Ag Bridges Research Articles

Redox Reactions Between Guanidine Electron Donors and Silver Dicyanamide: Synthesis of C,N Material Precursors and Coordination Polymers

AbstractReaction of the organic electron donors ttmgb [1,2,4,5‐tetrakis(tetramethylguanidinyl)benzene] and tdmegb [1,2,4,5‐tetrakis(N,N′‐dimethylethyleneguanidinyl)benzene] with two equivalents of silver dicyanamide, Ag(dca), yielded the two salts (ttmgb)(dca)2 (1a) and (tdmegb)(dca)2 (1b). Their relatively high N/C ratios motivated an analysis of their suitability as precursors to C,N materials. The compounds melt at around 200 °C and decompose at 220 (1a) and 245 °C (1b). Reactions of the organic electron donors with an excess of Ag(dca) resulted in coupled redox and coordination reactions, leading finally to coordination polymers with different dimensionalities. The oxidized guanidine units in the polymers are connected through Ag–dca–Ag bridges, leading to stacks of co‐planar C6 rings. The resulting cationic networks are neutralized by dca anions or anionic Ag(dca) chains.

Self-assembled hetero-bimetallic sandwich with Ag–Ag bridging using a flexible two-arm ferrocene amide linker

The tetradentate ferrocenyl sandwich molecule N,N′-bis(di-2-pyridylamine)-1,1′-ferrocenedicarboxamine (bdpfa) has been designed and synthesized as an organometallic ligand in order to construct hetero-bimetallic architectures. By combining the flexibility from the arm-like molecule bdpfa with AgI ions, a novel complex [Ag2(bdpfa)](ClO4)2·(H2O)2 (1) was obtained. Single crystal X-ray analysis has revealed that complex 1 forms a hetero-trinuclear sandwich-type complex with Ag–Ag bridging. Furthermore, the preliminary electrochemical properties of the ligand and complex were investigated.

Strong nonlinear optical effects from a new polymeric silver(I)-propyl-benzotriazole complex

A new polymeric Ag(I) complex {[Ag2(pbbt)3](NO3)2·0.5H2O}n [where pbbt = 1,1′-(1,3-propylene)-bis-1H-benzotriazole] was synthesized as a good optical limiter. Single-crystal X-ray diffraction determined that the complex assumed a rare, two-dimensional lamellar structural motif with isolated cavities of [Ag6(pbbt)6]. The Z-scan measurements of an 8-ns pulsed laser at 532 nm revealed that the presented complex in dimethylformamide exhibited very strong third-order optical nonlinear absorption. The optical-limiting (OL) performance was investigated in the sample of the complex induced by 8-ns and 30-ps pulses at 532 nm. The results show that this complex exhibited large OL effects with a very low limiting threshold value of 0.16 J/cm2 under 8-ns pulse and 0.56 J/cm2 under 30-ps pulses, among those of the best OL materials. Density functional theory calculation results further confirmed the cooperative contributions of Ag ions and bridge ligands to the optical nonlinearities of Ag(I)-containing metal–organic complex. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Catena-Poly[[tetrakis(hexamethylphosphoramide-κO)bis(nitrato-κ2O,O′)samarium(III)] [silver(I)-di-μ-sulfido-tungstate(VI)-di-μ-sulfido

The Sm atom in the cation of the title salt, {[Sm(NO3)2(C6H18N3OP)4][AgS4W]}n, is coordinated by eight O atoms derived from two chelating nitrate anions, and four hexamethylphos­phoramide ligand, defining a distorted square-antiprismatic geometry. The anions self-assemble into polymeric chains via W—S—Ag bridges having a [AgS4W] repeat unit; the W—Ag—W and Ag—W—Ag angles are 161.657 (17) and 153.978 (9)°, respectively. The title complex is isostructural with the Y, Yb, Eu, Nd, La and Dy isomorphs.

