Racemic Tartrate/Malate Anions Combine with Racemic Complex Cations to Form Optically Active Ionic Crystals.

Abstract

Spontaneous resolution has attracted continuing attention in various research fields since Pasteur's work on the crystallization behavior of racemic tartrate. Here, a unique example of this phenomenon is reported, involving ionic crystals generated from racemic RR/SS- tartrate or R/S-malate and racemic ΔΔ/ΛΛ-[Ag3 Rh2 (2-aminoethanethiolato)6 ]3+ (ΔΔ/ΛΛ-[1]3+ ) in water. RR- and SS-tartrate selectively recognize the ΛΛ and ΔΔ isomers of [1]3+ to produce ionic crystals of (ΛΛ-[1])2 (RR-tartrate)3 and (ΔΔ-[1])2 (SS-tartrate)3 , respectively, which can undergo spontaneous resolution. While spontaneous resolution also occurs when using R/S-malate, R- and S-malate select the opposite isomers of [1]3+ to give ionic crystals of (ΔΔ-[1])2 (R-malate)3 and (ΛΛ-[1])2 (S-malate)3 , respectively. In the presence of S-aspartate, (ΛΛ-[1])2 (R-tartrate)3 and (ΔΔ-[1])2 (S-tartrate)3 are preferentially crystallized from ΔΔ/ΛΛ-[1]3+ and RR/SS-tartrate at solution pH values of 6 and 10, respectively. This finding provides significant insight into the optical resolution of chemical species by spontaneous resolution and the origin of homochirality in nature.