Diastereoisomeric charge-transfer complexes. Measurement of thermodynamic constants by ¹H nuclear magnetic resonance spectroscopy

The addition of (-)-7,7′-bismethoxycarbonyl[6]helicene to a solution of (±)-2-(2,4,5,7-tetranitrofluoren-9-ylideneamino-oxy)propionic acid methyl ester (TAPM) results in the formation of two diastereoisomeric charge-transfer complexes. In the ¹H n.m.r. spectrum, this shifts the TAPM signals upfield and simultaneously splits them into separate groups of peaks. The chemical shifts relative to pure TAPM of solutions containing different helicene : TAPM ratios led to the determination of the equilibrium constants for the formation of the two complexes at room temperature. These measurements were then extended to a ca. 100°C temperature range, and from that the enthalpies and enthropies of a complex formation were obtained. It is shown that while at low temperatures, the (-)(+)-complex is more stable than the (-)(-)-one, this is reversed at higher temperatures with a crossover point at 16°C. These results are discussed by comparison with the chromatographic separation of helicene enantiomers.