Radical-substituted dihydrophenazine radical cation salts: Molecular packing structure and bulk magnetic property

Radical-substituted radical cation salts are exotic species that are potentially applicable as spin building blocks for molecular magnets. We recently found that these species, which are derived from a diphenyldihydrophenazine (DPP) framework, are stable under aerated conditions at room temperature. Of these species, nitronyl nitroxide (NN^•)-substituted DPP^•+ tetrachloroferrate (NNDPP^••+_·FeCl₄^–) showed antiferromagnetic interaction at low temperature (<150 K), whereas the tetrabromoferrate salt NNDPP^••+_·FeBr₄^– exhibited a magnetic phase transition at 6.7 K to produce a bulk ferrimagnet. Both salts had very similar molecular structures. The difference in the magnetic properties was ascribed to the difference in molecular packing structures. A significant difference in these two salts was observed at the (NNDPP^••+)–(NNDPP^••+) intermolecular contact, including the oxygen atom of the nitroxide moiety; NNDPP^••+·FeCl₄^– had a serious antiferromagnetic O–O (nitroxide oxygen, 3.02 Å) intermolecular contact, whereas NNDPP^••+·FeBr₄^– had a ferromagnetic O–HC (2.53 Å) intermolecular contact.