Extending Wallach's Rule: Quasi-Racemate Stability in Liquid Crystalline Metal Complexes

Wallach's rule states that racemic crystals are generally denser and thermodynamically stable than enantiopure forms. To test this principle in quasi-racemic liquid crystalline systems, we examined the thermal stability of Δ-Ru-C8 and Λ-Ir-C8 mixtures at various ratios. The quasi-racemate, 1:1 molar mixture of Δ-Ru-C8 and Λ-Ir-C8, exhibited the highest transition temperature and enthalpy among the mixed samples, confirming its superior stability. In contrast, mixtures with an excess of either enantiomer lost order and became unstable. Quasi-enantiomers with different alkyl chain lengths (Λ-Ru-C8 and Δ-Ru-C9) also assembled cooperatively, yielding a cubic liquid crystal phase. However, mixtures of the same optical isomers of Ru-C8 and Ir-C8-that is, Δ-Ru-C8/Δ-Ir-C8 and Λ-Ru-C8/Λ-Ir-C8-became amorphous and gradually crystallized over a month without forming liquid crystalline phases. These results demonstrate steric recognition between Δ and Λ isomers and extend Wallach's rule to quasi-racemic liquid crystalline materials.