Dimers of Ag2+ Ions – Synthesis and Characterization of the Quaternary Silver Fluoride Ag2ZnZr2F14 with [Ag2F7]3– Units

AbstractDeep blue‐violet colored powder samples of Ag2ZnZr2F14 were synthesized by heating Zn(NO3)2·4H2O, Ag and ZrOCl2·8H2O at 300 °C under fluorine atmosphere. The crystal structure of Ag2ZnZr2F14 was refined from X‐ray powder diffraction data using the Rietveld method (C2/m, a = 9.0206(1) Å, b = 6.6373(1) Å, c = 9.0563(1) Å, β = 90.44(1)°, Z = 2). The structure is derived from the isotypic Ag3Zr2F14 by replacing only one of the two crystallographically different Ag2+ ions with Zn2+ ions, thus leading to discrete Ag2F7 dimers. These dimers are connected via nearly linear Ag–F···F–Ag bridges with short F···F distances of 2.33 Å to form two‐legged ladders. Magnetic susceptibility measurements and density functional calculations show that the two Ag2+ ions in each Ag2F7 dimer are strongly coupled antiferromagnetically.

Silver(II) Fluorosulfate: A Thermally Fragile Ferromagnetic Derivative of Divalent Silver in an Oxa‐Ligand Environment

AbstractSeveral synthetic pathways and characterization of silver(II) fluorosulfate are reported. The title compound crystallizes in the monoclinic space group, P21/c, with a = 10.5130(4) Å, b = 7.7524(3) Å, c = 8.9366(4) Å, β = 117.867(2)° [V = 643.88(5) Å3, Z = 4, dcalcd. = 3.15 g cm–3] in a structure type related to that of AgF2. Puckered [Ag(SO3F)2] sheets are present in the crystal structure with two oxygen atoms of the fluorosulfate anions utilized for bonding within the sheet; the third oxygen atom serves as a linker to the adjacent sheet. Terminal fluorine atoms form small cavities in the structure. The S–O stretching region of the vibrational (IR and Raman) spectra is rich in bands, thus confirming the structural complexity of Ag(SO3F)2. Ag(SO3F)2 is a soft ferromagnet with a Curie temperature of 24.8 K and it shows a single broad electron spin resonance (ESR) with g = 2.183 at T = 293 K. The intrasheet magnetic superexchange constant, J, derived from magnetization measurements, equals +1.0 meV per formula unit. Density functional theory calculations suggest that the superexchange occurs through the OO moiety of the Ag–O–S–O–Ag bridge while omitting the S atom, and the yield is J = +1.1 meV. The Coulomb‐corrected local spin density approximation (LSDA+U) calculations predict a direct electronic band gap at the Fermi level of 1.05 eV. Large magnetic moments reside on O atoms attached to AgII; in consequence, Ag(SO3F)2 is thermally unstable; at room temperature or in the presence of strong acids its dark brown crystals slowly decompose at the surface to a black mixed‐valence Ag3(SO3F)4. Very fast exothermic decomposition of Ag(SO3F)2 with emission of a fluorosulfuryl radical (SO3F·) occurs above 120 °C as confirmed by simultaneous thermogravimetric, calorimetric and evolved gas analyses.

A 2D helical coordination polymer based on Keggin-type polyoxoanion: Synthesis, crystal structure and luminescent properties

A novel two-dimensional helical coordination polymer [Ag(CH3CN)4][Ag3(CH3CN)8(SiMo12O40)] (1) has been gained by the conventional self-assembly reaction, and characterized by IR, elemental analysis, UV, TG technique and X-ray single crystal diffraction. Each [SiMo12O40]4− cluster interconnects with three bimetallic cation subunits through three Mo–O–Ag bridges, meanwhile, each bimetallic cation subunit is surrounded by three [SiMo12O40]4− to form an unprecedented two-dimensional network structure. The metal–metal bonds between Ag(1) and Ag(2) atoms from the adjacent helical chains connects the chains together into a chiral 2D helical network. The 2D network structures are further interconnected with each other to form a 3D supramolecular network through multiform intermolecular hydrogen bonds. The luminescent properties of the coordination polymer in the solid state were investigated.

