Linear Metal Chains in Ca2M2X (M = Pd, Pt; X = Al, Ge): Origin of the Pairwise Distortion and Its Role in the Structure Stability

A series of four new analogue phases Ca2M2X (M = Pd, Pt and X = Al, Ge) were prepared by direct combination of the respective elements in stoichiometric mixtures at high temperature in order to analyze the impact of valence electron count (vec) and electronegativity differences (Δχ) on the structure selection and stability. Their crystal structures, as determined from single-crystal X-ray diffraction data, correspond to two different but closely related structure types. The first compound, Ca2Pd2Ge (I), is an unprecedented ternary ordered variant of the Zr2Al3-type (orthorhombic, Fdd2). The three other phases, Ca2Pt2Ge (II), Ca2Pd2Al (III) and Ca2Pt2Al (IV), adopt the Gd2Ge2Al-type structure (monoclinic, C2/c). All title structures feature linear chains of the noble metals (Pd or Pt). The Pd linear chains in I are undistorted with equidistant Pd···Pd atoms, whereas the metal chains in II–IV are pairwise distorted, resulting in short connected {Pd2} or {Pt2} dumbbells that are separated by longer M···M contacts. The occurrence and magnitude of the pairing distortion in these chains are controlled by the vec and the Δχ between the constituent elements, a result which is supported by analysis of the calculated Bader effective charges. The metal chains act as charge modulation units, critical for the stability and the electronic flexibility of the structures by an adequate adjustment of the metal–metal bond order to both the vec and the degree of charge transfer. Thus, Ca2Pd2Ge (28 ve/f.u) is a Zintl-like, charge optimized phase with formally zerovalent Pd atoms forming the undistorted metal chains; semimetallic properties are predicted by TB-LMTO calculations. In contrast, the isoelectronic Ca2Pt2Ge is predicted to be a good metal with the Fermi level located at a local maximum of the DOS, a fingerprint of potential electronic instability. This is due to greater charge transfer to the more electronegative Pt atoms forming the metal chains and probably to packing frustration in the well packed structure that may prevent a larger distortion of the Pt chains. However, the instability is suppressed in the aliovalent but isostructural phases Ca2M2Al (27 ve/f.u) with an enhancement of the pairing distortion within the metal chains but lower M–M bond order. Further reduction of the vec as in Ca2M2Cd (26 ve/f.u) may induce a transition toward the more geometrically flexible W2CoB2-type with a low dimensional structure, to create more room for a larger distortion of the metal chain as dictated by the shortage of valence electrons

A pyrazine amide – 4-aminoquinoline hybrid and its rhodium and iridium pentamethylcyclopentadienyl complexes; evaluation of anti-mycobacterial and anti-plasmodial activities

The synthesis and characterization of N-(2-((7-chloroquinolin-4-yl)amino)ethyl)pyrazine-2-carboxamide (L), an aminoquinoline – pyrazinamide hybrid, and the complexes (N-(2-((7-chloroquinolin-4-yl)amino)ethyl)pyrazine-2-carboxamide)(cyclopentadienyl) chlorido-rhodium or iridium hexafluorophosphate ([M(L)(Cp*)Cl] PF6; M = Rh, Ir) and the corresponding chlorido salts ([M(L)(Cp*) Cl]Cl; M = Rh, Ir) are described. The ligand and the hexafluorophosphate salts of the metal complexes have been evaluated for anti-plasmodial and anti-mycobacterial activity. The rhodium and the iridium complexes were significantly more active against M. tuberculosis than the free ligand. The crystallographically determined molecular structures of complexes (N-(2-((7-chloroquinolin-4-yl)amino)ethyl)pyrazine-2-carboxamide)(cyclopentadienyl)chlororhodium hexafluoro-phosphate and (N-(2-((7-chloroquinolin-4-yl)amino)ethyl)pyrazine-2-carboxamide)(cyclopentadienyl)chloro-iridium chloride are presented

