5-Fluoro-1-Methyl-Pyrazol-4-yl-Substituted Nitronyl Nitroxide Radical and Its 3d Metal Complexes: Synthesis, Structure, and Magnetic Properties

The metal-radical approach is a well-established synthetic way toward multi-spin systems that relies on the coordination of stable radical ligands with transition metal ions. The advantage offered by the use of paramagnetic ligands is that metal-radical magnetic exchange coupling is direct between the magnetic orbitals of the radical and metal ion. With the aim of further exploring this approach, crystals of four heterspin complexes, [M(hfac)2LF]2 {M = Mn, Co, or Ni and hfac = hexafluoroacetylacetonate} and [Cu(hfac)2LF]n, were obtained using a new fluorinated pyrazolyl-substituted nitronyl nitroxide radical, 4,4,5,5-tetramethyl-2-(5-fluoro-1-methyl-1H-pyrazol-4-yl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LF) as a ligand. The newly synthesized complexes were fully characterized, including X-ray crystallography and magnetometry. XRD analysis revealed that complexes [M(hfac)2LF]2 have similar dimer structures in which a metal ion is in a six-coordinated environment with four O atoms from the two hfac ligands, one radical O atom, and one pyrazole N atom from ligand LF. Nonetheless, the packing patterns of the complexes were found to be considerably different. In [Mn(hfac)2LF]2, there are no magnetically important short contacts between manganese dimers. By contrast, in [Co(hfac)2LF]2 and [Ni(hfac)2LF]2, there are short contacts between non-coordinate O atoms of nitronyl nitroxide moieties. Magnetic behaviors of [M(hfac)2LF]2 showed that the M ions and the directly coordinated radicals are strongly antiferromagnetically coupled (JMn-ON = -84.1 +/- 1.5 cm-1, JCo-ON = -134.3 +/- 2.6 cm-1, and JNi-ON = -276.2 +/- 2.1 cm-1; H<^>=-2JS ?<^>MS ?<^>NO). Notably, the magnetization of [Mn(hfac)2LF]2 having molecular structure proved to be accompanied by hysteresis. The [Cu(hfac)2LF]n complex has a chain-polymer structure with alternating magnetic fragments: three spin exchange clusters {ONO-Cu(II)-ONO} and {Cu(II)} ions. Despite the direct coordination of radicals, its magnetic properties are weakly ferromagnetic (JCu-ON = 14.8 +/- 0.3 cm-1).