ANSTO :: Browsing by Author "Edwards, AJ"

Browsing by Author "Edwards, AJ"

Now showing 1 - 20 of 23

Adventures in reciprocal space - from Laue to Bragg and back again
(Australian Institute of Physics, 2014-02-05) Edwards, AJ
The earliest X-ray diffraction experiments [1] employed radiation “as generated” without monochromation to produce the reciprocal space images we know as Laue patterns. The pioneering work of W.L. Bragg [2] using monochromated X-rays followed rapidly and provided the major simplification in the mathematics required to analyse X-ray diffraction patterns to derive data from which atomic resolution structural information can be deduced. In the following century, physicists, chemists and later biologists developed the Bragg methodology into a powerful tool which underpins the structure based paradigm at the core of modern chemistry and biology. Application of the Laue method to questions of structure determination at atomic resolution languished for many decades until the availability of fast computers and the technical challenges of Synchrotron sources led to a resurgence in this experimental approach [3]. With the cost of neutron beams being substantially more than that of X-ray beams, the applicability of this method to neutron diffraction studies was soon investigated [4] and today Laue neutron diffraction is the method of choice for the determination of structures where neutron diffraction is scientifically required to prove aspects of structure for which X-ray diffraction can only be “suggestive”. Chemists typically employ an array of physical methods to support their structural assertions, but a crystal structure is often presented as absolute proof and as justification of inferior characterization by other methods. This being the case it is of great concern that crystallographic studies be critically reviewed by both analyst and in the publication process. Checkcif is a fine tool but insufficient to ensure the integrity of the scientific literature – that is properly the role of the analyst and the reviewers.
The ambiguous origin of thermochromism in molecular crystals of dichalcogenides: chalcogen conds versus dynamic Se−Se/Te−Te bonds
(Wiley, 2023-11-06) Thomas, SP; Singh, A; Grosjean, A; Alhameedi, K; Grønbech, TBE; Piltz, RO; Edwards, AJ; Iversen, BB
We report thermochromism in crystals of diphenyl diselenide (dpdSe) and diphenyl ditelluride (dpdTe), which is at variance with the commonly known mechanisms of thermochromism in molecular crystals. Variable temperature neutron diffraction studies indicated no conformational change, tautomerization or phase transition between 100 K and 295 K. High‐pressure crystallography studies indicated no associated piezochromism in dpdSe and dpdTe crystals. The evolution of the crystal structures and their electronic band structure with pressure and temperature reveal the contributions of intramolecular and intermolecular factors towards the origin of thermochromism—especially the intermolecular Se⋅⋅⋅Se and Te⋅⋅⋅Te chalcogen bonds and torsional modes of vibrations around the dynamic Se−Se and Te−Te bonds. Further, a co‐crystal of dpdSe with iodine (dpdSe‐I2) and an alloy crystal of dpdSe and dpdTe implied a predominantly intramolecular origin of the observed thermochromism associated with vibronic coupling. © 1999-2024 John Wiley & Sons
Chinese puzzle molecule: A 15 hydride, 28 copper atom nanoball
(Wiley Online Library, 2014-05-06) Edwards, AJ; Dhayal, RS; Liao, PK; Liao, JH; Chiang, MH; Piltz, RO; Kahlal, S; Saillard, JY; Liu, CW
The syntheses of the first rhombicuboctahedral copper polyhydride complexes [Cu28(H)15(S2CNR)12]PF6 (NR=NnPr2 or aza-15-crown-5) are reported. These complexes were analyzed by single-crystal X-ray and one by neutron diffraction. The core of each copper hydride nanoparticle comprises one central interstitial hydride and eight outer-triangular-face-capping hydrides. A further six face-truncating hydrides form an unprecedented bridge between the inner and outer copper atom arrays. The irregular inner Cu4 tetrahedron is encapsulated within the Cu24 rhombicuboctahedral cage, which is further enclosed by an array of twelve dithiocarbamate ligands that subtends the truncated octahedron of 24 sulfur atoms, which is concentric with the Cu24 rhombicuboctahedron and Cu4 tetrahedron about the innermost hydride. For these compounds, an intriguing, albeit limited, H2 evolution was observed at room temperature, which is accompanied by formation of the known ion [Cu8(H)(S2CNR)6]+ upon exposure of solutions to sunlight, under mild thermolytic conditions, and on reaction with weak (or strong) acids. © 2014, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Finding the Goldilocks zone for chemical crystallography via Laue single-crystal neutron diffraction – what have we learned from KOALA to improve KOALA 2.0?
