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-Δ and Λ 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 Å, respectively, bonds that are unusually short and indicate a ¹A_1g low-spin ground state for iron(II). A sample of 1 prepared with iron-57 has been studied by Mössbauer 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 β₁ = 0.85 ± 0.1, log β₂ = 3.55 ± 0.15, and log β₃ = 6.36 ± 0.15. Both 1 and 2 exhibit irreversible oxidation of iron(II) at ∼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.