Six- and seven-coordinate Fe(II) and Zn(II) compounds ligated by unsymmetric xanthene-based ligands: Characterization and magnetic properties

The synthesis of a set of novel unsymmetric ligands, iXa and iXa-2, and of their corresponding Fe^II and Zn^II complexes are described. The ligands incorporate a redox-active α-diimine moiety alongside (2-pyridylmethyl)amino groups. Electrochemical data highlight ligand-centered reduction processes available in these systems. The molecular structures of the series of six- and seven-coordinate metal complexes reveal a pentagonal bipyramidal geometry for all compounds. The ligands and complexes were further characterized in the solution state by NMR spectroscopy, with assignments supported by DFT calculations. The ferrous-iXa and -iXa-2 complexes have a high spin, S = 2 configuration. Magnetic susceptibility studies reveal the unusual low-temperature ferromagnetism of these complexes. Intermolecular interactions between the iron centers are mediated by the PF₆ ^- counterions, as evidenced by ³¹P-NMR studies.