Strong optical limiting effects of two Ag(I)-bridged metal-organic polymers

Two Ag(I)-bridged metal-organic polymers: 2D double-crossed layer polymer {[Ag(bpfp)] · NO 3 · H 2O)} n (bpfp = N, N′-bis(3-pyridylformyl)-piperazine) ( 1) and 1D zigzag chain polymer {[Ag(btx)] · NO 3} n (btx = 1,4-bis(triazol-1-ylmethyl)-benzene) ( 2) have been prepared and structurally characterized by single crystal diffraction. Their third-order nonlinear optical (NLO) properties and optical limiting (OL) effects were determined by Z-scan technique. The results show that both polymers possess strong optical nonlinear absorption and optical nonlinear refraction, and exhibit strong OL effect to nanosecond and picosecond incident pulsed laser. By comparing the optical nonlinearities of the polymers and purely bpfp and btx ligands, we find that the incorporation of Ag ion can largely enhance the optical nonlinearities of organic molecules attached. DFT calculation further confirms the corporate contributions of Ag ions and bridge ligands to the optical nonlinearities.

Fully Room-Temperature-Fabricated Nonvolatile Resistive Memory for Ultrafast and High-Density Memory Application

Through a simple industrialized technique which was completely fulfilled at room temperature, we have developed a kind of promising nonvolatile resistive switching memory consisting of Ag/ZnO:Mn/Pt with ultrafast programming speed of 5 ns, an ultrahigh R(OFF)/R(ON) ratio of 10(7), long retention time of more than 10(7) s, good endurance, and high reliability at elevated temperatures. Furthermore, we have successfully captured clear visualization of nanoscale Ag bridges penetrating through the storage medium, which could account for the high conductivity in the ON-state device. A model concerning redox reaction mediated formation and rupture of Ag bridges is therefore suggested to explain the memory effect. The Ag/ZnO:Mn/Pt device represents an ultrafast and highly scalable (down to sub-100-nm range) memory element for developing next generation nonvolatile memories.

Excess-silver-induced bridge formation in a silver sulfide atomic switch

Structural properties and electron transport of a Ag2S atomic switch composed of Ag–Ag2S–Ag heterostructure are investigated by nonequilibrium Green’s function calculations considering the effect of excess Ag in the Ag2S layer. In addition to confirming experimentally the formation of the Ag bridge inside Ag2S, the bridge is found to consist of units having a structure similar to that of the Ag (111) face in the bulk Ag. The analyses of Mulliken population, transmission spectra, and current-voltage characteristics reveal that the bridge has a conductive and metallic nature.

Self‐Assembly of Silver(I) Coordination Polymers from AgX (X = BF4–, ClO4–, CF3COO–, and SO3CF3–) and a Rigid Bent 3,6‐Dicyano‐9‐phenylcarbazole Ligand: The Templating Effect of Anions

AbstractOne rigid bent bridging ligand with highly planar π‐conjugated spacers, 3,6‐dicyano‐9‐phenylcarbazole (dcphcz), was designed and synthesized. The coordination reactions of dcphcz with a series of AgI salts with different counterions has been investigated. Four coordination polymers were obtained by solution reactions and characterized by IR,elemental analysis, and single‐crystal X‐ray diffraction. The solid‐state structures of {[Ag(dcphcz)]BF4}n (1) and {[Ag(dcphcz)]ClO4}n (2), exhibit similar 1D “zigzag” patterns. The complex {[Ag(dcphcz)][Ag2(dcphcz)(H2O)2](SO3CF3)3·C6H6·(H2O)2}n (3) includes two different one‐dimensional (1D) chain units: one forms double‐layer two‐dimensional (2D) metal–organic frameworks (MOFs) through intermolecular Ag–O–Ag bridging interactions, the other features double‐layer 2D supramolecular networks through C–H···O hydrogen‐bonding interactions. The complex [Ag2(dcphcz)(CF3COO)2]n (4) features a bundle of novel nanotubular structures, which contain silver chains formed by carboxylate spacers. Furthermore, all these complexes are connected by face‐to‐face π···π stacking interactions between carbazolyl planes, affording a series of three‐dimensional (3D) architectures with different structural geometries. The structuraldiversities of these complexes demonstrated that counteranions and bridging ligands play essential roles in the construction of supramolecular frameworks. In addition, the luminescence properties of the free ligand dcphcz and complexes 1–4 were investigated.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Coordination Networks with Carborane Anions: Ag(I) and Nitrogen Bridging Ligands

