Dinuclear enantiopure Ln3+ complexes with (S-) and (R-) 2-phenylbutyrate ligands. Luminescence, CPL and magnetic properties

Abstract

The reaction of Ln(NO₃)₂·6H₂O (Ln = Nd, Sm, Eu, Tb, Dy, Tm and Yb) with the respective enantiopure (R)-(−)-2-phenylbutyric or (S)-(+)-2-phenylbutyric acid (R/S-2-HPhBut) and 4,7-diphenyl-1,10-phenanthroline (Bphen) allows the isolation of chiral dinuclear compounds of the formula [Ln₂(μ-R/S-2-PhBut)₄(R/S-2PhBut)₂(Bphen)₂] where Ln = Nd³⁺ (R/S-Nd-a), Sm³⁺ (R/S-Sm-a), Eu³⁺ (R/S-Eu-a), Tb³⁺ (R/S-Tb-a and R/S-Tb-b), Dy³⁺ (R/S-Dy-a and R/S-Dy-b), Tm³⁺ (R/S-Tm-b) and Yb³⁺ (R/S-Yb-b). Single crystal X-ray diffraction was performed for compounds S-Eu-a and S-Tm-b. Powder crystal X-ray diffraction was performed for all complexes. From the crystallographic data two different structural motifs were found which are referred to as structure type a and structure type b. In structure type a, the Ln³⁺ atoms are bridged through four R or S-2-PhBut ligands with two different kinds of coordination modes whereas in structure type b the two Ln³⁺ atoms are bridged through four R or S-2-PhBut ligands showing only one kind of coordination mode. For those lanthanide ions exhibiting both structure types, Tb³⁺ and Dy³⁺, a difference in the luminescence and magnetism behavior is observed. All compounds (except R/S-Tm-b) exhibit sensitized luminescence, notably the Eu³⁺ and Tb³⁺ analogues. Circular Dichroism (CD) and Circular Polarized Luminescence (CPL) in the solid state and in 1 mM dichloromethane (DCM) solutions are reported, leading to improved chiroptical properties for the DCM solutions. The asymmetry factor (g_lum) in 1 mM DCM is ±0.02 (+ for R-Eu-a) for the magnetically allowed transition ⁵D₀ → ⁷F₁ and ±0.03 (+ for R-Tb-a and R-Tb-b) for the ⁵D₄ → ⁷F₅ transition. Magnetic properties of all compounds were studied and the Dy³⁺ compound with the structural motif b (R-Dy-b) shows Single Molecular Magnet (SMM) behavior under a 0 T magnetic field. However, R-Dy-a is a field-induced SMM.