Uwe Ruschewitz

Binary and ternary carbides of alkali and alkaline-earth metals

Abstract The review surveys the preparation methods, crystallochemistry and physical properties (i.e. mainly spectroscopic properties) of binary and ternary carbides of Group I or Group II metals. Due to the low electronegativity of alkali and alkaline-earth metals the bonding in their binary carbides can be assumed to be ionic with carbon forming the anion. Therefore, the first part of this overview will be subdivided into chapters according to the type of the carbon anion, i.e. isolated C anions, C2 and C3 dumbbells. For the ternary carbides of alkali and alkaline-earth metals, which include an additional metal of other groups, the ionic description is not valid in all cases, as these compounds display interesting properties on the borderline between ionic and metallic behaviour.

Lanthanide coordination polymers with tetrafluoroterephthalate as a bridging ligand: thermal and optical properties.

By slow diffusion of triethylamine into a solution of 2,3,5,6-tetrafluoroterephthalic acid (H2tfBDC) and the respective lanthanide salt in EtOH/DMF single crystals of seven nonporous coordination polymers, (∞)(2)[Ln(tfBDC)(NO(3))(DMF)(2)]·DMF (Ln(3+) = Ce, Pr, Nd, Sm, Dy, Er, Yb; C2/c, Z = 8) have been obtained. In the crystal structures, two-dimensional square grids are found, which are composed of binuclear lanthanide nodes connected by tfBDC(2-) as a linking ligand. The coordination sphere of each lanthanide cation is completed by a nitrate anion and two DMF molecules (CN = 9). This crystal structure is unprecedented in the crystal chemistry of coordination polymers based on nonfluorinated terephthalate (BDC(2-)) as a bridging ligand; as for tfBDC(2-), a nonplanar conformation of the ligand is energetically more favorable, whereas for BDC(2-), a planar conformation is preferred. Differential thermal analysis/thermogravimetric analysis (DTA/TGA) investigations reveal that the noncoordinating DMF molecule is released first at temperatures of 100-200 °C. Subsequent endothermal weight losses correspond to the release of the coordinating DMF molecules. Between 350 and 400 °C, a strong exothermal weight loss is found, which is probably due to a decomposition of the tfBDC(2-) ligand. The residues could not be identified. The emission spectra of the (∞)(2)[Ln(tfBDC)(NO(3))(DMF)(2)]·DMF compounds reveal intense emission in the visible region of light for Pr, Sm, and Dy with colors from orange, orange-red, to warm white.

Metal–Organic Frameworks as Hosts for Photochromic Guest Molecules

Several metal-organic framework compounds (MOF-5, MIL-68(Ga), MIL-68(In), MIL-53(Al)) were loaded with azobenzene (AZB), as confirmed by XRPD measurements and elemental analysis. By IR spectroscopy, it was shown that the light-induced trans/cis isomerization of AZB in these hybrid host-guest compounds is improved compared to that of solid AZB. A population of the excited cis state up to 30% has been obtained for AZB0.66@MIL-68(In). However, no light-induced trans/cis isomerization was observed for AZB0.5@MIL-53(Al). Structural models obtained from high-resolution synchrotron powder diffraction data show that AZB molecules are densely packed within the channels of MIL-53(Al) so that no trans/cis isomerization can occur. A different situation was observed for AZB in the larger channels of MIL-68(Ga). Thus, this investigation shows the influence of the host material on the switching behavior of the embedded AZB molecules.

Structural phase transitions in CaC2.

Pure CaC2, free of CaO impurities, was obtained by the reaction of elemental calcium with graphite at 1,070 K. By means of laboratory X-ray and synchrotron powder diffraction experiments, the phase diagram was investigated in the temperature range from 10 K to 823 K; this confirmed the literature data that reported the partial coexistence of up to four modifications. Aside from a cubic high-temperature modification CaC2 IV (Fm3m, Z = 4) and the well-known tetragonal modification CaC2 I (I4/mmm, Z = 2), a low-temperature modification CaC2 II (C2/c, Z =4) that crystallizes in the ThC2 structure type and a metastable modification CaC2 III (C2/m, Z = 4) that crystallizes in a new structure type were found. It was shown that phase transition temperatures as well as the relative amounts of the various CaC2 modifications depend upon the size of the crystallites, the thermal treatment. and the purity of the sample, as a comparison with technical CaC2 confirmed.Pure CaC2, free of CaO impurities, was obtained by the reaction of elemental calcium with graphite at 1070 K. By means of laboratory X-ray and synchrotron powder diffraction experiments, the phase diagram was investigated in the temperature range from 10 K to 823 K; this confirmed the literature data that reported the partial coexistence of up to four modifications. Aside from a cubic high-temperature modification CaC2 IV (Fmm, Z=4) and the well-known tetragonal modification CaC2 I (I4/mmm, Z=2), a low-temperature modification CaC2 II (C2/c, Z=4) that crystallizes in the ThC2 structure type and a metastable modification CaC2 III (C2/m, Z=4) that crystallizes in a new structure type were found. It was shown that phase transition temperatures as well as the relative amounts of the various CaC2 modifications depend upon the size of the crystallites, the thermal treatment, and the purity of the sample, as a comparison with technical CaC2 confirmed.

