RESUMEN
Two new quaternary aluminum silicides, RE8Ru12Al49Si9(Al(x)Si12-x) (x approximately 4; RE = Pr, Sm), have been synthesized from Sm (or Sm2O3), Pr, Ru, and Si in molten aluminum between 800 and 1000 degrees C in sealed fused silica tubes. Both compounds form black shiny crystals that are stable in air and NaOH. The Nd analog is also stable. The compounds crystallize in a new structural type. The structure, determined by single-crystal X-ray diffraction, is cubic, space group Pm3m with Z = 1, and has lattice parameters of a = 11.510(1) A for Sm8Ru12Al49Si9(Al(x)Si12-x) and a = 11.553(2) A for Pr8Ru12Al49Si9(Al(x)Si12-x) (x approximately 4). The structure consists of octahedral units of AlSi6, at the cell center, Si2Ru4Al8 clusters, at each face center, SiAl8 cubes, at the middle of the cell edges, and unique (Al,Si)12 cuboctohedral clusters, at the cell corners. These different structural units are connected to each other either by shared atoms, Al-Al bonds, or Al-Ru bonds. The rare earth metal atoms fill the space between various structural units. The Al/Si distribution was verified by single-crystal neutron diffraction studies conducted on Pr8Ru12Al49Si9(Al(x)Si12-x). Sm8Ru12Al49Si9(Al(x)Si12-x) and Pr8Ru12Al49Si9(Al(x)Si12-x) show ferromagnetic ordering at Tc approximately 10 and approximately 20 K, respectively. A charge of 3+ can be assigned to the rare earth atoms while the Ru atoms are diamagnetic.
RESUMEN
A series of four structurally related cis-dithiolate-ligated Fe(III) complexes, [Fe(III)(DITpy)2]Cl (1), [Fe(III)(DITIm)2]Cl (2), [Fe(III)(ADIT)2]Cl (3), and [Fe(III)(AMIT)2]Cl (4), are described. The structural characterization of 3 as well as the spectroscopic properties of 3 and 4 has been previously reported. Crystal data for 1, 2, and 4 are as follows: 1.3H2O crystallizes in the orthorhombic space group Pca2(1) with a = 19.800(4) A, b = 18.450(4) A, c = 14.800(3) A, and Z = 8. 2.(1/2)EtOH.1/2H2O crystallizes in the monoclinic space group Cc with a = 24.792(4) A, b = 14.364(3) A, c = 17.527(3) A, beta = 124.91(2) degrees, and Z = 8. 4 crystallizes in the triclinic space group P1 with a = 8.0152(6) A, b = 10.0221(8) A, c = 11.8384(10) A, alpha = 73.460(3) degrees, beta = 71.451(5) degrees, gamma = 72.856(4) degrees, and Z = 2. Complexes 1-4 share a common S2N4 coordination environment that consists of two cis-thiolates, two trans-imines, and two cis-terminal nitrogen donors: Nterm = pyridine (1), imidazole (2), and primary amine (3 and 4). The crystallographically determined mean Fe-S bond distances in 1-4 range from 2.196 to 2.232 A and are characteristic of low-spin Fe(III)-thiolate complexes. The low-spin S = 1/2 ground state was confirmed by both EPR and magnetic susceptibility measurements. The electronic spectra of these complexes are characterized by broad absorption bands centered near approximately 700 nm that are consistent with ligand-to-metal charge-transfer (CT) bands. The complexes were further characterized by cyclic voltammetry measurements, and all possess highly negative Fe(III)/Fe(II) redox couples ( approximately -1 V vs SCE, saturated calomel electrode) indicating that alkyl thiolate donors are effective at stabilizing Fe(III) centers. Both the redox couple and the 700 nm band in the visible spectra show solvent-dependent shifts that are dependent upon the H-bonding ability of the solvent. The implications of these results with respect to the active site of the iron-containing nitrile hydratases are also discussed.
Asunto(s)
Hidroliasas/química , Modelos Moleculares , Sitios de Unión , Cristalografía por Rayos X , Electroquímica , Hidroliasas/metabolismo , Magnetismo , Estructura Molecular , Oxidación-ReducciónRESUMEN
Propagating modes of vibration of a bilipid membrane have been detected with light beating spectroscopy. The dependence of omega on q is consistent with a model of a fluid film of surface tension sigma = 2.5 +/- 0.5 dyn cm-1 surrounded by a medium with rho = 1 g cm-3 and eta = 1.01 X 10(-2) P.
