Glacial acetic acid as a resolution solvent for growing enantiopure crystals from racemic mixtures

Abstract

Chirality is pervasive in nature, yet chiral separation continues to be a real challenge. Herein we report the synthesis and structure of [Pd₄S₂(dppm)₃(CN^tBu)₂][H(OAc)₂]₂ (abbre. Pd₄S₂, HOAc = acetic acid), with an intrinsically chiral Pd–S core. The as-prepared clusters are racemates crystallized in the centric space group P2₁/n or as twins of acentric space groups P4₃2₁2 and P4₁2₁2, both with co-crystallized acetic acid molecules. Surprisingly, when re-crystallized from glacial acetic acid, optically pure enantiomeric crystals of P4₃2₁2 or P4₁2₁2 were obtained simultaneously, with the same amount in one pot. In this regard, glacial acetic acid functions both as a recrystallization solvent and as a resolution agent. It also effectively eliminates twinning. This unexpected finding suggests an easy and economical way to separate a racemic mixture into enantiomers which may find applications in chiral separation technologies.