Chiral recognition and resolution of the enantiomers of supramolecular triangular hosts:: Synthesis, circular dichroism, NMR, and X-ray molecular structure of [Li⊂(R,R,R)-{Cp*Rh(5-chloro-2,3-dioxopyridine)}3][Δ-trisphat]

The racemic triangular supramolecular host [Cp*Rh(5-chloro-2,3-dioxopyridine)](3) (1) was prepared in high yield. Treatment with LiCl followed by addition of silver salt AgOTf gave the triflate salt species [Lisubset of{Cp*Rh(5-chloro2,3-dioxopyridine)}(3)][OTf] (2). Subsequent anion metathesis using the optically pure chiral shift reagent [Cinchonidinium][Delta-Trisphat] produced a pair of diastereomers [Lisubset of(R,R,R)-{Cp*Rh(5-chloro-2,3-dioxopyridine)}(3)][Delta-Trisphat] (3a) and [Lisubset of(SSS)-{Cp*Rh(5-chloro-2,3-dioxopyridine)}(3)][Delta-Trisphat] (3b). The resolution of these diastereomers was achieved by fractional crystallization, and their stereochemistry relationship was established by circular dichroism studies. The X-ray molecular structure of 3a is reported and shows as an outstanding feature a chiral recognition between the Delta-Trisphat anion and a single enantiomer cation [Lisubset of(RRR)-{Cp*Rh(5-chloro-2,3-dioxopyridine)}(3)](+) manifested through a pi-pi interaction. H-1 NMR and circular dichroism studies in solution support the solid-state behavior.