Ca-Pd/Pt-Ge Compounds on the Zintl Border

Zintl compounds form a category in intermetallic chemistry somewhere between metallic and ionic compounds. Classical Zintl compounds form between electropositive and electronegative elements, such as the alkaline earth metals and the p-block elements, i.e. elements with a relatively large electronegativity difference. The concept is based on the idea of a complete charge transfer from the electropositive to the electronegative element, leaving the electropositive element with a complete valence shell. The electronegative element will behave like its isoelectronic counterpart and complete its valence shell by forming a covalent anionic network, if necessary. Due to this charge separation, Zintl compounds are semiconductors. This classification is a useful tool for predicting and describing a structure, but what happens when a third element with similar electronegativity as the p-block element is added? The work covered in this thesis is based on how well the structures formed in the Ca-Pd/Pt-Ge system can be rationalised by the Zintl concept. The system has shown a large structural variability ranging from Laves phases to Zintl-like cluster compounds. What they all have in common is the network formation between the transition metal and Ge, and the compounds are all more metallic rather than semiconducting. Despite this metallicity there is a tendency toward a Zintl-like behaviour. That is, when there are more electrons available for palladium, platinum and germanium, the tendency to form extended anionic network becomes less

Transferring Grains from Single-Grain Luminescence Discs to SEM Specimen Stubs

The grain transfer protocol presents a step-by-step guide on how to successfully transfer positioned grains from a single-grain luminescence disc to a scanning electron microscope (SEM) specimen stub and how to transport them between laboratories. Single-grain luminescence analysis allows the determination of luminescence characteristics for individual sand-sized grains.By combining such luminescence data with other grain properties such as geochemical composition, shape, or structure also at single-grain level, it is possible to investigate factors controlling luminescence signals or study other material properties. The non-luminescence properties are typically measuredin another instrument; thus, grains need to be transferred between machines and sample holders, and sometimes also between laboratories. It is then important that the position of each grain is known and stable so that the properties from the same grain are compared. By providing an easily observableorientation marker on the specimen stub, the hundred numbered grains from the single-grain disc can be transferred and later identified when analyzed in the SEM

Transferring grains from single-grain luminescence discs to SEM specimen stubs

The grain transfer protocol presents a step-by-step guide on how to successfully transfer positioned grains from a single-grain luminescence disc to a scanning electron microscope (SEM) specimen stub and how to transport them between laboratories. Single-grain luminescence analysis allows the determination of luminescence characteristics for individual sand-sized grains. By combining such luminescence data with other grain properties such as geochemical composition, shape, or structure also at single-grain level, it is possible to investigate factors controlling luminescence signals or study other material properties. The non-luminescence properties are typically measured in another instrument; thus, grains need to be transferred between machines and sample holders, and sometimes also between laboratories. It is then important that the position of each grain is known and stable so that the properties from the same grain are compared. By providing an easily observable orientation marker on the specimen stub, the hundred numbered grains from the single-grain disc can be transferred and later identified when analyzed in the SEM

Ca2Pd3Ge, a new fully ordered ternary Laves phase structure

The title compound, Ca2Pd3Ge, was prepared as a part of a systematic investigation of the Ca-Pd-Ge ternary phase diagram. The structure was determined and refined from single-crystal X-ray diffraction data. It is a new fully ordered ternary Laves phase with the space group R-3m, Z=3, a=5.6191 (5) angstrom, c=12.1674 (7) angstrom. wR(2)=0.054 (all data) and is isostructural to Mg2Ni3Si (Noreus et al., 1985 [17]) but due to the larger size of all elements in Ca2Pd3Ge, the cell axes are approximately 10% longer. The compound may formally be considered as a Zintl compound, with [Pd3Ge](4-) forming a poly-anionic network and divalent Ca cations located in truncated tetrahedral interstices. The electronic structure and chemical bonding of Ca2Pd3Ge is discussed in terms of LMTO band structure calculations and compared with CaPd2 (MgCu2-type). (C) 2012 Elsevier Inc. All rights reserved

Ca(10)Pt(7)Tt(3) (Tt = Si, Ge): New Platinide Phases Featuring Electron-Rich 4c-6e Bonded [Pt(7)Tt(3)](20-) Intermetalloid Clusters.