(International Union of Crystallography, 2021-08-14) Edwards, AJ; Piltz, RO
KOALA is a single-crystal Laue neutron diffractometer standing at the end of guide position of the supermirror guide TG3 at the OPAL reactor, ANSTO. The instrument was initially modelled closely on VIVALDI[1], an instrument available in the user program at the ILL from 2001-2010. The elegantly simple concept of the instrument employs a cylindrical neutron sensitised image plate detector which is used to record a series of diffraction images from a suitable number of crystal positions to provide a sufficient data set from which valid model parameters can be derived to answer questions regarding material properties which cannot be adequately derived from more readily available methods, most particularly X-ray diffraction and more recently the hybrid methodology of quantum crystallography. Our initial practice with the instrument adhered largely to that shared with us by the scientists at the ILL. This early experience[2] was the commencement of a steep learning curve which has, with a very limited number of other instruments brought single-crystal neutron diffraction into greater use in chemistry and chemical crystallography in the second decade of the 21st century. Key developments have been (i) the installation of an Oxford Cryosystems COBRA™ nitrogen cryostream which facilitates handling of oxygen and moisture sensitive compounds (which encompass a significant fraction of the proposals received for the instrument) and (ii) the development of a user accessible data reduction for the diffraction images. From the first proposal round for the instrument in 2009 exciting chemistry was proposed for experiments which exceeded the nominal maximum primitive unit cell volume for the recording of useful diffraction images. A simple work around for this has been to reduce the resolution of the images by manipulation of the temperature at which they are recorded – in order to obtain data against which a model may be refined. More commonly though, it is observed that crystals for which the unit cell volume is relatively large tend, where they can be grown to a size sufficient for Laue neutron diffraction, to have a mosaic spread which limits the resolution of the pattern observed without manipulating the temperature to further reduce the resolution. With careful attention to experimental detail and the availability of discretionary beam-time access it has been possible to undertake studies of important new materials in timeframes which have resulted in the publication of the single-crystal neutron diffraction study with the chemistry it underpins, rather than as a stand alone paper reporting only the neutron study result. It is of particular importance to note that in the case of hydride containing compounds, it can be critical to prove the location of the hydride via neutron diffraction and even a low resolution study can provide the necessary proof. In consequence of their publication with the chemistry, papers from KOALA are now submitted to and published in journals of the highest standing [4-7]. Having achieved a more routine applicability of neutron diffraction in chemical crystallography, we reached a point where electronic components of KOALA had exceeded their serviceable lifespans and contemplation of replacing this aspect of the instrument led us to realise that reworking the existing mechanical elements with new electronics posed significant challenges and would cost a large fraction of the potential cost of building a new instrument. We are fortunate that the decision was reached to design a new instrument which is allowing us to optimise key design elements to yield maximum flexibility of the instrument across all of its possible applications in chemistry, physics, materials science and crystallography. The instrument is currently under construction and should be available for users in the second half of 2022. © The Authors
Has crystallography lost the plot on gender equity, or has it been penalised because of its historically greater equity?
