Racemic and enantiomerically pure phenyl α-nitronyl nitroxide radicals:: influence of chirality on solution and solid state properties

Two chiral phenyl alpha-nitronyl nitroxidesdouble dagger substituted in the aromatic ring with a lactate moiety have been prepared in their enantiopure and racemic forms. The X-ray crystal structures of the racemic methyl ester and both the racemic and enantiopure carboxylic acid have been solved. The ester forms chains of molecules linked by weak hydrogen bonds, while the racemic acid also forms chains, but linked by strong hydrogen bonds between the acid OH group and one of the nitroxide oxygen atoms. In stark contrast, the racemic acid forms cyclic hydrogen bonded dimers by virtue of the same interaction. In addition, these dimers are observed in solution state for both enantiopure and racemic acids as witnessed by EPR spectroscopy. The ground state of the cyclic aggregates appears to be a triplet with a very weak magnetic interaction. Circular dichroism spectroscopy of the chiral compound reveals additional evidence for dimer formation, while LDI-TOF mass spectrometry indicates the presence of chains in amorphous films of the compounds. Magnetic susceptibility measurements of the solids reveal antiferromagnetic interactions in all cases: in the racemic ester and enantiopure acid they are weak, while in the racemic acid they are strong as a result interactions between cyclic dimers.