Enantiopure Crystallization From Achiral Building Blocks Under Supercooled Conditions.

Area of Science:

• Crystallization science
• Materials science
• Chemical synthesis

Background:

• Homochirality is crucial for abiogenesis and asymmetric synthesis but challenging to achieve without external influences.
• Spontaneous mirror-symmetry breaking (SMSB) is a key process in the origin of homochirality.

Purpose of the Study:

• To investigate melt supercooling as a solvent- and template-free strategy for producing homochiral materials.
• To elucidate the role of thermodynamic and kinetic factors in SMSB during crystallization.

Main Methods:

• Melt crystallization of (BTBA)FeCl4 supramolecular salt polymorphs at a cooling rate of 3600 K h⁻¹.
• Differential scanning calorimetry (DSC) to analyze phase transitions.
• Nucleation experiments at varying degrees of supercooling.

Main Results:

• Both conglomerate and achiral polymorphs of (BTBA)FeCl4 exclusively converted to homochiral forms (100% 1-P3₁ or 1-P3₂) upon melt crystallization.
• Racemic mixtures formed an intermediate achiral polymorph (3) before transformation, while the achiral polymorph (2) transformed directly.
• Increased supercooling suppressed secondary nucleation, allowing a single chiral nucleus to dominate crystal growth.

Conclusions:

• Melt supercooling is an effective strategy for accessing homochiral materials, bypassing the need for solvents or templates.
• The balance between thermodynamic stability and kinetic control, particularly suppressed diffusion at deeper supercooling, governs SMSB.