(Society of Crystallographers in Australia and New Zealand, 2017-12-03) Edwards, AJ
Crystallography as a science has a history that predates the discovery of the phenomenon of X-ray diffraction, having been an issue for students of mathematics, chemistry, biology, geology and physics—for example, Pasteur’s key observation of the chirality of crystals [1]. Could it be that this scientifically broadly based activity has contributed to the remarkable success of crystallographic studies in modern science? Is it this broad-based interest that saw the atypical evolution of crystallography in the 20th century as a science based in mathematics and physics in which women were welcome and, indeed, became key players as the science expanded into molecular and macromolecular studies? Whether by accident, or virtue, the prominence of women in crystallography compared to other physical sciences in the late 20th century was a fact clearly observed by many of our members—some of us still practising in the field today. At the most recent International Union of Crystallography (IUCr) meeting, it was notable that the representation of women at the conference, in particular the numbers of women presenting plenary lectures (none) and keynote and invited lectures, appeared to be diminished with respect to what appear to be healthy numbers of women practitioners in crystallography. It is currently fashionable in STEM subjects (Science, Technology, Engineering and Mathematics) for significant attention to be paid to gender balance within educational and scientific organisations. Concepts such as “male champions of change” are being advanced as a “solution” to the perceived “problem”. This author questions the validity of such an approach where an acknowledged outlier field is undergoing an apparent reversal from a situation of greater to one of lesser gender balance and indeed equity. Is something else at play here? Could it be that in the atypical gender balance evident in crystallography in the late 20th century, in the “primary subject areas” across which crystallographic endeavours are distributed, a perceived (whether real or not) level of feminisation of crystallography has actually led to the diminution of respect and career prospects for practitioners of crystallography for either gender? The relegation of crystallographic appointments in university chemistry departments to the status of service or peripheral activity, delivered in many instances by practitioners of limited experience and subject to limited tenure, has occurred widely in chemistry departments and anecdotally is now reported to be underway to some extent in the biological areas of crystallography. Is our science actually being diminished and downplayed by the primary subject areas because it has been "feminised” to a modest extent? It is up to us as the professional scientific crystallographers to understand what the drivers are that may have been reversing what was our more desirable gender balance. It is especially important to enquire and understand why our science itself has been so diminished as to be in many cases only a handmaiden to the main game. It is critical for the future of our science that we reflect on these questions of status and equity and act collectively to ensure our science is properly valued.
A hexagonal planar transition-metal complex
(Springer Nature Limited, 2019-10-09) Garçon, M; Bakewell, C; Sackman, GA; White, AJP; Cooper, RI; Edwards, AJ; Crimmin, MR
Transition-metal complexes are widely used in the physical and biological sciences. They have essential roles in catalysis, synthesis, materials science, photophysics and bioinorganic chemistry. Our understanding of transition-metal complexes originates from Alfred Werner's realization that their three-dimensional shape influences their properties and reactivity1, and the intrinsic link between shape and electronic structure is now firmly underpinned by molecular-orbital theory2-5. Despite more than a century of advances in this field, the geometries of transition-metal complexes remain limited to a few well-understood examples. The archetypal geometries of six-coordinate transition metals are octahedral and trigonal prismatic, and although deviations from ideal bond angles and bond lengths are frequent6, alternative parent geometries are extremely rare7. The hexagonal planar coordination environment is known, but it is restricted to condensed metallic phases8, the hexagonal pores of coordination polymers9, or clusters that contain more than one transition metal in close proximity10,11. Such a geometry had been considered12,13 for [Ni(PtBu)6]; however, an analysis of the molecular orbitals suggested that this complex is best described as a 16-electron species with a trigonal planar geometry14. Here we report the isolation and structural characterization of a simple coordination complex in which six ligands form bonds with a central transition metal in a hexagonal planar arrangement. The structure contains a central palladium atom surrounded by three hydride and three magnesium-based ligands. This finding has the potential to introduce additional design principles for transition-metal complexes, with implications for several scientific fields. © 2020 Springer Nature Limited
High-yield synthesis and crystal structure of a green Au30 cluster co-capped by thiolate and sulfide
(Royal Society of Chemistry, 2016-09-24) Yang, H; Wang, Y; Edwards, AJ; Yana, J; Zheng, N
A green gold-cluster, Au30S(StBu)18, was successfully prepared in high yield and crystallographically characterized. Each cluster consists of an Au22 core capped by a mixed layer of staple Au-thiolate units, bridging thiolates and a μ3-S2−.© 2014, The Royal Society of Chemistry.