New crystalline coordination networks based on Ag(i) and N-bridging ligands — pyrazine (pyz), 4,4′-bipyridine (bpy), and 2,3-bis-(2-pyridyl)pyrazine (bppz) — incorporating carborane monoanions (CB11H12)− or [Co(C2B9H11)2]− were isolated and characterized using single crystal X-ray diffraction, microanalysis, and infrared spectroscopy. All the complexes with the bpy and bppz ligands reveal 1D coordination infinite chains involving the Ag(i) and the ligands, and layered extended structures with rows of carborane anions between the layers of chains. Interestingly, two polymorphs of the complex [Ag(bpy)(CH3CN)][Co(C2B9H11)2] were observed. The complex [Ag(pyz)(CH3CN)2][Co(C2B9H11)2] shows a 1D coordination polymer, while the pyz complex with the smaller carborane anion {[Ag(pyz)](CB11H12)} exhibits a 3D coordination network structure with four Ag···H−B interactions between the silver centre and carborane anions.

How strong can the bend be on a DNA helix from cisplatin? DFT and MP2 quantum chemical calculations of cisplatin-bridged DNA purine bases.

The B3LYP/6-31G(d) level of theory was used for the optimization of [Pt(NH(3))(4)](2+), [Pt(NH(3))(3)(H(2)O)](2+), cis-[Pt(NH(3))(2)(H(2)O)(2)](2+), and related platinum complexes. In addition, water or ammonium ligands were replaced by DNA purine bases so that finally cis-diammineplatinum with two bases (Pt-bridged complexes) is obtained. Single point calculations using the MP2/6-31+G(d) method were performed on the obtained reference geometries and were utilized for estimating bond dissociation energies (BDEs) and stabilization energies, and for electron density analyses. After reoptimization, IR spectra were determined from HF second derivatives. It was found that replacement of both water and ammonium by the DNA base is an exothermic process (20-50 kcal/mol depending on the ligands present in the complex). Asymmetric structures with one interbase H-bond were obtained for cis-diammine[bond](N(7),N(7)'-diadenine)[bond]platinum and mixed cis-diammine[bond](N(7)-adenine)[bond](N(7)-guanine)[bond]platinum complexes. In the case of the diguanine Pt-bridge, a symmetrical complex with two ammonium...O(6) H-bonds was found. The higher stabilization energy of the di-guanine complex is linked to a larger component of the Coulombic interaction. However, the BDE of Pt[bond]N(7)(G) is smaller in this complex than the BDE of Pt[bond]N(7)(G) from the mixed Pt[bond]AG complex. Also, steric repulsion of the ligands is about 10 kcal/mol smaller for the asymmetrical Pt[bond]AA and Pt[bond]AG bridges. The influence of the trans effect on DBE can be clearly seen. Adenine exhibits the largest trans effect, followed by guanine, ammonium, and water. The strength of the H-bond can be determined from the IR spectra. The strongest H-bond is the interbase H-bridge between adenine and guanine in the mixed Pt[bond]AG complex; otherwise, the H-bonds of adenine complexes are weaker than in guanine complexes. BDE can be traced in the guanine-containing complexes. The nature of the covalent bonding is analyzed in terms of partial charges and MO. A general explanation of the lower affinity of transition metals to oxygen than nitrogen can be partially seen in the less favorable geometrical orientation of lone electron pairs of oxygen.

