Polymorphs of a pyrazole nitronyl nitroxide and its complexes with metal(II) hexafluoroacetylacetonates

The synthesis of pyrazol-4-yl nitronyl nitroxide is reported, along with the crystal structures of its polymorphs and coordination compounds with nickel(II) and cobalt( II) hexafluoroacetylacetonate. The polymorphic forms of the pure radical show very different magnetic properties: the alpha form shows only antiferromagnetic interactions while those of the beta form reveal competing ferro- and anti-ferromagnetic interactions. This phenomenon can be traced to the relative orientations of the spin carriers in the crystal. These, in turn, are determined in large measure by the hydrogen bonding networks, where a competition between [N - H center dot center dot center dot O] and [N - H center dot center dot center dot N] hydrogen bonds is observed. This delicate balance is also seen in the coordination chemistry of the radical acting as a ligand for the hexafluoroacetylacetonate complexes of nickel( II) and cobalt( II). In the former, a 2 : 1 radical : metal complex is formed because of coordination of the pyrazolyl nitrogen atom only. In the latter, a cyclic dimer is formed which involves both the nitrogen and oxygen as coordinating atoms. In both cases, the NH groups of the pyrazolyl moiety lead to the formation of hydrogen bonded networks in the crystals. The magnetic properties of the complexes reveal antiferromagnetic interactions. However, the pyrazolyl nitronyl nitroxide has been proven an interesting component for the formation of coordination compounds with hydrogen bonds in between the magnetic components.