Hirshfeld atom refinement for modelling strong hydrogen bonds
(International Union of Crystallography, 2014-01-01) Woinska, M; Jayatilaka, D; Spackman, MA; Edwards, AJ; Dominiak, DJ; Wozniak, PM; Nishibori, E; Sugimoto, K; Grabowsky, S
High-resolution low-temperature synchrotron X-ray diffraction data of the salt L-phenylalaninium hydrogen maleate are used to test the new automated iterative Hirshfeld atom refinement (HAR) procedure for the modelling of strong hydrogen bonds. The HAR models used present the first examples of Z' > 1 treatments in the framework of wavefunction-based refinement methods. L-Phenylalaninium hydrogen maleate exhibits several hydrogen bonds in its crystal structure, of which the shortest and the most challenging to model is the O-H...O intramolecular hydrogen bond present in the hydrogen maleate anion (O...O distance is about 2.41 Å). In particular, the reconstruction of the electron density in the hydrogen maleate moiety and the determination of hydrogen-atom properties [positions, bond distances and anisotropic displacement parameters (ADPs)] are the focus of the study. For comparison to the HAR results, different spherical (independent atom model, IAM) and aspherical (free multipole model, MM; transferable aspherical atom model, TAAM) X-ray refinement techniques as well as results from a low-temperature neutron-diffraction experiment are employed. Hydrogen-atom ADPs are furthermore compared to those derived from a TLS/rigid-body (SHADE) treatment of the X-ray structures. The reference neutron-diffraction experiment reveals a truly symmetric hydrogen bond in the hydrogen maleate anion. Only with HAR is it possible to freely refine hydrogen-atom positions and ADPs from the X-ray data, which leads to the best electron-density model and the closest agreement with the structural parameters derived from the neutron-diffraction experiment, e.g. the symmetric hydrogen position can be reproduced. The multipole-based refinement techniques (MM and TAAM) yield slightly asymmetric positions, whereas the IAM yields a significantly asymmetric position. © 2014, International Union of Crystallography.
Hydrogen Bonding of O-Ethylxanthate Compounds and Neutron Structural Determination of C–H···S Interactions
(CSIRO Publishing, 2014-10-10) Macreadie, LK; Edwards, AJ; Chesman, ASR; Turner, DR
A range of ethylxanthate (EtXn) salts, containing either protic or aprotic cations (guanidinium (1), methylammonium (2), dimethylammonium (3), trimethylammonium (4), tetramethylammonium (5), tetraethylammonium (6), and tetrapropylammonium (7)), have been synthesised and structurally characterised. The cations in these compounds differ in their degree of hydrogen-bonding ability, i.e. the number of donor groups, with significant structural consequences. Compounds 1–4 contain cations that are able to form N–H···S hydrogen bonds, with six, three, two, and one donor groups in 1–4 respectively. The number of donor atoms affects greatly the dimensionality of the hydrogen-bonding networks in the solid state. The structure of 1 has a 3-D hydrogen-bonding network, 2 and 3 form 2-D sheets and 1-D chains respectively, whereas the lone NH donor group in 4 has strong hydrogen bonding only within a discrete cation–anion pair. The tetraalkylammonium salts 5–7 have no strong hydrogen bonding, with only C–H···S and C–H···O interactions possible. To determine unambiguously the presence of such interactions, single-crystal Laue neutron diffraction data were obtained for compound 5, providing a fully anisotropic model, which can be used to rationalise potential close interactions in the other structures. The neutron structure of 5 confirms the existence of C–H···S hydrogen bonds, with the H···S distance falling well within the sum of the van der Waals radii of the atoms. The close-packing in 5–7 is mediated solely through these weak interactions, with the size of the cations influencing the structures. © CSIRO 1996-2020
Insights into the crystallisation process from anhydrous, hydrated and solvated crystal forms of diatrizoic acid
(Wiley, 2014-11-04) Fucke, K; McIntyre, GJ; Lemée-Cailleau, MH; Wilkinson, C; Edwards, AJ; Howard, JAK; Steed, JW
