Supramolecular selection in molecular alloys

Complexes of the type \[M(phen)3](PF6)2 (M = Ni(II), Fe(II), Ru(II) and phen = 1,10-phenanthroline) were found to co-crystallize to form molecular alloys (solid solutions of molecules) with general formula \[MAxMB1–x(phen)3](PF6)2·0.5H2O in which the relative concentrations of the metal complexes in the crystals closely match those in the crystallizing solution. Consequently, the composition of the co-crystals can be accurately predicted and controlled by modulating the relative concentrations of the metal complexes in the crystallizing solution. Although they are chemically and structurally similar, complexes of the type \[M(bipy)3](PF6)2 (M = Ni(II), Fe(II), Ru(II) and bipy = 2,2′-bipyridine) display markedly different behavior upon co-crystallization. In this case, the resulting co-crystals of general formula \[MAxMB1–x(bipy)3](PF6)2 have relative concentrations of the constituent complexes that are markedly different from the relative concentrations of the complexes initially present in the crystallizing solution. For example, when the nickel and iron complexes are co-crystallized from a solution containing a 50:50 ratio of each, the result is the formation of some crystals with a higher proportion of iron and others with a higher proportion of nickel. The relative concentrations of the metal complexes in the crystals can vary from those in the crystallizing solutions by as much as 15%. This result was observed for a range of combinations of metal complexes (Ni/Fe, Ni/Ru, and Fe/Ru) and a range of starting concentrations in the crystallizing solutions (90:10 through to 10:90 in 10% increments). To explain this remarkable result, we introduce the concept of “supramolecular selection”, which is a process driven by molecular recognition that leads to the partially selective aggregation of like molecules during crystallization.

Infrared and Raman spectroscopic characterization of the phosphate mineral kosnarite KZr2(PO4)3 in comparison with other pegmatitic phosphates

In this research, we have used vibrational spectroscopy to study the phosphate mineral kosnarite KZr2(PO4)3. Interest in this mineral rests with the ability of zirconium phosphates (ZP) to lock in radioactive elements. ZP have the capacity to concentrate and immobilize the actinide fraction of radioactive phases in homogeneous zirconium phosphate phases. The Raman spectrum of kosnarite is characterized by a very intense band at 1,026 cm−1 assigned to the symmetric stretching vibration of the PO4 3− ν1 symmetric stretching vibration. The series of bands at 561, 595 and 638 cm−1 are assigned to the ν4 out-of-plane bending modes of the PO4 3− units. The intense band at 437 cm−1 with other bands of lower wavenumber at 387, 405 and 421 cm−1 is assigned to the ν2 in-plane bending modes of the PO4 3− units. The number of bands in the antisymmetric stretching region supports the concept that the symmetry of the phosphate anion in the kosnarite structure is preserved. The width of the infrared spectral profile and its complexity in contrast to the well-resolved Raman spectrum show that the pegmatitic phosphates are better studied with Raman spectroscopy.

Hydrogen spillover mechanism on a pd-doped mg surface as revealed by ab initio density functional calculation

The hydrogenation kinetics of Mg is slow, impeding its application for mobile hydrogen storage. We demonstrate by ab initio density functional theory (DFT) calculations that the reaction path can be greatly modified by adding transition metal catalysts. Contrasting with Ti doping, a Pd dopant will result in a very small activation barrier for both dissociation of molecular hydrogen and diffusion of atomic H on the Mg surface. This new computational finding supports for the first time by ab initio simulationthe proposed hydrogen spillover mechanism for rationalizing experimentally observed fast hydrogenation kinetics for Pd-capped Mg materials.

Fuel additive

A fuel additive comprising one or more complex oxides having a nominal compn. as set out in formula (1): AxB1-yMyOn; wherein A is selected from one or more group III elements including the lanthanide elements or one or more divalent or monovalent cations; B is selected from one or more elements with at. no. 22 to 24, 40 to 42 and 72 to 75; M is selected from one or more elements with at. no. 25 to 30; x is defined as a no. where 0 < x ≤ l; y is defined as a no. where 0 ≤ y < 0.5. [on SciFinder(R)]

Electrochemically-induced TCNQ/Mn TCNQ 2 (H2O) 2 (TCNQ= 7, 7, 8, 8-Tetracyanoquinodimethane) solid-solid interconversion : two voltammetrically distinct processes that allow selective generation of nanofiber or nanorod network morphologies

