal quadruple bonds was studied. It was shown by the calculations that the probabilities for finding the σ-, π-, and δ-electrons between two metal atoms, under the influence of the ? orbitals on the metal–metal quadruple bonds, increased while their mean kinetic energy components along the metal bond axis decreased. In addition, the effects of the ? orbitals upon the σ, π, and δ metal–metal bonds were different. In general, σ < π < δ.     

A manganese sulfite with extended metal–oxygen–metal bonds exhibiting hydrogen uptake

A manganese sulfite of the formula Mn₅(OH)₄(SO₃)₃·2H₂O, I{a=7.5759(7) Å, b=8.4749(8) Å, c=10.852(1) Å, β=100.732(2)°, Z=2, space group=P2₁/m (no. 11), R₁=0.0399 and wR₂=0.1121 [for R indexes I>2σ(I)]}, comprising Mn₃O₁₄ units and extended Mn–O–Mn bonds along the three dimensions has been synthesized under hydrothermal conditions. It has narrow channels along the b-axis and exhibits hydrogen storage of 2.1 wt% at 300 K and 134 bar.     

Single-molecule junctions of multinuclear organometallic wires: long-range carrier transport brought about by metal–metal interaction

Here, we report multinuclear organometallic molecular wires having (2,5-diethynylthiophene)diyl-Ru(dppe)₂ repeating units. Despite the molecular dimensions of 2–4 nm the multinuclear wires show high conductance (up to 10⁻² to 10⁻³G₀) at the single-molecule level with small attenuation factors (β) as revealed by STM-break junction measurements. The high performance can be attributed to the efficient energy alignment between the Fermi level of the metal electrodes and the HOMO levels of the multinuclear molecular wires as revealed by DFT–NEGF calculations. Electrochemical and DFT studies reveal that the strong Ru–Ru interaction through the bridging ligands raises the HOMO levels to access the Fermi level, leading to high conductance and small β values.

Multinuclear organometallic molecular wires having (diethynylthiophene)diyl-Ru(dppe)₂ repeating units show high conductance with small attenuation factors. The strong Ru–Ru interaction is the key for the long-range carrier transport.     

The metal–solution interface

This article reviews our present understanding of the metal–solution interface, drawing comparisons between it and the metal–gas interface. The chief difference between these two systems is the solvent, which has a profound effect on the physics and chemistry of the interface. The solvent is never entirely inert, and if polar, as water is, will adsorb on the metal surface and stabilise ions in solution. However, the presence of ions in solution allows a current to be passed between two electrodes inserted in the solution, and one of these electrodes can be the metal under investigation. This is the field of electrochemistry, and much of our knowledge of the metal–solution interface is derived from electrochemical measurements. Complementary information is gained from many of the techniques used to study the metal–gas interface. The first type of electrochemical system to be considered is one in which the application of a potential across the interface does not lead to charge transfer. Such a system behaves as a capacitance due to the formation of a complex double layer. The effects of various types of adsorption is then described. The deposition of metals on the electrode surface is next considered, and this is followed by a discussion of the reverse metal dissolution reaction. The final section concerns the formation of films on the metal surface, a reaction which has its parallel in metal–gas reactions.     

The shortest metal‐metal bond

The synthesis and isolation of stable bimetallic complexes that contain formally quintuply bonded transition metals is a novel and emerging field of science. Efforts have been undertaken in designing and tuning the ligands to achieve a very short (actually the shortest) metal‐metal bond. The motivation for these efforts arose from the expectation that an increasing bond order may go along with a shortening of the bond length. In consequence, formally quintuply bonded bimetallics could have shorter metal‐metal distances than quadruply bonded ones. A chromium homo‐bimetallic complex with a Cr‐Cr bond length of 1.7293(12) Å has been synthesized, and a formal bond order of five was assigned. This compound holds the record for the shortest metal‐metal bond in a stable molecule to date. At this stage, there is no evidence that additional shortening is impossible. © 2010 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.201000028     

Microstructure of metallic copper nanoparticles/metallic disc interface in metal–metal bonding using them

This paper describes a metal–metal bonding technique using metallic Cu nanoparticles prepared in aqueous solution. A colloid solution of metallic Cu particles with a size of 54 ± 15 nm was prepared by reducing Cu²⁺ (0.01 M (CH₃COO)₂Cu) with hydrazine (0.6 M) in the presence of stabilizers (5 × 10^?4 M citric acid and 5 × 10^?3 M cetyltrimethylammonium bromide) in water at room temperature in air. Discs made of metallic materials (Cu, Ni/Cu, or Ag/Ni/Cu) were successfully bonded under annealing at 400 °C and pressurizing at 1.2 MPa for 5 min in H₂ gas with help of the metallic Cu particle powder. Shear strength required for separating the bonded discs was 27.9 ± 3.9 for Cu discs, 28.1 ± 4.1 for Ni/Cu discs, and 13.8 ± 2.6 MPa for Ag/Ni/Cu discs. Epitaxial crystal growth promotes on the discs with a good matching for the lattice constants between metallic nanoparticles and metallic disc surfaces, which leads to strong bonding. Copyright © 2013 John Wiley & Sons, Ltd.     

