Self-assembled Tetranuclear Eu<sup>III</sup> Complexes with <i>D</i><sub>2</sub>- and <i>C</i><sub>2h</sub>-Symmetrical Square Scaffold
Yan Bing Tan, Mihoko Yamada, Shohei Katao, Yoshiko Nishikawa, Fumio Asanoma, Junpei Yuasa, Tsuyoshi Kawai
Abstract
Three new tetranuclear europium(III) alternating circular helicates, (2-PhRRRR/2-PhSSSS) [(R)- or (S)-Ph-Pybox]4EuIII4(BPP)6, (2-iPrRRRR/2-iPrSSSS) [(R)- or (S)-iPr-Pybox]4EuIII4(BPP)6, and (3-PhRRRR/3-PhSSSS) [(R)- or (S)-Ph-Pybox]4EuIII4(BHP)6, are presented with their structural and chiroptical characterization (BPP and BHP = bis-β-diketonates; Pybox = chiral bis(oxazolinyl) pyridine ligand). X-ray crystallographic analysis revealed that different extents of interligand-contacting interactions between bis-β-diketonates and Pybox ligands produce different bis-β-diketonates arrangements around four EuIII ions and, thus, their specific symmetry in the final tetranuclear complexes. 2-PhRRRR/2-PhSSSS and 3-PhRRRR/3-PhSSSS are recognized as D2-symmetry, while 2-iPrRRRR/2-iPrSSSS self-assemblies possess pseudo C2h-symmetry. Due to different molecular symmetry, 2-PhRRRR/2-PhSSSS and 2-iPrRRRR/2-iPrSSSS crystals display different ligands orientation in their EuIII coordination spheres. The presence of pseudo σh-mirror symmetry in 2-iPrRRRR/2-iPrSSSS promotes a pair of distinguishable EuIII geometries. All the chiral self-assemblies exhibit almost identical photoluminescence emission spectra patterns of f–f transitions in the EuIII core. The EuIII self-assemblies exhibit intense CPL with different observed |glum| values (2-PhRRRR/2-PhSSSS ∼ |0.31|, 2-iPrRRRR/2-iPrSSSS ∼ |0.08|, 3-PhRRRR/3-PhSSSS ∼ |0.31| in chloroform).