Abstract:

Three pairs of N₂O₂-type Schiff base ligands were synthesized by condensing dehydroacetic acid (dha) with chiral 1, 2-diaminopropane (pn), trans-1, 2-diaminocyclohexane (chxn), and 1, 2-diphenylethylenediamine (dpen). These chiral ligands were used to coordinate copper(Ⅱ) ions to produce the corresponding Schiff base Cu(Ⅱ) complexes:[Cu(dha-R/S-pn)] (1a and 1b), [Cu(dha-R, R/S, S-chxn)] (2a and 2b), and[Cu(dha-R, R/S, S-dpen)] (3a and 3b). Detailed analyses using electronic circular dichroism (ECD) and vibrational circular dichroism (VCD) spectroscopies reveal that these Schiff base Cu(Ⅱ) complexes retain the main coordination modes and the absolute configurations of the metal centers, both in solution and the solid state. In addition, according to the crystal structures, the central Cu(Ⅱ) ions of 2a/2b and 3a/3b were found to not only coordinate to the chiral dha-en ligands, but were also axially coordinated to the carbonyl groups of the contiguous lactonic rings, providing one-dimensional supramolecular helical chains through self-assembly. In this work, we deeply studied the relationship between the chiral coordination units and the supramolecular helical structures of 2a/2b and 3a/3b. By comparing our experiment VCD spectroscopic data with related VCD spectral features reported in the literature, a specific correlation between the VCD spectral properties and absolute configurations was investigated, which provided fingerprint characteristics for chiral coordination structure.

Key words: Schiff base Cu(Ⅱ) complex, Electronic circular dichroism, Vibrational circular dichroism, Single-crystal structure, Supramolecular chiral assembly