Kathrin Hofmann

Probing for Structural Features of Boron-rich Solids with EELS†

Crystal structures of boron-rich solids are characterized by boron atom arrangements that are quite diverse: chains, sheets, and a variety of polyhedra like octahedra, pentagonal bipyramids, cuboctahedra, and icosahedra are observed. Probing by electron energy-loss spectroscopy (EELS), these different structural features are mirrored by a pronounced variation of the energy loss near-edge fine structure (ELNES) of the BK ionization edges. For identification, characteristics of these fine structures can be used as so-called “coordination fingerprints”, which is shown for solids like MgB2, TaB2, ZrB2, CaB6, SrB6, BaB6, NaB5C, KB5C, Na3B20, Na2B29, UB12, ZrB12, LaB2C2, CeB2C2, and CaB2C2. In addition, theoretical calculations of ELNES based on the density functional theory (FLAPW method) are presented for an example of boron-rich solids. Bestimmung struktureller Charakteristika in borreichen Festkorpern durch Elektronenenergieverlustspektroskopie Die Kristallstrukturen von borreichen Festkorpern zeigen stark variierende Boratom-Anordnungen: Man findet Ketten, Schichten und eine Vielzahl verschiedener Polyeder wie Oktaeder, pentagonale Bipyramiden, Kuboktaeder und Ikosaeder. Mittels der Elektronenenergieverlustspektroskopie (EELS) konnen diese strukturellen Charakteristika abgebildet werden, denn die Nahkanten-Feinstruktur (ELNES) der BK-Ionisierungskanten variiert signifikant mit der Boratom-Anordnung. Fur Verbindungen wie MgB2, TaB2, ZrB2, CaB6, SrB6, BaB6, NaB5C, KB5C, Na3B20, Na2B29, UB12, ZrB12, LaB2C2, CeB2C2 und CaB2C2 wird gezeigt, das die unterschiedlichen Feinstrukturen als “Fingerabdruck” der Koordination der Boratome und damit der Identifizierung von Verbindungen dienen konnen. Zusatzlich werden fur ein Beispiel borreicher Festkorper theoretische Berechnungen der ELNES, die auf der Dichtefunktionaltheorie basieren (FLAPW-Methode), vorgestellt.

A new 4c–2e bond in B6H7−

For N(C(4)H(9))(4)B(6)H(7) a proton is shown to be localised above one of the faces of the distorted octahedron, and rhomboid rings are formed as shown by topological analysis of charge densities.

Synthesis, Characterization, and Crystal Structures of [N(CH3)4]2[B12H12] and [N(CH3)4]2[B12H12] · CH3CN

Bis(tetramethylammonium) dodecahydrododecaborate, [(CH3)4N]2[B12H12], and bis(tetramethylammonium) dodecahydrododecaborate acetonitrile, [(CH3)4N]2[B12H12] · CH3CN, were synthesized and characterized via Infrared, 1H and 11B NMR spectroscopy. [(CH3)4N]2[B12H12] crystallizes isopunctual to the alkali metal dodecaborates. The crystal structure of [(CH3)4N]2[B12H12] · CH3CN was determined from single crystal data and refined in the orthorhombic crystal system (Pcmn, no. 62, a = 898.68(8), b = 1312.85(9) c = 1994.5(1) pm, R(|F| , 4σ) = 5.9%, wR(F2) = 18.3%). Here, the geometry of the dodecaborate anion is that of an almost ideal icosahedron, less distorted than most other dodecaborates known. By low-temperature Guinier-Simon diffractometry phase transitions were detected for [(CH3)4N]2[B12H12] and [(CH3)4N]2[B12H12] · CH3CN at –70 and –15 °C, respectively. Synthese, Charakterisierung und Kristallstrukturbestimmung von [N(CH3)4]2[B12H12] und [N(CH3)4]2[B12H12] · CH3CN Bis(tetramethylammonium)dodecahydrododecaborat, [(CH3)4N]2[B12H12], und Bis(tetramethylammonium)dodecahydrododecaborat Acetonitril, [(CH3)4N]2 [B12H12] · CH3CN, wurden mittels Infrarotspektroskopie, 1H- und 11B-NMR-Spektroskopie charakterisiert. [(CH3)4N]2- [B12H12] kristallisiert isopunktual zu den Alkalimetalldodecaboraten. Die Kristallstruktur von [(CH3)4N]2[B12H12] · CH3CN wurde mittels Einkristallrontgenstrukturanalyse bestimmt und im orthorhombischen Kristallsystem verfeinert (Pcmn, Nr. 62, a = 898,68(8), b = 1312,85(9), c = 1994,5(1) pm, R(|F| , 4σ) = 5,9%, wR(F2) = 18,3%). In dieser Verbindung ist die Geometrie des Dodecaborat-Anions die eines nahezu idealen Ikosaeders, also weniger verzerrt als in anderen Dodecaboraten. Mittels Tieftemperatur-Guinier-SimonDiffraktometrie wurden Phasenumwandlungen fur [(CH3)4N]2- [B12H12] und [(CH3)4N]2[B12H12](CH3CN bei –70 bzw. bei –15 °C detektiert.

