A pair of dinuclear Re(i) enantiomers: synthesis, crystal structures, chiroptical and ferroelectric properties

Abstract

The reaction of enantiomeric bis-bidentate bridging ligands (+)/(−)-2,5-bis(4,5-pinene-2-pyridyl)pyrazine (L_S/L_R) with [Re(CO)₅Cl] yielded a pair of dinuclear Re(I) enantiomers formulated as [Re₂(L_S/L_R)(CO)₆Cl₂]·4CH₂Cl₂ (R-1 and S-1, the isomers containing the respective L_R and L_S ligands). They were characterized by elemental analyses, IR spectra and X-ray crystallography. Circular dichroism spectra verified their chiroptical activities and enantiomeric nature. The measurements of second harmonic generation (SHG) and ferroelectric properties showed that R-1 displays a nonlinear optical (NLO) activity and ferroelectricity with a remnant polarization (P_r) of 1.6 μC cm⁻² under an applied field of 7.3 kV cm⁻¹ at room temperature. R-1 and S-1 represent the first example of polynuclear Re(I) complexes with ferroelectric properties. Notably, the P_r value is much larger than that of the reported mononuclear chiral Re(I) analogue. In particular, unlike mononuclear Re(I) complexes of the type [Re(CO)₃(N^N)(X)] (N^N = diimine and X = halide), which usually exhibit an intense emission in the visible range, R-1 and S-1 do not show any detectable emission at any temperature range and the reason for the nonluminescence of R-1 and S-1 was further elucidated in this work. Moreover, our research results also elucidated that Re nuclearity has a great influence on not only the emitting properties but also on ferroelectric behavior.