Combining metal–metal cooperativity,metal–ligand cooperativity and chemical non-innocence in diiron carbonyl complexes

Several metalloenzymes, including [FeFe]-hydrogenase, employ cofactors wherein multiple metal atoms work together with surrounding ligands that mediate heterolytic and concerted proton–electron transfer (CPET) bond activation steps. Herein, we report a new dinucleating PNNP expanded pincer ligand, which can bind two low-valent iron atoms in close proximity to enable metal–metal cooperativity (MMC). In addition, reversible partial dearomatization of the ligand''s naphthyridine core enables both heterolytic metal–ligand cooperativity (MLC) and chemical non-innocence through CPET steps. Thermochemical and computational studies show how a change in ligand binding mode can lower the bond dissociation free energy of ligand C(sp³)–H bonds by ∼25 kcal mol⁻¹. H-atom abstraction enabled trapping of an unstable intermediate, which undergoes facile loss of two carbonyl ligands to form an unusual paramagnetic (S = ) complex containing a mixed-valent iron(0)–iron(i) core bound within a partially dearomatized PNNP ligand. Finally, cyclic voltammetry experiments showed that these diiron complexes show catalytic activity for the electrochemical hydrogen evolution reaction. This work presents the first example of a ligand system that enables MMC, heterolytic MLC and chemical non-innocence, thereby providing important insights and opportunities for the development of bimetallic systems that exploit these features to enable new (catalytic) reactivity.

The PNNP expanded pincer ligand can bind two iron centers in close proximity and display heterolytic and homolytic metal–ligand cooperativity.     

Study of physicochemical interactions in metal–polymer systems

This article deals with the investigation of adhesive joints of unlike metals separated by layers of thermoplastics. Experimental data are presented on electrical conductivity of polymers at temperatures of 333–573 K in the electric field of 1–10² V/cm. Infrared (IR) spectroscopy data have been used to establish correlation between the parameters of the voltage generated on the metal–polymer specimens and the existence of groups that can form hydrogen bonds. Adhesive strength of the adhesive joints is discussed in connection with the performance of a metal–polymer–metal voltaic couple when the adhesive joints are being formed. The results presented allow a conclusion that polymeric dielectrics exhibit properties of electrolytes when heated in contact with metals. Therefore electrochemical interactions between components of metal–polymer systems should be taken into consideration when predictions are made of performance characteristics of industrial materials based on polymers and metals.     

Bimetallic Ni–Co‐based metal–organic framework: An open metal site adsorbent for enhancing CO2 capture

An open metal site framework named UTSA‐16 was synthesized and modified as a high‐capacity adsorbent for reversible CO₂ capture. Partial substitution of intrinsic Co²⁺ sites of UTSA‐16 with Ni²⁺ centres was realized in the molar composition range 0–75% Ni with the aim of increasing CO₂ uptake. Synthesized bimetallic Nix‐UTSA‐16 (x = 0, 20, 50, 75) materials were characterized using various techniques to assess the influence of chemical composition on CO₂ binding affinity and any subsequent physical change in morphology, crystal size and porosity on the total uptake. Experimental isotherm adsorption studies showed the following trend for CO₂ adsorption capacity employing the Nix‐UTSA‐16 series: Ni20‐UTSA‐16 > UTSA‐16 > Ni50‐UTSA‐16 > Ni75‐UTSA‐16. According to the dynamic breakthrough CO₂ profiles measured for a mixture of CO₂ and CH₄ (15/85 molar ratio), Ni20‐UTSA‐16 exhibited 2 times the breakthrough time with 1.5 times the loading capacity at 75 Nml min^?1 feed flow rate, compared to the parent UTSA‐16. In addition, the Ni20‐UTSA‐16 bimetallic metal–organic framework exhibited lower isosteric heat of adsorption compared to UTSA‐16 (ΔH_ave = 28.54 versus 46.85 kJ mol^?1). As a result, more than 95% of its capacity was restored by applying a partial vacuum for only 1 h at room temperature without involving any other time‐ and energy‐consuming regenerative step.     