Unlike the case with other divalent transition metal M\[TCNQ](2)(H(2)O)(2) (M = Fe, Co, Ni) analogues, the electrochemically induced solid-solid phase interconversion of TCNQ microcrystals (TCNQ = 7,7,8,8-tetracyanoquinodimethane) to Mn\[TCNQ](2)(H(2)O)(2) occurs via two voltammetrically distinct, time dependent processes that generate the coordination polymer in nanofiber or rod-like morphologies. Careful manipulation of the voltammetric scan rate, electrolysis time, Mn(2+)((aq)) concentration, and the method of electrode modification with solid TCNQ allows selective generation of either morphology. Detailed ex situ spectroscopic (IR, Raman), scanning electron microscopy (SEM), and X-ray powder diffraction (XRD) characterization clearly establish that differences in the electrochemically synthesized Mn-TCNQ material are confined to morphology. Generation of the nanofiber form is proposed to take place rapidly via formation and reduction of a Mn-stabilized anionic dimer intermediate, \[(Mn(2+))(TCNQ-TCNQ)(2)(*-)], formed as a result of radical-substrate coupling between TCNQ(*-) and neutral TCNQ, accompanied by ingress of Mn(2+) ions from the aqueous solution at the triple phase TCNQ/electrode/electrolyte boundary. In contrast, formation of the nanorod form is much slower and is postulated to arise from disproportionation of the \[(Mn(2+))(TCNQ-TCNQ)(*-)(2)] intermediate. Thus, identification of the time dependent pathways via the solid-solid state electrochemical approach allows the crystal size of the Mn\[TCNQ](2)(H(2)O)(2) material to be tuned and provides new mechanistic insights into the formation of different morphologies.

On the importance of an acid additive in the synthesis of pyrido[1,2-a]benzimidazoles by direct copper-catalyzed amination

Pyrido[1,2-a]benzimidazoles1, 2a are interesting compounds both from the viewpoint of medicinal chemistry2–7 (solubility,7 DNA intercalation3) and materials chemistry8 (fluorescence). Of note among the former is the antibiotic drug Rifaximin,5 which contains this heteroaromatic core. The classical synthetic approach for the assembly of pyrido[1,2-a]benzimidazoles is by [3+3] cyclocondensation of benzimidazoles containing a methylene group at C2 with appropriate bielectrophiles.2a However, these procedures are often low-yielding, involve indirect/lengthy sequences, and/or provide access to a limited range of products, primarily providing derivatives with substituents located on the pyridine ring (A ring, Scheme 1).2–4 Theoretically, a good alternative synthetic method for the synthesis of pyrido[1,2-a]benzimidazoles with substituents in the benzene ring (C ring) should be accessible by intramolecular transition-metal-catalyzed CN bond formation in N-(2-chloroaryl)pyridin-2-amines, based on chemistry recently developed in our research group.9 These substrates themselves are easily available through SNAr or selective Pd-catalyzed amination10 of 2-chloropyridine with 2-chloroanilines.11 If a synthetic procedure that eliminated the need for preactivation of the 2-position of the 2-chloroarylamino entity could be developed, this would be even more powerful, as anilines are more readily commercially available than 2-chloroanilines. Therefore the synthesis of pyrido[1,2-a]benzimidazoles (4) by a transition-metal-catalyzed intramolecular CH amination approach from N-arylpyridin-2-amines (3) was explored (Scheme 1).

Supramolecular selection in molecular and framework co-crystals

An ongoing challenge in chemistry and crystal engineering is the synthesis of functional materials with predictable structures and customisable properties. This may be achieved by crystallising mixtures of different compounds. Co-crystals formed through this method have predictable structures and their properties may be tuned by varying the ratio of the compounds in the crystallising solution. This thesis examines single crystals formed by the co-crystallisation of metal complexes that have similar structures but different physical or chemical properties. A variety of new compounds with interesting properties were prepared, characterised and their significance in the context of crystal engineering was explored.

