Wang Yiran, Nisbet Matthew L, Kamp Kendall R, Hiralal Emily, Gautier Romain, Halasyamani P Shiv, Poeppelmeier Kenneth R
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States.
Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, Nates cedex 3 F-44000, France.
J Am Chem Soc. 2023 Aug 2;145(30):16879-16888. doi: 10.1021/jacs.3c05380. Epub 2023 Jul 24.
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(HO)(bpy)][MF]·2HO (bpy = 2,2'=bipyridine; M = Ti, Zr, Hf) were shown to crystallize in the NCS space group 2, of polar, achiral crystal class 2. In this work, we synthesized five new racemic compounds with the formula [Cu(HO)(dmbpy)][MF]·HO (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)(HO) 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.
非中心对称（NCS）固态材料的设计，特别是如何打破对映体之间的反演对称性，多年来一直吸引着化学家、物理学家和材料科学家。由于分子外消旋构建单元的化学复杂性差异很大，因此对这些材料的研究还很有限。此前，三种化学式为[Cu(HO)(bpy)][MF]·2HO（bpy = 2,2'-联吡啶；M = Ti、Zr、Hf）的同构外消旋化合物被证明在极性非手性晶体类2的NCS空间群2中结晶。在这项工作中，我们合成了五种化学式为[Cu(HO)(dmbpy)][MF]·HO（dmbpy = 4,4'/5,5'-二甲基-2,2'-联吡啶；M = Ti、Zr、Hf）的新外消旋化合物。单晶X射线衍射表明，这五种新合成的化合物具有等量的Δ-和Λ-Cu(dmbpy)(HO)配合物组合，它们组装成的堆积模式与报道的NCS结构中的相似，但都在中心对称（CS）空间群中结晶。创建了七个结构描述符来分析CS和NCS结构中Cu外消旋体组装过程中的分子间相互作用。结构分析表明，在CS结构中，无论阳离子几何形状、氢键网络或层间结构如何，相邻Cu外消旋体之间平行的异手性π-π堆积相互作用导致了对称中心的形成；而在NCS结构中，相邻Cu外消旋体之间不平行的异手性π-π相互作用排除了对称中心的存在。CS结构中平行的异手性π-π相互作用可以通过甲基取代配体的受限几何形状来解释。这项工作表明，引入不平行堆积可以抑制NCS外消旋体对称中心的形成。针对所有NCS晶体类提出了一个将外消旋体中反演对称性缺失联系起来的概念框架和实用方法。
