Chiral Recognition and Resolution of the Enantiomers of Supramolecular Triangular Hosts: Synthesis, Circular Dichroism, NMR, and X-ray Molecular Structure of [Li⊂(<i>R,R,R</i>)-{Cp*Rh(5-chloro-2,3-dioxopyridine)}<sub>3</sub>][Δ-Trisphat]
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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 [Li⊂{Cp*Rh(5-chloro-2,3-dioxopyridine)}3][OTf] (2). Subsequent anion metathesis using the optically pure chiral shift reagent [Cinchonidinium][Δ-Trisphat] produced a pair of diastereomers [Li⊂(R,R,R)-{Cp*Rh(5-chloro-2,3-dioxopyridine)}3][Δ-Trisphat] (3a) and [Li⊂(S,S,S)-{Cp*Rh(5-chloro-2,3-dioxopyridine)}3][Δ-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 Δ-Trisphat anion and a single enantiomer cation [Li⊂(R,R,R)-{Cp*Rh(5-chloro-2,3-dioxopyridine)}3]+ manifested through a π−π interaction. 1H NMR and circular dichroism studies in solution support the solid-state behavior.
