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)2CrIII(μ-ox)LnIII(HBpz3)2] (Ln = Sm, Ho and Er)

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Abstract
A series of stereospecifically assembled configurational chiral heterometal dinuclear 3d–4f complexes (Λ-Δ)-[(acac)2Cr(ox)Ln(HBpz3)2] (acac− = acetylacetonate, ox2− = 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.