Diatrizoic acid (DTA), a clinically used X-ray contrast agent, crystallises in two hydrated, three anhydrous and nine solvated solid forms, all of which have been characterised by X-ray crystallography. Single-crystal neutron structures of DTA dihydrate and monosodium DTA tetrahydrate have been determined. All of the solid-state structures have been analysed using partial atomic charges and hardness algorithm (PACHA) calculations. Even though in general all DTA crystal forms reveal similar intermolecular interactions, the overall crystal packing differs considerably from form to form. The water of the dihydrate is encapsulated between a pair of host molecules, which calculations reveal to be an extraordinarily stable motif. DTA presents functionalities that enable hydrogen and halogen bonding, and whilst an extended hydrogen-bonding network is realised in all crystal forms, halogen bonding is not present in the hydrated crystal forms. This is due to the formation of a hydrogen-bonding network based on individual enclosed water squares, which is not amenable to the concomitant formation of halogen bonds. The main interaction in the solvates involves the carboxylic acid, which corroborates the hypothesis that this strong interaction is the last one to be broken during the crystal desolvation and nucleation process.© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Investigation of steric influences on hydrogen-bonding motifs in cyclic rreas by using x-ray, neutron, and computational methods
(Wiley-V C H Verlag GMBH, 2013-11-01) McCormick, LJ; McDonnell-Worth, C; Platts, JA; Edwards, AJ; Turner, DR
A series of urea-derived heterocycles, 5N-substituted hexahydro-1,3,5-triazin-2-ones, has been prepared and their structures have been determined for the first time. This family of compounds only differ in their substituent at the 5-position (which is derived from the corresponding primary amine), that is, methyl (1), ethyl (2), isopropyl (3), tert-butyl (4), benzyl (5), N,N-(diethyl)ethylamine (6), and 2-hydroxyethyl (7). The common heterocyclic core of these molecules is a cyclic urea, which has the potential to form a hydrogen-bonding tape motif that consists of self-associative R-2(2)(8) dimers. The results from X-ray crystallography and, where possible, Laue neutron crystallography show that the hydrogen-bonding motifs that are observed and the planarity of the hydrogen bonds appear to depend on the steric hindrance at the -carbon atom of the Nsubstituent. With the less-hindered substituents, methyl and ethyl, the anticipated tape motif is observed. When additional methyl groups are added onto the -carbon atom, as in the isopropyl and tert-butyl derivatives, a different 2D hydrogen-bonding motif is observed. Despite the bulkiness of the substituents, the benzyl and N,N-(diethyl)ethylamine derivatives have methylene units at the -carbon atom and, therefore, display the tape motif. The introduction of a competing hydrogen-bond donor/acceptor in the 2-hydroxyethyl derivative disrupts the tape motif, with a hydroxy group interrupting the NHOC interactions. The geometry around the hydrogen-bearing nitrogen atoms, whether planar or non-planar, has been confirmed for compounds 2 and 5 by using Laue neutron diffraction and rationalized by using computational methods, thus demonstrating that distortion of O-C-N-H torsion angles occurs to maintain almost-linear hydrogen-bonding interactions. © 2014, Wiley-VCH Verlag GmbH & Co. KGaA
Molecular origins of the high-performance nonlinear optical susceptibility in a phenolic polyene chromophore: electron density distributions, hydrogen bonding, and ab initio calculations
(American Chemical Society, 2013-05-09) Lin, TC; Cole, JM; Higginbotham, AP; Edwards, AJ; Piltz, RO; Pérez-Moreno, J; Seo, JY; Lee, SC; Clays, K; Kwon, OP
