Vapochromism in Complexes of Stoichiometry [Au2Ag2R4L2]n

Reaction of [NBu4][Au(C6X5)2] (C6X5 = 3,5-C6Cl2F3, C6Cl5) with AgOClO3 in an Et 2O/CH2Cl2 mixture affords [Au 2Ag2(C6X5)4(OEt 2)2]n [C6X5 = 3,5-C 6Cl2F3 (1a), C6Cl5 (1b)]. These compounds react with tetrahydrofuran, acetone, acetonitrile or toluene in solution and in the gas phase (with toluene only in solution) to the new complexes [Au2Ag2(C6X5) 4L2]n [L = THF (2a, 2b), (CH3) 2CO (3a, 3b), CH3CN (4a, 4b), C7H8 (5a, 5b)]. The crystal structures of 2a and 5a have been determined by X-ray diffraction methods, showing polymeric chains formed by the union of tetranuclear Au2Ag2 units via aurophilic interactions. The thermal stability and the vapochromic behaviour of these complexes have been studied by thermogravimetric analysis (TGA), X-ray powder diffraction and FT-IR spectroscopy. © 2009 Verlag der Zeitschrift für Naturforschung, Tübingen

Multiple Evidence for Gold(I)center dot center dot center dot Silver(I) Interactions in Solution

[AuAg3(C6F5)(CF3CO 2)3 (CH2PPh3)Jn (2) was prepared by reaction of [Au(C6F5)(CH2PPh3)] (1) and [Ag(CF3CO2)] (1:3). The crystal structures of complexes 1 and 2 were determined by X-ray diffraction, and the latter shows a polymeric 2D arrangement built by Au-Ag, Ag-Ag, and Ag-O contacts. The metallophilic interactions observed in 2 in the solid state seem to be preserved in concentrated THF solutions, as suggested by EXAFS, pulsed-gradient spin-echo NMR, and photophysical studies, which showed that the structural motif [AuAg3(C6F5)(CF3CO2) 3(CH2PPh3)] is maintained under such conditions. Time-dependent DFT calculations agree with the experimental photophysical energies and suggest a metalto-ligand charge-transfer phosphorescence process. Ab initio calculations give an e timated interaction energy of around 60 kJ mol-1 for each Au-Ag interaction. © 2009 Wiley-VCH. Verlag GmbH & Co. KGaA

Six3 repression of Wnt signaling in the anterior neuroectoderm is essential for vertebrate forebrain development

In vertebrate embryos, formation of anterior neural structures requires suppression of Wnt signals emanating from the paraxial mesoderm and midbrain territory. In Six3(−/−) mice, the prosencephalon was severely truncated, and the expression of Wnt1 was rostrally expanded, a finding that indicates that the mutant head was posteriorized. Ectopic expression of Six3 in chick and fish embryos, together with the use of in vivo and in vitro DNA-binding assays, allowed us to determine that Six3 is a direct negative regulator of Wnt1 expression. These results, together with those of phenotypic rescue of headless/tcf3 zebrafish mutants by mouse Six3, demonstrate that regionalization of the vertebrate forebrain involves repression of Wnt1 expression by Six3 within the anterior neuroectoderm. Furthermore, these results support the hypothesis that a Wnt signal gradient specifies posterior fates in the anterior neural plate