Selection of peptides binding to metallic borides by screening M13 phage display libraries

BackgroundMetal borides are a class of inorganic solids that is much less known and investigated than for example metal oxides or intermetallics. At the same time it is a highly versatile and interesting class of compounds in terms of physical and chemical properties, like semiconductivity, ferromagnetism, or catalytic activity. This makes these substances attractive for the generation of new materials. Very little is known about the interaction between organic materials and borides. To generate nanostructured and composite materials which consist of metal borides and organic modifiers it is necessary to develop new synthetic strategies. Phage peptide display libraries are commonly used to select peptides that bind specifically to metals, metal oxides, and semiconductors. Further, these binding peptides can serve as templates to control the nucleation and growth of inorganic nanoparticles. Additionally, the combination of two different binding motifs into a single bifunctional phage could be useful for the generation of new composite materials.ResultsIn this study, we have identified a unique set of sequences that bind to amorphous and crystalline nickel boride (Ni3B) nanoparticles, from a random peptide library using the phage display technique. Using this technique, strong binders were identified that are selective for nickel boride. Sequence analysis of the peptides revealed that the sequences exhibit similar, yet subtle different patterns of amino acid usage. Although a predominant binding motif was not observed, certain charged amino acids emerged as essential in specific binding to both substrates. The 7-mer peptide sequence LGFREKE, isolated on amorphous Ni3B emerged as the best binder for both substrates. Fluorescence microscopy and atomic force microscopy confirmed the specific binding affinity of LGFREKE expressing phage to amorphous and crystalline Ni3B nanoparticles.ConclusionsThis study is, to our knowledge, the first to identify peptides that bind specifically to amorphous and to crystalline Ni3B nanoparticles. We think that the identified strong binding sequences described here could potentially serve for the utilisation of M13 phage as a viable alternative to other methods to create tailor-made boride composite materials or new catalytic surfaces by a biologically driven nano-assembly synthesis and structuring.

The η6,η1-Coordination of Beryllium Atoms in the Graphite Analogue BeB2C2

The structure of BeB2C2 (see picture; Be white, B dark gray, C light gray) has been solved using near-edge fine-structure analysis of electron energy loss spectra and high-resolution powder diffractometry. It exhibits the unusual features of graphite-analogous B/C layers and a nonsymmetrical coordination of the beryllium atoms. The latter reveals a striking parallelism of the molecular and solid-state chemistries of Be compounds.

Metastable Ni7B3: A New Paramagnetic Boride from Solution Chemistry, Its Crystal Structure and Magnetic Properties

We trapped an unknown metastable boride by applying low-temperature solution synthesis. Single-phase nickel boride, Ni7B3, was obtained as bulk samples of microcrystalline powders when annealing the amorphous, nanoscale precipitate that is formed in aqueous solution of nickel chloride upon reaction with sodium tetrahydridoborate. Its crystal structure was solved based on a disordered Th7Fe3-type model (hexagonal crystal system, space group P63mc, no. 186, a = 696.836(4) pm, c = 439.402(4) pm), using synchrotron X-ray powder data. Magnetic measurements reveal paramagnetism, which is in accordance with quantum chemical calculations. According to high-temperature X-ray diffraction and differential scanning calorimetry this nickel boride phase has a narrow stability window between 300 and 424 °C. It crystallizes at ca. 350 °C, then starts decomposing to form Ni3B and Ni2B above 375 °C, and shows an exothermic effect at 424 °C.

Crystal structures of M2[B10H10] (M = Na, K, Rb) via real-space simulated annealing powder techniques

Abstract Alkali metal decahydro-closo-decaborates M2[B10H10] (M = Na, Rb, Cs) were synthesised by ion-exchange and characterised via infrared and 11B-NMR spectroscopy. The crystal structures of Na2[B10H10], K2[B10H10], and Rb2[B10H10] were determined from X-ray powder diffraction data using real-space techniques. The compounds crystallise in the monoclinic space group P21/n (no. 14) with the lattice constants: Na2[B10H10]: a = 1028.28(6) pm, b = 1302.18(5) pm, c = 667.34(3) pm, β = 93.754(3)°; K2[B10H10]: a = 1285.54(8) pm, b = 1117.84(7) pm, c = 682.27(4) pm, β = 93.357(3)°; Rb2[B10H10]: a = 1320.04(7) pm, b = 1136.88(6) pm, c = 704.23(4) pm, β = 94.158(3)°. The hydroborate anions are arranged almost hexa gonally with alkali metal cat ions in the tetrahedral interstices.

Analyzing ammonia bridges--and more about bonding in boron-rich solids.

Three types of boron-rich compounds in unusual bonding situations are described: First, salts that contain closo-hydroborate anions and exhibit hydrogen and dihydrogen bonds and a strong ammonia network; second, boron-rich metal borides with an unexpected metal-metal bond stabilized by Peierls distortion; and third, nanoscale metal borides that bind selectively to certain heptapeptides identified by the phage display technique.

Crystal Structure of Bis(triethylammonium)closo-decahydrodecaborate, [(C2H5)3NH]2[B10H10]

The crystal structure of bis(triethylammonium)closo-decahydrodecaborate [bis(triethylammonium) decaboranate(10)], [(C2H5)3NH]2[B10H10], was determined and refined (space group Pmmn, no. 59, a = 989.7, b = 1333.7, c = 903.7 pm). The compound is a versatile starting material for many substances containing the [BioHio]2- entity and its derivatives. The closo-[B10H10]2- cluster is a bicapped square antiprism which is only slightly distorted. Its deviation from D4d symmetry is smaller than that of the B10 cages in every other compound containing this entity that have been structurally characterised. The presence of additional (N )H ---B3 interactions in form of multiple-centre bonds between the cations and the anions, which were postulated earlier and which should influence the cage symmetry, could not be confirmed. At 55 °C, the transition into a high temperature phase was investigated by X-ray powder diffraction. The high temperature phase crystallises in the tetragonal crystal system (a = 946.9, c = 1351.0 pm).