The molecular and supramolecular origins of the superior nonlinear optical (NLO) properties observed in the organic phenolic triene material, OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile), are presented. The molecular charge-transfer distribution is topographically mapped, demonstrating that a uniformly delocalized passive electronic medium facilitates the charge-transfer between the phenolic electron donor and the cyano electron acceptors which lie at opposite ends of the molecule. Its ability to act as a "push-pull" pi-conjugated molecule is quantified, relative to similar materials, by supporting empirical calculations; these include bond-length alternation and harmonic-oscillator stabilization energy (HOSE) tests. Such tests, together with frontier molecular orbital considerations, reveal that OH1 can exist readily in its aromatic (neutral) or quinoidal (charge-separated) state, thereby overcoming the "nonlinearity-thermal stability trade-off". The HOSE calculation also reveals a correlation between the quinoidal resonance contribution to the overall structure of OH1 and the UV-vis absorption peak wavelength in the wider family of configurationally locked polyene framework materials. Solid-state tensorial coefficients of the molecular dipole, polarizability, and the first hyperpolarizability for OH1 are derived from the first-, second-, and third-order electronic moments of the experimental charge-density distribution. The overall solid-state molecular dipole moment is compared with those from gas-phase calculations, revealing that crystal field effects are very significant in OH1. The solid-state hyperpolarizability derived from this charge-density study affords good agreement with gas-phase calculations as well as optical measurements based on hyper-Rayleigh scattering (HRS) and electric-field-induced second harmonic (EFISH) generation. This lends support to the further use of charge-density studies to calculate solid-state hyperpolarizability coefficients in other organic NLO materials. Finally, this charge-density study is also employed to provide an advanced classification of hydrogen bonds in OH1, which requires more stringent criteria than those from conventional structure analysis. As a result, only the strongest OH center dot center NC interaction is so classified as a true hydrogen bond. Indeed, it is this electrostatic interaction that influences the molecular charge transfer: the other four, weaker, nonbonded contacts nonetheless affect the crystal packing. Overall, the establishment of these structure?property relationships lays a blueprint for designing further, more NLO efficient, materials in this industrially leading organic family of compounds. © 2013, American Chemical Society.
Neutron diffraction - recent applications to chhemical structure determination
(CSIRO Publishing, 2011-07-19) Edwards, AJ
A brief introduction to the capabilities of the modern Laue neutron diffraction technique is given, and recent examples of successful solutions of structural problems are highlighted.© 2011, CSIRO Publishing
A neutron diffraction study of hydrogen bonding in isostructural potassium and ammonium lanthanoidates
(Royal Society of Chemistry, 2014-10-08) Emerson, AJ; Edwards, AJ; Batten, SR; Turner, DR
Two isostructural series of compounds NH4[Ln(cdm)4(H2O)4]·18c6·3H2O (1Ln) and [K(18c6)(H2O)2][Ln(cdm)4(H2O)4]·H2O (2Ln) have been synthesised and structurally characterised (Ln = Gd, Dy, Er, cdm = C(CN)2(CONH2)−). These two classes of compounds are shown to be essentially isostructural to each other despite the change in counter-cation (ammonium vs. potassium) and the ensuing changes in coordination and hydrogen-bonding of water molecules in the structure. Structural data for 1Dy and 2Dy have been obtained using single crystal Laue neutron diffraction, allowing the precise location of all hydrogen atoms to be determined using fully anisotropic models. Both 1 and 2 contain the anionic complex [Ln(cdm)4(H2O)4]− in which the 8-coordinate lanthanoid is coordinated by a ring of four O-bound cdm ligands supported by inter-ligand N–H⋯O hydrogen bonds. The structures pack with complicated hydrogen-bonding networks in which both coordinated and non-coordinated water molecules are hydrogen-bond donors and all of the nitrile groups of the cdm ligands are hydrogen-bond acceptors. The change in cation between ammonium and potassium affects two water molecule sites which are both coordinated in 2Ln but not in 1Ln; in 1Ln one of these water molecules forms a hydrogen bond with NH4+ whilst the other has neither a coordination interaction nor a hydrogen bond to its oxygen atom. © The Royal Society of Chemistry.
