Solution NIR CD and MCD in 4f-4f transitions of a series of chiral 3d-4f dinuclear complexes: X-ray structures of (Λ-Δ)-[(acac)₂Cr^III(μ-ox)Ln ^III(HBpz₃)₂] (Ln = Sm, Ho and Er)

A series of stereospecifically assembled configurational chiral heterometal dinuclear 3d-4f complexes (Λ-Δ)-[(acac)₂-Cr(ox)Ln(HBpz₃) ₂] (acac^- = acetylacetonate, ox^2- = oxalate, HBpz₃^- = hydrotris(pyrazol-1-yl)borate; Ln(III) = La, Nd, Sm, Ho, Er and Tm: (Λ-Δ)-Cr(ox)Ln, where (Λ-Δ) implies the absolute configuration around the Cr and Ln moieties, respectively) was synthesized and three of them (Sm, Ho and Er) were structurally characterized by X-ray analysis. The (Λ-Δ)-Cr(ox)Ln (Ln = Ho, Er and Yb) complexes crystallize in the identical space group P2₁2₁2₁, whereas the (Λ-Δ)-Cr(ox)Sm complex crystallizes in the monoclinic space group P2₁. The absolute configurations of all of the Ln complexes were found to be identical with one another. Near infrared circular dichroism (NIR CD) spectra in the 4f-4f transitions of the (Λ-Δ)-Cr(ox)Ln (Ln = Nd, Sm, Ho, Er and Tm) complexes were studied for the first time in CH₂Cl₂ solution and were compared with those of the previously reported Yb(III) and Dy(III) ions in the (Λ-Δ)-Cr(ox)Ln. The dissymmetry factors g (= Δε/ε) of the NIR CD for the 4f-4f transitions of these complexes were found to be of the order of 10^-3 and 10^-4, confirming the stereospecific formation and retention of the configurational chirality around the Ln(III) ions of the (Δ-Λ)-Cr(ox)Ln complexes in solution. The solution NIR magnetic circular dichroism (MCD) spectra of the Cr(ox)Ln complexes (Ln = Nd, Sm, Dy, Ho, Er and Tm) were also studied in conjunction with the NIR CD spectra.