Steven W. Keller

Turning down the heat: design and mechanism in solid-state synthesis.

Solid-state compounds have historically been prepared through high-temperature solid-solid reactions. New mechanistic understanding of these reactions suggests possible routes to metastable compositions and structures as well as to thermodynamically stable, low-temperature phases that decompose at higher temperatures. Intermediate-temperature synthetic techniques, including flux and hydrothermal methods, as well as low-temperature intercalation and coordination reactions, have recently been developed and have been used to prepare unprecedented materials with interesting electronic, optical, and catalytic properties. The trend in modern solid-state synthesis resembles increasingly the approach used in small-molecule chemistry, in the sense that attention to reaction mechanism and the use of molecular building blocks result in an ability to prepare new materials of designed structure.

A host–guest complex between a metal–organic cyclotriveratrylene analog and a polyoxometalate: [Cu6(4,7-phenanthroline)8(MeCN)4]2PM12O40(M = Mo or W)

The synthesis and crystal structure of Cu(6)(4,7-phenanthroline)(8)(MeCN)(4)(6+) a novel Cu(i)-molecular hexamer is described in which the metal cations and phenanthroline molecules self-assemble into a dimer of shallow triangular-shaped bowls, within which are located large spherical polyoxometalate anions PM(12)O(40)(3-)(M = Mo or W).

Oxygen Isotope Effect in High-Temperature Oxide Superconductors

The effect of oxygen isotope substitution on the superconducting transition temperature, Tc, has been measured for BaBi0.25Pb0.75O3 (Tc, ≈ 11 K) and Lal1.85 Ca0.15CuO4 (Tc ≈ 20 K), and is compared to the shifts observed for La1.85Sr0.15CuO4 (Tc ≈ 37 K) and YBa2Cu3O7 (Tc ≈ 92 K). For all four materials, the transition temperature is shifted to lower temperature upon substitution of oxygen-18 for oxygen-16. The observed shifts demonstrate that phonons are involved in the electron-pairing mechanism in these oxide superconductors.

A polyoxometallate-templated coordination polymer: synthesisand crystal structure of [Cu3(4,4′-bipy)5(MeCN)2]PW12O40·2C6H5CN

The spherical phosphotungstate ion, PW12O40(3-), has been used as a non-coordinating anionic template for the construction of a novel, three-dimensional Cu(I) coordination polymer.

The use of temperature programmed desorption to study the removal of oxygen from the YBa2Cu3O7 superconductor

Abstract An interesting property of the new YBa 2 Cu 3 O 7 superconductor is that oxygen can be removed from this material at temperatures as low as 400°C. In this paper, temperature programmed desorption (TPD) is used to study the removal of oxygen as well as other impurities. During heating in flowing helium, 0.25 moles of O 2 per mole of YBa 2 Cu 3 O 7 desorb in a peak centered near 815 K. This peak is associated with the reduction of the sample to YBa 2 Cu 3 O 6.5 ; the reduced material does not superconduct. In addition, YBa 2 Cu 3 O 7 incorporates a considerable amount of H 2 O, CO 2 , and CO during exposure to air.

Crystal and molecular structures of several tetrakis (nitrile)copper(I) complexes

The crystal structures of several [Cu(RCN)4]X salts (R = Me, X = SO3CF3−; R = Ph, X = BF4−, ClO4−, and PF6−) were determined using single crystal X-ray diffraction. All of the compounds contain distorted tetrahedral Cu(I) centers and noncoordinating anions, with the acetonitrile and benzonitrile structures containing three and one unique CuL4+ complex in their respective asymmetric units. One important distortion is observed in the benzonitrile-Cu bonds, which are bent up to 23° away from linearity. The result is a flattened complex that maximizes the π–π tacking of the aromatic rings and is the dominant packing interactions between the complexes.

Subtle changes, profound effects: crystal engineering of one-dimensional helical copper(I):4,7-phenanthroline coordination polymers

Abstract The syntheses and crystal structures of four novel coordination polymers containing copper(I) and 4,7-phenanthroline (phen) are described. [Cu(phen)(MeCN)]SO3CF3, 1 and [Cu(phen)]SO3CF3·(C6H5NO2)0.75, 2 are prepared from an equimolar ratio of [Cu(MeCN)4]SO3CF3 and phen using acetonitrile (MeCN) and benzonitrile (BzCN), respectively, as solvent. Structures 3, [Cu(phen)2]SO3CF3, and 4, [Cu(phen)2)]SO3CF3·(C6H5NO2)(H2O) are also made from MeCN and BzCN, respectively, but using a tenfold excess of phen. All four compounds exhibit helical polymeric structures containing Cu(I) centers bridged by 4,7-phenanthroline ligands with a coordinated triflate counteranion completing the copper coordination sphere. In addition, 1 contains one terminal MeCN ligand, while 3 and 4 contain one terminal phenanthroline ligand in their copper(I) environments. Packing diagrams reveal π–π stacking between 4,7-phenanthrolines to be the primary intrachain interactions present. There is inclusion of noncoordinated nitrobenzene in both 2 and 4, and in each case the solvent molecules have very different influences on the chain stacking motifs.

Self-assembled thin films from lamellar metal disulfides and organic polymers

The intercalation/exfoliation reactions of MoS2 and SnS2 yield lamellar colloids, which may be grown layer-by-layer on cationic surfaces by alternate adsorption of cationic clusters or polymers.

Crystal and molecular structure of [Cu(pyrimidine)4]2BF4

The crystal structure of [Cu(pyrimidine)4]2BF4 was determined using single crystal X-ray diffraction. The compound crystallizes in the monoclinic space groupP21/n, with lattice parametersa=9.9736(8),b=13.872(1),c=16.568(1) Å. β=98.002(1), andZ=8, yielding a calculated density of 1.857 g/cm3. The Jahn-Teller distorted copper coordination sphere is composed of the four pyrimidine ligands in the equatorial positions (ave. Cu−N=2.02 Å) and the longer axial bonds from the copper to one fluorine (ave Cu−F=2.39 Å) on each of the BF4− anions. The crystal structure features layers of molecules having interleaved pyrimidine rings: the layers are connected through intermolecular H … F hydrogen bonds.

The production of high performance YBa sub 2 Cu sub 3 O sub 7 using nitrogen dioxide

Energy dispersive x-ray spectroscopy (EDS) was used to show that the elemental homogeneity of YBa{sub 2}Cu{sub 3}O{sub 7} powders can be improved substantially by heating the powder in a nitrogen dioxide-containing atmosphere (e.g., 950 {degree}C), followed by annealing in oxygen at 950 {degree}C, and slow-cooling to room temperature. The improved homogeneity results in a substantially larger flux exclusion signal for the NO{sub 2}-treated powder, as measured by both ac and dc techniques. The experimental results suggest a mechanism which involves the formation of a small amount of molten Ba(NO{sub 3}){sub 2}, which acts as a flux that dissolves the constituents and reprecipitates them as high purity YBa{sub 2}Cu{sub 3}O{sub 7}.