Length dependence of charge transport in nanoscopic molecular junctions incorporating a series of rigid thiol-terminated norbornylogs

Four tetrathiol-terminated norbornane homologues were synthesized and self-assembled monolayers (SAMs) of these molecules were formed on Au via adsorption from CH2Cl2. SAMs were characterized structurally via spectroscopic ellipsometry (SE), reflection-absorption infrared spectroscopy (RAIRS), Rutherford backscattering spectrometry (RBS), and X-ray photoelectron spectroscopy (XPS). Results of these analyses show that the rigid norbornylogs form monolayers that have a surface coverage slightly lower than that of alkanethiols, and that they exhibit a nonmonotonic dependence of film thickness on molecular length. Nanoscale molecular junctions incorporating these SAMs were formed and characterized electrically using conducting probe atomic force microscopy (CP-AFM). The resistances of these junctions scale exponentially with the contour length of the molecules, with beta = 0.9 A(-1), consistent with a nonresonant tunneling mechanism. Further, the resistance of norbornyl SAMs correlates well with the resistance of alkanedithiol SAMs of similar length, suggesting that the norbornyl molecules form sulfur-metal bonds on both ends of the junction.     

Energy‐level alignment at metal–organic and organic–organic interfaces

This article reports on the electronic structure at interfaces found in organic semiconductor devices. The studied organic materials are C₆₀ and poly (para‐phenylenevinylene) (PPV)‐like oligomers, and the metals are polycrystalline Au and Ag. To measure the energy levels at these interfaces, ultraviolet photoelectron spectroscopy has been used. It is shown how the energy levels at interfaces deviate from the bulk. Furthermore, it is demonstrated that the vacuum levels do not align at the studied interfaces. The misalignment is caused by an electric field at the interface. Several effects are presented that influence the energy alignment at interfaces, such as screening effects, dipole layer formation, charge transfer, and chemical interaction. The combination of interfaces investigated here is similar to interfaces found in polymer light‐emitting diodes and organic bulk heterojunction photovoltaic devices. The result, the misalignment of the vacuum levels, is expected to influence charge‐transfer processes across these interfaces, possibly affecting the electrical characteristics of organic semiconductor devices that contain similar interfaces. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2549–2560, 2003     

Analysis of Cu segregation to oxide–metal interface of Ni‐based alloy in a metal‐dusting environment

Hard X‐ray photoelectron spectroscopy (HX‐PES) has been realized using high‐brilliance synchrotron radiation. High‐energy photon excitation enables us to probe photoelectrons with larger escape depth compared to conventional PES. This allows us to conduct, without destruction, a study of the embedded interface of materials as the oxide‐ metal interface. We apply HX‐PES to investigate for Cu segregation in the oxide–metal interface during metal‐dusting corrosion. The effective concentration of Cu in the segregation was estimated a few times higher than the bulk concentration. These results on the interface layer can explain the variation in the corrosion resistance. Copyright © 2008 John Wiley & Sons, Ltd.     

Recent applications of metal–organic frameworks in sample pretreatment

Metal–organic frameworks are promising materials in diverse analytical applications especially in sample pretreatment by virtue of their diverse structure topology, tunable pore size, permanent nanoscale porosity, high surface area, and good thermostability. According to hydrostability, metal–organic frameworks are divided into moisture‐sensitive and water‐stable types. In the actual applications, both kinds of metal–organic frameworks are usually engineered into hybrid composites containing magnetite, silicon dioxide, graphene, or directly carbonized to metal–organic frameworks derived carbon. These metal–organic frameworks based materials show good extraction performance to environmental pollutants. This review provides a critical overview of the applications of metal–organic frameworks and their composites in sample pretreatment modes, that is, solid‐phase extraction, magnetic solid‐phase extraction, micro‐solid‐phase extraction, solid‐phase microextraction, and stir bar solid extraction.     

Tandem magnetization and post‐synthetic metal ion exchange of metal–organic framework: Synthesis,characterization and catalytic study

For the first time, metal‐exchange in a magnetic metal–organic framework (MOF) via tandem magnetization and post‐synthetic modification has been developed. The new magnetic mixed‐metal metal–organic framework nanocomposite, CoFe₂O₄/[Cu_0.63/Zn_0.37‐TMU‐17‐NH₂] (CoFe₂O₄/[Cu/Zn‐MOF]) has been synthesized by immersing the CoFe₂O₄/Zn‐TMU‐17‐NH₂ (CoFe₂O₄/Zn‐MOF) as a template in DMF solution of Cu (II) salts. CoFe₂O₄/[Cu/Zn‐MOF] showed to be a highly reactive and easily recoverable magnetic catalyst for the preparation of tetrazole derivatives via one‐pot three‐component reactions of different aldehydes with hydroxyl amine hydrochloride and sodium azide. Our results (Fourier transform‐infrared, inductively coupled plasma‐optical emission spectroscopy, powder X‐ray diffraction, field emission‐scanning electron microscopy, energy‐dispersive X‐ray spectroscopy‐mapping and vibrating‐sample magnetometer) show successful partial metal‐exchange in which the framework integrity remained intact during the metal‐exchange process.     

