Diversity of the coordination modes of Croconate Violet. Crystal structures, spectroscopic characterization and redox studies of mono-, di- and poly-nuclear iron(II) complexes

The crystal structures of three iron(II) complexes with Croconate Violet [3,5-bis(dicyanomethylene)cyclopentane-1,2,4-trionate dianion = L2-] have been determined: K2[FeL2(H2O)4]1, {[Fe(2,2-bipy)L(H2O)2]·H2O}22 and {[FeL(H2O)2]·2H2O}n3. Complex 1 consists of dianionic pseudo-octahedral mononuclear entities containing two N-mono-coordinated ligands in trans positions. Complex 2 consists of dimeric entities with metal atoms bridged by two N-coordinated Croconate Violet ligands. Complex 3 has a polymeric structure with both N- and O-coordinated Croconate Violet ligands: each iron(II) atom is bonded to three different Croconate Violet dianions and each ligand bonds three metal atoms. The magnetic properties have been investigated in the 2-300 K temperature range for complexes 2 and 3. Magnetic susceptibility measurements for these two complexes show the presence of a long distance magnetic coupling (ferromagnetic coupling in 2 with J=+0.18 cm^-1 and antiferromagnetic coupling in 3 with J=-0.19 cm^-1). The redox properties of the three complexes in dmf solution are compared to those of the free ligand.