NaRb₆(B₄O₅(OH)₄)₃(BO₂) Featuring Noncentrosymmetry, Chirality, and the Linear Anionic Group BO₂

Noncentrosymmetric (NCS) structures are of particular interest owing to their symmetry-dependent physical properties, e.g., pyroelectricity, ferroelectricity, piezoelectricity, and nonlinear optical (NLO) behavior. Among them, chiral materials exhibit polarization rotation and host topological properties. Borates often contribute to NCS and chiral structures via their triangular [BO₃] and tetrahedral [BO₄] units and their numerous superstructure motifs. However, no chiral compound with the linear [BO₂] unit has been reported to date. Herein, an NCS and chiral mixed-alkali-metal borate, NaRb₆(B₄O₅(OH)₄)₃(BO₂), with a linear BO₂^- unit in the structure was synthesized and characterized. The structure features a combination of three types of basic building units (BBUs), [BO₂], [BO₃], and [BO₄] with sp-, sp²-, and sp³-hybridization of boron atoms, respectively. It crystallizes in the trigonal space group R32 (No. 155), one of the 65 Sohncke space groups. Two enantiomers of NaRb₆(B₄O₅(OH)₄)₃(BO₂) were found, and their crystallographic relationships are discussed. These results not only expand the small family of NCS structures with the rare linear BO₂^- unit but also prompt recognition to the fact that NLO materials have generally overlooked the existence of two enantiomers in achiral Sohncke space groups.