Chiralité (chimie) | EPFL Graph Search

Le concept de la chiralité existe également dans d'autres domaines. En chimie, un composé est dit chiral (du grec χείρ : la main) s'il n'est pas superposable à son image dans un miroir plan. Il existe un certain nombre de raisons pour lesquelles une molécule peut être chirale :

la présence d'un ou plusieurs centres asymétriques (sauf certaines conditions particulières de symétrie) ;
une forme en hélice ;
un plan de chiralité.

Énantiomérie et diastéréoisomérie Énantiomérie Énantiomérie Si une molécule est chirale, elle possède au moins deux formes dites énantiomères qui se différencient par une configuration absolue opposée. Il est nécessaire que tous les éléments asymétriques soient opposés entre une molécule et son énantiomère. Diastéréomérie Diastéréoisomérie Lorsqu'une molécule A est totalement équivalente à une molécule B si ce n'est pour la configuration absolue de ses centres asymétriques, et que A et B ne sont pas énantiomères, alors

Enantiopure helical (supra)molecular arrays containing lanthanides

Marco Antonio Lama

The diastereoselective synthesis of chiral coordination compounds with d metals proved the effectiveness of the pinene-bipyridine approach in predetermining the chirality at the metal centre. The strong Lewis acidic character of lanthanide cations required the modification of this class of ligands in order to adapt their coordination properties to the binding affinity of Ln(III) toward hard donor atoms like oxygen. The pinene bipyridine carboxylic derivative (+)-L- (figure 1) has been designed and synthesized to form configurationally stable lanthanide complexes. This ligand proved its effectiveness as chiral building block for the synthesis of oligometallic supramolecular structures. Indeed a self-assembly process takes place with complete diastereoselectivity between the enantiomerically pure (-) or (+) L- and Ln(III) ions (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er) in methanol, giving rise to trinuclear enantiopure structures with general formula Ln3(L)6(µ3-OH)(H2O)32 , possessing predetermined helical chirality. The ligands in this case display a new version of supramolecular helical chirality since they are wrapping around a trimetallic core instead of one single metal centre. The same components afford a second polynuclear structure if the reaction is carried out in CH3CN. In this medium a three-dimensional tetranuclear array self-assembles (with general formula Ln4(L)9(µ3-OH)2). The nine ligands are forming three distinct helical domains, two supramolecular as in the case of the trinuclear array and one classical around a single metal centre, with predetermined configuration. The two classes of compounds have been structurally characterized in solid state (X-ray and IR) and in solution (ES-MS, NMR). Their chiroptical properties were investigated by means of Circular Dichroism and Circularly Polarized Luminescence (for the emitting compounds). Trinuclear and tetranuclear arrays are shown to interconvert in CH3CN, in a process mediated by water. This process was rationalized taking into account the nature of the Ln(III) ions, the initial concentration of the components and the addition or removal of water in solution. The availability of both the enantiomers of the ligand prompted us to study the self-recognition capabilities of the two systems. 1H-NMR and CD studies reveal an almost complete chiral recognition in the self-assembly leading to the trinuclear complex. The process leading to tetranuclear species appears instead perturbed, probably due to the higher number of fundamental components involved (4 metals and 9 ligands instead of 3 metals and 6 ligands). Figure 1: Reaction conditions leading to the synthesis of trinuclear and tetranuclear Ln(III) complexes with ligand (+)-L-. The introduction of a carboxylic moiety onto the 6' posititon of the (-)-5,6- and (-)-4,5- pinene bipyridine lead to the tridentate ligands (-)-HL1 and (-)-HL2, respectively. Mononuclear neutral complexes are obtained upon reaction with Ln(ClO4)3. Different extents of diastereoselectivity are pointed out in the two systems by X-ray analysis and CD spectroscopy. In particular the ligand (-)-HL1, in which the pinene moiety is next to the binding site, seems to be more effective in predetermining the configuration at the metal centre. A low diastereoselectivity is instead expressed in the reactions with (-)-HL2 as revealed by the X-ray structure of [Pr{(-)-L2}3] (both Δ and Λ isomers detected) and by the CD spectrum void of signal.

EPFL2006

Enantiopure, supramolecular helices containing three-dimensional tetranuclear lanthanide(III) arrays: Synthesis, structure, properties, and solvent-driven Trinuclear/Tetranuclear interconversion

Marco Antonio Lama

The enantiomerically pure pinene-bipyridine-based receptor, (-) or (+) L-, diastereoselectively self-assembles in dry acetonitrile in the presence of Ln(III) ions (Ln = La, Pr, Nd, Sm, Eu, Gd, and Tb) to give a C-3-symmetrical, pyramidal architecture with the general formula Ln(4)(L)(9)(mu(3)-OH)(2)) (abbreviated as tetra-Ln(4)L(9)). Three metal centers shape the base: an equilateral triangle surrounded by two sets of helically wrapping ligands with opposite configurations. This part of the structure is very similar to the species Ln(3)(L-6(mu(3)-OH)(H2O)(3)(2)) (recently reported by us and abbreviated as tris-LnL(2)) formed by the ligand and the Ln(III) ions when the reactions are performed in methanol. The tetranuclear structure is completed by a capping, helical unit LnL(3) whose chirality is also predetermined by the chirality of the ligand. A complete characterization of these isostructural, chiral compounds was performed in solid state (X-ray, IR) and in solution (ES-MS, NMR, CD, UV-vis and emission spectroscopies). The sign and the intensity of the CD bands in the region of the pi pi* transitions of the bipyridine (absolute Delta epsilon values at 327 nm are about 280 M-1.cm(-1)) are highly influenced by the helicity of the capping fragment LnL(3). The photophysical properties (lifetime, quantum yield) of the visible (Eu and Tb complexes) and NIR (Nd complex) emitters indicate a good energy transfer between the ligands and the metal centers. The two related superstructures tetra-Ln(4)L(9) and tris-LnL(2) can be interconverted in acetonitrile, the switching process depending on the amount of water present in the solvent, the size of the Ln(III) ion, and the concentration. The weak chiral recognition capabilities of the self-assembly leading to the formation of tetra-Ln(4)L(9) either by direct synthesis from a racemic mixture of the ligand and Ln(III) ions or by the conversion of a tris-Ln(+/-)-L racemate were likewise demonstrated.

2008

Enantiopure helical (supra)molecular arrays containing lanthanides

Marco Antonio Lama

EPFL2006

Enantiopure, supramolecular helices containing three-dimensional tetranuclear lanthanide(III) arrays: Synthesis, structure, properties, and solvent-driven Trinuclear/Tetranuclear interconversion

Marco Antonio Lama

2008