Two new phases Ca(10)Pt(7)Tt(3) (with Tt = Si, Ge) were obtained by reacting stoichiometric mixtures of the elements at high temperature. Their structures were refined from single crystal X-ray diffraction data. They are isostructural and crystallize in the Ba(10)Al(3)Ge(7) type structure, space group P6(3)/mcm (No. 193) with a = b = 8.7735(3) Å, c = 13.8260(5) Å, V = 921.66(6) Å(3), Z = 2 for Tt = Si, and a = b = 8.7995(6) Å, c = 13.9217(14) Å, V = 933.56(16) Å(3) for Tt = Ge phase. The most interesting structural features in these phases are the propeller shape {Pt(7)Tt(3)} (Tt = Si, Ge) intermetalloid clusters in a D(3h) local symmetry. LMTO electronic structure calculations and COHP analyses reveal that both Ca(10)Pt(7)Tt(3) (Tt = Si, Ge) phases are charge optimized, which is not predicted by the classical Zintl concept and the octet or Wade-Mingo's rules, but rather by a more complex bonding model based on the unprecedented electron-rich 4c-6e multicenter bonding. The clusters are best described as three-condensed trigonal planar [TtPt(3)](8-) units, resulting in a central Pt atom also with a trigonal planar coordination of three symmetrical equivalent Si/Ge atoms that are further connected to two terminal Pt atoms each. The "trefoil" electron-rich multicenter bonding is proposed here for the first time, and may be viewed as a unique bonding feature with potential relevance for the catalytic properties of the noble metal platinum

Synthesis, Crystal Structure, and Bonding Analysis of the Hypoelectronic Cubic Phase Ca5Pd6Ge6.

The title compound, Ca5Pd6Ge6, was obtained during a systematic investigation of the Ca-Pd-Ge ternary phase diagram. The crystal structure was determined and refined from single-crystal X-ray diffraction data. It crystallizes in a new structure variant of the Y4PdGa12-type structure (Im3̅m, a = 8.7764(4) Å) that features an arrangement of vertex-sharing body-centered cubes of calcium, Ca@Ca8, with a hierarchical bcc network, interpenetrating a second (Pd6Ge6) network consisting of Ge2 dumbbells surrounded by Pd in a strongly flattened octahedron with Pd(μ(2)-η(2),η(4)-Ge2)-like motifs. These octahedra are condensed through the Pd to form a 3D open fcc network. Theoretical band structure calculations suggested that the compound is hypoelectronic with predominantly multicenter-type interatomic interactions involving all three elements and essentially a Hume-Rothery-like regime of electronic stabilization. The similar electronegativity between germanium and palladium atoms has a decisive impact on the bonding picture of the system

Bonding Schemes for Polar Intermetallics through Molecular Orbital Models: Ca-Supported Pt–Pt Bonds in Ca10Pt7Si3

crystal

Structure and Bonding of an Intergrowth Phase Ca7Ag2+xGe7-x (x approximate to 2/3) Featuring a Zintl-Type Polyanionic Chain

Single crystals of a new ternary phase, Ca7Ag2+xGe7-x [x = 0.48(3)], were obtained from as side-product of high-temperature solid-state reactions, and its crystal structure determined by X-ray diffraction methods. Following the Zintl concept, the anionic substructure consists of a novel pentameric Zintl anion [Ge-5](12-) with C-2v local symmetry, as well as [AgxGe2-x]((6-3x)-) units accounting for [Ge-2](6-) dimers and isolated Ge4- (and Ag+) species sharing the same atomic sites. DFT-level band structure calculations were carried out on a hypothetical, fully ordered model (x = 0). We found that the electronic structure associated with the planar W-shaped [Ge-5](12-) polyanions is more suited to optimize the structure's stability than the helical configuration of the isoelectronic [Se-5](2-), in the context of incomplete charge transfer from the electropositive metal Ca. Thus, the antibonding states at the Fermi level that are centered on the two [Ge-n]((2n+2)-) oligomers can be depopulated by means of Ge-to-Ca electron back donation, strengthening at the same time the Ge-Ge bonds. These antibonding states also endow the system with substantial electronic flexibility, which may result in some phase width. Finally, plausible local ordering models of Ag/Ge mixing in [AgxGe2-x]((6-3x)-) units for x = 2/3, as expected from the Zintl concept, are briefly discussed within the coloring problem approach