Lanthanide Class of a Trinuclear Enantiopure Helical Architecture Containing Chiral Ligands: Synthesis, Structure, and Properties

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 Ln(III) 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 [Ln(3)(L)(6)(mu(3)-OH)(H(2)O)(3)](ClO(4))(2) (abbreviated as tris(Ln[L](2))). This class of C(3)-symmetrical compounds was structurally characterized in the solid state and solution. Electrospray (ES) mass spectrometric and (1)H NMR spectroscopic analyses indicated that the trinuclear species are maintained in solution (CH(2)Cl(2)) and are stable in the investigated concentration range (10(-2)-10(-6) m). The photophysical 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 pi-pi* transitions of bipyridine. CPL spectra of tris(Eu[(+)-L](2)) and tris(Tb[(+)-L](2)) present large dissymmetry factors g(em) for the sensitive transitions of Eu(III) ((5)D(0)-->(7)F(1), g(em)=-0.088) and Tb(III) ((5)D(4)-->(7)F(5), g(em)=-0.0806). The self-recognition capabilities of the system were tested in the presence of artificial enantiomeric mixtures of the ligand. (1)H 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.

Lanthanide Circularly Polarized Luminescence: Bases and Applications

“…CsEu(hfbc) 4 has an almost perfect antiprismatic geometry in solution with a twist angle of À41.4°, thus static coupling ZINNA AND DI BARI mechanism alone cannot account for the measured g lum = 1.38 value at 598 nm, 65 i.e., for the 5 D 0 → 7 F 1 transition, which has magnetic dipole but no electric dipole character. Indeed, in an ideal antiprismatic geometry the twist angle is 45°, which means that the coordination polyhedron is completely symmetric with respect to rotoreflection operations.…”

“…Camphor provides a rigid chiral backbone employable in the synthesis of β-diketones; aside from the exceptional performances mentioned for CsEu(hfbc) 4 , several other camphorate-based complexes have been investigated by CPL spectroscopists with interesting results.…”

“…10 we show a plot of g lum (without correction) and e g 1um (δ = 3 · 10 À4 , i.e., 2/100 of the maximum intensity of the band at 585 nm) for a CPL spectrum obtained for CsEu(hfbc) 4 . 10 we show a plot of g lum (without correction) and e g 1um (δ = 3 · 10 À4 , i.e., 2/100 of the maximum intensity of the band at 585 nm) for a CPL spectrum obtained for CsEu(hfbc) 4 .…”

Diastereoselective Self‐Assembly of a Homochiral Europium Triangle from a BipyoxazolineCarboxylate Ligand

“…[1,[4][5][6][7][8][9][10] Fewer examples of chiral polynuclear assemblies have been described for lanthanides. [3,[11][12][13][14][15][16] Some original examples of chiral assemblies were obtained from the controlled hydrolysis [11][12][13] or CO 2 fixation reactions [15,16] of lanthanide salts in the presence of chiral ligands. In these synthetic methods the hydroxo and the carbonate groups play an important role in the formation of the heteroleptic assemblies causing the rational design of the final assembly to be difficult.…”

Lanthanide Assemblies and Polymetallic Complexes

“…Additional binding sites can be used to extend the system. For instance, Piguet and co-workers have used extended variants upon these compartmental structures to prepare ligands such as 10 and 11, from the trinuclear and tetranuclear assemblies (Ln 3 2+ [33], in which the chiral ligand induces chirality into the assembly (Fig. 5).…”