Relaxation in NiFe/Ag giant magnetoresistive devices

Giant magnetoresistance was measured as a function of time and device size for patterned NiFe/Ag multilayer films. The sputtered NiFe/Ag multilayers were postannealed at 340 °C for 5 min in order to produce a change in resistivity Δρ/ρ of 5% in a saturating field of 4 kA/m (50 Oe). The microstructure of these films is believed to be discontinuous due to Ag bridging through the NiFe grain boundaries after the anneal. The films were fabricated into rectangular stripes with Au current lead, and then exposed to a magnetic field pulse to measure the time response of the resistance, characterized by a time constant τ, from the relation ΔR(t)=ΔR0e−t/τ. An apparatus was developed to produce a magnetic field pulse up to 8 kA/m (100 Oe) with a turn-on/off time constant of 10 μs. The response of the NiFe/Ag devices saturated quickly with the turn-on step with a time constant nearly equal to that of the field pulse. The response to the turn-off step, however, had a time constant of nearly 300 μs. When the field is first applied, the torque on the magnetic moments quickly aligns the magnetization. When the field is shut off, however, the torque due to the field drops to 0, so interacting magnetostatic fields from the grains and thermal energy dominate the relaxation process. The average time constant depends on the device size and the applied current density. Relaxation may be detrimental for using this type of material for read heads where very high data rates are required.

Microstructural origin of giant magnetoresistance in a new sensor structure based on NiFe/Ag discontinuous multilayer thin films

The origin of giant magnetoresistance (GMR) in a new sensor structure incorporating discontinuous multilayers (DML) of NiFe thin films separated by Ag interlayers is elucidated by means of x-ray diffraction, x-ray reflectivity, and cross-section transmission electron microscopy. It is shown that the observed magnetoresistance, ∼4%–6% in fields on the order of 5–10 Oe at 25 °C, is associated with the breakup upon annealing of the initially dense, columnar superlattice structure into discontinuous multilayers of NiFe due to highly mobile Ag forming Ag bridges between the Ag layers. This observation supports a micromagnetic model for these structures that is based on the concept of initially large ferromagnetic domains in the as-deposited NiFe structure, exhibiting negligible GMR, breaking up into discontinuous layers, exhibiting an appreciable GMR, with antiferromagnetic ordering across the Ag interlayers.

Easy access to a new class of anionic cyano-bridged di- and tri-nuclear organotin adducts. Crystal structure of [N(PPh3)2][ClPh3Sn(µ-NC)Ag(CN)

Cyanometallates [N(PPh3)2][Ag(CN)2]1 and [N(PPh3)2]2[Pd(CN)4]2 reacted with SnPh3Cl, producing anionic complexes characterized by IR and 119Sn NMR spectroscopy, an X-ray structural analysis of the silver complex evidencing a Sn–NC–Ag bridging interaction; the crystal structure of 1 has also been determined for comparison.

High-T C superconductor-normal metal-superconductor Josephson microbridges with high-resistance normal metal links

We have developed an in situ process for fabricating high transition temperature superconductor-normal metal-superconductor microbridges using a step edge to define the normal metal length. Critical current-normal resistance products over 1 mV have been measured at low temperature in devices with high-resistivity Ag-Au alloy bridges. Results on samples with Ag bridges are compared with the alloy data as an initial test of recent theories of SNS Josephson junctions. Josephson effects have been demonstrated in these devices at temperatures higher than 80 K. Clearly defined rf steps have been observed, with power dependence qualitatively similar to theoretical predictions.

Hydrogen chemisorption by copper, silver and gold overlayers on W{100}: Site blocking and alloy formation in surface reactivity

The reactivity of copper, silver and gold films (up to $ ̃ 2 monolayers in thickness) on W{100} towards hydrogen chemisorption has been examined using thermal desorption spectroscopy (TDS). For metal overlayers produced during deposition at 300 K, a linear decrease in the amount of hydrogen adsorbed at 300 K is obtained for increasing metal loadings up to one monolayer, by which time all hydrogen chemisorption ceases. It is believed that a site-blocking mechanism is responsible in which four-fold hollow sites are occupied by the metal adsorbate. In contrast, adsorption of hydrogen by metal films annealed at 800 K is more strongly inhibited than films produced at 300K. It is proposed that the irreversible formation of a two-dimensional surface alloy between the tungsten substrate and metal overlayer is the cause of this behaviour. We conclude that dissociative adsorption of room-temperature H 2 does not occur onto a surface containing only mixed W-Cu (Ag or Au) bridge sites. This behaviour is similar to that reported recently by Lambert and co-workers for N 2 adsorbed on annealed Pt/W{100} surface alloys.