Coordination polymeric structures in the sodium salt of 4-chloro-3-nitrobenzoic acid and the sodium and potassium salts of 4-nitroanthranilic acid

The structures of the hydrated sodium salts of 4-chloro-3-nitrobenzoic acid {poly[aqua(μ4-4-chloro-3-nitrobenzoato)sodium(I)], [Na(C7H3ClNO4)(H2O)]n, (I)} and 2-amino-4-nitrobenzoic acid {poly[μ-aqua-aqua(μ3-2-amino-4-nitrobenzoato)sodium(I)], [Na(C7H5N2O4)(H2O)2]n, (II)}, and the hydrated potassium salt of 2-amino-4-nitrobenzoic acid {poly[μ-aqua-aqua(μ5-2-amino-4-nitrobenzoato)potassium(I)], [K(C7H5N2O4)(H2O)]n, (III)} have been determined and their complex polymeric structures described. All three structures are stabilized by intra- and intermolecular hydrogen bonding and strong π–π ring interactions. In the structure of (I), the distorted trigonal bipyrimidal NaO5 coordination polyhedron comprises a monodentate water molecule and four bridging carboxylate O-atom donors, generating a two-dimensional polymeric structure lying parallel to (001). Intra-layer hydrogen-bonding associations and strong inter-ring π–π interactions are present. Structure (II) has a distorted octahedral NaO6 stereochemistry, with four bridging O-atom donors, two from a single carboxylate group and two from a single nitro group and three from the two water molecules, one of which is bridging. Na centres are linked through centrosymmetric four-membered duplex water bridges and through 18-membered duplex head-to-tail ligand bridges. Similar centrosymmetric bridges are found in the structure of (III), and in both (II) and (III) strong inter-ring π–π interactions are found. A two-dimensional layered structure lying parallel to (010) is generated in (II), whereas in (III) the structure is three-dimensional. With (III), the irregular KO7 coordination polyhedron comprises a doubly bridging water molecule, a single bidentate bridging carboxylate O-atom donor and three bridging O-atom donors from the two nitro groups. A three-dimensional structure is generated. These coordination polymer structures are among the few examples of metal complexes of any type with either 4-chloro-3-nitrobenzoic acid or 4-nitroanthranilic acid.

Enhanced coloration efficiency for electrochromic devices based on anodized N2O5 / Electrodeposited MoO3 Binary Systems

There is a continuous quest for developing electrochromic (EC)transition metal oxides (TMOs) with increased coloration efficiency. As emerging TMOs, Nb2O5 films, even those of ordered anodized nanochannels, have failed to produce the required EC performance for practical applications. This is attributed to limitations presented by its relatively wide bandgap and low capacity for accommodating ions. To overcome such issues, MoO3 was electrodeposited onto Nb2O5 nanochannelled films as homogeneously conformal and stratified α-MoO3 coatings of different thickness. The EC performance of the resultant MoO3 coated Nb2O5 binary system was evaluated. The system exhibited a coloration efficiency of 149.0 cm2 C−1, exceeding that of any previous reports on MoO3 and Nb2O5 individually or their compounds. The enhancement was ascribed to a combination of the reduced effective bandgap of the binary system, the increased intercalation probability from the layered α-MoO3 coating, and a high surface-tovolume ratio, while the Nb2O5 nanochannelled templates provided stability and low impurity pathways for charge transfer to occur.

Thin films and nanostructures of niobium pentoxide : fundamental properties, synthesis methods and applications

As one of the transition metal oxides, niobium pentoxide (Nb2O5) offers a broad variety of properties that make it a potentially useful and highly applicable material in many different areas. In comparison to many other transition metal oxides, Nb2O5 has received relatively little attention, which presents a significant opportunity for future investigations aimed at fundamentally understanding this material and finding new and interesting applications for it. In this article, a general overview of Nb2O5 is presented which focuses on its fundamental properties, synthesis methods and recent applications, along with a discussion on future research directions relevant to this material.

