Beyond π–π Stacking: Understanding Inversion Symmetry Breaking in Crystalline Racemates

Abstract

The design of noncentrosymmetric (NCS) solid state materials, specifically how to break inversion symmetry between enantiomers, has intrigued chemists, physicists, and materials scientists for many years. Because the chemical complexity of molecular racemic building units is so varied, targeting these materials is poorly understood. Previously, three isostructural racemic compounds with a formula of [Cu(H2O)(bpy)2]2[MF6]2·2H2O (bpy = 2,2’=bipyridine; M = Ti, Zr, Hf) were shown to crystallize in the NCS space group Pna21, of polar, achiral crystal class mm2. In this work, we synthesized five new racemic compounds with the formula [Cu(H2O)(dmbpy)2]2[MF6]2·xH2O (dmbpy = 4,4′/5,5′-dimethyl-2,2′-bipyridine; M = Ti, Zr, Hf). Single crystal X-ray diffraction reveals that the five newly synthesized compounds feature equimolar combinations of Δ- and Λ-Cu(dmbpy)2(H2O)2+ complexes that are assembled into packing motifs similar to those found in the reported NCS structure but all crystallize in centrosymmetric (CS) space groups. Seven structural descriptors were created to analyze the intermolecular interactions on the assembly of Cu racemates in the CS and NCS structures. The structural analysis reveals that in the CS structures, the inversion center results from parallel heterochiral π–π stacking interactions between adjacent Cu racemates regardless of cation geometries, hydrogen bonding networks, or interlayer architectures, whereas in the NCS structure, nonparallel heterochiral π–π interactions between the adjacent Cu racemates preclude an inversion center. The parallel heterochiral π–π interactions in the CS structures can be rationalized by the restrained geometries of the methyl-substituted ligands. This work demonstrates that the introduction of nonparallel stacking can suppress the formation of an inversion center for an NCS racemate. A conceptual framework and practical approach linking the absence of inversion symmetry in racemates is presented for all NCS crystal classes.

Highly Birefringent d0 Transition Metal Fluoroantimonite in the Mid Infrared Band: Order–Disorder Regulation by Cationic Size

“…In this work, the structures of A 2 SbMoO 2 F 7 (A = K, Rb, Cs) were constituted by the same functional units, namely, SbF 5 square pyramids and MoO 2 F 4 octahedrons. However, due to the different size of alkali metals, the MoO 2 F 4 octahedrons undergo a disorder to order transition in O/F occupation . Triggered by the cationic size, the structure of A 2 SbMoO 2 F 7 changed greatly from Rb 2 SbMoO 2 F 7 ( Pnma ) to K 2 SbMoO 2 F 7 ( Pbcm ) (Figure ).…”

“…However, due to the different size of alkali metals, the MoO 2 F 4 octahedrons undergo a disorder to order transition in O/F occupation. 50 Triggered by the cationic size, the structure of A 2 SbMoO 2 F 7 changed greatly from Rb 2 SbMoO 2 F 7 (Pnma) to K 2 SbMoO 2 F 7 (Pbcm) (Figure 4). To illuminate the impact of the structure on performance, we calculated the dipole moments of single chains in the unit cell of A 2 SbMoO 2 F 7 (A = K, Rb, Cs) (Table S4).…”

Dual-Anion Strategy Induces Dual Enhancement Toward Ultrashort Phase-Matching Wavelength in Deep-UV Transparent d0 Transition Metal Oxyfluorides

“…The anionic groups containing d 0 transition metals with second-order Jahn–Teller (SOJT) distortions are one of the most commonly used NLO-active groups in visible light region, as the out-of-center distortions (e.g., NbO 6 in LN and TiO 6 in KTP) arising from metal dπ–oxygen pπ orbital interactions have the potential to induce noncentrosymmetric (NCS) structures and generate strong SHG intensities. − The SOJT distortion also leads to increased polarization anisotropy, making birefringence capable of better PM ability . Extensive attempts including Nb 2 O 3 (IO 3 ) 2 (SO 4 ) (6 × KDP, Δ n = 0.22), LiZrTeO 6 (2.5 × KDP, Δ n = 0.06), BaTeMo 2 O 9 (15 × KDP), BaTeW 2 O 9 (12.5 × KDP), K­(VO) 2 O 2 (IO 3 ) 3 (20 × KDP), BaNbO­(IO 3 ) 5 (14 × KDP), and Cs 3 Nb 5 GeO 16 (Δ n = 0.19) demonstrate success in the visible to IR light regions.…”

Chiral and Polar Duality Design of Heteroanionic Compounds: Sr18Ge9O5S31 Based on [Sr3OGeS3]2+ and [Sr3SGeS3]2+ Groups

“…gesting that different types of racemic compounds are produced when the enantiomers exhibit distinct symmetries and emphasizing the importance of nonparallel packing of ligands. [6] This study provides valuable insights for achieving NCS racemates by breaking the inversion symmetry between enantiomers.…”