Glacial Acetic Acid as Resolution Solvent for Growing Enantiopure Crystals from Racemic Mixtures

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(CNtBu)2][H(OAc)2]2 (abbre. Pd4S2, HOAc = acetic acid), with an intrinsically chiral Pd-S core. As prepared clusters are racemates crystallized in 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 conglomerates which may find applications in chiral separation technology.