IRON(II) COMPLEXES OF ORTHO-FUNCTIONALIZED P-NAPHTHOQUINONES .2. CRYSTAL AND MOLECULAR-STRUCTURE OF BIS(AQUO)BIS(LAWSONATO)IRON(II) AND INTERMOLECULAR MAGNETIC EXCHANGE INTERACTIONS IN BIS(3-AMINOLAWSONATO)IRON(II)

Two related complexes [Fe(lawsonato)2(H2O)2] (1) and [Fe(3-aminolawsonato)2(CH3OH)] (2), wherein lawsonato is the anion of 2-hydroxy-1,4-naphthoquinone, have been synthesized and studied. 1 crystallizes in the monoclinic system, space group P21/c with Z = 2, a = 5.036 (3) Å, b = 16.448 (3) Å, c = 10.710 (3) Å, and β = 99.75 (4)°. The structure was solved by the heavy-atom method and refined to conventional agreement indices R = 0.051 and Rw = 0.057 with 1444 unique reflections for which I > 2σ. The three-dimensional network of the crystal results from the linking of infinite chains of [Fe(lawsonato)2(H2O)2] molecules through hydrogen bonds. These chains consist of complex molecules doubly hydrogen-bonded along the [100] direction. Each complex molecule comprises two bidentate lawsone ligands coordinated to the ferrous iron center in their fully oxidized p-quinone form and two trans water molecules. The resulting coordination sphere can be described as a rhombically distorted octahedron. The synthesis, IR, UV-visible, and Mössbauer spectra, and thermogravimetric, variable-temperature magnetic susceptibility, and cyclic voltammetry studies of 2 are described. Both 3-aminolawsone ligands are in their fully oxidized form, and the coordinating centers are the C(2) phenolate oxygen and C(3) amino nitrogen atoms. Variable-temperature magnetic susceptibility studies establish the presence of isotropic magnetic exchange interactions between the high-spin iron(II) centers of a dimeric species (7 = −1.6 cm-1) and crystalline field anisotropy of the ferrous ion (D = 4.9 cm−1). The dimeric structural arrangement of 2 implies a N2O4 ligand environment holding two p-naphthoquinone moieties at ca. 4 Å from the ferrous center and can therefore serve as a basis for studying the effect of quinone oxidation state changes on electronic and magnetic properties of the iron(II)-quinone couple in the “ferroquinone complex” of photosynthetic bacteria and photosystem 2. © 1990, American Chemical Society. All rights reserved.