Isolation and characterization of the first circular single-stranded polymetallic lanthanide-containing helicate

A thorough examination of the disassembly of bimetallic triple-stranded lanthanide helicates [Ln2(Li)3]6+ (stoichiometry S = m/n = 2/3 = 0.67, global complexity GC = m + n = 2 + 3 = 5) in excess of metals shows the competitive formation of standard linear bimetallic complexes [Ln2(Li)2]6+ (S = 1.0, GC = 4), and circular trimetallic single-stranded helicates [Ln3(Li)3]9+ (S = 1.0, GC = 6)

Connecting Terminal Carboxylate Groups in Nine-Coordinate Lanthanide Podates: Consequences on the Thermodynamic, Structural, Electronic, and Photophysical Properties

The hydrolysis of terminal tbutyl-ester groups provides the novel nonadentate podand tris{2-[N-methylcarbamoyl-(6-carboxypyridine-2)-ethyl]amine} (L13) which exists as a mixture of slowly interconverting conformers in solution. At pH = 8.0 in water, its deprotonated form [L13 − 3H]3- reacts with Ln(ClO4)3 to give the poorly soluble and stable podates [Ln(L13 − 3H)] (log(β110) = 6.7−7.0, Ln = La−Lu). The isolated complexes [Ln(L13 − 3H)](H2O)7 (Ln = Eu, 8; Tb, 9; Lu, 10) are isostructural, and their crystal structures show Ln(III) to be nine-coordinate in a pseudotricapped trigonal prismatic site defined by the donor atoms of the three helically wrapped tridentate binding units of L13. The Ln−O(carboxamide) bonds are only marginally longer than the Ln−O(carboxylate) bonds in [Ln(L13 − 3H)], thus producing a regular triple helix around Ln(III) which reverses its screw direction within the covalent Me−TREN tripod. High-resolution emission spectroscopy demonstrates that (i) the replacement of terminal carboxamides with carboxylates induces only minor electronic changes for the metallic site, (ii) the solid-state structure is maintained in water, and (iii) the metal in the podate is efficiently protected from interactions with solvent molecules. The absolute quantum yields obtained for [Eu(L13 − 3H)] ( = 1.8 × 10-3) and [Tb(L13 − 3H)] ( = 8.9 × 10-3) in water remain modest and strongly contrast with that obtained for the lanthanide luminescence step (ΦEu = 0.28). Detailed photophysical studies assign this discrepancy to the small energy gap between the ligand-centered singlet (1ππ*) and triplet (3ππ*) states which limits the efficiency of the intersystem crossing process. Theoretical TDDFT calculations suggest that the connection of a carboxylate group to the central pyridine ring prevents the sizable stabilization of the triplet state required for an efficient sensitization process. The thermodynamic and electronic origins of the advantages (stability, lanthanide quantum yield) and drawbacks (solubility, sensitization) brought by the "carboxylate effect" in lanthanide complexes are evaluated for programming predetermined properties in functional devices

Predictions, Synthetic Strategy, and Isolation of a Linear Tetrametallic Triple-Stranded Lanthanide Helicate

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Enregistrement fluviatile et paléoenvironnements au Pléistocène supérieur sur la bordure méridionale atlantique de l'Anti-Atlas (Oued Assaka, S-O marocain)

International audienceLaurent. Enregistrement fluviatile et paléoenvironnements au Pléistocène supérieur sur la bordure méridionale atlantique de l'Anti-Atlas (Oued Assaka, SO marocain)

LES DÉPÔTS HOLOCÈNES DE LA BASSE VALLÉE DE L'OUED ASSAKA, SUD-MAROC: RÉSULTATS PRÉLIMINAIRES

International audienceAt the southem side of the Anti-Atlas mountains, the lower section of the Assaka wadi valley joins the Atlantic ocean by crossing an Appalachian basin and range area. Holocene deposits of the lower section of the Assaka wadi show two main sedimentary formations: the upper sheet-fans and the lower terraces. The upper sheet-fans are probably from the lower Rharbian (Holocene) age, they are located above the Soltanian (Wünnian) formations at the outlets of tributaries. They consist of accumulations with silts, aeolian quartz and high palygorskite contents. After this period, a trench cut through the older formations took place between 8 000 years BP and 4 200 years BP. The lower river terraces, 5 to Il metres thick, are inset in the trench cut through the Soltanian formations and show four superimposed and fining upwards (from pebble to silt) cross-bedded units. This study shows many geodynamic and palaeoclimatic conditions. Lower Rharbian fans indicate that the climate was sufficiently arid to denude the slopes, where, during big storms, gullies and run-off supplied the Assaka wadi with silts and sands. Torrential floods eut the Soltanian formations and then created the Middle Rharbian low terrace. Finally, the Historie terrace, superimposed onto the Middle Rharbian one, shows that important flood events took place during the XVIIO and XVIIIo centuries, but nevertbeless with low frequency. However. this sequence indicates wetter conditions during an arid episode. The 14C age is contemporary with the "cold winters" in Western Europe; this fact proves that the synoptic conditions in South Morocco were the same at 280±30 years B.P. as at present.L'oued Assaka, exutoire d'un vaste bassin versant (7 000 km2) rejoint l'océan Atlantique sur le versant sud de l' Anti-Atlas en recoupant une zone de relief appalachien. Des cônes attribuables au Pléistocène supérieur ou au Rharbien ancien (Holocène ancien) surmontent les formations alluviales et colluviales soltaniennes (Würm), avant que celles-ci soient entaillées sur 20 à 25 mètres; ils sont circonscrits aux débouchés des foums que déterminent les affluents de l'oued Assaka. Ils sont constitués de limons sableux riches en grains de quartz éoJisés et par des teneurs en palygorskite qui s'accentuent de la base vers le sommet. Aux environs de 8 000 BP, une phase d'incision linéaire importante entailla les dépôts plus anciens sur une épaisseur de 20 mètres. De basses terrasses sont emboîtées dans les dépôts soltaniens. D'une épaisseur variant de 5 à Il mètres, elles sont constituées de séquences granoclassées qui mettent en évidence des géodynamiques et des ambiances climatiques contrastées. Le Rharbien ancien reflète une sécheresse suffi-sante pour dénuder en partie les versants, qui lors d'orages probablement assez localisés, alimentent les affluents de l'oued principal en limons sableux. Les crues torrentielles (charriant des blocs surdimensionnés) ont incisé les formations soltaniennes -essentiellement limoneuses-et ont mis en place les basses terrasses du Rharbien moyen. Enfin, la terrasse historique étagée sur celle du Rharbien moyen montre la permanence de crues importantes durant les XVIIème et XVIIIème siècles avec une fréquence cependant moins élevée. En outre, cette séquence indique l'existence de conditions humides durant un épisode aride sous cette latitude contemporaines des "hivers froids" de l'Europe de l'Ouest; ce qui prouve que les situations synoptiques dans le Sud du Maroc étaient les mêmes à 280±30 ans BP qu'aujourd'hui