Richard L. Harlow

The incommensurate structure of (Sr14−xCax)Cu24O41 (0 < x ∼ 8) a superconductor byproduct

Abstract The structures of two compositions of the title compound, Sr 8 Ca 6 Cu 24 O 41 and Sr 14 Cu 24 O 41 , have been determined by x-ray, single-crystal diffraction techniques. The structure of the first is best described as two interpenetrating structures: one with (Sr,Ca)(Cu 2 O 3 sheets)(Sr,Ca) layers in an orthorhombic cell with dimensions of a = 11.375(2), b = 12.9027(2), c = 3.910(1) A and a second structure with layers of (CuO 2 chains) in a cell having identical a and b values but with c = 2.753 A . The title formulation arises because these two subcells are nearly commensurate at 7 × c (sheet) = 27.372(2) and 10 × c (chain) = 27.534. The structure of the pure strontium phase is very similar but the adjacent CuO 2 chains in a given layer shift to a less constrained conformation: a = 11.459(2), b = 13.368(5), c (sheet) = 3.931(1), c ( chain ) = 2.749(1) A . X-ray powder diffraction studies have shown that x may vary from 0 to ∼ 8, with the layer axis, b, showing the most sensitivity to composition. Single crystals of these phases were found to be semiconductors.

Synthesis and Single-Crystal X-ray Structure of a Highly Symmetrical C60 Derivative, C60Br24

C60 and liquid bromine react to form C60Br24, a crystalline compound isolated as a bromine solvate, C60Br24(Br2)x, The x-ray crystal structure defines a new pattern of addition to the carbon skeleton that imparts a rare high symmetry. The parent C60 framework is recognizable in C60Br24, but sp3 carbons at sites of bromination distort the surface, affecting conformations of all of the hexagonal and pentagonal rings. Twenty-four bromine atoms envelop the carbon core, shielding the 18 remaining double bonds from addition. At 150� to 200�C there is effectively quantitative reversion of C60Br24 to C60 and Br2.

New chiral phospholanes; Synthesis, characterization, and use in asymmetric hydrogenation reactions

Abstract We describe the practical synthesis of enantiomerically pure trans-2,5-disubstituted-1-phenylphospholanes which are then employed in the preparation of a new series of C2-symmetric bis- and C3-symmetric tris(phospholane) ligands. A versatile three-step route to the important chiral 1,4-diol intermediates, used in the phosphine syntheses, is outlined. Rhodium complexes bearing the new phosphine ligands were prepared and shown to act as efficient catalyst precursors for the enantioselective hydrogenation of various unsaturated substrates.

Oxygen Isotope Effect and Structural Phase Transitions in La2CuO4-Based Superconductors

The oxygen isotope effect on the superconducting transition temperature (αo) varies as a function of x in La2-xSrxCuO4 and La2-xBaxCuO4, with the maximum αo values (αo ≥ 0.5) found for x near 0.12. This unusual x dependence implies that the isotope effect is influenced by proximity to the Abma → P42/ncm structural phase transition in these systems. Synchrotron x-ray difaction measurements reveal little change in lattice parameters or orthorhombicity due to isotope exchange in strontium-doped materials where αo > 0.5, eliminating static structural distortion as a cause of the large isotope effects. The anomalous behavior of αo in both strontium- and barium-doped materials, in combination with the previously discovered Abma → P42/ncm structural phase-transition in La1.88B0.12CuO4, suggests that an electronic contribution to the lattice instability is present and maximizes at ∼1/8 hole per copper atom. These observations indicate a dose connection between hole doping of the Cu-O sheets, tilting instabilities of the CuO6 octahedra, and superconductivity in La2CuO4-based superconductors.

