NaRb 6 (B 4 O 5 (OH) 4 ) 3 (BO 2 ) Featuring Noncentrosymmetry, Chirality, and the Linear Anionic Group BO 2 .

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 [BO 3 ] and tetrahedral [BO 4 ] units and their numerous superstructure motifs. However, no chiral compound with the linear [BO 2 ] unit has been reported to date. Herein, an NCS and chiral mixed-alkali-metal borate, NaRb 6 (B 4 O 5 (OH) 4 ) 3 (BO 2 ), with a linear BO 2 - unit in the structure was synthesized and characterized. The structure features a combination of three types of basic building units (BBUs), [BO 2 ], [BO 3 ], and [BO 4 ] with sp-, sp 2 -, and sp 3 -hybridization of boron atoms, respectively. It crystallizes in the trigonal space group R 32 (No. 155), one of the 65 Sohncke space groups. Two enantiomers of NaRb 6 (B 4 O 5 (OH) 4 ) 3 (BO 2 ) were found, and their crystallographic relationships are discussed. These results not only expand the small family of NCS structures with the rare linear BO 2 - unit but also prompt recognition to the fact that NLO materials have generally overlooked the existence of two enantiomers in achiral Sohncke space groups.