Preparation and characterization of new chiral pyrrolyl α-nitronyl nitroxide radicals in which the imidazolyl framework was directly bound to chiral center

摘要

Two pairs of nitronyl nitroxide radicals with specific molecular structures in which the imidazole radical moiety was directly bound to their chiral center have been prepared in their enantiopure forms. In these specific molecular structures, chirality may directly affect the unpaired spin electronic cloud. The X-ray crystal structures of them have been solved, which reveal that supermolecule helical chains are formed by intermolecular hydrogen bond interactions in the l-NNP crystal and the l-NNNBP is a 3D supermolecule structure shaped by means of intermolecular hydrogen bond interactions to link homochiral chains. Circular dichroism spectroscopy of the two pairs of chiral α-nitronyl nitroxides shows significant Cotton effects between 200 and 400 nm. The magnetic properties of the two pairs of nitronyl nitroxide radicals in the solid state were characterized by EPR spectroscopy and magnetic susceptibility measurements, respectively. Magnetic susceptibility measurements of the solids reveal antiferromagnetic interactions in all cases. However, in the l-NNNBP, the exchange interaction abruptly changes from untiferromagnetic to a ferromagnetic interaction at 40.0 k.