Nitrile groups as hydrogen-bond acceptors in a donor-rich hydrogen-bonding network
(Royal Society of Chemistry, 2012-01-01) Turner, DR; Edwards, AJ; Piltz, RO
A complex hydrogen-bonding network involving the guanidinium cation and the dinitrile anion C(CN)(2)(CONH2)(-) has been studied using Laue neutron diffraction. The role of nitrile groups as hydrogen bond acceptors is highlighted in this donor-rich system combined with a survey of the CSD exploring R-2(1)(6) synthons containing nitrile groups. © 2012, Royal Society of Chemistry
On the temperature dependence of H-Uiso in the riding hydrogen model
(Acta Crystallographica, 2014-07) Lübben, J; Volkmann, C; Grabowsky, S; Edwards, AJ; Morgenroth, W; Fabbiani, FPA; Sheldrick, GM; Dittrich, B
The temperature dependence of H-Uiso in N-acetyl-L-4-hydroxyproline monohydrate is investigated. Imposing a constant temperature-independent multiplier of 1.2 or 1.5 for the riding hydrogen model is found to be inaccurate, and severely underestimates H-Uiso below 100 K. Neutron diffraction data at temperatures of 9, 150, 200 and 250 K provide benchmark results for this study. X-ray diffraction data to high resolution, collected at temperatures of 9, 30, 50, 75, 100, 150, 200 and 250 K (synchrotron and home source), reproduce neutron results only when evaluated by aspherical-atom refinement models, since these take into account bonding and lone-pair electron density; both invariom and Hirshfeld-atom refinement models enable a more precise determination of the magnitude of H-atom displacements than independent-atom model refinements. Experimental efforts are complemented by computing displacement parameters following the TLS+ONIOM approach. A satisfactory agreement between all approaches is found. © International Union of Crystallography
Reduction of a chelating bis(NHC) palladium(II) complex to [{μbis( NHC)}2Pd2H]+: a terminal hydride in a binuclear palladium(I) species formed under catalytically relevant conditions
(Wiley-VCH Verlag Berlin, 2010-08-23) Boyd, PDW; Edwards, AJ; Gardiner, MG; Ho, CC; Lemée-Cailleau, MH; McGuinness, DS; Riapanitra, A; Steed, JW; Stringer, DN; Yates, BF
The first palladium(I) N-heterocyclic carbene complex has been isolated in high yield by the base-assisted reduction of a palladium(II) precursor. The location of the unique terminal hydride (see picture; PdI cyan, H white, N blue) was established by neutron crystal structure determination, and the solution fluxional behavior of the complex was explored. © 2010, Wiley-VCH Verlag Berlin
Short- and long-range modulated oxygen order in brownmillerite-type Sr2FeCoO5 and Ca2FeCoO5
(Asia-Oceania Neutron Scattering Association, 2015-07-23) Auckett, JE; Withers, RL; Studer, AJ; Avdeev, M; Edwards, AJ; McIntyre, GJ; Ling, CD
Sr2FeCoO5 and Ca2FeCoO5 belong to the perovskite-derived group of oxides known as the brownmillerites (A2B2O5), which are characterised by layers of BO6 octahedra alternating with layers of chain-linked BO4 tetrahedra separated by oxygen vacancy channels. Brownmillerites have potential applications as diverse as combustion catalysts, cements, and solid oxide fuel cell cathodes. Oxide-ionic conductivity in brownmillerites is associated with the presence of oxygen vacancies adjacent to the tetrahedral chains, and is therefore influenced by the relative arrangements of oppositely twisted chains throughout the structure. Recently, a variety of modulated chain-ordering schemes have been identified in several brownmillerites that were previously thought to adopt only simple ordered or completely disordered chain arrangements. In this work, a brownmillerite chain-ordering modulation first identified in Sr2FeCoO5 by electron diffraction has been confirmed by single-crystal neutron diffraction on a large crystal grown by the floating-zone (FZ) method. Although incommensurate modulation vectors are identified on the short electron diffraction length scale, the neutron diffraction data show clearly that a commensurate arrangement dominates the crystal on average. This behaviour closely follows that of the series end-member Sr2Fe2O5, an important ionic-conductive brownmillerite, implying that Sr2FeCoO5 may display similarly favourable ionic-conductive properties. We also investigated Ca2FeCoO5, a relatively new brownmillerite characterised in 2010. The presence of well-resolved satellite reflections in single-crystal and powder neutron diffraction data support the choice of a commensurate chain-ordered arrangement to describe the structure. However, attempts to refine this structure against either data set yielded poor results. A detailed examination of neutron and x-ray precession images obtained for FZ-grown single crystals reveal the presence of certain reflections that are forbidden by the expected Pbma symmetry. Consideration of the apparent pseudo-symmetry yields evidence for a micro-intergrowth of regions with slightly different chain-ordering schemes within the dominating Pbma matrix, similar to an arrangement reported previously for Ca2MnGaO5.