Unusual eight-membered rings with linear symmetric methyl groups from yttrium and lutetium methyl complexes

Abstract New examples of symmetric linear bridge bonding of methyl groups between two metal ions is demonstrated in the complexes [M(Cp*)2μ-Me2M′Me2]2 (1) (M = yttrium or lutetium; M′ = aluminum or gallium; Cp* = pentamethylcyclopentadienyl) which are formed by reversible dimerisation of M(Cp*)2μ-Me2M′Me2. X-ray crystallography shows the metal atoms in Y(Cp*)2μ-Me2AlMe2 to be arranged in a flat square array with alternating Y and A1 atoms at the vertices, and methyl groups along the sides. The linear YCAl bonds so formed subtend a bond angle of 176°(1). Equilibrium constants between +40 °C and −40 °C for dimerisation to 1 of Y(Cp*)2μ-Me2AlMe2, Y(Cp*)2μ-Me2GaMe2 and Lu(Cp*)2μ-Me2AlMe2 in toluene were obtained, giving the thermodynamic parameters ΔHeq = −10.4, −5.6 and −8.1(±0.2) kcal mol-1 and ΔSeq = −29.9, −18.0 and −27.4(±3) e.u. respectively. From NMR data, particularly comparison of coupling constants (Jch values = 118–124 Hz for linear bridge methyl groups in these systems), the products from reaction of M(Cp*)2μ-MeM(Cp*)2Me (M = yttrium or lutetium) and [Y(C5H5)2Me]2 are also linearly methyl-bridged tetra-metal complexes [M(Cp*)2MeY(C5H5)2Me-]2.

Single crystal structure determination of double layered strontium iridium oxide [Sr3Ir2O7]

Single crystals of Sr3Ir2O7 were grown for the first time from a SrCl2 flux using SrO2 and IrO2 in a sealed Pt tube under ambient pressure. The compound crystallizes in a tetragonal unit cell [a = 3.896 (1) and c = 20.879 (3) A] with space group I4/mmm (No. 139) and Z = 2. The crystal structure was solved by means of single crystal X-ray diffraction and refined to R = 0.014. The structural refinement showed that the IrO6 octahedra are rotated about the c-axis by about 12 degrees, similar to the rotational distortion recently found in Sr2IrO4. However, unlike in Sr2IrO4, the IrO6 rotations in Sr3Ir2O7 are not correlated, hence no superlattice reflections appear.

Activation of dichloromethane by basic rhodium(I) and iridium(I) phosphine complexes. Synthesis and structures of fac-[Rh(PMe3)3Cl2(CH2PMe3)]Cl·CH2Cl2 and trans-[Rh(Me2PCH2CH2PMe2)2Cl(CH2Cl)]Cl

Sixteen-electron RhI complexes of basic phosphines react with CH2Cl2 yielding either RhCH2Cl or RhCH2PMe3 complexes; an example of each has been structurally characterized.

Some well characterized chemical reactivities of buckminsterfullerene (C60)

Abstract The metalation, halogenation and free radical addition chemistry of C 60 is described. Vibrational spectroscopy is a useful tool in assigning the structures of the products. Several underlying principles emerge for C 60 chemistry, some of which are supported by molecular orbital calculations.

Structure of YSr2Cu3−xMxO7+δ (MMo, W and Re) from single-crystal X-ray and powder-neutron diffraction substituent site preference and oxygen defect structure

Abstract Unlike YBa 2 Cu 3 O 7 , the Sr analog YSr 2 Cu 3 O 7 cannot be made at ambient pressures without partial substitution of some small, higher-oxidation state ions for the Cu. We have prepared doped YSr 2 Cu 3− x M x O 7+δ (MMo, W and Re) samples with x values in the range 0.1 to 0.3 and with superconducting transition temperatures up to 50 K. We have determined the structures of YSr 2 Cu 2.82 Mo 0.18 O 7.25 and YSr 2 Cu 2.77 Re 0.23 O 7.26 using X-ray single-crystal diffraction and of YSr 2 Cu 3− x M x O 7+δ (MMo, W and Re) using neutron powder diffraction. We find that the M +6 ions substitute preferentially for the chain Cu +2 ions and that they substantially disrupt the CuO chain structure. The substitution of M +6 also increases the oxygen stoichiometry and leads to an increase in the effective oxidation state of the Cu ions. This increase in oxidation state plays the very important dual role of stabilizing the crystal structure with the smaller Cu +3 ions and of turning YSr 2 Cu 3− x M x O 7+δ into superconductor.

Structure and reactivity in the group 5B t-butylimido complexes (Me2N)3MNBut; X-ray crystal and molecular structure of N-t-butylimidotris(dimethylamido)tantalum

The novel complexes (Me2N)3MNBut(M = Nb or Ta) contain a linear MN–C unit and undergo reactions with electrophiles at the amido nitrogen; the structure of (Me2N)3MNBut has been determined by X-ray crystallography.