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

The design of noncentrosymmetric (NCS) solid state materials,specificallyhow to break inversion symmetry between enantiomers, has intriguedchemists, physicists, and materials scientists for many years. Becausethe chemical complexity of molecular racemic building units is sovaried, targeting these materials is poorly understood. Previously,three isostructural racemic compounds with a formula of [Cu(H2O)(bpy)(2)](2)[MF6](2)& BULL;2H(2)O (bpy = 2,2'=bipyridine; M = Ti, Zr,Hf) were shown to crystallize in the NCS space group Pna2(1), of polar, achiral crystal class mm2. In this work, we synthesized five new racemic compounds with theformula [Cu(H2O)(dmbpy)(2)](2)[MF6](2)& BULL;xH(2)O (dmbpy= 4,4 & PRIME;/5,5 & PRIME;-dimethyl-2,2 & PRIME;-bipyridine; M = Ti,Zr, Hf). Single crystal X-ray diffraction reveals that the five newlysynthesized compounds feature equimolar combinations of & UDelta;- and?-Cu(dmbpy)(2)(H2O)(2+) complexesthat are assembled into packing motifs similar to those found in thereported NCS structure but all crystallize in centrosymmetric (CS)space groups. Seven structural descriptors were created to analyzethe intermolecular interactions on the assembly of Cu racemates inthe CS and NCS structures. The structural analysis reveals that inthe CS structures, the inversion center results from parallel heterochiral & pi;-& pi; stacking interactions between adjacent Cu racematesregardless of cation geometries, hydrogen bonding networks, or interlayerarchitectures, whereas in the NCS structure, nonparallel heterochiral & pi;-& pi; interactions between the adjacent Cu racematespreclude an inversion center. The parallel heterochiral & pi;-& pi;interactions in the CS structures can be rationalized by the restrainedgeometries of the methyl-substituted ligands. This work demonstratesthat the introduction of nonparallel stacking can suppress the formationof an inversion center for an NCS racemate. A conceptual frameworkand practical approach linking the absence of inversion symmetry inracemates is presented for all NCS crystal classes.