On the synthesis and crystal structure of BaC2

Abstract By the reaction of BaCO 3 with graphite in an arc melting furnace single crystals of BaC 2 could be synthesized. Structural investigations confirm the tetragonal structure (space group: I 4/ mmm , Z = 2) given in the literature. In another approach polycrystalline BaC 2 of high purity could be prepared by the reaction of elemental barium and graphite at temperatures of about 1000 K. Temperature dependent X-ray powder investigations on this sample confirm the phase transition to a cubic phase (space group: F m 3 m , Z = 4) at about 520 K (1st order phase transition). At low temperatures a new monoclinic form of BaC 2 (space group: C 2/ c , Z = 4) was found which is isotypic to the crystal structure of ThC 2 at room temperature. This structure was refined from neutron TOF diffraction data at 20 K.

Optimized synthesis of tetrafluoroterephthalic acid: a versatile linking ligand for the construction of new coordination polymers and metal-organic frameworks.

Pure 2,3,5,6-tetrafluoroterephthalic acid (H2tfBDC) is obtained in high yields (95%) by reacting 1,2,4,5-tetrafluorobenzene with a surplus (>2 equiv) of n-butyllithium in tetrahydrofuran (THF) and subsequent carbonation with CO2 without any extensive purification procedure. A single crystal X-ray structure analysis of H2tfBDC (1) confirms former data obtained for a deuterated sample (P1̅, Z = 1). Recrystallization from water/acetone leads to single crystals of H2tfBDC·2H2O (2, P21/c, Z = 2), where an extensive hydrogen bonding network is found. By reacting H2tfBDC with an aqueous ammonia solution, single crystals of (NH4)2tfBDC (3, C2/m, Z = 2) are obtained. 3 is thermally stable up to 250 °C and shows an enhanced solubility in water compared to H2tfBDC. Monosubstituted 2,3,5,6-tetrafluorobenzoic acid (H2tfBC, 4) is obtained by reacting 1,2,4,5-tetrafluorobenzene with stoichiometric amounts (1 equiv) of n-butyllithium in THF. Its crystal structure (Fdd2, Z = 16) shows dimeric units as characteristic structural feature.

Ternary alkali metal transition metal acetylides A2MC2 (A = Na, K; M = Pd, Pt).

Ternary transition metal acetylides A2MC2 (A = Na, K; M = Pd, Pt) can be synthesised by reaction of the respective alkali metal acetylide A2C2 with palladium or platinum in an inert atmosphere at about 350 degrees C. The crystal structures are characterised by (infinity)1[M(C2)(2/2)2-] chains, which are separated by the alkali metals (P3m1, Z = 1). The refinement of neutron powder diffraction data gave C-C = 1.263(3) A for Na2PdC2 (Na2PtC2: 1.289(4) A), which is distinctively longer than the expected value for a C-C triple bond (1.20 A). On the basis of band-structure calculations this can be attributed to a strong back-bonding from the metal into the anti-bonding orbitals of the C2 unit. This was further confirmed by Raman spectroscopic investigations, which showed that the wavenumbers of the C-C stretching vibrations in Na2PdC2 and Na2PtC2 are about 100 cm(-1) smaller than in acetylene. 13C MAS-NMR spectra demonstrated that the acetylenic C2 units in the title compounds are very different from those in acetylene. Electrical conductivity measurements and band-structure calculations showed that the black title compounds are semiconductors with a small indirect band gap (approximately 0.2 eV).