Electron lifetimes in image-potential states at metal–dielectric interfaces

We present an overview of experimental and theoretical studies of image states dynamics at metal–dielectric interfaces. The interaction of an image-state electron with a metal substrate is largely altered by the presence of a dielectric adlayer. The electron affinity of the adsorbate determines, to a great extent, the evolution of image states upon adsorption. A large variety of adsorbates and surfaces have been studied, from both experimental and theoretical points of view. On the theoretical side, penetration approaches are not able to include all the physics involved in decay processes. A more realistic many-body calculation, which takes into account all the fundamental factors determining the lifetime, has been recently performed, and a fairly good agreement with experiments has been obtained.     

Interaction energy‐based drug–receptor interaction study of metal–bicyclam complexes

Bicyclams inhibit HIV replication by binding to the CXCR4 chemokine receptor, which is the main coreceptor for gp120 used by X4, T‐tropic strains of HIV for membrane fusion and cell entry. Bicyclam AMD3100 mainly interacts with the aspartic acid residues namely Asp171 and Asp262, which are located at the extracellular ends in the CXCR4 coreceptor. Incorporation of some metal ions by the macrocyclic rings of bicyclam enhances its binding affinity to the CXCR4 receptor and enhances their anti‐HIV activity because the acetate can make a strong coordination bond to the metal and one weaker hydrogen bond to nitrogen in the cyclam ring. The interaction energy (E_int) between 150 metal–bicyclam complexes and aspartic acid has been evaluated. The metal–bicyclam complexes are obtained by the incorporation of six metal ions namely Fe³⁺, Co³⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Pd²⁺ in 25 well‐known bicyclams including AMD3100. In most of the cases, Fe and Co–bicyclam complexes interact best with aspartic acid. The anti‐HIV activity of metal–bicyclam complexes can be predicted on the basis of interaction energy before the synthesis of the metal–bicyclam complex. On the basis of interaction energy, the anti‐HIV activity of bicyclam complexes can be predicted in advance to their synthesis. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Soluble metal–polymer catalysts in the hydrogenation of organic compounds

Literature describing the use of soluble metal–polymer catalysts in the molecular hydrogenation of alkenes, alkynes, aromatic and heterocyclic compounds has been reviewed. Special attention has been paid to coordinated metallic and organometallic catalytic systems and highly dispersed colloidal metals stabilized with polymers. The influence of metals, polymers and solvents on the structures of active sites is discussed. The prospects for the application of soluble metal–polymer catalysts in organic synthesis are also considered.     

An integrative approach for analyzing metal–polyelectrolyte interactions

An integrative approach based on the combined use of both experiments and modelling is discussed here aimed at investigating metal–polyelectrolyte interactions in solution. Electrochemical techniques are applied because of their potential to measure the actual speciation without disturbing the solution physico–chemical equilibrium. The experimental methodologies are complementary since the ranges of applicability depend on the solution composition itself. To complement and interpret the results of these experimental techniques, a physico–chemical association model, based on the so-called ‘chemical model’ of counterion condensation theory, is used. The model considers that, in addition to the usual electrostatic interactions and entropic effects, territorial affinity and chemical bonding interactions take place between the small counterions in solution and the polyelectrolyte. A number of particular cases of metal/polyelectrolyte systems are discussed aimed at showing that the integrative approach leads to additional information about the solution system which can not be deduced from experimental results solely. Future challenges with respect to the applications in the study of natural aquatic systems are pointed out.     

Hybrid metal–polymer composites from functional block copolymers

The combination of metals and polymers in hybrid materials is a research area of great current interest. A number of methods for controlling the positioning of metallic species within polymer matrices on the nanometer scale have been developed. This highlight focuses on the use of functional block copolymers for the localization of metal species, especially nanoparticles, on the nanometer scale through block copolymer phase segregation. Research from the author's group on the use of alkyne‐functional block copolymers for the preparation of cobalt‐containing materials is discussed in this context. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4323–4336, 2005     

A density functional study of pseudotetrahedral metal–nitrosyl complexes

Local and nonlocal density functional computations have been carried out to study the electronic structure and the equilibrium geometry of the isoelectronic series Cr(NC)₄, Mn(NO)₃(CO), Fe(NO)₂(CO)₂, and Co(NO)(CO)₃ and model compounds Fe(NO)₂L₂ (L = Cl, HCN, NH₃, PH₃, and C₂H₄). The structure of Fe(NO)₂(C₄H₆) is also described. The discussion is focused on structural modifications through a change of ligand, in particular those concerning the metal-nitrosyl conformation (linear vs. bent). Though this is a preliminary study of metal–nitrosyl properties by DFT methods and more computations are required to analyze the mechanism of homogeneous catalysis processes, our results support the hypothesis that structural reorganization from linear to bent metal–nitrosyl plays a key role in some reactions, such as in butadiene dimerization. © 1994 John Wiley & Sons, Inc.