Synthesis, structural characterization, and gas-phase unimolecular reactivity of the silver hydride nanocluster [Ag3((PPh2)2CH2)3(μ3-H)](BF4)2
(ACS Publications, 2014-07-03) Zavras, A; Khairallah, GN; Connell, TU; White, JM; Edwards, AJ; Mulder, RJ; O'Hair, RAJ; Donnelly, PS
A bis(diphenylphosphino)methane-ligated trinuclear silver hydride nanocluster, [Ag3((Ph2P)2CH2)3(μ3-H)](BF4)2, featuring three silver(I) ions coordinated to a μ3-hydride, and its deuteride analogue, [Ag3((Ph2P)2CH2)3(μ3-D)](BF4)2, have been isolated and structurally characterized using electrospray ionization mass spectrometry (ESI-MS), X-ray crystallography, NMR and IR spectroscopy. The position of the deuteride in [Ag3((Ph2P)2CH2)3(μ3-D)](BF4)2 was determined by neutron diffraction. ESI-MS of [Ag3L3(μ3-H/D)](BF4)2 [L = ((Ph2P)2CH2)2] produces [Ag3L3(μ3-H/D)]2+ and [Ag3L3(μ3-H/D)(BF4)]+. A rich gas-phase ion chemistry of [Ag3L3(μ3-H/D)]2+ is observed under conditions of collision-induced dissociation (CID) and electron-capture dissociation (ECD). CID gives rise to the following complementary ion pairs: [Ag3L2]+ and [L+(H/D)]+; [Ag2(H/D)L2]+ and [AgL]+; [Ag2(H/D)L]+ and [AgL2]+. ECD gives rise to a number of dissociation channels including loss of the bis(phosphine) ligand, fragmentation of a coordinated bis(phosphine) ligand via C–P bond activation, and loss of a hydrogen (deuterium) atom with concomitant formation of [Ag3L3]+. Under CID conditions, [Ag3L3(μ3-H/D)(BF4)]+ fragments via ligand loss, the combined loss of a ligand and [H,B,F4], and cluster fragmentation to give [Ag2(BF4)L2]+ and [Ag2(L-H)L]+ [where (L-H) = (Ph2P)2CH–]. © 2014, American Chemical Society
Synthesis, structure and gas-phase reactivity of a silver hydride complex ag-3{(PPh2)(2)CH2}(3)(mu(3)-H)(mu(3)-Cl) BF4
(John Wiley and Sons, 2013-08-05) Zavras, A; Khairallah, GN; Connell, TU; White, JM; Edwards, AJ; Donnelly, PS; O'Hair, RAJ
Mass spectrometry shows the way! MS analysis of silver salts treated with sodium borohydride in the presence of a bis(phosphino) ligand revealed the formation of novel silver hydride nanocluster cations instead of all silver nanocluster cations. This serendipitous discovery prompted the condensed-phase synthesis, isolation, and characterization of [Ag3{(Ph2P)2CH2}3(μ3-H)(μ3-Cl)]BF4⋅0.5 CHCl3. © 2013, Wiley‐Vch Verlag.

Browsing by Author "Edwards, AJ"

Results Per Page

Sort Options