Structural and electronic properties of layered arsenic and antimony arsenide

Layered materials exhibit intriguing electronic characteristics and the search for new types of two-dimensional (2D) structures is of importance for future device fabrication. Using state-of-art first principle calculations, we identify and characterize the structural and electronic properties of two 2D layered arsenic materials, namely, arsenic and its alloy AsSb. The stable 2D structural configuration of arsenic is confirmed to be the low-buckled two-dimensional hexagonal structure by phonon and binding energy calculations. The monolayer exhibits indirect semiconducting properties with gap around 1.5 eV (corrected to 2.2 eV by hybrid function), which can be modulated into a direct semiconductor within a small amount of tensile strain. These semiconducting properties are preserved when cutting into 1D nanoribbons, but the band gap is edge dependent. It is interesting to find that an indirect to direct gap transition can be achieved under strain modulation of the armchair ribbon. Essentially the same phenomena can be found in layered AsSb, except a weak Rashba induced band splitting is present in AsSb due to the nonsymmetric structure and spin orbit coupling. When an additional layer is added on the top, a semiconductor–metal transition will occur. The findings here broaden the family of 2D materials beyond graphene and transition metal dichalcogenides and provide useful information for experimental fabrication of new layered materials with possible application in optoelectronics.

Virtues of marginally metallic oxides

Transition-metal oxides at the metal-insulator boundary, especially those belonging to the perovskite family, exhibit fascinating phenomena such as insulator-metal transitions controlled by composition, high-temperature superconductivity and giant magnetoresistance (GMR), Interestingly, many of these marginally metallic oxides obey the established criteria for metallicity and have a finite density of states at the Fermi;level. The perovskite manganates exhibiting GMR, on the other hand, are unusual in that they possess very high resistivities in the 'metallic' state and show no significant density of states at the Fermi level, Marginal metallicity in oxide systems is a problem of great complexity and contemporary interest and its understanding is of crucial significance to the diverse phenomena exhibited by these materials.

Electronic structure studies and photocatalytic properties of cubic Bi1.5ZnNb1.5O7

The photocatalytic ability of cubic Bi1.5ZnNb1.5O7 (BZN) pyrochlore for the decolorization of an acid orange 7 (AO7) azo dye in aqueous solution under ultraviolet (UV) irradiation has been investigated for the first time. BZN catalyst powders prepared using low temperature sol-gel and higher temperature solid-state methods have been evaluated and their reaction rates have been compared.The experimental band gap energy has been estimated from the optical absorption edge and has been used as reference for theoretical calculations. The electronic band structure of BZN has been investigated using first-principles density functional theory (DFT) calculations for random, completely and partially ordered solid solutions of Zn cations in both the A and B sites of the pyrochlore structure.The nature of the orbitals in the valence band (VB) and the conduction band (CB) has been identified and the theoretical band gap energy has been discussed in terms of the DFT model approximations.

Synthesis, characterization, and electronic structure studies of cubic Bi1.5ZnTa1.5O7 for photocatalytic applications

Bi1.5ZnTa1.5O7 (BZT) has been synthesized using an alkoxide based sol-gel reaction route. The evolution of the phases produced from the alkoxide precursors and their properties have been characterized as function of temperature using a combination of thermogravimetric analysis (TGA) coupled with mass spectrometry (MS), infrared emission spectrometry (IES), X-ray diffraction (XRD), ultraviolet and visible (UV-Vis) spectroscopy, Raman spectroscopy, and N2 adsorption/desorption isotherms. The lowest sintering temperature (600∘C) to obtain phase pure BZT powders with high surface area (14.5m2/g) has been determined from the thermal decomposition and phase analyses.The photocatalytic activity of the BZT powders has been tested for the decolorization of organic azo-dye and found to be photoactive under UV irradiation.The electronic band structure of the BZT has been investigated using density functional theory (DFT) calculations to determine the band gap energy (3.12 eV) and to compare it with experimental band gap (3.02 eV at 800∘C) from optical absorptionmeasurements. An excellent match is obtained for an assumption of Zn cation substitutions at specifically ordered sites in the BZT structure.

Study of electron states of solids by techniques of electron spectroscopy

Studies of valence bands and core levels of solids by photoelectron spectroscopy are described at length. Satellite phenomena in the core level spectra have been discussed in some detail and it has been pointed out that the intensity of satellites appearing next to metal and ligand core levels critically depends on the metal-ligand overlap. Use of photoelectron spectroscopy in investigating metal-insulator transitions and spin-state transitions in solids is examined. It is shown that relative intensities of metal Auger lines in transition metal oxides and other systems provide valuable information on the valence bands. Occurrence of interatomic Auger transitions in competition with intraatomic transitions is discussed. Applications of electron energy loss spectroscopy and other techniques of electron spectroscopy in the study of gas-solid interactions are briefly presented.