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 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.