Optical Activity in the Near-IR Region: The λ=980 nm Multiplet of Chiral Yb3+ Complexes

Abstract

We used a very simplified electrostatic model based on charge and polarizability of atoms and groups on an organic ligand around a lanthanide ion to predict the near-infrared electronic circular dichroism (NIR ECD) spectra of Yb³⁺ (a monoelectronic ion). We tuned our method by using two widely different complexes. The first was the heterobimetallic species CsYb(hfbc)₄ [hfbc=(-)-3-heptafluorobutyrylcamphorate], in which the ligand is a diketonate and, as such, is endowed with a chromophore with strong UV absorption (π-π^* ). Its oxygen atoms define a square antiprism, which provides a symmetric coordination polyhedron. The second system was Yb DOTMA [DOTMA=(1R,4R,7R,10R)-α,α',α'',α'''-tetramethyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid], a chiral Yb analogue of Gd DOTA (DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), in which the ligand lacks relevant electronic transitions and provides a dissymmetric cage. The relative weights of dynamic (ligand polarization) and static contributions to Yb NIR ECD were evaluated, and the spectra appear to have been well predicted by theory through the introduction of a heuristic weight factor. To validate the approach and to confirm the value of the weight factor, we applied it to two other compounds, namely, Na₃ Yb(BINOLate)₃ and Yb(BINOLAM)₃ [BINOLate=2,2'-dihydroxy-1,1'-binaphthyl; BINOLAM=3,3'-bis(diethylaminomethyl)-1-1'-bi-2-naphthol].

Keywords: Richardson theory; chirality; circular dichroism; crystal-field splitting; ytterbium.