Synthesis and characterization of two intensely colored tris(benzoylcyanoxime)iron(II) anionic complexes

Two intensely blue-colored complexes, P(C6H5)(4)[Fe(BCO)(3)] (1) and Na[Fe(BCO)(3)] (2), where BCO- is the benzoylcyanoxime anion, have been prepared and characterized in solution and in the solid state. The crystal structure of 1 has been determined at several temperatures (100, 155, 225, and 293 K) and consists of layers of P(C6H5)(4)(+) cations and [Fe(BCO)(3)](-) anions. The latter exist as a pair of fac-Delta and Lambda enantiomers in a monoclinic unit cell in the P2(1)/n space group. Iron(II) has a trigonal-prismatic N3O3 coordination environment with average Fe-N and Fe-O bond distances of 1.866 and 1.956 angstrom, respectively, bonds that are unusually short and indicate a (1)A(1g) low-spin ground state for iron(II). A sample of 1 prepared with iron-57 has been studied by Mossbauer spectroscopy between 4.2 and 430 K and found to be low-spin iron(II) in studied temperature range. The stepwise formation constants for 1 in aqueous solution at 296 K and pH of 7 are log beta(1) = 0.85 +/- 0.1, log beta(2) = 3.55 +/- 0.15, and log beta(3) = 6.36 +/- 0.15. Both 1 and 2 exhibit irreversible oxidation of iron(II) at similar to 1.0 V, indicating a significant degree of the ligand-to-iron charge transfer. Thus, 1 and 2 are rare examples of highly colored iron(II) anionic complexes that do not contain aromatic heterocyclic amine ligands, such as bipyridine or phenanthroline.