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

Area of Science:

• Solid-state chemistry and materials science
• Crystallography
• Coordination chemistry

Background:

• Designing noncentrosymmetric (NCS) solid-state materials is challenging due to the complex nature of racemic building units.
• Previous work identified NCS structures in [Cu(H2O)(bpy)2]2[MF6]2·2H2O compounds, crystallizing in the NCS space group Pna21.
• Understanding how to break inversion symmetry in enantiomers is crucial for developing materials with specific properties.

Purpose of the Study:

• To synthesize and characterize new racemic copper complexes with dimethyl-bipyridine ligands.
• To investigate the structural factors influencing the formation of centrosymmetric (CS) versus noncentrosymmetric (NCS) crystal structures.
• To develop a framework for controlling inversion symmetry in NCS materials.

Main Methods:

• Synthesis of five new racemic compounds: [Cu(H2O)(dmbpy)2]2[MF6]2·xH2O.
• Single crystal X-ray diffraction to determine crystal structures.
• Analysis of intermolecular interactions using seven structural descriptors.

Main Results:

• Five new compounds crystallized in centrosymmetric (CS) space groups, unlike previously reported NCS analogs.
• CS structures were characterized by parallel heterochiral π-π stacking interactions between adjacent copper (Cu) racemates.
• NCS structures featured nonparallel heterochiral π-π stacking, which precluded the formation of an inversion center.

Conclusions:

• The geometry of methyl-substituted ligands, specifically dmbpy, favors parallel π-π stacking, leading to CS structures.
• Introducing nonparallel stacking interactions is a key strategy to suppress inversion centers and achieve NCS racemates.
• A conceptual framework is presented for designing NCS materials by controlling intermolecular stacking.