Lanthanide class of a trinuclear enantiopure helical architecture containing chiral ligands: Synthesis, structure, and properties

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 [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 g_cm for the sensitive transitions of Eu^III (⁵D₀→⁷F₁, gcm = -0.088) and Tb^III (⁵D₄→ ⁷F₅, g_cm = -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.