Lessons from the Microsoft case

International audienceCorrection: Divergence in the evolution of Paleolithic symbolic and technological systems: The shining bull and engraved tablets of Rocher de l’Impératric

Stereoselective Association of Binuclear Metallacycles in Coordination Polymers

A series of structurally related binuclear metallacycles Cd(NO3)2L]2, where L is an angular exobidentate ligand, have been synthesized. Each metallacycle contains two coordinatively unsaturated, chiral metal centers within a single molecule, and the assembly of these metallacycles into polymeric framework structures has been studied systematically for the first time. Stereoselective homochiral association of [Cd(NO3)2L]2 leads to the formation of helical coordination polymers, whereas meso type association results in nonhelical chain structures. The type of stereoselective aggregation depends on the conditions of self-assembly as well as on ligand functionality. Both helical and nonhelical polymeric complexes have been isolated for the metallacycle [Cd(NO3)2(2,4′-pyacph)]2 (2,4′-pyacph = 2,4′-(4-ethynylphenyl)bipyridyl). Homochiral association results in the formation of helical [Cd(NO3)]∞ chains which link the binuclear [Cd(NO3)2(2,4′-pyacph)]2 metallacycles into racemic two-dimensional sheets which contain both P and M [Cd(NO3)]∞ helices. In contrast, meso-association leads to the formation of nonhelical one-dimensional chains. It is shown that the product of homochiral association is predominately formed at room temperature and that of meso-association is generated at elevated temperatures. Thus, it may be concluded that the homochiral association appears to be energetically less favorable than the meso-association, a conclusion that has been confirmed by theoretical calculations of the crystal lattice energy. Several high-yield syntheses of bipyridyl-type ligands used for metallacyclic assembly are also reported

Take Shelter! Short-Term Occupations of the Late Paleolithic and the Mesolithic in the French Far West

International audienceDespite the important development of radiocarbon dating and Bayesian analysis these past years or our ability to interpret the archaeological materials and their spatial organization, the definition of occupation span is still very uncertain in prehistory. The techno-economic approach of lithic materials, using the concept of chaine opératoire segmentation, allows, however, to broach this complex topic. Research on Northwestern Lateglacial and Mesolithic sites essentially invested large open-air sites these past years. If these results brought important data, it remains reductive in the perspective of a diachronic study of paleo-economic systems since small sites, complementary to these large occupations, are not included in the models. These small sites have a low archaeological visibility and are particularly difficult to identify in survey or during rescue archaeology. A new research program entitled “Take shelter!” aims to search for these sites focusing on small Armorican rock shelters. Because of particular geological contexts, these cavities are particularly small and atypical. For these reasons, archaeologists neglected them. The survey, testing, and excavation of some of these rock shelters allow illustrating occupations with very specific techno-economic signals that are interpreted to be good evidence for short-term occupations. From these results, we discuss various archaeological evidences we consider diagnostic of short-term occupation and propose a first diachronic model for the evolution of paleo-economic systems in Northwestern France between the Lateglacial and the end of the Mesolithic