Alkali metal copper acetylides ACuC2 (A = Na-Cs): synthesis, crystal structures and spectroscopic properties

Abstract By reaction of CuI and A2C2 (A = K, Rb, Cs) suspended in liquid ammonia and subsequent heating of the remaining residue in vacuum ternary alkali metal copper acetylides ACuC2 were accessible. NaCuC2 could be obtained by decomposing NaCu5C6, which was synthesized from NaC2H and CuI in liquid ammonia. The crystal structures were determined by both X-ray and neutron powder diffraction. In all compounds 1 ∞ [ Cu ( C 2 ) 2/2 − ] chains are the characteristic structural motif. In NaCuC2 and β-RbCuC2 these chains are orientated parallel to the c axis of a tetragonal unit cell (KAgC2 type, P4/mmm, Z=1), whereas in KCuC2, α-RbCuC2 and CsCuC2 these chains are arranged in layers perpendicular to the c axis of a tetragonal unit cell (CsAgC2 type, P42/mmc, Z=2). These layers are staggered along the c axis by rotating them by 90° to each other. The alkali metal ions separate the copper carbon chains. Raman spectroscopic investigations indicate the existence of CC triple bonds, as the frequencies of the CC stretching vibration are comparable to those found for acetylene and ternary silver and gold acetylides. In the 13C MAS NMR spectra of KCuC2, RbCuC2 and CsCuC2 the isotropic signals are complicatedly split due to the coupling to the nearby quadrupolar copper nuclei, but the chemical shifts are in the range found for other acetylides with CC triple bonds.

Zur Kristallstruktur von Rb2C2 und Cs2C2

Durch Reaktion von in flussigem Ammoniak gelostem Rubidium bzw. Caesium mit Acetylen konnte AC2H mit A = Rb, Cs erhalten werden, das sich dann mit einem Uberschus des entsprechenden Alkalimetalls bei 520 K (Rb2C2) bzw. 470 K (Cs2C2) im Vakuum in das binare Acetylid A2C2 umwandeln lies. Die Kristallstrukturen der auserst luftempfindlichen Produkte konnten mit einer Kombination von Neutronen- und Rontgenbeugungsuntersuchungen an pulverformigen Praparaten gelost und verfeinert werden. Rb2C2 als auch Cs2C2 liegen in zwei Modifikationen vor, die miteinander koexistieren konnen. Die hexagonale Modifikation (P 6 2m, Z = 3) kristallisiert im bekannten Na2O2-Typ mit zwei kristallographisch unabhangigen Lagen fur die C22–-Hanteln. Fur die orthorhombische Modifikation (Pnma, Z = 4) wurde hingegen ein neuer Strukturtyp gefunden, der mit dem PbCl2-Typ verwandt ist, wobei die Pb-Lagen durch geordnete C22–-Einheiten ersetzt sind. Temperaturabhangige Untersuchungen zwischen 4 K und der Zersetzungstemperatur mit Hilfe der Neutronen- und der Rontgenbeugung ergaben ein sehr komplexes Bewegungsverhalten der C2-Hanteln, das noch nicht vollstandig verstanden ist. Die mit Hilfe der Ramanspektroskopie bestimmten Frequenzen der C–C-Streckschwingung passen gut in der Verlauf der bisher untersuchten Alkalimetallacetylide und Alkalimetallhydrogenacetylide, wobei die Elektronegativitat des Alkalimetalls einen grosen Einflus auf die Lage der C–C-Streckschwingungsfrequenz zu haben scheint. On the Crystal Structure of Rb2C2 and Cs2C2 By reaction of rubidium or caesium solved in liquid ammonia with acetylene AC2H with A = Rb, Cs was obtained, which was subsequently converted into the binary acetylide A2C2 in vacuum at temperatures of 520 K (Rb2C2) and 470 K (Cs2C2) using a surplus of the respective alkali metal. The crystal structures of the very air sensitive compounds were solved and refined by a combination of both neutron and X-ray powder diffraction data. Rb2C2 as well as Cs2C2 coexist in two modifications. The hexagonal modification (P 6 2m, Z = 3) crystallises in the known Na2O2 structure type with two crystallographic independent sites for the C22– dumbbells. For the orthorhombic modification (Pnma, Z = 4) a new structure type was found, which is related to the PbCl2 structure type with ordered C22– dumbbells occupying the Pb sites. Temperature dependent investigations between 4 K and the decomposition temperature by the means of neutron and X-ray powder diffraction resulted in a very complex dynamic disorder of the C2 dumbbells, which is still not completely understood. The frequencies of the C–C stretching vibration determined by the help of Raman spectroscopy fit nicely to the results obtained for other alkali metal acetylides and alkali metal hydrogen acetylides. These results seem to indicate that the electronegativity of the alkali metal has a strong influence on the frequency of the C–C stretching vibration.

Novel Ternary Alkali Metal Silver Acetylides MIAgC2 (MI=Li, Na, K, Rb, Cs)

Three different packings of (1)(infinity)[Ag(C(2))(2/2)(-)] chains (represented by rods in the picture) have been found in the crystal structures of the first ternary alkali metal silver acetylides, which were obtained by the reaction of M(I)C(2)H (M(I)=Li-Cs) with AgI in liquid ammonia and subsequent heating of the remaining residue to 120 degrees C.