Band 239, Heft 11-12

Well-shaped single crystals of Ta 11 Pt 4 formed through crucible attack during phase analytical studies in the Ca–Pt–Zn system. Phase-pure Ta 11 Pt 4 was subsequently obtained from the pure elements by arc-melting. The crystal structure of Ta 11 Pt 4 was refined from single crystal X-ray diffractometer data: P 4 2 / mnm , a  = 992.94(8), c  = 516.59(4) pm, wR 2 = 0.0672, 576 F 2 values and 27 variables. Ta 11 Pt 4 is a new coloring variant in the large family of sigma phase compounds. Of the five crystallographically independent sites, four are occupied by tantalum and the fifth one by platinum in a fully ordered manner. The platinum atoms have distorted icosahedral coordination by 11 tantalum and 1 platinum atom. Two of these Ta@Ta 11 Pt icosahedra are condensed by interpenetration forming a Pt 2 pair with 275 pm Pt–Pt distance. These double-icosahedra are the monomeric building unit of the Ta 11 Pt 4 structure and further condense via common edges to a three-dimensional dense sphere packing. The Ta–Ta distances cover a broader range from 256 to 329 pm, comparable to the structure of bcc tantalum. Temperature dependent magnetic susceptibility studies indicate Pauli paramagnetism.

The crystal structure of a new polymorph of metaboric acid, δ- HBO 2 (s.g. Pnma , a  = 14.7578(9), b  = 9.3973(5), c =  3.6352(2) Å) is described. The structure consists of flat six-membered B 3 O 3 (OH) 3 rings forming chains linked to each other by hydrogen bonds. Also, the stability of δ- HBO 2 phase was investigated.

The intermetallic compounds Eu 4 PdMg and Ca 4 PdMg form a complete solid solution Eu 4– x Ca x PdMg. Further phase analytical studies showed that Eu 4 PdMg does not allow substitution with strontium. The polycrystalline Eu 4– x Ca x PdMg samples were characterized by powder X-ray diffraction. The structure of Eu 2.233(6) Ca 1.767 PdMg was refined from single crystal X-ray diffractometer data: Gd 4 RhIn type, F 4 ‾ $\bar{4}$ 3 m , a = 1,475.42(10) pm, w R 2 = 0.0476, 674  F 2 values, 23 variables. Temperature dependent magnetic susceptibility measurements show a drastic decrease of the Curie temperature with increasing calcium substitution. 151 Eu Mössbauer spectra of Eu 4 PdMg and Eu 2 Ca 2 PdMg confirm the divalent ground state of europium.

The CeCr 2 Al 20 -type aluminides RE Ti 2 Al 20 ( RE = Eu, Gd, Tb, Dy and Yb), space group F d 3 ‾ m $Fd\overline{3}m$ , show the formation of solid solutions RE Ti 2 Al 20− x Ga x with gallium in the ranges 0 ≤ x ≤ 10. Polycrystalline samples were prepared from the elements in alumina crucibles or directly via arc-melting followed by annealing in order to increase crystallinity. The structures of six single crystals were refined from single crystal X-ray diffractometer data for determining the precise occupancy parameters of the aluminum and gallium atoms. The solubility limit of gallium was found at around x ≈ 10. The RE Ti 2 Al 20− x Ga x structures show a MgCu 2 -related substructure of the rare earth and titanium atoms. The Al/Ga atoms then form Frank-Kasper type polyhedra around the RE and Ti atoms, i. e., RE @(Al/Ga) 16 and Ti@(Al/Ga 12 ). These are the basic building units of the RE Ti 2 Al 20− x Ga x structures. Temperature-dependent magnetic susceptibility studies show Pauli paramagnetic behavior for the YbTi 2 Al 10 Ga 10 sample, i. e., a stable divalent ytterbium ground state.

The chiral ligand (R)-sirtinol and its ferric derivative, compound (I), have been used in a wide range of chemical and biological studies which should be sensitive to their chirality inasmuch as enantioselectivity is of primordial importance in such processes. It is, therefore, imperative that the chiral nature of the iron-containing molecule should be recognized; and, if the metal is chiral, the expectation is that its function(s) may be stereospecific. Surprisingly enough, until now little or nothing has been elucidated concerning its stereochemical character, probably because the chiral nature of a seven-fold coordinated environment is often overlooked and not commonly encountered in 3d compounds. Below, we address these issues because (I) crystallizes with Z ′ = 2.0; thus, the chiral nature of its two diastereoisomers must be clearly understood in order to properly interpret the results of future chemical and biochemical investigations. We found that there is a novel form of crystallization in (I) characterized by: (1) there are two distinct diastereoisomeric species in the asymmetric unit differing in the magnitude and/or sign of relevant torsional angles; (2) the sirtinol ligand itself bears an (R) carbon in both, as expected from Z ′ = 2.0. Thus, if the chirality of the ligand carbon is momentarily ignored , the crystals are simple examples of a kryptoracemate. If not, we have discovered a new crystalline form for which we propose a suitable nomenclature as this is now needed. In analogy to the classical example of “racemic mimics” recognized a long time ago, we name it kryptoracemic mimic. In essence, there are two different conditions to consider: the kryptoracemic character of the iron (in this case), and the pure enantiomeric character of the sirtinol ligand, and they must be properly described in the name of such species.