[Show abstract][Hide abstract] ABSTRACT: 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 C3-symmetrical, pyramidal architecture with the general formula [Ln4(L)9(mu3-OH)](ClO4)2) (abbreviated as tetra-Ln4L9). 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 [Ln3(L)6(mu3-OH)(H2O)3](ClO4)2) (recently reported by us and abbreviated as tris-LnL2) 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 LnL3 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) x cm(-1)) are highly influenced by the helicity of the capping fragment LnL3. 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-Ln4L9 and tris-LnL2 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-Ln4L9 either by direct synthesis from a racemic mixture of the ligand and Ln(III) ions or by the conversion of a tris-Ln[(+/-)-L]2 racemate were likewise demonstrated.
[Show abstract][Hide abstract] ABSTRACT: The pinene-bipyridine carboxylic derivatives (+)- and (-)-HL, designed to form configurationally stable lanthanide complexes, proved their effectiveness as chiral building blocks for the synthesis of lanthanide-containing superstructures. Indeed a self-assembly process takes place with complete diastereoselectivity between the enantiomerically pure ligand L- and LnIII ions (La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er), thus leading to the quantitative formation of a trinuclear supramolecular architecture with the general formula [Ln3(L)6(μ3-OH)-(H 2O)3](ClO4)2 (abbreviated as tris(Ln[L]2)). This class of C3-symmetrical compounds was structurally characterized in the solid state and solution. Electrospray (ES) mass spectrometric and 1H NMR spectroscopic analyses indicated that the trinuclear species are maintained in solution (CH2Cl2) and are stable in the investigated concentration range (10_2-10 -6M). The photo-physical properties of the ligand HL and its tris(Ln[L]2) complexes were studied at room temperature and 77 K, thus demonstrating that the metal-centered luminescence is well sensitized both for the visible and near-IR emitters. The chiroptical properties of tris(Ln[L]2) complexes were investigated by means of circular dichroism (CD) and circularly polarized luminescence (CPL). A high CD activity is displayed in the region of π-π* transitions of bipyridine. CPL spectra of tris(Eu[(+)-L]2) and tris(Tb[(+)-L]2) present large dissymmetry factors gcm for the sensitive transitions of EuIII (5D0→7F1, gcm = -0.088) and TbIII (5D4→ 7F5, gcm = -0.0806). The self-recognition capabilities of the system were tested in the presence of artificial enantiomeric mixtures of the ligand. 1H NMR spectra identical to those of the enantiomerically pure complexes and investigations by CD spectroscopic analysis reveal an almost complete chiral self-recognition in the self-assembly process, thus leading to mixtures of homochiral trinuclear structures.
[Show abstract][Hide abstract] ABSTRACT: Dreieck oder Pyramide? Durch solvensinduzierte divergente Selbstorganisation von PrIII-Ionen und einem chiralen Liganden entstehen diastereoselektiv zwei verschiedene, aber verwandte Überstrukturen, in denen die Liganden helixförmig an dreieckige Pr3- bzw. pyramidale Pr4-Metallgerüste binden (Pr grün). Die beiden Strukturen lassen sich einfach und reversibel durch Änderung der Wassermenge im Lösungsmittel CH3CN ineinander überführen.
[Show abstract][Hide abstract] ABSTRACT: (Chemical Equation Presented) A supramolecular propeller: A trinuclear EuIII helix with six chiral bipyridine-carboxylate ligands self-assembles with complete diastereoselectivity (see graphic; Eu green, O red, N blue). Ground-and excited-state chiroptical properties of this red-emissive complex reflect the helical arrangement of the ligands around the metallic core.
[Show abstract][Hide abstract] ABSTRACT: A new class of chiral pinene 2,2'-bipyridine compounds containing carboxylic or alcoholic groups have been synthesized using the Krohnke procedure followed by haloform or lithium mediated substitution reactions.